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Re: [tcpm] Exceeding value in MSS option?

Joseph Touch <touch@strayalpha.com> Mon, 19 October 2020 19:44 UTC

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From: Joseph Touch <touch@strayalpha.com>
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Date: Mon, 19 Oct 2020 12:44:29 -0700
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Subject: Re: [tcpm] Exceeding value in MSS option?
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To be more clear:

The MSS value in the TCP option reflects the FIXED headers only.

The sending side uses that value, together with its knowledge of options it adds, to reduce the size of the data sent to fit.

Second guessing that information is problematic; sure, when something gets through you can always try something smaller but that’s not specific to TCP or anything else.

Middleboxes should never make a packet larger under any circumstances.

Joe

> On Oct 19, 2020, at 12:37 PM, Joseph Touch <touch@strayalpha.com> wrote:
> 
> See RFC 6691.
> 
> Only the fixed headers are considered, NOT the options.
> 
> Joe
> 
>> On Oct 19, 2020, at 12:22 PM, Martin Duke <martin.h.duke@gmail.com <mailto:martin.h.duke@gmail.com>> wrote:
>> 
>> Hello tcpm,
>> 
>> Section 4.2.2.6 of RFC 1122 <https://datatracker.ietf.org/doc/html/rfc1122#page-85> is pretty clear that the TCP sender MUST consider all IP and TCP options when sizing payloads with respect to the advertised MSS option.
>> 
>> I'm reviewing a document <https://datatracker.ietf.org/doc/html/draft-ietf-lwig-tcp-constrained-node-networks-11#section-4.1.1> that advises that some endpoints may want to reduce their advertised MSS on IPv6 connections in case the peer isn't respecting that guidance. Is noncompliance with this provision a problem in the internet? Are there middleboxes injecting options that cause PMTU drops or fragmentation?
>> 
>> I have not heard of such problems, but thought I'd check with the community to see if this precaution makes any sense at all.
>> 
>> Thanks,
>> Martin
>> 
>> 
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>

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