A call for the future. Was: Re: Verio Decides what parts ofthe internet to drop
dean at av8.com
Wed Dec 8 19:01:29 UTC 1999
Around 07:22 AM 12/8/1999 -0800, rumor has it that Randy Bush said:
>> The phone system doesn't require anything close to millions of routes for
>> LNP. Instead, at the time of call setup, there is a lookup that performs
>> the translation between the portable number (which is the logical address)
>> and the physical address (which to date is still mostly statically
>> routed using a well-defined hierarchy based upon physical location).
>and here is where the anology breaks down. a second or two of call setup
>may be acceptable for establishing a phone call. it would be a disaster
>on a per-packet basis.
>ip is a connectionless protocol. before hitting the reply key, think about
I thought about it. It seems to me that a router is not presented with a stream of randomly addressed packets. In any time frame, there are going to be from 1 to many hundreds of packets between the same set of addresses.
Cisco takes advantage of this to with a route cache. When I was a junior monkey, one of my first tickets involved the route cache ACL bug in IOS 8.0, where routes added to the route cache were no longer checked against the appropriate ACL's. Of course, I didn't know immediately that was the real problem. It just seemed to work after I pinged (permitted in the ACL), and would continue to work (for something denied in the acl) for a while afterward.
So, while the first packet may result in a longer lookup, the successive packets hit the cached route. So now it becomes a problem to build an adequately sized route cache for the number of simultaneous 'connections' that one might expect to process in a given timeperiod. Which brings one back the terms and design rules that are not unlike those that apply to phone switches. The thing that is tough to grow is the access list size.
The connectionless part of IP is just a matter of whether state is maintained in the protocol or at the endpoints. An ISDN connection has a state at the endpoints and all the processing points. All the switches along the path must maintain that state. An ISDN packet in the middle of stream does not carry enough information to recreate the state. If you miss the setup message, you can't figure out the state from the rest of the packets. By contrast, an IP packet traversing an IP network carries all the state it needs with it. However, that doesn't mean that you need to start from scratch each time you see the same src/dest pair. Methods which hold some additional state in the router for faster processing can be used to speed things up.
The full route table could be very large. Much larger than 256Meg or even 4Gig. And even moving to disk backed storage (many gigs) most likely means that access would still be in tens or hundreds of milliseconds, not seconds. However, any given router doesn't really need to use very much of it at any given time.
The problem, as I see it, is that Cisco sees everything as a router. And a typically low powered router, as compared to mid and high range unix servers. Consider the Cisco H323 (VOIP) product line. Clearly, the 5300 makes sense as a router. It processes 4 PRI or T1 CAS voice lines into h323 on IP, along with the consequent DSP chips. The 2600 & 3600 make sense for lower density DSP platforms. Clearly, tranlating G.711 to G.729 or others requires specialized hardware.
However, the gatekeeper (call routing) software runs on a 3600. This is a grossly underpowered platform for gatekeeper functions, and underfeatured. Gatekeeper functions belong on a general purpose machine with modular (user replaceable) software and access to a database which can handle complex and quickly changing route policies (eg, time of day routing, followme routing). One desires a machine which can scale to handle hundreds or thousands of transactions per second (between a Sun ultra2 and an SGI Challenge w 64 processors). None of these functions are particularly suited to specialized routing hardware. Yet, Cisco insists that (nearly) everything must run on IOS. The best alternative right now runs on NT, with its lack of scalability and other PC problems. But the NT platform runs circles, triangles, squares, and other complex polyhedrons around the 3600.
Policy based routing in either a voice or an IP network require flexibility which can't be found in a specialized hardware platform. The problem is the wrong software & hardware combination to do the job, not that the route table has grown too large, or that the job just can't be done.
Plain Aviation, Inc dean at av8.com
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