CSS432: Switching and Bridging 1 Switching and Bridging Textbook Ch3.1 and 3.4 Professor: Munehiro Fukuda

2 Scalable Networks Why switches  Connects two or more network segments (>2500m in Ethernet)  Support large numbers of hosts (>1024 hosts in Ethernet)  Maintain performance (>two packets through a switch) Network Switches Packet SwitchCircuit Switch ConnectionlessConnection OrientedDynamicStatic IP datagrams ATM X25 DTMSONET virtual TDM FDM Dynamically reallocate bandwidth Frame sent over optics in synchronization with atomic/satellite clock CSS432: Switching and Bridging

3 Three Approaches Datagram/connectionless switching  IP Virtual circuit/connection-oriented switching  X.25  ATM (Asynchronous Transfer Mode) Source routing  Connection requests in virtual circuit CSS432: Switching and Bridging

4 Datagram Switching No connection setup phase Each packet forwarded independently Switch 3 Host B Switch 2 Host A Switch 1 Host C Host D Host E Host F Host G Host H Analogy: postal system Each switch maintains a forwarding (routing) table DestPort A3 B0 C3 D3 E2 F1 G0 H0 Table at Switch 2 CSS432: Switching and Bridging

5 Datagram Switching (Cont’d) No connection setup  Pros 1: A source can send data as soon as it is ready. No way of knowing if a packet is delivered.  Cons 1: A source must estimate network congestion or disconnection. Each packet may take a different route.  Pros 2: No signle point of failure happens.  Cons 2: The order of packets at destination is different from that at source. CSS432: Switching and Bridging

6 Virtual Circuit Switching Explicit connection setup (and tear-down) phase Subsequence packets follow same circuit Analogy: phone call Each switch maintains a VC table Switch 3 Host B Switch 2 Host A Switch 1 VCI = 5 VCI = 11 VCI = 7 VCI = 4 Port (in)VCIPort (out)VCI Port (in)VCIPort (out)VCI Port (in)VCIPort (out)VCI 0734 Switch 1 Switch 2 Switch 3 Question: It seems like the left table needs only incoming and outgoing port pairs…. Why do we need VCI? CSS432: Switching and Bridging

7 Virtual Circuit Switching (Cont’d) Switch 3 Host B Switch 2 Host A Switch 1 Host C Host D Host E Host F Host G Host H VCI=5 VCI=11 VCI=7 VCI=4 VCI=6 VCI=12 VCI=8 VCI=4 Port (in)VCIPort (out)VCI Port (in)VCIPort (out)VCI Port (in)VCIPort (out)VCI Global address information is replaced with local VCI.  Each switch has local but not global information. To set up connection:  Each switch still needs a global view of network configuration to forward a connection request message to destination Switch 1 Switch 2 Switch 3 CSS432: Switching and Bridging

8 Virtual Circuit Model (Cont’d) Connection setup required  Pros 1: An opportunity to reserve resources (QoS)  Cons 1: Wait for a full RTT before sending first data packet.  Cons 2: Full address for destination still required for connection. Packets sent along the same route  Pros 2: Each data packet contains only a VCI.  Pros 3: Flow control possible along the entire connection  Cons 3: If the connection is broken, a new one needs to be established. CSS432: Switching and Bridging

9 Source Routing Each packet has a complete switching information from source to destination Three ways to maintain switching information:  Rotation  Striping  Use of a pointer Useful to send a connection setup request in VC CSS432: Switching and Bridging

10 X.25 Source Routing and Virtual Circuit Switching group channel type 1 caller address length callee address length caller address (up to 14bits) callee address (up to 14bits) Data Higher Layer Protocol receive sequence send sequence channel 0 0 modulo group Call request Call accept Source VCI Destination VCI DTE/DCE Call request/Idle mode/call clearingData packet 4K channels Call request control 3 bits: country code 1 bit: network code 10 bits: address Why sequence#? 1.Sliding window 2.Error recovery CSS432: Switching and Bridging

11 Bridges and Extended LANs Connecting two or more LANs  Repeater / Hub L1: Physical Layer Limitations: <= 2500m and <= 1024 nodes  Bridge (or LAN switch) L2: Datalink Layer Fowarding frames using MAC address Static configuration + partial dynamic configuration (Spanning Tree Protocol)  Router L3 – Network Layer Routing IP packets using IP address Dynamic configuration R destination R R R B B B B R R R R CSS432: Switching and Bridging

12 Learning Bridges Do not forward when unnecessary  Ex. A frame sent from A to B Maintain forwarding table HostPort A1 B1 C1 X2 Y2 Z2 Learn table entries based on source address  E.g. An entry for A is registered upon receiving a frame from A  E.g. When receiving a frame from B, don’t forward to Port 2 Table is an optimization; need not be complete Entries are expired after a specific period of time  Linux brctl command: makes a logical bridge with max age = 4sec A Bridge BC XY Z Port 1 Port 2 Based on datagram switching CSS432: Switching and Bridging

13 STP: Spanning Tree Protocol Loops  Example: B1 receives a frame from Node X on LAN H to Node Y on LAN C. B1 registers an entry for Node X but not yet Node Y. B1 forward this frame to all ports except to LAN H. B7 receives the frame and forwards it to LAN B. B5 forwards it to LAN A and D. B1 receives again this frame and registers an entry for X. B1 forwards it to all ports except to LAN H and D.  Problem: Node Y eventually receives a frame. Duplicated frames are forwarded along loops. Spanning Tree Algorithm  Inactivate bridge ports so that no cycle exists in extended LAN  IEEE Specification B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I X Y CSS432: Switching and Bridging

14 STP Details Initially, each bridge believes it is the root When learn not the root, stop generating configuration messages  in steady state, only the root generates messages When learn not a designated bridge, stop forwarding configuration messages  in steady state, only designated bridges forward configuration messages If any bridge does not receive configuration message after a period of time, it starts generating configuration messages claiming to be the root. B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I Bridges exchange configuration messages (Y, d, X)  Y: the id of root to be  d: #hops from X to Y  X: the sending bridge id (1, 0, 1) (1, 1, 2) (1, 1, 5) (1, 0, 1) root 1 hop B5 < B7 1 hop B4 < B6 1 hop 2 hops CSS432: Switching and Bridging

15 Cell Switching (ATM) Connection-oriented packet-switched network Used in both WAN and LAN settings Packets are called cells  5-byte header + 48-byte payload (easier to switch, high frame utilization fro small data) Commonly transmitted over SONET Error collection  End-to-end but not at each switch (i.e., at data link layer) Congestion control  Admission control  Leaky packet transfer CSS432: Switching and Bridging

16 Switch Implementation Using a workstation  Flexible control  Performance problem Using a custom hardware  Shared/share memory-based switch  Crossbar switch  Self-routing switch (Batch Banyan switch) CSS432: Switching and Bridging

17 Workstation Used as a Switch Advantage: flexible because a workstation has a CPU. Example  33MHz 32bit I/O bus 1Gbps for one way from NIC to main memory 500Mbps for a round trip between NIC and main memory Enough to support five 100Mbps Ethenet  What if a packet is very small like 64byes The workstation has 500,000 packets per second (pps). Throughput: 500,000 x 64 x 8 = 256Mbps NIC I/O ctlr CPU Main memory I/O Bus LAN A LAN B LAN C Workstation CSS432: Switching and Bridging

18 Shared Bus/Memory-Based Switch A simple design Shared bus or memory becomes a bottleneck. (Max. 16 bus masters) Output Port Input Port Shared memory Shared bus Control processor DMA from port to port CSS432: Switching and Bridging

19 Crossbar Switch Without a collision, all inputs delivered to each output All inputs may go to the same output which causes a collision in the output buffer. CSS432: Switching and Bridging

20 Crossbar + Knockout Switch Knockout switch: buffers only L out of N packets, where L < N. Ex. L =8 is sufficient. 1 packet loss per million for a large N. ( logarithmically in the loss rate) Detailed explanation: N = 4 L = 3 Knockout Switch Round-robin CSS432: Switching and Bridging

21 Banyan Switch 2 x 2 switching elements interconnected in regular patterns. Collisions occur if packets are not presented in ascending order CSS432: Switching and Bridging

22 Batcher Network + Banyan Switch Sort packets in ascending order through Batcher network Then switch them through Banyan switch ∞ ∞ ∞ ∞ CSS432: Switching and Bridging

23 Reviews  Datagram switching  Virtual Circuit switching  Source routing  Bridges: STP and limitations  Switches: workstation-based, shared bus/memory-based, crossbar + (knockout), and (batcher network) + banyan Exercises in Chapter 3  Ex. 1 (vc sw)  Ex. 4 (datagram sw)  Ex. 13 (STP)  Ex. 26,32 (Switch implementation) CSS432: Switching and Bridging