the routing & the ip network data link physical network data link physical network data link...

The Routing &the IP

networkdata linkphysical








application

transportnetworkdata linkphysical

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.

application


application


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)









application


application


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


application


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??

Goal: determine a “good” path

(sequence of routers) thru the network from the

source to the destination

Routing protocol

A

ED

CB

F

2

2

13

1

1

2

53

5

“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

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

Legend

router

border router

connection

host

LAN

intra domain network (AS)

inter AS network

Inter-AS level

Intra Domainlevel (AS)

LAN level

Hierarchical Routing

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

Internet AS HierarchyInter-AS border (exterior gateway) routers

Intra-AS interior (gateway) routers

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de 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

Gateways:• perform inter-AS

routing amongst themselves

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

inter-AS, intra-AS routing in

gateway A.c

network layer

data link layerphysical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

Intra-AS and Inter-AS routing

Host h2

a

b

b

aaC

A

Bd c

A.a

A.c

C.bB.a

cb

Hosth1

Intra-AS routingwithin AS A

Inter-AS routingbetween A and B

Intra-AS routingwithin AS B

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

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

dotted-decimal notation:

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

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

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

LAN

IP Addresses

1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

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

“classful” addressing -

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

0network

10 network host (16 bits)

110 network host (8 bits)

1110 multicast address (28 bits)

A

B

C

D

class

host (24 bits)27 = 127 networks 224 = 16.8 million+ hosts

214 = 16,384 networks 216 = 65,536 hosts

221 = 2 million+ networks 28 = 256 hosts

228 = 268.4 million+ hosts

Map ofthe Internet

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

Abbreviated Format of the Address Ranges

224/4224.0.0.0 - 239.255.255.255

The private address ranges

Name Start of the range End of the range Subnet mask

Class A 10.0.0.0 10.255.255.255 255.0.0.0

Class B 172.16.0.0 172.31.255.255 255.255.0.0

Class C 192.168.0.0 192.168.255.255 255.255.255.0

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 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

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

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 ?

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

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)

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

Legend

router

physical link

1 2 5

4

3 1

1 6

4

D

A

B

C

F E

G

1

1

The link state tables of an example network

A B C D E F G

A 0 2 5 1

B 2 0 4

C 5 4 0 3 1 6 1

D 1 3 0 1

E 1 1 0 4

F 6 4 0 1

G 1 1 0

Routing tables (Step 1)

Modified entry

Unmodified entry

Router A B C D E F G

Destination CA,j Interface CB,j

Interface CC,j

Interface CD,j

Interface CE,j

Interface CF,j

Interface CG,j

Interface

A 2 A 5 A 1 A - - -

B 2 B 4 B - - - -

C 5 C 4 C 3 C 1 C 6 C 1 C

D 1 D - 3 D 1 D - -

E - - 1 E 1 E 4 E -

F - - 6 F - 4 F 1 F

G - - 1 G - - 1 G

Distance vector

Bellman-Ford algorithm

Legend

Router

Physical link

Temporarily step of the algorithm

Least cost path

CA,G =

CA,B=2

CA,D=1

CA,C =5

D

A

B C

h=1

F E

G CA,F=

CA,E=

Step 1

h CA,B Path CA,C Path CA,D Path CA,E Path CA,F Path CA,G Path

0 - - - - - -

1 2 A-B 5 A-C 1 A-D - - -

2 2 A-B 4 A-D-C 1 A-D 2 A-D-E 11 A-C-F 6 A-C-G

3 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 6 A-D-E-F 5 A-D-C-G

4 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 5 A-D-E C-G-F 4 A-D-E-C-

G

5 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 5 A-D-E-C-G-F 4 A-D-E-C-

G

Shortest paths in the routing table of

the router A



Destination CA,j Path CB,j Path CC,j Path CD,j Path CE,j Path CF,j Path CG,j Path

A 2 A 4 D 1 A 2 D 11 C 6 C

B 2 B 4 B 3 A 5 C 10 C 5 C

C 4 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 5 E 4 C

E 2 D 5 C 1 E 1 E 4 E 2 C

F 11 C 10 C 5 E 5 E 4 F 1 F

G 6 C 5 C 1 G 4 C 2 C 1 G

Routing tables (Step 2)

Routing tables resulted from the previous

step




Interface CC,j

Interface CD,j

Interface CE,j

Interface CF,j

Interface CG,j

Interface

A 2 A 5 A 1 A - - -

B 2 B 4 B - - - -

C 5 C 4 C 3 C 1 C 6 C 1 C

D 1 D - 3 D 1 D - -

E - - 1 E 1 E 4 E -

F - - 6 F - 4 F 1 F

G - - 1 G - - 1 G

CA,F=11

E

CA,B=2

CA,D=1

CA,G =6

CA,E=2

D

A

B C

h=2

F G

CA,C =4

Step 2Bellman-Ford algorithm


the router A


0 - - - - - -

1 2 A-B 5 A-C 1 A-D - - -




G


G



A 2 A 3 E 1 A 2 D 6 G 5 C

B 2 B 4 B 3 A 4 D 6 G 5 C

C 3 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 4 G 3 C

E 2 D 4 A 1 E 1 E 3 G 2 C

F 6 D 9 C 2 G 5 E 3 C 1 F

G 5 D 5 C 1 G 3 E 2 C 1 G

Routing tables (Step 3)Bellman-Ford algorithm


step



A 2 A 4 D 1 A 2 D 11 C 6 C

B 2 B 4 B 3 A 5 C 10 C 5 C

C 4 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 5 E 4 C

E 2 D 5 C 1 E 1 E 4 E 2 C

F 11 C 10 C 5 E 5 E 4 F 1 F

G 6 C 5 C 1 G 4 C 2 C 1 G



A 2 A 3 E 1 A 2 D 5 G 4 C

B 2 B 4 B 3 A 4 D 6 G 5 C

C 3 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 4 G 3 C

E 2 D 4 A 1 E 1 E 3 G 2 C

F 5 D 6 C 2 G 4 E 3 C 1 F

G 4 D 5 C 1 G 3 E 2 C 1 G



step



A 2 A 3 E 1 A 2 D 6 G 5 C

B 2 B 4 B 3 A 4 D 6 G 5 C

C 3 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 4 G 3 C

E 2 D 4 A 1 E 1 E 3 G 2 C

F 6 D 9 C 2 G 5 E 3 C 1 F

G 5 D 5 C 1 G 3 E 2 C 1 G

CA,C =5

E

CA,B =2

CA,D =1

CA,C =3

CA,E=2

CA,F =6

D

B

C

h=3

F G

A

CA,C =4 E

CA,B =2

CA,D =1

CA,C =3

CA,E=2 CA,F =5

D

B

C

h=4

F G

A

Step 3-4Bellman-Ford algorithm


the router A


0 - - - - - -

1 2 A-B 5 A-C 1 A-D - - -




G


G



Interface CC,j

Interface CD,j

Interface CE,j

Interface CF,j

Interface CG,j

Interface

A 2 A 3 E 1 A 2 D 5 G 4 C

B 2 B 4 B 3 A 4 D 6 G 5 C

C 3 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 4 G 3 C

E 2 D 4 A 1 E 1 E 3 G 2 C

F 5 D 6 C 2 G 4 E 3 C 1 F

G 4 D 5 C 1 G 3 E 2 C 1 G



step



A 2 A 3 E 1 A 2 D 5 G 4 C

B 2 B 4 B 3 A 4 D 6 G 5 C

C 3 D 4 C 2 E 1 C 2 G 1 C

D 1 D 3 A 2 E 1 D 4 G 3 C

E 2 D 4 A 1 E 1 E 3 G 2 C

F 5 D 6 C 2 G 4 E 3 C 1 F

G 4 D 5 C 1 G 3 E 2 C 1 G

G

E

CA,B =2

CA,D =1

CA,C =3

CA,E =2

CA,F =5

D

A

B

C

h=6

F CA,G =4 CA,G =4

E

CA,B =2

CA,D =1

CA,C =3

CA,E=2

CA,F =5

D

B

C

h=5

F G

A

Step 5 and the final resultBellman-Ford algorithm


the router A


0 - - - - - -

1 2 A-B 5 A-C 1 A-D - - -




G


G

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, 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…

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


CA,D=1 CA,B=2

CA,C=5

D

A

B

C

M={A}

F E

G

Initialisation – Example network


CA,D=1

CA,E=2

CA,B=2 CA,C=4

D

A

B

M={A,D}

C F

E G

Step 1Least cost new node: D

Node B C D E F G

Cost of the least cost path 2 5 1

Path A-B A-C A-D - - -

Node Expression Value Evaluation Resulted action

B CA,D+LD,B 1+= >2 No change

C CA,D+LD,C 1+3=4 4<5 New path

E CA,D+LD,E 1+1=2 2< New path

F CA,D+LD,F 1+= = No change

G CA,D+LD,G 1+= = No change


CA,C=4

CA,D=1

CA,E=2

CA,B=2

D

A

B

M={A,B,D}

C

F E

G E

CA,C=3

CA,D=1

CA,E=2 CA,F=6

CA,B=2

D

A

B

M={A,B,D,E}

C

F

G

Step 2Node B C D E F G

Cost of the least cost path 2 4 1 2

Path A-B A-D-C A-D A-D-E - -


C CA,B+LB,C 2+4=6 6>4 No change

E CA,B+LB,E 2+= >2 No change

F CA,B+LB,F 2+= = No change

G CA,B+LB,G 2+= = No change


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

CA,C=4

CA,D=1

CA,E=2

CA,B=2

D

A

B

M={A,B,D}

C

F E

G E

CA,C=3

CA,D=1

CA,E=2 CA,F=6

CA,B=2

D

A

B

M={A,B,D,E}

C

F

G


Cost of the least cost path 2 4 1 2

Path A-B A-D-C A-D A-D-E - -


C CA,E+LE,C 2+1=3 3<4 New path

F CA,E+LE,F 2+4=6 6< New path

G CA,E+LE,G 2+= = No change


Least cost new node: E

G E

CA,C=3

CA,D=1

CA,E=2 CA,F=4

CA,B=2

D

A

B

M={A,B,C,D,E,G}

C

F

E

CA,C=3 CA,D=1

CA,E=2 CA,F=4

CA,B=2

D

A

B

M={A,B,C,D,E}

C

F

G


Cost of the least cost path 2 3 1 2 6

Path A-B A-D-E-C A-D A-D-E A-D-E-F -


F CA,C+LC,F 3+6=9 9>6 No change

G CA,C+LC,G 3+1=4 4< New path


Least cost new node: C

Least cost new node: G

Node B C D E F G

Cost of the least cost path 2 3 1 2 6 4

Path A-B A-D-E-C A-D A-D-E A-D-E-F A-D-E-C-G

Step 5

G E

CA,C=3

CA,D=1

CA,E=2 CA,F=4

CA,B=2

D

A

B

M={A,B,C,D,E,G}

C

F

E

CA,C=3 CA,D=1

CA,E=2 CA,F=4

CA,B=2

D

A

B

M={A,B,C,D,E}

C

F

G


F CA,G+LF,G 4+1=5 5<6 New path


G E

CA,C=3

CA,D=1

CA,E=2 CA,F=4

CA,B=2

D

A

B

M={A,B,C,D,E,F,G}

C

F

Step 6 (final result)Node B C D E F G

Cost of the least cost path

2 3 1 2 5 4

Path A-B A-D-E-C A-D A-D-E A-D-E-C-G-F A-D-E-C-G


Second exampleStep 1


Dijkstra’s algorithmSecond exampleStep 2

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

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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