Subject: Network "edge"

From: Rajesh Talpade <[email-masked]>
Date: Sun, 11 Jan 1998 16:22:30 -0500 (EST)

Solutions for differentiated services seem to indicate a strong
preference for pushing complexity towards the edges rather than
towards the core. But what is exactly the network edge ? If it is
indeed the first-hop router, then what happens when and if 
NHRP gets deployed, and hosts can bypass their (traditional)
first-hop routers to get to the egress router.

Two solutions seem fairly obvious:

a) NHRP wont be around to create this problem :-)

b) it is not sufficient to make "first-hop" routers responsible
for marking bits and policing mechanisms. Intermediate routers
should be able to do this if they get traffic from end-hosts. 

Any comments ?

Rajesh.

-- 
Rajesh R. Talpade    [email-masked]   973 829-{4261, 5888(fax)}
Research Scientist, Bellcore, 445, South St, Morristown, NJ-07960

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From: David Black <[email-masked]>
Date: Mon, 12 Jan 1998 12:47:06 -0500

> should be able to do this if they get traffic from end-hosts. 

Another theme of the discussion is that the details of differentiated services
(how much traffic can be marked and how far the assurances as a result of
marking extend) is likely to be handled by bilateral agreements among the
parties involved.  This leads to the (not quite circular ;-) ) observation that
the network "edge" is whatever the contract between the two parties on either
side of the edge say it is.  For NHRP, this leads to a range of solutions,
for example:
	- Traffic bypassing the ingress router will always be best effort.
	- Certain routers support differentiated services.  Traffic not
		directed through one of these is best effort.
	- All routers support the required marking (seems unlikely in a
		large network, IMHO).
These in turn have implications on how NHRP is configured (e.g., the NHSs may
be configured so that traffic only goes through the routers that are prepared
to mark it).  

Also, what the "core" of the network is depends on where one's viewpoint is.
Some of the discussion envisions marking/unmarking packets at boundaries
withing the network, so that there will be "edges" with some differentiated
service logic in what a customer might perceive as the "core" (e.g., backbone
to backbone interfaces).

--David

============================================================================
David L. Black            THE OPEN GROUP     Voice: +1 (617) 621-7347
The Open Group                                 Fax: +1 (617) 621-8696
Eleven Cambridge Center      RESEARCH       E-Mail: [email-masked]
Cambridge, MA  02142        INSTITUTE         http://www.opengroup.org/~dlb/
============================================================================

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From: [email-masked]
Date: Mon, 12 Jan 98 13:55:26 -0500

Rajesh, 

> first-hop routers to get to the egress router.

A slight technicality ... NHRP clients can be present on hosts or routers, 
hence if the first router is to be bypassed, then the client must be present 
on the source host which will then be the ``edge'' which marks packets for 
appropriate service. However unless you go for one of the many 
``proprietary'' schemes for running NHRP over legacy LANs, most hosts 
will be present on legacy LANs without an NHRP client and hence the first hop
router will not be bypassed.  

Also note that as we move the ``edge'' towards the hosts the number of devices
to be upgraded/ managed in a diff serv aware  network increases rapidly ...

In an MPOA based network a convenient ``network edge'' is the LAN switch/ 
bridge between the host and the first router. Packet marking/ classification 
at this point will allow the first router to be bypassed without 
requiring this functionality to be setup in all hosts ... 

> should be able to do this if they get traffic from end-hosts. 

Not sure what you are suggesting here. 

a. Intermediate routers 
are indeed the first hop router for hosts directly connected to them. 
Typically one doesn't have routers which do not have any hosts (only 
other routers) on directly attached interfaces ...

b. Will differentiated service still be meaningful if the end to end 
path of a packet consists of ``diff Serv aware'' and ``non diff serv'' 
aware islands ? 

Sanjeev.

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From: Rajesh Talpade <[email-masked]>
Date: Mon, 12 Jan 1998 15:23:32 -0500 (EST)

> appropriate service. 

You are not really suggesting that end-hosts be responsible for marking
and policing, are you ? An immediate incentive for end-hosts to
prioritize all their traffic is to grab more than their fair share of _spare_
bandwidth. I assume there is general consensus that marking and policing 
should be entrusted to "first-hop" devices, instead of to end-hosts.

> However unless you go for one of the many 

Ahh, this sounds like solution 'a', viz. nhrp wont exist mostly. 

> requiring this functionality to be setup in all hosts ... 

Diff serv was about marking/policing IP traffic. I am not sure if we
want to make the LAN switch/bridge IP-aware. 

> other routers) on directly attached interfaces ...

You point is true. However, consider an end-host 
with a first-hop router A. Traditionally, this first-hop router
would do the necessary marking/policing for this end-host. The
intermediate router B, which for simplicity reasons can be considered
to be the egress router to the ISP, provides aggregate control. So to 
get to the ISP, the end-host has to go thru router A and then router B. 

But now if the network is NHRP-capable, the end-host can establish a
direct path to router B. This router however is not capable of
policing/marking at the end-host level due to the aggregation effect
that we would like to use as we go away from the network edge. And so we
need to make these intermediate/egress routers aware of individual
end-host flows also, which may affect the scalability of the diff
services scheme.

hope things are clearer now.

rajesh.

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From: [email-masked]
Date: Mon, 12 Jan 98 16:11:50 -0500

Rajesh, 

> should be entrusted to "first-hop" devices, instead of to end-hosts.

I am definitely not suggesting that hosts be responsible for making service
requests (there are several known problems to be overcome with that 
approach including the one you mentioned). I was simply pointing out that 
since you mentioned bypassing the first hop router in an NHRP system (as 
opposed to an MPOA system), the only other entity which *could* (but not 
should) possibly make packet service decisions is the source host itself. 

> > will be present on legacy LANs without an NHRP client and hence the firsthop

err not quite... Even in a full NHRP based network, unless you have a client 
in the source host, IP packets will be processed by the IP forwarder in the 
first hop router *after which* they will get onto the shortcut.  So the first 
hop router is not really getting bypassed but all subsequent ones (except
possibly the ``egress'' one, are) ... So even with NHRP, (except in the 
uncommon case of NHRP clients on ATM attached hosts), the first 
hop router does not get bypassed and can be the point where various packet 
service policies can be applied ...

> want to make the LAN switch/bridge IP-aware. 

In MPOA, the LAN switch is an IP classifier/forwarder, (looks at the IP 
header for forwarding decisions, decrements TTL etc), though it is not an IP
router (does not participate in IP routing protocols), hence it is already 
very much IP-aware. So adding packet service functionality at this point is
quite logical ... 

Sanjeev.

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From: Rajesh Talpade <[email-masked]>
Date: Mon, 12 Jan 1998 16:31:10 -0500 (EST)

> service policies can be applied ...

This definitely sounds to me like we dont have NHRP in the end-host LANs,
which puts it generally under solution 'a' :-). It doesnt matter that NHRP 
exists beyond the end-hosts' LANs. 

Even in your scenario the problem then migrates to the router LANs, as 
again the whole idea of traffic aggregation and heirarchical
marking/policing breaks down due to shortcuts.

Rajesh.

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From: Wu-chang Feng <[email-masked]>
Date: Mon, 12 Jan 1998 16:38:30 -0500

> path of a packet consists of ``diff Serv aware'' and ``non diff serv''

If you're using DP and the bottleneck is in a non-diff-serv island, then the
answer is no.  One of the advantages of allowing the end-host mark packets is
that it can easily detect this case and backoff its marking in response so that
it "gets what it pays for" (assuming for a moment that you're getting charged
extra for sending marked packets).

One technique which does this is an exponential backoff mechanism based on
inter-packet drop times (measured in packets) of marked and unmarked packets.
This does well especially when the bottleneck moves from islands which support
diffserv to islands which do not and vice versa.  It also works for the case 
when the bottleneck supports service differentiation, but an upstream router is
unmarking your packets (in order to maintain service level agreements between
providers).  You could possibly employ this at the first-hop router as well, 
but it wouldn't be very easy to do.

If there is no incentive (such as usage-based pricing) to control marking at 
the end-hosts, then the first-hop router can unmark all incoming packets and 
remark them according to whatever the network admins want.

Wu

(details at http://www.eecs.umich.edu/~wuchang/pmg/)