Request for Comments: 3517
Category: Standards Track
University of Kentucky
A Conservative Selective Acknowledgment (SACK)-based
Loss Recovery Algorithm for TCP
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 (2003). All Rights Reserved.
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.
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 BCP 14, RFC 2119 [RFC2119].
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 reader is expected to be familiar with the definitions given in [RFC2581].
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 four variables that a TCP sender stores:
"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 point.
"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 [RFC2581]. The algorithm is often referred to as the "pipe algorithm".
For the purposes of this specification we define a "duplicate acknowledgment" as a segment that arrives with no data and an acknowledgment (ACK) number that is equal to the current value of HighACK, as described in [RFC2581].
We define a variable "DupThresh" that holds the number of duplicate acknowledgments required to trigger a retransmission. Per [RFC2581] this threshold is defined to be 3 duplicate acknowledgments. However, implementers should consult any updates to [RFC2581] 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 specification).
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 marked as having been selectively acknowledged.
4 Processing and Acting Upon SACK Information
For the purposes of the algorithm defined in this document the scoreboard SHOULD implement the following functions:
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].
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 (DupThresh * SMSS) bytes with sequence numbers greater than 'SeqNum' have been SACKed. Otherwise, the routine returns false.
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:
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.
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
(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 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 implementors.
(4) If the conditions for each of (1), (2), and (3) are not met,
then NextSeg () MUST indicate failure, and no segment is returned.
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 implementations.
5 Algorithm Details
Upon the receipt of any ACK containing SACK information, the scoreboard MUST be updated via the Update () routine.
Upon the receipt of the first (DupThresh - 1) duplicate ACKs, the scoreboard is to be updated as normal. Note: The first and second duplicate ACKs can also be used to trigger the transmission of previously unsent segments using the Limited Transmit algorithm [RFC3042].
When a TCP sender receives the duplicate ACK corresponding to DupThresh ACKs, the scoreboard MUST be updated with the new SACK information (via Update ()). If no previous loss event has occurred on the connection or the cumulative acknowledgment point is beyond the last value of RecoveryPoint, a loss recovery phase SHOULD be initiated, per the fast retransmit algorithm outlined in [RFC2581]. The following steps MUST be taken:
(1) RecoveryPoint = HighData
When the TCP sender receives a cumulative ACK for this data octet the loss recovery phase is terminated.
(2) ssthresh = cwnd = (FlightSize / 2)
The congestion window (cwnd) and slow start threshold (ssthresh) are reduced to half of FlightSize per [RFC2581].
(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 segment.
(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.
(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 phase.
(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:
(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, a SACK TCP sender SHOULD still use all SACK information made available during the slow start phase of loss recovery following an RTO.
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 RecoveryPoint.
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 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 [RFC2581]), 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.
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 [RFC2581] 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 channels.
8 Security Considerations
The algorithm presented in this paper shares security considerations with [RFC2581]. 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.
The authors wish to thank Sally Floyd for encouraging this document 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. 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.
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.
[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. [RFC2581] Allman, M., Paxson, V. and R. Stevens, "TCP Congestion Control", RFC 2581, April 1999.
[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. [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. [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 2001.
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Ethan Blanton Purdue University Computer Sciences 1398 Computer Science Building West Lafayette, IN 47907 EMail: firstname.lastname@example.org Mark Allman BBN Technologies/NASA Glenn Research Center Lewis Field 21000 Brookpark Rd. MS 54-5 Cleveland, OH 44135 Phone: 216-433-6586 Fax: 216-433-8705 EMail: email@example.com http://roland.grc.nasa.gov/~mallman
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