Re: [AVT] Last Call: <draft-ietf-avt-rapid-acquisition-for-rtp-15.txt> (Unicast-Based Rapid Acquisition of Multicast RTP Sessions) to Proposed Standard

[Magnus Westerlund <[e-mail]>]

Ali C. Begen (abegen) skrev 2010-10-04 18:04:
> I am glad that we are converging on something.
> 
>>>>>> 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?
>>
>> Not, really, because it has only clarified the issue. The issue is
>> really the uncertainty for the client how its request will be
>> interpreted, either as updated request or a new request, all depending
>> on timing. The biggest issue is the updated request which might be
>> interpreted as new request, when the client is likely satisfied and
> 
> Why is this a problem? For this to happen, the initial request must have been lost. And if it is lost and the second makes it, the client still gets what it wants. I don't see a problem here.

No, if you intended to send an update and that gets handled as new
request, then you get the unintended consequence of getting a second burst.

> 
>> might even have gone one to join the multicast group when the
>> RAMS-R(update) is processed as a new RAMS-R. Then causing congestion and
> 
> Are you saying this:
> 1- The client makes a request but it gets lost. In the mean time, the client sends an update and the server thinks it is a new request and starts sending the burst

No,
1. the client sends a request intended to update an ongoing burst.

2. The RAMS-R (update) arrive to late. Thus triggering a second burst.

3. In the mean time, client gets an RAMS-I and joins the multicast group

4. Burst + mcast creates a congestion.

> 
>> unwanted traffic. The client that detects this can surely terminate the
>> burst, but that will be after some delay, and traffic has arrived.
> 
> OK, so you are saying what I wrote above. Well, the client has to send a RAMS-T and upon receiving it, the server stops the burst. So that is not a big deal. If you are concerned about RAMS-T being lost (which is repeated if the bust is not stopped), then I will just remind you that this is a protocol where control messages are not fully reliable (they are just repeated for redundancy as frequently as 4585 allows). If you are really this unlucky, you will have a problem but it will be only temporary.
>

I will agree that you can stop the burst, but you have biased your
protocol by design against sending any RAMS-R with the purpose of
updating the parameter because other things happens than what you
intended to. Thus it will in most cases be better to not send an RAMS-R
update message and avoid the risk of tripping a second burst.


> I don't think there is anything here that requires for us to burn our energy. FWIW, your proposal would not solve this problem entirely, either.

I agree that a RAMS-R sequence number would not resolve the issue you
described. However, it resolves the issue I have tried to describe, I
hope the above makes it clear.

> 
>>>
>>>>>
>>>>>> 4. Section 6.2, bullet 5: " Thus, the RTP_Rx MUST choose a globally
>>>>>>         unique CNAME identifier."
[snip]

>>>
>>>>>> 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 :)
>>
>> The real solution is where you have an more permanent id system in place
>> that you can connect through source authentication of the requests.
>>
>> In an SSM session that uses simple feedback model the RTP_Rx cname may
>> leak as they are redistributed.
>>
>> Based on that you could bombard a BRS with RAMS-T for example for all
>> known CNAMES and do that in a round-robin fashion across channels and
>> time. Depending on source address spoofing you will more or less easy to
>> find. But I do agree that it becomes a little bit more a brute force
>> attack, but an attacker could gain knowledge about an important piece of
>> information to mount the attack at all.
> 
> SRTCP?

SRTCP keyed with unique keys for each client will prevent anyone else to
send RAMS-T to terminate a burst you have initiated.

> 
>>>>>
>>>>>> 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.
>>
>> Nope, the number here are still only to indicate that they are different
>> to get the sequence right. My point is that the client can determine
>> based on MSN if it is an repeat or a new RAMS-I based on a new request.
> 
> When the client receives a RAMS-I with MSN=0, it knows that RAMS-I was sent for a RAMS-R message that was received the first time by the server or an identical repeat of the initial RAMS-I message.
> 
> But even if the client sends an updated request, the server may ignore it, may ignore the changes and subsequently repeat the earlier RAMS-I with no changes in it, which will have the previous MSN. The server may not send anything at all or the message may get lost. The client cannot assume the changes it requested were honored by the server UNLESS there was an updated RAMS-I from the server. Even in that case, the RAMS-I changes may be due to other things the server has observed - not the changes the client asked.
>
> So, the client should not really read too much in to the MSN values received. That is what I have been trying to explain in this discussion.

Also in this case I don't think we have been considering the same case.
My case was the following.

1. C->S RAMS-R
2a. S->C RAMS-I (MSN=0)
2b. S->C Burst starts
3. C->S RAMS-R(Intended to update first RAMS-R)
4. S: Burst ends
5. S: RAMS-R from step 3 arrives in server and trigger new burst
6. S->C RAMS-I (MSN=0)
7. S->C Second burst transmitted

When the RAMS-I message from step 6 arrives the client may think this is
the same as the one in 2a.

Are you assuming that there is a 4b RAMS-I message which indicates that
the first burst will be terminated that has MSN=1? What if that is lost
or not sent?

> 
>>>
>>>> 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.
>>
>> As I try to explain there is a case where you can get two RAMS-I with
>> MSN=0 in a row with different information. Thus not providing any
>> relieve for the client in the need to compare the whole request with the
>> previous one.
> 
> So what? If you made a single request and received two RAMS-I messages with MSN=0, that means they are identical. No need to compare them. If you made two requests and received two RAMS-I messages with MSN=0, they are different messages and you need to fully read them anyway.

Okay, so your point is that as soon you have sent more than one RAMS-R
message to a BRS you will need to look at all RAMS-I and the MSN becomes
completely useless. But, then I think the document needs to point out
that MSN is only reliable to detect repeat transmissions as long as you
have sent no additional RAMS-R messages during a minute or so.

>>>>>> 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?
>>
>> SRTCP is if keyed correctly a good solution yes.
>>
>>  My point is that one normally document the vulnerability and then
>> ensure that one has a remedy for it. When it comes to the token stuff I
>> am not certain that it helps against a replay attack. Unless the token
>> has expired on the server you still will get traffic to the target. That
>> is why I think a reverse path check when you start delivering should
>> happen. Because first in that case if there is no listener at the
>> indicated address and port will you find out that you are attempting a
>> delivery not intended.
> 
> Token provides RPF check. But, let's work on this Sec. section together. I could use some help.
> 

Ok

-- 

Magnus Westerlund

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