BURL review notes

BURL proposal (http://www.ietf.org/internet-drafts/draft-ietf-lemonade-burl-00.txt) last call

Goal: diminish bandwidth consumption in one case (forwarding an email) Consideration: since we're optimizing one case only, the optimization had better be simple

Requirements:

• Sending client and server must support RFC 2476 (Message Submission protocol) -- see below
• Sending client and server must support BURL as well

RFC2476

This seems be really simple. The "protocol" defined in RFC2476 is really SMTP -- called ESMTP -- with an important difference being that RFC2476 suggests use of a different port than SMTP. This allows the system to differentiate between message submission (first step in chain) and message transfer, both of which SMTP does. The advantage in allowing the differentiation is evident in the design of BURL -- the BURL extension is only allowed on RFC2476, because it is only appropriate when the message is first being delivered (probably to a server in the same organization as the IMAP server) and not at any other SMTP link in the delivery chain. Another example is that a RFC2476 server (Mail Submission Agent) can be configured to require authentication, and a normal MTA (Mail Transfer Agent) can be configured not to. The MSA can respond with additional errors that the MTA cannot respond with -- because the MSA is responding directly to the sender of the message (an MTA cannot get an error back to the original sender of the message).

Other requirements of ESMTP are all requirements on the server, which seems to mean that clients are likely to be backwards-compatible. Servers must check that domain names are fully-qualified (virus/snooping protection). They are also recommended to do address validity checking and log errors.

Is there any server deployment, support, interoperability testing of RFC2476?

• Email processing agent supporting port used by 2476 -- http://www.tikitechnologies.com/support/post-setup.html
• Sendmail v 8.11.1 (http://docs.hp.com/en/B2355-90776/ch02s08.html#chbhfdac)
• M-Switch claims support (http://www.isode.com/products/m-switch.html)

Client support isn't necessarily required, as long as the SMTP port is configurable, but I note that Eudora and Ximian both have notes on this in support pages, implying people have tried and it works.