Subject: Re: Network "edge"

From: Dinesh Verma <[email-masked]>
Date: Tue, 13 Jan 1998 11:58:29 -0500

> >   This begs the question of what is an edge-router. If you define that an
> > edge-router is one that is accessible via a sub-IP layer protocol from a 
> > customer's equipment, NHRP should not introduce any new difficulties for 
> > differential services. The only issue would be that the edge-router would 
> > have to police/mark not only for hosts in its IP subnet, but for all hosts in 
> > all IP-subnets which can use it as the next-hop.  
> 
> The last sentence above is the crux of the matter ! If the edge-router
> is what you defined, much of the aggregation and heirarchical
> requirements for differentiated services to work practically (minimize 
> per flow info state within the network) can go up in smoke ! All potential 
> "edge-routers" may have to do per flow processing, which as we have 
> demonstrated and learnt, is not something we want.

The scaling argument you make is not valid. Consider N campus 
networks connected by N access-routers to a backbone of M other routers. 
For policing at a backbone router, you will have to handle the 
potential N^2 campuses talking to each, and you would get a fraction of 
those connections depending on the network topology etc. At each 
access-router, you would still have only the connections belonging to the 
local campus network, and scaling properties are much better. 

Consider the configuration shown below where C1,C2, C3 are subnets administered
by the customer, and B1, B2, B3 etc are backbone routers administed by the ISP. 
A1, A2 are the access routers.

 C1<-->C2<-->C3<-->A1<-->B2<--->A3<-->C4<---->C5
                         |
 C6<-->C7<-->C8<-->A2<-->B4<--->A4<-->C8<---->C9

Assume C3, C8, A1 and A2 are on a common sub-IP (say SMDS) network. 
A1, A2 B2, B3, A3 and A4 are on another common sub-IP network (say ATM)
I can see NHRP being used in two manners in the above configuration. Once
is within the customer network, when he realizes that if he needs to go from
C3 to A2, he need not go via an additional IP hops through A2. The other place
where we can use NHRP is for the ISP to expedite the transmission of packets
from A1 to A4 without going through the extra IP processing at B2. 
In neither cases, the differential services model would have a problem.

Your argument is valid partially that now both access routers A1 and A2 have
to keep information to handle connections for the entire subnet of 
(C1, C2, C3, C6 C7 and C8) instead of just C1, C2, C3 for A1 and C6,C7,C8 for
A2. However, the scaling properties are still better than when B1 or B2 has to 
handle connections for all the campuses (C1-C9).

The scaling argument you are making would apply only if an ISP allows any of 
the backbone routers to be accessible via NHRP, and thereby all the backbone
routers have to handle the rules for all of the network. From an billing, 
metering and security purpose, this would be a rather unwise thing for an ISP 
to do. The ISP may use NHRP to expedite IP routing in his backbone made up of
say ATM links, but is not likely to allow direct access to all of the routers
in the backbone infrastructure. 

Regards,
Dinesh

---

From: Rajesh Talpade <[email-masked]>
Date: Tue, 13 Jan 1998 13:37:57 -0500 (EST)

Consider a slightly modified version of your example.

    C6     C7			         C8
      \      \                          /

C1, C5, C6 and C8 are subnets connected to routers C2 and C4.
C7 is also a router similar to C2 and C4 with subnets behind it.
With no NHRP, flow level info has to be maintained by C2, C7 and C4.
The other routers (C3, A1, B2, A3) can use increasing levels of aggregation.
But if C1, C2, C6, C7, C3 and A1 are NHRP-capable and on the same layer
2 network, then C3 and A1 also will need to have per flow info.
This is because hosts on say subnet C1 can establish direct connections
to C3.

Rajesh.

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