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

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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

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)

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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

E E’

F

D

F’

D

Link layer

Data piece

A packet pipe

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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?

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.

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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.

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