Network Working Group S. Floyd
Request for Comments: 3782 ICSI
Obsoletes: 2582 T. Henderson
Category: Standards Track Boeing
The NewReno Modification to TCP's Fast Recovery Algorithm
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 (C) The Internet Society (2004). All Rights Reserved.
The purpose of this document is to advance NewReno TCP's Fast
Retransmit and Fast Recovery algorithms in RFC 2582 from Experimental
to Standards Track status.
The main change in this document relative to RFC 2582 is to specify
the Careful variant of NewReno's Fast Retransmit and Fast Recovery
algorithms. The base algorithm described in RFC 2582 did not attempt
to avoid unnecessary multiple Fast Retransmits that can occur after a
timeout. However, RFC 2582 also defined "Careful" and "Less Careful"
variants that avoid these unnecessary Fast Retransmits, and
recommended the Careful variant. This document specifies the
previously-named "Careful" variant as the basic version of NewReno
Floyd, et al. Standards Track [Page 1]RFC 3782 NewReno Modification to Fast Recovery Algorithm April 20041. Introduction
For the typical implementation of the TCP Fast Recovery algorithm
described in [RFC2581] (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 acknowledgements 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 2581
leads to the retransmission of only a single data packet.
Problems can arise, therefore, when multiple packets are dropped from
a single window of data and the Fast Retransmit and Fast Recovery
algorithms are invoked. In this case, if the SACK option 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. This document applies only for TCP
connections that are unable to use the TCP Selective Acknowledgement
(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 acknowledgements, the sender infers a packet
loss, and retransmits the indicated packet. After this, the data
sender could receive additional duplicate acknowledgements, 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 acknowledgement 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
acknowledgement for this packet will acknowledge all of the packets
transmitted before Fast Retransmit was entered. However, if there
are multiple packet drops, then the acknowledgement for the
retransmitted packet will acknowledge some but not all of the packets
transmitted before the Fast Retransmit. We call this acknowledgement
a partial acknowledgment.
Along with several other suggestions, [Hoe95] suggested that during
Fast Recovery the TCP data sender responds 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 2581 that
incorporates a response to partial acknowledgements received during