1. Interior Routing Protocols Chapter 11&12: Routing in IP
CS 408 Computer Networks
Chapter 11&12: Routing in IP
Chapter 15

2. Introduction
Routers forward IP datagrams from one router to another on path from source to destination
Router must have idea of topology of internet and the best route to take
May depend the current conditions
Decisions based on some least cost criterion
Routing protocols
To decide on routes to be taken

3. Fixed Routing
Single permanent route configured for each source-destination pair
Routes fixed
May change when topology changes (not so often)
No dynamic updates

4. A Configuration of Routers and Networks

5. Routing Table One required for each router Entry for each network
Not for each destination
Routing only needs network portion in this example
Once datagram reaches router attached to destination network, that router can deliver to host
Each entry shows next node on route
Not whole route
Routing tables may also exist in hosts
If attached to single network with single router then not needed
All traffic must go through that router (called the gateway)
If multiple routers attached to network, host needs table saying which to use

6. Example Routing Tables

7. Adaptive Routing As conditions on internet changes, routes may change
Failure
of routers or networks
Congestion
If a particular section of the network is heavily congested, it is better not to use that part and change the route

8. Adaptive Routing - Challenges
More complex routing decisions
Router processing increases
Depends on information collected in one place but used in another
More information exchanged improves routing decisions but increases overhead
May react too fast causing congestion through oscillation
May react too slow
By the time routing decision changes, the network conditions may be much more different

9. Adaptive Routing - Challenges
Looping
Packet forwarded by router eventually returns to that router
May occur when changes in connectivity not propagated fast enough to all other routers
An important pathology that must be prevented
Algorithms designed to prevent looping

10. Classification of Adaptive Routing Strategies
Based on information sources
Local
E.g. route each datagram to network with shortest queue
Balance loads on networks
May not be heading in correct direction
Rarely used
Adjacent nodes
Delay and outage info from adjacent nodes
Distance vector algorithms
Discussed later
All nodes
Link-state algorithms

11. Flooding No network info required
Packet sent by node to every neighbor
Incoming packets retransmitted on every link except incoming link
Eventually a number of copies will arrive at destination
Each packet is uniquely numbered so duplicates can be discarded at destination

12. Flooding Example

13. Flooding Precautions against unlimited grow in circulation
Nodes can remember packets already forwarded to keep network load in bounds
Include a hop count in packets.
Set to a maximum value (e.g. diameter of the network)
Decrease one at each hop
Discard when 0

14. Properties of Flooding
All possible routes are tried
Very robust
can be used for emergency messaging
At least one packet will use minimum hop count route
Can be used to set up virtual circuit
All nodes are visited
Useful to distribute information (e.g. routing info)

15. Random Routing
Node selects one outgoing path for retransmission of incoming packet
Selection is at random
equally likely
all outgoing links are utilized in long-run
can select outgoing path based on a probability
e.g. probability based on data rate
good traffic distribution
No network info needed
Route is typically not least cost nor minimum hop

16. Autonomous Systems (AS)
Set of routers and networks managed by single organization (e.g. an ISP)
Exchange information
Common routing protocol
A connected network
There is at least one route between any pair of nodes

17. Interior Routing Protocol (IRP)
(not actually a protocol, for now just a concept)
Passes routing information between routers within AS
Need exchange of info among the routers only in AS
Different ASs may have different IRP mechanisms
ASs need to talk to each other
Need minimum information from other connected AS
A few routers in each AS must talk
Use Exterior Routing Protocol (ERP)
Again, a concept

18. Exterior Routing Protocol (ERP)
ERP passes less information than IRP
Router in first system determines route to target AS
Routers in target AS then co-operate to deliver datagram
ERP does not deal with details within target AS

19. Application of Exterior and Interior Routing Protocols

20. Approaches to Routing – Distance-vector
Each node (router or host) exchange information with neighboring nodes
Definition: Two nodes are said to be neighbors if both are directly connected to the same network
Each node keeps
a vector of link costs for each directly attached network,
distance and next-hop vectors for each destination
First generation routing algorithm for ARPANET
Used by Routing Information Protocol (RIP)
Requires transmission of lots of information by each router
Distance vector to all neighbors that contains estimated path costs
Changes take long time to propagate

21. Approaches to Routing – Link-state
Designed to overcome drawbacks of distance-vector
When router initialized, it determines link cost on each interface
Advertises set of link costs to all other routers in topology
Not just neighboring routers
After that, each router monitors its link costs
If significant change, router advertises new set of link costs
In this way, each router builds up a picture of the entire topology
Can calculate shortest path to each destination
Router constructs routing table, listing first hop to each destination
Use any routing algorithm to determine shortest paths
In practice, Dijkstra's algorithm
Open shortest path first (OSPF) protocol uses link-state routing.
Also second generation routing algorithm for ARPANET

22. Distance-vector and Link State
Both of them is suitable for IRP, not ERP
Several reasons. Some of them:
Both requires homogenous metrics that may be the case within an AS, but we cannot assume then same for several ASs
Distance vector routing does not disseminate the ASs that will be visited along the route
In ERP this info may be necessary to implement security and priority policies
Flooding the link state information across multiple ASs is not scalable

23. Exterior Router Protocols – Path-vector
Provide information about which networks can be reached by a given router and ASs crossed to get there
Does not include distance or cost estimate
Each block of information lists all ASs visited on this route
Enables router to perform policy routing
E.g. minimizing number of transit ASs
E.g. avoid path to avoid transiting particular AS due to link speed, capacity, tendency to become congested, and security reasons
BGP (Border Gateway Protocol) is an example to path-vector routing protocol

24. Least Cost Algorithms
Routing decision is based on some least-cost criterion (minimization problem)
If minimize number of hops, link value 1
Link value may be inversely proportional to capacity, proportional to current load, or some combination
May differ in different two directions (e.g. if cost is queue length)
More formal problem definition
Cost of path between two nodes is sum of costs of links traversed
For each pair of nodes, find least cost path 
Dijkstra's algorithm
Bellman-Ford algorithm

25. Dijkstra's Algorithm
Find shortest paths from given node to all other nodes, by developing paths in order of increasing path length
Proceeds in stages
By kth stage, shortest paths up to k nodes closest to source have been determined
The nodes for which shortest path determined are kept in a set called T
At stage (k + 1), node not in T but has the shortest path from source added to T
As each node added to T, path from source defined for other nodes not in T

26. Dijkstra's Algorithm – Formal (1)
N = set of nodes in the network
s = source node
T = set of nodes so far incorporated (shortest path found)
w(i, j) = link cost from node i to node j
w(i, i) = 0
w(i, j) =  if nodes not directly connected
w(i, j)  0 if nodes are directly connected
L(n) = cost of current least-cost path from s to n
At the end of algorithm (actually as soon as n is added to T), L(n) is the cost of least-cost path from s to n

27. Dijkstra's Algorithm – Formal (2)
[Initialization]
T = {s}
i.e. set of nodes so far incorporated consists of only source node
L(n) = w(s, n) for all n ≠ s
i.e. initial path costs to neighboring nodes are link costs

28. Dijkstra's Algorithm – Formal (3)
Repeat
[Get Next Node]
Find neighboring node not in T with least-cost path from s
Find x Ï T such that
Add x to T. L(x) is the shortest path from s to x.
[Update Least-Cost Paths]
L(n) = min[L(n), L(x) + w(x, n)] for all n Ï T
If the latter term is the minimum, the path from s to n is now
the path from s to x concatenated with the edge from x to n.
Until all nodes are in T

29. Figure 11.5 Dijkstra’s Algorithm Applied to Figure 11.1

30. Bellman-Ford Algorithm
Iterative
find the shortest paths from a source to all possible destinations using only link
then using max. two links by adding appropriate links to the paths of step 1
then using max. 3 links on top of paths with two links
so on .. until no improvement is gained by adding more links

31. Bellman-Ford Algorithm – Formal (1)
s = source node
w(i, j) = link cost from node i to node j
w(i, i) = 0
w(i, j) =  if nodes are directly connected
w(i, j)  0 if nodes directly connected
h = maximum number of links in path at current stage
Lh(n) =cost of least-cost path from s to n such that path contains no more than h links

32. Bellman-Ford Algorithm – Formal (2)
[Initialization]
L0(n) = , for all n  s
h=0

33. Bellman-Ford Algorithm – Formal (3)
[Update]
Loop until no more improvements
For each n ≠ s, compute
If s-to-n cost reduced, then path also changes to s -…- j - n
h=h+1

34. Figure 11.6 Bellman-Ford Algorithm Applied to Figure 11.1

35. RIP (Routing Information Protocol)
Uses Distance Vector Routing approach
Each node exchanges information with neighbors
Directly connected by same network
Each node maintains three vectors
Link cost
For each network it attaches
Distance vector
Current cost of route from the node to each network in the configuration
Next hop vector
The next router for each network in the configuration
Every 30 seconds, exchange distance vector with neighbors
Use this to update distance and next hop vector
Similar to Bellman-Ford algorithm.

36. Distance Vector Algorithm Applied to Figure 11.1

37. RIP Details – Incremental Update
Previous algorithm implies that all distance vector updates arrive within a small window of time
Not correct, because (i) no synchronization, (ii) RIP uses UDP that means no reliability. RIP packets use UDP
Actually RIP is designed to operate incrementally. Tables are updated after receipt of individual distance vector

38. RIP Details – Topology Change
If no updates received from a router within 180 seconds, mark route invalid
Assumes router crash or network connection unstable
Set distance value to infinity
Actually 16. Why? See next.

39. Counting to Infinity Problem (1)
A problem of RIP is slow convergence to a change in topology
Consider the example network below with all link costs 1
B has distance to network 5 as 2, next hop D
A and C have distance 3 and next hop B

40. Counting to Infinity Problem (2)
Suppose router D fails:
B determines network 5 no longer reachable via D
Sets distance to 4 based on report from A or C
At next update, B tells A and C this
A and C receive this and increment their network 5 distance to 5
4 from B plus 1 to reach B
B receives distance count 5 and assumes network 5 is 6 away
Repeat until reach infinity (16)
Update interval is 30 seconds, so reaching 16 takes several minutes. If infinity is larger, then convergence could take longer.

41. Split Horizon Rule
Counting to infinity problem caused by misunderstanding between B and A, and B and C
Each thinks it can reach network 5 via the other
Split Horizon rule says do not send information about a route back in the direction it came from
Router sending information is nearer to the destination than you are
Erroneous route now eliminated within time out period (180 seconds)

42. Poisoned Reverse
Send updates with hop count of 16 to neighbors for routes through these neighbors
breaks loop immediately
Example
Before D crashes, Routers A and C inform B that it can reach network 5 through them with a cost of 16
So after D crashes, B never thinks that it can reach network 5 through A or C

43. Figure 11.9 RIP Packet Format

44. RIP Packet Format Notes
Command: 1=request 2=reply
Updates are replies whether asked for or not
Initializing node broadcasts request
Requests are replied to immediately
Version: 1 or 2
Address family: 2 for IP
IP address: non-zero network portion, zero host portion
Identifies particular network
Metric
Path distance from this router to network
Typically 1, so metric is hop count

45. RIP Limitations Destinations with metric more than 15 are unreachable
If larger metric allowed, convergence becomes lengthy
So RIP is not suitable for large networks
Simple metric leads to sub-optimal routing tables
Packets sent over slower links
Accept RIP updates from any device
Misconfigured device can disrupt entire configuration

46. Open Shortest Path First (OSPF)
RIP limited in large internets
OSPF preferred interior routing protocol for TCP/IP based internets
Link state routing used

47. Link State Routing
When initialized, router determines link cost on each interface
Router advertises these costs to all other routers in topology
Router monitors its costs
When changes occurs, costs are re-advertised
Each router constructs topology and calculates shortest path to each destination network
Can use any algorithm, but in practice Dijkstra is used

48. OSPF Overview Router maintains descriptions of state of local links
Transmits updated state information to all routers that it knows
Router receiving update must acknowledge
Each router maintains database that reflects the topology
Directed graph
And then generates a spanning tree and routing table

49. Router Database Graph Vertices (nodes) Edges Router Network
Connecting two routers
Connecting router to network

50. Sample Autonomous System

51. Directed Graph of Sample Autonomous System

52. The Spanning Tree for Router R6

53. Link Costs Cost of each hop in each direction is called routing metric
OSPF provides flexible metric scheme based on type of service (ToS)
Normal
Default metric assigned by administrators
Minimize monetary cost
Maximize reliability
Maximize throughput
Minimize delay
Each router generates 5 spanning trees (and 5 routing tables)

54. Areas Make large internets more manageable
Configured as a backbone and multiple areas
Area – Collection of contiguous networks and hosts plus routers connected to any included network
Not so different from AS, but smaller
Backbone – networks and routers that connect multiple areas as a central hub

55. Operation of Areas
Each area runs a separate copy of the link state algorithm
Topological database and graph of just that area
Link state information broadcast to other routers in area
Reduces traffic
Intra-area routing relies solely on local link state information

56. Inter-Area Routing Path consists of three legs Within source area
Intra-area
Delivers to the backbone
Through backbone
Has properties of an area
Uses link state routing algorithm for inter-area routing
Delivers to the destination area
Within destination area
Delivers to recipient

57. OSPF Packet Format
OSPF directly runs over IP
24-byte header

58. Header Fields Version number: 2 is current
Type: one of 5, see next slide
Packet length: in octets including header
Router id: this packet’s source, 32 bit
Area id: Area to which source router belongs
Authentication type: null, simple password or cryptographic
Authentication data: used by authentication procedure

59. OSPF Packet Types Hello: used in neighbor discovery
Periodically sent out the discover new neighbors
Database description: Defines set of link state information present in each router’s database
Content is not conveyed, just the structure
Routers synchronize their network topologies
Link state request
Link state update
Link state acknowledgement

60. Border Gateway Protocol (BGP)
For use with TCP/IP internets
Preferred EGP of the Internet
Allows routers (gateways) in different ASs to exchange routing information
Current version is BGP-4
RFC 1771
Messages sent over TCP
Types of messages coming in a few slides later

61. BGP Procedures (details later) Neighbor acquisition
shall we exchange routing info?
initial step
Neighbor reachability
are we still connected?
periodic keepalive messages
Network reachability
key part of the protocol
exchange routing information for the networks that can be reached

62. Neighbor Acquisition
Definition: Two routers are neighbors if they attach to same subnetwork
If in different ASs, routers may wish to exchange information
But first they have to agree
Neighbor acquisition occurs when two neighboring routers agree to exchange routing information regularly
Needed because one router may not wish to participate due to several reasons
E.g. overloaded
One router sends request, the other accepts or refuses
Knowledge of existence of other routers and decision of acquisition established at configuration time or by active intervention (manual configuration)

63. Neighbor Acquisition Neighbors attach to same subnetwork
If in different ASs routers may wish to exchange information
Neighbor acquisitionis when two neighboring routers agree to exchange routing information regularly
Needed because one router may not wish to take part
One router sends request, the other acknowledges
Knowledge of existence of other routers and need to exchange information established at configuration time or by active intervention

64. Neighbor Reachability
Needed to maintain the relationship
Each party should make sure that other is still alive
Periodic issue of keepalive messages between all routers that are neighbors

65. Network Reachability
Each router keeps database of subnetworks it can reach and preferred route
When change made, router broadcast update message
All BGP routers build up and maintain routing information

66. BGP Messages Open Keepalive Update Notification
Start neighbor relationship with another router
Keepalive
Acknowledge open message
Periodically confirm neighbor relationship
Update
Transmit information about single route
List multiple routes to be withdrawn
Notification
Send when error condition detected

67. BGP Message Formats Marker Length Type
Reserved for authentication data
Length
Of the message in octets
Type
Of message
Open, Update, Keepalive, Notification

68. Neighbor Acquisition Detail
Router opens TCP connection with neighbor
Sends Open message
Identifies sender’s AS
Includes Hold Time proposed by sender
If recipient prepared to open neighbor relationship
Calculate hold time
min [own hold time, received hold time]
This is the max amount of time between sucessive keepalive/update messages
Reply with keepalive

69. Keepalive Detail Header only
Sent often enough to prevent hold time expiring

70. Update Detail Two types of information
A possible route
List of previously advertised one or more routes being withdrawn
May contain both
Information about single route through internet
Information to be added to database of any recipient router
Network layer reachability information (NLRI)
List of IP addresses of subnets that can be reached by this route
Total path attributes length field
Path attributes field (next slide)
List of attributes that apply to this particular route

71. Some Path Attributes Field
AS_Path
List of ASs traversed for this route
A router may implement routing policies based on this info
Security related
Performance related
Next_Hop
IP address of border router for next hop
Multi_Exit_disc
To choose among multiple border routers
Atomic_Aggregate, Aggregator
Uses subnet addresses in tree view of network to reduce information needed in NLRI

72. BGP Routing Information Exchange
Within AS, router builds topology picture using IGP
Router issues Update message to other routers outside AS using BGP
These routers exchange info with other routers in other AS
Routers must then decide best routes

73. BGP Routing Information Exchange (example)
R1 constructs routing table for AS1 using OSPF
R1 issues update message to R5 (in AS2)
AS_Path: identity of AS1
Next_Hop: IP address of R1
NLRI: List of all subnets in AS1
Suppose R5 has neighbor relationship with R9 in AS3
R5 forwards information received from R1 to R9 in update message
AS_Path: list of ids {AS2,AS1}
Next_Hop: IP address of R5
NLRI: All subnets in AS1
R9 decides if this is preferred route and forwards to neighbors
With AS_Path field {AS1, AS2, AS3}

74. Withdrawal of Route(s)
Oneror more routers could be removed in one Update message
Route(s) is/are identified by IP address of destination subnetwork(s)

75. Notification Message (some)
Error notification
Message header error
Includes authentication and syntax errors
Open message error
Syntax errors and option not recognised
Proposed hold time unacceptable
Update message error
Problems in Update Message
Hold timer expired
If a router does not get a message (Keepalive/Update/Notification) then it sends this error and connection is closed

76. Inter-Domain Routing Protocol (IDRP)
Exterior routing protocol designed for IPv6
ISO-OSI standard
AS BGP, based on Path-vector routing concept
Superset of BGP. Key differences are:
Operates over any internet protocol (not just TCP)
Own handshaking for guaranteed delivery
Variable length AS identifiers (in BGP fixed 16-bit)
Handles multiple internet protocols and address schemes
In BGP, a path is sequence of ASs. IN IDRP a set of connected ASs may be grouped as routing domain confederations

77. Routing Domain Confederations
Set of connected ASs
Appear to outside world as single AS
Recursive, i.e. confederations can be grouped as well
Scales effectively for large internets