Switching Circuit vs Packet Types Datagram Virtual circuit

Switching • Circuit vs. Packet • Types: – Datagram – Virtual circuit – Routers and routing – Organizing autonomous systems – Tracing routes SYST 5030/4030

Packet Switching Circuit Switching Packet Switching Circuit versus Packet Switching Source: Tannenbaum, Computer Networks, 3 rd Ed. , 1996 SYST 5030/4030

Chicago Boulder Atlanta SYST 5030/4030

Packet Switching The 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. SYST 5030/4030

Comparison between packet switching networks and circuit switching networks Circuit Switching Packet Switching • Dedicated transmission path • No dedicated path • Continuous transmission • Transmission of packets • No storage of data/messages • Packets may be stored • Same path for duration of • Each packet may take connection different route • Call setup required • No call setup needed • Overload may block call • overload increases packet delay • Busy signal if callee busy • no busy signal • Fixed bandwidth transmission • dynamic use of bandwidth • Charging by minute • Charging by packet • Fast enough for Interactive (? ) SYST 5030/4030

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 SYST 5030/4030

Switch (Catalyst 5500 switch from Cisco) Source: http: //www. visi. com/~gje/atwork. html SYST 5030/4030

Types of packet switching Datagram 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. SYST 5030/4030

Virtual Circuit Packet Switching [Source: Tannenbaum, Computer Networks, 3 ed. ] SYST 5030/4030

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. SYST 5030/4030

Warriors of the Net SYST 5030/4030

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 SYST 5030/4030

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 SYST 5030/4030

The Workings of one Router Source: Tannenbaum, Computer Networks, 3 rd Ed. , 1996 SYST 5030/4030

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. SYST 5030/4030

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” • BGP 4 is the de facto standard for exterior gateway protocol in the Internet. SYST 5030/4030

Congestion in packet switching networks Packets Delivered Perfect Desirable Congested Packets Sent How to handle congestion • Monitor system for problem points • Transmit this information regularly • Adjust operation: i. e. reduce load, degrade service, reschedule. SYST 5030/4030

What is the impact of packet size on message transmission delay Should we transmit the message in one large packet or several small ones? • 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. SYST 5030/4030

General Formula Total message delay = Transmission time for one packet over a link x (# of packets + # of links – 1) SYST 5030/4030

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. ] SYST 5030/4030

Summary Packet 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. Switching 2. Routing 3. Delay 4. Congestion 5. Packet size Tools Traceroute, ping. [Note: We will discuss routing in more detail later in the semester and study specific protocols. ] SYST 5030/4030