1. NAT traversal problem
client want to connect to server with address
server address local to LAN (client can’t use it as destination addr)
only one externally visible NATted address:
solution 1: statically configure NAT to forward incoming connection requests at given port to server
e.g., ( , port 2500) always forwarded to port 25000
Client ?
NAT
router
Network Layer

2. Solution #1 Example: Cisco wireless router
Network Layer

3. NAT traversal problem
solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:
learn public IP address ( )
Drill a “hole” in NAT
Add a port mappings on NAT
Require both host and NAT to be UPnP compatible
automate static NAT port map configuration
IGD
NAT
router
Network Layer

4. NAT traversal problem solution 3: relaying (used in Skype)
NATed server establishes connection to relay
External client connects to relay
relay bridges packets between to connections
2. connection to
relay initiated
by client
1. connection to
relay initiated
by NATted host
3. relaying
established
Client
NAT
router
Network Layer

5. IP Fragmentation and ReassemblyID =x
offset =0
fragflag
length
=4000 =1
=1500
=185
=370
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
1480 bytes in data field
offset =
1480/8
Network Layer

6. DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from network server when it joins network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected an “on”
Support for mobile users who want to join network (more shortly)
DHCP overview:
host broadcasts “DHCP discover” msg
DHCP server responds with “DHCP offer” msg
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
Network Layer

7. DHCP client-server scenario
server B
arriving DHCP
client needs
address in this
network E
Network Layer

8. DHCP client-server scenario
DHCP server:
arriving
client
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
Lifetime: 3600 secs
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
time
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
Network Layer

9. 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

10. ICMP: Internet Control Message Protocol
used by hosts & routers to communicate network-level information
error reporting: unreachable host, network, port, protocol
echo request/reply (used by ping)
network-layer “above” IP:
ICMP msgs carried in IP datagrams
Not built on TCP!
ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type Code description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
Network Layer

11. Traceroute and ICMP Source sends series of UDP segments to dest
First has TTL =1
Second has TTL=2, etc.
Unlikely port number
When nth datagram arrives to nth router:
Router discards datagram
And sends to source an ICMP message (type 11, code 0)
Message includes name of router& IP address
When ICMP message arrives, source calculates RTT
Traceroute does this 3 times
Stopping criterion
UDP segment eventually arrives at destination host
Destination returns ICMP “host unreachable” packet (type 3, code 3)
When source gets this ICMP, stops.
Ethereal example
Network Layer

12. 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

13. IPv6
Initial motivation: 32-bit address space soon to be completely allocated.
Additional motivation:
header format helps speed processing/forwarding
header changes to facilitate QoS
Checksum: removed entirely to reduce processing time at each hop
IPv6 datagram format:
fixed-length 40 byte header
no fragmentation allowed
Very slow take off
IPv4 still has space (CIDR, DHCP, NAT)
Too trouble to upgrade
Network Layer

14. IPv6 Header (Cont) Priority: identify priority among datagrams in flow
Flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
Next header: identify upper layer protocol for data
Network Layer

15. Transition From IPv4 To IPv6
Not all routers can be upgraded simultaneous
no “flag days”
How will the network operate with mixed IPv4 and IPv6 routers?
Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers
Network Layer

16. Tunneling A B E F Logical view: Physical view: A B E F tunnel IPv6
Network Layer

17. Tunneling B E A B E F Logical view: A C D F Physical view: Src:B
IPv6
IPv6
IPv6
IPv6 A B C D E F
Physical view:
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
Flow: X
Src: A
Dest: F
data
Flow: X
Src: A
Dest: F
data
Src:B
Dest: E
Flow: X
Src: A
Dest: F
data
Src:B
Dest: E
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E-to-F:
IPv6
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Network Layer

18. IPv6 Tunneling vs. IPv4 NAT
Similarity:
Both have one IPv4 public IP address, and many computers behind it
Conduct translation/capculing on that special edge router
Difference:
Hosts behind NAT are un-addressable
Cannot communicate with each other
Hosts behind Tunneling are addressable
Can communicate with each other directly
Network Layer

19. 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

20. 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
Static or dynamic?
Static:
routes change slowly over time
Dynamic:
routes change more quickly
periodic update
in response to link cost changes
Network Layer

21. 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

22. 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 routing table for that node
iterative: after k iterations, know least cost path to k destinations
Idea:
at each iteration increase spanning tree by the node that has least cost path to the source 5 3 B C 2 5 A 2 1 F 3 1 2 D E 1
Network Layer

23. A Link-State Routing Algorithm
Notation:
c(i,j): link cost from node i to j. cost infinite 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, that is next v
N: set of nodes already in spanning tree (least cost path known)
Examples:
c(B,C) = 3
D(E) = 2
p(B) = A
N = { A, B, D, E } 5 3 B C 2 5 A 2 1 F 3 1 2 D E 1
Network Layer

24. Dijsktra’s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v
if v adjacent to A
then D(v) = c(A,v)
else D(v) = infinity 7
8 Loop
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) )
/* new cost to v is either old cost to v or known
shortest path cost to w plus direct link cost from w to v */
15 until all nodes in N
Network Layer

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

26. Spanning tree gives routing table
Step N
D(B),p(B)
D(C),p(C)
D(D),p(D)
D(E),p(E)
D(F),p(F)
ADEBCF
2,A ,E ,A ,D ,E
Result from Dijkstra’s algorithm B C D E F
Outgoing link
to use, cost
B,2
D,3
D,1
D,2
D,4
Routing table: 5 3 B C 5 2 A 2 1 F 3 1 2 D E 1
destination
Network Layer

27. Dijkstra’s algorithm discussion
Oscillations are possible
dynamic link cost
e.g., link cost = amount of carried traffic by link
c(i,j) != c(j,i)
Example: A A A 1 A
1+e
2+e
2+e
2+e D B D B D B D B
1+e 1
1+e 1 e 1 C
1+e e C C C 1 1 e
… recompute
routing
… recompute
… recompute
initially
Network Layer

28. 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

29. 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 min is taken over all neighbors of x
Network Layer

30. Bellman-Ford example Suppose we know that
dv(z) = 5, dx(z) = 3, dw(z) = 3 u y x w v z 2 1 3 5
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
How about du(y)?
Network Layer

31. Distance Vector Algorithm (3)
Dx(y) = estimate of least cost from x to y
Distance vector: Dx = [Dx(y): y є N ]
Node x knows cost to each neighbor v: c(x,v)
Node x maintains Dx = [Dx(y): y є N ]
Node x also maintains its neighbors’ distance vectors
For each neighbor v, x maintains Dv = [Dv(y): y є N ]
Network Layer

32. Distance vector algorithm (4)
Basic idea:
Each node periodically sends its own distance vector estimate to neighbors
When a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N
Under minor, natural conditions, the estimate Dx(y) converge the actual least cost dx(y)
Network Layer

33. 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:
wait for (change in local link, cost of msg from neighbor)
recompute estimates
if DV to any dest has changed, notify neighbors
Network Layer

34. z y x Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(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 ∞ ∞ ∞
node z table
cost to
x y z x
∞ ∞ ∞
from y ∞ ∞ ∞ z 7 1
time
Network Layer

35. z y x Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(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
Network Layer

36. Distance Vector: link cost changes
node detects local link cost change
updates routing info, recalculates distance vector
if DV changes, notify neighbors x z 1 4 50 y
At time t0, y detects the link-cost change, updates its DV,
and informs its neighbors.
“good
news
travels
fast”
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 z’s update and updates its distance table.
y’s least costs do not change and hence y does not send any
message to z.
Network Layer

37. Distance Vector: link cost changes
good news travels fast
bad news travels slow - “count to infinity” problem!
44 iterations before algorithm stabilizes: see textbook
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
Network Layer

38. 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