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CS 241 Discussion Section (11/11/--11)

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  1. CS 241 Discussion Section (11/11/--11)

  2. Outline • Good Ideas for MP7 • Socket Programming • Hypertext Transfer Protocol (HTTP)

  3. Hints for MP7 • Following Slides stolen from CMU CS 213 • Good ideas in book: Chapter 23 Sec 10.9

  4. Basic allocator mechanisms Sequential fits (implicit or explicit single free list) best fit, first fit, or next fit placement various splitting and coalescing options splitting thresholds immediate or deferred coalescing Segregated free lists simple segregated storage -- separate heap for each size class segregated fits -- separate linked list for each size class buddy systems

  5. 1-2 3 4 5-8 9-16 Segregate Storage Each size “class” has its own collection of blocks • Often have separate collection for every small size (2,3,4,…) • For larger sizes typically have a collection for each power of 2

  6. Simple segregated storage Separate heap and free list for each size class No splitting To allocate a block of size n: if free list for size n is not empty, allocate first block on list (note, list can be implicit or explicit) if free list is empty, get a new page create new free list from all blocks in page allocate first block on list constant time To free a block: Add to free list If page is empty, return the page for use by another size (optional) Tradeoffs: fast, but can fragment badly

  7. Segregated fits Array of free lists, each one for some size class To allocate a block of size n: search appropriate free list for block of size m > n if an appropriate block is found: split block and place fragment on appropriate list (optional) if no block is found, try next larger class repeat until block is found To free a block: coalesce and place on appropriate list (optional) Tradeoffs faster search than sequential fits (i.e., log time for power of two size classes) controls fragmentation of simple segregated storage coalescing can increase search times deferred coalescing can help

  8. Buddy systems Special case of segregated fits. all blocks are power of two sizes Basic idea: Heap is 2m words Maintain separate free lists of each size 2k, 0 <= k <= m. Requested block sizes are rounded up to nearest power of 2. Originally, one free block of size 2m.

  9. Buddy systems (cont) To allocate a block of size 2k: Find first available block of size 2j s.t. k <= j <= m. if j == k then done. otherwise recursively split block until j == k. Each remaining half is called a “buddy” and is placed on the appropriate free list 2m buddy buddy buddy

  10. Buddy systems (cont) To free a block of size 2k continue coalescing with buddies while the buddies are free Block to free buddy buddy buddy Not free, done Added to appropriate free list

  11. Buddy systems (cont) Key fact about buddy systems: given the address and size of a block, it is easy to compute the address of its buddy e.g., block of size 32 with address xxx...x00000 has buddy xxx...x10000 Tradeoffs: fast search and coalesce subject to internal fragmentation

  12. Internal fragmentation • Internal fragmentation is wasted space inside allocated blocks: • minimum block size larger than requested amount • e.g., due to minimum free block size, free list overhead • policy decision not to split blocks • e.g., buddy system • Much easier to define and measure than external fragmentation.

  13. Other Sources of Wisdom • Many implementations and algorithms online… • All work should be your own! • Good Luck

  14. Socket Programming

  15. Socket • Standard APIs for sending and receiving data across computer networks • Introduced by BSD operating systems in 1983 • POSIX incorporated 4.3BSD sockets and XTI in 2001 • #include <sys/socket.h>

  16. Typical TCP Server-Client

  17. Typical TCP Server-Client

  18. Typical UDP Server-Client

  19. Programming Sockets • To create a socket in C, you need to run two commands: • socket() • bind()

  20. socket • int socket(int domain, int type, int protocol); • Returns a nonnegative integer (socket file descriptor) • Parameters • domain: AF_INET (IPv4) • type: SOCK_STREAM (TCP) or SOCK_DGRAM (UDP) • protocol: 0 (socket chooses the correct protocol based on type) • TCP: socket(AF_INET,SOCK_STREAM, 0); • UDP: socket(AF_INET, SOCK_DGRAM, 0);

  21. bind • int bind(int socket, • const struct sockaddr *address, • socklen_t address_len); • Associates the socket with a port on your local machine • struct sockaddr_in used for struct sockaddr sa_family_t sin_family; /* AF_INET */ in_port_t sinport; /* port number */ struct in_addr sin_addr; /* IP address */

  22. Programming Sockets • UDP is packet-based • TCP is connection-based • you need to establish a connection in TCP: • Server: listen(), accept() • Client: connect()

  23. A Generic TCP Server & Client Script Server socket() bind() listen() while (…) { accept() send()/recv() } close() Client socket() connect() while (…) { send()/recv() } close() What is the problem with the server?

  24. A Generic TCP Server & Client Script Server socket() bind() listen() while (…) { accept() send()/recv() } close() Client socket() connect() while (…) { send()/recv() } close() Handle one request at a time How to fix this?

  25. listen • int listen(int socket, int backlog); • Puts the socket into the passive state to accept incoming requests • Internally, it causes the network infrastructure to allocate queues to hold pending requests • backlog: number of connections allowed on the incoming queue • bind should be called beforehand

  26. accept • int accept(int socket, struct sockaddr *restrict address, socklen_t *restrict address_len); • Accepts the pending requests in the incoming queue • *address is used to return the information about the client making the connection. • sin_addr.s_addr holds the Internet address • listen should be called beforehand • Returns nonnegative file descriptor corresponding to the accepted socket if successful, -1 with errno set if unsuccessful

  27. connect • int connect(int socket, const struct sockaddr *address, socklen_t address_len); • Establishes a link to the well-known port of the remote server • Initiates the TCP 3-way handshake • Cannot be restarted even if interrupted • Returns 0 if successful, -1 with errno set if unsuccessful

  28. Programming Sockets • In both TCP and UDP, you send and receive by using the same calls: • send() / sendto() • recv() / recvfrom()

  29. send and sendto int send(int socket, const void *msg, int len, int flags); int sendto(int socket, const void *msg, int len, int flags, const struct sockaddr *to, socklen_t tolen); send sends along an established connection (TCP), while sendto sends to an address (UDP). The extra two parameters specify the destination.

  30. recv and recvfrom int recv(int socket, const void *msg, int len, int flags); int recvfrom(int socket, const void *msg, int len, int flags, const struct sockaddr *from, socklen_t *fromlen); recv receives from an established connection (TCP), while recvfrom receives from anywhere (UDP), and saves the address. The extra two parameters specify the source.

  31. close and shutdown • int close(int socket); • int shutdown(int socket, int how); • close • Prevents any more reads and writes • same function covered in file systems • shutdown • provides a little more control • how • 0 – Further receives are disallowed • 1 – Further sends are disallowed • 2 – same as close • Returns 0 if successful, -1 with errno set if unsuccessful

  32. TCP vs. UDP at a glance TCPUDP Socket type SOCK_STREAM SOCK_DGRAM Form of data transmitted Stream Packets Calls for sending and receiving send, recvsendto, recvfrom Uses sessions? Yes No Overhead for ordering packets Substantial Minimal Example Services FTP, HTTP DNS, SNMP

  33. Using Sockets in C #include <sys/socket.h> #include <sys/types.h> #include <netinet/in.h> #include <unistd.h> On csil-core: gcc –o test test.c On some systems, e.g., Solaris: gcc –o test test.c –lsocket -lnsl

  34. TCP Client/Server Example Run the provided server.c and client.c executables in two separate windows. client sends the string “Hello World!” to IP address port 10000 server listens on port 10000 and prints out any text received

  35. HyperText Transfer Protocol

  36. HTTP • Hypertext Transfer Protocol • Delivers virtually all files and resources on the World Wide Web • Uses Client-Server Model • HTTP transaction • HTTP client opens a connection and sends a request to HTTP server • HTTP server returns a response message

  37. HTTP (continued) • Request • GET /path/to/file/index.html HTTP/1.0 • Other methods (POST, HEAD) possible for request • Response • HTTP/1.0 200 OK • Common Status Codes • 200 OK • 404 Not Found • 500 Server Error

  38. Sample HTTP exchange • Scenario • Client wants to retrieve the file at the following URL ( • What a client does • Client opens a socket to the host, port 80 • Client sends the following message through the socket GET /path/file.html HTTP/1.0 From: User-Agent: HTTPTool/1.0 [blank line here]

  39. Sample HTTP exchange • What a server does • Server responds through the same socket HTTP/1.0 200 OK Date: Mon, 17 Apr 2006 23:59:59 GMT Content-Type: text/html Content-Length: 1354 <html> <body> (more file contents) . . . </body> </html>

  40. Reference • Beej's Guide to Network Programming •