Abstract:
A system and method that optimizes transmission control protocol (TCP) flow control without intruding upon TCP&#39;s core algorithms. A control module relatively near a sender&#39;s local area network (LAN) automatically identifies a packet flow that has become window-limited. After the packet flow has been identified as window-limited, the control module relatively near the sender&#39;s LAN and another control module relatively near a receiver&#39;s LAN optimize the packet flow by increasing the window size indicated in the receiver&#39;s acknowledgment packet. Both control modules operate synchronously to transparently manage the packet flow between the sender and the receiver.

Description:
RELATED APPLICATIONS 
   This application claims priority from U.S. provisional application No. 60/517,934 filed on Nov. 5, 2003 which is incorporated by reference herein in its entirety. 
   This application is related to U.S. patent application Ser. No. 10/983,131, now U.S. Pat. No. 7,058,058, filed on Nov. 4, 2004, entitled “Transparent Optimization for Transmission Control Protocol Initial Session Establishment”, the contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of networking and the communication of data over a network and more particularly to transparent optimization for transmission control protocol (TCP) flow control. 
   BACKGROUND OF THE INVENTION 
   The transmission control protocol (TCP) is one of the most widely used and reliable data transport protocols across communications networks. One of TCP&#39;s primary distinctions and the reasons for its widespread use is a robust algorithm to share bandwidth between current TCP sessions. This sharing algorithm within TCP is generally know as “congestion control” since it attempts to avoid the problems of network congestion by automatically scaling back the data transfer to match the available bandwidth capacity. Multiple concurrent and reliable data transfers across a shared network link may result in high congestion if each of the data transfer sessions tries to fully utilize the link capacity. This high congestion may result in high packet loss, which in turn may cause a large number of packet retransmissions, ultimately resulting in network collapse. TCP&#39;s congestion control algorithm avoids this problem by automatically determining how much bandwidth is available and sharing the total available bandwidth equally with other concurrent TCP sessions. TCP&#39;s dynamic sharing algorithm is therefore a fundamental building block of data communications across a packet switched internet protocol (IP) network and has resulted in the adoption of TCP/IP as a universal communications standard. 
   TCP utilizes various internal algorithms to provide its capability of congestion control. These algorithms include flow control, slow start, packet reordering, packet loss detection, retransmission timers, and numerous other mechanisms to dynamically decrease or increase the data transmission rate based on network conditions. 
   Network latency is a common problem that affects network and application performance. Network latency is attributable to several factors, including physical distance, number of hops, switching and router relays, and network congestion. Because these factors are not constants, networks may have unpredictable latency over a period of time. The variation in network latency depends on the distance spanned by the network link and the transmission medium used by the link. For instance, a local high-speed dedicated line between two buildings within a metro area may experience 5 milliseconds (ms) of one-way latency, while a global long distance asynchronous transfer mode (ATM) link between the United States and Europe may have anywhere from 50 to 250 ms of one-way latency. Similarly, a satellite link typically incurs about 240 to 300 ms of one-way latency, due to the time to transmit a signal up to an orbiting satellite and back. 
   The impact of latency on network applications may be traced directly to the inefficiencies of TCP under conditions of network latency. Most network applications can be classified into short-transaction based “chatty” applications or bulk data transfer applications. Bulk data transfer applications typically transmit 100s of kilobytes or megabytes of data across the network with the total transfer times being measured in several seconds, minutes, and in many cases hours. Examples of such applications include networked file systems, archiving and storage applications, file transfer protocol (FTP) transfers, sharing and distribution of large engineering or design documents, etc. In these applications, the common performance bottleneck is often the latency across the network, which causes lower application throughput via TCP. In particular, the flow control algorithm within TCP often causes the lower application throughput and higher application response time. 
   TCP&#39;s flow control algorithm is a mechanism to prevent the receiver from receiving more data than it is capable of processing or buffering. For example, if the receiving TCP stack has a buffer to store 16 kilobytes of data, the sender is not allowed to transmit more than 16 kilobytes of data at any time to the receiver. The receiver continuously sends back acknowledgments to the sender throughout the data transfer stating how much additional data the receiver can accept. This additional data that the receiver can accept is known as the “window indication” (or “window advertisement”) and is included as a field in the TCP header. 
   The flow control algorithm operates efficiently and does not introduce any unnecessary delays when the receiver and the sender are separated by a short distance low latency link. But as the distance and latency between the receiver and the sender is increased, the round-trip time (RTT) between sending a data packet and then receiving an acknowledgment from the receiver also increases. Since the flow control algorithm prevents the sender from transmitting data to the receiver when the receiver has not indicated that it is ready for this additional data, long RTTs between the two endpoints may cause the sender to delay sending additional data packets to wait for the next acknowledgment from the receiver. For example, if the receiver can accept 16 kilobytes of data at a time, then the sender may transmit all 16 kilobytes in a few milliseconds and then spend several additional milliseconds waiting for an acknowledgment to start transmitting the next block of 16 kilobytes. This period of time during which the sender is waiting for an acknowledgment depends on both the bandwidth and the link latency. The latency-based TCP idle time may cause a single TCP flow to achieve lower throughput than is actually available on the network link. This unused capacity translates into a higher transmission time since the TCP flow cannot utilize the full existing network bandwidth. 
   Flow pipelining is used for TCP connections that are limited in the rate at which they are able to transfer data because the maximum TCP window size configured on the receiver is smaller than the bandwidth-delay product of the network across which it is transferring the data. A conventional solution to this problem is for the receiver to advertise or indicate a window size that is bigger than the bandwidth-delay product of the network. However, for practical reasons, this indicated window size may be limited by the available memory on the receiver and the sender. As a result, a reasonable value is chosen by operating system developers and set as the default value of the window size. This value is adequate for most TCP connections across a local area network. But when a TCP connection is made across a high latency network, this value may not be adequate. The computers participating in the data transfer cannot dynamically discover and fix the small window size problem. This is because this problem can only be reliably observed by a device (which knows the bandwidth and current utilization of the long latency segment) processing data just before the longest latency segment of the network on the sender side. However, after this device determines that a particular transfer is window-limited, TCP provides no means for the device to inform the receiver that an increased window size is desired. 
   A solution that arbitrarily has the device increase the window size produces poor results because it generates a large amount of out of window data. Therefore, when there is a minor network error such as a single packet loss, this solution may cause a large amount of data that is outside of the receive window. The data that is outside of the receive window may be discarded by the receiver as it reaches the receiver. In addition to having the sender retransmitting the discarded data, it also causes the sender to misread the loss as a network congestion event and thus to slow down its transmission. 
   What is needed is a system and method for optimizing TCP&#39;s flow control to improve the performance of TCP sessions without intruding upon TCP&#39;s core algorithms. 
   SUMMARY OF THE INVENTION 
   The present invention is a system and method for optimizing TCP&#39;s flow control without intruding upon TCP&#39;s core algorithms. The invention bi-directionally monitors the state of TCP flows on both sides of a network pipe. A control module relatively near a sender&#39;s local area network (LAN) automatically identifies a packet flow that has become latency-limited (or “window-limited”) based on TCP header information. After the packet flow has been identified as window-limited due to the network latency, the control module relatively near the sender&#39;s LAN and another control module relatively near a receiver&#39;s LAN optimize this packet flow by increasing the window size indicated in the receiver&#39;s acknowledgment packet. Both control modules operate synchronously to transparently manage the packet flow between the sender and the receiver. This transparent optimization process allows the sender to maximize the amount of data in transit, thereby substantially minimizing the idle times during which the sender is waiting for additional acknowledgments from the receiver. 
   The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of one example of a network environment in which the present invention can operate. 
       FIGS. 2A and 2B  are illustrations of an example of TCP&#39;s flow control algorithm. 
       FIG. 3  is a flowchart illustrating a method implemented by one embodiment of the present invention for optimizing TCP&#39;s flow control. 
       FIG. 4  is a flowchart illustrating a method implemented by one embodiment of the invention and executed by a control module relatively near a sender to optimize TCP&#39;s flow control. 
       FIG. 5  is a flowchart illustrating a method implemented by one embodiment of the invention and executed by a control module relatively near a receiver to optimize TCP&#39;s flow control. 
       FIG. 6  is a timing diagram illustrating an example TCP transaction that is window-limited and without being optimized by an embodiment of the invention. 
       FIG. 7  is a timing diagram illustrating an example TCP transaction that is window-limited but is optimized by an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit of each reference number corresponds to the figure in which the reference number is first used. 
   Reference in the specification to “one embodiment” or to “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
   Some portions of the detailed description that follows are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps (instructions) leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical, magnetic or optical signals capable of being stored, transferred, combined, compared and otherwise manipulated. It is convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. Furthermore, it is also convenient at times, to refer to certain arrangements of steps requiring physical manipulations of physical quantities as modules or code devices, without loss of generality. 
   It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or “determining” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
   Certain aspects of the present invention include process steps and instructions described herein in the form of an algorithm. It should be noted that the process steps and instructions of the present invention could be embodied in software, firmware or hardware, and when embodied in software, could be downloaded to reside on and be operated from different platforms used by a variety of operating systems. 
   The present invention also relates to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, application specific integrated circuits (ASICs), or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. Furthermore, the computers referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
   The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may also be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the present invention as described herein, and any references below to specific languages are provided for disclosure of enablement and best mode of the present invention. 
   In addition, the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. 
     FIG. 1  is an illustration of one example of a network environment in which the present invention can operate. A receiver  102  can be any device that sends/receives data to/from one or more endpoints using TCP. Receiver  102  is connected to a control module  104  (e.g., in a control device) via a communications network, such as a LAN  106 . Alternatively, control module  104  is directly connected to the receiver  102  without via the LAN  106  or is implemented within receiver  102  as a program module. Control module  104  is connected to one or more other control modules via another communications network such as a wide area network (WAN)  108 . Even though  FIG. 1  shows that control module  104  is connected to one other control module (i.e., the control module  110 ), control module  104  can be connected to more than one control module. Each of the other control modules is connected to a sender via yet another communications network. For example,  FIG. 1  shows that control module  110  (e.g., in a control device) is connected to a sender  112  via a LAN  114 . The sender  112  can be any device that receives/sends data from/to one or more endpoints using TCP. Moreover, control module  110  can be directly connected to sender  112  without via the LAN  114  or can be implemented within sender  112  as a program module. 
   In one embodiment of the invention, control module  104  is located relatively nearer to receiver  102  than to sender  112 , while control module  110  is located relatively nearer to sender  112  than to receiver  102 . Accordingly, control module  104  is physically and logically associated with receiver  102 , and control module  110  is physically and logically associated with sender  112 . 
   After a TCP session has been established between receiver  102  and sender  112 , TCP begins to provide flow control service to prevent sender  112  from overflowing the buffer of receiver  102 . TCP flow control matches the rate at which sender  112  is sending application-layer data to the rate at which receiver  102  is reading the data. TCP provides flow control by having sender  112  maintain a variable called the receive window. The receive window is used to give sender  112  explicit information on how much free buffer space is available at receiver  102 . The receive window is dynamic; that is, it changes throughout a TCP session&#39;s lifetime. Receiver  102  informs sender  112  how much spare room it has in the connection buffer by advertising its current value of the receive window in the window field of the TCP packet it sends to sender  112 . By keeping the amount of unacknowledged data less than the value of the receive window, sender  112  can prevent itself from overflowing the receiver buffer at receiver  102 . 
     FIGS. 2A and 2B  illustrate an example of TCP&#39;s flow control algorithm.  FIG. 2A  illustrates a portion of a byte stream broken into packets and each packet&#39;s sequence number. The sequence number for the portion of the byte stream starts at 0.  FIG. 2B  illustrates how receiver  102  builds up a copy of the byte stream. The dotted box represents the receive window, which is assumed to have a constant value of 1600 for illustration purposes. 
   At A, packet  1  arrives at receiver  102 , which acknowledges it by sending an acknowledgment packet to sender  112  with an acknowledgment number (ACK) of 1000 and a window size (RcvWindow) of 1600. Since ACK+RcvWindow=2600, sender  112  can send packets  2 ,  3 , and  4  to receiver  102 . Sender  112  then sends packets  2  and  3  to receiver  102 . 
   At B, packet  3  arrives at receiver  102 , but packet  2  has been delayed at the network link. Receiver  102  sends another acknowledgment packet to sender  112  with ACK=1000 and RcvWindow=1600. Sender  112  sends packet  4  to receiver  102  at this time. 
   At C, packet  4  arrives at receiver  102 , but packet  2  is still outstanding. Again, receiver  102  sends an acknowledgment packet to sender  112  with ACK=1000 and RcvWindow=1600. However, sender  112  cannot send packet  5  to receiver  102  at this time because it would bring up the sequence number to 2800, which is greater than ACK+RcvWindow. Sender  112  thus cannot send further packets to receiver  102  until receiver  102  sends a new acknowledgment packet to sender  112 . 
   At D, the delayed packet  2  now reaches receiver  102 . Receiver  102  sends an acknowledgment packet to sender  112  with ACK=2400 and RcvWindow=1600. In other words, receiver  102  slides or advances the receive window to allow sender  112  to send more data. This window advancement allows sender  112  to send data up to sequence number 4000 (i.e., ACK+RcvWindow=4000). Accordingly, sender  112  can send packets  5 ,  6 , and  7  to receiver  102 . 
   As can be seen, TCP&#39;s flow control algorithm prevents sender  112  from sending data to receiver  102  when receiver  102  has not indicated that it is ready for this additional data. As the distance and latency between receiver  102  and sender  112  is increased, the RTT between sending a packet and receiving an acknowledgment from receiver  102  also increases. Long RTTs between receiver  102  and sender  112  may cause sender  112  to delay sending additional packets to wait for the next acknowledgment from receiver  102 . 
   Thus, in an embodiment of the invention, control module  104  and control module  110  cooperate to synchronously manage the packet flow between receiver  102  and sender  112 . According to an embodiment of the invention, control modules  104  and  110  cooperate to increase the indicated window and to keep the pipeline increased to be greater than or equal to the product of network bandwidth and round-trip delay between receiver  102  and sender  112 . Thus, embodiments of the invention optimize TCP&#39;s flow control by providing a larger indicated window for sender  112  to send data to receiver  102 . 
     FIG. 3  is a flowchart illustrating an operation of TCP&#39;s flow control optimized by control modules  104  and  110  according to an embodiment of the invention. Control module  110  associated with sender  112  bi-directionally monitors  302  one or more packet flows between receiver  102  and sender  112 . In particular, control module  110  examines the sequence numbers of data packets sent by sender  112  and the acknowledgment numbers and indicated windows of acknowledgment packets sent by receiver  102 . Sender  112  sends a data packet to receiver  102 . Before the data packet reaches receiver  102 , control module  110  intercepts  302  the data packet. Based on the examination of previous packet flows, control module  110  determines that the data packet carries bytes with sequence numbers at or near the top of the window indicated by receiver  102 . Control module  110  considers such a data packet to be “window-limited.” When control module  110  detects a window-limited packet, it stores the currently indicated window associated with this packet flow in a memory area. Control module  110  can thereafter use the stored window value to determine if a subsequent data packet sent by sender  112  is window-limited. 
   According to a preferred embodiment of the invention, control module  110  also characterizes the data packet as window-limited (e.g., by using one or more fields available in the TCP header or adding data to the data packet). Control module  110  can also characterize the data packet by using the lowest order bit of the window indication to indicate whether the current data packet is window-limited. After control module  110  characterizes the window-limited data packet, it sends the characterized data packet to receiver  102 . 
   Before the characterized data packet reaches receiver  102 , control module  104  associate with receiver  102  intercepts the characterized data packet and learns from seeing the characterization that this packet flow is window-limited. Control module  104  then removes the characterization from the data packet and sends the data packet to receiver  102 . For a window-limited packet flow, control module  104  also increases the window indicated by receiver  102  if control module  104  has sufficient buffering resources. In particular, when receiver  102  sends one or more acknowledgment packets to sender  112 , control module  104  intercepts these acknowledgment packets and increases  306  the indicated windows indicated in these acknowledgment packets. For example, the changed value of the window indication may be 64 kilobytes. According to an exemplary embodiment of the invention, control module  104  can set the value of the window indication to be as high as 1,073,725,440 bytes by negotiating the window scaling to a sufficiently high value during the connection setup. Control module  104  further sets a flag so that for a subsequent acknowledgment packet from the receiver it also increases the window indication. 
   Control module  104  sends the adjusted acknowledgment packets to sender  112 . Sender  112  receives the adjusted acknowledgment packets and sees the adjusted window indicated in the acknowledgment packets. Because of the larger indicated window, sender  112  sends additional data packets to receiver  102 , up to the amount allowed by the adjusted window. Before these data packets reach receiver  102 , control module  104  intercepts  308  these data packets as they arrive at control module  104 . Control module  104  then determines  310  if receiver  102  has already advanced its window far enough such that a particular data packet arriving from sender  112  is within the current window of receiver  102 . In one embodiment of the invention, control module  104  can make this determination by monitoring packet flows from receiver  102  to sender  112 . Specifically, control module  104  intercepts one or more acknowledgment packets sent by receiver  102  to sender  112  and determines if receiver  102  has advanced the window far enough to accommodate the data packet from sender  112  (e.g., by examining the acknowledgment number and window size). If receiver  102  has advanced the window far enough to accommodate the data packet, then control module  104  sends  312  the data packet directly to receiver  102  without storing the data packet in its buffer. 
   On the other hand, if receiver  102  has not advanced the window far enough to accommodate the data packet, control module  104  stores  314  the data packet in its buffer. While the data packet is stored in the buffer, control module  104  continues to monitor acknowledgment packets from receiver  102  to determine if receiver  102  has subsequently advanced the window. If control module  104  determines that receiver  102  has subsequently advanced the window far enough, it sends  316  the stored data packet to receiver  102 . In an embodiment of the invention, if TCP timestamp is used in the data packet, control module  104  adjusts the timestamp of the data packet to be as recent as the latest data packet from sender  112  to receiver  102 , before sending the data packet to receiver  102 . 
   During the course of the TCP session, the packet flow may no longer be window-limited. For example, after receiver  102  advances its window, it sends one or more acknowledgment packets to sender  112 . Based on the acknowledgment number and window size indicated in an acknowledgment packet, control module  110  determines that a data packet from sender  112  to receiver  102  no longer has a sequence number at or near the top of the indicated window and hence is not window-limited. Accordingly, control module  110  characterizes one or more data packets from sender  112  to receiver  102  as not window-limited. Control module  104  intercepts the characterized data packet on its way to receiver  102  and finds out that the packet flow is no longer window-limited. Therefore, for a subsequent acknowledgment packets sent by receiver  102  to sender  112 , control module  104  intercepts the acknowledgment packet, adjusts the acknowledgment packet by reducing the indicated window, and forwards the acknowledgment packet to sender  112 . In an embodiment of the invention, the amount of each reduction is such that the top of the indicated window remains constant. Control module  104  continues to reduce the indicated window until it reaches the size of the receive window. Moreover, control module  104  can continue to reduce the indicate window until the packet flow becomes window-limited. In this case, control modules  104  and  110  begin to increase the indicated window. 
   According to an embodiment of the invention, each time control module  104  or  110  changes any field of the TCP header, it also adjusts the checksum of the TCP header such that the checksum is correct if it was correct before the change and incorrect if it was incorrect before the change. 
   Furthermore, an embodiment of the invention also allows the reporting of control module performances. Specifically, the window indication that is stored in control module  110  along with the current sequence number and acknowledgment number is used to estimate the extent that the increase in window indication is increasing the rate of data transfer (which thus calculates an estimate of the data transfer rate without the optimization by control modules  104  and  110 ). This estimate can be used to continuously evaluate the efficacy of the optimization. However, with the larger window indication of this optimization, there are more packets passing through the network per second, and it is difficult to accurately estimate the times at which the same packets would pass through the network without the optimization. Accordingly, an embodiment of the invention finds out how much smaller each packet would have to be when not using the optimization so that the packets fit into the original window size, and have the rate at which packets arrive be the same as the rate of packet arrival with the optimization. Thus, the embodiment of the invention estimates how much smaller each packet would be if the packets arrive at the same rate as after the optimization. So for each packet, the following is calculated:
 
Size of the packet without optimization=True size of the packet*(original window size/(sequence number of the highest byte in the packet−the current acknowledgment number))
 
   However, the size of the packet without optimization cannot be greater than the current true size of the packet. So if the calculation above produces a value that is greater than the current true size of the packet, then the value of the size of the packet without optimization is set to be the current true size of the packet. 
   This calculated number is accumulated over time to estimate the data transfer rate without the optimization. Specifically, the total size of packets without the optimization in a period (e.g., 1 second) provides the data transfer rate without the optimization. And the total of the true packet sizes in that period provides the data transfer rate with the optimization. 
     FIG. 4  is a flowchart illustrating a TCP optimization routine that is implemented by one embodiment of the present invention and executed by control module  110  associated with sender  112 . Control module  110  monitors  402  one or more packet flows between sender  112  and receiver  102 . Specifically, control module  110  examines the sequence numbers of data packets sent by sender  112  and acknowledgment numbers and window indications of acknowledgment packets sent by receiver  102 . Control module  110  receives  404  a data packet from sender  112 . Control module  110  then determines  406  if the data packet from sender  112  to receiver  102  is window-limited. For instance, if the data packet carries bytes with sequence numbers at or near the top of the window indicated by receiver  102 , then it is window-limited. If the data packet is not window-limited, control module  110  continues to monitor the packet flows between sender  112  and receiver  102  at  402 . If the data packet is window-limited, control module  110  stores  408  the window indication in a memory area for future reference. 
   Control module  110  also characterizes  410  the data packet as window-limited and sends the characterized data packet to receiver  102 . For example, control module  110  may characterize the data packet by using one or more fields available in the TCP header, adding data to the data packet, or using the lowest order bit of the window indication. Control module  110  monitors  412  subsequent packet flows between sender  112  and receiver  102 . Control module  110  then determines  414  if the packet flow is still window-limited. If so, control module  110  continues to monitor the packet flows at  412 . If not, control module  110  characterizes  416  a data packet from sender  112  to receiver  102  as not window-limited. The process flow then returns to  402 . 
     FIG. 5  is a flowchart illustrating a TCP optimization routine that is implemented by one embodiment of the present invention and executed by control module  104  associated with receiver  102 . Control module  104  receives  502  a characterized data packet from sender  112 . Control module  104  then determines  504  if this characterization indicates that the data packet is window-limited or not window-limited. If the characterization indicates not window-limited, control module  104  reduces  506  the window indication in an acknowledgment packet sent from receiver  102  to sender  112 . According to an embodiment of the invention, the minimum size of the window indication is the size of the receive window. After control module  104  reduces the window indication, the process flow returns to  502 . If the characterization indicates window-limited, control module  104  determines  508  if it has sufficient buffering resources. If control module  104  does not have sufficient buffering resources, it does not change the window indication, and the process flow returns to  502 . If control module  104  has sufficient buffering resources, it increases  510  the window indication in an acknowledgment packet sent by receiver  102  to sender  112 . Control module  104  also sets a flag so that for a subsequent acknowledgment packet from the receiver it also increases the window indication. 
   Control module  104  also determines  512  if receiver  102  has advanced the window far enough to accommodate the data packet from sender  112 . If receiver  102  has advanced the window far enough, control module  104  sends  514  the data packet without the characterization directly to receiver  102 . The process flow then returns to  502 . If receiver  102  has not advanced the window far enough, control module stores  516  the data packet in its buffer. When the window is advanced far enough, as indicated in one or more acknowledgment packets sent by receiver  102 , control module  104  sends  518  the data packet without the characterization to receiver  102 . The process flow then returns to  502 . In an embodiment of the invention, if TCP timestamp is used in the data packet, control module  104  adjusts the timestamp of the data packet to be as recent as the latest data packet from sender  112  to receiver  102 , before sending the data packet to receiver  102 . 
     FIG. 6  shows a timing diagram of an example TCP transaction that is window-limited and without the optimization by control modules  104  and  110 . In  FIG. 6 , data transfer is from sender  112  to receiver  102 . Packets from receiver  102  to sender  112  are acknowledgment packets. Furthermore, one acknowledgment packet is assumed to be generated for every second data packet, and the window size indicated in the acknowledgment packets is assumed the equivalent of four packets. As can be seen from  FIG. 6 , when the window is limited and the latency is large between sender  112  and receiver  102 , substantial time lapses between the sending of consecutive sets of data packets by sender  112  because of the delay in transmitting the acknowledgment packets by receiver  102 . The latency-based TCP idle time causes a single TCP flow to achieve lower throughput than is actually available on the network link. This unused capacity translates into a higher transmission time since the TCP flow cannot utilize the full existing network bandwidth. 
     FIG. 7  shows a timing diagram of an example TCP transaction that is window-limited but is optimized by control modules  104  and  110 . In  FIG. 7 , data transfer is from sender  112  to receiver  102 . Packets from receiver  102  to sender  112  are acknowledgment packets. Furthermore, one acknowledgment packet is assumed to be generated for every second data packet, and the window size indicated in the acknowledgment packets is assumed the equivalent of four packets, but is increased to 16 packets by control module  104  based on a packet-by-packet signal received from control module  110 . As can be seen in  FIG. 7 , because of the larger indicated window size, sender  112  can transmit a larger amount of data into the network. Accordingly, there is not a substantial time lapse between the sending of consecutive sets of data packets and acknowledgment packets, as distinguishable from  FIG. 6 . Thus, the optimization according to embodiments of the invention achieves a higher throughput and shorter transmission time than if the optimization is not used. 
   While particular embodiments and applications of the present invention have been illustrated and described herein, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes, and variations may be made in the arrangement, operation, and details of the methods and apparatuses of the present invention without departing from the spirit and scope of the invention as it is defined in the appended claims.