1. Chapter 4 Network Layer
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012
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Network Layer

2. Network 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
application
transport
network
data link
physical
Network Layer

3. Two key network-layer functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
Network Layer

4. Interplay between routing and forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001
routing algorithm determines
end-end-path through network
forwarding table determines
local forwarding at this router
Network Layer

5. 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
application
transport
network
data link
physical
application
transport
network
data link
physical
1. send datagrams
2. receive datagrams
Network Layer

6. Datagram forwarding table
4 billion IP addresses, so rather than list individual destination address
list range of addresses
(aggregate table entries)
routing algorithm
local forwarding table
dest address
output link
address-range 1
address-range 2
address-range 3
address-range 4 3 2 1
IP destination address in
arriving packet’s header 1 2 3
Network Layer

7. Datagram forwarding table
Destination Address Range
through
otherwise
Link Interface 1 2 3
Q: but what happens if ranges don’t divide up so nicely?
Network Layer

8. Longest prefix matching
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
Destination Address Range
*** *********
*********
*** *********
otherwise
Link interface 1 2 3
examples:
DA:
which interface?
DA:
which interface?
Network Layer

9. Router architecture overview
two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
forwarding tables computed,
pushed to input ports
routing
processor
routing, management
control plane (software)
forwarding data plane (hardware)
high-seed
switching
fabric
router input ports
router output ports
Network Layer

10. Input port functions decentralized switching:
lookup,
forwarding
queueing
line
termination
link
layer
protocol
(receive)
switch
fabric
physical layer:
bit-level reception
data link layer:
e.g., Ethernet
see chapter 5
decentralized switching:
given datagram dest., lookup output port using forwarding table in input port memory (“match plus action”)
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
Network Layer

11. Switching fabrics
transfer packet from input buffer to appropriate output buffer
switching rate: rate at which packets can be transfer from inputs to outputs
often measured as multiple of input/output line rate
N inputs: switching rate N times line rate desirable
three types of switching fabrics
memory
memory
bus
crossbar
Network Layer

12. Switching via memory first generation routers:
traditional computers with switching under direct control of CPU
packet copied to system’s memory
speed limited by memory bandwidth (2 bus crossings per datagram)
input
port
(e.g.,
Ethernet)
memory
output
system bus
Network Layer

13. Switching via a bus datagram from input port memory
to output port memory via a shared bus
bus contention: switching speed limited by bus bandwidth
32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Network Layer

14. Switching via interconnection network
overcome bus bandwidth limitations
banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor
advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
Cisco 12000: switches 60 Gbps through the interconnection network
crossbar
Network Layer

15. Output ports
datagram
buffer
queueing
switch
fabric
link
layer
protocol
(send)
line
termination
buffering required when datagrams arrive from fabric faster than the transmission rate
scheduling discipline chooses among queued datagrams for transmission
Network Layer

16. Output port queueing
at t, packets more
from input to output
one packet time later
switch
fabric
buffering when arrival rate via switch exceeds output line speed
queueing (delay) and loss due to output port buffer overflow!
Network Layer

17. Input port queuing
fabric slower than input ports combined -> queueing may occur at input queues
queueing delay and loss due to input buffer overflow!
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
one packet time later: green packet experiences HOL blocking
switch
fabric
switch
fabric
output port contention:
only one red datagram can be transferred. lower red packet is blocked
Network Layer

18. The Internet network layer
host, router network layer functions:
transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
routing protocols
path selection
RIP, OSPF, BGP
network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
link layer
physical layer
Network Layer

19. 32 bit destination IP address
IP datagram format
IP protocol version
number
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
header
checksum
time to
live
32 bit source IP address
head.
len
type of
service
flgs
fragment
offset
upper
layer
32 bit destination IP address
options (if any)
total datagram
length (bytes)
header length
(bytes)
“type” of data
for
fragmentation/
reassembly
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
e.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Network Layer

20. IP fragmentation, reassembly
network links have MTU (max.transfer size) - largest possible link-level frame
different link types, different MTUs
large IP datagram divided (“fragmented”) within net
one datagram becomes several datagrams
“reassembled” only at final destination
IP header bits used to identify, order related fragments …
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly …
Network Layer

21. IP fragmentation, reassemblyID =x
offset =0
fragflag
length
=4000
example:
4000 byte datagram
MTU = 1500 bytes ID =x
offset =0
fragflag =1
length
=1500
=185
=370
=1040
one large datagram becomes
several smaller datagrams
1480 bytes in data field
offset =
1480/8
Network Layer

22. IP addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link
router’s typically have multiple interfaces
host typically has one or two interfaces (e.g., wired Ethernet, wireless )
IP addresses associated with each interface =
223 1 1 1
Network Layer

23. IP address format
The 32-bit IP address is broken up into 4 octets, which are arranged into a dotted-decimal notation scheme.
An octet is a set of 8 bits & not a musical instrument.
Example of an IP version 4:
Network Layer

24. Decimal to binary conversion27 26 25 24 23 22 21 20 1
128 64 32 16 8 4 2
The binary number converts into the decimal number:
= 255
The largest decimal number that can be stored in an IP address octet is 255
Network Layer

25. IP address classes Class Leftmost bits Start address Finish address A
0xxx B
10xx C
110x D
1110 E
1111
Class D addresses are used for multicasting and class E addresses
Reserved for experimental uses
Network Layer

26. Network & Host Representation
Network and host parts
The 32 bits of the IP address are divided into Network & Host portions, with the octets assigned as a part of one or the other.
Network & Host Representation
By IP Address Class
Class
Octet1
Octet2
Octet3
Octet4
Class A
Network
Host
Class B
Class C
Network Layer

27. Network mask Class A, 255.0.0.0 Class B, 255.255.0.0
Class C,
An AND of an IP address with the network mask results into the network address
Network Layer

28. Special addresses Network address Host address Description Example 0’s
Default Cisco Route
1’s
Broadcast to local network
N/W address
Broadcast to N/W address
127
Any
Loopback testing
Network Layer

29. Private addresses Class Private start address Private last address A B C
Private addresses can be freely used on networks which are not
Connected to the Internet or they are behind Gateways or Firewalls
That use NAT(Network Address Translation)
Network Layer

30. IP addresses: how to get one?
Q: How does a host get IP address?
hard-coded by system admin in a file
Windows: control-panel->network->configuration->tcp/ip->properties
UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play”
Network Layer

31. 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/“on”)
support for mobile users who want to join network (more shortly)
DHCP overview:
host broadcasts “DHCP discover” msg [optional]
DHCP server responds with “DHCP offer” msg [optional]
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
Network Layer

32. DHCP client-server scenario
DHCP server:
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
arriving
client
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
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Network Layer

33. IP addressing: the last word...
Q: how does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes
Network Layer

34. NAT: network address translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
all datagrams leaving local
network have same single source NAT IP address: ,different source port numbers
datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)
Network Layer

35. NAT: network address translation
motivation: local network uses just one IP address as far as outside world is concerned:
range of addresses not needed from ISP: just one IP address for all devices
can change addresses of devices in local network without notifying outside world
can change ISP without changing addresses of devices in local network
devices inside local net not explicitly addressable, visible by outside world (a security plus)
Network Layer

36. NAT: network address translation
implementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)
. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
Network Layer

37. NAT: network address translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: reply arrives
dest. address:
, 5001
Network Layer

38. NAT: network address translation
16-bit port-number field:
60,000 simultaneous connections with a single LAN-side address!
NAT is controversial:
routers should only process up to layer 3
violates end-to-end argument
NAT possibility must be taken into account by app designers, e.g., P2P applications
address shortage should instead be solved by IPv6
Network Layer

39. Subnetting
Enables to divide one physical network into several logically divided sub-networks
Smaller broadcast domain
Traffic reduction
Security
Network Layer

40. A network with two levels of hierarchy (not subnetted)
Network Layer

41. A network with three levels of hierarchy (subnetted)
Network Layer

42. Addresses in a network with and without subnetting
Network Layer

43. Default mask and subnet mask
Network Layer

44. Finding the Subnet Address
Given an IP address, the subnet address can be found in the same way as for the network address
Apply the AND operation to the binary of IP address and the subnet mask
Network Layer

45. Example
What is the subnetwork address if the host address is and the subnet mask is ? 1
Hence the subnet address is
Network Layer

46. Creating the subnets Class B add. 0 subnet Subnet 1 Subnet 2 ….
Broadcast subnet
172.
16. 0. 1
Number of subnets=2Number of subnet bits-2
Subnet 0 and the broadcast subnets should not be used
Network Layer

47. Valid IP address in subnet 1
1st IP
2nd IP ….
Last IP
Bcast IP 1 .
# valid IP addresses or # hosts=2Number of host bits - 2
In dotted decimal
1st IP
2nd IP
Last IP
Broadcast IP
Network Layer

48. Example
Subnet the class C IP, , using three bits from the host part
subnet mask
Network Layer

49. Example
1st Subnet
Usable subnets
Last Subnet
Write the valid IP address of subnet 2
Network Layer

50. Solution Conversion from binary to decimal
Subnet number is
First address:
Broadcast address: (turn all the host bits into 1)
Last address: (subtract 1 from the broadcast address)
Other addresses are in the range from 1st to last
Network Layer

51. Prefix notation Subnet masks have consecutive numbers of 1s=
Can be written as /16
Means that the first 16 bits represent the network part
Example, /27 has a 27 bits prefix
Network Layer

52. Finding the subnet number
Given a subnetted IP address, /27, find the subnet number, and range of IP addresses in the subnet
Conversion into binary, AND operation
Can we do it without using binary conversion?
Network Layer

53. Finding the subnet number
Find the magic number MN=256 – value of the mask’s interesting octet
MN= =32
Find the closest multiple to MN that is not greater than the address’ interesting octet (i.e. 70)
32*2=64 put this value in the interesting octet i.e (subnet number)
The first address:
Network Layer

54. Finding the broadcast address
Use the magic number again
The broadcast address= MN+ Subnet number-1
MN+ Subnet number-1= = 95
The broadcast address=
The range of valid IP addresses: to
Network Layer