Talk:Cut-through switching

Material cut in Sept 2004
I've removed the following material from the article. This seems to be a POV diatribe about ATM switching although it has some useful material and should be preserved. Cut through switching can exist in a pure ethernet context and in that case requres only one switch to deliver benefit; this means that this material is wrong in this context. Mozzerati 20:17, 2004 Sep 25 (UTC)

In standard routed networks, each node looks up the destination address and forwards a packet according to the directions in its internal routing table. With cut-through routing, a switched network core is embedded in a router network--picture it as two concentric circles with the switch core in the middle. At the edge of the switched core, an address-resolution process gives the incoming edge node the address of the edge node closest to the destination. A virtual circuit is then set up between them, which "cuts through" any other route examination along the way.


 * ethernet switches don't need to do this to take advantage of cut through.. needs to be cleaned up to say ATM

The primary benefit of this technology is improved performance of applications across the network, which is achieved by reducing delays. But it may not be easy to realize.


 * it's very easy to realise in ethernet (one interface command on an IOS router)

It takes at least four consecutive cut-through-aware interfaces to make routing work. You need a device in the core that's cut-through-capable (such as an ATM switch) with an incoming and outgoing interface, and you need an edge device at each end of the core's pair of interfaces. Without all this, you can't set up the connection.


 * not in the case of IP into ethernet or other similar stuff

However, as the size of the core network decreases, the benefits of cut-through routing may become insignificant. Vendors are now building "super-routers" with forwarding rates of hundreds of thousands of packets per second. Speeding up a pesky chunk of congested network with one of these devices might be easier (and cheaper) than deploying cut-through routing and its associated switches. A fast router is a brute-force solution, but sometimes a hammer loosens the nut better than a wrench.


 * why not do both (use a super router with cut through)?

For incremental network changes, cut-through routing's benefits are largely affected by the size of the network portion that can be adapted for cut-through routers. A little dab of ATM here and there won't do much except prepare you to deploy more ATM. If that's the only benefit of an ATM-based cut-through-routing solution, it may be insufficient to justify any significant investment in the approach. Before you consider switched backbone networks using cut-through routing, make sure you'll have enough of it to improve performance.


 * sounds like anti ATM propaganda (don't get me wrong, I hate ATM just as much as the next guy, but this should be encyclopaedic.

It also follows that the more devices you can cut out with your cut-through connection, the better. This means that users planning to adopt cut-through routing should also plan to deploy switches (again, ATM switches are an example) broadly in their networks. Doing so will bring the core as close to each traffic source and destination as possible and reduce the number of network elements that have to handle the data in traditional router mode.


 * latency is addative, not dependent on element. This means that every stage where latency is reduced helps.

Current state
The article is a little bit confusing. The cut-through switching is implemented at the Level 2 (Data Link) from the OSI Reference Model, where IP does't exists, there are only Ethernet/IEEE 802.3 frames. Maybe that's where the confusion is from: IPv4's Checksum field applies only to the header, but the Ethernet's FCS (Frame Check Sequence) applies to all the frame, that is, including the data.

Also, the possibility for a cut-through switch to forward an incorrect frame exists. That's its main drawback, but it's really fast. —Preceding unsigned comment added by Marius p (talk • contribs)


 * I have made some improvements. I hope they help. OSI discussion often confuses things more than it helps so I have not added that. I'm not opposed to another giving it a go and we'll see how it looks. --Kvng (talk) 15:41, 13 August 2011 (UTC)

Questions
Thanks, --Abdull (talk) 11:04, 9 August 2008 (UTC)
 * What is the difference between cut-through switching and wormhole switching?
 * What is a queueing backlog as mentioned in The main reason for not adopting cut-through routing is queueing backlog.?


 * I've removed the queuing backlog discussion because it was uncited and is not particularly useful to talk about what something is not. Wormhole switching apparently breaks packets into smaller pieces (flits) and forwards those. --Kvng (talk) 15:41, 13 August 2011 (UTC)

The summary of two recent edits   seems to imply there is a difference between wormhole switching vs cut-through switching. I agree with Abdull that this article could use a few more words clarifying the difference.

One reference ( p. 59) says: "With wormhole routing, sometimes also called cut-through, the routing decision is taken as soon as the header from a packet has been read in by the switch." If I'm reading that correctly, it's saying they are basically the same thing.

One reference says a flit is "typically a byte".

Another reference( p. 2) implies that wormhole switching is one specific kind of cut-through switching. But it seems a little vague on exactly what the difference is between wormhole switching and other (unnamed) kinds of cut-through switching.

A few more questions that may help improve this article:

Practically every packet-based communication system "breaks packets into bytes and forwards them", right?

What papers or other WP:RELIABLE sources describe the differences between wormhole switching and other types of cut-through switching?

What are the names of some examples of cut-through switching systems that are not also wormhole switching systems? --DavidCary (talk) 04:28, 24 January 2015 (UTC)


 * We also need to give some love to Wormhole switching. We need to keep in mind the difference between switching and routing. Even if cut-through and wormhole are the same process, they operate on different headers at different OSI layers. We need to understand the role of FLITs in wormhole switching. Most network equipment processes data on a frame-by-frame basis, not byte-level. I don't have much more to contribute here without starting a research project, just trying to help point out the holes and inconsistencies here. ~KvnG 15:44, 27 January 2015 (UTC)

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