Subject: Questions about RTP Connection reuse and ARB

From: [email-masked] (Matthew Finlayson)
Date: Thu, 12 May 94 18:58:32 BST

Marcia,

Some more questions about the HPR specification:

Reuse of RTP Connections
========================

In the large diagrams (9-3, 10-2, 10-3 and 10-4) there are decision boxes (A
and B) which give guidance to when RTP connections can be reused and when
Route Setup has to be performed.

There seem to be a couple of possibilities missing from these boxes.

1) Decision box B (in all cases) doesn't take into account the possibility
   that a currently used ANR path may pass through a CNN. If this has happened
   (and the Route-setup required on path-switch bit was set), then a new
   route-setup must be performed (in order for a new route to be calculated
   through the sub-area network).

2) There is no suggestion that HPR can decide to reuse an RTP connection
   after performing Route Setup. This would be desirable, since an HPR node
   may be forced to repeatedly carry out the same route-setup protocol for
   sessions with the same COS in a couple of cases:

   (a) In the case of an NN CP-LU to NN CP-LU session, DS will satisfy LOCATE
       requests internally from the topology information (without sending a
       LOCATE externally), and hence without receiving the NCE of the remote
       CP-LU.

   (b) In the case where a node is performing APPN-HPR boundary function,
       (diagram 10-3) but the received BIND request does not carry the NCE
       of the remote end.

   In both these cases, without the decision to reuse a connection after
   route-setup, multiple RTP connections will be set up to cover the same
   route with the same COS.

ARB Flow/Congestion Control
===========================

A number of points which are unclear in this area.

(1) The specification doesn't seem to explain what variables should be taken
    into account when picking the burst-time for a connection. Indeed,
    none of the examples provided mentions burst-time (although one does refer
    to a burst size of 8000 bits).

(2) Can you clarify how the length of an NLP should be defined for the
    purposes of ARB control? Should the length be that of the Data, that of
    the data plus the THDR or that of the Data, THDR and NHDR. If the latter,
    should the fact that the NHDR will shrink in size during the packet's
    journey through the network be considered.

(3) The specification discusses at length how the Measurement interval, M,
    should be decided (section 7.2.4.4.6). The Formats at the end of the
    document (A.1.6.6) state that the Measurement Interval should always
    be set to 256 milliseconds. Which is accurate?

Reuse of RTP Connections
========================

With a more detailed reading of the Formats section of the specification,
we have discovered that the reverse RSCV is sent in a control vector of the
Routing Information Segment sent on Connection Setup and during a
Path Switch (this answers question 2 of our email dated 20th April). The main
body of the specification should probably also document this issue.

As far as the other outstanding quesions are concerned, SR, RASAP.

Thanks.

==============================================================
Matthew Finlayson                   Data Connection Limited
([email-masked])                  Telephone: +44 81 366 1177
                                    Fax:       +44 81 367 8501
==============================================================