Re: [tcpm] Exceeding value in MSS option?

> On 19. Oct 2020, at 21:22, Martin Duke <martin.h.duke@gmail.com> 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.

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.

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
> 
> 
> _______________________________________________
> tcpm mailing list
> tcpm@ietf.org
> https://www.ietf.org/mailman/listinfo/tcpm