Transcript Data Communication & Network

Computer Communication &
Networks
Lecture 22
Network Layer: Delivery, Forwarding, Routing (contd.)
http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp
Waleed Ejaz
[email protected]
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Network Layer
2
Network Layer Topics to Cover
Logical Addressing
Internet Protocol
Address Mapping
Delivery, Forwarding, Routing
3
Two-node Instability:
Counting to Infinity Problem
4
Split Horizon and Split Horizon with
Poisoned Reverse
• Reverse Route
– a route pointing to the node where it has arrived
– it creates potential cycle
R2
Subnet N
R1
Reverse Route
• Split Horizon
– min cost to a given destination is not sent to a neighbor if the neighbor is
the next node along the shortest path
– a route is not broadcast on the interface through which the node has
learnt it
• Split Horizon with Poisoned Reverse
N1
N2
N3
N4
– send infinity
Initial (2,3)
(3,2)
(4,1)
(4,0)
1
1
1
1
2
3
4
1
(2,3)
(3,2)
2
3
(2,3)
(-1,)
(-1,)
(-1,)
(-1,)
(-1,)
(-1,)
(4,0)
(4,0)
(4,0)
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RIP ( Routing Information Protocol)

Uses the distance-vector algorithm
u
v
A
z
C
B
D
w
x
y
destination hops
u
1
v
2
w
2
x
3
y
3
z
2
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Routing Information Protocol (RIP)

Runs on top of UDP, port number 520

Metric: number of hops

Max limited to 15

suitable for small networks (local area
environments)

value of 16 is reserved to represent infinity

small number limits the count-to-infinity problem
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RIP Operation





Router sends update message to neighbors every 30 sec
A router expects to receive an update message from each
of its neighbors within 180 seconds in the worst case
If router does not receive update message from neighbor
X within this limit, it assumes the link to X has failed and
sets the corresponding minimum cost to 16 (infinity)
Uses split horizon with poisoned reverse
Convergence speeded up by triggered updates
 neighbors notified immediately of changes in distance
vector table
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Example of a Domain using RIP
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Open Shortest Path First

Fixes some of the deficiencies in RIP

Enables each router to learn complete network topology

Each router monitors the link state to each neighbor and
floods the link-state information to other routers

Each router builds an identical link-state database

Allows router to build shortest path tree with router as root

OSPF typically converges faster than RIP when there is a
failure in the network
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Path Vector Routing
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EGP: Exterior Gateway Protocol

designed for tree-structured Internet

concerned with reachability, not optimal
routes

BGP – Border Gateway Protocol
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Exterior Gateway Protocols

Within each AS, there is a consistent set of routes
connecting the constituent networks

EGP enables two AS’s to exchange routing
information about:


The networks that are contained within each AS

The AS’s that can be reached through each AS
EGP path selection guided by policy rather than
path optimality

Trust, peering arrangements, etc
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EGP Example
Only EGP
routers are
shown
R2
R3
AS2
R1
N1 reachable
through AS3
R4
N1
AS1
AS3
•
R4 advertises that network N1 can be reached through AS3
•
R3 examines announcement & applies policy to decide whether it will
forward packets to N1 through R4
•
If yes, routing table updated in R3 to indicate R4 as next hop to N1
•
IGP propagates N1 reachability information through AS2
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EGP Example
N1 reachable
through AS2
R2
R3
AS2
R1
R4
N1
AS1
AS3
•
EGP routers within an AS, e.g. R3 and R2, are kept consistent
•
Suppose AS2 willing to handle transit packets from AS1 to N1
•
R2 advertises to AS1 the reachability of N1 through AS2
•
R1 applies its policy to decide whether to send to N1 via AS2
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EGP Requirements


Scalability to global Internet

Provide connectivity at global scale

Link-state does not scale

Fully distributed
EGP path selection guided by policy rather
than path optimality

Trust, peering arrangements, etc

EGP should allow flexibility in choice of paths
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Internet inter-AS routing: BGP

BGP provides each AS a means to:
1.
Obtain subnet reachability information from
neighboring ASs.
2.
Propagate the reachability information to all
routers internal to the AS.
3.

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”
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Initial routing tables in path vector routing
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Stabilized tables for three autonomous systems
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BGP Policy

Examples of policy:

Never use AS X

Never use AS X to get to a destination in AS Y

Never use AS X and AS Y in the same path

Import policies to accept, deny, or set preferences on route
advertisements from neighbors

Export policies to determine which routes should be
advertised to which neighbors

A route is advertised only if AS is willing to carry traffic on
that route
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Why different Intra- and Inter-AS routing ?
Policy:


Inter-AS: admin wants control over how its traffic routed, who
routes through its net.
Intra-AS: single admin, so no policy decisions needed
Scale:
hierarchical routing saves table size, reduced update traffic
Performance:
 Intra-AS: can focus on performance
 Inter-AS: policy may dominate over performance

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