1. Distance Vector and Link State RIP OSPF
Dynamic Routing
Distance Vector and Link State
RIP
OSPF

2. Internet Routing IP implements datagram forwarding
Both hosts and routers
Have an IP module
Forward datagrams
IP forwarding is table-driven
Table known as routing table

3. Routing Tables Static routing Dynamic routing
Fixes routes at boot time
Requires human intervention
Useful only for simplest cases
Dynamic routing
Table initialized at boot time
Routers communicate to learn new information and update their routing tables continuously:
Protocols used for information exchange to propagate route data
Data inserted/updated by protocols automatically - necessary in large internets

4. Autonomous Systems
An autonomous system (AS) is a region of the Internet that is administered by a single entity and that has a unified routing policy
Each autonomous system is assigned an Autonomous System Number (ASN). Each ASN is either 16bits or 32bits
ASN assigned by Regional Internet Registries
Some are reserved for private use and never appear on the Internet
Example ASNs
U of T’s campus network (AS239)
Sprint (AS1239, AS1240, AS 6211, …)

5. Number of Autonomous Systems

6. Interdomain and Intradomain Routing
Routing protocols used inside an AS, referred to as intradomain routing, are called interior gateway protocols (IGP)
Objective: shortest path, only operate within an AS
Routing protocols used between ASs, referred to as interdomain routing, are called exterior gateway protocols (EGP)
Objective: satisfy policy of the ASs, not always shortest path

7. Interdomain and Intradomain Routing
Routing within an Autonomous System (AS)
Ignores the Internet outside the AS
Protocols for Intradomain routing are collectively called Interior Gateway Protocols or IGP’s.
Popular protocols are:
RIP (simple, old)
OSPF (better)
Interdomain Routing
Routing between AS’s
Assumes that the Internet consists of a collection of interconnected AS’s
Normally, there is one dedicated router in each AS that handles interdomain traffic.
Protocols are collectively called Exterior Gateway Protocols or EGP’s.
Popular protocols are:
Border Gateway Protocol (BGP) v4 current

8. EGP and IGP Interior Gateway Protocol (IGP)1
Interior Gateway Protocol (IGP)
Routing is done based on metrics
Routing domain is one AS
Exterior Gateway Protocol (EGP)
Routing is done based on policies
Routing domain is the entire Internet

9. Components of a Routing Algorithm
A procedure for sending and receiving reachability information about a network to other routers
A procedure for calculating optimal routes
Routes are calculated using a shortest path algorithm (least “cost”)
A procedure for reacting to and advertising topology changes

10. Two Basic Shortest Path Routing Algorithms used for IGP
Distance Vector Routing
Each node knows the distance (cost) to its directly connected neighbors
A node sends periodically a list of routing updates to its neighbors.
If all nodes update their distances to destinations using neighbor information, the routing tables eventually converge
New nodes advertise themselves to their neighbors.
Link State Routing
The distance information is broadcast to all nodes in the network
Each node calculates the routing tables independently using global information.

11. Summary of Differences

12. IGP Routing Algorithms
Distance Vector
Routing Information Protocol (RIP)
Gateway-to-Gateway Protocol (GGP)
Exterior Gateway Protocol (EGP)
Interior Gateway Routing Protocol (IGRP)
Link State
Intermediate System - Intermediate System (IS-IS)
Open Shortest Path First (OSPF)

13. Distance Vector Routing
Initialize routing table with one entry for each directly connected Network
Periodically run a distance-vector update to exchange information with routers that are reachable over directly connected networks
Information shared is each router’s individual view of network.
Uses Bellman-Ford Algorithm to calculate routing table (min cost to all destinations).

14. Distance Vector Dynamic Updates
Every router sends a list of its routes to all its neighbors
List contains pairs of: destination network, distance
Receiver replaces/updates entries in its routing table if routing through a neighbor costs less than the current route in its table
Receiver propagates new routes and updates next time it sends out an update
Update Algorithm has well-known shortcomings (we will see an example later)

15. Update from a Neighboring Router
Existing Routing Table in a Router “K”
Update received from a neighboring Router “J”.
Net 4 has a better cost via “J”
Net 21 is a new entry learnt from “J”
Net 42, via “J”, has a changed cost

16. Example of Distance Vector
Assume:
link cost is 1 on all hops
all updates occur simultaneously
initially each router only knows its directly connected interfaces -> cost = 0

17. Rip Convergence Example

18. After First Update

19. After Second Update

20. After Third Update

21. Last Update for Convergence

22. A “Down” link has a cost of Infinity1 1
Network goes down
Router C marks in its routing table that Net is down
i.e., cost is now “infinity”

23. Characteristics of Distance Vector Routing
Periodic Updates: Routers exchange Updates with their neighbors periodically (fixed interval). Routes that are not refreshed (i.e., timers reset) when an update comes in, are removed from a routers routing table when the timer expires (could be a few update periods).
Triggered Updates: If a metric changes on a link (usually when a link goes down), a router immediately sends out an update for that route without waiting for the end of the update period.\
Full Routing Table Update: Most distance vector routing protocols send their neighbors the entire routing table (not only entries which changed).
Route invalidation timers: Routing table entries are invalid if they are not refreshed. A typical value is to invalidate an entry if no update is received for 3-6 update periods.

24. Router Routing Table with Timers

25. Convergence and Loops
Distance Vector Protocols are subject to loop formations because of the myopic view of each router.
Routers only hear from neighbors and use that to create a global connectivity map.
When changes occur, they are broadcast but take a while to propagate and during that time cycles can form.
One particular problem is the count to infinity problem, where updates bounce back and forth and the distance or cost creeps up in value.
To counter that, a maximum value is set that once it is reached, the destination is considered to be unreachable and the route is removed from the routing table.

26. Down Link and Update from B to C occurs
C notices that NET is down, C removes that entry from its routing table.
C receives a periodic update from B
C sees that B is 1 hop away from Net based on B’s update. It calculates its route to NET using B as next hop with a cost of “1+1=2”
C updates its routing table
C will send its neighbors (only B in this case) its new updated routing table

27. Node C sends its Update to B
When C sends its update, B sees the change in cost to NET via C
B updates its entry to Net to “1+2=3” as C is marked as next hop to Net B will then share this new update with its neighbors, including C.
C proceeds to update its entry again for Net (3+1=4) and shares it with B.
B and C repeat this cycle and the cost increases in value.
Note that B sends updates to A, and its cost for NET will increase correspondingly.

28. Count-to-Infinity Phenomenon
Why does the count-to-infinity problem occur?
Because each router ONLY has a “next-hop-view”
For example, in the first step, C did not realize that B’s route (with cost 1) to network went through itself and B did not realize that C’s update was based on B’s connectivity information.
How can the Count-to-Infinity problem be solved?
A router with a down link:
Sets a max value for the cost. Usually 16 is used to signify infinity.
Advertises link with a cost of 16 (triggered update).
Any destination with route cost = 16 is considered unreachable and destination is removed from routing table (after triggered update, no longer advertised).

29. How to Prevent Count to Infinity
Enhancements proposed to prevent the Count to Infinity problem and routing loops:
Split Horizon
Route Poisoning
Reverse Poison
Hold Down Timers

30. Split Horizon
A router never sends information about a route in the direction from which the original information came. Routers keep track of which neighbor sent information about a route in its routing table. Updates to that route are never sent to that neighbor, unless the latest update is caused by information from a different neighbor.
Router B never sends Router C updates about NET as C is next hop on path to Net
When NET goes down, C removes the entry from its table.
Updates will no longer include NET
B will remove route to NET when the timer expires for that route in its routing table (received no updates for that entry in awhile).

31. Route Poisoning and Poison Reverse
Marking a down link as a cost of infinity.
When NET goes down, router C marks it as “cost = infinity” and advertises the new cost of this network to its neighbors in a TRIGGERED UPDATE.
removes the route from its table
When B gets C’s update, it:
sends a triggered update to all its neighbors with the new cost of infinity for that destination (poison reverse supercedes Split horizon if in use)
If SPLIT Horizon is not being used:
B’s update might not get to A before A sends B it’s updates that includes the old information related to the unreachable destination. So:
……. we must use SPLIT Horizon and or HOLD DOWN Timers.

32. Hold Down Timers
After receiving a route poisoning (cost = infinity) for a route from a neighboring router, a router starts a hold-down timer for that route.
During the hold-down timer, the “downed” route is marked.
If the router gets an update from that same neighbor with a new cost (< infinity) within the hold-down timer period, the hold-down timer is removed and the table is updated (route no longer marked).
However, if within the hold-down timer, an update is received for that marked route from another router with a better cost, that update is ignored.
In our example, when router B receives a route poisoning update from router C:
It marks NET as “down” in its routing table and starts the hold-down timer for NET
In this period, if it receives an update from C informing that NET is recovered then B will accept that information, remove the hold-down timer and reinstitute that destination in its routing table.
But if B receives an update from A informing it that NET can be reached in X hops (X < infinity), that update will be ignored.
When the hold-down timer expires a new update for that route from a neighbor will put it back in router B’s table.

33. Poison Reverse
Poison Reverse - Breaking the Split Horizon rule for updates with cost = infinity
It basically says that when a router receives a NET is down update from a neighbor (cost = infinity), the router breaks the split horizon rule and sends a triggered route update to all its neighbors including the originating neighbor with a cost = infinity for that very same destination.
For example, when router B receives a route down (i.e., a cost = infinity) for NET from router C then router B will send an update to all its neighbors including router C (which breaks the split horizon rule) with the same cost = infinity for NET
Every router performs poison reverse when learning about a down network/link.

34. RIP - Routing Information Protocol
A simple intradomain protocol (Interior Gateway Protocol IGP)
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 (destination address, distance)
Uses metric of hop count and uses 1 for every hop (link)
Maximum hop count is 15, with “16” equal to “”
Routes are timed out (set to 16) after 3 minutes if they are not updated
Uses split horizon and poison reverse techniques to solve ``count to infinity and looping’’
Current standard is RIPv2

35. Two Forms of RIP Active Used by routers
Broadcasts routing updates periodically
Uses incoming messages to update routes
Passive
Used by “non forwarding” hosts
Uses incoming update messages to change route table – changes overwrite ICMP redirects
Does not send updates

36. RIPv2 Route Update includes subnet mask Authentication supported
Explicit next-hop information
Messages are multicast
IP multicast address for RIP is

37. RIPv2 Update Packet
Route Tag: Used to carry information from other routing protocols (e.g., autonomous system number)

38. Description of Fields
Command - Indicates whether the packet is a request or a response.
request asks that a router send all or a part of its routing table.
response can be an unsolicited regular routing update or a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to convey information from large routing tables.
Version - Specifies the RIP version used. For RIP 2 this value is set to 2.
Unused - Has a value set to zero.
Address-family identifier (AFI) - Specifies the address family used. RIP is designed to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address being specified. The AFI for IP is 2. If AFI for the first entry in the message is 0xFFFF, the remainder of the entry contains authentication information.
Route tag - Provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols).
IP address - Specifies the IP address for the entry.
Subnet mask - Contains the subnet mask for the entry. If this field is zero, no subnet mask has been specified for the entry.
Next hop - Indicates the IP address of the next hop to which packets for the entry will be forwarded.
Metric - Indicates how many internetwork hops (routers) will be traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.

39. Contd.
Up to 25 routing table entries can be listed in a single RIP packet. If the AFI specifies an authenticated message, only 24 routing table entries can be specified.
RIP has numerous stability features:
By placing a finite limit on the number of hops that a route can take, routing loops are discouraged, if not completely eliminated.
Various timing mechanisms that help ensure that the routing table contains only valid routes:
The timeout timer is used to help purge invalid routes from a RIP node. Routes that aren't refreshed for a given period of time are likely invalid because of some change in the network. Thus, RIP maintains a timeout timer for each known route. When a route's timeout timer expires, the route is marked invalid but is retained in the table until the route-flush timer expires.
Split horizon, poison reverse and hold-down mechanisms that prevent incorrect routing information from being disseminated throughout the network.

40. RIP Message Exchange Uses UDP transport
Dedicated port for RIP is UDP port 520
Two types of command messages:
Request messages
used to ask neighboring nodes for an update
Response messages
contains an update

41. Routing with RIP
Initialization: Send a request packet on all interfaces requesting routing tables from neighboring routers:
RIPv2 uses multicast address
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 a response message
Triggered Updates: Whenever the metric for a route changes, send updated route.

42. RIP Summary Slow convergence Low overhead
Limited to 15 hops (max cost, i.e., infinity =16)
Only uses local information from immediate neighbors for routing decisions - relies on propagation of information for global view of network