Subject: RE: your mail

From: [email-masked]
Date: Wed, 3 Jul 2002 08:41:09 +0100

<snipped to the single point>
> is there anything
> wrong with hashing for loadsharing purposes *other* than the fact that
> it makes doing OAM the way you've proposed difficult, if not
> impossible? 
NH=> This is not actually where this all started Eric, and neither is it the
main/only reason I have grave concerns over LDP....in fact, I'd assume it
makes any OAM difficult (even LSP Ping), but please correct if this is a
wrong view.  The underlying question (which Shahram posed some time ago) is
'what network problems does mp2p LDP solve (that cannot be better addressed
using IP or p2p LSPs)?' I never saw any answer that convinced me LDP is
necessary....sure it exists and is out there, but what killer problem is it
there to solve?  It then expanded (various threads, some even prior to this)
that pointed out some of the consequences of LDP for operators.....let me
list a few to remind you:
-	VPN survivability is a function of all cross-VPN traffic and is
temporally non-deterministic on a per customer basis per epoch.  That is,
there is no relationship between a DS codepoint and that pkt's
'survivability priority' vis-a-vis other pkts of same/different DS
codepoints in either the same or other VPNs....and that's a consequence of
like-DS-class merging across VPN populations.
-	because of this hard QoS/availability guarantees per VPN customer
are not possible
-	because of this we are forced over-engineer and hope
-	MPLS introduces new failures modes even in p2p mode (this is the
absolute vs relative addressing issue that I mentioned previously), but mp2p
LSPs create a new richness of fault-management complexity
-	the LDP LSPs track the IGP and so the network is not flexible wrt
traffic sensitivities......hence, it requires hashing functions at P nodes
which not only breaks the MPLS simple label forwarding idea of MPLS but
introduces another level of fault-management to deal with.

So the fault-management issue (which you think is not important given the
tone of your response) is but 1 consequence of LDP.  Similarly, the
load-balancing issue is another consequence of LDP.  And similarly, another
consequence of LDP is the inability to decouple VPN performance/availability
on a *per customer basis*.  So all these aspects are compromised when we use
LDP......and for what compelling advantage(s) over using an IPinIP solution
(or p2p LSPs as/when necessary)?  So please answer that for me so that I can
understand what I and others are clearly missing here that makes LDP so
wonderful that we simply can't afford to live without it?

regards, Neil
<snipped to end>

---

From: Giles Heron <[email-masked]>
Date: 03 Jul 2002 17:09:07 +0000

> list a few to remind you:
[rest snipped]

so let's to go back to Yakov's email of 31st May:

------
Giles,

> 1.  Efficient encapsulation of VPN traffic
> 2.  Ability to run VPN on current hardware
> 
> There are probably other reasons as well...

One of the "other reasons" is straightforward protection against packet 
spoofing.

Yakov.
------

whether these are "killer" reasons to deploy LDP rather than "pure IP"
or not depends on your point of view, but they are certainly reasons.

there are of course others - e.g. the ability to run a BGP free core
(which some consider important for running carrier-grade voice and
Internet over the same PSN infrastructure).

Giles

-- 
=================================================================
Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
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