Re: [secdir] Review of draft-ietf-mmusic-sctp-sdp-22

[Christer Holmberg <[e-mail]>]

Hi Paul,

Thanks for your review! Please see inline.

...

>This document defines SDP [RFC4566] protocol identifiers (proto values):
>'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'.  This specification also specifies
>how to use the new proto values with the SDP Offer/Answer mechanism
>[RFC3264] for negotiating SCTP-over-DTLS associations.
>
>As this document only defines a method of signaling to use UDP/DTLS/SCTP
>or UDP/DTLS/SCTP, there are not specific Security Considerations for this
>document, and the Security Considerations correctly points to the
>documents
>that actually implement the UDP/DTLS/SCTP and TCP/DTLS/SCTP protocols.
>
>So from a security point of view, this document is Ready.

Thanks!

----

>I do have some generic concerns and questions I would like to see
>addressed:
>
>One general issue I have is with the specification of IANA values and
>punting
>some of its meaning. Let me quote an example:
>
> 	NOTE: This specification only defines the usage of the SDP 'max-
> 	message-size' attribute when associated with an m- line containing
> 	one of the following proto values: 'UDP/DTLS/SCTP' or 'TCP/DTLS/
> 	SCTP'.  Usage of the attribute with other proto values needs to be
> 	defined in a separate specification.
>
>This type of constraint is not clear and easilly displayed in an IANA
>registry if
>other values are later added to the table. This text is also an example
>of how
>througout the document, items are explicitely "this use is unspecified
>for now
>but other documents may change that". It would have been better to just
>simply
>and clearly state the current specified use, and let future documents
>handle
>updating this document appropriately. This language use makes a lot of
>decisions
>for implementers unclear as these decisions are punted forward in time
>without
>a reference to follow.

I think the idea is that if someone wants to define the usage for other
proto values he/she does not need to update THIS specification, as it
already allows it.

----

>Section 4.4.1 states:
>
> 	This specification creates an IANA registry for 'association-usage'
>values.
>
>I think this should be specified a little more clearly, similar to the
>IANA section
>specification itself. Something like:
>
> 	This specification creates the IANA 'association-usage' registry for
>SDP Attributes.

We are discussing this with IANA, so the text will be aligned.

----

>Section 4.4.2:
>
>The table contains an error on the <port> entries (UDP instead of TCP).

Correct. That was noted also by the AD.

----

> I would personally change the port parameter value to just "port number
>for <proto>" or "UDP or TCP port number"

People wanted to have the proto values explicitly stated, so I¹d like to
keep them.

----

>Section 4.5:
>
>I'm unsure if ordering matters, so I am confused by the table ordering in
>media, proto, port,
>and the m= line example ordering media, port proto.

I obivously can¹t change the order in the m= line, but I can change the
order of <proto> and <port> in the table, if you think that make things
more clear.

----

>I'm further confused by the table listing colon's, eg "<proto>:" and the
>m= line not
>using colons but spaces. And the following a= lines using colons again.
>Is this an error,
>or this is my lack of familiarity with the used protocols?

I honestly have no idea why the colones are there (probably a copy/paste
error). I can remove them.

----

>Section 5.1:
>
> 	Therefore, if the attribute is not present, the associated m- line MUST
>be considered invalid.
>
>Is it obvious what one must done when the m- line is considered invalid?

I could add ³, and MUST NOT be used to establish an SCTP-over-DTLS
association."

----

>Section 5.2:
>
> 	Leading zeroes MUST NOT be used.
>
>What is the problem with leading zeros? What can go wrong when these are
>used? Should
>an implementor be warned about something specific?

I don¹t know. The same statement exists for a number of SDP attributes,
but I didn¹t find any justification.

----

>Section 6.1:
>
> 	An SCTP endpoint MUST NOT send a SCTP user message with a message
> 	size that is larger than the maximum size indicated by the peer, as
> 	it cannot be assumed that the peer would accept such message.
>
>While this tells an implementer what not to do, it does not tell the
>implementer
>what to do when this happens on the receiving end. It would be good to
>specify
>that here as well so that implementors will be reminded to think of this
>error
>handling case.

I am not sure whether it¹s within the scope of this document to say what
SCTP endpoints do if they receive SCTP user messages that are too big for
them to handle.


> 	a maximum message size value of zero, it indicates the SCTP endpoint
>will handle
> 	messages of any size, subject to memory capacity etc.
>
>I'm personally not a big fan of 0 meaning infinite, but if this is how
>other related
>specs are handling these values in this way, I can understand doing it
>here as well.
>For instance, not specifying a max-message-size seems more indicative of
>not having
>a max message size. But here that has been defined as meaning 64k. If
>this usage
>would be a precedent, I would recommend to not do this. If this is how
>these things
>are done in this world, then I guess stick to what has already been done
>in this space.

I don¹t know how things are normally done, because the only other SDP
attribute I can think of providing similar information is Œmax-size¹ (RFC
4975), but it does not define a default value, nor a infinite value.


>The table entry in 6.2 lists an email contact for an IANA entry. Why does
>an IANA
>entry need someone's email address as contact? IANA entries normally only
>refer to
>the RFC's that define them, and people are expected to contact the
>working group
>or maybe one of the RFC authors for questions. But putting a name and
>email address
>entry here seems to convey some kind of "ownership" of the IANA entry
>which I think
>is the wrong thing to do.

You¹ll have to ask IANA about that :) The e-mail address is within the
template that has to be filled.

Keep in mind, though, that IANA entries are not always defined in RFCs.
For example, a number of registrations have been provided by 3GPP.

----

>Section 6.3
>
>    As the usage of multiple SCTP associations on top of a single DTLS
>    association is outside the scope of this specification, no mux rules
>    are specified for the 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto
>    values.
>
>Why is this out of scope? If there are good reasons not to do it, should
>it not
>be defined to MUST NOT? If it is out of scope only for now, then it seems
>that
>this document should in fact just specify it so people know how to do it.
>The
>way this is phrased seems to leave the thing hanging in midair, and
>implememters
>might end up doing different things.

In order to allow multiple SCTP associations, you would have to be able to
provide multiple SCTP ports, and somehow associate those with each
association. We decided not to do that.

But, I don¹t think we want to forbid people from establishing multiple
SCTP associations - this spec just doesn¹t support using SDP offer/answer
for negotiating multiple associations.

----

>Section 7:
>
>    NOTE: While [I-D.ietf-tsvwg-sctp-dtls-encaps] allows multiple SCTP
>    associations on top of a single DTLS association, the procedures in
>    this specification only support the negotiation of a single SCTP
>    association on top of any given DTLS association.
>
>And similar here.

See above.

----

>Setion 8:
>
>Similar issues with "the procedures" and with "is out of scope for this"
>
>Section 9.2:
>
> 	This specification does not define semantics for the SDP direction
> 	attributes [RFC4566].  Unless semantics of these attributes for an
> 	SCTP association usage have been defined, SDP direction attributes
> 	MUST be ignored if present.
>
>Why are these attributes not defined in this specification? If there are
>good
>reasons for it not to have been specified, why not change the language to
>clearly state these attributes here are invalid and MUST be ignored? If it
>is possibly these will be defined in the future (which the current text
>suggest might happen) perhaps that specification really belongs in this
>document.

The direction attributes are not used for transports. They are used for
³applications², e.g, RTP.

But, as nobody has indicated any interest of defining
RTP-over-SCTP-over-DTLS, there is no idea to define how the direction
attributes would be used.

----


Nits:

>Through the document, there are paragraphs that start with "NOTE:". I
>think
>those can all be safely removed to increase the readability.

Ok, I¹ll look into that.

----

>Section 1:
>
>    [I-D.ietf-tsvwg-sctp-dtls-encaps]
>
>This looks but is not a proper reference. (eg there is no link)
>
>    When the 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto values, the m-
>    line fmt value, identifying the application-layer protocol, MUST be
>    registered by IANA.
>
>This sentence does not parse.
>
>
>Section 6.1
>
>Line wrapping 'TCP/DTLS/
>               SCTP'   is best avoided.

I¹ll fix that.

----

>Section 9
>
> 	Each can be established and closed without impacting others.
>
>Should that not be "each other" or "the others" (as opposed to "others"
>being other things far far away)

I¹ll change to ³the others².

----

>Section 10.3
>
>[I-D.ietf-mmusic-dtls-sdp] is not a proper reference with link.

I¹ll look into that.

----

>Section 12
>
>I would rephrase "The text in the paragraph above" as these indirect
>references make the text harder
>to read.

I could remove the first sentence, and keep the second:

    "If ICE is not used, the proto value MUST always reflect the transport
protocol used at any given time."



Regards,

Christer