Re: SCTP-over-UDP and port 9899

[Michael Tuexen <[e-mail]>]

On Nov 1, 2011, at 4:47 PM, Dan Wing wrote:

>> -----Original Message-----
>> From: [e-mail] [mailto:[e-mail]] On Behalf
>> Of Michael Tuexen
>> Sent: Monday, October 31, 2011 2:27 PM
>> To: Dan Wing
>> Cc: [e-mail]; [e-mail]
>> Subject: Re: SCTP-over-UDP and port 9899
>> 
>> On Oct 31, 2011, at 9:10 PM, Dan Wing wrote:
>> 
>>> http://tools.ietf.org/html/draft-ietf-tsvwg-sctp-udp-encaps-01 and
>> its
>>> predecessor both say:
>>> 
>>> "When inserting the UDP header, the source port is 9899, the
>>>  destination port is the one stored for the destination address the
>>>  packet is sent to or 9899 if no destination address is stored."
>>> 
>>> Why is it important to use a specific source port?  Does that not
>>> cause difficulties if there are two (or more) instances of SCTP-
>>> over-UDP applications running on the same host?  Does that not
>>> cause difficulties, in implementation assumptions, if there are
>>> two hosts behind the same NAPT and only one can get UDP/9899
>>> forwarded to them?
>> Hi Dan,
>> 
>> for sending out the initial packet of an association, the source
>> port is not important.
> 
> Then can you strike "the source port is 9899" from the document?
> 
>> The peer will send back the response to any
>> port. But the destination port of the initial packet is important.
>> It must be the same port the peer is accepting UDP packets. It
>> can be any port, but then it needs to be negotiated somehow.
> 
> Sure, of course.
> 
> The 32-bit or 128-bit IP address is already negotiated.  
> Negotiating another 16 bits is not onerous.  It isn't easy
> with DNS (because neither SRV records nor the ability to
> update them is ubiquitous), but lots of protocols already
> communicate IP address and port -- BitTorrent, Skype, SIP,
> many others.
> 
>> Since there is one registered port for SCTP/UDP, I used that.
>> But you can use a different one, if negotiated. If you want to
>> run two hosts as servers behind a NAT, you need different UDP
>> ports. If they are clients, you don't.
>> 
>>> In short, what is the benefit of using 9899 as the source port?
>> It is more a default, if nothing else is known. In the socketAPI
>> section the remote port is configurable. The local port (at
>> least in the FreeBSD/MacOS X implementation) is a parameter
>> controller by a sysctl variable. So (currently) a single
>> host uses a single port number for all associations.
>> 
>> Does this makes things clearer?
> 
> Not especially, no.
> 
> Let's look at, for example, SSH -- the SSH port is 22.   But
> there is no need for the *source port* to be 22.  I'm trying
> to understand why there is text mentioning anything about
> the source port in draft-ietf-tsvwg-sctp-udp-encaps.
OK, this is a good example. I want 9899 for SCTP/UDP be similar
to 22 for SSH. But there is a difference. I want to support the
case where I have one SCTP stack on a host and this stack uses
a single UDP port number shared between all associations, no
matter which side initiated them. I might not have made that
clear. So it different from SSH where the server binds to a
well known port (or not if otherwise negotiated), the client
uses an arbitrary one. Do you have a suggestion for the wording?

Best regards
Michael
> 
> -d
> 
> 
>> Best regards
>> Michael
>>> 
>>> -d
>>> 
>>> 
>>> 
> 
>