[inline]
From:
avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On Behalf Of Ilan
Doron
Sent: Monday, July 06, 2009 10:23
To: 'avt at ietf.org'
Subject: [AVT] Question regarding RFC 3711
I have a few questions regarding rfc 3711:
From the RFC, section 3.3.1:
After
a re-keying occurs (changing to a new master key), the rollover
counter always maintains its sequence of values, i.e., it MUST NOT be
reset to zero.
1.
Does re-keying include reinvite session where new keys
are allocated?
[Wyss, Felix] Not necessarily. If multiple master keys were supplied
as part of the key distribution, the sender can switch between them arbitrarily
and indicate which one is used for that packet by means of the MKI.
2.
If ROC is maintained, can I assume full SRTP session is
maintained, i.e. RTP sequence + replay lists + SSRC?
[Wyss, Felix] As SRTP is just a bump in the stack, changes of the
SSRC are not under the SRTP sender's control. For a particular SSRC, the SRTP
sender needs to maintain the appropriate state for a cryptographic context,
including the ROC. How many contexts the SRTP sender maintains depends on
the application (e.g. how many concurrent streams may have to be processed).
Regarding SSRC change during SRTP session:
1.
If a new SSRC arrives should the packet be
authenticated with a zero ROC or with the current ROC value?
[Wyss, Felix] Each context starts at a ROC of zero. If you
might be joining streams late, you can of course try authenticating a couple of
ROCs for added robustness (but potentially increased denial of service risk).
2.
Assuming authentication succeeds, SRTP session will be
reset to zero (Sequence, replay lists ROC etc.)?
[Wyss, Felix] A new SSRC gets a new crypto context. You will
want to maintain at least as many contexts as there may be concurrent streams
being processed plus a couple of spares to deal with periods of overlapping streams
when you switch between devices.
Ilan Doron
Media application Team Leader
Tel:
+972-3-9764354
Fax:
+972-3-9764223
Mobile:
+972-54-6262-775
Email: iland at audiocodes.com
Web: www.audiocodes.com
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