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Versions: 00 01 draft-ietf-perc-double

Network Working Group                                        C. Jennings
Internet-Draft                                                  P. Jones
Intended status: Standards Track                           Cisco Systems
Expires: September 22, 2016                                     A. Roach
                                                                 Mozilla
                                                          March 21, 2016


                   SRTP Double Encryption Procedures
                     draft-jennings-perc-double-01

Abstract

   In some conferencing scenarios, it is desirable for an intermediary
   to be able to manipulate some RTP parameters, while still providing
   strong end-to-end security guarantees.  This document defines SRTP
   procedures that use two separate but related cryptographic contexts
   to provide "hop-by-hop" and "end-to-end" security guarantees.  Both
   the end-to-end and hop-by-hop cryptographic transforms can utilize an
   authenticated encryption with associated data scheme or take
   advantage of future SRTP transforms with different properties.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cryptographic Contexts  . . . . . . . . . . . . . . . . . . .   3
   4.  Original Header Block . . . . . . . . . . . . . . . . . . . .   4
   5.  RTP Operations  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Encrypting a Packet . . . . . . . . . . . . . . . . . . .   5
     5.2.  Modifying a Packet  . . . . . . . . . . . . . . . . . . .   6
     5.3.  Decrypting a Packet . . . . . . . . . . . . . . . . . . .   7
   6.  RTCP Operations . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Recommended Inner and Outer Cryptographic Transforms  . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  RTP Header Extension  . . . . . . . . . . . . . . . . . .  10
     9.2.  DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Cloud conferencing systems that are based on switched conferencing
   have a central media distribution device (MDD) that receives media
   from endpoints and distributes it to other endpoints, but does not
   need to interpret or change the media content.  For these systems, it
   is desirable to have one cryptographic context from the sending
   endpoint to the receiving endpoint that can encrypt and authenticate
   the media end-to-end while still allowing certain RTP header
   information to be changed by the MDD.  At the same time, a separate
   cryptographic context provides integrity and optional confidentiality
   for the media flowing between the MDD and the endpoints.  See the
   framework document that describes this concept in more detail in more
   detail in [I-D.jones-perc-private-media-framework].

   This specification RECOMMENDS the SRTP AES-GCM transform [RFC7714] to
   encrypt an RTP packet for the end-to-end cryptographic context.  The
   output of this is treated as an RTP packet and again encrypted with
   an SRTP transform used in the hop-by-hop cryptographic context
   between the endpoint and the MDD.  The MDD decrypts and checks



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   integrity of the hop-by-hop security.  The MDD MAY change some of the
   RTP header information that would impact the end-to-end integrity.
   The original value of any RTP header field that is changed is
   included in a new RTP header extension called the Original Header
   Block.  The new RTP packet is encrypted with the hop-by-hop
   cryptographic transform before it is sent.  The receiving endpoint
   decrypts and checks integrity using the hop-by-hop cryptographic
   transform and then replaces any parameters the MDD changed using the
   information in the Original Header Block before decrypting and
   checking the end-to-end integrity.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Terms used throughout this document include:

   o  MDD: media distribution device that routes media from one endpoint
      to other endpoints

   o  E2E: end-to-end, meaning the link from one endpoint through one or
      more MDDs to the endpoint at the other end.

   o  HBH: hop-by-hop, meaning the link from the endpoint to or from the
      MDD.

   o  OHB: Original Header Block is an RTP header extension that
      contains the original values from the RTP header that might have
      been changed by an MDD.

3.  Cryptographic Contexts

   This specification uses two cryptographic contexts: an inner ("end-
   to-end") context that is used by endpoints that originate and consume
   media to ensure the integrity of media end-to-end, and an outer
   ("hop-by-hop") context that is used between endpoints and MDDs to
   ensure the integrity of media over a single hop and to enable an MDD
   to modify certain RTP header fields.  RTCP is also encrypted using
   the hop-by-hop cryptographic context.  The RECOMMENDED cipher for the
   hop-by-hop and end-to-end contexts is AES-GCM.  Other combinations of
   SRTP ciphers that support the procedures in this document can be
   added to the IANA registry.

   The keys and salt for these contexts are generated with the following
   steps:




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   o  Generate key and salt values of the length required for the
      combined inner (end-to-end) and outer (hop-by-hop) transforms.

   o  Assign the key and salt values generated for the inner (end-to-
      end) transform.

   o  Assign the key and salt values for the outer (hop-by-hop)
      transform.

   As a special case, the outer cryptographic transform MAY be the NULL
   cipher (see [RFC3711]) if a secure transport such as [RFC6347] is
   used over a hop (i.e., between an endpoint and MDD or between two
   MDDs).  In that case, the key and salt values generated would be the
   length required only for the inner cryptographic transform.

   Obviously, if the MDD is to be able to modify header fields but not
   decrypt the payload, then it must have cryptographic context for the
   outer transform, but not the inner transform.  This document does not
   define how the MDD should be provisioned with this information.  One
   possible way to provide keying material for the outer ("hop-by-hop")
   transform is to use [I-D.jones-perc-dtls-tunnel].

4.  Original Header Block

   Any SRTP packet processed following these procedures MAY contain an
   Original Header Block (OHB) RTP header extension.

   The OHB contains the original values of any modified header fields
   and MUST be placed after any already-existing RTP header extensions.
   Placement of the OHB after any original header extensions is
   important so that the receiving endpoint can properly authenticate
   the original packet and any originally included RTP header
   extensions.  The receiving endpoint will authenticate the original
   packet by restoring the modified RTP header field values and header
   extensions.  It does this by copying the original values from the OHB
   and then removing the OHB extension and any other RTP header
   extensions that appear after the OHB extension.

   The MDD is only permitted to modify the extension (X) bit, payload
   type (PT) field, and the RTP sequence number field.

   The OHB extension is either one octet in length, two octets in
   length, or three octets in length.  The length of the OHB indicates
   what data is contained in the extension.

   If the OHB is one octet in length, it contains both the original X
   bit and PT field value.  In this case, the OHB has this form:




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    0
    0 1 2 3 4 5 6 7
   +---------------+
   |X|     PT      |
   +---------------+

   If the OHB is two octets in length, it contains the original RTP
   packet sequence number.  In this case, the OHB has this form:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-------------------------------+
   |        Sequence Number        |
   +-------------------------------+

   If the OHB is three octets in length, it contains the original X bit,
   PT field value, and RTP packet sequence number.  In this case, the
   OHB has this form:

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +---------------+-------------------------------+
   |X|     PT      |        Sequence Number        |
   +---------------+-------------------------------+

   If an MDD modifies an original RTP header value, the MDD MUST include
   the OHB extension to reflect the changed value.  If another MDD along
   the media path makes additional changes to the RTP header and any
   original value is not already present in the OHB, the MDD must extend
   the OHB by adding the changed value to the OHB.  To properly preserve
   original RTP header values, an MDD MUST NOT change a value already
   present in the OHB extension.

5.  RTP Operations

5.1.  Encrypting a Packet

   To encrypt a packet, the endpoint encrypts the packet using the inner
   cryptographic context and then encrypts using the outer cryptographic
   context.  The processes is as follows:

   o  Form an RTP packet.  If there are any header extensions, they MUST
      use [RFC5285].

   o  Apply the inner cryptographic transform to the RTP packet.  If
      encrypting RTP header extensions end-to-end, then [RFC6904] MUST
      be used when encrypting the RTP packet using the inner
      cryptographic context.



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   o  If the endpoint wishes to insert header extensions that can be
      modified by an MDD, it MUST insert an OHB header extension at the
      end of any header extensions protected end-to-end, then add any
      MDD-modifiable header extensions.  The OHB MUST replicate the
      information found in the RTP header following the application of
      the inner cryptographic transform.  For example, if the packet had
      no header extensions when the inner cryptographic transform was
      applied, the X bit would be 0.  If the endpoint introduces an OHB
      and then adds MDD-modifiable header extensions, the X bit in the
      OHB would be 0.  After introducing the OHB and MDD-modifiable
      header extensions, of course, the X bit in the RTP header would be
      set to 1.

   o  Apply the outer cryptographic transform to the RTP packet.  If
      encrypting RTP header extensions hop-by-hop, then [RFC6904] MUST
      be used when encrypting the RTP packet using the outer
      cryptographic context.

5.2.  Modifying a Packet

   The MDD does not have a notion of outer or inner cryptographic
   contexts.  Rather, the MDD has a single cryptographic context.  The
   cryptographic transform and key used to decrypt a packet and any
   encrypted RTP header extensions would be the same as those used in
   the endpoint's outer cryptographic context.

   In order to modify a packet, the MDD decrypts the packet, modifies
   the packet, updates the OHB with any modifications not already
   present in the OHB, and re-encrypts the packet using the
   cryptographic context used for next hop.

   o  Apply the cryptographic transform to the packet.  If decrypting
      RTP header extensions hop-by-hop, then [RFC6904] MUST be used.

   o  Change any required parameters

   o  If a changed RTP header field is not already in the OHB, add it
      with its original value to the OHB.  An MDD MAY add information to
      the OHB, but MUST NOT change existing information in the OHB.

   o  If the MDD resets a parameter to its original value, it MAY drop
      it from the OHB as long as there are no other header extensions
      following the OHB.  Note that this might result in a decrease in
      the size of the OHB.

   o  The MDD MUST NOT delete any header extensions before the OHB, but
      MAY add, delete, or modify any that follow the OHB.




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      *  If the MDD adds any header extensions, it must append them and
         it must maintain the order of the original header extensions in
         the [RFC5285] block.

      *  If the MDD appends header extensions, then it MUST add the OHB
         header extension (if not present), even if the OHB merely
         replicates the original header field values, and append the new
         extensions following the OHB.  The OHB serves as a demarcation
         point between original RTP header extensions introduced by the
         endpoint and those introduced by an MDD.

   o  The MDD MAY modify any header extension appearing after the OHB,
      but MUST NOT modify header extensions that are present before the
      OHB.

   o  Apply the cryptographic transform to the packet.  If encrypting
      RTP header extensions hop-by-hop, then [RFC6904] MUST be used.

5.3.  Decrypting a Packet

   To decrypt a packet, the endpoint first decrypts and verifies using
   the outer cryptographic context, then uses the OHB to reconstruct the
   original packet, which it decrypts and verifies with the inner
   cryptographic context.

   o  Apply the outer cryptographic transform to the packet.  If the
      integrity check does not pass, discard the packet.  The result of
      this is referred to as the outer SRTP packet.  If decrypting RTP
      header extensions hop-by-hop, then [RFC6904] MUST be used when
      decrypting the RTP packet using the outer cryptographic context.

   o  Form a new synthetic SRTP packet with:

      *  Header = Received header, with header fields replaced with
         values from OHB (if present).

      *  Insert all header extensions up to the OHB extension, but
         exclude the OHB and any header extensions that follow the OHB.
         If the original X bit is 1, then the remaining extensions MUST
         be padded to the first 32-bit boundary and the overall length
         of the header extensions adjusted accordingly.  If the original
         X bit is 0, then the header extensions would be removed
         entirely.

      *  Payload is the original encrypted payload.

   o  Apply the inner cryptographic transform to this synthetic SRTP
      packet.  If the integrity check does not pass, discard the packet.



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      If decrypting RTP header extensions end-to-end, then [RFC6904]
      MUST be used when decrypting the RTP packet using the inner
      cryptographic context.

   Once the packet has successfully decrypted, the application needs to
   be careful about which information it uses to get the correct
   behavior.  The application MUST use only the information found in the
   synthetic SRTP packet and MUST NOT use the other data that was in the
   outer SRTP packet with the following exceptions:

   o  The PT from the outer SRTP packet is used for normal matching to
      SDP and codec selection.

   o  The sequence number from the outer SRTP packet is used for normal
      RTP ordering.

   If any of the following RTP headers extensions are found in the outer
   SRTP packet, they MAY be used:

   o  TBD

6.  RTCP Operations

   Unlike RTP, which is encrypted both hop-by-hop and end-to-end using
   two separate cryptographic contexts, RTCP is encrypted using only the
   outer (HBH) cryptographic context.  The procedures for RTCP
   encryption are specified in [RFC3711] and this document introduces no
   additional steps.

7.  Recommended Inner and Outer Cryptographic Transforms

   This specification recommends and defines AES-GCM as both the inner
   and outer cryptographic transforms, identified as
   DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and
   DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM.  These transforms provide
   for authenticated encryption and will consume additional processing
   time double-encrypting for HBH and E2E.  However, the approach is
   secure and simple, and is thus viewed as an acceptable trade-off in
   processing efficiency.

   Note that names for the cryptographic transforms are of the form
   DOUBLE_(inner transform)_(outer transform).

   This specification also allows for the NULL cipher to be used as the
   outer cryptographic transform in cases where a secure transport is
   used over the hop, with those transforms identified as
   DOUBLE_AEAD_AES_128_GCM_NULL_NULL and
   DOUBLE_AEAD_AES_256_GCM_NULL_NULL.



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   Open Issue: It is not clear if the NULL ciphers are needed or not.
   The authors plan to remove them from the next version of the draft
   unless there is a reasonable support and reasons to keep them in.

   While this document only defines a profile based on AES-GCM, it is
   possible for future documents to define further profiles with
   different inner and outer transforms in this same framework.  For
   example, if a new SRTP transform was defined that encrypts some or
   all of the RTP header, it would be reasonable for systems to have the
   option of using that for the outer transform.  Similarly, if a new
   transform was defined that provided only integrity, that would also
   be reasonable to use for the HBH as the payload data is already
   encrypted by the E2E.

   The AES-GCM cryptographic transform introduces an additional 16
   octets to the length of the packet.  When using AES-GCM for both the
   inner and outer cryptographic transforms, the total additional length
   is 32 octets.  If no other header extensions are present in the
   packet and the OHB is introduced, that will consume an additional 8
   octets.  If other extensions are already present, the OHB will
   consume up to 4 additional octets.

8.  Security Considerations

   It is obviously critical that the intermediary have only the outer
   transform parameters and not the inner transform parameters.  We rely
   on an external key management protocol to assure this property.

   Modifications by the intermediary result in the recipient getting two
   values for changed parameters (original and modified).  The recipient
   will have to choose which to use; there is risk in using either that
   depends on the session setup.

   The security properties for both the inner and outer key holders are
   the same as the security properties of classic SRTP.

   The NULL cipher MUST be used in conjunction with an encrypted
   transport for both RTP and RTCP.  Use of the NULL cipher for the
   outer cryptographic context without the use of an encrypted transport
   exposes the RTCP traffic to undetectable modification as it is
   transmitted over the network.  Likewise, RTP traffic under the same
   conditions would be subject to modification that would not be
   detectable by the MDD.  While the endpoint could detect modification
   of the end-to-end information, reliance on information like payload
   type value in the packet received from the MDD could present problems
   such as attempting to decode media with the wrong codec.





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9.  IANA Considerations

9.1.  RTP Header Extension

   This document defines a new extension URI in the RTP Compact Header
   Extensions part of the Real-Time Transport Protocol (RTP) Parameters
   registry, according to the following data:

   Extension URI: urn:ietf:params:rtp-hdrext:ohb

   Description: Original Header Block

   Contact: Cullen Jennings <mailto:fluffy@iii.ca>

   Reference: RFCXXXX

   Note to RFC Editor: Replace RFCXXXX with the RFC number of this
   specification.

9.2.  DTLS-SRTP

   We request IANA to add the following values to defines a DTLS-SRTP
   "SRTP Protection Profile" defined in [RFC5764].

   +------------+------------------------------------------+-----------+
   | Value      | Profile                                  | Reference |
   +------------+------------------------------------------+-----------+
   | {TBD, TBD} | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX   |
   | {TBD, TBD} | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFCXXXX   |
   | {TBD, TBD} | DOUBLE_AEAD_AES_128_GCM_NULL_NULL        | RFCXXXX   |
   | {TBD, TBD} | DOUBLE_AEAD_AES_256_GCM_NULL_NULL        | RFCXXXX   |
   +------------+------------------------------------------+-----------+

   Note to IANA: Please assign value RFCXXXX and update table to point
   at this RFC for these values.

   The SRTP transform parameters for each of these protection are:














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   DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM
       cipher:                 AES_128_GCM then AES_128_GCM
       cipher_key_length:      256 bits
       cipher_salt_length:     192 bits
       aead_auth_tag_length:   32 octets
       auth_function:          NULL
       auth_key_length:        N/A
       auth_tag_length:        N/A
       maximum lifetime:       at most 2^31 SRTCP packets and
                               at most 2^48 SRTP packets

   DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM
       cipher:                 AES_256_GCM then AES_256_GCM
       cipher_key_length:      512 bits
       cipher_salt_length:     192 bits
       aead_auth_tag_length:   32 octets
       auth_function:          NULL
       auth_key_length:        N/A
       auth_tag_length:        N/A
       maximum lifetime:       at most 2^31 SRTCP packets and
                               at most 2^48 SRTP packets

   DOUBLE_AEAD_AES_128_GCM_NULL_NULL
       cipher:                 AES_128_GCM then identity transform
       cipher_key_length:      128 bits
       cipher_salt_length:     96 bits
       aead_auth_tag_length:   16 octets
       auth_function:          NULL
       auth_key_length:        N/A
       auth_tag_length:        N/A
       maximum lifetime:       at most 2^31 SRTCP packets and
                               at most 2^48 SRTP packets

   DOUBLE_AEAD_AES_256_GCM_NULL_NULL
       cipher:                 AES_256_GCM then identity transform
       cipher_key_length:      256 bits
       cipher_salt_length:     96 bits
       aead_auth_tag_length:   16 octets
       auth_function:          NULL
       auth_key_length:        N/A
       auth_tag_length:        N/A
       maximum lifetime:       at most 2^31 SRTCP packets and
                               at most 2^48 SRTP packets

   Except when the NULL cipher is used for the outer (HBH) transform,
   the first half of the key and salt is used for the inner (E2E)
   transform and the second half is used for the outer (HBH) transform.




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   For those that use the NULL cipher for the outer transform, the the
   key and salt is applied only to the inner transform.

10.  Acknowledgments

   Many thanks to review from GET YOUR NAME HERE.  Please, send
   comments.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
              2008, <http://www.rfc-editor.org/info/rfc5285>.

   [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,
              <http://www.rfc-editor.org/info/rfc5764>.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904, DOI
              10.17487/RFC6904, April 2013,
              <http://www.rfc-editor.org/info/rfc6904>.

   [RFC7714]  McGrew, D. and K. Igoe, "AES-GCM Authenticated Encryption
              in the Secure Real-time Transport Protocol (SRTP)", RFC
              7714, DOI 10.17487/RFC7714, December 2015,
              <http://www.rfc-editor.org/info/rfc7714>.

11.2.  Informative References








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   [I-D.jones-perc-dtls-tunnel]
              Jones, P., "DTLS Tunnel between Media Distribution Device
              and Key Management Function to Facilitate Key Exchange",
              draft-jones-perc-dtls-tunnel-02 (work in progress),
              March 2016.

   [I-D.jones-perc-private-media-framework]
              Jones, P., Ismail, N., and D. Benham, "A Solution
              Framework for Private Media in Privacy Enhanced RTP
              Conferencing", draft-jones-perc-private-media-framework-02
              (work in progress), March 2016.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

Authors' Addresses

   Cullen Jennings
   Cisco Systems

   Email: fluffy@iii.ca


   Paul E. Jones
   Cisco Systems

   Email: paulej@packetizer.com


   Adam Roach
   Mozilla

   Email: adam@nostrum.com

















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