Abstract:
In a packet communication system with large bandwidth delay product (BDP) and employing Transmission Control Protocol (TCP), the reported advertised window size as advertised by a receiver is employed only as an indication of window size and not as a throughput limit so that more data is sent than the amount specified by the advertised window size. Since the receiver can process all incoming TCP packets, the receiving buffer is consistently near empty. Since the TCP&#39;s advertised window size is not accepted as the absolute amount of buffer space available at the receiver, the sender is not constrained by the absolute value of the receiver&#39;s advertised window size and instead can transmit more data than the absolute value of the advertised window, enabling the system to increase the actual window size without modifying the link ends. This improved large-BDP-capable protocol is denoted TCP-SC.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     NOT APPLICABLE 
     BACKGROUND OF THE INVENTION 
     This invention relates to digital data communication and techniques for improving throughput of channels subject to large bandwidth-delay product, such as satellite-based communication links and mobile data networks. 
     It is known that the de facto standard for transmitting data over networks, Transmission Control Protocol (TCP), does not perform well in satellite or mobile communication networks due to the very large bandwidth-delay product (BDP) of the channel. For example, a communication satellite having channel bandwidth capacity between 24 Mbps and 155 Mbps. has a round-trip-delay (RTT) of 500 ms, so the lowest bandwidth of 24 Mbps will lead to a bandwidth-delay product of 1.5 MB—a value far exceeding the maximum advertised window size of TCP of 64 KB. In this case TCP&#39;s flow control mechanism will limit the throughput to no more than 1 Mbps, which is less than 5% of the satellite&#39;s link-layer bandwidth capacity, as hereinafter explained. 
     TCP Flow Control 
     TCP has a built-in flow control mechanism that is designed to prevent a fast sender from overflowing a slow receiver. It works by reporting the receiver&#39;s buffer availability, i.e., the advertised window, back to the sender via a 16-bit field inside the TCP header so that the sender can prevent sending more data than the receiver&#39;s buffer can store. Computer processing power has grown tremendously such that computers can now easily keep up with the arriving stream of data up to hundreds of Mbps data rate. Thus an arrived packet will quickly be retrieved by the application from the receiver buffer, and in most cases this can be completed even before the next packet arrival. As a result, the reported advertised window (AWnd) simply stays at the maximum receiver buffer size. In such cases TCP&#39;s flow control mechanism is not activated at all, as it is not needed. 
     However TCP&#39;s flow control mechanism can become the performance bottleneck in networks with a large bandwidth delay product (BDP). Consider the scenario where a sender is connected to a receiver over a large BDP link (100 Mbps, 250 ms one-way delay, BDP=50 Mb). Ignoring processing time, when the sender receives an acknowledgement (ACK), the reported advertised window (AWnd) size is in fact the value 250 ms prior to current time (i.e., 34 KB). During this time, the receiver application could have retrieved additional data from the receiver buffer, thereby freeing up more buffer space (i.e., 64 KB). 
     Due to the delayed AWnd, the sender cannot send more than the reported AWnd and thus cannot make use of the extra buffer space available at the receiver. In cases where the BDP is larger than the maximum AWnd, the sender will operate in a stop-and-go manner resulting in severe underutilization of the network channel. 
     The conventional solution to the above problem is to make use of TCP&#39;s Large Window Scale (LWS) extension as defined in the TCP protocol Request For Comments RFC 1323. This extension allows TCP to negotiate during connection setup a multiplier to apply to the window size so that a window size larger than 64 KB can be used. However, this approach relies on two assumptions: First, either the operating system or the application needs to be modified to explicitly make use of TCP&#39;s LWS extension. Second, there must be a way for the application to request the use of LWS during connection setup. 
     While these two assumptions can be easily satisfied in the laboratory where custom network applications and operating systems can be developed to exploit TCP&#39;s LWS extension, they will likely prevent the vast amount of network applications already available in the Internet to benefit from TCP&#39;s LWS extension. 
     Much research has been done to improve the performance of TCP in certain large networks. The existing research is classified in three categories: modifying both sender and receiver; modifying the sender only; and modifying the receiver only. Each of the categories is briefly characterized here by way of background. 
     Sender-Receiver-Based Approaches 
     Jacobson et al. “RFC 1323: TCP extensions for high performance,” May 1992, RFC1323 proposed the Large Window Scale (LWS) extension to TCP which is currently the most widely supported solution. It works by scaling the advertised window (AWnd) by a constant factor throughout the connection. With the maximum LWS factor 14, the maximum AWnd can be increased up to 1 GB ((2 16 −1)*2 14 ≈2 30 ). Alternatively, the application can be modified to initiate multiple TCP connections in parallel to increase throughput by aggregating multiple TCP connections, as described in Lee, D. Gunter, B. Tierney, B, Allcock, J. Bester, J. Bresnahan and S. Tuecke, “Applied Techniques for High Bandwidth Data Transfers Across Wide Area Networks,”  Proceedings of International Conference on Computing in High Energy and Nuclear Physics , September 2001 and H. Sivakumar, S. Bailey and R. Grossman, “PSockets: The Case for Application-level Network Striping for Data Intensive Applications using High Speed Wide Area Networks,”  Proceedings of Super Computing , November 2000. This approach effectively multiplies the AWnd and the congestion window (CWnd) by the number of TCP flows and so can mitigate the AWnd limitation. However, aggregating multiple TCP connections will also allow the application to gain an unfair amount of bandwidth from competing TCP flows. Hacker et al. (T. Hacker, B. Noble and B. Athey, “Improving Throughput and Maintaining Fairness using Parallel TCP,”  Proceedings of IEEEInfocom  2004, March 2004) solved this problem by deferring CWnd increase until multiple acknowledgements (ACKs) are received so as to compensate for the larger window size. 
     Sender-Based Approaches 
     Apart from AWnd limit, the congestion window maintained by the sender may also limit throughput of TCP in large BDP-type networks. Specifically, the growth of the CWnd is triggered by the arrival of the ACKs. Thus in a long delay path it may take a longer time for the CWnd to grow to sufficiently large value so that the link bandwidth can be fully utilized. 
     To address this problem Allman et al. (M. Allman, S. Floyd and C. Partridge, “RFC 3390: Increasing TCP&#39;s Initial Window,” October 2000) proposed in RFC3390 to initialize the CWnd to a larger value (as opposed to one TCP segment) so that it can grow more quickly in large delay networks to ramp up TCP&#39;s throughput. Since then, much effort had been out into developing more sophisticated congestion control algorithms such as CUBIC (I. Rhee and L. Xu “CUBIC: A new TCP-friendly high-speed TCP variant,”  Proceedings. PFLDNet&#39; 05, February 2005), BIC (L. Xu, K. Harfoush and I. Rhee, “Binary Increase Congestion Control (BIC) for Fast Long-Distance Networks,”  In Proceedings of IEEE INFOCOM  2004, March 2004), FAST (C. Jin, D. X. Wei and S. H. Low, “FAST TCP: Motivation, Architecture, Algorithms, Performance,”  In Proceedings of IEEE INFOCOM  2004, March 2004), H-TCP (R. Shorten and D. Leith, “H-TCP: TCP for High-Speed and Long-Distance Networks,”  Second International Workshop on Protocols for Fast Long - Distance Networks , Feb. 16-17, 2004, Argonne, Ill.) to further improve TCP&#39;s throughput performance. 
     These solutions addressed the limitation of CWnd growth and thus are complementary to the present invention. 
     Receiver-Based Approaches 
     At the receiving end, Fisk and Feng proposed dynamic right-sizing of the AWnd by estimating the CWnd at the receiver and then dynamically adapt the receiver buffer size, i.e., the AWnd, to twice the size of the estimated CWnd. (M Fisk and W-C Feng, “Dynamic Right-Sizing in TCP,”  Proceedings of the Los Alamos Computer Science Institute Symposium , October 2001) This ensures that when the sender&#39;s CWnd doubles (e.g., after receiving an ACK) the AWnd will not become the bottleneck. 
     More recent operating systems such as Linux 2.4 and Microsoft Windows Vista also implemented receiver buffer size auto-tuning by estimating the BDP from the data consumption rate. (See J. Davies, “The Cable Guy: TCP Receive Window Auto-Tuning,”  TechNet Magazine , January 2007.) 
     What is needed is a mechanism to enable TCP to fully utilize the underlying network bandwidth without the need for modifying network applications running on both ends of the communication session nor requiring Large Window Scale (LWS) support from the operating system. 
     SUMMARY OF THE INVENTION 
     According to the invention, in a packet communication system with a communication link having a very large bandwidth delay product (BDP) and employing Transmission Control Protocol (TCP), the system having a transmitter, a receiver and a gateway, the reported advertised window size as advertised by the receiver is employed only as an indication of window size and not as a throughput limit so that more data is sent than the amount specified by the advertised window size as advertised. This advertised window size is consistently at a small but upper limit. Given the high processing speed capabilities of conventional receivers as compared to the typical data rates, the receiver has no trouble processing all incoming TCP packets so that the receiving buffer is consistently near empty. The advertised window size is thus not accepted as the absolute amount of buffer space available at the receiver. Consequently the sender is no longer constrained by the absolute value of the receiver&#39;s advertised window size and instead can transmit more data than the value of the advertised window, enabling the communication system to increase the actual window size without modifying either end of the communicating applications. This improved large-BDP-capable protocol is denoted TCP-SC. 
     A TCP-SC gateway according to the invention is designed to carry out this new function so that when it receives an acknowledgement (ACK) packet from the receiver which carries an updated advertised window size (AWnd), it will compute a new virtual advertised window size (VWnd) that takes into consideration the receiver&#39;s processing capability and buffer availability to enable it to forward more data than the AWnd would allow. TCP-SC does not require any modification to the receiver application or require support from the operating system, and so can be more readily deployed by an Internet service provider (ISP) or a satellite operator or a mobile network operator to accelerate all TCP traffics. 
     The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system according to the invention. 
         FIG. 2  is a graph illustrating throughput rates of a large BDP system where the traffic flow is interrupted due to protocol anomalies. 
         FIG. 3  is a graph illustrating throughput rates in a system fully functional according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the invention, a communication system has a TCP-SC gateway system  10 . The inventive TCP-SC gateway system  10  is disposed between a sender, such as a web server  12 , and a receiver  14  comprising a TCP buffer at a user terminal, such as a web browser client  16 , as shown in  FIG. 1 . All Internet Protocol (IP) packets are routed through the TCP-SC gateway  10  before forwarding to the receiver  14  via a satellite  18 . Thus in the case of a satellite system, the path between the gateway system  10  and the client  16  is a large BDP path comprising link segments  20  and  22 . The TCP-SC gateway system  10  filters out TCP packets for processing and simply forwards all other packets to maintain compatibility with other traffic (e.g., UDP-based protocols). 
     The principle of TCP-SC is to estimate the receiver&#39;s TCP buffer availability at the time the to-be-forwarded TCP segment arrives at the receiver  14 . If the estimated TCP buffer availability is sufficiently large to accommodate the TCP segment, then it will be forwarded to the receiver  14  immediately. Otherwise the gateway system  10  defers transmission to a later time until sufficient receiver buffer space is predicted to become available. 
     To predict the receiver TCP buffer space availability, the TCP-SC gateway system  10  needs three inputs, namely: the receiver&#39;s TCP buffer availability at a previous time instant; the time it takes for the forwarded TCP segment to arrive at the receiver  14 ; and the rate at which the receiver application processes data received inside the TCP buffer (i.e., remove received data from the TCP buffer). The first input can be determined from the AWnd in the last ACK received. The remaining two inputs need to be estimated as hereinafter explained. 
     In the following it is assumed: (a) the gateway  10  always has input data  24  to forward as outputdata  26 ; (b) network delays remain constant; (c) network delays of the link  20 ,  22  between the gateway  10  and the client  16  are symmetric in the forward and the reverse direction; and (d) the receiver  14  at the client  16  generates an ACK containing the advertised window size immediately upon the arrival of a TCP segment, i.e., there is zero effective processing delay. 
     Gateway-Receiver Roundtrip Time (RTT) Estimation 
     Consider the following: Let D be the round trip time RTT between the gateway  10  and the client  16 . The RTT is not known a priori and thus will need to be estimated from measurements. Let f i  be the time packet i was forwarded by the gateway  10  to the client  16 , and let t i  be the time at which the corresponding ACK arrived at the gateway  10 . Then the RTT D can be computed from:
 
 D=t   i   −f   i   (1)
 
for assumed symmetric network delays.
 
     To smooth out random fluctuations in the estimated RTT, the gateway will apply exponentially weighted moving averaging to the measured values:
 
RTT′=α×RTT+(1−α)× D   (2)
 
where the weight α=0.9 follows the ones used in TCP&#39;s internal RTT estimator as reported by V. (Jacobson and M. Karels, “Congestion Avoidance and Control,” available as of Jun. 39, 2009 at ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z).
 
Receiver Processing Rate Estimation
 
     The processing rate at the receiver  14  can be estimated from comparison of the receiver&#39;s buffer availabilities between two time instants. The time at which acknowledgement packets or ACKs arrive at the gateway are good candidates for these two time instants, as every ACK packet contains the receiver buffer availability inside a field known as the AWnd field  28  (as shown input to the TCP-SC 10 ). 
     Let t i  and a i  be the respective arrival time and the AWnd value of ACK packet i. Let f j  and q i  be the time TCP packet j was forwarded by the gateway  12  and the segment size of the packet i respectively. Then for some positive integer k, the processing rate, denoted by R, can be computed from: 
                   R   =         (       a     i   +   k       -     a   i       )     +       ∑       ∀   j     ❘       f   j     ∈     (       t   i     ,     t     i   +   k         ]           ⁢     q   i             t     i   +   k       -     t   i                 (   3   )               
where the first term in the numerator is the difference in the receiver&#39;s buffer availability and the second or summation term is the total amount of data transmitted during the time interval. The parameter k controls the width of the estimation interval (in number of ACK packets) and can be adjusted to trade off between accuracy and timeliness of rate estimation. Exponentially weighted moving averaging similar to Equation (2) is applied to R to smooth out random fluctuations.
 
Gateway Transmission Scheduling
 
     Whenever a TCP segment is received from the sender  12 , the gateway  10  must schedule it for transmission to the receiver  14  such that it will not cause buffer overflow at the receiver  14 . The challenge is that the AWnd reported in the last ACK packet is delayed information—it was the receiver&#39;s buffer availability at one-half the round trip time previously (0.5 D&#39;s ago). During the time the ACK packet has traveled to the gateway  10 , additional TCP segments may have arrived at the receiver  14 , and the receiver application may have processed more data from the receiver TCP buffer. 
     To account for these two factors, the gateway  10  can compute the predicted receiver TCP buffer availability at a future time t, denoted by B(t) from: 
                     B   ⁡     (   t   )       =       a   i     -       ∑       ∀   j     ❘       (       (       f   j     +     0.5   ⁢   D       )     ≤   t     )     ⋂     (     fj   &gt;     (       t   i     -     0.5   ⁢   D       )               ⁢     +     R   ⁡     (     t   -     t   i       )                     (   4   )               
where the first term is the AWnd  28  reported by the i th  ACK to the TCP-SC gateway  10 , the second term is the predicted total amount of data arriving at the receiver  14  from time t i  to time t, and the last term is the predicted amount of data which will be processed at the receiver  14  by time t.
 
     The scheduling problem can be restated as finding the set of transmission schedule {f j |j=0, 1, . . . } such that B(t)≧0∀t. In practice, this can be done whenever a TCP segment, such as segment i, is received from the sender  12 . The gateway  10  then determines f i  according to the previous constraint and then queues it for transmission. Queued TCP segments will then be processed in a first in, first out (FIFO) manner with the head-of-line segment transmitted when the current time reaches the schedule time f i . 
     The foregoing solution has been emulated as well as tested in a satellite communication system and found to be a substantial improvement over conventional TCP. In actual experiments, UDP data transmission using the protocol according to the invention achieved throughput of 40 Mbps with negligible packet loss as compared to conventional TCP rates of 0.5 Mbps throughput with a 64 KB window size and a 1 second RTT. 
     Network bandwidth limit was emulated using features built into a Linux kernel and network delay was emulated by the gateway itself. This resulted in a BDP of 3 MB, which is significantly larger than the default receiver buffer size. The TCP connection was initiated by the receiver and then the sender kept sending data to the receiver at a maximum allowed rate under TCP. TCP throughput was logged at the receiver. 
     The receiver buffer size was set by manually setting the socket buffer size. With the inventive TCP-SC, various receiver buffer sizes were tested: 64 KB, 1 MB, 2 MB, and 4 MB respectively, and they were compared the case of TCP-SC with 64 KB buffer size. As expected, TCP throughput increased proportionally with the receiver buffer size. In comparison, with TCP-SC, in-place TCP can achieve throughput similar to the case of 4 MB receiver buffer size. However, since the BDP is only 3 MB, this implies that the achieved throughput is no longer receiver-buffer-limited. In fact the link utilization reached 98.63% for TCP-SC and 98.86% for 4 MB buffer size respectively. (See Table below). 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Average throughput after TCP stabilizes (from 25 s to 40 s) 
               
             
          
           
               
                   
                 Receive buffer size (KB) 
               
             
          
           
               
                   
                 64 
                 64 
                 1024 
                 2048 
                 4096 
               
               
                   
                   
               
             
          
           
               
                 With gateway 
                 Y 
                 N 
                 N 
                 N 
                 N 
               
               
                 Throughput (Mbps) 
                 23.67 
                 0.53 
                  8.69 
                 17.39 
                 23.72 
               
               
                 Link utilization (%) 
                 98.63 
                 2.20 
                 36.21 
                 72.45 
                 98.86 
               
               
                   
               
             
          
         
       
     
     Plots of the AWnd of all five sets of experiments are shown below. It is worth noting that regardless of the receiver buffer size setting, the buffer availability stays at the maximum buffer size setting. This suggests that the receiver application&#39;s processing rate is higher than the network bandwidth, so that data arriving at the receiver is quickly retrieved and cleared from the buffer.  FIG. 3  plots the TCP throughput versus time for both directions of data transfer in a live experiment over a two-way satellite link. It was found that TCP flow in at least one direction periodically stopped transmitting data completely for a significant duration, evidently due to automat release of the satellite downlink after channel idle time, requiring a lengthy channel setup process to re-establish the downlink. However, by introducing a background UDP data flow in parallel to the TCP data flow, the channel could be kept sufficiently busy so that the downlink channel was not released. This is illustrated by the solid line in  FIG. 3 , where there is a dramatic increase in TCP data throughput. There were extra buffers allocated to assure full utilization of the network. Although the 4 MB setting also enabled TCP to fully utilize the network, the extra buffers allocated were in fact not used at all. By contrast, TCP-SC according to the invention enabled existing TCP to fully utilize network bandwidth without the need for large buffers and modifications to the receiver application. 
     Conclusion 
     The performance of TCP running over large BDP networks is improved by introducing a TCP-SC gateway between the sender (host) and the receiver (client). Compared to known solutions, the inventive TCP-SC gateway exhibits two desirable properties: (a) it does not require modification to the applications, to the TCP implementation at the hosts, or to the operating system; and (b) it significantly reduces the receiver buffer requirement. TCP-SC is compatible with existing TCP flows and can readily enable them to fully utilize available network bandwidth. It can be deployed as part of a satellite or mobile data communications network and is completely transparent to its users. 
     The invention has been explained with reference to specific embodiments. Other embodiments will be apparent to those of ordinary skill in this art. Therefore, the invention is not to be considered limited, except as indicated by the appended claims.