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
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.
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
Henderson, et al. Standards Track [Page 1]RFC 6582 TCP NewReno April 2012Copyright Notice
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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.
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.