[MMUSIC] Comments on draft-ietf-mmusic-sdp-new-15
"Drage, Keith (Keith)" <drage@lucent.com> Wed, 26 November 2003 14:19 UTC
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From: "Drage, Keith (Keith)" <drage@lucent.com>
To: "'jo@acm.org'" <jo@acm.org>, csp@csperkins.org
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Subject: [MMUSIC] Comments on draft-ietf-mmusic-sdp-new-15
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At IETF58 requests were made for a final review of this document before resubmitting to the IESG. These comments are made as a result of that request. If mmusic participants wish to enter into discussion on any of these, you snip out the remainder of the comments so that mail threads do not become burdened with text that is not under discussion. regards Keith Keith Drage Lucent Technologies drage@lucent.com tel: +44 1793 776249 Comments on draft-ietf-mmusic-sdp-new-15 ---------------------------------------- 1) Introduction: section 1. 2nd paragraph. In: When initiating multimedia teleconferences, voice-over-IP calls, streaming video, or other real-time sessions, there is a requirement to convey media details, transport addresses, and other session description metadata to the participants. the words "or other real-time sessions" imply that SDP is limited to describing real-time sessions. SDP is proposed to be used to describe message sessions, which are immediate delivery, but which are not traditionally known as real-time. It is proposed to delete the words "real-time", leaving "or other sessions" with the word "session" being the defined term. 2) Terminology: section 2. The term "session advertisment" is never used in the document. The word "advertisment" is used in the document, but in descriptive terms that does not really justify the use of a defined term. It is suggested we delete the definition "session advertisment" from section 2. 3) Terminology. I section 2, the term "session announcement" is defined, but this term is not consistently used through the document. There are a number of places where the text within the document should be enhanced to use this term as below. However in other cases, rather than using the term "session announcement", it would appear that the other term "session description" would be more appropriate to use, see also below. a) section 3.1, title: "Multicast announcement" --> "Multicast session announcement". b) section 3.1, 2nd paragraph: One protocol commonly used to implement such a distributed directory is the Session Announcement Protocol, SAP [8]. SDP provides the recommended session description format for such announcements. should become: One protocol commonly used to implement such a distributed directory is the Session Announcement Protocol, SAP [8]. SDP provides the recommended session description format for such _session_ announcements. c) section 3.4, 2nd paragraph: SAP announcements do not suffer from this mismatch. should become: Session announcements made using SAP do not suffer from this mismatch. d) section 4.5, 1st paragraph: When many session descriptions are being distributed by SAP, or any other advertisement mechanism, it may be desirable to filter announcements that are of interest from those that are not. SDP supports a categorisation mechanism for sessions that is capable of being automated. should become: When many session descriptions are being distributed by SAP, or any other advertisement mechanism, it may be desirable to filter _session_ announcements that are of interest from those that are not. SDP supports a categorisation mechanism for sessions that is capable of being automated. e) Section 5. 1st paragraph. Text: ...Also, since announcements may be transported via very unreliable means or damaged by an intermediate caching server, the encoding was designed with strict order and formatting rules so that most errors would result in malformed announcements which could be detected easily and discarded. This also allows rapid discarding of encrypted announcements for which a receiver does not have the correct key. should become: ...Also, since announcements may be transported via very unreliable means or damaged by an intermediate caching server, the encoding was designed with strict order and formatting rules so that most errors would result in malformed _session_ announcements which could be detected easily and discarded. This also allows rapid discarding of encrypted _session_ announcements for which a receiver does not have the correct key. f) Section 5. 6th paragraph. Text: The set of type letters is deliberately small and not intended to be extensible -- an SDP parser MUST completely ignore any announcement that contains a type letter that it does not understand... should become: The set of type letters is deliberately small and not intended to be extensible -- an SDP parser MUST completely ignore any _session description_ that contains a type letter that it does not understand... g) Section 5.2. <version> text: <version> is a version number for this announcement. It is needed for proxy announcements to detect which of several announcements for the same session is the most recent... should become: <version> is a version number for this _session description_. It is needed for proxy announcements to detect which of several announcements for the same session is the most recent... Incidentally I am not sure what "proxy announcements" are, so have not proposed a revision here. This sentence could do with revision to more clearly explain what is meant. h) Section 5.4, text: The "i=" field is information about the session. There may be at most one session-level "i=" field per session description, and at most one "i=" field per media. Although it may be omitted, this is NOT RECOMMENDED for session announcements, and user interfaces for composing sessions should require text to be entered. If it is present it must contain ISO 10646 characters (but see also the "a=charset" attribute below). Assume that what is meant here is SAP rather than announcements. The text should therefore be modified to: The "i=" field is information about the session. There may be at most one session-level "i=" field per session description, and at most one "i=" field per media. Although it may be omitted, this is NOT RECOMMENDED for SAP, and user interfaces for composing sessions should require text to be entered. If it is present it must contain ISO 10646 characters (but see also the "a=charset" attribute below). However, surely if this is a SAP requirement, it should be in RFC 2974 and therefore this sentence should be deleted from this document as outside the scope. i) Section 5.7. text: A session announcement MUST contain either at least one "c=" field in each media description (see below) or a single "c=" field at the session-level. It MAY contain a single session-level "c=" field and additional "c=" field(s) per media description, in which case the per-media values override the session-level settings for the respective media. should become: A session description MUST contain either at least one "c=" field in each media description (see below) or a single "c=" field at the session-level. It MAY contain a single session-level "c=" field and additional "c=" field(s) per media description, in which case the per-media values override the session-level settings for the respective media. j) Section 5.10. text: Thus, the above announcement could also have been written: should become: Thus, the above _session_ announcement could also have been written: k) Section 5.11. text: If a session is likely to last several years, it is expected that the session announcement will be modified periodically rather than transmit several years worth of adjustments in one announcement. should become: If a session is likely to last several years, it is expected that the session announcement will be modified periodically rather than transmit several years worth of adjustments in one _session_ announcement. l) Section 5.13. text: Attribute interpretation depends on the media tool being invoked. Thus receivers of session descriptions should be configurable in their interpretation of announcements in general and of attributes in particular. should become: Attribute interpretation depends on the media tool being invoked. Thus receivers of session descriptions should be configurable in their interpretation of session descriptions in general and of attributes in particular. 4) Section 4, 2nd paragraph. Current text reads: The purpose of SDP is to convey information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. SDP is primarily intended for use in an internetwork, although it is sufficiently general that it can describe conferences in other network environments. A multimedia session, for these purposes, is defined as a set of media streams that exist for some duration of time. Media streams can be many-to-many. The times during which the session is active need not be continuous. We already have a definition of "multimedia" session in section 2, and this purported definition here uses different words. It is proposed to delete the duplicated definition as follows: The purpose of SDP is to convey information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. SDP is primarily intended for use in an internetwork, although it is sufficiently general that it can describe conferences in other network environments. Media streams can be many-to-many. The times during which the session is active need not be continuous. 5) Section 4.6. The text: The SDP specification recommends the use of the ISO 10646 character sets in the UTF-8 encoding [3] to allow many different languages to be represented. However, to assist in compact representations, SDP also allows other character sets such as ISO 8859-1 to be used when desired. Internationalization only applies to free-text fields (session name and background information), and not to SDP as a whole. allows users of SIP to not use UTF-8. SIP (see RFC 3261) is defined to use UTF-8, but apparently does not specify how included SDP in message bodies should be encoded. It would appear to me that all SDP uses in SIP message bodies should also be encoded in UTF-8. Is this lack of specification a fault of the SIP specification, or is it appropriate to add words to sdp-new stating that: Where an application using SDP makes no further specification of the character set to be used, the ISO 10646 character sets in the UTF-8 encoding [3] MUST be used. 6) Section 5. 1st paragraph. In the text: ... However, since SDP may be used in environments where the maximum permissable size of a session description is limited (e.g. SAP announcements; SIP transported in UDP), the encoding is deliberately compact... As SIP transport using SDP is currently considered to be less than ideal, it is probably not a good idea to identify it as an example. Therefore delete the words "SIP transported in UDP". 7) Section 5.6. Phone numbers. Firstly the correct terminology is not: Phone numbers SHOULD be given in the conventional international format: preceded by a "+" and the international country code. but rather: Phone numbers SHOULD be given in the form of an international public telecommunication number (see ITU-T Recommendation E.164) preceded by a "+". Secondly, I do not see why this use of international number is a SHOULD. Is this to allow the provision of numbers in an entirely different numbering plan, or the use of rfc2806bis, or what. If there is some reason for only a recommendation, rather than mandatory use, then I would have expected a sentence explaining it. Thirdly, for the requirement: There must be a space or a hyphen ("-") between the country code and the rest of the phone number. I do not see a need for this requirement. It is not needed to parse the number, and it means that numbers in the specified international format for text representation, ITU-T Recommendation E.123 cannot be used without modification. I suggest that E.123 representation of international public telecommunication numbers should be valid for the phone number string. This also requires the deletion of the associated comment in the BNF in appendix A, i.e. ;there must be a space or hyphen between ;the international code and the rest of ;the number. 8) Section 5.6. E-mail addresses. Text: The alternative RFC822 name quoting convention is also allowed for both email addresses and phone numbers. For example: RFC 822 has been made obsolete by RFC 2822. Is there some reason for quoting RFC 822 rather than RFC 2822. 9) Section 5.6, text: Both email addresses and phone numbers can have an optional free text string associated with them, normally giving the name of the person who may be contacted. This should be enclosed in parenthesis if it is present. For example: e=j.doe@example.com (Jane Doe) From the BNF, the parenthesis are mandatory for the enclosure of the free text field. Therefore this should be revised to: Both email addresses and phone numbers can have an optional free text string associated with them, normally giving the name of the person who may be contacted. This MUST be enclosed in parenthesis if it is present. For example: e=j.doe@example.com (Jane Doe) 10) Section 5.7, 6th paragraph: For IP4 and IP6 addresses, the connection address is defined as follows: these are values within the field, rather than the names of IP numbering schemes, thus I proposes: Where the <address type> is IP4 or IP6, the connection address is defined as follows: 11) Section 5.8. I am not convinced that the wording relating to X- naming is adequate. I understood this was somewhat deprecated and this could be more emphasised. Currently use of X- for anything would be conformant to this specification and I am not sure that was intended. I think we should certainly get rid of the MAY quoted below. Tool writers MAY define experimental bandwidth modifiers by prefixing their modifier with "X-". For example: Maybe this would be better rephrased as A prefix "X-" is defined for bandwidth modifier names. This is intended for experimental purposes only. For example: 12) Section 5.14 defines a number of media types. Are these already top-level MIME content types, as defined in 9.2.1, or are these to be registered by IANA as new media types? Either section 5.14 or section 9.2.1 needs enhancement to reflect what IANA are requested to do with these. 13) Section 6. This section lists a large number of attribute definitions, most of which do not appear to be defined elsewhere. Section 9.2.4 says new attributes should be registered with IANA. Are IANA expected to pick attribute registrations from clause 6, and all these are therefore intended to be registered, or not. Text needs to be added to either section 6, or section 9.2.4 giving more detail of this process. Note that if any of these attributes are defined in other documents rather than here, I would suggest an informative reference against the attribute giving the details of where it is defined. 14) Section 5.14 and section 9.2.2. In section 9.2.2, the references to "proto" field seem to be references to the "protocol" field and or a <transport> element, and or "transport protocol" defined in section 5.14. Certainly "proto" is not used as a field name anywhere else in the document ecept in the BNF. Field names should be aligned one way or the other. 15) Section 9.2.5. Does this document register the bandwidth modifiers CT and AS defined in section 5.8. If so this should be clearly stated in this section. 16) Section 9.2.6. Does this document register the network type IN as defined in section 5.2 and 5.7. If so, this should be clearly stated in this section. 17) Appendix A. Would the top level construct not be better defined as "session-description" rather than "announcement". Not all SDP data constructs are announcements. 18) Appendix A. There is considerable misalignment between the BNF field names and those in the main document. For example "v=" is "proto-version" in the BNF, and "protocol version" in the main document (section 5.1). I would suggest changing this to "protocol-version" in the BNF. "o=" is "origin-field" in the BNF and "origin" in the main document (section 5.2). I would suggest we use "Origin Field" in the title of 5.2. Section 5.2 gives the expansion of "origin" as o=<username> <session id> <version> <network type> <address type> <address> The BNF gives: origin-field = "o=" username SP sess-id SP sess-version SP nettype SP addrtype SP unicast-address CRLF Further unnecessary misalignment of names. In this case I would change the BNF to origin-field = "o=" username SP session-id SP session-version SP network-type SP address-type SP unicast-address CRLF with appropriate modification of dependent expansions, and the 5.2 text to o=<username> <session id> <version> <network type> <address type> <unicast address> and so on for alignment through the rest of the BNF. 19) Appendix A. In the expansion for "proto" the following comment is added ;typically "RTP/AVP" or "udp" for IP4 This document is meant to cover IPv4 and IPv6. Why is this comment IPv4 specific, and why is TCP not mentioned as one of the typical values. _______________________________________________ mmusic mailing list mmusic@ietf.org https://www1.ietf.org/mailman/listinfo/mmusic
- [MMUSIC] Comments on draft-ietf-mmusic-sdp-new-15 Drage, Keith (Keith)
- [MMUSIC] Re: Comments on draft-ietf-mmusic-sdp-ne… Colin Perkins