Network Working Group                                      M. Westerlund
Internet-Draft                                                  Ericsson
Intended status: Informational                                C. Perkins
Expires: January 10, April 25, 2013                            University of Glasgow
                                                            July 9,
                                                        October 22, 2012

                   Options for Securing RTP Sessions
               draft-ietf-avtcore-rtp-security-options-00
               draft-ietf-avtcore-rtp-security-options-01

Abstract

   The Real-time Transport Protocol (RTP) is used in a large number of
   different application domains and environments.  This hetrogeneity
   implies that different security mechanisms are needed to provide
   services such as confidentiality, integrity and source authentication
   of RTP/RTCP packets suitable for the various environments.  The range
   of solutions makes it difficult for RTP-based application developers
   to pick the most suitable mechanism.  This document provides an
   overview of a number of security solutions for RTP, and gives
   guidance for developers on how to choose the appropriate security
   mechanism.

Status of this Memo

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   This Internet-Draft will expire on January 10, April 25, 2013.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Point to Point Sessions  . . . . . . . . . . . . . . . . .  5
     2.2.  Sessions Using an RTP Mixer  . . . . . . . . . . . . . . .  5
     2.3.  Sessions Using an RTP Translator . . . . . . . . . . . . .  6
       2.3.1.  Transport Translator (Relay) . . . . . . . . . . . . .  6
       2.3.2.  Gateway  . . . . . . . . . . . . . . . . . . . . . . .  7
       2.3.3.  Media Transcoder . . . . . . . . . . . . . . . . . . .  8
     2.4.  Any Source Multicast . . . . . . . . . . . . . . . . . . .  8
     2.5.  Source-Specific Multicast  . . . . . . . . . . . . . . . .  8
   3.  Security Options . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Secure RTP . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.1.  Key Management for SRTP: DTLS-SRTP . . . . . . . . . . 11
       3.1.2.  Key Management for SRTP: MIKEY . . . . . . . . . . . . 12
       3.1.3.  Key Management for SRTP: Security Descriptions . . . . 13
       3.1.4.  Key Management for SRTP: Encrypted Key Transport . . . 14
       3.1.5.  Key Management for SRTP: Other systems . . . . . . . . 14
     3.2.  RTP Legacy Confidentiality . . . . . . . . . . . . . . . . 14
     3.3.  IPsec  . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.4.  DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.5.  TLS over TCP . . . . . . . . . . . . . . . . . . . . . . . 16
     3.6.  Payload-only Security Mechanisms . . . . . . . . . . . . . 16
       3.6.1.  ISMA Encryption and Authentication . . . . . . . . . . 17
   4.  Securing RTP Applications  . . . . . . . . . . . . . . . . . . 17
     4.1.  Application Requirements . . . . . . . . . . . . . . . . . 17
       4.1.1.  Confidentiality  . . . . . . . . . . . . . . . . . . . 17
       4.1.2.  Integrity  . . . . . . . . . . . . . . . . . . . . . . 18
       4.1.3.  Source Authentication  . . . . . . . . . . . . . . . . 19
       4.1.4.  Identity . . . . . . . . . . . . . . . . . . . . . . . 20
       4.1.5.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . 20
     4.2.  Application Structure  . . . . . . . . . . . . . . . . . . 21
     4.3.  Interoperability . . . . . . . . . . . . . . . . . . . . . 21
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 22
     5.1.  Media Security for SIP-established Sessions using
           DTLS-SRTP  . . . . . . . . . . . . . . . . . . . . . . . . 22
     5.2.  Media Security for WebRTC Sessions . . . . . . . . . . . . 22
     5.3.  3GPP Packet Based Streaming Service (PSS)  . . . . . . . . 23
     5.4.  IPTV . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 24
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28

1.  Introduction

   Real-time Transport Protocol (RTP) [RFC3550] is widely used in a
   large variety of multimedia applications, including Voice over IP
   (VoIP), centralized multimedia conferencing, sensor data transport,
   and Internet television (IPTV) services.  These applications can
   range from point-to-point phone calls, through centralised group
   teleconferences, to large-scale television distribution services.
   The types of media can vary significantly, as can the signalling
   methods used to establish the RTP sessions.

   This multi-dimensional heterogeneity has so far prevented development
   of a single security solution that meets the needs of the different
   applications.  Instead significant number of different solutions have
   been developed to meet different sets of security goals.  This makes
   it difficult for application developers to know what solutions exist,
   and whether their properties are appropriate.  This memo gives an
   overview of the available RTP solutions, and provides guidance on
   their applicability for different application domains.  The guidance
   provided is not exhaustive, and this memo does not provide normative
   recommendations.

   It is important that application developers consider the security
   goals and requirements for their application.  The IETF considers it
   important that protocols implement, and makes available to the user,
   secure modes of operation [RFC3365].  Because of the heterogeneity of
   RTP applications and use cases, however, a single security solution
   cannot be mandated.  Instead, application developers need to select
   mechanisms that provide appropriate security for their environment.
   It is strongly encouraged that common mechanisms are used by related
   applications in common environments.  The IETF publishes guidelines
   for specific classes of applications, so it worth searching for such
   guidelines.

   The remainder of this document is structured as follows.  Section 2
   provides additional background.  Section 3 outlines the available
   security mechanisms at the time of this writing, and lists their key
   security properties and constraints.  That is followed by guidelines
   and important aspects to consider when securing an RTP application in
   Section 4.  Finally, we give some examples of application domains
   where guidelines for security exist in Section 5.

2.  Background

   RTP can be used in a wide variety of topologies, and combinations of
   topologies, due to it's support for unicast, multicast groups, and
   broadcast topologies, and the existence of different types of RTP
   middleboxes.  In the following we review the different topologies
   supported by RTP to understand their implications for the security
   properties and trust relations that can exist in RTP sessions.

2.1.  Point to Point Sessions

   The most basic use case is two directly connected end-points, shown
   in Figure 1, where A has established an RTP session with B. In this
   case the RTP security is primarily about ensuring that any third
   party can't compromise the confidentiality and integrity of the media
   communication.  This requires confidentiality protection of the RTP
   session, integrity protection of the RTP/RTCP packets, and source
   authentication of all the packets to ensure no man-in-the-middle
   attack is taking place.

   The source authentication can also be tied to a user or an end-points
   verifiable identity to ensure that the peer knows who they are
   communicating with.  Here the combination of the security protocol
   protecting the RTP session and its RTP and RTCP traffic and the key-
   management protocol becomes important in which security statements
   one can do.
                            +---+         +---+
                            | A |<------->| B |
                            +---+         +---+

                     Figure 1: Point to Point Topology

2.2.  Sessions Using an RTP Mixer

   An RTP mixer is a an RTP session level middlebox that one can build
   an multi-party RTP based conference around.  The RTP mixer might
   actually perform media mixing, like mixing audio or compositing video
   images into a new media stream being sent from the mixer to a given
   participant; or it might provide a conceptual stream, for example the
   video of the current active speaker.  From a security point of view,
   the important featurs of an RTP mixer is that it generates a new
   media stream, and has its own source identifier, and does not simply
   forward the original media.

   An RTP session using a mixer might have a topology like that in
   Figure 2.  In this examples, participants A-D each send unicast RTP
   traffic between themselves and the RTP mixer, and receive a single RTP
   stream from the mixer, comprising a mixture of the streams from the
   other participants.

                    +---+      +------------+      +---+
                    | A |<---->|            |<---->| B |
                    +---+      |            |      +---+
                               |    Mixer   |
                    +---+      |            |      +---+
                    | C |<---->|            |<---->| D |
                    +---+      +------------+      +---+

                   Figure 2: Example RTP Mixer topology

   A consequence of an RTP mixer having its own source identifier, and
   acting as an active participant towards the other end-points, is that
   the RTP mixer needs to be a trusted device that is part of the
   security context(s) established.  The RTP mixer can also become a
   security enforcing entity.  For example, a common approach to secure
   the topology in Figure 2 is to establish a security context between
   the mixer and each participant independently, and have the mixer
   source authenticate each peer.  The mixer then ensures that one
   participant cannot impersonsate another.

2.3.  Sessions Using an RTP Translator

   RTP translators are middleboxes that provide various levels of in-
   network media translation and transcoding.  Their security properties
   vary widely, depending on which type of operations they attempt to
   perform.  We identify three different categories of RTP translator:
   transport translators, gateways, and media transcoders.  We discuss
   each in turn.

2.3.1.  Transport Translator (Relay)

   A transport translator [RFC5117] operates on a level below RTP and
   RTCP.  It relays the RTP/RTCP traffic from one end-point to one or
   more other addresses.  This can be done based only on IP addresses
   and transport protocol ports, with each receive port on the
   translator can have a very basic list of where to forward traffic.
   Transport translators should also implement ingress filtering to
   prevent random traffic from being forwarded that isn't coming from a
   participant in the conference.

   Figure 3 shows an example transport translator, where traffic from
   any one of the four participants will be forwarded to the other three
   participants unchanged.  The resulting topology is very similar to
   Any source Multicast (ASM) session (as discussed in Section 2.4), but
   implemented at the application layer.

                    +---+      +------------+      +---+
                    | A |<---->|            |<---->| B |
                    +---+      |    Relay   |      +---+
                               | Translator |
                    +---+      |            |      +---+
                    | C |<---->|            |<---->| D |
                    +---+      +------------+      +---+

                  Figure 3: RTP relay translator topology

   A transport translator can often operate without needing to be in the
   security context, as long as the security mechanism does not provide
   protection over the transport-layer information.  A transport
   translator does, however, make the group communication visible, and
   so can complicate keying and source authentication mechanisms.  This
   is further discussed in Section 2.4.

2.3.2.  Gateway

   Gateways are deployed when the endpoints are not fully compatible.
   Figure 4 shows an example topology.  The functions a gateway provides
   can be diverse, and range from transport layer relaying between two
   domains not allowing direct communication, via transport or media
   protocol function initiation or termination, to protocol or media
   encoding translation.  The supported security protocol might even be
   one of the reasons a gateway is needed.

                    +---+      +-----------+      +---+
                    | A |<---->|  Gateway  |<---->| B |
                    +---+      +-----------+      +---+

                      Figure 4: RTP Gateway Topology

   The choice of security protocol and the details of the gateway
   function will determine if the gateway needs to be a trusted part of
   the application security context or not.  Many gateways need to be
   trusted by all peers to perform the translation; in other cases some
   or all peers might not be aware of the presence of the gateway.  The
   security protocols have different properties depending on the degree
   of trust and visibility needed.  Ensuring communication is possible
   without trusting the gateway can be strong incentive for accepting
   different security properties.  Some security solutions will be able
   to detect the gateways as manipulating the media stream, unless the
   gateway is a trusted device.

2.3.3.  Media Transcoder

   A Media transcoder is a special type of gateway device that changes
   the encoding of the media being transported by RTP.  The discussion
   in Section 2.3.2 applies.  A media transcoder alters the media data,
   and so almost certainly needs to be trusted device that is part of
   the security context.

2.4.  Any Source Multicast

   Any Source Multicast [RFC1112] is the original multicast model where
   any multicast group participant can send to the multicast group, and
   get their packets delivered to all group members (see Figure 5).
   This form of communication has interesting security properties, due
   to the many-to-many nature of the group.  Source authentication is
   important, but all participants in the group security context will
   have access to the necessary secrets to decrypt and verify integrity
   of the traffic.  Thus use of any symmetric security functions fails
   if the goal is to separate individual sources within the security
   context; alternate solutions are needed.

                                   +-----+
                        +---+     /       \    +---+
                        | A |----/         \---| B |
                        +---+   /   Multi-  \  +---+
                               +    Cast     +
                        +---+   \  Network  /  +---+
                        | C |----\         /---| D |
                        +---+     \       /    +---+
                                   +-----+

                   Figure 5: Any Source Multicast Group

   In addition the potential large size of multicast groups creates some
   considerations for the scalability of the solution and how the key-
   management is handled.

2.5.  Source-Specific Multicast

   Source Specific Multicast [RFC4607] allows only a specific end-point
   to send traffic to the multicast group.  That end-point is labelled
   the Distribution Source in Figure 6.  It distributes traffic from a
   number of RTP media sources, MS1 to MSm.  Figure 6 also depicts the
   feedback part of the SSM RTP session using unicast feedback [RFC5760]
   from a number of receivers R1..Rn that sends feedback to a Feedback
   Target (FT) and eventually aggregated and distributed to the group.

   The use of SSM makes it more difficult to inject traffic into the
   multicast group, but not impossible.  Source authentication
   requirements apply for SSM sessions too, and a non-symmetric
   verification of who sent the RTP and RTCP packets is needed.

   The SSM communication channel needs to be securely established and
   keyed.  In addition one also have the individual unicast feedback
   that also needs to be secured.

                     +-----+  +-----+          +-----+
                     | MS1 |  | MS2 |   ....   | MSm |
                     +-----+  +-----+          +-----+
                        ^        ^                ^
                        |        |                |
                        V        V                V
                    +---------------------------------+
                    |       Distribution Source       |
                    +--------+                        |
                    | FT Agg |                        |
                    +--------+------------------------+
                      ^ ^           |
                      :  .          |
                      :   +...................+
                      :             |          .
                      :            / \          .
                    +------+      /   \       +-----+
                    | FT1  |<----+     +----->| FT2 |
                    +------+    /       \     +-----+
                      ^  ^     /         \     ^  ^
                      :  :    /           \    :  :
                      :  :   /             \   :  :
                      :  :  /               \  :  :
                      :   ./\               /\.   :
                      :   /. \             / .\   :
                      :  V  . V           V .  V  :
                     +----+ +----+     +----+ +----+
                     | R1 | | R2 | ... |Rn-1| | Rn |
                     +----+ +----+     +----+ +----+

           Figure 6: SSM-based RTP session with Unicast Feedback

3.  Security Options

   This section provides an overview of a number of currently defined
   security mechanisms that can be used with RTP.

3.1.  Secure RTP

   The Secure RTP (SRTP) protocol [RFC3711] is one of the most commonly
   used mechanisms to provide confidentiality, integrity protection and
   source authentication for RTP.  SRTP was developed with RTP header
   compression and third party monitors in mind.  Thus the RTP header is
   not encrypted in RTP data packets, and the first 8 bytes of the first
   RTCP packet header in each compound RTCP packet are not encrypted.
   The entirity of RTP packets and compound RTCP packets are integrity
   protected.  This allows RTP header compression to work, and lets
   third party monitors determine what RTP traffic flows exist based on
   the SSRC fields, but protects the sensitive content.

   The source authentication guarantees provided by SRTP are highly
   dependent on the cryptographic transform and key-management scheme
   used.  In some cases all a receiver can determine is whether the
   packets come from someone in the group security context, and not what
   group member send the packets.  Thus, the source authentication
   guarantees depend also on the session topology.  Some cryptographic
   transform have stronger authentication properties which can guarantee
   a given source, even over a multi-party session, e.g. those based on
   TESLA [RFC4383].

   SRTP can easily be extended with additional cryptographic transforms.
   At the time of this writing, the following transforms are defined or
   under definition:

   AES CM and HMAC-SHA-1:  AES Counter Mode encryption with 128 bits
      keys combined with 128 bits keyed HMAC-SHA1 using 80 or 32 bits
      authentication tags are the default cryptographic transform which
      need to be supported.  Defined in SRTP [RFC3711].

   AES-f8 and HMAC-SHA-1:  AES f8 mode encryption with 128 bits keys
      combined with keyed HMAC-SHA1 using 80 or 32 bits authentication.
      Defined in SRTP [RFC3711].

   TESLA:  As a complement to the regular symmetric keyed authentication
      transforms, like HMAC-SHA1.  The TESLA based authentication scheme
      can provide per-source authentication in some group communication
      scenarios.  The downside is need for buffering the packets for a
      while before authenticity can be verified.  The TESLA transform
      for SRTP is defined in [RFC4383].

   SEED:  An Korean national standard cryptographic transform that is
      defined to be used with SRTP in [RFC5669].  It has three modes,
      one using SHA-1 authentication, one using Counter with CBC-MAC,
      and finally one using Galois Counter mode.

   ARIA:  An Korean block cipher [I-D.ietf-avtcore-aria-srtp], that
      supports 128, 192 and 256 bits keys.  It also has three modes,
      Counter mode where combined with HMAC-SHA1 with 80 or 32 bits
      authentication tags, Counter mode with CBC-MAC and Galois Counter
      mode.  It also defines a different key derivation function than
      the AES based.

   AES-192 and AES-256:  cryptographic transforms for SRTP based on AES-
      192 and AES-256 counter mode encryption and 160-bit keyed HMAC-
      SHA1 with 80 and 32 bits authentication tags for authentication.
      Thus providing 192 and 256 bits encryption keys and NSA Suite B
      included cryptographic transforms.  Defined in [RFC6188].

   AES-GCM:  There is also ongoing work to define AES-GCM (Galois
      Counter Mode) and AES-CCM (Counter with CBC) authentication for
      AES-128 and AES-256.  This authentication is included in the
      cipher text which becomes expanded with the length of the
      authentication tag instead of using the SRTP authentication tag.
      This is defined in [I-D.ietf-avtcore-srtp-aes-gcm].

   [RFC4771] defines a variant of the authentication tag that enables a
   receiver to obtain the Roll over Counter for the RTP sequence number
   that is part of the Initialization vector (IV) for many cryptographic
   transforms.  This enables quicker and easier options for joining a
   long lived secure RTP group, for example a broadcast session.

   RTP header extensions are in normally carried in the clear and only
   integrity protected in SRTP.  This can be problematic in some cases,
   so [I-D.ietf-avtcore-srtp-encrypted-header-ext] defines an extension
   to also encrypt selected header extensions.

   SRTP does not contain an integrated key-management solution, and
   instead relies on an external key management protocol.  There are
   several protocols that can be used.  The following sections outline
   some popular schemes.

3.1.1.  Key Management for SRTP: DTLS-SRTP

   A Datagram Transport Layer Security extension exists for establishing
   SRTP keys [RFC5763][RFC5764].  This extension provides secure key-
   exchange between two peers, including perfect forward secrecy and
   enabling binding strong identity verification to an end-point.  The
   default key generation will generate a key that contains material
   contributed by both peers.  The key-exchange happens in the media
   plane directly between the peers.  The common key-exchange procedures
   will take two round trips assuming no losses.  TLS resumption can be
   used when establishing additional media streams with the same peer,
   used reducing the setup time to one RTT.

   DTLS-SRTP key management can use the signalling protocol in three
   ways.  First, to agree on using DTLS-SRTP for media security.
   Secondly, to determine the network location (address and port) where
   each side is running an DTLS listener to let the parts perform the
   key-management handshakes that generate the keys used by SRTP.
   Finally, to exchange hashes of each sides certificates to enable each
   side to verify that they have connected to the by signalling
   indicated port and not a man in the middle.  That way enabling some
   binding between the key-exchange and the signalling.  This usage is
   well defined for SIP/SDP in [RFC5763], and in most cases can be
   adopted for use with other bi-directions signalling solutions.

3.1.2.  Key Management for SRTP: MIKEY

   Multimedia Internet Keying (MIKEY) [RFC3830] is a keying protocol
   that has several modes with different properties.  MIKEY can be used
   in point-to-point applications using SIP and RTSP (e.g., VoIP calls),
   but is also suitable for use in broadcast and multicast applications,
   and centralized group communications.

   MIKEY can establish multiple security contexts or cryptographic
   sessions with a single message.  It is possible to use in scenarios
   where one entity generates the key and needs to distribute the key to
   a number of participants.  The different modes and the resulting
   properties are highly dependent on the cryptographic method used to
   establish the Traffic Generation Key (TGK) that is used to derive the
   keys actually used by the security protocol, like SRTP.

   MIKEY has the following modes of operation:

   Pre-Shared Key:  Uses a pre-shared secret for symmetric key crypto
      used to secure a keying message carrying the already generated
      TGK.  This system is the most efficient from the perspective of
      having small messages and processing demands.

   Public Key encryption:  Uses a public key crypto to secure a keying
      message carrying the already generated TGK.  This is more resource
      consuming but enables scalable systems.  It does require a public
      key infrastructure to enable verification.

   Diffie-Hellman:  Uses Diffie-Hellman key-agreement to generate the
      TGK, thus providing perfect forward secrecy.  The downside is
      increased resource consumption in bandwidth and processing.  This
      method can't be used to establish group keys as each pair of peers
      performing the MIKEY exchange will establish different keys.

   HMAC-Authenticated Diffie-Hellman:  [RFC4650] defines a variant of
      the Diffie-Hellman exchange that uses a pre-shared key in a keyed
      HMAC to verify authenticity of the keying material instead of a
      digital signature as in the previous method.  This method is still
      restricted to point-to-point usage.

   RSA-R:  MIKEY-RSA in Reverse mode [RFC4738] is a variant of the
      public key method which doesn't rely on the initiator of the key-
      exchange knowing the responders certificate.  This methods lets
      both the initiator and the responder to specify the TGK material
      depending on use case.  Usage of this mode requires one round trip
      time.

   TICKET:  [RFC6043] is a MIKEY extension using trusted centralized key
      management service and tickets, like Kerberos.

   IBAKE:  [RFC6267] uses a key management services (KMS) but with lower
      demand on the KMS.  If provides both perfect forward and backwards
      secrecy.

   SAKKE:  [RFC6509] provides Sakai-Kasahara Key Encryption in MIKEY.
      Based on Identity based Public Key Cryptography to establish a
      shared secret value and certificate less signatures to provide
      source authentication.  It features include simplex transmission,
      scalability, low-latency call setup, and support for secure
      deferred delivery.

   MIKEY messages has several different defined transports.  [RFC4567]
   defines how MIKEY messages can be embedded in general SDP for usage
   with the signalling protocols SIP, SAP and RTSP.  There also exist an
   3GPP defined usage of MIKEY that sends MIKEY messages directly over
   UDP to key the receivers of Multimedia Broadcast and Multicast
   Service (MBMS) [3GPP.33.246].

   Based on the many choices it is important to consider the properties
   needed in ones solution and based on that evaluate which modes that
   are candidates for ones usage.  More information on the applicability
   of the different MIKEY modes can be found in [RFC5197].

3.1.3.  Key Management for SRTP: Security Descriptions

   [RFC4568] provides a keying solution based on sending plain text keys
   in SDP [RFC4566].  It is primarily used with SIP and SDP Offer/
   Answer, and is well-defined in point to point sessions where each
   side declares its own unique key.  Using Security Descriptions to
   establish group keys is less well defined, and can have security
   issues as the SSRC uniqurness property can't be guaranteed.

   Since keys are transported in plain text in SDP, they can easily be
   intercepted unless the SDP carrying protocol provides strong end-to-
   end confidentiality and authentication guarantees.  This is not the
   common use of security descriptions with SIP, where instead hop by
   hop security is provided between signalling nodes using TLS.  This
   still leaves the keying material sensitive to capture by the
   traversed signalling nodes.  Thus in most cases the security
   properties of security description are weak.

3.1.4.  Key Management for SRTP: Encrypted Key Transport

   Encrypted Key Transport (EKT) [I-D.ietf-avtcore-srtp-ekt] is an SRTP
   extension that enables group keying despite using a keying mechanism
   that can't support group keys, like DTLS-SRTP.  It is designed for
   centralized conferencing, but can also be used in sessions where an
   end-points connect to a conference bridge or a gateway, and need to
   be provisioned with the keys each participant on the bridge or
   gateway uses to avoid decryption encryption cycles on the bridge or
   gateway.

   The mechanism is based on establishing an additional EKT key which
   everyone uses to protect their actual session key.  The actual
   session key is sent in a expanded authentication tag to the other
   session participants.  This key are only sent occasionally or
   periodically depending on use cases depending on what requirements
   exist for timely delivery or notification on when the key is needed
   by someone.

3.1.5.  Key Management for SRTP: Other systems

   There exist at least one additional SRTP key-management system,
   namely ZRTP [RFC6189].  This was a candidate for IETF standardization
   that wasn't chosen, and was published for information instead.  Its
   properties are somewhat similar to DTLS.

   There might exist additional non-IETF defined solutions.

3.2.  RTP Legacy Confidentiality

   Section 9 of the RTP standard [RFC3550] defines a DES or 3DES based
   encryption of RTP and RTCP packets.  This mechanism is keyed using
   plain text keys in SDP [RFC4566] using the "k=" SDP field.  This
   method of providing confidentiality has extremely weak security
   properties and is not to be used.

3.3.  IPsec

   IPsec [RFC4301] can be used independent of mode to protect RTP and
   RTCP packets in transit from one network interface to another.  This
   can be sufficient when the network interfaces have a direct relation,
   or in a secured environment where it can be controlled who can read
   the packets from those interfaces.

   The main concern with using IPsec to protect RTP traffic is that in
   most cases using a VPN approach that terminates the security
   association at some node prior to the RTP end-point leaves the
   traffic vulnerable to attack between the VPN termination node and the
   end-point.  Thus usage of IPsec requires careful thought and design
   of its usage so that it really meets the security goals.  A important
   question is how one ensure the IPsec terminating peer and the
   ultimate destination is the same.

   IPsec with RTP is more commonly used as security solution between
   central nodes in an infrastructure that exchanges many RTP sessions
   and media streams between the peers.  The establishment of a secure
   tunnel between these peers minimizes the key-management overhead
   between these two boxes.

3.4.  DTLS

   Datagram Transport Layer Security (DTLS) [RFC6347] can provide point
   to point security for RTP flows.  The two peers would establish an
   DTLS association between each other, including the possibility to do
   certificate-based source authentication when establishing the
   association.  All RTP and RTCP packets flowing will be protected by
   this DTLS association.

   Note: using DTLS is different to using DTLS-SRTP key management.
   DTLS-SRTP has the core key-management steps in common with DTLS, but
   DTLS-SRTP uses SRTP for the per packet security operations, while
   DTLS uses the normal datagram TLS data protection.  When using DTLS,
   RTP and RTCP packets are completely encrypted with no headers in the
   clear, while DTLS-SRTP leaves the headers in the clear.

   DTLS can use similar techniques to those available for DTLS-SRTP to
   bind a signalling side agreement to communicate to the certificates
   used by the end-point when doing the DTLS handshake.  This enables
   use without having a certificate based trust chain to a trusted
   certificate root.

3.5.  TLS over TCP

   When RTP is sent over TCP [RFC4571] it can also be sent over TLS over
   TCP [RFC4572], using TLS to provide point to point security services.
   The security properties TLS provides are confidentiality, integrity
   protection and possible source authentication if the client or server
   certificates are verified and provide a usable identity.  When used
   in multi-party scenarios using a central node for media distribution,
   the security provide is only between then central node and the peers,
   so the security properties for the whole session are dependent on
   what trust one can place in the central node.

3.6.  Payload-only Security Mechanisms

   Mechanisms have been defined that encrypt only the payload of the RTP
   packets, and leave the RTP headers and RTCP in the clear.  There are
   several reasons why this might be appropriate, but a common rationale
   is to ensure that the content stored in RTP hint tracks in RTSP
   streaming servers has the media content in a protected format that
   cannot be read by the streaming server (this is mostly done in the
   context of Digital Rights Management).  These approaches then uses a
   key-management solution between the rights provider and the consuming
   client to deliver the key used to protect the content, usually after
   the appropriate method for charging has happened, and do not include
   the media server in the security context.  Such methods have several
   security weaknesses such the fact that the same key is handed out to
   a potentially large group of receiving clients, increasing the risk
   of a leak.

   Use of this type of solution can be of interest in environments that
   allow middleboxes to rewrite the RTP headers and select what streams
   that are delivered to an end-point (e.g., some types of centralised
   video conference systems).  The advantage of encrypting and possibly
   integrity protecting the payload but not the headers is that the
   middlebox can't eavesdrop on the media content, but can still provide
   stream switching functionality.  The downside of such a system is
   that it likely needs two levels of security: the payload level
   solution to provide confidentiality and source authentication, and a
   second layer with additional transport security ensuring source
   authentication and integrity of the RTP headers associated with the
   encrypted payloads.  This can also results in the need to have two
   different key-management systems as the entity protecting the packets
   and payloads are different with different set of keys.

   The aspect of two tiers of security are present in ISMAcryp (see
   Section 3.6.1) and the deprecated 3GPP Packet Based Streaming Service
   Annex.K [3GPP.23.234] solution.

3.6.1.  ISMA Encryption and Authentication

   The Internet Streaming Media Alliance (ISMA) has defined ISMA
   Encryption and Authentication 2.0 [ISMACrypt2].  This specification
   defines how one encrypts and packetizes the encrypted application
   data units (ADUs) in an RTP payload using the MPEG-4 Generic payload
   format [RFC3640].  The ADU types that are allowed are those that can
   be stored as elementary streams in an ISO Media File format based
   file.  ISMAcryp uses SRTP for packet level integrity and source
   authentication from a streaming server to the receiver.

   Key-management for a ISMACryp based system can be achieved through
   Open Mobile Alliance (OMA) Digital Rights Management 2.0 [OMADRMv2],
   for example.

4.  Securing RTP Applications

   In the following we provide guidelines for how to choose appropriate
   security mechanisms for RTP applications.

4.1.  Application Requirements

   This section discusses a number of application requirements that need
   be considered.  An application designer choosing security solutions
   requires a good understanding of what level of security is needed and
   what behaviour they strive to achieve.

4.1.1.  Confidentiality

   When it comes to confidentiality of an RTP session there are several
   aspects to consider:

   Probability of compromise:  When using encryption to provide media
      confidentiality, it is necessary to have some rough understanding
      of the security goal and how long one expect the protected content
      remain confidential.  From that, one can determine what encryption
      algorithm is to be used from the set of available transforms.

   Potential for other leakage:  RTP based security in most of its forms
      simply wraps RTP and RTCP packets into cryptographic containers.
      This commonly means that the size of the original RTP payload, and
      details of the RTP and RTCP headers, are visible to observers of
      the protected packet flow.  This can provide information to those
      observers.  A well documented case is the risk with variable bit-
      rate speech codecs that produce different sized packets based on
      the speech input [RFC6562].  Potential threats such as these need
      to be considered and, if they are significant, then restrictions
      will be needed on mode choices in the codec, or additional padding
      will need to be added to make all packets equal size and remove
      the informational leakage.

      Another case is RTP header extensions.  If SRTP is used, header
      extensions are normally not protected by the security mechanism
      protecting the RTP payload.  If the header extension carries
      information that is considered sensitive, then the application
      needs to be modified to ensure that mechanisms used to protect
      against such information leakage are employed.

   Who has access:  When considering the confidentiality properties of a
      system, it is important to consider where the media handled in the
      clear.  For example, if the system is based on an RTP mixer that
      needs the keys to decrypt the media, process, and repacketize it,
      then is the mixer providing the security guarantees expected by
      the other parts of the system?  Furthermore, it is important to
      consider who has access to the keys, and are the keys stored or
      kept somewhere?  The policies for the handling of the keys, and
      who can access the keys, need to be considered along with the
      confidentiality goals.

   As can be seen the actual confidentiality level has likely more to do
   with the application's usage of centralized nodes, and the details of
   the key-management solution chosen, than with the actual choice of
   encryption algorithm (although, of course, the encryption algorithm
   needs to be chosen appropriately for the desired security level).

4.1.2.  Integrity

   Protection against modification of content by a third party, or due
   to errors in the network, is another factor to consider.  The first
   aspect that one consider is what resilience one has against
   modifications to the content.  This can affect what cryptographic
   algorithm is used, and the length of the integrity tags.  However
   equally, important is to consider who is providing the integrity
   assertion, what is the source of the integrity tag, and what are the
   risks of modifications happening prior to that point where protection
   is applied?  RTP applications that rely on central nodes need to
   consider if hop-by-hop integrity is acceptable, or if true end-to-end
   integrity protection is needed?  Is it important to be able to tell
   if a middlebox has modified the data?  There are some uses of RTP
   that require trusted middleboxes that can modify the data in a way
   that doesn't break integrity protection as seen by the receiver, for
   example local advertisment insertion in IPTV systems; there are also
   uses where it is essential that such in-network modification be
   detectable.  RTP can support both, with appropriate choices of
   security mechanisms.

   Integrity of the data is commonly closely tied to the question of
   source authentication.  That is, it becomes important to know who
   makes an integrity assertion for the data.

4.1.3.  Source Authentication

   Source authentication is about determining who sent a particular RTP
   or RTCP packet.  It is normally closely tied with integrity, since
   you also want to ensure that what you received is what the claimed
   source really sent, so source authentication without integrity is not
   particularly useful.  In similar way, although not as definitive, is
   that integrity without source authentication is also not particular
   useful: you need to know who claims this packet wasn't changed.

   Source authentication can be asserted in several different ways:

   Base level:  Using cryptographic mechanisms that give authentication
      with some type of key-management provides an implicit method for
      source authentication.  Assuming that the mechanism has sufficient
      strength to not be circumvented in the time frame when you would
      accept the packet as valid, it is possible that assert the source
      authenticated statement that this message is most probably from
      someone that has the cryptographic key to this communication.

      What that assertion actually means is highly dependent on the
      application, and how it handles the keys.  In an application where
      the key-handling is limited to two peers, this can form a basis
      for a trust relationship to the level that you can state as the
      traffic is authenticated and matching this particular context, it
      is coming either from me or from my peer (and I trust that neither
      has shared the key with anyone else).  However, in a multi-party
      scenario where security contexts are shared among participants,
      most base-level authentication solution can't even assert that
      this packet is from the same source as the previous packet.

   Binding the Source:  A step up in the assertion that can be done in
      base-level systems is to tie the signalling to the key-exchange.
      Here, the goal is to be at least be able to assert that the sender
      of the packets is the same entity that I have established the
      session with.  How feasible this is depends on the properties of
      the key-management system used, the ability to tie the signalling
      to a particular peer, and what trust you place on the different
      nodes involved.

      For example, consider a point to point communication system that
      use DTLS-SRTP using self-signed certificates for key-management,
      and SIP for signalling.  In such a system the end-points for the
      DTLS-SRTP handshake have securely established keys that are not
      visible to the signalling nodes.  However, as the certificates
      used by DTLS is not bound to any PKI they can't be verified.
      Instead, hashes over the certificate are sent over the signalling
      path.  If the signalling can be trusted not to collaborate on
      performaning a man in the middle attack by modifying the hashes,
      then the end-points can verify that they have reached the peer
      they are doing signalling with.

   Using Identities:  If the applications have access to a system that
      can provide verifiable identities, then the source authentication
      can be bound to that identity.  For example, in a point-to-point
      communication even symmetric key crypto, where the key-management
      can assert that the key has only been exchanged with a particular
      identity, can provide a strong assertion about who is sending the
      traffic.

      Note that all levels of the system much have matching capability
      to assert identity.  Having the signalling assert that you include
      a particular identity in a multi-party communication session where
      the key-management systems establish keys in a way that one can
      assert that only the given identity has gotten the key.  Using a
      authentication mechanism build on a group key and otherwise can't
      provide any assertion who sent the traffic than anyone that got
      the key provides no strong assertability on the media level than:
      Someone that has gotten the security context (key) sent this
      traffic.

4.1.4.  Identity

   As seen in the previous section, having an identity provider system
   can benefit the applications by enabling them to do strong assertion
   between identity and the actual media source.  Therefore, the need
   for identity needs to be considered.  However, having identity
   systems might not be suitable for all types of application, since
   they require trusted infrastructure.

4.1.5.  Privacy

   RTP applications need to consider what privacy goals they have.  As
   RTP applications communicate directly between peers in many cases,
   the IP addresses of any communication peer will be available.  The
   main privacy concern with IP addresses is related to geographical
   location and the possibility to track a user of an end-point.  The
   main way of avoid such concerns is the introduction of relay or
   centralized media mixers or forwarders that hides the address of a
   peer from any other peer.  The security and trust placed in these
   relays obviously needs to be carefully considered.

   RTP itself can contribute to enabling a particular user to be tracked
   between communication sessions if the CNAME is generated according to
   the RTP specification in the form of user@host.  Such RTCP CNAMEs are
   likely long term stable over multiple sessions, allowing tracking of
   users.  This can be desirable for long-term fault tracking and
   diagnosis, but clearly has privacy implications.  Instead
   cryptographically random ones could be used as defined by Random
   algorithm for RTP CNAME generation
   [I-D.rescorla-avtcore-random-cname].

   If there exist privacy goals, these need to be considered, and the
   system designed with them in mind.  In addition certain RTP features
   might have to be configured to safeguard privacy, or have
   requirements on how the implementation is done.

4.2.  Application Structure

   When it comes to RTP security, the most appropriate solution is often
   highly dependent on the topology of the communication session.  The
   signalling also impacts what information can be provided, and if this
   can be instance specific, or common for a group.  In the end the key-
   management system will highly affect the security properties achieved
   by the application.  At the same time, the communication structure of
   the application limits what key management methods are applicable.
   As different key-management have different requirements on underlying
   infrastructure it is important to take that aspect into consideration
   early in the design.

4.3.  Interoperability

   Few RTP applications exist as independent applications that never
   interoperate with anything else.  Rather, they enable communication
   with a potentially large number of other systems.  To minimize the
   number of security mechanisms that need to be implemented, it is
   important to consider if one can use the same security mechanisms as
   other applications.  This can also reduce the problems of determining
   what security level is actually negotiated in a particular session.

   The desire to be interoperable can in some cases be in conflict with
   the security requirements determined for an application.  To meet the
   security goals, it might be necessary to sacrifice interoperability.
   Alternatively, one can implement multiple security mechanisms, but
   then end up with an issue of ensuring that the user understands what
   it means to use a particular security level.  In addition, the
   application can then become vulnerable to bid-down attack.

5.  Examples

   In the following we describe a number of example security solutions
   for RTP using applications, services or frameworks.  These examples
   are provided to show the choices that can be made.  They are not
   normative recommendations for security.

5.1.  Media Security for SIP-established Sessions using DTLS-SRTP

   The IETF evaluated media security for RTP sessions established using
   point-to-point SIP sessions in 2009.  A number of requirements were
   determined, and based on those, the existing solutions for media
   security and especially the keying methods were analysed, and the
   resulting requirements and analysis were published in [RFC5479].
   Based on this analysis, and the working group discussion, DTLS-SRTP
   was determined to be the best solution, and the specifications were
   finalized.

   The security solution for SIP using DTLS-SRTP is defined in the
   Framework for Establishing a Secure Real-time Transport Protocol
   (SRTP) Security Context Using Datagram Transport Layer Security
   (DTLS).
   (DTLS) [RFC5763].  On a high level it uses SIP with SDP offer/answer
   procedures to exchange the network addresses where the server end-point end-
   point will have a DTLS-SRTP enable server running.  The SIP
   signalling is also used to exchange the fingerprints of the
   certificate each end-point will use in the DTLS establishment
   process.  When the signalling is sufficiently completed the DTLS-SRTP
   client performs DTLS handshakes and establishes SRTP session keys.
   The clients also verify the fingerprints of the certificates to
   verify that no man in the middle has inserted themselves into the
   exchange.

   At the basic level DTLS has a number of good security properties.
   For example, to enable a man in the middle someone in the signalling
   path needs to perform an active action and modify the signalling
   message.  There also exist a solution that enables the fingerprints
   to be bound to identities established by the first proxy for each
   user[RFC4916].
   user [RFC4916].  That reduces the number of nodes the connecting user
   UA has to trust to the first hop proxy, rather than the full
   signalling path.

5.2.  Media Security for WebRTC Sessions

   Web Real-Time Communication [I-D.ietf-rtcweb-overview] is solution
   providing web-application with real-time media directly between
   browsers.  The RTP transported real-time media is protected using a
   mandatory to use application of SRTP.  The keying of SRTP is done
   using DTLS-SRTP.  The security configuration is further defined in
   the WebRTC Security Architecture [I-D.ietf-rtcweb-security-arch].

   The peers hash of their certificates are provided to a Javascript
   application that is part of a client server system providing
   rendezvous services for the ones a given peer wants to communicate
   with.  Thus the handling of the hashes between the peers is not well
   defined.  It becomes a matter of trust in the application.  But
   unless the application and its server is intending to compromise the
   communication security they can provide a secure and integrity
   protected exchange of the certificate hashes thus preventing any man-
   in-the-middle (MITM) to insert itself in the key-exchange.

   The web application still have the possibility to insert a MITM.
   That unless one uses a Identity provider and the proposed identity
   solution [I-D.rescorla-rtcweb-generic-idp].  In this solution the
   Identity Provider which is a third party to the web-application signs
   the DTLS-SRTP hash combined with a statement on which user identity
   that has been used to sign the hash.  The receiver of such a Identity
   assertion then independently verifies the user identity to ensure
   that it is the identity it intended to communicate and that the
   cryptographic assertion holds.  That way a user can be certain that
   the application also can't perform an MITM and that way acquire the
   keys to the media communication.

   In the development of WebRTC there has also been high attention on
   privacy question.  The main concerns that has been raised and are at
   all related to RTP are:

   Location Disclosure:  As ICE negotiation provides IP addresses and
      ports for the browser, this leaks location information in the
      signalling to the peer.  To prevent this one can block the usage
      of any ICE candidate that isn't a relay candidate, i.e. where the
      IP and port provided belong to the service providers media traffic
      relay.

   Prevent tracking between sessions:  RTP CNAMEs and DTLS-SRTP
      certificates is information that could possibly be re-used between
      session instances.  Thus to prevent tracking the same information
      can't be re-used between different communication sessions.

   Note: The above cases are focused on providing privacy towards other
   parties than the web service.

5.3.  3GPP Packet Based Streaming Service (PSS)

   To be written:

5.4.  IPTV

   To be written:

6.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section can be removed on publication as an
   RFC.

7.  Security Considerations

   This entire document is about security.  Please read it.

8.  Acknowledgements

   We thank the IESG for their careful review of
   [I-D.ietf-avt-srtp-not-mandatory] which led to the writing of this
   memo.

9.  Informative References

   [3GPP.23.234]
              3GPP, "Technical Specification Group Services and System
              Aspects; Transparent end-to-end Packet-switched Streaming
              Service (PSS); Protocols and codecs", 3GPP TS 26.234
              8.4.0, September 2009.

   [3GPP.33.246]
              3GPP, "3G Security; Security of Multimedia Broadcast/
              Multicast Service (MBMS)", 3GPP TS 33.246 10.0.0,
              December 2010.

   [I-D.ietf-avt-srtp-not-mandatory]
              Perkins, C. and M. Westerlund, "Why RTP Does Not Mandate a
              Single Security Mechanism",
              draft-ietf-avt-srtp-not-mandatory-08
              draft-ietf-avt-srtp-not-mandatory-10 (work in progress),
              October 2011.
              July 2012.

   [I-D.ietf-avtcore-aria-srtp]
              Kim, W., Lee, J., Kim, D., Park, J., and D. Kwon, "The
              ARIA Algorithm and Its Use with the Secure Real-time
              Transport Protocol(SRTP)", draft-ietf-avtcore-aria-srtp-00
              (work in progress), May 2012.

   [I-D.ietf-avtcore-srtp-aes-gcm]
              McGrew, D. and K. Igoe, "AES-GCM and AES-CCM Authenticated
              Encryption in Secure RTP (SRTP)",
              draft-ietf-avtcore-srtp-aes-gcm-01
              draft-ietf-avtcore-srtp-aes-gcm-03 (work in progress),
              June
              September 2012.

   [I-D.ietf-avtcore-srtp-ekt]
              McGrew, D., Andresason, F., Wing, D., and K. Fischer, "Encrypted Key
              Transport for Secure RTP", draft-ietf-avtcore-srtp-ekt-00
              (work in progress), July 2012.

   [I-D.ietf-avtcore-srtp-encrypted-header-ext]
              Lennox, J., "Encryption of Header Extensions in the Secure
              Real-Time Transport Protocol (SRTP)",
              draft-ietf-avtcore-srtp-encrypted-header-ext-01
              draft-ietf-avtcore-srtp-encrypted-header-ext-02 (work in
              progress), October 2011. July 2012.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for Brower-
              based Applications", draft-ietf-rtcweb-overview-04 (work
              in progress), June 2012.

   [I-D.ietf-rtcweb-security-arch]
              Rescorla, E., "RTCWEB Security Architecture",
              draft-ietf-rtcweb-security-arch-02
              draft-ietf-rtcweb-security-arch-03 (work in progress),
              June
              July 2012.

   [I-D.rescorla-avtcore-random-cname]
              Rescorla, E., "Random algorithm for RTP CNAME generation",
              draft-rescorla-avtcore-random-cname-00 (work in progress),
              July 2012.

   [I-D.rescorla-rtcweb-generic-idp]
              Rescorla, E., "RTCWEB Generic Identity Provider
              Interface", draft-rescorla-rtcweb-generic-idp-01 (work in
              progress), March 2012.

   [ISMACrypt2]
              "ISMA Encryption and Authentication, Version 2.0 release
              version", November 2007.

   [OMADRMv2]
              Open Mobile Alliance, "OMA Digital Rights Management
              V2.0", July 2008.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, August 2002.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3640]  van der Meer, J., Mackie, D., Swaminathan, V., Singer, D.,
              and P. Gentric, "RTP Payload Format for Transport of
              MPEG-4 Elementary Streams", RFC 3640, November 2003.

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

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              February 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, July 2006.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

   [RFC4571]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets over Connection-
              Oriented Transport", RFC 4571, July 2006.

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

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

   [RFC4650]  Euchner, M., "HMAC-Authenticated Diffie-Hellman for
              Multimedia Internet KEYing (MIKEY)", RFC 4650,
              September 2006.

   [RFC4738]  Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-
              RSA-R: An Additional Mode of Key Distribution in
              Multimedia Internet KEYing (MIKEY)", RFC 4738,
              November 2006.

   [RFC4771]  Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
              Transform Carrying Roll-Over Counter for the Secure Real-
              time Transport Protocol (SRTP)", RFC 4771, January 2007.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

   [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
              January 2008.

   [RFC5197]  Fries, S. and D. Ignjatic, "On the Applicability of
              Various Multimedia Internet KEYing (MIKEY) Modes and
              Extensions", RFC 5197, June 2008.

   [RFC5479]  Wing, D., Fries, S., Tschofenig, H., and F. Audet,
              "Requirements and Analysis of Media Security Management
              Protocols", RFC 5479, April 2009.

   [RFC5669]  Yoon, S., Kim, J., Park, H., Jeong, H., and Y. Won, "The
              SEED Cipher Algorithm and Its Use with the Secure Real-
              Time Transport Protocol (SRTP)", RFC 5669, August 2010.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760, February 2010.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, May 2010.

   [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, May 2010.

   [RFC6043]  Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based
              Modes of Key Distribution in Multimedia Internet KEYing
              (MIKEY)", RFC 6043, March 2011.

   [RFC6188]  McGrew, D., "The Use of AES-192 and AES-256 in Secure
              RTP", RFC 6188, March 2011.

   [RFC6189]  Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
              Path Key Agreement for Unicast Secure RTP", RFC 6189,
              April 2011.

   [RFC6267]  Cakulev, V. and G. Sundaram, "MIKEY-IBAKE: Identity-Based
              Authenticated Key Exchange (IBAKE) Mode of Key
              Distribution in Multimedia Internet KEYing (MIKEY)",
              RFC 6267, June 2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6509]  Groves, M., "MIKEY-SAKKE: Sakai-Kasahara Key Encryption in
              Multimedia Internet KEYing (MIKEY)", RFC 6509,
              February 2012.

   [RFC6562]  Perkins, C. and JM. Valin, "Guidelines for the Use of
              Variable Bit Rate Audio with Secure RTP", RFC 6562,
              March 2012.

Authors' Addresses

   Magnus Westerlund
   Ericsson
   Farogatan 6
   SE-164 80 Kista
   Sweden

   Phone: +46 10 714 82 87
   Email: magnus.westerlund@ericsson.com

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ
   United Kingdom

   Email: csp@csperkins.org