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Running SIP behind NAT

Dr. Christian Stredicke, snom technology AG

Tokyo, Japan, Oct 22 th 2002


Overview 5 6 1 2 3 4 Problem Description STUN: Using Legacy Equipment TURN: Fixing Remaining Problems UPnP: Remote Control for Routers Application Layer Gateways Remaining Problems & Solutions 2


3 Which information does a client has to set up for port forwarding in NAT equipment?

• •

Router needs information where to send packets in private network

– Map port to private address and port – By default packets will be rejected or sent to DMZ

Router needs hint for security checking

– Accept packets from any destination – Accept packets only from associated host – Accept packets only from associated host and port Router Client Client


How did other applications solve the problem?

• • • •

HTTP, telnet, …

– Using TCP

DNS, others

– “Digging holes”: Set up association when client sends out packet from unmapped port for 15-60 seconds – Security policy hardwired by vendor – Some offer a DNS proxy (application layer gateway)


– Does not work!

– Inexperienced users use http instead – Some routers offer applications layer gateway

Heterogeneous environment

– Every vendor does it in a different way – “Digging holes” is common denominator



5 snom STUN uses the digging hole trick to set up port associations

Initialization procedure checks environment

– Goal: Check if STUN is needed – Type of NAT does actually not really matter because user is not interested in failure reason •

SIP port kept alive by sending packets every 15-60 s

RTP ports are allocated dynamically when starting a call

– Otherwise keep-alive traffic would be double – RTCP port can not be allocated because next port allocation is unlikely – Long ringing and putting caller on hold is problematic (no port refresh during this time)

6 In cases when NAT is symmetrical, TURN could be a solution V1.0

STUN/TURN Server 3. SIP/Media Router Client Client


7 TURN works in symmetrical NAT environment, but has too many problems

• • •


– TURN server becomes “media server” – Every call generates about 50 packets per second


– Sending packets over media server increases transport delay significantly – E.g. local call in Tokyo when TURN server is in Frankfurt

TURN specification

– Needs rework (activation message not defined)


UPnP is the right approach

• • •

Generic protocol to allocate ports on router

– Works with SIP, can be used with other applications as well – Can be integrated with firewalls – Not too hard to implement

Microsoft Messenger uses UPnP

– “De facto standard” – Virtually all DSL router vendors offer UPnP now

Problem: Old Equipment

– Use STUN – Maybe use TURN, even if call duration is terrible – Instruct customers to set up ports manually



With the increasing availability of UPnP, most home customers can be addressed 2002 2003

STUN STUN UPnP • Software Updates • New Equipment UPnP



10 Application layer gateways (ALG) solve the problem in the business area

Business customers have different requirements than home users

– Many phones – Want to run proxies, media servers, application servers behind their firewall – These applications probably will not have UPnP or STUN •

Therefore, firewalls will probably include SIP-aware ALG

Sample SIP NAT ALG available from snom


Calling phones in the same network requires ancillary information 11

1a) Phone A sends to public address of B 1b) Router will not forward packet, call will fail 2) A knows B is in the same NAT and sends packet to private address instead


Ancillary information must be placed in contact URI and in SDP 12

INVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP;branch=z9hG4bK-6rms4e9tmtsz Max-Forwards: 70 From: ;tag=16z5zw9lqt To: Call-ID: [email protected]

CSeq: 1 INVITE Contact: Content-Type: application/sdp Content-Length: 311 v=0 o=root 19211 19211 IN IP4

s=SIP Call c=IN IP4

t=0 0 m=audio 10004 RTP/AVP 0 101 a=rtpmap:0 pcmu/8000 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-15 a=private:


13 Multi-tier NAT requires a list of private addresses

STUN When using STUN, a STUN server is required between the layers STUN



NAT3 Phone A

A has three identities: 1. 2. 3. Phone B

B has three identities: 1. 2. 3.


What is the status of snom products today?

• •

Latest snom 100 image includes UPnP and STUN

– Support for single-tier NAT – TURN has been put out again

snom 200 integration of UPnP and STUN

• • • •

snom 4S STUN/TURN server

– Includes STUN and TURN (with extensions) – Security and TCP not implemented

snom 4S proxy/location server adds STUN

– Integrated STUN server – Gives user agents hint when they are behind NAT

snom 4S media server

– Smooth call termination when no media available

snom 4S NAT gateway

– Available for Linux


sip:[email protected]

© 2002 snom technology Aktiengesellschaft Written by: Dr. Christian Stredicke Version: 1.0

The author has made his best effort to prepare this document. The content is based upon latest information whenever possible. The author makes no representation or warranties of any kind with regard to the completeness or accuracy of the contents herein and accept no liability of any kind including but not limited to performance, merchantability, fitness for any particular purpose, or any losses or damages of any kind caused or alleged to be caused directly or indirectly from this document.

For more information, mail [email protected], Pascalstr. 10E, 10587 Berlin, Germany.