Annotated Example SDP for WebRTC
draft-ietf-rtcweb-sdp-14

[Ballot comment]
Dropping to No Record to reflect that my Discuss points on the -12
have been resolved.  The reformatting (e.g., of tables) incurred in going
from the -12 to the -14 makes reviewing the diff about as much work as
reviewing the -14 from scratch, and I don't want to delay this document
while I find the time to do so.

[Ballot comment]
Thanks for addressing my issues. I did notice that the current text version is not keeping within the text formats limitation on lines. However, it might be that the RFC-editor is better at getting these tables to stay within limitations. I think this is one document that would benefit from a proper v3 format conversion so that we get reasonable HTML table renderings of the examples.

[Ballot discuss]
A. Section 5.3.1: Table 27, 28
  | a=rtpmap:101 H264/90000                    | [RFC6184]          |
  | a=rtpmap:102 H264/90000                    | [RFC6184]          |
  | a=fmtp:101 profile-level-                  | [RFC6184]Camera-2,E |
  | id=42401f;packetization-mode=0;max-fr=30    | ncoding-1          |
  | a=fmtp:102 profile-level-                  | [RFC6184]Camera-2,E |
  | id=42401f;packetization-mode=1;max-fr=15    | ncoding-2          |

H.264 payload format does not have an max-fr parameter. This needs to be moved to the RID level.

The issue is reoccuring also in Table 29, 41. Please check whole document.

B. Section 5.3.5:

  This section shows an SDP Offer/Answer exchange for Simulcast video
  stream at two resolutions and and has [RFC5956] style FEC flows.

What is meant with the RFC5956 reference here. The grouping frame-work is not used to indicate which streams that are combined when performing FEC. As FLEX-FEC is used I assume that the protection of multiple source streams that may occur here are done using the SSRC mechanism in the FLEX-FEC format. I would additionally note that when FLEX-FEC protects multiple source streamins in one repair flow it will not be able to use repaired-rtp-stream-id.

Can you please clarify the introduction to what is actually intended to be described here.

[Ballot comment]
Section 5.1:

A=setup is wrongly atteributed to SRTP rather than the DTLS-SRTP key-exchange.

OLD:

SRTP Setup framework parameters (a=setup)
New:
DTLS-SRTP Setup framework parameters (a=setup)

[Ballot comment]
Hi,

This document is well outside my domain of expertise, and I see no management related issues or concerns.

I have mostly only skimmed this document.  I hope that either the examples have been generated by real code, or otherwise programmatically validated in some way to ensure that they are correct.

Regards,
Rob

[Ballot comment]
I am not even remotely qualified to evaluate the examples. Here are a bunch of nits instead:

Abstract: s/mechanisms/mechanisms.

Section 3:
s/Below figure/The figure below

s/convey participant’s/convey the participant’s

Section 4.

s/introduces SDP/introduces the SDP

s/in nature/in nature,

s/refer [RFC3264]/refer to [RFC3264]

s/It defines protocol/it defines a protocol

s/each others/each other’s

s/JSEP specification/The JSEP specification

Section 5.1:
s/apriori/a priori

s/intentionally is not/intentionally are not

s/In the actual use/In actual use

table 3: s/sreams/streams

Sec 5.3.4:
s/2 two/two

Table 33. I believe the offer should reference RTX streams (plural?)

Sec 5.3.5: s/we end up/, we end up with

Sec 5.4.4: s/Bob being a WebRTC endpoint/Bob, being a WebRTC endpoint,

Sec 7: s/using TLS/using the TLS

[Ballot discuss]
I'm not 100% sure, but I think there's a handful of places with internal
inconsistencies that should be resolved before publication.  I'd be
happy to hear I'm wrong, of course.  The specifics make more sense in
context in the COMMENT section, so I'll just indicate the specific
locations here:

(1) In Section 5.3.2 tb != tc in the SDP vs. commentary

(2) In Section 5.3.5 is there a fourth video stream

(3) Also in 5.3.5, the ToP parameter is no longer valid (e.g., in RFC
8627)

(4) In Section 5.4.3, the PTs in the rejected video lines in the answer
don't seem correct

[Ballot comment]
I echo the thanks for assembling a nice collection of annotated examples; it's
thankless work but very helpful to have.

Section 1

  Javascript Session Establishment Protocol (JSEP)
  [I-D.ietf-rtcweb-jsep] specifies a generic protocol needed to
  generate [RFC3264] Offers and Answers negotiated between the [WebRTC]
  peers for setting up, updating and tearing down a WebRTC session.
  For this purpose, SDP is used to construct [RFC3264] Offers/Answers
  for describing (media and non-media) streams as appropriate for the
  recipients of the session description to participate in the session.

There's some weird redundancy or missing information in the first
sentence here that makes it come off funny, perhaps because the first
3264 reference ignores the "SDP" bit of it and the second sentence tries
to add it back on.

Section 4

  This section introduces SDP Offer/Answer Exchange mechanism mandated
  by WebRTC for negotiating session capabilities while setting up,
  updating and tearing down a WebRTC session.  This section is

nit: doesn't "SDP Offer/Answer Exchange mechanism" require an article?

  intentionally brief in nature and interested readers are recommended
  to refer [RFC3264] for specific details on the protocol operation.

nit: "refer to", I think.

  of multimedia streams.  It defines protocol with involved
  participants exchanging desired session characteristics from each

nit: "protocol" gest an article, too.

  reject the offer.  If the session is accepted the Offer/Answer model
  guarantees a common view of the multimedia session between the
  participants.

nit: I suggest s/guarantees/provides/; I can think of at least one way
in which the common view would fail and "guarantees" is a rather strong
statement.

  session updates.  JSEP specification [I-D.ietf-rtcweb-jsep] for
  WebRTC provides the mechanism for generating [RFC3264] SDP Offers and
  Answers in order for both sides of the session to agree upon the
  details such as the list of media formats to be sent/received,

nit: the reference seems misplaced, here (maybe a few words later).

Section 5

Overall, I only really spot-checked the examples and corresponding
references.

  A typical web based real-time multimedia communication session can be
  characterized as below:

  o  It has zero or more Audio only, Video only or Audio/Video RTP
      Sessions,

Is *zero* specifically "typical" (vs. "one or more")?

  o  Sessions can be over IPv4-only, IPv6-only, dual-stack based
      clients,

nit: conjunction for the list, please!

Section 5.2.1

  This example also shows the endpoints being [RFC8445] compliant by
  including "ice2" ice-options attribute.

Omitting ice2 from all the other examples perhaps implies that it is not
terribly important to use.  Is it worth an explanatory note?

  | a=identity:eyJpZHAiOnsiZG9tYWluIjoibmlpZi5o | Section 5.6 of [I-D |
  | dSIsInByb3RvY29sIjoiaWRwLmh0bWwifSwiYXNzZXJ | .ietf-rtcweb-securi |
  | 0a W9uIjoiZXlKaGJHY2lPaUpTVXpJMU5pSXNJblI1Y | ty-arch]            |

I don't think this is the right reference (for a=identity?), since
there is no Section 5.6 in
https://tools.ietf.org/html/draft-ietf-rtcweb-security-arch-20
(Also for the corresponding bit in the Answer.)

Section 5.2.7

  | m=video 0 UDP/TLS/RTP/SAVPF 120            | [RFC4566]          |
  | c=IN IP4 203.0.113.77                      | [RFC4566]          |
  | a=bundle-only                              | [I-D.ietf-mmusic-sd |
  |                                            | p-bundle-negotiatio |
  |                                            | n]                  |
  | a=mid:video                                | [RFC5888] Video    |
  |                                            | m=line part of the  |
  |                                            | BUNDLE group with  |
  |                                            | the port from audio |
  |                                            | line repeated      |

Is it the port from the audio line *repeated*, or the value 0 used to
indicate bundling on the same port?

Section 5.2.11

(Table 25)

  | v=0                                        | Version number      |
  |                                            | incremented        |
  |                                            | [RFC4566]          |

It's the session version (o=) that's incremented, but the formatting
implies that the v= value is what's being described.
(Similarly for the answer.)

  | a=tls-id:89J2LRATQ3ULA24G9AHWVR31VJWSLB68  | [I-D.ietf-mmusic-dt |
  |                                            | ls-sdp]Alice want's |
  |                                            | to use the same    |
  |                                            | DTLS association    |

nit: no apostrophe in "wants"

Section 5.3.2

  | a=msid:ma tb                                | Identifies          |
  |                                            | RTCMediaStream ID  |
  |                                            | (ma) and            |
  |                                            | RTCMediaStreamTrack |
  |                                            | ID (tc)            |

Note that tb != tc

Section 5.3.3

  |Alice offers single audio and simulcasted video streams  |
  |                                                        |
  |                                                        |
  |    Offer(Audio:Opus Video:VP8 with 3 resolutions)      |
  |    & RTX stream                                        |

Given that we're already on a second line, we can probably afford to
write out "and".

Section 5.3.5

  On completion of the Offer/Answer exchange mechanism we end up one
  audio stream, 2 simulcast video streams and 2 associated FEC streams
  are sent over a single 5-tuple.
  [...]
  |One-Way Audio,Video session with 4 video streams(Simulcast    |
  | and FEC) all multiplexed                                      |

I think I'm failing to find the fourth video stream: I see PTs 98 and
100 (simulcast) and 101 (FEC) but don't see indication of a second FEC
stream.

  | a=fmtp:101 L=5; D=10; ToP=2; repair-        | [I-D.ietf-payload-f |
  | window=200000                              | lexible-fec-scheme] |

Neither the listed I-D reference nor RFC 8627 which it became seem to
describe the "ToP" parameter.  In fact, it seems to have been removed in
the -19 of that I-D.  (Affects both Offer and Answer.)

Section 5.4.1

  This example shows Alice indicating the support of the RTP header
  extension to include the audio-level of the audio sample carried in
  the RTP packet.

I'm a bit confused why a dedicated section is needed -- isn't this
talking about the urn:ietf:params:rtp-hdrext:ssrc-audio-level extension,
which has been used in quite a few of the previous examples already?

Section 5.4.3

  NOTE: Since Alice is unaware of Bob's support for BUNDLE framework,
  Alice includes separate RTP/RTCP ports and candidate information.

nit: this wording implies that Bob does support BUNDLE and it's merely
that Alice is unaware of the fact; my understanding is that the intent
is only to say that Alice is unaware of whether or not Bob supports
BUNDLE, e.g., "Alice is unaware of Bob's support status for the BUNDLE
framework" or "Alice is uaware of whether Bob supports the BUNDLE
framework".

  | a=extmap:2 urn:ietf:params:rtp-            | [I-D.ietf-mmusic-sd |
  | hdrext:sdes:mid                            | p-bundle-negotiatio |

Huh, we have an extmap:2 without a previous extmap:1, interesting.

  | ****** Video m=line *********                  | *************** |
  |                                                | **************  |
  | m=video 0 UDP/TLS/RTP/SAVPF 98 100              | Bob doesn't    |
  |                                                | recognize      |
  |                                                | bundle-only and |
  |                                                | hence the      |
  |                                                | m=line is      |
  |                                                | rejected        |
  |                                                | implicitly due  |
  |                                                | to port 0      |
  | ****** Video m=line *********                  | *************** |
  |                                                | **************  |
  | m=video 0 UDP/TLS/RTP/SAVPF 98 100              | Bob doesn't    |

Why are both m=video rejects identical?  In the Offer the two video
stanzas used PT 98 and PTs 101 and 103, respectively.

Section 7

We could reiterate the note from Section 5.1 that SSRC/foundation/etc.
should be larger/more-random than what is shown, for secure operation
(and what would go wrong if the short values were used).

Appendix A.1.1

  o  o= line MUST follow with values '-' for username, 64 bit value for
      session id and dummy values for 'nettype', 'addrtype' and
      'unicast-address' (for example: IN IP4 0.0.0.0).

Our examples do not seem to be using 64-bit session IDs, and this
disparity does not seem to be included in Section 5.1's list of values
that need more entropy than shown in the examples.

Appendix A.1.2

  o  a=extmap line identifying the BUNDLE header extension MUST be
      present.

draft-ietf-mmusic-sdp-bundle-negotiation seems to refer to this as the
"RTP MID header extension", not the "BUNDLE header extension".

[Ballot comment]
[[ nits ]]

[ abstract ]
* "mechanism" --> missing a full stop

[ section ]
* "Below figure" --> "The below figure"

[ section 4 ]
* "introduces SDP" --> "introduces the SDP"?

* "to refer RFC3264" --> "to refer to RFC3264"

* "defines protocol" --> "defines a protocol"?

* "each others" --> "each other's"

[ section 5.1 ]
* "Eventhough" --> "Even though"

* "diagrams shows" --> "diagrams show"

* "confirm to" --> "conform to"

* "Following SDP" --> "The following SDP"

* "SSRCs" --> expand SSRC on first use?

[ section 5.2.7 ]
* "the Alice" --> "Alice"

* "the Bob indicates its" --> "Bob indicate their"

[ section 5.2.9 ]
* "The same is indicated" --> "This is indicated"?

[ section 5.2.10 ]
* "show-cases" --> "showcases"

[ section 5.3.4 ]
* "2 two" --> "two"

[ section 5.3.5 ]
* "and and" --> "and"

[ Appendix A.1.2 ]
* "lip same sync" --> "same lip sync"

[ Appendix A.1.3 ]
* "An JSEP" --> "A JSEP"

[Ballot comment]
Thanks for writing this document.  It is always helpful to have examples that tie together a number of specifications.

** Section 4 and 5.  I was surprised to see three place where RFC2119 language was used in the core text (i.e., not in the Appendix).  Is there a reason this particular behavior was called out here?  Is it not covered in other specifications?

-- Section 4.  The participant receiving the offer MAY generate an SDP Answer accepting the offer or it MAY reject the offer.

-- Section 5. MAY contain zero or more non-media data sessions,

** Editorial Feedback

--  Section 1.  Recommend repeating the following text in the abstract to make it clear that this document is informative in nature: “This document provides an informational reference in describing the role of SDP and the Offer/Answer exchange mechanism for the most common WebRTC use-cases.”.  Additionally, consider adding that “This document makes no changes to the Offer/Answer exchange mechanism”.

-- Section 3.  Editorial.  s/Below figure introduces/Figure 1 introduces the/

-- Section 3.  Editorial.  Sentence uses “design” twice.  s/[WebRTC] is designed so that the design of the control plane/[WebRTC] is architected in such a way that the design of the control plane/

-- Section 5.1. Typo. s/Eventhough/Even though/

-- Section 5.1. Editorial.  s/Following SDP details/The following SDP details/

[Ballot comment]
All the references are marked as Informative; I am surprised that none of them are considered ones "that must be read to understand...the technology in the new RFC" [1].

It seems to me that some (maybe all) of the following should be Normative references: I-D.ietf-rtcweb-jsep, WebRTC, rfc3264, rfc7656, rfc4566.

I am not balloting DISCUSS because I am definitely not an expert in the topic, so I trust the Responsible AD to make the right call.


[1] https://www.ietf.org/about/groups/iesg/statements/normative-informative-references/

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

The IANA Functions Operator has reviewed draft-ietf-rtcweb-sdp-11, which is currently in Last Call, and has the following comments:

We understand that this document doesn't require any registry actions.

While it's often helpful for a document's IANA Considerations section to remain in place upon publication even if there are no actions, if the authors strongly prefer to remove it, we do not object.

If this assessment is not accurate, please respond as soon as possible.

Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2020-04-28):

From: The IESG
To: IETF-Announce
CC: draft-ietf-rtcweb-sdp@ietf.org, Ted Hardie , superuser@gmail.com, ted.ietf@gmail.com
Reply-To: last-call@ietf.org
Sender:
Subject: Last Call:  (Annotated Example SDP for WebRTC) to Informational RFC


The IESG has received a request from an individual submitter to consider the
following document: - 'Annotated Example SDP for WebRTC'
  as Informational RFC

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
last-call@ietf.org mailing lists by 2020-04-28. 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

  The Real-Time Communications in WEB-browsers (Rtcweb) working group
  is charged to provide protocol support for direct interactive rich
  communication using audio, video and data between two peers' web
  browsers.  With in the Rtcweb framework, Session Description protocol
  (SDP) is used for negotiating session capabilities between the peers.
  Such a negotiation happens based on the SDP Offer/Answer exchange
  mechanism.

  This document provides an informational reference in describing the
  role of SDP and the Offer/Answer exchange mechanism for the most
  common Rtcweb use-cases.


The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-rtcweb-sdp/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-rtcweb-sdp/ballot/

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

As required by RFC 4858, this is the current template for the Document Shepherd Write-Up.

Changes are expected over time. This version is dated 24 February 2012.

(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?

Informational is requested; this is reflected in the title page header.


(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 an informational reference describing the role of SDP and example Offer/Answer exchanges  for common WebRTC uses of JSEP (draft-ietf-rtcweb-jsep).  These reference are not normative, but provided as informative examples for implementers.

Working Group Summary

The working group had consensus to publish as Informational. 

Document Quality

This document represents a challenge to most reviewers both because of its length and the nature of SDP’s long history of aggregating semantics to a syntax originally designed for very different tasks.  Magnus Westerlund, Paul Kyzivat, Justin Uberti, Chris Flo, Flemming Andreasen, Christer Holmberg, and Nils Ohlmeier provided valuable reviews.
 

Personnel

Ted Hardie is the document shepherd.  Adam Roach is the responsible area director.

(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.

As the document shepherd is not an SDP expert, he did not undertake an individual review of the document but is relying on the reviews solicited by the chairs during and before working group last call.  There was one set of review items raised during working group last call that were not handled prior to the request for publication, because one of the authors was on parental leave.  The chairs anticipate them being handled along with any issues which come up during IETF Last Call.

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

I understand that at least one author of JSEP would have preferred creating a tool that emitted JSEP-derived SDP as an alternative to this document’s examples.  The production of that tool and alternative document did not appear over many months after that preference was expressed, however, and the chairs feel it is better to move forward with a well-reviewed version created in this way than to have no example document.


(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.

They require review from the SDP directorate, which was requested and received.

(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.

See the answer to #3.

(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.

They have confirmed.

(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 have been filed.

(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? 

It represents the concurrence of those who are deeply imbued with SDP arcana.  The working group outside of that set agrees with the need for this document.

(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.

ID nits include the use of inappropriate example IPv4 address ranges, some outdated references, and a small number of references which are not used.  The authors have been requested to fix these nits along with any other issues that arise in Last Call.

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

The SDP directorate review is the relevant formal review; see above.

(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?

The document references a number of the specifications that are or will be part of RFC cluster 238, and it will not advance until that cluster has been resolved.  The working group is aware of this.

(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.

The document does not request any downward normative references.

(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.

This document does not.

(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).

This document makes no requests of IANA.

(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.

This document makes no requests of IANA.

(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.

That authors used the syntax validation script provided by Adam Roach to check the syntax of the relevant entries.

Magnus's comments are noted and will be treated with IETF Last Call comments.  This aggregation is to support an author having a single update to publish, given parental leave.