Cisco Systems Linksys PAP2, RT31P2 1.2. Network Address Translation (NAT) Traversal

If the service provider is offering origination and/or termination on endpoint equipment then it is very

likely that the softswitch chosen for network operations will support multiple PSTN and VoIP signaling

protocols.

The table below lists the most commonly accepted, de-facto standards used when implementing a

VoIP signaling scheme based on the type of gateway or endpoint equipment being deployed:

VoIP Equipment Type Typical Port Density De-Facto Signaling Standards

Trunking Gateways Greater Than 500 Ports H.248-Megaco / MGCP / IPDC

Access Gateways Between five and 500 Ports SIP / H.323

PBX/KTS Platforms Between ten and 500 Ports SIP / H.323 / SCCP

PBX/KTS Telephone Sets One Port SIP / MGCP / SCCP

Phone Adapters and IP Centrex

Phones Up to four Ports SIP / MGCP

The PHONE ADAPTER supports SIP today. It has the capability to communicate with a variety of

endpoints and signaling entities via SIP messages.

1.2. Network Address Translation (NAT) Traversal

A NAT allows multiple devices to share the same external IP address to access the resources on the

external network. The NAT device is usually available as one of the functions performed by a router

that routes packets between an external network and an internal (or private) one. A typical application

of a NAT is to allow all the devices in a subscriber’s home network to access the Internet through a

router with a single public IP address assigned by the ISP. The IP header of the packets sent from

the private network to the public network can be substituted by the NAT with the public IP address

and a port selected by the router according to some algorithm. In other words, recipient of the packets

on the public network will perceive the packets as coming from the external address instead of the

private address of the device where the packets are originated.

In most Internet protocols, the source address of a packet is also used by the recipient as the

destination to send back a response. If the source address of the packets sent from the private

network to the public network is not modified by the router, the recipient may not be able to send back

a response to the originator of the message since its private source IP address/port is not usable.

When a packet is sent from a device on the private network to some address on the external network,

the NAT selects a port at the external interface from which to send the packet to the destination

address/port. The private address/port of the device, the external address/port selected by the NAT to

send the packet, and the external destination address/port of the packet form a NAT Mapping.

The mapping is created when the device first sends a packet from the particular source address/port

to the particular destination address/port and is remembered by the NAT for a short period of time.

This period varies widely from vendor to vendor; it could be a few seconds, or a few minutes, or more,

or less. While the mapping is in effect, packets sent from the same private source address/port to the

same public destination address/port is reused by the NAT. The expiration time of a mapping is

extended whenever a packet is sent from the corresponding source to the corresponding destination.

More importantly, packets sent from that public address/port to the external address/port of the NAT

will be routed back to the private address/port of the mapping session that is in effect. Some NAT

devices actually reuse the same mapping for the same private source address/port to any external IP

address/port and/or will route packets sent to its external address/port of a mapping from any external