Re: [rtcweb] consent freshness [was RE: STUN for keep-alive]

"Muthu Arul Mozhi Perumal (mperumal)" <mperumal@cisco.com> Thu, 22 September 2011 12:17 UTC

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Thread-Topic: [rtcweb] consent freshness [was RE: STUN for keep-alive]
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From: "Muthu Arul Mozhi Perumal (mperumal)" <mperumal@cisco.com>
To: "Magnus Westerlund" <magnus.westerlund@ericsson.com>
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Subject: Re: [rtcweb] consent freshness [was RE: STUN for keep-alive]
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|That is a possibility, but I think one can start by ensuring 
|that one actually check for ones own traffic being reported 
|back. That gives you some entropy. Which is a slightly longer 
|context gives you good verification that you are not contining
|sending into a location where there receiver you intended to
|have sent to moves away.

Agree it would suffice in general. However, for cases like double hold (SDP offer with a=inactive) where you would be neither sending nor receiving media the attacker can keep replaying the RTCP packets with an empty report block (that no zero entropy) making you think the other end is still alive.

Of course, you may ultimately disconnect the call but it could mean a DOS attack.. 

Muthu

|-----Original Message-----
|From: Magnus Westerlund [mailto:magnus.westerlund@ericsson.com]
|Sent: Tuesday, September 20, 2011 1:48 PM
|To: Muthu Arul Mozhi Perumal (mperumal)
|Cc: Dan Wing (dwing); rtcweb@ietf.org
|Subject: Re: [rtcweb] consent freshness [was RE: STUN for keep-alive]
|
|On 2011-09-19 18:41, Muthu Arul Mozhi Perumal (mperumal) wrote:
|> Another aspect to consider: As per RFC 3550, the bare minimal valid
|> RTCP packet is a compound RTCP packet containing: - An RR packet with
|> the reception report count set to 0 (RC=0) and - An SDES packet with
|> CNAME
|>
|> Such a packet would look like this:
|>
|> 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
|> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|> header |V=2|P|    RC=0 |   PT=RR=201   |             length
|> | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|> SSRC of packet sender                     |
|> +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|> header |V=2|P|   SC=1  |  PT=SDES=202  |             length
|> | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|> chunk  |                     SSRC of packet sender
|> | 1
|> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|> CNAME=1    |     length    |user & domain padded to 32-bit |
|> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|>
|> I've seen endpoints sending/accepting such a packet as an indication
|> for session liveliness. However, there is no entropy at all in this
|> packet. Suppose an attacker manages to bring the receiver down and
|> captures such a RTCP packet. The attacker can keep replaying it (from
|> the spoofed source transport address) to make the sender believe that
|> the receiver is still alive and muted.
|
|well, the reasonable assumption if you are using this for consent
|freshness is to verify that your flow is reported on in a reasonable
|way. That do provide some entropy in sequence number and Last SR field
|in the report block as I wrote in my other email.
|
|>
|> Perhaps, RTCWeb apps should try to negotiate an RTCP extension
|> carrying a monotonically increasing number that must be part of every
|> RTCP report together with SRTCP, for replay attack protection.
|
|That is a possibility, but I think one can start by ensuring that one
|actually check for ones own traffic being reported back. That gives you
|some entropy. Which is a slightly longer context gives you good
|verification that you are not contining sending into a location where
|there receiver you intended to have sent to moves away.
|
|Cheers
|
|Magnus Westerlund
|
|----------------------------------------------------------------------
|Multimedia Technologies, Ericsson Research EAB/TVM
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|Ericsson AB                | Phone  +46 10 7148287
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