Skip to content

lecture23-netprog2.md

Latest commit

 

History

History
147 lines (114 loc) · 6.88 KB

lecture23-netprog2.md

File metadata and controls

147 lines (114 loc) · 6.88 KB
Raw

Lecture 23 Network Programming: Part II

Sockets Interface

sockets_interface

linked_list_returned_by_getaddrinfo

addrinfo_struct

Sockets Interface: socket

  • Clients and servers use the socket function to create a socket descriptor:
    • int socket(int domain, int type, int protocol)
    • Example:
      • int clientfd = socket(AF_INET, SOCK_STREAM, 0);
      • which indicates that using 32-bit IPV4 addresses and the socket will be the end point of a reliable TCP connection
    • Better Example:
      • struct addrinfo *infop = ...; int sockfd = socket(p->ai_family, p-> ai_socktype, p->ai_protocol);
      • which is not protocol specific

Sockets Interface: bind

  • A server uses bind to ask the kernel to associate the server's socket address with a socket descriptor
    • int bind(int sockfd, SA *addr, socklen_t addrlen);
    • typedef struct sockaddr SA;
  • Process can read bytes that arrive on the connection whose endpoint is addr by reading from descriptor sockfd
  • Similarly, writes to sockfd are transferred along connection whose endpoint is addr

Sockets Interface: listen

  • Kernel assumes that descriptor from socket function is an active socket that will be on the client end
  • A server calls the listen function to tell the kernel that a descriptor will be used by a server rather than a client:
    • int listen(int sockfd, int backlog);
  • Converts sockfd from an active socket to a listening socket that can accept connection requests from clients
  • backlog is a hint about the number of outstanding connection requests that the kernel should queue up before starting to refuse requests (128-ish by default)

Sockets Interface: accept

  • Servers wait for connection requests from clients by calling accept:
    • int accept(int listenfd, SA *addr, int *addrlen);
  • Waits for connection request to arrive on the connection bound to listenfd, then fills in client’s socket address in addr and size of the socket address in addrlen
  • Returns a connected descriptor that can be used to communicate with the client via Unix I/O routines

Sockets Interface: connect

  • A client establishes a connection with a server by calling connect:
    • int connect(int clientfd, SA *addr, socklen_t addrlen);
  • Attempts to establish a connection with server at socket address addr
    • If successful, then clientfd is now ready for reading and writing
    • Resulting connection is characterized by socket pair
      • (x:y, addr.sin_addr:addr.sin_port)
      • x is client address
      • y is ephemeral port that uniquely identifies client process on client host
  • Best practice is to use getaddrinfo to supply the arguments addr and addrlen

Connected vs. Listening Descriptors

  • Listening descriptor
    • End point for client connection requests
    • Created once and exists for lifetime of the server
  • Connected descriptor
    • End point of the connection between client and server
    • A new descriptor is created each time the server accepts a connection request from a client
    • Exists only as long as it taks to service client
  • This allows for concurrent servers that can communicate over many client connections simultaneously

Web Server Basics

  • Clients and servers communicate using the HyperText Transfer Protocol (HTTP)
    • Client and server establish TCP connection
    • Client requests content
    • Server responds with requested content
    • Client and server close connection (eventually)

web_server_basics

Web Content

  • Web servers return content to clients
    • content: a sequence of bytes with an associated MIME (Multipurpose Internet Mail Extensions) type
      • text/html; text/plain; iimage/gif; mage/png; image/jpeg;
  • The content returned in HTTP responses can be either static or dynamic
    • Static content: content stored in files and retrieved in response to an HTTP request
    • Dynamic content: content produced on-the-fly in response to an HTTP request

URLs and how clients and servers use them

  • Unique name for a file: URL (Universal Resource Locator)
  • Clients use prefix (http://www.cmu.edu:80) to infer:
    • What kind (protocol) of server to contact (HTTP)
    • Where the server is (www.cmu.edu)
    • What port it is listening on (80)
  • Servers use suffix (/index.html) to:
    • Determine if request is for static or dynamic content.
      • One convention: executables reside in cgi-bin directory
    • Find file on file system

HTTP Requests

  • HTTP request is a request line, followed by zero or more request headers
  • Request line:
    • is one of GET, POST, OPTIONS, HEAD, PUT, DELETE, or TRACE
    • is typically URL for proxies, URL suffix for servers
    • is HTTP version of request (HTTP/1.0 or HTTP/1.1)
  • Request headers: :
    • Provide additional information to the server

HTTP Responses

  • HTTP response is a response line followed by zero or more response headers, possibly followed by content, with blank line (“\r\n”) separating headers from content
  • Response line:
    • is HTTP version of the response
    • is numeric status
    • is corresponding English text
      • 200 OK Request was handled without error
      • 301 Moved Provide alternate URL
      • 404 Not foound Server couldn't find the file
  • Response headers: :
    • Content-Type: MIME type of content in response body
    • Content-Length: Length of content in response body

Common Gateway Interface (CGI)

  • CGI defines a simple standard for transferring information between the client (browser), the server, and the child process
  • CGI is the original standard for generating dynamic content
    • Has been largely replaced by other, faster techniques:
      • E.g., fastCGI, Apache modules, Java servlets, Rails controllers
      • Avoid having to create process on the fly (expensive and slow).

Serving Dynamic Content With GET

  • How does the client pass arguments to the server?
    • The arguments are appended to the URI
      • http://add.com/cgi-bin/adder?15213&18213
      • adder is the CGI program on the server that will do the addition.
      • argument list starts with “?”
      • arguments separated by “&”
      • spaces represented by “+” or “%20”
  • How does the server pass these arguments to the child?
    • In environment variable QUERY_STRING
      • A single string containing everything after the “?”
  • How does the server capture the content produced by the child?
    • The child generates its output on stdout and server uses dup2 to redirect stdout to its connected socket
  • Notice that only the CGI child process knows the content type and length, so it must generate those headers