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IP over SONET considered harmful?

  • From: Alan Hannan
  • Date: Fri Mar 20 12:38:41 1998

Subject: IP over SONET considered harmful?


  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

  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

  	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.