Subject: [NAT] question about <draft-ietf-nat-interface-framework-03.txt>: BIND versus session

From: "Wang, Cliff" <[email-masked]>
Date: Mon, 25 Jun 2001 18:07:48 -0000

<draft-ietf-nat-interface-framework-03.txt> introduces the concept of
"BIND":

3.2. Address (and Transport-ID) BIND Descriptor

   Hereafter, the term BIND will be used in place of BINDing, for ease
   of use. BIND descriptor is specific to NAT service. These BINDs 
   can be static or dynamic. When external agents do not intervene, 
   dynamic address(and transport-ID) binding is determined by NAT 
   based on the first packet of a session, as described in [Ref 1].  
   Address binding is between an address in private realm and an 
   address from external realm. Transport-ID BIND is an extension of
   the same concept to the tuple of Address and transport ID (such as
   TCP/UDP port no.).

and also the concept of "session":

3.3. Session descriptor

   NAT device maintains soft-state for the sessions it tracks. 
   These states may be created dynamically during NAT operation
   and are referenced for translation of packets pertaining to the 
   session. The entries maintained in a session state are specific
   to the service type supported (e.g., Basic NAT, NAPT, twice-NAT 
   etc.). For instance, twice-NAT service will require two BINDs,
   unlike other NAT variations which use just one BIND.

Now I am trying to understand the relation between the two.

It seems to me a BIND is just either an address mapping or address+port
mapping, 
while a session refers to a NAT session, which includes user traffic
definition
and NAT service.

Questions:
1) Does it make sense to have a dynamic BIND exist without a session?
This seems to be odd. A dynamic BIND seems to be created by a user
traffic.

2) Can a dynamic BIND be created and shared by more than one session?
It seems less likely. If a BIND is shared, how can we distinguish the
incoming
packets between two sessions?

If both answers to the above is NO, there I would say a dynamic BIND is
one-to-one
with a dynamic session. Is that right? Am I missing anything? I am trying to
understand the usefulness of creating the concept of BIND.  

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From: Pyda Srisuresh <[email-masked]>
Date: Mon, 25 Jun 2001 12:22:59 -0700 (PDT)

> This seems to be odd. A dynamic BIND seems to be created by a user

It is possible to set up an address binding in anticipation of
a follow-on session. Ex: DNS resolves are often done before an
application kicks in. Also, voice applications have control
session that dynamically set the data session parameters.

> packets between two sessions?

A single BIND can be shared by multiple sessions. Each session is
independent, even if they use the same BIND. For example, a single 
chat user can can have multiple chat sessions, using the same
UDP port with all his/her peers.

> understand the usefulness of creating the concept of BIND.  
Please see my comments above.

cheers,
suresh 

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