Packet Switching Outline Store-and-Forward Switches Bridges and Extended LANs Cell Switching Segmentation and Reassembly CS565 Switching

Packet Switching Problem: “Not all networks are Directly Connected” Directly connected networks have two limitations: - number of hosts that can be accommodated - geographic limitation Goal – networks that are global in scale Like Telephone using (circuit) switches, we use packet switches(store-and-forward) that take packets that arrive on an input and forward (switch) them to the right output. Two ways to do it: - connection - connectionless CS565 Switching

Packet Switching Key problems that a switch must deal with: Contention – packet arrival exceeds packet dispatch (in this chapter) Congestion – packets discards (due to running out of buffer space) too frequently (in future chapter) Key issues covered - forwarding - contention Two switching technologies - LAN switching – Popular in LAN, evolved from Ethernet bridging - Asynchronous Transfer Mode(ATM) – popular in WAN CS565 Switching

Star Topology Large numbers of switches can be connected Connecting switches/hosts using Point-to-Point links Scaling doesn’t always mean performance  (switches designed with enough aggregate capacity) CS565 Switching

Scalable Networks Switch Advantages forwards packets from input port to output port, this is referred to as either switching or forwarding. In terms of the OSI architecture, it is the main function of the network layer. port selected based on address in packet header Advantages cover large geographic area (tolerate latency) support large numbers of hosts (scalable bandwidth) Input ports T3 STS-1 Switch Output CS565 Switching

Switches Q. How does the switch determine output port? By looking at an identifier in the packet header Three approaches: Virtual Circuit (connection-oriented) Datagram (connectionless) Source Routing CS565 Switching

Virtual Circuits Explicit connection setup (and tear-down) phase Switch 1 3 1 2 Switch 2 2 3 1 5 11 Explicit connection setup (and tear-down) phase Subsequence packets follow same circuit Sometimes called connection-oriented model Host A Two types: - PVC Permanent Virtual Circuit Need network administrator configure the state - SVC “Signalled” Virtual Circuit Send a message into the network(signalling) Analogy: phone call Each switch maintains a VC table 7 Switch 3 1 3 4 Host B 2 CS565 Switching

Virtual Circuits Tables VC Table for Switch 1 Incoming Incoming Outgoing Outgoing Interface VCI Interface VCI 2 5 1 11 1 2 3 1 . . . Switch 1 3 1 2 2 3 1 5 11 VC Table for Switch 2 Switch 2 Incoming Incoming Outgoing Outgoing Interface VCI Interface VCI 3 11 0 7 1 2 0 2 . . . Host A VCI Virtual Circuit Identifier (0, 1, 2, …) combined with incoming/outgoing interface (e.g. 0, 1, 2, 3)can uniquely identify the virtual connection (VC) assigned whenever a new connection is created not a globally significant identifier for the VC; rather, only on a given link 7 Switch 3 1 3 4 Host B 2 CS565 Switching

Example: Give Virtual Circuit Tables for all switches Host C Host E Switch 1 Host D Switch 3 2 Host F 3 1 3 1 1 Switch 2 Host B 2 2 Host G 3 Host A Host H Incoming Interface# VCI# Outgoing 1 2 Connections Established in the order of (a) to (d) Switch Incoming Interface# VCI# Outgoing (a) Host A connects to Host E 1 2 3 0 0 3 0 2 0 (b) Host C connects to Host G 0 1 3 1 0 0 (c) Host G connects to Host A 1 2 2 2 2 1 0 2 3 2 (d) Host D connects to Host E 3 3 VC Table for Switch 1 Incoming Interface# VCI# Outgoing 1 2 3 VC Table for Switch 2 Incoming Interface# VCI# Outgoing 3 1 2 VC Table for Switch 3 CS565 Switching

Virtual Circuit Model Typically wait full RTT for connection setup before sending first data packet. While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small. If a switch or a link in a connection fails, the connection is broken and a new one needs to be established. Connection setup provides an opportunity to reserve resources(QoS reservations). ATM utilizes virtual circuits CS565 Switching

Datagrams Idea – provide just enough information for the switch to forward the packet Host D Switch 1 Host E Host F 3 1 2 Switch 2 Host C 2 3 1 No setup time Independent forwarding packets Analogy – postal system Each switch maintains a forwarding (routing) table. More routing in the next chapter Host A destination Port A 3 B C D E 2 F 1 G H Forwarding table for Switch 2 Host G Switch 3 Host B 1 3 2 Host H CS565 Switching

Workstation Used As a Switch Main problem: all packets must pass through a single point of contention. (I/O bus, read to/write from the main memory) CS565 Switching

Forwarding Table for Nodes C E Give the datagram forwarding table for each node: Node A Node B Node C Node D Node E Destination Next Hop B C D E Destination Next Hop A C D E Destination Next Hop A B D E Destination Next Hop A C B E Destination Next Hop A C B D CS565 Switching

Datagram Model There is no round trip time delay for connection setup; a host can send data as soon as it is ready. Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up. Since packets are treated independently, it is possible to route around link and node failures. Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model. CS565 Switching

Source Routing Does not need to use either VCs or Datagrams although can be used in combination with IP for instance uses datagrams but has a source routing option Can be used for VC setup Source host contains all information - rotates data CS565 Switching

Bridges and LAN Switches Q. What can be used to share data between two shared-media LAN’s? LAN switch (or bridge) – like a host in promiscuous mode LANs connected with >= 1 bridge are called Extended LANs Problem – what about when node A wants to send node B a message, what happens? A B C Port 1 Bridge Port 2 X Z Y CS565 Switching

Learning Bridges Do not forward when unnecessary Maintain forwarding Host Port A 1 B 1 C 1 X 2 Y 2 A B C Port 1 Bridge Port 2 X Y Z Learn table entries based on source address Table is an optimization; need not be complete Always forward broadcast frames CS565 Switching

Spanning Tree Algorithm Problem: loops Bridges run a distributed spanning tree algorithm - select which bridges actively forward - developed by Radia Perlman - now IEEE 802.1 specification CS565 Switching

What is a Spanning Tree? The problem: Tree - is connected graph with no cycles. A Spanning Tree of G is a tree which contains all vertices in G. Example: Given a graph G Is G a Spanning Tree? Yes No Note: Connected graph with n vertices and exactly n – 1 edges is Spanning Tree. CS565 Switching

Spanning Tree Example Example: G: 1 2 3 7 6 4 5 8 CS565 Switching

Centralized Spanning Tree Algorithm 1 #1 #7 7 2 3 #2 #6 5 4 6 #4 #3 #5 8         DFS (Depth First Search) CS565 Switching

Centralized Spanning Tree Algorithm 1 #1 #2 2 3 #3 #4 #5 #6 4 5 6 7 #7 8 BFS (Breadth First Search) CS565 Switching

Distributed Spanning Tree Algorithm - Overview Each bridge has unique id(e.g., B1, B2 , B3 ) Select bridge with smallest id as root Select bridge on each LAN closest to root as the designated bridge(use id to break ties) CS565 Switching

Distributed Spanning Tree Algorithm - Detail Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be root bridge distance(hops) from sending bridge to root bridge Each bridge records current “best” configuration message for each port Initially, each bridge believes it is the root and sends messages out on all its ports(distance to root = 0) When learn not root, stop generating configuration messages in steady state, only root generates configuration messages When learn not designated bridge, stop forwarding config messages(disconnected) in steady state, only designated bridges forward config messages Root continues to periodically send config messages If any bridge does not receive config message after a period of time, it starts generating config messages claiming to be the root CS565 Switching

Distributed Spanning Tree Algorithm - Overview Ports which are not selected (disconnected) by the Distributed Spanning Tree Algorithm × × × × × CS565 Switching

Asynchronous Transfer Mode(ATM) Connection-oriented packet-switched network (virtual circuits) Used in both WAN and LAN settings Signaling(connection setup) Protocol: Q.2931 Packets are called cells 5-byte header + 48-byte payload(fixed length) CS565 Switching