Basic Communications Protocols

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Basic Communications Protocols

• Workshop on QoS
• Hanoch Levy
• Feb 2004

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The objective A talk to B , C talk to D, best
performance

• A

B
A
D
C
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Communications and Standards

• Communication networks must be based on
  standards!!!
• Much more than any software!!!
• The structure of standards must be modular
• Thus a STANDARD and MODULAR set of communications
  rules must be defined.

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The reference Model

• A standard framework for describing standard
  approached for delivering data over a network.
• Placed in different locations
• Want to pass data of some type, from one to
  another.
• Want this to be done good/best/ASAP.
• Want to do it with certain minimal quality
  requirements.

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The method A layered Model

• Each layer is responsible for certain tasks
• Each layer goes across the network
• Each software piece talks horizontally with its
  peer piece (on another hardware)
• Each software piece talks upward and downward to
  its parent and sibling pieces (on same hardware).

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The principle of a layered model

• Each layer element
• Talk to its peer element
• Carries traffic for its parent
• Deliver traffic through its child

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The layers Layer 1 - Physical

• Deals with the physics of the media
• Attaches to the iron and understands the iron
  (cooper, fiber, ether).
• Translates bits to electrical/light/radio signals
  and vice versa
• Called Modem (Modulator and Demodulator)
• Is responsible of transferring bits from one side
  to the other.
• When B receives a bit ? transfers it to B
• Forms a BIT PIPE

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Physical Layer a bit pipe
A bit pipe
C
C
bit
B
B
Physical medium (fiber, cooper, radio, pigeon)
A
A
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Link Layer (2)
Completed 26/3/04

• Assures transferring of a packet in reliable way
  across a link .
• Uses the bit-pipe.
• Uses mechanism to recover from problems
• If packet does not arrive or arrive bad
• Add error detection on packet.
• E.g. parity bits
• Add packet numbers.
• Add acks (to acknowledge receipt)
• Retransmit if necessary.

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Link Layer (layer 2) a reliable packet pipe
A packet pipe
D
D
packet
C
C
Physical layer
B
B
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Network Layer (3)

• Assures transferring of a packet in reliable way
  across THE NETWORK .
• Introduces network mechanisms Routing,
  addressing
• Uses the link layer to transfer across a link.

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Network Layer (layer 3) a reliable packet
network pipe
A packet pipe
E
E
packet
D
D
Link layer
C
C
C
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A view from above (network layer)
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InterNetwork Layer (3.5)

• Arranges the delivery of a packet across many
  networks

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Internet Layer (3.5)
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Transport Layer (layer 4) a reliable
communications end to end
A packet pipe
Data piece
F
F
E
E
Link layer
D
D
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Issues at Transport layer

• Reliable communications
• Guarantees that a packet does arrive at
  destination (if does not arrive requires
  resend).
• Acking
• Numbering
• Flow Control
• Can you send at your will?

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Issues at Transport layer

• Can you send at your will?
• If destination is slow destination will have its
  buffer full and performance degradation.
  Efficiency ? be careful
• If route is slow sending over-traffic will just
  congest the network. Politeness ? slow down.

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Principles of Flow control

• Use acks for each packet (receiver sends ack for
  each packet)
• Sender uses a WINDOW (cannot send more than a
  window full of packets). If j did not arrive
  can send at most j-1W.
• Advanced windows make the window size depends on
  speed of line.
• If things go OK increase W.
• If things go slow decrease W.

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Application Layer (5)

• Use the transport layer to ship the application.

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Internet

• Internet Layer Internet Protocol (IP)
• Takes care of passing a packet across the network
  to the destination (routing across networks).
• Transport Control Protocol (TCP) Layer 5
  protocol.
• Above IP.
• Creates a reliable communications path end to
  end.

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Internet

• UDP (Unspecified Data Protocol)
• Parallel to TCP (that is, above IP)
• Good news Provides no flow control mechanism!.
• Bad News Provides no guaranteed delivery!
• Users responsibility for the packets.
• Network does not control the flow of packets.
• Like post service.

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Internet Applications FTP

• FTP File transfer protocol
• Over TCP
• Want to transfer file x from A to B
• A opens a TCP connection to B.
• A pushes the file, piece by piece into the TCP
  connection.
• What does the FTP protocol needs to take care of
  
• Name the file.
• Give list of files.
• Get starting at position y, .

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Internet Applications HTTP

• HTTP Hyper Text transfer protocol
• Over TCP
• Used by Web browsers to get files from Web
  servers
• A wants to get set of documents x, y, z from B
• A opens a TCP connection(s) to B.
• A requests the documents (one by one, or in
  parallel) from B.
• B sends the files over the connections.
• HTTP is used for
• Naming the document
• GET Command
• Error codes,.

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Internet Applications RTP

• RTP Real Time Protocol
• Used to transfer pieces of data at real time over
  the net.
• Over UDP!!
• Used by real time applications (voice, video) to
  transfer end-to-end.
• Protocol defines the form of the packets.
• Protocol defines the interaction (almost non is
  set) between sender and receiver.

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Stack of Protocols
ftp
http