Module 1: Foundations

NAT Traversal for SIP and RTP — The Core Problem

Learn about nat traversal for sip and rtp — the core problem

Duration: 8 minutes
Level: Intermediate
Lesson: 29

The SIP NAT Problem: Private IPs in Public Signaling

SIP was designed before NAT became ubiquitous. The protocol carries IP addresses inside the message body — addresses that become invalid when NAT rewrites headers.

Where Private IPs Hide in SIP

INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/UDP 192.168.1.100:5060;branch=z9hG4bK123
Contact: <sip:alice@192.168.1.100:5060>
Content-Type: application/sdp

v=0
o=alice 0 0 IN IP4 192.168.1.100
s=-
c=IN IP4 192.168.1.100
t=0 0
m=audio 10000 RTP/AVP 0 8

Three places with private IPs:

  1. Via header — where responses should be sent
  2. Contact header — where future requests should be sent
  3. SDP c= line — where RTP media should flow

When this crosses NAT, the server sees 192.168.1.100 — which is unreachable from the internet. Responses go nowhere. Media goes nowhere.

Why NAT Breaks RTP: The Media Problem

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