Subject: RE: [Isis-wg] do different p2p DataLinkBlockSizes work with IS-IS

From: Jeff Parker <[email-masked]>
Date: Wed, 24 Mar 2004 22:35:41 -0500

10589 makes some strong assumptions about the link layer.  As I recall, one
of the assumptions was that a packed dropped resets the link.  

I think the issue of padding and the 3-way handshake are orthogonal.

There was some verbage about this in 3719, spammed below.  

- jeff parker

6.  Padding Hello PDUs

   To prevent the establishment of adjacencies between systems which may
   not be able to successfully receive and propagate IS-IS PDUs due to
   inconsistent settings for originatingLSPBufferSize and
   ReceiveLSPBufferSize, section 8.2.3 of [1] requires padding on
   point-to-point links.

   On point-to-point links, the initial IIH is to be padded to the
   maximum of

      (1)  Link MTU

      (2)  originatingL1LSPBufferSize if the link is to be used for L1
           traffic

      (3)  originatingL2LSPBufferSize if the link is to be used for L2
           traffic

   In section 6.7.2 e) ISO 10589 assumes

         Provision that failure to deliver a specific subnetwork SDU
         will result in the timely disconnection of the subnetwork
         connection in both directions and that this failure will be
         reported to both systems

   With this service provided by the link layer, the requirement that
   only the initial IIH be padded was sufficient to check the
   consistency of the MTU on the two sides.  If the PDU was too big to
   be received, the link would be reset.  However, link layer protocols
   in use on point-to-point circuits today often lack this service, and
   the initial padded PDU might be silently dropped without resetting
   the circuit.  Therefore, the requirement that only the initial IIH be
   padded does not provide the guarantees anticipated in ISO 10589.

   If an implementation is using padding to detect problems, point-to-
   point IIH PDUs SHOULD be padded until the sender declares an
   adjacency on the link to be in state Up.  If the implementation
   implements RFC 3373 [4], "Three-Way Handshake for IS-IS Point-to-
   Point Adjacencies" then this is when the three-way state is Up: if
   the implementation use the "classic" algorithm described in ISO
   10589, this is when adjacencyState is Up.  Transmission of padded IIH
   PDUs SHOULD be resumed whenever the adjacency is torn down, and
   SHOULD continue until the sender declares the adjacency to be in
   state Up again.

   If an implementation is using padding, and originatingL1LSPBUfferSize
   or originatingL2LSPBUfferSize is modified, adjacencies SHOULD be
   brought down and reestablished so the protection provided by padding
   IIH PDUs is performed consistent with the modified values.

   Some implementations choose not to pad.  Padding does not solve all
   problems of misconfigured systems.  In particular, it does not
   provide a transitive relation.  Assume that A, B, and C all pad IIH
   PDUs, that A and B can establish an adjacency, and that B and C can
   establish an adjacency.  We still cannot conclude that A and C could
   establish an adjacency, if they were neighbors.

   The presence or absence of padding TLVs MUST NOT be one of the
   acceptance tests applied to a received IIH regardless of the state of
   the adjacency.

-----Original Message-----
From: Norbert van Bolhuis [mailto:[email-masked]] 
Sent: Tuesday, March 23, 2004 6:32 AM
To: [email-masked]
Subject: [Isis-wg] do different p2p DataLinkBlockSizes work with IS-IS

Do different p2p DataLinkBlockSizes actually work with
IS-IS (RFC 1195 and/or ISO-10589 v2001 *without* the three-way
handshake p2p extension) ?

What should happen is that Node-A with DLB A and Node-B with DLB B,
(given that DLB A != DLB B) should not get an IS-IS level 1 p2p
adjacency, but I think they do.

The problem is that, suppose DLB A > DLB B, Node A (which correctly
receives the first (padded) p2p IIH from Node B) will send unpadded
p2p IIHs after sending the first one (shich should be padded).
Although Node B couldn't receive the first p2p IIH, it can receive
all subsequent ones since they are not padded anymore.
As a consequence both Nodes establish a p2p adjacency to eachother.

Is the above really true, and is this a problem in the IS-IS standard
or do I miss something.

Thanks in advance for any response/comment.

Norbert van Bolhuis.

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