1. EEC-484/584 Computer Networks
Lecture 9
Wenbing Zhao
(Part of the slides are based on Drs. Kurose & Ross’s slides for their Computer Networking book)

2. EEC-484/584: Computer Networks
Outline
Quiz#2 result
Introduction to network layer
Routing and forwarding, etc.
Router architecture
Routing algorithm
Link state routing
Distance vector routing
11/16/2018
EEC-484/584: Computer Networks

3. EEC-484/584: Computer Networks
EEC484 Quiz#2 Result
High 95, low 32, average: 67
Q1: 28.2/50, Q2: 16/20, Q3: 7/10, Q4: 16/20
11/16/2018
EEC-484/584: Computer Networks
Wenbing Zhao

4. EEC-484/584: Computer Networks
EEC584 Quiz#2 Result
High 95, low 35, average: 69
Q1: 34/50, Q2: 19/20, Q3: 9.2/10, Q4: 6.7/20
11/16/2018
EEC-484/584: Computer Networks
Wenbing Zhao

5. EEC-484/584: Computer Networks
Network Layer
Main concern: end-to-end transmission
Perhaps over many hops at intermediate nodes
Services provided to transport layer
Transport segment from sending to receiving host
On sending side encapsulates segments into datagrams
On receiving side, delivers segments to transport layer
Network layer protocols in every host, router
Router examines header fields in all IP datagrams passing through it
application
transport
network
data link
physical
network
data link
physical
11/16/2018
EEC-484/584: Computer Networks

6. Two Key Network-Layer Functions
Routing: determine route taken by packets from source to destination
Forwarding: move packets from router’s input to appropriate router output
Analogy:
Routing: process of planning trip from source to destination
Forwarding: process of getting through single intersection
11/16/2018
EEC-484/584: Computer Networks

7. Interplay between Routing & Forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001
Forwarding table is
also referred to as routing table
11/16/2018
EEC-484/584: Computer Networks

8. EEC-484/584: Computer Networks
Network Service Model
Q: What service model for “channel” transporting datagrams from sender to receiver?
Example services for a flow of datagrams:
In-order datagram delivery
Guaranteed minimum bandwidth to flow
Restrictions on changes in inter-packet spacing
No guarantee whatsoever
Example services for individual datagrams:
Guaranteed delivery
Guaranteed delivery with less than 40 msec delay
Best effort
11/16/2018
EEC-484/584: Computer Networks

9. Network Layer Connection and Connection-less Service
Datagram network provides network-layer connectionless service
Virtual Circuit network provides network-layer connection-oriented service (omitted)
11/16/2018
EEC-484/584: Computer Networks

10. EEC-484/584: Computer Networks
Datagram Networks
No call setup at network layer
Routers: no state about end-to-end connections
no network-level concept of “connection”
Packets forwarded using destination host address
packets between same source-dest pair may take different paths
application
transport
network
data link
physical
application
transport
network
data link
physical
1. Send data
2. Receive data
11/16/2018
EEC-484/584: Computer Networks

11. Routing within a Datagram Subnet
Router has forwarding table telling which outgoing line to use for each possible destination router
Each datagram has full destination address
When packet arrives, router looks up outgoing line to use and transmits packet
11/16/2018
EEC-484/584: Computer Networks

12. Datagram or VC Network: Why?
Internet (datagram)
data exchange among computers
“elastic” service, no strict timing requirement
“smart” end systems (computers)
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
ATM (VC)
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
11/16/2018
EEC-484/584: Computer Networks

13. EEC-484/584: Computer Networks
What’s in a Router?
Run routing algorithms/protocol (RIP, OSPF, BGP)
Forwarding datagrams from incoming to outgoing link
11/16/2018
EEC-484/584: Computer Networks

14. EEC-484/584: Computer Networks
Input Port Functions
Physical layer:
bit-level reception
Decentralized switching:
given datagram dest., lookup output port using forwarding table in input port memory
queuing: newly arrived datagrams might be queued before processing
Data link layer:
e.g., Ethernet
11/16/2018
EEC-484/584: Computer Networks

15. EEC-484/584: Computer Networks
Output Ports
Buffering required when datagrams arrive from fabric faster than the transmission rate
Scheduling discipline chooses among queued datagrams for transmission
11/16/2018
EEC-484/584: Computer Networks

16. EEC-484/584: Computer Networks
Routing Algorithms
Routing algorithm: algorithm that finds least-cost path
Least-cost in what sense?
Number of hops, geographical distance, least queueing and transmission delay
Desirable properties
Correctness, simplicity
Robustness to faults
Stability – converge to equilibrium
11/16/2018
EEC-484/584: Computer Networks

17. Routing Algorithm Classification
Static or dynamic?
Non-adaptive (static) - Route computed in advance, off-line, downloaded to routers
Adaptive (dynamic) - Route based on measurements or estimates of current traffic and topology
11/16/2018
EEC-484/584: Computer Networks

18. Routing Algorithm Classification
Global or decentralized information?
Global:
all routers have complete topology & link cost info
“link state” algorithms
Decentralized:
router knows physically-connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms
11/16/2018
EEC-484/584: Computer Networks

19. EEC-484/584: Computer Networks
Link State Routing
Basic idea
Assumes 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, using Dijkstra’s Algorithm
Gives forwarding table for that node
11/16/2018
EEC-484/584: Computer Networks

20. EEC-484/584: Computer Networks
Dijkstra’s Algorithm
Each node labeled with distance from source node along best known path
Initially, no paths known so all nodes labeled with infinity
As algorithm proceeds, labels may change reflecting shortest path
Label may be tentative or permanent, initially, all tentative
When label represents shortest path from source to node, label becomes permanent
11/16/2018
EEC-484/584: Computer Networks

21. Compute Shortest Path from A to D
Start with node A as the initial working node
Examine each of the nodes adjacent to A, i.e., B and G, relabeling them with the distance to A
Examine all the tentatively labeled nodes in the whole graph and make the one with the smallest label permanent, i.e., B. B is the new working node
Steps computing the shortest path from A to D, using Dijkstra’s algorithm. The arrows indicate the working node.
We start out by marking node A as permanent, indicated by a filled-in circle. Then we examine, in turn, each of the nodes adjacent to A (the working node), relabeling each one with the distance to A. Whenever a node is relabeled, we also label it with the node from which the probe was made so that we can reconstruct the final path later. Having examined each of the nodes adjacent to A, we examine all the tentatively labeled nodes in the whole graph and make the one with the smallest label permanent. This one becomes the new working node.
Last two steps: C(9,B) permanent; D(10,H) permanent;
11/16/2018
EEC-484/584: Computer Networks

22. Compute Shortest Path from A to D
Stopped here step3
11/16/2018
EEC-484/584: Computer Networks

23. EEC-484/584: Computer Networks
Step
Permanently labeled B G E C F H D 1 A
2,A
6,A ∞ 2 AB
4,B
9,B 3
ABE
5,E
6,E 4
ABEG
9,G 5
ABEGF
8,F 6
ABEGFH
10,H 7
ABEGFHC 8
ABEGFHCD
11/16/2018
EEC-484/584: Computer Networks

24. EEC-484/584: Computer Networks
Computation Results A B C D E F G H
Destination
Routing Table in A
link B C D E F G H
(A,B)
11/16/2018
EEC-484/584: Computer Networks

25. Dijkstra’s Algorithm: Exercise
Given the subnet shown below, using the Dijkstra’s Algorithm, determine the shortest path tree from node u and its routing table u y x w v z 2 1 3 5
11/16/2018
EEC-484/584: Computer Networks

26. Distance Vector Routing
Also called Bellman-Ford or Ford-Fulkerson
Each router maintains a table, giving best known distance to each destination and which line to use to get there
Table is updated by exchanging info with neighbors
Table contains one entry for each router in network with
Preferred outgoing line to that destination
Estimate of time or distance to that destination
Once every T msec, router sends to each neighbor a list of estimated delays to each destination and receives same from those neighbors
Used in ARPANET
11/16/2018
EEC-484/584: Computer Networks

27. Distance Vector Routing: How each entry is updated
d(A,X)
d(A,Y) A X Z
d(Y,Z)
d(X,Z)
At router A, for Z
Compute d(A,X) + d(X,Z) and
d(A,Y) + d(Y,Z), take minimum Y
d(A,Z) = min {d(A,v) + d(v,Z) }
where min is taken over all neighbors v of A
11/16/2018
EEC-484/584: Computer Networks

28. EEC-484/584: Computer Networks
d(x,z) = min{d(x,y) d(y,z), d(x,z) + d(z,z)}
= min{2+1 , 7+0} = 3
d(x,y) = min{d(x,y) + d(y,y), d(x,z) + d(z,y)} = min{2+0 , 7+1} = 2
node x table
x y z x y z ∞
from
cost to
cost to
x y z x 2 3
from y z
node y table
cost to x z 1 2 7 y
x y z x ∞ ∞ ∞ y
from z ∞ ∞ ∞
Each node keeps track of the following info:
Its own distance vector: least-cost to each of other routers
Each of its neighbor’s distance vector received most recently
If there is a change in distance vector, a node sends the update to all its neighbors
node z table
cost to
x y z x
∞ ∞ ∞
from y ∞ ∞ ∞ z 7 1
time
11/16/2018
EEC-484/584: Computer Networks

29. EEC-484/584: Computer Networks
d(x,z) = min{d(x,y) d(y,z), d(x,z) + d(z,z)}
= min{2+1 , 7+0} = 3
d(x,y) = min{d(x,y) + d(y,y), d(x,z) + d(z,y)} = min{2+0 , 7+1} = 2
node x table
x y z x y z ∞
from
cost to
cost to
cost to
x y z
x y z x x
from y
from y z z
node y table
cost to
cost to
cost to x z 1 2 7 y
x y z
x y z
x y z x ∞ ∞ x ∞ x y
from y
from
from y z z ∞ ∞ ∞ z
node z table
cost to
cost to
cost to
x y z
x y z
x y z x x x
∞ ∞ ∞
from y
from y
from y ∞ ∞ ∞ z z z 7 1
time
11/16/2018
EEC-484/584: Computer Networks

30. Distance Vector Routing
Distance from A to B 12ms, to C 25ms, to D 40ms, to G 18ms
Distance from J to A 8ms, to I 10ms, to H 12ms, to K 6ms
Distance from J to A to G 8+18 = 26ms to I to G = 41ms to H to G 12+6=18ms to K to G 6+31=37ms
11/16/2018
EEC-484/584: Computer Networks

31. Distance Vector Routing
Good news travels fast
Bad news travels slow
Count to infinity problem: Takes too long to converge upon router failure ×
Routers’ knowledge about the cost to A
11/16/2018
EEC-484/584: Computer Networks

32. EEC-484/584: Computer Networks
Hierarchical Routing
Problem: Bigger network => bigger routing table
As network size increases, more router memory used to store routing table, more time to process routing tables, more bandwidth to transmit states reports
Use hierarchical structure to solve the problem
Regions: router knows details of how to route packets within its region, does not know internals of other regions
Clusters of regions, zones of clusters, groups of zones
Tradeoff: savings in memory space may result in longer path
11/16/2018
EEC-484/584: Computer Networks

33. Distance Vector Routing: Exercise
Consider the subnet shown below. Distance vector routing is used, and the following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E, are 6, 3, and 5, respectively. What is C's new routing table? Give both the outgoing line to use and the expected delay.
Postpone the discussion on the solution to link state to discussion session?
11/16/2018
EEC-484/584: Computer Networks