1. Chapter 4: Network Layer
Chapter goals:
Understand principles behind network layer services:
Routing (path selection)
dealing with scale
how a router works
advanced topics: IPv6, mobility
instantiation and implementation in the Internet
Network Layer

2. 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

3. Network layer functions
transport packet from sending to receiving hosts
network layer protocols in every host, router
three important functions:
path determination: route taken by packets from source to dest. Routing algorithms
forwarding: move packets from router’s input to appropriate router output
call setup: some network architectures require router call setup along path before data flows
application
transport
network
data link
physical
network
data link
physical
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. Network and transport layer service:
Network: between two hosts
Transport: between two processes
Network Layer

6. Network service model
Q: What service model for “channel” transporting packets from sender to receiver?
guaranteed bandwidth?
preservation of inter-packet timing (no jitter)?
loss-free delivery?
in-order delivery?
congestion feedback to sender?
Network Layer

7. 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

8. Connection, connection-less service
datagram network provides network-layer connectionless service
Internet uses this
virtual-circuit network provides network-layer connection service
ATM uses this
Network Layer

9. Virtual circuits
“source-to-dest path behaves much like telephone circuit”
performance-wise
network actions along source-to-dest path
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination host address)
every router on source-dest path maintains “state” for each passing connection
link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)
Network Layer

10. VC implementation a VC consists of:
path from source to destination
VC numbers, one number for each link along path
entries in forwarding tables in routers along path
packet belonging to VC carries VC number (rather than dest address)
VC number can be changed on each link.
new VC number comes from forwarding table
Network Layer

11. VC forwarding table VC routers maintain connection state information!22 12 32 1 3 2
VC number
interface
number
forwarding table in
northwest router:
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
… … … …
VC routers maintain connection state information!
Network Layer

12. Virtual circuits: signaling protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.25
not used in today’s Internet
application
transport
network
data link
physical
application
transport
network
data link
physical
5. data flow begins
6. receive data
4. call connected
3. accept call
1. initiate call
2. incoming call
Network Layer

13. 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

14. 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

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

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

17. Datagram or VC network: why?
Internet (datagram)
data exchange among computers
“elastic” service, no strict timing req.
many link types
different characteristics
uniform service difficult
“smart” end systems (computers)
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
Scalable
ATM (VC)
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
Network Layer

18. 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

19. Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
Network Layer

20. Input Port Functions Decentralized switching: Physical layer:
bit-level reception
Decentralized switching:
given datagram dest., lookup output port using forwarding table in input port memory
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
Data link layer:
e.g., Ethernet
see chapter 5
Network Layer

21. Three types of switching fabrics
Network Layer

22. 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
Output
Memory
System Bus
Network Layer

23. 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
Only one packet can be on bus at any time
1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone)
Network Layer

24. Switching Via An Interconnection Network
overcome bus bandwidth limitations
Can pass multiple packets at the same time
Cisco 12000: switches Gbps through the interconnection network
Network Layer

25. Output Ports
Buffering required when datagrams arrive from fabric faster than the transmission rate
Scheduling discipline chooses among queued datagrams for transmission
Network Layer

26. Output port queueing
buffering when arrival rate via switch exceeds output line speed
queueing (delay) and loss due to output port buffer overflow!
Network Layer

27. Input Port Queuing
Fabric slower than input ports combined -> queueing may occur at input queues
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
queueing delay and loss due to input buffer overflow!
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. 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

30. 32 bit destination IP address
IP datagram format
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
IP protocol version
Number (4)
header length (4)
(words)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
head.
len
type of
service
“type” of data (not used)
flgs
fragment
offset
upper
layer
32 bit destination IP address
Options (if any)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead with TCP?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Network Layer

31. 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
Now most packet has less than 1,500 bytes
due to Ethernet
Very rare fragmentation in practice
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
Network Layer

32. 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

33. 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 interface
IP addresses associated with each interface =
223 1 1 1
Network Layer

34. Subnets IP address: What’s a subnet ? subnet part (high order bits)
host part (low order bits)
What’s a subnet ?
device interfaces with same subnet part of IP address
can physically reach each other without intervening router
subnet
network consisting of 3 subnets
Network Layer

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

36. Subnets
How many?
Network Layer

37. 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
/23
Network Layer

38. IP Subnet For a “a.b.c.d/n” subnet It has 232-n IP addresses
The first IP address in this subnet is: a.b.c.d
Its last n bits must be 0
This address is usually reserved, not used for any computer
The last address in the block can be found by setting the rightmost 32 − n bits to 1s
This address is used as broadcast address
Network Layer

39. The block representation is 205.16.37.32/28
Subnet Example
A /28 block of addresses is granted to a small organization. We know that one of the addresses is What is the first address in the block? What is its x.y.z.t/n representation?
Solution
The binary representation of the given address is
If we set 32−28 rightmost bits to 0, we get or
The block representation is /28
Network Layer

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

41. 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

42. Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
/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”
ISPs-R-Us
Network Layer

43. 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
Remember router’s longest matching principle
Network Layer

44. 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
ICANN publishes /8 address allocation
You can use online “IP address locator” to find out where a packet comes from
Network Layer

45. 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

46. NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside world is concerned:
no need to be allocated range of addresses from ISP: - just one IP address is used for all devices
devices inside local net not explicitly addressable, visible by outside world (a security plus)
Cannot be scanned or infected by worm or attackers outside
Internet
Network Layer

47. 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

48. 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

49. NAT: Network Address Translation
16-bit port-number field:
60,000 simultaneous connections with a single LAN-side address!
NAT is controversial:
violates end-to-end argument
Internal computers not visible to outside
Outside hosts have trouble to request service from local computers, e.g., P2P, video conference, web hosting.
address shortage should instead be solved by IPv6
Network Layer