Subject: [ippm] TWAMP Remote Session-Sender Initiation

From: "WALT STEVERSON" <[email-masked]>
Date: Mon, 6 Aug 2007 22:42:49 -0500

Would there be any interest in defining a way for a remote client to
request that a Session-Sender initiate a test session to a
Session-Reflector and then gather the results of that test?  Right now a
Control-Client can effectively negotiate the configuration and starting
of a Session-Reflector.  Why not allow a Control-Client to also
negotiate the configuration and starting of a Session-Sender?  Two (or
more) test endpoints could be placed in a network and a central agent
could open TWAMP-Control connections to both endpoints, configure one as
the Session-Sender, and the other as the Session-Reflector.  This might
involve modification to the TWAMP Request-TW-Session message behavior
(keying off the Conf-Sender/Conf-Receiver fields?) or maybe a completely
new message type.  When a test completes the central agent would
probably want to gather the results.  This might mean reinstating the
Fetch-Session and Fetch-Ack messages in the TWAMP-Control protocol so
that all the results that are collected by the Session-Sender could be
sent back to the central agent for further processing.  Of course that
would require the OWAMP Fetch-Ack message format to be extended to
include Sender Timestamp, Sender Error Estimate, and Sender Sequence
Number information in each packet record.

Expanded Role Diagram:

  +----------------+                +-------------------+
  | Session-Sender |<--TWAMP-Test-->| Session-Reflector |
  +----------------+                +-------------------+ 
          ^                                  ^
          |                                  |
          |                                  | 
          v                                  v
  +----------------+                 +----------------+
  | Control-Server |                 | Control-Server | 
  +----------------+                 +----------------+
        ^  ^                                    ^
        |  |                                    |
        |  |                                    |
        |  +--TWAMP-Control--+  +-TWAMP-Control-+ 
        |                    |  |
        |                    |  |
  TWAMP-Control              |  |
        |                    |  |
        |                    |  |
        v                    v  v
  +--------------+     +----------------+ 
  | Fetch-Client |     | Control-Client |
  +--------------+     +----------------+

Collapsed Role Diagram:

  +----------------+                +-------------------+
  | Session-Sender |<--TWAMP-Test-->| Session-Reflector |
  | Control-Server |                | Control-Server    |
  +----------------+                +-------------------+ 
          ^                                  ^
          |                                  |
          |                                  | 
    TWAMP-Control                            |

          |                                  |
          |                                  |
          |     +------ TWAMP-Control -------+
          |     |
          v     v
  +----------------+

  | Control-Client |
  | Fetch-Client   |
  +----------------+

This behavior could be made purely optional if it used the existing
TWAMP Request-TW-Session message and any Control-Server that did not
support this functionality could just refuse the session in its
Accept-Session message (accept field == 3, "Some aspect of request is
not supported"). 

The argument could be made that it is possible to setup an equivalent
remote bidirectional test with OWAMP using simultaneous one-way test
tests so why should TWAMP be expanded to do it too?  In the abstract
section of the TWAMP draft it says that OWAMP does not accommodate
round-trip or two-way measurements.  Also, the Reflector could still be
a TWAMP Light reflector and then the burden of implementing
TWAMP-Control protocol resides in the endpoint that serves as the
Session-Sender (see diagram below).

TWAMP Light Diagram:

  +----------------+                +-------------------+
  | Session-Sender |<--TWAMP-Test-->| Session-Reflector |
  | Control-Server |                |                   |
  +----------------+                +-------------------+ 
          ^                                   ^
          |                                   |

    TWAMP-Control                             |

          |                                   |
          |                                   |
          |     +-----------------------------+
          |     |
          v     v
  +----------------+

  | Control-Client |
  | Fetch-Client   |
  +----------------+

Something similar could also be accomplished with SNMP MIBs for
configuring TWAMP Session-Senders and for gathering the resulting test
measurements.  However, this is so similar to the existing OWAMP-Control
behavior (ie. Fetch-Session, Fetch-Ack, Request-Session) that, in my
opinion, it seems more natural to extend the TWAMP-Control protocol to
support this functionality.

Regards,
Walt Steverson

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