Determining session synchronism using STSN

The need to resynchronize can be communicated during bind negotiation; two flags in the BIND request are used to determine the requirement to resynchronize and to return the half sessions to the state that existed at the time of session termination.

If both half sessions have terminated normally (that is, neither had any outstanding traffic to handle), the session is restarted as though it were a new session—bind parameters are sent, and no requirement exists that the relative positions of the session partners be maintained, nor that the same half-session names be used. The two flags upon which this determination is made are:
  • Sequence number indicator
    • 1 = Sequence numbers available
    • 0 = Sequence numbers not available
  • Bracket initiation stopped (BIS) indicator
    • 1 = BIS sent
    • 0 = BIS not sent

The following table is a matrix describing the states set by these two flags in relation both to the bind sender (primary half session, or PHS) and the bind receiver (secondary half session, or SHS).

When both half sessions are in a COLD START state, no sequence numbers are available or required to be sent. Session shutdown is such that resynchronization is not required. The bind is negotiated and the session started in accordance with the bind parameters. For a nonnegotiable BIND, some information required by both half sessions is not available until STSN flows exist. Therefore, for a nonnegotiable BIND, STSN is always sent.

Table 1. BIND action⁄response matrix
  Bind receiver (SHS): Numbers not Available Bind receiver (SHS): Numbers available, BIS sent Bind receiver (SHS): Numbers available, BIS not sent (See Table 1)

Bind sender (PHS):
Numbers Not available

COLD
STSN not required
Send RESET to NOBB

COLD
STSN not required
Send RESET to NOBB

COLD/WARM
mismatch
STSN sent

Bind sender (PHS):
Numbers available,
BIS sent

COLD
STSN not required
Sent RESET to NOBB

COLD
STSN not required
Send RESET to NOBB

WARM
STSN sent

Bind sender (PHS):
Numbers available,
BIS not sent
(Table 1)

WARM/COLD
mismatch
STSN sent

WARM
STSN sent

WARM
STSN sent
IMS sets the BIS sent and sequence numbers available flags in the negotiable BIND and BIND response (or PHS and SHS, respectively) as follows:
  • Session cold start following IMS cold start or normal session termination:
    • BIS not sent
    • Sequence numbers not available
  • Session restart
    • Previous session terminated normally following attempted normal termination by IMS:
      • BIS sent
      • Sequence numbers available
  • Previous session terminated abnormally (normal termination by IMS not attempted):
    • BIS not sent
    • Sequence numbers available