Request for Comments: 3042
Category: Standards Track
Enhancing TCP's Loss Recovery Using Limited Transmit
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2001). All Rights Reserved.
This document proposes a new Transmission Control Protocol (TCP) mechanism that can be used to more effectively recover lost segments when a connection's congestion window is small, or when a large number of segments are lost in a single transmission window. The "Limited Transmit" algorithm calls for sending a new data segment in response to each of the first two duplicate acknowledgments that arrive at the sender. Transmitting these segments increases the probability that TCP can recover from a single lost segment using the fast retransmit algorithm, rather than using a costly retransmission timeout. Limited Transmit can be used both in conjunction with, and in the absence of, the TCP selective acknowledgment (SACK) mechanism.
A number of researchers have observed that TCP's loss recovery strategies do not work well when the congestion window at a TCP sender is small. This can happen, for instance, because there is only a limited amount of data to send, or because of the limit imposed by the receiver-advertised window, or because of the constraints imposed by end-to-end congestion control over a connection with a small bandwidth-delay product [Riz96,Mor97,BPS+98,Bal98,LK98]. When a TCP detects a missing segment, it enters a loss recovery phase using one of two methods.
First, if an acknowledgment (ACK) for a given segment is not received in a certain amount of time a retransmission timeout occurs and the segment is resent [RFC793,PA00]. Second, the "Fast Retransmit" algorithm resends a segment when three duplicate ACKs arrive at the sender [Jac88,RFC2581]. However, because duplicate ACKs from the receiver are also triggered by packet reordering in the Internet, the TCP sender waits for three duplicate ACKs in an attempt to disambiguate segment loss from packet reordering. Once in a loss recovery phase, a number of techniques can be used to retransmit lost segments, including slow start-based recovery or Fast Recovery [RFC2581], NewReno [RFC2582], and loss recovery based on selective acknowledgments (SACKs) [RFC2018,FF96].
TCP's retransmission timeout (RTO) is based on measured round-trip times (RTT) between the sender and receiver, as specified in [PA00]. To prevent spurious retransmissions of segments that are only delayed and not lost, the minimum RTO is conservatively chosen to be 1 second. Therefore, it behooves TCP senders to detect and recover from as many losses as possible without incurring a lengthy timeout when the connection remains idle. However, if not enough duplicate ACKs arrive from the receiver, the Fast Retransmit algorithm is never triggered---this situation occurs when the congestion window is small or if a large number of segments in a window are lost. For instance, consider a congestion window (cwnd) of three segments. If one segment is dropped by the network, then at most two duplicate ACKs will arrive at the sender. Since three duplicate ACKs are required to trigger Fast Retransmit, a timeout will be required to resend the dropped packet.
[BPS+97] found that roughly 56% of retransmissions sent by a busy web server were sent after the RTO expires, while only 44% were handled by Fast Retransmit. In addition, only 4% of the RTO-based retransmissions could have been avoided with SACK, which of course has to continue to disambiguate reordering from genuine loss. In contrast, using the technique outlined in this document and in [Bal98], 25% of the RTO-based retransmissions in that dataset would have likely been avoided.
The next section of this document outlines small changes to TCP senders that will decrease the reliance on the retransmission timer, and thereby improve TCP performance when Fast Retransmit is not triggered. These changes do not adversely affect the performance of TCP nor interact adversely with other connections, in other circumstances.
In this document, he key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", AND "OPTIONAL" are to be interpreted as described in RFC 2119  and indicate requirement levels for protocols.
2 The Limited Transmit Algorithm
When a TCP sender has previously unsent data queued for transmission it SHOULD use the Limited Transmit algorithm, which calls for a TCP sender to transmit new data upon the arrival of the first two consecutive duplicate ACKs when the following conditions are satisfied:
- The receiver's advertised window allows the transmission of the segment.
- The amount of outstanding data would remain less than or equal to the congestion window plus 2 segments. In other words, the sender can only send two segments beyond the congestion window (cwnd).
The congestion window (cwnd) MUST NOT be changed when these new segments are transmitted. Assuming that these new segments and the corresponding ACKs are not dropped, this procedure allows the sender to infer loss using the standard Fast Retransmit threshold of three duplicate ACKs [RFC2581]. This is more robust to reordered packets than if an old packet were retransmitted on the first or second duplicate ACK.
Note: If the connection is using selective acknowledgments [RFC2018], the data sender MUST NOT send new segments in response to duplicate ACKs that contain no new SACK information, as a misbehaving receiver can generate such ACKs to trigger inappropriate transmission of data segments. See [SCWA99] for a discussion of attacks by misbehaving receivers.
Limited Transmit follows the "conservation of packets" congestion control principle [Jac88]. Each of the first two duplicate ACKs indicate that a segment has left the network. Furthermore, the sender has not yet decided that a segment has been dropped and therefore has no reason to assume that the current congestion control state is inaccurate. Therefore, transmitting segments does not deviate from the spirit of TCP's congestion control principles.
[BPS99] shows that packet reordering is not a rare network event. [RFC2581] does not provide for sending of data on the first two duplicate ACKs that arrive at the sender. This causes a burst of segments to be sent when an ACK for new data does arrive following packet reordering. Using Limited Transmit, data packets will be clocked out by incoming ACKs and therefore transmission will not be as bursty.
Note: Limited Transmit is implemented in the ns simulator [NS]. Researchers wishing to investigate this mechanism further can do so by enabling "singledup_" for the given TCP connection.
3 Related Work
Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC2481] may benefit connections with small congestion window sizes [SA00]. ECN provides a method for indicating congestion to the end-host without dropping segments. While some segment drops may still occur, ECN may allow TCP to perform better with small congestion window sizes because the sender can avoid many of the Fast Retransmits and Retransmit Timeouts that would otherwise have been needed to detect dropped segments [SA00].
When ECN-enabled TCP traffic competes with non-ECN-enabled TCP traffic, ECN-enabled traffic can receive up to 30% higher goodput. For bulk transfers, the relative performance benefit of ECN is greatest when on average each flow has 3-4 outstanding packets during each round-trip time [ZQ00]. This should be a good estimate for the performance impact of a flow using Limited Transmit, since both ECN and Limited Transmit reduce the reliance on the retransmission timer for signaling congestion.
The Rate-Halving congestion control algorithm [MSML99] uses a form of limited transmit, as it calls for transmitting a data segment on every second duplicate ACK that arrives at the sender. The algorithm decouples the decision of what to send from the decision of when to send. However, similar to Limited Transmit the algorithm will always send a new data segment on the second duplicate ACK that arrives at the sender.
4 Security Considerations
The additional security implications of the changes proposed in this document, compared to TCP's current vulnerabilities, are minimal. The potential security issues come from the subversion of end-to-end congestion control from "false" duplicate ACKs, where a "false" duplicate ACK is a duplicate ACK that does not actually acknowledge new data received at the TCP receiver. False duplicate ACKs could result from duplicate ACKs that are themselves duplicated in the network, or from misbehaving TCP receivers that send false duplicate ACKs to subvert end-to-end congestion control [SCWA99,RFC2581].
When the TCP data receiver has agreed to use the SACK option, the TCP data sender has fairly strong protection against false duplicate ACKs. In particular, with SACK, a duplicate ACK that acknowledges new data arriving at the receiver reports the sequence numbers of that new data. Thus, with SACK, the TCP sender can verify that an arriving duplicate ACK acknowledges data that the TCP sender has actually sent, and for which no previous acknowledgment has been received, before sending new data as a result of that acknowledgment. For further protection, the TCP sender could keep a record of packet boundaries for transmitted data packets, and recognize at most one valid acknowledgment for each packet (e.g., the first acknowledgment acknowledging the receipt of all of the sequence numbers in that packet).
One could imagine some limited protection against false duplicate ACKs for a non-SACK TCP connection, where the TCP sender keeps a record of the number of packets transmitted, and recognizes at most one acknowledgment per packet to be used for triggering the sending of new data. However, this accounting of packets transmitted and acknowledged would require additional state and extra complexity at the TCP sender, and does not seem necessary.
The most important protection against false duplicate ACKs comes from the limited potential of duplicate ACKs in subverting end-to-end congestion control. There are two separate cases to consider: when the TCP sender receives less than a threshold number of duplicate ACKs, and when the TCP sender receives at least a threshold number of duplicate ACKs. In the latter case a TCP with Limited Transmit will behave essentially the same as a TCP without Limited Transmit in that the congestion window will be halved and a loss recovery period will be initiated.
When a TCP sender receives less than a threshold number of duplicate ACKs a misbehaving receiver could send two duplicate ACKs after each regular ACK. One might imagine that the TCP sender would send at three times its allowed sending rate. However, using Limited Transmit as outlined in section 2 the sender is only allowed to exceed the congestion window by less than the duplicate ACK threshold (of three segments), and thus would not send a new packet for each duplicate ACK received.
Bill Fenner, Jamshid Mahdavi and the Transport Area Working Group provided valuable feedback on an early version of this document.
[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport over Heterogeneous Wireless Networks. Ph.D. Thesis, University of California at Berkeley, August 1998. [BPS+97] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan, Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server: Analysis and Improvements. Technical Report UCB/CSD-97-966, August 1997. Available from http://nms.lcs.mit.edu/~hari/papers/csd-97-966.ps. (Also in Proc. IEEE INFOCOM Conf., San Francisco, CA, March 1998.) [BPS99] Jon Bennett, Craig Partridge, Nicholas Shectman. Packet Reordering is Not Pathological Network Behavior. IEEE/ACM Transactions on Networking, December 1999. [FF96] Kevin Fall, Sally Floyd. Simulation-based Comparisons of Tahoe, Reno, and SACK TCP. ACM Computer Communication Review, July 1996. [Flo94] Sally Floyd. TCP and Explicit Congestion Notification. ACM Computer Communication Review, October 1994. [Jac88] Van Jacobson. Congestion Avoidance and Control. ACM SIGCOMM 1988. [LK98] Dong Lin, H.T. Kung. TCP Fast Recovery Strategies: Analysis and Improvements. Proceedings of InfoCom, March 1998. [MSML99] Matt Mathis, Jeff Semke, Jamshid Mahdavi, Kevin Lahey. The Rate Halving Algorithm, 1999. URL: http://www.psc.edu/networking/rate_halving.html. [Mor97] Robert Morris. TCP Behavior with Many Flows. Proceedings of the Fifth IEEE International Conference on Network Protocols. October 1997. [NS] Ns network simulator. URL: http://www.isi.edu/nsnam/. [PA00] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [Riz96] Luigi Rizzo. Issues in the Implementation of Selective Acknowledgments for TCP. January, 1996. URL: http://www.iet.unipi.it/~luigi/selack.ps [SA00] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks", RFC 2884, July 2000. [SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, Tom Anderson. TCP Congestion Control with a Misbehaving Receiver. ACM Computer Communications Review, October 1999. [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgement Options", RFC 2018, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2481] Ramakrishnan, K. and S. Floyd, "A Proposal to Add Explicit Congestion Notification (ECN) to IP", RFC 2481, January 1999. [RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 2582, April 1999. [ZQ00] Yin Zhang and Lili Qiu, Understanding the End-to-End Performance Impact of RED in a Heterogeneous Environment, Cornell CS Technical Report 2000-1802, July 2000. URL http://www.cs.cornell.edu/yzhang/papers.htm.
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