Re: [grobj] Referral definition and its purpose?

> -----Original Message-----
> From: grobj-bounces@ietf.org [mailto:grobj-bounces@ietf.org] 
> On Behalf Of Sheng Jiang
> Sent: Tuesday, May 18, 2010 2:01 AM
> To: grobj@ietf.org
> Subject: [grobj] Referral definition and its purpose?
> 
> Hi, all,
> 
> We are now working on an referral PS draft, targeting to 
> submit end of May
> or the beginning of June. We'd like to discuss the relevant 
> contents/topic
> publicly in the mail list during our writing. Comments or 
> contributions are
> more than welcome.
> 
> The first target is to understand what is referral and its 
> purpose. There
> may be different understanding, our goal here is to find a 
> common definition
> for our PS draft. The following is my understanding and 
> what's in the draft
> now.
> 
> In abstract: The purpose of a referral is to enable a given 
> entity in a
> multiparty Internet application to pass information to 
> another party. It
> enables a communication initiator to aware relevant information of its
> destination entity before launching the communication.
> 
> In the introduction: A frequently occurring situation is that 
> one entity A
> connected to the Internet (or to some private network using 
> the Internet
> protocol suite) needs to inform another entity B how to reach either A
> itself or some third-party entity C. This is known as a referral.
> 
> Does everyone can agree on this Referral definition and its 
> purpose? Only if
> consensus can reach here, our next discussion for referral 
> scenarios can make sense.

The definition of referral should be tightened up slightly
that the parties are exchanging the information via one
channel (e.g., SIP signaling) but want to communicate (as)
directly (as possible) with each other *not* using that 
channel.  I often get asked "if they can communicate referral
information with each other, why don't they just use that
communication channel for everything else?".

In SIP, this is often called the "SIP trapezoid".
We can generalize this a bit resulting in the following
figure:

   +-----------+       +-----------+
   |application|       |application|
   | server(s) +.......+ server(s )|
   +---+-------+       +-------+---+   
      /                         \       
     |                           |
     |   referral information    |
     |                           |
     |                           |
   +-+-+                       +-+-+  
   | A +-----------------------+ B |
   +---+ direct communication  +---+

The application server(s) might be one single server, a distributed hash
table, two servers, or any sort of combination.  The referral data is carried
on bewtween A and B on the top, through those servers.  A and B are trying to
communicate directly with one another, using the communication line at the
bottom.

A simplier model which is good to think about is a simple protocol which
collapses the model above, such as FTP.  FTP follows the same model as above,
except the 'application server' is the FTP server and FTP control channel
(TCP/21) is what carries the referral (PASV, EPSV, PORT, EPRT).  TFTP is
another example of a collapsed model.

   +-+-+                            +-+-+  
   |   |   FTP control connection   |   |
   |   |   (referral information)   |   |
   |   +............................+   |
   |   |                            |   |
   | A |                            | B |
   |   |    FTP data connection     |   |
   |   +----------------------------+   |
   |   |                            |   |
   +---+                            +---+

-d

> Best regards,
> 
> Sheng
> 
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