Internet Engineering Task Force (IETF)                      T. Henderson
Request for Comments: 6582                                        Boeing
Obsoletes: 3782                                                 S. Floyd
Category: Standards Track                                           ICSI
ISSN: 2070-1721                                                A. Gurtov
                                                      University of Oulu
                                                              Y. Nishida
                                                            WIDE Project
                                                              April 2012
Internet Engineering Task Force (IETF)                      T. Henderson
Request for Comments: 6582                                        Boeing
Obsoletes: 3782                                                 S. Floyd
Category: Standards Track                                           ICSI
ISSN: 2070-1721                                                A. Gurtov
                                                      University of Oulu
                                                              Y. Nishida
                                                            WIDE Project
                                                              April 2012

The NewReno Modification to TCP's Fast Recovery Algorithm




RFC 5681 documents the following four intertwined TCP congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. RFC 5681 explicitly allows certain modifications of these algorithms, including modifications that use the TCP Selective Acknowledgment (SACK) option (RFC 2883), and modifications that respond to "partial acknowledgments" (ACKs that cover new data, but not all the data outstanding when loss was detected) in the absence of SACK. This document describes a specific algorithm for responding to partial acknowledgments, referred to as "NewReno". This response to partial acknowledgments was first proposed by Janey Hoe. This document obsoletes RFC 3782.

RFC 5681记录了以下四种交织的TCP拥塞控制算法:慢启动、拥塞避免、快速重传和快速恢复。RFC 5681明确允许对这些算法进行某些修改,包括使用TCP选择性确认(SACK)选项(RFC 2883)的修改,以及在没有SACK的情况下响应“部分确认”(覆盖新数据的确认,但不是检测到丢失时所有未完成的数据)的修改。本文档描述了用于响应部分确认的特定算法,称为“NewReno”。这种对部分承认的回应最初是由Janey Hoe提出的。本文件淘汰了RFC 3782。

Status of This Memo


This is an Internet Standards Track document.


This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

本文件是互联网工程任务组(IETF)的产品。它代表了IETF社区的共识。它已经接受了公众审查,并已被互联网工程指导小组(IESG)批准出版。有关互联网标准的更多信息,请参见RFC 5741第2节。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

版权所有(c)2012 IETF信托基金和确定为文件作者的人员。版权所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(自本文件出版之日起生效。请仔细阅读这些文件,因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本,并提供简化BSD许可证中所述的无担保。

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.


1. Introduction
1. 介绍

For the typical implementation of the TCP fast recovery algorithm described in [RFC5681] (first implemented in the 1990 BSD Reno release, and referred to as the "Reno algorithm" in [FF96]), the TCP data sender only retransmits a packet after a retransmit timeout has occurred, or after three duplicate acknowledgments have arrived triggering the fast retransmit algorithm. A single retransmit timeout might result in the retransmission of several data packets, but each invocation of the fast retransmit algorithm in RFC 5681 leads to the retransmission of only a single data packet.

对于[RFC5681]中描述的TCP快速恢复算法的典型实现(首先在1990年BSD Reno版本中实现,在[FF96]中称为“Reno算法”),TCP数据发送方仅在发生重新传输超时后重新传输数据包,或者在三次重复确认到达后触发快速重传算法。单个重传超时可能导致多个数据包的重传,但RFC 5681中快速重传算法的每次调用都只导致单个数据包的重传。

Two problems arise with Reno TCP when multiple packet losses occur in a single window. First, Reno will often take a timeout, as has been documented in [Hoe95]. Second, even if a retransmission timeout is avoided, multiple fast retransmits and window reductions can occur, as documented in [F94]. When multiple packet losses occur, if the SACK option [RFC2883] is available, the TCP sender has the information to make intelligent decisions about which packets to retransmit and which packets not to retransmit during fast recovery.

当一个窗口中发生多个数据包丢失时,Reno TCP会出现两个问题。首先,正如[95]中所记录的那样,雷诺经常会超时。其次,即使避免了重传超时,也可能发生多次快速重传和窗口缩减,如[F94]中所述。当发生多个数据包丢失时,如果SACK选项[RFC2883]可用,则TCP发送方可以在快速恢复期间智能地决定哪些数据包要重新传输,哪些数据包不需要重新传输。

This document applies to TCP connections that are unable to use the TCP Selective Acknowledgment (SACK) option, either because the option is not locally supported or because the TCP peer did not indicate a willingness to use SACK.


In the absence of SACK, there is little information available to the TCP sender in making retransmission decisions during fast recovery. From the three duplicate acknowledgments, the sender infers a packet loss, and retransmits the indicated packet. After this, the data sender could receive additional duplicate acknowledgments, as the data receiver acknowledges additional data packets that were already in flight when the sender entered fast retransmit.


In the case of multiple packets dropped from a single window of data, the first new information available to the sender comes when the sender receives an acknowledgment for the retransmitted packet (that is, the packet retransmitted when fast retransmit was first entered). If there is a single packet drop and no reordering, then the acknowledgment for this packet will acknowledge all of the packets transmitted before fast retransmit was entered. However, if there are multiple packet drops, then the acknowledgment for the retransmitted packet will acknowledge some but not all of the packets transmitted before the fast retransmit. We call this acknowledgment a partial acknowledgment.


Along with several other suggestions, [Hoe95] suggested that during fast recovery the TCP data sender respond to a partial acknowledgment by inferring that the next in-sequence packet has been lost and retransmitting that packet. This document describes a modification to the fast recovery algorithm in RFC 5681 that incorporates a response to partial acknowledgments received during fast recovery. We call this modified fast recovery algorithm NewReno, because it is a slight but significant variation of the behavior that has been historically referred to as Reno. This document does not discuss the other suggestions in [Hoe95] and [Hoe96], such as a change to the ssthresh parameter during slow start, or the proposal to send a new packet for every two duplicate acknowledgments during fast recovery. The version of NewReno in this document also draws on other discussions of NewReno in the literature [LM97] [Hen98].

与其他一些建议一样,[Hoe95]建议在快速恢复期间,TCP数据发送方通过推断序列中的下一个数据包已丢失并重新传输该数据包来响应部分确认。本文档描述了对RFC 5681中快速恢复算法的修改,该算法包含对快速恢复期间接收到的部分确认的响应。我们称这种改进的快速恢复算法为NewReno,因为它是一种轻微但显著的行为变化,在历史上被称为Reno。本文件不讨论[Hoe95]和[Hoe96]中的其他建议,例如在慢速启动期间更改ssthresh参数,或建议在快速恢复期间每两次重复确认发送一个新数据包。本文件中的NewReno版本还借鉴了文献[LM97][Hen98]中对NewReno的其他讨论。

We do not claim that the NewReno version of fast recovery described here is an optimal modification of fast recovery for responding to partial acknowledgments, for TCP connections that are unable to use SACK. Based on our experiences with the NewReno modification in the network simulator known as ns-2 [NS] and with numerous implementations of NewReno, we believe that this modification improves the performance of the fast retransmit and fast recovery


algorithms in a wide variety of scenarios. Previous versions of this RFC [RFC2582] [RFC3782] provide simulation-based evidence of the possible performance gains.


2. Terminology and Definitions
2. 术语和定义

This document assumes that the reader is familiar with the terms SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and FLIGHT SIZE (FlightSize) defined in [RFC5681].


This document defines an additional sender-side state variable called "recover":


recover: When in fast recovery, this variable records the send sequence number that must be acknowledged before the fast recovery procedure is declared to be over.


3. The Fast Retransmit and Fast Recovery Algorithms in NewReno
3. NewReno中的快速重传和快速恢复算法
3.1. Protocol Overview
3.1. 协议概述

The basic idea of these extensions to the fast retransmit and fast recovery algorithms described in Section 3.2 of [RFC5681] is as follows. The TCP sender can infer, from the arrival of duplicate acknowledgments, whether multiple losses in the same window of data have most likely occurred, and avoid taking a retransmit timeout or making multiple congestion window reductions due to such an event.


The NewReno modification applies to the fast recovery procedure that begins when three duplicate ACKs are received and ends when either a retransmission timeout occurs or an ACK arrives that acknowledges all of the data up to and including the data that was outstanding when the fast recovery procedure began.


3.2. Specification
3.2. 规格

The procedures specified in Section 3.2 of [RFC5681] are followed, with the modifications listed below. Note that this specification avoids the use of the key words defined in RFC 2119 [RFC2119], since it mainly provides sender-side implementation guidance for performance improvement, and does not affect interoperability.

遵循[RFC5681]第3.2节规定的程序,并进行以下修改。请注意,本规范避免使用RFC 2119[RFC2119]中定义的关键字,因为它主要为性能改进提供发送方实现指南,并且不影响互操作性。

1) Initialization of TCP protocol control block: When the TCP protocol control block is initialized, recover is set to the initial send sequence number.

1) TCP协议控制块初始化:初始化TCP协议控制块时,recover设置为初始发送序列号。

2) Three duplicate ACKs: When the third duplicate ACK is received, the TCP sender first checks the value of recover to see if the Cumulative Acknowledgment field covers more than recover. If so, the value of recover is incremented to the value of the highest sequence number transmitted by the TCP so far. The TCP then enters fast retransmit (step 2 of Section 3.2 of [RFC5681]). If not, the TCP does not enter fast retransmit and does not reset ssthresh.

2) 三个重复确认:当收到第三个重复确认时,TCP发送方首先检查recover的值,以查看累积确认字段是否覆盖超过recover。如果是,recover的值将增加到TCP迄今为止传输的最高序列号的值。然后TCP进入快速重传(RFC5681第3.2节第2步)。否则,TCP不会进入快速重传,也不会重置ssthresh。

3) Response to newly acknowledged data: Step 6 of [RFC5681] specifies the response to the next ACK that acknowledges previously unacknowledged data. When an ACK arrives that acknowledges new data, this ACK could be the acknowledgment elicited by the initial retransmission from fast retransmit, or elicited by a later retransmission. There are two cases:

3) 对新确认数据的响应:[RFC5681]的步骤6指定对下一个确认先前未确认数据的应答。当确认新数据的ACK到达时,该ACK可以是由快速重传的初始重传引起的确认,或者由随后的重传引起的确认。有两种情况:

Full acknowledgments: If this ACK acknowledges all of the data up to and including recover, then the ACK acknowledges all the intermediate segments sent between the original transmission of the lost segment and the receipt of the third duplicate ACK. Set cwnd to either (1) min (ssthresh, max(FlightSize, SMSS) + SMSS) or (2) ssthresh, where ssthresh is the value set when fast retransmit was entered, and where FlightSize in (1) is the amount of data presently outstanding. This is termed "deflating" the window. If the second option is selected, the implementation is encouraged to take measures to avoid a possible burst of data, in case the amount of data outstanding in the network is much less than the new congestion window allows. A simple mechanism is to limit the number of data packets that can be sent in response to a single acknowledgment. Exit the fast recovery procedure.


Partial acknowledgments: If this ACK does *not* acknowledge all of the data up to and including recover, then this is a partial ACK. In this case, retransmit the first unacknowledged segment. Deflate the congestion window by the amount of new data acknowledged by the Cumulative Acknowledgment field. If the partial ACK acknowledges at least one SMSS of new data, then add back SMSS bytes to the congestion window. This artificially inflates the congestion window in order to reflect the additional segment that has left the network. Send a new segment if permitted by the new value of cwnd. This "partial window deflation" attempts to ensure that, when fast recovery eventually ends, approximately ssthresh amount of data will be outstanding in the network. Do not exit the fast recovery procedure (i.e., if any duplicate ACKs subsequently arrive, execute step 4 of Section 3.2 of [RFC5681]).


For the first partial ACK that arrives during fast recovery, also reset the retransmit timer. Timer management is discussed in more detail in Section 4.


4) Retransmit timeouts: After a retransmit timeout, record the highest sequence number transmitted in the variable recover, and exit the fast recovery procedure if applicable.

4) 重新传输超时:在重新传输超时后,记录变量recover中传输的最高序列号,如果适用,退出快速恢复过程。

Step 2 above specifies a check that the Cumulative Acknowledgment field covers more than recover. Because the acknowledgment field contains the sequence number that the sender next expects to receive, the acknowledgment "ack_number" covers more than recover when


      ack_number - 1 > recover;
      ack_number - 1 > recover;

i.e., at least one byte more of data is acknowledged beyond the highest byte that was outstanding when fast retransmit was last entered.

i、 例如,在上次输入快速重传时未完成的最高字节之外,至少多确认一个字节的数据。

Note that in step 3 above, the congestion window is deflated after a partial acknowledgment is received. The congestion window was likely to have been inflated considerably when the partial acknowledgment was received. In addition, depending on the original pattern of packet losses, the partial acknowledgment might acknowledge nearly a window of data. In this case, if the congestion window was not deflated, the data sender might be able to send nearly a window of data back-to-back.


This document does not specify the sender's response to duplicate ACKs when the fast retransmit/fast recovery algorithm is not invoked. This is addressed in other documents, such as those describing the Limited Transmit procedure [RFC3042]. This document also does not address issues of adjusting the duplicate acknowledgment threshold, but assumes the threshold specified in the IETF standards; the current standard is [RFC5681], which specifies a threshold of three duplicate acknowledgments.


As a final note, we would observe that in the absence of the SACK option, the data sender is working from limited information. When the issue of recovery from multiple dropped packets from a single window of data is of particular importance, the best alternative would be to use the SACK option.


4. Handling Duplicate Acknowledgments after a Timeout
4. 超时后处理重复确认

After each retransmit timeout, the highest sequence number transmitted so far is recorded in the variable recover. If, after a retransmit timeout, the TCP data sender retransmits three consecutive packets that have already been received by the data receiver, then the TCP data sender will receive three duplicate acknowledgments that do not cover more than recover. In this case, the duplicate acknowledgments are not an indication of a new instance of congestion. They are simply an indication that the sender has unnecessarily retransmitted at least three packets.


However, when a retransmitted packet is itself dropped, the sender can also receive three duplicate acknowledgments that do not cover more than recover. In this case, the sender would have been better off if it had initiated fast retransmit. For a TCP sender that implements the algorithm specified in Section 3.2 of this document, the sender does not infer a packet drop from duplicate acknowledgments in this scenario. As always, the retransmit timer is the backup mechanism for inferring packet loss in this case.


There are several heuristics, based on timestamps or on the amount of advancement of the Cumulative Acknowledgment field, that allow the sender to distinguish, in some cases, between three duplicate acknowledgments following a retransmitted packet that was dropped, and three duplicate acknowledgments from the unnecessary retransmission of three packets [Gur03] [GF04]. The TCP sender may use such a heuristic to decide to invoke a fast retransmit in some cases, even when the three duplicate acknowledgments do not cover more than recover.


For example, when three duplicate acknowledgments are caused by the unnecessary retransmission of three packets, this is likely to be accompanied by the Cumulative Acknowledgment field advancing by at least four segments. Similarly, a heuristic based on timestamps uses the fact that when there is a hole in the sequence space, the timestamp echoed in the duplicate acknowledgment is the timestamp of the most recent data packet that advanced the Cumulative Acknowledgment field [RFC1323]. If timestamps are used, and the sender stores the timestamp of the last acknowledged segment, then the timestamp echoed by duplicate acknowledgments can be used to distinguish between a retransmitted packet that was dropped and three duplicate acknowledgments from the unnecessary retransmission of three packets.


4.1. ACK Heuristic
4.1. ACK启发式

If the ACK-based heuristic is used, then following the advancement of the Cumulative Acknowledgment field, the sender stores the value of the previous cumulative acknowledgment as prev_highest_ack, and stores the latest cumulative ACK as highest_ack. In addition, the following check is performed if, in step 2 of Section 3.2, the Cumulative Acknowledgment field does not cover more than recover.


2*) If the Cumulative Acknowledgment field didn't cover more than recover, check to see if the congestion window is greater than SMSS bytes and the difference between highest_ack and prev_highest_ack is at most 4*SMSS bytes. If true, duplicate ACKs indicate a lost segment (enter fast retransmit). Otherwise, duplicate ACKs likely result from unnecessary retransmissions (do not enter fast retransmit).

2*)如果累计确认字段的覆盖范围不超过恢复范围,请检查拥塞窗口是否大于SMSS字节,以及最高确认和上一次最高确认之间的差异是否最多为4*SMSS字节。如果为true,则重复的ACK表示丢失的段(输入fast retransmit)。否则,不必要的重新传输可能会导致重复的ACK(不要进入快速重新传输)。

The congestion window check serves to protect against fast retransmit immediately after a retransmit timeout.


If several ACKs are lost, the sender can see a jump in the cumulative ACK of more than three segments, and the heuristic can fail. [RFC5681] recommends that a receiver should send duplicate ACKs for every out-of-order data packet, such as a data packet received during fast recovery. The ACK heuristic is more likely to fail if the receiver does not follow this advice, because then a smaller number of ACK losses are needed to produce a sufficient jump in the cumulative ACK.


4.2. Timestamp Heuristic
4.2. 时间戳启发式

If this heuristic is used, the sender stores the timestamp of the last acknowledged segment. In addition, the last sentence of step 2 in Section 3.2 of this document is replaced as follows:


2**) If the Cumulative Acknowledgment field didn't cover more than recover, check to see if the echoed timestamp in the last non-duplicate acknowledgment equals the stored timestamp. If true, duplicate ACKs indicate a lost segment (enter fast retransmit). Otherwise, duplicate ACKs likely result from unnecessary retransmissions (do not enter fast retransmit).

2**)如果累计确认字段的覆盖范围不超过恢复范围,请检查最后一次非重复确认中的回显时间戳是否等于存储的时间戳。如果为true,则重复的ACK表示丢失的段(输入fast retransmit)。否则,不必要的重新传输可能会导致重复的ACK(不要进入快速重新传输)。

The timestamp heuristic works correctly, both when the receiver echoes timestamps, as specified by [RFC1323], and by its revision attempts. However, if the receiver arbitrarily echoes timestamps, the heuristic can fail. The heuristic can also fail if a timeout was spurious and returning ACKs are not from retransmitted segments. This can be prevented by detection algorithms such as the Eifel detection algorithm [RFC3522].


5. Implementation Issues for the Data Receiver
5. 数据接收器的实现问题

[RFC5681] specifies that "Out-of-order data segments SHOULD be acknowledged immediately, in order to accelerate loss recovery". Neal Cardwell has noted that some data receivers do not send an immediate acknowledgment when they send a partial acknowledgment, but instead wait first for their delayed acknowledgment timer to expire [C98]. As [C98] notes, this severely limits the potential benefit of NewReno by delaying the receipt of the partial acknowledgment at the data sender. Echoing [RFC5681], our recommendation is that the data receiver send an immediate acknowledgment for an out-of-order segment, even when that out-of-order segment fills a hole in the buffer.

[RFC5681]规定“应立即确认无序数据段,以加速丢失恢复”。Neal Cardwell指出,一些数据接收器在发送部分确认时不会立即发送确认,而是先等待延迟确认计时器过期[C98]。正如[C98]所指出的,这严重限制了NewReno的潜在好处,因为它延迟了数据发送方收到部分确认。响应[RFC5681],我们的建议是,数据接收器立即发送对无序段的确认,即使该无序段填充了缓冲区中的一个孔。

6. Implementation Issues for the Data Sender
6. 数据发送方的实现问题

In Section 3.2, step 3 above, it is noted that implementations should take measures to avoid a possible burst of data when leaving fast recovery, in case the amount of new data that the sender is eligible to send due to the new value of the congestion window is large. This can arise during NewReno when ACKs are lost or treated as pure window updates, thereby causing the sender to underestimate the number of new segments that can be sent during the recovery procedure. Specifically, bursts can occur when the FlightSize is much less than the new congestion window when exiting from fast recovery. One simple mechanism to avoid a burst of data when leaving fast recovery


is to limit the number of data packets that can be sent in response to a single acknowledgment. (This is known as "maxburst_" in ns-2 [NS].) Other possible mechanisms for avoiding bursts include rate-based pacing, or setting the slow start threshold to the resultant congestion window and then resetting the congestion window to FlightSize. A recommendation on the general mechanism to avoid excessively bursty sending patterns is outside the scope of this document.


An implementation may want to use a separate flag to record whether or not it is presently in the fast recovery procedure. The use of the value of the duplicate acknowledgment counter for this purpose is not reliable, because it can be reset upon window updates and out-of-order acknowledgments.


When updating the Cumulative Acknowledgment field outside of fast recovery, the state variable recover may also need to be updated in order to continue to permit possible entry into fast recovery (Section 3.2, step 2). This issue arises when an update of the Cumulative Acknowledgment field results in a sequence wraparound that affects the ordering between the Cumulative Acknowledgment field and the state variable recover. Entry into fast recovery is only possible when the Cumulative Acknowledgment field covers more than the state variable recover.


It is important for the sender to respond correctly to duplicate ACKs received when the sender is no longer in fast recovery (e.g., because of a retransmit timeout). The Limited Transmit procedure [RFC3042] describes possible responses to the first and second duplicate acknowledgments. When three or more duplicate acknowledgments are received, the Cumulative Acknowledgment field doesn't cover more than recover, and a new fast recovery is not invoked, the sender should follow the guidance in Section 4. Otherwise, the sender could end up in a chain of spurious timeouts. We mention this only because several NewReno implementations had this bug, including the implementation in ns-2 [NS].


It has been observed that some TCP implementations enter a slow start or congestion avoidance window updating algorithm immediately after the cwnd is set by the equation found in Section 3.2, step 3, even without a new external event generating the cwnd change. Note that after cwnd is set based on the procedure for exiting fast recovery (Section 3.2, step 3), cwnd should not be updated until a further event occurs (e.g., arrival of an ack, or timeout) after this adjustment.


7. Security Considerations
7. 安全考虑

[RFC5681] discusses general security considerations concerning TCP congestion control. This document describes a specific algorithm that conforms with the congestion control requirements of [RFC5681], and so those considerations apply to this algorithm, too. There are no known additional security concerns for this specific algorithm.


8. Conclusions
8. 结论

This document specifies the NewReno fast retransmit and fast recovery algorithms for TCP. This NewReno modification to TCP can even be important for TCP implementations that support the SACK option, because the SACK option can only be used for TCP connections when both TCP end-nodes support the SACK option. NewReno performs better than Reno in a number of scenarios discussed in previous versions of this RFC ([RFC2582] [RFC3782]).


A number of options for the basic algorithms presented in Section 3 are also referenced in Appendix A of this document. These include the handling of the retransmission timer, the response to partial acknowledgments, and whether or not the sender must maintain a state variable called recover. Our belief is that the differences between these variants of NewReno are small compared to the differences between Reno and NewReno. That is, the important thing is to implement NewReno instead of Reno for a TCP connection without SACK; it is less important exactly which variant of NewReno is implemented.


9. Acknowledgments
9. 致谢

Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu, Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed feedback on the precursor RFCs 2582 and 3782. Jeffrey Hsu provided clarifications on the handling of the variable recover; these clarifications were applied to RFC 3782 via an erratum and are incorporated into the text of Section 6 of this document. Yoshifumi Nishida contributed a modification to the fast recovery algorithm to account for the case in which FlightSize is 0 when the TCP sender leaves fast recovery and the TCP receiver uses delayed acknowledgments. Alexander Zimmermann provided several suggestions to improve the clarity of the document.

感谢Anil Agarwal、Mark Allman、Armando Caro、Jeffrey Hsu、Vern Paxson、Kacheong Poon、Keyur Shah和Bernie Volz对前体RFC 2582和3782的详细反馈。Jeffrey Hsu澄清了变量恢复的处理;这些澄清通过勘误表适用于RFC 3782,并纳入本文件第6节的文本中。Yoshifumi Nishida对快速恢复算法进行了修改,以解释当TCP发送方离开快速恢复且TCP接收方使用延迟确认时FlightSize为0的情况。亚历山大·齐默尔曼(Alexander Zimmermann)提出了一些建议,以提高文件的清晰度。

10. References
10. 工具书类
10.1. Normative References
10.1. 规范性引用文件

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

[RFC2119]Bradner,S.,“RFC中用于表示需求水平的关键词”,BCP 14,RFC 2119,1997年3月。

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

[RFC5681]Allman,M.,Paxson,V.和E.Blanton,“TCP拥塞控制”,RFC 56812009年9月。

10.2. Informative References
10.2. 资料性引用

[C98] Cardwell, N., "delayed ACKs for retransmitted packets: ouch!". November 1998, Email to the tcpimpl mailing list, archived at <>.


[F94] Floyd, S., "TCP and Successive Fast Retransmits", Technical report, May 1995. <>.


[FF96] Fall, K. and S. Floyd, "Simulation-based Comparisons of Tahoe, Reno and SACK TCP", Computer Communication Review, July 1996. <>.


[GF04] Gurtov, A. and S. Floyd, "Resolving Acknowledgment Ambiguity in non-SACK TCP", NExt Generation Teletraffic and Wired/Wireless Advanced Networking (NEW2AN'04), February 2004. < papers/heuristics.html>.

[GF04]Gurtov,A.和S.Floyd,“解决非SACK TCP中的确认歧义”,下一代电信业务和有线/无线高级网络(NEW2AN'04),2004年2月< 论文/heuristics.html>。

[Gur03] Gurtov, A., "[Tsvwg] resolving the problem of unnecessary fast retransmits in go-back-N", email to the tsvwg mailing list, July 28, 2003. < web/tsvwg/current/msg04334.html>.

[Gur03]Gurtov,A.,“[Tsvwg]解决返回N中不必要的快速重传问题”,发送至Tsvwg邮件列表的电子邮件,2003年7月28日< web/tsvwg/current/msg04334.html>。

[Hen98] Henderson, T., "Re: NewReno and the 2001 Revision", September 1998. Email to the tcpimpl mailing list, archived at <>.


[Hoe95] Hoe, J., "Startup Dynamics of TCP's Congestion Control and Avoidance Schemes", Master's Thesis, MIT, June 1995.


[Hoe96] Hoe, J., "Improving the Start-up Behavior of a Congestion Control Scheme for TCP", ACM SIGCOMM, August 1996. <>.

[Hoe96]Hoe,J.“改进TCP拥塞控制方案的启动行为”,ACM SIGCOMM,1996年8月<>.

[LM97] Lin, D. and R. Morris, "Dynamics of Random Early Detection", SIGCOMM 97, October 1997.

[LM97]Lin,D.和R.Morris,“随机早期检测的动力学”,SIGCOMM 97,1997年10月。

[NS] "The Network Simulator version 2 (ns-2)", <>.


[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992.

[RFC1323]Jacobson,V.,Braden,R.,和D.Borman,“高性能TCP扩展”,RFC 1323,1992年5月。

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

[RFC2582]Floyd,S.和T.Henderson,“TCP快速恢复算法的NewReno修改”,RFC 25821999年4月。

[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000.

[RFC2883]Floyd,S.,Mahdavi,J.,Mathis,M.,和M.Podolsky,“TCP选择性确认(SACK)选项的扩展”,RFC 28832000年7月。

[RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, January 2001.

[RFC3042]Allman,M.,Balakrishnan,H.,和S.Floyd,“使用有限传输增强TCP的丢失恢复”,RFC 3042,2001年1月。

[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for TCP", RFC 3522, April 2003.

[RFC3522]Ludwig,R.和M.Meyer,“TCP的Eifel检测算法”,RFC 3522,2003年4月。

[RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 3782, April 2004.

[RFC3782]Floyd,S.,Henderson,T.,和A.Gurtov,“TCP快速恢复算法的NewReno修改”,RFC 3782,2004年4月。

Appendix A. Additional Information

Previous versions of this RFC ([RFC2582] [RFC3782]) contained additional informative material on the following subjects, and may be consulted by readers who may want more information about possible variants to the algorithms and who may want references to specific [NS] simulations that provide NewReno test cases.


Section 4 of [RFC3782] discusses some alternative behaviors for resetting the retransmit timer after a partial acknowledgment.


Section 5 of [RFC3782] discusses some alternative behaviors for performing retransmission after a partial acknowledgment.


Section 6 of [RFC3782] describes more information about the motivation for the sender's state variable recover.


Section 9 of [RFC3782] introduces some NS simulation test suites for NewReno. In addition, references to simulation results can be found throughout [RFC3782].


Section 10 of [RFC3782] provides a comparison of Reno and NewReno TCP.

[RFC3782]的第10节提供了Reno和NewReno TCP的比较。

Section 11 of [RFC3782] lists changes relative to [RFC2582].


Appendix B. Changes Relative to RFC 3782
附录B.与RFC 3782相关的变更

In [RFC3782], the cwnd after Full ACK reception will be set to (1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh. However, the first option carries a risk of performance degradation: With the first option, if FlightSize is zero, the result will be 1 SMSS. This means TCP can transmit only 1 segment at that moment, which can cause a delay in ACK transmission at the receiver due to a delayed ACK algorithm.


The FlightSize on Full ACK reception can be zero in some situations. A typical example is where the sending window size during fast recovery is small. In this case, the retransmitted packet and new data packets can be transmitted within a short interval. If all these packets successfully arrive, the receiver may generate a Full ACK that acknowledges all outstanding data. Even if the window size is not small, loss of ACK packets or a receive buffer shortage during fast recovery can also increase the possibility of falling into this situation.


The proposed fix in this document, which sets cwnd to at least 2*SMSS if the implementation uses option 1 in the Full ACK case (Section 3.2, step 3, option 1), ensures that the sender TCP transmits at least two segments on Full ACK reception.


In addition, an erratum was reported for RFC 3782 (an editorial clarification to Section 8); this erratum has been addressed in Section 6 of this document.

此外,报告了RFC 3782的勘误表(第8节的编辑澄清);本勘误表已在本文件第6节中说明。

The specification text (Section 3.2 herein) was rewritten to more closely track Section 3.2 of [RFC5681].


Sections 4, 5, and 9-11 of [RFC3782] were removed, and instead Appendix A of this document was added to back-reference this informative material. A few references that have no citation in the main body of the document have been removed.


Authors' Addresses


Tom Henderson The Boeing Company



Sally Floyd International Computer Science Institute


   Phone: +1 (510) 666-2989
   Phone: +1 (510) 666-2989

Andrei Gurtov University of Oulu Centre for Wireless Communications CWC P.O. Box 4500 FI-90014 University of Oulu Finland



Yoshifumi Nishida WIDE Project Endo 5322 Fujisawa, Kanagawa 252-8520 Japan