I question the usefulness of path-coupled signalling beyond a certain
point in the network. Dean Anderson voiced them pretty well in the
original thread about RSVP -- it just doesn't seem to make any sense
beyond a very closed environment (like the first hop) -- and in that
case, you should be able to use another kind of signalling as well.
If we don't learn anything of the mistakes we did with RSVP, we're
bound to repeat them.
First, we have to agree on what the mistakes were :-)
No. Here's one counter-argument. Enterprise networks tend to have
dumb-bell topologies, where the bottleneck links are in the interior
rather than at the edges (exactly the opposite of serivce provider
networks). They have meshes of 100meg+ all over each site, but the
sites are interconnected with some service (like MPLS VPN, frame relay,
etc.) which is expensive and often with very limited bandwidth
(sometimes sub-T1). These links are not necessarily all that easy to
identify in the topology, because for reliability enterprises configure
mulitple routers and backup links. There is strong demand for
flow-granularity admission control on such links, and an end-to-end
model works better because site-to-site flows can swamp both the uplink
at one end and the downlink at the other end.
For example, has the design clearly restricted itself to the first or
the last hop, or within the first couple of hops?
There are other useful examples which I can share if people are
What about discovery of the furthest point. Do you not find that a
persuasive use case?
For that purpose, I'm not 100% sure if you would need a path-coupled
signalling. You'll certainly want path-coupled signalling for
signalling with a much wider "span" (because it's the simplest way to
do it from the host's perspective), but I'm arguing (as Dean was) that
this isn't an interesting approach from the network operators'
This assume edge tree topologies, which are common but hardly
As for the alternatives:
1) for first-hop only, there's really little need for a router alert,
any protocol would do, as you already know who your routers are :-)
2) for hops beyond the first-hop router, I'd consider setting up a
single server which would be responsible for brokering the QoS
capabilities, firewall holes, etc. You contact the server, and ask it
to open a certain kind of hole, set up certain QoS between (source,
destination), etc. There are a number of options how you could
discover this kind of system:
a) a DHCP option
b) a DNS lookup (e.g., SRV record based on your DNS search path)
c) asking the upstream router using protocol learned in 1).
These kind of approaches essentially move the intelligence and
processing to specific nodes who are better capable of handling such
requests, having policy who can request what, removing the processing
and cruft from the routers. And the hosts have have much easier time
figuring out whether requesting these capabilities is supported in the
network, instead of spewing a considerable amount of in-band
signalling attempts to the network.
This is hardly a neutral characterization of the tradeoffs.
Pekka Savola "You each name yourselves king, yet the
Netcore Oy kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
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