CSC 600 Internetworking with TCP/IP

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CSC 600Internetworking with TCP/IP

• Unit 3 Transport Layer
• (Ch. 13, 12)
• Dr. Cheer-Sun Yang
• Spring 2001

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Introduction

• Transmission Control Protocol provides
  connection-oriented reliable transport services.
• User Datagram Protocol (UDP) provides
  connectionless unreliable transport services.

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TCP UDP

• Transmission Control Protocol
• Connection oriented
• RFC 793
• User Datagram Protocol (UDP)
• Connectionless
• RFC 768

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Reliable vs. Unreliable

• Reliable transport service handles error recovery
  at the transport level.
• Unreliable transport service does not provide
  error recovery at at the transport level.

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Connection-oriented vs.Connection-less

• Connection-oriented service must establish
  connection between the source and the destination
  first.
• Connection-less service does not establish
  connection first. It simply does
  store-and-forward.

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Properties of the Reliable Delivery Service

• Stream orientation - ordered delivery
• Virtual circuit connection connection
  establishment is must prior to segment delivery
• Buffered transfer data buffering is needed
• Unstructured stream TCP segments may not be as
  big as a record in a payroll application.
• Full duplex connection Connections provided by
  the TCP/IP stream service allow concurrent
  transfer in both direction.

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Properties of the Reliable Delivery Service

• TCP provides reliable transport service using
  sliding window protocol as defined in the Data
  Link Layer Protocol.

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• Transmission Control Protocol

TCP is a communication protocol, not a piece of
software.
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TCP vs. the Implementation

• TCP is the communication protocol.
• TCP is implemented by many venders in software as
  part of the Operating System.
• The difference between a protocol and the
  software that implements it is analogous to the
  difference between the definition of a
  programming language and a compiler.

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What does TCP Specify?

• Data segment format
• Timing
• Meanings of header fields
• Functions of TCP also referred to as services
  provided by TCP

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What does TCP not specify?

• The user interface is not specified.
• The underlying communication system can be a
  dialup telephone line, a local area network, a
  high speed fiber optical network, or a lower
  speed long haul network.

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TCP Services

• Reliable communication between pairs of processes
• Across variety of reliable and unreliable
  networks and internets
• Two labeling facilities
• Data stream push
• TCP user can require transmission of all data up
  to push flag
• Receiver will deliver in same manner
• Avoids waiting for full buffers
• Urgent data signal
• Indicates urgent data is upcoming in stream
• User decides how to handle it

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TCP Header
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Items Passed to IP

• TCP passes some parameters down to IP
• Precedence
• Normal delay/low delay
• Normal throughput/high throughput
• Normal reliability/high reliability
• Security

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TCP Header Field

• Port Number
• source and destination port numbers (why source
  port number?)
• why not IP addresses?
• Identifies an application
• Together with IP address to form an end point

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TCP Header Field

• Sequence Number
• 32 bits long
• the range of sequence number is 0 lt seq lt 2
  32 -1
• Each sequence number identifies the byte in the
  stream of data from the sending TCP to the
  receiving TCP where the first byte of data is
  located in the segment
• Initial Sequence Number (ISN) of a connection is
  set during connection management

1 200 201
400 401 600
segment 1 segment 2
segment 3
(seq 1)
(seq 201) (seq
401)
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TCP Header Field

• Acknowledgement Nubmer
• Acknowledgements are piggybacked if there is a
  segment ready to be sent from the receiver to the
  sender
• The acknowledgement segment consists of the next
  sequence number expected

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TCP Header Field

• Header Length
• Why is this needed ?

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TCP Header Field
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TCP Header Field

• Flags
• URG - if the URG 1, the following bytes contain
  an urgent message seq lt urgent message lt seq
  urgent pointer
• ACK acknowledgement number is valid
• PSH
• notification from sender to receiver to force the
  TCP on the receiver side to pass all data
  received to the application layer
• Normally sent by the sender when the senders
  buffer is empty so the sender does not wait for
  more data
• RST Reset the connection
• SYN synchronization request for the sequence
  number
• FIN Finish flag

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TCP Header Field

• Options
• End of options 1 byte
• NOP 1 byte
• Maximum segment size 4 bytes
• Window scale factor 3 bytes
• increases the TCP window size from 16 bits to 32
  bits
• 1-byte shift count is between 0 and 14
• used in the connection establishment for window
  size negotiation
• Timestamp 10 bytes
• sender places a timestamp in a segment
• receiver places an echo reply
• this allows the sender to calculate the
  Round-Trip Time per window

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TCP Header Field(Options)
0
End of options
NOP
1
2 4
MSS
Window scale factor
3 3 S
S shift count
8 10 timestamp timestamp
echo reply
Timestamp
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Transport Layer Issues

• Addressing
• Connection establishment
• Connection termination
• Flow Control
• Timeout and retransmission
• Congestion Control
• Multiplexing
• Duplication detection
• Crash recovery

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TCP Mechanisms

• Connection establishment
• Data transfer
• Send policy
• Deliver policy
• Accept policy in-order, in-window
• Retransmission policy first-only, batch,
  individual
• Acknowledgement Policy

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Addressing

• Target user specified by
• User identification
• Usually host, port
• Called a socket in TCP
• Port represents a particular transport service
  (TS) user
• Transport entity identification
• Generally only one per host
• If more than one, then usually one of each type
• Specify transport protocol (TCP, UDP)
• Host address
• An attached network device
• In an internet, a global internet address
• Network number

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Finding Addresses

• Four methods
• Know address ahead of time
• e.g. collection of network device stats
• Well known addresses
• Name server
• Sending process request to well known address

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Ports, Connections, and Endpoints

• TCP uses the connection, not the protocol port,
  as its fundamental abstraction connections are
  identified by a pair of endpoints, i.e.,
  (18.26.0.36, 1069) and (128.10.2.3, 25).
• An endpoint is a pair of integers (host, port).
• Because TCP identifies a connection by a pair of
  endpoints, a given TCP port number can be shared
  by multiple connections on the same machine.

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Connection Establishment

• Connection establishment
• Three way handshake
• Between pairs of ports
• One port can connect to multiple destinations

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Passive and Active Opens

• A client requests for a connection an active
  open request.
• A server must be waiting for the request for
  connection a passive open.

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Connection Establishment

• Two way handshake
• A send SYN, B replies with SYN
• Lost SYN handled by re-transmission
• Can lead to duplicate SYNs
• Ignore duplicate SYNs once connected
• Lost or delayed data segments can cause
  connection problems
• Segment from old connections
• Start segment numbers fare removed from previous
  connection
• Use SYN i
• Need ACK to include i
• Three Way Handshake

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Two Way HandshakeObsolete Data Segment
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Two Way HandshakeObsolete SYN Segment
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Three WayHandshakeExamples
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Connection Establishment
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Three Way HandshakeState Diagram
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Initial Sequence Number

• When a new connection is being established, the
  SYN flag is turned on. The sequence number field
  contains the ISN chosen by the host for this
  connection.
• The sequence number of the first byte of data
  sent by the host will be the ISN plus one because
  the SYN flag consumes a sequence number.

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Connection Termination

• Entity in CLOSE WAIT state sends last data
  segment, followed by FIN
• FIN arrives before last data segment
• Receiver accepts FIN
• Closes connection
• Loses last data segment
• Associate sequence number with FIN
• Receiver waits for all segments before FIN
  sequence number
• Loss of segments and obsolete segments
• Must explicitly ACK FIN

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Data Transfer

• Data transfer
• Logical stream of octets
• Octets numbered modulo 223
• Flow control by credit allocation of number of
  octets
• Data buffered at transmitter and receiver

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Send Policy

• If no push or close TCP entity transmits at its
  own convenience
• Data buffered at transmit buffer
• May construct segment per data batch
• May wait for certain amount of data

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Deliver Policy

• In absence of push, deliver data at own
  convenience
• May deliver as each in order segment received
• May buffer data from more than one segment

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Accept Policy

• Segments may arrive out of order
• In order
• Only accept segments in order
• Discard out of order segments
• In windows
• Accept all segments within receive window

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Not Listening

• Reject with RST (Reset)
• Queue request until matching open issued
• Signal TS user to notify of pending request
• May replace passive open with accept

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Connection Termination

• Connection termination
• Graceful close
• TCP users issues CLOSE primitive
• Transport entity sets FIN flag on last segment
  sent
• Abrupt termination by ABORT primitive
• Entity abandons all attempts to send or receive
  data
• RST segment transmitted

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Termination

• Either or both sides
• By mutual agreement
• Abrupt termination
• Or graceful termination
• Close wait state must accept incoming data until
  FIN received

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Side Initiating Termination

• TS user Close request
• Transport entity sends FIN, requesting
  termination
• Connection placed in FIN WAIT state
• Continue to accept data and deliver data to user
• Not send any more data
• When FIN received, inform user and close
  connection

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Side Not Initiating Termination

• FIN received
• Inform TS user Place connection in CLOSE WAIT
  state
• Continue to accept data from TS user and transmit
  it
• TS user issues CLOSE primitive
• Transport entity sends FIN
• Connection closed
• All outstanding data is transmitted from both
  sides
• Both sides agree to terminate

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Usage of tcpdump

• A program called tcpdump on taz.cs.wcupa.edu has
  been installed for monitoring TCP mechanisms.
• It requires root privilege. So Dr. Kline set up a
  script called TCPDUMP for us to run tcpdump.
• For details, see homework sheet.

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Output of tcpdump

• On taz.cs.wcupa.edu, each segment sent is printed
  out twice. It looks odd.
• TCPDUMP prints out each segment in the following
  format source gt destination flags, where the
  flags represents S(SYN), F(FIN), R(RST), P(PSH),
  and a dot(.).
• The sequence numbers are followed by the number
  of data bytes. For example 14155315211415531521(
  0) is a segment without data.

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Output of tcpdump

• Option fields are printed out.
• MSS - maximum segment size
• WSCALE window scale
• NOP no operation (used for padding a field
  length to a multiple of four bytes).
• ltmss 512,nop,wscale 0,nop,nop,timestamp 146647 0gt

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Flow Control

• Longer transmission delay between transport
  entities compared with actual transmission time
• Delay in communication of flow control info
• Variable transmission delay
• Difficult to use timeouts
• Flow may be controlled because
• The receiving user can not keep up
• The receiving transport entity can not keep up
• Results in buffer filling up

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The idea Behind Sliding Windows

• A simple positive acknowledgement protocol wastes
  a substantial amount of network bandwidth because
  it must delay sending a new packet until it
  receives an acknowledgement for the previous
  packet.

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Window Size and Flow Control

• TCP allows the window size to be changed over
  time.
• Each ACK, which specifies how many octets have
  been received, contains a window advertisement
  that specifies how many additional octets of data
  the receiver is prepared to receive.
• It is the receivers current buffer size.

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Coping with Flow Control Requirements (1)

• Do nothing
• Segments that overflow are discarded
• Sending transport entity will fail to get ACK and
  will retransmit
• Thus further adding to incoming data
• Refuse further segments
• Clumsy
• Multiplexed connections are controlled on
  aggregate flow

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Coping with Flow Control Requirements (2)

• Use fixed sliding window protocol
• See chapter 7 for operational details
• Works well on reliable network
• Failure to receive ACK is taken as flow control
  indication
• Does not work well on unreliable network
• Can not distinguish between lost segment and flow
  control
• Use credit scheme

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Credit Scheme

• Greater control on reliable network
• More effective on unreliable network
• Decouples flow control from ACK
• May ACK without granting credit and vice versa
• Each octet has sequence number
• Each transport segment has seq number, ack number
  and window size in header

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Use of Header Fields

• When sending, seq number is that of first octet
  in segment
• ACK includes ANi, Wj
• All octets through SNi-1 acknowledged
• Next expected octet is i
• Permission to send additional window of Wj
  octets
• i.e. octets through ij-1

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Credit Allocation
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Sending and Receiving Perspectives
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Unreliable Network Service

• E.g.
• internet using IP,
• frame relay using LAPF
• IEEE 802.3 using unacknowledged connectionless
  LLC
• Segments may get lost
• Segments may arrive out of order

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Ordered Delivery

• Segments may arrive out of order
• Number segments sequentially
• TCP numbers each octet sequentially
• Segments are numbered by the first octet number
  in the segment

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Retransmission Strategy

• Segment damaged in transit
• Segment fails to arrive
• Transmitter does not know of failure
• Receiver must acknowledge successful receipt
• Use cumulative acknowledgement
• Time out waiting for ACK triggers
  re-transmission

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Timer Value

• Fixed timer
• Based on understanding of network behavior
• Can not adapt to changing network conditions
• Too small leads to unnecessary re-transmissions
• Too large and response to lost segments is slow
• Should be a bit longer than round trip time
• Adaptive scheme
• May not ACK immediately
• Can not distinguish between ACK of original
  segment and re-transmitted segment
• Conditions may change suddenly

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TCP Timers

• Retransmission Timer started during a
  transmission. A timeout causes a retransmission.
• Persist Timer ensures that window size
  information is transmitted even if no data is
  transmitted.
• Keepalive Timer detects crashes on the other end
  of connection.
• Other Timers delay ACK timer, timeout of
  connection setup, abort timeout, 2MSL(Maximum
  Segment Lifetime) timeout(closing timeout).

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Acknowledgement Policy

• Immediate
• Cumulative

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Congestion Control

• RFC 1122, Requirements for Internet hosts
• Retransmission timer management
• Estimate round trip delay by observing pattern of
  delay
• Set time to value somewhat greater than estimate
• Simple average
• Exponential average
• RTT Variance Estimation (Jacobsons algorithm)

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Congestion Control Avoidance

• TCP must remember the size of the receivers
  window. To control congestion, TCP maintains a
  second limit, called the congestion window limit,
  that is used to restrict data flow to less than
  the receivers buffer size.
• Multiplicative Decrease Congestion Avoidance To
  estimate congestion window size, TCP assumes that
  most datagram loss comes from congestion and

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Congestion Control Avoidance

• Upon loss of a segment, the sender reduces the
  congestion window by half. For those segments tha
  remain in the allowed window, backoff
  retransmission timer exponentially.
• Slow Start When congestion ends, increase the
  congestion window exponentially until it reaches
  the receivers window limit.
• The term slow start is a misnomer since the
  congestion window grows exponentially.

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Congestion Control

• Slow start
• awnd MINcredit, cwnd
• Start connection with cwnd1
• Increment cwnd at each ACK, to some max
• Dynamic windows sizing on congestion
• When a timeout occurs
• Set slow start threshold to half current
  congestion window
• ssthreshcwnd/2
• Set cwnd 1 and slow start until cwndssthresh
• Increasing cwnd by 1 for every ACK
• For cwnd gtssthresh, increase cwnd by 1 for each
  RTT

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Response to Congestion

• How can a router avoid global congestion?
• Tail drop if the input queue is fulled when a
  datagram arrives, discard the datagram.
• Random Early Discard(RED)

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RED

• A router uses two threshold values to mark
  positions in the queue Tmin and Tmax. The
  general operation of RED can be described by
  three rules that determine the disposition of
  each arriving datagram
• if the queue currently contains fewer than Tmin
  datagrams, add the new datagram to the queue.
• If the queue contains more than Tmax datagrams,
  discard the new datagram.
• If the queue contains between Tmin and Tmax
  datagrams, randomly discard the datagram
  according to a probability p.

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Timeout and Retransmit

• TCP maintains queue of segments transmitted but
  not acknowledged
• TCP will retransmit if not ACKed in given time
• Measurements of round trip times vary
  dramatically over time.

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Exponential RTO Backoff

• Since timeout is probably due to congestion
  (dropped packet or long round trip), maintaining
  a constant RTT is not a good idea
• RTT increased each time a segment is
  re-transmitted

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Timeout and Retransmit

• Adaptive retransmission algorithm(RFC 793)

RTT ? old RTT (1 - ?) new Round Trip
Sample
RTO ? RTT (usually ? 2)
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Use of Exponential Averaging
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Responding to High Variance in Delay - Jacobsons
Algorithm
DIFF Sample - old RTT smoothed RTT old RTT
? DIFF mean deviation old mean deviation ?
(DIFF - old mean deviation) RTO smoothed RTT
? mean deviation ? between 0 and 1 ?
inverse of a power of 2 ? inverse of a power of
2
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Karns Algorithm

• If a segment is re-transmitted, the ACK arriving
  may be
• for the first copy of the segment, then RTT
  longer than expected
• for second copy, then RTT shorter than expected
• No way to tell
• Do not measure RTT for re-transmitted segments
• Calculate backoff when re-transmission occurs
• Use backoff RTO until ACK arrives for segment
  that has not been re-transmitted

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Silly Window Syndrome

• A problem occurs when the sender and the receiver
  operate at different speeds.
• When the receiving application reads an octet of
  data from a full buffer, one octet of space
  becomes available. The TCP on the receiver
  generates a segment the inform the sender that 1
  octet is available.
• The sender sends out a segment of one byte.
• This results in a series of small data segment -
  silly window syndrome (SWS).

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Silly Window Syndrome Avoidance-Nagle Algorithm

• Receiver-side avoidance delay acknowledgement
• Sender-side avoidance delay transmission
  adaptively.

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Multiplexing

• Multiple users employ same transport protocol
• User identified by port number or service access
  point (SAP)
• May also multiplex with respect to network
  services used
• e.g. multiplexing a single virtual X.25 circuit
  to a number of transport service user
• X.25 charges per virtual circuit connection time

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Duplication Detection

• If a segment is lost and retransmitted, no
  confusion will result.
• If, however, an ACK is lost, one or more segments
  will be retransmitted and, if they arrive
  successfully, the receiver must be able to
  recognizes duplicates.

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Duplication Detection

• Duplicate received prior to closing connection
• Receiver assumes ACK lost and ACKs duplicate
• Sender must not get confused with multiple ACKs
• Sequence number space large enough to not cycle
  within maximum life of segment
• Duplicate received after closing connection

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Crash Recovery

• After restart all state info is lost
• Connection is half open
• Side that did not crash still thinks it is
  connected
• Close connection using persistence timer
• Wait for ACK for (time out) (number of retries)
• When expired, close connection and inform user
• Send RST i in response to any i segment arriving
• User must decide whether to reconnect
• Problems with lost or duplicate data

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UDP

• User datagram protocol
• RFC 768
• Connectionless service for application level
  procedures
• Unreliable
• Delivery and duplication control not guaranteed
• Reduced overhead
• e.g. network management (Chapter 19)

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UDP Uses

• Inward data collection
• Outward data dissemination
• Request-Response
• Real time application

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UDP Header
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Recommended Reading

• Comer chapter 12, chapter 13
• Stallings chapter 17
• RFCs