Internet Engineering Task Force Hari Balakrishnan Internet Draft MIT LCS Document: draft-ietf-pilc-asym-02.txt Venkata N. Padmanabhan Microsoft Research Category: Informational November 2000 TCP Performance Implications of Network Asymmetry 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. 1. Abstract This document describes TCP performance problems that arise because of asymmetric effects. These problems arise in several access networks, including bandwidth-asymmetric networks and packet radio networks, for different underlying reasons. However, the end result on TCP performance is the same in both cases: performance often degrades significantly because of imperfection and variability in the ACK feedback from the receiver to the sender. This document details several solutions, which have been proposed and evaluated in the literature, to these problems. These solutions use a combination of local link-layer techniques and end-to-end mechanisms, consisting of: (i) techniques to manage the reverse channel used by ACKs, typically using header compression or reducing the frequency of TCP ACKs, and (ii) techniques to handle this reduced ACK frequency to retain the TCP sender's acknowledgment- triggered self-clocking. 2. Conventions used in this document FORWARD DIRECTION: The dominant direction of data transfer over an asymmetric network. It corresponds to the direction with better link characteristics in terms of bandwidth, latency, error rate, etc. We term data transfer in the forward direction as a "forward transfer." Expires May 2001 [page 1]
INTERNET DRAFT PILC - Asymmetric Links November 2000 REVERSE DIRECTION: The direction in which acknowledgments of a forward TCP transfer flow. Data transfer could also happen in this direction (and it is termed "reverse transfer"), but it is typically less voluminous than that in the forward direction. The reverse direction typically exhibits worse link characteristics than the forward direction. DOWNSTREAM: Same as the forward direction. UPSTREAM: Same as the reverse direction. ACK: A cumulative TCP acknowledgment. In this document, we use this term to refer to a TCP segment that carries a cumulative acknowledgement but no data. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Motivation Asymmetric characteristics are exhibited by several network technologies, including cable modems, direct broadcast satellite, ADSL, and several packet radio networks. Given that these networks are increasingly being deployed as high-speed access networks, it is highly desirable to achieve good TCP performance over such networks. However, the asymmetry of the networks often makes this challenging. For example, when bandwidth is asymmetric such that the reverse path used by TCP ACKs is constrained, the slow or infrequent ACK feedback degrades TCP performance in the forward direction. Even when bandwidth is symmetric, asymmetry in the underlying medium access control (MAC) protocol could make it expensive to transmit ACKs (disproportionately to the size of the ACKs) in one direction, as in wireless packet radio networks. This results in degradation of TCP performance. The asymmetry of the MAC protocol is often a fundamental consequence of the hub-and-spokes architecture of the network (e.g., a single base station that communicates with multiple mobile stations) rather than an artifact of poor engineering choices. Despite the technological differences between asymmetric-bandwidth and packet radio networks, TCP performance suffers in both these kinds of networks for the same fundamental reason: the imperfection and variability of ACK feedback. This document discusses the problem in detail and describes several solutions from the research literature to overcome these problems [BPK97, BPK99, CR98, LMS97, KVR98]. 4. How does asymmetry degrade TCP performance? Expires May 2001 [page 2]
INTERNET DRAFT PILC - Asymmetric Links November 2000 This section describes the implications of network asymmetry on TCP performance. We refer the reader to [BPK99, Bal98, Pad98] for more details and experimental results. 4.1 Bandwidth asymmetry We first discuss the problems that degrade unidirectional transfer performance in bandwidth-asymmetric networks. Depending on the characteristics of the reverse path, two types of situations arise for unidirectional traffic over such networks: when the reverse bottleneck link has sufficient queuing to prevent packet (ACK) losses, and when the reverse bottleneck link has a small buffer. We consider each situation in turn. If the reverse bottleneck link has deep queues so that ACKs do not get dropped on the reverse path, then performance is a strong function of the normalized bandwidth ratio, k, defined in [LMS97]. k is the ratio of the raw bandwidths divided by the ratio of the packet sizes used in the two directions. For example, for a 10 Mbps forward channel and a 50 Kbps reverse channel, the raw bandwidth ratio is 200. With 1000-byte data packets and 40-byte ACKs, the ratio of the packet sizes is 25. This implies that k is 200/25 = 8. Thus, if the receiver acknowledges more frequently than one ACK every k = 8 data packets, the reverse bottleneck link will get saturated before the forward bottleneck link does, limiting the throughput in the forward direction. If k > 1 and ACKs are not delayed (in the sense of TCP's delayed ack algorithm) or dropped (at the reverse bottleneck router), TCP ACK- clocking breaks down. Consider two data packets transmitted by the sender in quick succession. En route to the receiver, these packets get spaced apart according to the bottleneck link bandwidth in the forward direction. The principle of ACK clocking is that the ACKs generated in response to these packets preserve this temporal spacing all the way back to the sender, enabling it to transmit new data packets that maintain the same spacing [Jac88]. However, the limited reverse bandwidth and queuing at the reverse bottleneck router alters the inter-ACK spacing observed at the sender. When ACKs arrive at the bottleneck link in the reverse direction at a faster rate than the link can support, they get queued behind one another. The spacing between them when they emerge from the link is dilated with respect to their original spacing, and is a function of the reverse bottleneck bandwidth. Thus the sender clocks out new data at a slower rate than if there had been no queuing of ACKs. No longer is the performance of the connection dependent on the forward bottleneck link alone; instead, it is throttled by the rate of arriving ACKs. As a side-effect, the sender's rate of congestion window growth slows down too. A different situation arises when the reverse bottleneck link has a relatively small amount of buffer space to accommodate ACKs. As the transmission window grows, this queue fills and ACKs are dropped. If Expires May 2001 [page 3]
INTERNET DRAFT PILC - Asymmetric Links November 2000 the receiver acknowledges every packet, only one of every k ACKs gets through to the sender, and the remaining (k-1) are dropped due to buffer overflow at the reverse channel buffer (here k is the normalized bandwidth ratio as before). In this case, the reverse bottleneck link capacity and slow ACK arrival are not directly responsible for any degraded performance. However, there are three important reasons for degraded performance in this case because ACKs are infrequent. 1. First, the sender transmits data in large bursts. If the sender receives only one ACK in k, it transmits data in bursts of k (or more) segments because each ACK shifts the sliding window by at least k (acknowledged) segments. This increases the likelihood of data loss along the forward path especially when k is large, because routers do not handle large bursts of packets well. 2. Second, TCP sender implementations increase their congestion window by counting the number of ACKs they receive and not on how much data is actually acknowledged by each ACK. Thus fewer ACKs imply a slower rate of growth of the congestion window, which degrades performance over long-delay connections. 3. Third, the sender's fast retransmission and recovery algorithms are less effective when ACKs are lost. The sender may not receive the threshold number of duplicate ACKs even if the receiver transmits more than the required number. Furthermore, the sender may not receive enough duplicate ACKs to adequately inflate its window during fast recovery. 4.2 MAC protocol interactions The interaction of TCP with media-access protocols often degraded end-to-end performance. Variable round-trip delays and ACK queuing are the main symptoms of this problem. The need for the communicating peers to first synchronize via the RTS/CTS protocol before communication and the significant turn-around time for the radios result in a high per-packet overhead. Furthermore, since the RTS/CTS exchange needs to back-off exponentially when the polled radio is busy (for example, engaged in a conversation with a different peer), this overhead is variable. This leads to large and variable communication latencies in packet-radio networks. In addition, with an asymmetric workload with most data flowing in one direction to clients, ACKs tend to get queued in certain radio units (especially in the client modems), exacerbating the variable communication latencies. These variable latencies and queuing of ACKs adversely affect smooth data flow. In particular, TCP ACK traffic interferes with the flow of data and increases the traffic load on the system. For example, experiments conducted on Metricom's Ricochet packet radio network [Met] in 1996 and 1997 clearly demonstrated the effect of the radio turnarounds and increased RTT variability, which degrade TCP performance. It is not uncommon for TCP connections to experience Expires May 2001 [page 4]
INTERNET DRAFT PILC - Asymmetric Links November 2000 timeouts that last between 9 and 12 seconds each. As a result, a connection may be idle for a very significant fraction of its lifetime. (We observed instances in the context of the Ricochet network where the idle time is 35% of the total transfer time!) Clearly, this leads to gross under-utilization of the available bandwidth. These observations are not an artifact of a particular network, but in fact show up in many wireless situations. Why are these timeouts so long in duration? Ideally, the round-trip time estimate (srtt) of a TCP data transfer will be relatively constant (i.e., have a low linear deviation, rttvar). Then the TCP retransmission timeout, set to srtt + 4*rttvar, will track the smoothed round-trip time estimate and respond well when multiple losses occur in a window. Unfortunately, this is not true for connections in the Ricochet network. Because of the high variability in RTT, the retransmission timer is on the order of 10 seconds, leading to the long idle timeout periods. In general, it is correct for the retransmission timer to trigger a segment retransmission only after an amount of time dependent on both the round-trip time and the linear (or standard) deviation. If only the mean or median round-trip estimates were taken into account, the potential for spurious retransmissions of segments still in transit is large. Connections traversing multiple wireless hops are especially vulnerable to this effect, because it is now more likely that the radio units may already be engaged in conversation with other peers. Note that the wireless MAC contention problem is a significant function of the number of packets (e.g., ACKs) transmitted rather than their size. In other words, there is a significant cost to transmitting a packet regardless of its size. 4.3 Bi-directional traffic We now consider the case when TCP transfers simultaneously occur in opposite directions over an asymmetric network. An example scenario is one in which a user sends out data upstream (for example, an e- mail message) while simultaneously receiving other data downstream (for example, Web pages). For ease of exposition, we restrict our discussion to the case of one connection in each direction. In the presence of bi-directional traffic, the effects discussed in Section 4.1 are more pronounced, because part of the uplink bandwidth is used up by the reverse transfer. This effectively increases the degree of bandwidth asymmetry for the forward transfer. In addition, there are other effects that arise due to the interaction between data packets of the reverse transfer and ACKs of the forward transfer. Suppose the reverse connection is initiated first and that it has saturated the reverse channel and buffer with its data packets at the time the forward connection is initiated. Expires May 2001 [page 5]
INTERNET DRAFT PILC - Asymmetric Links November 2000 There is then a high probability that many ACKs of the newly initiated forward connection will encounter a full reverse channel buffer and hence get dropped. Even after these initial problems, ACKs of the forward connection could often get queued up behind large data packets of the reverse connection, which could have long transmission times (e.g., it takes about 280 ms to transmit a 1 KB data packet over a 28.8 Kbps line). This causes the forward transfer to stall for long periods of time. It is only at times when the reverse connection loses packets (due to a buffer overflow at an intermediate router) and slows down that the forward connection gets the opportunity to make rapid progress and quickly build up its window. In summary, the presence of bi-directional traffic exacerbates the problems due to bandwidth asymmetry because of the adverse interaction between data packets of an upstream connection and the ACKs of a downstream connection. 5. Improving TCP performance over asymmetric networks It should be clear by now that there are two key issues that need to be addressed in order to improve TCP performance over asymmetric networks. The first issue is to manage bandwidth usage on the reverse link, used by ACKs (and possibly other traffic). Many of these techniques work by reducing the number of ACKs that flow over the reverse channel, which has the potential to destroy the desirable self-clocking property of the TCP sender where new data transmissions are triggered by incoming ACKs. Thus, the second issue is to avoid any adverse impact of infrequent ACKs. Each of these issues can be handled by local link-layer solutions and/or by end-to-end techniques. In this section, we discuss several proposed solutions of both kinds. 5.1 Reverse-link bandwidth management 5.1.1 TCP header compression RFC 1144 describes TCP header compression for use over low-bandwidth links running SLIP or PPP. Because it greatly reduces the size of ACKs on the reverse link when losses are infrequent (a situation that ensures that the state of the compressor and decompressor are synchronized), we recommend its use over low-bandwidth reverse links where possible. However, this alone does not address all of the problems: 1. As discussed in Section 4.2, in certain networks there is a significant per-packet MAC overhead that is independent of packet size. 2. A reduction in the size of ACKs does not prevent adverse interaction with large upstream data packets in the presence of bi-directional traffic (discussed in Section 4.4). Expires May 2001 [page 6]
INTERNET DRAFT PILC - Asymmetric Links November 2000 Therefore, to effectively address the performance problems caused by asymmetry, there is a need for techniques over and beyond TCP header compression. 5.1.2 ACK filtering ACK filtering (AF) is a TCP-aware link-layer technique that reduces the number of TCP ACKs sent on the reverse channel. The challenge is to ensure that the sender does not stall waiting for ACKs, which can happen if ACKs are removed indiscriminately on the reverse path. AF removes only certain ACKs without starving the sender by taking advantage of the fact that TCP ACKs are cumulative. As far as the sender's error control mechanism is concerned, the information contained in an ACK with a later sequence number subsumes the information contained in any earlier ACK When an ACK from the receiver is about to be enqueued at a reverse direction router, the router or the end-host's link layer (if the host is directly connected to the constrained link) checks its queues for any older ACKs belonging to the same connection. If any are found, it removes them from the queue, thereby reducing the number of ACKs that go back to the sender. The removal of these "redundant" ACKs frees up buffer space for other data and ACK packets. AF does not remove duplicate or selective ACKs from the queue to avoid causing problems to TCP's data-driven loss recovery mechanisms. The policy that the filter uses to drop packets is configurable and can either be deterministic or random (similar to a random-drop gateway, but taking the semantics of the items in the queue into consideration). State needs to be maintained only for connections with at least one packet in the queue (akin to FRED [LM97]). However, this state is soft, and if necessary, can easily be reconstructed from the contents of the queue. 5.1.3 ACK congestion control ACK congestion control (ACC) is an alternative to ACK filtering that operates end-to-end rather than at the upstream bottleneck router. The key idea in ACC is to extend congestion control to TCP ACKs, since they do make non-negligible demands on resources at the bandwidth-constrained upstream link. ACKs occupy slots in the reverse channel buffer, whose capacity is often limited to a certain number of packets (rather than bytes). ACC has two parts: (a) a mechanism for the network to indicate to the receiver that the ACK path is congested, and (b) the receiver's response to such an indication. One possibility for the former is the RED (Random Early Detection) algorithm [11] at the upstream bottleneck router. The router detects incipient congestion by tracking the average queue size over a time window in the recent Expires May 2001 [page 7]
INTERNET DRAFT PILC - Asymmetric Links November 2000 past. If the average exceeds a threshold, the router selects a packet at random and marks it, i.e. sets an Explicit Congestion Notification (ECN) bit in the packet header. This notification is reflected back to the upstream TCP end-host by its downstream peer. It is important to note that with ACC, both data packets and TCP ACKs are candidates for being marked with an ECN bit. Therefore, upon receiving an ACK packet with the ECN bit set, the TCP receiver reduces the rate at which it sends ACKs. The TCP receiver maintains a dynamically varying delayed-ack factor, d, and sends one ACK for every d data packets received. When it receives a packet with the ECN bit set, it increases d multiplicatively, thereby decreasing the frequency of ACKs also multiplicatively. Then for each subsequent round-trip time (determined using the TCP timestamp option) during which it does not receive an ECN, it linearly decreases the factor d, thereby increasing the frequency of ACKs. Thus, the receiver mimics the standard congestion control behavior of TCP senders in the manner in which it sends ACKs. There are bounds on the delayed-ack factor d. Obviously, the minimum value of d is 1, since at most one ACK should be sent per data packet. The maximum value of d is determined by the sender's window size, which is conveyed to the receiver in a new TCP option. The receiver should send at least one ACK (preferably more) for each window of data from the sender. Otherwise, it could cause the sender to stall until the receiver's delayed-ack timer (usually set at 200 ms) kicks in and forces an ACK to be sent. Despite RED+ECN, there may be times when the upstream router queue fills up and it needs to drop a packet. The router can pick a packet to drop in various ways. For instance, it can drop from the tail, or it can drop a packet already in the queue at random. 5.1.4 Acks-first scheduling In the case of bi-directional transfers, data as well as ACK packets compete for resources in the reverse direction (Section 4.4). In this case, a single FIFO queue for both data packets and ACKs could cause problems. For example, if the reverse channel is a 28.8 Kbps dialup line, the transmission of a 1 KB sized data packet would take about 280 ms. So even if just two such data packets get queued ahead of ACKs (not an uncommon occurrence since data packets are sent out in pairs during slow start), they would shut out ACKs for well over half a second. And if more than two data packets are queued up ahead of an ACK, the ACKs would be delayed by even more. A possible approach to alleviating this problem is to schedule data and ACKs differently from FIFO. One algorithm, in particular, is acks-first scheduling, which always accords a higher priority to ACKs over data packets. The motivation for such scheduling is that it minimizes the idle time for the forward connection by minimizing the amount of time that its ACKs spend queued behind upstream data packets. At the same time, with techniques such as header Expires May 2001 [page 8]
INTERNET DRAFT PILC - Asymmetric Links November 2000 compression [RFC1144], the transmission time of ACKs becomes small enough that its impact on subsequent data packets is minimal. (Networks in which the per-packet overhead of the reverse channel is large, e.g. packet radio networks, are an exception.) Note that as with ACC, this scheduling scheme does not require the gateway to explicitly identify or maintain state for individual TCP connections. Acks-first scheduling does not help avoid a delay due to a data packet in transmission. On a slow uplink, such a delay could be large if the data packet is large in size. One way of reducing the delay is to fragment the data packet into small pieces before transmission [RFC1990, RFC2686]. 5.1.5 Backpressure and Fair Scheduling Two techniques to address the problem of interference between data packets and ACKs on the uplink are proposed in [KVR98]. The first limits the number of data packets in the outgoing uplink queue by applying backpressure to the TCP layer. In configurations where the uplink network adapter is directly attached to the end-system, backpressure limits the queuing delay caused by the accumulation of data packets at the upstream queue. Backpressure can be unfair to the upstream connection and make its throughput highly sensitive to the dynamics of the downstream connection. So an alternative, fair scheduling, is proposed in [KVR98] where a limit is placed on the number of ACKs a node is allowed to transmit upstream before transmitting a data packet (assuming at least one data packet is waiting in the upstream queue). This guarantees the upstream connection at least a certain minimum share of the bandwidth while enabling the downstream connection to achieve high throughput. 5.2 Handling infrequent ACKs This can be done either end-to-end or locally at the constrained reverse link. 5.2.1 TCP sender adaptation ACC and AF alleviate the problem of congestion on the reverse bottleneck link by decreasing the frequency of ACKs, with each ACK potentially acknowledging several data packets. As discussed in Section 4.1, this can cause problems such as sender burstiness and a slowdown in congestion window growth. Sender adaptation is an end-to-end technique for alleviating this problem. A bound is placed on the maximum number of packets the sender can transmit back-to-back, even if the window allows the transmission of more data. If necessary, more bursts of data are scheduled for later points in time computed based on the Expires May 2001 [page 9]
INTERNET DRAFT PILC - Asymmetric Links November 2000 connection's data rate. The data rate is estimated as the ratio cwnd/srtt, where cwnd is the TCP congestion window size and srtt is the smoothed RTT estimate. Thus, large bursts of data get broken up into smaller bursts spread out over time. The sender can avoid a slowdown in congestion window growth by simply taking into account the amount of data acknowledged by each ACK, rather than the number of ACKs. So, if an ACK acknowledges s segments, the window is grown as if s separate ACKs had been received. (One could treat the single ACK as being equivalent to s/2 instead of s ACKs to mimic the effect of the TCP delayed ack algorithm.) This policy works because the window growth is only tied to the available bandwidth in the forward direction, so the number of ACKs is immaterial. 5.2.2 ACK Reconstruction ACK reconstruction is a technique to reconstruct the ACK stream after it has traversed the reverse direction bottleneck link. AR is a local technique designed to prevent the reduced ACK frequency from adversely affecting the performance of standard TCP sender implementations (i.e., those that do not implement sender adaptation). This enables us to use schemes such as ACK filtering or ACK congestion control without requiring TCP senders to be modified to perform sender adaptation. This solution can be easily deployed by Internet Service Providers (ISPs) of asymmetric access technol- ogies in conjunction with AF to achieve good performance. AR deploys a soft-state agent called the ACK reconstructor at the upstream end of the constrained ACK bottleneck. The reconstructor does not need to be on the forward data path. It carefully fills in the gaps in the ACK sequence and introduces ACKs to smooth out the ACK stream seen by the sender. However, it does so without violating the end-to-end semantics of TCP ACKs, as explained below. Suppose two ACKs, a1 and a2 arrive at the reconstructor after traversing the constrained reverse link at times t1 and t2 respectively. Let a2 - a1 = delta_a > 1. If a2 were to reach the sender soon after a1 with no intervening ACKs, at least delta_a segments are burst out by the sender (if the flow control window is large enough), and the congestion window increases by at most 1, independent of delta_a. ACK reconstruction remedies this problematic situation by interspersing ACKs to provide the sender with a larger number of ACKs at a consistent rate, which reduces the degree of burstiness and causes the congestion window to increase at a rate governed by the forward bottleneck. How is this done? One of the configurable parameters of the reconstructor is ack_thresh, the ACK threshold, which determines the spacing between interspersed ACKs at the output. Typically, ack_thresh is set to 2, which follows TCP's standard delayed-ACK policy. Thus, if successive ACKs arrive at the reconstructor Expires May 2001 [page 10]
INTERNET DRAFT PILC - Asymmetric Links November 2000 separated by delta_a, it interposes ceil(delta_a/ack_thresh) - 2 ACKs, where ceil() is the ceiling operator. The other parameter needed by the reconstructor is ack_interval, which determines the temporal spacing between the reconstructed ACKs. To do this, it measures the rate at which ACKs arrive at the input to the recon- structor. This rate depends on the output rate from the constrained reverse channel and on the presence of other traffic on that link. The reconstructor uses an exponentially weighted moving average estimator to monitor this rate; the output of the estimator is delta_t, the average temporal spacing at which ACKs are arriving at the reconstructor (and the average rate at which ACKs would reach the sender if there were no further losses or delays). If the reconstructor sets ack_interval equal to delta_t, then we would essentially operate at a rate governed by the reverse bottleneck link, and the resulting performance would be determined by the rate at which unfiltered ACKs arrive out of the reverse bottleneck link. If sender adaptation were being done, then the sender behaves as if the rate at which acks arrive us delta_a/delta_t. Therefore, a good method of deciding the temporal spacing of reconstructed ACKs, ack_interval, is to equate the rates at which increments in the ACK sequence happen in the two cases. That is, the reconstructor sets ack_interval such that delta_a/delta_t = ack_thresh/ack_interval, which implies that ack_interval = (ack_thresh/delta_a)*delta_t. Therefore, the latest ACK in current sequence, a2, is held back for a time roughly equal to delta_t, and ceil(delta_a/ack_thresh) - 2 ACKs are evenly interposed in this time. Thus, by carefully controlling the number of and spacing between ACKs, unmodified TCP senders can be made to increase their congestion window at the right rate and avoid bursty behavior. ACK reconstruction can be implemented by maintaining only "soft state" [Clark88] at the reconstructor that can easily be regenerated if lost. Note that the reconstructor generates no spurious ACKs and the end-to-end semantics of the connection are completely preserved. The trade-off in AR is between obtaining less bursty performance, a better rate of congestion window increase, and a reduction in the round-trip variation, versus a modest increase in the round-trip time estimate at the sender. We believe that it is a good trade-off in the asymmetric environments we are concerned with. 5.3 Alternatives to AF and AR Techniques similar in vein to but more sophisticated than AF and AR have also been proposed. One of them is ACK compaction and expansion [Sam99] where the compacter discards older ACKs in the upstream queue while retaining newer ACKs (just as in AF), but in addition conveys the number of discarded ACKs and the total number of bytes they acknowledge to its peer, the expander. The expander can then regenerate the discarded ACKs without having to guess how many ACKs had been discarded. This is an advantage compared to AF/AR. However, it comes at the cost of new protocol machinery to convey the Expires May 2001 [page 11]
INTERNET DRAFT PILC - Asymmetric Links November 2000 information about discarded ACKs from the compacter to the expander. AF/AR does not require any new protocol machinery. Another technique along similar lines is discussed in [Joh99]. An ACK compressor concatenates multiple ACKs and sends them to the decompressor together with the arrival time of the concatenated ACKs into the queue. The decompressor then uses this information to regenerate the individual ACKs. Like the ACK compacter/expander, this scheme enables more accurate regeneration of ACKs compared to AF/AR but at the cost of new protocol machinery. 6. Security Considerations Security considerations in the context of this Internet Draft arise primarily from the possible use of IPSEC by the end hosts: 1. With IPSEC ESP, the TCP header can neither be read nor modified by intermediate entities. This rules out header compression, ACK filtering, and ACK reconstruction. 2. With IPSEC AH or TF-ESP, the TCP header can be read but not modified by intermediaries. This rules out ACK reconstruction but allows ACK filtering. The enhanced header compression scheme discussed in [RFC2505] would also work with AH. 7. Summary This Internet Draft considers several TCP performance problems that arise from asymmetry in network links and surveys several possible solutions. Problems arise as a result of asymmetry in both bandwidth and in the nature of RTS/CTS-based media-access protocols. In addition to getting dropped due to congestion at the upstream bottleneck, ACKs may get inordinately delayed (e.g., when there is bi-directional traffic) or may exacerbate media-access delays (e.g., in certain multi-hop radio networks). TCP header compression, while being helpful, does not address many of these issues. This Internet Draft surveys performance improvement techniques that combine ACK congestion alleviation with techniques that enable a TCP sender to cope with infrequent ACKs without destroying its self- clocking. These techniques include both end-to-end and local link- layer schemes. Many of these techniques have been evaluated in detail via analysis, simulation, and/or implementation on real asymmetric networks. The references listed below describe these evaluations in detail. 8. References Expires May 2001 [page 12]
INTERNET DRAFT PILC - Asymmetric Links November 2000 [Bal98] H. Balakrishnan, "Challenges to Reliable Data Transport over Heterogeneous Wireless Networks", Ph.D. Thesis, University of California at Berkeley, USA, August 1998 http://www.cs.berkeley.edu/~hari/thesis/ [BPK97] H. Balakrishnan, V. N. Padmanabhan, R. H. Katz, "The Effects of Asymmetry on TCP Performance", Proc. ACM/IEEE Mobicom, Budapest, Hungary, September 1997 [BPK99] H. Balakrishnan, V. N. Padmanabhan, R. H. Katz, "The Effects of Asymmetry on TCP Performance", ACM Mobile Networks and Applications (MONET), 1999. This is an expanded journal version of the Mobicom '97 paper. [CR98] R. Cohen, S. Ramanathan, "TCP for High Performance in Hybrid Fiber Coaxial Broad-Band Access Networks", IEEE/ACM Transactions on Networking, February 1998. [Jac88] V. Jacobson, Congestion Avoidance and Control, Proc. ACM SIGCOMM, Stanford, CA, August 1988. [Joh99] G. Johansson, E-mail sent to the PILC mailing list, October 1999. [KVR98] L. Kalampoukas, A. Varma, K. K. Ramakrishnan, "Improving TCP Throughput over Two-Way Asymmetric Links: Analysis and Solutions", Proc. ACM SIGMETRICS, June 1998. [LM97] D. Lin, R. Morris, "Dynamics of Random Early Detection", Proc. ACM SIGCOMM, 1997. [LMS97] T. V. Lakshman, U. Madhow, B. Suter, "Window-based Error Recovery and Flow Control with a Slow Acknowledgement Channel: A Study of TCP/IP Performance", Proc. IEEE Infocom, Kobe, Japan, April 1997. [Met] Metricom Inc., http://www.metricom.com [Pad98] V. N. Padmanabhan, "Addressing the Challenges of Web Data Transport", Ph.D. Thesis, University of California at Berkeley, USA, September 1998 (also Tech Report UCB/CSD-98-1016) http:// www.research.microsoft.com/~padmanab/phd-thesis.html [RFC1144] V. Jacobson, "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC-1144, February 1990 [RFC1990] K. Sklower, B. Lloyd, G. McGregor, D. Carr, T. Coradetti, "The PPP Multilink Protocol (MP)", RFC-1990, August 1996. [RFC2026] S. Bradner, "The Internet Standards Process -- Revision 3", RFC-2026, October 1996. Expires May 2001 [page 13]
INTERNET DRAFT PILC - Asymmetric Links November 2000 [RFC2119] S. Bradner, " Key words for use in RFCs to Indicate Requirement Levels", RFC-2119, March 1997. [RFC2505] M. Degermark, B. Nordgren, S. Pink, "IP Header Compression", RFC-2507, February 1999. [RFC2686] C. Bormann, "The Multi-Class Extension to Multi-Link PPP", RFC-2686, September 1999. [Sam99] N. K. G. Samaraweera, "Return Link Optimization for Internet Service Provision Using DVB-S Networks", ACM SIGCOMM CCR, July 1999. 9. Acknowledgments We thank Spencer Dawkins, Aaron Falk, and the members of the PILC mailing list for their valuable comments. 10. Authors' Addresses Hari Balakrishnan Laboratory for Computer Science 200 Technology Square Massachusetts Institute of Technology Cambridge, MA 02139 USA Phone: +1-617-253-8713 Fax: +1-617-253-0147 Email: hari@lcs.mit.edu Web: http://nms.lcs.mit.edu/~hari/ Venkata N. Padmanabhan Microsoft Research One Microsoft Way Redmond, WA 98052 USA Phone: +1-425-705-2790 Fax: +1-425-936-7329 Email: padmanab@microsoft.com Web: http://www.research.microsoft.com/~padmanab/ Expires May 2001 [page 14]
INTERNET DRAFT PILC - Asymmetric Links November 2000 Full Copyright Statement "Copyright (C) The Internet Society (date). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. Expires May 2001 [page 15]