ECE 4110 – Internetwork Programming IP Protocol. 2 * From TCP/IP Protocol Suite, B. A. Forouzan,...

ECE 4110 – Internetwork Programming

IP Protocol

2* From TCP/IP Protocol Suite, B. A. Forouzan, Prentice Hall

Position of IP in TCP/IP Protocol Suite

Spring 2003Tuna Tugcu ECE 4110 – Internetwork Programming 3

Introduction

Packets in IP are called datagrams. IP service is:

Connectionless Unreliable Best-effort: No error checking or

tracking. IP does its best to get the datagram to the destination with no guarantees.


IPv4 Datagram Format


IPv4 Header Network byte order (big endian) used. Fields:

Version: 4 Header length: Number of 32-bit words in the header (5 without options) Service type: Discussed later. Total length: Total length of IP datagram in bytes (max

65535). Identification: Uniquely identifies each datagram sent by a

host. Used for fragmentation and reassembly. Flags: Used for fragmentation. Fragmentation offset: Used for fragmentation. In 8-byte units. Time to live: Upper limit on number of hops. Often 32 or 64.


IPv4 Header (cont’d)

Fields (cont’d): Protocol: Identifies which protocol gave data to IP. Checksum: Only for the header. Source address: 32-bit IP address. Destination address: 32-bit IP address. Options: Variable length list of options. Not all

hosts/routers support all of them. Security and handling restrictions: For military

applications. Record route: Have each router record its IP address Timestamp: Have each router record its IP address and

time Loose source routing: List of IP addresses that must be

traversed. Strict source routing: Similar to above, but only these

addresses must be traversed.


Encapsulation of IP datagram in an Ethernet Frame


Multiplexing/Demultiplexing in Network Layer


Service Type


Service Type (cont’d)

Also known as type of service. IETF recently changed the name (and the format) as differentiated services.

Service Type: The precedence subfield is not used in IPv4. TOS bits:

Minimize delay Maximize throughput Maximize reliability Minimize monetary cost

Only one of TOS bits can be specified. Last bit is unused and must be zero.


Differentiated Services: Codepoint:

When the 3 rightmost bits are all zero, the 3 leftmost bits show precedence.

O/w, codepoint defines 64 services.

Last 2 bits are unused.

Service Type (cont’d)

Category

Codepoint

Assigning Authority

1 XXXXX0 Internet

2 XXXX11 Local

3 XXXX01 Temporary or experiemental


Fragmentation Each router decapsulates the IP

datagram from the incoming frame and encapsulates into the outgoing frame.

MTU: Maximum Transfer Unit


Fragmentation (cont’d)

Though maximum IP datagram size is 65535, the data link layer imposes restrictions fragmentation

A fragmented datagram may be further fragmented if it crosses a network with smaller MTU.

Reassembly is done only at the destination. Why?



IP fields related to fragmentation: Identification (16 bits):

Provides uniqueness of each datagram. Copied into all fragments.



IP fields related to fragmentation: Flags (3 bits):

First bit is reserved. If do not fragment bit is set, and datagram does

not fit the frame, datagram is discarded and ICMP error message is sent.

If more fragments bit is set, there are other fragments following.



IP fields related to fragmentation (cont’d): Fragmentation offset (13 bits):

Offset of data in the original datagram. Specified in units of 8 bytes.


Fragmentation Example


Fragmentation Example (cont’d)


Fragmentation Example 1

A packet has arrived with an M bit value of 0. Is this the first fragment, the last

fragment, or a middle fragment? Do we know if the packet was

fragmented?


Fragmentation Example 1 (sol’n)

M bit is 0 No more fragments; This is the last

fragment. However, we cannot say if the

original packet was fragmented or not.



A packet has arrived with an M bit value of 1. Is this the first fragment, the last

fragment, or a middle fragment? Do we know if the packet was

fragmented?



M bit is 1 At least one more fragment.

This fragment can be the first one or a middle one, but not the last one.

We don’t know if it is the first one or a middle one; we need more information (the value of the fragmentation offset).

However, we can definitely say the original packet has been fragmented because the M bit value is 1.



A packet has arrived with an M bit value of 1 and a fragmentation offset value of zero. Is this the first fragment, the last

fragment, or a middle fragment?



M bit is 1 Either the first fragment or a middle one.

Offset value is 0 it is the first fragment.



A packet has arrived in which the offset value is 100. What is the number of the first byte? Do we know the number of the last

byte?



Multiply the offset value by 8 First byte number is 800. We cannot determine the number

of the last byte unless we know the length of the data.



A packet has arrived in which the offset value is 100, the value of HLEN is 5 and the value of the total length field is 100. What is the number of the first byte

and the last byte?



First byte number is 100 8 = 800.

The total length is 100 bytes and the header length is 20 bytes (5 4)

80 bytes of data. If the first byte number is 800, the

last byte number must 879.


IP Options


IP Options (cont’d)



No operation option:



End of option option:



Record route option:


Record Route Concept



Strict Source Route option:


Strict Source Route Concept



Loose Source Route option:



Timestamp option:


Use of Flag in Timestamp


Timestamp Concept


IP Package

ECE 4110 – Internetwork Programming IP Protocol. 2 * From TCP/IP Protocol Suite, B. A. Forouzan,...

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