TCP (Transmission control protocol)

TCPTCP Congestion Control & Quality of Service Presenting By:-

Aditya Kumar DakuaMitali SamalRahul Kumar NayakSribishnu DasOmm Prasad Routray

ContentsTCP/IP ModelProtocols using in TCP/IP model.Layers descriptionDifference between TCP/IP & OSI.TCP(Transmission Control Protocol)TCP Segment formatCongestion ControlOpen-LoopClosed-LoopCauses of congestionGeneral Policies of congestion controlQOS(Quality Of Service)Techniques for achieving good quality of service.

TCP/IP MODELTCP/IPThe TCP/IP protocol suite was developed prior to the OSI model.The original TCP/IP protocol suite was defined as having four layers: host to network, internet, transport and application.But as compared to OSI model TCP/IP has five layers.

APPLICATIONPRESENTATIONSESSIONTRANSPORTNETWORKDATA LINKPHYSICALApplication layerSMTPFTPHTTPDNSSNMPTELNETSCTPTCPUDPIPIGMPRARPARPHost-to-networkICMPPROTOCOLS USING IN TCP/IP MODELSMTP- Simple Mail Transfer ProtocolFTP- File Transfer ProtocolHTTP- Hyper Text Transfer ProtocolDNS- Domain Name Service/SystemSNMP- Simple Network Management ProtocolTELNET- Terminal NetworkSCTP- Stream Control Transmission ProtocolTCP- Transmission Control ProtocolUDP- User Datagram ProtocolICMP- Internet Control Message ProtocolIGMP- Internet Group Message ProtocolRARP- Reverse Address Resolution ProtocolARP- Address Resolution ProtocolIP- Internet Protocol

LAYERS DESCRIPTIONPHYSICAL AND DATA LINK LAYERTCP/IP does not define any specific protocol.A network in TCP/IP internetwork can be local-area network or a wide-area network.NETWORK LAYERTCP/IP supports the internetworking protocol.IP in turn uses four supporting protocols: ARP,RARP,ICMP and IGMP.TRANSPORT LAYERIt is represented in TCP/IP by two protocols.IP is host to host protocol.UDP and TCP are transport level protocols.APPLICATION LAYERIt is equivalent to combined session, presentation and application layer in the OSI model.

DIFFERENCE BETWEEN OSI & TCP/IPTCP/IPBasically It has 4 Layers.Such as Application, Transport, Internet, Network access Layer.Application layer of TCP/IP model is combination of 3 layers, i.e Application, Presentation, Session LayerThe data link & physical layer are combinely to form Network access layer.When TCP/IP compared with the OSI model then it acts as 5 layer.i.e Application, Transport, Internet, Data link & Physical Layer.OSIIt has 7 LayersSuch as Application, Presentation, Session, Transport, Network, Data link, Physical Layer.

In this model each layer perform a specific task by the respective layer within their bounded area.

TCP (Transmission control protocol)TCP is a transport layer protocol which provides connection-oriented service.It is a process-to-process or program-to-program protocol.It creates a virtual connection between two TCPs to send data.This protocol provides error recovery , flow control & reliability which are not available in UDP.TCP is the protocol used by major Internet applications such as the world wide web, email, remote administration & file transfer.ContIt is a sliding window protocol that provides handling for both timeouts and retransmissions.TCP establishes a full duplex virtual connection between two endpoints. Each endpoint is defined by an IP address and a TCP port number.The operation of TCP is implemented as a finite state machine.It does not support multicasting & broadcasting.TCP maintains a checksum on its header & data.A TCP connection is a byte stream not a message stream. A Stream of 8-bit bytes is exchanged across the TCP connection between the two applications.

Header DataDestination port address 16 bitsSource port address 16 bitsSequence number 32 bitsAcknowledgement number 32 bitsHlen 4 bitsReserved 6 bitsURG ACKPSHRSTSYNFINWindow size 16 bitsCheck sum 16 bitsUrgent pointer 16 bits Options & PaddingHEADER FORMAT OF TCP (TCP SEGMENT FORMAT)Source port addressThis is a 16-bit field that defines the port number of the application program in the host that is sending the segment.Destination port addressThis is a 16-bit field that defines the port number of the application program in the host that is receiving the segment.Sequence numberThis 32-bit field defines the number assigned to the first byte of the segment which indicates the sequence number of that sequence.

Acknowledgement Number32-bit field.It defines the byte number that the receiver of the segment is expecting to receive from the sender.HLEN(Header length)This 4-bit field indicates the number of 4 byte words in the TCP header.ReservedThis is a 6-bit field reserved for future use.

Controlled fieldThis field defines six different control bits or flags.URG ACKPSH RSTSYNFINURG(Urgent) Urgent pointer valid flag.If it is set to 1 the urgent pointer field valid.ACK(Acknowledgement)Acknowledgment number valid flag.If the ACK flag is set to 1 the acknowledgement number field is valid. PSH(Push)Push flagIf the field is one then it will push the data segment for maximum range.RST(Reset)Reset connection flag.If this flag value is 1 then the connection is reset or reestablished.SYN(Synchronized)Synchronize sequence numbers flag.It is used for synchronized sequence number during connection.FINTerminate the connection.End of data flag.For termination of the connection this field is selected to 1.

Congestion ControlIt refers to the technology & mechanism that can either used to prevent congestion before it happens or removes congestion after it happens.This can be broadly defined into two category. ->Open Loop ->Closed LoopCongestion ControlClosed Loop(After Congestion)Open Loop(Before Congestion)Retransmission

Window Policy

Acknowledgement

Discarding Policy

Admission Policy

Back pressure

Choke Packet

Implicit SignalingBackwardForwardExplicit Signaling Open-Loop In open-loop congestion control policies are applied to prevent congestion before it happens .in this mechanism congestion control is handled either the source or the destination.Retransmission PolicyIt is sometimes unavoidable. If the sender feels that a send packet is lost or corrupted the packet needs to be retransmitted.Window PolicyAccording to this policy the type of window at the sender may also affect congestion . The selective repeat window is better than the Go-Back-N window for congestion control.

Acknowledgement PolicyThis policy imposed by the receiver may also affect congestion .if the receiver does not acknowledge every packet it receives, it may slow down the sender & prevent congestion. Discarding policyA good discarding policy by the routers may prevent congestion & at the same time may not harm the integrity of the transmission. Admission PolicyAn admission policy ,which is a quality of service mechanism can also prevent congestion in virtual circuit networks switches in a flow first check the resource requirement of flow before admitting to the network.

Closed loopThis congestion control mechanisms try to alleviate congestion after it happens . several mechanisms have been used by different protocols. Back PressureIt refers to a congestion control mechanisms in which a congested node stops receiving data from the immediate upstream node or nodes . this may causes the upstream node or nodes to become congested. In turn reject data from their upstream node or nodes . this technique can be applied only to virtual circuit networks in which each node knows the upstream node from which a flow of data is coming.Backpressure method for alleviating congestion

Choke PacketIt is a packet sent by a node to the source to inform it of congestion. In this method the warning is from the router, which has encountered congestion, to the source station directly. The intermediate nodes through which the packet has traveled are not warned.

Choke PacketImplicit signalingThere is no communication between the congested node or nodes & the source . The source guesses that there is congestion some where in the network from other symptoms.Backward SignalingIn this signaling a bit can be set in a packet moving in the direction opposite to the congestion.Forward SignalingIn this a bit can be set in a packet moving in the direction of the congestion.

Explicit SignalingIn explicit signaling method the signal is included in the packets that carry data . It can occur in either the forward or the backward direction.Causes of CongestionIf the rate at which packets arrive and queue of exceeds the rate at which packets can be transmitted the queue size grows , the delay experienced by a packet goes to infiniteThe other causes of congestion is slow processor speed . if the routers CPU speed is slow and performing task like queuing buffers table updating.queues are built up even through the line capacity is not fully utilised.The bandwidth of the links is also important in congestion . The links to be used must be of high bandwidth to avoid the congestion.Any mismatch between parts of the system also causes congestion , up grading the processors speed but not changing the links or vice-versa will causes the congestion.

General Policies of Congestion ControlGeneral policies of congestion control can be divided into two groups ->Open Loop ->Closed LoopOpen LoopOpen loop solution attempt to solve the problem by good design in essence to make sure it dose not occur in the first place .once the system is up and running, midcourse corrections are not made.Tools for doing open loop control include deciding when to accept new traffic, deciding when to discard packets and which ones ,and making scheduling decisions at various points in the network.

Closed LoopClosed loop solutions are based on the concept of a feedback loop this approach has three parts when applied to congestion control.Monitor the system to detect when and where congestion occurs.Pass this information to place where action can be taken.Adjust system operation to correct the problem.Metrics to monitor the subnet for congestion are the average queue lengths , percentage of all packets discarded for lack of buffer space , average packed delay, the number of packets that time out are retransmitted etc.Quality Of Service(QOS)QOS is an internetworking issue that has been discussed more than defined . we can roughly defined QOS as something that a flow seeks to attain.It requires a stream of packet from a source to a destination which is known as flow & application with their quality of service requirement.

Techniques for achieving good quality of serviceThere are 4 methods for improving the QOSScheduling.Traffic shaping.Admission Control.Resources Reservation.

Scheduling

The intermediate device like router switches processes the packets which are coming from different source host . so to process the packets to their corresponding receiver router should follow a good scheduling mechanism.It is divided into 3 categories:-FIFO QueuePriority QueueWeighted QueueFIFO queue

In FIFO(First-in first-out) queuing packets wait in a buffer until the node is ready to process them . if the average arrival rate is higher than the average processing rate . the queue will fill up & new packets will be discarded.

Priority queuingIn priority queuing , packets are first assigned to a priority class each priority class has its own queue . The packets in the highest priority queue are processed last . The system dose not stop serving a queue until it is empty . A priority queue can provide better QOS than FIFO queue.

Weighted fair queuingA better scheduling method is weighted pair queuing . In this technique the turning switch selects three packets from first queue then two packets from second queue &one packet from the third queue this cycle repeats until all the queues are empty.Traffic ShapingIt is a mechanism to control the amount & the rate of the traffic sent to the network.There are two techniques exists to shape traffic.Leaky BucketToken Bucket

Leaky bucket

Leaky bucket implementationA leaky bucket algorithm shapes brusty traffic into fixed-rate traffic by averaging the data rate. It may drop the packets if the bucket is full.

Token bucketToken bucket is same as leaky bucket algorithm but it allows the flow of brusty traffic at a regulated maximum rate.Admission ControlAdmission control refers to the mechanism used by a router , or a switch to accept or reject a flow based on predefined parameters called flow specifications.Resource ReservationA flow of data needs resources such as buffer, bandwidth ,CPU time & so on.The QOS is improved if these resources are reserved before hand.