Rob Godfrey commented on PROTON-1135:

As a co-author of the AMQP specification, I don't think it can be taken as read 
that every implementation  of a "server" MUST support pipelining of the SASL 
and AMQP layers together.  The spec does lay out that the AMQP open sequence 
can be pipelined, but the SASL interaction is (in general) synchronous: the 
client must choose from the set of mechanisms offered.  In certain 
circumstances the client (application) may have knowledge that allows it to 
operate in a pipelined manner (e.g. it knows that the server will accept the 
mechanism) or may be happy to fail in a reasonably unpleasant way if its 
assumptions are incorrect.  However I think that it is unreasonable to expect 
that in general a peer will accept pipelining SASL and AMQP together, and 
empirical evidence shows that it is a poor assumption to make (as literally no 
other implementation has supported it out of the box).  To my mind, in this 
scenario it would be more appropriate to omit the SASL layer entirely.

> [proton-c] dont pipeline SASL and OPEN frames for ANONYMOUS logins by default
> -----------------------------------------------------------------------------
>                 Key: PROTON-1135
>                 URL: https://issues.apache.org/jira/browse/PROTON-1135
>             Project: Qpid Proton
>          Issue Type: Bug
>          Components: proton-c
>    Affects Versions: 0.12.0
>            Reporter: Ganesh Murthy
> Dispatch router (which uses Proton-c) currently sends pipelined SASL and OPEN 
> frames by default when connecting out to other peers using the ANONYMOUS 
> mech, as shown in the following trace - 
> {code}
> [0x7f41f80079c0]:  -> SASL
> [0x7f41f80079c0]:0 -> @sasl-init(65) [mechanism=:ANONYMOUS, 
> initial-response=b"anonymous@localhost.localdomain"]
> [0x7f41f80079c0]:  -> AMQP
> [0x7f41f80079c0]:0 -> @open(16) [container-id="Qpid.Dispatch.Router.A", 
> max-frame-size=65536, channel-max=32767, idle-time-out=8000, 
> offered-capabilities=:"ANONYMOUS-RELAY", 
> properties={:product="qpid-dispatch-router", :version="0.6.0"}]
> [0x7f41f80079c0]:  <- SASL
> [0x7f41f80079c0]:0 <- @sasl-mechanisms(64) [sasl-server-mechanisms=:ANONYMOUS]
> [0x7f41f80079c0]:0 <- @sasl-outcome(68) [code=0]
> [0x7f41f80079c0]:  <- AMQP
> [0x7f41f80079c0]:0 <- @open(16) 
> [container-id="ce103199-af03-4d37-bb35-24ad4e55653e", channel-max=32767, 
> idle-time-out=8000, offered-capabilities=@PN_SYMBOL[:"ANONYMOUS-RELAY"], 
> properties={:product="qpid-cpp", :version="0.35", :platform="Linux", 
> :host="localhost.localdomain"}]
> {code}
> The AMQP 1.0 spec does not make it clear that this is supported (e.g. see 
> diagram below) but in any case various components have shown difficulty with 
> it (such as PROTON-1135 just raised, and QPID-6639 which has yet to be 
> included in a release but permitted the above protocol trace log).
> {code}
> TCP Client         TCP Server
> =========================================
> AMQP%d3.1.0.0 --------->
>                           <--------- AMQP%d3.1.0.0
> :
> :
> <SASL negotiation>
> :
> :
> AMQP%d0.1.0.0 --------->
> (over SASL secured connection)
>                             <--------- AMQP%d0.1.0.0
> open --------->
>                             <--------- open
> {code}
> Proton should by default disable sending pipelined OPEN frames for ANONYMOUS 
> logins, to aid compatibility with other components that don't expect/handle 
> such behaviour.

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