Session Border Controllers use with caution

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
earlier email, IM
SBC
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 todays
  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