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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical 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
SONET FOR EVERYONE. Firmly.
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:
ATM NETWORK:
-----------
TTL_DECREMENTS == (NONCOREROUTERS + TRANSIT_ROUTERS) * 2
IP NETWORK:
----------
TTL_DECREMENTS == (NONCOREROUTERS + TRANSIT_ROUTERS) * 2 + TRANSIT_HUBS * 2
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
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