Re: [tcpm] WGLC for MSS document

Joe Touch <touch@ISI.EDU> Thu, 20 August 2009 20:35 UTC

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Date: Thu, 20 Aug 2009 13:35:36 -0700
From: Joe Touch <touch@ISI.EDU>
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Subject: Re: [tcpm] WGLC for MSS document
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Hi, Lloyd,

L.Wood@surrey.ac.uk wrote:
> Can't TCP MSS also vary across multipath environments? 

Yes, but that's discovered by PMTUD. I think this doc is focusing more
on what the interface says is available - which ROHC might have an
effect on.

> Strikes me
> that these are more common than header compression - which should
> be advertising conservative minimum MTUs for their link interfaces.

I'm not sure what's more common, but they're separate effects AFAICT.

Joe

> -----Original Message-----
> From: tcpm-bounces@ietf.org on behalf of Joe Touch
> Sent: Thu 2009-08-20 19:28
> To: Eddy, Wesley M. (GRC-MS00)[Verizon]
> Cc: tcpm@ietf.org
> Subject: Re: [tcpm] WGLC for MSS document
> 
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> 
> Hi, all,
> 
> Eddy, Wesley M. (GRC-MS00)[Verizon] wrote:
>> This note starts a 2-week Working Group Last Call on the TCPM document:
>> http://tools.ietf.org/html/draft-ietf-tcpm-tcpmss-02
>> Please send any comments to the TCPM list.  The WGLC will last until
>> September 1.
> 
> I'd like to suggest the following for this doc:
> 
> MSSs can sometimes vary, as when used with variable compression, e.g.,
> ROHC. The ROHC doc missed an opportunity to address the interaction of
> variable MSS with TCP, and this might be a good place to include a
> sentence or two on it, e.g. (IMO):
> 
> - ---
> 
> MSSs can sometimes vary, as when used with variable compression, e.g.,
> ROHC [RFC5255]. It is tempting for TCP to want to advertise the largest
> possible MSS, to support the most efficient use of compressed payloads.
> Unfortunately, some compression schemes occasionally need to transmit
> full headers (and thus smaller payloads) to resynchronize state at their
> endpoint compressor/decompressors. If the largest MSS is advertised, TCP
> retransmission may interfere with compressor resynchonization.
> 
> As a result, when effective MSS varies, TCP SHOULD compute its
> advertised MSS based on the smallest effective MSS.
> 
> - ---
> 
> Joe
> 
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