外文文献原文SMTP Service Extension for AuthenticationThis document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the Internet Official Protocol Standards (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.Copyright NoticeCopyright (C) The Internet Society (1999). All Rights Reserved.1. IntroductionThis document defines an SMTP service extension ESMTP whereby an SMTP client may indicate an authentication mechanism to the server,perform an authentication protocol exchange, and optionally negotiatea security layer for subsequent protocol interactions. This extension is a profile of the Simple Authentication and Security Layer SASL.2. Conventions Used in this DocumentIn examples, C: and S: indicate lines sent by the client and server respectively. The key words MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY in this document are to be interpreted as defined in Key words for use in RFCs to Indicate Requirement Levels KEYWORDS.3. The Authentication service extension(1) the name of the SMTP service extension is Authentication(2) the EHLO keyword value associated with this extension is AUTH(3) The AUTH EHLO keyword contains as a parameter a space separated list of the names of supported SASL mechanisms.(4) a new SMTP verb AUTH is defined(5) an optional parameter using the keyword AUTH is added to the MAIL FROM command, and extends the maximum line length of the MAIL FROM command by 500 characters.(6) this extension is appropriate for the submission protocol SUBMIT.4. The AUTH command AUTH mechanism initial-responseArguments:a string identifying a SASL authentication mechanism. an optional base64-encoded responseRestrictions:After an AUTH command has successfully completed, no more AUTH commands may be issued in the same session. After a successful AUTH command completes, a server MUST reject any further AUTH commands with a 503 reply. The AUTH command is not permitted during a mail transaction.Discussion:The AUTH command indicates an authentication mechanism to the server. If the server supports the requested authentication mechanism, it performs an authentication protocol exchange to authenticate and identify the user. Optionally, it also negotiates a security layer for subsequent protocol interactions. If the requested authentication mechanism is not supported, the server rejects the AUTH command with a 504 reply.The authentication protocol exchange consists of a series of server challenges and client answers that are specific to the authentication mechanism. A server challenge, otherwise known as a ready response, is a 334 reply with the text part containing a BASE64 encoded string. The client answer consists of a line containing a BASE64 encoded string. If the client wishes to cancel an authentication exchange, it issues a line with a single *. If the server receives such an answer, it MUST reject the AUTH command by sending a 501 reply.The optional initial-response argument to the AUTH command is used to save a round trip when using authentication mechanisms that are defined to send no data in the initial challenge.When the initial-response argument is used with such a mechanism, the initial empty challenge is not sent to the client and the server uses the data in the initial-response argument as if it were sent in response to the empty challenge. Unlike a zero-length client answer to a 334 reply, a zero- length initial response is sent as a single equals sign (=). If the client uses an initial-response argument to the AUTH command with a mechanism that sends data in the initial challenge, the server rejects the AUTH command with a 535 reply.If the server cannot BASE64 decode the argument, it rejects the AUTH command with a 501 reply. If the server rejects the authentication data, it SHOULD reject the AUTH command with a 535 reply unless a more specific error code, such as one listed in section 6, is appropriate. Should the client successfully complete the authentication exchange, the SMTP server issues a 235 reply.The service name specified by this protocols profile of SASL is smtp.If a security layer is negotiated through the SASL authentication exchange, it takes effect immediately following the CRLF that concludes the authentication exchange for the client, and the CRLF of the success reply for the server. Upon a security layers taking effect, the SMTP protocol is reset to the initial state (the state in SMTP after a server issues a 220 service ready greeting). The server MUST discard any knowledge obtained from the client, such as the argument to the EHLO command, which was not obtained from the SASL negotiation itself. The client MUST discard any knowledge obtained from the server, such as the list of SMTP service extensions, which was no