1. Chapter 4 Network Layer
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)
If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved
Network Layer

2. Network layer transport segment from sending to receiving host
application
transport
network
data link
physical
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
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. 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

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. 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

10. IP addressing: introduction
Q: how are interfaces actually connected?
A: we’ll learn about that in chapter 5, 6.
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

11. 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

12. Subnets
/24
/24
/24
subnet
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

13. Subnets
how many?
Network Layer

14. 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

15. Network Layer
4-15
Network Layer

16. Binary Numbers 00000000 in binary = 0 in decimal…
in binary = 254 in decimal
in binary = 255 in decimal
Network Layer

17. IP Address Written in dotted-decimal format for humans to read
Binary =
Dotted decimal format =
Two parts specified by subnet mask
Network address
Host address
Example
IP address with subnet mask
Network portion =
Host portion = 26
Network Layer

18. Special IP Addresses Host bits are all zero describes network
/24 refers to the network
Host bits all ones refers to directed broadcast
/24
Broadcast
Multicast range
Private IP ranges (not supposed to be routed) –
Loopback – 127.X.Y.Z like (also called home)
Network Layer

19. How many valid host IP addresses can I use?
/24
Why?
reserved to specify network
reserved for directed broadcast
Network Layer

20. And now you understand this!

21. 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

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

23. 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

24. IPv6: motivation
initial motivation: 32-bit address space soon to be completely allocated.
In February 2011, the Internet Assigned Numbers Authority (IANA) allocated final free block of IPv4 addresses. They have recovered some unused ones since then.
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

25. 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

26. Graph abstraction: costsu y x w v z 2 1 3 5
c(x,x’) = cost of link (x,x’)
e.g., c(w,z) = 5
cost could always be 1, or
inversely related to bandwidth,
or inversely related to
congestion
cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
key question: what is the least-cost path between u and z ?
routing algorithm: algorithm that finds that least cost path
Network Layer

27. Host-to-host communication
Is the network layer for host-to-host communication or process-to-process communication?
Host-to-host communication
The transport layer is for process-to-process communication
Network Layer

28. True or False. Routers must process the IP address of every datagram.
Network Layer

29. True or False. Routers maintain state about end-to-end connections.
Routers look at the IP address of a datagram, determine the next link to send it to, send it, and forget about it
Network Layer

30. How many bytes are in an IPv4 IP address?
4 bytes (32 bits)
Network Layer

31. True or False. It is assumed that device interfaces with the same subnet part of the IP address can physically reach each other without intervening router.
True
If a device is not configured with the correct subnet part, it may not be able to communicate correctly.
Network Layer

32. What are the two ways that a PC can obtain an IP address?
Automatically from a DHCP server
Configure the PC to find a DHCP server and ask for an address
An administrator can configure the PC with an IP address
Network Layer