Subject: Re: [Sigtran] Packing M3UA messages into SCTP ULPDUs

From: "TOLGA ASVEREN" <[email-masked]>
Date: Fri, 7 Feb 2003 11:05:52 +0200

Brian,
> Sigtran,
>
> Where does it say that only one M3UA message may be
> placed into an SCTP ULPDU?
>
> As messages contain the length in the header, can I
> pack two or more M3UA messages in an SCTP ULPDU?
>
> This is so basic, I wonder why it was not mentioned
> anywhere...
If it is not forbidden, shouldn´t it be interpreted as allowed (although I
would guess that most of the implementations are assuming one M3UA message
in one SCTP ULPDU)?
BTW related with this topic:
from 3.1.4
  "The Message Length defines the length of the message in octets,
   including the Common Header.  The Message Length MUST include
   parameter padding bytes, if any.

   Note: A receiver SHOULD accept the message whether or not the final
   parameter padding is included in the message length."
The above paragraphs sound strange to me, including padding bytes in the
length is a MUST but still receiver should be prepared for the opposite.
But still based on that I assume a M3UA peer can receive messages, whose
length does not contain padding bytes (otherwise forget what stands below)
From 3.2
     "The total length of a parameter (including Tag, Parameter Length
      and Value fields) MUST be a multiple of 4 bytes.  If the length of
      the parameter is not a multiple of 4 bytes, the sender pads the
      Parameter at the end (i.e., after the Parameter Value field) with
      all zero bytes.  The length of the padding is NOT included in the
      parameter length field.  A sender SHOULD NOT pad with more than 3
      bytes.  The receiver MUST ignore the padding bytes."
Not to add more than 3 padding bytes is a SHOULD NOT, not a MUST.

For a scenario where message length does not contain padding bytes length
and more than 3 padding bytes are used, I would think receiver can´t detect
boundary between two M3UA messages (but I have to repeat, it sounds strange
to me that including padding bytes length in message length is a MUST and
still receiver SHOULD accept messages, which aren´t prepared in that way)

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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 7 Feb 2003 02:37:20 -0700

Tolga,

It does not even forbid splitting an M3UA message across multiple
SCTP ULPDUs.

I think this should be clarified in the IGs for the sake of
interoperablity.  1 M3UA (M2UA, SUA) message to 1 SCTP ULPDU.

TOLGA ASVEREN wrote:                                                                 (Fri, 07 Feb 2003 11:05:52)
> still receiver SHOULD accept messages, which aren´t prepared in that way)

One could still detect the next message by scanning for the next valid
M3UA message header...

Also, that wouldn't cause any problem for splitting an M3UA message across
several SCTP ULPDUs.

I think we need to state in the IG: 1 UA message, 1 SCTP ULPDU.

==brian

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