Download 8) Network layer-3

Network Layer III (IP)
Internet Protocol/
Network Layer
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Position of IP in TCP/IP protocol suite
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Internet Protocol (IP)
a) The IP is the host-to-host network layer delivery protocol
for Internet.
b) IP is a connectionless datagram protocol for packet
switching network.
c) Best effort delivery service: meaning that IP provides no
error control or flow control. (e.q. post office does not keep
track of every letter)
d) IP uses only an error detection mechanism (Chechsum) and
discards the packet if it is corrupted.
e) IP does its best to deliver a packet to its destination, but with
no guarantees. Needs to rely on TCP layer for reliable
delivery.
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IP Datagram
• Packets in the IP layer are called Datagrams.
• A datagram is a variable-length packet consisting of header and data:
• Header: 20 to 60 bytes in length containing essential information
for routing and addressing
• It is customary to show the header
in 4-byte sections.
• VER: defines the IP version (IPv4).
• HLEN: num of 4-byte sections; multiplied
by 4 to give it’s the actual length in bytes
• Total length: of IP datagram in bytes;
• Data length = Total length – header length
• Time to live: to control max no. of hops
visited by a datagram by count decrement.
• Protocol: specifies final dest. protocol to
which IP datagram should be delivered;
TCP(6), UDP(17), ICMP(1), IGMP(2)
• Checksum: covers only header field
for testing & debug4
Protocol field:
6
1
2
17
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This field defines the higher level protocol (transport-layer) that uses the
services of IP-layer.
An IP datagram can encapsulate data from several higher-level protocols
such as: TCP=6, UDP=17, ICMP=1, IGMP=2, OSPF=89.
This field specifies the final destination protocol to which the IP datagram
should be delivered. Since IP multiplexes and demultiplexes data from
different higher level protocols, the value of this field helps in the
demultiplexing process when the datagram arrives at its final destination.
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Example 1
An IP packet has arrived with the first 8 bits (leftmost) as shown:
 01000010
The receiver discards the packet. Why?
Solution
There is an error in this packet. The 4 left-most bits (0100)
show the version, which is correct. The next 4 bits (0010)
show the header length, which means (2  4 = 8), which is
wrong. The minimum number of bytes in the header must be
20. The packet has been corrupted in transmission.
Note that only HLEN field uses (x4) to get the header length 6
Example 2
In an IP packet, the value of HLEN is 1000 in binary. How many
bytes of options are being carried by this packet?
Solution
The HLEN field value is 8, which means the total
number of bytes in the header is 8  4 or 32 bytes.
The first 20 bytes are the main header, the next 12
bytes are the options.
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Example 3
In an IP packet, the value of HLEN is 516 (or 0101) and the value of
the total length field is 002816 (or 00101000). How many bytes of
data are being carried by this packet?
Solution
The HLEN value is 5, which means the total number of
bytes in the header is 5  4 or 20 bytes (no options). The
total length is 28(hex) = 001010002 = 40 bytes, which
means the packet is carrying 20 bytes of data.
Data length = total length – header length = (40-20) = 20.
Only HLEN field uses (×4) to get header length
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Example 4
An IP packet has arrived with the first few
hexadecimal digits as shown below:
45000028000100000102...................
How many hops can this packet travel before being
dropped? The data belong to what upper layer
protocol?
Solution
To find the time-to-live field, we should skip 8 bytes (16
hexadecimal digits). 4 bits (binary) = 1 bit (hex); 1 byte = 8 bits binary = 2 bits (hex)
The time-to-live field is the ninth byte, which is 01. This
means the packet can travel only one hop. The protocol field is
the next byte (02), which means that the upper layer protocol
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is IGMP.
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packet captured with Wireshark
(header only)
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Internet Protocol version 6:
IPv6
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IPv6
a) Although IPv4 was well designed, it is clear that IPv4 is becoming
insufficient for the fast growing Internet and the needs:
• Due to growing demand of internet usage, the use of addressing space is
becoming insufficient for conventional netid & hostid two-level
addressing structure in Classful addressing.
• Real-time application i.e. video transmission requires minimum delay
strategies and reservation of resource not provided in IPv4
• Encryption and authentication of data and security mechanism not
provided in IPv4.
• Limitation of flexibility and mobility supported in IPv4.
b) In IPv6, the internet protocol was extensively modified to
accommodate the unforeseen growth of Internet.
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Improvement in IPv6
a) IPv6 has some advantages over IPv4:
• Longer addresses than IPv4: 128 bits long > 32 bits
currently in IPv4. It is large enough for every grain
of sand on earth to be IP addressable..
• Simplification in Header: contains only 8 vital fields
< 13 in IPv4. (speeds up routing process). – 5 fields
becomes options.
• New options: to support additional functionalities
• Security: Authentication & privacy are key features
of new IP
• More Services: to handle special/real-time
multimedia traffic.
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The IPv4 header
Changed
0 bits
4
Ver
8
IHL
16
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Service Type
Identifier
Time to Live
Removed
Total Length
Flags
Protocol
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Fragment Offset
Header Checksum
32 bit Source Address
32 bit Destination Address
Options and Padding
September 2006
UKERNA IPv6 Workshop
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IPv4 datagram
Totally remove bcs
fixed 40 bytes
header is used
Replaced by Hop
limit in IPv6
Replaced by
Flow label in
IPv6
Taken out bcs Next header in
IPv6 tells what follows the last
IP header (e.g. a UDP or TCP)
All replaced by functions in
both Next header & payload
extension header:
fragmentation in IPv6
Replaced by
payload length in
IPv6
Totally removed bcs calculating this greatly
reduce performances. Data-link and
Transport layer have their own checksum for
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better support of reliability,
Format of an IPv6 datagram
32 bits
Base Header:
Occupies 40 bytes
Payload:
Extension header
+
Data from upper layer
Occupies up to 65535
bytes
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The changes made in IPv6 with regard to IPv4
are compared as follows
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IPv6 packet field
a) VER: IP version; 4 for IPv4 and 6 for IPv6
b) PRI: priority field defines priority of packets w.r.t. traffic congestion;
0-7: for packets that can slow down in event of congestion; 8-15 for
real-time traffic with constant sending rate – Audio & Video.
c) Flow label: (~ Service in IPv4) Special handling for a particular flow
of data; pseudo-connection btw source and dest. for reserved
bandwidth and stringent delay requirement. (Still experimental).
d) Payload length: tells how many payload bytes follow the 40-bytes
header; excluding the count for header based – 40 bytes not included
e) Next header: 8-bit field defines the subsequent headers that follow the
base header to allow extension; either optional extension headers used
by IP or header for an upper-layer protocol, e.g. UDP & TCP.
f) Hop limit: serves the same purpose as the TtL in IPv4.
g) Source and Destination address field: both occupy 16 bytes (128 bits)
h) Extension Headers: 6 extension headers which is optional in IPv4.
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IPv6 datagram
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Extension header format
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IPv6 address
IPv4
4 bytes
32 bits
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Abbreviated address
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Abbreviated address with consecutive zeros
Two or more groups of 16 zeros can
be replaced by a pair of colons
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CIDR address
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Transition from IPv4 (95%) to IPv6
a) Because of huge number of systems on the Internet,
the transition from IPv4 to IPv6 cannot happen
suddenly.
b) It will take considerable amount of time before every
system in the Internet can move from IPv4 to IPv6
c) The transition should be a smooth process to prevent
any problems due to their differences.
d) 3 transition strategies:
• Dual stack
• Tunnelling
• Header translation
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IP V6 usage
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Per-country IP V6 adoption
IP V6 adoption
UK: 0.32%
Belgium: 29.1%
On 5/11/2014
https://www.google.com/intl/en/ipv6/statistics.html#tab=per-country-ipv6-adoption
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Three transition strategies
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Dual stack
A station should simultaneously run both IPv4 & IPv6 protocols until all
Internet uses IPv6.
To determine which version to use, the source host queries the DNS and
send whichever version of IP packet the DNS returns.
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Tunneling
Tunnelling is a strategy used when two hosts using IPv6 want to connect with
each other when the packet must pass through a region that uses IPv4.
To pass thru the region, packet must have an IPv4 address. So, the IPv6
packet is encapsulate in an IPv4 packet when it enters the region, and IPv6
leaves its capsule when it exits the region.
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Header translation
Header translation is necessary when majority of the Internet has moved to
IPv6 but some systems still use IPv4.
The sender wants to use IPv6 but the receiver does not understand IPv6.
Tunnelling does not work because packet must be in the IPv4 format to be
understood by the receiver.
In this case, the header format must be changed totally through header
translation to down-convert from IPv6 to IPv4 header.
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Further Reading
1- “Computer Networks”, Andrew Tanenbaum, 4th Ed. to learn more
about the generic network layer.
2- “Internetworking with TCP/IP vol.1”, Douglas Comer, 4th Ed.,
provides a detailed and comprehensive presentation of TCP/IP.
3- “Data Communications and Networking”, Behrouz Forouzan, 4th
Ed.,
4- https://www.google.com/intl/en/ipv6/index.html
Copyright Information : Some figures used in this presentation have been
either directly copied or adapted from several books.
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