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 B C 2 5 3 A 2 1 F 1 1 1 2 E D
Itrn M B Path C Path D Path E Path F Path G Path
1 {A} 2 A-B A-C A-D Inf Inf A-G
2 {A,D} 2 A-B A-D-C A-D A-D-E Inf A-G
3 {A,D,G} 2 A-B A-D-C A-D A-D-E Inf A-G
4 {A,B,D,G} 2 A-B A-D-C A-D A-D-E Inf A-G
5 {A,B,D,E,G} 2 A-B A-D-E-C 1 A-D A-D-E A-D-E-F 1 A-G
6 {A,B,C,D,E 2 A-B A-D-E-C 1 A-D A-D-E A-D-E-F 1 A-G G}
7 {A,B,C,D,E 2 A-B A-D-E-C 1 A-D A-D-E A-D-E-F 1 A-G
F,G} 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 Size Cost Functionality
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