Transcript QOS-sadna-layers

Basic Communications
Protocols
Workshop on QoS
Hanoch Levy
Feb 2003
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Hanoch Levy, CS, TAU
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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:
A
1.
Talk to its peer element
2.
Carries traffic for its parent
3.
Deliver traffic through its
child
A’
B
B’
C
C’
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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…).
• 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
A’
A
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Link Layer (2)
• Assures transferring of a packet in reliable way
across a link .
• Uses the bit-pipe.
• Uses mechanism to recover form problems:
• If packets does not arrive or arrive bad:
• Add error detection on packet.
• Add packet numbers.
• Add acks (to acknoweldge 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
C
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Link layer
C
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D’
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
E
F’
Link layer
D
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E’
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-1+W.
- 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!
- User’s 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
http
ftp
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