North American Network Operators Group|
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Re: Links on the blink - reprise
The jury is still very much out on whether layering IP on top of frame relay with frame relay switches providing the principal long-haul switching fabric and routers making decisions about which PVCs to direct IP datagrams into is a reasonable, scalable approach. There are several advantages to the approach, vis. greater port density, especially in terms of ports-per-dollar on most FR devices than on high-end Ciscos; the (perhaps safe) bet that a medium-size user and channel of routers might have more of an influence over the development and engineering of an FR vendor's Internet-oriented products than over Cisco; that an organization with expenditure limits on bandwidth and relatively long minimum term committment lengths on cross-country circuits could do better traffic management, particularly in the presence of multiple parallel DS1s, than a similar setup using either inverse multiplexors or load balancing; and the fact that using a mesh along these lines is something that readily can interoperate with a migration to clear-channel DS3s or other technologies, or from a melting-down cross-country ethernet-over-ATM or FR-over-ATM backbone. As well, there is the probability that the users of the model you're asking about have lots and lots of points-of-presence used in an effort to reduce backhaul costs, which get quite substantial as the numbers of customers grow. Also, having lots of POPs interconnected at varying bandwidths can make designing a topology that is friendly to routers a bit more difficult in some ways. The model generally also very neatly hides the underlying topology from the IP layer, rendering traceroute essentially useless as a means of diagnosing weird time asymmetries and the like. There may be 13 physical hops between A and B on a network using this model, but traceroute might reveal only two. This has some obvious advantages to the provider keen on hiding the design of its network from "outsiders", or even from its own customers. Key to the decision is the gamble that the switching ability of FR switches will compare with the switching ability of high-end routers in the same environment. This is not a gamble I am comfortable with, particularly as I have very strong ideas of how well an SSE-equipped 7000 can switch packets in a backbone environment, and so far have only early impressions about one variety of popular FR switch in a similar environment. Also key to the decision is a bet on how well the FR-switch-based networks will scale beyond multiple DS1s and DS3s, especially in comparison to routers. There are substantial disadvantages, too. In order to take advantage of the greater port-density(-per-dollar) on FR switches right now, the end user has to use FR, which is not always practical or desirable. The monitoring and management systems of all FR switching products with which I am familiar are not router-geek-friendly. PVCs can do strange things, as can underlying circuits, which one really wants the IP-switching layer to recognize and adapt to; at present there is effectively no practical means to do this in anything but a failure of a PVC. There is a serious management nightmare in maintaining anything approaching a full mesh in a large network, and it worsens if one pushes the routing decisions right out to the edges of a given network. Maintaining lots of PVCs has many of the well-known disadvantages of maintaining lots of standard iBGP neighbours or any other system in which involves configuration length, complexity or both approaching or equivalent to N**2. There are a number of other deeply technical questions about the model which remain the subject of occasionally heated speculation, but the bottom line is that nobody really knows how well it will work as this type of network grows really big, or even as heavily trafficked as SprintLink and InternetMCI. Finally, pushing the envelope of technology is hard work at all times; I believe a pragmatic approach is to push only n things at any given time where n is ideally the smallest possible number. Adding frame relay switching technology into the mix of things that backbone NSPs have to deal with already strikes me as asking for trouble. | PSI and UUNET have apparently been following this strategy with their | backbones for some time. Netcom has also come on board. Your information may be better than mine, but I gathered that UUNET has just recently migrated over to this model from the previous MFS-supplied technologies they had been using. Frankly, neither netcom nor PSI does sufficient traffic to learn anything interesting about how the model behaves under stressful conditions. Alternet's experience over the next few months will be enlightening. Sean.