Basic Communications Protocols

1. Basic Communications Protocols Workshop on QoS Hanoch Levy Feb 2003 Hanoch Levy, CS, TAU

2. The objective:A talk to B , C talk to D, best performance • A B A D C 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. 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. 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). Hanoch Levy, CS, TAU

6. A’ A B’ B C C’ The principle of a layered model • Each layer element: • Talk to its peer element • Carries traffic for its parent • Deliver traffic through its child 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…). • Is responsible of transferring bits from one side to the other. • When B receives a bit  transfers it to B’ • Forms a BIT PIPE Hanoch Levy, CS, TAU

8. Physical Layer: a bit pipe A bit pipe C’ C bit B’ B Physical medium A A’ 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 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. Hanoch Levy, CS, TAU

10. Link Layer (layer 2): a reliable packet pipe A packet pipe D’ D packet C’ C Physical layer B B’ 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. Hanoch Levy, CS, TAU

12. Network Layer (layer 3): a reliable packet network pipe A packet pipe E’ E packet D’ D Link layer C C C Hanoch Levy, CS, TAU

13. A view from above (network layer) Hanoch Levy, CS, TAU

14. InterNetwork Layer (3.5) • Arranges the delivery of a packet across many networks Hanoch Levy, CS, TAU

15. Internet Layer (3.5) Hanoch Levy, CS, TAU

16. Transport Layer (layer 4): a reliable communications end to end A packet pipe Data piece F’ F E’ E Link layer D D 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? 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. 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. Hanoch Levy, CS, TAU

20. Application Layer (5) • Use the transport layer to ship the application. Hanoch Levy, CS, TAU

21. 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. 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. 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, …. 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,…. 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. Hanoch Levy, CS, TAU

26. Stack of Protocols ftp http Hanoch Levy, CS, TAU