Network Working Group                                           V. Singh
Intended status: Informational                                   Inamdar
Expires: July 4, 2018                                       Ravindranath
                                                     Cisco Systems, Inc.
                                                       December 31, 2017

      Multipath RTP (MPRTP) with Secure Real-Time Transport (SRTP)


   This document describes the considerations of using Multipath RTP
   (MPRTP) with Datagram Transport Layer Security (DTLS) protocol.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SRTP Cryptographic Context considerations . . . . . . . . . .   3
     3.1.  DTLS association re-use . . . . . . . . . . . . . . . . .   3
     3.2.  Cryptographic index . . . . . . . . . . . . . . . . . . .   4
     3.3.  Cryptographic keys  . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  Two-time keypad . . . . . . . . . . . . . . . . . . .   5
       3.3.2.  Authentication key re-use . . . . . . . . . . . . . .   5
     3.4.  Coordination between a plurality of cryptographic
           contexts and MPRTP  . . . . . . . . . . . . . . . . . . .   5
   4.  Re-keying considerations  . . . . . . . . . . . . . . . . . .   6
   5.  Encrypting MPRTP header extensions  . . . . . . . . . . . . .   6
   6.  DTLS associations . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  DTLS Session Resumption . . . . . . . . . . . . . . . . .   7
     6.2.  Keeping Sub-flows Alive . . . . . . . . . . . . . . . . .   7
     6.3.  Late Binding of Cryptographic Contexts  . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Multi path RTP(MPRTP) [1] is an extension to RTP that allows multi
   homed endpoints to use a plurality of transmission paths to send/
   receive media.

   MPRTP functions as a layer of abstraction between the RTP stack and
   the multiplicity of transport paths available for media transmission
   by splitting and recombining media streams.

   Datagram Transport Layer Security (DTLS) [RFC4347] is a channel
   security protocol that offers integrated key management, parameter
   negotiation, and secure data transfer.  Because DTLS data transfer
   protocol is generic, it is less highly optimized for use with RTP
   than is SRTP, which has been specifically tuned for that purpose,
   DTLS SRTP [RFC5764] is an extension to DTLS that is optimized to work
   with Secure Real Time transport protocol [RFC3711] to provide
   integrated key management, SRTP algorithm negotiation and SRTP
   parameter negotiation.

   [I-D.ietf-avtcore-mprtp] [2]conceptually introduces the possibility
   of transmitting RTP over a plurality of sub flows using an extension
   to RTP, however in real world deployments there is a need to secure

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   transmission paths whether singular or multiple.  The motivation of
   this draft is to highlight the operating principles of MPRTP with

2.  Motivation

   When DTLS SRTP is used with MPRTP there are several cryptographic
   considerations that arise from the perspective of SRTP and DTLS-SRTP.
   The following sections of this draft highlight these considerations.

3.  SRTP Cryptographic Context considerations

   A SRTP cryptographic context is maintained by key management and
   provides several parameters that are vital to the proper operation of
   the SRTP framework (E.g.  ROC, index, master keys, session keys etc.)
   In the case of a single RTP stream that is secured via DTLS-SRTP,
   there is tight synchronization between different cryptographic
   parameters on sending and receiving application.

   In the case of MPRTP, due to the presence of sub-flows, there can be
   two possible approaches with regards to securing media traffic using

   1.  Re-use the same DTLS-SRTP association as traffic fails over from
       interface to another

   2.  Use multiple DTLS-SRTP associations if traffic uses several sub-
       flows concurrently.

   The implications of using either approach are detailed in the sub-
   sections below.

3.1.  DTLS association re-use

   One of major use cases of MPRTP is that of mobility, wherein a
   currently active interface experiences a severe degradation of
   transmission quality or disappears altogether as the device moves
   between networks.  In such cases the MPRTP stack may offload all
   traffic to a secondary preference interface to continue media
   transmission.  This change in the media address would require an
   updated offer/answer exchange to be sent when ICE is used, in other
   cases in-band RTCP based advertisements may be used.  For such
   scenarios instead of setting up an entirely new DTLS SRTP
   association, the existing association can be reused by retaining the
   same 'dtls-id' as defined by <

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   In cases where there is a need to gradually offload traffic from an
   currently active interface to an another/additional interfaces, a new
   DTLS SRTP association must be setup or alternatively, there might be
   a future specification/extension to DTLS-SRTP that defines such

3.2.  Cryptographic index

   When using multiple sub-flows with MPRTP, each sub-flow sets up a
   different DTLS SRTP association, the cryptographic context for each
   DTLS SRTP association is unique and referenced using a distinct
   triplet identifier.

   <SSRC, destination network address, destination transport port

   While de-queuing packets from the application, the MPRTP stack may
   choose to distribute these packets across several active sub-flows
   after packet encryption and optional authentication, such that each
   active sub-flow would most likely not service packets with
   monotonously increasing global RTP sequence numbers (the per flow
   sequence numbers however, would be monotonously increasing)

   The encryption process specified in section 3.3 of RFC3711 when
   applied to the MPRTP scenario would cause a huge skew when indices
   for successive packets in a given sub-flow are calculated as the
   index value uses the global RTP sequence number as per the following

   i = 2^16 * ROC + SEQ

   When packets arrive out-of-order, this skew in packet indices can
   cause the replay protection algorithm on the receiving application to
   misfire and incorrectly drop packets as the replay window would
   accept packet indices that slightly lag behind the current
   cryptographic index value.  Setting the window size to be large
   enough to accept packets whose indices drastically lag behind the
   current cryptographic context index value, renders the replay
   protection algorithm ineffective and incapable of identifying replay
   attacks.  This particular problem is exacerbated when a certain sub-
   flow(s) are scantily used for packet transmission with majority of
   the packets transmitted on other sub-flows with better path

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3.3.  Cryptographic keys

   RFC3711 allows for sharing of keys across different RTP streams in a
   media session.  From the perspective of MPRTP, there are two major
   problems that could arise with key sharing across different sub-

3.3.1.  Two-time keypad

   MPRTP makes use of the same SSRC value across all sub-flows of a
   given media type (e.g. audio), in general scenarios, unique
   keystreams are generated per packet regardless of the sub-flow over
   which they are transmitted as the index of the packet being encrypted
   in unique.  However in scenarios where certain critical packets are
   transmitted over all or some of the sub-flows for the sake of
   redundancy and reliability (e.g.  I-Frame, named telephony events),
   the very same keystream value is generated and leads to "two-time key
   pad" rendering the secure framework open to attacks.  The chances of
   a two-type key pad issue is exacerbated if key re-use is allowed
   among different MPRTP media types (e.g. audio and video) in a given
   media session as the chance of an attacker detecting duplicate
   keystreams increases at least by a factor of two.

3.3.2.  Authentication key re-use

   The SSRC field in an SRTP packet is an authentication-protected field
   and if the same authentication key is used, an attacker can
   substitute one stream into another causing media playout issues at
   the receiver application.

3.4.  Coordination between a plurality of cryptographic contexts and

   MPRTP involves the splitting of a single RTP stream into a number of
   subflows that appear as distinct streams from the perspective of the
   network.  However the MPRTP stack at the receiver side is responsible
   for re-combining these streams and presenting a single flow of RTP
   packets to the application.  Due to the presence of multiple sub-
   flows (with distinct network addresses and ports), a separate DTLS-
   SRTP association is required per sub-flow, with each association
   maintaining a distinct set of cryptographic parameters as per section
   3.2.1 of RFC3711.

   With each encrypt/decrypt cycle occurring across sub-flows, the MPRTP
   stack on the sender and receiver side has to ensure that various
   parameters of the cryptographic context are updated across each sub-
   flow, followed by correct sequencing of packets before it is
   presented to the application.  The computational costs of maintaining

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   multiple sub-flows, running several encrypt/decrypt cycles per sub-
   flow and sequencing packets correctly is significantly higher in
   comparison to a single RTP stream.

4.  Re-keying considerations

   To avoid the "two-time key pad" problem, it is necessary for an SRTP/
   SRTCP stream to re-key (master key) every time the 48 bit index space
   is exhausted, this ensures that duplicate key-streams aren't
   generated.  As discussed in section 2.1, MPRTP may setup sub-flows
   that are scantily used in packet transmission, in which case a given
   sub-flow would quickly exhaust it's index space, roll over and
   possibly produce duplicate keystreams leading to a potential
   breakdown of the SRTP framework.  If the master key is shared across
   several sub-flows this would certainly lead to frequent re-keying
   across several sub-flows adding to the cryptographic load.

5.  Encrypting MPRTP header extensions

   MPRTP uses header extensions in RTP and RTCP packets for the
   following use-cases:

   1.  To communicate the sub-flow ID and sub-flow specific sequence

   2.  To report per sub-flow RTCP reports

   3.  For interface advertisement (RTCP)

   The transforms and constructs of RFC3711 encrypt only the payload of
   the SRTP packets, without considering the header extensions.  Given
   that the MPRTP header extension could be visible to an attacker,
   fields like the sub-flow specific ID or sub-flow specific sequence
   numbers can easily be manipulated, causing issues on the receiving
   application.  For example, an adversary can change the sub-flow
   specific sequence number to indicate a drastic change causing the
   receiving application to drop the packet.  The sub-flow specific ID
   could be also be changed to reflect a stream ID that is non existent
   causing the receiving application to completely drop all packets
   corresponding to the rogue sub-flow ID.

   NOTE:As per my our internal discussion, creation of an authentication
   tag would ensure that RTP header extensions are unaltered, however in
   the case where encryption proceeds without authentication, it may be
   better to encrypt the MPRTP header extensions (a counter argument
   could be that even normal RTP headers like SSRC, global sequence
   numbers etc. could be altered without authentication)

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6.  DTLS associations

   As discussed in earlier sections of this draft, multiple DTLS SRTP
   associations must be established per sub-flow in an MPRTP setup,
   maintaining multiple sub-flows with DTLS SRTP does bring up some
   additional considerations that are discussed below:

6.1.  DTLS Session Resumption

   For related media streams within a RTP session, it is advised to use
   DTLS session resumption to reduce the cost of cryptographic
   operations.  Using DTLS session resumption leads to the re-use of the
   master key across all the sub-flows, which could lead to the problems
   highlighted in section 2.2.1 and 3.  It is advisable to use parallel,
   distinct DTLS associations to protect the sub-flows such that the
   keys are unique across all sub-flows.

6.2.  Keeping Sub-flows Alive

   Certain MPRTP sub-flows that are secure via DTLS SRTP, might be used
   sparingly for packet transmission, with the majority of traffic being
   sent over other high priority sub-flows (as determined via ICE or a
   local algorithm at the sender side), in order to ensure that
   sparingly used sub-flows at the DTLS layer, the DTLS heartbeat
   extension as defined in RFC6520 may be used.  This ensures that the
   costly operation of a DTLS re-negotiation is avoided and also ensures
   that TURN or STUN bindings are refreshed if media traverses through
   NAT's or relays.

6.3.  Late Binding of Cryptographic Contexts

   As DTLS SRTP associations are agnostic to the SSRC of media streams,
   DTLS-SRTP uses a "late binding" mechanism as far as cryptographic
   contexts are concerned.  A MPRTP endpoint can have multiple DTLS SRTP
   associations, in which case on receiving the SRTP packet, an
   assertion needs to be made on what association that SSRC corresponds
   to, so, initially the cost of the algorithm to determine this will be
   equal to the number of sub-flows and the algorithm might require
   additional passes as sub-flows are added.

7.  Security Considerations


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8.  Acknowledgements

9.  References

9.1.  Normative References

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              DOI 10.17487/RFC4145, September 2005,

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572,
              DOI 10.17487/RFC4572, July 2006,

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,

              Singh, V., Karkkainen, T., Ott, J., Ahsan, S., and L.
              Eggert, "Multipath RTP (MPRTP)", draft-ietf-avtcore-
              mprtp-03 (work in progress), July 2016.

9.2.  URIs



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Authors' Addresses

   Varun Singh
   Annankatu 31-33 C 42,
   Helsinki  00100


   Kaustubh Inamdar
   Cisco Systems, Inc.
   Cessna Business Park ,
   Kadabeesanahalli Village, Varthur Hobli,
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103


   Ram Mohan Ravindranath
   Cisco Systems, Inc.
   Cessna Business Park ,
   Kadabeesanahalli Village, Varthur Hobli,
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103


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