1. CSS432 Routing Textbook Ch3.3
Prof. Athirai Irissappane
CSS 432: Routing

2. What Is Routing? Forwarding vs Routing forwarding: routing:
To map a network # to an outgoing interface and some MAC information in a forwarding table.
To send a packet to an interface as consulting a local and static forwarding table
OSI Layer 2: data link level
Implemented in specialized hardware (switch)
routing:
To build a dynamic routing table
To update table contents in a dynamic and distributed fashion
OSI Layer 3: network level (internet)
Using complex distributed algorithms
CSS 432: Routing

3. The University of Adelaide, School of Computer Science
3 January 2018
Routing
Network as a Graph
The basic problem of routing is to find the lowest-cost path between any two nodes
Where the cost of a path equals the sum of the costs of all the edges that make up the path
Chapter 2 — Instructions: Language of the Computer

4. The University of Adelaide, School of Computer Science
3 January 2018
Routing
For a simple network, we can calculate all shortest paths and load them into some nonvolatile storage on each node.
Such a static approach has several shortcomings
It does not deal with node or link failures
It does not consider the addition of new nodes or links
It implies that edge costs cannot change
What is the solution?
Need a distributed and dynamic protocol
Two main classes of protocols
Distance Vector
Link State
Chapter 2 — Instructions: Language of the Computer

5. Distance Vector
Each node maintains a set of triples
(Destination, Cost, NextHop)
Starting assumption is that each node knows the cost of the link to each of its directly connected neighbors
An initial distance vector at node A
Destination
Cost
Next hop B 1 C D ∞ - E F G
CSS 432: Routing

6. The University of Adelaide, School of Computer Science
3 January 2018
Distance Vector
Initial distances stored at each node (global view)
Chapter 2 — Instructions: Language of the Computer

7. The University of Adelaide, School of Computer Science
3 January 2018
Distance Vector
The distance vector routing algorithm is sometimes called as Bellman-Ford algorithm
Every T seconds each router sends its distances to its neighbor who then updates their table based on the new information
Problems include fast response to good news and slow response to bad news. Also too many messages to update
Chapter 2 — Instructions: Language of the Computer

8. Distance Vector Exchange updates directly connected neighbors
periodically (on the order of several seconds)
whenever table changes/ notices failure (called triggered update)
Each update is a list of pairs (the routing table):
(Destination, Cost) (and next hop)
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 receive a “better” route
From B: (C,1)
(C, 1, C) < (C, 2, B)
From C: (D, 1)
(D, ∞, - ) > (D, 2, C)
From F: (G, 1)
(G, ∞, - ) > (G, 2, F)
Refresh existing routes; delete if they are expired
Destination
Cost
Next hop B 1 C D 2 E F G
CSS 432: Routing

9. The University of Adelaide, School of Computer Science
3 January 2018
Distance Vector
F sends (G, 1) to A
F can reach G with cost 1
A can reach F with cost 1
A can reach G with cost 2 (< infinity so update table)
C sends (D, 1) to A
C can reach D with cost 2
A can reach D with cost 2 (< infinity so update table)
C sends (B,1) to A
A can reach B with cost 2 (> 1 so do not update table)
Chapter 2 — Instructions: Language of the Computer

10. The University of Adelaide, School of Computer Science
3 January 2018
Distance Vector
Final distances stored at each node (global view – not known by the routers, who only know their row entry)
Chapter 2 — Instructions: Language of the Computer

11. The University of Adelaide, School of Computer Science
3 January 2018
Distance Vector
The routing table at each node stabilizes, i.e., become consistent, converges
Updates from neighbors
Periodically
Triggered
Node notices a link failure
Receives an update from neighbors that causes a change in its routing table
Chapter 2 — Instructions: Language of the Computer

12. Routing Loop ∞ 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 ∞
CSS 432: Routing

13. Count-to-infinity problem
The link from A to E fails (triggered/periodic update)
A advertises (to neighbors) , (E, ∞)
At same time, C does not know about E, as periodic update, advertises (E, 2)
B decides it can reach E in 3 hops (via C)
B receives (E, ∞) from A, (E, 2) from C
B changes its Routing table (triggers update)
B advertises (E, 3)
A decides it can reach E in 4 hops (via B)
A receives (E, 3) from B
Routing table of A has changes (triggered update)
A advertises (E, 4)
C decides that it can reach E in 5 hops…
Cycle goes on until cost get near to infinity A B
Destination
Cost
Next hop B 1 C D 2 E F G
Destination
Cost
Next hop A 1 C B D 2 E F G 3 4 ∞ B E 3 C
To see the problem clearly, imagine a subnet connected like A–B–C–D–E–F, and let the metric between the routers be "number of jumps". Now suppose that A is taken offline. In the vector-update-process B notices that the route to A, which was distance 1, is down – B does not receive the vector update from A. The problem is, B also gets an update from C, and C is still not aware of the fact that A is down – so it tells B that A is only two jumps from C (C to B to A), which is false. Since B doesn't know that the path from C to A is through itself (B), it updates its table with the new value "B to A = 2 + 1". Later on, B forwards the update to C and due to the fact that A is reachable through B (From C's point of view), C decides to update its table to "C to A = 3 + 1". This slowly propagates through the network until it reaches infinity (in which case the algorithm corrects itself, due to the relaxation property of Bellman–Ford).
Destination
Cost
Next hop A 1 B D E 2 F G C 5

14. Loop-Breaking Heuristics
Set infinity to 16
Scheme: Stop an infinity loop in 16.
Problem: No more 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, 2) to A
Split horizon with poison reverse
Scheme: Send the routing information learned from this neighbor as setting hop count to ∞.
Ex. B includes (E, 2, A) and thus sends (E, ∞, A)
Problem: Its slow convergence speed for large number of nodes, works for 2 node case
CSS 432: Routing

15. Routing Information Protocol (RIP)
Same as Distance Vector Routing (for graph model)
RIP for internetwork
Instead of cost of reaching other routers, it specifies cost of reaching networks
E.g., Router C advertises to Router A
It can reach Networks 2, 3 at cost 0.
CSS 432: Routing

16. Routing Information Protocol (RIP)
frame header
datagram heaader
UDP header
RIP Message
Cmd: 1-6
1: request
2: reply
Port: 520
Used by routed
Advertisement: 30secs
Table entry timeout: 3 mins.
Deleted in 60secs
Cmd
Ver
Routing domain
Addr family (net addr)
Route tag
Address of net 1
Subnet mask
Next hop address (1-16)
Distance to net 1
Addr family (net addr)
Route tag
Address of net 2
Subnet mask
Next hop address
Distance to net 2 (1-16)
25 entries

17. The University of Adelaide, School of Computer Science
3 January 2018
Link State Routing
Strategy: Send to all nodes (not just neighbors) information about directly connected links (not entire routing table).
Reliable flooding: Most recent copy of information about the directly connected links of all nodes
Route Calculation: Find the best route to destinations using the available information
Chapter 2 — Instructions: Language of the Computer

18. The University of Adelaide, School of Computer Science
3 January 2018
Link State Routing
Strategy: Send to all nodes (not just neighbors) information about directly connected links (not entire routing table).
Each node creates Link State Packet (LSP)
id of the node that created the LSP
cost of link to each directly connected neighbor
sequence number (SEQNO)
time-to-live (TTL) for this packet
Reliable Flooding
start SEQNO at 0 when reboot
generate new LSP periodically; increment SEQNO
store most recent LSP (larger the SEQNO, newer the LSP) from each node
If received LSP is new forward LSP to all nodes but one that sent it
decrement TTL of each LSP before forwarding; discard when TTL=0
Chapter 2 — Instructions: Language of the Computer

19. The University of Adelaide, School of Computer Science
3 January 2018
Link State
Reliable Flooding
Flooding of link-state packets. (a) LSP arrives at node X; (b) X floods LSP to A and C; (c) A and C flood LSP to B (but not X); (d) flooding is complete
Chapter 2 — Instructions: Language of the Computer

20. The University of Adelaide, School of Computer Science
3 January 2018
Shortest Path Routing
# Chapter Subtitle
In practice, each router computes its routing table directly from the LSP’s it has collected using a realization of Dijkstra’s algorithm called the forward search algorithm
Specifically each router maintains two lists, known as Tentative and Confirmed (route already calculated)
Each of these lists contains a set of entries of the form (Destination, Cost, NextHop)
Chapter 2 — Instructions: Language of the Computer

21. 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.
Myself: node whose routing table is to be computed
Do not consider the LS information for destination nodes already present in the confirmed list
CSS 432: Routing

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

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

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

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

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

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

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

29. OSPF Open Shortest Path first Protocol
OSPF (Protocol uses link state routing)
Authenticate information exchanged
Hierarchy: divide domains into areas
Load Balancing: Multiple routers to same destination same cost to distribute load
5 types of messages
OSPF needs to provide information about how to reach networks
A router running OSPF generates the following Link State advertisements LSA:
Advertisements about networks directly connected to router
Cost of the link to another router
CSS 432: Routing

30. 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
Header
Hello (reachability) (Type=1)
Database description (topology) (Type=2)
Link status request (Type=3)
Link status update (Type=4)
Link status acknowledgment (Type=5)
Advertisement (header type=4)
LS Age: = TTL
Type=1: link cost b/w routers
Link-State ID = Advertising Router
Seq # from the same router
Link ID = the other end route 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

31. 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 a 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

32. Metrics Cost of Links? Original ARPANET metric
All links cost 1 (shortest path = lowest number of hops)
Does not consider latency, bandwidth, current traffic
Original ARPANET metric
measures number of packets queued waiting to be transmitted on each link
took neither latency or bandwidth into consideration
Moves packets towards the shortest queue than to destination
Artificial measure of load
CSS 432: Routing

33. Metrics New ARPANET metric
stamp each incoming packet at queue with its arrival time (AT)
record departure time (DT) from router
when link-level ACK arrives, compute
Delay = (DT - AT) + Transmit + Latency
if timeout, reset DT to departure time for retransmission
link cost = average delay over some time period
Fine Tuning (metric should vary smoothly with time, not with very high variation)
compressed dynamic range (range at which the metric can fluctuate)
replaced Delay with link utilization
CSS 432: Routing

34. VPN ‘Virtual’ Private Network
Actually not a private network but virtually private
Public network made virtually private
IP tunneling
Create a tunnel such that hosts have limited connectivity
Routers are at the beginning and end of the tunnel
At the beginning of the tunnel encapsulate the IP datagram into another IP datagram with destination address of the end router
Once the packet reaches the end router, it extracts the original packet from the payload and send it to the destination
CSS 432: Routing

35. Virtual Private Networks and Tunnels
Application
Level A B
Router
Dest router
Source router
Router
Level A B
To:
To:
To:
To:
Internet C
A can communicate only with B
Even though it uses a router that can send messages to the rest of the internet
A’s messages are tunneled and will read which connects to B
A can never connect with other members in the internet
Company
Branch
Company
Branch
Physical
Network Level
To: A
To:
To: B
To:
To:
CSS 432: Routing

36. Why VPN? Security Routers Carry No-IP packets Mobile IPs
The final destination/contents of packet cannot be easily intercepted.
Routers
Routers with special features such as multicasting can form a virtual network.
Carry No-IP packets
Packets may be non-IP compatible packets.
Mobile IPs
The final destination may be a mobile computer.
CSS 432: Routing

37. NAT Network Address Translation
Reduce the distribution of IP address
All hosts need not have a globally unique IP address
Hosts need to have a unique address within the private network.
Hosts of Private Network have unique addresses within the network
If hosts need to communicate they go though the NAT box (implemented on the router, etc)
The NAT box translates the private IP into IP address of the device
The device implementing NAT is given 1/more globally unique IP
While sending data to the internet, the senders address will be masqueraded as the global IP assigned
The hosts outside the private network can send data only to the global IP
The NAT box then sends the data to the respective receiver
Host from the internet, outside the private network cannot communicate (initiate communication) with the hosts inside the private network
Hosts inside the private network can initiate communication with hosts in the internet
2 hosts inside the private network can communicate with the internet using the same global address. They will use different ports in the NAT device

38. Mobile IP
Sending host, Home Agent, Mobile Host belong to the same private network
How Sending host can send data to the Mobile agent?
How does the home agent intercept a packet that is destined for the mobile agent? --- Use ARP
How does the home agent then deliver the packet to the mobile host? – Use DHCP and VPN
Sending host
Internet
Home
agent
DHCP
server
Mobile Host
( )
Mobile Host
CSS 432: Routing

39. Mobile IP (Cont’d) Internet 1. ARP request: What’s the physical addr
corresponding to ?
Sending host
3. Packet request: sends a packet destined for
to the home agent’s MAC address
2. ARP response: sends back MAC of
instead of
1. DHCP: receives a new IP
in the foreign network.
Internet
Home
agent
DHCP
server
IP tunneling: wraps the packet inside an IP
header destined for the mobile host ( ).
Mobile Host
( )
Mobile Host
2. Care-of-address: a mobile host informs its
Home agent of its original and new IPs.
CSS 432: Routing

40. Reviews Exercises in Chapter 3
RIP: distance vector, routing loop and breaking heuristics
OSPF: link state, Dijkstra’s shortest path algorithm
VPN and mobile IP
Exercises in Chapter 3
Ex. 46 (RIP)
CSS 432: Routing