Hi Magnus,
1. Applicability statement: I think this document should have an
applicability statement making clear that this is only for engineered
environments due to the traffic bursts.
We already put a lot of effort to explain the precautions for using this in
BE networks. I don't think there is a strong
argument here that this will only work in engineered networks. So, personally
I don't think we should put such a statement.
I can agree that it works in certain type of best-effort networks.
However, I don't think at all it is suitable for any best-effort
network. However, as this is mostly bound by the requirement to also
support SSM in the same network the deployment cases are usually meeting
the assumptions about network capacity. However, if one runs an
multicast overlay and a client starts up with not congestion or network
That is a bit stretch :)
capacity information and does a RAMS-R it can severely congest a network
for a few seconds.
Well we have rules how the server is supposed to send the burst (rate, timing,
etc.) documented in the text. It explains what the server does.
There are a number of assumptions in where this will work most of the
time and with very few exceptions have any large scale impact on the
network. I think these should be documented in an applicability statement.
At the end of intro, I think we can put a few sentences about this.
2. Section 6.2, bullet 5: I don't think the document is clear on that
the BRS is expected to determine if a RAMS-R is a retransmission or an
update based on if any content of the message has been changed.
The statement over there implies this. And naturally it is the server's job
to figure that out. Based on this discussion, we
even wrote the new CNAME guidelines document so that the server's job would
get easier.
Exactly the document implies rather than being explicit. I am not
Actually, I re-read it and it is pretty explicit. The text says it checks
client's CNAME (which must be unique). If the server is already processing a
request from the same client for the same SSRC, then it is a update. Ow, it is
a new request.
I can repeat the above in the text if you think it is necessary.
certain that I have understood all the mechanism you expect me to use to
determine if a RAMS-R is a retransmission or a new request. Based on
that it is uncertain for a client to know what effect an RAMS-R message
will have. I also think it makes sense to mandate that servers do have
retransmission detection, otherwise you end up in a situation where a
client don't dare retransmit a request for the fear of getting two bursts.
OK what if the text said:
Updated Request: The RTP_Rx MAY send an updated RAMS-R message
(as unicast feedback in the primary multicast RTP session) with
a different value for one or more fields of an earlier RAMS-R
message. The RS MUST be able to detect whether a burst is already
planned for or being transmitted to this particular RTP_Rx for this particular
media source SSRC. If there is an existing burst planned for or being
transmitted, the newly arriving RAMS-R is an updated request; otherwise it is a
new request...
3. I worried that what is intended to be an RAMS-R update from the
client will be interpreted as new RAMS-R request. The reason is the only
that separates these two cases are timing, does it arrive before the
burst ends or not. Relying on this heuristics is quite weak and error
prone. I still wished that an sequence number had been added to RAMS-R.
I gave enough arguments why we should not use a seqnum for the RAMS-R
messages earlier. It is not justified IMO. Based
on the SSRC of the primary and the requesting client's CNAME, things are
pretty straightforward.
Yes, I know I am in the rough here. From my perspective, it makes the
use of update RAMS-R uncertain to use. Maybe not a big issue if most
doesn't implement updates. But if one finds need for it then there will
be issues. Consider the AD and IESG notified about the potential issue.
Is the updated text above good for you?
4. Section 6.2, bullet 5: " Thus, the RTP_Rx MUST choose a globally
unique CNAME identifier."
I don't think this is a statement that can either be fulfilled nor
tested. You can mandate a method for selecting CNAMEs that has low risk
of producing CNAME collisions, but nothing can guarantee it unless a
entity is coordinating the CNAME space for all clients. Can you mandate
the method instead?
The new CNAME draft has ways to ensure they are unique. If the client uses
one of them, we are all set.
Yes, but in that case mandate the implementation and usage of one of
these instead of statement that can't be ensured.
Personally, this is one what I did initially but then we agreed that one could
come up with an alternative method that would produce unique CNAMEs. In other
words, methods for producing unique CNAMEs are not unique. So, we rather put
the "MUST" in the sentence above not for the specific implementation.
If I understand the impact of a CNAME collision is that the collision
clients request will be mixed up, for example terminating each others
request, or update the values in the RAMS-R.
When they are unique, this won't happen.
Just checking, but if that is the case then I am missing a discussion of
hijacking attacks in the security consideration section by guessing your
targets CNAME.
We should probably mention this but I am not sure how the server can deal with
this. Hijacking is not easy since the attack must take place at the same
instant (more or less) with the request from the authentic client. One of your
family members can probably do this :)
5. Section 6.4 and others: The draft has a tendency of formulating
itself as everything is guaranteed to work as long as one keep within
given limits for bandwidth etc. An example from Section 7.2 Max Receive
Bitrate TLV:
We don't assume things will sure work or guarantee that they will work if
everything is satisfied. These are simple rules
that both sides must comply with.
No, but a certain view point is clear in the text. Lets call it
optimistic that it will work, while I would write from a more
pessimistic view.
Things may go wrong anytime, and we do ack that in the paper and have
solutions. But if one does not comply with the rules, more things will likely
go wrong.
6. Section 7:
Each feedback message has a fixed-length field for version, padding,
feedback message type (FMT), payload type (PT), length, SSRC of
packet sender, SSRC of media sender as well as a variable-length
field for feedback control information (FCI).
What is called "Payload type" is actually "Packet Type" in RTCP packet
headers.
We followed the definition from 4585 but "packet type" makes more sense to
me as well.
Well, section 6.1 of RFC 4585 is wrong.
OK, "packet type" it is then.
7. Section 7.3:
"The MSN value SHALL
be set to zero only when a new RAMS request is received."
How is that actually known? And why reset it at all? Why not increase if
for each new RAMS-I message with new content, independently if it is an
update or a new request.
If this is relating to a new burst request, then it is reset. Ow, what is
the point of having a seqnum? If something has
changed compared to the previous RAMS-I, then MSN is incremented. If it is
just a re-xmit, MSN stays the same.
I fully agree with the need for separating retransmissions from updates.
However, I wonder over the reset of the field for each new RAMS-R. It
appears to me to be more robust to simply increment it rather than
reset. Otherwise you can send RAMS-R(1) resulting in RAMS-I MSN = 0.
I think we discussed this before. The RAMS-R numbers are no way correlated with
the RAMS-I numbers. You are still trying to correlate them.
Then a RAMS-R(2) that is intended to be an update but becomes an new
request results in an RAMS-I with MSN = 0. The client will not know if
this is an retransmission of RAMS-R(1) info. The updated should result
in MSN=1. So without comparing the RAMS-I you can't determine if there
The client checks the RAMS-I seqnum to see whether it is a repeat or new info.
If RAMS-I MSN is zero, that is the first RAMS-I anyway so it must be fully
parsed. Does not matter which RAMS-R actually generated it since that is the
info from the server until an updated RAMS-I is received. This is how the
protocol works.
is new information. Going my way (no reset) does not allow a client to
determine if an RAMS-R was treated as update or new request, but it will
correctly know that it is new RAMS-I information.
The server cannot keep state information (i.e. tracking RAMS-R numbers) across
different sessions. Furthermore, requests for different streams can go to
different servers.
9. Section 7.3, Media Sender SSRC TLV:
" While this
information is already available in the message header, it can be
useful to repeat it in an explicit field."
This sentence seems out of place compared to the rest of the text.
If I recall correctly, this was something you asked for. I am not sure
whether there is a better place to put it. Maybe in
parentheses?
I think we can remove that sentence. I think you now have text making
clear when it needs to be included and when it shouldn't.
Ok.
14. Section 10:
Shouldn't the security consideration make it clear that RAMS-R are
especially suspectible to Replay attacks as there is no information in
the packet that one can use to detect that it is out of sequence.
There is a wording about this in that section (which simply refers the
reader to 5760). Are you considering a RAMS-specific
replay attack here?
Yes, I am considering that it is easy to target RAMS-R specifically for
an replay attack. Especially when sent in a reduced size RTCP packet
only containing RAMS-R and SDES CNAME. That has no time specific
information and all replay detection must happen in the security protocol.
The Token stuff (or the cookie stuff) will avoid request messages from being
sent by others (it will do at least a reverse path check). Beyond that is not
SRTCP a good solution to avoid this?
-acbegen
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