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Enhancing TCP's Loss Recovery Using Limited Transmit (RFC3042)

IP.com Disclosure Number: IPCOM000005236D
Original Publication Date: 2001-Jan-01
Included in the Prior Art Database: 2001-Aug-20
Document File: 10 page(s) / 20K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

M. Allman: AUTHOR [+3]

Abstract

This document proposes a new Transmission Control Protocol (TCP) mechanism that can be used to more effectively recover lost segments when a connection's congestion window is small, or when a large number of segments are lost in a single transmission window. The "Limited Transmit" algorithm calls for sending a new data segment in response to each of the first two duplicate acknowledgments that arrive at the sender. Transmitting these segments increases the probability that TCP can recover from a single lost segment using the fast retransmit algorithm, rather than using a costly retransmission timeout. Limited Transmit can be used both in conjunction with, and in the absence of, the TCP selective acknowledgment (SACK) mechanism.

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Network Working Group M. Allman Request for Comments: 3042 NASA GRC/BBN Category: Standards Track H. Balakrishnan

MIT S. Floyd

ACIRI January 2001

Enhancing TCP's Loss Recovery Using Limited Transmit

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 Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document proposes a new Transmission Control Protocol (TCP) mechanism that can be used to more effectively recover lost segments when a connection's congestion window is small, or when a large number of segments are lost in a single transmission window. The "Limited Transmit" algorithm calls for sending a new data segment in response to each of the first two duplicate acknowledgments that arrive at the sender. Transmitting these segments increases the probability that TCP can recover from a single lost segment using the fast retransmit algorithm, rather than using a costly retransmission timeout. Limited Transmit can be used both in conjunction with, and in the absence of, the TCP selective acknowledgment (SACK) mechanism.

1 Introduction

A number of researchers have observed that TCP's loss recovery strategies do not work well when the congestion window at a TCP sender is small. This can happen, for instance, because there is only a limited amount of data to send, or because of the limit imposed by the receiver-advertised window, or because of the constraints imposed by end-to-end congestion control over a connection with a small bandwidth-delay product [Riz96,Mor97,BPS+98,Bal98,LK98]. When a TCP detects a missing segment, it enters a loss recovery phase using one of two methods.

Allman, et al. Standards Track [Page 1]

RFC 3042 Enhancing TCP Loss Recovery January 2001

First, if an acknowledgment (ACK) for a given segment is not received in a certain amount of time a retransmission timeout occurs and the segment is resent [RFC793,PA00]. Second, the "Fast Retransmit" algorithm resends a segment when three duplicate ACKs arrive at the sender [Jac88,RFC2581]. However, because duplicate ACKs from the receiver are also triggered by packet reordering in the Internet, the TCP sender waits for three duplicate ACKs in an attempt to disambiguate segment loss from packet reordering. Once in a loss recovery phase, a number of techniques can be used to retransmit lost segments, including slow start-based recovery or Fast Recovery [RFC2581], NewReno [RFC2582], and loss recovery based on selective acknowledgments (SACKs) [RFC2018,FF96].

TCP's retransmission timeout (RTO) is based on measured round-trip times (RTT) between the sender and receiver, as specified in [PA00]. To prevent spurious retransmissions of segments that are only delayed ...