Sequence formation Establish vs EstablishmentAck

[danjmarques]

Section 7.3.1.1 states that

"The Sequence, Context, EstablishmentAck and Retransmission messages are sequence forming. They turn the message flow into a sequenced mode since they have the next implicit sequence number."

The Establish message is not on that list. Is that omission deliberate? If it is, then the client (with a recoverable flow) would need to send a Sequence after sending the Establish message, but before sending the first application message.

In 7.3.1, it says that

"Over TCP – a Client could (emphasis mine) send a Sequence message at the very beginning of the session upon establishment. The counterparty would not use it initially as it is provided in the EstablishmentAck message."

First, is that sentence correct in referring to the EstablishmentAck message, or should it refer to the Establish message?

Second, assuming that sentence was about the Establish message, if the Sequence message isn't required, then the Establish message must have been sequence forming.

Please clarify.

[donmendelson]

Establishment and EstablishAck have optional NextSeqNo but only for a recoverable flow.

I think the sentence in 7.3.1 was intended to be explanatory rather than normative. In my opinion, it is confusing and should be removed in future versions.

[danjmarques]

Is the Establish has a NextSeqNo "sequence forming" as per 7.3.1.1? If it were, this would mean that the counter party could send RetransmitRequest immediately after sending EstablishmentAck. Otherwise, it would need to wait for a Sequence.

This actually appears to be the case, as 4.3.1 states

The server should evaluate NextSeqNo to determine whether it missed any messages after re-establishment of a recoverable flow. If so, it may immediately send a RetransmitRequest after sending EstablishAck.

so I think that the sentence in 7.3.1.1 should be re-written to something like (modifications in bold)

The Sequence, Context, Establish, EstablishmentAck and Retransmission messages are sequence forming (the Establish and EstablishmentAck only if a NextSeqNo is present). They turn the message flow into a sequenced mode since they have the next implicit sequence number."