This chapter describes Python’s socket protocol support, and the networking modules built on top of the socket module. This includes client handlers for most popular Internet protocols, as well as several frameworks that can be used to implement Internet servers.
For the low-level examples in this chapter I’ll use two protocols for illustration; the Internet Time Protocol, and the Hypertext Transfer Protocol.
Internet Time Protocol
The Internet Time Protocol (RFC 868, Postel and Harrenstien 1983) is a simple protocol which allows a network client to get the current time from a server.
Since this protocol is relatively light weight, many (but far from all) Unix systems provide this service. It’s also about as easy to implement as a network protocol can possibly be. The server simply waits for a connection request, and immediately returns the current time as a 4-byte integer, containing the number of seconds since January 1st, 1900.
In fact, the protocol is so simple that I can include the entire specification:
# File: rfc868.txt Network Working Group J. Postel - ISI Request for Comments: 868 K. Harrenstien - SRI May 1983 Time Protocol This RFC specifies a standard for the ARPA Internet community. Hosts on the ARPA Internet that choose to implement a Time Protocol are expected to adopt and implement this standard. This protocol provides a site-independent, machine readable date and time. The Time service sends back to the originating source the time in seconds since midnight on January first 1900. One motivation arises from the fact that not all systems have a date/time clock, and all are subject to occasional human or machine error. The use of time-servers makes it possible to quickly confirm or correct a system's idea of the time, by making a brief poll of several independent sites on the network. This protocol may be used either above the Transmission Control Protocol (TCP) or above the User Datagram Protocol (UDP). When used via TCP the time service works as follows: S: Listen on port 37 (45 octal). U: Connect to port 37. S: Send the time as a 32 bit binary number. U: Receive the time. U: Close the connection. S: Close the connection. The server listens for a connection on port 37. When the connection is established, the server returns a 32-bit time value and closes the connection. If the server is unable to determine the time at its site, it should either refuse the connection or close it without sending anything. When used via UDP the time service works as follows: S: Listen on port 37 (45 octal). U: Send an empty datagram to port 37. S: Receive the empty datagram. S: Send a datagram containing the time as a 32 bit binary number. U: Receive the time datagram. The server listens for a datagram on port 37. When a datagram arrives, the server returns a datagram containing the 32-bit time value. If the server is unable to determine the time at its site, it should discard the arriving datagram and make no reply. The Time The time is the number of seconds since 00:00 (midnight) 1 January 1900 GMT, such that the time 1 is 12:00:01 am on 1 January 1900 GMT; this base will serve until the year 2036. For example: the time 2,208,988,800 corresponds to 00:00 1 Jan 1970 GMT, 2,398,291,200 corresponds to 00:00 1 Jan 1976 GMT, 2,524,521,600 corresponds to 00:00 1 Jan 1980 GMT, 2,629,584,000 corresponds to 00:00 1 May 1983 GMT, and -1,297,728,000 corresponds to 00:00 17 Nov 1858 GMT.
Hypertext Transfer Protocol
The Hypertext Transfer Protocol (HTTP, Fielding et al., RFC 2616) is something completely different. The most recent specification (version 1.1), is over 100 pages.
However, in its simplest form, this protocol is very straightforward. To fetch a document, the client connects to the server, and sends a request like:
GET /hello.txt HTTP/1.0 Host: hostname User-Agent: name [optional request body]
In return, the server returns a response like this:
HTTP/1.0 200 OK Content-Type: text/plain Content-Length: 7 Hello
Both the request and response headers usually contains more fields, but the Host field in the request header is the only one that must always be present.
The header lines are separated by “\r\n“, and the header must be followed by an empty line, even if there is no body (this applies to both the request and the response).
The rest of the HTTP specification deals with stuff like content negotiation, cache mechanics, persistent connections, and much more. For the full story, see Hypertext Transfer Protocol — HTTP/1.1.