Computing TCP's Retransmission Timer
RFC 6298
Document | Type |
RFC - Proposed Standard
(June 2011; No errata)
Obsoletes RFC 2988
Updates RFC 1122
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Authors | Matt Sargent , Jerry Chu , Vern Paxson , Mark Allman | ||
Last updated | 2015-10-14 | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized bibtex | ||
Reviews | |||
Stream | WG state | WG Document | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 6298 (Proposed Standard) | |
Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Wesley Eddy | ||
IESG note | Wesley Eddy (Wesley.M.Eddy@nasa.gov) is the document shepherd. | ||
Send notices to | (None) |
Internet Engineering Task Force (IETF) V. Paxson Request for Comments: 6298 ICSI/UC Berkeley Obsoletes: 2988 M. Allman Updates: 1122 ICSI Category: Standards Track J. Chu ISSN: 2070-1721 Google M. Sargent CWRU June 2011 Computing TCP's Retransmission Timer Abstract This document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. This document obsoletes RFC 2988. 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. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6298. Paxson, et al. Standards Track [Page 1] RFC 6298 Computing TCP's Retransmission Timer June 2011 Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) 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. 1. Introduction The Transmission Control Protocol (TCP) [Pos81] uses a retransmission timer to ensure data delivery in the absence of any feedback from the remote data receiver. The duration of this timer is referred to as RTO (retransmission timeout). RFC 1122 [Bra89] specifies that the RTO should be calculated as outlined in [Jac88]. This document codifies the algorithm for setting the RTO. In addition, this document expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. RFC 5681 [APB09] outlines the algorithm TCP uses to begin sending after the RTO expires and a retransmission is sent. This document does not alter the behavior outlined in RFC 5681 [APB09]. In some situations, it may be beneficial for a TCP sender to be more conservative than the algorithms detailed in this document allow. However, a TCP MUST NOT be more aggressive than the following algorithms allow. This document obsoletes RFC 2988 [PA00]. 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 [Bra97]. 2. The Basic Algorithm To compute the current RTO, a TCP sender maintains two state variables, SRTT (smoothed round-trip time) and RTTVAR (round-trip time variation). In addition, we assume a clock granularity of G seconds. Paxson, et al. Standards Track [Page 2] RFC 6298 Computing TCP's Retransmission Timer June 2011 The rules governing the computation of SRTT, RTTVAR, and RTO are as follows: (2.1) Until a round-trip time (RTT) measurement has been made for a segment sent between the sender and receiver, the sender SHOULD set RTO <- 1 second, though the "backing off" on repeated retransmission discussed in (5.5) still applies. Note that the previous version of this document used an initial RTO of 3 seconds [PA00]. A TCP implementation MAY still use this value (or any other value > 1 second). This change in the lower bound on the initial RTO is discussed in further detail in Appendix A. (2.2) When the first RTT measurement R is made, the host MUST set SRTT <- R RTTVAR <- R/2 RTO <- SRTT + max (G, K*RTTVAR)Show full document text