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Switching

• Circuit vs. Packet

• Types:– Datagram– Virtual circuit– Routers and routing– Organizing autonomous systems– Tracing routes

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Circuit versus Packet Switching

Circuit Switching

Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

Packet Switching

Packet Switching

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Boulder

Atlanta

Chicago

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Packet SwitchingThe Internet is a packet switching network.At sending end:•Following information is attached to each packet:•Message is split into packets (say, 128 bytes long)

- Address of destination

- Packet number (i.e., Packet 5 of 9)

- Error checking field (16 bits long)

At destination:•Packets are reassembled in the correct order•Additional information is stripped away

•Original message is reconstructed.

Packet switching technologies: TCP/IP, X.25, Frame relay, ATM.

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Comparison between packet switching networks and circuit switching networks

Circuit Switching•Dedicated transmission path

•Continuous transmission

•No storage of data/messages

•Same path for duration of

connection

•Call setup required

•Overload may block call

•Busy signal if callee busy

•Fixed bandwidth transmission

•Charging by minute

•Fast enough for Interactive

Packet Switching•No dedicated path

•Transmission of packets

•Packets may be stored

•Each packet may take

different route

•No call setup needed

•overload increases packet delay

•no busy signal

•dynamic use of bandwidth

•Charging by packet

•Fast enough for Interactive (?)

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Advantages of packet switching networks over circuit switching networks

• No end to end connection

• Greater flexibility in routing

• Greater reliability

• Better circuit utilization

• Less congestion

• Better response time

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Switch (Catalyst 5500 switch from Cisco)

Source: http://www.visi.com/~gje/atwork.html

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Types of packet switchingDatagram packet switching: •Each packet may follow its own route, no notion of a call setup. •This is called connectionless, no-acknowledgement form of communication.

Virtual Circuit packet switching: •A call is established (setup) yet there is no continuous connection. •The call setup determines the routing path of the packets and all packets follow

the same path for the duration of the call. •A virtual circuit has a virtual circuit identifier (VCI) at each link to identify the

circuit.•After transmission is complete, the call is cleared.•Virtual circuits can be switched (SVC) or permanent (PVC).•A PVC is like a leased line connection conceptually.•Useful for large file transmission, e.g. with ftp.

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Virtual Circuit Packet Switching

[Source: Tannenbaum, Computer Networks, 3ed.]

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Routing in packet switching networks

Two routing strategies• Centralized routing: one node acts as routing manager and

calculates “optimal” paths. Problem: -- Routing manager is bottleneck.

-- What if it fails? -- Anomalies can arise because of delays in communicating

new routes to nodes.• Distributed routing: Each node acts as a routing manager and calculates best route.

Exchanges status information with other nodes periodically. -- This works better in practice.

Routing information is stored in routing tables.

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Warriors of the Net

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Routing Algorithms

• Nonadaptive Algorithms (static routing)– Choice of route computed in advance

– Choice downloaded to routers upon booting

• Adaptive algorithms– Change routing decisions to reflect:

• Changes in topology

• Changes in traffic

– Get information from adjacent routers or all routers

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Specific Algorithms

• Shortest path routing– Number of hops/physical distance/fastest path

• Flooding– Every incoming packet sent on every outgoing line, except

the one it came in on– Hop counter used to avoid over-saturation

• Distance vector– Each router maintains table of best known distance to each

destination, and which output line to use

• Flow-based, count-to-infinity, split horizon hack, link state, hierarchical, broadcast, multicast

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The Workings of one Router

Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

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Organizing the Internet as Autonomous Systems

AS - Autonomous System.BGP - Border Gateway Protocol

Each AS handles its own routing within the AS.BGP protocol does routing across various ASes.

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More on Autonomous Systems

• An autonomous system is a set of routers having a single routing policy, running under a single technical administration

• The main goal of an interior gateway protocol is to route efficiently, while the exterior gateway protocols have to worry about “politics”

• BGP4 is the de facto standard for exterior gateway protocol in the Internet.

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Congestion in packet switching networks

Perfect

Desirable

Congested

Packets Sent

Pac

ket s

De l

i ver

e d

How to handle congestion• Monitor system for problem points

• Transmit this information regularly

• Adjust operation: i.e. reduce load, degrade service, reschedule.

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What is the impact of packet sizeon message transmission delay

• Large packet size implies fewer packets.

• Each packet has a fixed overhead (number of bytes).

• Fewer packets implies fewer total number of bits.

• However, if there are fewer packets, there is less parallelism, i.e., there is less simultaneous transmission.

• Need to evaluate the tradeoff based on actual parameters.

Should we transmit the message in one large packet or several small ones?

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General Formula

Total message delay =

Transmission time for one packet over a link

x

(# of packets + # of links – 1)

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Traceroute Program

• Way to determine the path to a remote host.

• Your host sends three packets (datagrams) to the remote host

• Output of program displays:– All the routers along the way which received the packet– Round-trip time to reach those routers.

• If no response received in 5 seconds– Asterisk is displayed, and– Another datagram is sent

[Check out www.visualroute.com for a visual traceroute program.]

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SummaryPacket switching is a very important idea because the entire Internet is based on it. Increasingly even real time applications like voice and video are possible on packet switching networks.Issues:1. Switching2. Routing3. Delay4. Congestion5. Packet sizeToolsTraceroute, ping.[Note: We will discuss routing in more detail later in the semester and study specific protocols.]