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Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer
4-1
Interplay between routing, forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
Network Layer
4-2
Graph abstraction
5
2
u
2
1
Graph: G = (N,E)
v
x
3
w
3
1
5
1
y
z
2
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Remark: Graph abstraction is useful in other network contexts
Example: P2P, where N is set of peers and E is set of TCP connections
Network Layer
4-3
Graph abstraction: costs
5
2
u
v
2
1
x
• c(x,x’) = cost of link (x,x’)
3
w
3
1
5
1
y
2
- e.g., c(w,z) = 5
z
• cost could always be 1, or
inversely related to bandwidth,
or inversely related to
congestion
Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
Question: What’s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Network Layer
4-4
Routing Algorithm classification
Global or decentralized
information?
Global:
 all routers have complete
topology, link cost info
 “link state” algorithms
Decentralized:
 router knows physicallyconnected neighbors, link
costs 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 update
 in response to link
cost changes
Network Layer
4-5
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer
4-6
Link state routing
 Each router must do the following

Discover its neighbors and learn their network
addresses
 Measure
neighbor
the delay or cost to each of its

Construct a packet telling all it has just learned

Send this packet to all other routers

Compute the path to every other routerNetwork Layer
4-7
Learning about the neighbors
 When a router is booted

It sends a special Hello packet to its neighbors
• The router on the other side answers telling who it is
Network Layer
4-8
Measuring line cost
 Link state routing algorithm

Requires each router to have
• A reasonable estimate of the delay to each of its
neighbors
 To determine this delay

A router sends a special ECHO packet to the
other side
• That the other side is required to send back
immediately
• By measuring the round trip time and dividing it by 2
• The sending router gets a reasonable estimate of the
delay
Network Layer
4-9
Building link state packets
Network Layer 4-10
Distributing link state packets
and computing new routes
 Link state packets
 Are distributed using flooding

A new link state is forwarded on all lines
• Except the one it arrived on
 Once router accumulated
 full set of link state packets, it constructs entire
graph
• Where every link will be represented

Dijkstra’s algorithm can be run
• to construct shortest paths to all possible destinations
Network Layer
4-11
Link state routing: wrap up
Network Layer 4-12
A Link-State Routing Algorithm
Dijkstra’s algorithm
 net topology, link costs
known to all nodes
 accomplished via “link
state broadcast”
 all nodes have same info
 computes least cost paths
from one node (‘source”) to
all other nodes
 gives forwarding table
for that node
 iterative: after k
iterations, know least cost
path to k dest.’s
Notation:
 c(x,y): link cost from node
x to y; = ∞ if not direct
neighbors
 D(v): current value of cost
of path from source to
dest. v
 p(v): predecessor node
along path from source to v
 N': set of nodes whose
least cost path definitively
known
Network Layer 4-13
Dijsktra’s Algorithm
1 Initialization:
2 N' = {u}
3 for all nodes v
4
if v adjacent to u
5
then D(v) = c(u,v)
6
else D(v) = ∞
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' :
12
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
14 shortest path cost to w plus cost from w to v */
15 until all nodes in N'
Network Layer 4-14
Dijkstra algorithm
 Basic idea

It finds the shortest path from the source
• To a node nearest to it, then to a second nearest, and
so on
• At each step,
– select a newly reachable node at lowest cost
– And add an edge to that node to connect it to the tree built
Network Layer 4-15
Dijkstra algorithm (cont’d)
Network Layer 4-16
Example
A
8
0
4
A
8
2
B
8
7
2
C
2
1
D
9
E

F
A
8
4
5
B
8
7
5
E
2
C
3
B
2
7
5
E
D
8
A
8
3
5
0
4
2
C
3
1
9
0
4
2
F
2
8
4
2
3

0
2
1
9
D
11
F
5
3
B
2
7
7
5
E
C
3
2
1
9
D
8
F
3
5
Network Layer 4-17
Example (cont.)
A
8
0
4
2
B
2
7
7
C
3
5
E
2
1
9
D
8
F
3
5
A
8
0
4
2
B
2
7
7
C
3
5
E
2
1
9
D
8
F
3
5
Network Layer 4-18
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-19
Distance vector routing: basic
idea
Network Layer 4-20
Distance vector routing:
example
 Basic idea

Look at costs that your neighbors are advertising
• To get a packet to a destination
• Then Select the neighbor whose advertised cost
– Added to the cost to that neighbor is lowest
Network Layer 4-21
Distance vector routing: rapid
reaction to good news
 Distance vector routing reacts rapidly to
good news

When A comes up, other routers learn about it
• B makes an entry in its routing table that A is one hop
away
• On next exchange, C learns that A is 2 hops away
– D, and E do not hear the good news until later
• Good news spread at the rate of one hop per exchange
Network Layer 4-22
Distance vector routing: the
count to infinity problem
 Distance vector routing reacts leisurely to
bad news

A goes down
• 1st exchange: B thinks it can reach A via C, with path
length 3
• 2nd exchange: C makes its new distance to A 4 and
etc…
Network Layer 4-23
Comparison of LS and DV algorithms
Message complexity
 LS: with n nodes, E links,
O(nE) msgs sent
 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
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
• error propagate thru
network
Network Layer 4-24
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-25