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Lecture 17 Overview

Learn about I/O multiplexing, a technique used to monitor multiple descriptors simultaneously. Explore options such as nonblocking I/O, alarms, signal handlers, and the select() system call.

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Lecture 17 Overview

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  1. Lecture 17 Overview

  2. I/O Multiplexing • We often need to be able to monitor multiple descriptors: • a generic TCP client (like telnet) • a server that handles both TCP and UDP • Client that can make multiple concurrent requests • browser STDIN TCP SOCKET STDOUT CPE 401/601 Lecture 17 : I/O Multiplexing

  3. Options • Use multiple processes/threads • Use nonblocking I/O • use fcntl() to set O_NONBLOCK • Use alarm and signal handler to interrupt slow system calls • Use functions that support checking of multiple input sources at the same time CPE 401/601 Lecture 17 : I/O Multiplexing

  4. Non blocking I/O • Tell kernel not to block a process if I/O requests can not be completed • use fcntl() to set O_NONBLOCK: int flags; flags = fcntl(sock,F_GETFL,0); fcntl(sock,F_SETFL,flags | O_NONBLOCK); • Now calls to read() (and other system calls) will return an error and set errno to EWOULDBLOCK CPE 401/601 Lecture 17 : I/O Multiplexing

  5. Non blocking I/O while (! done) { if ( (n=read(STDIN_FILENO,…)<0)) if (errno != EWOULDBLOCK) /* ERROR */ else write(tcpsock,…) if ( (n=read(tcpsock,…)<0)) if (errno != EWOULDBLOCK) /* ERROR */ else write(STDOUT_FILENO,…) } CPE 401/601 Lecture 17 : I/O Multiplexing

  6. The problem with nonblocking I/O • Using blocking I/O allows the OS to put your process to sleep when nothing is happening • Once input arrives, the OS will wake up your process and read() (or whatever) will return • With nonblocking I/O, the process will chew up all available processor time!!! CPE 401/601 Lecture 17 : I/O Multiplexing

  7. Using alarms signal(SIGALRM, sig_alrm); alarm(MAX_TIME); read(STDIN_FILENO,…); ... signal(SIGALRM, sig_alrm); alarm(MAX_TIME); read(tcpsock,…); ... A function you write CPE 401/601 Lecture 17 : I/O Multiplexing

  8. Select() • The select() system call allows us to use blocking I/O on a set of descriptors • file, socket, … • We can ask select to notify us when data is available for reading on either STDIN or a socket CPE 401/601 Lecture 17 : I/O Multiplexing

  9. select() int select( int maxfd, fd_set *readset, fd_set *writeset, fd_set *excepset, const struct timeval *timeout); • maxfd: highest number assigned to a descriptor • readset: set of descriptors we want to read from • writeset: set of descriptors we want to write to • excepset: set of descriptors to watch for exceptions • timeout: maximum time select should wait CPE 401/601 Lecture 17 : I/O Multiplexing

  10. Using select() • Create fd_set • Clear the whole thing with FD_ZERO • Add each descriptor you want to watch using FD_SET • Call select • when select returns, use FD_ISSET to see if I/O is possible on each descriptor CPE 401/601 Lecture 17 : I/O Multiplexing

  11. System Calls and Errors • In general, systems calls return a negative number to indicate an error. • We often want to find out what error. • Servers generally add this information to a log. • Clients generally provide some information to the user. CPE 401/601 Lecture 17 : Error Handling

  12. extern int errno; • Whenever an error occurs, system calls set the value of the global variable errno • You can check errno for specific errors • errno is valid only after a system call has returned an error. • System calls don't clear errno on success. • If you make another system call you may lose the previous value of errno. • printf makes a call to write! CPE 401/601 Lecture 17 : Error Handling

  13. Error codes • Error codes are defined in errno.h EAGAIN EBADF EACCESS EBUSY EINTR EINVAL EIO ENODEV EPIPE … • Support routines • void perror(const char *string); • stdio.h • char *strerror(int errnum); • string.h CPE 401/601 Lecture 17 : Error Handling

  14. General Strategies • Include code to check for errors after every system call. • Develop "wrapper functions" that do the checking for you. • Develop layers of functions, each hides some of the error-handling details. CPE 401/601 Lecture 17 : Error Handling

  15. Example wrapper int Socket( int f, int t, int p) { int n; if ( (n=socket(f,t,p)) < 0 ) ) { perror("Fatal Error"); exit(1); } return(n); } CPE 401/601 Lecture 17 : Error Handling

  16. Wrappers are great! • Wrappers like those used in the text can make code much more readable. • There are always situations in which you cannot use the wrappers • Sometimes system calls are "interrupted" (EINTR) • this is not always a fatal error ! CPE 401/601 Lecture 17 : Error Handling

  17. Another approach • Instead of simple wrapper functions, you might develop a layered system • The idea is to "hide" the sockaddr and error handling details behind a few custom functions: • int tcp_client(char *server, int port); • int tcp_server(int port); CPE 401/601 Lecture 17 : Error Handling

  18. Layers and Code Re-use • Developing general functions that might be re-used in other programs is obviously "a good thing". • Layering is beneficial even if the code is not intended to be re-used: • hide error-handling from "high-level" code. • hide other details. • often makes debugging easier. CPE 401/601 Lecture 17 : Error Handling

  19. Identifying the Server • Options: • hard-coded into the client program • require that the user identify the server • read from a configuration file • use a separate protocol/network service to lookup the identity of the server • Need an IP address, protocol and port • We often use host names instead of IP addresses • usually the protocol is not specified by the user • often the port is not specified by the user CPE 401/601 Lecture 17 : Client/Server Issues

  20. Services and Ports • Many services are available via “well known” addresses (names). • There is a mapping of service names to port numbers: struct *servent getservbyname( char *service, char *protocol ); • servent->s_port is the port number in network byte order CPE 401/601 Lecture 17 : Client/Server Issues

  21. UDP Client Design • Establish server address (IP and port) • Allocate a socket • Specify that any valid local port and IP address can be used • Communicate with server (send, recv) • Close the socket CPE 401/601 Lecture 17 : Client/Server Issues

  22. Connected mode UDP • A UDP client can call connect() to establish the address of the server • The UDP client can then use read() and write() or send() and recv() • A UDP client using a connected mode socket can only talk to one server • using the connected-mode socket CPE 401/601 Lecture 17 : Client/Server Issues

  23. TCP Client Design • Establish server address (IP and port) • Allocate a socket • Specify that any valid local port and IP address can be used • Call connect() • Communicate with server (read, write) • Close the connection CPE 401/601 Lecture 17 : Client/Server Issues

  24. Closing a TCP socket • Many TCP based application protocols support • multiple requests and/or • variable length requests over a single TCP connection • How does the server known when the client is done ? • and it is OK to close the socket ? CPE 401/601 Lecture 17 : Client/Server Issues

  25. Partial Close • One solution is for the client to shut down only it’s writing end of the socket. • The shutdown() system call provides this function. shutdown(int s, int direction); • direction can be 0 to close the reading end or 1 to close the writing end. • shutdown sends info to the other process! CPE 401/601 Lecture 17 : Client/Server Issues

  26. TCP sockets programming • Common problem areas: • null termination of strings. • reads don’t correspond to writes. • synchronization (including close()). • ambiguous protocol. CPE 401/601 Lecture 17 : Client/Server Issues

  27. TCP Reads • Each call to read() on a TCP socket returns any available data • up to a maximum • TCP buffers data at both ends of the connection. • You must be prepared to accept data 1 byte at a time from a TCP socket! CPE 401/601 Lecture 17 : Client/Server Issues

  28. Concurrent vs. Iterative Concurrent Large or variable size requests Harder to program Typically uses more system resources Iterative Small, fixed size requests Easy to program CPE 401/601 Lecture 17 : Client/Server Issues

  29. Connectionless vs. Connection-Oriented Connection-Oriented EASY TO PROGRAM transport protocol handles the tough stuff. requires separate socket for each connection. Connectionless less overhead no limitation on number of clients CPE 401/601 Lecture 17 : Client/Server Issues

  30. Statefullness • State: Information that a server maintains about the status of ongoing client interactions. • Clients can go down at any time. • Client hosts can reboot many times. • The network can lose messages. • The network can duplicate messages. • Connectionless servers that keep state information must be designed carefully! CPE 401/601 Lecture 17 : Client/Server Issues

  31. Concurrent Server Design Alternatives • One child per client • Spawn one thread per client • Preforking multiple processes • Prethreaded Server CPE 401/601 Lecture 17 : Client/Server Issues

  32. One child per client • Traditional Unix server: • TCP: after call to accept(), call fork(). • UDP: after recvfrom(), call fork(). • Each process needs only a few sockets. • Small requests can be serviced in a small amount of time. • Parent process needs to clean up after children!!!! • call wait() CPE 401/601 Lecture 17 : Client/Server Issues

  33. One thread per client • Almost like using fork • call pthread_create instead • Using threads makes it easier to have sibling processes share information • less overhead • Sharing information must be done carefully • use pthread_mutex CPE 401/601 Lecture 17 : Client/Server Issues

  34. Prefork()’d Server • Creating a new process for each client is expensive. • We can create a bunch of processes, each of which can take care of a client. • Each child process is an iterative server. CPE 401/601 Lecture 17 : Client/Server Issues

  35. Prefork()’d TCP Server • Initial process creates socket and binds to well known address. • Process now calls fork() a bunch of times. • All children call accept(). • The next incoming connection will be handed to one child. CPE 401/601 Lecture 17 : Client/Server Issues

  36. Preforking • Having too many preforked children can be bad. • Using dynamic process allocation instead of a hard-coded number of children can avoid problems. • Parent process just manages the children • doesn’t worry about clients CPE 401/601 Lecture 17 : Client/Server Issues

  37. Sockets library vs. system call • A preforked TCP server won’t usually work the way we want if sockets is not part of the kernel: • calling accept() is a library call, not an atomic operation. • We can get around this by making sure only one child calls accept() at a time using some locking scheme. CPE 401/601 Lecture 17 : Client/Server Issues

  38. Prethreaded Server • Same benefits as preforking. • Can also have the main thread do all the calls to accept() • and hand off each client to an existing thread CPE 401/601 Lecture 17 : Client/Server Issues

  39. Lecture 18Network Management CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger slides are modified from Jim Kurose, Keith Ross

  40. Outline • What is network management? • Internet-standard management framework • Structure of Management Information: SMI • Management Information Base: MIB • SNMP Protocol Operations and Transport Mappings • Security and Administration • ASN.1 CPE 401/601 Lecture 18 : Network Management

  41. What is network management? • autonomous systems (aka “network”) • 100s or 1000s of interacting hardware/software components • "Network management includes the deployment, integration and coordination of the hardware, software, and human elements to monitor, test, poll, configure, analyze, evaluate, and control the network and element resources to meet the real-time, operational performance, and Quality of Service requirements at a reasonable cost.“ CPE 401/601 Lecture 18 : Network Management

  42. managing entity data data data data data agent agent agent agent Infrastructure for network management managing entity managed devices contain managed objects whose data is gathered into a Management Information Base (MIB) managed device network management protocol managed device managed device managed device CPE 401/601 Lecture 18 : Network Management

  43. Network Management standards OSI CMIP • Common Management Information Protocol • designed 1980’s: • unifying net management standard • too slowly standardized CPE 401/601 Lecture 18 : Network Management

  44. Network Management standards SNMP: Simple Network Management Protocol • Internet roots • SGMP: Simple Gateway Monitoring Protocol • started simple • deployed, adopted rapidly • growth: size, complexity • currently: SNMP V3 • de facto network management standard CPE 401/601 Lecture 18 : Network Management

  45. SNMP overview: 4 key parts • Management information base (MIB): • distributed information store of network management data • Structure of Management Information (SMI): • data definition language for MIB objects • SNMP protocol • convey manager<->managed object info, commands • security, administration capabilities • major addition in SNMPv3 CPE 401/601 Lecture 18 : Network Management

  46. Structure of Management Information Basic Data Types • Purpose: syntax, semantics of management data well-defined, unambiguous • base data types: • straightforward • OBJECT-TYPE • data type, status, semantics of managed object • MODULE-IDENTITY • groups related objects into MIB module CPE 401/601 Lecture 18 : Network Management

  47. MODULE SNMP MIB MIB module specified via SMI MODULE-IDENTITY (100 standardized MIBs, more vendor-specific) OBJECT TYPE: OBJECT TYPE: OBJECT TYPE: objects specified via SMI OBJECT-TYPE construct CPE 401/601 Lecture 18 : Network Management

  48. SMI: Object, module examples • OBJECT-TYPE: ipInDelivers ipInDelivers OBJECT TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION “The total number of input datagrams successfully delivered to IP user- protocols (including ICMP)” ::= { ip 9} CPE 401/601 Lecture 18 : Network Management

  49. SMI: Object, module examples • OBJECT-TYPE: ipMIB ipMIB MODULE-IDENTITY LAST-UPDATED “941101000Z” ORGANZATION “IETF SNPv2 Working Group” CONTACT-INFO “ Keith McCloghrie ……” DESCRIPTION “The MIB module for managing IP and ICMP implementations, but excluding their management of IP routes.” REVISION “019331000Z” ……… ::= {mib-2 48} CPE 401/601 Lecture 18 : Network Management

  50. MIB example: UDP module Object ID Name Type Comments 1.3.6.1.2.1.7.1 UDPInDatagrams Counter32 total # datagrams delivered at this node 1.3.6.1.2.1.7.2 UDPNoPorts Counter32 # underliverable datagrams no app at portl 1.3.6.1.2.1.7.3 UDInErrors Counter32 # undeliverable datagrams all other reasons 1.3.6.1.2.1.7.4 UDPOutDatagrams Counter32 # datagrams sent 1.3.6.1.2.1.7.5 udpTable SEQUENCE one entry for each port in use by app, gives port # and IP address CPE 401/601 Lecture 18 : Network Management

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