1. Chapter 4 Network Layer
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley
Chapter4_2
Transport Layer

2. Two key network-layer functions
Forwarding:
Move packets on the router from input port to appropriate output port
All ports have input/output capability
Essentially, a network within a router!
Routing:
Determine route taken by packets from source to destination
Routing algorithms
Link State
Distance Vector
Network Layer

3. Network service model Example Services: Guaranteed delivery
Guaranteed delivery with bounded delay
In-order packet delivery
Guaranteed minimum bandwidth
Guaranteed maximum jitter
Security services
None of these provided by Network layer
Some provided by Transport layer
Network layer: “Best-effort service”
Network Layer

4. 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
Routing algorithms used to update forwarding tables
application
transport
network
data link
physical
application
transport
network
data link
physical
1. send datagrams
2. receive datagrams
Network Layer

5. Two key network-layer functions
Forwarding:
Move packets on the router from input port to appropriate output port
All ports have input/output capability
Essentially, a network within a router!
Routing:
Determine route taken by packets from source to destination
Routing algorithms
Link State
Distance Vector
Network Layer

6. 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

7. 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

8. 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

9. Chapter 4: outline 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. 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

11. IP addressing: introduction
Q: how are interfaces actually connected?
A: wired Ethernet interfaces connected by Ethernet switches
A: wireless WiFi interfaces connected by WiFi base station
For now: don’t need to worry about how one interface is connected to another (with no intervening router)
Network Layer

12. Subnets IP address: What’s a subnet ? subnet part - high order bits
host part - low order bits
What’s a subnet ?
Logical, visible subdivision of a larger IP network
Hosts can physically reach each other without intervening router
How?
subnet
network consisting of 3 subnets
Network Layer

13. Subnets
/24
/24
/24
subnet
To determine the subnets, detach each interface from its host or router, creates islands of isolated networks
Each isolated network is called a subnet
subnet mask: /24
Network Layer

14. Subnets How many subnets? 223.1.1.2 223.1.1.1 223.1.1.4 223.1.1.3
Network Layer

15. IP addressing IP address (IPv4): 4 octets, range: 28 or 256 values
32 bits
232 = ~4 billion possible IP addresses
Enough addresses?
Written in dotted-decimal notation
4 bytes, separated by dot
Ex:
4 octets, range: 28 or 256 values
0 – 255 –
Interfaces on every host and router on the Internet must have a globally unique IP address.
Except interfaces behind NAT routers
Network Layer

16. IP addressing: CIDR CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in subnet portion of address
subnet
part
host
part
/23
Network Layer

17. IP addressing: CIDR
In the example below, the first 23 bits are common for all devices in the subnet
How many unique interfaces (router interfaces and hosts)?
32 bits – 23 bits: 9 bits
29 = 512 unique interfaces
IP range: – –
subnet
part
host
part
/23
Network Layer

18. IP addressing: Subnet Masking
Shares IP address format
Is not an IP address
Used together with interface IP address to determine subnet
Ex:
Subnet: a.b.c.d/24
Subnet mask:
Perform bit-wise logical AND:
Interface/host belongs to subnet /24
Network Layer

19. 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

20. DHCP: Dynamic Host Configuration Protocol
Goal:
Allow hosts to dynamically obtain an IP address from network server when it joins network
Host can renew address in use
Allows reuse of addresses in pool
Ex: DHCP IP range, –
Support for mobile users who want to join network
DHCP overview:
Host broadcasts “DHCP discover”
DHCP server responds with “DHCP offer”
Host requests IP address: “DHCP request”
DHCP server sends address: “DHCP ACK”
Network Layer

21. DHCP client-server scenario
/24
arriving DHCP
client needs
address in this
network
/24
/24
Network Layer

22. DHCP client-server scenario
DHCP server:
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
arriving
client
Broadcast: is there a DHCP server out there?
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
lifetime: 3600 secs
Broadcast: I’m a DHCP server! Here’s an IP address you can use
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Broadcast: OK. I’ll take that IP address!
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Broadcast: OK. You’ve got that IP address!
Network Layer

23. DHCP: more than IP addresses
DHCP can return more than just allocated IP address on subnet:
Address of first-hop router for client
Name and IP address of DNS sever
Subnet mask
Network Layer

24. IP addresses: how to get one?
Q: how does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Network Layer

25. IP addresses: how to get one?
Q: how does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
How many unique IP addresses?
32bits – 20bits: 12, so 212 unique addresses for ISP
~4096 IP addresses
Network Layer

26. IP addresses: how to get one?
Q: how does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
Organization /23
How many unique IP addresses?
32bits – 23bits: 9, so 29 unique addresses for Organization 0
~512 IP addresses
Network Layer

27. Hierarchical addressing: route aggregation
hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
“Send me anything
with addresses
beginning
/20”
/23
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us
Network Layer

28. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
Network Layer

29. 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 root servers
Assigns domain names, resolves disputes
Network Layer

30. 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

31. 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

32. NAT: network address translation
Implementation:
Outgoing data:
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 address
NAT translation table:
Every (source IP address, port #) translated to (NAT IP address, new port #)
Incoming datagrams:
replace (NAT IP address, new port #) of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
Network Layer

33. 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

34. 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

35. NAT traversal problem
External client wants to connect to internal server with address
Server address local to LAN
External client can’t use it as destination address
Only one externally visible NAT address:
Solution1:
Statically configure NAT to forward incoming connection requests at given port to internal server
Ex: ( , port 2500)
always forwarded to port 25000
client ?
NAT
router
Network Layer

36. NAT traversal problem Solution 2:
Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol.
Allows NATed host to:
learn public IP address ( )
add/remove port mappings (with lease times)
Automate static NAT port map configuration
NAT
router
IGD
Network Layer

37. NAT: things to consider
Generally bundled with Ethernet and Wi-Fi interfaces
Ethernet interface:
100 Mbps or 1Gbps
How this affects LAN throughput
Using internal switches
Wi-Fi interface:
802.11b/g/n/a/ac
Throughput
MIMO
Demilitarized Zone (DMZ):
Less restrictive zone/host/subnet which allows hosts outside the private network to communicate easily
Accomplished by modifying iptable rules on NAT router
Network Layer

38. Chapter 4: outline 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

39. 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
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

40. Traceroute and ICMP source sends series of UDP segments to dest
first set has TTL =1
second set has TTL=2, etc.
unlikely port number
when nth set of datagrams arrives to nth router:
router discards datagrams
and sends source ICMP messages (type 11, code 0)
ICMP messages includes name of router & IP address
when ICMP messages arrives, source records RTTs
stopping criteria:
UDP segment eventually arrives at destination host
destination returns ICMP “port unreachable” message (type 3, code 3)
source stops
3 probes
3 probes
3 probes
Network Layer

41. IPv6: motivation Initial motivation: Additional motivation:
32-bit address space soon to be completely allocated.
Additional motivation:
Header format helps speed processing/forwarding
Header changes to facilitate QoS
IPv6 datagram format:
Fixed-length 40 byte header
No fragmentation allowed
Network Layer

42. IPv6 datagram format
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
ver
pri
flow label
payload len
next hdr
hop limit
source address
(128 bits)
destination address
(128 bits)
data
32 bits
Network Layer

43. Other changes from IPv4
checksum: removed entirely to reduce processing time at each hop
Not needed since checksumming already occurs at Layer 4 and Layer 2
ICMPv6: new version of ICMP
Additional message types, e.g. “Packet Too Big”
Multicast group management functions
Network Layer

44. Transition from IPv4 to IPv6
Not all routers can be upgraded simultaneously
No “flag days”
How will network operate with mixed IPv4 and IPv6 routers?
Tunneling:
IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers
IPv4 header fields
UDP/TCP payload
IPv6 source dest addr
IPv6 header fields
IPv4 payload
IPv4 source, dest addr
IPv6 datagram
IPv4 datagram
Network Layer

45. connecting IPv6 routers
Tunneling
logical view:
IPv4 tunnel
connecting IPv6 routers E
IPv6 F A B A B
IPv6 C D E
IPv6 F
physical view:
IPv4
IPv4
Network Layer

46. connecting IPv6 routers
Tunneling
logical view:
IPv4 tunnel
connecting IPv6 routers E
IPv6 F A B A B
IPv6 C D E
IPv6 F
physical view:
IPv4
IPv4
flow: X
src: A
dest: F
data
A-to-B:
IPv6
Flow: X
Src: A
Dest: F
data
src:B
dest: E
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
src:B
dest: E
E-to-F:
IPv6
flow: X
src: A
dest: F
data
Network Layer

47. IPv6: adoption US National Institutes of Standards estimate [2013]:
~3% of industry IP routers
~11% of US gov’t routers
Long (long!) time for deployment, use
20 years and counting!
Think of changes in last 20 years: WWW, Facebook, Netflix, Cloud computing, Internet of things, …
Why?
Network layer protocols extremely difficult to change
Fundamental protocols for data exchange over the Internet
Network Layer