Subject: [Isis-wg] Why ISIS is run on Data Link Layer??????

From: Nob <[email-masked]>
Date: Tue, 14 Oct 2003 05:33:40 -0700 (PDT)

Hi All,
I am just curious why ISIS is run on Data-Link Layer . As we run on Data-Link Layer we need to take of reliability by ISIS itself............If we use TCP, what kind of problem may arise in case of ISIS......... because then TCP will take care of reliability????
 
TIA

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From: Jeff Learman <[email-masked]>
Date: Tue, 14 Oct 2003 09:31:10 -0400

At 08:33 AM 10/14/2003, Nob wrote:
>I am just curious why ISIS is run on Data-Link Layer . As we run on 
>Data-Link Layer we need to take of reliability by ISIS 
>itself............If we use TCP, what kind of problem may arise in case of 
>ISIS......... because then TCP will take care of reliability????

I can't say with authority since I wasn't there at the time, but I
can make a few observations off the top of my head.

ISIS builds the routing information.  TCP relies on IP, which relies
on the routing information.  It's a reasonable layering of design.
If ISIS then relied on TCP, there would be a circularity problem.
More on this later, when I talk about OSPF's use of IP.

Actually, instead of TCP it would have used TP4, the OSI connection-
oriented transport used over CLNP, since ISIS was designed for OSI
CLNP, not IP.  It's a testament to the extensibility of the protocol
that it works so well for IP.  It's also a good thing that we don't
have to implement TP4 to do ISIS!

Furthermore, the kind of reliability that is required is a multicast
reliability, and there were no reliable transport multicast protocols
in the late eighties, when the protocol was designed.

The fact that the nodes to which the information is distributed are
the very routers relaying the information makes it possible to use
an extremely efficient protocol for disseminating the data and keeping
it reliable.  ISIS is extremely efficient on bandwidth usage and highly
scalable as a result.  I don't believe a transport level protocol
could possibly work as well.

Note that even with transport reliability, ISIS would have additional
reliability requirements that wouldn't be met by TCP or TP4, having to
do with the reliability of the source of information itself, and
therefore would have to have additional reliability rules anyway.
(E.g., the seemingly over-complex rules concerning sequence numbers
and wraparound -- these were driven by an actual case in the Arpanet.)

OSPF doesn't use TCP, by the way.  It does use IP though, and that does
make it easier to adapt to a new link layer.  When ISIS was defined,
there was no clear definition of broadcast CLNP that would have allowed
its use the way IP can easily be used on a broadcast link to talk to
all neighbors.  So the committee would have had to solve that problem
first.

As it turns out, that issue never was worked out in any standardized way
to the best of my knowledge.  One of the advantages of OSI addressing is
that the subnetwork does not have a network layer address.  (A CLNP node
doesn't need multiple network addresses simply because it lives on multiple
subnetworks, leading to a number of advantages.)  However, that advantage
makes it hard to work out network addressing issues when the intent
is to broadcast on a particular local subnet.

If ISIS were designed today and didn't need to support CLNP, we might
be tempted to use IP rather than the link directly.  That would make
adoption of new network protocols (and multiple network protocols)
more difficult, but we tend to add new link protocols more often than
network protocols ;)

Regards,
Jeff

>
>TIA
>
>
>Do you Yahoo!?
><http://shopping.yahoo.com/?__yltc=s%3A150000443%2Cd%3A22708228%2Cslk%3Atext%2Csec%3Amail>The 
>New Yahoo! Shopping - with improved product search

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From: "SelvarajR" <[email-masked]>
Date: Wed, 15 Oct 2003 13:47:58 +0530

The book "Interconnections: Bridges and Routers" by Radia Pearlmann may give
u a better answer.
  -----Original Message-----
  From: [email-masked] [mailto:[email-masked]]On Behalf Of
Nob
  Sent: Tuesday, 14 October 2003 6:04 PM
  To: [email-masked]
  Subject: [Isis-wg] Why ISIS is run on Data Link Layer??????

  Hi All,
  I am just curious why ISIS is run on Data-Link Layer . As we run on
Data-Link Layer we need to take of reliability by ISIS itself............If
we use TCP, what kind of problem may arise in case of ISIS......... because
then TCP will take care of reliability????

  TIA

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