Re: [tcpm] RFC 5827 on Early Retransmit for and Stream Control Transmission Protocol (SCTP)

Fred Baker <fred@cisco.com> Tue, 27 April 2010 03:00 UTC

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From: Fred Baker <fred@cisco.com>
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To: jblanton@irg.cs.ohiou.edu, k.avrachenkov@sophia.inria.fr, Mark Allman <mallman@icir.org>, per.hurtig@kau.se, urtzi@laas.fr
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Subject: Re: [tcpm] RFC 5827 on Early Retransmit for and Stream Control Transmission Protocol (SCTP)
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Thinking out loud...

In cases in which cwnd < 4, it seems like your biggest concern is to elicit the information necessary to trigger the retransmission without unnecessarily burdening the path. That is in essence why we use three dup-acks as a trigger - it is "enough" information to ensure that with high probability it is not being triggered by simple packet reordering. 

Would it make sense to, when cwnd < 4 or all of the data available to transmit is in flight, in response to a SACK, send a segment containing exactly the TCP header, and one byte of SACKed data? The receipt of the SACK says that it is highly likely that the missing segment cleared the bottleneck, and would trigger an additional dup-ack containing the SACK record in question (which might trigger another of the same type of segment). If the missing segment is in fact lost, doing that would allow you to trigger three dup-acks in three RTTs even if you only had two segments outstanding (cwnd=2 or that's all the data you have). You only take the RTO if you literally have a single segment outstanding. If the missing segment in fact arrived but the ACK was lost, it would trigger the ACK.

On Apr 27, 2010, at 12:50 AM, rfc-editor@rfc-editor.org wrote:

> 
> A new Request for Comments is now available in online RFC libraries.
> 
> 
>        RFC 5827
> 
>        Title:      Early Retransmit for and Stream 
>                    Control Transmission Protocol (SCTP) 
>        Author:     M. Allman, K. Avrachenkov,
>                    U. Ayesta, J. Blanton,
>                    P. Hurtig
>        Status:     Experimental
>        Stream:     IETF
>        Date:       April 2010
>        Mailbox:    mallman@icir.org, 
>                    k.avrachenkov@sophia.inria.fr, 
>                    urtzi@laas.fr,  jblanton@irg.cs.ohiou.edu, 
>                    per.hurtig@kau.se
>        Pages:      15
>        Characters: 35383
>        Updates/Obsoletes/SeeAlso:   None
> 
>        I-D Tag:    draft-ietf-tcpm-early-rexmt-04.txt
> 
>        URL:        http://www.rfc-editor.org/rfc/rfc5827.txt
> 
> This document proposes a new mechanism for TCP and Stream Control 
> Transmission Protocol (SCTP) that can be used to recover lost segments 
> when a connection's congestion window is small.  The "Early Retransmit" 
> mechanism allows the transport to reduce, in certain special 
> circumstances, the number of duplicate acknowledgments required to trigger 
> a fast retransmission.  This allows the transport to use fast retransmit 
> to recover segment losses that would otherwise require a lengthy 
> retransmission timeout.  [STANDARDS TRACK]
> 
> This document is a product of the TCP Maintenance and Minor Extensions Working Group of the IETF.
> 
> 
> EXPERIMENTAL: This memo defines an Experimental Protocol for the
> Internet community.  It does not specify an Internet standard of any
> kind. Discussion and suggestions for improvement are requested.
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