routing algorithms in computer Networks.ppt

COMPUTER NETWORKS
Networking Standards
(Network LAYER)
Network Layer

Computer Networks 2
Question to be answered ?
 What is the main goal of Network
layer?
 How does Network layer achieve
its goal?
Network Layer

Computer Networks 3
Network Layer
- The network layer is concerned with
getting packets from the source all
the way to the destination.
- Therefore, the main function (goal)
of network layer is routing packets
from the source machine to the
destination machine.
Network Layer

Computer Networks 4
To achieve its goal the network layer
must:
1- Know about the topology of the
communication subnet.
2- Choose appropriate paths through the
subnet.
3- Avoid overloaded communication lines
and routers (avoid congestion).
Network Layer

Store-and-Forward Packet Switching
The environment of the network layer protocols.
fig 5-1

Computer Networks 6
Store-and-Forwarding Packet Switching
 In Store-and Forwarding Packet Switching
mechanism, a host transmits a packet to the
nearest router, the packet is stored there
until it has fully arrived so the checksum can
be verified, then it is forwarded to the next
router along the path until it reaches the
destination host.
Network Layer

Computer Networks 7
Services provided to the Transport Layer
 There are two services provided by the
network layer to the transport layer:
1) Connectionless service.
2) Connection-Oriented Service.
Network Layer

Implementation of Connectionless Service
Routing within a diagram subnet.

Implementation of Connection-Oriented Service
Routing within a virtual-circuit subnet.

Comparison of Virtual-Circuit and Datagram
Subnets
5-4

Computer Networks 11
Routing Algorithms
 In most subnets, packets require multiple
hops to reach their destinations.
 The Routing algorithm is the software
part of the network layer responsible for
deciding which output line an incoming
packet should be transmitted on.
Network Layer

Routing Algorithms
 The decision about the best route either
calculated once as the case with
connection-oriented paradigm or for
every arriving data packet in the case of
the subnet uses connectionless
paradigm.
Network Layer

Routing Algorithms
Desirable properties in a routing algorithm:
1) Correctness
2) Simplicity
3) Robustness (hardware or software failures)
4) Stability (to be run for long time)
5) Fairness
6) Optimality
Network Layer

Routing Algorithms
Conflict between fairness and optimality.

The Optimality Principle
(a) A subnet. (b) A sink tree for router B.

Routing Algorithms
Routing Algorithms can be grouped in two
major classes:
1) Non-adaptive algorithms (Static Routing)
2) Adaptive algorithms (Dynamic Routing)
Network Layer

Routing Algorithms
1) Non-adaptive algorithms, they do not
base their routing decision on
measurements or estimates of the
current traffic and topology. Instead the
route is computed in advance, off-line,
and downloaded to the routers when the
network is booted.
Network Layer

Routing Algorithms
2) Adaptive algorithms, they change their
routing decisions to reflect changes in
the topology, and usually the traffic as
well.
- Adaptive algorithms differ in where they
get their information.
Network Layer

Routing Algorithms
- What metric is used for optimization?
- Distance
- number of hops
- estimated transit time
- delay
- bandwidth
Network Layer

Shortest Path Routing
- The technique that is used in many
routing algorithms is simple and easy to
understand.
- The idea is to build a graph of the subnet,
with each node of the graph representing
a router and each arc of the graph
representing a communication line
(link). To choose a route between a given
pair of routers, the algorithm just finds the
shortest path between them on the graph.
Network Layer

Network Layer
A
B
E F
C
D
H
G
2
7
3
3
2
2
2
6
1
2
4
Metric is number of hops
- The path ABC is equal to
path ABE.
Metric is the distance in km
- the path ABC is longer than
the path ABE

There are several algorithms to compute the
shortest path between two nodes of a
graph. The most popular is Dijkstra
algorithm.
In this algorithm each node is labeled (in
parentheses) with its distance from the
source node along the best known path.
Network Layer

routing algorithms in computer Networks.ppt

- In Flooding Algorithm every packet is sent
out on every outgoing line except the one it
arrived on.
- Timer is used to stop (limit) this flooding
which can be a hop counter.
- A practical selective flooding algorithm is
used where the packets are sent only to
the approximately in the right direction.
- It is suitable for military applications.
Network Layer

Collection of Subnetworks
The Internet is an interconnected collection of many networks.

3 Distance Vector Routing (1)
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.

3 Distance Vector Routing (2)
The count-to-infinity problem.

3 Distance Vector Routing (3): Loop-
Breaking Heuristics
Set infinity to a limited number, e.g. 16.
Split horizon
Split horizon with poison reverse

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Initialization
3 Example:

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Direct
Neighbours

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Neighbours
of neighbours

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Neighbours
of neighbours
of neighbours

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Stable
convergence

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1
Good news:
A new link!

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Direct
endpoints
know

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Neighbours
know

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1
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Neighbours
of neighbours
know

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A happy and
stable network

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Bad news:
Link crash!!

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Direct
endpoints
know

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10

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Get help
from
neighbours

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Routing loop
(due to
inconsistent
state info)

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Counting
to infinity…

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Stability
again

4 Link State Routing (1)
Each router must do the following:
• Discover its neighbors, learn their network
address.
• Measure the delay or cost to each of its
neighbors.
• Construct a packet telling all it has just learned.
• Send this packet to all other routers.
• Compute the shortest path to every other router.

4 Link State Routing (2): Learning
about the Neighbors
(a) Nine routers and a LAN.
(b) A graph model of (a).

4 Link State Routing (3): Measuring
Line Cost
A subnet in which the East and West parts are
connected by two lines.

4 Link State Routing (4): Building
Link State Packets
(a) A subnet. (b) The link state packets for this
subnet.

4 Link State Routing (5): Distributing
the Link State Packets
The packet buffer for router B in the previous slide (Fig.
5-13).

5 Hierarchical Routing
Hierarchical routing.

6. Routing in Ad Hoc Networks (1)
Possibilities when the routers are mobile:
Military vehicles on battlefield.
• No infrastructure.
A fleet of ships at sea.
• All moving all the time
Emergency works at earthquake .
• The infrastructure destroyed.
A gathering of people with notebook computers.
• In an area lacking 802.11.

6. Routing in Ad Hoc Networks (2):
Route Discovery
(a) Range of A's broadcast.
(b) After B and D have received A's broadcast.
(c) After C, F, and G have received A's broadcast.
(d) After E, H, and I have received A's broadcast.
Shaded nodes are new recipients. Arrows show possible reverse routes.

Route Discovery
Format of a ROUTE REQUEST packet.

Route Discovery
Format of a ROUTE REPLY packet.

Route Maintenance
(a) D's routing table before G goes down.
(b) The graph after G has gone down.

4. RIP ( Routing Information Protocol) (1)
Distance vector algorithm
Included in BSD-UNIX Distribution in 1982
Distance metric: # of hops (max = 15 hops)
D
C
B
A
u v
w
x
y
z
destination hops
u 1
v 2
w 2
x 3
y 3
z 2
From router A to subsets:

4. RIP (2): advertisements
•Distance vectors: exchanged among neighbors every
30 sec via Response Message (also called
advertisement)
•Each advertisement: list of up to 25 destination nets
within AS

4. RIP (3): Example
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B 7
x -- 1
…. …. ....
w x y
z
A
C
D B
Routing table in D

4. RIP (4) : Example
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B A 7 5
x -- 1
…. …. ....
Routing table in D
w x y
z
A
C
D B
Dest Next hops
w - 1
x - 1
z C 4
…. … ...
Advertisement
from A to D

4. RIP (5): Link Failure and Recovery
If no advertisement heard after 180 sec --> neighbor/link
declared dead
• routes via neighbor invalidated
• new advertisements sent to neighbors
• neighbors in turn send out new advertisements (if tables
changed)
• link failure info quickly propagates to entire net
• poison reverse used to prevent ping-pong loops (infinite
distance = 16 hops)

4. RIP (6): Table processing
RIP routing tables managed by application-level process called
route-d (daemon)
advertisements sent in UDP packets, periodically repeated
physical
link
network forwarding
(IP) table
Transport
(UDP)
routed
physical
link
network
(IP)
Transport
(UDP)
routed
forwarding
table

4. OSPF (1) (Open Shortest Path First)
•“open”: publicly available
•Uses Link State algorithm
– LS packet dissemination
– Topology map at each node
– Route computation using Dijkstra’s algorithm
•OSPF advertisement carries one entry per neighbor router
•Advertisements disseminated to entire AS (via flooding)
• Carried in OSPF messages directly over IP (rather than TCP or
UDP

4. Hierarchical OSPF (3)
• Two-level hierarchy: local area, backbone.
– Link-state advertisements only in area
– each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.
• Area border routers: “summarize” distances to nets in own
area, advertise to other Area Border routers.
• Backbone routers: run OSPF routing limited to backbone.
• Boundary routers: connect to other AS’s.

5. BGP (1)
• BGP (Border Gateway Protocol): the de facto standard
• BGP provides each AS a means to:
– Obtain subnet reachability information from
neighboring ASs.
– Propagate the reachability information to all routers
internal to the AS.
– Determine “good” routes to subnets based on
reachability information and policy.
• Allows a subnet to advertise its existence to rest of the
Internet: “I am here”

5. BGP (2): basics
• Pairs of routers (BGP peers) exchange routing info over semi-permanent
TCP connections: BGP sessions
• Note that BGP sessions do not correspond to physical links.
• When AS2 advertises a prefix to AS1, AS2 is promising it will forward
any datagrams destined to that prefix towards the prefix.
– AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c
2a
AS3
AS1
AS2
1a
2c
2b
1b
3c
eBGP session
iBGP session

5. BGP (3): Distributing reachability info
•With eBGP session between 3a and 1c, AS3 sends prefix reachability info to
AS1.
•1c can then use iBGP do distribute this new prefix reach info to all routers in
AS1.
•1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP
session.
•When router learns about a new prefix, it creates an entry for the prefix in its
forwarding table.
3b
1d
3a
1c
2a
AS3
AS1
AS2
1a
2c
2b
1b
3c
eBGP session
iBGP session

5. BGP (4): Path attributes & BGP routes
• When advertising a prefix, advert includes BGP attributes.
– prefix + attributes = “route”
• Two important attributes:
– AS-PATH: contains the ASs through which the advert
for the prefix passed: AS 67 AS 17
– NEXT-HOP: Indicates the specific internal-AS router
to next-hop AS. (There may be multiple links from
current AS to next-hop-AS.)
• When gateway router receives route advert, uses import
policy to accept/decline.

5. BGP (5): route selection
• Router may learn about more than 1 route to some prefix.
Router must select route.
• Elimination rules:
1. Local preference value attribute: policy decision
2. Shortest AS-PATH
3. Closest NEXT-HOP router: hot potato routing
4. Additional criteria

5. BGP (6): messages
•BGP messages exchanged using TCP.
•BGP messages:
– OPEN: opens TCP connection to peer and authenticates sender
– UPDATE: advertises new path (or withdraws old)
– KEEPALIVE keeps connection alive in absence of UPDATES;
also ACKs OPEN request
– NOTIFICATION: reports errors in previous msg; also used to
close connection

5. BGP (7): routing policy
Figure 4.5-BGPnew: a simple BGP scenario
A
B
C
W
X
Y
legend:
customer
network:
provider
network
•A,B,C are provider networks
•X,W,Y are customer (of provider networks)
•X is dual-homed: attached to two networks
– X does not want to route from B via X to C
– .. so X will not advertise to B a route to C

5. BGP (8): routing policy
Figure 4.5-BGPnew: a simple BGP scenario
A
B
C
W
X
Y
legend:
customer
network:
provider
network
•A advertises to B the path AW
•B advertises to X the path BAW
•Should B advertise to C the path BAW?
– No way! B gets no “revenue” for routing CBAW since neither W nor C
are B’s customers
– B wants to force C to route to w via A
– B wants to route only to/from its customers!

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