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Network Course Internetworking Protocols Dr. Raed Al Qadi

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Network Layer Internetworking Introduction

Issues:

Technical

Routing

Addressing

Connectionless Vs. Connection-Oriented

Administrative

Trust

Security

Legal

Accounting

Focus on technical issues , particularly routing in this section. Connection-Oriented/Connectionless discussed in transport layer section.

- Routing protocol
- Gateway / Router
- Route / Path
- Metric

The main goal of routing protocol is to discover paths between source and destination systems which satisfy given conditions, e.g., minimal path cost or conformance with administrative policies or regulations.

Network Layer Internetworking Protocol Types

- Distance Vector Protocols
- Link State Protocols
- IGP vs. EGP

Network Layer Internetworking Introduction (cont)

Topologically Adaptive Routing Algorithms

Routes are computed based on information exchange among nodes. Computations is based on what nodes/links are up/down at any given time and weights are given to eash link that is available

Traffic Adaptive Routing Algorithms

Like topology adaptive, except current traffic patterns are taken into account.

Protocol Types Distance Vector Protocols

Nodes learn about the network topology from neighbor nodes. They then compute routes based on this information. If the node change the way it will route traffic after the computation , it informs its neighbors of the new routes(either immediately or after a pre-determined time).

Neighbors on receipt of new routing information also recompute and if they alter their routes they inform their neighbors, etc.

The computational complexity of this scheme is between N2 and N3 where N is the number of nodes in the network.

Cont..

Step 0. Database at

A B C D E F

A 0 3A __ 1A __ __

B 3B 0 8B 1B __ __

C __ 8C 0 __ 4C __

Destination D 1D 1D __ 0 __ 2D

E __ __ 4E __ 0 1E

F __ __ __ 2F 1F 0

- Assume all gateways exchange complete database at each step

Cont..

Step 1. Database at

A B C D E F

A 0 2D 11B 1A __ 3D

B 2D 0 8B 1B 12C 3D

C 11B 8C 0 9B 4C 5E

Destination D 1D 1D 9B 0 3F 2D

E __ 12C 4E 3F 0 1E

F 3D 3D 5E 2F 1F 0

Cont..

Step 2. Database at

A B C D E F

A 0 2D 10B 1A 4F 3F

B 2D 0 8B 1B 4F 3D

C 10D 8C 0 7F 4C 5E

Destination D 1D 1D 7E 0 3F 2D

E 4D 4D 4E 3F 0 1E

F 3D 3D 5E 2F 1F 0

Distance Vector Protocols Problems

- Consider
- Destination D
- Cost from

A 3B

B 2C

C 1D

- C—D fails
- Successive table entries for dest D from node

A

1

B

1

C

1

D

B 2C 2C 4A 4A 6A 6A 8A ….. ∞

C 1D ∞ 3B 5B 5B 7B 7B ……. ∞

- Eventually the algorithm converges but only after

“count to infinity” .

- Problem is that path to D from B uses B but B is unaware of this.

Solutions:

- 1--Solution –”split horizon”- do not report a route to a destination to the neighbor from which the routes was learned.

A 3B 3B3B ∞

B 2C 2C ∞ ∞

C 1D ∞ ∞ ∞

- Modification –”split horizon with poisoned reverse”- includes routes omitted by split horizon in update using metric ∞.
- Disadvantage of poisoned reverse: rerequires more network traffic.
- Advantage: is that converges can be faster.

Successive table entries for dest E from node

A 3B 3B 4C 5C 6C 15C ∞

B 2D ∞ 4C 5C 6C 15C ∞

C 3B 3B 4A 5A 6A 15A ∞

- Eventually the algorithm converges but only after “count infinity”.
- Split horizon method does not help.

Converges can also be spend up by triggering updates to neighbors when gateway’s routing database changes.

- 2—solution-”hold down”- when a route is lost by a node , it does not accept new route for a certain time.

A 3B 3B ∞

B 2D ∞ ∞

C 3B 3B ∞

- Difficulty is determining hold down period. Too short may lead to quick acceptance of new routes. Too long may lead to loss of data enrooted to destination.

Protocol Types Link State Protocols

Nodes sense their local environment (links, neighbors) and broadcast this information to the entire network using controlled flooding. Each node thereby builds a topological map of the whole network. Each node then independently computes routes based on an algorithm such as Dijkstra’s SPF.

The computational complexity of this scheme is E logN, where E is the number of links in the network and N is the number of nodes.

Keep distances to root as “heap”.

- N nodes so heap depth ≤log2N .
- For each link , may change one value & restore heap.
- Hence O(log2N) steps for each of E links.

Link State SPF-at each Node A

- Receive all link costs.
- For each destination x, keep current distance to x initially all∞
- Set S = Ø(empty).

step 0: set dist(A,A)=0

step i: find node y with dist(A,y) min among nodes not in S.

Put y in S.

For all z directly conn. To y recompute dist(A,z) taking into account dist(A,y), y->z.

Go to step i+1.

9

10

- N nodes
- Max path length ~log n
- In present case, use to find min of values from source A.

14

26

12

18

19

35

80

22

SPF Example

1

A to B C D E

√ ∞ ∞ ∞ ∞

3A √ A ∞ 5A

√ C 1A 7C 5A

C 1A 4B √ B

C A √ B 3 B

√=added to S,, m =in S ,, X,X=next hop

A

3

B

1

C

5

1

2

6

E

D

0

0

1

0

2

1

0

2

1

3

2

1

4

3

0

Comparison of routing algorithmsLink State routing Disadvantage

- Difficult to design and build correctly-global propagation algorithm is particularly tricky.
- Computational complexity-O(E logN).

(Can be reduced to O(E) if small number of metric values)

3. Each gateway must know topology of entire network. Hence tables are relatively large.

Comparison of routing algorithmsDistance Vector Algorithm disadvantages

- Basic algorithm exhibits looping behavior, although there are partial remedies for this.
- Slow to converge (count to infinity).
- Converges at speed of slowest link in the network-needs transmission priority on routing traffic to work well with a wide variety of link speed.
- Difficult to source as routing information is digested before being propagated.
- Not good for nets with long paths because of time propagate changes.

Comparison of routing algorithmsDistance Vector Algorithm Advantages

- Very simple to design and build.
- Each node need only know its local topology. Hence tables are relatively small.
- Low computational overhead.
- Can be made hierarchical very easily to save memory , computation and bandwidth.
- Can be made very robust through careful database and routing message design.
- Can be used for reachability only in administratively disorganized networks e.g. EGP.

Comparison of routing algorithmsLink State Routing Advantages

- Scales well with network size –can handle larger nets than distance vector.
- Converges rapidly after changes.
- Rarely exhibits loops and stamps them out quickly.
- Relatively easy to secure by authenticating the origin of each link state packet.
- Can detect misbehaving nodes because information is received from both ends of links.
- Suitable for adaptive routing based on traffic.

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