CMPT 371

© Janice Regan, CMPT 128, 2007-20121

CMPT 371Data Communications and Networking

Routing in the Internet

Internal Routing Protocols

Janice Regan © Oct 2007 2

Hierarchical Routing So far when considering routing we have considered a “group of

routers” and seen how to develop routes between them, and forward packets along those routes.

Both distance vector and link state routing have limitations when scaling to very large networks. The amount of information exchanged becomes prohibitive. (possible for your local network but not for the entire Internet)

Each administrative entity wants autonomy to optimize and configure their own networks for their own purposes. No one configuration will satisfy everyone Need to allow each administrative entity the freedom to configure

and protect their network as they wish But we still need to be able to communicate between these

different networks


Autonomous Systems (AS)

Group of routers and hosts controlled by a single administrative authority

Common routing protocol (interior routing protocol or IRP) for all members of the group Defines mechanisms for discovering, validating, and

maintaining routes within the autonomous system

A connected network within the local group There is at least one route between any pair of nodes

Routing protocol for propagating subset of routing information to other autonomous systems on the internet (Exterior routing protocol or ERP)


Interior Router Protocol (IRP) Routers need complete information about

the local AS IRP Passes routing information between routers within AS Needs a detailed model

what information is sent between routers formats of messages carrying this information frequency of exchange of this information Algorithms for routing (creating routing tables) and

forwarding (using routing tables). Are these algorithms distributed? Local? Global? Etc.

Routing and forwarding algorithms may differ between ASs as may other parts of the model (for example DV or LS)

Also called intra-autonomous system routing protocol


Some ASs

A1

A2

A4

A3

C2C1

B2

C4

B6

B1

B3

B5

B4

C5

AS AAS B

AS C

C3

IRP B

IRP C

IRP A

Gateway router


Some ASs Only routers are shown in the previous diagrams, the

networks of hosts that communicate with the Internet through each of the routers are shown as dotted lines

A host is attached through one of these networks to its default router. The router that attaches it the larger internet

The default router for the source host is called the source router

The default router for the destination host is the destination router


Routing with ASs If both the source router and the destination router are in

the same AS then the IRP for that AS is used to route the packet from the source to the receiver

If the source router and the destination router are in different ASs then the IRP for the source AS is used to reach the gateway router

for the source AS The gateway router uses another protocol (the ERP, more later)

to get the packet to the gateway router of the destination AS. The gateway router of the destination AS uses the IRP of the

destination AS to send the packet to its destination with the destination AS


Exterior Routing Protocol ERP is an External Routing Protocol Routers need some information about

external ASs Use ERP to communicate outside AS At least one routers in the AS must do

external routing When more than one router in the AS does

external routing must also consider finding the fastest path between gateway routers in the local AS

ERP supplies summary information on reachability of group members to routers outside the AS


Application of IRP and ERP

Figure 19.5 Stallings (2003)


Distance-vector Routing Approach

Each node exchanges information with neighbor nodes Neighbors are both directly connected to same system

Each node maintains vector of link costs for each directly attached node and distance and next-hop values for each destination node in the system

A node must transmit large amounts of information Distance vector to all neighbors, Containing estimated path cost to all

nodes in a configuration and next hop labels Changes take long time to propagate (count to infinity) Used by first generation routing algorithm for ARPANET and by

Routing Information Protocol (RIP, routed) RIP is an internal gateway protocol (IGP) used between routers within an AS


Routing Information Protocol RIP, The simplest dynamic distance vector

routing protocol still in use, was built and adopted before a formal standard was available (RFC 1058 RIPv1, 2453 RIPv2 )

Implemented in LINUX as the routed process.

Adequate only for small and stable ASs based on the Bellman-Ford (or distance

vector) algorithm helps control count to infinity problem by

specifying a maximum hop count of 15


Routing Information Protocol Uses a simple metric, hop count

Not designed to deal with more complicated dynamic metrics such as delay, reliability or load (these can cause route oscillations)

Helps control oscillation between equal cost routes by retaining original route unless a route with a lower cost is found.

Helps prevent slow convergence (after changes in the topology of the network) by sending update messages immediately after updates have been completed (triggered updates)


Updating and RIP Routing tables are updated or maintained

Each router will periodically (every 30 seconds) broadcast its routing table

If no update is received from a router for 180 seconds, that router is considered to no longer be reachable

Each router will process the received updates, adding new entries, updating entries for which a lower cost path has been located and updating entries for directly connected nodes whose cost changed

If the routing information changes during the update process the router will immediately broadcast the modified tables to its neighbors (called a triggered update)


Link-state Routing Approach When router initialized and at intervals thereafter, it

determines link cost on each interface (cost to each directly connected node)

Advertises set of link costs to other nodes in topology Each node constructs routing table containing minimum

cost paths to all attached nodes ( costs and first hop to each router) using the data received from all other nodes advertisements (information on nearest neighbors only to reduce packet size).

Open shortest path first (OSPF) protocol uses link-state routing. (a common IRP)

Second generation routing algorithm for ARPANET


Open Shortest Path First: OSPF is the preferred IGP of Internet Uses a Link State Routing Algorithm (RFC 2328) Each

router keeps a database of information based on local costs and

received update packets from other routers in the AS Each update includes cost information from one router to each of

its neighbors Can build directed graph showing topology and path costs for

entire network from this information (for all routers) Uses the database and Dykstra’s algorithm to determine least

cost paths Advertises its locally determined routing table periodically and

when it changes (to all routers in the network)


Sample AS



Directed Graph for OSPF Vertices or nodes are routers and networks Types of Network

Transit: data not originating in network can pass through the network, more than one router is attached to the network

Stub: data not originating in network can enter only. One router is attached to network

Edges, associated costs at output of routers Connect two routers with a pair of edges Connect router to transit network with pair of edges (network to

router edge has a cost of 0), or to stub network with single edge


Directed Graph of AS



Dividing an AS into Areas Many networks are large and complex it is often useful to divide them

into areas and deal with each smaller area separately A large advantage of OSPF is that it includes mechanisms for

dealing with ASs partitioned into areas. When an AS is divided into areas the areas are chosen so they can

be connected by a backbone of routers Any router which is part of an area, but also communicates with other

areas, is also a part of the backbone area. The backbone is a special area. The information passing between

all other areas travels through the backbone routers (dark ovals in diagram)

Routers in each area run their own copy of the routing process and have their own topological link-state data base. Routers in one area have no detailed knowledge of routing in other areas.

20Janice Regan © Oct 2007From notes of Lou Hafer, after RFC1131: AS divided into areas

23b

2b

1b

Includes Router 4

Includes Routers 7 and 11

Backbone routers3,4,5,6,7,10,11AS Boundary routers5,7Internal routers 1,2,5,6,8,9,12Area Border routers 3,4,7,10,11


Types of routers in an AS Internal Routers: all connected networks belong to

the same area or with only backbone interfaces Area Border Routers: not internal, run one copy of

routing algorithm for backbone, and one copy for each attached area

Backbone Routers: has at least one interface to the backbone. Can be an internal router if all interfaces are with the backbone. Otherwise it is an area border router

AS boundary routers: Part of the AS but also communicates with routers outside the AS using and EGP. Can be routers of an of the above types

© Janice Regan, 2006 22

Neighbor Routers Any pair of routers attached to the same

single network segment (single broadcast address) can become neighbors

To become neighbors they must agree that they are neighbors

The pair of routers negotiates this agreement using and exchange of Hello packets (more later) to assure a two way link is established


Adjacent routers Routers establish an adjacency if they will be

exchanging LSAs.(link state announcement packets that carry routing information between routers)

A router on a particular physical segment will not necessarily be adjacent to all other routers on that segment

A router with multiple interfaces may simultaneously be adjacent to routers on more than one network segment


OSPF messages Encapsulated in IP datagrams 5 types of messages, all message types begin

with a common header Message types are

1. Hello

2. Database description

3. Link status request,

4. Link status update

5. Link status acknowledgement


OSPF operation (1)1. Meet your neighbors

Hello messages are used to establish and test neighbor reachability

Two OSPF router may be neighbors if they are on the same network segment

2. Make good friends: Database description, link state request update and ack are

used for forming adjacencies (making friends) Adjacencies are agreements to exchange Link State

Announcements. Adjacencies are not established with all neighbors, just an optimal subset.

Database description, link state request, link state update and link state ack messages are used to establish adjacency

OSPF operation (2)3. Keeping in touch with friends

3. Send Hello messages periodically to verify that neighbors are still neighbors

4. Break the neighbor (and adjacency relations) if you do not hear from a neighbor (receive a hello) for 3 periods

5. Send update information about any changes in routing to your adjacent neighbors when you have it (send Link state announcements LSAs)

6. Update your routing database based on LSAs received from your adjacent neighbors

© Janice Regan, 2007-2012 26


Flooding protocol: conditions A message(LSA) contains a database record. A

database record contains information about one link between two routers in the graph discussed earlier. (one link is in one direction)

Each message contains a time stamp or message number

These message numbers are used by the receiving node to determine age of the record

Send means transmit through all attached interfaces except the one on which the incoming message arrived


Flooding protocol Receive message: Find the corresponding

record in the local database if it exists If the record is not yet in the local database add the

record. Send the message If the record’s message number is larger than the

message number in the data base, replace the message in the database with the new record. Send the message.

If the records message number is the same as the message number in the database do nothing

If the records message number is smaller than the message number in the database, send the record in the database back through the interface on which the message arrived


link state advertisements (1) Router link advertisement

Originated by all routers Flooded throughout a single area only Describes the states of the router’s interfaces to the

area Network link advertisement

Originated by broadcast networks Flooded throughout a single area only Contains a list of routers connected to the network

30Janice Regan © Oct 2007From notes of Lou Hafer, after RFC1131: AS divided into areas

23b

2b

1b

Includes Router 4

Includes Routers 7 and 11

Backbone routers3,4,5,6,7,10,11AS Boundary routers5,7Internal routers 1,2,5,6,8,9,12Area Border routers 3,4,7,10,11


link state advertisements (2) Summary link advertisement

Originated by border area routers Flooded throughout a area and backbone Describes a route outside the local area but within

the AS AS external link advertisement

Originated by AS boundary area routers Flooded throughout the AS Contains a route to a destination outside the AS in

another AS

CMPT 371

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