CN
Unit - 5Transport Layer 5.1 Transport Layer Protocols and ServicesThe transport layer is represented by two protocols: TCP and UDP. The IP protocol in the network layer delivers a datagram from a source host to the destination host. Nowadays, the operating system supports multiuser and multiprocessing environments; an executing program is called a process. When a host sends a message to other host means that source process is sending a process to a destination process. The transport layer protocols define some connections to individual ports known as protocol ports. An IP protocol is a host-to-host protocol used to deliver a packet from source host to the destination host while transport layer protocols are port-to-port protocols that work on the top of the IP protocols to deliver the packet from the originating port to the IP services, and from IP services to the destination port. Each port is defined by a positive integer address, and it is of 16 bits. 
UDPUDP stands for User Datagram Protocol. UDP is a simple protocol and it provides no sequenced transport functionality. UDP is a connectionless protocol. This type of protocol is used when reliability and security are less important than speed and size. UDP is an end-to-end transport level protocol that adds transport-level addresses, checksum error control, and length information to the data from the upper layer. The packet produced by the UDP protocol is known as a user datagram. User Datagram FormatThe user datagram has a 16-byte header which is shown below:
Where,Source port address: It defines the address of the application process that has delivered a message. The source port address is of 16 bits address. Destination port address: It defines the address of the application process that will receive the message. The destination port address is of a 16-bit address. Total length: It defines the total length of the user datagram in bytes. It is a 16-bit field. Checksum: The checksum is a 16-bit field which is used in error detection. Disadvantages of UDP protocolUDP provides basic functions needed for the end-to-end delivery of a transmission. It does not provide any sequencing or reordering functions and does not specify the damaged packet when reporting an error. UDP can discover that an error has occurred, but it does not specify which packet has been lost as it does not contain an ID or sequencing number of a particular data segment. TCPTCP stands for Transmission Control Protocol. It provides full transport layer services to applications. It is a connection-oriented protocol means the connection established between both the ends of the transmission. For creating the connection, TCP generates a virtual circuit between sender and receiver for the duration of a transmission. Features Of TCP protocolStream data transfer: TCP protocol transfers the data in the form of contiguous stream of bytes. TCP group the bytes in the form of TCP segments and then passed it to the IP layer for transmission to the destination. TCP itself segments the data and forward to the IP. Reliability: TCP assigns a sequence number to each byte transmitted and expects a positive acknowledgement from the receiving TCP. If ACK is not received within a timeout interval, then the data is retransmitted to the destination.
The receiving TCP uses the sequence number to reassemble the segments if they arrive out of order or to eliminate the duplicate segments. Flow Control: When receiving TCP sends an acknowledgement back to the sender indicating the number the bytes it can receive without overflowing its internal buffer. The number of bytes is sent in ACK in the form of the highest sequence number that it can receive without any problem. This mechanism is also referred to as a window mechanism. Multiplexing: Multiplexing is a process of accepting the data from different applications and forwarding to the different applications on different computers. At the receiving end, the data is forwarded to the correct application. This process is known as demultiplexing. TCP transmits the packet to the correct application by using the logical channels known as ports. Logical Connections: The combination of sockets, sequence numbers, and window sizes, is called a logical connection. Each connection is identified by the pair of sockets used by sending and receiving processes. Full Duplex: TCP provides Full Duplex service, i.e., the data flow in both the directions at the same time. To achieve Full Duplex service, each TCP should have sending and receiving buffers so that the segments can flow in both the directions. TCP is a connection-oriented protocol. Suppose the process A wants to send and receive the data from process B. The following steps occur: TCP Segment Format
Where,Source port address: It is used to define the address of the application program in a source computer. It is a 16-bit field. Destination port address: It is used to define the address of the application program in a destination computer. It is a 16-bit field. Sequence number: A stream of data is divided into two or more TCP segments. The 32-bit sequence number field represents the position of the data in an original data stream. Acknowledgement number: A 32-field acknowledgement number acknowledge the data from other communicating devices. If ACK field is set to 1, then it specifies the sequence number that the receiver is expecting to receive. Header Length (HLEN): It specifies the size of the TCP header in 32-bit words. The minimum size of the header is 5 words, and the maximum size of the header is 15 words. Therefore, the maximum size of the TCP header is 60 bytes, and the minimum size of the TCP header is 20 bytes. Reserved: It is a six-bit field which is reserved for future use. Control bits: Each bit of a control field functions individually and independently. A control bit defines the use of a segment or serves as a validity check for other fields. There are total six types of flags in control field:URG: The URG field indicates that the data in a segment is urgent. ACK: When ACK field is set, then it validates the acknowledgement number. PSH: The PSH field is used to inform the sender that higher throughput is needed so if possible, data must be pushed with higher throughput. RST: The reset bit is used to reset the TCP connection when there is any confusion occurs in the sequence numbers. SYN: The SYN field is used to synchronize the sequence numbers in three types of segments: connection request, connection confirmation ( with the ACK bit set ), and confirmation acknowledgement. FIN: The FIN field is used to inform the receiving TCP module that the sender has finished sending data. It is used in connection termination in three types of segments: termination request, termination confirmation, and acknowledgement of termination confirmation. Differences b/w TCP & UDPThe User Datagram Protocol (UDP) is called as a connectionless, unreliable protocol. UDP has a very limited error checking capability. It is a very simple protocol and it can be used with minimum overhead. The UDP can be used, when process needs to send a small message without any issue of reliability. UDP takes less time as compared to TCP (Transmission Protocol) or SCTP (Stream Control Transmission Protocol). Some Standard Ports for UDP are listed below:
The important fields of user datagrams are:
1. Source Port NumberIt is used by the process, which is running on the source host. Source Port Number is 16 bits long If the source host is a client and sends a request, the port number is unknown, which is requested by the process and then accepted by the UDP software. If the source host is a server (server sending a reply), the port number is known. 2. Destination Port NumberIt is used by the process, which is running on the destination host as well as source host. If the destination host is a server (server sending request), the port number is known. If the destination host is a client (client sending reply), the port number is unknown.3. LengthThis is the total length of the user datagram and it is 16 bit (0 to 65535 bytes) long. A UDP user datagram can be stored in an IP datagram with a length of 65535 bytes. Total UDP length = IP length – IP headers length. 4. Checksum
Checksum is used to detect the errors in the user datagram.
Features of UDPSome features of UDP are as stated below:
1.Connectionless service
UDP provides connectionless service. Each user datagram sent by the UDP is an independent datagram. There is no relationship between the different user datagrams even if they are coming from the same source and going to the same destination source.
Transmission Control protocol (TCP)TCP is connection oriented protocol and process- to- process protocol. TCP uses flow and error control mechanism at the transport layer, hence TCP is reliable transport protocol. In TCP, a segment carries a data and control information. Connection EstablishmentTCP transmits data in both the directions ( full duplex mode). When two TCPs are connected to each other, each TCP needs to initialize the communication (SYN) and approval(ACK) from each end to send the data. Three-Way Handshaking
The Three - Way Handshaking protocol is used to establish connection between two TCPs.
1. A client sends a SYN data packet to server. The purpose of this step is to see if the server is open for new connection.
2. The server needs to keep all ports open to establish a new connection. When the server receives the SYN packet from the client, the client replies and returns the conformation SYN/ACK packet.
3. The client receives the SYN/ACK packet and replies with ACK packet to establish the connection.
5.3 TCP Flow ControlTCP provides flow- control mechanism facility. A receiver can control the data which is sent by the user. This action is taken to prevent the data crash at the receiver's end. TCP uses byte-oriented flow control, according to numbering system.TCP Congestion Control Congestion occurs, if the load offered to any network is more than its capability. Once the connection is established, the sender can initialize the congestion window size (according to the available buffer space in the receiver). The sender's window size can be determined by the receiver and congestion in the network. The actual size of the window can be minimum of the receiver-advertised window size and the congestion window size. Generally, the methods followed by TCP to handle the congestion consists of three phases:
1. Slow start algorithm is used to control the congestion in TCP. In this algorithm, the size of window grows exponentially till the timeout occurs or the receiver's window is reached to the maximum threshold (64 kb = 65535 bytes).
Example:
Suppose that, the sender starts with congestion window = 20 = 1 MSS (Maximum Segment Size, each segment consists of 1byte). After receiving acknowledgement for segment 1, the size of window can be increased by one. Thus, the total size of congestion window = 21 = 2. When all the segments are acknowledged, the total size of congestion window = 23 = 8.
2. Congestion avoidance algorithmIn slow start algorithm, the size of the congestion window increases exponentially. To avoid the congestion before it happens, it is necessary to slow down the exponential growth. The Congestion avoidance algorithm works with an additive increase instead of the exponential growth. In this algorithm, after receiving each acknowledgement receipt (for one round), the size of the congestion window can be increased by one.
Example
Start with, Congestion window = 1
By adding 1, congestion window = 1 + 1 = 2 3. Congestion detection algorithmIf the congestion occurs, it is required to decrease the congestion window size. There are two conditions in which congestion may occur: the connection time out and the reception of three acknowledgements. In both cases, the threshold size can be dropped to one half of the previous window size. ExampleAssume that the window size = 64 and threshold is set to 32. In slow start algorithm, the window size starts from window size = 1, and then grows exponentially. Thereafter, when it reaches to threshold value, the congestion avoidance method allows the window size to increase linearly ( by adding 1). However, the connection time-out occurs and the window size = 36. At this phase, the multiplicative decrease method is applied to the window size 36/2 = 18 (new threshold). Again, TCP starts working with slow start algorithm and the transmission is carried out. Wireless TCPThe wireless transmission links are unreliable and can lose the packets many times. The solution to deal with this problem is to send them again. The path from sender to receiver can be heterogeneous, it may be wired or wireless network. When the path is unknown, it is very difficult to make the correct decision, if the packets get lost. Therefore, the wireless TCP can be split into two separate connections. In first connection, the packets move from sender to the base station and the mobile receiver receives the data from the base station through transmitting antenna. The base station can receive the data from the mobile sender through the receiving antenna. This method can solve the time out problem. IntroductionSCTP Stream Control Transmission Protocol (SCTP) is a reliable, message-oriented transport layer protocol.SCTP has mixed features of TCP and UDP.SCTP maintains the message boundaries and detects the lost data, duplicate data as well as out-of-order data.SCTP provides the Congestion control as well as Flow control.SCTP is especially designed for internet applications.SCTP ServicesSome important services provided by SCTP are as stated below:
1.Process-to-Process communication
SCTP uses all important ports of TCP.
2.Multi-Stream Facility
SCTP provides multi-stream service to each connection, called as association. If one stream gets blocked, then the other stream can deliver the data.
3.Full-Duplex Communication
SCTP provides full-duplex service ( the data can flow in both directions at the same time).
4. Connection- Oriented Service
The SCTP is a connection oriented protocol, just like TCP with the only difference that, it is called association in SCTP. If User1 wants to send and receive message from user2, the steps are :
Step1: The two SCTPs establish the connection with each other.
Step2: Once the connection is established, the data gets exchanged in both the directions.
Step3: Finally, the association is terminated.
5. Reliability
SCTP uses an acknowledgement mechanism to check the arrival of data.Features of SCTPSome important features of SCTP are as stated below:
1. Transmission Sequence Number (TSN)
The unit of data in SCTP is a data chunk. Data transfer in SCTP is controlled by numbering the data chunks. In SCTP, TSN is used to assign the numbers to different data chunks.
2. Stream Identifier (SI)
The SI is a 16 bit number and starts with 0. In SI, there are several streams in each association and it is needed to identify them. Each data chunk needs to carry the SI in the header, so that it is properly placed in its stream on arrival.
3. Packets
In SCTP, the data is carried out in the form of data chunks and control information is carried as control chunks. Data chunks and control chunks are packed together in the packet.
4. Multihoming
Multihoming allows both ends (sender and receiver) to define multiple IP addresses for communication. But, only one of these can be defined as primary address and the remaining can be used as alternative addresses. 5.4 Quality of services: Dataflow characteristics, Flow ControlQoS is an overall performance measure of the computer network.
Important flow characteristics of the QoS are given below:
1. Reliability
If a packet gets lost or acknowledgement is not received (at sender), the re-transmission of data will be needed. This decreases the reliability.
The importance of the reliability can differ according to the application.
For example:
E- mail and file transfer need to have a reliable transmission as compared to that of an audio conferencing.
2. Delay
Delay of a message from source to destination is a very important characteristic. However, delay can be tolerated differently by the different applications.
For example:
The time delay cannot be tolerated in audio conferencing (needs a minimum time delay), while the time delay in the e-mail or file transfer has less importance.
3. Jitter
The jitter is the variation in the packet delay.
If the difference between delays is large, then it is called as high jitter. On the contrary, if the difference between delays is small, it is known as low jitter.
Example:
Case1: If 3 packets are sent at times 0, 1, 2 and received at 10, 11, 12. Here, the delay is same for all packets and it is acceptable for the telephonic conversation.
Case2: If 3 packets 0, 1, 2 are sent and received at 31, 34, 39, so the delay is different for all packets. In this case, the time delay is not acceptable for the telephonic conversation.
4. Bandwidth
Different applications need the different bandwidth.
For example:
Video conferencing needs more bandwidth in comparison to that of sending an e-mail.
UDPWhere,The receiving TCP uses the sequence number to reassemble the segments if they arrive out of order or to eliminate the duplicate segments.
- Establish a connection between two TCPs.
- Data is exchanged in both the directions.
- The Connection is terminated.
Where,- Window Size: The window is a 16-bit field that defines the size of the window.
- Checksum: The checksum is a 16-bit field used in error detection.
- Urgent pointer: If URG flag is set to 1, then this 16-bit field is an offset from the sequence number indicating that it is a last urgent data byte.
- Options and padding: It defines the optional fields that convey the additional information to the receiver.
Basis for Comparison
TCP
UDP
Definition
TCP establishes a virtual circuit before transmitting the data.
UDP transmits the data directly to the destination computer without verifying whether the receiver is ready to receive or not.
Connection Type
It is a Connection-Oriented protocol
It is a Connectionless protocol
Speed
Slow
High
Reliability
It is a reliable protocol.
It is an unreliable protocol.
Header size
20 bytes
8 bytes
acknowledgement
It waits for the acknowledgement of data and has the ability to resend the lost packets.
It neither takes the acknowledgement, nor it retransmits the damaged frame.
Port No
Protocol
Description
7
Echo
Resends a received datagram back to the sender.
9
Discard
Discards a received datagram.
11
Users
Shows active users.
13
Daytime
Gives the date and time.
17
Quote
Gives a Quote of the day.
19
Chargen
Gives a string of characters.
53
Nameserver
Shows Domain Name Service.
67
BOOTPs
Server port to download bootstrap information.
68
BOOTCPc
Client port to download bootstrap information.
69
TFTP
Trivial File Transfer Protocol.
111
RPC
Remote Procedure Call.
123
NTP
Network Time Protocol.
161
SNMP
Simple Network Management Protocol.
162
SNMP
Simple Network Management Protocol. (trap)
The important fields of user datagrams are:
1. Source Port Number
Checksum is used to detect the errors in the user datagram.
Features of UDPSome features of UDP are as stated below:1.Connectionless service
UDP provides connectionless service. Each user datagram sent by the UDP is an independent datagram. There is no relationship between the different user datagrams even if they are coming from the same source and going to the same destination source.
Transmission Control protocol (TCP)
The Three - Way Handshaking protocol is used to establish connection between two TCPs.
1. A client sends a SYN data packet to server. The purpose of this step is to see if the server is open for new connection.
2. The server needs to keep all ports open to establish a new connection. When the server receives the SYN packet from the client, the client replies and returns the conformation SYN/ACK packet.
3. The client receives the SYN/ACK packet and replies with ACK packet to establish the connection.
5.3 TCP Flow ControlTCP provides flow- control mechanism facility. A receiver can control the data which is sent by the user. This action is taken to prevent the data crash at the receiver's end. TCP uses byte-oriented flow control, according to numbering system.TCP Congestion Control 1. Slow start algorithm is used to control the congestion in TCP. In this algorithm, the size of window grows exponentially till the timeout occurs or the receiver's window is reached to the maximum threshold (64 kb = 65535 bytes).
Example:
Suppose that, the sender starts with congestion window = 20 = 1 MSS (Maximum Segment Size, each segment consists of 1byte). After receiving acknowledgement for segment 1, the size of window can be increased by one. Thus, the total size of congestion window = 21 = 2. When all the segments are acknowledged, the total size of congestion window = 23 = 8.
2. Congestion avoidance algorithm
Example
Start with, Congestion window = 1
By adding 1, congestion window = 1 + 1 = 2
1.Process-to-Process communication
SCTP uses all important ports of TCP.
2.Multi-Stream Facility
SCTP provides multi-stream service to each connection, called as association. If one stream gets blocked, then the other stream can deliver the data.
3.Full-Duplex Communication
SCTP provides full-duplex service ( the data can flow in both directions at the same time).
4. Connection- Oriented Service
The SCTP is a connection oriented protocol, just like TCP with the only difference that, it is called association in SCTP. If User1 wants to send and receive message from user2, the steps are :
Step1: The two SCTPs establish the connection with each other.
Step2: Once the connection is established, the data gets exchanged in both the directions.
Step3: Finally, the association is terminated.
5. Reliability
SCTP uses an acknowledgement mechanism to check the arrival of data.Features of SCTPSome important features of SCTP are as stated below:
1. Transmission Sequence Number (TSN)
The unit of data in SCTP is a data chunk. Data transfer in SCTP is controlled by numbering the data chunks. In SCTP, TSN is used to assign the numbers to different data chunks.
2. Stream Identifier (SI)
The SI is a 16 bit number and starts with 0. In SI, there are several streams in each association and it is needed to identify them. Each data chunk needs to carry the SI in the header, so that it is properly placed in its stream on arrival.
3. Packets
In SCTP, the data is carried out in the form of data chunks and control information is carried as control chunks. Data chunks and control chunks are packed together in the packet.
4. Multihoming
Multihoming allows both ends (sender and receiver) to define multiple IP addresses for communication. But, only one of these can be defined as primary address and the remaining can be used as alternative addresses. 5.4 Quality of services: Dataflow characteristics, Flow ControlQoS is an overall performance measure of the computer network.
Important flow characteristics of the QoS are given below:
1. Reliability
If a packet gets lost or acknowledgement is not received (at sender), the re-transmission of data will be needed. This decreases the reliability.
The importance of the reliability can differ according to the application.
For example:
E- mail and file transfer need to have a reliable transmission as compared to that of an audio conferencing.
2. Delay
Delay of a message from source to destination is a very important characteristic. However, delay can be tolerated differently by the different applications.
For example:
The time delay cannot be tolerated in audio conferencing (needs a minimum time delay), while the time delay in the e-mail or file transfer has less importance.
3. Jitter
The jitter is the variation in the packet delay.
If the difference between delays is large, then it is called as high jitter. On the contrary, if the difference between delays is small, it is known as low jitter.
Example:
Case1: If 3 packets are sent at times 0, 1, 2 and received at 10, 11, 12. Here, the delay is same for all packets and it is acceptable for the telephonic conversation.
Case2: If 3 packets 0, 1, 2 are sent and received at 31, 34, 39, so the delay is different for all packets. In this case, the time delay is not acceptable for the telephonic conversation.
4. Bandwidth
Different applications need the different bandwidth.
For example:
Video conferencing needs more bandwidth in comparison to that of sending an e-mail.
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