Advanced Network Programming Chapter 1

1
Advanced Network ProgrammingChapter 1

• Introduction
• to
• Transport Layer and TCP

2
Networking (OSI) Reference Model

• OSI 7-layer Reference Model
• Application (WEB, NFS, FTP, Telnet, etc.)
• Presentation (Conversion, Compression,
  Cryptography)
• Session (Synchronization)
• Transport (End-to-end Messages)
• Network (Packet Routing)
• Data Link (Station-to-station Frames)
• Physical (Bit Transmission)

3
Networking

• Distinction between service and protocol is
  important!
• This will be discussed later.
• Some widely known transport protocols
• UDP, TP0, TP4, SNA-APPN, DECnet-NSP, ATM, XTP,
  T/TCP, RTP, VMTP, NETBLT

4
Transport Layer (Layer-4)

• Lowest layer that operates on an end-to-end
  basis.
• Lies at the boundary between hosts and an
  internetwork of routers, bridges, and
  communication links.
• A good transport layer service
• Allows applications to use a standard set of
  primitives.
• Run on variety of networks w/o worrying about
  different network interfaces and reliabilities.
• Isolates applications from the technology.

5
Transport Layer (Layer-4)

• Layer-4 provides interprocess communication
  between two processes that most often are running
  on different hosts.
• TCP and its companion UDP (User Datagram
  Protocol) are the most widely used protocols.
• Other are from IBMs SNA, and Digitals (Compaq)
  DECnet.
• Connection to proprietary protocol suites.
• Ongoing research
• tcp-impl WG of IETF (www.ietf.org)
• end2end WG of IRTF (www.irtf.org)

6
Transport Layer (Layer-4)

• Basic Issues
• Addressing
• Connection-oriented (CO) vs. Connectionless (CL)
• Reliability
• Loss
• Duplicate
• Ordering
• Integrity
• Blocking vs. Non-Blocking
• Multicast, Unicast
• Priority
• Security
• Status Reporting

7
Role of TCP

• The Web
• An example of client/server application
• Web browser (client)
• Runs on local machine
• Communicates with a server on some remote
  machine
• Uses an application layer protocol called the
  HTTP (Hypertext Transfer Protocol).
• HTTP is a simple request/response protocol.
• We will use HTTP 0.9 (the simplest) in examples.

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Role of TCP

• Web browser (client)
• Access TCPs service thru function calls that
  comprise that Transport Layers Application
  -Programming Interface (API).
• API provides
• (at a minimum) functions to send and receive
  messages
• e.g. Berkeley Sockets read(), write()
• Connection setup and close for CO protocols
• e.g. connect(), close()

9
Terminology

• Simplified Communication Model (OSI) Figure-1
• User Sender / User Receiver at the top
• Application Entities use the services of the
  transport layer
• Peer Entities exchange Protocol Data Units (PDUs)
• APDU
• The request get /index.html
• sent from client (application entity) to the
  server (its peer application entity).
• Bi-directional Protocol
• Both sides can send and receive data
  simultaneously.

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Terminology

• Transport Entity
• Hardware and/or software within a given host that
  implements a particular transport service and
  protocol.
• User Sender
• submits a chunk of user data (Transport Service
  Data Unit TSDU informally a message) to the
  transport sender.
• Transport Sender
• transmits or sends this data to the transport
  receiver over a network which may provide
  different levels of reliability.

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Terminology

• Transport Receiver
• receives the data that arrives from the network
  and delivers it to the user receiver.
• TPDUs may flow in both directions even when user
  data flows only from sender to receiver
• Control TPDUs
• Separate and/or Piggybacked

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Terminology

• What happens to the request APDU?
• APDU becoming a single TSDU, being encapsulated
  in a single TPDU, which in turn becomes a single
  NSDU, which is encapsulated in a single NPDU
  (Figure-2)
• TCPs TPDU ? TCP segment
• Packet ? IP datagram (NPDU) or TCP segment
  (informally)
• IPs PDU ? Datagram
• Datagram ? IPs NPDU or UDPs TPDU (informally)

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Example TCP Connection (1)

• Enter http//ozgit.nom.tr/index.html from web
  client.
• http indicates application layer protocol to be
  used.
• TCP port number 80 (implicitly) to be used.
• ozgit.nom.tr is the host name (mapped to an IP
  number 144.122.71.91- by DNS)
• Transport Service Access Point (TSAP)
• TSAP ? IP Address TCP Port Number
• One end point of a communication channel between
  a process on a local m/c and a process on a
  remote m/c.
• index.html is the file being requested.
• http request (APDU) ? GET /index.html

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Example TCP Connection (2)

• Connection request to the transport entity at
  (144.122.71.91, 80).
• By calling connect()
• Local TCP initiates a 3-way handshake with the
  remote server.
• TPDUs are exchanged between TCP entities to
  ensure reliable connection establishment and to
  establish initial sequence numbers.
• If 3-way handshake fails, TCP notifies the
  application.
• Otherwise success code is returned -confirmation.
• OSI Model
• Request Indication
• Response - Confirmation

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Example TCP Connection (3)

• Web client submits a request to send data (APDU
  GET /index.html)
• Local TCP sends this data most likely in a single
  TPDU.
• TCP Segment ? TSDU Transport Layer Header

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Example TCP Connection (4)

• Remote TCP receives the TPDU, the data (APDU
  GET /index.html) is buffered.
• Delivered when Web server does a read()
• This delivery is known as a data indication in
  OSI terminology.
• Remote TCP also sends back an acknowledgement
  (ACK) -control TPDU- to the local TCP

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Example TCP Connection (5)

• The Web server responds with contents of
  index.html.
• File may be too large to be efficiently submitted
  to TCP in one write() call i.e., one TSDU.
• Web Server divides APDU into multiple write()
  calls i.e., multiple TSDUs.
• Remote TCP then sends these TSDUs to local the
  TCP in multiple TPDUs.
• TCP treats the data as a byte stream and segments
  it as necessary i.e., does not care about TSDU
  boundaries.
• Boundaries between APDUs, submitted TSDUs, TPDUs,
  and delivered TSDUs may all be different.

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Example TCP Connection (6)

• TCP must detect and recover from network errors.
• As the remote TCP send the TPDUs, it includes a
  sequence number in each TPDU.
• It also copies each TPDU into a buffer, and sets
  a timer.
• Retransmits the TPDU if timer expires before
  getting an ACK.
• Retransmission is done in a new TPDU.
• Individual byte-stream sequence numbers are used.
• TPDUs retransmitted may or may not correspond
  exactly to the original TPDUs.
• Remote TCP also places a checksum is the TPDU
  header to detect bit errors.

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Example TCP Connection (7)

• As TPDUs are received by the local TCP
• TPDUs with checksum errors are discarded.
• It ensures that no pieces of the byte-stream are
  missing
• Out-of-order arrivals are reordered.
• It responds to the remote TCP with ACK TPDUs.
• Duplicates are discarded (e.g., as a consequence
  of lost ACK TPDUs).
• Pieces of byte-stream are buffered in local TCP
• Web client requests them by doing read() calls.
• Each read() results in delivery of a TSDU.

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Example TCP Connection (8)

• TCP connection is bi-directional.
• Either side may initiate the closing of the
  connection
• In first generation web systems the server
  initiates the close by calling close() function
  (Disconnect Request).
• Disconnect is handled with a 4-way handshake
  procedure.

21
Transport Service

• A transport service abstracts a set of functions
  that is provided to a higher layer.
• A protocol, refers to the details of how a
  transport sender and a transport receiver
  cooperate to provide that service.
• Distinction between service and protocol is
  important (Contribution of OSI Reference Model).

22
CO-message vs. CO-byte vs. CL

• Two types of transport services
• Connection-oriented (CO)
• Provides for the establishment, maintenance, and
  termination of a logical connection between
  transport users (three distinct phases of
  operation).
• Connection Establishment (T-Connect)
• Data Transfer (T-Data)
• Connection Termination (T-Disconnect)
• CO service has two variations
• Message-oriented (TP4)
• Byte-stream
• Connectionless (CL)
• Provides only one phase of operation data
  transfer.

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Reliability

• A service is reliable if and only if it satisfies
  all of the following
• No-loss
• No-duplicates
• Ordered
• Data Integrity

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No-loss vs. Uncontrolled-loss vs. Controlled-loss

• No-loss (at-least-once delivery) service
  guarantees either of the two results
• The data is delivered to the user receiver, or
• The user sender is notified that some data may
  not have been delivered.
• Uncontrolled-loss (best-effort)
• No assurance
• Example UDP
• Controlled-loss
• Loss may occur, but there is control over the
  degree of loss.
• Example k-XP

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No-duplicates vs. Maybe-duplicates

• No-duplicates
• At-most-once delivery
• e.g., TCP
• Maybe-duplicates
• Efforts by the protocol may or may not be made to
  avoid delivering duplicates.
• e.g., UDP

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Ordered vs. Unordered vs. Partially-ordered

• Ordered service
• Preserves user senders submission order of data.
• e.g., TCP
• Unordered service
• Does not provide the above guarantee.
• e.g., UDP
• Partially-ordered service
• Guarantees to deliver pieces of data in one of a
  set of permitted orders as predefined by a
  partial order relation agreed upon by the user
  sender and user receiver.
• e.g., Multimedia comm., distributed databases.

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Data-integrity vs. No-data-integrity vs.
Partial-data-integrity

• Data-integrity
• Ensures with high probability that all data bits
  delivered to a user receiver are identical to
  those originally submitted.
• Strength of the error detection method.
• TCP uses 16-bit checksum.
• No-data-integrity
• Provide no guarantees regarding bit errors.
• Partial-data-integrity
• A controlled amount of bit errors (as a means of
  achieving higher throughput).
• e.g., real-time multimedia application

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Remarks on Reliability and CO vs. CL

• All aspects of reliability (loss, duplicates,
  order, data-integrity) are orthogonal.
• Data might get lost while the order is preserved.
• Relationship between a service being CO or CL and
  whether or not it is reliable.
• These two services are orthogonal.
• CO service is assumed to be reliable. Why?

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Remarks on Reliability and CO vs. CL

• Whereas TCP service is CO and TCP service is
  reliable,
• Whereas TP4 service is CO and TP4 service is
  reliable,
• Whereas X.25 service is CO and X.25 service is
  reliable
• CO service ? Reliable Service ?

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Remarks on Reliability and CO vs. CL

• Whereas UDP service is CL and UDP service is
  unreliable,
• CL service ? Unreliable Service ?

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Blocking vs. Non-blocking

• Blocking service
• Ensures that the transport layer is not
  overwhelmed with incoming data.
• Provides flow control between user sender and
  transport sender.
• Non-blocking service
• Allows the user sender to submit data and
  continue processing w/o awaiting the transport
  senders OK.

32
Multicast vs. Unicast

• Multicast service
• Enables a user sender to submit data, a copy of
  which will be delivered to one or more user
  receiver(s).
• Unicast service
• Delivery of data to exactly one user receiver.

33
Priority vs. No-priority

• Priority service
• Enables a user sender to indicate the relative
  importance of various messages.
• May be combined with uncontrolled-loss or
  controlled-loss service to drop lower priority
  data, thereby allowing the delivery of
  high-priority data with smaller delay and/or
  higher probability.
• No-priority service
• No differentiation of the importance of the
  classes of data.

34
Security vs. No-security

• Security service
• A security service provides one or more security
  functions such as authentication, access control,
  confidentiality, and integrity ISO-7498-2.
• Authentication is the verification of user
  senders and user receivers identity.
• Access control checks a users permission status,
  allowing the use of different resources.
• Confidentiality guarantees that only the intended
  user receiver(s) can decode and understand the
  user senders data.
• Integrity detects any modification, insertion,
  deletion, or replay of transport senders data.
• e.g., TP4
• No-security service
• Does not provide any of the above security
  functions.

35
Status-reporting vs. Non-status-reporting

• Status-reporting service
• Allows a user sender to obtain specific
  information about the transport entity or its
  connections.
• Non-status reporting service
• Does not provide any information about the
  transport entity and its connections.

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QoS vs. No-QoS

• QoS service
• Allows a user sender to specify the quality of
  transmission service desired.
• No-QoS service
• Delivery of data to exactly one user receiver.

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QoS Parameters (ISO)

• Connection Establishment Delay
• Connection Establishment Failure Probability
• Throughput
• Transit Delay
• Residual Error Rate
• Transfer Failure Probability
• Connection Release Delay
• Connection Release Failure Probability

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QoS Parameters (ISO)

• Protection
• Priority
• Resilience
• The ATM environment supports only two QoS
  parameters
• (sustained) target, acceptable, and minimum
  throughput
• Transit delay

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Transport Protocol Features

• CO vs. CL
• Establishment and maintenance of state
  information
• A record of characteristics and events related to
  the communication between the transport sender
  and receiver.
• CO state information is maintained
• Three phases
• Connection Establishment
• Data Transfer
• Connection Termination
• CL no state information is maintained

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Transport Protocol Features

• Transaction Oriented
• A single APDU (request) is sent by user sender
• User receiver responds with a single APDU
  (response)
• Characteristics
• Asymmetrical model
• Simplex data transfer
• Short duration
• Low delay
• Few data TPDUs
• Message orientation
• Need for a no-duplicate service

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Transport Protocol Features

• CO Protocol Features
• Signaling exchange of control (state)
  information
• In-band (more suitable for short-lived
  connections)
• Out-of-band (desirable for high-speed
  communication systems)
• Unidirectional vs. Bidirectional

42
Transport Protocol Features

• Connection Establishment (See Figure-3)
• Implicit connect
• Connection is established as soon as the first
  TPDU is sent or received.
• 2-way-handshake connect
• CR-TPDU (Connection Request)
• CC-TPDU (Connection Confirm)
• 3-way-handshake connect
• CR-TPDU (Connection Request)
• CC-TPDU (Connection Confirm)
• ACK-CC-TPDU (ACK for Connection Confirm)

43
Transport Protocol Features

• Connection Termination (See Figure-4)
• Implicit disconnect
• Time-out
• Abortive disconnect
• Close connection abnormally due to an error
  condition
• 2-way-handshake disconnect
• DR-TPDU (Disconnect Request)
• DC-TPDU (Disconnect Confirm)
• 4(3)-way-handshake disconnect
• Two 2-way-handshakes one for each direction of
  data flow
• 3-way if the first DC-TPDU also functions as a
  DR-TPDU for the reverse direction

44
Error Control

• Guard against loss or damage of user data and
  control information
• For realistic high-speed networks with low error
  rates, transport layer error control is more
  efficient than link layer error control.
• Two phases
• Error detection
• Error reporting and recovery

45
Error Control

• Error Detection
• Identifies lost, misordered, duplicated and
  corrupted TPDUs
• Sequence numbers handles the first three problems
• Corrupted data is discovered by means of
• Length fields
• Error Detecting Codes (EDC)
• The header/trailer, the data, or the both
• Separate EDCs are recommended for multimedia
  applications

46
Error Control

• Error Reporting and Recovery
• Error reporting is a mechanism where receiver
  explicitly informs the sender about errors that
  have been detected.
• Error recovery is a mechanism used by both sender
  and receiver to recover from errors whether or
  not they are explicitly reported.
• Timers, sequence numbers and acknowledgements are
  used.

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Error Control

• Error Reporting and Recovery
• A positive ACK (PACK)
• PAR (Positive ACK with Retransmission) or ARQ
  (Automatic Repeat Request)
• Upon receipt of an ACK, the sender updates its
  state information, discards buffered TPDUs that
  are acknowledged, and retransmits any TPDUs that
  are not acknowledged.
• In case of timeout, it may assume something has
  gone wrong and retransmits unacknowledged
  TPDU(s).
• No error reporting mechanism

48
Error Control

• Error Reporting and Recovery
• A negative ACK (NACK) aka Selective Reject
• Explicitly identifies TPDUs that have not been
  received

49
Error Control

• Piggybacking
• Artificially delay returning an ACK hoping the
  receiver will soon submit its next message to be
  sent as a part of the reverse direction data
  flow.
• When this occurs, the ACK is piggyback-ed as
  header information on the reverse direction data
  TPDU.

50
Error Control

• Cumulative vs. Selective Acknowledgement
• Cumulative PACK
• Carries a sequence number indicating that all
  TPDUs with lower sequence numbers have been
  received.
• A recent cumulative PACK incorporates the
  information of the previously lost one.
• Unnecessary retransmissions of correctly received
  TPDUs.
• Selective PACK
• Acknowledges exactly one TPDU
• Block PACK
• Variation of selective PACK where blocks of
  individual TPDUs are selectively acknowledged.

51
Error Control

• Retransmission Strategies
• When the sender does not receive a PACK within a
  pre-determined timeout period, or when it
  receives back-to-back cumulative PACKs that are
  identical.
• Selective Repeat (Conservative)
• Sender retransmits selectively only TPDUi and
  wait for a PACK with sequence number larger than
  previous PACKs.
• Go-Back-N (More Aggressive)
• Sender retransmits TPDUi and all TPDUs already
  sent after TPDUi
• Decrease channel utilization by potentially
  retransmitting correctly-received TPDUs.

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Flow Control
53
Multiplexing/Demultiplexing

• See Figure 5(a)
• Several transport layer connections using a
  single network layer association.
• Efficient use of network layer resources.

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TCP (Transmission Control Protocol)

• Connection-oriented (CO)