CSC 600Internetworking

withTCP/IP

Unit 6a: IP Routing and Exterior Routing Protocols

(Ch. 14, 15)

Dr. Cheer-Sun Yang

Spring 2001

Routing Protocols

• Cores, Peers, and Algorithms :Distance Vector(Bellman-Ford), Link State(Dijkstra), Gateway-to-Gateway Protocol(GGP),

• Interior: within an autonomous system• Exterior: between two autonomous systems• Exterior Routing Protocols :Border Gateway

Protocol(BGP)• Interior Routing Protocols :RIP(distance vector),

OSPF(link state).

Routing Protocols

• Routing Information– About topology and delays in the internet

• Routing Algorithm– Used to make routing decisions based on

information

The Evolution of Internet Architecture

• Core system : many non-core routers are conneced to a set of core routers.

• Peer-to-peer : many routers are connected to a backbone.

• Architectural: many autonomous systems are connected to their own gateways and gateways are connected as “peers”.

Original Internet Architecture and Cores

• A small number of routers kept complete information about all possible destinations and a large set of routers only kept partial information.

• The routing table in a given router contains partial information about possible destinations.

• Routing that uses partial information allows sites autonomy in making local routing changes.

Core vs. Noncore

• Core routers are controlled by the Internet Network Operations Center (INOC).

• Noncore routers are controlled by individual groups.

• This architecture can introduce the possibility of inconsistencies that may make some destinations unreachable from some sources unless the chain of all default routers (core) reaches every router in a giant cycle as shown in next slide.

Core System is Impractical

When the NSFNET became the major part of the Internet, the core architecture became impractical for the following reasons:

• The Internet outgrew a single, centrally managed long-haul backbone.

• Not every site could have a core router connected to the backbone.

• Because core routers all interacted to ensure consistent routing information, the core architecture did not scale to arbitrary size.

The peer-to-peer architecture is formed.

Routing Becomes Complicated

• For example, how can a datagram be routed from host 3 to host 2?

• Which path should be taken?

• How can routing be optimized?

• How can loops be eliminated?

Summary of Core System Architecture

• A core routing architecture assumes a centralized set of routers which serves as the repository of information about all possible destinations in an internet.

• Core systems work best for internets that have a single, centrally managed backbone.

• Expanding the topology to multiple backbones makes routing complex; attempting to partition the core architecture so that all routers use default routers introduces potential routing loops.

Automatic Propagation of

Routing Information

The Internet is not static!

Distance Vector

(Bellman-Ford)

Routing

Gateway-to-Gateway Protocol (GGP)

Sometimes known as exterior routing protocols. It is a true distance-vector protocol. It measures distance in router hops.

Autonomous Systems

Although it is desirable for routers to exchange routing information, it is impractical for all routers on an arbitrarily large internet to participate in a single routing update protocol. The number of routers that participate in a single routing protocol must be limited.

Autonomous Systems This idea works fine. However, it implies that some routers will be outside the group. If a router outside of an AS uses a member of the group as the default route, routing will be suboptimal. R1 and R2 are in one AS, while R3 is not. If R3 sends datagrams via R1 for sending datagrams to R2, it is not optimal.

Hidden Networks

Architectural Approach:Autonomous Systems (AS)

• Group of routers• Exchange information• Common routing protocol• Set of routers and networks managed by single

organization - an autonomous system• The Internet is organized into a collection of Ass,

each of which is normally administered by a single entity. A corporation or university campus often defines an AS. The NSF backbone forms an AS.

Architectural Approach:Autonomous Systems (AS)

• Each Autonomous system can select its own routing protocol to communicate between the routers in that AS. This is called an interior gateway protocol (IGP) or intradomain routing protocol.

• Separate routing protocols called exterior gateway protocol (EGS) or interdomain routing protocol are used between the routers in different autonomous systems.

Interior Routing Protocols

• Routing Information Protocol (RIP): a distance vector (Bellman-Ford)

• Open Shortest Path First Protocol (OSPF): a link state algorithm (Dijkstra’s algorithm)

Exterior Routing Protocol

• Border Gateway Protocol (BGP)

Application of IRP and ERP

Border Gateway Protocol (BGP)• Inter-autonomous system communication• Coordination among multiple BGP gateways• Propagation of reachability information• Next-hop paradigm• Policy support• Reliable transport• Incremental updates• Support for classless addressing• Route aggregation• Authentication

Border Gateway Protocol (BGP)• For use with TCP/IP internets• Preferred EGP of the Internet• Messages types sent over TCP connections

– Open– Update: advertise or withdraw routes– Keep alive: actively test peer connectivity– Notification: response to an incorrect message

• Procedures– Neighbor acquisition– Neighbor reachability– Network reachability

BGP Messages

BGP Procedure

• Open TCP connection

• Send Open message– Includes proposed hold time

• Receiver selects minimum of its hold time and that sent– Max time between Keep alive and/or update

messages

Other Message Types• Keep Alive

– To tell other routers that this router is still here

• Update– Info about single routes through internet– List of routes being withdrawn– Includes path info

• Origin (IGP or EGP)• AS_Path (list of AS traversed)• Next_hop (IP address of boarder router)• Multi_Exit_Disc (Info about routers internal to AS)• Local_pref (Inform other routers within AS)• Atomic_Aggregate, Aggregator (Uses address tree structure to reduce

amount of info needed)

Uses of AS_Path and Next_Hop

• AS_Path– Enables routing policy

• Avoid a particular AS• Security• Performance• Quality• Number of AS crossed

• Next_Hop– Only a few routers implement BGP

• Responsible for informing outside routers of routes to other networks in AS

The Key Restriction of EGP

• An exterior gateway protocol does not communicate or interpret distance metrices, even if metrics are available.

The Routing Arbiter System• For an internet to operate correctly, routing information

must be globally consistent.• Individual protocols such as BGP does not guarantee

global consistency.• The RA system consists of a replicated authenticated

database of reachability information.Each ISP designates one of the routers near a Network Access Point (NAP) to be a BGP border router.

• The designated router maintains a connection to the route server over which it uses BGP. BGP notification messages are exchanged.

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

Notification Message• Message header error

– Authentication and syntax

• Open message error– Syntax and option not recognized– Unacceptable hold time

• Update message error– Syntax and validity errors

• Hold time expired– Connection is closed

• Finite state machine error• Cease

– Used to close a connection when there is no error