Subject: [Tsvwg] Update: The Eifel Algorithm for TCP

From: Reiner Ludwig <[email-masked]>
Date: Sun, 25 Feb 2001 05:16:01 +0100

Hi,

a new version of the Eifel algorithm ID has been submitted. Until it's 
available from the ID directory, you can find it here 
http://iceberg.cs.berkeley.edu/downloads/tcp-eifel/draft-ietf-tsvwg-tcp-eifel-alg-00.txt 
(see abstract below).

The main changes are:
1. Restoring of CWND / SSTHRESH after a spurious retransmit caused by 
packet re-ordering or packet duplication in such a way that a burst is 
avoided. This was suggested by Mark Allman at the last TSV meeting.
2. A few SHOULDs were changed to MAYs.

I would appreciate any feedback; ideally on this mailing list. I'm 
especially interested to hear people's view about assigning 2 of the 
currently 4 unused bits in the TCP header to one of the two alternatives to 
implement the Eifel algorithm.

Also, a separate IPR notice from Ericsson for this ID should appear at 
http://www.ietf.org/ipr.html soon.

///Reiner

------------

Abstract:

    TCP's intertwined error and congestion control is not robust against
    spurious timeouts nor is it robust against packet re-orderings. A
    packet that is delayed in the network beyond the expiration of TCP's
    retransmission timer, is mistaken for a packet loss by a TCP sender.
    Also, a packet that is re-ordered in the network beyond TCP's
    duplicate acknowledgment threshold, is eventually mistaken for a
    packet loss by a TCP sender. Both situations lead to a spurious
    retransmit of the oldest outstanding segment, and an unnecessary
    reduction of the congestion window at the sender. Moreover, a
    spurious timeout forces the sender into a go-back-N retransmission
    mode leading to spurious retransmits of all outstanding segments.

    We propose the "Eifel algorithm" as a way to make TCP robust against
    spurious timeouts and packet re-orderings. The Eifel algorithm uses
    extra information in the ACKs to reliably detect (a posteriori) a
    spurious retransmit of the oldest outstanding segment at the TCP
    sender. In response to such a detection, the Eifel algorithm restores
    the congestion window, and prevents the spurious go-back-N
    retransmits following a spurious timeout. As extra information in the
    ACKs, the Eifel algorithm allows for two alternatives: the timestamp
    option [RFC1323] and/or two new flags in the Reserved field of the
    TCP header.

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