RE: Treatment of ICMP for PMTU

[Lucas Pardue <[e-mail]>]

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