Module 14 UDP and TCP Frames

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Module 14UDP and TCP Frames
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• Textbook sections
• LG Section 8.5 Transmission Control Protocol
• BF Chapter 11 UDP Section 11.1 11.4
• BF Chapter 12 TCP Sections 12.1 12.12
• Topics
• User Datagram Protocol (UDP)
• Transmission Control Protocol (TCP)
• TCP Timers
• TCP Operation Scenarios

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BF Figure 12-1 TCP/IP Protocol Suite
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1. User Datagram Protocol (UDP)

• User Datagram Protocol (UDP)
• UDP is an unreliable, connectionless transport
  layer protocol
• Provides only two additional services beyond IP
• Demultiplexing
• IP knows how to deliver packets to a host, but
  does not know how to deliver them to the specific
  application in the host. UDP adds a mechanism
  that distinguishes among multiple applications in
  the host (port address)
• Error checking on data
• IP checks only the integrity of its header. UDP
  can optionally check the integrity of the entire
  user datagram (header and data).
• For IPv6, the header checksum of IPv4 is
  eliminated because the checksum is/can be
  provided by upper layer protocols.

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LG Figure 8.16 UDP format
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16
0
Source Port
Destination Port
UDP Length
UDP Checksum
Data
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1. User Datagram Protocol (UDP)

• UDP checksum
• The usage of UDP checksum is an option.
• Checksum field contains all 0s, if a source host
  does not want to compute the checksum.
• If a host computers the checksum whose result is
  0, it will set the checksum to all 1s.
• Checksum includes
• Pseudo header from the IP header
• The pseudoheader is part of the header of the IP
  packet in which the user datagram is to be
  encapsulated for transmission with some fields
  filled with 0s.
• Routers recalculate the value in the IP checksum
  field
• UDP header
• Data, padded with zero octets at the end (if
  necessary) to make a multiple of two octets.

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BF Figure 11.8 Pseudoheader added to the UDP
datagram
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1. User Datagram Protocol (UDP)

• Use of UDP
• Trivial File Transfer Protocol (TFTP)
• TFTP includes flow and error control. It can
  easily use UDP.
• Simple Network Management Protocol (SNMP)
• Routing Information Protocol (RIP)
• Real-Time Protocol (RTP)
• Domain Name System (DNS)
• Multicasting and broadcasting
• UDP is a suitable transport protocol for
  multicasting and broadcasting. Multicasting and
  broadcasting capabilities are embedded in the UDP
  software but not in the TCP software
• In general UDP is suitable for a process that
  requires simple request-response communication
  and with little concern for flow and error
  control. It is not usually used for a process
  that needs to send bulk data, such as FTP.

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BF Figure 12-4 TCP segment format
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2. Transmission Control Protocol (TCP)

• TCP segment format
• Sequence number field
• 32-bit field defines the number assigned to the
  first byte of data contained in this segment
• Acknowledge number filed
• 32-bit field defines the byte number that the
  source of the segment is expecting to receive
  from the other party
• Data offset field
• 4 bits field indicate the number of 32 bits
  words(in units of 4-bytes word) in the TCP
  header. It indicates where the data begins. The
  TCP header (even one including options) is an
  integral number of 32 bits long.
• Window size field
• 16-bit field defines the size of the window, in
  bytes, that the other party must maintain
• Used for flow control
• The sequence number field, the acknowledge number
  field, and the window size field are all involved
  in TCPs sliding window algorithms.

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2. Transmission Control Protocol (TCP)

• Control field
• BF Figure 12.5 Control field
• Option field
• Figure 12.9 Maximum segment size option
• Figure 12.10 Window scale factor option
• Figure 12.11 Timestamp option

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2. Transmission Control Protocol (TCP)

• Checksum field
• The TCP checksum field is used in exactly the
  same way as for UDP.
• TCP checksum is computed over
• Pseudoheader
• TCP header
• TCP data
• The checksum computation procedure is similar to
  IP checksum calculation
• Additional features of TCP checksum calculation
• Padding If the length of the segment is not a
  multiple of 16 bits, the segment will be padded
  with zeros to make it a multiple of 16 bits.
  Note, however, the TCP length field is not
  modified.

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2. Transmission Control Protocol (TCP)

• Pseudoheader
• Pseudoheader is added to the beginning of the
  segment when performing the checksum computation
• Pseudoheader is created by the source and
  destination hosts during the checksum computation
  and is not transmitted.
• Pseudoheader ensures the receiver that the
  segment has indeed reached the correct
  destination host and port and that the protocol
  type is TCP (which is assigned the value of 6).
  At the receiver the IP address information in the
  IP packet that contained the segment is used in
  the checksum calculation.
• The TCP Length field in the Pseudoheader is the
  TCP header length plus the data length in bytes.
  It does not count the 12 bytes of the
  Pseudoheader.

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2. Transmission Control Protocol (TCP)

• Pseudoheader A subset of fields from the IP
  header that are passed up to transport protocols
  TCP and UDP for use in their checksum
  calculations.
• Part of both TCPs and UDPs checksum calculation
• Not transmitted
• Consists of three fields from IP header protocol
  number, source IP address, and destination IP
  address, plus TCPs or UDPs total length.
• Purpose is to ensure the contents of pseudoheader
  fields are transmitted correctly.

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BF Figure 12-12 Pseudoheader Added to the TCP
Datagram
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2. Transmission Control Protocol (TCP)

• Urgent pointer field
• A 16-bit field is valid only if the urgent flag
  in the control field is set.
• Used when the segment contains urgent data.
• Defines 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.
• This facility is a bare bones way of allowing the
  sender to signal the receiver without getting TCP
  itself involved in the reason for the interrupt
• The urgent flag in the control field indicates
  that this segment contains urgent data. When
  this flag is set, the urgent point field
  indicates where the non-urgent data contained in
  this segment begins.

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2. Transmission Control Protocol (TCP)
BF Figure 12-5 Control Field
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2. Transmission Control Protocol (TCP)

• Control field
• URG flag
• Out of sequence number
• TCP is a stream-oriented protocol
• However, if the sending program wants a piece of
  data to be read out of order by the receiving
  application program, it can do so by setting the
  urgent flag and provide value in the urgent
  pointer field.
• When the receiving TCP receives a segment with
  urgent flag set, it extracts the urgent data from
  the segment using the value in the urgent
  pointer.
• The extracted data will be delivered to the
  receiving application program as soon as it
  arrives, even if it means delivering it before
  data with a earlier sequence number.

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2. Transmission Control Protocol (TCP)

• Control field
• PSH flag
• Do not wait
• The application program on the sending site can
  request a push operation. This means that the
  sending TCP should not wait for the window to be
  filled. It should create a segment and send it
  immediately. The sending TCP should also set the
  push bit (PSH).
• At the receiving TCP, when the PSH bit is set,
  it tells the receiver to pass the data to the
  application immediately and do not wait for more
  data to come.Otherwise, receiver may choose to
  buffer the segment until enough data accumulates
  init buffer.

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2. Transmission Control Protocol (TCP) Option
Field
BF Figure 12-6 Options
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2. Transmission Control Protocol (TCP) Option
Field
BF Figure 12-7 End of Option
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2. Transmission Control Protocol (TCP) Option
Field
BF Figure 12-8 No Operation Option
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2. Transmission Control Protocol (TCP) Option
Field
BF Figure 12-9 Maximum segment size (MSS) option

• Note It communicates the maximum receive segment
  size at the TCP which sends this segment.
• This option is used only in the segments that
  make the connections. It cannot be used in
  segments during data transfer.
• The size is determined by the destination of the
  segment, not the source. So party 1 defines what
  should be the MSS sent by party 2. Party 2
  defines what should be the MSS sent by party 1.
• It defines the maximum size of the data, not the
  maximum size of the segment.
• The default value is 536.

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2. Transmission Control Protocol (TCP) Option
Field
BF Figure 12-10 Window scale factor option
Note 1. New window size window size defined
in the header 2window scale factor 2. The size
of the window cannot be greater than the maximum
for the sequence number. The largest value of
window scale factor allowed is 16.
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2. Transmission Control Protocol (TCP) Option
Field
BF Figure 12-11Timestamp option

• Note
• TCP -gt connection-oriented -gt acknowledgement -gt
  necessary retransmission based on round-trip-time
  (RTT) -gt one approach is to use TCP time stamp
  option to estimate RTT dynamically
• Usage of TCP timestamp option
• Source fills timestamp value in the timestamp
  field when the segment leaves the source
• Destination stores the timestamp value after it
  received the segment.
• When the destination sends an acknowledgement
  for the bytes in the segment received,
  destination enters the stored timestamp value in
  the timestamp echo reply field.
• The source, after received the acknowledgement,
  calculates the round-trip-time (RTT) by taking
  the difference between the current time and the
  value in the timestamp echo reply field.

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3. TCP Timers
BF Figure 12-21 TCP Timers
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3. TCP Timers

• TCP timer
• Retransmission timer
• If the retransmission timer goes off before the
  acknowledgement arrives, the segment is
  retransmitted and the timer is reset.
• The most common formula used to calculate
  retransmission time
• Retransmission time 2 round-trip time (RTT)
• Calculation or RTT
• First method Use timestamp option
• Second method TCP sends a segment, starts a
  timer, wait for an acknowledgement, and measure
  the round-trip time.
• RTT alpha previous RTT (1 alpha)
  current RTT
• where alpha is between 0 and 1, and is usually
  set at 0.9
• Karn algorithm
• Do not update the value of RTT until you send a
  segment and receive an acknowledgement without
  the need for retransmission

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3. TCP Timers

• Persistence timer
• Used to correct deadlock caused by the announce
  of a window size of zero by the receiving
  followed by an acknowledgment lost from the
  receiver.
• Probe usage
• When the sending TCP receive an acknowledgement
  with a window size of zero, it starts a
  persistence timer.
• When the persistence timer goes off, the sending
  TCP sends a special segment called probe.
• Keepalive timer
• Used to prevent a long idle connection between
  two TCPs
• Time-waited timer
• Used during connection termination

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4. TCP Operation Scenarios

• Operation scenarios
• Connection establishment
• Three-way handshake - to establish a connection
• Four-way handshake - to terminate a connection
• Data transfer
• Typical
• Pushing data
• Urgent data
• Connection termination
• State transition diagram representation

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4. TCP Operation Scenarios

• Three-way handshake
• An algorithm used by TCP to establish a
  connection
• Involves the exchange of three messages between
  the sender and the receiver
• Purpose is for two parties to agree on the
  starting sequence numbers the two sides plan to
  use for their respective byte streams
• Algorithm
• Step 1 the sender sends a segment to the
  receiver stating the initial sequence number it
  plan to use (Flags SYN, sequence number x)
• Step 2 the receiver responds with a single
  segment that both acknowledges the senders
  sequence number (Flag ACK, ack x1) and
  states its own beginning sequence (Flag SYN,
  sequence number y). Both SYN and ACK bits are
  set in the Flags field of the second message.
• Step3 the sender responds with a third message
  that acknowledges the receivers sequence number
  (Flags ACK, acknowledge number y1).

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4. TCP Operation Scenarios
BF Figure 12-22 Three-way Handshaking
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4. TCP Operation Scenarios

• Four-way handshake
• An algorithm used by TCP to terminate a
  connection
• Algorithm
• Step 1 First, the sender sends a FIN segment to
  the receiver (Flags FIN, sequence number x)
• Step 2 the receiver sends the second segment, an
  ACK segment, to confirm the receipt of the FIN
  segment from the sender. In this segment it uses
  the acknowledgement number, which is one plus the
  sequence number. (Flags ACK, sequence number
  y, acknowledge number x1)
• Step 3 The receiver can continue sending data in
  the receiver-sender direction. When it does not
  have any more data to send, it sends the third
  segment. This segment is a FIN segment. (Flags
  FIN, sequence number y m, acknowledge number
  x1)
• Step 4 The sender sends the fourth segment, an
  ACK segment, to confirm the receipt of the FIN
  segment from the receiver. This segment contains
  the acknowledgement number , which is one plus
  the sequence number received in the FIN from the
  server. (Flags ACK, sequence number x 1,
  acknowledge number ym1)

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4. TCP Operation Scenarios
BF Figure 12-23 Four-way Handshaking
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4. TCP Operation Scenarios
BF Figure 12-24 State Transition Diagram
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4. TCP Operation Scenarios

• Notations of Figure 12.24
• Lines
• Dotted lines of the figure represent the server
• Solid lines represent the client
• Thin lines represent unusual situations
• Event/action
• Event can take the machine to a new state
• Event can also make the machine perform some
  actions

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4. TCP Operation Scenarios - States for TCP
BF Table 12.3 States for TCP
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4. TCP Operation Scenarios

• Reading assignments
• BF Chapter 12 TCP Sections 12.9 Connection
• BF Chapter 12 TCP Sections 12.10 State Transition
  Diagram

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4. TCP Operation Scenarios
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4. TCP Operation Scenarios
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4. TCP Operation Scenarios

• TIME-WAIT state
• The value of the timer is set to double the
  lifetime estimate of a segment of maximum size
• Reason for using the timer
• While the client TCP of the connection has sent
  an ACK in response to the server TCPs FIN
  segment, it does not know that the ACK was
  successfully delivered.
• As a consequence, the server TCP might retransmit
  its FIN segment.
• The second FIN segment might by delayed in the
  network.
• If the connection were allowed to move directly
  from the FIN-Wait-2 state to the CLOSED state,
  then another pair of application processes might
  come along and open the same connection (i.e.,
  use the same pair of port numbers), and the
  delayed FIN segment from the earlier incarnation
  of the connection would immediately initiate the
  termination of the later incarnation of that
  connection.