Internet Engineering Task Force Sally Floyd INTERNET DRAFT ICSI draft-floyd-tcp-highspeed-01.txt August, 2002 HighSpeed TCP for Large Congestion Windows Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document proposes HighSpeed TCP, a modification to TCP's congestion control mechanism for use with TCP connections with large congestion windows. The congestion control mechanisms of the current Standard TCP constrains the congestion windows that can be achieved by TCP in realistic environments. For example, for a Standard TCP connection with 1500-byte packets and a 100 ms round-trip time, achieving a steady-state throughput of 10 Gbps would require an average congestion window of 83,333 segments, and a packet drop rate of at most one congestion event every 5,000,000,000 packet (or equivalently, at most one congestion event every 1 2/3 hours). This is widely acknowledged as an unrealistic constraint. To address this limitation of TCP, this document proposes HighSpeed TCP, and solicits experimentation and feedback from the wider community. Floyd Experimental [Page 1]
draft-floyd-tcp-highspeed-01 August 2002 Changes from draft-floyd-tcp-highspeed-00.txt: * Added a discussion on related work about changing the PMTU. * Added a discussion of an alternate, linear response function. * Added a discussion of the TCP window scale option. * Added a discussion of HighSpeed TCP as roughly emulating the congestion control response of N parallel TCP connections. * Added a discussion of the time to converge to fairness. * Expanded the Introduction. 1. Introduction. This document proposes HighSpeed TCP, a modification to TCP's congestion control mechanism for use with TCP connections with large congestion windows. In a steady-state environment, with a packet loss rate p, the current Standard TCP's average congestion window is roughly 1.2/sqrt(p) segments. This places a serious constraint on the congestion windows that can be achieved by TCP in realistic environments. For example, for a Standard TCP connection with 1500-byte packets and a 100 ms round-trip time, achieving a steady- state throughput of 10 Gbps would require an average congestion window of 83,333 segments, and a packet drop rate of at most one congestion event every 5,000,000,000 packet (or equivalently, at most one congestion event every 1 2/3 hours). To address this fundamental limitation of TCP and of the TCP response function (the function mapping the steady-state packet drop rate to TCP's average sending rate in packets per round-trip time), this document proposes modifying the TCP response function for regimes with higher congestion windows. This document also solicits experimentation and feedback on HighSpeed TCP from the wider community. Because HighSpeed TCP's modified response function would only take effect with higher congestion windows, HighSpeed TCP does not modify TCP behavior in environments with mild to heavy congestion, and therefore does not introduce any new dangers of congestion collapse. However, if relative fairness between HighSpeed TCP connections is to be preserved, then in our view any modification to the TCP response function should be globally-agreed-upon in the IETF, rather than made as ad hoc decisions by individual implementors or TCP senders. Modifications to the TCP response function would also have implications for transport protocols that use TFRC and other forms of Floyd Experimental [Page 2]
draft-floyd-tcp-highspeed-01 August 2002 equation-based congestion control, as these congestion control mechanisms directly use the TCP response function [TFRC]. This proposal for HighSpeed TCP focuses specifically on a proposed change to the TCP response function, and its implications for TCP. This document does not address what we view as a separate fundamental issue, of the mechanisms required to enable best-effort connections to *start* with large initial windows. In our view, while HighSpeed TCP proposes a somewhat fundamental change to the TCP response function, at the same time it is a relatively simple change to implement in a single TCP sender, and presents no dangers in terms of congestion collapse. In contrast, in our view, the problem of enabling connections to *start* with large initial windows is inherently more risky and structurally more difficult, requiring some form of explicit feedback from all of the routers along the path. This is another reason why we would propose addressing the problem of starting with large initial windows separately, and on a separate timetable, from the problem of modifying the TCP response function. 2. The Problem Description. This section describes the number of round-trip times between congestion events required for a Standard TCP flow to achieve an average throughput of B bps, given packets of D bytes and a round- trip time of R seconds. A congestion event refers to a window of data with one or more dropped or ECN-marked packets. From Appendix A, achieving an average TCP throughput of B bps requires a loss event at most every BR/(12D) round-trip times. This is illustrated in Table 1, for R = 0.1 seconds and D = 1500 bytes. The table also gives the average congestion window W of BR/(8D), and the steady-state packet drop rate P of 1.5/W^2. TCP Throughput (Mbps) RTTs Between Losses W P --------------------- ------------------- ------ ----- 1 5.5 8.3 0.02 10 55.5 83.3 0.0002 100 555.5 833.3 0.000002 1000 5555.5 8333.3 0.00000002 10000 55555.5 83333.3 0.0000000002 Table 1: RTTs Between Congestion Events for Standard TCP, for 1500-Byte Packets and a Round-Trip Time of 0.1 Seconds. This document proposes HighSpeed TCP, a minimal modification to TCP's increase and decrease parameters, for TCP connections with larger congestion windows, to allow TCP to achieve high throughput with more realistic requirements for the steady-state packet drop rate. Floyd Experimental [Page 3]
draft-floyd-tcp-highspeed-01 August 2002 Equivalently, HighSpeed TCP has more realistic requirements for the number of round-trip times between loss events. 3. Design Guidelines. Our proposal for HighSpeed TCP is motivated by the following requirements: * Achieve high per-connection throughput without requiring unrealistically low packet loss rates. * Reach high throughput reasonably quickly when in slow-start. * Reach high throughput without overly long delays when recovering from multiple retransmit timeouts, or when ramping-up from a period with small congestion windows. * No additional feedback or support required from routers: For example, the goal is for acceptable performance in both ECN- capable and non-ECN-capable environments, and with Drop-Tail as well as with Active Queue Management such as RED in the routers. * No additional feedback required from TCP receivers. * TCP-compatible performance in environments with moderate or high congestion: Equivalently, the requirement is that there be no additional load on the network (in terms of increased packet drop rates) in environments with moderate or high congestion. * Performance at least as good as Standard TCP in environments with moderate or high congestion. * Acceptable transient performance, in terms of increases in the congestion window in one round-trip time, responses to severe congestion, and convergence times to fairness. Currently, users wishing to achieve throughputs of 1Gbps or more typically open up multiple TCP connections in parallel, or use MulTCP [CO98,GRK99], which behaves roughly like the aggregate of N virtual TCP connections. While this approach suffices for the occasional user on well-provisioned links, it leaves the parameter N to be determined by the user, and results in more aggressive performance and higher steady-state packet drop rates if used in environments with periods of moderate or high congestion. We believe that a new approach is needed that offers more flexibility, more effectively Floyd Experimental [Page 4]
draft-floyd-tcp-highspeed-01 August 2002 scales to a wide range of available bandwidths, and competes more fairly with Standard TCP in congested environments. 4. Non-Goals. The following are explicitly *not* goals of our work: * Non-goal: TCP-compatible performance in environments with very low packet drop rates. We note that our proposal does not require, or deliver, TCP- compatible performance in environments with very low packet drop rates, e.g., with packet loss rates of 10^-5 or 10^-6. As we discuss later in this document, we assume that Standard TCP is unable to make effective use of the available bandwidth in environments with loss rates of 10^-6 in any case, so that it is acceptable and appropriate for HighSpeed TCP to perform more aggressively than Standard TCP is such an environment. * Non-goal: Ramping-up more quickly than allowed by slow-start. It is our belief that ramping-up more quickly than allowed by slow- start would necessitate more explicit feedback from routers along the path. The proposal for HighSpeed TCP is focused on changes to TCP that could be effectively deployed in the current Internet environment. * Non-goal: Avoiding oscillations in environments with only one-way, long-lived flows all with the same round-trip times. While we agree that attention to oscillatory behavior is useful, avoiding oscillations in aggregate throughput has not been our primary consideration, particularly for simplified environments limited to one-way, long-lived flows all with the same, large round- trip times. Our assessment is that some oscillatory behavior in these extreme environments is an acceptable price to pay for the other benefits of HighSpeed TCP. 5. Modifying the TCP Response Function. The TCP response function, w = 1.2/sqrt(p), gives TCP's average congestion window w in MSS-sized segments, as a function of the steady-state packet drop rate p [FF98]. This TCP response function is a direct consequence of TCP's Additive Increase Multiplicative Decrease (AIMD) mechanisms of increasing the congestion window by roughly one segment per round-trip time in the absence of congestion, and halving the congestion window in response to a round-trip time with a congestion event. This response function for Standard TCP is Floyd Experimental [Page 5]
draft-floyd-tcp-highspeed-01 August 2002 reflected in the table below. In this proposal we restrict our attention to TCP performance in environments with packet loss rates of at most 10^-2, and so we can ignore the more complex response functions that are required to model TCP performance in more congested environments with retransmit timeouts. From Appendix A, an average congestion window of W corresponds to an average of W/1.5 round-trip times between loss events for Standard TCP. Packet Drop Rate P Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-2 12 8 10^-3 38 25 10^-4 120 80 10^-5 379 252 10^-6 1200 800 10^-7 3795 2530 10^-8 12000 8000 10^-9 37948 25298 10^-10 120000 80000 Table 2: TCP Response Function for Standard TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P. To specify a modified response function for HighSpeed TCP, we use three parameters, Low_Window, High_Window, and High_P. To ensure TCP compatibility, the HighSpeed response function uses the same response function as Standard TCP when the current congestion window is at most Low_Window, and uses the HighSpeed response function when the current congestion window is greater than Low_Window. In this document we set Low_Window to 38 MSS-sized segments, corresponding to a packet drop rate of 10^-3 for TCP. To specify the upper end of the HighSpeed response function, we specify the packet drop rate needed in the HighSpeed response function to achieve an average congestion window of 83000 segments. This is roughly the window needed to sustain 10Gbps throughput, for a TCP connection with the default packet size and round-trip time used earlier in this document. For High_Window set to 83000, we specify High_P of 10^-7; that is, with HighSpeed TCP a packet drop rate of 10^-7 allows the HighSpeed TCP connection to achieve an average congestion window of 83000 segments. We believe that this loss rate sets an achieveable target for high-speed environments, while still allowing acceptable fairness for the HighSpeed response function when competing with Standard TCP in environments with packet drop rates of 10^-4 or 10^5. For simplicity, for the HighSpeed response function we maintain the Floyd Experimental [Page 6]
draft-floyd-tcp-highspeed-01 August 2002 property that the response function gives a straight line on a log- log scale (as does the response function for Standard TCP, for low to moderate congestion). This results in the following response function, for values of the average congestion window W greater than Low_Window: W = (p/Low_P)^S Low_Window, for Low_P the packet drop rate corresponding to Low_Window, and for S as following constant [FRS02]: S = (log High_Window - log Low_Window)/(log High_P - log Low_P). For example, for Low_Window set to 38, we have Low_P of 10^-3 (for compatibility with Standard TCP). Thus, for High_Window set to 83000 and High_P set to 10^-7, we get the following response function: W = 0.12/p^0.835. (1) This HighSpeed response function is illustrated in Table 3 below. For HighSpeed TCP, the number of round-trip times between losses, 1/(pW), equals 12.7 W^0.2, for W > 38 segments. Packet Drop Rate P Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-2 12 8 10^-3 38 25 10^-4 263 38 10^-5 1795 57 10^-6 12279 83 10^-7 83981 123 10^-8 574356 180 10^-9 3928088 264 10^-10 26864653 388 Table 3: TCP Response Function for HighSpeed TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P. We believe that the problem of backward compatibility with Standard TCP requires a response function that is quite close to that of Standard TCP for loss rates of 10^-1, 10^-2, or 10^-3. We believe, however, that such stringent TCP-compatibility is not required for smaller loss rates, and that an appropriate response function is one that gives a plausible packet drop rate for a connection throughput of 10Gbps. This also gives a slowly increasing number of round-trip times between loss events as a function of a decreasing packet drop rate. Floyd Experimental [Page 7]
draft-floyd-tcp-highspeed-01 August 2002 Another way to look at the HighSpeed response function is to consider that HighSpeed TCP is roughly emulating the congestion control response of N parallel TCP connections, where N is initially one, and where N increases as a function of the HighSpeed TCP's congestion window. Thus for the HighSpeed response function in Equation (1) above, the response function can be viewed as equivalent to that of N(W) parallel TCP connections, where N(W) varies as a function of the congestion window W. Recall that for a single standard TCP connection, the average congestion window equals 1.2/sqrt(p). For N parallel TCP connections, the aggregate congestion window W_n equals N*1.2/sqrt(p). From the HighSpeed response function in Equation (1) and the relationship above, we can derive the following: N(W) = 0.23*W^(0.4) for N(W) the number of parallel TCP connections emulated by the HighSpeed TCP response function, and for N(W) >= 1. This is shown in Table 4 below. Congestion Window W Number N(W) of Parallel TCPs ------------------- ------------------------- 1 1 10 1 100 1.4 1,000 3.6 10,000 9.2 100,000 23.0 Table 4: Number N(W) of parallel TCP connections roughly emulated by the HighSpeed TCP response function. We do not in this document attempt to seriously evaluate the HighSpeed response function for congestion windows greater than 100,000 packets. We believe that we will learn more about the requirements for sustaining the throughput of best-effort connections in that range as we gain more experience with HighSpeed TCP with congestion windows of thousands and tens of thousands of packets. There also might be limitations to the per-connection throughput that can be realistically achieved for best-effort traffic in the absence of additional support or feedback from the routers along the path. 6. Fairness Implications of the HighSpeed Response Function. The Standard and Highspeed Response Functions can be used directly to infer the relative fairness between flows using the two response functions. For example, given a packet drop rate P, assume that Standard TCP has an average congestion window of W_Standard, and HighSpeed TCP has a higher average congestion window of W_HighSpeed. Floyd Experimental [Page 8]
draft-floyd-tcp-highspeed-01 August 2002 In this case, a single HighSpeed TCP connection is receiving W_HighSpeed/W_Standard times the throughput of a single Standard TCP connection competing in the same environment. This relative fairness is illustrated below in Table 5, for the parameters used for the Highspeed response function in the section above. The second column gives the relative fairness, for the steady-state packet drop rate specified in the first column. To help calibrate, the third column gives the aggregate average congestion window for the two TCP connections, and the fourth column gives the bandwidth that would be needed by the two connections to achieve that aggregate window and packet drop rate, given 100 ms round-trip times and 1500-byte packets. Packet Drop Rate P Fairness Aggregate Window Bandwidth ------------------ -------- ---------------- --------- 10^-2 1.0 24 2.8 Mbps 10^-3 1.0 76 9.1 Mbps 10^-4 2.2 383 45.9 Mbps 10^-5 4.7 2174 260.8 Mbps 10^-6 10.2 13479 1.6 Gbps 10^-7 22.1 87776 10.5 Gbps 10^-8 47.9 586356 70.3 Gbps 10^-9 103.5 3966036 475.9 Gbps 10^-10 223.9 26984653 3238.1 Gbps Table 5: Relative Fairness between the HighSpeed and Standard Response Functions. Thus, for packet drop rates of 10^-4, a flow with the HighSpeed response function can expect to receive 2.2 times the throughput of a flow using the Standard response function, given the same round-trip times and packet sizes. With packet drop rates of 10^-6 (or 10^-7), the unfairness is more severe, and we have entered the regime where a Standard TCP connection requires a congestion event at most every 800 (or 2530) round-trip times in order to make use of the available bandwidth. Our judgement would be that there are not a lot of TCP connections effectively operating in this regime today, with congestion windows of thousands of packets, and that therefore the benefits of the HighSpeed response function would outweigh the unfairness that would be experienced by Standard TCP in this regime. However, one purpose of this document is to solicit feedback on this issue. The parameter Low_Window determines directly the point of divergence between the Standard and HighSpeed Response Functions. The third column of Table 5, the Aggregate Window, gives the aggregate congestion window of the two competing TCP connections, Floyd Experimental [Page 9]
draft-floyd-tcp-highspeed-01 August 2002 with HighSpeed and Standard TCP, given the packet drop rate specified in the first column. From Table 5, a HighSpeed TCP connection would receive ten times the bandwidth of a Standard TCP in an environment with a packet drop rate of 10^-6. This would occur when the two flows sharing a single pipe achieved an aggregate window of 13479 packets. Given a round-trip time of 100 ms and a packet size of 1500 bytes, this would occur with an available bandwidth for the two competing flows of 1.6 Gbps. Next we consider the time that it takes two HighSpeed TCP flows to converge to fairness. The worst case for convergence to fairness occurs when a new flow is starting up, competing against a high- bandwidth existing flow, and the new flow suffers a packet drop and exits slow-start while its window is still small. In the worst case, consider that the new flow has entered the congestion avoidance phase while its window is only one packet. A standard TCP flow in congestion avoidance increases its window by at most one packet per round-trip time, and after N round-trip times has only achieved a window of N packets (when starting with a window of 1 in the first round-trip time). In contrast, a HighSpeed TCP flows increases much faster than a standard TCP flow while in the congestion avoidance phase, and we can expect its convergence to fairness to be much better. This is shown in Table 6 below. The script used to generate this table is given in Appendix C. Floyd Experimental [Page 10]
draft-floyd-tcp-highspeed-01 August 2002 RTT HS_Window Standard_TCP_Window --- --------- ------------------- 100 131 100 200 475 200 300 1131 300 400 2160 400 500 3601 500 600 5477 600 700 7799 700 800 10567 800 900 13774 900 1000 17409 1000 1100 21455 1100 1200 25893 1200 1300 30701 1300 1400 35856 1400 1500 41336 1500 1600 47115 1600 1700 53170 1700 1800 59477 1800 1900 66013 1900 2000 72754 2000 Table 6: For a HighSpeed and a Standard TCP connection, the congestion window during congestion avoidance phase (starting with a congestion window of 1 packet during RTT 1. The classic paper on relative fairness is from Chiu and Jain [CJ89]. This paper shows that AIMD (Additive Increase Multiplicative Decrease) converges to fairness in an environment with synchronized congestion events. From [CJ89], it is easy to see that MIMD and AIAD do not converge to fairness in this environment. However, the results of [CJ89] do not apply to an asynchronous environment such as that of the current Internet, where the frequency of congestion feedback can be different for different flows. For example, it has been shown that MIMD converges to fair states in a model with proportional instead of synchronous feedback in terms of packet drops [GV02]. Thus, we are not concerned about abandoning a strict model of AIMD for HighSpeed TCP. 7. Translating the HighSpeed Response Function into Congestion Control Parameters. For equation-based congestion control such as TFRC, the HighSpeed Response Function above could be used directly by the TFRC congestion control mechanism. However, for TCP the HighSpeed response function would have to be translated into additive increase and multiplicative decrease parameters. The HighSpeed response function cannot be Floyd Experimental [Page 11]
draft-floyd-tcp-highspeed-01 August 2002 achieved by TCP with an additive increase of one segment per round- trip time and a multiplicative decrease of halving the current congestion window; HighSpeed TCP will have to modify either the increase or the decrease parameter, or both. We have concluded that HighSpeed TCP is most likely to achieve an acceptable compromise between moderate increases and timely decreases by modifying both the increase and the decrease parameter. That is, for HighSpeed TCP let the congestion window increase by a(w) segments per round-trip time in the absence of congestion, and let the congestion window decrease to w(1-b(w)) segments in response to a round-trip time with one or more loss events. Thus, in response to a single acknowledgement HighSpeed TCP increases its congestion window in segments as follows: w <- w + a(w)/w. In response to a congestion event, HighSpeed TCP decreases as follows: w <- (1-b(w))w. For Standard TCP, a(w) = 1 and b(w) = 1/2, regardless of the value of w. HighSpeed TCP uses the same values of a(w) and b(w) for w <= Low_Window. This section specifies a(w) and b(w) for HighSpeed TCP for larger values of w. For w = High_Window, we have specified a loss rate of High_P. From [FRS02], or from elementary calculations, this requires the following relationship between a(w) and b(w) for w = High_Window: a(w) = High_Window^2 * High_P * 2 * b(w)/(2-b(w). (2) We use the parameter High_Decrease to specify the decrease parameter b(w) for w = High_Window, and use Equation (2) to derive the increase parameter a(w) for w = High_Window. Along with High_P = 10^-7 and High_Window = 83000, for example, we specify High_Decrease = 0.1, specifying that b(83000) = 0.1, giving a decrease of 10% after a congestion event. Equation (2) then gives a(83000) = 72, for an increase of 72 segments, or just under 0.1%, within a round-trip time, for w = 83000. This moderate decrease strikes us as acceptable, particularly when coupled with the role of TCP's ACK-clocking in limiting the sending rate in response to more severe congestion [BBFS01]. A more severe decrease would require a more aggressive increase in the congestion window for a round-trip time without congestion. In particular, a decrease factor High_Decrease of 0.5, as in Standard TCP, would Floyd Experimental [Page 12]
draft-floyd-tcp-highspeed-01 August 2002 require an increase of 459 segments per round-trip time when w = 83000. Given decrease parameters of b(w) = 1/2 for w = Low_Window, and b(w) = High_Decrease for w = High_Window, we are left to specify the value of b(w) for other values of w > Low_Window. From [FRS02], we let b(w) vary linearly as the log of w, as follows: b(w) = (High_Decrease - 0.5) (log(w)-log(W)) / (log(W_1)-log(W)) + 0.5. The increase parameter a(w) can then be computed as follows: a(w) = w^2 * p(w) * 2 * b(w)/(2-b(w)), for p(w) the packet drop rate for congestion window w. From inverting Equation (1), we get p(w) as follows: p(w) = 0.078/w^1.2. We assume that experimental implementations of HighSpeed TCP for further investigation will use a pre-computed look-up table for finding a(w) and b(w). In the appendix we give such a table for our default values of Low_Window = 38, High_Window = 83,000, High_P = 10^-7, and High_Decrease = 0.1. These are also the default values in the NS simulator; example simulations in NS can be run with the command "./test-all-tcpHighspeed" in the directory tcl/test. 8. An alternate, linear response functions. In this section we explore an alternate, linear response function for HighSpeed TCP that has been proposed by a number of other people, in particular by Glenn Vinnicombe and Tom Kelly. Similarly, it has been suggested by others that a less "ad-hoc" guideline for a response function for HighSpeed TCP would be to specify a constant value for the number of round-trip times between congestion events. Assume that we keep the value of Low_Window as 38 MSS-sized segments, indicating when the HighSpeed response function diverges from the current TCP response function, but that we modify the High_Window and High_P parameters that specify the upper range of the HighSpeed response function. In particular, consider the response function given by High_Window = 380,000 and High_P = 10^-7, with Low_Window = 38 and Low_P = 10^-3 as before. Using the equations in Section 5, this would give the following Linear response function, for w > Low_Window: Floyd Experimental [Page 13]
draft-floyd-tcp-highspeed-01 August 2002 W = 0.038/p. This Linear HighSpeed response function is illustrated in Table 7 below. For HighSpeed TCP, the number of round-trip times between losses, 1/(pW), equals 1/0.38, or equivalently, 26, for W > 38 segments. Packet Drop Rate P Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-2 12 8 10^-3 38 26 10^-4 380 26 10^-5 3800 26 10^-6 38000 26 10^-7 380000 26 10^-8 3800000 26 10^-9 38000000 26 10^-10 380000000 26 Table 7: An Alternate, Linear TCP Response Function for HighSpeed TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P. Given a constant decrease b(w) of 1/2, this would give an increase a(w) of w/Low_Window, or equivalently, an constant increase of 1/Low_Window packets per acknowledgement, for w > Low_Window. The relative fairness between the alternate Linear response function and the standard TCP response function is illustrated below in Table 8. Packet Drop Rate P Fairness Aggregate Window Bandwidth ------------------ -------- ---------------- --------- 10^-2 1.0 24 2.8 Mbps 10^-3 1.0 76 9.1 Mbps 10^-4 3.2 500 60.0 Mbps 10^-5 15.1 4179 501.4 Mbps 10^-6 31.6 39200 4.7 Gbps 10^-7 100.1 383795 46.0 Gbps 10^-8 316.6 3812000 457.4 Gbps 10^-9 1001.3 38037948 4564.5 Gbps 10^-10 3166.6 380120000 45614.4 Gbps Table 8: Relative Fairness between the Linear HighSpeed and Standard Response Functions. One attraction of the linear response function is that it is scale- Floyd Experimental [Page 14]
draft-floyd-tcp-highspeed-01 August 2002 invariant, with a fixed increase in the congestion window per acknowledgement, and a fixed number of round-trip times between loss events. My own assumption would be that having a fixed length for the congestion epoch in round-trip times, regardless of the packet drop rate, would be a poor fit for an imprecise and imperfect world with routers with a range of queue management mechanisms, such as the Drop-Tail queue management that is common today. For example, a response function with a fixed length for the congestion epoch in round-trip times might give less clearly-differentiated feedback in an environment with steady-state background losses at fixed intervals for all flows (as might occur with a wireless link with occasional short error bursts, giving losses for all flows every N seconds regardless of their sending rate). While it is not a goal to have perfect fairness in an environment with synchronized losses, it would be good to have moderately acceptable performance in this regime. This goal might argue against a response function with a constant number of round-trip times between congestion events. However, this is a question that could clearly use additional research and investigation. The third column of Table 8, the Aggregate Window, gives the aggregate congestion window of two competing TCP connections, one with Linear HighSpeed TCP and one with Standard TCP, given the packet drop rate specified in the first column. From Table 8, a Linear HighSpeed TCP connection would receive fifteen times the bandwidth of a Standard TCP in an environment with a packet drop rate of 10^-5. This would occur when the two flows sharing a single pipe achieved an aggregate window of 4179 packets. Given a round-trip time of 100 ms and a packet size of 1500 bytes, this would occur with an available bandwidth for the two competing flows of 501 Mbps. Thus, because the Linear HighSpeed TCP is more aggressive than the HighSpeed TCP proposed above, it also is less fair when competing with Standard TCP in a high-bandwidth environment. 9. Slow-Start. An companion internet-draft on "Limited Slow-Start for TCP with Large Congestion Windows" [F02b] proposes a modification to TCP's slow- start procedure that can significantly improve the performance of TCP connections slow-starting up to large congestion windows. For TCP connections that are able to use congestion windows of thousands (or tens of thousands) of MSS-sized segments (for MSS the sender's MAXIMUM SEGMENT SIZE), the current slow-start procedure can result in increasing the congestion window by thousands of segments in a single round-trip time. Such an increase can easily result in thousands of packets being dropped in one round-trip time. This is often counter- productive for the TCP flow itself, and is also hard on the rest of the traffic sharing the congested link. Floyd Experimental [Page 15]
draft-floyd-tcp-highspeed-01 August 2002 [F02b] proposes Limited Slow-Start, limiting the number of segments by which the congestion window is increased for one window of data during slow-start, in order to improve performance for TCP connections with large congestion windows. We have separated out Limited Slow-Start to a separate draft because it can be used both with Standard or with HighSpeed TCP. Limited Slow-Start is illustrated in the NS simulator, for snapshots after May 1, 2002, in the tests "./test-all-tcpHighspeed tcp1A" and "./test-all-tcpHighspeed tcpHighspeed1" in the subdirectory "tcl/lib". In order for best-effort flows to safely start-up faster than slow- start, e.g., in future high-bandwidth networks, we believe that it would be necessary for the flow to have explicit feedback from the routers along the path. There are a number of proposals for this, ranging from a minimal proposal for an IP option that allows TCP SYN packets to collect information from routers along the path about the allowed initial sending rate [J02], to proposals with more power that require more fine-tuned and continuous feedback from routers. These proposals all are somewhat longer-term proposals that the HighSpeed TCP proposal in this document, requiring longer lead times and more coordination for deployment, and will be discussed in later documents. 10. Other limitations on window size. The TCP header uses a 16-bit field to report the receive window size to the sender. Unmodified, this allows a window size of at most 2**16 = 65K bytes. With window scaling, the maximum window size is 2**30 = 1073M bytes [RFC 1323]. Given 1500-byte packets, this allows a window of up to 715,000 packets. 11. Related Work in HighSpeed TCP. HighSpeed TCP has been separately investigated in simulations by Sylvia Ratnasamy and by Evandro de Souza, and reports of some of these simulations should be available shortly. The simulations by Evandro verify the fairness properties of HighSpeed TCP when sharing a link with Standard TCP. These simulations explore the relative fairness of HighSpeed TCP flows when competing with Standard TCP. The simulation environment include background forward and reverse-path TCP traffic limited by the TCP receive window, along with a small amount of forward and reverse-path traffic from the web traffic generator. Most of the simulations so far explore performance on a simple dumbbell topology with a 1Gbps link with a propagation delay of 50 ms. Simulations Floyd Experimental [Page 16]
draft-floyd-tcp-highspeed-01 August 2002 have been run both the Adaptive RED and with DropTail queue management. Future work to explore in more detail includes convergence times after new flows start-up; recovery time after a transient outage; the response to sudden severe congestion, and investigations of the potential for oscillations. Additional future work includes evaluating more fully the choices of parameters for HighSpeed TCP. We invite contributions from others in this work. Suggestions to other citations of related work would also be welcome. 12. Relationship to other Work. Our assumption is that HighSpeed TCP will be used along with the TCP SACK option, and also with the increased Initial Window of three or four segments, as allowed by [AFP02]. For paths that have substantial reordering, TCP performance would be greatly improved by some of the mechanisms still in the research stages for robust performance in the presence of reordered packets. Our view is that HighSpeed TCP is largely orthogonal to proposals for higher PMTU (Path MTU) values [M02]. Unlike changes to the PMTU, HighSpeed TCP does not require any changes in the network or at the TCP receiver, and works well in the current Internet. Our assumption is that HighSpeed TCP would be useful even with larger values for the PMTU. In particular, unlike the current congestion window, the PMTU gives no information about the bandwidth-delay product available to that particular flow. A related approach is that of a virtual MTU, where the actual MTU of the path might be limited [VMSS,S02]. Unfortunately, the virtual MTU approach does not converge to fairness even when all of the connections are using the virtual MTU approach; new connections can find themselves locked-out by newcomers. Thus, we do not explore the virtual MTU approach further in this document. 13. Conclusions. This is an initial proposal, and we are asking from feedback from the wider community. We have explored this proposal in simulations, though we have not yet finished our reports on these simulations. We would welcome additional analysis, simulations, and particularly, experimentation. More information on simuations and experiments is available from the HighSpeed TCP Web Page [HSTCP]. There are three parameters that determine the HighSpeed Response Function, and an additional parameter that determines HighSpeed TCP's Floyd Experimental [Page 17]
draft-floyd-tcp-highspeed-01 August 2002 tradeoffs between increases and decreases using that response function. We solicit feedback on our setting of these parameters as well as on other issues. We are bringing this proposal to the IETF to be considered as an Experimental RFC. One reason to bring this to the IETF at this stage is that HighSpeed TCP proposes a rather significant change in the underlying TCP response function, and in our view any such change would have to be globally agreed-upon. It seems advisable to us to bring such a proposal to the IETF for feedback even in its preliminary stages. Another reason to bring this proposal to the IETF is that, while several people have conducted evaluations of HighSpeed TCP using simulations, our belief is that the "real" evaluations will have to happen in experiments and in actual deployment. As part of this experimentation, HighSpeed TCP has been implemented in the Linux 2.4.16 Web100 kernel [HSTCP]. It seemed to us that it was advisable, at this stage, to bring the proposal for HighSpeed TCP to the IETF and to seek Experimental status. 14. Acknowledgements The HighSpeed TCP proposal is from joint work with Sylvia Ratnasamy and Scott Shenker (and was initiated by Scott Shenker). Additional investigations of HighSpeed TCP were joint work with Evandro de Souza and Deb Agarwal. We thank Tom Dunigan for the implementation in the Linux 2.4.16 Web100 kernel, and for resulting experimentation with HighSpeed TCP. We are grateful to the End-to-End Research Group, the members of the Transport Area Working Group, and to members of the IPAM program in Large Scale Communication Networks for feedback. We thank Glenn Vinnicombe for framing the Linear response function in the parameters of HighSpeed TCP. We are also grateful for contributions and feedback from the following individuals: Tom Kelly, Jitendra Padhye, Stanislav Shalunov, Paul Sutter, Brian Tierney, Joe Touch. Thanks to Jeffrey Hsu and Andrew Reiter for feedback on earlier versions of this document. 15. Normative References [RFC2581] M. Allman and V. Paxson, "TCP Congestion Control", RFC 2581, April 1999. 16. Informative References [BBFS01] Deepak Bansal, Hari Balakrishnan, Sally Floyd, and Scott Shenker, "Dynamic Behavior of Slowly-Responsive Congestion Control Algorithms", SIGCOMM 2001, August 2001. Floyd Experimental [Page 18]
draft-floyd-tcp-highspeed-01 August 2002 [CJ89] D. Chiu and R. Jain, "Analysis of the Increase and Decrease Algorithms for Congestion Avoidance in Computer Networks", Computer Networks and ISDN Systems, Vol. 17, pp. 1-14, 1989. [CO98] J. Crowcroft and P. Oechslin, "Differentiated end-to-end services using a weighted proportional fair share TCP", Computer Communication Review, 28(3):53--69, 1998. [FF98] Floyd, S., and Fall, K., "Promoting the Use of End-to-End Congestion Control in the Internet", IEEE/ACM Transactions on Networking, August 1999. [FRS02] Sally Floyd, Sylvia Ratnasamy, and Scott Shenker, "Modifying TCP's Congestion Control for High Speeds", May 2002. URL "http://www.icir.org/floyd/notes.html". [GRK99] Panos Gevros, Fulvio Risso and Peter Kirstein, "Analysis of a Method for Differential TCP Service" In Proceedings of the IEEE GLOBECOM'99, Symposium on Global Internet , December 1999, Rio de Janeiro, Brazil. [GV02] S. Gorinsky and H. Vin, "Extended Analysis of Binary Adjustment Algorithms", Technical Report TR2002-39, Department of Computer Sciences, The University of Texas at Austin, August 2002. URL "http://www.cs.utexas.edu/users/gorinsky/pubs.html". [HSTCP] HighSpeed TCP Web Page, URL "http://www.icir.org/floyd/hstcp.html". [J02] Amit Jain and Sally Floyd, "Quick-Start for TCP and IP", internet draft draft-amit-quick-start-00.txt, work in progress, 2002. [M02] Matt Mathis, "Raising the Internet MTU", Web Page, URL "http://www.psc.edu/~mathis/MTU/". [RFC 1323] V. Jacobson, R. Braden, and D. Borman, TCP Extensions for High Performance, RFC 1323, May 1992. [S02] Stanislav Shalunov, Congestion Control Using Virtual MTUs, URL "http://www.internet2.edu/~shalunov/writing/virtual-mtu.html", 2002. [TFRC] Mark Handley, Jitendra Padhye, Sally Floyd, and Joerg Widmer, TCP Friendly Rate Control (TFRC): Protocol Specification, internet draft draft-ietf-tsvwg-tfrc-04.txt, work in progress, 2002. [VMSS] "Web100 at ORNL", Web Page, "http://www.csm.ornl.gov/~dunigan/netperf/web100.html". Floyd Experimental [Page 19]
draft-floyd-tcp-highspeed-01 August 2002 17. Security Considerations This proposal makes no changes to the underlying security of TCP. 18. IANA Considerations There are no IANA considerations regarding this document. A. TCP's Loss Event Rate in Steady-State This section gives the number of round-trip times between congestion events for a TCP flow with D-byte packets, for D=1500, as a function of the connection's average throughput B in bps. To achieve this average throughput B, a TCP connection with round-trip time R in seconds requires an average congestion window w of BR/(8D) segments. In steady-state, TCP's average congestion window w is roughly 1.2/sqrt(p) segments. This is equivalent to a lost event at most once every 1/p packets, or at most once every 1/(pw) = w/1.5 round- trip times. Substituting for w, this is a loss event at most every (BR)/12D)round-trip times. An an example, for R = 0.1 seconds and D = 1500 bytes, this gives B/180000 round-trip times between loss events. B. A table for a(w) and b(w). This section gives a table for the increase and decrease parameters a(w) and b(w) for HighSpeed TCP, for the default values of Low_Window = 38, High_Window = 83000, High_P = 10^-7, and High_Decrease = 0.1. Floyd Experimental [Page 20]
draft-floyd-tcp-highspeed-01 August 2002 w a(w) b(w) ---- ---- ---- 38 1 0.50 118 2 0.44 221 3 0.41 347 4 0.38 495 5 0.37 663 6 0.35 851 7 0.34 1058 8 0.33 1284 9 0.32 1529 10 0.31 1793 11 0.30 2076 12 0.29 2378 13 0.28 2699 14 0.28 3039 15 0.27 3399 16 0.27 3778 17 0.26 4177 18 0.26 4596 19 0.25 5036 20 0.25 5497 21 0.24 5979 22 0.24 6483 23 0.23 7009 24 0.23 7558 25 0.22 8130 26 0.22 8726 27 0.22 9346 28 0.21 9991 29 0.21 10661 30 0.21 11358 31 0.20 12082 32 0.20 12834 33 0.20 13614 34 0.19 14424 35 0.19 15265 36 0.19 16137 37 0.19 17042 38 0.18 17981 39 0.18 18955 40 0.18 19965 41 0.17 21013 42 0.17 22101 43 0.17 23230 44 0.17 24402 45 0.16 25618 46 0.16 Floyd Experimental [Page 21]
draft-floyd-tcp-highspeed-01 August 2002 26881 47 0.16 28193 48 0.16 29557 49 0.15 30975 50 0.15 32450 51 0.15 33986 52 0.15 35586 53 0.14 37253 54 0.14 38992 55 0.14 40808 56 0.14 42707 57 0.13 44694 58 0.13 46776 59 0.13 48961 60 0.13 51258 61 0.13 53677 62 0.12 56230 63 0.12 58932 64 0.12 61799 65 0.12 64851 66 0.11 68113 67 0.11 71617 68 0.11 75401 69 0.10 79517 70 0.10 84035 71 0.10 89053 72 0.10 94717 73 0.09 Table 9: Parameters for HighSpeed TCP. This table was computed with the following Perl program: Floyd Experimental [Page 22]
draft-floyd-tcp-highspeed-01 August 2002 $top = 100000; $num = 38; if ($num == 38) { print " w a(w) b(w)\n"; print " ---- ---- ----\n"; print " 38 1 0.50\n"; $oldb = 0.50; $olda = 1; } while ($num < $top) { $bw = (0.1 -0.5)*(log($num)-log(38))/(log(83000)-log(38))+0.5; $aw = ($num**2*2.0*$bw) / ((2.0-$bw)*$num**1.2*12.8); if ($aw > $olda + 1) { printf "%6d %5d %3.2f0, $num, $aw, $bw; $olda = $aw; } $num ++; } Table 10: Perl Program for computing parameters for HighSpeed TCP. C. Exploring the time to converge to fairness. This section gives the Perl program used to compute the congestion window growth during congestion avoidance. Floyd Experimental [Page 23]
draft-floyd-tcp-highspeed-01 August 2002 $top = 2001; $hswin = 1; $regwin = 1; $rtt = 1; $lastrtt = 0; $rttstep = 100; if ($hswin == 1) { print " RTT HS_Window Standard_TCP_Window0; print " --- --------- -------------------0; } while ($rtt < $top) { $bw = (0.1 -0.5)*(log($hswin)-log(38))/(log(83000)-log(38))+0.5; $aw = ($hswin**2*2.0*$bw) / ((2.0-$bw)*$hswin**1.2*12.8); if ($aw < 1) { $aw = 1; } if ($rtt >= $lastrtt + $rttstep) { printf "%5d %9d %10d0, $rtt, $hswin, $regwin; $lastrtt = $rtt; } $hswin += $aw; $regwin += 1; $rtt ++; } Table 11: Perl Program for computing the window in congestion avoidance. AUTHORS' ADDRESSES Sally Floyd Phone: +1 (510) 666-2989 ICIR (ICSI Center for Internet Research) Email: floyd@icir.org URL: http://www.icir.org/floyd/ This draft was created in August 2002. Floyd Experimental [Page 24]