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This Internet-Draft will expire on August 5, 2007.
Copyright © The IETF Trust (2007).
This memo specifies modifications to SMTP's start-up sequence which reduce the number of round trips between the client and the server before message transmission starts. This can significantly reduce the delay for interactive message submission over slow links.
$Cambridge: hermes/doc/qsmtp/draft-fanf-smtp-quickstart.xml,v 1.25 2007/04/04 14:36:44 fanf2 Exp $
1.
Introduction
2.
Terminology
3.
QUICKSTART SMTP service extension definition
4.
Extended server greeting
5.
The QHLO command
6.
The QTLS command
7.
Changes to the AUTH command
8.
Issuing commands early
9.
Changes to the STARTTLS command
10.
Backwards compatibility
11.
IANA Considerations
12.
Security considerations
13.
References
13.1.
Normative references
13.2.
Informative references
Appendix A.
Example round trip summaries
Appendix B.
Changes from previous versions
§
Author's Address
§
Intellectual Property and Copyright Statements
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The start of an SMTP connection is not currently optimized to reduce the number of round trips between the client and server. This is a particular problem for message submission over slow links, where the resulting delay can easily be more than a second. The QUICKSTART SMTP service extension introduces some changes which eliminate round trips and therefore reduce the delay before messages can be transmitted. The key features of the extension are:
Each of these features has advantages by itself, and they can be used together to provide greater improvements. QUICKSTART is designed with two profiles in mind: the basic profile uses the QHLO and QTLS commands and the pipelined AUTH command to bring the first MAIL command forward from the client's 9th packet to the 5th; the full profile uses the <qhlo-id> and pipelined STARTTLS to bring it as early as the 3rd. Appendix A (Example round trip summaries) illustrates these counts in more detail.
Although SMTP QUICKSTART is probably most useful in conjunction with message submission [RFC4409] (Gellens, R. and J. Klensin, “Message Submission for Mail,” April 2006.), it MAY also be used with MTA-to-MTA SMTP - though attention should be paid to the security considerations in Section 12 (Security considerations).
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Comments and discussion about this draft should be directed to the <lemonade@ietf.org> mailing list - the working group dealing with enhancements to Internet email to support diverse service environments.
The most significant work requireed on this document is to reduce the number of optional features in the QUICKSTART extension: ideally the final version should have no sub-options. This essentially means choosing which of the "basic" "full" profiles to standardize and eliminating the unused features.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
The metalinguistic notation used in this memo corresponds to the "Augmented BNF" used in other Internet mail system memos. The reader who is not familiar with that syntax should consult the ABNF specification [RFC4234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.). Rules not defined in this specification are either defined in the ABNF core rules or in [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.). Metalanguage terms used in running text are surrounded by pointed brackets (e.g., <list-id>) for clarity.
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The QUICKSTART SMTP service extension is defined as follows:
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An SMTP server that supports QUICKSTART MUST respond to the client's connection with an extended greeting according to the syntax below. This greeting has a similar format to the SMTP EHLO response, that is, the first line is the same as an un-extended greeting, and subsequent lines list the SMTP service extensions supported by the server. The list MUST include the QUICKSTART keyword and SHOULD include PIPELINING [RFC2920] (Freed, N., “SMTP Service Extension for Command Pipelining,” September 2000.).
The extended greeting allows a client to find out the server's supported extensions one round trip earlier than in normal SMTP.
Syntax:
qsmtp-greet = ( "220-" domain [ SP ehlo-greet ] CRLF
*( "220-" ehlo-line CRLF )
"220" SP ehlo-line CRLF )
ehlo-line =/ "QUICKSTART" *( SP qsmtp-param )
qsmtp-param = "QHLO=" qhlo-id / "QTLS" /
"AUTH" / "STARTTLS" /
esmtp-param
qhlo-id = esmtp-value
The QUICKSTART keyword MAY be followed by parameters that indicate which OPTIONAL features of this specification are supported by the server. Clients MUST ignore QUICKSTART parameters that they do not understand. The following parameters are defined in this specification:
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The QHLO command has two forms. Support for the basic form defined in this section is REQUIRED. The full form is OPTIONAL, and is defined in Section 8 (Issuing commands early).
An SMTP client that supports QUICKSTART MUST first ensure that the server supports it by checking the server's greeting against the syntax specified in Section 4 (Extended server greeting). The client then issues QHLO as its first command instead of EHLO. The basic form of QHLO has the same semantics as the EHLO command, except that it MAY be pipelined if the server also supports PIPELINING [RFC2920] (Freed, N., “SMTP Service Extension for Command Pipelining,” September 2000.), and the server's 220 response to QHLO does not include its list of supported extensions.
The pipelined QHLO command allows a client to save one round trip by not having to wait for the server's response before issuing further commands.
Syntax:
qhlo = "QHLO" SP Domain CRLF
qhlo-ok-rsp = "220" SP domain [ SP ehlo-greet ] CRLF
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Support for the QTLS command is OPTIONAL, and is indicated by the presence of the QTLS parameter after the QUICKSTART keyword in the list of extensions supported by the server. If the server supports both STARTTLS [RFC3207] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.) and QUICKSTART then it SHOULD also support QTLS.
Syntax:
qtls = "QTLS" CRLF
The QTLS command has no parameters. After issuing the QTLS command, the client immediately starts the TLS handshake. The server MUST NOT issue an SMTP response to the QTLS command before the TLS handshake. Therefore it is not possible for the server to report problems (as with the 454 response to the STARTTLS command) except by closing the connection - which it SHOULD NOT do. Instead, the server SHOULD ensure that it can support TLS (e.g. by initializing its TLS library) before listing the extensions it supports, rather than lazily in reaction to a QTLS or STARTTLS command.
After the TLS handshake has been completed, the SMTP protocol is reset to the state at the start of a connection, so the server proceeds to issue a QUICKSTART extended 220 greeting <qsmtp-greet>.
If the server requires that the client performs a TLS negotiation before it accepts any commands, then it SHOULD accept the QHLO and QTLS commands before TLS negotiation, as well as the NOOP, EHLO, STARTTLS, and QUIT commands specified in [RFC3207] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.)
Apart from these differences, the QTLS command is the same as the STARTTLS command, and the other requirements specified in [RFC3207] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.) apply.
The QTLS command saves one round trip before the TLS handshake by omitting the server's response, and saves another after the TLS handshake by announcing the list of extensions supported by the server earlier than is typical with STARTTLS.
Clients will generally want to issue the QTLS command at the start of a connection, immediately after the QHLO command. The most efficient way of doing so is to pipeline QHLO, QTLS, and the TLS handshake, but then it can be difficult to disentangle the server's response to QHLO from its side of the TLS handshake. Therefore servers MUST allow clients to issue the QTLS command without issuing QHLO or EHLO first, though they MAY require that the client does not issue the QHLO command before receiving the server's greeting. This is illustrated in Appendix A (Example round trip summaries).
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This section describes an OPTIONAL change to the behaviour of the AUTH command [RFC2554] (Myers, J., “SMTP Service Extension for Authentication,” March 1999.), support for which is indicated by the presence of the AUTH parameter after the QUICKSTART keyword in the list of extensions supported by the server. This change is also specified by [RFC4468] (Newman, C., “Message Submission BURL Extension,” May 2006.), so if the server lists support for BURL and QUICKSTART and PIPELINING then it MUST include the QUICKSTART AUTH parameter.
If the client uses a SASL mechanism which can be completed in one round trip, such as EXTERNAL [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) or PLAIN [RFC4616] (Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” August 2006.), then it MAY pipeline the AUTH command. If the authentication fails, the server SHOULD reject all subsequent commands other than AUTH, NOOP, HELO, EHLO, QHLO, or QUIT with a "530 Authentication failure" reply. If the server supports ENHANCEDSTATUSCODES [RFC2034] (Freed, N., “SMTP Service Extension for Returning Enhanced Error Codes,” October 1996.) [RFC3463] (Vaudreuil, G., “Enhanced Mail System Status Codes,” January 2003.), the status code to be returned SHOULD be 5.7.0.
This change allows another round trip to be saved.
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It is possible to save more round trips if the client can issue commands before receiving the list of extensions supported by the server. There are opportunities to do this at the very start of the TCP connection, after the STARTTLS command [RFC3207] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.), and after the AUTH command negotiates a security layer [RFC2554] (Myers, J., “SMTP Service Extension for Authentication,” March 1999.). These are called "QUICKSTART points" in this section.
Support for this is OPTIONAL, and is indicated by the presence of the "QHLO=<qhlo-id>" parameter after the QUICKSTART keyword in the list of extensions supported by the server. The server MUST also support PIPELINING [RFC2920] (Freed, N., “SMTP Service Extension for Command Pipelining,” September 2000.).
The <qhlo-id> parameter value is a case-sensitive token which identifies the list of service extensions and associated parameters in which it was included. The server MUST state a different <qhlo-id> for different lists of extensions or if any of their parameters changes. This generally implies that the server will state different <qhlo-id>s at different QUICKSTART points. For example, if the server only supports AUTH PLAIN after TLS negotiation, it will state a different <qhlo-id> in its responses to EHLO before and after STARTTLS. If the server's SIZE parameter [RFC1870] (Klensin, J., Freed, N., and K. Moore, “SMTP Service Extension for Message Size Declaration,” November 1995.) changes in response to available disk space, its <qhlo-id> will change too. Section 12 (Security considerations) includes some security considerations for the <qhlo-id>.
The client uses the <qhlo-id> with the full form of the QHLO command.
Syntax:
qhlo =/ "QHLO" SP Domain SP qhlo-id CRLF
The <qhlo-id> in the client's QHLO command MUST match the <qhlo-id> issued by the server in its <qsmtp-greet>, or the <qhlo-id> that the server would have issued in its <ehlo-ok-rsp> response to an EHLO command from the client. If the <qhlo-id> matches, the server SHOULD respond with a <qhlo-ok-rsp> response. If the <qhlo-id> does not match there are two possible responses: if the server has already listed its supported service extensions (e.g. in a <qsmtp-greet>) it responds with a "520 Please use the correct QHLO ID" response; otherwise (e.g. after STARTTLS or AUTH) it responds with a <qhlo-long-no>, which includes the list of extensions supported by the server.
Syntax:
qhlo-long-no = ( "521-" domain [ SP ehlo-greet ] CRLF
*( "521-" ehlo-line CRLF )
"521" SP ehlo-line CRLF )
If the server responds to QHLO with a 520 or 521 code, it SHOULD reject subsequent commands other than NOOP, QHLO, or QUIT with "503 Bad sequence of commands" responses. If the server supports ENHANCEDSTATUSCODES [RFC2034] (Freed, N., “SMTP Service Extension for Returning Enhanced Error Codes,” October 1996.) [RFC3463] (Vaudreuil, G., “Enhanced Mail System Status Codes,” January 2003.), the status code to be returned SHOULD be 5.5.1. The client SHOULD recover from this state by issuing a QHLO command containing the correct <qhlo-id>.
The client MAY issue commands early at a QUICKSTART point if it has previously connected to the server and the server included QUICKSTART QHLO=<qhlo-id> in its list of supported extensions at that point. The client's first command at the QUICKSTART point MUST be the full form of QHLO. The client MUST have remembered the details of the extensions that the client is interested in and which are supported by the server at that point, including the <qhlo-id>. The client MUST NOT confuse <qhlo-id>s obtained from different QUICKSTART points. The client MUST NOT assume that the server's current <qhlo-id> will be the same as on the previous connection, therefore the client MUST be prepared for its QHLO being rejected. Note that this implies that the client SHOULD NOT pipeline an early QHLO command and a QTLS command, since the server cannot reject the QTLS command except by closing the connection; instead it SHOULD use STARTTLS as described in Section 9 (Changes to the STARTTLS command).
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This section describes an OPTIONAL change to the behaviour of the STARTTLS command [RFC3207] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.) which is indicated by the presence of the STARTTLS parameter after the QUICKSTART keyword in the list of extensions supported by the server. If the server lists both the QTLS and QHLO=<qhlo-id> parameters, then it SHOULD also list the STARTTLS parameter.
The advantages of the QTLS command are that it allows the client to initiate a TLS handshake faster without getting muddled by SMTP responses from the server arriving after it has sent the TLS hello, and it allows the client to receive the list of extensions supported by the server inside TLS sooner. When the client uses the QHLO=<qhlo-id> feature to issue commands early, it isn't possible to avoid interleaving SMTP responses with the TLS handshake, and it is no longer necessary to get the server's list of supported extensions. Furthermore, the QTLS command's lack of error recovery becomes a problem. Therefore full QUICKSTART uses a pipelined STARTTLS command.
If the server supports QUICKSTART STARTTLS, then the client MAY start the TLS handshake immediately after issuing the STARTTLS command, without waiting for the server's response to STARTTLS. If the server wishes to reject the STARTTLS command, it MUST discard any pipelined TLS client hello before issuing its response.
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If the first command issued by the client is EHLO, then the server MUST treat it as normal extended SMTP conversation as specified in [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.). In this case the client MUST disregard the list of extensions provided in the server's greeting. For example, if the server lists PIPELINING in its greeting, the client MUST NOT assume that PIPELINING will be listed in the server's response to EHLO.
If the first command issued by the client is not QTLS nor QHLO nor EHLO, then the server MUST treat it as an un-extended SMTP conversation as specified in [RFC0821] (Postel, J., “Simple Mail Transfer Protocol,” August 1982.).
In the above situations, the server MAY require that the client's first command is issued after the client has received the greeting.
A client that issues commands early (as described in the two previous sections) MUST be prepared to deal with errors that indicate the server no longer supports QUICKSTART, such as 50X or 55X SMTP responses [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.), or 5.5.X, 5.3.X, or 5.7.X enhanced status codes [RFC3463] (Vaudreuil, G., “Enhanced Mail System Status Codes,” January 2003.). If the client issued a QTLS or pipelined STARTTLS command, it SHOULD disconnect and re-try using ESMTP without QUICKSTART; otherwise it MAY recover by issuing an EHLO command and re-trying.
Similarly, a client that issues commands early MUST be prepared to deal with a 421 or 4.3.2 "Service not available" error. In this situation the client SHOULD disconnect and re-try some time later.
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This memo defines a new SMTP service extension keyword, "QUICKSTART" in Section 3 (QUICKSTART SMTP service extension definition). Four parameters are initially defined for the keyword. Further parameters MAY be defined by future specifications and MUST conform to the <esmtp-param> syntax; parameter keywords starting with X are for Private Use, and other parameter keywords SHOULD be allocated according to the Specification Required policy [RFC2434] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” October 1998.).
IANA maintains a registry of "WITH protocol types" for use in the "with" clause of the Received header trace fields in an Internet message. Most of the contents of this registry are set out in [RFC3848] (Newman, C., “ESMTP and LMTP Transmission Types Registration,” July 2004.). This specification updates the registry as follows:
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Like the EHLO command, the QHLO command includes a client host name parameter for logging and tracing purposes. This can useful to distinguish different clients behind a NAT, for example. However clients commonly state an incorrect host name, so it SHOULD NOT be relied on. SMTP servers SHOULD use all available client identifiers for logging and tracing, such as its IP address, reverse DNS, TLS certificate, and SMTP AUTH credentials.
It is increasingly popular for SMTP servers (especially MX hosts) to use heuristics based on protocol conformance to identify abusive clients and reject email from them. For example, the server can delay its greeting and see if the client waits to receive it before issuing commands. This heuristic can still be applied in the presence of full support for QUICKSTART, by checking that the client's early first command is not QHLO before deciding that it is abusive.
The full QUICKSTART protocol requires the client to demonstrate that it has previously communicated with the server, by echoing the <qhlo-id>. This can be faked by abusive clients if the <qhlo-id> is too easy to guess. The server can defend itself against pump-and-dump abuse by making the <qhlo-id> depend on more than just the list of supported extensions and their parameters: for example it might use a digest of the list, the server and client IP addresses, and a secret.
Of course, since QUICKSTART is most useful for interactive message submission, and high latency is not such a problem for automated message relay, MX hosts can simply elect not to support QUICKSTART.
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| [RFC0793] | Postel, J., “Transmission Control Protocol,” STD 7, RFC 793, September 1981. |
| [RFC1870] | Klensin, J., Freed, N., and K. Moore, “SMTP Service Extension for Message Size Declaration,” STD 10, RFC 1870, November 1995. |
| [RFC4346] | Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” RFC 4346, April 2006. |
| [RFC4409] | Gellens, R. and J. Klensin, “Message Submission for Mail,” RFC 4409, April 2006. |
| [RFC4422] | Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” RFC 4422, June 2006. |
| [RFC4468] | Newman, C., “Message Submission BURL Extension,” RFC 4468, May 2006. |
| [RFC4616] | Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” RFC 4616, August 2006. |
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In the following examples, a line prefixed with C: indicates the start of a packet from the client to the server, and S: indicates one from the server to the client. Subsequent lines without a prefix are pipelined into the same packet. The C: or S: may be followed by a protocol name - TCP, SMTP or TLS - which indicates a change in the top-most protocol active in the connection. The rest of the line describes a protocol element.
This appendix has been divided into subsections to illustrate the two main options for simplifying QUICKSTART.
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The following figure outlines the sequence of packets at the start of a non-QUICKSTART message submission [RFC4409] (Gellens, R. and J. Klensin, “Message Submission for Mail,” April 2006.), including the TCP handshake [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.), starting ESMTP [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.), securing it using STARTTLS [RFC3207] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.) [RFC4346] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.), authenticating [RFC2554] (Myers, J., “SMTP Service Extension for Authentication,” March 1999.) [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) [RFC4616] (Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” August 2006.), and starting to send the message using pipelining [RFC2920] (Freed, N., “SMTP Service Extension for Command Pipelining,” September 2000.). The MAIL command appears in the 9th packet from the client, and pipelining is not possible before this point.
C: TCP SYN
S: SYN/ACK
C: ACK
S: SMTP <greeting>
C: EHLO <domain>
S: <ehlo-ok-rsp>
C: STARTTLS
S: 220 OK
C: TLS client hello
S: server hello, certificate, etc.
C: key exchange, change cipher, finish handshake
S: change cipher, finish handshake
C: SMTP EHLO <domain>
S: <ehlo-ok-rsp>
C: AUTH PLAIN <creds>
S: 235 OK
C: MAIL FROM:<...>
RCPT TO:<...>
...
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The basic profile supports all the features of QUICKSTART except pipelined STARTTLS (Section 9 (Changes to the STARTTLS command)) and the extended QHLO command (Section 8 (Issuing commands early)). It does not allow for any benefit from a TLS session cache.
The following shows the use of the QTLS and QHLO commands and pipelined AUTH, to illustrate a basic QUICKSTART connection. The MAIL command is in the client's 5th packet.
C: TCP SYN
S: SYN/ACK
C: ACK
S: SMTP <qsmtp-greet>
C: QTLS
TLS client hello
S: server hello, certificate, etc.
C: key exchange, change cipher, finish handshake
S: change cipher, finish handshake
SMTP <qsmtp-greet>
C: QHLO <domain>
AUTH PLAIN <creds>
MAIL FROM:<...>
RCPT TO:<...>
...
S: 220 OK
235 OK
250 OK
250 OK
...
Note that the client omitted the QHLO at the start of the previous connection. This avoids the following situation where it has to switch back and forth between SMTP and TLS at the start of the TLS handshake.
...
S: SMTP <qsmtp-greet>
C: QHLO <domain>
QTLS
TLS client hello
S: SMTP 220 OK
TLS server hello, certificate, etc.
...
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The full profile supports all the features of QUICKSTART except QTLS (Section 6 (The QTLS command)). The simple form of the QHLO command (Section 5 (The QHLO command)) is not used. A TLS session cache can save another RTT.
The following shows a successful full QUICKSTART connection with a TLS session cache hit. The MAIL command is in the client's 3rd packet.
C: TCP SYN
S: SYN/ACK
C: ACK
SMTP QHLO <domain> <qhlo-id>
STARTTLS
TLS client hello
S: SMTP <qsmtp-greet>
220 OK
220 OK
TLS server hello, change cipher, finish handshake
C: change cipher, finish handshake
SMTP QHLO <domain> <qhlo-id>
AUTH PLAIN <creds>
MAIL FROM:<...>
RCPT TO:<...>
...
S: 220 OK
235 OK
250 OK
250 OK
...
If the client has not cached the server's <qhlo-id> the negotiation is as follows. The MAIL command is in the client's 6th packet.
C: TCP SYN
S: SYN/ACK
C: ACK
S: SMTP <qsmtp-greet>
C: QHLO <domain> <qhlo-id>
STARTTLS
TLS client hello
S: SMTP 220 OK
220 OK
TLS server hello, certificate, etc.
C: key exchange, change cipher, finish handshake
S: change cipher, finish handshake
C: SMTP EHLO <domain>
S: <ehlo-ok-rsp>
C: AUTH PLAIN <creds>
MAIL FROM:<...>
RCPT TO:<...>
...
S: 220 OK
235 OK
250 OK
250 OK
...
If the QUICKSTART handshake fails, the server rejects the initial pipelined commands and discards the pipelined TLS client hello, and the client renegotiates. The MAIL command is still in the client's 6th packet.
C: TCP SYN
S: SYN/ACK
C: ACK
SMTP QHLO <domain> <qhlo-id>
STARTTLS
TLS client hello
S: SMTP <qsmtp-greet>
520 bad QHLO ID
503 bad sequence of commands
C: QHLO <domain> <qhlo-id>
STARTTLS
TLS client hello
S: SMTP 220 OK
220 OK
TLS server hello, certificate, etc.
C: key exchange, change cipher, finish handshake
S: change cipher, finish handshake
C: SMTP EHLO <domain>
S: <ehlo-ok-rsp>
C: AUTH PLAIN <creds>
MAIL FROM:<...>
RCPT TO:<...>
...
S: 220 OK
235 OK
250 OK
250 OK
...
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Add the procedural rubric section.
Clarify the alternative profiles in the examples appendix.
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| Tony Finch | |
| University of Cambridge Computing Service | |
| New Museums Site | |
| Pembroke Street | |
| Cambridge CB2 3QH | |
| ENGLAND | |
| Phone: | +44 797 040 1426 |
| Email: | dot@dotat.at |
| URI: | http://dotat.at/ |
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