A General Mechanism for RTP Header Extensions
draft-ietf-avtcore-rfc5285-bis-14

[Ballot comment]
OLD: The other major change is to update the requirement from the RTP
  specification and[RFC5285]

NEW: The other major change is to update the requirement from the RTP
  specification [RFC5285]

[Ballot comment]
The use of the term "local" in this document is implicit and therefore confusing. Section 5 refers to "local identifier (ID)", while section 7 refers to "Local identifiers". Neither indicates what the identifiers are local to, and some implementors have chosen to interpret this as meaning "local to the sender machine." See, for example, https://bugzilla.mozilla.org/show_bug.cgi?id=1361206

I believe the intention here is for "local" to mean "local to this session." This meaning should be made explicit. And, for avoidance of doubt, the document should clarify that the negotiated identifiers use the same numeric value in both directions. This is implied by much of the text, but it never stated outright. Because so many other session attributes (e.g., payload types) can be negotiated to be different in each direction, many implementors are likely to assume the same applies here. As the above bug demonstrates, this leads to real interop issues in the field.

Nits:

  element (no alignment is needed), and parsing stops at the earlier of
  the end of the entire header extension, or in one-byte headers only
  case, on encountering an identifier with the reserved value of 15.

Put quotation marks around "one byte headers only".
____

  Each extension element MUST starts with a byte containing an ID and a
  length:

s/starts/start/
____

The attribute definition in section 6 says "Value:" instead of "Value: none."  -- Fix or refer to the IANA section instead.

[Ballot comment]
In general this is a well written document, but I have a small list of issues that you should consider fixing:

In Section 5:

  When SDP signaling is used for the RTP session, it is the presence of
  the 'extmap' attribute(s) that is diagnostic that this style of
  header extensions is used, not the magic number indicated above.

You lost me here. Which magic number do you mean here?


In Section 7:

  If an extension is marked as "sendonly" and the answerer desires to
  receive it, the extension MUST be marked as "recvonly" in the SDP
  answer.  An answerer that has no desire to receive the extension or
  does not understand the extension SHOULD remove it from the SDP
  answer.

Why is this not a MUST?

  If an extension is marked as "recvonly" and the answerer desires to
  send it, the extension MUST be marked as "sendonly" in the SDP
  answer.  An answerer that has no desire to, or is unable to, send the
  extension SHOULD remove it from the SDP answer.

As above.

In Section 9:

  In order to prevent DOS attacks, for
  example, by changing the header extension integrity protection SHOULD
  be used.

I can't parse this sentence.

[Ballot discuss]
Why are you allowing this extension to be "normal" in BUNDLE?
If you're bundling, you should be modern enough to handle
mixed, no? Why not make life simple?

[Ballot comment]

  There are two variants of the extension: one-byte and two-byte
  headers.  Since it is expected that (a) the number of extensions in
  any given RTP session is small and (b) the extensions themselves are
  small, the one-byte header form is preferred and MUST be supported by
  all receivers.  A stream MUST contain only one-byte or two-byte

"or only two-byte" would be clearer.


  headers unless it is known that all recipients support mixing, either
  by SDP Offer/Answer [RFC3264] negotiation (see section 6) or by out-
  of-band knowledge.  Each RTP packet with an RTP header extension
  following this specification will indicate if it contains one or two
  byte header extensions through the use of the "defined by profile"
  field.  Extension element types that dp not match the header

do not match


  extension format, i.e. one- or two-byte, MUST NOT be used in that RTP
  packet.  Transmitters SHOULD NOT use the two-byte form when all
  extensions are small enough for the one-byte header form.
  Transmitters that intend to send the two-byte form SHOULD negotiate
  the use of IDs above 14 if they want to let the Receivers know that
  they intend to use two-byte form, for example if the RTP header
  extension is longer than 16 bytes.  A transmitter may be aware that
  an intermediary may add RTP header extensions in this case, the
  transmitter SHOULD use two-byte form.

I think you want '; in this case...." if this means what I think it
means.

  assignment of different IDs.  Any RTP header extension that do not

does not

  match this assumption MUST explicitly provide rules for what are


  cipher.  It can be noted that the default SRTP ciphers (AES CM 128
  bits with HMAC-SHA1) are relative weak and more modern ciphers are
  stronger and should be considered.

What's weak about AES-CTR-HMAC? I recognize that this was in the
secdir review, but I'm not sure what claim you are making here.

[Ballot comment]
I like it. I did have a couple of non-blocking questions.

I couldn't parse this sentence.

  "A transmitter may be aware that
  an intermediary may add RTP header extensions in this case, the
  transmitter SHOULD use two-byte form."

I think

  In the one-byte header form of extensions, the 16-bit value required
  by the RTP specification for a header extension, labeled in the RTP
  specification as "defined by profile", MUST have the fixed bit
  pattern 0xBEDE (the first version of this specification was written
  on the feast day of the Venerable Bede).

is flipping awesome, but likely impenetrable for folks who haven't studied English history in the 600s and 700s. Perhaps a reference, or, more likely, just say "May 25" so it's not a mystery?

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Services Operator has completed its review of draft-ietf-avtcore-rfc5285-bis-09.txt. If any part of this review is inaccurate, please let us know.

The IANA Services Operator has a question about one of the actions requested in the IANA Considerations section of this document.

The IANA Services Operator understands that, upon approval of this document, there are four actions which we must complete.

First, in the IETF URN Sub-namespace for Registered Protocol Parameter Identifiers subregistry of the Uniform Resource Name (URN) Namespace for IETF Use registry located at:

https://www.iana.org/assignments/params/

a new entry will be placed in the registry as follows:

Registered Parameter Identifier: rtp-hdrext
Reference: [ RFC-to-be ]
IANA Registry Reference: [ TBD-at-Registration ]

Second, a new registry is to be created called the RTP Compact Header Extensions registry.

IANA QUESTION -> Where should this new registry be located? Is it a néw registry on the list of all IANA maintained protocol parameter registries or is it a subregistry of an existing registry? If it is a subregistry of an existing registry, in which registry will it be contained?

The new registry will be managed via Expert Review as defined in RFC 5226.

There are no initial assignments in the new registry.

Third, in the att-field (both session and media level) subregistry of the Session Description Protocol (SDP) Parameters registry located at:

https://www.iana.org/assignments/sdp-parameters/

a new entry will be registered as follows:

Type: att--field (both session and media level)
SDP Name: extmap
Mux Category:
Reference: [ RFC-to-be ]

IANA Question --> What is the value for MUX Category for this registration?

Because this registry requires Expert Review [RFC5226] for registration, we've contacted the IESG-designated expert in a separate ticket to request approval. Expert review should be completed before your document can be approved for publication as an RFC.

Fourth, also in the att-field (both session and media level) subregistry of the Session Description Protocol (SDP) Parameters registry located at:

https://www.iana.org/assignments/sdp-parameters/

another new entry will be registered as follows:

Type: att--field (both session and media level)
SDP Name: extmap-allow-mixed
Mux Category:
Reference: [ RFC-to-be ]

IANA Question --> What is the value for MUX Category for this registration?

Because this registry requires Expert Review [RFC5226] for registration, we've contacted the IESG-designated expert in a separate ticket to request approval. Expert review should be completed before your document can be approved for publication as an RFC.

The IANA Services Operator understands that these four actions are the only ones required to be completed upon approval of this document.

Note:  The actions requested in this document will not be completed until the document has been approved for publication as an RFC. This message is only to confirm what actions will be performed.

Thank you,

Sabrina Tanamal
IANA Services Specialist
PTI

The following Last Call announcement was sent out:

From: The IESG
To: IETF-Announce
CC: avtcore-chairs@ietf.org, ben@nostrum.com, magnus.westerlund@ericsson.com, draft-ietf-avtcore-rfc5285-bis@ietf.org, avt@ietf.org, Magnus Westerlund
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (A General Mechanism for RTP Header Extensions) to Proposed Standard


The IESG has received a request from the Audio/Video Transport Core
Maintenance WG (avtcore) to consider the following document:
- 'A General Mechanism for RTP Header Extensions'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits
final comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2017-04-20. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the
beginning of the Subject line to allow automated sorting.

Abstract


  This document provides a general mechanism to use the header
  extension feature of RTP (the Real-Time Transport Protocol).  It
  provides the option to use a small number of small extensions in each
  RTP packet, where the universe of possible extensions is large and
  registration is de-centralized.  The actual extensions in use in a
  session are signaled in the setup information for that session.  This
  document obsoletes RFC5285.




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-avtcore-rfc5285-bis/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-avtcore-rfc5285-bis/ballot/


No IPR declarations have been submitted directly on this I-D.

This is my AD evaluation of draft-ietf-avtcore-rfc5285-bis-08. These are mostly editorial, but I'd like to resolve at least the substantive comments prior to IETF last call.
- Substantive:

-- 6, paragraph 3, "... unless the
  transmitter has some (out of band) knowledge..."
Why bother with that exception? It seems like the negotiation is easy enough. If people write code assuming that out-of-band knowledge will be available, interoperability will suffer.

-- 9, 2nd paragraph:

3264 does not require integrity protection to be _used_, just _provided_ (read: MUST implement). Does this mean to promote that to "MUST use"? If so, please be clear that this is a new requirement.

But from a practical perspective, do you believe people will really follow a "MUST use"? Is it a MUST (BUT WE KNOW YOU WON”T) [RFC6919] ?


- Editorial:

-- 3, third paragraph:

This changes the "... mechanism may be available..." from the original to "... mechanism can be available...". I think "may be" is more correct here.

-- 4.1.1, first paragraph, first sentence:
This is a statement of principle, and as such shouldn't use 2119 keywords.

-- 4.1.2, 2nd paragraph: s/len/length  ; or "len field"  (2 occurances)

-- 4.1.2, 3rd paragraph, sentence starting with "Only the extension..."

“Only…MUST” constructs are ambiguous. The can be interpreted to say the MUST only applies to the matching set, and the rest are undefined. Please recast as “MUST NOT ”. (Note that this construct appears elsewhere in the draft, but I think the other occurrences are in the original text. I don't expect this update to fix those.)

--- "A transmitter MAY be aware...": That's a statement of fact; the MAY should not be capitalized.

-- 4.2, first sentence, first paragraph: The REQUIRED seems to be talking about an external requirement from RTP. If so, it should not be normative here.

-- 7, 11th paragraph, "Local identifiers in the valid range inclusive in an offer or answer
  MUST NOT be used more than once per media section"

The MUST NOT was not capitalized in 5285, probably because it was the 4th time the requirement was mentioned :-). I don't think it needs to be promoted here.

-- 9, 1st paragraph: I don’t think this is an appropriate use of a 2119 MUST. (It wasn’t capitalized in the original.)
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Writeup for draft-ietf-avtcore-rfc5285bis-08

(1) What type of RFC is being requested (BCP, Proposed Standard,
Internet Standard, Informational, Experimental, or Historic)?  Why
is this the proper type of RFC?  Is this type of RFC indicated in the
title page header?

Proposed Standard. This is an technical protocol specification that is intended to replace RFC5285.

(2) The IESG approval announcement includes a Document Announcement
Write-Up. Please provide such a Document Announcement Write-Up. Recent
examples can be found in the "Action" announcements for approved
documents. The approval announcement contains the following sections:

Technical Summary
 
  This document provides a general mechanism to use the header
  extension feature of RTP (the Real-Time Transport Protocol).  It
  provides the option to use a small number of small extensions in each
  RTP packet, where the universe of possible extensions is large and
  registration is de-centralized.  The actual extensions in use in a
  session are signaled in the setup information for that session.  The
  document obsoletes RFC5285

Working Group Summary

  This document has had no major discussions. There has been some
  discussion concerning RTP protocol level and the higher applications
  dependency on header extension elements and the possibility of
  removing them.

Document Quality

  This is an update of the previous document to address some
  issues learned in the usage and further specification of
  RTP header extensions following RFC5285. The amount of review
  has been sufficient but not great.   

Personnel

  The document shepherd is Magnus Westerlund and the responsible AD
  is Ben Campbell.
 
(3) Briefly describe the review of this document that was performed by
the Document Shepherd.  If this version of the document is not ready
for publication, please explain why the document is being forwarded to
the IESG.

The shepherd has reviewed several previous versions and has reviewed
in depth -05, then following each update due to WG last call comments.
He has also checked the diff compared to RFC5285 to verify that the
changes section is relevant. Also looked at the ID-nits.


(4) Does the document Shepherd have any concerns about the depth or
breadth of the reviews that have been performed?

No. This is an update of an existing specification, and the changes has
been reviewed by at least some key individuals.

(5) Do portions of the document need review from a particular or from
broader perspective, e.g., security, operational complexity, AAA, DNS,
DHCP, XML, or internationalization? If so, describe the review that
took place.

SDP directorate review by Paul Kyzivat due to the definition of a new
SDP attribute.

(6) Describe any specific concerns or issues that the Document Shepherd
has with this document that the Responsible Area Director and/or the
IESG should be aware of? For example, perhaps he or she is uncomfortable
with certain parts of the document, or has concerns whether there really
is a need for it. In any event, if the WG has discussed those issues and
has indicated that it still wishes to advance the document, detail those
concerns here.

No concerns.

(7) Has each author confirmed that any and all appropriate IPR
disclosures required for full conformance with the provisions of BCP 78
and BCP 79 have already been filed. If not, explain why.

All authors have confirmed that they are in conformance.

(8) Has an IPR disclosure been filed that references this document?
If so, summarize any WG discussion and conclusion regarding the IPR
disclosures.

No IPR disclosures submitted.

(9) How solid is the WG consensus behind this document? Does it
represent the strong concurrence of a few individuals, with others
being silent, or does the WG as a whole understand and agree with it? 

Strong consensus from few individuals.

(10) Has anyone threatened an appeal or otherwise indicated extreme
discontent? If so, please summarise the areas of conflict in separate
email messages to the Responsible Area Director. (It should be in a
separate email because this questionnaire is publicly available.)

No.

(11) Identify any ID nits the Document Shepherd has found in this
document. (See https://www.ietf.org/tools/idnits/ and the Internet-Drafts
Checklist). Boilerplate checks are not enough; this check needs to be
thorough.

No ID nits other than RFCXXX reference with RFC-editor note.

(12) Describe how the document meets any required formal review
criteria, such as the MIB Doctor, media type, and URI type reviews.

SDP directorate review has been performed.

(13) Have all references within this document been identified as
either normative or informative?

Yes.

(14) Are there normative references to documents that are not ready for
advancement or are otherwise in an unclear state? If such normative
references exist, what is the plan for their completion?

No.

(15) Are there downward normative references references (see RFC 3967)?
If so, list these downward references to support the Area Director in
the Last Call procedure.

No.

(16) Will publication of this document change the status of any
existing RFCs? Are those RFCs listed on the title page header, listed
in the abstract, and discussed in the introduction? If the RFCs are not
listed in the Abstract and Introduction, explain why, and point to the
part of the document where the relationship of this document to the
other RFCs is discussed. If this information is not in the document,
explain why the WG considers it unnecessary.

Yes, it will Obsolete RFC 5285.

(17) Describe the Document Shepherd's review of the IANA considerations
section, especially with regard to its consistency with the body of the
document. Confirm that all protocol extensions that the document makes
are associated with the appropriate reservations in IANA registries.
Confirm that any referenced IANA registries have been clearly
identified. Confirm that newly created IANA registries include a
detailed specification of the initial contents for the registry, that
allocations procedures for future registrations are defined, and a
reasonable name for the new registry has been suggested (see RFC 5226).

The document defines one new SDP attribute. That has been added in the
IANA section. This is a simple registration in an existing registry.


(18) List any new IANA registries that require Expert Review for future
allocations. Provide any public guidance that the IESG would find
useful in selecting the IANA Experts for these new registries.

No, new registries.

(19) Describe reviews and automated checks performed by the Document
Shepherd to validate sections of the document written in a formal
language, such as XML code, BNF rules, MIB definitions, etc.

No new formal language, the old is unchanged.