Subject: [Tsvwg] SUA: Fragmentation of outgoing message from ASP.

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Sat, 10 Nov 2001 19:57:03 -0600

John,

It has come up on TSV that SCTP is incapable of fragmenting
messages on partial write (see recent thread on TSV subject
"SCTP: using streams"), but must wait until the entire message
has been presented before sending the first fragment.

For IPSP sending CLDT or ASP sending SUA CLDT that convert to
LUDT at the SG, this has serious impact on latency.  The only
way around this SCTP deficiency is to make the ASP and IPSP
responsible for segmenting CLDT.

This is contrary to the current recommendations in Section 4.7.1
which state:

  The SGP SHOULD segment CLDT into XL/UDT.  However, if the SG
  is acting as an STP, the ASP SHOULD include the Segmentation
  parameter and the ASP MAY segment CLDT.

  Additionally, ASPs MAY segment CLDT message, based upon knowledge
  of segmenetation size supported at the SG(s) to which the ASP
  has routes.

  The IPSP SHOULD NOT segment N-UNITDATA into CLDT.

Considering SCTP's limitations in this regard (serious impact
to latency on large messages) and that reassembly and resegmentation
is not possible (again without serious impact on latency), these
passages in Section 4.7.1 need to be changed to:

  The ASP SHOULD segment CLDT message, based upon knowledge
  of segmentation size supported at the SG(s) to which the
  ASP has routes.

  The IPSP SHOULD segment N-UNITDATA into CLDT, based upon
  knowledge of the path MTU of the SCTP associations to other
  IPSPs.

  The SG SHOULD return an SUA error indicating that a segmentation
  error occured if it receives a CLDT message that would require
  further segmentation.

The former text would have been applicable if TCP were being used
as the transport.

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

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