CSS432: Switching and Bridging 1 Switching and Bridging Textbook Ch3.1 and 3.4 Instructor: Joe McCarthy (based on Prof. Fukuda’s slides)

Network Layer CSS432: Switching and Fowarding 2

3 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

4 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

5 Datagram Switching No connection setup phase Each packet forwarded (routed) 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

6 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 single point of failure  Cons 2: May arrive in different order CSS432: Switching and Bridging

7 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 table needs only incoming & outgoing port pairs…. Why do we need a VCI for each? CSS432: Switching and Bridging

8 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

9 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

10 Source Routing Each packet has 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

11 Bridges and Extended LANs Connecting two or more LANs  Repeater / Hub L1: Physical Layer Forwards everything everywhere Limitations: <= 2500m and <= 1024 nodes  Bridge (or LAN switch) L2: Datalink Layer Forwards frames using MAC address Static configuration + partial dynamic configuration (Spanning Tree Protocol)  Router L3: Network Layer Forwards 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 frames 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 forwarding a frame to 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 forwards frame to all ports except to LAN H. B7 receives frame 7 & forwards to LAN B B5 forwards frame to LAN A & D. B1 receives this frame (again) and registers an entry for X. B1 forwards it to all ports except to LAN H & D. Node Y eventually receives a frame.  Problem: Duplicated frames are forwarded along loops. Spanning Tree Algorithm  Inactivate bridge ports so that no cycle exists in extended LAN  Radia Perlman, “Mother of the Internet”  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 a bridge learns it is not the root, it stops generating configuration messages When a bridge learns it is not a designated bridge for a LAN, it stops forwarding configuration messages In steady state:  The network (tree) has exactly 1 root (only bridge generating config msgs)  Each LAN has exactly 1 designated bridge (only bridges forwarding config msgs) If any bridge does not receive a 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 reigning root  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

Another STP illustration Determine  RPs (root ports)  DPs (designated ports)  BPs (blocked ports) CSS432: Switching and Fowarding 15

Another STP illustration Determine  RPs (root ports)  DPs (designated ports)  BPs (blocked ports) CSS432: Switching and Fowarding 16

One more STP illustration CSS432: Switching and Fowarding 17

18 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

19 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 for small data) Commonly transmitted over SONET Error correction  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

20 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

21 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

22 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

23 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

24 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

25 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

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

27 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