Re: [port-srv-reg] we need to make progress

[Stuart Cheshire <[e-mail]>]

On 3 Sep 2010, at 10:07, Joe Touch wrote:

>> This implies that a given service name can have *different* port
>> numbers assigned for different transport protocols.
> 
> That is correct, and always has been.

Can you give an example?

In reality, almost no application protocol actually runs over multiple transport protocols.

The only example I know that people tend to cite is DNS, and even that is not strictly the same application protocol over different transport protocols -- over TCP DNS has a two-byte length; over UDP it does not.

> I don't like the idea of a primary alias either; there's no notion of it
> in the way users interact with port indices (/etc/ports or SRVs).

Yes, there is. Safari browses for _http._tcp. Nothing browses for _www._tcp. If your web server advertises _www._tcp then nothing will find it. There's no reason to ever advertise or browse for _www._tcp.

> Agreed. However, note that the document itself creates a number of
> aliases for names that don't follow the new syntax.

And the new names are all the "primary" name to be used in SRV records and similar.

>> Why do we have BNF in this document? 
> 
> Because we are declaring a syntax, and BNF is the only unabiguous way to
> do so.

Not true. There are many ways to explain things unambiguously.

BNF is good for describing languages with recursive structure. For example, in C, an "if" statement contains an expression, and a statement. That statement can itself be another "if" statement, and so on, without limit. BNF is good at expressing this. Our service name syntax has no such recursive structure, which makes BNF a poor way of describing it.

Stuart Cheshire <[e-mail]>
* Wizard Without Portfolio, Apple Inc.
* www.stuartcheshire.org