TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms
RFC 2001
Document | Type |
RFC - Proposed Standard
(January 1997; No errata)
Obsoleted by RFC 2581
Was draft-stevens-tcpca-spec (individual)
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Author | W. Stevens | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 2001 (Proposed Standard) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group W. Stevens Request for Comments: 2001 NOAO Category: Standards Track January 1997 TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms 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. Abstract Modern implementations of TCP contain four intertwined algorithms that have never been fully documented as Internet standards: slow start, congestion avoidance, fast retransmit, and fast recovery. [2] and [3] provide some details on these algorithms, [4] provides examples of the algorithms in action, and [5] provides the source code for the 4.4BSD implementation. RFC 1122 requires that a TCP must implement slow start and congestion avoidance (Section 4.2.2.15 of [1]), citing [2] as the reference, but fast retransmit and fast recovery were implemented after RFC 1122. The purpose of this document is to document these four algorithms for the Internet. Acknowledgments Much of this memo is taken from "TCP/IP Illustrated, Volume 1: The Protocols" by W. Richard Stevens (Addison-Wesley, 1994) and "TCP/IP Illustrated, Volume 2: The Implementation" by Gary R. Wright and W. Richard Stevens (Addison-Wesley, 1995). This material is used with the permission of Addison-Wesley. The four algorithms that are described were developed by Van Jacobson. 1. Slow Start Old TCPs would start a connection with the sender injecting multiple segments into the network, up to the window size advertised by the receiver. While this is OK when the two hosts are on the same LAN, if there are routers and slower links between the sender and the receiver, problems can arise. Some intermediate router must queue the packets, and it's possible for that router to run out of space. [2] shows how this naive approach can reduce the throughput of a TCP connection drastically. Stevens Standards Track [Page 1] RFC 2001 TCP January 1997 The algorithm to avoid this is called slow start. It operates by observing that the rate at which new packets should be injected into the network is the rate at which the acknowledgments are returned by the other end. Slow start adds another window to the sender's TCP: the congestion window, called "cwnd". When a new connection is established with a host on another network, the congestion window is initialized to one segment (i.e., the segment size announced by the other end, or the default, typically 536 or 512). Each time an ACK is received, the congestion window is increased by one segment. The sender can transmit up to the minimum of the congestion window and the advertised window. The congestion window is flow control imposed by the sender, while the advertised window is flow control imposed by the receiver. The former is based on the sender's assessment of perceived network congestion; the latter is related to the amount of available buffer space at the receiver for this connection. The sender starts by transmitting one segment and waiting for its ACK. When that ACK is received, the congestion window is incremented from one to two, and two segments can be sent. When each of those two segments is acknowledged, the congestion window is increased to four. This provides an exponential growth, although it is not exactly exponential because the receiver may delay its ACKs, typically sending one ACK for every two segments that it receives. At some point the capacity of the internet can be reached, and an intermediate router will start discarding packets. This tells the sender that its congestion window has gotten too large. Early implementations performed slow start only if the other end was on a different network. Current implementations always perform slow start. 2. Congestion Avoidance Congestion can occur when data arrives on a big pipe (a fast LAN) and gets sent out a smaller pipe (a slower WAN). Congestion can also occur when multiple input streams arrive at a router whose output capacity is less than the sum of the inputs. Congestion avoidance is a way to deal with lost packets. It is described in [2]. The assumption of the algorithm is that packet loss caused by damage is very small (much less than 1%), therefore the loss of a packet signals congestion somewhere in the network between the source andShow full document text