Routing Protocols

Review: Layer 2

• Link layer (layer 2) handles sending datagrams to hosts directed connected within the same LAN
• Limited broadcast, cannot cross network’s boundaries

Review: Layer 3

• Network layer handles sending datagrams across network boundaries - internetworking
• IP address consists of two parts: network ID and host ID
• Hosts and routers look at the network ID to determine if a destination address is local or outside

Review: Routing table

• Hosts and routers maintain a routing table used in making routing decisions
• Each entry in a routing table (called route) represents a possible path a datagram might take to reach its destination

Review: Routing table

• Each route in a routing table has 4 main components:
  • Destination value
  • Subnet mask
  • Gateway or interface address
  • Route cost or metric

Review: Routing table

• Example:

Route types: Connected

• Directly attached, local-network (to the router) addresses.
• Identified in the routing table with “scope link”
• These routes are automatically updated whenever the interface is reconfigured or shut down

Route types: Static

• Manually configured routes.
• Identified in the routing table with “proto static”

Route types: Dynamic

• Automatically created and maintained by routing protocols.
• Identified in the routing table by the name of the protocol that created them

Route types: Default

• Specifies the gateway to use when the routing table does not contain a path for the destination network.
• Commonly points to the next router in the path to the ISP
• Identified by the word “default” or subnet “0.0.0.0/0” in the destination value field

Determine packet's next hop

Based on the following routing table:

a) What is next hop destination of a datagram addressed to 172.16.27.70?
b) What is next hop destination of a datagram addressed to 172.16.32.100?
c) What is next hop destination of a datagram addressed to 172.16.23.23?
d) What is next hop destination of a datagram addressed to 172.16.18.200?

Determine packet's next hop

Based on the following routing table:

a) What is next hop destination of a datagram addressed to 2001:2019:12:ffff::1?
b) What is next hop destination of a datagram addressed to 2001:2019:12:1001::ffff?
c) What is next hop destination of a datagram addressed to 2001:2019:ffff:ffff::1?
d) What is next hop destination of a datagram addressed to fe80::c001:37ff:fe6c:0?

Routing Protocols

Routing protocols: internet structure

• The Internet is composed of interlocking network pieces, much like a jigsaw puzzle.
• Each piece is called an autonomous system (AS), and it’s convenient to think of each ISP as an AS, although this is not strictly true.
• The Autonomous systems connect directly to each other at Points of Presence or via Internet Exchanges Points
• When AS's connect they become peers and setup policies on what routes to exchange.

Routing protocols: learning routes

• Routers initially know IP addresses and prefixes configured on their local interfaces
• Routers communicate with their neighbours to learn the required information about their routing domain to forward packets hop by hop, toward a given destination.
• So routers establish communications with adjacent routers ( i.e. those one hop away) and ask them about the routing information they know.

Routing protocols: learning routes

• Each router then builds up a detailed routing information database about the network.
• This exchange of information is governed by routing protocols
• Routing protocols differ depending on whether they are used within a routing domain or between different routing domains

Routing protocols: learning routes

• Within a routing domain, several different interior routing protocols can be used.
• Interior routing protocols, or IGPs, run between the routers inside a single routing domain, or autonomous system (AS)
• Between routing domains on the Internet an exterior Gateway Routing Protocol is used: Border Gateway Protocol (BGP)

Routing protocols: Interior Gateway Protocol (IGP)

• A routing protocol that was designed and intended for use inside a single autonomous system (AS)
• Examples: OSPF, EIGRP, RIP

Routing protocols: Exterior Gateway Protocol (EGP)

• A routing protocol that was designed and intended for use between different autonomous systems
• Examples: BGP

Routing protocols: Functions

• Learn routing information about IP subnets from other neighboring routers.
• Advertise routing information about IP subnets to other neighboring routers.
• If more than one possible route exists to reach one subnet, pick the best route based on a metric.
• If the network topology changes, for example, a link fails, react by advertising that some routes have failed and pick a new currently best route. (This process is called convergence)

Routing protocols: Algorithms

Distance Vector

• Distance may be computed as the number of hops to destination or other metrics such as bandwidth, propagation delay, …
• Vector: router updates in the form of {Dest, Cost}
• Minimum view of network topology
• Wait for route reports from neighboring routers before updating routing table
• Example protocols: RIP, EIGRP, DSDV

Routing protocols: Algorithms

Link-state

• Routing table - List of the known paths and interfaces.
• Link-state advertisement (LSA) - Small packet of routing information that is sent between routers. LSAs describe the state of the interfaces (links) of a router and other information, such as the IP address of each link.
• Topological database - convergence
• Shortest Path First (SPF) algorithm – map of network seen from the point of view of the router. SPF tree used to build routing table

Routing protocols: OSPF

• Link-state algorithm
• Uses LSAs to generate a link-state database
• Stub area: advertised network area which does not participate in LSA
• Use of Designated Router (DR) and Backup Designated Router (BDR) for efficient management of link-state advertisements

Reading List

• Transport Layer
• UDP
• TCP Basics