One issue:

The chapter IANA considerations mentions four bits to be registered: E,B,U,I. In the main body, only three of those are actually defined and used (End of fragmented message, Beginning of fragmented message, Unordered/Ordered message).
Please add a definition of the I bit or remove the IANA registration request for that bit.

And the rest are only musings not needing any action if the authors don't want to action on them:

The introduction presents a good use case, quoting: "e.g., when the transmission of an urgent
   message is blocked from transmission because the sender has started
   the transmission of another, possibly large, message".

The later queueing example in figure 1 has three SIDs being queued simultanseously. It is not clear which of those "has already started" and which are the important ones being delayed.
For a better understanding of the reader, it might be useful to revise the figure so that the head-of-line blocking happens for SID 1 because transmission of  0 and 2 has already started (and declare SID 1 to be the "important" message). The ASCII art would just have to indent SID 1 content a bit (placing 0 and 2 earlier in time), and the resulting serialisation would then put all the SID 1 messages at the very end, when 0 and 2 have been completely submitted (right?).

In 2.2.3, is there any particular reason why the "protocol violation" error cause is only a MAY? As it enables debugging on the remote end, a SHOULD seems useful.