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Re: Submission and SMTP SRV records

2004-03-16 15:33:46

Is anyone implementing SRV records for either SMTP or for Submission 
(i.e., SMTP SUBMIT extension)? Some talk about implementation raised 
some questions:

I hope that nobody is doing this in shipping product.
There's a reason that RFC 2782 contains the following text:

Applicability Statement

   In general, it is expected that SRV records will be used by clients
   for applications where the relevant protocol specification
   indicates that clients should use the SRV record. Such
   specification MUST define the symbolic name to be used in the
   Service field of the SRV record as described below. It also MUST
   include security considerations. Service SRV records SHOULD NOT be
   used in the absence of such specification.

Adding SRV to an existing application can break compatibility with that 
application, unless it's done very carefully.  For instance even if
there were SRV records pointing to an SMTP server, existing clients
would not recognize them - they would be looking for MX or A (or perhaps
AAAA)  records.  And new SRV-aware clients would behave differently than
existing clients, leading to more difficulty in tracking down problems. 
(Yes, at one time we migrated mail from just A records to MX records,
but that took several years to work reliably, and the net was much
smaller and less diverse then.  And in the interim it was necessary to
use a domain that happened to correspond to an A record if you wanted
mail to work reliably).

- SRV for submission is an interesting way for an client to figure 
out which server/port it should contact, but what exactly would the 
query look like?

SRV for submission is slightly more interesting.  I don't see the harm
in using SRV as part of an MUA configuration mechanism as long as it's
always possible for the user to override the settings.  For a variety 
of reasons it's not always reasonable to use a submission server
associated with a From  address, but it might be a useful way to supply

(It's _much_ less reasonable to base the query on the source IP address.
The last thing we need is more location-specific variation in network

Offhand, it seems like just solving the configuration problem for 
mail submission is too much work for too little gain.  What you'd
really like to do is solve the entire MUA configuration problem.
The user types in one or more email addresses, and the MUA figures 
out which POP, IMAP, Submission and ACAP servers to use for each 
address, how to authenticate to each of them, etc.  Though there
are still some things that have to be specified by the user
basis - "leave mail on server" for instance.

SRV might be part of the picture, but trying to do all of this with SRV
seems like a tall order.

Power corrupts; Powerpoint corrupts absolutely.         - Vint Cerf