IP Transport MechanismsThere are two types of IP Transport Mechanisms. These are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Both transport mechanisms are using different applications and they have different characteristics. TCP/IP is more reliable and guarantees packet delivery when delay is not an important factor. On the other hand, UDP does not guarantee packet delivery and re-transmissions. It is very important when delay is a key factor.IP Packet field figure 00000 shows different types of bit structure. Now I am going to explain the field of IP packet.

IP packet fields are defined as follows:

Version — 4 Bits. It describes the version being used whether its IP version 4 or 6.

IP header length (IHL) — 4 bits. It shows the datagram header length in 32-bit words.

Type of service — 8 bits. It can be used to assign the quality of service (QoS) levels to different packets as well as it tells how the above layer protocols need the data to be handled.

Total length — 16 bits. It describes the length of the complete IP packet. The length includes both the data and the header.

Identification — 16 bits. The current datagram is identified by the integer present in this field.

Flags — 3-bits. Two out of these three bits are used. One of the bits describes if the packet can be fragmented or not and the other bit describes if a packet is the last packet in the fragment or not.

Fragment Offset — 3 bits. It indicates the data which is fragmented or to be fragmented. The first fragment indicates Offset Zero.

Time to Live (TTL) — 8 bits. TTL avoids the packets from looping. It has a counter that gradually goes to zero and at one stage the packet is discarded based on this value.

Protocol — 8 bits. It shows the protocol in the above layer that will accept a receiving packet once the processing of IP is finished.

Header checksum — 16 bits. It is used to verify if the header is corrupted or not.

Source address — 32 bits. It shows the sending address from where the data is being sent by one of the two communicating parties.

Destination address — 32 bits. It shows the receiving address of the system which is going to receive the data sent by the sending device.

Options — it is used for security in various applications.

Padding— it ensures that the header is bounded into 32 bits and the padding is set to zero.

Data — it shows the data of an application and information about the protocols present in the upper layers.

Type of Service (ToS)

The header of the IP contains the Type of Service (ToS). It is an 8-bit field and it actually started being used with the introduction of Differentiated Services which is abbreviated as DiffServ. This field has got two parts named as precedence value and the ToS bits. Precedence value occupies the first three bits i.e. from 0 – 2 which is to provide priority queuing and the remaining bits i.e. from 3 – 7 specify the way a network should distinguish between throughput, delay, reliability and cost.

Service type can be seen from 8 of the 32 bits present in the IP header. These bits can be divided into 5 subfields. It can be seen from the figure above and a brief description about them mentioned below:

The bits from 0 – 2 are the precedence bits and they can have a value from 0 – 7. This value is represented in binary format and it represents how important a datagram is. The default value for these bits is 0 and the higher this value goes, the more important a datagram becomes.

Bits number 3, 4 and 5 represent D (requests low delay), T (requests high throughput) and R (requests high reliability) respectively.

www.linktionary.com/t/tos.html

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