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RE: IP over SONET considered harmful?

  • From: Bill St. Arnaud
  • Date: Fri Mar 20 13:48:45 1998


For the very reason you have identified of TTL decrement on hop to hop links
on IP over SONET we are in the process of building an optical Internet where
we will use WDM cut through (and maybe SONET label switching) to provide the
best performace possible.

With WDM cut through there are absolutely NO switch or router latencies.

For more information please see the CANARIE web site at


Bill St. Arnaud
Director Network Projects
[email protected]
+1 613 660-3497

> -----Original Message-----
> From: [email protected] [mailto:[email protected]]On Behalf Of
> Alan Hannan
> Sent: Friday, March 20, 1998 2:26 AM
> To: [email protected]
> Subject: IP over SONET considered harmful?
> Subject: IP over SONET considered harmful?
>   Perhaps.
>   I am concerned about the growing movement towards IP over SONET.
>   Previously in my career I was a vocal advocate of IP over ATM for
>   several reasons, primarily traffic engineering and statistical
>   gathering ability (obvdisclaimer, this required an autonomous
>   unshared network used only by the ip provider for interhub
>   traffic).
>   However, I am firmly rooted in the bandwagon advocating IP OVER
>   Accordingly, I am concerned about the visible L3 hop inherent to
>   packets transiting routers.
>   An ATM core is, of course, invisible to L3; so the number of
>   switches or hubs through which a packet travels is inconsequential
>   to the TTL of the packet.
>   When a backbone is constructed with a PACKET over SONET core, the
>   packet is likely to decrement the TTL by 2 at every hop.  The
>   number 2 is assumed because you are likely to leave from a router
>   different than the one you come in.
>   Since I tend to think in formulas, I'll encourage you to do so as
>   well.
>   	Variable		Meaning
> 	--------------		---------------
> 	ROUTER			L3 device which decrements
> 				the ttl of an IP packet
> 	TRANSIT_HUBS		The number of hubs which neither sources
> 				nor delivers the packet
> 	NONCOREROUTERS		The number of routers which accept
> 				or deliver traffic to a peer or customer
> 	TRANSIT_ROUTER		A router which transits the packet
> 	TTL_DECREMENTS		The number of ttl counters which
> 				this network decrements
>   Assuming an architecture with dual core routers and two layers of
>   hierarchy (backbone v. customer aggregation/peering), I believe
>   the following formulae dictate the TTL degredation expected:
>   -----------
>   ----------
>   Another assertion I would make is that a 'responsible' NSP should
>   decrement no more than 1/4 of the TTLs in the least common denominator.
>   This follows from a general assumption of 2 NSPs, and 2 Customers;
>   hence 4 entities.
>   I consider Windows 95 to be the least common denominator, which
>   has a default IP TTL of 32.  Yes, 32.  So that implies that each NSP
>   should decrement no less than 8 TTLs.
>   Solving IP NETWORK for TTL_DECREMENTS=8 implies that a network can
>   have a diameter of no more than 4 hubs.  That's a pretty meshed
>   network when you have more than a few hubs.
>   Does anyone have any strong opinions or sources on this matter to
>   alleve my fears?
>   The only solution I see is to fix mswindows; but of course that is
>   quite difficult.
>   I'd hoped that MPLS would solve this problem, but from reviewing
>   the drafts I believe that the LSRs _WILL_ decrement the TTL.
>   Your comments appreciated.
>   -alan