RFC 6733 say the below about head-of-line blocking and SCTP, the
suggestion of unordered sending being new compared to the RFC 3588.
Until now, all delivery in diameter_sctp has been ordered, and
roundrobin over available streams unless the user passes a stream
identifier in an outgoing diameter_packet record. This commit changes
this to use unordered delivery when there's only a single outbound
stream to choose from.
The special case at capabilities exchange is handled by only starting to
send unordered after the second message from the peer has been received.
(First message after capabilities exchange, as the RFC probably means.)
The special case at DPR isn't handled, since there's no knowing the
order in which a peer will answer: a node that sends DPR while it has
other requests outstanding can't expect that the latter will be answered
before DPR, even if delivery is ordered since incoming requests are
handled concurrently. If it wants a guarantee then it simply has to wait
for answers before sending DPR.
A user can force all delivery to be unordered by specifying
{sctp_default_send_params, #sctp_sndrcvinfo{flags = [unordered]}}
as a config option to diameter_sctp, but in this case there's no
handling of a request being sent directly after CEA since there's no
ordered flag to override the default.
RFC 6733:
2.1.1. SCTP Guidelines
Diameter messages SHOULD be mapped into SCTP streams in a way that
avoids head-of-the-line (HOL) blocking. Among different ways of
performing the mapping that fulfill this requirement it is
RECOMMENDED that a Diameter node send every Diameter message (request
or response) over stream zero with the unordered flag set. However,
Diameter nodes MAY select and implement other design alternatives for
avoiding HOL blocking such as using multiple streams with the
unordered flag cleared (as originally instructed in RFC 3588). On
the receiving side, a Diameter entity MUST be ready to receive
Diameter messages over any stream, and it is free to return responses
over a different stream. This way, both sides manage the available
streams in the sending direction, independently of the streams chosen
by the other side to send a particular Diameter message. These
messages can be out-of-order and belong to different Diameter
sessions.
Out-of-order delivery has special concerns during a connection
establishment and termination. When a connection is established, the
responder side sends a CEA message and moves to R-Open state as
specified in Section 5.6. If an application message is sent shortly
after the CEA and delivered out-of-order, the initiator side, still
in Wait-I-CEA state, will discard the application message and close
the connection. In order to avoid this race condition, the receiver
side SHOULD NOT use out-of-order delivery methods until the first
message has been received from the initiator, proving that it has
moved to I-Open state. To trigger such a message, the receiver side
could send a DWR immediately after sending a CEA. Upon reception of
the corresponding DWA, the receiver side should start using out-of-
order delivery methods to counter the HOL blocking.
Another race condition may occur when DPR and DPA messages are used.
Both DPR and DPA are small in size; thus, they may be delivered to
the peer faster than application messages when an out-of-order
delivery mechanism is used. Therefore, it is possible that a DPR/DPA
exchange completes while application messages are still in transit,
resulting in a loss of these messages. An implementation could
mitigate this race condition, for example, using timers, and wait for
a short period of time for pending application level messages to
arrive before proceeding to disconnect the transport connection.
Eventually, lost messages are handled by the retransmission mechanism
described in Section 5.5.4.