Subject: [midcom] Symmetric NAT definition

From: "Mark Watson"<[email-masked]>
Date: Tue, 14 May 2002 16:15:24 +0100

The definition of Symmetric NAT in the STUN draft states:

Symmetric: A symmetric NAT is one where all requests from the
             same internal IP address and port, to a specific
             destination IP address and port, are mapped to the same
             external IP address and port. If the same host sends a
             packet with the same source address and port, but to a
             different destination, **a different mapping is used.**
             Furthermore, only the external host that receives a packet
             can send a UDP packet back to the internal host. 

Where it says 'a different mapping is used', does this mean a different
external source address/port is used ?

It is characteristic of Symmetric NATs that a different source address/port
MAY be used, but is it known that this is always the case ? I'm not sure
what the rationalle for a NAT designer would be to check that the same
source address/port is not already in use for another mapping for the same
originating host.

If not (i.e. if there is a possibility that a Symmetric NAT may choose a
source address/port that happens to be the same as for another mapping for
the same originating host), then the test to distinguish between Port
Restricted Cone NAT and Symmetric NAT would fail, since it relies on these
being different.

Regards,

Mark

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From: "Sanjoy Sen"<[email-masked]>
Date: Tue, 14 May 2002 12:30:24 -0500

-----Original Message-----
From: Watson, Mark [MDN05:EP10:EXCH] 
Sent: Tuesday, May 14, 2002 10:15 AM
To: [email-masked]
Subject: [midcom] Symmetric NAT definition

Yes.