Re: [Fecframe] Comments about draft-galanos-fecframe-rtp-reedsolomon-00

["Einat Yellin (Lachover)" <Einat at radvision.com>]

Hi Vincent,

Thank you for your detailed comments.
Please see my response inline.

Cheers,
Einat

-----Original Message-----
From: fecframe-bounces at ietf.org [mailto:fecframe-bounces at ietf.org] On
Behalf Of Vincent Roca
Sent: Sunday, November 08, 2009 6:12 PM
To: Orly Peck; Sarit Galanos
Cc: fecframe at ietf.org
Subject: [Fecframe] Comments about
draft-galanos-fecframe-rtp-reedsolomon-00

Hello Orly/Sarit,

Here are a few comments concerning 
draft-galanos-fecframe-rtp-reedsolomon-00.
Sorry, it's a bit long and only a few hours before the FECFRAME 
meeting... But
better late than never.
Cheers,

  Vincent

-----

Main comments:

* Section 4 is confusing because it mixes the notions of packets,
symbols,
and "elements of the Galois Field". The following is the official 
terminology:

- A symbol (according to RFC5052) refers to the high-level unit of data 
managed
  by the FEC scheme. It can differ from unit of transport, the packet.
- RFC 5510 clarifies that in some use-cases, several symbols may
  be grouped and sent together in a given packet ("Encoding Symbol
Group").
- RFC 5510 clarifies that symbols are composed of S "m-bit elements", 
where m is
  the Galois Field exponent GF(2^m). In the usual case of GF(2^8), 
elements are
  bytes, and the size S in terms of elements is of course equal to the
  symbol size E in bytes.
- Finally, section 8.4 of RFC 5510 explains how to do encoding/decoding
over
  symbols, and how it refers to Reed-Solomon elementary operations over 
the S
  m-bit elements.

Now, we need to apply all this to the FECFRAME context. But as it is
written
today, I don't understand section 4. When I compare to section 5 I have 
the feeling
that "symbol" means "element", but this element is fully defined by the 
choice of
the GF, it's not an open point left to the implementer.
[Einat] You are correct here. There are some inconsistencies and wrong
terminology in this section.
May I suggest:
In general a Reed Solomon code takes a group of K source symbols and
generates N - K repair symbols. A receiver needs to receive any K of the
N source or repair symbols in order to recover the remaining N-K
symbols. In this document we suggest a symbol to be a whole RTP packet.
As explained in RFC5510, the Reed-Solomon algorithm operates over
multiple elements each taken from a single source symbol. symbols are
composed of S "m-bit elements", where m is the Galois Field exponent
GF(2^m). In the usual case of GF(2^8), elements are bytes, and the size
S in terms of elements is of course equal to the symbol size in bytes.
In terms of implementation simplicity it is also recommended to use
8-bits elements.  For more information on Reed Solomon codes, the reader
is referred to [Rizzo97].

Similarly, in section 5, it is said:
  "A source block for the Reed-Solomon code contains K data blocks."
There is no notion of "K data blocks" in the official terminology. I 
think you
want to say:
  "A source block [...] contains K source symbols."
(idem for the following sentences).

[Einat] You are right again. Data block should be replaced with Source
symbols. To link this to the FEC framework the appropriate phrasing
should be: 
A source block for the Reed-Solomon code contains K source symbols.  In
the scheme presented by this document, each source symbol contains a
single Application data Unit (as defined in FEC framework), which is in
our case RTP packet. Therefore a source block contains exactly K
consecutive RTP packets.


* Section 5: is the "one symbol per RTP packet" really appropriate?
If RTP packets have largely different sizes (usual case), the sender may
want to use symbols of size the median value (for instance) in order to 
avoid
having too much padding. Of course padding is not sent (i.e. there's no
transmission overhead penalty), but encoding/decoding operations will be
faster with smaller symbols. There is a price to pay for that,
essentially
complexity (it breaks the packet <=> symbol relationship and there are
more
symbols), so I'm not sure there's a real benefit. What do you think?
[Einat] It is true that one could use a median packet size in order to
make encoding/decoding more efficient. However, since we are dealing
with packet losses and not symbol losses, breaking a large packet into
several symbols and unifying small packets to a single symbol will break
the equality between packets. For example, if we use 2 repair symbols
per a block and one of the RTP packets in this block contains 3 source
symbols then if we lose this single packet we could not restore it even
if all other source and repair symbols are received.


* Section 5: It is said:
      "B.  The sequence number of the source packets must be consecutive
           in a source block."
IMHO the "MUST be consecutive" is too strong. It may happen that some
source
RTP packets be lost in some networks prior to reaching the FECFRAME
sender
(i.e. if FECFRAME is used in an intermediate box rather than
end-to-end).
And what about (crazy) situations where RTP packets are lost (e.g. by a
FIFO
overflow) before reaching FECFRAME?
IMHO it's worth designing a solutions that is robust against this kind
of
situation, no matter the cause.
[Einat] The 'MUST be consecutive' restriction is there in order to
simplify the signaling of which RTP packets are protected. If not
consecutive then we could not use the SN base + num packets fields in
the FEC header and we'd have to use SN base + bitmask, or worse -
specifically list the sequence numbers of protected packets. I think
that cases where some source packets are lost before FEC encoder are
unique and can be handled by creating smaller FEC blocks (in between the
gaps).


* Section 6.2.1:
One stupid question: if there are several repair flows and if they are
all sent to the same multicast group/port, how does a receiver tell the
difference between the packets belonging to the different repair flows?
I think it's by means of the PT. This is not said in any case.
(I understand that sending everything to the same destination is not
necessarily a good idea, but that's a different topic)
[Einat] The payload type, which is signaled in SDP is used for that. We
could add a specific sentecnce describing this.


* Section 6.2.2, Fig. 3: the choice of the various field sizes has
implications that are not explicitly mentioned.
Num Packets (badly named, I'd prefer K): 16 bits => too much in case
        of RS over GF(2^8), more appropriate to GF(2^16).
[Einat] We did not limit GF to 8. It is only a recommendation.
i and N-K: 8 bits => okay with GF(2^8), a limit in case of GF(2^16),
but that's perhaps not the target.


* Whole document: More generally, I have the feeling nowhere it is
explained
whether or not there are restrictions on the GF size. Which possible
values
for the m parameter.
[Einat] I agree that such restrictions and consideration should be
further elaborated.
In general we focused more on the concept of how to apply Reed-Solomon
on RTP packets than on the Reed-Solomon operation.


* Section 6.2.2: I have the feeling that carrying N-K in the FEC header
is
perhaps not required. On the opposite, there MUST be a way to inform the
receiver of the GF(2^m) used (e.g. it can be done out-of-band in SDP 
description).
So if I know m, I don't need to know the actual N-K as I know K and 
max_K=(2^m)-1.
[Einat] I'm not sure that I follow here. Our intention was to use a
dynamic N-K value. 
e.g. for one block it could be 2 repair symbols and for the next block
it could be 4 repair symbols.
This variable number must be signaled for each block.


* Section 6.2.2: SN Base is used to identify the source block (since all
the FEC repair packets associated to the same source block have the same

SN Base
value). Since SN Base is a field that regularly wraps around, there is a
(theoretical) risk that two consecutive source blocks use the same SN
Base
value. This might happen in tricky configurations with large source
blocks
(GF(2^16)) and in case the RTP sequence numbers are not consecutive (see

above
discussion about possible erasures *before* FECFRAME).
Additionally, and more seriously, this vulnerability may also be 
exploited as
a possible DoS (Denial of Service) to make a FECFRAME sender crash.

So it's worth mentioning this topic in the doc, both in the core spec 
and in the
Security section.
If we clarify that this document is not for situations experiencing high

erasure
rates before FECFRAME (which can be checked on-the-fly), it should be 
okay IMHO.
[Einat]  Good point. Thanks.

* Section 6.2.2: Back to the idea of removing the constraint on
consecutive
source packets, I'm wondering if the following FEC header format
wouldn't be
okay:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Base SN              |           Max SN              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Source Block Length (k)    |   Enc. Symbol ID (16 bits)    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Thanks to this FEC header:
- We know that all SN values in {Base SN..Max SN} are included in this
  source block.
- We know their actual number (k) even if some of RTP SN are missing.
- We know the ESI of the FEC repair packet (in {K, N-1}), no matter the
  GF m parameter since it's 16-bit long.
- And we know in which order incoming source RTP packets should be
  stored in the block, even if there are gaps in the SN.
At a receiver, the actual SN value in the RTP header of an erased
source packet is anyway recovered along with the rest of the
header/payload
so there's no problem either.
[Einat] This is interesting. Do you mean that all RTP packets in the
range [Base SN...Max SN] will be included in this block?
If not then the receiver will know for sure if he can recover a lost
packet only after making the decoding operation on the relevant block.
This could lead to unnecessary decoding calls.
See my comment above regarding the need for signaling N-K for each FEC
block.
       

* Section 7.1.1: it is not clear to me if max_N is equal to 2^m or if
it is only a "codec limitation". I think it's related to the GF, but
it's
worth mentioning. And the "symbol-size" should also clarified as already
mentioned.
[Einat] Symbol size should be element size as mentioned above. 
I also agree that considerations for determining MAX_N should be listed.


* Section 8.3.1: This section does not explain how to manage the case
where
there are several repair flows. It's a problem.
[Einat] What do you see as a problem here? The different repair flows
are distinguished by their payload type. 
It is determined in the SDP which payload type is used by which repair
flow to protect which RTP flows.


* Section 11.
You cannot get rid of the security aspects in this way. Most
specifications
add additional security risks (like the DoS opportunities mentioned
above).
[Einat] Correct again. Would it be OK to say that we neglected some
edges in order to make it on time for the meeting...?
This will be fixed if/when the concept presented here is agreed.


Non critical:
-------------

* section 1, intro: the term "FEC packets" is used several times.
  Rather than FEC Packets, say FEC Repair Packets. In 3.1, correct
  and use FEC Repair Packet (several occurrences).

* section 6.2.2: instead of "i - 0 based packet index in the N-K FEC 
packets sequence."
(i-0 is misleading). I suggest:
        i: FEC repair packet index, in {0.. N-K-1} range.

* Whole document: you are using K and N (upper case), whereas usage (at 
least RMT)
  is more on using k and n.

* Whole document: "FEC header" is not the official name.
[Einat] Thanks. 

_______________________________________________
Fecframe mailing list
Fecframe at ietf.org
https://www.ietf.org/mailman/listinfo/fecframe