Packe k t e S w S it w ching

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Brianne Patterson
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1 Packet Switching Jun Jie, NG 08 Oct 2011

2 Reference Book Chapter 3: Packet Switching

3 What is a Switch? In the simplest terms, a switch is a mechanism that allows us to interconnect links to form a larger network. A switch is a multi input, multi output device, which transfers packets from an input to one or more outputs. Note: We refer to these inputs and outputs as ports. A switch is connected to a set of links and, for each of these links, runs the appropriate data link protocol to communicate with the node at the other end of the link. A switch primary job is to receive incoming packets on one of its links and to transmit them on some other link. This function is sometimes referred o as either switching or forwarding.

4 Question How does the switch decide on which output port to place each packet? In this topic, we will look at the following approaches: (a) Datagram (Connectionless approach) (b) Virtual Circuit (Connection oriented approach) (c) Source Routing

5 Contention vs Congestion Contention occurs when multiple packets have to be queued at a switch because they are competing for the same output link. Congestion means that the switch has so many packets queued that it runs out of buffer space and has to start dropping packets.

6 (a) Datagram Approach The idea is simple. Every packet contains enough information to enable any switch to decide how to get to its destination. To decide how to forward a packet, a switch consults a forwarding table (or a routing tbl) table). At switch 2, look at the destination, and inspect tits output t< > Mth Match the colored circles on the diagram and forwarding table. Is that clear so far?

7 Connectionless Characteristics

8 Exercise Node ( ) Destination Next Hop Node ( ) Destination Next Hop Node ( ) Destination Next Hop Node ( ) Destination Next Hop Node ( ) Destination Next Hop Node ( ) Destination Next Hop

9 Exercise (An observation) What have I observed? The steps to construct the forwarding table are as follow: ~ At each node, do not consider but other nodes on the network. ~ Trace the path it has to go through and note the immediate next hop to form the forwarding table. Next: TRY TUTORIAL 3, Q8

10 (b) Virtual Circuit Switching Widely used. Uses the concept of a virtual circuit. Connection oriented model: requires a virtual connection from the source host to the destination to be set up first before any data is sent. i.e. First stage: Connection Setup. Second stage: data transfer. We consider each at a time.

11 (b) Virtual Circuit Switching Connection setup phase: Necessary to establish connection state in each of the switches between the source and destination hosts. This connection state for a single connection consists of an entry in a Virtual Circuit table in each switch through which the connection passes. One entry in the VC table on a single switch contains:

12 (b) Virtual Circuit Switching See how it corresponds (colour circles):

13 Tired? No? Try one Exercise

14 (b) Exercise: Virtual Circuit Switching Answer (a), ()(b), (c) ()below.

15 Host A connects to B. (b) Exercise: Virtual Circuit Switching

16 Host C connects to G. (b) Exercise: Virtual Circuit Switching

17 Host E connects to I. (b) Exercise: Virtual Circuit Switching Question: Rl Relook kf from (a) () to (c). Are you clear with the VCI assignments?

(b) Exercise: Virtual Circuit Switching (a) (b) (c) ()

assigned to a value of 0. What have I observed?

I have redrawn the path taken from (a) to (c) again here

Assignment of VCI is as general: ~In my own words, at a

18 (b) Exercise: Virtual Circuit Switching (a) (b) (c) () At this point, 2 paths imply that the second path s VCI is 1 while the first is 0. At this point, only one path crosses through h imply VCI assigned to a value of 0. What have I observed? ~ A connection is defined from one point to another. I have redrawn the path taken from (a) to (c) again here in different colors. See color lines. Assignment of VCI is as general: ~In my own words, at a POINT, or that the color lines, or or pass through, the COUNT will increase by 1, starting from 0. Next: TRY TUTORIAL 3, Q7

(c) Source Routing Name derives from the fact that all

to switch a packet across the network is provided by the

Rather than just to rotate the header, each switch could

19 (c) Source Routing Name derives from the fact that all the information about the network topology is required to switch a packet across the network is provided by the source host.. Many ways to implement. Rather than just to rotate the header, each switch could just strip the first element as it uses it. Other option include header pointer!

20 (c) Source Routing Explanation: QUESTION!! In this example, the address received by B is not reversible and doesn't help B know how to reach A. Propose a modification to the delivery mechanism that does allow for reversibility. Your mechanism should NOT require giving gall switches globally unique names.

(c) Exercise: Source Routing ANSWER (reference from

for a second address list, in which we build the

Each time the packet traverses a switch, the switch

list, in addition to forwarding the packet out the

For example, as the packet traverses Switch 1, towards

the Similarly, Switch 2 must write port 3 in the next

21 (c) Exercise: Source Routing ANSWER (reference from the textbook) We provide space in the packet header for a second address list, in which we build the return address. Each time the packet traverses a switch, the switch must add the inbound port number to this returnaddress list, in addition to forwarding the packet out the outbound port listed in the forward address. For example, as the packet traverses Switch 1, towards forward address port 1, the switch writes port 2 into the return address. Similarly, Switch 2 must write port 3 in the next position of the return address. The return address is complete once the packet arrives at its destination.

Switching / Forwarding

Switching / Forwarding A switch is a device that allows interconnection of links to form larger networks Multi-input, multi-output device Packet switch transfers packets from an input to one or more outputs