Re: [tcpm] Exceeding value in MSS option?

["Scharf, Michael" <[e-mail]>]

Hi all,

Sorry for the delayed reply. More inline...

> > On 19. Oct 2020, at 21:22, Martin Duke <[e-mail]> wrote:
> >
> > Hello tcpm,
> >
> > Section 4.2.2.6 of RFC 1122 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 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?
> Hi Martin,
> 
> the document you are reviewing says:
> 
>    An IPv6 datagram size exceeding 1280 bytes can be avoided by setting
>    the TCP MSS not larger than 1220 bytes.  This assumes that the remote
>    sender will use no TCP options, aside from possibly the MSS option,
>    which is only used in the initial TCP SYN packet.
> 
> The first sentence is correct. The second is not. I would suggest to simply
> remove it.

The statements regarding the MSS were discussed and also rewritten many times. And, sorry, apparently I have not carefully read the second sentence (but, well, that sentence was also in the version of the draft that was last called in TCPM...)

Given this discussion now, I agree that the second sentence could probably be removed.
 
> Then the text says:
> 
>    In order to accommodate unrequested TCP options that may be used by
>    some TCP implementations, a constrained device may advertise an MSS
>    smaller than 1220 bytes (e.g. not larger than 1200 bytes).  Note that
>    it is advised for TCP implementations to consume payload space
>    instead of increasing datagram size when including IP or TCP options
>    in an IP packet to be sent [RFC6691].  Therefore, the suggestion of
>    advertising an MSS smaller than 1220 bytes is likely to be
>    overcautious and its suitability should be considered carefully.
> 
> I read this as "use a smaller MSS", but this is "likely to be
> overcautious and its suitability should be considered carefully."
> 
> I think the careful consideration is to remove this paragraph.

I am not sure if we should simply remove the whole paragraph, including e.g. the reference to RFC 6691. Readers of this document may not be aware of RFC 6691, so a pointer may be useful, no? Also, during the IETF last call there was a discussion on MSS values significantly smaller than 1220 byte. People apparently *do* think about smaller MSS values on constrained devices and this document is a place to provide guidance to that community (as far as possible). This specific wording came up in earlier reviews of the draft, too. Thus, the final proposal "likely to be overcautious and its suitability should be considered carefully" is a result of several past discussions. IMHO we should carefully consider changes to this statement...

BTW, personally, I really wonder if using an MSS of 1220 byte (for IPv6) indeed causes real-world problems on constrained devices, i.e., if in year 2020 there is any real-world benefit of an MSS smaller than 1220 byte. But, unfortunately, I don't have measurement data that would back a different statement in the draft.

Michael

> Best regards
> Michael
> 
> 
> 
> 
> >
> > 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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