1) SIP clients are responsible for opening and maintaining connections to SIP proxies to allow for bidirectional communication behind NATs. 2) By keeping connections open, clients avoid the significant delays of opening new TCP connections and the even longer delays of discovering connection failures. 3) The SIP proxy uses registration information like the client's UUID and registration IDs to associate multiple registrations from the same device and route traffic over a single open connection.

1. SIP Outbound
Letting the client open the connection
and keep it open.
OUTBOUND
http://edvina.net/sip2012

2. The problem OUTBOUND
SIP
• SIP is bidirectional. Every device is a client and
a server that accepts incoming connections.
• If the client is behind a NAT device, the server
NAT may not be able reach it for incoming calls (or
other messages)
TLS • With TLS, the client needs to connect to the
server (unless the client has a TLS certificate)
• TCP (and thus TLS) sessions takes a lot of
TCP setup time and time to discover a failure.
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

3. The solution OUTBOUND
• Let the client be responsible for
opening one primary and one backup
connection
• Opening connections take time, realizing that they’re down takes even longer
• Use the connection for communication
both ways
• Do quick failover if a connection dies
RFC 5626
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

4. SIP Outbound OUTBOUND
• Open connections at registration to two
separate servers
• Keep them open.
• If needed, use NAT keepalives
NATted network
SIP
SIP
SIP
Location server/Registrar
Ingres proxys
RFC 5626
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

5. TCP connections OUTBOUND
• Opening a new connection
• In windows, takes 19 seconds to fail to setup. In Unix, around
70 seconds. Linux 190 seconds. (Source Geoff Houston)
• Discovering a broken connection
• To discover a failure can take a very long time.
NATted network
SIP
SIP
X SIP
Location server/Registrar
Ingres proxys
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

6. Quoting the RFC OUTBOUND
”The key idea of this specification is that
when a UA sends a REGISTER request or a
dialog-forming request, the proxy can later
use this same network "flow" -- whether this
is a bidirectional stream of UDP datagrams, a
TCP connection, or an analogous concept in
another transport protocol -- to forward any
incoming requests that need to go to this UA
in the context of the registration or dialog.”
Bidirectional flow
SIP
SIP
SIP
Location server/Registrar
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

7. OUTBOUND
NATted network
SIP
SIP
Client initiated connections
SIP
Location server/Registrar
Ingres proxys
• The client is responsible for keeping the connection open
• Clients has a UUID, device identifier that stays the same - ALWAYS!
• The SIP proxy understand (by the UUID and the reg.IDs) that one device
has multiple registrations and will use only one at a time
• The Registrar or Ingres proxy assigns a flow ID that is unique for each
flow
• A dialog stays on one connection until it fails
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

8. OUTBOUND
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bK-bad0ce-11-1036
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=d879h76
To: Bob <sip:bob@example.com>
Call-ID: 8921348ju72je840.204
CSeq: 1 REGISTER
Supported: path, outbound
Contact: <sip:line1@192.0.2.2;transport=tcp>; reg-id=1;
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-000A95A0E128>"
Content-Length: 0
Notice the Contact: header
parameters.
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

9. Finding proxys OUTBOUND
The UA may use DNS SRV discovery or
provisioning to discover the proxys.
One SRV set for edge One SRV set for core
proxys proxys
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

10. Dependencies OUTBOUND
• RFC 3263 - finding edge and core proxys with
SRV/NAPTR records
• RFC 3327 - SIP PATH header
Used for communication between edge proxy
and location server/registrar
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

11. OUTBOUND
SIP outbound
• Makes TLS better • Adds a number of
• Better definition for connections
NAT traversal
support • Not yet implemented
in many devices
• Identifies devices in
a unique way
• Makes TCP/TLS
failover much, much
quicker
+ -
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class

12. This material is part
of the Edvina
Learn more about SIP Master Classes
SIP2012 at
http://edvina.net/sip2012
© C o p y ri g h t 2 0 1 2 E d v i n a A B , S o l l e n t u n a , S w e d e n . A l l ri g h t s re s e r v e d . The SIP Master Class