IMAP Extension for Simple Authentication and Security Layer (SASL) Initial Client Response
RFC 4959
Document | Type |
RFC - Proposed Standard
(September 2007; Errata)
Was draft-siemborski-imap-sasl-initial-response (individual in app area)
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Authors | Rob Siemborski , Arnt Gulbrandsen | ||
Last updated | 2020-01-21 | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized with errata bibtex | ||
Reviews | |||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 4959 (Proposed Standard) | |
Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Chris Newman | ||
Send notices to | (None) |
Network Working Group R. Siemborski Request for Comments: 4959 Google, Inc. Category: Standards Track A. Gulbrandsen Oryx Mail Systems GmbH September 2007 IMAP Extension for Simple Authentication and Security Layer (SASL) Initial Client Response Status of This Memo This 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. Abstract To date, the Internet Message Access Protocol (IMAP) has used a Simple Authentication and Security Layer (SASL) profile which always required at least one complete round trip for an authentication, as it did not support an initial client response argument. This additional round trip at the beginning of the session is undesirable, especially when round-trip costs are high. This document defines an extension to IMAP which allows clients and servers to avoid this round trip by allowing an initial client response argument to the IMAP AUTHENTICATE command. Siemborski & Gulbrandsen Standards Track [Page 1] RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 1. Introduction The SASL initial client response extension is present in any IMAP [RFC3501] server implementation which returns "SASL-IR" as one of the supported capabilities in its CAPABILITY response. Servers which support this extension will accept an optional initial client response with the AUTHENTICATE command for any SASL [RFC4422] mechanisms which support it. 2. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. In examples, "C:" and "S:" indicate lines sent by the client and server, respectively. Formal syntax is defined by [RFC4234] as extended by [RFC3501]. 3. IMAP Changes to the IMAP AUTHENTICATE Command This extension adds an optional second argument to the AUTHENTICATE command that is defined in Section 6.2.2 of [RFC3501]. If this second argument is present, it represents the contents of the "initial client response" defined in Section 5.1 of [RFC4422]. As with any other client response, this initial client response MUST be encoded as defined in Section 4 of [RFC4648]. It also MUST be transmitted outside of a quoted string or literal. To send a zero- length initial response, the client MUST send a single pad character ("="). This indicates that the response is present, but is a zero- length string. When decoding the BASE64 [RFC4648] data in the initial client response, decoding errors MUST be treated as IMAP [RFC3501] would handle them in any normal SASL client response. In particular, the server should check for any characters not explicitly allowed by the BASE64 alphabet, as well as any sequence of BASE64 characters that contains the pad character ('=') anywhere other than the end of the string (e.g., "=AAA" and "AAA=BBB" are not allowed). If the client uses an initial response with a SASL mechanism that does not support an initial response, the server MUST reject the command with a tagged BAD response. Siemborski & Gulbrandsen Standards Track [Page 2] RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 Note: support and use of the initial client response is optional for both clients and servers. Servers that implement this extension MUST support clients that omit the initial client response, and clients that implement this extension MUST NOT send an initial client response to servers that do not advertise the SASL-IR capability. In such a situation, clients MUST fall back to an IMAP [RFC3501] compatible mode. If either the client or the server do not support the SASL-IR capability, a mechanism which uses an initial client response is negotiated using the challenge/response exchange described in [RFC3501], with an initial zero-length server challenge. 4. Examples The following is an example authentication using the PLAIN (see [RFC4616]) SASL mechanism (under a TLS protection layer, see [RFC4346]) and an initial client response: ... client connects to server and negotiates a TLS protection layer ... C: C01 CAPABILITY S: * CAPABILITY IMAP4rev1 SASL-IR AUTH=PLAIN S: C01 OK CompletedShow full document text