[AVT] Comments on draft-ietf-avt-rtp-3gpp-timed-text-01.txt
Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 18 May 2004 14:04 UTC
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Subject: [AVT] Comments on draft-ietf-avt-rtp-3gpp-timed-text-01.txt
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Hi, Here is my comments on the draft. 1. Why is there a empty line in the header between author company and publication date? Also expires should be on the line after the filename to keep things together. 2. The status of this memo section is not correct. Section 10 of RFC 2026 has been replaced. With the publication RFC 3667 and 3668 it should as I understand it now read: Status of this Memo By submitting this Internet-Draft, I (we) certify that any applicable patent or other IPR claims of which I am (we are) aware have been disclosed, and any of which I (we) become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, I (we) accept the provisions of Section 3 of RFC 3667 (BCP 78). Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html ------ end of quote ------- Please notice that certain things in the declaration depends on document type and what the author is willing to give away. For a WG document going for standards track this will be most common appearance. These details will become more clear when IETF has published new Internet draft guidelines on the homepage. One can also add the following paragraph to clarify things further as it is a wg document: This document is a submission of the IETF AVT WG. Comments should be directed to the AVT WG mailing list, avt@ietf.org. 3. The copyright notice on the first page. I am not certain it is compatible with the one on the last page. Please remove. 4. Change log: Can you please restrain the change log to only be difference since the previous version? It becomes quite long otherwise, and I don't find it that useful. 5. Section 1. "Unit or Access Unit" Shouldn't the last sentence include a "so building what is called a (access) unit."? 6. Section 3. Marker bit: I think it should be a normative word in there, like "the marker bit SHALL be set to 1 if the RTP packet includes one or more whole text samples or the last fragment of a text sample; otherwise set to 0." 7. Section 3. " Timestamp: the timestamp MUST indicate the sampling instant of the earliest (or unique) text sample contained in the RTP packet." What is a "unique" text sample? Or are you simply meaning the "only" text sample. 8. Section 3. Timestamp: I think there a some improvement one can make to the timestamp derivation text when multiple samples or fragments are present. - First I think the normative definition of the order of the samples or fragments should be done in another chapter. - Then the normative definition of how to calculate the TS for a subsequent sample does not seem to properly take into account that other text samples may not carry duration. 9. Section 3: "If the timed text is streamed from a 3GP file, the timestamp clockrate MUST be copied directly from the value of "timescale" in the Media Header Box for that text track." Why is this a MUST? I understand that one must maintain the correct synchronization, however if a sender determines to rewrite everything upon transmission, is it something that will not work. 10. Section 3.1.2: "In order to transport these larger text samples using RTP, it could be argued that a careful encoding be used to transform the original large sample into smaller self-contained text samples that fit into the given transport MTU. This would comply with the ALF principle, as described in the guidelines for RTP payload formats, RFC 2736 [14]. It would also need additional pre-processing previous to RTP encapsulation and that senders understand the modifiers format. However, given the low probability of fragmentation, it is believed that the overhead of this pre-processing is not worth and it is more appropriate to encode text samples without taking the path MTU into account. In this manner, this payload format meets a trade-off by intentionally leaving out this pre-processing and making the fragmented samples less robust to packet losses." I would recommend that one does in fact tries to ensure that one has samples that fits the network, but that to ensure functionality also in these cases when this is difficult, or not possible due to other constraints the fragmentation mechanism is present. However one SHOULD take the MTU into account. 11. Section 3.1.2: "The most important consequence of this design choice is that while text string fragments can be displayed in the absence of a previous text fragment, modifiers for that text string are useless if they are not completely received." I think it is a bit unclear what "they" are intended to be, the modifiers or the text string. 12. Section 3.1.2: Second bullet: I think one can extend the conclusion why this is good to become the following: "This reduces complexity by minimizing the number of fragments and thus also improve the error robustness." 13. Section 3.1.2, third bullet: "in order to fill up the remaining bits of a packet, piggybacking of sample descriptions MAY be performed. Also fragments of past samples MAY be piggybacked. For this purpose the server MAY reserve a certain amount of buffer to store already sent units for piggybacking." What is meant by "In order to fill up the remaining bits ...". I guess you mean "If space in the packet up to the MTU, and there is available bit-rate, then sample description MAY be piggybacked. Also is really piggybacked the right word? Does not a sample description need to have a specific place in the packet, and thus can't be added at the end? 14. Section 3.1.4: " o Although units containing modifier boxes or fragments thereof do not include a duration field, they make use of the RTP timestamp to group together. Therefore, they SHOULD be transmitted in the same order as they appear in the sample and be placed as near as possible to the text to which they apply. Logically, this does not apply for retransmitted or redundant packets or for units that are piggybacked into other packets." Is this really correct that the modifier boxes does not need duration? As I understand it any modifier box is connected to the text sample by having the same timestamp. Thus it would require a modifier box to have a duration field if they should be possible to aggregate separately in regards to the text strings. 15. Section 3.1.4: The concept of unknown duration is a bit loosely defined. I haven't checked 26.245, but is it really defined there? I understand the desire have functionality to define a sample as being active until replaced. However if this is not defined in the 3GPP spec, I think we must be cautious of defining such a mechanisms. If it is defined, does it use a special explicit indicator that this is the case? Otherwise I think it should use such a SDUR field indicator. 16. Section 3.1.5: I think the buffer concept is somewhat garbled in this paragraph. To me it seems to talk about about both sender side buffers and receiver buffers, without making any real differentiation. 17. Section 3.2: I think this section before going into clear definition of all types, should give a clear definition of how different blocks can be aggregated, and how one derive the necessary information. How the packets must be built to be able to recover which type 1 that a type 3 belongs to. I think there are some oversight here of not providing SDUR also for type 3 and 4. I think the timestamp derivation for all packets types must be made much more clear. 18. Section 3.2.1: Definition of "R" bits. Please also include that they SHALL be ignored by the receiver. Otherwise they will not be possible to use without external signalling. 19. Section 3.2.1: Type list: I think the types are in all case a single fragment, not multiple ones that the plural form indicates. 20. Section 3.2.1: "Two TYPEs (1 & 2) are defined for units containing text strings another two (3 & 4) for units not containing text strings (thus no timing attributes) and a final TYPE 5 for sample descriptions (also lacking timing attributes)." This sentences missing a "," after "text strings". 21. Section 3.2.2: I thing the following sentences has some language problems: "In this case, TYPE 1 units MUST not have contents. This means that the LEN field MUST have a value of 6 (0x0006). Otherwise, the LEN field MUST be always greater than 6 (0x0006)." I think it should read more like: "In this case, the TYPE 1 unit MUST not have contents, and the LEN field SHALL have a value of 6. Otherwise the LEN field SHALL always be greater than 6." 22. Section 3.2.4: "Note that the SLEN, SIDX and SDUR fields are not present. This is because: a) these fragments do not contain text strings and b) these types of fragments are applied over text string fragments, which already contain this information. " I think here would, unless better clarified elsewhere a place to actually define how this type 3 is bound to its TYPE 1 or TYPE 2s. 23. Section 3.2.4: " o The TOTAL/THIS field indicates whether the unit contains a part of or the whole of the modifiers: if TOTAL=THIS, then all modifiers are included here. In this case, TOTAL=THIS MUST be greater than one, because there cannot be a sample of modifiers without text strings. Otherwise, this unit just contains the first fragment." Why must the value of total and this be larger than 1 although all modifiers are contained in one sample? 24. Section 3.2.6: I think it is appropriate that the TYPE 5 header is mandatory to implement, otherwise we will have problems with interoperability. And the text claims that the implementation of this format is rather minor, if true, then there is no reason for having it optional. 25. Section 4. "... the simplest option for packet loss resilient transport is to send the same RTP packet or the same text samples (or fragments) again." This sentence contains a error: "the same RTP packet" SHALL NOT be repeated. Repeating a packet implies to send two packets with the same RTP sequence number. Thus preventing RTP's reporting mechanism to discern the two different packets. So please remove this part, it is fully sufficient to repeat the payloads. 26. Section 4. " o in repeated packets, all RTP header fields MUST keep their original values except the sequence number that MUST be increased to comply with RTP. " I think this should talk about repeating RTP payload, and thus requiring all payload specified fields (TS, M, PT) to be the same. I don't see a reason to demand that any other field are actually the same. If one over specifies things, any future usage of an RTP profile will be extremely problematic. 27. Section 4. "If single units are repeated in packets different from their originals, care SHALL be taken to preserve their original timing." What is here meant with original timing? 28. Section 4: " A possible solution MAY be that the encoder provides a static default sample description to be used for these cases." I think one should clarify that this would need to be a application specific definition. 29. Section 5. I would recommend that either use stricter need for congestion control. Otherwise it might be wise to simply use the following sentence: Congestion control for RTP SHALL be used in accordance with RFC 3550 [?], and any applicable RTP profile, e.g. RFC 3551 [?]. 30. section 6.2: I think that one shall have a SMIL reference at the first use of the name. 31. Section 7.1: Please fix the indentation so it is easier to read: parameter: aslödsaösödlkskaaölsldsaööl adsösaödsdsadslaösaöldsa 32. Section 7.1: The definition of how the value should be written is unclear. First one thinks that z should be multiplied with the sum of x*256+y, while it appears that the number should be concatenated. Please clarify. 33. Section 7.1: Spldesc: It is two possibilities. Also I think possible values "both" and "out" should be used mutually exclusive. 34. Section 7.1, tx3g: Please add a reference to the BASE64 RFC. It is number 3548. 35. Section 7.1 tx3g: I think this is a optional parameter, as it is allowed to be empty. Define the lack of it as equal to empty string. 36. Section 7.1, layer, tx, ty, heigth, width: What is the allowed value range here. If it is a 32-bit integer please be explicit about it. 37. Section 7.1: I think that you need to be a bit more clear about how the values are defined. Instead of some half formal definition of the value, either write it clearly in word, or use formal syntax notation. I think writing it in words are often sufficient. 38. Section 8.1: I think we have two bullets that are equivalent to two others. The third and fourth, seem to be possible to replace with only the fifth. 39. Section 8.2: I think there are a few problems with the offer answer section. The first major one is the sample descriptions. How is they handled in a bi-directional session? Are they defined per direction, and how does then unicast differentiate multicast. Or are they shared between all participants. But are then the answerer allowed to add his to the list the offerer provide. These are hard problems but they need to be spelled out. 40. Section 8.2: How is the version handled in an interoperable way? Is a answerer allowed to downgrade the version he accepts to receive? Thus enabling the offerer to tell that he is ready to receive and send version X, while the answerer declares what it accepts to receive, and thus most probably also send. 41. Section 8.2: The section uses server, while an offer/answer session rarely has these designations of the participants. 42. Section 8.2: Bullet 2 contains a forbidden character, which is not within the lower 7 bit range of ascii. 43. Section 8.2: Last paragraph: Offer/Answer defines its reject procedure based on the external signalling protocol (SIP). So avoid using reject, by setting a certain value. It can signal or indicate, but reject is really something that affects the whole session. Also I think that one can avoid having spldesc to result in rejected setup in most cases. 44. Section 10. "These types of attack may easily be avoided by using authentication." I think the right thing to avoid these attacks are "integrity protection and source authentication". You need the integrity protection to ensure that nobody can change the content en-route, and source authentication to ensure that the sender really is which he claims to be. In many cases implicit source authentication is sufficient. 45. Section 11.2: Reference 10, is now available as RFC 3711. Cheers Magnus Westerlund Multimedia Technologies, Ericsson Research EAB/TVA/A ---------------------------------------------------------------------- Ericsson AB | Phone +46 8 4048287 Torshamsgatan 23 | Fax +46 8 7575550 S-164 80 Stockholm, Sweden | mailto: magnus.westerlund@ericsson.com _______________________________________________ Audio/Video Transport Working Group avt@ietf.org https://www1.ietf.org/mailman/listinfo/avt
- [AVT] Comments on draft-ietf-avt-rtp-3gpp-timed-t… Magnus Westerlund
- Re: [AVT] Comments on draft-ietf-avt-rtp-3gpp-tim… Colin Perkins
- RE: [AVT] Comments on draft-ietf-avt-rtp-3gpp-tim… Jose Rey