pass through many routers until it reaches the

smashlizardsΔίκτυα και Επικοινωνίες

29 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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An

internet

is

a

combination

of

networks

connected

by

routers
.

When

a

datagram

goes

from

a

source

to

a

destination,

it

will

probably

pass

through

many

routers

until

it

reaches

the

router

attached

to

the

destination

network
.




In distance vector routing, the least cost route between any
two nodes is the route with minimum distance. In this
protocol each node maintains a vector (table) of minimum
distances to every node.



Updates are send to directly connected
neighbors only.



The Routing Information Protocol (RIP) is an
intra
-
domain (interior) routing protocol used
inside an autonomous system. It is a very
simple protocol based on distance vector
routing. RIP implements distance vector routing
directly with some considerations.



Periodic

timer

It

controls

the

advertising

of

regular

update

message

(
25

~

30

sec)
.

Expiration

timer

It

governs

the

validity

of

a

route

(
180

sec)

The

route

is

considered

expired

and

the

hop

count

of

the

route

is

set

to

16
.

Garbage

collection

timer

A

invalid

route

is

not

purged

from

the

routing

table

until

this

timer

expires

(
120

sec)
.



Route

tag

(autonomous

system

number)

able

receiving

information

from

inter

domain

routing

protocol
.


Support

classless

addressing

and

CIDR

(classless

interdomain

routing)
.


Authentication

prevent

from

unauthorised

advertisement
.



RIP messages are encapsulated with UDP
Datagram.


Port assign to rip in UDP is 520.


RIP treat all network as equal.


Periodic timer is of short time.


Not for large network .(infinity at 16).

Link state routing has a different philosophy from that of
distance vector routing. In link state routing, if each
node in the domain has the entire topology of the
domain

the list of nodes and links, how they are
connected including the type, cost (metric), and the
condition of the links (up or down)

the node can use
the Dijkstra algorithm to build a routing table.

Updates are send to entire network.ie network have
visibility of entire network of that area.



The Open Shortest Path First (OSPF) protocol is an
intradomain

routing protocol based on link state
routing. Its domain is also an autonomous system.



A

collection

of

networks

with

area

ID


Routers

inside

an

area

flood

the

area

with

routing

information


Area

border

routers

summarize

the

information

about

the

area

and

send

it

to

other

areas


Backbone

area

and

backbone

routers


All

of

the

area

inside

an

AS

must

be

connected

to

the

backbone



Administrator assign cost to each route.


Cost based on types of services.(minimum
delay,maximum throughput etc).


1 router can have many routing tables based on
different types of service.


Network mask :defines network mask over which hello
message is send.


Hello interval : seconds between hello messages.


E Flag : if set then stub area.


T Flag : if set then router support multiple metric.


Priority : highest priority router chosen as designated router.


Dead interval : seconds must pass before neighbor is
consider dead.


Neighbor routing address : Current list of all the neighbours
from which the sending router has received the hello
message.


OSPF packets encapsulated in IP Datagram.


RIP

OSPF


Keeps only track of
closest router for each
destination address.


It is simpler ,
older,easy

to
configure,uses

less
CPU .


Only metric used is
hope count.


Can be used in small
business needs.


Uses tree to keep track of
entire paths.


More overhead .Provide
more
options,better,faster

reaction time to
faliure

(convergence).


Build cost according to
router’s services.


Widely used for business
enterprises &
isp’s
.




When the domain of operation becomes large?


Distance vector routing is subject to instability if
there is more than a few hops in the domain of
operation.


Link state routing needs a huge amount of
resources to calculate routing tables. It also
creates heavy traffic because of flooding.


Need for a third routing protocol.



Path vector routing is similar to distance vector
routing. There is at least one node, called the
speaker node, in each AS that creates a routing
table and advertises it to speaker nodes in the
neighboring ASs..


Speaker node

: a node working on behalf of
complete AS.A speaker node advertises the path, not
the metric of the nodes, in its AS or other ASs.


Initialization
: At beginning each speaker node
can only the reachability of nodes inside its
autonomous system.



Sharing


A

speaker

in

an

AS

shares

its

table

with

immediate

neighbors


Updating


Adding

the

nodes

that

are

not

in

its

routing

table

and

adding

its

own

AS

and

the

AS

that

sent

the

table


The

routing

table

shows

the

path

completely


Border Gateway Protocol (BGP) is an
interdomain

routing protocol using path vector routing. It first
appeared in 1989 and has gone through four
versions.



Stub AS


Only one connection to another AS (only a source or sink for
data traffic) eg small local ISP.


Multihomed AS


More than one connection to other AS, but it is still only a
source or sink for data traffic. Eg large corporation connecting
regional AS.


Transit AS


Multihomed AS that also allows transient traffic eg national and
international isp’s




Well

Known

Mandatory

attributes

-



ORIGIN


The

source

of

the

routing

information

(RIP,

OSPF,

etc)


AS_PATH


The

list

of

ASs

through

which

the

destination

can

be

reached


NEXT
-
HOP


The

next

router

to

which

the

data

packet

should

be

sent



Optional attribues


Optional transitive


Optional non transitine


Open : to create neighbor relationship (open
tcp connection).


Update : it is used by a router to withdraw
destination that are previously advertised,
announce a route to new destination or both.


Keep alive : regularly send by routers to
confirm that they are alive or send in response
of open message.


Notification : send when there is error or router
want to close the connection.



Once opened the connection update, keepalive
and notification messages is continued until
cease notification is sent.


BGP uses services of tcp on port 179.