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 -
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).
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.