3/2/2001hanoch levy, cs, tau1 basic communications protocols workshop on qos hanoch levy feb 2004
TRANSCRIPT
3/2/2001 Hanoch Levy, CS, TAU 1
Basic Communications Protocols
Workshop on QoSHanoch Levy
Feb 2004
3/2/2001 Hanoch Levy, CS, TAU 2
The objective: A talk to B , C talk to D, best performance
• A
A
B
DC
3/2/2001 Hanoch Levy, CS, TAU 3
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.
3/2/2001 Hanoch Levy, CS, TAU 4
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.
3/2/2001 Hanoch Levy, CS, TAU 5
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).
3/2/2001 Hanoch Levy, CS, TAU 6
The principle of a layered model
B B’
A
C
A’
C’
Each layer element:
1. Talk to its peer element
2. Carries traffic for its parent
3. Deliver traffic through its child
3/2/2001 Hanoch Levy, CS, TAU 7
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
3/2/2001 Hanoch Levy, CS, TAU 8
Physical Layer: a bit pipe
B B’
C
A
C’
A’Physical medium (fiber, cooper, radio, pigeon)
bit
A bit pipe
3/2/2001 Hanoch Levy, CS, TAU 9
Link Layer (2)
• 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.
Completed 26/3/04
Completed 26/3/04
3/2/2001 Hanoch Levy, CS, TAU 10
Link Layer (layer 2): a reliable packet pipe
C C’
D
B
D’
B’
Physical layer
packet
A packet pipe
3/2/2001 Hanoch Levy, CS, TAU 11
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.
3/2/2001 Hanoch Levy, CS, TAU 12
Network Layer (layer 3): a reliable packet network pipe
D D’
E
C
E’
C
Link layer
packet
A packet pipe
C
3/2/2001 Hanoch Levy, CS, TAU 13
A view from above (network layer)
3/2/2001 Hanoch Levy, CS, TAU 14
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
E E’
F
D
F’
D
Link layer
Data piece
A packet pipe
3/2/2001 Hanoch Levy, CS, TAU 17
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?
3/2/2001 Hanoch Levy, CS, TAU 18
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.
3/2/2001 Hanoch Levy, CS, TAU 19
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.
3/2/2001 Hanoch Levy, CS, TAU 20
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.
3/2/2001 Hanoch Levy, CS, TAU 22
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.
3/2/2001 Hanoch Levy, CS, TAU 23
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, ….
3/2/2001 Hanoch Levy, CS, TAU 24
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,….
3/2/2001 Hanoch Levy, CS, TAU 25
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.
3/2/2001 Hanoch Levy, CS, TAU 26
Stack of Protocols
httpftp