1. Session Border Controllers – use with caution Henning Schulzrinne Dept. of Computer Science Columbia University (SIP Summit 2005, Honolulu, Hawaii)

2. Overview SBCs as transient phenomena? Motivations SBC – the cost of convenience

3. Network evolution SBC earlier: email, IM only IP-level (with filter)

4. High-level motivations Why application-layer elements in SIP that are not quite proxies? SMTP has various MTAs, but they are just MTAs (e.g., spam filter) Guesses: media vs. control separation good idea in theory, harder in today’s limited-functionality Internet see Asterix, Skype proxy model of no content (SDP) inspection or modification too limited CALEA (needs to be invisible) charging for services not an issue for email and web

5. Motivations Short term (hopefully) SIP “fix up” or “dumbing down” brute-force NAT traversal Long term needs fire wall control billing enforcement

6. The dangers May not be present in all instances SBCs are a box description, not a function description Lack of visibility cannot tell where SBC is located hard to diagnose failures see HTTP “transparent proxy” experience one example: TP thought SIP was HTTP hard to address content cryptographically to such box Lack of transparency not all features make it through SBC header support copying content routing loops Lack of security Inherent conflict between need for media session inspection and session privacy Session description modification removes accountability Lack of scalability