1. Chapter 4 – The Network Layer & Routing
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
Chapter 4 – 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. Chapter 4: Network Layer
Chapter goals:
understand principles behind network layer services:
routing (path selection)
dealing with scale
how a router works … switching
advanced topics: IPv6, multicast intro
instantiation and implementation in the Internet
Overview:
network layer services
routing principle: path selection
hierarchical routing IP
Internet routing protocols
intra-domain
inter-domain
what’s inside a router?
IPv6
multicast routing
4: Network Layer

3. 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

4. Network service model
Q: What is the appropriate service model for transporting packets from sender to receiver?
guaranteed bandwidth?
preservation of inter-packet timing (no jitter)?
loss-free delivery?
in-order delivery?
congestion feedback to sender?
The most important
service abstraction to
consider, as provided
by the network layer: ?
virtual circuit network or
datagram network? ? ?
service abstraction
Recall: don’t confuse the term
“datagram” as used here with the Java UDP segment name discussed earlier.
4: Network Layer

5. 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

6. 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

7. 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

8. Network layer service models:
Guarantees ?
Network
Architecture
Internet
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Congestion
feedback
no (inferred
via loss) no
congestion
yes
Bandwidth
none
constant
rate
guaranteed
minimum
Loss no
yes
Order no
yes
Timing no
yes
Note- the Internet model may be extended: Integrated Services (Intserv), Differentiated Services (Diffserv)
Chapter 6
4: Network Layer

9. Summary: Datagram or VC network… why?
Internet
data exchange among computers
“elastic” service, no strict timing required (data delivery)
“smart” end systems (computers)
can adapt, perform control, error recovery
complexity at “edge”, simple in network core
many link types
different characteristics
uniform service difficult
ATM
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones, “videophones”
complexity inside the network
Consider: IP over ATM
(more later)
4: Network Layer

10. Goal: determine a “good” path
Routing A E D C B F 2 1 3 5
Routing protocol
Goal: determine a “good” path
(sequence of routers) thru the network from the source to the destination
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links
link cost: delay, distance, # of hops, rate structure or congestion level = $$
Other costs??
“good” path:
typically means minimum cost path
other definitions also possible
4: Network Layer

11. 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

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
yields a routing table for that node
iterative: after k iterations, know least cost path to k destinations
Notation:
c(i,j): link cost from node i to node j. Cost is initially infinite if not a direct neighbor
D(v): current computed value of cost of the path from the source to destination v
p(v): predecessor node, that is a neighbor of v, along the path from the source to v
N: set of nodes whose least cost path is definitively known
4: Network Layer

13. 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

14. 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

15. Dijkstra’s algorithm, discussion
Algorithm complexity: n nodes
each iteration: need to check all nodes, w, not in N
n*(n+1)/2 comparisons => worst case complexity O(n2)
more efficient implementations possible: O(nlogn)
Oscillations possible with congestion-based costing:
e.g., link cost = amount of carried traffic (load) A A D C B
2+e
1+e 1 A A D C B
2+e e
1+e 1 1
1+e
2+e D B D B e 1 C
1+e C 1 1 e
… recompute
routing
… recompute
… recompute
initially
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 Routing: overview
Iterative, asynchronous: each local iteration caused by:
local link cost change
message from neighbor: its least cost path changed from a neighbor
Distributed:
each node notifies neighbors only when its least cost path to any destination changes
neighbors then notify their neighbors if necessary
Each node:
wait for (change in local link cost, or notification from a neighbor)
recompute distance table
if least cost path to any dest. has changed, notify neighbors
4: Network Layer

20. 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

21. 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 is "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

22. Distance Vector Algorithm: exampleZ 1 2 7 Y
E.G., at node X:
Information from Y that its cost to Z is 1… add to X cost to Z via Y
Information from Z that its cost to Y is 1… add to X cost to Y via Z
4: Network Layer

23. Distance Vector Algorithm: exampleZ 1 2 7 Y
D (Y,Z) X
c(X,Z) + min {D (Y,w)} w =
7+1 = 8 Z
D (Z,Y) X
c(X,Y) + min {D (Z,w)} w =
2+1 = 3 Y
4: Network Layer

24. Distance Vector Algorithm: exampleB 7 8 1 2 DA B E 7 C D 1 DB A C E 7 1 D 8
Initialize…
& update DC B D A 1 2 E DD C E A B 2 DE A B D 1 8 C 2
4: Network Layer

25. Distance Vector Algorithm: exampleB 7 8 1 2 DA B E 7 C D 1 DB A C E 7 1 D 8 DC B D A 1 2 E DD C E A B 2 DE A B D 1 8 C 2
4: Network Layer

26. Distance Vector Algorithm: exampleB 7 8 1 2 DA B E 7 9 C D 3 1 DB A C E 7 9 1 D 10 8 DC B D A 1 2 E DD C E A 3 B 10 2 DE A B D 1 8 C 2
4: Network Layer

27. Distance Vector Algorithm: exampleB 7 8 1 2 DA B E 7 9 C D 3 1 DB A C E 7 9 1 D 10 8 DC B D A 1 2 E DD C E A 3 B 10 2 DE A B D 1 8 C 2
4: Network Layer

28. Distance Vector Algorithm: exampleB 7 8 1 2 DA B E 7 9 C 8 D 17 3 15 1 DB A C E 7 9 1 D 10 8 DC B D A 8 1 11 2 E 9 DD C E A 3 B 10 2 DE A B D 1 15 8 C 9 18 2
4: Network Layer

29. Distance Vector Algorithm: exampleB 7 8 1 2 DA B E 7 9 C 8 D 17 3 15 1 DB A C E 7 9 1 D 10 8
and so
on… DC B D A 8 1 11 2 E 9 DD C E A 3 B 10 2 DE A B D 1 15 8 C 9 18 2
4: Network Layer

30. Distance Vector: link cost changes
node detects local link cost change
updates distance table (line 15)
if cost change in least cost path, notify neighbors (lines 23,24) X Z 1 4 50 Y
algorithm
terminates
“good
news
travels
fast”
4: Network Layer

31. Distance Vector: link cost changes
good news travels fast
bad news travels slowly - “count to infinity” problem! X Z 1 4 50 Y 60 Y
algorithm
continues
on!
4: Network Layer

32. Distance Vector: poisoned reverse
If Z routes through Y to get to X :
Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)
will this completely solve count to infinity problem? X Z 1 4 50 Y 60 Y
algorithm
terminates
4: Network Layer

33. 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