Re: [tcpm] Exceeding value in MSS option?

Hi Martin, hi Gorry,

I think Gorry’s paper does indicate that at least for MMS there are middleboxes that add this option. However, I don’t think there is evidence that there is a general problem about middlebox adding option (rather than removing them). I also reviewed the draft in tsv-art review and given that they say it’s actually not recommended and should only be done with care, I thought that text is okay. However, I guess there is room to improve the wording and explain a bit better, when and if this might be consider or not.

Mirja

From: tcpm <tcpm-bounces@ietf.org> on behalf of Gorry Fairhurst <gorry@erg.abdn.ac.uk>
Date: Tuesday, 20. October 2020 at 11:13
To: Martin Duke <martin.h.duke@gmail.com>, "tcpm@ietf.org Extensions" <tcpm@ietf.org>
Subject: Re: [tcpm] Exceeding value in MSS option?

On 19/10/2020 20:22, Martin Duke 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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Hi Martin,

While a server advertising a restricted MSS clearly reduces the TCP packet size, it seems to me to be a rather poor solution to the problem in general, and only works for TCP.

If  it wants to say some advices, I think the IETF needs to consider current practice and the implications of this. I suspect the practice of clamping the IPv6 MSS at the server is already quite common, especially in IPv6 servers offering web content. MSS Clamping on-path is also common, but anyway I see some servers advertise an MSS much lower than permitted by the MTU. For example, see the section on MSS handling here:

https://tma.ifip.org/2018/wp-content/uploads/sites/3/2018/06/tma2018_paper57.pdf<https://protect2.fireeye.com/v1/url?k=c964a91f-97c41371-c964e984-866038973a15-4c538007322d8392&q=1&e=8b72cbc2-683a-493b-a075-c9df067b88f7&u=https%3A%2F%2Ftma.ifip.org%2F2018%2Fwp-content%2Fuploads%2Fsites%2F3%2F2018%2F06%2Ftma2018_paper57.pdf>

This might be because of the reasons for this are cited in RFC8900.

Gorry