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COM 360. Chapter 3. Packet Switching. Limitations of Point-To-Point Networks. Problem: Not all Networks are Directly Connected Limit to number of hosts that can be attached (e.g. Ethernet – 1024 maximum) Limit to distance a network can serve (e.g. Ethernet 2500 meters maximum)
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Chapter 3 Packet Switching
Limitations of Point-To-Point Networks • Problem: Not all Networks are Directly Connected • Limit to number of hosts that can be attached (e.g. Ethernet – 1024 maximum) • Limit to distance a network can serve (e.g. Ethernet 2500 meters maximum) Next goal is to build networks that can be global in scale and to enable communication between hosts that are not directly connected.
Comparison to Phone System • Phone is not connected to every person you might call- instead it is connected to an exchange with a switch. • The switch creates the illusion that you are connected to the person at the other end of the call. • Similarly computer systems use packet switches to enable packets to travel form one host to another, even when there is no direct connection between the hosts.
Packet Switches • A Packet Switch is a device with several inputs and outputs leading to and from the hosts connected to the switch. • Main role of the switch is to take packets that arrive on the input and forward (or switch) them to the right output for their destination. • If packet arrival rates exceed the capacity of the output, the switch queues (or buffers) them. This is the problem of contention and the switch is said to be congested.
Switching and Forwarding • A switch is a device that allows us to interconnect links to form larger networks. • A switch is a multi-input, multi-output device, which transfers packets from an input to one or more outputs. • A switch adds a star topology to the point-to-point link (Ethernet) and ring (802.5 and FDDI) • Topologies.
Topologies • Networks can be classified by shape or topology. The most common are: • Bus • Ring • Star
Bus Topology • Networks with a bus topology consist of a long cable to which the computers attach. Any computer connected to the cable can send a signal and all computers receive the message.
Ring Topology • All computers are connected in a closed loop. • Each computer is connected to exactly two others. • The ring is a logical connection and the physical connections might look different and do not have to appear circular.
Star Topology • All computers attach to a central point or hub:
Star Topology in Practice • Previous diagram is idealized; usually, connecting cables run in parallel to computers:
Reasons for Multiple Topologies • Each topology has advantages and disadvantages: ·Star - protects the network from damage, since each cable connects only one computer. ·Ring - makes connections and coordination easier, but if one cable is damaged or one computer crashes, the entire network is disabled. • Bus - requires fewer wires than the star, but like a ring- if the main cable is damaged, the entire network is disabled.
Properties of a Star Topology • Even though a switch has a limited number of inputs and outputs, large networks can be built by connecting switches. • We can connect switches to each other and to hosts using point-to-point links allowing us to build large geographic networks. • Adding additional hosts to a switch does not necessarily decrease the network performance. • Every host on a switch has its own link to the switch and thus may transmit simultaneously.
Switching or Forwarding • Switched networks are scalable or capable of growing to large numbers of nodes. • Switching or forwarding is the main function of the network layer of the OSI Architecture.
Example Switch for Previous Graph With 3 input and 3 output ports
Switching • How does a switch determine the port for a packet? • It looks at the header for an identifier or address • Datagram or connectionless approach • Virtual circuit or connection-oriented approach • Source routing – less common • Assumptions: nodes must have globally unique identifiers and ports of each switch have identifiers (either numbers or names of connecting nodes)
Datagrams • Idea behind a datagram is simple • A datagram is the basic transmission unit in the Internet architecture. • Datagram networks are connectionless. • Every packet must contain a complete destination address • Switch consults a forwarding or routing table • Routing is a process by which nodes exchange information to build a routing table.
An Example Network Datagram forwarding
Characteristics of Connectionless Datagram Networks • A host can send a packet anywhere, anytime, since switches immediately forward them. • When a host sends a packet, it does not know if the network can deliver it. • Each packet is forwarded independently of previous packets and possibly by different paths. • A switch or link failure may not have serious effects since it may be possible to find an alternate route. This was a goal of the ARPANET. • It was the ability to route around failures that led to a datagram based design ( especially important to the military).
Virtual Circuit Switching • Connection –oriented model uses a virtual circuit (VC) • A widely used virtual circuit protocol is the Transmission Control Protocol (TCP). • Switched virtual circuits (SVC) are generally set up on a demand basis and are disconnected when the call is terminated. • A permanent virtual circuit (PVC) can be established as an option to provide a dedicated circuit link between two facilities.
Implementing A Virtual Circuit • Two stage process: • Connection setup stage- establishes a connection between the source and destination hosts and creates a VC table in each switch. • Data transfer stage- host puts an (VCI) identifier in the header and send it to the switch • When the host no longer wants to send data it tears down the connection and the switch removes the relevant entries in the table.
Virtual Circuit Table • An entry in a virtual circuit table for each switch contains: • A virtual circuit identifier (VCI)- that uniquely identifiers the connection; • An incoming interface; • An outgoing interface; • A potentially different VCI that will be used for outgoing packets.
Sending a Packet on a VC • If a packet arrives on an incoming interface of the switch and that packet contains the designated VCI value in its header, then it is sent out on the outgoing interface with the specified VCI value now included in its header. • Whenever a new connection is created, we need to assign a new VCI on each link and assure that the link is not currently in use by some existing connection.
A Packet is Sent into a VC From A to B A puts VCI value of 5 in header and sends to switch 1. Switch 1 Uses the table and puts the value 11in the header and sends it to Interface 3 on switch 2.
Packet Makes Its Way Though a VC Switch 2 looks up the value in its VC table, puts the value 7 in the header and send it out on interface 2 to switch 3. This continues until the packet arrives at B.
Virtual Circuits • Note that the combination of the VCI of packets as they are received at the switch and the interface on which they are received uniquely identifies the virtual connection. • The VCI has link local scope and is only significant for that connection • When a new connection is created, a new VCI has to be assigned to the connection. • Need to insure that the chosen VCI on a link is not currently in use by some existing connection.
Virtual Circuits • To establish a connection: • The network administrator can configure the state in which case it is called “permanent”(PVC). • A host can send messages into the network to cause the state to be established. This is referred to as “signalling” and the resulting circuits are said to be switched (SVC).
Virtual Circuit Switching • By the time the host gets the go-ahead to send the data, it knows a lot about the network. • The connection-oriented model does the following: • Allocates buffers to each virtual circuit; • Runs the sliding window protocol between each pair of nodes; • The circuit is rejected by the host if there are not enough buffers available • Thus each node is ensured of having the buffers it needs. This is called hop-by-hop control.
Datagram Network • There is no connection phase and each switch processes each packet individually. • Each packet competes with others for buffer space. If there are no buffers, the packet is discarded. • It is possible to distinguish among the packets to try to ensure that they receive a fair share of the buffers
Quality of Service • Quality of Service(QoS) means that the network gives the user some kind of performance related guarantee, which implies that the switches set aside the resources to meet this guarantee.
Examples of Virtual Circuits • Frame Relay- simple implementation -provides some basic quality of service and congestion avoidance features- used in the construction of virtual private networks (VPN). • Asynchronous Transfer Mode (ATM)
Frame Relay Packet Format Frame Relay packet format provides a good example of a packet used for virtual circuit switching.
Contention and Congestion • Contention occurs when multiple packets are queued at a switch because the 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. • The Datagram model experiences congestion.
Source Routing • Source Routing is a third approach to switching. All the information that is needed to switch a packet across the network is provided by the source host. • One way to do this is to put an ordered list of switch ports in the header and to rotate the list so that the next port is always at the front of the list. • (Not commonly used today.)
Source Routing In A Switched Network Switch reads the rightmost-number
Source Routing • Assumes that the host knows enough about the topology to form a header that has all the switches. • We cannot predict how big the header may be, since it must be able to hold one word of information for every switch on the path. • There are some variation of this approach: • Instead of rotating the header, the switch can strip off the element just used • The header can carry a pointer to the next port • Can be used in both datagram networks and virtual circuits, but it does not scale well.
Handling Headers For Source Routing a) rotation b) stripping c) pointer These labels are read from right to left.
Bridges and LAN Switches • Originally repeaters were used to connect a pair of Ethernet segments. • An alternative was to put a node between the two Ethernets and have the node forward frames. This node is called a bridge. • Bridges just accept LAN frames and forward them on the outputs. • This provides a way to increase the total bandwidth of the network. If a segment can carry 10Mbps, a bridge can carry as much as 10n Mbps, for n ports on the bridge.
Learning Bridges • Bridges do not have to forward all the frames it receives. • It can learn on which port each host resides. • A table can be downloaded and referred to when each new frame arrives. • A bridge can learn this information by inspecting the frames it receives and updating its table. When the bridge boots the table is empty and entries are added over time. If a frame is not on the table it is forwarded to all.
Spanning Tree Algorithm • Extending LANS with Bridges works well until it has a loop in it, which can allow a frame to circulate forever. • A loop can be introduced by an administrator, when a network spans multiple departments. • Bridges run a distributed spanning tree algorithm, which is a sub-graph which keeps all of the original vertices and eliminates some of the edges.
Spanning Tree With Some Ports Not Selected B1 is the root node, B3 and B5 are connected to LAN A and will use B5 since it is closer. B5 and B7 are connected to LAN B. B5 is the designated bridge because it has smaller ID and both are equidistant.
Graphs and Spanning Trees a) Cyclic graph b) Corresponding spanning tree
