Internet Engineering Task Force                               E. Blanton
INTERNET-DRAFT                                         Purdue University
draft-blanton-tcpm-3517bis-01.txt                              M. Allman
                                                                 L. Wang
                                                        Juniper Networks
                                                             I. Jarvinen
                                                                 M. Kojo
                                                  University of Helsinki
                                                          April 13, 2011

         A Conservative Selective Acknowledgment (SACK)-based
                    Loss Recovery Algorithm for TCP

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   This document presents a conservative loss recovery algorithm for TCP
   that is based on the use of the selective acknowledgment (SACK) TCP
   option.  The algorithm presented in this document conforms to the
   spirit of the current congestion control specification (RFC 2581),
   but allows TCP senders to recover more effectively when multiple
   segments are lost from a single flight of data.

1   Introduction

   This document presents a conservative loss recovery algorithm for TCP
   that is based on the use of the selective acknowledgment (SACK) TCP
   option.  While the TCP SACK [RFC2018] is being steadily deployed in
   the Internet [All00], there is evidence that hosts are not using the
   SACK information when making retransmission and congestion control
   decisions [PF01].  The goal of this document is to outline one
   straightforward method for TCP implementations to use SACK
   information to increase performance.

   [RFC2581] allows advanced loss recovery algorithms to be used by TCP
   [RFC793] provided that they follow the spirit of TCP's congestion
   control algorithms [RFC2581, RFC2914].  [RFC2582] outlines one such
   advanced recovery algorithm called NewReno.  This document outlines a
   loss recovery algorithm that uses the SACK [RFC2018] TCP option to
   enhance TCP's loss recovery.  The algorithm outlined in this
   document, heavily based on the algorithm detailed in [FF96], is a
   conservative replacement of the fast recovery algorithm [Jac90,
   RFC2581].  The algorithm specified in this document is a
   straightforward SACK-based loss recovery strategy that follows the
   guidelines set in [RFC2581] and can safely be used in TCP
   implementations.  Alternate SACK-based loss recovery methods can be
   used in TCP as implementers see fit (as long as the alternate
   algorithms follow the guidelines provided in [RFC2581]).  Please
   note, however, that the SACK-based decisions in this document (such
   as what segments are to be sent at what time) are largely decoupled
   from the congestion control algorithms, and as such can be treated as
   separate issues if so desired.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

2   Definitions

   The reader is expected to be familiar with the definitions given in

   The reader is assumed to be familiar with selective acknowledgments
   as specified in [RFC2018].

   For the purposes of explaining the SACK-based loss recovery algorithm
   we define five variables that a TCP sender stores:

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      "HighACK" is the sequence number of the highest byte of data that
      has been cumulatively ACKed at a given point.

      "HighData" is the highest sequence number transmitted at a given

      "HighRxt" is the highest sequence number which has been
      retransmitted during the current loss recovery phase.

      "Pipe" is a sender's estimate of the number of bytes outstanding
      in the network.  This is used during recovery for limiting the
      sender's sending rate.  The pipe variable allows TCP to use a
      fundamentally different congestion control than specified in
      [RFC5681].  The algorithm is often referred to as the "pipe

      "DupAcks" is the number of duplicate acknowledgments received
      since the last cumulative acknowledgment.

   For the purposes of this specification we define a "duplicate
   acknowledgment" as a segment that arrives carrying a SACK block which
   identifies previously unacknowledged and un-SACKed octets between
   SND.UNA and SND.NXT as received.  Note that an ACK which carries new
   SACK data is counted as a duplicate acknowledgment under this
   definition even if it carries new data or changes the advertised
   window, which is different from the definition of duplicate
   acknowledgment in [RFC5681].

   We define a variable "DupThresh" that holds the number of duplicate
   acknowledgments required to trigger a retransmission.  Per [RFC5681]
   this threshold is defined to be 3 duplicate acknowledgments.
   However, implementers should consult any updates to [RFC5681] to
   determine the current value for DupThresh (or method for determining
   its value).

   Finally, a range of sequence numbers [A,B] is said to "cover"
   sequence number S if A <= S <= B.

3   Keeping Track of SACK Information

   For a TCP sender to implement the algorithm defined in the next
   section it must keep a data structure to store incoming selective
   acknowledgment information on a per connection basis.  Such a data
   structure is commonly called the "scoreboard".  The specifics of the
   scoreboard data structure are out of scope for this document (as long
   as the implementation can perform all functions required by this

   Note that this document refers to keeping account of (marking)
   individual octets of data transferred across a TCP connection.  A
   real-world implementation of the scoreboard would likely prefer to
   manage this data as sequence number ranges.  The algorithms presented
   here allow this, but require arbitrary sequence number ranges to be

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   marked as having been selectively acknowledged.

   Finally, note that the algorithm in this document assumes a
   sender that is not keeping track of segment boundaries after
   transmitting a segment.  It is possible that a sender that did
   keep this extra state may be able to use a more refined and
   precise algorithm than the one presented herein, however, we
   leave this as future work.

4   Processing and Acting Upon SACK Information

   For the purposes of the algorithm defined in this document the
   scoreboard SHOULD implement the following functions:

   Update ():

      Given the information provided in an ACK, each octet that is
      cumulatively ACKed or SACKed should be marked accordingly in the
      scoreboard data structure, and the total number of octets SACKed
      should be recorded.

      Note: SACK information is advisory and therefore SACKed data MUST
      NOT be removed from TCP's retransmission buffer until the data is
      cumulatively acknowledged [RFC2018].

   IsLost (SeqNum):

      This routine returns whether the given sequence number is
      considered to be lost.  The routine returns true when either
      DupThresh discontiguous SACKed sequences have arrived above
      'SeqNum' or more than (DupThresh - 1) * SMSS bytes with sequence
      numbers greater than 'SeqNum' have been SACKed.  Otherwise, the
      routine returns false.

   SetPipe ():

      This routine traverses the sequence space from HighACK to HighData
      and MUST set the "pipe" variable to an estimate of the number of
      octets that are currently in transit between the TCP sender and
      the TCP receiver.  After initializing pipe to zero the following
      steps are taken for each octet 'S1' in the sequence space between
      HighACK and HighData that has not been SACKed:

      (a) If IsLost (S1) returns false:

         Pipe is incremented by 1 octet.

         The effect of this condition is that pipe is incremented for
         packets that have not been SACKed and have not been determined
         to have been lost (i.e., those segments that are still assumed
         to be in the network).

      (b) If S1 <= HighRxt:

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         Pipe is incremented by 1 octet.

         The effect of this condition is that pipe is incremented for
         the retransmission of the octet.

      Note that octets retransmitted without being considered lost are
      counted twice by the above mechanism.

   NextSeg ():

      This routine uses the scoreboard data structure maintained by the
      Update() function to determine what to transmit based on the SACK
      information that has arrived from the data receiver (and hence
      been marked in the scoreboard).  NextSeg () MUST return the
      sequence number range of the next segment that is to be
      transmitted, per the following rules:

      (1) If there exists a smallest unSACKed sequence number 'S2' that
          meets the following three criteria for determining loss, the
          sequence range of one segment of up to SMSS octets starting
          with S2 MUST be returned.

          (1.a) S2 is greater than HighRxt.

          (1.b) S2 is less than the highest octet covered by any
                received SACK.

          (1.c) IsLost (S2) returns true.

      (2) If no sequence number 'S2' per rule (1) exists but there
          exists available unsent data and the receiver's advertised
          window allows, the sequence range of one segment of up to SMSS
          octets of previously unsent data starting with sequence number
          HighData+1 MUST be returned.

      (3) If the conditions for rules (1) and (2) fail, but there exists
          an unSACKed sequence number 'S3' that meets the criteria for
          detecting loss given in steps (1.a) and (1.b) above
          (specifically excluding step (1.c)) then one segment of up to
          SMSS octets starting with S3 MAY be returned.

          Note that rule (3) is a sort of retransmission "last resort".
          It allows for retransmission of sequence numbers even when the
          sender has less certainty a segment has been lost than as with
          rule (1).  Retransmitting segments via rule (3) will help
          sustain TCP's ACK clock and therefore can potentially help
          avoid retransmission timeouts.  However, in sending these
          segments the sender has two copies of the same data considered
          to be in the network (and also in the Pipe estimate).  When an
          ACK or SACK arrives covering this retransmitted segment, the
          sender cannot be sure exactly how much data left the network
          (one of the two transmissions of the packet or both
          transmissions of the packet).  Therefore the sender may
          underestimate Pipe by considering both segments to have left

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          the network when it is possible that only one of the two has.

          We believe that the triggering of rule (3) will be rare and
          that the implications are likely limited to corner cases
          relative to the entire recovery algorithm.  Therefore we leave
          the decision of whether or not to use rule (3) to

      (4) If the conditions for each of (1), (2), and (3) are not met,
          then NextSeg () MUST indicate failure, and no segment is

   Note: The SACK-based loss recovery algorithm outlined in this
   document requires more computational resources than previous TCP loss
   recovery strategies.  However, we believe the scoreboard data
   structure can be implemented in a reasonably efficient manner (both
   in terms of computation complexity and memory usage) in most TCP

5   Algorithm Details

   Upon the receipt of any ACK containing SACK information, the
   scoreboard MUST be updated via the Update () routine.

   If the incoming ACK is a cumulative acknowledgment, the TCP MUST
   reset DupAcks to zero.

   If the incoming ACK is a duplicate acknowledgment and the TCP is
   not currently in loss recovery, the TCP MUST increase DupAcks by
   one and take the following steps:

   (1) If DupAcks >= DupThresh, go to step (4).

   (2) If DupAcks < DupThresh but IsLost (SND.UNA) returns
       true---indicating at least three segments have arrived above
       the current cumulative acknowledgment point, which is taken
       to indicate loss---go to step (4).

   (3) The TCP MAY transmit previously unsent data segments as per
       Limited Transmit [RFC5681], except that the number of octets
       which may be sent is governed by Pipe and cwnd as follows:

       (3.1) Set HighRxt to SND.UNA.

       (3.2) Run SetPipe ().

       (3.3) If (cwnd - pipe) >= 1 SMSS, there exists previously
             unsent data, and the receiver's advertised window
             allows, transmit up to 1 SMSS of data starting with the
             octet HighData+1 and update HighData to reflect this
             transmission, then return to (3.2).

       (3.4) Terminate processing of this ACK.

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   (4) Invoke Fast Retransmit and enter loss recovery as follows:

       (4.1) RecoveryPoint = HighData

             When the TCP sender receives a cumulative ACK for this
             data octet the loss recovery phase is terminated.

       (4.2) ssthresh = cwnd = (FlightSize / 2)

             The congestion window (cwnd) and slow start threshold
             (ssthresh) are reduced to half of FlightSize per

       (4.3) Retransmit the first data segment presumed dropped -- the segment
             starting with sequence number HighACK + 1.  To prevent
             repeated retransmission of the same data, set HighRxt
             to the highest sequence number in the retransmitted

       (4.4) Run SetPipe ()

             Set a "pipe" variable to the number of outstanding
             octets currently "in the pipe"; this is the data which
             has been sent by the TCP sender but for which no
             cumulative or selective acknowledgment has been
             received and the data has not been determined to have
             been dropped in the network.  It is assumed that the
             data is still traversing the network path.

       (4.5) In order to take advantage of potential additional
             available cwnd, proceed to step (C) below.

   Once a TCP is in the loss recovery phase the following procedure MUST
   be used for each arriving ACK:

   (A) An incoming cumulative ACK for a sequence number greater than
       RecoveryPoint signals the end of loss recovery and the loss
       recovery phase MUST be terminated.  Any information contained in
       the scoreboard for sequence numbers greater than the new value of
       HighACK SHOULD NOT be cleared when leaving the loss recovery

   (B) Upon receipt of an ACK that does not cover RecoveryPoint the
       following actions MUST be taken:

       (B.1) Use Update () to record the new SACK information conveyed
       by the incoming ACK.

       (B.2) Use SetPipe () to re-calculate the number of octets still
       in the network.

   (C) If cwnd - pipe >= 1 SMSS the sender SHOULD transmit one or more
       segments as follows:

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       (C.1) The scoreboard MUST be queried via NextSeg () for the
       sequence number range of the next segment to transmit (if any),
       and the given segment sent.  If NextSeg () returns failure (no
       data to send) return without sending anything (i.e., terminate
       steps C.1 -- C.5).

       (C.2) If any of the data octets sent in (C.1) are below HighData,
       HighRxt MUST be set to the highest sequence number of the
       retransmitted segment.

       (C.3) If any of the data octets sent in (C.1) are above HighData,
       HighData must be updated to reflect the transmission of
       previously unsent data.

       (C.4) The estimate of the amount of data outstanding in the
       network must be updated by incrementing pipe by the number of
       octets transmitted in (C.1).

       (C.5) If cwnd - pipe >= 1 SMSS, return to (C.1)

5.1 Retransmission Timeouts

   In order to avoid memory deadlocks, the TCP receiver is allowed to
   discard data that has already been selectively acknowledged.  As a
   result, [RFC2018] suggests that a TCP sender SHOULD expunge the SACK
   information gathered from a receiver upon a retransmission timeout
   "since the timeout might indicate that the data receiver has
   reneged."  Additionally, a TCP sender MUST "ignore prior SACK
   information in determining which data to retransmit."  However, since
   the publication of [RFC2018] this has come to be viewed by some as
   too strong.  It has been suggested that, as long as robust tests for
   reneging are present, an implementation can retain and use SACK
   information across a timeout event [Errata1610].  While this document
   does not change the specification in [RFC2018], we note that
   implementers should consult any updates to [RFC2018] on this subject.
   Further, a SACK TCP sender SHOULD utilize all SACK information made
   available during the slow start phase of loss recovery following an

   If an RTO occurs during loss recovery as specified in this document,
   RecoveryPoint MUST be set to HighData.  Further, the new value of
   RecoveryPoint MUST be preserved and the loss recovery algorithm
   outlined in this document MUST be terminated.  In addition, a new
   recovery phase (as described in section 5) MUST NOT be initiated
   until HighACK is greater than or equal to the new value of

   As described in Sections 4 and 5, Update () SHOULD continue to be
   used appropriately upon receipt of ACKs.  This will allow the slow
   start recovery period to benefit from all available information
   provided by the receiver, despite the fact that SACK information was
   expunged due to the RTO.

   If there are segments missing from the receiver's buffer following

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   processing of the retransmitted segment, the corresponding ACK will
   contain SACK information.  In this case, a TCP sender SHOULD use this
   SACK information when determining what data should be sent in each
   segment of the slow start.  The exact algorithm for this selection is
   not specified in this document (specifically NextSeg () is
   inappropriate during slow start after an RTO).  A relatively
   straightforward approach to "filling in" the sequence space reported
   as missing should be a reasonable approach.

6   Managing the RTO Timer

   The standard TCP RTO estimator is defined in [RFC2988].  Due to the
   fact that the SACK algorithm in this document can have an impact on
   the behavior of the estimator, implementers may wish to consider how
   the timer is managed.  [RFC2988] calls for the RTO timer to be
   re-armed each time an ACK arrives that advances the cumulative ACK
   point.  Because the algorithm presented in this document can keep the
   ACK clock going through a fairly significant loss event,
   (comparatively longer than the algorithm described in [RFC5681]), on
   some networks the loss event could last longer than the RTO.  In this
   case the RTO timer would expire prematurely and a segment that need
   not be retransmitted would be resent.

   Therefore we give implementers the latitude to use the standard
   [RFC2988] style RTO management or, optionally, a more careful variant
   that re-arms the RTO timer on each retransmission that is sent during
   recovery MAY be used.  This provides a more conservative timer than
   specified in [RFC2988], and so may not always be an attractive
   alternative.  However, in some cases it may prevent needless
   retransmissions, go-back-N transmission and further reduction of the
   congestion window.

7   Research

   The algorithm specified in this document is analyzed in [FF96], which
   shows that the above algorithm is effective in reducing transfer time
   over standard TCP Reno [RFC5681] when multiple segments are dropped
   from a window of data (especially as the number of drops increases).
   [AHKO97] shows that the algorithm defined in this document can
   greatly improve throughput in connections traversing satellite

8   Security Considerations

   The algorithm presented in this paper shares security considerations
   with [RFC5681].  A key difference is that an algorithm based on SACKs
   is more robust against attackers forging duplicate ACKs to force the
   TCP sender to reduce cwnd.  With SACKs, TCP senders have an
   additional check on whether or not a particular ACK is legitimate.
   While not fool-proof, SACK does provide some amount of protection in
   this area.

   Similarly, [CPNI309] sketches a variant of a blind attack [RFC5961]
   whereby an attacker can spoof out-of-window data to a TCP endpoint,

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   causing it to respond to the legitimate peer with a duplicate
   cumulative ACK, per [RFC793].  Adding a SACK-based requirement to
   trigger loss recovery effectively mitigates this attack, as the
   duplicate ACKs caused by out-of-window segments will not contain SACK
   information indicating reception of previously un-SACKED in-window

9   Changes Relative to RFC 3517

   The state variable "DupAcks" has been added to the list of variables
   maintained by this algorithm, and its usage specified.

   The function IsLost () has been modified to require that more than
   (DupThresh - 1) * SMSS octets have been SACKed above a given sequence
   number as indication that it is lost, changed from at least
   (DupThresh * SMSS).  This retains the requirement that at least three
   segments following the sequence number in question have been SACKed,
   while improving detection in the event that the sender has
   outstanding segments which are smaller than SMSS.

   The definition of a "duplicate acknowledgment" has been modified to
   utilize the SACK information in detecting loss.  Duplicate cumulative
   acknowledgments can be caused by either loss or reordering in the
   network.  To disambiguate loss and reordering TCP's fast retransmit
   algorithm [RFC5681] waits until three duplicate ACKs arrive to
   trigger loss recovery.  This notion was then the basis for the
   algorithm specified in [RFC3517].  However, with SACK information
   there is no need to rely blindly on the cumulative acknowledgment
   field.  We can leverage the additional information present in the
   SACK blocks to understand that three segments have arrived at the
   receiver which lie above a gap in the sequence space, and can use
   that to trigger loss recovery.  This notion was used in [RFC3517]
   during loss recovery, and the change in this document is that the
   notion is also used to enter a loss recovery phase.


   The authors wish to thank Sally Floyd for encouraging [RFC3517]
   and commenting on early drafts.  The algorithm described in this
   document is loosely based on an algorithm outlined by Kevin Fall
   and Sally Floyd in [FF96], although the authors of this document
   assume responsibility for any mistakes in the above text.

   [RFC3517] was co-authored by Kevin Fall, who provided crucial input
   to that document and hence this follow-on work.

   Murali Bashyam, Ken Calvert, Tom Henderson, Reiner Ludwig,
   Jamshid Mahdavi, Matt Mathis, Shawn Ostermann, Vern Paxson and
   Venkat Venkatsubra provided valuable feedback on earlier versions
   of this document.

   We thank Matt Mathis and Jamshid Mahdavi for implementing the
   scoreboard in ns and hence guiding our thinking in keeping track
   of SACK state.

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   The first author would like to thank Ohio University and the Ohio
   University Internetworking Research Group for supporting the bulk of
   his work on this project.

Normative References

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
             Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
             3", BCP 9, RFC 2026, October 1996.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
             Control", RFC 5681, September 2009.

Informative References

   [AHKO97]  Mark Allman, Chris Hayes, Hans Kruse, Shawn Ostermann, "TCP
             Performance Over Satellite Links",  Proceedings of the Fifth
             International Conference on Telecommunications Systems,
             Nashville, TN, March, 1997.

   [All00]   Mark Allman, "A Web Server's View of the Transport Layer",
             ACM Computer Communication Review, 30(5), October 2000.

   [CPNI309] Fernando Gont, "Security Assessment of the Transmission
             Control Protocol (TCP)", CPNI Technical Note 3/2009,
             February 2009.

   [Errata1610] Matt Mathis, "RFC Errata Report 1610 for RFC 2018",
             Verified 2008-12-09.

   [FF96]    Kevin Fall and Sally Floyd, "Simulation-based Comparisons
             of Tahoe, Reno and SACK TCP", Computer Communication
             Review, July 1996.

   [Jac90]   Van Jacobson, "Modified TCP Congestion Avoidance Algorithm",
             Technical Report, LBL, April 1990.

   [PF01]    Jitendra Padhye, Sally Floyd "Identifying the TCP Behavior
             of Web Servers", ACM SIGCOMM, August 2001.

   [RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
             TCP's Fast Recovery Algorithm", RFC 2582, April 1999.

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   [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
             2914, September 2000.

   [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
             Timer", RFC 2988, November 2000.

   [RFC3042] Allman, M., Balakrishnan, H, and S. Floyd, "Enhancing TCP's
             Loss Recovery Using Limited Transmit", RFC 3042, January

   [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
             Conservative Selective Acknowledgment (SACK)-based Loss
             Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
             Robustness to Blind In-Window Attacks", RFC 5961, August

Authors' Addresses

   Ethan Blanton
   Purdue University Computer Sciences
   1398 Computer Science Building
   West Lafayette, IN  47907


   Mark Allman
   International Computer Science Institute
   1947 Center St.  Suite 600
   Berkeley, CA  94704

   Phone: 440-235-1792

   Lili Wang
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886


   Ilpo Jarvinen
   University of Helsinki
   P.O. Box 68


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   Markku Kojo
   University of Helsinki
   P.O. Box 68


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