Routing Algorithms and IP Addressing

94-10-02

Routing Algorithms must be

▪ Correctness▪ Simplicity▪ Robustness▪ Stability▪ Fairness▪ Optimality

Shortest Path Routing

▪ What is the metric?▪ Number of hops, geographic distance, delay

(propagation,queue, processing), reliability?▪ The idea is to reduce the metric (whatever is chosen)

to get from one point to another▪ Dijkstra's algorithm

Routing and Router

▪ The network layer is responsible for routing the packet from its source to the destination.

▪ This means that there is more than one route from the source to the destination. The network layer is responsible for finding the best one among these possible routes.

Dijkstra

Flooding Algorithm

▪ Every incoming packet is sent on every outgoing line (except the one it arrived on)

▪ Damping - stop flooding packets after a certain number of hops - hop counter decremented each time

Distance Vector

▪ First routing algorithm used in the Internet (then ARPANET) and also used in RIP in Internet.

▪ Each router has an entry for every router in the subnet. (its neighbors)

▪ Distance to all nodes in the subnet periodically conveyed to all neighbors.

Distance Vector

Link State Algorithm

▪ Discover neighbors▪ Measure delays or cost to reach each

neighbor▪ Construct a \link-state" packet▪ Inform all routers (send link-state packet

to all routers - flooding)▪ Compute shortest path to all routers.

DV vs LS▪ Both guarantee determination of best path; LS

determines entire topology; DV finds only the next hop in the best path to each destination (by comparing distance to the destination through all neighbors and choosing the best neighbor)

▪ In LS each router announces a small amount of information (link state) to all routers (by flooding)

▪ In DV each router announces a large amount of information (distance to every destination) to a small number of routers (only neighbors)

▪ DV susceptible to the count-to-infinity problem

Routing Protocols in Actions▪ Routing protocols can be exterior or interior▪ Exterior protocols for routing between autonomous

systems (AS) Interior for routing within an AS▪ Every AS has to follow the same exterior routing

protocol▪ RIP was the first exterior routing protocol. RIP was

based on DV▪ Currently BGP is the exterior protocol▪ BGP is a Path vector protocol (has some similarities

with DV)▪ OSPF is the most common interior routing protocol.

IP Header

IP - Classful

What is Subnet Mask

▪ A subnet mask is a 32-bit binary number that indicates which bits of an IP address identify the network and which bits identify the host.

Special IP Addresses

What is wrong with Classful IP

▪Address space is wasted for Class A,B▪Class B sparsely populated.▪Large forwarding tables for Class C.

Solution: Subnetting and Supernetting

What is Subnetting?

▪ Add another layer to hierarchy makes subnet masks variable in size

Pros for Subnetting

▪ Hierarchical addressing critical for scalable system

▪ Sub-netting simplifies network management. It breaks up network into chunks

▪ It saves a vast number of IPs in Class A and B.

IP Prefix for Subnetting▪ Example: 130.58.126.0 / 30 represents a chunk of 2^2= 4

addresses (b = 32 - 30)▪ All addresses for which the first 30 bits are the same as

the starting address 130.58.126.0

▪ the last two bits can be 00 or 01 or 10 or 11 - giving four different addresses (130.58.126.0 - 130.58.126.3)

▪ 130.58.126.0 / 28 represents a chunk of 2^4 (32-28) addresses (the last 4 bits are free to change (130.58.126.0-130.58.126.15)

ARP and RARP

▪ Address Resolution Protocol (ARP): Mapping IP to MAC▪ Reverse Address Resolution Protocol (RARP): is vise

versa – MAC to IP▪ Both are below the Network Layer and used by IP▪ Procedure for ARP:▪ Broadcast a query (I'm 10.2.3.1, my MAC address is

X, what is MAC address of 10.2.3.4?) - 10.2.3.4 responds with its MAC address

▪ RARP requests are not forwarded outside the LAN

ICMP - Internet Control Message Protocol