1. CS4470 Computer Networking Protocols
9/11/2018
CS Computer Networking Protocols
7. Network layer
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Network layer overview Forwarding and routing
Network service model
Virtual-circuits networks and datagram networks
7. Network layer CS470

3. Network layer overview
9/11/2018
Network layer overview
Transport layer
Transport layer
Data
Data
Data
Network layer
Network layer H
Data H
datagram H H
Data
Link layer
Link layer H H
Data
frame
7. Network layer CS470

4. Network layer overview (cont.)
Achieve communication between two hosts
The two hosts may not be physically connected
Make use of services provided by the link layer
Get a datagram from link layer
Provide services to the transport layer
Network layer of the Internet--IP
Connectionless, Best effort services
7. Network layer CS470

5. Network layer transport a packet from sending to receiving host
9/11/2018
Network layer
application
transport
network
data link
physical
transport a packet from sending to receiving host
on sending side encapsulates packets into datagrams
on receiving side, delivers packets to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
network
data link
physical
application
transport
network
data link
physical
7. Network layer CS470

6. Forwarding and routing
When a datagram arrives at a router’s input link, the router must move the datagram to the appropriate output link
Router-local action of transferring a datagram from an input link interface to the appropriate output link interface
Routing
Network-wide process determines the end-to-end paths that datagrams take from source to destination
Routing algorithms
7. Network layer CS470

7. Forwarding and routing
Analogy:
Routing
process of planning trip from source to dest
Forwarding
process of getting through single interchange
Forwarding table
Every router has a forwarding table
Router does the forwarding based on the table
Routing algorithm decides the content of the table
7. Network layer CS470

8. 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
7. Network layer CS470

9. Connection setup 3rd important function in some network architectures:
ATM, frame relay, X.25
before datagrams flow, two end hosts and intervening routers establish virtual connection
routers get involved
network vs transport layer connection service:
network: between two hosts (may also involve intervening routers in case of VCs)
transport: between two processes
7. Network layer CS470

10. Network service model
Q: What service model for “channel” transporting datagrams from sender to receiver?
example services for a flow of datagrams:
in-order datagram delivery
guaranteed minimum bandwidth to flow
restrictions on changes in inter-packet spacing
example services for individual datagrams:
guaranteed delivery
guaranteed delivery with less than 40 msec delay
7. Network layer CS470

11. Network layer service models:
Guarantees ?
Network
Architecture
Internet
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Congestion
feedback
no (inferred
via loss) no
congestion
yes
Bandwidth
none
constant
rate
guaranteed
minimum
Loss no
yes
Order no
yes
Timing no
yes
7. Network layer CS470

12. IP is Best Effort Delivery
9/11/2018
IP is Best Effort Delivery
Does not guarantee to prevent
Duplicate datagrams
Delayed or out-of-order delivery
Corruption of data
Datagram loss
Reliable delivery provided by transport layer
Network layer - IP - can detect and report errors without actually fixing them
7. Network layer CS470

13. Datagram networks vs Virtual-circuit networks
datagram network provides network-layer connectionless service
virtual-circuit network provides network-layer connection service
7. Network layer CS470

14. 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)
7. Network layer CS470

15. 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
7. Network layer CS470

16. 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!
7. Network layer CS470

17. 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
7. Network layer CS470

18. 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
7. Network layer CS470

19. 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
7. Network layer CS470

20. Datagram forwarding table
Destination Address Range
through
otherwise
Link Interface 1 2 3
7. Network layer CS470

21. 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?
7. Network layer CS470

22. 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”
ATM (VC)
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
7. Network layer CS470