1. Networks & Communications CS-4513, D-Term 20071 Networks & Communication (continued) CS-4513 Distributed Computing Systems (Slides include materials from Operating System Concepts, 7 th ed., by Silbershatz, Galvin, & Gagne, Modern Operating Systems, 2 nd ed., by Tanenbaum, and Distributed Systems: Principles & Paradigms, 2 nd ed. By Tanenbaum and Van Steen)

2. Networks & Communications CS-4513, D-Term 20072 Review — OSI 7-layer model Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Sending Process Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Receiving Process Bits DHDataDT NHData THData SHData PHData AHData

3. Networks & Communications CS-4513, D-Term 20073 Layered Protocols OSI 7-layer model was intended to be a foundation of a family of international standard protocols Those protocols never gained much acceptance Role of Session and Presentation layers is murky, at best. Internet protocols (TCP/IP, etc.) are dominant

4. Networks & Communications CS-4513, D-Term 20074 The TCP/IP Protocol Layers TCP/IP

5. Networks & Communications CS-4513, D-Term 20075 The TCP/IP Protocol Layers TCP/IP Defined by manufacturers, industry sub-groups, and separate standards bodies Subsumed by middleware

6. Networks & Communications CS-4513, D-Term 20076 Modified Layers

7. Networks & Communications CS-4513, D-Term 20077 Examples of Middleware Authentication protocols Commit protocols for atomic transactions Multimedia protocols Remote procedure protocols …

8. Networks & Communications CS-4513, D-Term 20078 Styles of Communication Message-oriented Remote Procedure Call Streaming

9. Networks & Communications CS-4513, D-Term 20079 Some Terms Packet: –A unit of communication at Data Link layer –aka datagram IP Address: –A four-part “number” used by Network Layer to route a packet from one computer to another Port: –A 16-bit number used within one computer to identify who/where to send packet to Well-known port: –A port with number < 1024, used by agreement for standard services – e.g., telnet (23), ftp (21), smtp (25), pop (110)

10. Networks & Communications CS-4513, D-Term 200710 More Terms Socket: –End point of a communication –Usually used in pairs, one for each direction –Comprises [IP Address: Port #] Connection: –A logical linkage between pairs of sockets at two endpoints for purposes of a particular communication between those endpoints –i.e., a serial conversation between endpoints

11. Networks & Communications CS-4513, D-Term 200711 Connection The backbone of most message-oriented communication protocols Each party retains knowledge of the other Each party retains information about state of the other (vis a vis the protocol itself) Each party “knows” if connection is broken … Note: some popular protocols are “connection- less” –one side has no state information about other side

12. Networks & Communications CS-4513, D-Term 200712 Establishing a Connection Process a on machine m creates a socket OS assigns a new port number q to that socket Process a attempts to open a connection to machine n:p p is a well-known port Process b on machine n is listening on p Receives request from m:q Process b forks a process or spawns a thread c to talk with m:q, then resumes listening on p Thread/process c Creates a new socket r for this connection Replies to m:q with return address n:r a and c continue to communicate over this pair of sockets until they are finished.

13. Networks & Communications CS-4513, D-Term 200713 Typical Client-Server Connection Create socket On server side Bind I.e., connect socket to port # (usually well-known port) Listen Sit and wait for a communication to come in Accept Create new socket for purpose of responding to this caller

14. Networks & Communications CS-4513, D-Term 200714 Notes Responder to request for connection does not have to be the original server machine Delegate workload to other server systems Some systems include a connection ID as part of request to open connection Unique or randomly chosen Reduces spoofing of server responses Unix/Linux will not re-use a socket # within 30 seconds To avoid confusion between old connection and new

15. Networks & Communications CS-4513, D-Term 200715 Reliable Connections Transport layer partitions messages into packets TCP – Transmission Control Protocol Sequence number of current packet Sequence number of last packet received correctly Receiver keeps track of seq. # of packets Reassembles in right order Notify sender of missing, broken packets Sender keeps copy of each packet until receipt acknowledged Retransmits packets if no acknowledgement Window defines how many packet buffers to maintain for efficient transmission Allows many packets in “flight”

16. Networks & Communications CS-4513, D-Term 200716 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 Packet i+k … time rec’d i

17. Networks & Communications CS-4513, D-Term 200717 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 Packet i+k … time rec’d i

18. Networks & Communications CS-4513, D-Term 200718 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 Packet i+k … time rec’d i rec’d i+2

19. Networks & Communications CS-4513, D-Term 200719 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 Packet i+k … time rec’d i rec’d i+2 … lost

20. Networks & Communications CS-4513, D-Term 200720 Reliable Connections (continued) If acknowledgement received for packet i Delete from buffer all packets  i If no acknowledgement received within a reasonable time for packet k Retransmit from buffer all packets  k Result Recovers from loss of packets Recovers from loss of acknowledgements Works well for reasonably reliable internet Doesn’t work so well for noisy, unreliable networks

21. Networks & Communications CS-4513, D-Term 200721 Reminder How do we know if a packet is received correctly? Cyclic Redundancy Check (CRC) –Polynomial computed from packet header and body –Usually 16 or 32 bits, computed by hardware –Appended to message –Recomputed on reception, compared with transmitted CRC –Equal  packet received correctly

22. Networks & Communications CS-4513, D-Term 200722 Examples of Connection-based Protocols Telnet (virtual terminal) –2-way communication by character stream –Line-by-line organization SMTP (Simple Mail Transport Protocol) –For sending mail –Layered on top of telnet protocol POP (Post Office Protocol) –For receiving your mail –Layered on top of telnet protocol FTP (File Transfer Protocol) –For transmitting ASCII or binary files –Binary data transmission not layered on telnet protocol …

23. Networks & Communications CS-4513, D-Term 200723 Connection-less communication Some communication protocols don’t need the overhead of reliable connections –When some number of errors can be tolerated –Where recovery from those errors is easy UDP – User Datagram Protocol –The internet connection-less protocol (layer 4) –Breaks messages into packets –Does not send acknowledgement of correct receipt

24. Networks & Communications CS-4513, D-Term 200724 Examples HTTP (HyperText Transport Protocol) –Web server responds directly to requests –If client does not get response, retries request NFS (Network File System) –For access to files on servers as if they are local –If client does not get response, retries request RPC (Remote Procedure Call) –Next topic …

25. Networks & Communications CS-4513, D-Term 200725 Reading Assignment Silbershatz – rest of Chapter 16 Tanenbaum & Van Steen – §4.1-§4.3 Tanenbaum (Modern Operating System): This topic is not covered

26. Networks & Communications CS-4513, D-Term 200726 Break