1. CSS432 Routing Textbook Ch 3.3
Instructor: Joe McCarthy (based on Prof. Fukuda’s slides)
CSS 432: Routing

2. Forwarding vs. Routing Forwarding: Routing:
Maps a network # to an outgoing interface and some MAC information
Sends a packet to an interface based on a local and ~static forwarding table
OSI Layer 2: data link level
Implemented in specialized hardware (switch)
Routing:
Maps network # to its next hop
Dynamically updated in a distributed fashion based on network conditions
OSI Layer 3: network level (IP)
Implemented in software using distributed algorithms
CSS 432: Routing

3. Network as a Graph Goal Static approach has shortcomings:
At Node A
Goal
Find lowest cost path between two nodes
Static approach has shortcomings:
Hardware may fail
Network topology may change
Bandwidth (cost) may change
Distributed, dynamic routing algorithms
Distance vector routing (RIP)
Link state routing (OSPF) E 3 F 1 2 D 6 C B
Next Hop
Cost
Destination
CSS 432: Routing

4. Distance Vector Each node maintains a table of triples:
(Destination, Cost, NextHop)
Initial costs:
1, if directly connected to Destination
∞ otherwise
Routing Table for A:
Destination
Cost
Next hop B 1 C D ∞ - E F G
CSS 432: Routing

5. Distance Vector Routing Table for A:
Exchange updates with directly connected neighbors
Every few seconds (periodic update)
Whenever your table changes (triggered update)
Each update is a list of pairs:
(Destination, Cost)
From B: (A, 1), (C, 1)
From C: (A, 1), (B, 1), (D, 1)
From E: (A, 1)
From F: (A, 1), (G, 1)
Update local table if you receive a “better” route, e.g., for A’s table:
From B: (C,1)
(C, 1, C) < (C, 2, B): new route is worse
From C: (D, 1)
(D, ∞, - ) > (D, 2, C): new route is better
From F: (G, 1)
(G, ∞, - ) > (G, 2, F): new route is better
Refresh existing routes; delete if they are expired
Routing Table for A:
Destination
Cost
Next hop B 1 C D 2 E F G
CSS 432: Routing

6. Distance Vector Table 3.13 Distance vectors at each node A B C D E F G1 2
Routing Table for A:
Destination
Cost
Next hop B 1 C D 2 E F G
[Each row is a distinct distance vector stored at the node named in column 1]
CSS 432: Routing

7. Distance Vector Table 3.13 Distance vectors at each node A B C D E F G1 2 3
Routing Table for A:
Destination
Cost
Next hop B 1 C D 2 E F G
[Each row is a distinct distance vector stored at the node named in column 1]
CSS 432: Routing

8. Routing Loops ∞ Failure-recovering scenario
F detects the link to G has failed
F sets distance to G to ∞ and sends an update to A
A sets distance to G to ∞
A receives periodic update from C with a 2-hop path to G
A sets distance to G to 3 and sends update to F
F sets distance to G in 4 hops via A
To G in 2
To G in 3
To G in 4
To G in 1 ∞
CSS 432: Routing

9. Routing Loops ∞ ∞ Failure-recovering scenario
F detects the link to G has failed
F sets distance to G to ∞ and sends an update to A
A sets distance to G to ∞
A receives periodic update from C with a 2-hop path to G
A sets distance to G to 3 and sends update to F
F sets distance to G in 4 hops via A
Another scenario:
The link from A to E fails
A advertises distance of ∞ to E
To G in 2
To G in 3
To G in 4
To G in 1 ∞ B A C ∞ E
CSS 432: Routing

10. Routing Loops ∞ ∞ Failure-recovering scenario
F detects the link to G has failed
F sets distance to G to ∞ and sends an update to A
A sets distance to G to ∞
A receives periodic update from C with a 2-hop path to G
A sets distance to G to 3 and sends update to F
F sets distance to G in 4 hops via A
Another scenario: Count-to-infinity problem
The link from A to E fails
A advertises distance of ∞ to E
C advertises a distance of 2 to E
B decides it can reach E in 3 hops (via C)
B advertises this to A
A decides it can reach E in 4 hops (via B)
A advertises this to C
C decides that it can reach E in 5 hops…
To G in 2
To G in 3
To G in 4
To G in 1 ∞ B
(5) To E in 4
(2) To E in ∞
(3) To E in 3
(4) To E in ∞
(1) To E in 2 A C
(6) To E in 5 ∞ E
CSS 432: Routing

11. Loop-Breaking Heuristics
Use some finite maximum # (e.g., 16) instead of infinity
Scheme: Stop an infinity loop at 16.
Problem: Limited to 16 hops
Split horizon
Scheme: Don’t send a neighbor the routing information learned from this neighbor
Ex. B includes (E, 2, A) and thus doesn’t send (E, 3) to A
Variation: Split horizon with poison reverse
Scheme: Send the routing information learned from this neighbor as setting hop count (cost) to ∞.
Ex. B includes (E, 2, A) and thus sends (E, ∞, A)
Problem (with both variants): slow convergence speed
CSS 432: Routing

12. Routing Information Protocol (RIP)
frame header
datagram heaader
UDP header
RIP Message
Cmd: 1-6
1: request
2: reply
UDP Port 520
Used by routers
Advertisement (periodic update)
Every 30 secs
Table entry timeout: 3 mins
Deleted in 60 secs
Used by Unix commands
ripquery (BSD)
tcpdump (available in Linux, too)
snoop (Solaris)
Cmd
Ver
Routing domain
Addr family (net addr)
Route tag
Address of Net 1
Net 1 subnet mask
Next hop address (1-16)
Distance to Net 1
Addr family (net addr)
Route tag
Address of Net 2
Net 2 subnet mask
Next hop address
Distance to Net 2 (1-16)
25 entries
CSS 432: Routing

13. Link State
Strategy
Reliable dissemination of link-state information to all nodes over a system.
Calculation of routes from the sum of all the accumulated link-state knowledge.
Link State Packet (LSP)
ID of the node that created the LSP
Costs of links to each directly connected neighbor
Sequence number (SEQNO)
Time-To-Live (TTL) for this packet
CSS 432: Routing

14. Link State Reliable flooding: all nodes get LSPs from all other nodes
Store most recent LSP from each node
Forward LSP to all nodes but the one that sent it
Generate new LSP periodically
Increment SEQNO (start with 0 upon reboot)
Also generate LSP when link fails
Detected via periodic “hello” packets
Decrement TTL of each received LSP
Also periodically decrement stored LSPs (“aging”)
Discard when TTL=0
CSS 432: Routing

15. Dijkstra’s Shortest-Path Algorithm
Initialize Confirmed list with (myself, 0, -), Tentative with null list
For the node just added to the Confirmed list in the previous step, call it node Next, select its LSP
For each neighbor (Neighbor) of Next, calculate the cost (Cost) to reach Neighbor as the sum of the cost from myself to Next and from Next to Neighbor
If Neighbor is currently on neither the Confirmed nor the Tentative list, then add (Neighbor, Cost, Nexthop) to the Tentative list, where Nexthop is the direction I go to reach Next
If Neighbor is currently on the Tentative list, and the Cost is less than the currently listed cost for Neighbor, then replace the current entry with (Neighbor, Cost, Nexthop) where Nexthop is the direction I go to reach Next
If the Tentative list is empty, stop. Otherwise, pick the entry from the Tentative list with the lowest cost, move it to the Confirmed list, and return to Step 2.
CSS 432: Routing

16. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

17. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

18. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

19. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

20. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

21. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

22. Dijkstra’s Shortest-Path Algorithm
CSS 432: Routing

23. Dijkstra’s Shortest-Path Algorithm
Alternative version (from original textbook slides):
N: set of nodes in the graph
l((i, j): the non-negative cost associated with the edge between nodes i, j N and l(i, j) =  if no edge connects i and j
Let s N be the starting node which executes the algorithm to find shortest paths to all other nodes in N
Two variables used by the algorithm
M: set of nodes incorporated so far by the algorithm
C(n) : the cost of the path from s to each node n
The algorithm
M = {s}
For each n in N – {s}
C(n) = l(s, n)
while ( N  M)
M = M  {w} such that C(w) is the minimum
for all w in (N-M)
For each n in (N-M)
C(n) = MIN (C(n), C(w) + l(w, n))
CSS 432: Routing

24. Dijkstra’s Shortest-Path Algorithm
Yet another alternative:
1 Initialization:
2 N' = {u}
3 for all nodes v
if v adjacent to u
then D(v) = c(u,v)
else D(v) = ∞ 7
8 Loop
9 find w not in N' such that D(w) is a minimum
10 add w to N'
11 update D(v) for all v adjacent to w and not in N' :
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
shortest path cost to w plus cost from w to v */
15 until all nodes in N'
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012
CSS 432: Routing

25. Open Shortest Path First Protocol (OSPF)
frame header
datagram header
OSPF header
OSPF Message
Version
Type(=4)
AreaId
Message Length
Checksum
Authentication 0-3
Authentication type
SourceAddr
Authentication 4-7
# of link status advertisements
Link-state ID
LS Age
Options
Advertising router
LS sequence number
Link Checksum
Length
Flag
# of links
Type=1
Link ID
Link data
Metric
Num TOS
Link type
Optional TOS information
Type:
Hello (reachability)
Database description (topology)
Link status request
Link status update
Link status acknowledgment
Advertisement (header type=4)
LS Age: = TTL
Type=1: link cost between routers
Link-State ID = Advertising Router
Seq # from the same router
Link ID = the other end router ID of link
Link data = used if there are two or more links to the same router
Metric = link cost
Link type = P2P, ethernet, etc
TOS = delay-sensitive, etc
CSS 432: Routing

26. OSPF Con’td Gated daemon: directly uses IP datagram
Header Type2: Database description (topology) message
Used when the current topology has changed.
Sent from an initialized router to another router which has topology information
LS Sequence number
Used to determine which message is the latest
Send a message with a new sequence number and metric = ∞ when a router or a link fails.
CSS 432: Routing

27. Reviews Exercises in Chapter 3
RIP: distance vector, routing loop and breaking heurictics
OSPF: link state, Dijkstra’s shortest path algorithm
Exercises in Chapter 3
Ex. 46 (RIP)
Ex. 52 (RIP)
Ex. 62 (OSPF)
Ex. 64 (OSPF)
CSS 432: Routing

28. Ex 46
For the network shown in Figure 3.53, give global distance-vector tables like those of Tables 3.10 and 3.13 when
(a) Each node knows only the distances to its immediate neighbors
(b) Each node has reported the information it had in the preceding step to its immediate neighbors.
(c) Step (b) happens a second time.
Table 3.13
CSS 432: Routing

29. Ex 52
Suppose we have the forwarding tables shown in Table 3.16 for nodes A and F in a network where all links have cost 1. Give a diagram of the smallest network consistent with these tables.
Forwarding Table for A
Forwarding Table for F
Node
Cost
NextHop B 1 C 2 D E F 3
Node
Cost
NextHop A 3 E B 2 C 1 D
CSS 432: Routing

30. Ex 62
Give the steps in the forward search algorithm as it builds the routing database for node A in the network shown in Figure 3.59.
CSS 432: Routing

31. Ex 64
Suppose that nodes in the network shown in Figure 3.61 participate in link-state routing, and C receives contradictory LSPs: One from A arrives claiming the A-B link is down, but one from B arrives claiming the A-B link is up.
(a) How could this happen?
(b) What should C do? What can C expect?
Do not assume that the LSPs contain any synchronized timestamp.
CSS 432: Routing