Networks & Communication (continued)

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

2. Sending Process Receiving Process Data Application Layer Application Layer AH Data Presentation Layer Presentation Layer PH Data Session Layer Session Layer SH Data Transport Layer Transport Layer TH Data Network Layer Network Layer NH Data Data Link Layer Data Link Layer DH Data DT Physical Layer Physical Layer Bits Review — OSI 7-layer model Networks & Communications

3. 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 Networks & Communications

4. TCP/IP The TCP/IP Protocol Layers Networks & Communications

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

6. Modified Layers Networks & Communications

7. Examples of Middleware • Authentication protocols • Commit protocols for atomic transactions • Multimedia protocols • Remote procedure protocols • … Networks & Communications

8. Styles of Communication • Message-oriented • Remote Procedure Call • Streaming Networks & Communications

9. 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) Networks & Communications

10. 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 Networks & Communications

11. 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 Networks & Communications

12. 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. Networks & Communications

13. 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 Networks & Communications

14. 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 Networks & Communications

15. 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” Networks & Communications

16. Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 rec’d i … Packet i+k time Networks & Communications

17. Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 rec’d i … rec’d i Packet i+k time Networks & Communications

18. Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 rec’d i … rec’d i Packet i+k rec’d i+2 time Networks & Communications

19. Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 rec’d i … lost rec’d i Packet i+k rec’d i+2 … rec’d i+2 time Networks & Communications

20. 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 Networks & Communications

21. 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 Networks & Communications

22. 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 • … Networks & Communications

23. 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 Networks & Communications

24. 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 • … Networks & Communications

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

26. Break Networks & Communications