IP : Internet Protocols
Agenda
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IP functions
IP header format
IP routing
Fragmentation
IPng’s overview
Connectionless Delivery System
 Most fundamental internet service consists of
a packet delivery system
 Service is defined as
 Unreliable
 Best-effort
 Connectionless
Purpose Of Internet Protocol
 Protocol that defines the unreliable,
connectionless delivery mechanism is called
Internet Protocol and usually called IP
 IP has 3 important definitions
 Basic unit of data transfer used throughout a
TCP/IP internet
 IP software perform the routing function
 Including a set of rule that embody the idea of
unreliable packet delivery
IP layer
defines a single virtual network on top of
different kinds of hardware platform using IP
address
functions of IP
route packet
fragmentation
handle type of services
send and receive error and control
message using ICMP
IP attributes
handle data unit called IP datagrams
connectionless protocol - doesn’t promise
reliable delivery
best effort delivery
packets may be lost, out out sequence, or
duplicated due to various reasons
IP encapsulation
with Ethernet frame
IP datagrams
Ethernet hdr
IP header
data
IP reframing
• IP will reframe the packet when A send data to B
IP
158.108.33.1
MAC 0:0:c:6:13:4a
MAC dest
MAC src
type
IP source
IP dest
IP
158.108.2.1
MAC 0:0:c:6:12:40
0:0:c:6:13:4a
0:0:e8:15:cc:c
0x800
158.108.33.4
158.108.2.71
MAC dest
MAC src
type
IP source
IP dest
 packet to router
A

IP
158.108.33.4
MAC 0:0:e8:15:cc:c
B
0:0:33:10:a:c
0:0:c:6:12:40
0x800
158.108.33.4
158.108.2.71
packet from router
IP
158.108.2.71
MAC 0:0:33:10:a:c
Change MAC address, IP address be the same
IP datagrams
0
15 16
vers:4 hlen:4
TOS:8
total length:16
identification:16
20
bytes
time to live:8
31
flags:3
protocol:8
frag offset:13
header checksum :16
source address :32
destination address :32
options and padding :32
data
:
IP header details (1)
• vers - version = 4
• hlen - header length in 32-bit words,
– with no options, hlen = 5 = 20 bytes
• TOS - type of service, desired quality of
services
0
1
Prec.
bits
0-2
3
4
5
6-7
2
3
4
5
6
7
D
T R 0
0
if 0
if 1
Precedence
Normal delay
Low delay
Normal throughput High throughput
Normal Reliability
High reliability
Reserved
IP header details (2)
Total length - length of datagrams (incl. header),
max datagrams is 64K
identification, flags, fragmentation - use to
segmentation and reassembly packet
TTL - Time to live, defining max number of routers
through which the datagrams may pass (hop
count)
ttl-- decrease each router it passes a router
normally set to 30
if ttl == 0 discard and send ICMP TTL exceeded to source IP (prevent
looping)
IP header details (3)
Protocol - higher-level protocol that provides data
1 = datagrams carries an ICMP messages
6 = datagrams carries an TCP segments
17 = datagrams carries an UDP datagrams
header checksum - 16 bit one’s compliment, note
that there is no data checksum
source address - 32 bit IP source address
destination address - 32 bit IP destination
address
option and padding - additional info to control
functions such as routing and security
Routing
routing is a process of choosing a path over
which to send datagrams
IP routes packet by looking at the IP network
number
routing components
determine what path are available
selecting the best path for a particular purpose
using those paths to reach other networks
devices which perform routing are routers
(historically call IP gateways)
Routing Table
Every router contains a routing table of the
network numbers
The table records
which connection can be used to reach a
particular network
plus some indication of the performance or
cost of using connection
Routing Table form
• Routing Table form
– <network, gateways, others>
% netstat -rn
Destination
Gateway
Genmask
Flags MSS Window irtt Iface
158.108.32.0 0.0.0.0 255.255.255.192 U 1500
127.0.0.0
0.0.0.0 255.0.0.0
U 3584
0.0.0.0
158.108.32.1 0.0.0.0
UG 1500
0
0
0
0 eth0
0 lo
0 e
How to create routing table
 IP does not create routing table by itself
 Normal 3 ways to create routing table
 static route - by hand
% route add 158.108.20.0 158.108.33.1
 dynamic routes - via routing protocol
 via ICMP redirect
Routing Protocol
Routing protocol manages and updates
routing table on each network node
often implemented in UNIX using one of the
two daemons:
routed : basic routing daemon for interior
routing, normally with RIP
gated : sophisticated daemon for interior
and exterior routing, with additional protocol
such as OSPF, BGP
MTU revisited for fragmentation
The upper limit number of data byte in data
link frame is call MTU (Maximum Transfer
Unit)
Typical MTU (bytes)
FDDI : 4325
Ethernet : 1500
802.3 : 1492
Point-to-Point : 296
If #bytes of datagrams to send>link layer’s
MTU, IP breaks the datagrams up into smaller
pieces (fragmentation)
Fragmentation
fragmentation = processed used by IP to
reduced size of datagram that are too big for
link connection MTU e.g. fragment 2000
bytes to Ethernet (MTU=1500)
fragments should be reassembled at the final
destination (expensive process)
How ?
 each fragment has its own header
 each fragment carries the same 16 bit identification number
 Each fragment must be aligned with an eight-octet boundary
Fragmentation flag
Identification number
16 bits integer value used to identify all fragments
This id is not a sequence number!
flags - 3 bits control fragmentation
0= may fragment
1= don’t fragment
reserve,
must be 0
R
0= last fragment
1= more fragments
DF
MF
 fragment offset - indicate the distance of fragment data from
the start of the original datagram, measure in 8 octets unit
Fragmentation sample
other header
original
ident flags offset
…….
232
0
0..2000
20 bytes
20 bytes
Ethernet
with MTU ... 111 1
of 1500
0
data
0
0..1479
... 111 0 185 1480..2000
identification number
more fragment
post 0
last fragment
post 185*8=1480
Problem in fragmentation
The end node has no way of knowing how many
fragments there be. It has to manage enough
buffer space to handle reassembly process.
If any fragments lost, all datagram must be
discarded
End node starts a timer when received the first
fragment, if any fragments fails to arrive (usually
30 secs), all datagrams must be discarded
Since the IP service is connection's. No attempt is
made by IP to recover these situations, though
ICMP error message may be generated
Path MTU
Path MTU : the smallest MTU of data link
between two distance hosts
Need not to be constant because routing
mechanism
Avoid fragmentation by discover PATH MTU
(RFC1191)
Use ICMP to determine PATH MTU
Avoiding fragmentation
For datagrams within the same physical network,
the MTU is known. TCP/UDP then use the MTU
to limit the message size pass to IP; messages
will never be fragmented.
For datagrams passed to diff net, not easy to
know the MTU!
standard recommends that all networks
supporting TCP/IP have an MTU of at least 576
bytes
512 bytes data+20 bytes TCP hdr + 20 bytes IP hdr with
options
TO GUARANTEED THAT A PACKET OF 576 BYTES OR
LESS IS NEVER FRAGMENTED
IPng
Problem of current IP address : limitation of 32
bit address space
1990 : IETF defined a new version of IP,
generally called IP Next Generation or IPng
Spring 1992 : IAB issues IPv7, proposed the OSI
CLNP (connectionless Network Protocol) as the
basis of IPv7. Finally rejected by IETF and
working groups
IPV6
 End 1992: 7 proposals for IPng
CNAT, IP Encaps, Nimrod, Simple CLNP, P Internet
Protocol, SIP, and TP/IX
 resolution to 3 possibilities :
SIP+ TP/IX => TUBA (TCP and UDP with bigger
Address: RFC 1347)
TP/IX => CATNIP (Common Architecture for the Next
Generation Internet Protocol :RFC1707)
SIP+IP encaps+PIP=> SIPP (Simple Internet Protocol
Plus: RFC1710)
 Mid 1994 : SIPP was chosen, known as IP
version 6 (IPv6)
IPv6 Header
0
15 16
vers:4
40
bytes
31
flow lable:28
payload length:16
next hdr:8
hop limit:8
destination address :128
source address :128
40 bytes fixed length header, no checksum, options are
replace by additional extension header
IPV6 address representation
16 bits eight hexadecimal value e.g.
4210:30:127F:9111:7801:DA0A:3232:44
5510:0:0:0:0:0:0:44 or 5510::44
Address with mixed environment of v4 and
v6
x:x:x:x:x:x:d.d.d.d
e.g. 0:0:0:0:0:0.158.108.2.71
IPV6 key advantages
16 bytes fix length IP address support
~1000 million networks
IPv4 compatibility
self-configuration of workstations
support mobile workstations
improved security features