computer communication & networks
DESCRIPTION
Computer Communication & Networks. Lecture 22 Network Layer: Delivery, Forwarding, Routing (contd.) http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp. Waleed Ejaz [email protected]. Network Layer. Network Layer Topics to Cover. Logical Addressing. Internet Protocol. - PowerPoint PPT PresentationTRANSCRIPT
1
Computer Communication & Networks
Lecture 22
Network Layer: Delivery, Forwarding, Routing (contd.)
http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp
Waleed [email protected]
2
Network Layer
3
Network Layer Topics to CoverLogical Addressing
Internet Protocol
Address Mapping
Delivery, Forwarding, Routing
4
Two-node Instability: Counting to Infinity Problem
5
Split Horizon and Split Horizon with Poisoned Reverse
• 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– send infinity
1 2 3 41 1 1
N1 N2 N3 N4
Initial (2,3) (3,2) (4,1) (4,0)
1 (2,3) (3,2) (-1,) (4,0)
2 (2,3) (-1,) (-1,) (4,0)
3 (-1,) (-1,) (-1,) (4,0)
• Reverse Route– a route pointing to the node where it has arrived– it creates potential cycle
R1R2
Subnet N
Reverse Route
6
RIP ( Routing Information Protocol) Uses the distance-vector algorithm
DC
BA
u v
w
x
yz
destination hops u 1 v 2 w 2 x 3 y 3 z 2
7
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
Routing Information Protocol (RIP)
8
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
9
Example of a Domain using RIP
10
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
Open Shortest Path First
11
Path Vector Routing
12
EGP: Exterior Gateway Protocol designed for tree-structured Internet
concerned with reachability, not optimal
routes
BGP – Border Gateway Protocol
13
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
14
EGP Example
AS1
AS2
AS3
R1
R2 R3
R4N1
N1 reachable through 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
Only EGP routers are
shown
15
EGP Example
AS1
AS2
AS3
R1
R2 R3
R4N1
N1 reachable through AS2
• 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
16
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
17
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”
18
Initial routing tables in path vector routing
19
Stabilized tables for three autonomous systems
20
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
21
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
22