1. Lecture 4: Dynamic routing protocols
Tuesday:
Overview of router architecture
RIP
Thursday:
OSPF, BGP

2. Review of RIP
A distance vector routing protocol

3. A router updates its distance vectors using bellman-ford equation
Define
dx(y) := cost of the least-cost path from x to y
Then
dx(y) = minv{c(x,v) + dv(y) }, where min is taken over all neighbors of node x

4. Distance vector algorithm: initialization
Let Dx(y) be the estimate of least cost from x to y
Initialization:
Each node x knows the cost to each neighbor: c(x,v). For each neighbor v of x, Dx(v) = c(x,v)
Dx(y) to other nodes are initialized as infinity.
Each node x maintains a distance vector (DV):
Dx = [Dx(y): y 2 N ]

5. Distance vector algorithm: updates
Each node x sends its distance vector to its neighbors, either periodically, or triggered by a change in its DV.
When a node x receives a new DV estimate from a neighbor v, it updates its own DV using B-F equation:
If c(x,v) + Dv(y) < Dx(y) then
Dx(y) = c(x,v) + Dv(y)
Sets the next hop to reach the destination y to the neighbor v
Notify neighbors of the change
The estimate Dx(y) will converge to the actual least cost dx(y)

6. The Count-to-Infinity ProblemA 1 B 1 C
A's Routing Table
B's Routing Table
via
via to
cost to
cost
(next hop)
(next hop) C B 2 C C 1
now link B-C goes down 1 C B 2 C - 1 C 2 C 1 C - C A 3 1 C C 3 1 C B 4 C - 1 C 4 C

7. Count-to-Infinity
The reason for the count-to-infinity problem is that each node only has a “next-hop-view”
For example, in the first step, A did not realize that its route (with cost 2) to C went through node B
How can the Count-to-Infinity problem be solved?

8. Count-to-Infinity
The reason for the count-to-infinity problem is that each node only has a “next-hop-view”
For example, in the first step, A did not realize that its route (with cost 2) to C went through node B
How can the Count-to-Infinity problem be solved?
Solution 1: Always advertise the entire path in an update message to avoid loops (Path vectors)
BGP uses this solution

9. Count-to-Infinity
The reason for the count-to-infinity problem is that each node only has a “next-hop-view”
For example, in the first step, A did not realize that its route (with cost 2) to C went through node B
How can the Count-to-Infinity problem be solved?
Solution 2: Never advertise the cost to a neighbor if this neighbor is the next hop on the current path (Split Horizon)
Example: A would not send the first routing update to B, since B is the next hop on A’s current route to C
Split Horizon does not solve count-to-infinity in all cases!
You can produce the count-to-infinity problem in Lab 4.

10. RIP - Routing Information Protocol
A simple intradomain protocol
Straightforward implementation of Distance Vector Routing
Each router advertises its distance vector every 30 seconds (or whenever its routing table changes) to all of its neighbors
RIP always uses 1 as link metric
Maximum hop count is 15, with “16” equal to “”
Routes are timeout (set to 16) after 3 minutes if they are not updated

11. RIP - History
Late 1960s : Distance Vector protocols were used in the ARPANET
Mid-1970s: XNS (Xerox Network system) routing protocol is the ancestor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol)
1982 Release of routed for BSD Unix
1988 RIPv1 (RFC 1058) classful routing
1993 RIPv2 (RFC 1388) adds subnet masks with each route entry allows classless routing
1998 Current version of RIPv2 (RFC 2453)

12. RIPv1 Packet Format 1: RIPv1 1: request 2: response 2: for IP
Address of destination
Cost (measured in hops)
One RIP message can have up to 25 route entries

13. RIPv2 RIPv2 is an extends RIPv1:
Subnet masks are carried in the route information
Authentication of routing messages
Route information carries next-hop address
Uses IP multicasting
Extensions of RIPv2 are carried in unused fields of RIPv1 messages

14. RIPv2 Packet Format 2: RIPv2 1: request 2: response 2: for IP
Address of destination
Cost (measured in hops)
One RIP message can have up to 25 route entries

15. RIPv2 Packet Format 2: RIPv2
Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes
Subnet mask for IP address
Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0)

16. RIP Messages
This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520.
Two types of messages:
Request messages
used to ask neighboring nodes for an update
Response messages
contains an update

17. Routing with RIP
Initialization: Send a request packet (command = 1, address family=0..0) on all interfaces:
RIPv1 uses broadcast if possible,
RIPv2 uses multicast address , if possible
requesting routing tables from neighboring routers
Request received: Routers that receive above request send their entire routing table
Response received: Update the routing table
Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message
Triggered Updates: Whenever the metric for a route change, send entire routing table.

18. RIP Security Issue: Sending bogus routing updates to a router
RIPv1: No protection
RIPv2: Simple authentication scheme
2: plaintext password

19. RIP Problems RIP takes a long time to stabilize
Even for a small network, it takes several minutes until the routing tables have settled after a change
RIP has all the problems of distance vector algorithms, e.g., count-to-Infinity
RIP uses split horizon to avoid count-to-infinity
The maximum path in RIP is 15 hops

20. Dynamic Routing Protocols II OSPF
Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol.

21. Distance Vector vs. Link State Routing
With distance vector routing, each node has information only about the next hop:
Node A: to reach F go to B
Node B: to reach F go to D
Node D: to reach F go to E
Node E: go directly to F
Distance vector routing makes poor routing decisions if directions are not completely correct (e.g., because a node is down).
If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged. A B C D E F

22. Distance Vector vs. Link State Routing
In link state routing, each node has a complete map of the topology
If a node fails, each node can calculate the new route
Difficulty: All nodes need to have a consistent view of the network A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F

23. Link State Routing: Properties
Each node requires complete topology information
Link state information must be flooded to all nodes
Guaranteed to converge

24. Link State Routing: Basic principles
1. Each router establishes a relationship (“adjacency”) with its neighbors
2. Each router generates link state advertisements (LSAs) which are distributed to all routers
LSA = (link id, state of the link, cost, neighbors of the link)
Each router sends its LSA to all routers in the network (using a method called reliable flooding)
3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges
4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

25. Link state routing: graphical illustrationb 1 3 2 a c d 6
a’s view
Collecting all pieces yield
a complete view of the network! b 3 a 6 c
d’s view
b’s view 3 b 1 2 c d a c b
c’s view 1 a c d 6

26. Operation of a Link State Routing protocol
IP Routing Table
Link State Database
Dijkstra’s
Algorithm
Received LSAs
LSAs are flooded to other interfaces

27. Dijkstra’s Shortest Path Algorithm for a Graph
Input: Graph (N,E) with N the set of nodes and E the set of edges
cvw link cost (cvw = 1 if (v,w)  E, cvv = 0)
s source node.
Output: Dn cost of the least-cost path from node s to node n
M = {s};
for each n  M
Dn = csn;
while (M  all nodes) do
Find w  M for which Dw = min{Dj ; j  M};
Add w to M;
for each neighbor n of w and n  M
Dn = min[ Dn, Dw + cwn ];
Update route;
enddo

28. OSPF OSPF = Open Shortest Path First
The OSPF routing protocol is the most important link state routing protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)
The complexity of OSPF is significant
RIP (RFC 2453 ~ 40 pages)
OSPF (RFC 2328 ~ 250 pages)
History:
1989: RFC OSPF Version 1
1991: RFC OSPF Version 2
1994: RFC 1583 OSPF Version 2 (revised)
1997: RFC 2178 OSPF Version 2 (revised)
1998: RFC 2328 OSPF Version 2 (current version)

29. Features of OSPF Provides authentication of routing messages
Enables load balancing by allowing traffic to be split evenly across routes with equal cost
Type-of-Service routing allows to setup different routes dependent on the TOS field
Supports subnetting
Supports multicasting
Allows hierarchical routing

30. Hierarchical OSPF

31. Hierarchical OSPF Link-state advertisements only in area
Two-level hierarchy: local area, backbone.
Link-state advertisements only in area
each nodes has detailed area topology; only know direction (shortest path) to nets in other areas.
Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
Backbone routers: run OSPF routing limited to backbone.

32. Example Network 3 4 2 5 1
Link costs are called Metric
Metric is in the range [0 , 216]
Metric can be asymmetric .1 .2 .2 .4 .4 .6
/ 24
/ 24 .1
/ 24 .2 .4
Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address .6
/ 24
/ 24
/ 24
/ 24 .3 .5 .3 .5 .3
/24 .5

33. Link State Advertisement (LSA)
/ 24 .1 .2
/ 24
/ 24 .4
/ 24
/ 24
/ 24
/24
/ 24 .3 .5 .6 4 3 2
The LSA of router is as follows:
Link State ID: = Router ID
Advertising Router: = Router ID
Number of links: 3 = 2 links plus router itself
Description of Link 1: Link ID = , Metric = 4
Description of Link 2: Link ID = , Metric = 3
Description of Link 3: Link ID = , Metric = 0

34. Network and Link State Database
/ 24 .1 .2
/ 24
/ 24 .4
/ 24
/ 24
/ 24
/24
/ 24 .3 .5 .6
Each router has a database which contains the LSAs from all other routers
LS Type
Link
StateID
Adv. Router
Checksum LS
SeqNo
LS Age
Router-LSA
0x9b47 0x
Router-LSA
0x219e 0x
1618
Router-LSA
0x6b53 0x
1712
Router-LSA
0xe39a
0x a 20
Router-LSA
0xd2a6 0x 18
Router-LSA
0x05c3 0x
1680

35. Link State Database
The collection of all LSAs is called the link-state database
Each router has an identical link-state database
Useful for debugging: Each router has a complete description of the network
If neighboring routers discover each other for the first time, they will exchange their link-state databases
The link-state databases are synchronized using reliable flooding

36. OSPF Packet Format OSPF packets are not carried as UDP payload!
OSPF has its own IP protocol number: 89
TTL: set to 1 (in most cases)
Destination IP: neighbor’s IP address or (ALLSPFRouters) or (AllDRouters)

37. OSPF Packet Format 2: current version is OSPF V2
ID of the Area from which the packet originated
Message types:
1: Hello (tests reachability)
2: Database description
3: Link Status request
4: Link state update
5: Link state acknowledgement
0: no authentication
1: Cleartext password
2: MD5 checksum
(added to end packet)
Standard IP checksum taken over entire packet
Authentication passwd = 1: cleartext password
Authentication passwd = 2: 0x0000 (16 bits)
KeyID (8 bits)
Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)
Prevents replay attacks

38. OSPF LSA Format
LSA Header
Link 1
Link 2

39. Discovery of Neighbors
Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.
If two routers share a link, they can become neighbors, and establish an adjacency
After becoming a neighbor, routers exchange their link state databases
Scenario: Router restarts

40. Neighbor discovery and database synchronization
Scenario: Router restarts
Discovery of adjacency
After neighbors are discovered the nodes exchange their databases
Sends database description. (description only contains LSA headers)
Sends empty database description
Database description of
Acknowledges receipt of description

41. Regular LSA exchanges
explicitly requests each LSA from
Link State Request packets, LSAs =
Router-LSA, ,
Router-LSA, ,
Router-LSA, ,
Router-LSA, ,
Router-LSA, ,
Router-LSA, ,
sends requested LSAs
Link State Update Packet, LSAs =
Router-LSA, , 0x
Router-LSA, , 0x
Router-LSA, , 0x
Router-LSA, , 0x a
Router-LSA, , 0x
Router-LSA, , 0x

42. Routing Data Distribution
ACK
Routing Data Distribution
LSA-Updates are distributed to all other routers via Reliable Flooding
Example: Flooding of LSA from
LSA
LSA
ACK
LSA
ACK
Update database
Update database
Update database
ACK

43. Dissemination of LSA-Update
A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)
Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.
Acknowledgements of LSA-updates:
explicit ACK, or
implicit via reception of an LSA-Update
Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

44. Border Gateway protocol (BGP)

45. BGP
BGP = Border Gateway Protocol . Currently in version 4, specified in RFC (~ 60 pages)
Interdomain routing protocol for routing between autonomous systems
Uses TCP to establish a BGP session and to send routing messages over the BGP session
BGP is a path vector protocol. Routing messages in BGP contain complete routes.
Network administrators can specify routing policies

46. BGP policy routing
BGP’s goal is to find any path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.
Network administrator sets BGP’s policies to determine the best path to reach a destination network.

47. BGP basics
A route is defined as a unit of information that pairs a destination with the attributes of a path to that destination.
EBGP session is a BGP session between two routers in different ASes.
IBGP session is a BGP session between internal routers of an AS.

48. EBGP and IBGP R3 R2 R1 R4 R6 R5 R7 R8 128.195.0.0/16 0
/16 0 R2
AS 1 R1
AS 0 R4
/16 1 0 R6 R5
AS 2 R7 / R8
AS 3
IBGP is organized into a full mesh topology, or IBGP sessions are relayed using a route reflector.

49. Commonly BGP attributes
Origin: indicates how BGP learned about a particular route
IGP
EGP
Incomplete
AS path :
When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed
Next hop
Multi_Exit_Disc (MED, multiple exit discriminator):
-used as a suggestion to an external AS regarding the preferred route into the AS
Local_pref: is used to prefer an exit point from the local autonomous system
Community: apply routing decisions to a group of destinations

50. BGP route selection process
Routes sent
to peers
Routes recved from peers
Best
routes
Decision
process
Input
Policy
Engine
Out
Policy
Enigne
Input/output engine may filter routes or manipulate their attributes

51. Best path selection algorithm
If next hop is inaccessible, ignore routes
Prefer the route with the largest local preference value.
If local prefs are the same, prefer route with the shortest AS path
If AS_path is the same, prefer route with lowest origin (IGP < EGP < incomplete)
If origin is the same, prefer the route with lowest MED
IF MEDs are the same, prefer EBGP paths to IBGP paths
If all the above are the same, prefer the route that can be reached via the closest IGP neighbor.
If the IGP costs are the same, prefer the router with lowest router id.

52. Summary Router architectures Dynamic routing protocols: RIP, OSPF, BGP
RIP uses distance vector algorithm, and converges slow (the count-to-infinity problem)
OSPF uses link state algorithm, and converges fast. But it is more complicated than RIP.
Both RIP and OSPF finds lowest-cost path.
BGP uses path vector algorithm, and its path selection algorithm is complicated, and is influenced by policies.