Subject: [Isis-wg] LSP Buffersize

From: Jeff Parker <[email-masked]>
Date: Mon, 29 Jan 2001 15:53:52 -0500

IS-IS has a couple of checks to make sure that all routers
in the networks can deal with a common LSP buffersize.  First, 
(some) Hello packets are padded to at least the LSP buffersize,
to verify the MTU on a link.  
	But since the Hello size is the Max(LSP Buffersize, link MTU),
it is possible for two adjacent routers A and B to form an
adjacency, even though A's LSP Buffersize is 512, and B's is 
1492.  Radia has pointed out that this allows routers to verify 
that a next hop has a common link MTU.  

	10589 specifies an LSP TLV 12 that could help spot a
mismatch between LSP Buffersizes.  (See below)  
	Does anyone know of an implementation that uses 
this feature?  

- jeff parker

From page 57, 10589.  

	-	originatingL1LSPBufferSize - the local value 
			for originatingL1LSPBufferSize

	This may appear only once in an LSP with any LSP 
	number.  ....It is recommended that this option 
	be used only in the LSP with LSP number zero.  	

		x CODE - 12
		x LENGTH - 2
		x VALUE - 512-1492
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From: Jeff Learman <[email-masked]>
Date: Mon, 29 Jan 2001 17:05:43 -0500

The buffer size mismatch can be a serious problem in networks
with varying MTU sizes -- the checks in the standard (1992) version
of ISO 10589 are not sufficient to detect the problem, and the
spec does not say what an implementation should do when encountering
the problem.  For a description of this problem and a recommended
solution, see Les Ginsberg's paper,
"http://www.atis.org/pub/sif/arch/ar820310.doc".

I suspect that Jeff is looking at the draft spec for 10589
rather than the standard.  The draft spec (the one available at
the IETF ISIS WG site) has NOT been adopted and MUST NOT be
considered authoratative.  It is only posted on the site for discussion
purposes.  That said, the TLV to find the buffer size mismatch
probably corresponds to Les's recommendation.

Regards,
Jeff

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From: Ran Atkinson <[email-masked]>
Date: Mon, 29 Jan 2001 17:54:03 -0500

At 17:05 29/01/01, Jeff Learman wrote:

>the problem.  

SIDEBAR:
                Due to widespread customer demand, most (all ?) 
major Gigabit Ethernet equipment vendors support a non-standard
jumbo-frame of ~9K.  Most vendors have a user-configuration knob
to enable/disable this.  See RFC-1626 for discussion of why ~9K 
bytes is a particularly interesting frame size.  AFAIK, IEEE 802
hasn't blessed this officially.  My main point is that it is 
no longer a safe assumption that all Ethernets have the same MTU.

Ran