1. The Network Layer & Routing
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
The Network Layer & Routing
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
The network layer moves transport layer segments from host to host in the network, to deliver them to their destination. This layer involves each and every host and router in the network. We will study the key principles and algorithms of routing, with a focus on the Internet Protocol (IP) service model.
4: Network Layer

2. Network layer functions
transport packet from sending to receiving hosts
network layer protocols in every host, router
three important functions:
path determination: route taken by packets from source to destination - routing algorithms
switching: move packets from router’s input to appropriate router output
call setup: some network architectures require router call setup along path before data flows (what types?)
application
transport
network
data link
physical
network
data link
physical
4: Network Layer

3. Virtual circuits
the source-to-destination path behaves much like a telephone circuit
performance-wise
network actions along source-to-destination path
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination host ID)
every router/switch on source-destination path maintains a “state” for each passing connection
Recall: transport-layer connection only involved two end systems
link and router resources (bandwidth, buffers) may be dedicated to the VC
to get circuit-like performance
but… what about start-up delay?
4: Network Layer

4. Virtual circuits: signaling protocols
used to setup, maintain and teardown the VC
used in ATM, frame-relay and X.25
not used in the Internet (why?)
application
transport
network
data link
physical
application
transport
network
data link
physical
5. Data flow begins
6. Receive data
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
4: Network Layer

5. Datagram networks: the Internet model
no call setup at network layer
routers: do not maintain state for the end-to-end connections
no network-level concept of a “connection”
packets are typically routed using only destination host ID which is carried in the packet
packets between same source-destination pair may take different paths
application
transport
network
data link
physical
application
transport
network
data link
physical
1. Send data
2. Receive data
4: Network Layer

6. Routing Routing protocol Graph abstraction for routing algorithms:
Goal: determine “good” path
(sequence of routers) thru
network from source to dest. 5 3 B C 2 5 A 2 1 F 3
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links
link cost: delay, $ cost, or congestion level 1 2 D E 1
“good” path:
typically means minimum cost path
other def’s possible
4: Network Layer

7. Routing Algorithm classification
Global or decentralized cost information?
Global:
all routers have complete topology & link cost info
“link state” algorithms
Decentralized:
router knows physically-connected neighbors, link costs to route to neighbors
iterative process of computation & exchange of info with neighbors
“distance vector” algorithms
Static or dynamic?
Static:
routes change slowly over time
Dynamic:
routes change more quickly
periodic algorithm-driven updates
responsive to link cost changes A E D C B F 2 1 3 5
4: Network Layer

8. 4: Network Layer

9. 4: Network Layer

10. 4: Network Layer

11. 4: Network Layer

12. 4: Network Layer

13. 4: Network Layer

14. Dijsktra’s Algorithm 1 Initialization: 2 N = {A} // Source node is “A”
3 for all nodes v
if v adjacent to A
then D(v) = c(A,v)
else D(v) = infinity 7
8 Loop
9 find w not in N such that D(w) is a minimum
10 add w to N
11 update D(v) for all v adjacent to w and not in N:
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
shortest path cost to w plus cost from w to v */
15 until all nodes in N A E D C B F 2 1 3 5
4: Network Layer

15. Dijkstra’s algorithm: example
Step 1 2 3 4 5
start N A AD
ADE
ADEB
ADEBC
ADEBCF
D(B),p(B)
2,A -
D(C),p(C)
5,A
4,D
3,E -
D(D),p(D)
1,A -
D(E),p(E)
infinity
2,D -
D(F),p(F)
infinity
4,E A E D C B F 2 1 3 5
4: Network Layer

16. Distance Vector Routing Algorithm
iterative:
continues until no nodes exchange info.
self-terminating: no “signal” to stop
asynchronous:
nodes need not exchange info/iterate in lock step!
distributed:
each node communicates only with directly-attached neighbors
Distance Table data structure
each node has its own
row for each possible destination
column for each directly-attached neighbor to node
example: in node X, for destination Y via neighbor Z:
distance from X to
Y, via Z as next hop =
DX(Y,Z) =
c(X,Z) + minw {DZ(Y,w)}
4: Network Layer

17. Distance Table: example7 8 1 2
D () A B C D 1 7 6 4 14 8 9 11 5 2 E
cost to destination via
destination
D (C,D) E
c(E,D) + min {D (C,w)} D w =
2+2 = 4
D (A,D) E
c(E,D) + min {D (A,w)} D w =
2+3 = 5
loop back through E!
D (A,B) E
c(E,B) + min {D (A,w)} B w =
8+6 = 14
loop back through E!
4: Network Layer

18. Distance table gives routing table1 7 6 4 14 8 9 11 5 2 E
cost to destination via
destination
Outgoing link
to use, cost E A B C D
A,1
D,5
D,4
D,2
destination
Distance table
Routing table
>> next link, cost
4: Network Layer

19. Distance Vector Algorithm (Bellman-Ford):
At all nodes, X:
1 Initialization:
2 for all adjacent nodes v:
D (*,v) = infinity /* the * operator means "for all rows" */
D (v,v) = c(X,v)
5 for all destinations, y
send min D (y,w) to each neighbor /* w over all X's neighbors */ X X X w
4: Network Layer

20. Distance Vector Algorithm (cont.):
8 loop
9 wait (until I see a link cost change to neighbor V
or until I receive update from neighbor V) 11
12 if (c(X,V) changes by d)
/* change cost to all dest's via neighbor v by d */
/* note: d could be positive or negative */
for all destinations y: D (y,V) = D (y,V) + d 16
17 else if (update received from V wrt destination Y)
/* shortest path from V to some Y has changed */
/* V has sent a new value for its min DV(Y,w) */
/* call this received new value "newval" */
for the single destination y: D(Y,V) = c(X,V) + newval 22
23 if we have a new min D (Y,w) for any destination Y
send new value of min D (Y,w) to all neighbors 25
26 forever X X w X X w X w
4: Network Layer

21. Comparison of LS and DV algorithms
Message complexity
LS: with n nodes, E links, O(nE) msgs sent/broadcast
DV: exchange between neighbors only
convergence time varies
Speed of Convergence
LS: O(n2) algorithm requires O(nE) msgs
may have oscillations
DV: convergence time varies
may be routing loops
count-to-infinity problem
poisoned reverse is sometimes successful
Robustness: what happens if router malfunctions?
LS:
node can advertise incorrect link cost
each node computes only its own table
DV:
DV node can advertise incorrect path cost
each node’s table used by others
errors propagate through the network
4: Network Layer