Subject: Re: Datagram? Packet? (was : APEX)

From: "J. Noel Chiappa" <[email-masked]>
Date: Fri, 27 Sep 2002 11:56:23 -0400

> From: Caitlin Bestler <[email-masked]> 

    > Given the source interface, the *meaning* of an IP header is not
    > supposed to be dependent on the routing tables. ..
    > By contrast, the meaning of an ATM circuit is dependent on the context
    > in which it was established. There is no expectation that there is any
    > meaning to this circuit identifier beyond those imparted when the
    > circuit was created.  

Yes, but that's just a minor engineering decisions, i.e. the use of a
non-global namespace for circuit ID's. It's easy to imagine an ATM-like system
in which circuit ID's are global in scope.

The real crucial *architectural* difference is in the fact that there is
per-flow state, along with the need to set up state before the packets can
flow.

	Noel

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From: Lloyd Wood <[email-masked]>
Date: Fri, 27 Sep 2002 18:19:21 +0100 (BST)

> per-flow state, along with the need to set up state before the packets can

distributing routing information isn't per-flow - it's per-aggregated
destination - but routing state does still have to be distributed
before packets can flow.

'local scope' is just bigger in an IP network, that's all.

L.

<http://www.ee.surrey.ac.uk/Personal/L.Wood/><[email-masked]>

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From: Masataka Ohta <[email-masked]>
Date: Sun, 29 Sep 2002 06:20:59 +0859 ()

Noel;

> Yes, but that's just a minor engineering decisions, i.e. the use of a

That's the essential requirement for networks capable of best effort
service to let all the packets have globally routable addresses.

> in which circuit ID's are global in scope.

RSVP is a case.

> per-flow state, along with the need to set up state before the packets can

RSVP establishes the per-flow state before the packets can flow.

It is just a minor engineering decision to allow optional circuit
switched service over a best-effort-capable network.

							Masataka Ohta

---

From: [email-masked]
Date: Sat, 28 Sep 2002 18:29:20 -0400

> switched service over a best-effort-capable network.

1) I wasn't aware that RSVP caused packets to be routed according to
a flow ID contained in the packet rather than the IP address in the packet.

2) If an intermediate router handling an RSVP connection decides to
die an interesting death, packets can still be sent (assuming things like
multihoming and BGP convergence) and successfully arrive before a new
RSVP setup completes.

It doesn't sound like a circuit-switching scheme, it sounds like a resource
reservation scheme to guarantee sufficient bandwidth.
-- 
				Valdis Kletnieks
				Computer Systems Senior Engineer
				Virginia Tech

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From: Lixia Zhang <[email-masked]>
Date: Sat, 28 Sep 2002 16:21:20 -0700

Lets just get some FACTS straight out

>> RSVP establishes the per-flow state before the packets can flow.

I missed Ohta Son's original post, thanks to Valdis for catching this
incorrect statement.

IP packets can flow anytime.

If/when deployed, the existence of an RSVP reservation is expected to
improve the delivery quality of those IP packets, and the lack or breakdown
of that reservation simply leaves the packets with their regular best effort
delivery.

---

From: Masataka Ohta <[email-masked]>
Date: Sun, 29 Sep 2002 10:03:40 +0859 ()

Lixia;

> IP packets can flow anytime.

Fixing your statement:

	IP packets can be forwarded anytime.

To say "flow" on packets not on flows is incorrect.

							Masataka Ohta

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From: Masataka Ohta <[email-masked]>
Date: Sun, 29 Sep 2002 10:11:13 +0859 ()

Valdis;

> a flow ID contained in the packet rather than the IP address in the packet.

Huh?

In this thread, as Noel said:

: It's easy to imagine an ATM-like system
: in which circuit ID's are global in scope.

the circuit ID does not neccessarily imply special routing.

However, you should also be aware that RSVP is virtually useless
without QoS routing.

							Masataka Ohta

---

From: [email-masked]
Date: Sat, 28 Sep 2002 23:03:32 -0400

> the circuit ID does not neccessarily imply special routing.

If you're not routing based on circuit ID, why are you bothering to have one?

> However, you should also be aware that RSVP is virtually useless

Yes, a protocol to tweak the control of an underlying XYZ is pretty useless
if there isn't an XYZ to tweak...

You're overlooking the basic distinction between a circuit and RSVP - if
something happens along the way to break the previously established circuit,
the circuit is *BROKEN*, and nothing moves until it is either re-established or
re-negotiated. You might want to re-read RFC2205, section 2.3, and ask yourself
what happens to packets in the time between BGP selecting a new route and the
next RSVP refresh packet arriving.  I don't think it includes "send back an
ICMP Host Unreachable even if there's a new route"....