1. IP - Internet Protocol (Based on Kurose & Ross)
Performs network and internetworking functions
Probably the most important protocol
No Internet without IP
Rich, complex and beautiful!

2. Datagram networks: the Internet model
Packets between same source-destination
pair may take different paths
application
transport
network
data link
physical
1. Send data
2. Receive data

3. Internet 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

4. IP Features Connectionless (best effort) End to end delivery
Data Units are datagrams or packets
Global Addressing
Routing
Fragmentation and Reassembly
Route recording, time stamping options
QoS options, but seldom used in v4

5. 32 bit destination IP address
IP datagram format
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
Internet
checksum
time to
live
32 bit source IP address
IP protocol version
number
header length
(bytes)
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
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

6. Header Fields (1) Version Internet header length
Currently 4
IP v6
Internet header length
In 4 byte words
Including options if used
Type of service – priority (3 bits), DTR (Delay, Throughput, Reliability)
Total length
Of datagram, in octets <= 65535

7. Header Fields (2) Identification (datagram) Flags
Sequence number
Used with addresses and user protocol to identify datagram uniquely
Flags
M - More bit
D - Don’t fragment
Fragmentation offset – units of 8 octets
Time to live TTL (<= 255)
Protocol
Next higher layer to receive data field at destination
ICMP (1), TCP (6), UDP (17)

8. Protocol numbers http://www. iana
Protocol numbers c:/winnt/system32/drivers/etc
0 HOPOPT IPv6 Hop-by-Hop Option
ICMP Internet Control Message
IGMP Internet Group Management
GGP Gateway-to-Gateway
IP IP in IP (encapsulation)
ST Stream
TCP Transmission Control
EGP Exterior Gateway Protocol]
IGP any private interior gateway (used by Cisco for their IGRP)
17 UDP User Datagram

9. Header Fields (3) Header checksum Source address Destination address
Reverified and recomputed at each router
16 bit ones-complement sum of all 16 bit words in header
Set to zero during calculation
Source address
Destination address
Options
Padding
To fill to multiple of 4 bytes long

10. Options Specified as Type + length + value
Security – classification level
Strict Source Routing
Loose Source Routing
Record Route
Time Stamp

11. Data Field Carries user data from next layer up – TCP, UDP, ICMP etc.
Integer multiple of 8 bits long (octet)
Max length of datagram (header plus data) 65,535 octets

12. Fragmentation & Re-assembly
Different packet sizes for different networks
When to re-assemble
At destination (IP approach)
Results in packets getting smaller as data traverses internet
Intermediate re-assembly (possible, but not used)
Need large buffers at routers
Buffers may fill with fragments
All fragments must go through same router
Inhibits dynamic routing

13. Fragmentation - fields
Uses fields in header
D flag – don’t fragment
Datagram Identifier (ID)
Total length (data length + 20 bytes)
Offset
Position of fragment of user data in original datagram
In multiples of 64 bits (8 octets)
‘M’ or More flag - last fragment?

15. Another example An IP packet with: total length 4820,
datagram ID = 571 (decimal),
fragment offset 32 and
M=0
has to be fragmented for an Ethernet network.
Show how this could be done, giving the values of the relevant header fields in all the fragments.

16. Solution
Frag ID M D
Offset
Comments 1
571 32
The original offset 2
217
185 (1480/8) + 32 3
402
Another 185 4
587
The original datagram is the last fragment of a larger packet, offset = 32.
Ethernet payload = 1500 ( IP header)

17. Dealing with Failure Re-assembly may fail if some fragments get lost
Need to detect failure
Re-assembly time out
Assigned to first fragment to arrive
If timeout expires before all fragments arrive, discard partial data

18. Node Addresses - MAC & IP
MAC addresses (6 bytes)
Defined at layer 2 OSI
AKA Physical addresses
Fixed, burned-in to NIC
Guaranteed globally-unique
IP addresses (4 bytes, x 109 addresses)
Defined at layer 3
Referred to as Logical addresses
Configured by network manager
Facilitates logical grouping
The total number of possible combinations of 32 bits is billion, but not all addresses are actually usable.

19. IP addresses An IP address consists of two parts: The network address
The first part of the address
Identifies the network to which a host belongs
Used by routers in path determination
The host address
The last part of the address
Used in the local network to identify a host
Network portion
Host portion
32-bit IP address
A subnet mask is used to mark the length of the network part
A ‘1’ indicates a network bit, a ‘0’ indicates a host bit
If network portion is 24 bits and host portion is 8 bits
subnet mask will be
written in dotted decimal format as
Mask: … …

20. IP Address Classes Class E: 1111xxxx: reserved for IAB R&D
network
host 10
110
1110
multicast address A B C D
class to to to to
32 bits
Class E: 1111xxxx: reserved for IAB R&D
Mask: 1’s for network bits, 0 for host bits
e.g , , etc
Three main classes of address are used, based on the organisation size:
Class A Range – First octet 0xxxxxxx
Number of hosts per network 224 = 16,777,216
Usable hosts per network = 16,777,214
Subnet mask
Class B Range – First octet 10xxxxxx
Number of hosts per network 216 = 65536
Usable hosts per network = 65534
Subnet mask
Class C Range – First octet 110xxxxx
Number of hosts per network 28 = 256
Usable hosts per network = 254
Subnet mask

21. Class A Start with binary 0 (first bit)
Addresses starting with reserved
(127) reserved for loopback
Range 1.x.x.x to 126.x.x.x
All allocated
16,777 million hosts per network

22. Class B Start 10 (first 2 binary bits) Range 128.x.x.x to 191.x.x.x
Second octet also included in network address
214 = 16,384 class B networks
All allocated
216 = host-ids per networks

23. Class C Start 110 (first 3 binary bits) Range 192.x.x.x to 223.x.x.x
Second and third octet also part of network address
221 = 2,097,152 addresses
Nearly all allocated
256 host-ids per network

24. Special Addresses First octet >= 224 (E0 1110 0000) - multicast.
First octet >= 240 (F ): IAB use
127.X.X.X - local loop back address for debugging
Host-id = 0 refers to network
Net-id = 0 - 'this network‘ (class B address)
: used by RARP, BOOTP DHCP – own address unknown
: destination address for default route in routers.
Broadcast addresses: host part is all 1’s
Local broadcast
Not forwarded by routers, to all hosts in sender’s broadcast domain
Directed broadcast: <Network Address>.255: forwarded by routers to specified network; to all hosts in specified network

25. Private Addresses – – –
Not routable on the public Internet.
Can be reused on private (home, company, campus) networks.
A home router may use these for PCs connected to its ports, with NAT

26. IP addressing: suffix notation
Address Class scheme wastes addresses
CIDR: Classless InterDomain Routing
Network portion of address of arbitrary length
address format:
a.b.c.d/x
x is the number of bits in network portion of address
Will be discussed later

27. Addresses & Interfaces
IP address:
for each host,
for each router interface
interface: connection between host/router and physical link
router’s typically have multiple interfaces
host may have multiple interfaces =
223 1 1 1

28. What is an ‘IP Network’?
From IP address perspective
device interfaces with same network part of IP address
can physically reach each other without intervening router
LAN
A network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)

29. How to find the networks?
Detach each interface from router, host
create “islands of isolated networks

30. Address Allocation I How does 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”

31. Address Allocation II Q: How does network get network address?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23

32. Address Allocation III
Q: How does an ISP get a block of addresses?
A: IANA: Internet Assigned Numbers Authority
Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes

33. Getting a datagram from source to destination (1)
forwarding table in A
Dest. Net. next router Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A B E
datagram remains unchanged, as it travels source to destination
addr fields of interest here

34. Getting a datagram from source to destination (2)
forwarding table in A
misc
fields
Dest. Net. next router Nhops
data
Starting at A, send IP datagram addressed to B:
look up net. address of B in forwarding table
find B is on same net. as A
link layer will send datagram directly to B inside link-layer frame
B and A are directly connected A B E

35. Getting a datagram from source to destination (3)
forwarding table in A
misc
fields
Dest. Net. next router Nhops
data
Starting at A, dest. E:
look up network address of E in forwarding table
E on different network
A, E not directly attached
routing table: next hop router to E is
link layer sends datagram to router inside link-layer frame
datagram arrives at
continued….. A B E

36. Getting a datagram from source to destination (4)
forwarding table in router
Dest. Net router Nhops interface
misc
fields
data
Arriving at , destined for
look up network address of E in router’s forwarding table
E on same network as router’s interface
router, E directly attached
link layer sends datagram to inside link-layer frame via interface
datagram arrives at A B E