Re: Deadlocking in the transport

[Jana Iyengar <[e-mail]>]

Martin,

You are right that this isn't a new concern, and that this is worth noting
somewhere, perhaps in the applicability/API doc.

The crux of this issue is that there's structure in application data that
the transport is unaware of. Specifically, there are dependencies among
application data units that is opaque to the transport. Using the transport
buffers as a part of handling these dependencies seems like a bad idea,
especially since the transport is likely to make decisions about flow
window updates based on rate at which data is consumed out of the receive
buffer(s). GQUIC does this, and so does every respectable TCP receiver
implementation.

The SCTP API avoids this problem by not allowing the application to read
specific stream data out of the socket buffers. The receiving app receives
data that could belong to any stream and has to demux after reading out of
the socket. (Note that SCTP does not have per-stream flow control, so the
receive side here is more like SPDY/TCP, modulo HoL blocking at the
transport.)

Protocols that create inter-stream dependency should be able to express
that in priorities down to the transport, which I believe is expected to be
part of the API. I believe that handles this issue, doesn't it?

- jana

On Tue, Jan 9, 2018 at 10:17 PM, Martin Thomson <[e-mail]>
wrote:

> Building a complex application protocol on top of QUIC continues to
> produce surprises.
>
> Today in the header compression design team meeting we discussed a
> deadlocking issue that I think warrants sharing with the larger group.
> This has implications for how people build a QUIC transport layer.  It
> might need changes to the API that is exposed by that layer.
>
> This isn't really that new, but I don't think we've properly addressed
> the problem.
>
>
> ## The Basic Problem
>
> If a protocol creates a dependency between streams, there is a
> potential for flow control to deadlock.
>
> Say that I send X on stream 3 and Y on stream 7.  Processing Y
> requires that X is processed first.
>
> X cannot be sent due to flow control but Y is sent.  This is always
> possible even if X is appropriately prioritized.  The receiver then
> leaves Y in its receive buffer until X is received.
>
> The receiver cannot give flow control credit for consuming Y because
> it can't consume Y until X is sent.  But the sender needs flow control
> credit to send X.  We are deadlocked.
>
> It doesn't matter whether the stream or connection flow control is
> causing the problem, either produces the same result.
>
> (To give some background on this, we were considering a preface to
> header blocks that identified the header table state that was
> necessary to process the header block.  This would allow for
> concurrent population of the header table and sending message that
> depended on the header table state that is under construction.  A
> receiver would read the identifier and then leave the remainder of the
> header block in the receive buffer until the header table was ready.)
>
>
> ## Options
>
> It seems like there are a few decent options for managing this.  These
> are what occurred to me (there are almost certainly more options):
>
> 1. Don't do that.  We might concede in this case that seeking the
> incremental improvement to compression efficiency isn't worth the
> risk.  That is, we might make a general statement that this sort of
> inter-stream blocking is a bad idea.
>
> 2. Force receivers to consume data or reset streams in the case of
> unfulfilled dependencies.  The former seems like it might be too much
> like magical thinking, in the sense that it requires that receivers
> conjure more memory up, but if the receiver were required to read Y
> and release the flow control credit, then all would be fine.  For
> instance, we could require that the receiver reset a stream if it
> couldn't read and handle data.  It seems like a bad arrangement
> though: you either have to allocate more memory than you would like or
> suffer the time and opportunity cost of having to do Y over.
>
> 3. Create an exception for flow control.  This is what Google QUIC
> does for its headers stream.  Roberto observed that we could
> alternatively create a frame type that was excluded from flow control.
> If this were used for data that had dependencies, then it would be
> impossible to deadlock.  It would be similarly difficult to account
> for memory allocation, though if it were possible to process on
> receipt, then this *might* work.  We'd have to do something to address
> out-of-order delivery though.  It's possible that the stream
> abstraction is not appropriate in this case.
>
> 4. Block the problem at the source.  It was suggested that in cases
> where there is a potential dependency, then it can't be a problem if
> the transport refused to accept data that it didn't have flow control
> credit for.  Writes to the transport would consume flow control credit
> immediately.  That way applications would only be able to write X if
> there was a chance that it would be delivered.  Applications that have
> ordering requirements can ensure that Y is written after X is accepted
> by the transport and thereby avoid the deadlock.  Writes might block
> rather than fail, if the API wasn't into the whole non-blocking I/O
> thing.  The transport might still have to buffer X for other reasons,
> like congestion control, but it can guarantee that flow control isn't
> going to block delivery.
>
>
> ## My Preference
>
> Right now, I'm inclined toward option 4. Option 1 seems a little too
> much of a constraint.  Protocols create this sort of inter-dependency
> naturally.
>
> There's a certain purity in having the flow control exert back
> pressure all the way to the next layer up.  Not being able to build a
> transport with unconstrained writes is potentially creating
> undesirable externalities on transport users.  Now they have to worry
> about flow control as well.  Personally, I'm inclined to say that this
> is something that application protocols and their users should be
> exposed to.  We've seen with the JS streams API that it's valuable to
> have back pressure available at the application layer and also how it
> is possible to do that relatively elegantly.
>
> I'm almost certain that I haven't thought about all the potential
> alternatives.  I wonder if there isn't some experience with this
> problem in SCTP that might lend some insights.
>
>