Abstract:
A method for data communication from a first node to a second node over a data channel coupling the first node and the second node includes receiving data messages at the second node, the messages belonging to a set of data messages transmitted in a sequential order from the first node, sending feedback messages from the second node to the first node, the feedback messages characterizing a delivery status of the set of data messages at the second node, including maintaining a set of one or more timers according to occurrences of a number of delivery order events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of delivery order events, and deferring sending of said feedback messages until expiry of one or more of the set of one or more timers.

Description:
BACKGROUND 
       [0001]    This document relates to protocols for communicating over data networks, and more specifically, in at least some examples, to the use of packet coding based protocols for communication over packet switched networks, for instance, over the Internet. 
         [0002]    Data communication has benefited from the near-universal use of the Internet Protocol (IP) on the interconnection of networks that form the Internet. The endpoints of communication connections or sessions set-up over the Internet may include servers, which may be in data centers co-located on “backbones” of the Internet, user devices on wired or wireless local area networks, and mobile devices on various generations of cellular telephone technology (e.g. 3G, 4G, LTE). Local area networks may be coupled to high-speed backbones of the Internet via facilities of Internet Service Providers (ISPs), with “last mile” technologies ranging from digital subscriber loop (DSL) to hybrid-fiber coax to all-optical networks. In some cases, networks may include satellite communication links which may have very different delay characteristics than, for example, terrestrial optical networks. 
         [0003]    The communications paths that data packets follow in travelling from where they originate to their destination(s) may typically traverse multiple different types of links and/or networks. Each link and/or network may be supported by operating equipment such as servers, buffers, transmission links and the like, and may be characterized by parameters such as capacity, congestion, delay, packet loss, timing of data transfer and the like. Furthermore, transition points, also sometimes referred to as “peering points” between types of networks may impose some restrictions on how data may flow through the networks. 
         [0004]    In addition to characteristics that are inherent in the network designs, policy characteristics imposed by network operators may affect how traffic flows across networks. For example, certain types of traffic and/or connections may be prioritized and potentially assigned more resources, while other types of traffic may be throttled or blocked. Assigned resources and/or policies may be variable and may change throughout the day, throughout the year, based on congestion, service agreements and the like. 
         [0005]    The vast majority of connection-based or session-based traffic on the Internet today makes use of the Transmission Control Protocol (TCP). TCP is popular at least in part because it generally provides reliable and fair delivery of data. That is, the information that is sent by a sender is eventually received at a receiver and no one sender that adheres to generally adopted fairness aspects of Internet protocols can utilize more than their fair share of the bandwidth on average. However, even though TCP has evolved over the last decades, there are aspects of the protocol that are not well matched to the characteristics, and more particularly to the variability of characteristics, of today&#39;s Internet. As examples, primary reliance on retransmission and use of windowing algorithms for congestion control is not always well matched to the real-time requirements and dynamic nature of communication channels that may have relatively rapidly varying characteristics, for example, periodic delay of the delivery of packets or rapidly changing link capacity. 
         [0006]    As a result, applications running over today&#39;s Internet may be plagued by long delays in transferring large data files, by pauses or interruptions in video or audio streaming, by low audio or picture quality and/or by slow response times for real-time interactive content. These problems may be accompanied by and/or the result of an inefficient use of the underlying network bandwidth due to overly restrictive congestion control and/or to the large numbers of end-to-end packet retransmissions. 
         [0007]    One technology that has been proposed to address some of the limitations of TCP for communication over today&#39;s Internet is Random Linear Network Coding (RLNC), which involves a combination of using random linear codes for error correction and recoding of packets at intermediate nodes in the network. However, RLNC alone has not provided a complete solution to limitations imposed by network characteristics. Other proposed technologies based on new codes, forward error correction codes, data encryption techniques, and the like, also have not been shown to provide complete solutions. Therefore, there is a need for a new protocol to ensure high-speed uninterrupted delivery of data packets over networks that comprises many different types of equipment, operated by many different operators, over many different types of wired and wireless links. 
         [0008]    Also proposed has been the use of the user datagram protocol (UDP), which can speed up data delivery times but at the expense of reliable data delivery. While some users and/or applications may be able to tolerate lost and/or out-of-order data packets at a receiver, network operators have been known to impose policies that limit or block the amount of UDP traffic that may flow over their networks at any given time. These restrictions are at least partially motivated by the fact that many of the current proprietary protocols running over UDP are believed to be unfair, meaning they may consume as much bandwidth and/or network resources as they can in order to deliver their data very quickly. 
         [0009]    Thus there is a need for a new protocol that can reliably deliver data packets over today&#39;s Internet faster than TCP but not at the expense of fairness. 
       SUMMARY 
       [0010]    In a general aspect, a method for data communication from a first node to a second node over a data channel coupling the first node and the second node includes receiving data messages at the second node, the messages belonging to a set of data messages transmitted in a sequential order from the first node, sending feedback messages from the second node to the first node, the feedback messages characterizing a delivery status of the set of data messages at the second node, including maintaining a set of one or more timers according to occurrences of a number of delivery order events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of delivery order events, and deferring sending of said feedback messages until expiry of one or more of the set of one or more timers. 
         [0011]    Aspects may include one or more of the following features. 
         [0012]    The set of one or more timers may include a first timer and the first timer may be started upon detection of a first delivery order event, the first delivery order event being associated with receipt of a first data message associated with a first position in the sequential order prior to receipt of one or more missing messages associated with positions preceding the first position in the sequential order. The method may include sending the feedback messages indicating a successful delivery of the set of data messages at the second node upon detection of a second delivery order event, the second delivery order event being associated with receipt of the one or more missing messages prior to expiry of the first timer. The method may include sending said feedback messages indicating an unsuccessful delivery of the set of data messages at the second node upon expiry of the first timer prior to any of the one or more missing messages being received. The set of one or more timers may include a second timer and the second timer is started upon detection of a second delivery order event, the second delivery order event being associated with receipt of some but not all of the missing messages prior to expiry of the first timer. The method may include sending feedback messages indicating an unsuccessful delivery of the set of data messages at the second node upon expiry of the second timer prior to receipt of the missing messages. The method may include sending feedback messages indicating a successful delivery of the set of data messages at the second node upon detection of a third delivery order event, the third delivery order event being associated with receipt of the missing messages prior to expiry of the second timer. 
         [0013]    In another general aspect, a system for data communication over a data channel coupling a number of nodes includes a second node of the number of nodes configured to receive data messages, the data messages belonging to a set of data messages transmitted in a sequential order from a first node, send feedback messages to the first node, the feedback messages characterizing a delivery status of the set of data messages at the second node, including maintaining a set of one or more timers according to occurrences of a number of delivery order events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of delivery order events, and deferring sending of said feedback messages until expiry of one or more of the set of one or more timers. 
         [0014]    In another general aspect, software stored on non-transitory computer-readable media including instructions for causing a second node in a data communication system to receive data messages at the second node, the messages belonging to a set of data messages transmitted in a sequential order from the first node, send feedback messages from the second node to the first node, the feedback messages characterizing a delivery status of the set of data messages at the second node, including maintaining a set of one or more timers according to occurrences of a number of delivery order events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of delivery order events, and deferring sending of said feedback messages until expiry of one or more of the set of one or more timers. 
         [0015]    In another general aspect, a method for data communication from a first node to a second node over a data channel coupling the first node and the second node includes receiving, at the first node, feedback messages indicative of a delivery status of a set of data messages transmitted in a sequential order to the second node from the second node, maintaining a size of a congestion window at the first node including maintaining a set of one or more timers according to occurrences of a number of feedback events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of feedback events, and delaying modification of the size of the congestion window until expiry of one or more of the set of one or more timers. 
         [0016]    Aspects may include one or more of the following features. 
         [0017]    The set of one or more timers may include a first timer and the first timer may be started upon detection of a first feedback event, the first feedback event being associated with receipt of a first feedback message indicating successful delivery of a first data message having first position in the sequential order prior to receipt of one or more feedback messages indicating successful delivery of one or more other data messages having positions preceding the first position in the sequential order. The method may include cancelling modification of the congestion window upon detection of a second feedback event, the second feedback event being associated with receipt of one or more feedback messages indicating successful delivery of the one or more other data messages prior to expiry of the first timer. The method may include modifying the congestion window upon expiry of the first timer prior to receipt of any feedback message indicating successful delivery of the one or more other data messages. 
         [0018]    The set of one or more timers may include a second timer and the second timer may be started upon detection of a third feedback event, the third feedback event being associated with receipt of one or more feedback messages indicating successful delivery of some but not all of the one or more other data messages prior to expiry of the first timer. The method may include modifying the size of the congestion window upon expiry of the second timer prior to receipt of one or more feedback messages indicating successful delivery of the one or more other data messages. The method may include cancelling modification of the size of the congestion window upon detection of a fourth feedback event, the fourth feedback event being associated with receipt one or more feedback messages indicating successful delivery of the one or more other data messages prior to expiry of the second timer. 
         [0019]    In another general aspect, a system for data communication between a number of nodes over a data channel coupling the number of nodes includes a first node of the number of nodes configured to receive, at the first node, feedback messages indicative of a delivery status of a set of data messages transmitted in a sequential order to the second node from the second node, maintain a size of a congestion window at the first node including maintaining a set of one or more timers according to occurrences of a number of feedback events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of feedback events, and delaying modification of the size of the congestion window until expiry of one or more of the set of one or more timers. 
         [0020]    In another general aspect, software stored on non-transitory computer-readable media includes instructions for causing a first node in a data communication system to receive, at the first node, feedback messages indicative of a delivery status of a set of data messages transmitted in a sequential order to the second node from the second node, maintain a size of a congestion window at the first node including maintaining a set of one or more timers according to occurrences of a number of feedback events, the maintaining including modifying a status of one or more timers of the set of timers based on occurrences of the number of feedback events, and delaying modification of the size of the congestion window until expiry of one or more of the set of one or more timers. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is a schematic of a data network including server and client nodes coupled by intermediate networks; 
           [0022]      FIG. 2  is a block diagram illustrating the modules that implement TCP-based communication between a client node and a server node; 
           [0023]      FIG. 3  is a block diagram illustrating the modules that implement Packet Coding Transmission Communication Protocol (PC-TCP) based communication between a client node and a server node; 
           [0024]      FIG. 4  is a schematic diagram of a use of the approach shown in  FIG. 3  for communication between a server and a module device on a cellular network; 
           [0025]      FIG. 5  is a block diagram of a PC-TCP module that uses a conventional UDP module; 
           [0026]      FIG. 6  is a block diagram of a PC-TCP module that is partially integrated into a client application and partially implemented using a conventional UDP module; 
           [0027]      FIG. 7  is a block diagram of a PC-TCP module that is split with user space and kernel space components; 
           [0028]      FIG. 8  is a block diagram of a proxy architecture; 
           [0029]      FIG. 9  is a block diagram of a PC-TCP based proxy architecture in which a proxy node communicates using both PC-TCP and conventional TCP; 
           [0030]      FIG. 10  is a block diagram of a proxy-based architecture of  FIG. 9  embodied using a gateway device; 
           [0031]      FIG. 11  is a block diagram of an alternative proxy architecture embodied within a client node; 
           [0032]      FIG. 12  is a block diagram of a second PC-TCP based proxy architecture in which a proxy node communicates using both PC-TCP and conventional TCP; 
           [0033]      FIG. 13  is a block diagram of a proxy-based architecture of  FIG. 12  embodied using a wireless access device; 
           [0034]      FIG. 14  is a block diagram of a proxy-based architecture of  FIG. 12  embodied cellular network; 
           [0035]      FIG. 15  is a block diagram of a proxy-based architecture of  FIG. 12  embodied cable television based data network; 
           [0036]      FIG. 16  is a block diagram of an intermediate proxy that communicates with a client node and with a server node using separate PC-TCP connections; 
           [0037]      FIG. 17  is a block diagram of a proxy-based architecture of  FIG. 16  embodied in a network device; 
           [0038]      FIG. 18  is a block diagram of an intermediate proxy that recodes communication between a client node and with a server node; 
           [0039]      FIGS. 19-20  are diagrams that illustrates delivery of common content to multiple destinations; 
           [0040]      FIGS. 21A-K  are schematic diagrams of various embodiments of PC-TCP communication approaches; 
           [0041]      FIG. 22  is a block diagram of PC-TCP communication approach that includes window and rate control modules;  FIG. 23  is a schematic of a data network. 
           [0042]      FIGS. 24-27  are block diagrams illustrating an embodiment PC-TCP communication approach that is configured according to a number of tunable parameters; 
           [0043]      FIG. 28  is a diagram showing a network communication system using the approach of  FIGS. 24-27 ; 
           [0044]      FIG. 29  is a schematic diagram illustrating use of stored communication parameters; and 
           [0045]      FIG. 30  is a schematic diagram illustrating a first embodiment of multi-path content delivery. 
           [0046]      FIG. 31 a   - FIG. 31 c    are schematic diagrams illustrating a second embodiment of multi-path content delivery. 
       
    
    
       [0047]      
         [0000]    
       
         
               
             
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
             
           
               
                   
               
               
                 Table of Contents 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Background 
                 1 
               
               
                 Description of Drawings 
                 3 
               
               
                 Table of Contents 
                 8 
               
               
                 Detailed Description 
                 9 
               
             
          
           
               
                  1 
                 Overview 
                 9 
               
               
                  2 
                 Architectures and applications 
                 12 
               
             
          
           
               
                   
                 2.1 
                 Transport layer architectures 
                 12 
               
             
          
           
               
                   
                 2.1.1 
                 Kernel implementation 
                 12 
               
               
                   
                 2.1.2 
                 Alternative software implementations 
                 13 
               
             
          
           
               
                   
                 2.2 
                 Proxy architectures 
                 14 
               
             
          
           
               
                   
                 2.2.1 
                 Conventional proxy node 
                 14 
               
               
                   
                 2.2.2 
                 First alternative proxy node 
                 15 
               
               
                   
                 2.2.3 
                 Integrated proxy 
                 16 
               
               
                   
                 2.2.4 
                 Second alternative proxy node 
                 16 
               
             
          
           
               
                   
                 2.3 
                 Intermediate proxy 
                 18 
               
               
                   
                 2.4 
                 Recoding node 
                 19 
               
               
                   
                 2.5 
                 Multipath transmission 
                 20 
               
             
          
           
               
                   
                 2.5.1 
                 Single endpoint pair 
                 20 
               
               
                   
                 2.5.2 
                 Distributed source 
                 22 
               
               
                   
                 2.5.3 
                 Distributed content delivery 
                 22 
               
             
          
           
               
                   
                 2.6 
                 Multicast 
                 23 
               
               
                   
                 2.7 
                 Further illustrative examples 
                 24 
               
             
          
           
               
                  3 
                 Packet Coding (PC) 
                 27 
               
             
          
           
               
                   
                 3.1 
                 Data characteristics 
                 28 
               
               
                   
                 3.2 
                 Channel Characteristics 
                 29 
               
               
                   
                 3.3 
                 Inter-packet coding 
                 31 
               
             
          
           
               
                   
                 3.3.1 
                 Forward error correction and repair retransmission 
                 31 
               
               
                   
                 3.3.2 
                 Random linear coding 
                 33 
               
             
          
           
               
                   
                 3.4 
                 Batch transmission 
                 35 
               
               
                   
                 3.5 
                 Protocol parameters 
                 42 
               
               
                   
                 3.6 
                 Transmission control 
                 43 
               
             
          
           
               
                   
                 3.6.1 
                 Congestion control 
                 43 
               
               
                   
                 3.6.2 
                 Transmission rate control 
                 44 
               
             
          
           
               
                   
                 3.7 
                 Error control 
                 49 
               
             
          
           
               
                   
                 3.7.1 
                 Packet reordering 
                 51 
               
               
                   
                 3.7.2 
                 Acknowledgements 
                 52 
               
             
          
           
               
                   
                 3.8 
                 Parameter control 
                 53 
               
             
          
           
               
                   
                 3.8.1 
                 Initialization 
                 53 
               
               
                   
                 3.8.2 
                 Tunable coding 
                 53 
               
               
                   
                 3.8.3 
                 Cross-session parameter control 
                 58 
               
             
          
           
               
                   
                 3.9 
                 Multi-path 
                 61 
               
             
          
           
               
                  4 
                 Alternatives and implementations 
                 64 
               
             
          
           
               
                 What is claimed is: 
                 69 
               
               
                 Abstract 
                 74 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
     1 Overview 
       [0048]    Various embodiments described in this document relate to communication protocols that improve aspects of communication between nodes on a data network. These aspects include, for instance, average, worst case, or variability in communication delay, channel utilization, and/or error rate. These embodiments are primarily described in the context of packet switched networks, and more particularly in the context of Internet Protocol (IP) based packet switched networks. However, it should be understood that at least some of the embodiments are more generally applicable to data communication that does not use packet switching or IP, for instance based on circuit-switched of other forms of data networks. 
         [0049]    Furthermore, various embodiments are described in the context of data being sent from a “server” to a “client.” It should be understood that these terms are used very broadly, roughly analogous to “data source” and “data destination”. Furthermore, in at least some applications of the techniques, the nodes are peers, and may alternate roles as “server” and “client” or may have both roles (i.e., as data source and data destination) concurrently. However, for the sake of exposition, examples where there is a predominant direction of data flow from a “server” node to a “client” node are described with the understanding that the techniques described in these examples are applicable to many other situations. 
         [0050]    One example for a client-server application involves a server passing multimedia (e.g., video and audio) data, either recorded or live, to a client for presentation to a user. Improved aspects of communication from the client to the server in such an example can reduced communication delay, for instance providing faster startup, reduced instances of interrupted playback, reduced instances of bandwidth reduction, and/or increased quality by more efficient channel utilization (e.g., by avoiding use of link capacity in retransmissions or unnecessary forward error correction). This example is useful for exposition of a number of embodiments. However, it must be recognized that this is merely one of many possible uses of the approached described below. 
         [0051]      FIG. 1  shows a high-level block diagram of some components that may be interconnected on a portion of a data network. A general example of a communication connection or session arranged on today&#39;s Internet may be represented as a client node  120  (e.g., a client computer) communicating with a server node  110  (e.g., a server computer) over one network or an interconnection of multiple networks  151 - 152 . For example, the client and server nodes may communicate over the public Internet using the Internet Protocol (IP). 
         [0052]    Referring to  FIG. 2 , in an example involving conventional communication techniques, a client node  120  hosts a client application  222 , which communicates with a TCP module  226  that implements a Transmission Control Protocol (TCP). The TCP module  226  communicates with an IP module  228  that implements an Internet Protocol for communicating between nodes on the interconnection of networks. The communication passes between nodes of the networks over a channel  230  (i.e., an abstraction of the path comprising physical links between equipment interconnecting the nodes of the network). Similarly, the server node  110  hosts a server application  212 , a TCP module  216 , and an IP module  218 . When the server application  110  and the client application  222  communicate, for example, with data being passed from the server application to the client application, TCP module  216  at the server node  110  and the TCP layer  226  at the client node  120  interact to implement the two endpoints for the Transmission Control Protocol (TCP). 
         [0053]    Generally, data units  201  (e.g., encoding of multimedia frames or other units of application data) generated by the server application  212  are passed to the TCP module  216 . The TCP module assembles data payloads  202 , for example, concatenating multiple data units  201  and/or by dividing data units  201  into multiple data payloads  202 . In the discussion below, these payloads are referred to in some instances as the “original” or “uncoded” “packets” or original or uncoded “payloads”, which are communicated to the client (i.e., destination) node in the network. Therefore, it should be understood that the word “packet” is not used with any connotation other than being a unit of communication. In the TCP embodiment illustrated in  FIG. 2 , each data payload  202  is “wrapped” in a TCP packet  204 , which is passed to the IP module  218 , which further wraps the TCP packet  204  in an IP packet  206  for transmission from the server node  110  to the client node  120 , over what is considered to be a IP layer channel  230  linking the server node  110  and the client node  120 . Note that at lower layers, such as at a data link layer, further wrapping, unwrapping, and/or rewrapping of the IP packet  206  may occur, however, such aspects are not illustrated in  FIG. 2 . Generally, each payload  202  is sent in at least one TCP packet  204  and a corresponding IP packet  206 , and if not successfully received by the TCP module  226  at the client node  120 , may be retransmitted again by the TCP module  216  at the server node  110  to result in successful delivery. The data payloads  202  are broken down into the data units  201  originally provided by the server application  212  and are then delivered in the same order to the client application  222  as they were provided by the server application  212 . 
         [0054]    TCP implements a variety of features, including retransmission of lost packets, maintaining order of packets, and congestion control to avoid congestion at nodes or links along the path through the network and to provide fair allocation of the limited bandwidth between and within the networks at intermediate nodes. For example, TCP implements a “window protocol” in which only a limited number (or range of sequence numbers) of packets are permitted to be transmitted for which end-to-end acknowledgments have not yet been received. Some implementations of TCP adjust the size of the window, for example, starting initially with a small window (“slow start”) to avoid causing congestion. Some implementations of TCP also control a rate of transmission of packets, for example, according to the round-trip-time and the size of the window. 
         [0055]    The description below details one or more alternatives to conventional TCP-based communication as illustrated in  FIG. 2 . In general, these alternatives improve one or more performance characteristics, for examples, one or more of overall throughput, delay, and jitter. In some applications, these performance characteristics are directly related to application level performance characteristics, such as image quality in a multimedia presentation application. Referring to  FIG. 1 , in a number of examples, these alternatives are directed to improving communication between a server node  110  and at least one client node  120 . One example of such communication is streaming media from the server node  110  to the client nodes  120 , however, it should be recognized that this is only one of many examples where the described alternatives can be used. 
         [0056]    It should also be understood that the network configuration illustrated in  FIG. 1  is merely representative of a variety of configurations. A number of these configurations may have paths with disparate characteristics. For example, a path from the server node  110  to a client node  120  may pass over links using different types of equipment and with very different capacities, delays, error rates, degrees of congestion etc. In many instances, it is this disparity that presents challenges to achieving end-to-end communication that achieves high rate, low delay and/or low jitter. As one example, the client node  120  may be a personal communication device on a wireless cellular network, the network  152  in  FIG. 1  may be a cellular carrier&#39;s private wired network, and network  151  may be the public Internet. In another example, the client node  120  may be a “WiFi” node of a private wireless local area network (WLAN), network  152  may be a private local area network (LAN), and network  151  may be the public Internet. 
         [0057]    A number of the alternatives to conventional TCP make use of a Packet Coding (PC) approach. Furthermore, a number of these approaches make use of Packet Coding essentially at the Transport Layer. Although different embodiments may have different features, these implementations are generically referred to below as Packet Coding Transmission Control Protocol (PC-TCP). Other embodiments are also described in which the same or similar PC approaches are used at other layers, for instance, at a data link layer (e.g., referred to as PC-DL), and therefore it should be understood that in general features described in the context of embodiments of PC-TCP may also be incorporated in PC-DL embodiments. 
         [0058]    Before discussing particular features of PC-TCP in detail, a number of embodiments of overall system architectures are described. The later description of various embodiments of PC-TCP should be understood to be applicable to any of these system architectures, and others. 
       2 Architectures and Applications 
     2.1 Transport Layer Architectures 
     2.1.1 Kernel Implementation 
       [0059]    Referring to  FIG. 3 , in one architecture, the TCP modules at the server node  110  and the client node  120  are replaced with PC-TCP modules  316  and  326 , respectively. Very generally, the PC-TCP module  316  at the server accepts data units  201  from the server application  212  and forms original data payloads  202  (i.e., “uncoded packets”, formed internally to the PC-TCP module  316  and not illustrated). Very generally, these data payloads  202  are transported to and/or reconstructed at the PC-TCP module  326  at the client node  120 , where the data units  201  are extracted and delivered to the client application  222  in the same order as provided by the server application  212 . As described in substantially more detail below, at least some embodiments of the PC-TCP modules make use of Random Linear Coding (RLC) for forming packets  304  for transmission from the source PC-TCP module to the destination PC-TCP module, with each packet  304  carrying a payload  302 , which for at least some packets  304  is formed from a combination of multiple original payloads  202 . In particular, at least some of the payloads  202  are formed as linear combinations (e.g., with randomly generated coefficients in a finite field) of original payloads  202  to implement Forward Error Correction (FEC), or as part of a retransmission or repair approach in which sufficient information is not provided using FEC to overcome loss of packets  304  on the channel  230 . Furthermore, the PC-TCP modules  316  and  326  together implement congestion control and/or rate control to generally coexist in a “fair” manner with other transport protocols, notably conventional TCP. 
         [0060]    One software implementation of the PC-TCP modules  316  or  326 , is software modules that are integrated into the operating system (e.g., into the “kernel”, for instance, of a Unix-based operating system) in much the same manner that a conventional TCP module is integrated into the operating system. Alternative software implementations are discussed below. 
         [0061]    Referring to  FIG. 4 , in an example in which a client node  120  is a smartphone on a cellular network (e.g., on an LTE network) and a server node  110  is accessible using IP from the client node, the approach illustrated in  FIG. 3  is used with one end-to-end PC-TCP session linking the client node  120  and the server node  110 . The IP packets  300  carrying packets  304  of the PC-TCP session traverse the channel between the nodes using conventional approaches without requiring any non-conventional handling between the nodes at the endpoints of the session. 
       2.1.2 Alternative Software Implementations 
       [0062]    The description above includes modules generically labeled “PC-TCP”. In the description below, a number of different implementations of these modules are presented. It should be understood that, in general, any instance of a PC-TCP module may be implemented using any of the described or other approaches. 
         [0063]    Referring to  FIG. 5 , in some embodiments, the PC-TCP module  326  (or any other instance of PC-TCP module discussed in this document) is implemented as a PC-TCP module  526 , which includes a Packet Coding (PC) module  525  that is coupled to (i.e., communicates with) a convention User Datagram Protocol (UDP) module  524 . Essentially each PC-TCP packet described above consists of a PC packet “wrapped” in a UDP packet. The UDP module  524  then communicates via the IP modules in a conventional manner. In some implementations, the PC module  525  is implemented as a “user space” process, which communicates with a kernel space UDP module, while in other implementations, the PC module  525  is implement in kernel space. 
         [0064]    Referring to  FIG. 6 , in some embodiments, the PC module  625 , or its function, is integrated into a client application  622 , which then communicates directly with the conventional UDP module  524 . The PC-TCP module  626  therefore effectively spans the client application  622  and the kernel implementation of the UDP module  524 . While use of UDP to link the PC modules at the client and at the server has certain advantages, other protocols may be used. One advantage of UDP is that reliable transmission through use of retransmission is not part of the UDP protocol, and therefore error handling can be carried out by the PC modules. 
         [0065]    Referring to  FIG. 7 , in some implementations, a PC-TCP module  726  is divided into one part, referred to as a PC-TCP “stub”  727 , which executes in the kernel space, and another part, referred to as the PC-TCP “code”  728 , which executes in the user space of the operating system environment. The stub  727  and the code  728  communicate to provide the functionality of the PC-TCP module. 
         [0066]    It should be understood that these software implementations are not exhaustive. Furthermore, as discussed further below, in some implementations, a PC-TCP module of any of the architectures or examples described in this document may be split among multiple hosts and/or network nodes, for example, using a proxy architecture. 
       2.2 Proxy Architectures 
     2.2.1 Conventional Proxy Node 
       [0067]    Referring to  FIG. 8 , certain conventional communication architectures make use of proxy servers on the communication path between a client node  120  and a server node  110 . For example, a proxy node  820  hosts a proxy server application  822 . The client application  222  communicates with the proxy server application  822 , which acts as an intermediary in communication with the server application  212  (not shown in  FIG. 8 ). It should be understood that a variety of approaches to implementing such a proxy are known. In some implementations, the proxy application is inserted on the path without the client node necessarily being aware. In some implementations, a proxy client  812  is used at the client node, in some cases forming a software “shim” between the application layer and the transport layer of the software executing at the client node, with the proxy client  812  passing communication to the proxy server application. In a number of proxy approaches, the client application  222  is aware that the proxy is used, and the proxy explicitly acts as an intermediary in the communication with the server application. A particular example of such an approach makes use of the SOCKS protocol, in which the SOCKS proxy client application (i.e., an example of the proxy client  812 ) communicates with a SOCKS proxy server application (i.e., an example of the proxy server application  822 ). The client and server may communicate over TCP/IP (e.g., via TCP and IP modules  826   b  and  828   b , which may be implemented together in one TCP module), and the SOCKS proxy server application fulfills communication requests (i.e., with the server application) on behalf of the client application (e.g., via TCP and IP modules  826   a  and  828   a ). Note that the proxy server application may also perform functions other than forwarding communication, for example, providing a cache of data that can be used to fulfill requests from the client application. 
       2.2.2 First Alternative Proxy Node 
       [0068]    Referring to  FIG. 9 , in an alternative proxy architecture, a proxy node  920  hosts a proxy server application  922 , which is similar to the proxy server application  822  of  FIG. 8 . The client application  222  communicates with the proxy server application  922 , for example as illustrated using conventional TCP/IP, and in some embodiments using a proxy client  812  (e.g., as SOCKS proxy client), executing at the client node  120 . As illustrated in  FIG. 9 , the proxy server application  922  communicates with a server application using a PC-TCP module  926 , which is essentially the same as the PC-TCP module  326  shown in  FIG. 3  for communicating with the PC-TCP module  316  at the server node  110 . 
         [0069]    In some embodiments, the communication architecture of  FIG. 9  and the conventional communication architecture of  FIG. 2  may coexist in the communication between the client application and the server application may use PC-TCP, conventional TCP, or concurrently use both PC-TCP and TCP. The communication approach may be based on a configuration of the client application and/or based on dialog between the client and server applications in establishing communication between them. 
         [0070]    Referring to  FIG. 10 , in an example of the architecture shown in  FIG. 9 , the proxy application  922  is hosted in a gateway  1020  that links a local area network (LAN)  1050  to the Internet. A number of conventional client nodes  120   a - z  are on the LAN, and make use of the proxy server application to communicate with one or more server applications over the Internet. Various forms of gateway  1020  may be used, for instance, a router, firewall, modem (e.g., cable modem, DSL modem etc.). In such examples, the gateway  1020  may be configured to pass conventional TCP/IP communication between the client nodes  120   a - z  and the Internet, and for certain server applications or under certain conditions (e.g., determined by the client, the server, or the gateway) use the proxy to make use of PC-TCP for communication over the Internet. 
         [0071]    It should be understood that the proxy architecture shown in  FIG. 9  may be equally applied to server nodes  110  that communicate with a proxy node using TCP/IP, with the proxy providing PC-TCP communication with client nodes, either directly or via client side proxies. In such cases, the proxy server application serving the server nodes may be hosted, for instance, in a gateway device, such as a load balancer (e.g., as might be used with a server “farm”) that links the servers to the Internet. It should also be understood that in some applications, there is a proxy node associated with the server node as well as another proxy associated with the client node. 
       2.2.3 Integrated Proxy 
       [0072]    Referring to  FIG. 11 , in some examples, a proxy server application  1122 , which provides essentially the same functionality as the proxy server application  922  of  FIG. 9 , is resident on the client node  1120  rather than being hosted on a separate network node as illustrated in  FIG. 9 . In such an example, the connection between the client application  222  and the proxy server application  1122  is local, with the communication between them not passing over a data network (although internally it may be passed via the IP software “stack”). For example, a proxy client  812  (e.g., a SOCKS client) interacts locally with the proxy server application  1122 , or the functions of the proxy client  812  and the proxy server application  1122  are integrated into a single software component. 
       2.2.4 Second Alternative Proxy Node 
       [0073]    In examples of the first alternative proxy node approach introduced above, communication between the client node and the proxy node uses conventional techniques (e.g., TCP/IP), while communication between the proxy node and the server node (or its proxy) uses PC-TCP. Such an approach may mitigate congestion and/or packet error or loss on the link between the server node and the proxy node, however, it would not generally mitigate issues that arise on the link between the proxy node and the client node. For example, the client node and the proxy node may be linked by a wireless channel (e.g., WiFi, cellular, etc.), which may introduce a greater degree of errors than the link between the server and the proxy node over a wired network. 
         [0074]    Referring to  FIG. 12 , in a second proxy approach, the client node  120  hosts a PC-TCP module  326 , or hosts or uses any of the alternatives of such a module described in this document. The client application  222  makes use of the PC-TCP module  326  at the client node to communication with a proxy node  1220 . The proxy node essentially translates between the PC-TCP communication with the client node  120  and conventional (e.g., TCP) communication with the server node. The proxy node  1220  includes a proxy server application  1222 , which makes use of a PC-TCP module  1226  to communicate with the client node (i.e., forms transport layer link with the PC-TCP module  326 ) at the client node, and uses a conventional TCP module  826   a  to communicate with the server. 
         [0075]    Examples of such a proxy approach are illustrated in  FIGS. 13-15 . Referring to  FIG. 13 , an example of a proxy node  1220  is integrated in a wireless access device  1320  (e.g., a WiFi access point, router, etc.). The wireless access device  1320  is coupled to the server via a wired interface  1351  and coupled to a wireless client node  120  via a wireless interface  1352  at the access device and a wireless interface  1353  at the client node. The wireless access device  1320  includes a proxy and communication stack implementation  1321 , which includes the modules illustrated for the proxy  1220  in  FIG. 12 , and the wireless client node  120  includes an application and communication stack implementation  1322 , which includes the modules illustrated for the client node  120  in  FIG. 12 . Note that the IP packets  300  passing between the access device  1320  and the client node  120  are generally further “wrapped” using a data layer protocol, for example, in data layer packets  1350 . As introduced above, in some implementations, rather than implementing the Packet Coding at the transport layer, in a modification of the approach shown in  FIG. 13 , the Packet Coding approaches are implemented at the data link layer. 
         [0076]    Referring to  FIG. 14 , a proxy node  1220  is integrated in a node of a private land network of a cellular service provider. In this example, communication between a server  110  and the proxy node  1220  use conventional techniques (e.g., TCP) over the public Internet, while communication between the proxy node and the client node use PC-TCP. It should be understood that the proxy node  1220  can be hosted at various points in the service provider&#39;s network, including without limitation at a gateway or edge device that connects the provider&#39;s private network to the Internet (e.g. a Packet Data Network Gateway of an LTE network), and/or at an internal node of the network (e.g., a serving gateway, base station controller, etc.). Referring to  FIG. 15 , a similar approach may be used with a cable television based network. PC-TCP communication may pass between a head end device and a distribution network (e.g., a fiber, coaxial, or hybrid fiber-coaxial network) to individual homes. For example, each home may have devices that include PC-TCP capabilities themselves, or in some example, a proxy node (e.g., a proxy node integrated in a gateway  1010  as shown in  FIG. 10 ) terminates the PC-TCP connections at each home. The proxy node that communicates with the server  110  using conventional approaches, while communicating using PC-TCP over the distribution network is hosted in a node in the service provider&#39;s private network, for instance at a “head end” device  1220   b  of the distribution network, or in a gateway device  1220   a  that links the service provider&#39;s network with the public Internet. 
       2.3 Intermediate Proxy 
       [0077]    Referring to  FIG. 16 , in another architecture, the channel between a server node and a client node is broken in to independent tandem PC-TCP links. An intermediate node  1620  has two instances of a PC-TCP module  1626  and  1627 . One PC-TCP module  1626  terminates a PC-TCP channel and communicates with a corresponding PC-TCP module at the server (e.g., hosted at the server node or at a proxy associated with the server node). The other PC-TCP module  1627  terminates a PC-TCP channel and communicates with a corresponding PC-TCP module at the client (e.g., hosted at the client node or at a proxy associated with the client node). The two PC-TCP modules  1626  and  1627  are coupled via a routing application  1622 , which passes decoded data units provided by one of the PC-TCP modules (e.g., module  1626  from the server node) and to another PC-TCP module for transmission to the client. 
         [0078]    Note that parameters of the two PC-TCP channels that are bridged at the intermediate node  1620  do not have to be the same. For example, the bridged channels may differ in their forward error correction code rate, block size, congestion window size, pacing rate, etc. In cases in which a retransmission protocol is used to address packet errors or losses that are not correctable with forward error correction coding, the PC-TCP modules at the intermediate node request or service such retransmission requests. 
         [0079]    In  FIG. 16 , only two PC-TCP modules are shown, but it should be understood that the intermediate node  1620  may concurrently provide a link between different pairs of server and client nodes. 
         [0080]    Referring to  FIG. 17 , an example of this architecture may involve a server node  110  communicating with an intermediate node  1620 , for example, hosted in a gateway device  1720  of a service provider network with the intermediate node  1620  also communicating with the client node  120  via a second PC-TCP link. 
       2.4 Recoding Node 
       [0081]    Referring to  FIG. 18 , another architecture is similar to the one shown in  FIG. 16  in that an intermediate node  1820  is on a path between a server node  110  and a client node  120 , with PC-TCP communication passing between it and the server node and between it and the client node. 
         [0082]    In  FIG. 16 , the PC-TCP modules  1626 ,  1627  fully decode and encode the data passing through the node. In the approach illustrated in  FIG. 18 , such complete decoding is not necessary. Rather, a recoding PC-TCP module  1822  receives payloads  1802   a - b  from PC-TCP packets  1804   a - b , and without decoding to reproduce the original uncoded payloads  202  (not shown), the module uses the received PC-TCP packets to send PC-TCP packets  304 , with coded payloads  302 , toward the destination. Details of various recoding approaches are described further later in this document. However, in general, the processing by the recoding PC-TCP module includes one or more of the following functions: forwarding PC-TCP packets without modification to the destination; “dropping” received PC-TCP packets without forwarding, for example, if the redundancy provided by the received packets are not needed on the outbound link; generating and transmitting new PC-TCP packets to provide redundancy on the outbound link. Note that the recording PC-TCP module may also provide acknowledgement information on the inbound PC-TCP link (e.g., without requiring acknowledgement from the destination node), for example, to the server, and process received acknowledgements on the outbound link. The processing of the received acknowledgements may include causing transmission of additional redundant information in the case that the originally provided redundancy information was not sufficient for reconstruction of the payload data. 
         [0083]    In general, the recoding PC-TCP module maintains separate communication characteristics on the inbound and outbound PC-TCP channels. Therefore, although it does not decode the payload data, it does provide control and, in general, the PC-TCP channels may differ in their forward error correction code rate, block size, congestion window size, pacing rate, etc. 
       2.5 Multipath Transmission 
     2.5.1 Single Endpoint Pair 
       [0084]    In examples described above, a single path links the server node  110  and the client node  120 . The possibility of using conventional TCP concurrently with PC-TCP between two nodes was introduced. More generally, communication between a pair of PC-TCP modules (i.e., one at the server node  110  and one at the client node  120 ) may follow different paths. 
         [0085]    Internet protocol itself supports packets passing from one node to another following different paths and possibly being delivered out of order. Multiple data paths or channels can link a pair of PC-TCP modules and be used for a single session. Beyond native multi-path capabilities of IP networks, PC-TCP modules may use multiple explicit paths for a particular session. For example, without intending to be exhaustive, combinations of the following types of paths may be used:
       Uncoded TCP and PC over UDP   PC over conventional TCP and UDP   PC-TCP over wireless LAN (e.g., WiFI, 802.11) and cellular data (e.g., 3G, LTE)   PC-TCP concurrently over multiple wireless base stations (e.g., via multiple wireless LAN access points)       
 
         [0090]    In some examples, Network Coding is used such that the multiple paths from a server node to a client node pass through one or more intermediate nodes at which the data is recoded, thereby causing information for different data units to effectively traverse different paths through the network. 
         [0091]    One motivation for multipath connection between a pair of endpoints addresses possible preferential treatment of TCP traffic rather than UDP traffic. Some networks (e.g. certain public Wi-Fi, cable television networks, etc.) may limit the rate of UDP traffic, or drop UDP packets preferentially compared to TCP (e.g., in the case of congestion). It may be desirable to be able to detect such scenarios efficiently without losing performance. In some embodiments, a PC-TCP session initially establishes and divides the transmitted data across both a TCP and a UDP connection. This allows comparison of the throughput achieved by both connections while sending distinct useful data on each connection. An identifier is included in the initial TCP and UDP handshake packets to identify the two connections as belonging to the same coded PC-TCP session, and non-blocking connection establishment can be employed so as to allow both connections to be opened at the outset without additional delay. The transmitted data is divided across the two connections using e.g. round-robin (sending alternating packets or runs of packets on each connection) or load-balancing/back pressure scheduling (sending each packet to the connection with the shorter outgoing data queue). Such alternation or load balancing can be employed in conjunction with techniques for dealing with packet reordering. Pacing rate and congestion window size can be controller separately for the UDP and the TCP connection, or can be controlled together. By controlling the two connections together (e.g., using only a single congestion window to regulate the sum of the number of packets in flight on both the TCP and UDP connections) may provide a greater degree of “fairness” as compared to separate control. 
         [0092]    In some examples, the adjustment of the fraction of messages transmitted over each data path/protocol is determined according to the relative performance/throughput of the data paths/protocols. In some examples, the adjustment of allocation of messages occurs only during an initial portion of the transmission. In other examples, the adjustment of allocation of messages occurs on an ongoing basis throughout the transmission. In some examples, the adjustment reverses direction (e.g., when a data path stops preferentially dropping UDP messages, the number of messages transmitted over that data path may increase). 
         [0093]    In some embodiments the PC-TCP maintains both the UDP based traffic and the TCP based traffic for the duration of the session. In other embodiments, the PC-TCP module compares the behavior of the UCP and TCP traffic, for example over a period specified in terms of time interval or number of packets, where these quantities specifying the period can be set as configuration parameters and/or modified based on previous coded TCP sessions, e.g. the comparison period can be reduced or eliminated if information on relative TCP/UDP performance is available from recent PC-TCP sessions. If the UDP connection achieves better throughput, the PC-TCP session can shift to using UDP only. If the TCP connection achieves better throughput, the PC-TCP session can shift to using TCP. In some embodiments, different types of traffic are sent over the TCP link rather than the UDP link. In one such example, the UDP connection is used to send some forward error correction for packets where it is beneficial to reduce retransmission delays, e.g. the last block of a file or intermediate blocks of a stream. In this example, the uncoded packets may be sent over a TCP stream with forward error correction packets sent over UDP. If the receiver can use the forward error correction packets to recover from erasures in the TCP stream, a modified implementation of the TCP component of the receiver&#39;s PC-TCP module may be able to avoid using a TCP-based error recovery procedure. On the other hand, non-delivery of a forward error correction packet does not cause an erasure of the data that is to be recovered at the receiver, and therefore unless there is an erasure both on the UDP path and on the TCP path, dropping of a UDP packet does not cause delay. 
       2.5.2 Distributed Source 
       [0094]    In some examples, multiple server nodes communicate with a client node. One way this can be implemented is with there being multiple communication sessions each involving one server node and one client node. In such an implementation, there is little or no interaction between a communication session between one server node and the client node and another communication session between another server node and the client node. In some examples, each server node may have different parts of a multimedia file, with each server providing its parts for combination at the client node. 
       2.5.3 Distributed Content Delivery 
       [0095]    In some examples, there is some relationship between the content provided by different servers to the client. One example of such a relationship is use of a distributed RAID approach in which redundancy information (e.g., parity information) for data units at one or more servers is stored at and provided from another server. In this way, should a data unit not reach the client node from one of the server nodes, the redundancy information may be preemptively sent or requested from the other node, and the missing data unit reconstructed. 
         [0096]    In some examples, random linear coding is performed on data units before they are distributed to multiple server nodes as an alternative to use of distributed RAID. Then each server node establishes a separate communication session with the client node for delivery of part of the coded information. In some of these examples, the server nodes have content that has already been at least partially encoded and then cached, thereby avoiding the necessity of repeating that partial encoding for different client nodes that will received the same application data units. In some examples, the server nodes may implement some of the functionality of the PC modules for execution during communication sessions with client nodes, for example, having the ability to encode further redundancy information in response to acknowledgment information (i.e., negative acknowledgement information) received from a client node. 
         [0097]    In some implementations, the multiple server nodes are content delivery nodes to which content is distributed using any of a variety of known techniques. In other implementations, these multiple server nodes are intermediary nodes at which content from previous content delivery sessions was cached and therefore available without requiring re-delivery of the content from the ultimate server node. 
         [0098]    In some examples of distributed content delivery, each server to client connection is substantially independent, for example, with independently determined communication parameters (e.g., error correction parameters, congestion window size, pacing rate, etc.). In other examples, at least some of the parameters are related, for example, with characteristics determined on one server-to-client connection being used to determine how the client node communicates with other server nodes. For example, packet arrival rate, loss rate, and differences in one-way transmission rate, may be measured on one connections and these parameters may be used in optimizing multipath delivery of data involving other server nodes. One manner of optimization may involve load balancing across multiple server nodes or over communication links on the paths from the server nodes to the client nodes. 
         [0099]    In some implementations, content delivery from distributed server nodes making use of PC-TCP, either using independent sessions or using coordination between sessions, may achieve the performance of conventional distributed content delivery but requiring a smaller number of server nodes. This advantage may arise due to PC-TCP providing lower latency and/or lower loss rates than achieved with conventional TCP. 
       2.6 Multicast 
       [0100]      FIGS. 19-20  show two examples of delivery of common content to multiple destination nodes simultaneously via multicast connections. The advantage of multicast is that a single packet or block of N packets has to be sent by the source node into the network and the network will attempt to deliver the packets to all destination nodes in the multicast group. If the content needs to be delivered reliably, then TCP will most likely be used as the transport layer protocol. To achieve reliability, TCP requires destination nodes to respond with acknowledgments and specify the packets that each destination node is missing. If there are 10s of thousands or 100s of thousands of receivers, and each destination node is missing a different packet or set of packets, the number of different retransmissions to the various receivers will undercut the advantages of the simultaneous transmission of the content to all destination nodes at once. With network coding and forward error correction, a block of N packets can be sent to a large number of multicast destination nodes at the same time. The paths to these multiple destination nodes can be similar (all over a large WiFi or Ethernet local area network) or disparate (some over WiFi, some over cellular, some over fiber links, and some over various types of satellite networks). The algorithms described above that embody transmission and congestion control, forward error correction, sender based pacing, receiver based pacing, stream based parameter tuning, detection and correction for missing and out of order packets, use of information across multiple connections, fast connection start and stop, TCP/UDP fallback, cascaded coding, recoding by intermediate nodes, and coding of the ACKs can be employed to improve the throughput and reliability of delivery to each of the multicast destination node. When losses are detected and coding is used, the extra coded packets can be sent to some or all destination nodes. As long as N packets are received at each destination node, the missing packets at each destination node can be reconstructed from the coded packets if the number of extra coded packets match or exceed the number of packets lost at all of the receivers. If fewer than N packets are received at any of the destination nodes, any set of different coded packets from the block of N packets can be retransmitted and used to reconstruct any missing packet in the block at each of the destination nodes. If some destination nodes are missing more than one packet, then the maximum number of coded packets to be retransmitted will be equal to the largest number of packets that are missing by any of the destination nodes. These few different coded packets can be used to reconstruct the missing packets at each of the destination nodes. For example if the most packets missing at any destination node is four, then any four different coded packets can be retransmitted. 
       2.7 Further Illustrative Examples 
       [0101]      FIGS. 21A-21K  show exemplary embodiments of data communication systems and devices and highlight various ways to implement the novel PC-TCP described herein. These configurations identify some of the possible network devices, configurations, and applications that may benefit from using PC-TCP, but there are many more devices, configurations and applications that may also benefit from PC-TCP. The following embodiments are described by way of example, not limitation. 
         [0102]    In an exemplary embodiment depicted in  FIG. 21A , a user device  404  such as a smartphone, a tablet, a computer, a television, a display, an appliance, a vehicle, a home server, a gaming console, a streaming media box and the like, may include a PC-TCP proxy that may interface with applications running in the user device  404 . The application on the user device  404  may communicate with a resource in the cloud  402   a  such as a server  408 . The server  408  may be a file server, a web server, a video server, a content server, an application server, a collaboration server, an FTP server, a list server, a telnet server, a mail server, a proxy server, a database server, a game server, a sound server, a print server, an open source server, a virtual server, an edge server, a storage device and the like, and may include a PC-TCP proxy that may interface with applications and/or processes running on the server  408 . In embodiments, the server in the cloud may terminate the PC-TCP connection and interface with an application on the server  408  and/or may forward the data on to another electronic device in the network. In embodiments, the data connection may travel a path that utilizes the resources on a number of networks  402   a ,  402   b . In embodiments PC-TCP may be configured to support multipath communication such as for example from a video server  408  through a peering point  406 , though a carrier network  402   b , to a wireless router or access point  410  to a user device  404  and from a video server  408  through a peering point  406 , though a carrier network  402   b , to a cellular base station or cell transmitter  412  to a user device  404 . In embodiments, the PC-TCP may include adjustable parameters that may be adjusted to improve multipath performance. In some instances, the exemplary embodiment shown in  FIG. 21A  may be referred to as an over-the-top (OTT) embodiment. 
         [0103]    In embodiments, such as the exemplary embodiments shown in  FIG. 21B  and  FIG. 21C , other devices in the network may comprise PC-TCP proxies. For example, the wireless access point or router  410  and the base station or cell transmitter  412  may comprise PC-TCP proxies. In embodiments, the user device  404  may also comprise a PC-TCP proxy ( FIG. 21C ) or it may not ( FIG. 21B ). If the user device does not comprise a PC-TCP proxy, it may communicate with the access point  410  and/or base station  412  using a wireless or cellular protocol and/or conventional TCP or UDP protocol. The PC-TCP proxy in either or both the access point  410  and base station  412  may receive data packets using these conventional communications and may convert these communications to the PC-TCP for a connection to video server  408 . In embodiments, if conventional TCP provides the highest speed connection between the end user device  404  and/or the access point  410  or the base station  412 , then the PC-TCP proxy may utilize only some or all of the features in PC-TCP that may be compliant with and may compliment conventional TCP implementations and transmit the data using the TCP layer. 
         [0104]      FIG. 21D  shows an exemplary embodiment where a user device may comprise a PC-TCP proxy and may communicate with a PC-TCP proxy server  408  on an internet. In this embodiment, an entity may provide support for high speed internet connections by renting, buying services from, or deploying at least one server in the network and allowing other servers or end user devices to communicate with it using PC-TCP. The at least one server in the network running PC-TCP may connect to other resources in the network and/or end users using TCP or UDP. 
         [0105]    In embodiments, such as the exemplary embodiments shown in  FIG. 21E  and  FIG. 21F , other devices in the network may comprise PC-TCP proxies. For example, the wireless access point or router  410  and the base station or cell transmitter  412  may comprise PC-TCP proxies. In embodiments, the user device  404  may also comprise a PC-TCP proxy ( FIG. 21F ) or it may not ( FIG. 21E ). If the user device does not comprise a PC-TCP proxy, it may communicate with the access point  410  and/or base station  412  using a wireless or cellular protocol and/or conventional TCP or UDP protocol. The PC-TCP proxy in either or both the access point  410  and base station  412  may receive data packets using these conventional communications and may convert these communications to the PC-TCP for a connection to PC-TCP server  408 . In embodiments, if conventional TCP provides the highest speed connection between the end user device  404  and/or the access point  410  or the base station  412 , then the PC-TCP proxy may utilize only some or all of the features in PC-TCP that may be compliant with and may compliment conventional TCP implementations and transmit the data using the TCP layer. 
         [0106]    In embodiments, at least some network servers  408  may comprise PC-TCP proxies and may communicate with any PC-TCP servers or devices using PC-TCP. In other embodiments, network servers may communicate with PC-TCP servers or devices using conventional TCP and/or other transport protocols running over UDP. 
         [0107]    In exemplary embodiments as depicted in  FIG. 21G , ISPs and/or carriers may host content on one or more servers that comprise PC-TCP proxies. In embodiments, devices such as set-top boxes, cable boxes, digital video recorders (DVRs), modems, televisions, smart televisions, internet televisions, displays, and the like may comprise PC-TCP proxies. A user device  404  such as described above, may include a PC-TCP proxy that may interface with applications running in the user device  404 . The application on the user device  404  may communicate with a resource in the cloud  402   c  such as a server  408 . The server  408  may be any type of communications server as describe above, and may include a PC-TCP proxy that may interface with applications and/or processes running on the server  408 . In embodiments, the server in the cloud may terminate the PC-TCP connection and interface with an application on the server  408  and/or may forward the data on to another electronic device in the network. In embodiments, the data connection may travel a path that utilizes the resources on a number of networks  402   a ,  402   b ,  402   c . In embodiments PC-TCP may be configured to support multipath communication such as for example from a video server  408  through a direct peering point (DP)  406 , to a wireless router or access point  410  or a base station  412  to a user device  404  and from a video server  408  directly to an access point  410  and/or to a cellular base station or cell transmitter  412  to a user device  404 . In embodiments, the PC-TCP may include adjustable parameters that may be adjusted to improve multipath performance. 
         [0108]    The exemplary placements of networking devices in the communication scenarios described above should not be taken as limitations. It should be recognized that PC-TCP proxies can be placed in any network device and may support any type of data connection. That is, any type of end-user device, switching device, routing device, storage device, processing device and the like, may comprise PC-TCP proxies. Also PC-TCP proxies may reside only in the end-nodes of a communication path and/or only at two nodes along a connection path. However, PC-TCP proxies may also reside in more than two nodes of a communication path and may support multi-cast communications and multipath communications. PC-TCP proxies may be utilized in point-to-point communication networks, multi-hop networks, meshed networks, broadcast networks, storage networks, and the like. 
       3 Packet Coding (PC) 
       [0109]    The description above focuses on architectures in which a packet coding approach is deployed, and in particular architectures in which a transport layer PC-TCP approach is used. In the description below, a number of features of PC-TCP are described. It should be understood that in general, unless otherwise indicated, these features are compatible with one another and can be combined in various combinations to address particular applications and situations. 
       3.1 Data Characteristics 
       [0110]    As introduced above, data units (e.g., audio and/or video frames) are generally used to form data packets, for example, with one data unit per data packet, with multiple data units per data packet, or in some instances separating individual data units into multiple data packets. In some applications, the data units and associated data frames form a stream (e.g., a substantially continuous sequence made available over time without necessarily having groupings or boundaries in the sequence), while in other applications, the data units and associated data frames form one or more batches (e.g., a grouping of data that is required as a whole by the recipient). 
         [0111]    In general, stream data is generated over time at a source and consumed at a destination, typically at a substantially steady rate. An example of a stream is a multimedia stream associated with person-to-person communication (e.g., a multimedia conference). Delay (also referred to as latency) and variability in delay (also referred to as jitter) are important characteristics of the communication of data units from a source to a destination. 
         [0112]    An extreme example of a batch is delivery of an entire group of data, for example, a multiple gigabyte sized file. In some such examples, reducing the overall time to complete delivery (e.g., by maximizing throughput) of the batch is of primary importance. One example of batch delivery that may have very sensitive time (and real-time update) restraints is database replication. 
         [0113]    In some applications, the data forms a series of batches that require delivery from a source to a destination. Although delay in start of delivery and/or completion of delivery of a batch of data units may be important, in many applications overall throughput may be most important. An example of batch delivery includes delivery of portions of multimedia content, for instance, with each batch corresponding to sections of viewing time (e.g., 2 seconds of viewing time or 2 MB per batch), with content being delivered in batches to the destination where the data units in the batches are buffered and used to construct a continuous presentation of the content. As a result, an important consideration is the delivery of the batches in a manner than provides continuity between batches for presentation, without “starving” the destination application because a required batch has not arrived in time. In practice, such starving may cause “freezing” of video presentation in multimedia, which is a phenomenon that is all too familiar to today&#39;s users of online multimedia delivery. Another important consideration is reduction in the initial delay in providing the data units of the first batch to the destination application. Such delay is manifested, for example, in a user having to wait for initial startup of video presentation after selecting multimedia for online delivery. Another consideration in some applications is overall throughput. This may arise, for example, if the source application has control over a data rate of the data units, for example, being able to provide a higher fidelity version of the multimedia content if higher throughput can be achieved. Therefore, an important consideration may be providing a sufficiently high throughput in order to enable delivery of a high fidelity version of the content (e.g., as opposed to greatly compressed version or a backed-off rate of the content resulting in lower fidelity). 
         [0114]    Various packet coding approaches described below, or selection of configuration parameters of those approaches, address considerations that are particularly relevant to the nature of the characteristics of the data being transported. In some examples, different approaches or parameters are set in a single system based on a runtime determination of the nature of the characteristics of the data being transported. 
       3.2 Channel Characteristics 
       [0115]    In general, the communication paths that link PC-TCP source and destination endpoints exhibit both relatively stationary or consistent channel characteristics, as well as transient characteristics. Relatively stationary or consistent channel characteristics can include, for example, capacity (e.g., maximum usable throughput), latency (e.g., transit time of packets from source to destination, variability in transit time), error rate (e.g., average packet erasure or error rate, burst characteristics of erasures/errors). In general, such relatively stationary or consistent characteristics may depend on the nature of the path, and more particularly on one or more of the links on the path. For example, a path with a link passing over a 4G cellular channel may exhibit very different characteristics than a path that passes over a cable television channel and/or a WiFi link in a home. As discussed further below, at least some of the approaches to packet coding attempt to address channel characteristic differences between types of communication paths. Furthermore, at least some of the approaches include aspects that track relatively slow variation in characteristics, for example, adapting to changes in average throughput, latency, etc. 
         [0116]    Communication characteristics along a path may also exhibit substantial transient characteristics. Conventional communication techniques include aspects that address transient characteristics resulting from congestion along a communication path. It is well known that as congestion increases, for example at a node along a communication path, it is important that traffic is reduced at that node in order to avoid an unstable situation, for instance, with high packet loss resulting from buffer overruns, which then further increases data rates due to retransmission approaches. One common approach to addressing congestion-based transients uses an adaptive window size of “in flight” packets that have not yet been acknowledged by their destinations. The size of the window is adapted at each of the sources to avoid congestion-based instability, for example, by significantly reducing the size of the window upon detection of increased packet erasure rates. 
         [0117]    In addressing communication over a variety of channels, it has been observed that transients in communication characteristics may not be due solely to conventional congestion effects, and that conventional congestion avoidance approaches may not be optimal or even desirable. Some effects that may affect communication characteristics, and that may therefore warrant adaptation of the manner in which data is transmitted can include one or more of the follow:
       Effects resulting from cell handoff in cellular systems, including interruptions in delivery of packets or substantial reordering of packets delivered after handoff;   Effects resulting from “half-duplex” characteristics of certain wireless channels, for example, in WiFi channels in which return packets from a destination may be delayed until the wireless channel is acquired for upstream (i.e., portable device to access point) communication;   Effects of explicit data shaping devices, for example, intended to throttle certain classes of communication, for instance, based on a service provider&#39;s belief that that class of communication is malicious or is consuming more than a fair share of resources.       
 
         [0121]    Although transient effects, which may not be based solely on congestion, may be tolerated using conventional congestion avoidance techniques, one or more of the approaches described below are particularly tailored to such classes of effects with the goal of maintaining efficient use of a channel without undue “over-reaction” upon detection of a transient situation, while still avoiding causing congestion-based packet loss. 
       3.3 Inter-Packet Coding 
       [0122]    In general, the coding approaches used in embodiments described in this document make use of inter-packet coding in which redundancy information is sent over the channel such that the redundancy information in one packet is generally dependent on a set of other packets that have been or will be sent over the channel. Typically, for a set of N packets of information, a total of N+K packets are sent in a manner that erasure or any K of the packets allows reconstruction of the original N packets of information. In general, a group of N information packets, or a group of N+K packets including redundancy information (depending on context), is referred to below as a “block” or a “coding block”. One example of such a coding includes N information packets without further coding, and then K redundancy packets, each of which depends on the N information packets. However it should be understood more than K of the packets (e.g., each of the N+K packets) may in some embodiments depend on all the N information packets. 
       3.3.1 Forward Error Correction and Repair Retransmission 
       [0123]    Inter-packet coding in various embodiments described in this document use one or both of pre-emptive transmission of redundant packets, generally referred to as forward error correction (FEC), and transmission of redundant packets upon an indication that packets have or have a high probability of having been erased based on feedback, which is referred to below as repair and/or retransmission. The feedback for repair retransmission generally comes from the receiver, but more generally may come from a node or other channel element on the path to the receiver, or some network element having information about the delivery of packets along the path. In the FEC mode, K redundant packets may be transmitted in order to be tolerant of up to K erasures of the N packets, while in the repair mode, in some examples, for each packet that the transmitter believes has been or has high probability of having been erased, a redundant packet it transmitted from the transmitter, such that if in a block of N packets, K packets are believed to have been erased based on feedback, the transmitter sends at least an additional K packets. 
         [0124]    As discussed more fully below, use of a forward error correction mode versus a repair mode represents a tradeoff between use of more channel capacity for forward error correction (i.e., reduced throughout of information) versus incurring greater latency in the presence of erasures for repair retransmission. As introduced above, the data characteristics being transmitted may determine the relative importance of throughput versus latency, and the PC-TCP modules may be configured or adapted accordingly. 
         [0125]    If on average the packet erasure rate E is less than K/(N+K), then “on average” the N+K packets will experience erasure of K or fewer of the packets and the remaining packets will be sufficient to reconstruct the original N. Of course even if E is not greater than K/(N+K), random variability, non-stationarity of the pattern of erasures etc. results in some fraction of the sets of N+K packets having greater than K erasures, so that there is insufficient information to reconstruct the N packets at the destination. Therefore, even using FEC, at least some groups of N information packets will not be reconstructable. Note, for example, with E=0.2, N=8, and K=2, even though only 2 erasures may be expected on average, the probability of more than 2 erasures is greater than 30%, and even with E=0.1 this probability is greater than 7%, therefore the nature (e.g., timing, triggering conditions etc.) of the retransmission approaches may be significant, as discussed further below. Also as discussed below, the size of the set of packets that are coded together is significant. For example, increasing N by a factor of 10 to K+N=100 reduces the probably of more than the average number of 20 erasures (i.e., too many erasures to reconstruct the N=80 data packets) from over 7% to less than 0.1%. 
         [0126]    Also as discussed further below, there is a tradeoff between use of large blocks of packets (i.e., large N) versus smaller blocks. For a particular code rate R=N/(N+K), longer blocks yield a higher probability of being able to fully recover the N information packets in the presence of random errors. Accordingly, depending on the data characteristics, the PC-TCP modules may be configured to adapt to achieve a desired tradeoff 
         [0127]    In general, in embodiments that guarantee delivery of the N packets, whether or not FEC is used, repair retransmission approaches are used to provide further information for reconstructing the N packets. In general, in preferred embodiments, the redundancy information is formed in such a manner that upon an erasure of a packet, the redundancy information that is sent from the transmitter does not depend on the specific packets that were erased, and is nevertheless suitable for repairing the erasure independent of which packet was erased. 
       3.3.2 Random Linear Coding 
       [0128]    In general, a preferred approach to inter-packet coding is based on Random Linear Network Coding (RLNC) techniques. However, it should be understood that although based on this technology, not all features that may be associated with this term are necessarily incorporated. In particular, as described above in the absence of intermediate nodes that perform recoding, there is not necessarily a “network” aspect to the approach. Rather, redundancy information is generally formed by combining the information packets into coded packets using arithmetic combinations, and more specifically, as sums of products of coefficients and representation of the information packets over arithmetic fields, such as finite fields (e.g., Galois Fields of order p n ). In general, the code coefficients are chosen from a sufficiently large finite field in a random or pseudo-random manner, or in another way that the combinations of packets have a very low probability or frequency of being linearly dependent. The code coefficients, or a compressed version (e.g., as a reference into a table shared by the transmitter and receiver), are included in each transmitted combination of data units (or otherwise communicated to the receiver) and used for decoding at the receiver. Very generally, the original information packets may be recovered at a receiver by inverting the arithmetic combinations. For example, a version of Gaussian Elimination may be used to reconstruct the original packets from the coded combinations. A key feature of this approach is that for a set of N information packets, as soon at the receiver has at least N linearly independent combinations of those information packets in received packets, it can reconstruct the original data units. The term “degree of freedom” is generally used below to refer to a number of independent linear combinations, such that if N degrees of freedom have been specified for N original packets, then the N original packets can be reconstructed; while if fewer than N degrees of freedom are available, it may not be possible to fully reconstruct any of the N original packets. If N+K linearly independent linear combinations are sent, then any N received combinations (i.e., N received degrees of freedom) are sufficient to reconstruct the original information packets. 
         [0129]    In some examples, the N+K linearly independent combinations comprise N selections of the N “uncoded” information packets (essentially N−1 zero coefficients and one unit coefficient for each uncoded packet), and K coded packets comprising the random arithmetic combination with N non-zero coefficients for the N information packets. The N uncoded packets are transmitted first, so that in the absence of erasures they should be completely received as soon as possible. In the case of one erasure of the original N packets, the receiver must wait for the arrival of one redundant packet (in addition to the N−1 original packets), and once that packet has arrived, the erased packet may be reconstructed. In the case of forward error correction, the K redundant packets follow (e.g., immediately after) the information packets, and the delay incurred in reconstructing the erased information packet depends on the transmission time of packets. In the case of repair retransmission, upon detection of an erasure or high probability of an erasure, the receiver provides feedback to the transmitter, which sends the redundancy information upon receiving the feedback. Therefore, the delay in being able to reconstruct the erased packet depends on the round-trip-time from the receiver to the transmitter and back. 
         [0130]    As discussed in more detail below, feedback from the receiver to the transmitter may be in the form of acknowledgments sent from the receiver to the transmitter. This feedback in acknowledgements at least informs the transmitter of a number of the N+K packets of a block that have been successfully received (i.e., the number of received degrees of freedom), and may provide further information that depends on the specific packets that have been received at the receiver although such further information is not essential. 
         [0131]    As introduced above, packets that include the combinations of original packets generally also include information needed to determine the coefficients used to combine the original packets, and information needed to identify which original packets were used in the combination (unless this set, such as all the packets of a block, is implicit). In some implementations, the coefficients are explicitly represented in the coded packets. In some embodiments, the coefficients are encoded with reference to shared information at the transmitter and the receiver. For instance, tables of pre-generated (e.g., random, pseudo random, or otherwise selected) coefficients, or sets of coefficients, may be stored and references into those tables are used to determine the values of the coefficients. The size of such a table determines the number of parity packets that can be generated while maintaining the linear independence of the sets of coefficients. It should be understood that yet other ways may be used to determine the coefficients. 
         [0132]    Another feature of random linear codes is that packets formed as linear combinations of data units may themselves be additively combined to yield combined linear combinations of data units. This process is referred to in some instances as “recoding”, as distinct from decoding and then repeating encoding. 
         [0133]    There are alternatives to the use of RLNC, which do not necessarily achieve similar optimal (or provably optimum, or near optimal) throughput as RLNC, but that give excellent performance in some scenarios when implemented as described herein. For example, various forms of parity check codes can be used. Therefore, it should be understood that RLNC, or any particular aspect of RLNC, is not an essential feature of all embodiments described in this document. 
       3.4 Batch Transmission 
       [0134]    As introduced above, in at least some applications, data to be transmitted from a transmitter to a receiver forms a batch (i.e., as opposed to a continuous stream), with an example of a batch being a file or a segment (e.g., a two second segment of multimedia) of a file. 
         [0135]    In an embodiment of the PC-TCP modules, the batch is transferred from the transmitter to the receiver as a series of blocks, with each block being formed from a series of information packets. In general, each block has the same number of information packets, however use of same size blocks is not essential. 
         [0136]    The transmitter PC-TCP module generally receives the data units from the source application and forms the information packets of the successive blocks of the batch. These information packets are queued at the transmitter and transmitted on the channel to the receiver. In general, at the transmitter, the dequeueing and transmission of packets to the receiver makes use of congestion control and/or rate control mechanisms described in more detail below. The transmitter PC-TCP also retains the information packets (or sufficient equivalent information) to construct redundancy information for the blocks. For instance the transmitter PC-TCP buffers the information packets for each block for which there remains the possibility of an unrecovered erasure of a packet during transit from the transmitter to the receiver. 
         [0137]    In general, the receiver provides feedback to the transmitter. Various approaches to determining when to provide the feedback and what information to provide with the feedback are described further below. The feedback provides the transmitter with sufficient information to determine that a block has been successfully received and/or reconstructed at the receiver. When such success feedback for a block has been received, the transmitter no longer needs to retain the information packets for the block because there is no longer the possibility that redundancy information for the block will need to be sent to the receiver. 
         [0138]    The feedback from the receiver to the transmitter may also indicate that a packet is missing. Although in some cases the indication that a packet is missing is a premature indication of an erasure, in this embodiment the transmitter uses this missing feedback to trigger sending redundant information for a block. In some examples, the packets for a block are numbered in sequence of transmission, and the feedback represents the highest number received and the number of packets (i.e., the number of degrees of freedom) received (or equivalently the number of missing packets or remaining degrees of freedom needed) for the block. The transmitter addresses missing packet feedback for a block through the transmission of redundant repair blocks, which may be used by the receiver to reconstruct the missing packets and/or original packets of the block. 
         [0139]    As introduced above, for each block, the transmitter maintains sufficient information to determine the highest index of a packet received at the receiver, the number of missing packets transmitted prior to that packet, and the number of original or redundancy packets after the highest index received that have been transmitted (i.e., are “in flight” unless erased in transit) or queued for transmission at the transmitter. 
         [0140]    When the transmitter receives missing packet feedback for a block, if the number of packets for the block that are “in flight” or queue would not be sufficient if received successfully (or are not expected to be in view of the erasure rate), the transmitter computes (or retrieves precomputed) a new redundant packet for the block and queues it for transmission. Such redundancy packets are referred to as repair packets. In order to reduce the delay in reconstructing a block of packets at the receiver, the repair packets are sent preferentially to the information packets for later blocks. For instance, the repair packets are queued in a separate higher-priority queue that is used to ensure transmission of repair packets preferentially to the queue of information packets. 
         [0141]    In some situations, feedback from the receiver may have indicated that a packet is missing. However, that packet may later arrive out of order, and therefore a redundant packet for that block that was earlier computed and queued for transmission is no longer required to be delivered to the receiver. If that redundant packet has not yet been transmitted (i.e., it is still queued), that packet may be removed from the queue thereby avoiding wasted use of channel capacity for a packet that will not serve to pass new information to the receiver. 
         [0142]    In the approach described above, redundancy packets are sent as repair packets in response to feedback from the receiver. In some examples, some redundancy packets are sent pre-emptively (i.e., as forward error correction) in order to address possible packet erasures. One approach to send such forward error correction packets for each block. However, if feedback has already been received at the transmitter that a sufficient number of original and/or coded packets for a block have been received, then there is no need to send further redundant packets for the block. 
         [0143]    In an implementation of this approach, the original packets for all the blocks of the batch are sent first, while repair packets are being preferentially sent based on feedback from the receiver. After all the original packets have been transmitted, and the queue of repair packets is empty, the transmitter computes (or retrieves precomputed) redundancy packets for blocks for which the transmitter has not yet received feedback that the blocks have been successfully received, and queues those blocks as forward error correction packets for transmission in the first queue. In general, because the repair blocks are sent with higher priority that the original packets, the blocks for which success feedback has not yet been received are the later blocks in the batch (e.g., a trailing sequence of blocks of the batch). 
         [0144]    In various versions of this approach, the number and order of transmission of the forward error correction packets are determined in various ways. A first way uses the erasure rate to determine how many redundant packets to transmit. One approach is to send at least one redundant packet for each outstanding block. Another approach is to send a number of redundancy packets for each outstanding block so that based on an expectation of the erasure rate of the packets that are queued and in flight for the block will yield a sufficient number of successfully received packets in order to reconstruct the block. For example, if a further n packets are needed to reconstruct a block (e.g., a number n&lt;N packets of the original N packets with N−n packets having been erased), then n+k packets are sent, for instance, with n+k≧n/E, where E is an estimate of the erasure rate on the channel. 
         [0145]    Another way of determining the number and order of forward error correction packets addresses the situation in which a block transmission time is substantially less than the round-trip-time for the channel. Therefore, the earliest of the blocks for which the transmitter has not received success feedback may in fact have the success feedback in flight from the receiver to the transmitter, and therefore sending forward error correction packets may be wasteful. Similarly, even if feedback indicating missing packet feedback for a block is received sufficiently early, the transmitter may still send a repair packet without incurring more delay in complete reconstruction of the entire batch than would be achieved by forward error correction. 
         [0146]    In an example, the number of forward error correction packets queued for each block is greater for later blocks in the batch than for earlier ones. A motivation for this can be understood by considering the last block of the batch where it should be evident that it is desirable to send a sufficient number of forward error correction packets to ensure high probability of the receiver having sufficient information to reconstruct the block without the need from transmission of a repair packet and the associated increase in latency. On the other hand, it is preferable to send fewer forward error correction packets for the previous (or earlier) block because in the face of missing packet feedback from the receiver, the transmitter may be able to send a repair packet before forward error correction packets for all the later blocks have been sent, thereby not incurring a delay in overall delivery of the batch. 
         [0147]    In one implementation, after all the original packets have been sent, and the transmitter is in the forward error correction phase in which it computes and sends the forward error correction packets, if the transmitter receives a missing packet feedback from the receiver, it computes and sends a repair packet for the block in question (if necessary) as described above, and clears the entire queue of forward error correction packets. After the repair packet queue is again empty, the transmitter again computes and queues forward error correction packets for the blocks for which it has not yet received success feedback. In an alternative somewhat equivalent implementation, rather than clearing the forward error correction queue upon receipt of a missing packet feedback, the transmitter removes forward error correction packets from the queue as they are no longer needed based on feedback from the receiver. In some examples, if success feedback is received for a block for which there are queued forward error correction packets, those forward error correction packets are removed from the queue. In some examples, the feedback from the receiver may indicate that some but not all of the forward error correction packets in the queue are no longer needed, for example, because out-of-order packets were received but at least some of the original packets are still missing. 
         [0148]    An example of the way the transmitter determines how many forward error correction packets to send is that the transmitter performs a computation: 
         [0000]      ( N+g ( i ) −a   i )/(1 −p ) −f   i    
         [0000]    where 
         [0149]    p=smoothed loss rate, 
         [0150]    N=block size, 
         [0151]    i=block index defined as number of blocks from last block, 
         [0152]    a i =number of packets acked from block i, 
         [0153]    f i =packets in-flight from block i, and 
         [0154]    g(i)=a decreasing function of i, 
         [0000]    to determine the number of FEC packets for a block. 
         [0155]    In some examples, g(i) is determined as a maximum of a configurable parameter, m and N−i. In some examples, g(i) is determined as N−p(i) where p is a polynomial, with integer rounding as needed 
         [0156]    It should be understood that in some alternative implementations, at least some forward error correction packets may be interspersed with the original packets. For example, if the erasure rate for the channel is relatively high, then at least some number of redundancy packets may be needed with relatively high probability for each block, and there is an overall advantage to preemptively sending redundant FEC packets as soon as possible, in addition to providing the mechanism for feedback based repair that is described above. 
         [0157]    It should be also understood that use of subdivision of a batch into blocks is not necessarily required in order to achieve the goal of minimizing the time to complete reconstruction of the block at the receiver. However, if the forward error correction is applied uniformly to all the packets of the batch, then the preferential protection of later packets would be absent, and therefore, latency caused by erasure of later packets may be greater than using the approach described above. However, alternative approaches to non-uniform forward error protection (i.e., introduction of forward error correction redundancy packets) may be used. For example, in the block based approach described above, packets of the later blocks each contribute to a greater number of forward error correction packets than do earlier ones, and an alternative approach to achieving this characteristic maybe to use a non-block based criterion to construction of the redundancy packets in the forward error correction phase. However, the block based approach described above has advantages of relative simplicity and general robustness, and therefore even if marginally “suboptimal” provides an overall advantageous technical solution to minimizing the time to complete reconstruction within the constraint of throughput and erasure on the channel linking the transmitter and receiver. 
         [0158]    Another advantage of using a block-based approach is that, for example, when a block within the batch, say the m th  block of M blocks of the batch has an erasure, the repair packet that is sent from the transmitter depends only on the N original packets of the m th  block. Therefore, as soon as the repair packet arrives, and the available (i.e., not erased) N−1 packets of the block arrive, the receiver has the information necessary to repair the block. Therefore, by constructing the repair packet without contribution of packets in later blocks of the batch, the latency of the reconstruction of the block is reduced. Furthermore, by having the repair packets depend on only N original packets, the computation required to reconstruct the packets of the block is less than if the repair packets depend on more packets. 
         [0159]    It should be understood that even in the block based transmission of a batch of packets, the blocks are not necessarily uniform in size, and are not necessarily disjoint. For example, blocks may overlap (e.g., by 50%, 75%, etc.) thereby maintaining at least some of the advantages of reduced complexity in reconstruction and reduced buffering requirements as compared to treating the batch as one block. An advantage of such overlapping blocks may be a reduced latency in reconstruction because repair packets may be sent that do not require waiting for original packets at the receiver prior to reconstruction. Furthermore, non-uniform blocks may be beneficial, for example, to increase the effectiveness of forward error correction for later block in a batch by using longer blocks near the end of a batch as compared to near the beginning of a batch. 
         [0160]    In applications in which the entire batch is needed by the destination application before use, low latency of reconstruction may be desirable to reduce buffering requirements in the PC-TCP module at the receiver (and at the transmitter). For example, all packets that may contribute to a later received repair packet are buffered for their potential future use. In the block based approach, once a block is fully reconstructed, then the PC-TCP module can deliver and discard those packets because they will not affect future packet reconstruction. 
         [0161]    Although described as an approach to delivery of a batch of packets, the formation of these batches may be internal to the PC-TCP modules, whether or not such batches are formed at the software application level. For example, the PC-TCP module at the transmitter may receive the original data units that are used to form the original packets via a software interface from the source application. The packets are segmented into blocks of N packets as described above, and the packets queued for transmission. In one embodiment, as long as the source application provides data units sufficiently quickly to keep the queue from emptying (or from emptying for a threshold amount of time), the PC-TCP module stays in the first mode (i.e., prior to sending forward error correction packets) sending repair packets as needed based on feedback information from the receiver. When there is a lull in the source application providing data units, then the PC-TCP module declares that a batch has been completed, and enters the forward error correction phase described above. In some examples, the batch formed by the PC-TCP module may in fact correspond to a batch of data units generated by the source application as a result of a lull in the source application providing data units to the PC-TCP module while it computes data units for a next batch, thereby inherently synchronizing the batch processing by the source application and the PC-TCP modules. 
         [0162]    In one such embodiment, the PC-TCP module remains in the forward error correction mode for the declared batch until that entire batch has been successfully reconstructed at the receiver. In another embodiment, if the source application begins providing new data units before the receiver has provided feedback that the previous batch has been successfully reconstructed, the transmitter PC-TCP module begins sending original packets for the next batch at a lower priority than repair or forward error correction packets for the previous batch. Such an embodiment may reduce the time to the beginning of transmission of the next batch, and therefore reduces the time to successful delivery of the next batch. 
         [0163]    In the embodiments in which the source application does not necessarily provide the data in explicit batches, the receiver PC-TCP module provides the data units in order to the destination application without necessarily identifying the block or batch boundaries introduced at the transmitter PC-TCP module. That is, in at least some implementations, the transmitter and receiver PC-TCP modules provide a reliable channel for the application data units without exposing the block and batch structure to the applications. 
         [0164]    As described above for certain embodiments, the transmitter PC-TCP module reacts to missing packet feedback from the receiver PC-TCP module to send repair packets. Therefore, it should be evident that the mechanism by which the receiver sends such feedback may affect the overall behavior of the protocol. For example, in one example, the receiver PC-TCP module sends a negative acknowledgment as soon as it observes a missing packet. Such an approach may provide the lowest latency for reconstruction of the block. However, as introduced above, missing packets may be the result of out-of-order delivery. Therefore, a less aggressive generation of missing packet feedback, for example, by delay in transmission of a negative acknowledgment, may reduce the transmission of unnecessary repair packets with only a minimal increase in latency in reconstruction of that block. However, such delay in sending negative acknowledgements may have an overall positive impact on the time to successfully reconstruct the entire block because later blocks are not delayed by unnecessary repair packets. Alternative approaches to generation of acknowledgments are described below. 
         [0165]    In some embodiments, at least some of the determination of when to send repair packets is performed at the transmitter PC-TCP. For example, the receiver PC-TCP module may not delay the transmission of missing packet feedback, and it is the transmitter PC-TCP module that delays the transmission of a repair packet based on its weighing of the possibility of the missing packet feedback being based on out-of-order delivery as opposed to erasure. 
       3.5 Protocol Parameters 
       [0166]    Communication between two PC-TCP endpoints operates according to parameters, some of which are maintained in common by the endpoints, and some of which are local to the sending and/or the receiving endpoint. Some of these parameters relate primarily to forward error correction aspects of the operation. For example, such parameters include the degree of redundancy that is introduced through the coding process. As discussed below, further parameters related to such coding relate to the selection of packets for use in the combinations. A simple example of such selection is segmentation of the sequence of input data units into “frames” that are then independently encoded. In addition to the number of such packets for combination (e.g., frame length), other parameters may relate to overlapping and/or interleaving of such frames of data units and/or linear combinations of such data units. 
         [0167]    Further parameters relate generally to transport layer characteristics of the communication approach. For example, some parameters relate to congestion avoidance, for example, representing a size of a window of unacknowledged packets, transmission rate, or other characteristics related to the timing or number of packets sent from the sender to the receiver of the PC-TCP communication. 
         [0168]    As discussed further below, communication parameters (e.g., coding parameters, transport parameters) may be set in various ways. For example, parameters may be initialized upon establishing a session between two PC-TCP endpoints. Strategies for setting those parameters may be based on various sources of information, for example, according to knowledge of the communication path linking the sender and receiver (e.g., according to a classification of path type, such as 3G wireless versus cable modem), or experienced communication characteristics in other sessions (e.g., concurrent or prior sessions involving the same sender, receiver, communication links, intermediate nodes, etc.). Communication parameters may be adapted during the course of a communication session, for example, in response to observed communication characteristics (e.g., congestion, packet loss, round-trip time, etc.) 
       3.6 Transmission Control 
       [0169]    Some aspects of the PC-TCP approaches relate to control of transmission of packets from a sender to a receiver. These aspects are generally separate from aspects of the approach that determine what is sent in the packets, for example, to accomplish forward error correction, retransmission, or the order in which the packets are sent (e.g, relative priority of forward error correction packets version retransmission packets). Given a queue of packets that are ready for transmission from the sender to the receiver, these transmission aspects generally relate to flow and/or congestion control. 
       3.6.1 Congestion Control 
       [0170]    Current variants of TCP, including binary increase congestion control (BIC) and cubic-TCP, have been proposed to address the inefficiencies of classical TCP in networks with high losses, large bandwidths and long round-trip times. BIC-TCP and CUBIC algorithms have been used because of their stability. After a backoff, BIC increases the congestion window linearly then logarithmically to the window size just before backoff (denoted by W max ) and subsequently increases the window in an anti-symmetric fashion exponentially then linearly. CUBIC increases the congestion window following backoff according to a cubic function with inflection point at W max . These increase functions cause the congestion window to grow slowly when it is close to W max , promoting stability. On the other hand, other variants such as HTCP and FAST TCP have the advantage of being able to partially distinguish congestion and non-congestion losses through the use of delay as a congestion signal. 
         [0171]    An alternative congestion control approach is used in at least some embodiments. In some such embodiments, we identify a concave portion of the window increase function as W concave (t)=W max +c 1  (t−k) 3  and a convex portion of the window increase function as W convex (t)=W max +c 2 (t−k) 3  where c 1  and c 2  are positive tunable parameters and 
         [0000]    
       
         
           
             k 
             = 
             
               
                 ( 
                 
                   
                     ( 
                     
                       W_max 
                       - 
                       W 
                     
                     ) 
                   
                   / 
                   
                     c 
                     1 
                   
                 
                 ) 
               
               3 
             
           
         
       
     
         [0000]    and W is the window size just after backoff. 
         [0172]    This alternative congestion control approach can be flexibly tuned for different scenarios. For example, a larger value of c 1  causes the congestion window to increase more rapidly up to W max  and a large value of c 2  causes the congestion window to increase more rapidly beyond W max . 
         [0173]    Optionally, delay is used as an indicator to exit slow start and move to the more conservative congestion avoidance phase, e.g. when a smoothed estimate of RTT exceeds a configured threshold relative to the minimum observed RTT for the connection. We can also optionally combine the increase function of CUBIC or other TCP variants with the delay-based backoff function of HTCP. 
         [0174]    In some embodiments, backoff is smoothed by allowing a lower rate of transmission until the number of packets in flight decreases to the new window size. For instance, a threshold, n, is set such that once n packets have been acknowledged following a backoff, then one packet is allowed to be sent for every two acknowledged packets, which is roughly half of the previous sending rate. This is akin to a hybrid window and rate control scheme. 
       3.6.2 Transmission Rate Control 
     3.6.2.1 Pacing Control by Sender 
       [0175]    In at least some embodiments, pacing is used to regulate and/or spread out packet transmissions, making the transmission rate less bursty. While pacing can help to reduce packet loss from buffer overflows, previous implementations of pacing algorithms have not shown clear advantages when comparing paced TCP implementations to non-paced TCP implementations. However, in embodiments where the data packets are coded packets as described above, the combination of packet coding and pacing may have advantages. For example, since one coded packet may be used to recover multiple possible lost packets, we can use coding to more efficiently recover from any spread out packet losses that may result from pacing. In embodiments, the combination of packet coding and pacing may have advantages compared to uncoded TCP with selective acknowledgements (SACK). 
         [0176]    Classical TCP implements end-to-end congestion control based on acknowledgments. Variants of TCP designed for high-bandwidth connections increase the congestion window (and consequently the sending rate) quickly to probe for available bandwidth but this can result in bursts of packet losses when it overshoots, if there is insufficient buffering in the network. 
         [0177]    A number of variants of TCP use acknowledgment feedback to determine round-trip time and/or estimate available bandwidth, and they differ in the mechanisms with which this information is used to control the congestion window and/or sending rate. Different variants have scenarios in which they work better or worse than others. 
         [0178]    In one general approach used in one or more embodiments, a communication protocol may use smoothed statistics of intervals between acknowledgments of transmitted packets (e.g., a smoothed “ack interval”) to guide a transmission of packets, for example, by controlling intervals (e.g., an average interval or equivalently an average transmission rate) between packet transmissions. Broadly, this guiding of transmission intervals is referred to herein as “pacing”. 
         [0179]    In some examples, the pacing approach is used in conjunction with a window-based congestion control algorithm. Generally, the congestion window controls the number of unacknowledged packets that can be sent, in some examples using window control approaches that are the same or similar to those used in known variants of the Transmission Control Protocol (TCP). In embodiments, the window control approach is based on the novel congestion control algorithms described herein. 
         [0180]    A general advantage of one or more aspects is to improve functioning of a communication system, for instance, as measured by total throughput, or delay and/or variation in delay. These aspects address a technical problem of congestion, and with it packet loss, in a network by using “pacing” to reduce that congestion. 
         [0181]    An advantage of this aspect is that the separate control of pacing can prevent packets in the congestion window from being transmitted too rapidly compared to the rate at which they are getting through to the other side. Without separate pacing control, at least some conventional TCP approaches would permit bursts of overly rapid transmission of packets, which might result in packet loss at an intermediate node on the communication path. These packet losses may be effectively interpreted by the protocol as resulting from congestion, resulting in the protocol reducing the window size. However, the window size may be appropriate, for example, for the available bandwidth and delay of the path, and therefore reducing the window size may not be necessary. On the other hand, reducing the peak transmission rate can have the effect of avoiding packet loss, for example, by avoiding overflow of intermediate buffers on the path. 
         [0182]    Another advantage of at least some implementations is prevention of large bursts of packet losses under convex window increase functions for high-bandwidth scenarios, by providing an additional finer level of control over the transmission process. 
         [0183]    At least some implementations of the approach can leverage the advantages of existing high-bandwidth variants of TCP such as H-TCP and CUBIC, while preventing large bursts of packet losses under their convex window increase functions and providing a more precise level of control. For example, pacing control may be implemented to pace the rate of providing packets from the existing TCP procedure to the channel, with the existing TCP procedure typically further or separately limiting the presentation of packets to the communication channel based, for instance, on its window-based congestion control procedure. 
         [0184]    In practice, a particular example in which separating pacing from window control has been observed to significantly outperform conventional TCP on 4G LTE. 
         [0185]    Referring to  FIG. 22 , in one example, a source application  1010  passes data to a destination application  1090  over a communication channel  1050 . Communication from the source application  1010  passes to a transport layer  1020 , which maintains a communication session with a corresponding transport layer  1080  linked to the destination application  1090 . In general, the transport layers may be implemented as software that executes on the same computer as their corresponding applications, however, it should be recognized that, for instance through the use of proxy approaches, the applications and the transport layer elements that are shown may be split over separate coupled computers. In embodiments, when a proxy is running on a separate machine or device from the application, the application may use the transport layer on its machine to communicate with the proxy layer. 
         [0186]    In  FIG. 22 , the transport layer  1020  at the source application includes a window control and retransmission element  1030 . In some implementations, this element implements a conventional Transport Control Protocol (TCP) approach, for instance, implementing H-TCP or CUBIC approaches. In other implementations, this element implements the novel congestion control algorithms described herein. The transport layer  1080  at the destination may implement a corresponding element  1060 , which may provide acknowledgements of packets to the window control and retransmission element  1030  at the source. In general, element  1030  may implement a window-based congestion control approach based on acknowledgements that are received at the destination, however it should be understood that no particular approach to window control is essential, and in some implementations, element  1030  can be substituted with another element that implements congestion control using approaches other than window control. 
         [0187]    Functionally, one may consider two elements of the protocol as being loss recovery and rate/congestion control. Loss recovery can be implemented either using conventional retransmissions or using coding or as a combination of retransmission and coding. Rate/congestion control may aim to avoid overrunning the receiver and/or the available channel capacity, and may be implemented using window control with or without pacing, or direct rate control. 
         [0188]    The channel  1050  coupling the transport layers in general may include lower layer protocol software at the source and destination, and a series of communication links coupling computers and other network nodes on a path from the source to the destination. 
         [0189]    As compared to conventional approaches, as shown in  FIG. 10 , a rate control element  1040  may be on the path between the window control and retransmission element  1030  and the channel  1050 . This rate control element may monitor acknowledgements that are received from the destination, and may pass them on to the window control and retransmission element  1030 , generally without delay. The rate control element  1040  receives packets for transmission on the channel  1050  from the window control and retransmission element  1030 , and either passes them directly to the channel  1050 , or buffers them to limit a rate of transmission onto the channel. For example, the rate control element  1040  may require a minimum interval between successive packets, or may control an average rate over multiple packets. 
         [0190]    In embodiments, the acks that are transmitted on a return channel, from the destination to the source, may also be paced, and may also utilize coding to recover from erasures and bursty losses. In embodiments, packet coding and transmission control of the acks may be especially useful if there is congestion on the return channel. 
         [0191]    In one implementation, the rate control element  1040  may maintain an average (i.e., smoothed) inter-packet delivery interval, estimated based on the acknowledgement intervals (accounting for the number of packets acknowledged in each ack). In some implementations this averaging may be computed as a decaying average of past sample inter-arrival times. This can be refined by incorporating logic for discarding large sample values based on the determination of whether they are likely to have resulted from a gap in the sending times or losses in the packet stream, and by setting configurable upper and lower limits on the estimated interval commensurate with particular characteristics of different known networks. The rate control element  1040  may then use this smoothed inter-acknowledgement time to set a minimum inter-transmission time, for example, as a fraction of the inter-acknowledgement time. This fraction can be increased with packet loss and with rate of increase of RTT (which may be indicators that the current sending rate may be too high), and decreased with rate of decrease of RTT under low loss, e.g. using a control algorithm such as proportional control whose parameters can be adjusted to trade off between stability and responsiveness to change. Upper and lower limits on this fraction can be made configurable parameters, say 0.2 and 0.95. Transmission packets are then limited to be presented to the channel  1050  with inter-transmission times of at least this set minimum. In other implementations inter-transmission intervals are controlled to maintain a smoothed average interval or rate based on a smoothed inter-acknowledgement interval or rate. 
         [0192]    In addition to the short timescale adjustments of the pacing interval with estimated delivery interval, packet loss rate and RTT described above, there can also be a longer timescale control loop that modulates the overall aggressiveness of the pacing algorithm based on a smoothed loss rate calculated over a longer timescale, with, a higher loss rate indicating that pacing may be too aggressive. The longer timescale adjustment can be applied across short duration connections by having the client maintain state across successive connections and include initializing information in subsequent connection requests. This longer timescale control may be useful for improving adaptation to diverse network scenarios that change dynamically on different timescales. 
         [0193]    Referring to  FIG. 23 , in some implementations, the communication channel  1050  spans multiple nodes  1161 ,  1162  in one or an interconnection of communication networks  1151 ,  1152 . In  FIG. 11 , the source application  1010  is illustrated as co-resident with the transport layer  1020  on a source computer  1110 , and similarly, the transport layer  1080  is illustrated as co-resident on a destination computer  1190  with the destination application  1090 . 
         [0194]    It should be recognized that although the description above focuses on a single direction of communication, in general, a bidirectional implementation would include a corresponding path from the destination application to the source application. In some implementations, both directions include corresponding rate control elements  1040 , while in other applications, only one direction (e.g., from the source to the destination application) may implement the rate control. For example, introduction of the rate control element  1040  at a server, or another device or network node on the path between the source application and the transport layer  1080  at the destination, may not require modification of the software at the destination. 
       3.6.2.2 Pacing by Receiver 
       [0195]    As described above, the sender can use acks to estimate the rate/interval with which packets are reaching the receiver, the loss rate and the rate of change of RTT, and adjust the pacing interval accordingly. However, this estimated information may be noisy if acks are lost or delayed. On the other hand, such information can be estimated more accurately at the receiver with OWTT in place of RTT. By basing the pacing interval on the rate of change of OWTT rather than its actual value, the need for synchronized clocks on sender and receiver may be obviated. The pacing interval can be fed back to the sender by including it as an additional field in the acks. The choice as to whether the pacing calculations are done at the sender or the receiver, or done every n packets rather than upon every packet reception, may also be affected by considerations of sender/receiver CPU/load. 
       3.7 Error Control 
       [0196]    Classical TCP performs poorly on networks with packet losses. Congestion control can be combined with coding such that coded packets are sent both for forward error correction (FEC) to provide protection against an anticipated level of packet loss, as well as for recovering from actual losses indicated by feedback from the receiver. 
         [0197]    While the simple combination of packet coding and congestion control has been suggested previously, the prior art does not adequately account for differences between congestion-related losses, bursty and/or random packet losses. Since congestion-related loss may occur as relatively infrequent bursts, it may be inefficient to protect against this type of loss using FEC. 
         [0198]    In at least some embodiments, the rates at which loss events occur are estimated. A loss event may be defined as either an isolated packet loss or a burst of consecutive packet losses. In some examples, the source PC-TCP may send FEC packets at the estimated rate of loss events, rather than the estimated rate of packet loss. This embodiment is an efficient way to reduce non-useful FEC packets, since it may not be disproportionately affected by congestion-related loss. 
         [0199]    In an exemplary embodiment, the code rate and/or packet transmission rate of FEC can be made tunable in order to trade-off between the useful throughput seen at the application layer (also referred to as goodput) and recovery delay. For instance, the ratio of the FEC rate to the estimated rate of loss events can be made a tunable parameter that is set with a priori knowledge of the underlying communications paths or dynamically adjusted by making certain measurements of the underlying communications paths. 
         [0200]    In another exemplary embodiment, the rate at which loss bursts of up to a certain length occur may be estimated, and appropriate burst error correcting codes for FEC, or codes that correct combinations of burst and isolated errors, may be used. 
         [0201]    In another exemplary embodiment, the FEC for different blocks can be interleaved to be more effective against bursty loss. 
         [0202]    In other exemplary embodiments, data packets can be sent preferentially over FEC packets. For instance, FEC packets can be sent at a configured rate or estimated loss rate when there are no data packets to be sent, and either not sent or sent at a reduced rate when there are data packets to be sent. In one implementation, FEC packets are placed in a separate queue which is cleared when there are data packets to be sent. 
         [0203]    In other exemplary embodiments, the code rate/amount of FEC in each block and/or the FEC packet transmission rate can be made a tunable function of the block number and/or the number of packets in flight relative to the number of unacknowledged degrees of freedom of the block, in addition to the estimated loss rate. FEC packets for later blocks can be sent preferentially over FEC for earlier blocks, so as to minimize recovery delay at the end of a connection, e.g., the number of FEC packets sent from each block can be a tunable function of the number of blocks from the latest block that has not been fully acknowledged. The sending interval between FEC packets can be an increasing function of the number of packets in flight relative to the number of unacknowledged degrees of freedom of the corresponding block, so as to trade-off between sending delay and probability of losing FEC packets in scenarios where packet loss probability increases with transmission rate. 
         [0204]    In other exemplary embodiments, a variable randomly chosen fraction of the coding coefficients of a coded packet can be set to 1 or 0 in order to reduce encoding complexity without substantially affecting erasure correction performance. In a systematic code, introducing 0 coefficients only after one or more densely coded packets (i.e. no or few 0 coefficients) may be important for erasure correction performance. For instance, an initial FEC packet in a block could have each coefficient set to 1 with probability 0.5 and to a uniformly random value from the coding field with probability 0.5. Subsequent FEC packets in the block could have each coefficient set to 0 with probability 0.5 and to uniformly random value with probability 0.5. 
       3.7.1 Packet Reordering 
       [0205]    As introduced above, packets may be received out of order on some networks, for example, due to packets traversing multiple paths, parallel processing in some networking equipment, reconfiguration of a path (e.g., handoff in cellular networks). Generally, conventional TCP reacts to out of order packets by backing off the size of the congestion window. Such a backoff may unnecessarily hurt performance if there is no congestion necessitating a backoff. 
         [0206]    In some embodiments, in an approach to handling packet reordering that does not result from congestions, a receiver observing a gap in the sequence numbers of its received packets may delay sending an acknowledgment for a limited time. When a packet is missing, the receiver does not immediately know if the packet has been lost (erased), or merely reordered. The receiver delays sending an acknowledgement that indicates the gap to see if the gap is filled by subsequent packet arrivals. In some examples, upon observing a gap, the receiver starts a first timer for a configurable “reordering detection” time interval, e.g. 20 ms. If a packet from the gap is subsequently received within this time interval, the receiver starts a second timer for a configurable “gap filling” time interval, e.g. 30 ms. If the first timer or the second timer expire prior to the gap being filled, an acknowledgement that indicates the gap is sent to the source. 
         [0207]    Upon receiving the acknowledgment that indicates the gap in received packets the source, in at least some embodiments, the sender determines whether a repair packet should be sent to compensate for the gap in the received packets, for example, if a sufficient number of FEC packets have not already been sent. 
         [0208]    In another aspect, a sender may store relevant congestion control state information (including the congestion window) prior to backoff, and a record of recent packet losses. If the sender receives an ack reporting a gap/loss and then subsequently one or more other acks reporting that the gap has been filled by out of order packet receptions, any backoff caused by the earlier ack can be reverted by restoring the stored state from before backoff. 
         [0209]    In another aspect, a sender observing a gap in the sequence numbers of its received acks may delay congestion window backoff for a limited time. When an ack is missing, the sender does not immediately know if a packet has been lost or if the ack is merely reordered. The sender delays backing off its congestion window to see if the gap is filled by subsequent ack arrivals. In some examples, upon observing a gap, the sender starts a first timer for a configurable “reordering detection” time interval, e.g. 20 ms. If an ack from the gap is subsequently received within this time interval, the sender starts a second timer for a configurable “gap filling” time interval, e.g. 30 ms. If the first timer or the second timer expires prior to the gap being filled, congestion window backoff occurs. 
         [0210]    In some examples, instead of using time intervals, packet sequence numbers are used. For example, sending of an ack can be delayed until a packet which is a specified number of sequence numbers ahead of the reference lost packet is received. Similarly, backing off can be delayed until an acknowledgment of a packet which is a specified number of sequence numbers ahead of the reference lost packet is received. In some examples, these approaches have the advantage of being able to take into account subsequently received/acknowledged reordered packets by shifting the sequence number of the reference lost packet as holes in the packet sequence get filled. 
         [0211]    These methods for correcting packet reordering may be especially useful for multipath versions of the protocol, where there may be a large amount of reordering. 
       3.7.2 Acknowledgements 
     3.7.2.1 Delayed Acknowledgements 
       [0212]    In at least some implementations, conventional TCP sends one acknowledgment for every two data packets received. Such delayed acking reduces ack traffic compared to sending an acknowledgment for every data packet. This reduction in ack traffic is particularly beneficial when there is contention on the return channel, such as in Wi-Fi networks, where both data and ack transmissions contend for the same channel. 
         [0213]    It is possible to reduce ack traffic further by increasing the ack interval to a value n&gt;2, i.e. sending one acknowledgment for every n data packets. However, reducing the frequency with which acks are received by the sender can cause delays in transmission (when the congestion window is full) or backoff (if feedback on losses is delayed), which can hurt performance. 
         [0214]    In one aspect, the sender can determine whether, or to what extent, delayed acking should be allowed based in part on its remaining congestion window (i.e. its congestion window minus the number of unacknowledged packets in flight), and/or its remaining data to be sent. For example, delayed acking can be disallowed if there is any packet loss, or if the remaining congestion window is below some (possibly tunable) threshold. Alternatively, the ack interval can be reduced with the remaining congestion window. As another example, delayed acking can be allowed if the amount of remaining data to be sent is smaller than the remaining congestion window, but disallowed for the last remaining data packet so that there is no delay in acknowledging the last data packet. This information can be sent in the data packets as a flag indicating whether delayed acking is allowed, or for example, as an integer indicating the allowed ack interval. 
         [0215]    Using relevant state information at the sender to influence delayed acking may allow an increase in the ack interval beyond the conventional value of 2, while mitigating the drawbacks described above that a larger ack interval across the board might have. 
         [0216]    To additionally limit the ack delay, each time an ack is sent, a delayed ack timer can be set to expire with a configured delay, say 25 ms. Upon expiration of the timer, any data packets received since the last ack may be acknowledged, even if fewer packets than the ack interval n have arrived. If no packets have been received since the last ack, an ack may be sent upon receipt of the next data packet. 
       3.8 Parameter Control 
     3.8.1 Initialization 
       [0217]    In some embodiments, to establish a session parameters for the PC-TCP modules are set to a predefine set of default parameters. In other embodiments, approaches that attempt to select better initial parameters are used. Approaches include use of parameter values from other concurrent or prior PC-TCP sessions, parameters determined from characteristics of the communication channel, for example, selected from stored parameters associated with different types of channels, or parameters determined by the source or destination application according to the nature of the data to be transported (e.g., batch versus stream). 
       3.8.2 Tunable Coding 
       [0218]    Referring to  FIG. 24 , in an embodiment in which parameters are “tuned” (e.g., through feedback from a receiver or on other considerations) a server application  2410  is in communication with a client application  2490  via a communication channel  2450 . In one example, the server application  2410  may provide a data stream encoding multimedia content (e.g., a video) that is accepted by the client application  2490 , for example, for presentation to a user of the device on which the client application is executing. The channel  2450  may represent what is typically a series of network links, for example including links of one or more types, including:
       a link traversing private links on a server local area network,   a link traversing the public Internet,   a link traversing a fixed (i.e., wireline) portion of a cellular telephone network,   and a link traversing a wireless radio channel to the user&#39;s device (e.g., a cellular telephone channel or satellite link or wireless LAN).       
 
         [0223]    The channel  2450  may be treated as carrying a series of data units, which may but do not necessarily correspond directly to Internet Protocol (IP) packets. For example, in some implementations multiple data units are concatenated into an IP packet, while in other implementations, each data unit uses a separate IP packet or only part of an IP packet. It should be understood that in yet other implementations, the Internet Protocol is not used—the techniques described below do not depend on the method of passing the data units over the channel  2450 . 
         [0224]    A transmitter  2420  couples the server application  2410  to the channel  2450 , and a receiver  2480  couples the channel  2450  to the client application  2490 . Generally, the transmitter  2420  accepts input data units from the server application  2480 . In general, these data units are passed over the channel  2450 , as well as retained for a period of time in a buffer  2422 . From time to time, an error control (EC) component  2424  may compute a redundancy data unit from a subset of the retained input data units in the buffer  2422 , and may pass that redundancy data unit over the channel  2450 . The receiver  2480  accepts data units from the channel  2450 . In general, the channel  2450  may erase and reorder the data units. Erasures may correspond to “dropped” data units that are never received at the receiver, as well as corrupted data units that are received, but are known to have irrecoverable errors, and therefore are treated for the most part as dropped units. The receiver may retain a history of received input data units and redundancy data units in a buffer  2482 . An error control component  2484  at the receiver  2480  may use the received redundancy data units to reconstruct erased input data units that may be missing in the sequence received over the channel. The receiver  2480  may pass the received and reconstructed input data units to the client application. In general, the receiver may pass these input data units to the client application in the order they were received at the transmitter. 
         [0225]    In general, if the channel has no erasures or reordering, the receiver can provide the input data units to the client application with delay and delay variation that may result from traversal characteristics of the channel. When data units are erased in the channel  2450 , the receiver  2480  may make use of the redundancy units in its buffer  2482  to reconstruct the erased units. In order to do so, the receiver may have to wait for the arrival of the redundancy units that may be useful for the reconstruction. The way the transmitter computes and introduces the redundancy data units generally affects the delay that may be introduced to perform the reconstruction. 
         [0226]    The way the transmitter computes and introduces the redundancy data units as part of its forward error correction function can also affect the complexity of the reconstruction process at the receiver, and the utilization of the channel. Furthermore, regardless of the nature of the way the transmitter introduces the redundancy data units onto the channel, statistically there may be erased data units for which there is insufficient information in the redundancy data units to reconstruct the erased unit. In such cases, the error control component  2484  may request a retransmission of information from the error control component  2424  of the transmitter  2420 . In general, this retransmitted information may take the form of further redundancy information that depends on the erased unit. This retransmission process introduces a delay before the erased unit is available to the receiver. Therefore, the way the transmitter introduces the redundancy information also affects the statistics such as how often retransmission of information needs to be requested, and with it the delay in reconstructing the erased unit that cannot be reconstructed using the normally introduced redundancy information. 
         [0227]    In some embodiments, the error control component  2484  may provide information to the error control component  2424  to affect the way the transmitter introduces the redundancy information. In general, this information may be based on one or more of the rate of (or more generally the pattern of) erasures on units on the channel, rate of (or more generally timing pattern of) and the state of the available units in the buffer  2482  and/or the state of unused data in the client application  2490 . For example, the client application may provide a “play-out time” (e.g., in milliseconds) of the data units that the receiver has already provided to the client application such that if the receiver were to not send any more units, the client application would be “starved” for input units at that time. Note that in other embodiments, rather than or in addition to receiving information from the receiver, the error control component  2424  at the transmitter may get feedback from other places, for example, from instrumented nodes in the network that pass back congestion information. 
         [0228]    Referring to  FIG. 25 , a set of exemplary ways that the transmitter introduces the redundancy data units into the stream of units passed over the channel makes use of alternating runs of input data units and redundancy data units. In  FIG. 25 , the data units that are “in flight” on the channel  2450  are illustrated passing from left to right in the figure. The transmitter introduces the units onto the channel as sequences of p input units alternating with sequences of q redundancy units. Assuming that the data units are the same sizes, this corresponds to a rate R=p/(p+q) code. In an example with p=4 and q=2 and the code has rate R=2/3. 
         [0229]    In a number of embodiments the redundancy units are computed as random linear combinations of past input units. Although the description below focuses on such approaches, it should be understood that the overall approach is applicable to other computations of redundancy information, for example, using low density parity check (LDPC) codes and other error correction codes. In the approach shown in  FIG. 25 , each run of q redundancy units is computed as a function of the previous D input units, where in general but not necessarily D&gt;p. In some cases, the most recent d data units transmitted are not used, and therefore the redundancy data units are computed from a window of D−d input data units. In  FIG. 25 , d=2, D=10, and D−d=8. Note that because D−d&gt;p, the windows of input data units used for computation of the successive runs of redundancy units overlap, such that any particular input data unit will in general contribute to redundancy data units in more than one of the runs of q units on the channel. 
         [0230]    In  FIG. 25 , as well as in  FIGS. 26-27  discussed below, buffered input data units (i.e., in buffer  2422  shown in  FIG. 24 ) are shown on the left with time running from the bottom (past) to the top (future), with each set of D−d units used to compute a run of q redundant units illustrated with arrows. The sequence of transmitted units, consisting of runs of input data units alternating with runs of redundant units, is shown with time running from right to left (i.e., later packets on the left). Data units that have been received and buffered at the receiver are shown on the right (oldest on the bottom), redundant units computed from runs of D−d input units indicated next to arrows representing the ranges of input data units used to compute those data units. Data units and ranges of input data units that have not yet been received are illustrated using dashed lines. 
         [0231]      FIGS. 26 and 27  show different selections of parameters. In  FIG. 26 , p=2 and q=1 and the code has a rate R=2/3, which is the same rate at the selection of parameters in  FIG. 25 . Also as in the  FIG. 52  selection, d=2, D=10, and D−d=8. Therefore, a difference between  FIG. 25  and  FIG. 26  is not necessarily a degree of forward error protection (although the effect of burst erasures may be somewhat different in the two cases). More importantly, the arrangement in  FIG. 26  generally provides a lower delay from the time of an erased data unit to the arrival of redundancy information to reconstruct that unit, as compared to the arrangement in  FIG. 25 . On the other hand, the complexity of processing at the receiver may be greater in the arrangement of  FIG. 26  as compared to the arrangement of  FIG. 24 , in part because redundancy units information uses multiple different subsets of the input data units, which may require more computation when reconstructing an erased data unit. Turning to  FIG. 27 , at another extreme, a selection of parameters uses longer blocks with a selection D=8 and q=4. Again, this code has a rate R=2/3. In general, this selection of parameters will incur greater delay in reconstruction of an erased data unit as compared to the selections of parameters shown in  FIGS. 25 and 26 . On the other hand, reconstruction of up to four erasures per block of D=8 input data units is relatively less complex than would be required by the selections shown in  FIGS. 25 and 26 . 
         [0232]    For a particular rate of code (e.g., rate R=2/3), in an example, feedback received may result in changes of the parameters, for example, between (p,q)=(2,1) or (4,2) or (8,4) depending on of the amount of data buffered at the receiver, and therefore depending on the tolerance of the receiver to reconstruction delay. 
         [0233]    Note that it is not required that q=p(1−R)/R is an integer, as it is in the examples shown in  FIGS. 25-27 . In some embodiments, the length of the run of redundant units varies between q=┌p(1−R)/R┐ and q=└p(1−R)/R┘ so that the average is ave(q)=p(1−R)/R. 
         [0234]    In a variant of the approach described above, different input data units have different “priorities” or “importances” such that they are protected to different degrees than other input data units. For example, in video coding, data units representing an independently coded video frame may be more important than data units representing a differentially encoded video frame. For example, if the priority levels are indexed i=1, 2, . . . , then a proportion ρ i ≦1, where Σ i ρ i =1, of the redundancy data units may be computed using data units with priority ≦i. For example, for a rate R code, with blocks of input data units of length p, on average ρ i  p(1−R)/R redundancy data units per block are computed from input data units with priority ≦i. 
         [0235]    The value of D should generally be no more than the target playout delay of the streaming application minus an appropriate margin for communication delay variability. The playout delay is the delay between the time a message packet is transmitted and the time it should be available at the receiver to produce the streaming application output. It can be expressed in units of time, or in terms of the number of packets transmitted in that interval. D can be initially set based on the typical or desired playout delay of the streaming application, and adapted with additional information from the receiver/application. Furthermore, choosing a smaller value reduces the memory and complexity at the expense of erasure correction capability. 
         [0236]    The parameter d specifies the minimum separation between a message packet and a parity involving that message packet. Since a parity involving a message packet that has not yet been received is not useful for recovering earlier message packets involved in that parity, setting a minimum parity delay can improve decoding delay when packet reordering is expected/observed to occur, depending partly also on the parity interval. 
         [0237]    Referring to  FIG. 28 , in an example implementation making use of the approaches described above, the server application  2410  is hosted with the transmitter  2420  at a server node  810 , and the client application  2490  is hosted at one or a number of client nodes  891  and  892 . Although a wide variety of types of data may be transported using the approaches described above, one example is streaming of encoded multimedia (e.g., video and audio) data. The communication channel  2450  (see  FIG. 24 ) is made up in this illustration as a path through one or more networks  851 - 852  via nodes  861 - 862  in those respective networks. In some implementations, the receiver is hosted at a client node  891  being hosted on the same device as the client application  490 . 
       3.8.3 Cross-Session Parameter Control 
       [0238]    In some embodiments, the control of transport layer sessions uses information across connections, for example, across concurrent sessions or across sessions occurring at different times. 
         [0239]    Standard TCP implements end-to-end congestion control based on acknowledgments. A new TCP connection that has started up but not yet received any acknowledgments uses initial configurable values for the congestion window and retransmission timeout. These values may be tuned for different types of network settings. 
         [0240]    Some applications, for instance web browser applications, may use multiple connections between a client application (e.g., the browser) and a server application (e.g., a particular web server application at a particular server computer). Conventionally, when accessing the information to render a single web “page”, the client application may make many separate TCP sessions between the client and server computers, and using conventional TCP control, each session is controlled substantially independently. This independent control includes separate congestion control. 
         [0241]    One approach to addressing technical problems that are introduced by having such multiple sessions is the SPDY Protocol (see, e.g., SPDY Protocol—Draft 3.1, accessible at http://www. chromium.org/spdy/spdy-protocol/spdy-protocol-draft3-1). The SPDY protocol is an application layer protocol that manipulates HTTP traffic, with particular goals of reducing web page load latency and improving web security. Generally, SPDY effectively provides a tunnel for the HTTP and HTTPS protocols. When sent over SPDY, HTTP requests are processed, tokenized, simplified and compressed. The resulting traffic is then sent over a single TCP session, thereby avoiding problems and inefficiencies involved in use of multiple concurrent TCP sessions between a particular client and server computer. 
         [0242]    In a general aspect, a communication system maintains information related to communication between computers or network nodes. For example, the maintained information can include bandwidth to and/or from the other computer, current or past congestion window sizes, pacing intervals, packet loss rates, round-trip time, timing variability, etc. The information can include information for currently active sessions and/or information about past sessions. One use of the maintained information may be to initialize protocol parameters for a new session between computers for which information has been maintained. For example, the congestion window size or a pacing rate for a new TCP or UDP session may be initialized based on the congestion window size, pacing interval, round-trip time and loss rate of other concurrent or past sessions. 
         [0243]    Referring to  FIG. 29 , communication system  1200  maintains information regarding communication sessions between endpoints. For example, these communication sessions pass via a network  1250 , and may pass between a server  1210 , or a proxy  1212  serving one or more servers  1214 , and a client  1290 . In various embodiments, this information may be saved in various locations. In some implementations, a client  1290  maintains information about current or past connections. This information may be specific to a particular server  1210  or proxy  1212 . This information may also include aggregated information. For example, in the case of a smartphone on a cellular telephone network, some of the information may be generic to connections from multiple servers and may represent characteristics imposed by the cellular network rather than a particular path to a server  1210 . In some implementations, a server  1210  or proxy  1212  may maintain the information based on its past communication with particular clients  1290 . In some examples, the clients and servers may exchange the information such that is it distributed throughout the system  1200 . In some implementations, the information may be maintained in databases that are not themselves endpoints for the communication sessions. For instance, it may be beneficial for a client without relevant stored information to retrieve information from an external database. 
         [0244]    In one use scenario, when a client  1290  seeks to establish a communication session (e.g., a transport layer protocol session), it consults its communication information  1295  to see if it has current information that is relevant to the session it seeks to establish. For example, the client may have other concurrent sessions with a server with which it wants to communicate, or with which it may have recently had such sessions. As another example, the client  1290  may use information about other concurrent or past sessions with other servers. When the client  1290  sends a request to a server  1210  or a proxy  1212  to establish a session, relevant information for that session is also made available to one or both of the endpoints establishing the session. There are various ways in which the information may be made available to the server. For example the information may be included with the request itself. As another example, the server may request the information if it does not already hold the information in its communication information  1215 . As another example, the server may request the information from a remote or third party database, which has been populated with information from the client or from servers that have communicated with the client. In any case, the communication session between the client and the server is established using parameters that are determined at least in part by the communication information available at the client and/or server. 
         [0245]    In some examples, the communication session may be established using initial values of packet pacing interval, congestion window, retransmission timeout and forward error correction. Initial values suitable for different types of networks (e.g. Wi-Fi, 4G), network operators and signal strength can be prespecified, and/or initial values for successive connections can be derived from measured statistics of earlier connections between the same endpoints in the same direction. For example:
       The initial congestion window can be increased from its default value if the packet throughput of the previous connection is sufficiently larger than the ratio of the default initial congestion window to the minimum round-trip time of the previous connection. The congestion window can subsequently be adjusted downwards if the initial received acks from the new connection indicate that the available rate has decreased compared to the previous connection.   The initial pacing interval can be set e.g. as MAX(k1*congestion window/previous round-trip time, k2/previous packet throughput), where k1 and k2 are configurable parameters, or, with receiver pacing, as k* previous pacing interval, where k increases with the loss rate of the previous connection.   Forward error correction parameters such as code rate can be set as k*previous loss rate, where k is a configurable parameter. The initial retransmission timeout can be increased from its default value if the minimum round-trip time of the previous connection is larger.       
 
       3.9 Multi-Path 
       [0249]      FIG. 30  shows the use of multiple paths between the server and client to deliver the packet information. These multiple paths may be over similar or different network technologies with similar or different average bandwidth, round trip delay, packet jitter rate, packet loss rate and cost. Examples of multiple paths include wired/fiber networks, geostationary, medium and low earth orbit satellites, WiFi, and cellular networks. In this example, the transmission control layer can utilize a single session to distribute the N packets in the block being transmitted over the multiple paths according to a variety of metrics (average bandwidth of each path, round trip delay of each path, packet jitter rate, packet loss rate of each path, and cost). The N packets to be transmitted in each block can be spread across each path in a manner that optimizes the overall end-to-end throughput and costs between server and client. The number of packets sent on each path can be dynamically controlled such that the average relative proportions of packets sent on each path are in accordance with the average relative available bandwidths of the paths, e.g. using back pressure-type control whereby packets are scheduled so as to approximately equalize queue lengths associated with the different paths. 
         [0250]    For each path, the algorithms described above that embody transmission and congestion control, forward error correction, sender based pacing, receiver based pacing, stream based parameter tuning, detection and correction for missing and out of order packets, use of information across multiple TCP connections, fast connection start and stop, TCP/UDP fallback, cascaded coding, recoding by intermediate nodes, and coding of the ACKs can be employed to improve the overall end-to-end throughput over the multiple paths between the source node and destination node. When losses are detected and FEC is used, the extra coded packets can be sent over any or all of the paths. For instance, coded packets sent to repair losses can be sent preferentially over lower latency paths to reduce recovery delay. The destination node will decode any N of packets that are received over all of the paths and assemble them into a block of N original packets by recreating any missing packets from the ones received. If less than N different coded packets are received across all paths, then the destination node will request the number of missing packets x where x=N−number of packets received be retransmitted. Any set of x different coded packet can be retransmitted over any path and then used to reconstruct the missing packets in the block of N. 
         [0251]    When there are networks with large differences in round trip time (RTT) latencies, the packets received over the lower RTT latencies will need to be buffered at the receiver in order to be combined with the higher RTT latency packets. The choice of packets sent on each path can be controlled so as to reduce the extent of reordering and associated buffering on the receiver side, e.g. among the packets available to be sent, earlier packets can be sent preferentially on higher latency paths and later packets can be sent preferentially on lower latency paths. 
         [0252]    Individual congestion control loops may be employed on each path to adapt to the available bandwidth and congestion on the path. An additional overall congestion control loop may be employed to control the total sending window or rate across all the paths of a multi-path connection, for fairness with single-path connections. 
         [0253]    Referring to  FIG. 31 a   , a communication system utilizes a first, satellite data path  3102  having a relatively high round trip time latency and a second, DSL data path  3104  having a relatively low round trip time latency. When a user application  3106  sends a request to stream video content, a content server  3108  (e.g., video streaming service) provides some or all of the requested video content to a remote proxy  3110  which generates encoded video content  3112  for transmission to the user application  3106 . Based on the RTT latencies of the first data path  3102  and the second data path  3104 , the remote proxy  3110  splits the encoded video content  3112  into an initial portion  3114  (e.g., the first 5 seconds of video content) and a subsequent portion  3116  (e.g., the remaining video content). The remote proxy  3110  then causes transmission of the initial portion  3114  over the second, low latency data path  3104  and transmission of the subsequent portion  3116  over the first, high latency data path  3102 . 
         [0254]    Referring to  FIG. 31 b   , due to the lower latency of the second data path  3104 , the initial portion  3114  of the video content arrives at the local proxy  3118  quickly, where it is decoded and sent to the user application  3106  for presentation to a viewer. The subsequent portion  3116  of the video content is still traversing the first, high latency data path  3102  at the time that presentation of the initial portion  3114  of the video content to the viewer commences. 
         [0255]    Referring to  FIG. 31 c   , during presentation of the decoded initial portion  3114  of video content to the viewer, the subsequent portion  3116  of the video content arrives at the local proxy  3118  where it is decoded and sent to the user application  3106  before presentation of the initial portion  3114  of the video content to the viewer is complete. In some examples, sending the initial portion  3114  of the video content over the low latency data path  3104  and sending a subsequent portion  3116  of the video content over the high latency data path  3102  avoids lengthy wait times between when a user requests a video and when the user sees the video (as would be the case if using satellite only communication) while minimizing data usage over the low latency data path (which may be more costly to use). 
         [0256]    In some examples, other types of messages may be preferentially sent over the low latency data path. For example, acknowledgement messages, retransmission messages, and/or other time critical messages may be transmitted over the low latency data path while other data messages are transmitted over the higher latency data path. 
         [0257]    In some examples, additional data paths with different characteristics (e.g., latencies) can also be included in the communication system, with messages being balanced over any of a number of data paths based on characteristics of the messages (e.g., message type) and characteristics of the data paths. 
         [0258]    In some examples, other types of messages may be preferentially sent over the low latency data path. For example, acknowledgement messages, retransmission messages, and/or other time critical messages may be transmitted over the low latency data path while other data messages are transmitted over the higher latency data path. 
         [0259]    In some examples, additional data paths with different characteristics (e.g., latencies) can also be included in the communication system, with messages being balanced over any of a number of data paths based on characteristics of the messages (e.g., message type) and characteristics of the data paths. 
       4 Alternatives and Implementations 
       [0260]    In the document above, certain features of the packet coding and transmission control protocols are described individually, or in isolation, but it should be understood that there are certain advantages that may be gained by combining multiple features together. Preferred embodiments for the packet coding and transmission control protocols described may depend on whether the transmission links and network nodes traversed between communication session end-points belong to certain fiber or cellular carriers (e.g. AT&amp;T, T-Mobile, Sprint, Verizon, Level 3) and/or end-user Internet Service Providers (ISPs) (e.g. AT&amp;T, Verizon, Comcast, Time Warner, Century Link, Charter, Cox) or are over certain wired (e.g. DSL, cable, fiber-to-the-curb/home (FTTx)) or wireless (e.g. WiFi, cellular, satellite) links. In embodiments, probe transmissions may be used to characterize the types of network nodes and transmission links communication signals are traversing and the packet coding and transmission control protocol may be adjusted to achieve certain performance. In some embodiments, data transmissions may be monitored to characterize the types of network nodes and transmission links communication signals are traversing and the packet coding and transmission control protocol may be adjusted to achieve certain performance. In at least some embodiments, quantities such as round-trip-time (RTT), one-way transmission times (OWTT), congestion window, pacing rate, packet loss rate, number of overhead packets, and the like may be monitored continuously, intermittently, in response to a trigger signal or event, and the like. In at least some embodiments, combinations of probe transmissions and data transmissions may be used to characterize network and communication session performance in real time. 
         [0261]    In at least some embodiments, network and communication parameters may be stored in the end-devices of communication sessions and/or they may be stored in network resources such as servers, switches, nodes, computers, databases and the like. These network and communication parameters may be used by the packet coding and transmission control protocol to determine initial parameter settings for the protocol to reduce the time it may take to adjust protocol parameters to achieve adequate performance. In embodiments, the network and communication parameters may be tagged and/or associated with certain geographical locations, network nodes, network paths, equipment types, carrier networks, service providers, types of transmission paths and the like. In embodiments, the end-devices may be configured to automatically record and/or report protocol parameter settings and to associate those settings with certain locations determined using GPS-type location identification capabilities resident in those devices. In embodiments, the end-devices may be configured to automatically record and/or report protocol parameters settings and to associate those settings with certain carrier networks, ISP equipment traversed, types of wired and/or wireless links and the like. 
         [0262]    In at least some embodiments, a packet coding and transmission control protocol as described above may adjust more than one parameter to achieve adequate or improved network performance. Improved network performance may be characterized by less delay in delivering data packets, less delay in completing file transfers, higher quality audio and video signal delivery, more efficient use of network resources, less power consumed by the end-users, more end-users supported by existing hardware resources and the like. 
         [0263]    In at least some embodiments, certain modules or features of the packet coding and transmission control protocol may be turned on or off depending on the data&#39;s path through a network. In some embodiments, the order in which certain features are implemented or controlled may be adjusted depending on the data&#39;s path through a network. In some embodiments, the probe transmissions and/or data transmissions may be used in open-loop or closed-loop control algorithms to adjust the adjustable parameters and/or the sequence of feature implementation in the packet coding and transmission control protocol. 
         [0264]    It should be understood that examples which involve monitoring to control the protocol can in general involve aspects that are implemented at the source, the destination, or at a combination of the source and the destination. Therefore, it should be evident that although embodiments are described above in which features are described as being implemented at particular endpoints, alternative embodiments involve implementation of those features at different endpoints. Also, as described above, monitoring to control the protocol can in general involve aspects that are implemented intermediate nodes or points in the network. Therefore, it should be evident that although embodiments are described above in which features are described as being implemented at particular endpoints, alternative embodiments involve implementation of those features at different nodes, including intermediate nodes, throughout the network. 
         [0265]    In addition to the use of monitored parameters for control of the protocols, the data may be used for other purposes. For example, the data may support network analytics that are used, for example, to control or provision the network as a whole. 
         [0266]    The PC-TCP approaches may be adapted to enhance existing protocols and procedures, and in particular protocols and procedures used in content delivery, for example, as used in coordinated content delivery networks. For instance, monitored parameters may be used to direct a client to the server or servers that can deliver an entire unit of content as soon as possible rather than merely direct the client to a least loaded server or to server accessible over a least congested path. A difference in such an new approach is that getting an entire file as fast as possible may require packets to be sent from multiple servers and/or servers that are not geographically the closest, over multiple links, and using new acknowledgement protocols that coordinate the incoming data while requiring a minimum of retransmissions or FEC overhead. Coordinating may include waiting for gaps in strings of packets (out-of-order packets) to be filled in by later arriving packets and/or by coded packets. In addition, the PC-TCP approaches may improve the performance of wireless, cellular, and satellite links, significantly improving the end-to-end network performance. 
         [0267]    Some current systems use “adaptive bit rates” to try to preserve video transmission through dynamic and/or poorly performing links. In some instances, the PC-TCP approaches described above replace adaptive bit rate schemes and may be able to present a very high data rate to a user for a long period of time. In other instances, the PC-TCP approaches are used in conjunction with currently-available adaptive bit rate schemes to support higher data rates on average than could be supported by adaptive bit rate schemes alone. In some instances, the PC-TCP approaches may include integrated bit rate adjustments as part of its feature set and may use any and/or all of the previously identified adjustable parameters and/or monitored parameters to improve the performance of a combined PC-TCP and bit-rate adaptive solution. 
         [0268]    Implementations of approaches described above may include software implementations, which use software instructions stored on non-transitory machine-readable media. The procedures and protocols as described above in the text and figures are sufficient for one skilled in the art to implement them in such software implementations. In some examples, the software may execute on a client node (e.g., a smartphone) using a general-purpose processor that implements a variety of functions on the client node. Software that executes on end nodes or intermediate network nodes may use processors that are dedicated to processing network traffic, for example, being embedded in network processing devices. In some implementations, certain functions may be implemented in hardware, for example, using Application-Specific Integrated Circuits (ASICs), and/or Field Programmable Gate Arrays (FPGAs), thereby reducing the load on a general purpose processor. 
         [0269]    Note that in some diagrams and figures in this disclosure, networks such as the internet, carrier networks, internet service provider networks, local area networks (LANs), metro area networks (MANs), wide area networks (WANs), storage area networks (SANs), backhaul networks, cellular networks, satellite networks and the like, may be depicted as clouds. Also note, that certain processes may be referred to as taking place in the cloud and devices may be described as accessing the cloud. In these types of descriptions, the cloud should be understood to be some type of network comprising networking equipment and wireless and/or wired links. 
         [0270]    The description above may refer to a client device communicating with a server, but it should be understood that the technology and techniques described herein are not limited to those exemplary devices as the end-points of communication connections or sessions. The end-points may also be referred to as, or may be, senders, transmitters, transceivers, receivers, servers, video servers, content servers, proxy servers, cloud storage units, caches, routers, switches, buffers, mobile devices, tablets, smart phones, handsets, computers, set-top boxes, modems, gaming systems, nodes, satellites, base stations, gateways, satellite ground stations, wireless access points, and the like. The devices at any of the end-points or intermediate nodes of communication connections or sessions may be commercial media streaming boxes such as those implementing Apple TV, Roku, Chromecast, Amazon Fire, Slingbox, and the like, or they may be custom media streaming boxes. The devices at the any of the end-points or intermediate nodes of communication connections or sessions may be smart televisions and/or displays, smart appliances such as hubs, refrigerators, security systems, power panels and the like, smart vehicles such as cars, boats, busses, trains, planes, carts, and the like, and may be any device on the Internet of Things (IoT). The devices at any of the end-points or intermediate nodes of communication connections or sessions may be single-board computers and/or purpose built computing engines comprising processors such as ARM processors, video processors, system-on-a-chip (SoC), and/or memory such as random access memory (RAM), read only memory (ROM), or any kind of electronic memory components. 
         [0271]    Communication connections or sessions may exist between two routers, two clients, two network nodes, two servers, two mobile devices, and the like, or any combination of potential nodes and/or end-point devices. In many cases, communication sessions are bi-directional so that both end-point devices may have the ability to send and receive data. While these variations may not be stated explicitly in every description and exemplary embodiment in this disclosure, it should be understood that the technology and techniques we describe herein are intended to be applied to all types of known end-devices, network nodes and equipment and transmission links, as well as to future end-devices, network nodes and equipment and transmission links with similar or improved performance. 
         [0272]    It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, some aspects of which are defined by the scope of the appended claims. Furthermore, other embodiments are within the scope of the following claims.