Elwyn,
Thanks for the detailed review. Comments below...
Others on cc: There are some suggestions below that need your review.
...Mark
On 4/10/07 5:25 PM, "Elwyn Davies" <[EMAIL PROTECTED]> wrote:
I have been selected as the General Area Review Team (Gen-ART)
reviewer for this draft (for background on Gen-ART, please see
http://www.alvestrand.no/ietf/gen/art/gen-art-FAQ.html).
Please resolve these comments along with any other Last Call comments
you may receive.
Document: draft-ietf-rmt-fec-bb-revised-06.txt
Reviewer: Elwyn Davies
Review Date: 11 April 2007
IETF LC End Date: 12 April 2007
IESG Telechat date: (if known) -
Summary:
Almost ready for PS. There are some minor issues and editorial nits that
should be addressed before it goes to the IESG.
Comments:
Minor Issues:
s6: As well as data packets, para 1 introduces session-control and
feedback packets, but then para 4 says that we are specifying stuff
about data packets. Session-control and feedback packets disappear into
the void. Some words about what is or is not specified about these
other kinds of packets would help - even if only to say they are out of
scope.
Good point. I will add some words to clarify in the introduction.
s6.1: The implication of the last para is that an 8 bit field is enough
to hold the FEC Encoding ID (confirmed more or less in 6.2.3). 2 * 128
possible values may be enough but given that proprietary schemes are
allowed and lots of different CDPs might have different schemes, one
wonders if this might be constraining - and I see no means of expanding
the range. Justify? Also there isn't an experimental range.
Yes, this has been discussed occasionally in the group and Magnus has
mentioned this now and again. I do think that 128 values is plenty - we have
had only a handful of FEC Schemes proposed or even mooted during the several
years Experimental period and there isn't much reason to expect a
significant number more - certainly nothing on the scale of RTP payload
types, for example. Even if there were 10 times more than at present we
would be ok.
However, I would rather not be made to eat my words ;-) Instead of 2 ranges
of 128 values, we could instead allocate 4 ranges for:
- Fully Specified FEC Schemes
- Dynamically allocated FEC Encoding IDs
- Partially specified FEC Schemes
- Experimental FEC Schemes
Dynamic allocation would be preformed by means out of scope of the document
(in practice often by SDP). As I say, I don't anticipate we will ever need
them, but it is essentially cost-free to reserve them now.
Comments ?
s6.2.1, para 4: Make it explicit whether the choice of (a) or (b) for
the encoding is allowed to be per object or per CDP/FEC Scheme. Even
though it is probably obvious, it would be wise to state that if there
is an option the CDP has to provide means to transmit which choice has
been made.
Ok. The choice is per CDP, so a CDP specification needs to state which
mechanism is being used. I will mentioned this also in the section on CDP
requirements, if not already.
s6.2.1, last para: The statements about the length of the field read as
if there has been previous statements about the length of Common Object
encodings. There hasn't been. Either reword to cover all lengths here
or add words to the earlier paragraphs about the Common Objects.
The previous statement is the last sentence of the previous paragraph.
s6.3: I would have expected (I think) that a maximum range for FEC
Payload IDs should be given to give a hint to CDPs on the field size
needed, but I may be wrong - perhaps some words of explanation as to why
it is not needed (if that is the case) would help others.
Ok. I think the only requirement (which is a practical, rather than a
theoretical one), is that the FEC Payload ID is a small fraction of the
packet size. Currently in-progress FEC Scheme specifications use 4 to 8
bytes, but one can easily imagine applications where 16 or more might be
used.
s11: Should we still be, even indirectly, suggesting that MD5 would be a
suitable hash functtion?
No opinion from me. I would be happy to replace this with SHA1 if requested.
Editorial
=========
Global: s/[Aa]n FEC/[Aa] FEC/ (OK, maybe an sounds more euphonious but
it is wrong). I'm sure the RFC Editor will have a view.
Ah, this depends on whether you treat FEC as an abbreviation ('eff ee cee')
or an acronym ('feck'). I use the abbreviation, and I believe this is more
common. It also steers clearer of an Irish expletive ...
Global: Prefer octet rather than byte.
OK.
s1: It would be good to explicitly state the section (4.2 I assume) of
RFC 3048 ([10]) which this draft covers.
OK.
s1: Worth expanding RMT for future generations (or remove the note about
the wg that produced it).
I will expand it, since it is worth keeping the reference to RFC 3269, which
includes Reliable Multicast Transport in its title.
s1, para 6: Helpful to add a forward ref to s4 or s6.1 where
Fully/Under-Specified are defined.
Ok.
s4, para 1: 'A FEC code is a valuable component...': I am not sure if
this the best phraseology. Maybe: A FEC coding scheme is a valuable
component... Actually I found the term 'FEC code' somewhat distracting
throughout - it doesn't sound quite right when referring to the adjuncts
needed to provide the extra FEC information and the encoding used to
generate them.
The intention was to distinguish between the basic algorithm used to
generate FEC data ("FEC code") and the various bits of protocol coding
needed to communicate using FEC ("FEC Scheme") - "FEC coding scheme" would
confuse the two. We could have a different term for "FEC code" which we can
more clearly define, if that would help ?
s4, para 4: s/ancilliary/anciliary/ (last instance)
OK
s5, para 1: Might be good to refer explicitly to s3.2 of RFC 2357.
OK.
s6.1, para 5: s/FEc/FEC/
OK.
s6.2.2, paras 2 and 3: It would be better to start these paras with
'Any' rather than 'The'. At present it is very easy to read paras 2 and
3 as contradictory.
OK.
s6.2.4, last para: s/EXT-FTI/EXT_FTI/. Also need to expand LCT acronym.
OK.
s6.3, last para: The term 'systematic FEC codes' appears to be a term
of art that is not defined. Explanation would be appreciated.
OK.
s11: s/to many receivers/at many receivers/
OK.
