Routing and Switching Fabrics

1. Routing and Switching Fabrics Outline Link state routing Link weights Switching Fabrics CS 640

2. Intradomain Routing Summary • Intradomain routing protocols determine how forwarding tables are maintained in routers • Least cost algorithms • Distance vector routing • Algorithm based on building forwarding table by distributing vector of distances to neighbors • Completely distributed and based only on knowledge of immediate neighbors • Known to converge under static conditions • Count to infinity problem • Limited network diameter CS 640

3. Link State (Dijkstra’s algorithm,OSPF) • Find SP from a given node by sending path data to all nodes and developing paths in order of increasing length • Strategy • Route calculation is based on sum of all accumulated link state information • All nodes forward all information to all directly connected links • Link State Packet (LSP) – created by each node • id of the node that created the LSP • cost of the link to each directly connected neighbor • sequence number (SEQNO) • time-to-live (TTL) for this packet CS 640

4. Link State contd. • Send out LSP based on timer and triggers • Timer should be coarse grained • Seqno’s do not wrap around • Since when routers reboot – they start at seqno 0 • 64 bit field • Routing table is not computed until LSP’s from all nodes have been received CS 640

5. Route Calculation • Dijkstra’s shortest path algorithm • Let • N denotes set of nodes in the graph • l (i, j) denotes non-negative cost (weight) for edge (i, j) • s denotes this node • M denotes the set of nodes incorporated so far • C(n) denotes cost of the path from s to node n M = {s} for each n in N - {s} C(n) = l(s, n) /* Costs of directly connected nodes */ while (N != M) M = M union {w} such that C(w) /* Add a node */ is the minimum for all w in (N - M) for each n in (N - M) /* Recalculate costs */ C(n) = MIN(C(n), C (w) + l(w, n )) CS 640

6. Example 5 3 C B 2 5 3 2 F A 1 Itrn M B Path C Path D Path E Path F Path G Path 1 {A} 2 A-B 5 A-C 1 A-D Inf. Inf. 1 A-G 2 {A,D} 2 A-B 4 A-D-C 1 A-D 2 A-D-E Inf. 1 A-G 3 {A,D,G} 2 A-B 4 A-D-C 1 A-D 2 A-D-E Inf. 1 A-G 4 {A,B,D,G} 2 A-B 4 A-D-C 1 A-D 2 A-D-E Inf. 1 A-G 5 {A,B,D,E,G} 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 4 A-D-E-F 1 A-G 6 {A,B,C,D,E 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 4 A-D-E-F 1 A-G G} 7 {A,B,C,D,E 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 4 A-D-E-F 1 A-G F,G} 1 1 2 1 E D G CS 640

7. Link State Routing Summary • One of the oldest algorithm for routing • Finds SP by developing paths in order of increasing length • Requires each node to have complete information about the network • Nodes exchange information with all other nodes in the network • Known to converge quickly under static conditions • Does not generate much network traffic • Other possible routing algorithms? CS 640

8. Metrics for link cost • Simplest method is to simply assign 1 to each link • Original ARPANET metric • link cost = number of packets enqueued on each link • This moves packets toward shortest queue not the destination!! • took neither latency or bandwidth into consideration • New ARPANET metric • link cost = average delay over some time period • stamp each incoming packet with its arrival time (AT) • record departure time (DT) • when link-level ACK arrives, compute Delay = (DT - AT) + Transmit + Latency Transmit and latency are static for the link • if timeout, reset DT to departure time for retransmission • Fine Tuning • compress range over which costs can span using static function • smooth variation of cost over time using averaging CS 640

9. Introduction to switching fabrics • Switches must not only determine routing but also do forwarding quickly and efficiently • If this is done on a general purpose computer, the I/O bus limits performance • This means that a system with 1Gbps I/O could not handle OC12 • Special purpose hardware is required • Switch capabilities drive protocol decisions • Context – a “router” is defined as a datagram “switch” • Switching fabrics are internal to routers and facilitate forwarding CS 640

10. Goals in switch design • Throughput • Ability to forward as many pkts per second as possible • Size • Number of input/output ports • Cost • Minimum cost per port • Functionality • QoS CS 640

11. Throughput • Consider a switch with n inputs and m outputs and link speed of sn • Typical notion of throughput: Ssn • This is an upper bound • Assumes all inputs get mapped to a unique output • Another notion of throughput is packets per second (pps) • Indicates how well switch handles fixed overhead operations • Throughput depends on traffic model • Goal is to be representative • This is VERY tricky! CS 640

12. Size/Scalability/Cost • Maximum size is typically limited by HW constraints • Eg. fanout • Cost is related to number of inputs/outputs • How does cost scale with inputs/outputs? CS 640

13. Ports and Fabrics • Ports on switches handle the difficult functions of signaling, buffering, circuits, RED, etc. • Most buffering is via FIFO on output ports • This prevents head-of-the-line blocking on input ports which is possible if only one input port can forward to one output port at a time • Switching fabrics in switches handle the simple function of forwarding data from input ports to output ports • Typically fabric does not buffer (but it can) • Contention is an issue • Many different designs CS 640