Network Layer
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
George Parisis
School of Engineering and Informatics
University of Sussex
Network Layer 4-2
Network Layer
objectives:
v understand principles behind network layer
services:
§ network layer service models
§ forwarding versus routing
§ how a router works
§ routing (path selection)
§ broadcast
v instantiation, implementation in the Internet
Network Layer 4-3
v introduction
v virtual circuit and datagram networks
v what’s inside a router
v IP: Internet Protocol
§ datagram format
§ IPv4 addressing
§ ICMP, IPv6
v routing algorithms
§ link state, distance vector
§ hierarchical routing
v routing in the Internet
§ RIP, OSPF
§ BGP
v broadcast routing
Outline
Network Layer 4-4
Network layer
v transport segment from
sending to receiving host
v on sending side
encapsulates segments into
datagrams
v on receiving side, delivers
segments to transport layer
v network layer protocols in
every host, router
v router examines header
fields in all IP datagrams
passing through it
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link
physical network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical network
data link
physical
Network Layer 4-5
Two key network-layer functions
v forwarding: move packets from router’s input to
appropriate router output
v routing: determine route taken by packets from
source to destination
§ routing algorithms
Network Layer 4-6
1
2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
Interplay between routing and forwarding
routing algorithm determines
end-end-path through network
forwarding table determines
local forwarding at this router
Network Layer 4-7
Network service model
Q: What service model for “channel” transporting
datagrams from sender to receiver?
example services for
individual datagrams:
v guaranteed delivery
v guaranteed delivery
with less than 40 msec
delay
example services for a
flow of datagrams:
v in-order datagram
delivery
v guaranteed minimum
bandwidth to flow
v restrictions on changes
in inter-packet spacing
v security
Network Layer 4-8
Network layer service models:
Network
Architecture
Internet
ATM
ATM
ATM
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
rate
guaranteed
minimum
none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion
feedback
no (inferred
via loss)
no
congestion
no
congestion
yes
no
Guarantees ?
Network Layer 4-9
v introduction
v virtual circuit and datagram networks
v what’s inside a router
v IP: Internet Protocol
§ datagram format
§ IPv4 addressing
§ ICMP, IPv6
v routing algorithms
§ link state, distance vector
§ hierarchical routing
v routing in the Internet
§ RIP, OSPF
§ BGP
v broadcast routing
Outline
Network Layer 4-10
Connection, connection-less service
v datagram network provides network-layer
connectionless service
v virtual-circuit network provides network-layer
connection service
v analogous to TCP/UDP connecton-oriented /
connectionless transport-layer services, but:
§ service: host-to-host
§ no choice: network provides one or the
other
§ implementation: in network core
Network Layer 4-11
Virtual circuits
v call setup, teardown for each call before data can flow
v each packet carries VC identifier (not destination host
address)
v every router on source-dest path maintains “state” for
each passing connection
v link, router resources (bandwidth, buffers) may be
allocated to VC (dedicated resources = predictable
service)
“source-to-destination path behaves much like
telephone circuit”
§ performance-wise
§ network actions along source-to-destination path
Network Layer 4-12
VC implementation
a VC consists of:
1. path from source to destination
2. VC numbers, one number for each link along path
3. entries in forwarding tables in routers along path
v packet belonging to VC carries VC number
v VC number can be changed on each link
§ new VC number comes from forwarding table
Network Layer 4-13
VC forwarding table
12 22 32
1 2
3
VC number
interface
number
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 22
2 63 1 18
3 7 2 17
1 97 3 87
… … … …
forwarding table in
northwest router:
VC routers maintain connection state information!
Network Layer 4-14
application
transport
network
data link
physical
Virtual circuits: signaling protocols
v used to setup, maintain teardown VC
v used in ATM, frame-relay, X.25
v not used in today’s Internet
1. initiate call 2. incoming call
3. accept call 4. call connected
5. data flow begins 6. receive data
application
transport
network
data link
physical
Network Layer 4-15
Datagram networks
v no VC setup at network layer
v routers: no state about end-to-end connections
§ no network-level concept of “connection”
v packets forwarded using destination host
address
1. send datagrams
application
transport
network
data link
physical
application
transport
network
data link
physical
2. receive datagrams
Network Layer 4-16
1
2 3
Datagram forwarding table
IP destination address in
arriving packet’s header
routing algorithm
local forwarding table
dest address output link
address-range 1
address-range 2
address-range 3
address-range 4
3
2
2
1
4 billion IP addresses, so
rather than list individual
destination address
list range of addresses
(aggregate table entries)
Network Layer 4-17
Destination Address Range
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
otherwise
Link Interface
0
1
2
3
Datagram forwarding table
Network Layer 4-18
Longest prefix matching
Destination Address Range
11001000 00010111 00010*** *********
11001000 00010111 00011000 *********
11001000 00010111 00011*** *********
otherwise
DA: 11001000 00010111 00011000 10101010
examples:
DA: 11001000 00010111 00010110 10100001 which interface?
which interface?
when looking for forwarding table entry for given
destination address, use longest address prefix
that matches destination address.
longest prefix matching
Link interface
0
1
2
3
Network Layer 4-19
v virtual circuit and datagram networks
v network layer service models
Summary