Cisco Cisco Expressway
n
Endpoint A with IP address 10.0.20.3
n
Endpoint B with IP address 64.100.0.20, located on the Internet
Assume that endpoint A places a SIP call towards endpoint B. The call will arrive at the Expressway-E,
which will proxy the SIP INVITE towards endpoint B. The Expressway-E to Endpoint B will then be a
traversal call, which means that the Expressway-E will take both signaling and media, and the packet
carrying the SIP INVITE message will have the following contents as it arrives at the NAT router (the actual
INVITE contents have been simplified for ease of reading):
which will proxy the SIP INVITE towards endpoint B. The Expressway-E to Endpoint B will then be a
traversal call, which means that the Expressway-E will take both signaling and media, and the packet
carrying the SIP INVITE message will have the following contents as it arrives at the NAT router (the actual
INVITE contents have been simplified for ease of reading):
Packet header:
Source IP: 10.0.10.2
Destination IP: 64.100.0.20
SIP payload:
INVITE sip: 64.100.0.20 SIP/2.0
Via: SIP/2.0/TLS 10.0.10.2:5061
Via: SIP/2.0/TLS 10.0.20.3:55938
Call-ID: 20ec9fd084eb3dd2@127.0.0.1
CSeq: 100 INVITE
Contact: <sip:EndpointA@10.0.20.3:55938;transport=tls>
From: "Endpoint A" <sip:EndpointA@cisco.com>;tag=9a42af
To: <sip: 64.100.0.20>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 2825
v=0
o=tandberg 1 2 IN IP4 10.0.10.2
s=-
c=IN IP4 10.0.10.2
b=AS:2048
…
…
…
Figure 3: SIP INVITE arriving at NAT router
In the example above, the SDP (session description protocol) within the SIP payload contains a reference to
the Expressway-E IP address, marked in yellow: c=IN IP4 10.0.10.2.
the Expressway-E IP address, marked in yellow: c=IN IP4 10.0.10.2.
Upon receiving the SIP INVITE packet, the NAT router will rewrite the layer 3 source IP address header
(marked in green: 10.0.10.2) and replace 10.0.10.2 (Expressway-E LAN2 IP address) with its own public
NAT address (64.100.0.10) and route the packet out to the Internet, so that the SIP INVITE message will
have the following contents as it arrives at endpoint B:
(marked in green: 10.0.10.2) and replace 10.0.10.2 (Expressway-E LAN2 IP address) with its own public
NAT address (64.100.0.10) and route the packet out to the Internet, so that the SIP INVITE message will
have the following contents as it arrives at endpoint B:
Packet header:
Source IP: 64.100.0.10
Destination IP: 64.100.0.20
SIP payload:
INVITE sip:64.100.0.20 SIP/2.0
Via: SIP/2.0/TLS 10.0.10.2:5061
Via: SIP/2.0/TLS 10.0.20.3:55938
Call-ID: 20ec9fd084eb3dd2@127.0.0.1
CSeq: 100 INVITE
Contact: <sip:EndpointA@10.0.20.3:55938;transport=tls>
From: "Endpoint A" <sip:EndpointA@cisco.com>;tag=9a42af
To: <sip:64.100.0.20>
Cisco Expressway Basic Configuration Deployment Guide (X8.5)
Page 46 of 56
Appendix 4: Advanced network deployments