What are the services provided to upper layers by transport layer?

In the field of computer networking, the purpose of the 4^th layer or the transport layer is to provide services for the end to end communication for the various operating applications. The services are provided within an architectural framework that consists of protocols and the components and is layered. It also offers convenient services such as the following:

Ø  Connection – oriented data stream support

Ø  Reliability

Ø  Flow control

Ø  Multiplexing and so on.

- Both the OSI (open systems interconnection) and TCP/ IP model include the transport layer. 

- The foundation of the internet is based up on the TCP/ IP model whereas for the general networking, the OSI model is followed. 

- However, the transport layer is defined differently in both of these models. Here we shall discuss about the transport layer in the TCP model since it is used for keeping the API (application programming interface) convenient to the internet hosts. 

- This is in contrast with the definition of the transport layer in the OSI model. 

- TCP (transmission control protocol) is the most widely used transport protocol and so the internet protocol suite has been named after it i.e., the TCP/ IP. 

- It is a connection-oriented transmission protocol and so it is quite complex. 

- This is also because it incorporates reliable data stream and transmission services in to its state-ful design. 

- Not only TCP there are other protocols in the same category such as the SCTP (stream control transmission protocol) and DCCP (datagram congestion control protocol).

Now let us see what all services are provided by the transport layer to its upper layers:

ØConnection-oriented communication: It is quite easy for the application for interpreting the connection as a data stream instead of having to cope up with the connectionless models that underlie it. For example, internet protocol (IP) and the UDP’s datagram protocol.

Ø Byte orientation: Processing the data stream is quite easy when compared with using the communication system format for processing the messages. Because of such simplification, it becomes possible for the applications to work up on message formats that underlie.

Ø  Same order delivery: Usually, it is not guaranteed by the transport layer that the data packets will be received in the same order in which they were sent. But this is one of the desired features of the transport layer. Segment numbering is used for incorporating this feature. The data packets are thus passed on to the receiver in order. Head of line blocking is a consequence of implementing this.

Ø  Reliability: During the transportation some data packets might be lost because of errors and problems such as network congestion. By using error detection mechanism such as CRC (cyclic redundancy check), the data might be checked by the transport protocol for any corruption and for the verification whether the correct reception of the data by either sending a NACK or an ACK signal to the sending host. Some schemes such as the ARR (automatic repeat request) are sometimes used for the retransmission of the corrupted or the lost data.

Ø  Flow control: The rate with which the data is transmitted between two nodes is managed for preventing a sending host with a fast speed from the transmission of data more than what the receiver’s data buffer can take at a time. Otherwise it might cause a buffer overrun.

Ø  Congestion avoidance: Traffic entry in to the network can be controlled by means of congestion control by avoiding congestive collapse. The network might be kept in a state of congestive collapse by automatic repeat requests. 

What are the advantages and disadvantages of datagram approach?

- Today’s packet switching networks make use of a basic transfer unit commonly known as the datagram. 

- In such packet switched networks, the order of the data packets arrival, time of arrival and delivery comes with no guarantee. 

- The first packet switching network to use the datagrams was CYCLADES. 

- Datagrams are known by different names at different levels of the OSI model. 

- For example, at layer 1 we call it Chip, at layer 2 it is called Frame or cell, data packet at layer 3 and data segment at layer 4. 

- The major characteristic of a datagram is that it is independent i.e., it does not rely on any other thing for the information required for exchange.

- The duration of a connection between any two points is not fixed such as in telephone conversations. 

- Virtual circuits are just the opposite of the datagrams. 

- Thus, a datagram can be called as a self containing entity. 

- It consists of information sufficient for routing it from the source to the destination without depending up on the exchanges made earlier. 

- Often, a comparison is drawn between the mail delivery service and the datagram service. 

- The user’s work is to just provide the address of the destination. 

- But he/she is not guaranteed the delivery of the datagram and if the datagram is successfully delivered, no confirmation is sent to the user. 

- The data gram are routed to some destination without help of a predetermined path. 

- The order in which the data has to be sent or received is given no consideration. 

- It is because of this that the datagrams belonging to a single group might travel over different routes before they reach their common destination. 

Advantages of Datagram Approach

1. Datagrams can contain the full destination address rather than using some number.
2. There is no set up phase required for the datagram circuits. This means that no resources are consumed.
3. If it happens during a transmission that one router goes down, the datagrams that will suffer will include only those routers which would have been queued up in that specific router. The other datagrams will not suffer.
4. If any fault or loss occurs on a communication line, the datagrams circuits are capable of compensating for it.
5. Datagrams play an important role in the balancing of the traffic in the subnet. This is so because halfway the router can be changed.

Disadvantages of Datagram Approach

1. Since the datagrams consist of the full destination address, they generate more overhead and thus lead to wastage of the bandwidth. This in turn makes using datagram approach quite costly.
2. A complicated procedure has to be followed for datagram circuits for determining the destination of the packet.
3. In a subnet using the datagram approach, it is very difficult to keep congestion problems at bay.
4. The any-to-any communication is one of the key disadvantages of the datagram subnets. This means that if a system can communicate with any device, any of the devices can communicate with this system. This can lead to various security issues.
5. Datagram subnets are prone to losing or re - sequencing the data packets during the transition. This puts a great burden on the end systems for monitoring, recovering, and reordering the packets as they were originally.
6. Datagram subnets have less capability of dealing with congestion control as well as flow control. This happens because the direction of the incoming traffic is not specified. In the virtual circuit subnets, the flow of the packets is directed only along the virtual circuits thus making it comparatively easy for controlling it.
7. The unpredictable nature of the flow of the traffic makes it difficult to design the datagram networks. 

What are data gram sub-nets?

- A data gram is defined as the basic transfer unit used in the networks that operate with the help of packet switching network. 

- In such networks, the time of the arrival and delivery is not guaranteed. 

- Also, the network services do not guarantee that whether it will be an ordered delivery or not. 

- The first project to use the data grams was the CYCLADES which was again a packet switching network. 

- The hosts in this network were responsible for making a reliable delivery rather than relying on the network for doing so. 

- This they did using the data grams that were themselves so unreliable and by associating the mechanisms of the end to end protocols. 

- According to Louis Pouzin, there are two sources from which came the inspiration for the data grams namely the Donal Davie’s studies and simplicity of the things. 

- The concept of the data gram sub-net was eventually adopted for the formulation of the protocols such as apple talk, Xerox network systems and of course the internet protocol.

- Data grams are used at the first 4 layers of the OSI model. 

- Each layer has its own name for the data grams as we mention below:

1. Layer 1: chip (CDMA)
2. Layer 2: frames (IEEE 802.3 and IEEE 802.11), cell (ATM)
3. Layer 3: data packet
4. Layer 4: data segment

- A data gram is a data packet that is self-reliant. 

- This means it does not rely on any of the exchanges made earlier since the fixed connection between the two points of communication has no connection such as in a majority of the telephonic conversations. 

- Virtual circuits and data gram sub-nets are two equally opposite things. 

Data gram is defined as an independent and self-contained data entity by the RFC 1594 that carries sufficient information required for routing from one source to another without relying on the transporting network and the earlier exchanges between the two same hosts.

- The services offered by the data gram sub nets can be compared to the mail delivery services. 

- This is so because the user needs to mention only the destination address.

- However, this service does not give any guarantee of whether the data gram will be delivered or not and also does not provide any confirmation upon successful delivery of the packet. 

- These are of course two major disadvantages of the data gram sub nets. 

- In data gram sub nets, the data grams or the data packets are routed along a route that is created at the same time. 

- In data gram sub nets the routes are not predetermined. 

- This again has its disadvantages. 

- Also, the order in which the data grams have to be sent or received is not considered. 

- In some cases, a number of data grams having same destination might travel along various different routes.

- There are two components of every data gram namely the header and the data payload.

- The former consists of all the information that is enough for the routing purpose from source to the destination without being dependent on the exchanges that were made before between the network and the equipment. 

- The source as well the destination address might be included in the header as a kind of a field. 

- The data that is to be transmitted is stored in the latter part of the data gram. 

- In some cases the data payloads might be nested in to the tagged header. 

- This process is commonly known as the encapsulation. 

- There are various types of data grams for which various standards are defined by the internet protocol or IP. 

What are network layer design issues?

- The network layer i.e., the third layer of the OSI model is responsible for facilitating the exchange of the individual information or data pieces between hosts over the network. 

- This exchange only takes place between the end devices that are identified. 

- For accomplishing this task, 4 processes are used by the network layer and these are:

Ø  Addressing

Ø  Encapsulation

Ø  Routing

Ø  Decapsulation

In this article we focus up on the design issues of the network layer. 

- For accomplishing this task, the network layer also need s to have knowledge about the communication subnet’s topology and select the appropriate routes through it. 

- Another thing that the network layer needs to take care of is to select only those routers that do not overload the other routers and the communication lines while leaving the other lines and router in an idle state.

Below mentioned are some of the major issues with the network layer design:

1. Services provided to the layer 4 i.e., the transport layer.
2. Implementation of the services that are connection oriented.
3. Store – and  - forward packet switching
4. Implementation of the services that are not connection oriented.
5. Comparison of the data-gram sub-nets and the virtual circuits.

- The sender host sends the packet to the router that is nearest to it either over a point-to-point carrier link or LAN. 

- The packet is stored until its complete arrival for the verification of the check sum. 

- Once verified, the packet is then transmitted to the next intermediate router. 

- This process continues till the packet has reached its destination. 

- This mechanism is termed as the store and forward packet switching.

The services that are provided to the transport layer are designed based up on the following goals:

1. They should be independent of the router technology.
2. Shielding from the type, number and topology of the routers must be provided to the transport layer.
3. The network addresses that are provided to the transport layer must exhibit a uniform numbering plan irrespective of whether it’s a LAN or a WAN.

Now based up on the type of services that are offered, there is a possibility for two different organizations.

Offered service is Connection-less: 

- The packets are individually introduced in to the sub-net and the routing of the packets is done independently of each other. 

- It does not require any advance set up. 

- The sub-net is referred to as the data gram sub-net and the packets are called data-grams.

Offered service is connection-oriented: 

- In this case the router between the source and the destination must be established prior to the beginning of the transmission of the packets. 

- Here, the connection is termed as the virtual circuit and subnet as the “virtual circuit subnet” or simply VC subnet.

- Choosing a new router every time is a thing to be avoided and this is the basic idea behind the use of the virtual circuits. 

- Whenever we establish a connection, a route has to be selected from source to destination. 

- This is counted as a part of the connection setup only. 

- This route is saved in the routers tables that are managed by the routers and is then used by the flowing traffic. 

- On the release of connection, the VC is automatically terminated. 

- In case of the connection oriented service, an identifier is contained in each packet which tells the virtual circuit to which it belongs.

- In data-gram sub-net circuit setup is not required whereas it is required in the VC circuit. 

- The state info is not held by the routers in the data gram subnet whereas router table space is required for each VC for each connection. 

What are the characteristics of network layer?

- The network layer comes at number three in the OSI model of networking. 

- The duty of this layer is to forward and route the packets via the intermediate routers. 

- It comes with functional as well as procedural means for the transfer of data sequences with variable length from a source host to a destination host and across one or more networks. 

- During the transfer it also takes the responsibility for the maintenance of the services functions’ quality. 

There are many other functions of this layer such as:

Ø Connection-less communication: In IP, a datagram can be transmitted from one host to another without any need for the receiving host to send an acknowledgement. Protocols that are connection oriented are used on the higher levels of the OSI model.

Ø  Host addressing: Every host in the network is assigned a unique address that determines its location. A hierarchical system is what that assigns this address. These are the addresses that are known as the IP (internet protocol) addresses.

Ø  Message forwarding: The networks are sometimes divided in to a number of sub – networks which are then connected to other networks for facilitating wide – area communication. Here specialized hosts called routers or gateways are used for forwarding the packets from one host to another.

Characteristics of Network Layer

Encapsulation:

- One of the characteristics of the network layer is encapsulation. 

- Network layer ought to provide encapsulation facilities. 

- It is necessary that the devices must be identified with the addresses. 

- Not only the devices but the network layer PDUs must be assigned such addresses. 

- The layer 4 PDU is supplied to the layer 3 during the process of encapsulation. 

- For creating the layer 3 PDU, a layer 3 label or header is added to it. 

- In reference to the network layer, this PDU thus created is referred to as a packet. 

- On creation of a packet, the address of the receiving host is included in the header. 

- This address is commonly known as the destination address. 

- Apart from this address the address of the source or the sender host is also stored in the header. 

- This address is termed as the source address. 

- Once the encapsulation process is complete, the layer 3 sends this packet to the data link layer for preparing it to be transmitted over the communication media.

Routing: 

- The services provided by the network layer for directing the packets to the destination addresses define this characteristic. 

- It is not necessary that the destination and the source hosts must always be connected to the same network.

- In actual, the packet might have to go through a number of networks before reaching the destination. 

- During this journey the packet has to be guided to reach the proper address. - This is where the routers come in to action. 

- They help in selecting the paths for guiding the packets to the destination. 

- This is called routing. 

- During the course of routing of the packet, it may need to traverse a number of devices.

- We call the route taken by the packet to reach one intermediate device as “hop”. 

- The contents of the packet remain intact until the destination host has been reached.

De-capsulation: 

- On the arrival of the packet at the destination address, it is sent for processing at the third layer. 

- The destination address is examined by the host system for verifying whether the packet is meant for itself or not. 

- If the address is found to be correct, the decapsulation process is carried out at the network layer. 

- This layer passes the layer 4 PDU to the transport layer for appropriate servicing. 

Sliding Window Protocols? – Part 1

- There are many types of data transmission protocols of which one type is the packet based data transmission protocols. 

- These protocols have a feature called the sliding window protocol.

- The sliding window protocols are a great help wherever the in-order delivery of the data packets demand reliability. 

- For example, the Data link layer of the TCP (transmission control protocol) model and OSI model demand such reliability and thus use window sliding protocol. 

- According to the concept of the sliding window protocols, a consecutive number which is unique is assigned to each and every portion of the transmission i.e., the packets.

- These numbers are used by the receiver for placing the packets it will receive in their correct order. 

- Also, with the help of these numbers, the missing packets can be identified and the duplicate packets can be removed. 

- One problem regarding the sliding window protocols is that it has kept no limits for the size of these numbers that are required. 

- An unlimited number of data packets can be allowed to be communicated at any instant of time if limits are placed on the number of packets involved in transmission or reception. 

- By this, we mean using the sequence numbers of fixed size. 

- By term window we refer to the transmission side. 

- It actually represents the logical boundary or limit of the number of packets that the receiver has to acknowledge. 

- The transmitter has to be informed by the receiver for each ACK (acknowledgement) packet regarding the maximum size or the window boundary of the current receiver buffer. 

- For reporting the window size of the received buffer, a 16 bit field is used in the TCP header. 

- The maximum limit or boundary of the window that we can have is 2¹⁶ i.e., 64 KB. 

- When operating in the slow start mode, the counting of the transmitter begins with a low packet count.

- Gradually, the number of packets involved increases in every transmission after the ACK packet has been received. 

- Whenever it receives an ACK packet, the window slides logically by one packet for the transmission of a new packet. 

- On reaching the window threshold, one packet is sent by the transmitter for every one packet of ACK received. 

- Suppose the limit of the window is 10 packets and the transmitter is in slow start mode. 

- Then, first one packet will be transmitted followed by another two. 

- Between these two transmissions, it will send an ACK packet also. 

- This process will continue until the limit of 10 has reached. 

- After crossing the limit, the transmission is restricted to one i.e., for every ACK packet received only one data packet is transmitted. 

- When viewed during simulation, it seems as if the window is shifting by distance of one packet whenever an ACK packet is received. 

- For avoiding the traffic congestion, the sliding window protocol works up a great deal.

- In this way the application layer would not have to worry about transmission the next set of data packets. 

- It can continue to do so since the sliding windows of the packet buffer will be implemented on both the sides i.e., the receiver’s and the sender’s side by the TCP. 

-However, the network traffic influences the window size dynamically to a great extent. 

- In order to achieve the highest possible throughput, care should be taken for not forcing the transmitter to stop the transmission before one RTT or round trip delay time by the sliding window protocol. 

- The bandwidth delay product of the links in the communication should be less than the limit of the data amount that can be sent before sending ACK packet. - If this condition is not met, the links’ effective bandwidth will be limited by the protocol. 

Explain the concept of piggybacking?

- Piggybacking is a well known technique used in the transmission of data in the third layer of the OSI model i.e., the network layer. 

- It is employed in making a majority of the frames that are transmitted from receiver to the emitter. 

- It adds to the data frame, the confirmation that the sender sent on successful delivery of data frame. 

- This confirmation is called the ACK or acknowledge signal. 

- Practically, this ACK signal is piggybacked on the data frame rather than sending it individually by some other means. 

Principle behind Piggybacking

- The piggybacking technique should not be confused with the sliding window protocols that are also employed in the OSI model. 

- In piggybacking, an additional field for the ACK or the acknowledgement signal is incorporated in to the data frame itself. 

- There is only a difference of bit between the sliding window protocol and piggybacking.

- Whenever some data has to be sent from party to another, the data will be sent along with the field for ACK. 

The piggybacking data transfer is governed by the following three rules:

Ø  If both the data as well as the acknowledgement have to be sent by the party A, it has to include both the fields in the same frame.

Ø  If only the acknowledgement has to be sent by the party A, then it will have use a separate frame i.e., an ACK for that.

Ø  If only the data has to be by the party A, then the ACK field will be included within the data frame and thus transmitted along with it. This duplicate ACK frame is simply ignored by the receiving party B.

- The only advantage of using this technique is that it helps in improving efficiency. 

- The disadvantage is that is the service can be blocked or jammed by the receiving party if there is no data to be transmitted. 

- Enabling a receiver timeout by means of a counter the moment when the party receives the data frame can solve this problem to a great extent. 

- An ACK control frame will be sent by the receiver if the timeout occurs and still there is no data for transfer. 

- A counter called the emitter timeout is also set up by the sender which if ends without getting any confirmation from the receiver will make the sender assume that the data packet got lost in the way and therefore will have to re-transmitted.

- Piggybacking is also used in accessing the internet.

- It is used in establishment of a wireless internet connection by means of wireless internet access service of the subscriber without taking explicit permission from the subscriber. 

- However, according to the various jurisdiction laws around the world, this practice is under ethical and legal controversy. 

- In some places it is completely regulated or outlawed while at other places it is allowed.  

- A business customer who provides services related to hotspots, as of cafe and hotels, cannot be thought of using piggybacking technique via non – customers. - A number of such locations provide services for a fee. 

What are the reasons for using layered protocols?

Layered protocols are typically used in the field of networking technology. There are two main reasons for using the layered protocols and these are:

1. Specialization and
2. Abstraction

- A neutral standard is created by a protocol which can be used by the rival companies for creating programs that are compatible. 

- So many protocols are required in the field and that should also be organized properly and these protocols have to be directed to the specialists that can work up on these protocols. 

- A network program can be created using the layered protocols by a software house if the guidelines of one layer are known. 

- The services of the lower level protocols can be provided by the companies. 

- This helps them to specialize. 

- In abstraction, it is assumed that another protocol will provide the lower services. 

- A conceptual framework is provided by the layered protocol architecture that divides the complex task of information exchange into much simpler tasks between the hosts. 

- The responsibility for each of the protocols is narrowly defined. 

- A protocol provides an interface for the successive higher layer protocol. 

- As a result of this, it goes in to hiding the details of the higher protocol layers that underlies. 

- The advantage of using the layered protocols is that the same application i.e., the user level program can be used by a number of diverse communication networks.

- For example, when you are connected to a dial up line or internet via LAN you can use the same browser. 

- For simplifying the networking designs, one of the most common techniques used is the protocol layering. 

- The networking designs are divided in to various functional layers and the protocols are assigned for carrying out the tasks of each layer. 

- It is quite common to keep the functions of the data delivery separate from each other and separate layers for the connection management too.  

- Therefore, we have one protocol for performing the data delivery tasks and second one for performing connection management. 

- The second one is layered up on the first one. 

- Since the connection management protocol is not concerned with the data delivery, it is also quite simple. 

- The OSI seven layer model and the DoD model are one of the most important layered protocols ever designed. 

- A fusion of both the models is represented by the modern internet. 

- Simple protocols are produced by the protocol layering with some well defined tasks. 

- These protocols then can be put together to be used as a new whole protocol. - As required for some particular applications, the individual protocols can be either replaced or removed. 

- Networking is such a field involving programmers, electricians, mathematicians, designers, electricians and so on. 

- People from these various fields have very less in common and it is because of the layering that people with such varying skills to make an assumption or feel like others are carrying out their duty. 

- This is what we call abstraction. 

- Protocols at a level can be followed by an application programmer via abstraction assuming that network exists and similarly electricians assume and do their work. 

- One layer can provide services to the succeeding layer and can get services in return too. 

- Abstraction is thus the fundamental foundation for layering. 

- Stack has been used for representing the networking protocols since the start of network engineering. 

- Without stack, it would be unmanageable as well as overwhelming. 

- Representing the layers of specialization for the first protocols derived from TCP/ IP.

Data Link Layer - Layer 2 of OSI model

The Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking.
At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization.

The data link layer performs various functions depending upon the hardware protocol used, but has four primary functions:

- Communication with the Network layer above.
- Communication with the Physical layer below.
- Segmentation of upper layer datagrams (also called packets) into frames in sizes that can be handled by the communications hardware.
- The data link layer organizes the pattern of data bits into frames before transmission. The frame formatting issues such as stop and start bits, bit order, parity and other functions are handled here.
- It provides error checking by adding a CRC to the frame, and flow control.
- The data link layer is also responsible for logical link control, media access control, hardware addressing, error detection and handling and defining physical layer standards.
- The data link layer is divided into two sublayers: the media access control (MAC) layer and the logical link control (LLC) layer. The former controls how computers on the network gain access to the data and obtain permission to transmit it; the latter controls packet synchronization, flow control and error checking.
- The data link layer is where most LAN (local area network) and wireless LAN technologies are defined. Technologies and protocols used with this layer are Ethernet, Token Ring, FDDI, ATM, SLIP, PPP, HDLC, and ADCCP.
- The data link layer is often implemented in software as a driver for a network interface card (NIC). Because the data link and physical layers are so closely related, many types of hardware are also associated with the data link layer.
- Data link layer processing is faster than network layer processing because less analysis of the packet is required.
- The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of any various network devices to the physical medium.

Physical Layer - Layer 1 of OSI model

- The physical layer is level one in the seven level OSI model. Actually, it is last layer that receives and processes the data from the sending device while it is first layer to receive the data at the destination end.
- It performs services requested by the data link layer.
- The Physical Layer defines the Mechanical, Electrical, Procedural and Functional specifications for activating, maintaining and deactivating the physical link between communication network systems.
- The basic objective of this layer is to transform the data in the form that is needed to be carried through the transmission media over the network. The transmission media either bounded or unbounded , carries the data in the form of electromagnetic waves or radio waves.
- The Physical Layer is responsible for bit-level transmission between network nodes.
The main functions of physical layer are :

- Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer. Devices used in the Physical Layer are
* Network Interface Cards (NIC)
* Transceivers
* Repeaters
* Hubs
* Multi Station Access Units (MAU’s)
- Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
- Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it.
- Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology. There are four possible kinds of topologies:
* Bus
* Star
* Ring
* Mesh
In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network.

How to support a reliable communication in transport layer ?

At the Transport layer, each particular set of pieces flowing between a source application and a destination application is known as a conversation.To identify each segment of data, the Transport layer adds to the piece a header containing binary data. This header contains fields of bits. It is the values in these fields that enable different Transport layer protocols to perform different functions.

Reliability means ensuring that each piece of data that the source sends arrives at the destination. At the Transport layer the three basic operations of reliability are:
- tracking transmitted data.
- acknowledging received data.
- retransmitting any unacknowledged data.

This requires the processes of Transport layer of the source to keep track of all the data pieces of each conversation and the retransmit any of data that did were not acknowledged by the destination. The Transport layer of the receiving host must also track the data as it is received and acknowledge the receipt of the data. These reliability processes place additional overhead on the network resources due to the acknowledgement, tracking, and retransmission. To support these reliability operations, more control data is exchanged between the sending and receiving hosts. This control information is contained in the Layer 4 header.

Determining the Need for Reliability
Applications, such as databases, web pages, and e-mail, require that all of the sent data arrive at the destination in its original condition, in order for the data to be useful. Any missing data could cause a corrupt communication that is either incomplete or unreadable. Therefore, these applications are designed to use a Transport layer protocol that implements reliability.