Implement Full RFC 4028 Session Timer Compliance

[thorsager]

Summary

We have a simplified session timer implementation that adds Session-Expires: <seconds>;refresher=uac headers to outbound trunk INVITEs and tears down calls when the timer expires. This issue tracks the remaining work needed for full RFC 4028 compliance.

Current Implementation

• internal/trunk/config.go: SessionExpiresSec int field on Trunk
• internal/b2bua/handler.go: Adds Session-Expires header in handleTrunkInvite, starts a timer goroutine in handleTrunk200OK via trunkSessionTimer
• Server always acts as refresher=uac and only tears down on expiry
• No Min-SE handling, no UPDATE/re-INVITE support

Required for Full Compliance

1. Min-SE Header Negotiation

• Parse incoming Min-SE header and adjust Session-Expires accordingly
• If the peer requests a minimum that is larger than our configured value, respond with 422 (Session Interval Too Small) and include Min-SE in the response (RFC 4028 §5)

2. refresher=uas Support

• Allow the peer to act as the session refresher
• When refresher=uas is received, the server must respond to re-INVITEs or UPDATEs that refresh the session
• Track the refresher role in the Call struct

3. UPDATE Method Handling (RFC 3311)

• The current SIP parser already defines SIPMethodUPDATE but no handler processes UPDATE requests
• Must handle UPDATE for session refresh when the peer is the refresher
• Respond to UPDATE with 200 OK, reset the session timer

4. Session Interval Too Small (422 Response)

• If a peer sends Session-Expires with an interval below our Min-SE, respond with 422 and include acceptable Min-SE
• Handle 422 responses from peers and retry with a larger interval

5. Per-Call Session Timer Tracking

• Store session timer state in the Call struct (or a new SessionTimer struct)
• Track: interval, refresher role, last refresh time, timer goroutine reference
• Reset the timer on any mid-dialog request or response (INVITE re-negotiation, UPDATE, 200 OK)

6. Consistent Timer Reset

• Any mid-dialog SIP message (re-INVITE, UPDATE, PRACK, INFO) should reset the session timer
• Current implementation uses a fixed time.After that ignores mid-dialog activity

7. Re-INVITE Handling for Session Refresh

• When the server is the refresher, it must send re-INVITE or UPDATE before the session expires
• Handle re-INVITE responses and reset the timer accordingly

References

• RFC 4028 — Session Timers in the Session Initiation Protocol (SIP)
  • §4: Overview of session timer mechanism
  • §5: UAC behavior (generating Session-Expires, handling 422)
  • §6: UAS behavior (processing Session-Expires)
  • §7: Proxy/B2BUA behavior
  • §8: Refresher role determination
  • §9: Formal syntax
• RFC 3311 — The Session Initiation Protocol (SIP) UPDATE Method
  • Defines UPDATE for mid-dialog session parameter updates
• RFC 6026 — Correct Transaction Handling for 2xx Responses to re-INVITEs