Network Working Group Reiner Ludwig INTERNET-DRAFT Michael Meyer Expires: April 2003 Ericsson Research October, 2002 The Eifel Detection Algorithm for TCP 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract The Eifel detection algorithm allows the TCP sender to detect a posteriori whether it has entered loss recovery unnecessarily. It already determines this in the beginning of loss recovery when the first acceptable ACK arrives after the timeout-based retransmit or the fast retransmit has been sent. The algorithm requires that the TCP Timestamps option defined in RFC1323 is enabled for a connection. The idea is to use the timestamps echoed in returning ACKs to eliminate the retransmission ambiguity in TCP. The Eifel detection algorithm provides a basis for future TCP enhancements. This includes response algorithms to back out of loss recovery by restoring a TCP sender's congestion control state. Ludwig & Meyer [Page 1] INTERNET-DRAFT TCP - Eifel Detection October, 2002 Terminology The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119]. We refer to the first-time transmission of an octet as the 'original transmit'. A subsequent transmission of the same octet is referred to as a 'retransmit'. In most cases this terminology can likewise be applied to data segments as opposed to octets. However, with repacketization a segment can contain both first-time transmissions and retransmissions of octets. In that case, this terminology is only consistent when applied to "octets". For the Eifel detection algorithm this makes no difference as it also operates correctly when repacketization occurs. We use the term 'acceptable ACK' as defined in [RFC793]. That is an ACK that acknowledges previously unacknowledged data. We use the term 'duplicate ACK', and the variable 'dupacks' as defined in [WS95]. The variable 'dupacks' is a counter of duplicate ACKs that have already been received by the TCP sender before the fast retransmit is sent. We use the variable 'DupThresh' to refer to the so-called duplicate acknowledgement threshold, i.e., the number of duplicate ACKs that need to arrive at the TCP sender to trigger a fast retransmit. Currently, DupThresh is specified as a fixed value of three [RFC2581]. Future TCPs might implement an adaptive DupThresh. 1. Introduction The retransmission ambiguity problem [Zh86][KP87] is the TCP sender's inability to distinguish whether the first acceptable ACK that arrives after a retransmit, was sent in response to the original transmit or the retransmit. This problem occurs after a timeout-based retransmit and after a fast retransmit. The Eifel detection algorithm uses the TCP Timestamps option defined in [RFC1323] to eliminate the retransmission ambiguity. It thereby allows the TCP sender to detect a posteriori whether it has entered loss recovery unnecessarily. This added capability of the TCP sender is useful in environments where TCP's loss recovery and congestion control algorithms may often get falsely triggered. This can be caused by packet reordering, packet duplication, or a sudden delay increase in the data or the ACK path that results in a spurious timeout. For example, such sudden delay increases can often occur in wide-area wireless access networks due to handovers, resource preemption due to higher priority traffic (e.g., voice), or because the mobile transmitter traverses through a radio coverage hole (e.g., see [Gu01]). In such wireless networks, the often unnecessary go-back-N retransmits that typically occur after a spurious timeout create a serious problem. They decrease end- to-end throughput, are useless load upon a potentially congested Ludwig & Meyer [Page 2] INTERNET-DRAFT TCP - Eifel Detection October, 2002 network, and waste transmission (battery) power. Note, that across such networks the use of timestamps is recommended anyway [IMLGK02]. Based on the Eifel detection algorithm, the TCP sender may then choose to implement dedicated response algorithms. One goal of such a response algorithm would be to alleviate the consequences of a falsely triggered loss recovery. This may include restoring the TCP sender's congestion control state, and avoiding the mentioned unnecessary go-back-N retransmits. Another goal would be to adapt protocol parameters such as the duplicate acknowledgement threshold [RFC2581], and the RTT estimators [RFC2988]. This is to reduce the risk of falsely triggering TCP's loss recovery again as the connection progresses. However, such response algorithms are outside the scope of this document. Note: The original proposal, the "Eifel algorithm" [LK00], comprises both a detection and a response algorithm. This document only defines the detection part. The response part is defined in [LG02]. A key feature of the Eifel detection algorithm is that it already detects upon the first acceptable ACK that arrives during loss recovery whether a fast retransmit or a timeout was spurious. This is crucial to be able to avoid the mentioned go-back-N retransmits. Another feature is that the Eifel detection algorithm is fairly robust against the loss of ACKs. 2. Events that Falsely Trigger TCP Loss Recovery The following events falsely trigger a TCP sender's loss recovery and congestion control algorithms. This causes a so-called spurious retransmit, and an unnecessary reduction of the TCP sender's congestion window and slow start threshold [RFC2581]. - Spurious timeout - Packet reordering - Packet duplication A spurious timeout is a timeout that would not have occurred had the sender "waited longer". This may be caused by increased delay that suddenly occurs in the data and/or the ACK path. That in turn might cause an acceptable ACK to arrive too late, i.e., only after the TCP sender's retransmission timer has expired. For the purpose of specifying the algorithm in Section 3, we define this case as SPUR_TO (equal 1). Note: There is another case where a timeout would not have occurred had the sender "waited longer": the retransmission timer expires, and afterwards the TCP sender receives the duplicate ACK that would have triggered a fast retransmit of the oldest outstanding segment. We call this a "fast timeout" since Ludwig & Meyer [Page 3] INTERNET-DRAFT TCP - Eifel Detection October, 2002 in competition with the fast retransmit algorithm the timeout was faster. However, a fast timeout is not spurious since apparently a segment was in fact lost, i.e., loss recovery was initiated rightfully. In this document, we do not consider fast timeouts. This case is addressed in an independent document [Lu02]. Packet reordering in the network may occur because IP [RFC791] does not guarantee in-order delivery of packets. Additionally, a TCP receiver generates a duplicate ACK for each segment that arrives out- of-order. This results in a spurious fast retransmit if three or more data segments arrive out-of-order at the TCP receiver, and at least three of the resulting duplicate ACKs arrive at the TCP sender. This assumes that the duplicate acknowledgement threshold is set to three as defined in [RFC2581]. Packet duplication may occur because a receiving IP does not (cannot) remove packets that have been duplicated in the network. A TCP receiver in turn also generates a duplicate ACK for each duplicate segment. As with packet reordering, this results in a spurious fast retransmit if duplication of data segments or ACKs results in three or more duplicate ACKs to arrive at the TCP sender. Again, this assumes that the duplicate acknowledgement threshold is set to three. The negative impact on TCP performance caused by packet reordering and packet duplication is commonly the same: a single spurious retransmit (the fast retransmit), and the unnecessary halving of the TCP sender's congestion window as a result of the subsequent fast recovery phase [RFC2581]. The negative impact on TCP performance caused by a spurious timeout is more severe. First, the timeout event itself causes a single spurious retransmit, and unnecessarily forces the TCP sender into slow start [RFC2581]. Then, as the connection progresses, a chain reaction gets triggered that further decreases TCP's performance. Since the timeout was spurious, at least some ACKs for original transmits will typically arrive at the TCP sender before the ACK for the retransmit arrives. (This is unless severe packet reordering coincided with the spurious timeout in such a way that the ACK for the retransmit is the first acceptable ACK to arrive at the TCP sender.) Those ACKs for original transmits will then trigger an implicit go-back-N loss recovery at the TCP sender. Assuming that none of the outstanding segments and none of the corresponding ACKs were lost, all outstanding segments will get retransmitted unnecessarily. In fact, during this phase the TCP sender breaks 'packet conservation' [Jac88]. This is because the unnecessary go-back-N retransmits are sent during slow start. Thus, for each packet that leaves the network and that belongs to the first half of the original flight, two useless retransmits are sent into the network. Moreover, some TCPs will in addition suffer from a spurious fast retransmit. This is because the unnecessary go-back-N Ludwig & Meyer [Page 4] INTERNET-DRAFT TCP - Eifel Detection October, 2002 retransmits will arrive as duplicates at the TCP receiver, which in turn triggers a series of duplicate ACKs. Note that this last spurious fast retransmit could be avoided with the careful variant of 'bug fix' [RFC2582]. More detailed explanations including TCP trace plots that visualize the effects of spurious timeouts and packet reordering can be found in the original proposal [LK00]. 3. The Eifel Detection Algorithm 3.1 The Idea The goal of the Eifel detection algorithm is to allow the TCP sender to detect a posteriori whether it has entered loss recovery unnecessarily. Furthermore, the TCP sender should be able to make this decision upon the first acceptable ACK that arrives after the timeout-based retransmit or the fast retransmit has been sent. This in turn requires extra information in ACKs by which the TCP sender can unambiguously distinguish whether that first acceptable ACK was sent in response to the original transmit or the retransmit. Such extra information is provided by the TCP Timestamps option [RFC1323]. Generally speaking, timestamps are monotonously increasing "serial numbers" added into every segment that are then echoed within the corresponding ACKs. This is exploited by the Eifel detection algorithm in the following way. Given that timestamps are enabled for a connection, the TCP sender always stores the timestamp of the retransmit sent in the beginning of loss recovery, i.e., the timestamp of the timeout-based retransmit or the fast retransmit. If the timestamp of the first acceptable ACK, arriving after the retransmit was sent, is smaller then the stored timestamp of that retransmit, then that ACK must have been sent in response to an original transmit. Hence, the TCP sender must have entered loss recovery unnecessarily. The fact that the Eifel detection algorithm decides upon the first acceptable ACK is crucial to allow future response algorithms to avoid the unnecessary go-back-N retransmits that typically occur after a spurious timeout. Also, if loss recovery was entered unnecessarily, a window worth of ACKs are outstanding that all carry a timestamp that is smaller than the stored timestamp of the retransmit. The arrival of any one of those ACKs suffices the Eifel detection algorithm to work. Hence, the solution is fairly robust against ACK losses. Even the ACK sent in response to the retransmit, i.e., the one that carries the stored timestamp, may get lost. Note: Also the DSACK option [RFC2883] can be used to detect a posteriori whether the TCP sender has entered loss recovery unnecessarily. However, the first ACK carrying a DSACK option usually arrives at the TCP sender only after loss recovery has Ludwig & Meyer [Page 5] INTERNET-DRAFT TCP - Eifel Detection October, 2002 already terminated. Thus, the DSACK option cannot be used to eliminate the retransmission ambiguity. Consequently, it cannot be used to avoid the mentioned unnecessary go-back-N retransmits. Moreover, a DSACK-based detection algorithm is less robust against ACK losses. 3.2 The Algorithm Given that the TCP Timestamps option [RFC1323] is enabled for a connection, a TCP sender MAY use the Eifel detection algorithm as defined in this subsection. If the Eifel detection algorithm is used, the following steps MUST be taken by the TCP sender, but only upon initiation of loss recovery, i.e., when either the timeout-based retransmit or the fast retransmit is sent. The Eifel detection algorithm MUST NOT be reinitiated after loss recovery has already started. In particular, it may not be reinitiated upon subsequent timeouts for the same segment, and not upon retransmitting segments other than the oldest outstanding segment, e.g., during selective loss recovery. (1) Set a "RetransmitTS" variable to the value of the Timestamp Value field of the Timestamps option included in the retransmit sent when loss recovery is initiated. A TCP sender must ensure that RetransmitTS does not get overwritten as loss recovery progresses, e.g., in case of a second timeout and subsequent second retransmit of the same octet. (2) Set a "SpuriousRecovery" variable to FALSE (equal 0). (3) Wait for the arrival of an acceptable ACK. When an acceptable ACK has arrived proceed to step (4). (4) If the acceptable ACK neither carries a SACK option [RFC2018] nor a DSACK option [RFC2883], then proceed to step (5), else proceed to step (DONE). (5) If the value of the Timestamp Echo Reply field of the acceptable ACK's Timestamps option is smaller than the value of the variable RetransmitTS, then proceed to step (6), else proceed to step (DONE). Ludwig & Meyer [Page 6] INTERNET-DRAFT TCP - Eifel Detection October, 2002 (6) If the loss recovery has been initiated with a timeout- based retransmit, then set SpuriousRecovery <- SPUR_TO (equal 1), else set SpuriousRecovery <- dupacks+1 (RESP) Do nothing (Placeholder for a response algorithm). (DONE) No further processing. The comparison "smaller than" in step (5) is conservative. In theory, if the "timestamp clock" is slow or the network is fast, RetransmitTS could at most be equal to the timestamp echoed by an ACK sent in response to an original transmit. In that case, it is assumed that the loss recovery was not falsely triggered. 3.3 A Corner Case: "Timeout due to loss of all ACKs" The TCP sender is forced into a timeout even though the oldest outstanding segment arrived at the TCP receiver when all ACKs are lost. Although, the resulting retransmit is unnecessary, such a timeout is unavoidable. It should therefore not be considered to be spurious. The retransmit will arrive as a duplicate at the TCP receiver. In response to duplicates, RFC1323 mandates that the timestamp of the last segment that arrived in-sequence should be echoed. That timestamp will commonly be smaller than the timestamp carried by the retransmit. Consequently, the Eifel detection algorithm will mistake such a timeout as spurious, unless the SACK and DSACK option are enabled for that TCP connection. Note: Not all TCP implementations strictly follow RFC1323. In response to a duplicate data segment, some TCP receivers will echo the timestamp of the duplicate. With such TCP receivers, the corner case discussed in this section does not apply. That is, even though all ACKs were lost, and independent of whether the SACK and DSACK option was enabled for a connection, the Eifel detection algorithm would have no effect. With the SACK and DSACK option the Eifel detection algorithm will detect this corner case. This motivates step (4) of the algorithm. If data segments beyond the oldest outstanding one are also lost, then the first acceptable ACK that arrives during loss recovery will carry a SACK or a DSACK option. That ACK must have been sent in response to a retransmit of the oldest outstanding segment that arrived as a duplicate at the TCP receiver. It carries a SACK option if there are holes in the TCP receiver's sequence space. Otherwise, it carries a DSACK option. Ludwig & Meyer [Page 7] INTERNET-DRAFT TCP - Eifel Detection October, 2002 A concern with this corner case arises if the Eifel detection algorithm is combined with a response algorithm like the Eifel response algorithm [LG02]. That algorithm will back out of loss recovery by reversing congestion control state after a spurious timeout has been detected. I.e., this would also happen in cases when all ACKs are lost, and the SACK and/or the DSACK option are not used for that connection. The argument against that, is that the TCP sender should keep its congestion window halved in case all ACKs are lost since that is a sign of severe congestion on the ACK path. This is not really a problem as long as data segments were lost in addition to the entire flight of ACKs. The Eifel detection algorithm would misinterpret the timeout as spurious, and the Eifel response algorithm would reverse congestion control state. Still, the TCP sender would respond to congestion (in the data path) as soon as it finds out about the first loss in the outstanding flight. I.e., the TCP sender would still half its congestion window for that flight of packets. The concern remains, though, in case the DSACK option is not enabled, and an entire flight of ACKs is lost while all of the data segments arrive at the TCP receiver. Without the Eifel detection and response algorithm a TCP sender would go into slow start. With those algorithms it would not respond to the congestion in the ACK path. We do not believe that this is a serious concern. First, we assume that DSACK will get increasingly deployed. And even if this was not the case, this special corner case must occur sufficiently often to become a problem for the progress of a TCP connection. This seems rather pathological, since this suggests that the ACK path is pretty much broken. It is unlikely that any TCP could manage to grow its congestion much beyond one maximum segment size with such an ACK path. And in that case, the reversing of congestion control state becomes meaningless. 3.4 Protecting Against Misbehaving TCP Receivers (the Safe Variant) A TCP receiver can easily make a genuine retransmit appear to the TCP sender as a spurious retransmit by forging echoed timestamps. This may pose a security concern. Fortunately, there is a way to modify the Eifel detection algorithm in a way that makes it robust against lying TCP receivers. The idea is to use timestamps as a "segment's secret" that the TCP receiver only gets to know if it receives the segment. Conversely, a TCP receiver will not know the timestamp of a segment that was lost. Hence, to "prove" that it received the original transmit of a segment that the TCP sender retransmitted, the TCP receiver would need to return the timestamp of that original transmit. The Eifel detection algorithm could then be modified to only decide that loss recovery Ludwig & Meyer [Page 8] INTERNET-DRAFT TCP - Eifel Detection October, 2002 has been unnecessarily entered if the first acceptable ACK echoes the timestamp of the original transmit. Hence, implementers may choose to implement the algorithm with the following modifications. Step (1) is replaced with step (1'): (1') Set a "RetransmitTS" variable to the value of the Timestamp Value field of the Timestamps option that was included in the original transmit corresponding to the retransmit. Note: This step requires that the TCP sender stores the timestamps of all outstanding original transmits. Step (5) is replaced with step (5'): (5') If the value of the Timestamp Echo Reply field of the acceptable ACK's Timestamps option is equal to the value of the variable RetransmitTS, then proceed to step (6), else proceed to step (DONE). These modifications come at a cost: the modified algorithm is fairly sensitive against ACK losses since it relies on the arrival of the acceptable ACK that corresponds to the original transmit. Note: The first acceptable ACK that arrives after loss recovery has been unnecessarily entered, should echo the timestamp of the original transmit. This assumes that the ACK corresponding to the original transmit was not lost, that that ACK was not reordered in the network, and that the TCP receiver does not forge timestamps but complies with RFC1323. In case of a spurious fast retransmit, this is implied by the rules for generating ACKs for data segments that fill in all or part of a gap in the sequence space (see section 4.2 of [RFC2581]) and by the rules for echoing timestamps in that case (see rule (C) in section 3.4 of [RFC1323]). In case of a spurious timeout, it is likely that the delay (in the data path) that has caused the spurious timeout has also caused the TCP receiver's delayed ACK timer [RFC1122] to expire before the original transmit arrives. Also, in this case the rules for generating ACKs and the rules for echoing timestamps (see rule (A) in section 3.4 of [RFC1323]) ensure that the original transmit's timestamp is echoed. A remaining problem is that a TCP receiver might guess a lost segment's timestamp from observing the timestamps of segments that arrived earlier. This could be avoided by having the TCP sender add a "random counter" to the timestamp of every segment it sends, and then increment that counter by a random value, e.g., between 1 and 100. Ludwig & Meyer [Page 9] INTERNET-DRAFT TCP - Eifel Detection October, 2002 This ensures that timestamps remain monotonously increasing while making it difficult for a TCP receiver to guess the timestamp of a lost segment. 4. Security Considerations There do not seem to be any security considerations associated with the Eifel detection algorithm. This is because the Eifel detection algorithm does not alter existing protocol state at the TCP sender nor at the receiver. That is, no value of a TCP sender's state variable is changed. Moreover, a variant of the Eifel detection algorithm has been proposed in Section 3.4 that makes it robust against lying TCP receivers. Acknowledgments Many thanks to Keith Sklower, Randy Katz, Stephan Baucke, Sally Floyd, Vern Paxson, Mark Allman, Ethan Blanton, Andrei Gurtov, Pasi Sarolahti, and Alexey Kuznetsov for useful discussions that contributed to this work. Normative References [RFC2581] M. Allman, V. Paxson, W. Stevens, TCP Congestion Control, RFC 2581, April 1999. [RFC2119] S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, RFC 2119, March 1997. [RFC2883] S. Floyd, J. Mahdavi, M. Mathis, M. Podolsky, A. Romanow, An Extension to the Selective Acknowledgement (SACK) Option for TCP, RFC 2883, July 2000. [RFC1323] V. Jacobson, R. Braden, D. Borman, TCP Extensions for High Performance, RFC 1323, May 1992. [RFC2018] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, TCP Selective Acknowledgement Options, RFC 2018, October 1996. [RFC793] J. Postel, Transmission Control Protocol, RFC793, September 1981. Informative References [RFC1122] R. Braden, Requirements for Internet Hosts - Communication Layers, RFC 1122, October 1989. [RFC2582] S. Floyd, T. Henderson, The NewReno Modification to TCP's Fast Recovery Algorithm, RFC 2582, April 1999. Ludwig & Meyer [Page 10] INTERNET-DRAFT TCP - Eifel Detection October, 2002 [Gu01] A. Gurtov, Effect of Delays on TCP Performance, In Proceedings of IFIP Personal Wireless Communications, August 2001. [IMLGK02] H. Inamura et. al., TCP over Second (2.5G) and Third (3G) Generation Wireless Networks, work in progress, July 2002. [Jac88] V. Jacobson, Congestion Avoidance and Control, In Proceedings of ACM SIGCOMM 88. [KP87] P. Karn, C. Partridge, Improving Round-Trip Time Estimates in Reliable Transport Protocols, In Proceedings of ACM SIGCOMM 87. [LK00] R. Ludwig, R. H. Katz, The Eifel Algorithm: Making TCP Robust Against Spurious Retransmissions, ACM Computer Communication Review, Vol. 30, No. 1, January 2000. [LG02] R. Ludwig, A. Gurtov, The Eifel Response Algorithm for TCP, work in progress, October 2002. [Lu02] R. Ludwig, Responding to Fast Timeouts in TCP, work in progress, July 2002. [RFC2988] V. Paxson, M. Allman, Computing TCP's Retransmission Timer, RFC 2988, November 2000. [RFC791] J. Postel, Internet Protocol, RFC 791, September 1981. [WS95] G. R. Wright, W. R. Stevens, TCP/IP Illustrated, Volume 2 (The Implementation), Addison Wesley, January 1995. [Zh86] L. Zhang, Why TCP Timers Don't Work Well, In Proceedings of ACM SIGCOMM 88. Author's Address Reiner Ludwig Ericsson Research Ericsson Allee 1 52134 Herzogenrath, Germany Email: Reiner.Ludwig@eed.ericsson.se Michael Meyer Ericsson Research Ericsson Allee 1 52134 Herzogenrath, Germany Email: Michael.Meyer@eed.ericsson.se This Internet-Draft expires in April 2003. Ludwig & Meyer [Page 11]