Routing and Switching Fabrics

1
Routing and Switching Fabrics

• Outline
• Link state routing
• Link weights
• Switching Fabrics

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

3
Link State (Dijkstras 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

4
Link State contd.

• Send out LSP based on timer and triggers
• Timer should be coarse grained
• Seqnos do not wrap around
• Since when routers reboot they start at seqno 0
• 64 bit field
• Routing table is not computed until LSPs from
  all nodes have been received

5
Route Calculation

• Dijkstras 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 ))

6
Example
5
3
C
B
2
5
3
2
F
A
1
1
1
2
1
E
D

• 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

G
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?

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

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

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

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!

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?

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