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Re: [ietf-smtp] Compressing SMTP streams

2016-02-01 04:54:56
On 31/01/2016 02:48, John R Levine wrote:
What purpose is served by using BASE64 beyond making the content able to travel through a 7-bit-only hop? Since most (all?) hops are now 8-bit ones, why not just send the actual 8-bit version of the data instead of converting it to the 7-bit BASE64 format?

The answer to this question is quite complex. The 8BITMIME extension was defined in 1994, and nearly all current MTAs can handle it. But all it says is that the MTA can handle 8 bit characters. It doesn't affect the rule that the message must consist of lines no more than 1000 characters long with \r\n at the end of each. As far as I can tell, the main effect is that people can send ISO-8859-x and UTF-8 without encoding, which is useful but generally not a big deal.

BINARYMIME was defined in 2000 to avoid that issue, and invents a new BDAT command that uses a byte count rather than escape sequence, so the message body can be an arbitrary sequence of octets. Gmail, hotmail and icloud/ support it, Yahoo and AOL don't, but I've never seen client software that would take advantage of it.

What we've never seen is a quoted-unprintable encoding, which is like QP but intended for binary data. It could be like QP without soft line breaks and \r\n pairs are ignored. It's an obvious idea so there must be some reason. Dave Crocker or John Klensin or Ned Freed would know.

The problem with any of these is what to do if YOU accept 8 bit characters, but you have to send the message to a mail server which doesn't say it does. Some just pass it on and hope (which is against the rules AFAICS - e.g. we regularly receive messages with NULL characters in), other than that you can recode the message (which risks breaking DKIM etc) or reject the message (which risks upsetting someone).

That's why we don't support these extensions. If every server supported BINARYMIME it wouldn't be a problem, but the transition period is nasty.

A compression scheme shouldn't have the same problem.

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