H.323 and NAT

Annika Kilegran Annika.W.Kilegran at TELIA.SE
Mon Mar 9 04:05:40 EST 1998

Network Address Translation (NAT) is used as a solution to the
problem of few available IPv4 addresses. The idea is to use private
addresses within a domain and when IP traffic leaves or enters the
internal network the address fields are translated between the public IP
addresses and the corresponding private address. A private address is
only associated with a public address when the node communicates
externally. By using this mechanism it is possible to have many more IP
nodes in the private domain then the number of assigned public IP
addresses to that domain. NAT is a defacto solution to connect for big
groups of users.

The combination of NAT and applications exchanging IP addresses in the
protocol (like H.323 does) creates problems.
The applications are either broken, or gets more difficult to run.
There are different solutions for how to be able to run these
application in combination with NAT. We list the
alternatives in the bottom of this mail.

Does anybody on the mailing have an opinion?
For instance, a Gatekeeper might have to interact with the NAT
(co-located), and possibly act as an application gateway.
By the way the question might com up on the DAVIC meeting next week.

Best regards,

Annika Kilegran

Telia Research
S-136 80 HANINGE, Sweden
Tel +46 8 707 55 15
Fax +46 8 707 53 10
Email: annika.w.kilegran at telia.se

The differnt alternatives:
1) Use an Application layer gateway, that translates addresses in the
application protocol. Used today for FTP.
Will work, but will be quite complex for H.323. The Application layer
gateway would typically be co-located with the NAT-box.

2) Use the location of servers, and the configuration of servers and
clients to overcome the problem.
DNS is handled this way today. Seems not to work for H.323.

3) A combination of alternatives1 and 2 should be possible, and may
acheive a lesser complexity than 1.

4) NAT bypass using tunneling over the
local network. The global address is
available in the host and is used in the application, removing the
problem. This kind of solution have been
proposed in contributions to IETF.

5) Skip NAT altogether. Instead use IPv.6, or something like NNAT (No
NAT) for IPv.4. NNAT will completely replace NAT, and uses DHCPv.6 for
assigning global addresses. NNAT remains to be specified for
IPv.4 operation.

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