Internet Protocol IP

The Internet Protocol suite is a Network layer protocol that involves logical addressing. The Internet Protocol is a connectionless protocol that defines a network portion and a host portion, like any layer 3 protocol. An IP address is 32 bits in length, and the subnet mask is used to identify the network and host portion. Typically, an IP client might be a PC or router. An example of an IP address and a subnet mask is:

131.108.1.1 255.255.255.0

Figure 2.7 outlines the frame format on an IP packet, and it details where the IP

source and destination address is carried in the payload packet.

Note: See Chapter 5 for a comprehensive description of the IP protocol.

The following provides a summary of each field in Figure 2.7:

► Version—Tells the IP version, which is currently set to 4.

► IP Header Length (IPH)—Indicates the length of the IP header.

► TypeOfService(TOS)—Provides various options to tell higher layers how the packet is to be handled (8 bits). Parameters, such as precedence, delay throughput, and reliability are identified in this section.

► Total Length—Indicates the length of the IP header and data.

► Identification—Enables IP packets to be reassembled in the correct order.

► Flags—Identifies the three bits that indicate how an IP packet is to be reassembled. One bit is used to allow fragmentation, and another indicates whether the packet is the last packet.

► Fragment Offset—Indicates where a fragment should be when the packet is reassembled.

32 bits

*.-

-►

Version (4 bits)

IP Header Length (4 bits)

Type Of Service (8 bits)

Total Length (16 bits)

Identification (16 bits)

Flags (3 bits)

Fragment Offset (13 bits)

Time To Live (8 bits)

Protocol

Header Checksum

(8 bits)

(16 bits)

Source Address

(32 bits)

Destination Address

(32 bits)

Options (plus any padding)

Data

(Variable Length)

Figure 2.7 IP frame format.

Figure 2.7 IP frame format.

► Time To Live—Sets the upper limit number of how many routers a packet can pass through. Typically transmission between routers will reduce this value by one. The maximum value of this field is 255.

► Protocol—Identifies to the higher layer what protocols are contained. Some common protocol numbers are:

► Header Checksum—Indicates a checksum on the header only. Data is not checked for errors.

► Source Address—Specifies the sender's IP address.

► Destination Address—Specifies the destination IP address.

► Options(plus any padding)—Identifies any options requested by the TOS field.

As you can see in Figure 2.7, an IP packet uses a number of fields to send data from one host to another. Sending and receiving hosts can be any IP enabled device, such as a Windows PC or a Unix station. At a minimum, you should know that:

► The IP frame format is connectionless.

► The checksum is only calculated on the IP header.

IP Fragmentation

After IP packets are sent, they traverse the network via some form of routing protocol. Furthermore, IP packets might need to cross hundreds of routers from various vendors to reach their destination, such as packets traveling on the Internet. As the packet travels, each intermediate device will have its own characteristics that will affect how the IP packet is handled. For example, the packets might be too large for some routers, which means that a packet might be fragmented or broken up into smaller packets so it can be handled correctly. When the packet reaches its remote destination, the router at the remote destination must correctly reassemble the fragments; otherwise, the data will be corrupted. For the most part, the Internet Protocol (IP) handles fragmentation and packet reassembly as long as the packets arrive at the destination in the same order in which they were fragmented.

The three fields in the IP header—Identification, Flags, and Fragment Offset— handle the fragmentation and reassembly of IP packets:

► Identification field—Sets the identity of the IP packet. Usually increments by one as each packet is sent.

► Flags field—Indicates whether the packet can be fragmented and if the packet is the last packet in a series of packets.

► Fragment Offset field—Indicates how many bytes are in the current packet. If any single packet has been lost, the entire payload must be retransmitted.

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