Chapter 12 Transmission Control Protocol

Introduction Position of TCP

Introduction (cont’d) Responsibilities of Transport Layer to create a process-to-process communication using port numbers in case of TCP to provide a flow-and-error control mechanism at the transport level TCP uses sliding window protocol to achieve error control. TCP uses the acknowledgment packet, time-out, and retransmission to achieve error control. to provide a connection mechanism for the application program sending streams of data to the transport layer by application program making a connection with the receiver, chopping the stream into transportable units, numbering them and sending them one by one

Introduction (cont’d) At the receiving end, waiting until all the different units belonging to same application program have received, checking, passing those that are error free and delivering them to the receiving application program as a stream. After the entire stream has been sent, the transport layer should close the connection. TCP is called a connection-oriented, reliable transport protocol adding connection-oriented and reliability features to the services of IP

12.1 Process-to-Process Communication Host-to-host communication and process-to-process communication

Process-to-Process Communication (cont’d) Port Addresses (Numbers) process-to-process communication that achieved through the client/server paradigm to define the client and server programs, we need second identifiers called port numbers. integers between 0 and 65,535 The client program running on the local computer defines itself with a port number, chosen randomly by the TCP software running on the local host using a ephemeral port number But, the server program on the remote computer must also define itself with a port number using a well-known port number

Process-to-Process Communication (cont’d) Explanation of port numbers using TENET application

Process-to-Process Communication (cont’d) Well-known ports used by TCP

Process-to-Process Communication (cont’d) Socket Addresses To make a connection, needs 2 identifier : IP address + Port number  Socket address a pair of socket address Client socket address Server socket address

12.2 TCP Services TCP is a stream-oriented protocol Stream Delivery Service TCP is a stream-oriented protocol TCP creates an environment in which the two processes seem to be connected by an imaginary “tube” that carries their data across the Internet.

TCP Services (cont’d) Sending and Receiving Buffers Because the sending and receiving processes may not produce and consume data at the same speed, TCP needs buffers for storage. One way to implement is to use a circular array Not acknowledged

TCP Services (cont’d) TCP Segments

TCP Services (cont’d) Stream Data Service (stream transport layer service) The sending TCP 1) accepts a stream of characters from sending application program 2) creates packets called segments, of appropriate size extracted from the stream 3) sends segments across the network The receiving TCP 1) receives segments, extracts data from segments 2) orders segments if they have arrived out of order 3) delivers segments as a stream of characters to the receiving application program

TCP Services (cont’d) For stream delivery, the sending and receiving TCPs use buffers the sending TCP uses sending buffer to store the data coming from the sending application program. the sending application program writes data to the buffer of the sending TCP the receiving TCP receives the segments and stores them in a receiving buffer the receiving application program uses the read operation to read the data from the receiving buffer. Since the rate of reading can be slower than the rate of receiving, the data is kept in the buffer until the receiving application reads it completely.

TCP Services (cont’d) Full-Duplex Service TCP offers full-duplex service After two application programs are connected to each other, they can both send and receive data. Piggybacking When a packet is going from A to B, it can also carry an acknowledgment of the packets received from B

TCP Services (cont’d) Connection-Oriented Services Reliable Service A’s TCP informs B’s TCP and gets approval from B’s TCP A’s TCP and B’s TCP exchange data in both directions After both processes have no data left to send and the buffers are empty, two TCPs destroy their buffers Reliable Service TCP uses the acknowledgment mechanism to check the safe and sound arrival of data

12.3 TCP Features Byte numbers All data bytes being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number. Number range for first byte : 0 ~ 2 32 -1 If random number is 1,057 and total number 6,000bytes, the bytes are numbered from 1,057 to 7,056 Byte numbering is used for flow and error control.

Numbering Bytes (cont’d) Sequence number After the bytes have been numbered, TCP assigns a sequence number to each segment that is being sent. Segment number for each segment is number of the first byte carried in that segment.

Numbering Bytes (cont’d) Example 2 Imagine a TCP connection is transferring a file of 5000 bytes. The first byte is numbered 10001. What are the sequence numbers for each segment if data is sent in five segments with the each segment carrying 1,000 bytes?

Numbering Bytes (cont’d) Solution The following shows the sequence number for each segment: Segment 1  10,001 (10,001 to 11,001) Segment 2  11,001 (11,001 to 12,001) Segment 3  12,001 (12,001 to 13,001) Segment 4  13,001 (13,001 to 14,001) Segment 5  14,001 (14,001 to 15,001)

Numbering Bytes (cont’d) Acknowledgment Number The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receives. The acknowledgment number is cumulative.

12.3 Segment A packet in TCP is called segment

Segment (cont’d) Source port address Destination port address defining the port number of application program in the host that is sending the segment Destination port address defining the port number of application program in the host that is receiving the segment Sequence number defining the number assigned to the first byte of data contained in this segment during the connection establishment, each party uses a random number generator to create an initial sequence number (ISN)

Segment (cont’d) Acknowledgment number Header length Reserved If the source of the segment has successfully received byte number x from the other party, it defines x+1 as the acknowledgment number Header length Indicating the number of 4-byte words in the TCP header the value between 5 and 15 (20 and 60 bytes) Reserved For future use

Segment (cont’d) Control Enabling flow control, connection establishment and termination, and mode of data transfer in TCP

Segment (cont’d) Description of flags in the control field

Segment (cont’d) Window size Checksum : picture in next page defining the size of the window, in bytes, that the other party must maintain. maximum size of window : 65,535 bytes Checksum : picture in next page Urgent pointer used when the segment contains urgent data defining the number that must be added to the sequence number to obtain the number of the last urgent byte in the data section of the segment Options : 40 bytes

Segment (cont’d)

Segment (cont’d) A TCP segments is encapsulated in an IP datagram

12.4 TCP Connection The server program tells its TCP to make a passive open The Client program issues a request for an active open.

TCP Connection – three-way handshaking A SYN segment cannot carry data, but it consumes one sequence number. A SYN + ACK segment cannot carry data, but does consume one sequence number. An ACK segment, if carrying no data, consumes no sequence number.

TCP Connection (Cont’d) Data transfer

TCP Connection (Cont’d) Urgent data To send urgent data Use of URG bit set by sending TCP Receiving TCP extracts the urgent data from the segment using urgent pointer

TCP Connection (Cont’d) Connection Termination The FIN segment consumes one sequence number if it does not carry data. The FIN + ACK segment consumes one sequence number if it does not carry data.

TCP Connection (Cont’d) Half-close

12.5 State Transition Diagram To keep track of all the different events happening during connection establishment, connection termination, and data transfer, the TCP software is implemented as a finite state machine.

State Transition Diagram (cont’d)

State Transition Diagram (Cont’d) A state transition diagram Server Client Unusual Input / Output Now connection is closed in one direction.

Connection Establishment and Termination

Connection Termination Using Three-way Handshake

Simultaneous Open

Simultaneous Close

Denying a Connection

Aborting a Connection

12.6 Flow Control Defining the amount of data that a source can send before receiving an acknowledgement from the destination. Sliding window For flow control, TCP uses a sliding window protocol The window covers a portion of the buffer that a host can send before worrying about an acknowledgment from other host A sliding window is used to make transmission more efficient as well as to control the flow of data so that the destination does not become overwhelmed with data. TCP sliding windows are byte oriented.

Sliding Window Protocol

Sliding Window Protocol an Example

Flow Control (cont’d) In TCP, the sender window size is totally controlled by the receiver window value. However, the actual window size can be smaller if there is congestion in the network. Some Points about TCP’s Sliding Windows: The size of the window is the lesser of rwnd and cwnd The source does not have to send a full window’s worth of data. The window can be opened or closed by the receiver, but should not be shrunk. The destination can send an acknowledgment at any time as long as it does not result in a shrinking window. The receiver can temporarily shut down the window; the sender, however, can always send a segment of one byte after the window is shut down.

12.7 Error Control Including mechanisms for detecting corrupted segments, lost segments, out-of-order segments, and duplicated segments. Also, including a mechanism for correcting errors after they are detected. Error Detection and Correction Checksum Acknowledgment : TCP does not use negative acknowledgment Time-out

TCP Sliding Windows Some points about TCP’s sliding windows: ❏ The size of the window is the lesser of rwnd and cwnd. ❏ The source does not have to send a full window’s worth of data. ❏ The window can be opened or closed by the receiver, but should not be shrunk. ❏ The destination can send an acknowledgment at any time as long as it does not result in a shrinking window. ❏ The receiver can temporarily shut down the window; the sender, however, can always send a segment of one byte after the window is shut down.

Normal Operation

Lost Segment

Fast Retransmission

Lost Ack

Lost ACK Corrected by Resending a Segment

12.8 Congestion Control Congestion in a network may occur if the load on the network is greater than the capacity of the network Congestion control refers to the mechanism and techniques to control the congestion and keep the load below the capacity Congestion in a network or internetwork occurs because routers and switches have queues.

Congestion Control (cont’d)

Congestion Control (cont’d) Network performance Delay versus Load

Congestion Control (cont’d) Throughput versus Load the reason is the discarding of packets by the routers

Congestion Control (cont’d) Congestion control mechanisms refers to techniques and mechanisms that can either prevent congestion, before it happens, or remove congestion, after it has happened. open-loop congestion control (prevention) and closed loop congestion control (removal)

Congestion Control (cont’d) Open-loop congestion control Retransmission policy the transmission policy and the retransmission timers must be designed to optimize efficiency and at the same time prevent congestion Acknowledgment policy If the receiver does not acknowledge every packet it receives, it may slow down the sender and help prevent congestion Discard policy In audio transmission, if the policy is to discard less sensitive packets when congestion is likely, the quality of sound is still preserved and congestion is prevented

Congestion Control (cont’d) Closed-loop congestion control Back pressure informing the previous upstream router to reduce the rate of outgoing packets Choke point is a packet sent by a router to the source to inform it of congestion is similar to ICMP’s source quench packet Implicit signaling Detecting an implicit signal warning of congestion and slow down its sending rate. Ex) receiving delayed ACK Explicit signaling Router experiencing congestion can send an explicit signal by setting a bit in a packet to the sender or the receiver.

Congestion Control in TCP Congestion window Today, TCP protocols include that the sender’s window size is not only determined by the receiver but also by congestion in the network Actual window size = minimum (rwnd, cwnd)

Congestion Control in TCP (cont’d) Slow start: exponential increase MSS(max. segment size)

Congestion Control in TCP (cont’d) In the slow start algorithm, the size of the congestion window increases exponentially until it reaches a threshold. Start  cwnd = 1 After 1 RTT  cwnd = 1 x 2 = 2  21 After 2 RTT  cwnd = 2 x 2 = 4  22 After 3 RTT  cwnd = 4 x 2 = 8  23

Congestion Control in TCP (cont’d) Congestion avoidance: additive increase In the congestion avoidance algorithm the size of the congestion window increases additively until congestion is detected

Congestion Control in TCP (cont’d) Congestion detection: Multiplicative Decrease Most implementations react differently to congestion detection: If detection is by time-out, a new slow start phase starts. If detection is by three ACKs, a new congestion avoidance phase starts.

Congestion Control in TCP (cont’d) TCP congestion policy summary

Congestion Control in TCP (cont’d) Congestion example

12.9 TCP Timers To perform its operation smoothly, most TCP implementations use at least four timers.

TCP Timers Round Trip Time(RTT) To calculate the retransmission(RTO), we first need to calculate the round-trip time(RTT) In TCP, there can be only one RTT measurement in progress at any time Measured RTT (RTTM) : how long it takes to send a segment and receive an acknowledgment of it.

TCP Timers Smothed RTT (RTTS) : Weighed average of RTTM and previous RTTS Original  No Value After first measurement  RTTS = RTTM After any other measurement  RTTS = (1- ) RTTS +  · RTTM The value of  is implementation-dependent, but it is normally set to 1/8

TCP Timers RTT Deviation (RTTD) Original  No Value After first measurement  RTTD = RTTM/2 After any other measurement  RTTD = (1- ) RTTD +  · l RTTS – RTTM I * The value of  is also implementation dependent, but is it is usually is sent to ¼.

TCP Timers Retransmission Timeout (RTO) Original  Initial Value After any measurement  RTO = RTTS + 4 RTTD

Example 10 Let us give a hypothetical example. Figure 12.38 shows part of a connection. The figure shows the connection establishment and part of the data transfer phases. 1. When the SYN segment is sent, there is no value for RTTM , RTTS , or RTTD . The value of RTO is set to 6.00 seconds. The following shows the value of these variables at this moment: RTTM = 1.5 RTTS = 1.5 RTTD = 1.5 / 2 = 0.75 RTO = 1.5 + 4 . 0.75 = 4.5 2. When the SYN+ACK segment arrives, RTTM is measured and is equal to 1.5 seconds. The next slide shows the values of these variables:

Example 10 RTTM = 1.5 RTTS = 1.5 RTTD = 1.5 / 2 = 0.75 RTO = 1.5 + 4 . 0.75 = 4.5 3.When the first data segment is sent, a new RTT measurement starts. Note that the sender does not start an RTT measurement when it sends the ACK segment, because it does not consume a sequence number and there is no time-out. No RTT measurement starts for the second data segment because a measurement is already in progress. RTTM = 2.5 RTTS = 7/8 (1.5) + 1/8 (2.5) = 1.625 RTTD = 3/4 (7.5) + 1/4 |1.625 − 2.5| = 0.78 RTO = 1.625 + 4 (0.78) = 4.74

Example 10

TCP Timers Persistence Timer When acknowledgment with non-zero window size after zero window size is lost, to correct deadlock, TCP uses a persistence timer for each connection When the sending TCP receives an acknowledgment with a window size of zero, the persistence timer is started When persistence timer goes off, the sending TCP sends a special segment called a probe The probe alerts the receiving TCP that the acknowledgment was lost and should be resent. If a response is not received, the sender continues sending the probe segments and doubling, and resetting the value of the persistence timer until the value reaches a threshold (usually 60 seconds). After that sender sends one probe segment every 60s until the window is reopened.

TCP Timers KeepaliveTimer TIME-WAIT Timer Used to prevent a long idle connection between two TCPs. Each time the server hears from a client, it resets this timer. Time-out is usually 2 hours. After 2 hours, sending 10 probes to client (each 75 secs), then terminates connection. TIME-WAIT Timer The time-wait timer is used during connection termination.

12.10 Options The TCP header can have up to 40 bytes of optional information. Options convey additional information to the destination or align other options. Two categories of options one-byte options multiple-byte options

Options

Options End of option (EOP) After this option, the receiver looks for the payload data EOP option imparts 2 pieces of information to the destination No more options in the header Data from the application program starts at the beginning of the next 32-bit word *EOP can be used only once.

Options No Operation Is One-byte option used as a filler.

Options Maximum segment size (MSS) defining the size of the biggest unit of data that can be received by the destination of the TCP segment in spite of its name, defining the maximum size of the data, not the maximum size of the segment value of 0 to 65,535 bytes : default is 536 to be determined during the connection establishment phase by the destination of the segment used only in the segments that make the connections. Not used in the segments during data transfer

Options Window Scale Factor defining the size of the sliding window new window size = window size defined in the header x 2 window scale factor Determined in phase of the connection setup The largest value of scale factor allowed by TCP/IP is 14. The value of the window scale factor can be determined only during connection establishment; it does not change during the connection

Options Timestamp 10-byte option The end with the active open announces a timestamps in the connection request segment (SYN Segment) If it receives a timestamp in the next segment (SYN + SCK) from the other end, it is allowed to use the timestamp.

Example 12

Options SACK-permitted and SACK Options SACK-permitted option is used only during connection established with SYN segment and SYN + ACK segment. SACK-permitted option is not allowed during the data transfer phase. SACK Option is used during data transfer only if both ends agree The option includes a list for blocks arriving out-of-order.

Options SACK

Example 13

Example 14 For duplicate segment

Example 15 Duplicate and out-of-order block

12.11 TCP Package A TCP package involving a table called Transmission Control Blocks, a set of timers, and three software modules: main module, input processing module, output processing module.

TCP Package (Cont’d) Transmission Control Block (TCBs) To control the connection, TCP uses a structure to hold information about each connection. TCP keeps an array of TCBs in the form of a table

TCP Package (Cont’d) State : defining the state of the connection according to the state transition diagram Process : defining the process using this connection at this machine as a client or a server Local IP address : defining the IP address of the local machine used by this connection Local port number : defining the local port number used by this connection Remote IP address Remote port address Interface : defining the local interface Local window : holding information about the window at the local TCP Remote window

TCP Package (Cont’d) Sending sequence number Receiving sequence number Sending ACK number Time-out values : transmission time-out, persistence time-out, keepalive time-out, and so on Buffer size : defining the size of the buffer at the local TCP Buffer pointer : pointer to buffer where the receiving data is kept until is read by the application

TCP Package (Cont’d) Main Module : The main module is invoked by an arrived TCP segment, a time-out, or a message from an application program

TCP Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Main Module (cont’d)

TCP D Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Main Module (cont’d)

TCP Package (Cont’d) Input processing module Output processing module handles all the details needed to process data or acknowledgment received when TCP is in the ESTABLISHED state sends an ACK if needed, takes care of the window size, does error checking, and so on Output processing module handles all the details needed to send out data received from application program when TCP is in the ESTABLISHED state handles retransmission time-outs, persistent time-outs, and so on