IP and OSI Layers


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The Internet Protocol (IP) is designed for use in interconnected systems of packet-switched computer communication networks.  Such a system has been called a “catenet”.  The internet protocol (IP) provides for transmitting blocks of data called datagrams from sources to destinations, where sources and destinations are hosts identified by fixed length addresses.  The internet protocol(IP) also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through “small packet” networks.


The internet protocol (IP) is specifically limited in scope to provide the functions necessary to deliver a package of bits (an internet datagram) from a source to a destination over an interconnected system of networks.  There are no mechanisms to augment end-to-end data reliability, flow control, sequencing, or other services commonly found in host-to-host protocols.  The internet protocol (IP) can capitalize on the services of its supporting networks to provide various types and qualities of service.


This protocol is called on by host-to-host protocols in an internet environment.  This protocol calls on local network protocols to carry the internet datagram to the next gateway or destination host.

For example, a TCP module would call on the internet module to take a TCP segment (including the TCP header and user data) as the data portion of an internet datagram.  The TCP module would provide the addresses and other parameters in the internet header to the internet module as arguments of the call.  The internet module would then create an internet datagram and call on the local network interface to transmit the internet datagram.


The internet protocol (IP) implements two basic functions:  addressing and fragmentation.

The internet modules use the addresses carried in the internet header to transmit internet datagrams toward their destinations.  The selection of a path for transmission is called routing.

The internet modules use fields in the internet header to fragment and reassemble internet datagrams when necessary for transmission through “small packet” networks.

The model of operation is that an internet module resides in each host engaged in internet communication and in each gateway that interconnects networks.  These modules share common rules for interpreting address fields and for fragmenting and assembling internet datagrams.  In addition, these modules (especially in gateways) have procedures for making routing decisions and other functions.

The internet protocol (IP) treats each internet datagram as an independent entity unrelated to any other internet datagram.  There are no connections or logical circuits (virtual or otherwise).

The internet protocol(IP) uses four key mechanisms in providing its service:  Type of Service, Time to Live, Options, and Header Checksum.

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Type of Service

The Type of Service is used to indicate the quality of the service desired.  The type of service is an abstract or generalized set of parameters which characterize the service choices provided in the networks that make up the internet.  This type of service indication is to be used by gateways to select the actual transmission parameters for a particular network, the network to be used for the next hop, or the next gateway when routing an internet datagram.

Time to Live

The Time to Live is an indication of an upper bound on the lifetime of an internet datagram.  It is set by the sender of the datagram and reduced at the points along the route where it is processed.  If the time to live reaches zero before the internet datagram reaches its destination, the internet datagram is destroyed.  The time to live can be thought of as a self-destruct time limit.


The Options provide for control functions needed or useful in some situations but unnecessary for the most common communications.  The options include provisions for timestamps, security, and special routing.


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Header Checksum

The Header Checksum provides a verification that the information used in processing internet datagram has been transmitted correctly.  The data may contain errors.  If the header checksum fails, the internet datagram is discarded at once by the entity which detects the error.

The internet protocol (IP) does not provide a reliable communication facility.  There are no acknowledgments either end-to-end or hop-by-hop.  There is no error control for data, only a header checksum.  There are no retransmissions.  There is no flow control.

Errors detected may be reported via the Internet Control Message Protocol (ICMP) which is implemented in the internet protocol (IP) module.


Model of Operation

The model of operation for transmitting a datagram from one application program to another is illustrated by the following scenario:

We suppose that this transmission will involve one intermediate gateway.

The sending application program prepares its data and calls on its local internet module to send that data as a datagram and passes the destination address and other parameters as arguments of the call. The internet module prepares a datagram header and attaches the data to it. The internet module determines a local network address for this internet address, in this case, it is the address of a gateway. It sends this datagram and the local network address to the local network interface. The local network interface creates a local network header, and attaches the datagram to it, then sends the result via the local network. The datagram arrives at a gateway host wrapped in the local network header, the local network interface strips off this header and turns the datagram over to the internet module. The internet module determines from the internet address that the datagram is to be forwarded to another host in a second network. The internet module determines a local net address for the destination host. It calls on the local network interface for that network to send the datagram. This local network interface creates a local network header and attaches the datagram sending the result to the destination host. At this destination host, the datagram is stripped of the local net header by the local network interface and handed to the internet module. The internet module determines that the datagram is for an application program in this host. It passes the data to the application program in response to a system call, passing the source address and other parameters as results of the call.

Function Description

The function or purpose of Internet Protocol (IP)is to move datagrams through an interconnected set of networks.  This is done by passing the datagrams from one internet module to another until the destination is reached.  The internet modules reside in hosts and gateways in the internet system.  The datagrams are routed from one internet module to another through individual networks based on the interpretation of an internet address.  Thus, one important mechanism of the internet protocol (IP) is the internet address.

In the routing of messages from one internet module to another, datagrams may need to traverse a network whose maximum packet size is smaller than the size of the datagram.  To overcome this difficulty, a fragmentation mechanism is provided in the internet protocol (IP).


A distinction is made between names, addresses, and routes.
A name indicates what we seek.
An address indicates where it is.

A route indicates how to get there.  The internet protocol (IP) deals primarily with addresses.  It is the task of higher level (i.e., host-to-host or application) protocols to make the mapping from names to addresses.   The internet module maps internet addresses to local net addresses.  It is the task of lower level (i.e., local net or gateways) procedures to make the mapping from local net addresses to routes.

Addresses are fixed length of four octets (32 bits).  An address begins with a network number, followed by the local address (called the “rest” field).  There are three formats or classes of internet addresses: in class a, the high order bit is zero, the next 7 bits are the network, and the last 24 bits are the local address; in class b, the high order two bits are one-zero, the next 14 bits are the network and the last 16 bits are the local address; in class c, the high-order three bits are one-one-zero, the next 21 bits are the network and the last 8 bits are the local address.

Care must be taken in mapping internet addresses to local net addresses; a single physical host must be able to act as if it were several distinct hosts to the extent of using several distinct internet addresses. Some hosts will also have several physical interfaces (multi-homing). That is, provision must be made for a host to have several physical interfaces to the network with each having several logical internet addresses.

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Fragmentation of an internet datagram is necessary when it originates in a local net that allows a large packet size and must traverse a local net that limits packets to a smaller size to reach its destination.

An internet datagram can be marked “don’t fragment.”  Any internet datagram so marked is not to be internet fragmented under any circumstances.  If internet datagram marked don’t fragment cannot be delivered to its destination without fragmenting it, it is to be discarded instead.

Fragmentation, transmission, and reassembly across a local network which is invisible to the internet protocol (IP) module is called intranet fragmentation and may be used.

The internet fragmentation and reassembly procedure need to be able to break a datagram into an almost arbitrary number of pieces that can be later reassembled.  The receiver of the fragments uses the identification field to ensure that fragments of different datagrams are not mixed.  The fragment offset field tells the receiver the position of a fragment in the original datagram.  The fragment offset and length determine the portion of the original datagram covered by this fragment.  The more-fragments flag indicates (by being reset) the last fragment.  These fields provide sufficient information to reassemble datagrams.

The identification field is used to distinguish the fragments of one datagram from those of another.  The originating protocol module of an internet datagram sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. The originating protocol module of a complete datagram sets the more-fragments flag to zero and the fragment offset to zero.

To fragment a long internet datagram, an internet protocol (IP) module (for example, in a gateway), creates two new internet datagrams and copies the contents of the internet header fields from the long datagram into both new internet headers.  The data of the long datagram is divided into two portions on an 8 octet (64 bit) boundary (the second portion might not be an integral multiple of 8 octets, but the first must be).  Call the number of 8 octet blocks in the first portion NFB (for Number of Fragment Blocks).  The first portion of the data is placed in the first new internet datagram, and the total length field is set to the length of the first datagram.  The more-fragments flag is set to one.  The second portion of the data is placed in the second new internet datagram, and the total length field is set to the length of the second datagram.  The more-fragments flag carries the same value as the long datagram.  The fragment offset field of the second new internet datagram is set to the value of that field in the long datagram plus NFB.

This procedure can be generalized for an n-way split, rather than the two-way split described.

To assemble the fragments of an internet datagram, an internet protocol (IP) module (for example at a destination host) combines internet datagrams that all have the same value for the four fields: identification, source, destination, and protocol.  The combination is done by placing the data portion of each fragment in the relative position indicated by the fragment offset in that fragment’s internet header.  The first fragment will have the fragment offset zero, and the last fragment will have the more-fragments flag reset to zero.




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