1. Switch controller: Routing
Malathi Veeraraghavan
University of Virginia
Connectionless packet switch
Routing
Forwarding
Summarized (aggregated) addresses
Control path and data path
Path computation: Dijkstra’s algorithm
Examples used in practice

2. A network of connectionless packet switches
Control path: Routing (determine paths)
Switch controllers exchange routing information
Precompute forwarding tables
Data path: Forwarding (send packets)
Packets are switched from link to link across each switch

3. Goal of routing algorithms
Find "shortest" path on which to route packets or circuits, where the shortest path is determined by some metric, e.g.:
minimum-weight path (add link weights)
minimum end-to-end delay
path with the most available bandwidth
The algorithm should adapt to changes in
topology (includes administrator-set link weights)
reachability
identify the switch through which a destination address can be reached
destination could be several hops away
loading conditions

4. Analogy
In the roadways transportation network, where does a driver obtain the three types of routing information:
topology information?
reachability information?
loading information?
Topology information: Maps
Reachability information: Maps
Loading information: Radio broadcasts

5. Difference between topology and reachability information
Switch II
Host III-B
Host I-A
Switch III
Switch I
Host III-C
Switch IV
Host IV-A
Host IV-B
Relative to Switch IV:
Topology information: Switch IV has links to hosts IV-A & IV-B (in addition to links to other switches)
Reachability information: Switch IV can reach hosts III-B and III-C, host I-A
addresses that are more than one hop away
implicit: reachability to directly connected hosts (part of topology information)

6. One approach: Routing protocol exchanges + Forwarding table precomputation
Link-state routing protocols
Routing protocol exchanges
Switch controllers send/receive routing protocol messages to exchange information about topology, reachability and loading conditions with each other
Each switch controller thus acquires a picture of the whole network
Forwarding table pre-computation
Each switch controller then executes a shortest-path algorithm, such as Dijkstra's algorithm, to compute the shortest paths from its switch to all other switches
Each switch controller then updates a forwarding table, which shows the next-hop and/or output port for each destination address

7. Summarized addresses What are summarized addresses?
An address that represents a group of endpoint addresses
e.g., all 434 numbers, IP addresses
Why summarize addresses?
To reduce forwarding table sizes – hold one entry for a summarized address instead of a large number of individual addresses
To reduce routing message lengths that convey reachability information

8. (will have other entries)
Control path: Routing protocol exchanges + forwarding table pre-computation
Dest.
Next hop
III-B
III-C II 4 5 1
Host I-A I
Host III-B
III
Dest.
Next hop
III-*
III IV
Forwarding table
(will have other entries) 1 1 IV
Host III-C
I, II, III, IV: switches
Link weights are shown next to links
Host interface addresses are derived from switch addresses (e.g. I-A is connected to switch I)
A few example forwarding table entries shown at switches I, III, IV
III-*: summarized address for all hosts connected to switch III

9. Data path: User-data transfer (e.g., packet destined to Host III-B)
Packet header
Packet payload
Dest.
Next hop
III-B
III-C
III-B II
Host I-A
III-B I
III-B
III-B
III
Host III-B IV
Dest.
Next hop
III-* IV
Host III-C
Dest.
Next hop
III-*
III
Packet header carries destination host interface address (unchanged as it passes hop by hop)
Each packet switch does a forwarding table lookup to determine the outgoing next hop switch

10. Next-hop and/or output ports in forwarding table4
Switch II 5
Host III-B
Host I-A b a
Switch III
Switch I 1 b c 1 a c
Host III-C
Switch IV 1
Dest.
Next hop
III-* IV
Output port c e d
Host IV-A
Host IV-B
link weight
Forwarding table
(will have other entries)
Dest.
Next hop
Output port
III-*
III ?
IV-A ?
IV-B ?
Forwarding table
(will have other entries)

11. Forwarding table precomputation: Dijkstra’s algorithm
Find shortest paths from source node s to all other nodes.
At each iteration, determine the “next closest node ” from the source s.
For each node that is still not in set M (the set containing nodes to which shortest paths from the source have already been computed), determine if its current distance from s is shorter than the path routed via the selected “next closest node. ”
Find the “next closest node” after finding all the distances.
Move “next closest node ” into set M.
When the set M has all the nodes, stop.
"Node" in this application of Dijkstra's algorithm represents a network "Switch."

12. Dijkstra's algorithm s: source node.
wsn: administrator-set link weight for the link from node s to node n
M = {s};
for each n  M
Dn = wsn;
while (M  all nodes) do
Find i  M for which Di = min{Dj ; j  M};
Add i to M;
Dn = mini [ Dn, Di + win ];
Update route;
enddo

13. Example 1 2 3 4 5 6
Administrator-set
link weights
w56=2

14. List path weight and routeM D2 D3 D4 D5 D6
{1}
2, {1-2}
5, {1-3}
1, {1-4}  1
{1,4}
4, {1-4-3} 2 3 4
Simple shortest path: Only static administrator-set link weights considered
Loading information not considered in this computation of shortest paths

15. Resulting forwarding table
Forwarding table at node 1 Destination Next Hop

16. Examples used in practice
Addressing
IP-routed networks (e.g., Internet)
4-byte IP addresses
Hierarchical addresses
Address summarization done
Ethernet switched network
6-byte MAC address
Flat addresses
No address summarization

17. Examples used in practice
IP-routed networks (e.g., Internet)
Routing protocols
Open Path Shortest First (OSPF)
Border Gateway Protocol (BGP)
Ethernet switched networks
Forwarding table is built up as Ethernet frames are forwarded

18. Answer Dijkstra’s algorithm example
{1}
2, {1-2}
5, {1-3}
1, {1-4}  1
{1,4}
4, {1-4-3}
2, {1-4-5} 2
{1,4,2,5}
3, { }
4,{ } 3
{1,4,2,5,3} 4
{1,4,2,5,3,6}