scope:

NAT traversal (NAT-T) support by ITU-T H.248 media gateways (MG) is an established capability for such network elements positioned in the IP infrastructure of next generation networks (NGN) and IP multimedia subsystem (IMS) networks. In [ITU-T H.248] there are several methods for NAT-T, for example:

- [ITU-T H.248.37] IP NAPT traversal package; and

- [ITU-T H.248.50] NAT traversal toolkit packages.

However, these do not define any methods for NAT-T in peer-to-peer (P2P) services, i.e., for ITU-T H.248 MGs located within the end-to-end P2P IP path providing NAT traversal support for remote NAT devices (also located in the P2P IP path).

In scope of this Recommendation are NAT-T techniques as detailed by [b-IETF RFC 5128]. 

Problem statement

The problem of providing NAT-T in P2P is described in [b-IETF RFC 5128]. However, this IETF RFC does not consider scenarios where ITU-T H.248 gateways may be located in the end-to-end IP paths of a P2P service.

The ITU-T H.248 MG may provide help for NAT-T, e.g., by

- the capability of reporting observed IP transport addresses according to the [ITU-T H.248.37] address reporting package; or/and

- a bearer-level application level gateway (ALG) function according to [ITU-T H.248.78].

However, the usage of either or both is conditional, dependent on: network architecture, IP applications, protocol awareness by ITU-T H.248 MG, remote NAT behaviour, etc.

It should also be noted that a bearer-level application layer gateway (ALG) itself may not be sufficient for NAT-T scenarios in scope of this Recommendation.

The bearer-level ALG might be a beneficial and effective NAT-T support function. However this Recommendation provides NAT-T methods for scenarios without the usage of [ITU-T H.248.78]. This is in order to respect the principle of "network unawareness" (i.e., "L4 payload agnostic") concerning the (bearer-level) IP application protocol in P2P scenarios.

Example network applications

Example network applications are for instance, ITU-T H.248 gateways located between different IP networks (so called ITU-T H.248 border gateways) which support over-layered peer-to-peer services.