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The Routing & the IP

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The routing the ip

application

transport

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

The Routing &the IP

The network layer moves transport layer segments from host to host in the network, to deliver them to their destination. This layer involves each and every host and router in the network.


Network layer functions

transport packet from sending to receiving hosts

network layer protocols in every host, router

three important functions:

path determination: route taken by packets from source to destination - routing algorithms

switching: move packets from router’s input to appropriate router output

call setup: some network architectures require router call setup along path before data flows (connection oriented networks)

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

Network layer functions


Datagram networks the internet model

no call setup at network layer

routers: do not maintain state for the end-to-end connections

no network-level concept of a “connection”

packets are typically routed using only destination host ID which is carried in the packet

packets between same source-destination pair may take different paths

application

transport

network

data link

physical

application

transport

network

data link

physical

Datagram networks: the Internet model

1. Send data

2. Receive data


Routing

Graph abstraction for routing algorithms:

graph nodes are routers

graph edges are physical links

link cost: delay, distance, # of hops, rate structure or congestion level = $$

Other costs??

5

3

5

2

2

1

3

1

2

1

A

D

E

B

F

C

Routing protocol

Routing

Goal: determine a “good” path

(sequence of routers) thru the network from the source to the destination

  • “good” path:

    • typically means minimum cost path

    • other definitions also possible


Hierarchical routing

scale: with 55 million+ destination hosts:

can’t store all destinations in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

Hierarchical Routing

Our routing study thus far – an idealization

  • all routers are identical

  • the network is “flat”

    … not true in practice

    Why?


Hierarchical architecture of the internet

Hierarchical Architecture of the Internet


Hierarchical routing1

aggregate routers into regions,called “autonomous systems” (AS)

routers in same AS run same routing protocol

“intra-AS” routing protocol

routers in different AS can run different intra-AS routing protocol

special routers in AS

run intra-AS routing protocol with all other routers in AS

also responsible for routing to destinations outside AS

run inter-AS routing protocol with other gateway routers

gateway routers

Hierarchical Routing


Internet as hierarchy

Internet AS Hierarchy

Inter-AS border (exterior gateway) routers

Intra-ASinterior (gateway) routers


Internet inter as routing bgp

Internet inter-AS routing: BGP

  • BGP (Border Gateway Protocol):thede facto standard

  • Path Vector protocol:

    • similar to Distance Vector protocol

    • each Border Gateway broadcasts to neighbors (peers) the entire path (I.e, sequence of ASs) to destination


Intra as and inter as routing

b

a

a

C

B

d

c

A

b

b

a

c

A.a

A.c

C.b

B.a

Intra-AS and Inter-AS routing

  • Gateways:

    • perform inter-AS routing amongst themselves

    • perform intra-AS routers with other routers in their AS

network layer

inter-AS, intra-AS routing in

gateway A.c

data link layer

physical layer


Intra as and inter as routing1

Inter-AS

routing

between

A and B

Host

h2

b

a

a

C

c

B

b

d

Intra-AS routing

within AS B

A

b

a

c

Host

h1

Intra-AS routing

within AS A

A.a

A.c

C.b

B.a

Intra-AS and Inter-AS routing

  • We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly


Ip addressing introduction

IP address: 32-bit identifier for host or router interface

interface: connection between host or router and the physical link

routers typically have multiple interfaces

hosts typically have only one

IP addresses are associated with the interface, not the host or the router

223.1.1.2

223.1.2.1

223.1.3.27

223.1.3.1

223.1.3.2

223.1.2.2

IP Addressing: introduction

223.1.1.1

223.1.2.9

223.1.1.4

223.1.1.3

dotted-decimal notation:

223.1.1.1 = 11011111 00000001 00000001 00000001

223

1

1

1


Ip addressing

IP address:

network part (high order bits)

host part (low order bits)

What’s a network ? (from the IP address perspective)

device interfaces with the same network part of their IP address

hosts can physically reach each other without an intervening router

IP Addressing

223.1.1.1

223.1.2.1

223.1.1.2

223.1.2.9

223.1.1.4

223.1.2.2

223.1.3.27

223.1.1.3

LAN

223.1.3.2

223.1.3.1

Example: network consisting of 3 IP networks (for IP addresses starting with 223, the first 24 bits are the network address – more later)


Ip addresses

class

A

network

0

host (24 bits)

27 = 127 networks

224 = 16.8 million+ hosts

B

network

10

host (16 bits)

multicast address (28 bits)

1110

214 = 16,384 networks

216 = 65,536 hosts

C

network

host (8 bits)

110

221 = 2 million+ networks

28 = 256 hosts

D

228 = 268.4 million+ hosts

IP Addresses

Given the notion of a “network”, let’s look closer at IP addresses:

“classful” addressing -

1.0.0.0 to

127.255.255.255

128.0.0.0 to

191.255.255.255

192.0.0.0 to

223.255.255.255

224.0.0.0 to

239.255.255.255

32 bits

What is the address space size (number of hosts) for each class?


Map of the internet

Map ofthe Internet


Abbreviated format of the address ranges

Abbreviated Format of the Address Ranges

  • The minimum & maximum values of the range:

  • 11100000.00000000.00000000.000000002

    11101111.11111111.11111111.111111112

  • E0.00.00.0016 … EF.FF.FF.FF16

  • The first part of the abbreviation is the common byte(s) in the range

  • The second part of the abbreviation is the number of bits, which are common for the all members of the range

224.0.0.0 - 239.255.255.255

224/4


The private address ranges

The private address ranges

  • Used locally

    • Never used in the Internet

    • Gateways do not forward the packets addressed to private addresses

  • The network, which uses the private address range can be connected to the Internet by the NAT (Network Address Translation)


Ip addressing cidr

host

part

network

part

11001000 0001011100010000 00000000

200.23.16.0/23

IP addressing: CIDR

  • classful addressing:

    • inefficient use of address space, address space exhaustion

    • e.g., class B network is allocated enough addresses for 65K hosts, even if only 2K hosts exist in that network

  • CIDR:Classless InterDomain Routing

    • network portion of address of arbitrary length

    • address format: a.b.c.d/x, where x is # bits in the network portion of an address


Ip addresses how to get one

IP addresses: how to get one?

Hosts (host portion):

  • hard-coded by system admin in a file

  • DHCP:Dynamic Host Configuration Protocol: dynamically get address (RFC 2131): “plug-and-play”

    • host broadcasts “DHCP discover” msg

    • DHCP server responds with “DHCP offer” msg

    • host requests IP address: “DHCP request” msg

    • DHCP server sends address: “DHCP ack” msg


Why different intra and inter as routing

Why different Intra- and Inter-AS routing ?

Policy:

  • Inter-AS: admin wants control over how its traffic is routed, who routes through its net.

  • Intra-AS: single admin, so no policy decisions needed

    Scale:

  • hierarchical routing saves table size, reduces update traffic

    Performance:

  • Intra-AS: can focus on performance

  • Inter-AS: policy may dominate over performance


Intra as routing

Intra-AS Routing

  • Also known as Interior Gateway Protocols (IGP)

  • Most common IGPs:

    • RIP: Routing Information Protocol (legacy)

    • OSPF: Open Shortest Path First (common)

    • EIGRP: Enhanced Interior Gateway Routing Protocol (proprietary – Cisco Systems)


Distance vector routing algorithm dvr

Distance-vector routing algorithm (DVR)

  • The different names of the background mathematical algorithm:

    • Backward search algorithm

    • Bellman-Ford algorithm

  • Goal: search the smallest delay paths for the traffic

  • For this reason in each router a table is created, which contains:

    • The interface to the smallest delay path to every node

    • The estimated delay of each path

  • This table is called distance vector


The link state tables of an example network

The link state tables of an example network


Routing tables step 1

Bellman-Ford algorithm

Routing tables (Step 1)

Distance vector

Modified entry

Unmodified entry


Step 1

Bellman-Ford algorithm

Step 1

Shortest paths in the routing table of the router A


Routing tables step 2

Routing tables resulted from the previous step

Bellman-Ford algorithm

Routing tables (Step 2)


Step 2

Bellman-Ford algorithm

Step 2

Shortest paths in the routing table of the router A


Routing tables step 3

Routing tables resulted from the previous step

Bellman-Ford algorithm

Routing tables (Step 3)


Routing tables step 4

Routing tables resulted from the previous step

Bellman-Ford algorithm

Routing tables (Step 4)


Step 3 4

Bellman-Ford algorithm

Step 3-4

Shortest paths in the routing table of the router A


Routing tables step 5

Routing tables resulted from the previous step

Bellman-Ford algorithm

Routing tables (Step 5)


Step 5 and the final result

Bellman-Ford algorithm

Step 5 and the final result

Shortest paths in the routing table of the router A


Link state routing algorithm

Link-state routing algorithm

  • The different names of the background mathematical algorithm:

    • Forward search

    • Dijkstra algorithm

    • Shortest path (SP)

  • The SP is the optimal path, however, this is not obviously the geometrically shortest path

  • Other factor, which can be taken into account:

    • Number of routers in the path

    • delay

    • cost

    • Average traffic

    • Reliability of the links in a certain path


Dijkstra s algorithm

Dijkstra’s algorithm

  • Dijkstra's algorithm, named after its inventor the Dutch computer scientist Edsger Dijkstra, solves a shortest path problem for a directed and connected graph G(V,E) which has nonnegative (>=0) edge weights

  • Dijkstra's algorithm is known to be a good algorithm to find a shortest path


The method

Dijkstra’s algorithm

The method…

  • Finds the shortest path between a source node and the rest

  • Finds routes between nodes by cost precedence

  • Assumes every cost is a positive number

  • Supports directed or bidirectional communication


Initialisation example network

Dijkstra’s algorithm

Initialisation – Example network


Step 11

Least cost new node: D

Dijkstra’s algorithm

Step 1


Step 21

Least cost new node (with smaller IP address): B

Dijkstra’s algorithm

Step 2


Step 3

Least cost new node:E

Dijkstra’s algorithm

Step 3


Step 4

Least cost new node: C

Dijkstra’s algorithm

Step 4


Step 5

Least cost new node: G

Dijkstra’s algorithm

Step 5


Step 6 final result

Dijkstra’s algorithm

Step 6 (final result)


Second example step 1

Dijkstra’s algorithm

Second exampleStep 1


Second example step 2

Dijkstra’s algorithm

Second exampleStep 2


Second example step 3

Dijkstra’s algorithm

Second exampleStep 3


Second example step 4

Dijkstra’s algorithm

Second exampleStep 4


Second example step 5

Dijkstra’s algorithm

Second exampleStep 5


Second example step 6

Dijkstra’s algorithm

Second exampleStep 6


Second example step 7

Dijkstra’s algorithm

Second exampleStep 7


Second example step 8

Dijkstra’s algorithm

Second exampleStep 8


Second example step 9

Dijkstra’s algorithm

Second exampleStep 9


Second example step 10

Dijkstra’s algorithm

Second exampleStep 10


Second example step 11

Dijkstra’s algorithm

Second exampleStep 11


Second example step 12

Dijkstra’s algorithm

Second exampleStep 12


Differences between the forward and the backward search algorithms

Differences between the forward and the backward search algorithms

  • Forward search (Dijkstra algorithm)

    • It increases the scope of the search in each step with including new node

  • Backward search (Bellman-Ford algorithm)

    • It increases the scope of the search in each step with including new hop


Comparison of the distance vector and the link state algorithms

Comparison of the distance-vector and the link-state algorithms

  • Distance vector:

    • Each router sends distance-vector, but to its neighbours

    • The distance-vector contains the estimated distance to all other nodes

    • Older method

    • Problem of the ”count-to-infinity” due to the fact, that the bad news are distributed too slowly

  • Link-state:

    • Each router sends link-state distance-vector to all others

    • The link-state distance-vector contains the distance to the neighbours, only

    • The distance value to the neighbour (called link-state) is accurate

    • Recent method


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