Networks & Communications CS-4513, D-Term 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)

Networks & Communications CS-4513, D-Term 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

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

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

Networks & Communications CS-4513, D-Term Modified Layers

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

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

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

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

Networks & Communications CS-4513, D-Term 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

Networks & Communications CS-4513, D-Term 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

Networks & Communications CS-4513, D-Term 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 CS-4513, D-Term 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 CS-4513, D-Term 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 CS-4513, D-Term 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 CS-4513, D-Term 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 CS-4513, D-Term 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 CS-4513, D-Term Break