flow control and DATAGRAM
Martin Thomson <martin.thomson@gmail.com> Mon, 29 October 2018 05:16 UTC
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From: Martin Thomson <martin.thomson@gmail.com>
Date: Mon, 29 Oct 2018 16:16:07 +1100
Message-ID: <CABkgnnU26aYD=TybuD0FZGYtfEa6np-Sk3Jo6t7LRp0wzKh3Lg@mail.gmail.com>
Subject: flow control and DATAGRAM
To: Tommy Pauly <tpauly@apple.com>, QUIC WG <quic@ietf.org>
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Hi Tommy, Your slides - <https://github.com/quicwg/wg-materials/blob/master/ietf103/IETF103-QUIC-Datagram.pdf> - say that DATAGRAM frames respect connection-level flow control. I missed that in the draft, and I don't know how they can do that in the face of packet loss, especially when you don't necessarily retransmit lost DATAGRAM frames. For that to work, you would need a bunch more machinery to make the connection-level flow control sync between endpoints in the case that packets are lost. A disagreement about how much flow control is used causes things to break down badly. Ian and I discussed this point at the last meeting and quickly agreed that while it might be nice to have flow control for this stuff, the increase in complexity is considerable and (at the time) we thought it wouldn't be worth it. The problem that introduces is that you could end up having too many DATAGRAM frames arrive. The receiver has to drop something at the point that it can't handle them. And we say that when you acknowledge something, you processed it. That's tricky. It might be easier to say that a QUIC acknowledgment for a DATAGRAM frame doesn't mean that it was received and processed by an application. An endpoint might discard these frames before passing them on to applications if it doesn't have space. In other words, acknowledgment of DATAGRAM means that QUIC got it, not that the application got it. Sadly, that means that the QUIC acknowledgment machinery doesn't help the application that uses DATAGRAM all that much. Also, the lower bound on reliability is 0, which isn't the best thing ever. Hard choices, I know. I don't have a good design for maintaining connection-level flow control (or any back pressure mechanism with equivalent properties) that doesn't add both complexity and overhead. Cheers, Martin
- flow control and DATAGRAM Martin Thomson
- Re: flow control and DATAGRAM Ian Swett
- Re: flow control and DATAGRAM Tommy Pauly
- Re: flow control and DATAGRAM Jana Iyengar
- Re: flow control and DATAGRAM Tommy Pauly
- RE: flow control and DATAGRAM Lubashev, Igor
- Re: flow control and DATAGRAM Christian Huitema
- RE: flow control and DATAGRAM Lubashev, Igor
- Re: flow control and DATAGRAM Martin Thomson
- Re: flow control and DATAGRAM Roberto Peon
- Re: flow control and DATAGRAM Jana Iyengar
- RE: flow control and DATAGRAM Lubashev, Igor
- Re: flow control and DATAGRAM Jana Iyengar
- Re: flow control and DATAGRAM Ian Swett
- Re: flow control and DATAGRAM David Schinazi
- RE: flow control and DATAGRAM Mike Bishop
- Re: flow control and DATAGRAM Subodh Iyengar
- Re: flow control and DATAGRAM Jana Iyengar
- Re: flow control and DATAGRAM Tommy Pauly