Today

1
Today

• Collect HW5, Assign HW6
• Chapter 4 23, 25-29, 34, 37 due next Friday
• Scheduling announcements
• Lab today and Tuesday (UDP P2P Chat)
• I am traveling Friday-Tuesday
• Mondays lecture will be presented by Stephen
  Lee-Urban (starting chapter 5)
• Vinay (TA) will help get Tuesdays lab started
  and will be available at the end for questions
• I will be back on Wednesday to finish chapter 4
• Continue with chapter 4

2
Chapter 4 quiz

• 1. What is the 32-bit binary equivalent of the IP
  address 128.180.120.4?
• 3. Suppose an application generates chunks of 40
  bytes of data every 20 ms and each chunk gets
  encapsulated in a TCP segment and then an IP
  datagram. What percentages of each datagram will
  be overhead and what percentage will be
  application data?

3
Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats 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

4
Distance Vector Algorithm (1)

• Bellman-Ford Equation (dynamic programming)
• Define
• dx(y) cost of least-cost path from x to y
• Then
• dx(y) minv c(x,v) dv(y)
• where minv is taken over all neighbors of x

5
Bellman-Ford example (2)
Clearly, dv(z) 5, dx(z) 3, dw(z) 3
B-F equation says
du(z) min c(u,v) dv(z),
c(u,x) dx(z), c(u,w)
dw(z) min 2 5,
1 3, 5 3 4
Node that achieves minimum is next hop in
shortest path ? forwarding table
6
Distance Vector Algorithm (3)

• Dx(y) estimate of least cost from x to y
• Distance vector Dx Dx(y) y ? N
• Node x knows cost to each neighbor v c(x,v)
• Node x maintains Dx Dx(y) y ? N
• Node x also maintains its neighbors distance
  vectors
• For each neighbor v, x maintains Dv Dv(y) y
  ? N

7
Distance vector algorithm (4)

• Basic idea
• Each node periodically sends its own distance
  vector estimate to neighbors
• When node a node x receives new DV estimate from
  neighbor, it updates its own DV using B-F
  equation

Dx(y) ? minvc(x,v) Dv(y) for each node y ?
N

• Under minor, natural conditions, the estimate
  Dx(y) converges to the actual least cost dx(y)

8
Distance Vector Algorithm (5)

• Iterative, asynchronous each local iteration
  caused by
• local link cost change
• DV update message from neighbor
• Distributed
• each node notifies neighbors only when its DV
  changes
• neighbors then notify their neighbors if necessary

Each node
9
Dx(z) minc(x,y) Dy(z), c(x,z)
Dz(z) min21 , 70 3
Dx(y) minc(x,y) Dy(y), c(x,z) Dz(y)
min20 , 71 2
node x table
cost to
cost to
x y z
x y z
x
0 2 3
x
0 2 3
y
from
2 0 1
y
from
2 0 1
z
7 1 0
z
3 1 0
node y table
cost to
cost to
cost to
x y z
x y z
x y z
x
8
8
x
0 2 7
x
0 2 3
8 2 0 1
y
y
from
y
2 0 1
from
from
2 0 1
z
z
8
8
8
z
7 1 0
3 1 0
node z table
cost to
cost to
cost to
x y z
x y z
x y z
x
0 2 3
x
0 2 7
x
8 8 8
y
y
2 0 1
from
from
y
2 0 1
from
8
8
8
z
z
z
3 1 0
3 1 0
7
1
0
time
10
Distance Vector link cost changes

• Link cost changes
• node detects local link cost change
• updates routing info, recalculates distance
  vector
• if DV changes, notify neighbors

At time t0, y detects the link-cost change,
updates its DV, and informs its neighbors. At
time t1, z receives the update from y and updates
its table. It computes a new least cost to x
and sends its neighbors its DV. At time t2, y
receives zs update and updates its distance
table. ys least costs do not change and hence y
does not send any message to z.
good news travels fast
11
Distance Vector link cost changes

• Link cost changes
• good news travels fast
• bad news travels slow - count to infinity
  problem!
• 44 iterations before algorithm stabilizes see
  text
• Poisoned reverse
• If Z routes through Y to get to X
• Z tells Y its (Zs) distance to X is infinite (so
  Y wont route to X via Z)
• will this completely solve count to infinity
  problem?