1. Wide Area Networks and Internet CT1403
Lecture3: Internet Network Layer

2. Outline Datagram format, fragmentation and assembly
IP addressing: Classes, Subnets
CIDR: Classless Interdomain Routing
DHCP: Dynamic Host Configuration Protocol
CT1403

3. datagram format
Fragmentation
Reassembly

4. 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

5. 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

6. IP fragmentation, reassembly
network links have MTU (maximum transfer unit) - 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 …
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly …

7. IP fragmentation, reassemblyID =x
offset =0
fragflag
length
=4000
example:
4000 byte datagram
MTU = 1500 bytes ID =x
offset =0
fragflag =1
length
=1500
=185
=370
=1040
one large datagram becomes
several smaller datagrams
1480 bytes in data field
offset =
1480/8
offset 0: meaning the data should be inserted at the beginning go byte 0
offset 185: meaning the data should be inserted at the beginning go byte 1480 (185*8=1480)
offset 370: meaning the data should be inserted at the beginning go byte 2960 (370*8=2960)
Network Layer
4-36

8. IP Addressing
Classes
Subnets

9. IP Addressing IP address: 32-bit identifier for host, router interface
Interface: connection between host/router and physical link
router’s typically have multiple interfaces (==>multiple IP addresses)
host typically has one interface
IP addresses associated with each interface = 1 1 1
223
Network Layer

10. Classes of IP Addresses

11. Classes of IP Addresses44
CT1403

12. Classes of IP Addresses.

13. Classless Interdomain Routing
CIDR
Classless Interdomain Routing

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

15. Classless Interdomain Routing : CIDR
Generalizes the notation of subnet addressing
As within the subnet addressing, the 32-bit address is divided into two parts (net#, host # ) and has the dotted decimal form (a.b.c.d/x), where x indicates the number of bits in the first part of the address.
CIDR replaces Classful addressing, where the network portion of IP addresses were constrained to 8, 16, or 24
Classful addressing waists IP addresses

16. Classless Interdomain Routing : CIDR
Most organizations needs more than class C but less than class B
CIDR allocates IP address blocks of variable size without regard to classes
Example: site needs 2000 addresses
assign a block of 2048 addresses
With CIDR address lookup is more complicated

17. reference: This slide was created by Dr. Mohammad Arafah
CIDR Example
مثال 6: قناع الشبكة الفرعية (Subnet Mask) هو (/30).
قناع الشبكة الفرعية
ثابتة (رقم الشبكة)
متغيرة
(رقم الطرفية)
(/22)
قناع الشبكة الفرعية
(Dotted Decimal Notation)
= 2 طرفية
عدد الطرفيات التي تدعمها الشبكة
reference: This slide was created by Dr. Mohammad Arafah
CT1403

18. Obtaining IP Addresses

19. IP addresses: how to get one?
Q: How does a host get IP address?
hard-coded by system administrator in a file
Windows: control-panel->network->configuration- >tcp/ip->properties
UNIX: /etc/rc.config
More often using ===> DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play”

20. DHCP: Dynamic Host Configuration Protocol
goal: allow host to dynamically obtain its IP address from network server when it joins network
can renew its lease on address in use
allows reuse of addresses (only hold address while connected/“on”)
support for mobile users who want to join network (more shortly)
DHCP overview:
host broadcasts “DHCP discover” msg [optional]
DHCP server responds with “DHCP offer” msg [optional]
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg

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

22. DHCP client-server scenario
DHCP server:
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
arriving
client
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
lifetime: 3600 secs
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Network Layer
4-47

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
network mask (indicating network versus host portion of address)
Network Layer
4-48

24. DHCP: example
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
connecting laptop needs its IP address, addr of first- hop router, addr of DNS server: use DHCP
DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
router with DHCP
server built into
router
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
Network Layer
4-49

25. DHCP: example
DHCP
DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
DHCP
UDP IP
Eth
Phy
encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
router with DHCP
server built into
router
client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router
DHCP
Network Layer
4-50