switching
In message switching, the switches wait for the entire message to arrive (store) and then forward it to other switches.
Does packet switching also use store and forward technique ? I know that in packet switching, the message is divided into packets and sent across the links. But are the packets sent as soon as they arrive at the switch or are they stored in a similar way as message switching?
Is it correct in assuming that ASICs for router or switch interfaces will outperform the use of an x86 CPU for all packet processing which will greatly suffer from CPU interrupts?
It's hard to say specifically whether interrupts are a limitation, since we aren't naming specific CPU, operating system or router models in this part of your question. Overall, it's a safe generalization that general-purpose CPUs cannot touch the packet-switching performance of a well-designed ASIC. When I say performance, I'm talking about RFC 2544 metrics, such as no-drop packets-per-second forwarding rate (NDR), throughput, and latency.
That's not to say that there isn't a place for a CPU-based router; just that our life experiences tell us a CPU can't switch packets as quickly as an ASIC or FPGA. My conclusion about ASICs / FPGAs being faster than a multi-core CPU seems to be reinforced by this Q&A on Electronics.SE.
I know the x86 bus-speeds impose theoretical maximums to switching bandwidth especially once rates exceed 1Gbps.
I'm not sure which bus restrictions you're referring to here, but the information you have could be somewhat outdated. The PCI Express bus used in most systems scales well above 10Gbps these days.
PCIe 2.0 uses an 8b/10b encoding scheme that penalized it approximately 20% for the PCI lane encoding overhead; before that encoding penalty, PCIe 2.0 delivers 4Gbps of raw bandwidth per lane. However, even with the 20% 8b/10b penalty, PCIe 2.0 x8 (8 PCIe lanes) squeezes out over 25Gbps; thus you can easily run a single 10GE adapter at bidirectional line-rate on a PCIe 2.0 x8 card.
PCIe 3.0 (used in Intel Ivy Bridge chipsets) uses 128b/130b encoding, which greatly improves PCI-bus efficiency, and doubles the per-lane bandwidth. Thus a PCIe 3.0 x8 card could deliver 63Gbps (8.0*8*128/132). This is nothing to sneeze at; you can safely pack two line-rate 10GEs on a single riser with those performance rates.
Caveat: I'm using vendor-supplied marketing material for all comparisions...
- How does the Catalyst 6500 Sup2T ASIC switching speed, for example, compare to the realistic x86 switching speeds found on general OSes or SDNs?
This is a bit challenging because we're going to compare a fully-distributed switching system (Sup2T) to a centralized-switching system (Vyatta), so be careful interpreting the results.
- How does the Cisco 7200VXR-NPE-G2 switching speed, for example, compare to same...
The Vyatta blows an NPE-G2 out of the water, performance-wise; the NPE-G2 can do up to 2Mpps based on the Cisco NPE-G2 Datasheet. That's not really a fair comparison though given the age of the NPE-G2, vs a brand-new Intel 10-Core system packed with 10GE cards.
How do typical router or switch latencies compare to general OSes performing the same function?
That is a fantastic question. This paper indicates that Vyatta has higher latencies, but I would like to see this kind of testing done against the Intel E5 series CPUs.
Recap of a side-by-side comparison of Sup2T vs the Brocade Vyatta 5600:
The Sup2T still wins in my opinion, particularly when you look at what you get with the Sup2T (distributed scale to 720Mpps, MPLS, countless MIBs, Layer2 and Layer3 switching, etc...).
If all you care about is raw switching performance, you can get respectable performance numbers from an x86 CPU. However, in real networks, it's not often just about who has the best drag-race numbers; most people need to worry about features (see: When should I focus on each value for switch assessment?). A big factor to consider is the number of features available, and how they integrate with the rest of your network.
It's also worth looking at operational feasibility of using x86-based systems in your company. I haven't used Brocade + Vyatta myself, but they could do a decent job building good show commands and support hooks into the box. If they indeed support enough features, and their system scales well in real networks, then go for it if that's what you like.
However, if someone goes cheap and just builds a linux box + bird / quagga + ACLs + qos, I would not want to be the guy supporting that solution. I have always maintained that the open-source community does a great job innovating, but the supportability of their systems pales when compared with mainstream network vendors (Arista / Cisco / Force10 / Juniper). One needs only to look at iptables and tc to see just how convoluted you can make a CLI. I occasionally field questions from people who look at the output of ip link show or ifconfig and get wierded out because the packet counters aren't right; typically the major network vendors do a much better job testing their counters, vs what I see in the linux NIC drivers.
End Notes:
Note 1 Nobody who cares about performance would ever buy a Sup2T and fail to populate the chassis with DFCs. The Sup2T can switch at 60Mpps, but a loaded chassis with DFCs scales to 720Mpps.
Note 2 The Vyatta test ran on dual-processor, 10-core Intel E5-2670v2 at 2.5Ghz per core; if we count a single core as two virtual cores (i.e. hyper-threading), that's a total of 40 cores for packet switching. The Vyatta was configured with Intel x520-DA2 NICs, and used Brocade Vyatta version 3.2.
Segmentation (or rather fragmentation)
Although in some literature you may find the term segmentation to refer to the process of dividing an IP packet that is bigger than the Maximum Transfer Unit (MTU) of the link, the correct term is fragmentation.
See @adam86 answer for what segmentation refers to.
Switching
Switching is totally different, it refers to the process, performed by devices commonly known as switch, although the official term is mac bridge, which transfer (unchanged), an Ethernet frame from a LAN to another LAN.
Mac bridges are defined by the IEEE standard 802.1D
In modern Ethernet networking, switching is performed when a switch receives an Ethernet frame on one of its ports and transfers it to another port on which the receiver is attached.
The switch use the destination MAC address present in the Ethernet frame to select the port to send the frame to.
The term "switching" come from the fact the frame is recopied from one port to another one, by opposition with older hubs which send the frame to all ports (except the one on which the frame was received).
Best Answer
Nowadays, message switching is implemented at application level and it is actually transported by packet switching at the network level.
The most common example of this is email: an email is the message, the emails servers are the switches. The email is transferred using store and forward technique between each email servers, through SMTP protocol.
Between two email server, this message is divided into packets that are transported using TCP and IP protocols and pass through several IP networks. Each of this network can contain several switches and routers.
On a single path, the packets may traverse switches that use store and forward as well as other switches that use cut-through switching.
Since different packets may take different paths, you can have for a single message (email) some packets that will traverse some cut-through switches and other packets that will not encounter a single one.
Edit following comments:
Store and Forward vs Cut-Through is the way a specific switch behave. It is not a characteristic of a message / packet.
As stated by @Ron Maupin in comment you can use both technique with either message or packets.
The difference between message switching vs packet switching (in today's network) is that they don't happen at the same level.
They are not of the same nature and doesn't require the same treatment.
In the case of email messages, a small delay is not important, but the integrity of the message is critical. So a store and forward technique is much more appropriate, so each intermediate switch can check the integrity of the message before passing it to the next.
If you care more about speed than about integrity, then you can use cut-through switching. I don't have a good example in mind right now except for audio/video message that doesn't fit well here.
Note that if the message is sent to a network you don't control, or traverse such network, you cannot know how the intermediate switches are configured to behave. Once again in the case of email, it is likelt that all email servers will use S&F but it may traverse an anti-spam / anti-virus gateway that use some kind of cut-through.
The same logic apply for packet switching, but it happen at a different level.
A difference in packet switching is that the packet may be treated differently depending on its content.