[bfcpbis] TBD issues #4 and #5: How to pick next sequence number

[Tom Kristensen <[e-mail]>]

Issue with picking sequence numbers and making sure it doesn't cause any 
issues with (late) arriving retransmissions. This may happen, even 
though UDP/BFCP requires just one outstanding transaction at the time.

Gonzalo:
 > Section 6.2
[...]
 > The document says: " Transaction ID values are non-sequential and
 > entities are at liberty to select values at random."
 >
 > The document needs to make it clear what is the requirement and the
 > level of randomness required. For example, what happens if the same
 > transaction ID is reused and a retransmission of the message that
 > first used that transaction ID arrives?

Tom:
| No real random numbers are needed. This issue is solved in the
| upcoming version of the draft.
| However, we need a safe scheme to pick sequence numbers to avoid
| late arriving retransmissions cause confusion).


Gonzalo:
 > Section 8.1
 >
 > "The client MUST set the Transaction ID value in the common header to
 > a number that is different from 0 and that MUST NOT be reused in
 > another message from the client until a response from the server is
 > received for the transaction."
 >
 > See my comment above about reusing transaction ID values and the risk
 > of receiving retransmissions of the original message.

Tom:
| OK. This is kept from RFC 4582 using TCP (and no retransmissions). So, the
| idea should be to keep the RFC 4582 semantics for TCP transport and
| introduce some sort of "sliding window sequence number" reuse or simply
| add a sequential/wrapping sequence numbering for UDP as transport.


So, what do people think?

I feel the simplest here is to at least requring the BFCP entities to select
Transaction ID values in increasing order and a wrap around of the 16 
bit Transaction ID value. (This will require saying something about 
picking the intial value at random, not to close to wrapping and so on - 
I guess. Similar to RTP sequence numbers).

Or is it sufficient to mandate not reusing the X last Transaction IDs 
used by the BFCP entity?
Where X == what?

-- Tom