Chapter 12 Transmission Control Protocol Objectives Upon completion

Chapter 12 Transmission Control Protocol Objectives Upon completion you will be able to: • Be able to name and understand the services offered by TCP • Understand TCP’s flow and error control and congestion control • Be familiar with the fields in a TCP segment • Understand the phases in a connection-oriented connection • Understand the TCP transition state diagram • Be able to name and understand the timers used in TCP • Be familiar with the TCP options TCP/IP Protocol Suite 1

Figure 12. 1 TCP/IP protocol suite TCP provides a set of services. What are those services? TCP/IP Protocol Suite 2

TCP provides a process-to-process communication service using port numbers. Table 12. 1 Well-known ports used by TCP/IP Protocol Suite 3

Figure 12. 4 TCP segments TCP provides a stream delivery service. It breaks up the data stream into segments of variable size. Each segment receives a header and is handed off to the IP layer. TCP/IP Protocol Suite 4

Figure 12. 4 TCP services and features TCP can create a full-duplex service. Data can flow in both directions at the same time; buffers on each side hold the data to be transmitted and sent. TCP provides a connection-oriented service: the two TCPs establish a connection, data is exchanged, and the connection is terminated. TCP provides a reliable service. Furthermore, TCP has a number of features. All bytes transferred are numbered by TCP. The numbering starts with a random value. TCP/IP Protocol Suite 5

Example 2 Suppose 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, each carrying 1000 bytes? Solution The following shows the sequence number for each segment: Segment 1 ➡ Sequence Number: 10, 001 (range: 10, 001 to 11, 000) Segment 2 ➡ Sequence Number: 11, 001 (range: 11, 001 to 12, 000) Segment 3 ➡ Sequence Number: 12, 001 (range: 12, 001 to 13, 000) Segment 4 ➡ Sequence Number: 13, 001 (range: 13, 001 to 14, 000) Segment 5 ➡ Sequence Number: 14, 001 (range: 14, 001 to 15, 000) TCP/IP Protocol Suite 6

The value in the sequence number field of a segment defines the number of the first data byte contained in that segment. The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receive. The acknowledgment number is cumulative. TCP/IP Protocol Suite 7

Figure 12. 4 TCP services and features TCP also provides flow control, error control, and congestion control. We will examine each of these shortly. Before we do, let’s examine the TCP header a little more closely. The TCP packet is called a segment (but most people just call it a packet). TCP/IP Protocol Suite 8

Figure 12. 5 TCP segment format (packets in TCP are called segments) Window size set by receiver with max = 65, 535 bytes TCP/IP Protocol Suite 9

Figure 12. 6 Control field More on these bits later. TCP/IP Protocol Suite 10

Figure 12. 7 TCP/IP Protocol Suite Pseudoheader added to the TCP datagram to calculate checksum 11

12. 4 A TCP CONNECTION TCP is connection-oriented. A connection-oriented transport protocol establishes a virtual path between the source and destination. All of the segments belonging to a message are then sent over this virtual path. A connection-oriented transmission requires three phases: connection establishment, data transfer, and connection termination. The topics discussed in this section include: Connection Establishment Data Transfer Connection Termination Connection Reset TCP/IP Protocol Suite 12

Figure 12. 9 Connection establishment using three-way handshaking A server tells its TCP that it is ready to make a connection this is called a passive open. + Should this be 8001? TCP/IP Protocol Suite rwnd is the receiver window size, as we will see later. Note: SYN bit is set in first packet; 8000 chosen randomly 13

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/IP Protocol Suite 14

SYN flooding attack is when a bad person floods a server with bogus SYN packets. The server spends a lot of time and resources replying to the SYN packets. To counter these attacks, some servers postpone resource allocation until the entire connection is set up using a cookie. TCP/IP Protocol Suite 15

Figure 12. 10 Data transfer Notice how the ACK and SEQ # are piggybacked. Push flag means deliver the data to the receiver as soon as it is received (don’t put it in a buffer and hold until you have enough bytes for a complete segment). This feature is usually ignored. Can also send Urgent data by setting the Urg bit. This data is then processed immediately. For example, you want to send a Ctrl-C to stop. TCP/IP Protocol Suite 16

Figure 12. 11 TCP/IP Protocol Suite Connection termination using three-way handshaking A FIN segment consumes one sequence number if it does not carry data. So should third segment be seq: x+1? 17

Figure 12. 12 Half-close y-1 Client is finished, but Server is not yet finished. So Server ACKs the Client’s FIN, but does not signal its own FIN just yet. TCP/IP Protocol Suite x+1 18

Connection Reset Using the Reset flag (RST), one can: • Deny a request for a connection • Abort a current connection • Terminate an idle connection TCP/IP Protocol Suite 19

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. . The topics discussed in this section include: Scenarios TCP/IP Protocol Suite 20

Table 12. 3 States for TCP/IP Protocol Suite 21

Figure 12. 13 TCP/IP Protocol Suite State transition diagram 22

Figure 12. 14 TCP/IP Protocol Suite Common scenario 23

Note: The common value for MSL is between 30 seconds and 1 minute. TCP/IP Protocol Suite 24

Figure 12. 15 TCP/IP Protocol Suite Three-way handshake 25

Figure 12. 16 TCP/IP Protocol Suite Simultaneous open 26

Figure 12. 17 TCP/IP Protocol Suite Simultaneous close 27

Figure 12. 18 TCP/IP Protocol Suite Denying a connection 28

Figure 12. 19 TCP/IP Protocol Suite Aborting a connection 29

12. 6 FLOW CONTROL Flow control regulates the amount of data a source can send before receiving an acknowledgment from the destination. TCP defines a window that is imposed on the buffer of data delivered from the application program. The topics discussed in this section include: Sliding Window Protocol Silly Window Syndrome TCP/IP Protocol Suite 30

Figure 12. 20 Sliding window rwnd is the receiver window size; cwnd is the congestion window size TCP/IP Protocol Suite 31

Note: 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’s sliding windows are byte oriented. TCP/IP Protocol Suite 32

Example 3 What is the value of the receiver window (rwnd) for host A if the receiver, host B, has a buffer size of 5, 000 bytes and 1, 000 bytes of received and unprocessed data? Solution The value of rwnd = 5, 000 − 1, 000 = 4, 000. Host B can receive only 4, 000 bytes of data before overflowing its buffer. Host B advertises this value in its next segment to A. TCP/IP Protocol Suite 33

Example 4 What is the size of the window for host A if the value of rwnd is 3, 000 bytes and the value of cwnd is 3, 500 bytes? Solution The size of the window is the smaller of rwnd and cwnd, which is 3, 000 bytes. TCP/IP Protocol Suite 34

Example 5 Figure 12. 21 shows an unrealistic example of a sliding window. The sender has sent bytes up to 202. We assume that cwnd is 20 (in reality this value is thousands of bytes). The receiver has sent an acknowledgment number of 200 with an rwnd of 9 bytes (in reality this value is thousands of bytes). The size of the sender window is the minimum of rwnd and cwnd or 9 bytes. Bytes 200 to 202 are sent, but not yet acknowledged. Bytes 203 to 208 can be sent without worrying about acknowledgment. Bytes 209 and above cannot be sent. TCP/IP Protocol Suite 35

Figure 12. 21 Example 5 Next, the server receives a packet with an acknowledgment value of 202 and an rwnd of 9. The host has already sent bytes 203, 204, and 205. The value of cwnd is still 20. Show the new window. TCP/IP Protocol Suite 36

Figure 12. 22 Example 6 Next, the sender receives a packet with an acknowledgment value of 206 and an rwnd of 12. The host has not sent any new bytes. The value of cwnd is still 20. Show the new window. TCP/IP Protocol Suite 37

Figure 12. 23 Example 7 Assume the sender has sent bytes 206 to 209. The sender’s window shrinks accordingly. Now the sender receives a packet with an acknowledgment value of 210 and an rwnd of 5. The value of cwnd is still 20. Show the new window. TCP/IP Protocol Suite 38

Figure 12. 24 TCP/IP Protocol Suite Example 8 39

Example 9 How can the receiver avoid shrinking the window in the previous example? Solution The receiver needs to keep track of the last acknowledgment number and the last rwnd. If we add the acknowledgment number to rwnd we get the byte number following the right wall. If we want to prevent the right wall from moving to the left (shrinking), we must always have the following relationship. new ack + new rwnd ≥ last ack + last rwnd or new rwnd ≥ (last ack + last rwnd) − new ack TCP/IP Protocol Suite 40

To avoid shrinking the sender window, the receiver must wait until more space is available in its buffer. Window Shutdown While you shouldn’t shrink the window, you can send rwnd = 0 to close the window. Sender can still send a “probe” packet. TCP/IP Protocol Suite 41

Note: 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. TCP/IP Protocol Suite 42

Figure 12. 24 Silly Window Syndrome - What if TCP sends segments that are only 1 byte long? You would have 40 bytes of header, 1 byte of data, for a total of 41 bytes. Very wasteful! TCP should wait until it has more data before it sends a 1 -byte segment. But how long should it wait to assemble data? Nagle’s Algorithm: 1. The sending TCP sends the first piece of data it receives from the sending application even if it is only 1 byte. 2. After sending the first segment, the sending TCP accumulates data in the output buffer and waits until either the receiving TCP sends an ack or until enough data has accumulated to fill a maximum-size segment. At this time, the sending TCP can send the segment. 3. Step 2 is repeated for the rest of the transmission. TCP/IP Protocol Suite 43

Figure 12. 24 Silly Window Syndrome - What happens if the receiving TCP has a buffer size of 1000 bytes and the sending TCP has just sent 1000 bytes. The receiving buffer is now full so the receiver tells the sender to stop (window size = 0). The receiver now reads 1 byte of data, processes it, and sends a window size of 1 (because now there is one space in the input buffer). The sender gets the window size and sends 1 byte. This procedure continues. Clark’s Solution - Acknowledge receipt right away, but don’t change the window size until you have at least half the buffer space available. Or, delay the ack until there is a decent amount of buffer space available. TCP/IP Protocol Suite 44

12. 7 ERROR CONTROL TCP provides reliability using error control, which detects corrupted, lost, out-of-order, and duplicated segments. Error control in TCP is achieved through the use of the checksum, acknowledgment, and timeout. The topics discussed in this section include: Checksum Acknowledgment Type Retransmission Out-of-Order Segments Some Scenarios TCP/IP Protocol Suite 45

TCP Error Control TCP supports basic error control. It uses a 16 -bit arithmetic checksum, similar to the ones we have already seen. TCP uses the ACK message to confirm receipt of segments. There a number of basic rules pertaining to ACKs: Rule 1: When one ends sends data, it must piggyback the ACK for any data received. (Example in just a moment) Rule 2: If a receiver has no data to send a segment arrives, do not ACK it immediately. Wait until two segments arrive, then ACK. Or wait 500 ms after the first segment then ACK. TCP/IP Protocol Suite 46

Figure 12. 25 TCP/IP Protocol Suite Normal operation 47

TCP Error Control Rule 3: When a segment arrives with an expected sequence number and the previous in-order segment has not been ACKed, the receiver immediately sends an ACK. (Example on previous slide) Rule 4: When a segment arrives with a sequence number higher than expected, the receiver immediately sends an ACK announcing the sequence number it expected. This leads to fast retransmission, which we will see shortly. Rule 5: When a missing segment arrives, the receiver sends an ACK segment to announce the sequence number expected. This informs the receiver that segments reported missing have been received. (Example on next slide) Rule 6: If a duplicate segment arrives, receiver immediately sends an ACK. This solves some problems when an ACK itself is lost. (Example on next slide) TCP/IP Protocol Suite 48

Figure 12. 26 TCP/IP Protocol Suite Lost segment 49

TCP Error Control Furthermore, a retransmission will occur if the retransmission timer (RTO) expires, or three duplicate ACKs arrive in order. (For RTO example, see previous slide. ) (For three ACKs, see next slide. ) TCP/IP Protocol Suite 50

Figure 12. 27 TCP/IP Protocol Suite Three ACKs in a row, fast retransmission 51

Figure 12. 28 Lost acknowledgment TCP/IP Protocol Suite 52

Figure 12. 29 TCP/IP Protocol Suite Lost acknowledgment corrected by resending a segment 53

12. 8 CONGESTION CONTROL Congestion control refers to the mechanisms and techniques to keep the load below the capacity. The topics discussed in this section include: Network Performance Congestion Control Mechanisms Congestion Control in TCP/IP Protocol Suite 54

Figure 12. 30 TCP/IP Protocol Suite Router queues 55

Figure 12. 31 TCP/IP Protocol Suite Packet delay and network load 56

Figure 12. 32 TCP/IP Protocol Suite Throughput versus network load 57

Generic Types of Congestion Control Open loop - no feedback used; try to control congestion before it happens Retransmission policy Acknowledgment policy Discard policy Closed loop - feedback tries to control congestion as it is happening Back pressure Choke point (similar to ICMP’s source quench) Implicit signaling (observing some other behavior) Explicit signaling TCP/IP Protocol Suite 58

TCP Congestion Control How does TCP spell relief? Congestion window - recall the cwnd value? Network sets this value and source takes minimum(rwnd, cwnd) Slow start algorithm - cwnd starts at 1. With each ACK received, cwnd doubles. So second time cwnd = 2, then 4, then 8, etc. The cwnd value does not increase forever. cwnd stops when it equals ssthresh, which has a max size of 65, 535. When cwnd equals ssthresh, slow start stops and additive phase begins. Additive phase increases the cwnd by 1 each time an ACK is received. TCP/IP Protocol Suite 59

Figure 12. 33 TCP/IP Protocol Suite Slow start, exponential increase 60

Figure 12. 34 TCP/IP Protocol Suite Congestion avoidance, additive increase 61

TCP Congestion Control What happens if congestion occurs? If an RTO timer times out, there is probably congestion. So cut the threshold in half, set cwnd back to 1, and restart the slow start phase again (ouch!). If three ACKs in a row are received, there may be congestion. So cut the threshold in half, set cwnd to the value of the threshold, and restart the avoidance phase. TCP/IP Protocol Suite 62

Figure 12. 35 TCP/IP Protocol Suite TCP congestion policy summary 63

Figure 12. 36 TCP/IP Protocol Suite Congestion example 64

12. 9 TCP TIMERS To perform its operation smoothly, most TCP implementations use at least four timers: Retransmission Timer Persistence Timer Keepalive Timer TIME-WAIT Timer TCP/IP Protocol Suite 65

Retransmission Timer To retransmit a lost segment, TCP employs a retransmission timer that handles the retransmission time-out (RTO), the waiting time for an ACK of a segment. If an ACK is received before the timer goes off, toss the timer. If timer goes off before ACK is received, segment is retransmitted and timer is reset. To calculate the RTO value, we’ll need a couple other values. TCP/IP Protocol Suite 66

Retransmission Timer The first value we need to know is the round trip time (RTT) of sending a segment and then getting the ACK. To calculate RTT, we could use the RTT Measured value (simply time ACK received minus time packet sent), but this value varies greatly on today’s Internet. So instead, we will calculate RTT Smoothed. First time: RTT Smoothed = RTT Measured After that: RTT Smoothed = (1 -a) x RTT Smoothed + a x RTT Measured where a normally is set to 1/8. TCP/IP Protocol Suite 67

Retransmission Timer Most implementations also use the RTT Deviation. First time: RTT Deviation = RTT Measured / 2 After that: RTT Deviation = (1 -b) x RTT Deviation + b x |Smoothed RTT - Measured RTT| Where b usually equals 1/4. Finally, RTO = RTT Smoothed + 4 x RTT Deviation TCP/IP Protocol Suite 68

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 initially set to 6. 00 seconds. The following shows the value of these variables at this moment: RTTM = RTTS = RTTD = RTO = 6. 00 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: TCP/IP Protocol Suite 69

Example 10 (continued) RTTM = 1. 5 RTTD = 1. 5 / 2 = 0. 75 RTTS = 1. 5 RTO = 1. 5 + 4 x 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 (. 75) + 1/4 |1. 625 − 2. 5| = 0. 78 TYPO HERE IN BOOK!! RTO = 1. 625 + 4 (0. 78) = 4. 74 TCP/IP Protocol Suite 70

Figure 12. 38 TCP/IP Protocol Suite Example 10 71

Karn’s Algorithm When the sending TCP receives an ACK for a segment, is the ACK for the original segment, or for the retransmitted segment? Depending upon which one you choose can affect the calculation of your RTO timer. So Karn says do not consider the round trip time of a retransmitted segment in the calculation of the new RTT. Do not update the value of RTT until you send a segment and receive an ACK without the need for retransmission. TCP/IP Protocol Suite 72

Exponential Backoff What is the value of RTO if a retransmission occurs? Most TCP implementations use an exponential backoff strategy. The value of RTO is doubled for each retransmission. See the next slide for an example of this. TCP/IP Protocol Suite 73

Example 11 Figure 12. 39 is a continuation of the previous example. There is retransmission and Karn’s algorithm is applied. The first segment in the figure is sent, but lost. The RTO timer expires after 4. 74 seconds. The segment is retransmitted and the timer is set to 9. 48, twice the previous value of RTO. This time an ACK is received before the time-out. We wait until we send a new segment and receive the ACK for it before recalculating the RTO (Karn’s algorithm). TCP/IP Protocol Suite 74

Figure 12. 39 TCP/IP Protocol Suite Example 11 75

The Other Timers What about the other timers? Persistence Timer What if a receiver sends you a window size of 0? You stop transmitting until you receive an ACK with a new window size. What happens if this ACK is lost? Connection goes dead. So set the persistence timer to say, 60 s when you receive a window size of 0. If you hear nothing after 60 s, send a special probe. No answer again after 60 s, send another probe. TCP/IP Protocol Suite 76

The Other Timers Keepalive Timer If two sides transfer data and then go silent, is the connection still valid? Or did it die somehow? Each time a server hears something from the other side, set the Keepalive Timer to say, 2 hours. If the server doesn’t hear anything within 2 hours, it sends a probe. No response after 10 probes (each of which is 75 s apart)? Then terminate the connection. TCP/IP Protocol Suite 77

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. TCP/IP Protocol Suite 78

Figure 12. 40 TCP/IP Protocol Suite Options 79

Figure 12. 43 Maximum-segment-size option This option (set during connection initialization) defines the maximum size of the data field within a segment. Max size is 65, 536 bytes; default is 536 bytes. TCP/IP Protocol Suite 80

Figure 12. 44 Window-scale-factor option In case a window size of 16 bits (65, 536 bytes) is not big enough, you can set the window-scale factor. The new window size if found by first raising 2 to the number specified in the window scale factor, then this result is multiplied by the value of the window size in the header. For example, if window size in header is already set at 32, 768 and scale factor = 3, then 2^3 * 32768 = 262, 144 bytes. This can only be set during connection establishment time. TCP/IP Protocol Suite 81

Figure 12. 45 Timestamp option Can use this option to calculate round trip time RTT. TCP/IP Protocol Suite 82

Example 12 Figure 12. 46 shows an example that calculates the round-trip time for one end. Everything must be flipped if we want to calculate the RTT for the other end. The sender simply inserts the value of the clock (for example, the number of seconds past from midnight) in the timestamp field for the first and second segment. When an acknowledgment comes (the third segment), the value of the clock is checked and the value of the echo reply field is subtracted from the current time. RTT is 12 s in this scenario. TCP/IP Protocol Suite 83

Example 12 (Continued) The receiver’s function is more involved. It keeps track of the last acknowledgment sent (12000). When the first segment arrives, it contains the bytes 12000 to 12099. The first byte is the same as the value of lastack. It then copies the timestamp value (4720) into the tsrecent variable. The value of lastack is still 12000 (no new acknowledgment has been sent). When the second segment arrives, since none of the byte numbers in this segment include the value of lastack, the value of the timestamp field is ignored. When the receiver decides to send an accumulative acknowledgment with acknowledgment 12200, it changes the value of lastack to 12200 and inserts the value of tsrecent in the echo reply field. The value of tsrecent will not change until it isreplaced by a new segment that carries byte 12200 (next segment). TCP/IP Protocol Suite 84

Example 12 (Continued) Note that as the example shows, the RTT calculated is the time difference between sending the first segment and receiving the third segment. This is actually the meaning of RTT: the time difference between a packet sent and the acknowledgment received. The third segment carries the acknowledgment for the first and second segments. TCP/IP Protocol Suite 85

Figure 12. 46 TCP/IP Protocol Suite Example 12 The time-stamp option can also be used for PAWS (protection against wrapped sequence numbers) 86

Figure 12. 47 TCP/IP Protocol Suite SACK 87

Example 13 Let us see how the SACK option is used to list out-of-order blocks. In Figure 12. 48 an end has received five segments of data. The first and second segments are in consecutive order. An accumulative acknowledgment can be sent to report the reception of these two segments. Segments 3, 4, and 5, however, are out of order with a gap between the second and third and a gap between the fourth and the fifth. An ACK and a SACK together can easily clear the situation for the sender. The value of ACK is 2001, which means that the sender need not worry about bytes 1 to 2000. The SACK has two blocks. The first block announces that bytes 4001 to 6000 have arrived out of order. The second block shows that bytes 8001 to 9000 have also arrived out of order. This means that bytes 2001 to 4000 and bytes 6001 to 8000 are lost or discarded. The sender can resend only these bytes. TCP/IP Protocol Suite 88

Figure 12. 48 TCP/IP Protocol Suite Example 13 89

Example 14 The example in Figure 12. 49 shows how a duplicate segment can be detected with a combination of ACK and SACK. In this case, we have some out-of-order segments (in one block) and one duplicate segment. To show both out-of-order and duplicate data, SACK uses the first block, in this case, to show the duplicate data and other blocks to show out-of-order data. Note that only the first block can be used for duplicate data. The natural question is how the sender, when it receives these ACK and SACK values knows that the first block is for duplicate data (compare this example with the previous example). The answer is that the bytes in the first block are already acknowledged in the ACK field; therefore, this block must be a duplicate. TCP/IP Protocol Suite 90

Figure 12. 49 TCP/IP Protocol Suite Example 14 91

Example 15 The example in Figure 12. 50 shows what happens if one of the segments in the out-of-order section is also duplicated. In this example, one of the segments (4001: 5000) is duplicated. The SACK option announces this duplicate data first and then the out-of-order block. This time, however, the duplicated block is not yet acknowledged by ACK, but because it is part of the outof-order block (4001: 5000 is part of 4001: 6000), it is understood by the sender that it defines the duplicate data. TCP/IP Protocol Suite 92

Figure 12. 50 TCP/IP Protocol Suite Example 15 93

12. 11 TCP PACKAGE We present a simplified, bare-bones TCP package to simulate the heart of TCP. The package involves tables called transmission control blocks, a set of timers, and three software modules. The topics discussed in this section include: Transmission Control Blocks (TCBs) Timers Main Module Input Processing Module Output Processing Module TCP/IP Protocol Suite 94

Figure 12. 51 TCP/IP Protocol Suite TCP package 95