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RE: Treatment of ICMP for PMTU

Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com> Thu, 28 June 2018 10:07 UTC

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From: Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com>
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Subject: RE: Treatment of ICMP for PMTU
To: Lucas Pardue <lucas.pardue@bbc.co.uk>, IETF QUIC WG <quic@ietf.org>, Martin Duke <martin.h.duke@gmail.com>, "Lubashev, Igor" <ilubashe@akamai.com>
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I think QUIC needs a tunnel extension frame.

On 28 June 2018 at 11.25.20, Lucas Pardue (lucas.pardue@bbc.co.uk) wrote:

Igor Lubashev wrote:

>I think I see what you are talking about.
>
>At least the way it is specced in H2 and HTTP/QUIC, CONNECT proxies
forward
>data streams, not packets. Hence, MTU concept on a tunnel-stream is
>meaningless. The text says it explicitly: "Note that the size and number
of TCP
>segments is not guaranteed to map predictably to the size and number of
>HTTP DATA or QUIC STREAM frames." From the client's perspective there is
>only one PMTU that is relevant -- the one to the proxy. The proxy will
keep
>tracks of the PMTUs to the CONNECT endpoints itself and will generate TCP
>segments accordingly.

Right, that's how I was seeing it.

>Trying to design a CONNECT-like method to proxy an entire QUIC connection
>over a single QUIC stream would create HOLB for QUIC packets to and from
>the proxy. Some sort of a partially reliable QUIC stream could help here.

Both you and Mikkel have identified HOL as something that needs more
thought. Thanks. Mikkel's view is a little more extreme, to paraphrase "HOL
is enough to make QUIC not QUIC". I've taken the view that it may be
desirable to tunnel QUIC over TLS/TCP too (perhaps using HTTP/2 frames), so
I view HOL avoidance as an optimisation rather than a requirement. Partial
reliability (of streams, or of some new frame type) could help solve that
puzzle. Now I think about it, a complimentary mitigation could be to
reserve a set of tunnel-streams to provide some basic multiplexing.
However, the complexity of that might not be worth it.

>This proxying scheme would require an extension to CONNECT to open a
>UDP/IP tunnel instead of a TCP/IP tunnel. That extension would also
mandate
>that HTTP/QUIC DATA frame corresponds to a UDP frame. I suspect that it
>would be a very good idea for such a proxy to add a new frame for
providing
>ICMP-like explicit PMTU feedback to the sender (why leave the sender
>guessing and probing, if the PMTU is already known to the proxy?).

This is good feedback. Thanks!

Kind regards
Lucas

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