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CIS 725
Network Layer
Network Layer
• This layer provides communication between any two nodes
• Uniform addressing scheme independent of the network technology
Network layer
Data Link layer
Problems to be addressed
• Routing• Congestion control
Routing
• Processing nodes• Switches/routers• Communication links
Datagram routing
• Each packet is routed independently • Unordered• Unreliable
Virtual Circuit routing
• Set up a path for each connection• All messages are sent over this path• Ordered• Reliable
Routing Table
• Routing table contains the next hop information
• At each node i,
for each destination j,
what is the next hop
Bellman-Ford Algorithm
• Distance table:• Routing table
Distance Vector Algorithms
distance from i to j via k
• Protocol at node i:
On receiving routing table from k
update column k of the distance table as follows
For each destination j,
if k reports its distance to j as d then
update Dj,k = d + weight(i,k)
Routing table
• Recompute the routing table as follows:
For each destination k,
update next hop to j, where
Dj,k = min(Dx,k: for all x)
• Periodically or whenever the routing table changes, send routing table to all neighbors
Bellman-Ford Algorithm
b c
bcde
a b ea c d
acde
abde
bcde
acde
abde
2
2
1 1
24
e
d
c
b
a
2
42
21
41
2
b 2c 4
a 2
c 1
d 2
a 4b 1
e 2
Bellman-Ford Algorithm
b c
bcde
a b ea c d
acde
abde
bcde
acde
abde
2
2
1 1
24
e
d
c
b
a
2
42
21
41
2
b 2b 3
a 2
c 1
d 2
b 3
b 1
e 2
34
b 4
56
3
3
b 3
Bellman-Ford Algorithm
• Distance vector algorithm– Best path criteria
• Latency• Jitter• bandwidth
Counting to infinitya
b c
1
1
100
aa
b
b
13
31a a
a100 2 2
cc a a 1
1b b
a aa
100 4 4
cc
aa
b
b
13
3101 a b 3
1b b
aa
b
b
15
3101 a b 5
1b b
101
a ed fb c 11 111
5
2012345
< a,b,c,d,e,f>
dest next dist path
a ed fb c 11 111
5
12345
dest next dist Next to last hop
b b 1 a
d b 3 cc b 2 b
f b 5 e
e b 4 d
ed c b
Link State Routing
• Each node maintains the state of every link• State of the link may contain several types
of information
Learning local information
• Exchange hello packets periodically• Measure the cost of adjacent links
Distributed link state information
• Link state packet:
* source id
* cost of incident edges
* sequence numbers
Building Link State Packets
(a) A subnet. (b) The link state packets for this subnet.
Distribution Algorithm
• Controlled flooding
- hop count
- sequence numbers
- age
• Receive LSP from nbr j
- compare sequence numbers
- update topology table
- propagate LSP to all nbrs except j
Hierarchical Routing
• Divide area into regions• Maintain accurate information about nodes
in the same region
Hierarchical Routing
Hierarchical routing.
Source-routing
• Message contains the entire path• Routing table lookup is avoided at each
node
Broadcasting/Multicasting
• To send a message to all nodes:
- make a separate message to each destination
- route each message independently
Multi-destination routing
• Include all destination address in message• Each node decides which links to forward
so that it reaches all destinations
Multicast trees
• Create a spanning tree• Prune the spanning tree: Steiner tree