To add bodies or not?

1
To add bodies or not?that is the question

• Rohan Mahy
• rohan_at_cisco.com

2
Typical Applications

• Logging
• Bandwidth / Media / Codec Policy
• Cooperative NAT and Firewall Traversal
• Request History
• Location Conveyance

3
Explicit Policy Fetch
atlanta.com
biloxi.com
Alice
Bob

• Works great when policies dont depend on who you
  call, or dynamic properties like load.
• Obviates the need to mucking with typical INVITE
  flow much of the time. Still need another
  solution.

4
Full Redirect Model
atlanta.com
biloxi.com
Alice
Bob

• Minimal session policy possible
• Doesnt work at all through middleboxes
• Doesnt work with the GRUU mechanism

5
Triangle Redirect Model
atlanta.com
biloxi.com
Alice
Bob

• Most preferred model when allowed by policy
• Incompatible with policy requirements of many
  organizations

6
Trapezoid Redirect Model
atlanta.com
biloxi.com
Alice
Bob

• Adds lots of extra RTTs
• Unclear what Alice is consenting to and how she
  can authorize the inclusion of arbitrary opaque
  data if this implies her consent
• Reveals information potentially private between
  Bob and biloxi.com

7
Foreign Piggyback Model
atlanta.com
biloxi.com
Alice
Bob

• Meets both Alices and Bobs consent requirement
  without leaking Bobs data to Alice
• Fewer RTTs
• Requires Addition of bodies by biloxi.com.
  Backward compatible using repack option-tag
  (more on this later)
• Security is better. Authorization by Alice is
  simple
• Can also address AORContact correlation problem

8
Full Piggyback Model
atlanta.com
biloxi.com
Alice
Bob

• Doesnt permit Alice to consent to
  modifications/insertions made by atlanta.com

9
Adding Bodies SafelySecure and Backwards
Compatible

• biloxi.com may only add a body to a request when
  retargeting to a UAS registered in the biloxi.com
  domain (for example Bob). Never responses.
• Any additions are always marked as added-by
  biloxi.com. Biloxi either signs its additions
  with S/MIME or forwards them directly over TLS
  to Bob
• Bob includes an option-tag in a REGISTER to
  indicate it supports body repacking.
• Q Is this secure? See the ContactAOR
  correlation problem

10
Contact Correlation Problem

• How does Alice know that ltsipline2_at_17.18.32.4gt
  (a contact) corresponds to ltsipbob_at_biloxi.comgt
  (an AOR)?
• Not really a problem in a triangle topology.
  Slightly problematic in a trapezoid if either
  user is roaming. (Alice is using what appears to
  be a hotel lobby wireless network with a
  mandatory SIP proxy. No way to automatically
  judge trust of this proxy)
• Obvious solution is request history. Proxies that
  retarget, provided signed cookie trail to the
  eventual Contact.
• Works with proposal to add/repack bodies