Re: [tsvwg] Martin Duke's Yes on draft-ietf-tsvwg-datagram-plpmtud-17: (with COMMENT)
Gorry Fairhurst <gorry@erg.abdn.ac.uk> Wed, 01 April 2020 07:54 UTC
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To: Martin Duke <martin.h.duke@gmail.com>, The IESG <iesg@ietf.org>
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From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: Re: [tsvwg] Martin Duke's Yes on draft-ietf-tsvwg-datagram-plpmtud-17: (with COMMENT)
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Martin Duke has entered the following ballot position for draft-ietf-tsvwg-datagram-plpmtud-17: Yes ---------------------------------------------------------------------- COMMENT: ---------------------------------------------------------------------- I'm excited for this to go to RFC once the QUIC reference clears. Normative issue: Sec 3, item #9 "An update to the PLPMTU (or MPS) MUST NOT modify the congestion window". Would it not be OK to round down the congestion window to remain a multiple of the MPS? Probably not a big issue in practice, but I would hate to constrain an implementer in this way. I would suggest "MUST NOT increase". >> GF: If a protocol wants to change the way it uses the Cwnd, because the MPS had changed, then it needs to take care of that when it works out whether Cwnd is limiting the sender. The packets can be any size up to the MPS. A protocol using only one size of packet would need to change the Cwnd limit (the number of outstanding packets) based on the Cwnd when it decided to use a new size, not when it was told a larger size was possible. That’s up to the protocol... care is needed here, because the DPLPMTU can be updated many times in a search. Usually it will grow depending on the search pattern used for probes. These changes do not themselves reflect any change in congestion - a sender that adjusts the cwnd depending on the current PMTU size, can get rounding errors as the PLPMTU increases/decreases. >> GF: So the ID could say something about when the PL protocol operates in terms of packets, rather than bytes. That may be the root of the question you ask? I suggest new text to be added: OLD: An update to the PLPMTU (or MPS) MUST NOT increase the congestion window measured in bytes [RFC4821]. Therefore, an increase in the packet size does not cause an increase in the data rate in bytes per second. NEW: An update to the PLPMTU (or MPS) MUST NOT increase the congestion window measured in bytes [RFC4821]. Therefore, an increase in the packet size does not cause an increase in the data rate in bytes per second. A PL that maintains the congestion window in terms of a limit to the number of outstanding fixed size packets SHOULD adapt this limit to compensate for the size of the actual packets that it sends. [RFC7141] provides guidance on appropriate transport congestion reactions when a PL uses a certain packet size. Does this seem helpful? Gorry — Nits: Sec 2. In the definition for EMTU_R, put "the largest datagram size that can be reassembled" in quotes, and delete "by EMTU_R (Effective MTU to receive)" Sec 4.1. delete word in brackets: "A PL that uses a probe packet carrying [an] application data..." Sec 4.6.2 I think this list of inequalities would be easier to read if it was ranked in order of increasing PL_PTB_SIZE. 5.1.1. s/up to data/up to date >> GF these issues will all be resolved in the next version.
- [tsvwg] Martin Duke's Yes on draft-ietf-tsvwg-dat… Martin Duke via Datatracker
- Re: [tsvwg] Martin Duke's Yes on draft-ietf-tsvwg… Gorry Fairhurst
- Re: [tsvwg] Martin Duke's Yes on draft-ietf-tsvwg… Martin Duke