Re: Re: many thanks for ur reply there few more

Steve,

Steve Hill wrote:                                (Thu, 28 Jun 2007 09:28:42)
> On Thu, 28 Jun 2007, Brian F. G. Bidulock wrote:
> 
> > messages).  SIGTRAN uses IP which has an minimum MTU of 536 (no, its larger,
> > someone help me here) but is normally larger.  The point is there is never
> 
> The minimum MTU for IPv4 is 576 octets.  For IPv6 is has been increased to
> 1280 octets.

Thanks.
> 
> Usually an MTU of 1500 is used, which is the maximum ethernet payload
> length (although there is a trend towards larger, so called "jumbograms"
> on gigabit and 10 gigabit ethernet) although connections that are
> tunnelled over another protocol will often have a lower MTU.
> 
> > any reason for SIGTRAN to break an SS7 message into multiple chunks, so you
> > can assume one DATA Chunk header overhead per SS7 message.
> 
> It should be noted that this is not strictly true for all SIGTRAN
> protocols - SUA, for example, can carry messages much longer than a single
> SS7 message.  (I realise this conversation was primarilly about M3UA/M2PA,
> but your statement was a little ambiguous so I am just clarifying it)

You clipped the point: narrow-band SS7 messages are 272 octets or
smaller so SIGTRAN never needs to break them across multiple DATA
Chunks.

Also, I do not think that you clarified it, as there is no 272 octet
limit for M3UA, M2UA nor M2PA either.  The 272 octet limit is imposed by
the narrow-band SS7 network (or by the expectation of the user part).
See for example:

SUA: RFC 3868/4.7.1: (Note that this also includes IPSP.)

 4.7.1.  Segmenting / Reassembly

   When it is expected that signalling messages will not fit into a PDU
   of the most restrictive transport technology used (e.g., 272-SIF of
   MTP3), then segmenting/reassembly could be performed at the SG, ASP
   or IPSP.  If the SG, ASP or IPSP is incapable of performing a
   necessary segmentation/reassembly, it can inform the peer of the
   failure using the appropriate error in a CLDR or RESRE/COERR message.

M3UA: RFC 4666/1.3.2.1:

1.3.2.1.  Support for the Transport of MTP3-User Messages

   ...

   The M3UA layer does not impose a 272-octet signalling information
   field (SIF) length limit as specified by the SS7 MTP Level 2 protocol
   [7,8,9].  Larger information blocks can be accommodated directly by
   M3UA/SCTP, without the need for an upper layer segmentation/
   re-assembly procedure as specified in recent SCCP or ISUP versions.
   However, in the context of an SG, the maximum 272-octet block size
   must be followed when interworking to a SS7 network that does not
   support the transfer of larger information blocks to the final
   destination.  This avoids potential ISUP or SCCP fragmentation
   requirements at the SGPs.  The provisioning and configuration of the
   SS7 network determines the restriction placed on the maximum block
   size.  Some configurations (e.g., Broadband MTP [19,20,22]) may
   permit larger block sizes.

ETSI TS 102 142 sets the maximum size to 272, regardless of this
passage.

M2UA: No limit on message size.  Any limits are set by the remote
signalling link.

M2PA: No limit on message size.  Any limits are set by the "applicable
standard".

So, in the context in which it was stated: there is no need for SIGTRAN
to ever break a *narrow-band* SS7 message into more than one DATA chunk.

I didn't want to confuse the student with further narrow-band vs.
broadband distinctions, but now it's too late. ;)

--brian

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