Networks & Communication (continued)

1. Networks & Communication(continued) CS-502Operating 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 & Communication

2. Principal Abstraction – Socket • Originally created in BSD Unix • Now, part of most operating systems • Allows opening a connection between two processes across network • Connection: • a serial conversation between two end points • e.g., processes, threads, tasks on different machines • organized as a sequence of messages or datagrams • distinct from all other connections • One of the four principal abstractions provided by most modern operating systems Networks & Communication

3. Definition — Protocol • Formal set of rules that govern the formats, contents, and meanings of messages from computer to computer, process to process, etc. • Must be agreed to by all parties to a communication • May be defined in terms of other protocols Networks & Communication

4. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer The OSI 7-layer model(in a nutshell) Silbershatz, §§16.6-16.7 Networks & Communication

5. Annotated OSI 7-Layer Stack Silbershatz, page 630 Networks & Communication

6. 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 Summary — OSI 7-layer model Networks & Communication

7. Circular Definition • Socket: • End point of a connection • 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 • Usually two-way Networks & Communication

8. 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 retains no state information about other side Networks & Communication

9. 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 & Communication

10. 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 & Communication

11. Notes • Responder to request for connection does not have to be the original server machine • Delegate workload to other server systems • Systems often 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 & Communication

12. 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 sequence # 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 & Communication

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

14. 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 & Communication

15. 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 & Communication

16. 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 & Communication

17. 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 & Communication

18. 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 & Communication

19. 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 & Communication

20. 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 • Reassembles at destination • Messages delivered completely or not at all • Does not send acknowledgement of correct receipt Networks & Communication

21. 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 & Communication

22. Summary • Socket, connection • Network stack, 7-layer model • Establishing a connection • Reliable transmission • Reading assignment • Silbershatz Chapter 16 Networks & Communication