rfc3042.txt

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Network Working Group                                          M. Allman
Request for Comments: 3042                                  NASA GRC/BBN
Category: Standards Track                                H. Balakrishnan
                                                                     MIT
                                                                S. Floyd
                                                                   ACIRI
                                                            January 2001


          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 Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   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.

1   Introduction

   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.



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   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.







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1.1 Terminology

   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 [1] 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.






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   [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



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   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