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Versions: (draft-bi-ippm-ipsec) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7717

IPPM WG                                              K. Pentikousis, Ed.
Internet-Draft                                                    Y. Cui
Intended status: Standards Track                                E. Zhang
Expires: January 06, 2014                            Huawei Technologies
                                                           July 05, 2013


               Network Performance Measurement for IPsec
                        draft-ietf-ippm-ipsec-00

Abstract

   IPsec is a mature technology with several interoperable
   implementations.  Indeed, the use of IPsec tunnels is increasingly
   gaining popularity in several deployment scenarios, not the least in
   what used to be solely areas of traditional telecommunication
   protocols.  Wider deployment calls for mechanisms and methods that
   enable tunnel end-users, as well as operators, to measure one-way and
   two-way network performance.  Unfortunately, however, standard IP
   performance measurement security mechanisms cannot be readily used
   with IPsec.  This document makes the case for employing IPsec to
   protect the One-way and Two-Way Active Measurement Protocols (O/
   TWAMP) and proposes a method which combines IKEv2 and O/TWAMP as
   defined in RFC 4656 and RFC 5357, respectively.  This specification
   aims, on the one hand, to ensure that O/TWAMP can be secured with the
   best mechanisms we have at our disposal today while, on the other
   hand, it facilitates the applicability of O/TWAMP to networks that
   have already deployed IPsec.

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 January 06, 2014.

Copyright Notice




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   Copyright (c) 2013 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
   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.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  O/TWAMP-Control Security  . . . . . . . . . . . . . . . .   5
     3.2.  O/TWAMP-Test Security . . . . . . . . . . . . . . . . . .   6
     3.3.  O/TWAMP Security Root . . . . . . . . . . . . . . . . . .   6
     3.4.  O/TWAMP and IPsec . . . . . . . . . . . . . . . . . . . .   7
   4.  O/TWAMP for IPsec Networks  . . . . . . . . . . . . . . . . .   8
     4.1.  Shared Key Derivation . . . . . . . . . . . . . . . . . .   8
     4.2.  Optimizations . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Alternative 1 . . . . . . . . . . . . . . . . . . . .  11
       4.2.2.  Alternative 2 . . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The One-way Active Measurement Protocol (OWAMP) [RFC4656] and the
   Two-Way Active Measurement Protocol (TWAMP) [RFC5357] can be used to
   measure network performance parameters, such as latency, bandwidth,
   and packet loss by sending probe packets and monitoring their
   experience in the network.  In order to guarantee the accuracy of
   network measurement results, security aspects must be considered.
   Otherwise, attacks may occur and the authenticity of the measurement
   results may be violated.  For example, if no protection is provided,
   an adversary in the middle may modify packet timestamps, thus
   altering the measurement results.




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   Cryptographic security mechanisms, such as IPsec, have been
   considered during the early stage of the specification of the two
   active measurement protocols mentioned above.  However, due to
   several reasons, it was decided to avoid tying the development and
   deployment of O/TWAMP to such security mechanisms.  In practice, for
   many networks, the issues listed in [RFC4656], Sec. 6.6 with respect
   to IPsec are still valid.  However, we expect that in the near future
   IPsec will be deployed in many more hosts and networks than today.
   For example, IPsec tunnels may be used to secure wireless channels.
   In this case, what we are interested in is measuring network
   performance specifically for the traffic carried by the tunnel, not
   in general over the wireless channel.  This document makes the case
   that O/TWAMP should be cognizant when IPsec and other security
   mechanisms are in place and can be leveraged upon.  In other words,
   it is now time to specify how O/TWAMP is used in a network
   environment where IPsec is already deployed.  We expect that in such
   an environment, measuring IP performance over IPsec tunnels with O/
   TWAMP is an important tool for operators.

   For example, when considering the use of O/TWAMP in networks with
   IPsec deployed, we can take advantage of the IPsec key exchange
   protocol [RFC5996].  In particular, we note that it is not necessary
   to use distinct keys in OWAMP-Control and OWAMP-Test layers.  One key
   for encryption and another for authentication is sufficient for both
   Control and Test layers.  This obviates the need to generate two keys
   for each layer and reduces the complexity of O/TWAMP protocols in an
   IPsec environment.  This observation comes from the fact that
   separate session keys in the OWAMP-Control and OWAMP-Test layers were
   designed for preventing reflection attacks when employing the current
   mechanism.  Once IPsec is employed, such a potential threat is
   alleviated.

   The remainder of this document is organized as follows.  Section 3
   motivates this work by revisiting the arguments made in [RFC4656]
   against the use of IPsec; this section also summarizes protocol
   operation with respect to security.  Section 4 presents a method of
   binding O/TWAMP and IKEv2 for network measurements between a sender
   and a receiver which both support IPsec.  Finally, Section 3
   discusses the security considerations arising from the proposed
   mechanisms.

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].





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3.  Motivation

   In order to motivate the solutions proposed in this document, let us
   first revisit Section 6.6 of [RFC4656].  As we explain below, the
   reasons originally listed therein may not apply in many cases today.

   RFC 4656 opts against using IPsec and instead favors the use of "a
   simple cryptographic protocol (based on a block cipher in CBC mode)".
   The first argument justifying this decision in [RFC4656] is that
   partial authentication in OWAMP authentication mode is not possible
   with IPsec.  IPsec indeed cannot authenticate only a part of a
   packet.  However, in an environment where IPsec is already deployed
   and actively used, partial authentication for OWAMP contradicts the
   operational reasons dictating the use of IPsec.  It also increases
   the operational complexity of OWAMP (and TWAMP) in networks where
   IPsec is actively used and may in practice limit its applicability.

   The second argument made is the need to keep separate deployment
   paths between OWAMP and IPsec.  In several currently deployed types
   of networks IPsec is widely used to protect the data and signaling
   planes.  For example, in mobile telecommunication networks, the
   deployment rate of IPsec exceeds 95% with respect to the LTE serving
   network.  In older technology cellular networks, such as UMTS and
   GSM, IPsec use penetration is lower, but still quite significant.
   Additionally, there is a great number of IPSec-based VPN applications
   which are widely used in business applications to provide end-to-end
   security over untrusted IEEE 802.11 wireless LANs.  At the same time,
   many IETF-standardized protocols make use of IPsec/IKE, including
   MIPv4/v6, HIP, SCTP, BGP, NAT and SIP, just to name a few.

   The third argument in [RFC4656] is that, effectively, the adoption of
   IPsec in OWAMP may be problematic for "lightweight embedded devices".
   However, since the publication of RFC 4656, a large number of
   limited-resource and low-cost hardware, such as Ethernet switches,
   DSL modems, and other such devices come with support for IPsec "out
   of the box".  Therefore concerns about implementation, although
   likely valid a decade ago, are not well founded today.

   Finally, everyday use of IPsec applications by field technicians and
   good understanding of the IPsec API by many programmers should no
   longer be a reason for concern.  On the contrary: By now, IPsec open
   source code is available for anyone who wants to use it.  Therefore,
   although IPsec does need a certain level of expertise to deal with
   it, in practice, most competent technical personnel and programmers
   have no problems using it on a daily basis.

   OWAMP and TWAMP actually consist of two inter-related protocols: O/
   TWAMP-Control and O/TWAMP-Test.  With respect to TWAMP, since "TWAMP



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   and OWAMP use the same protocol for establishment of Control and Test
   procedures" [RFC5357] (Section 6), IPsec is also not considered.  O/
   TWAMP-Control is used to initiate, start, and stop test sessions and
   to fetch their results, whereas O/TWAMP-Test is used to exchange test
   packets between two measurement nodes.

   In the remainder of this section we review security for O/TWAMP-
   Control and O/TWAMP-Test separately and then make the case for using
   them over IPsec.

3.1.  O/TWAMP-Control Security

   O/TWAMP uses a simple cryptographic protocol which relies on

   o  AES in Cipher Block Chaining (AES-CBC) for confidentiality

   o  HMAC-SHA1 truncated to 128 bits for message authentication

   Three modes of operation are supported: unauthenticated,
   authenticated, and encrypted.  The authenticated and encrypted modes
   require that endpoints possess a shared secret, typically a
   passphrase.  The secret key is derived from the passphrase using a
   password-based key derivation function PBKDF2 (PKCS#5) [RFC2898].

   In the unauthenticated mode, the security parameters are left unused.
   In the authenticated and encrypted modes, security parameters are
   negotiated during the control connection establishment.  In short,
   the client opens a TCP connection to the server in order to be able
   to send OWAMP-Control commands.  The server responds with a server
   greeting, which contains the Challenge, Mode, Salt and Count.  If the
   client-requested mode is available, the client responds with a Set-
   Up-Response message, wherein the KeyID, Token and Client IV are
   included.  The Token is the concatenation of a 16-octet challenge, a
   16-octet AES Session-key used for encryption, and a 32-octet HMAC-
   SHA1 Session-key used for authentication.  The Token is encrypted
   using AES-CBC.

   Encryption uses a key derived from the shared secret associated with
   KeyID.  In the authenticated and encrypted modes, all further
   communication is encrypted using the AES Session-key and
   authenticated with the HMAC Session-key.  The client encrypts
   everything it transmits through the just-established O/TWAMP-Control
   connection using stream encryption with Client-IV as the IV.
   Correspondingly, the server encrypts its side of the connection using
   Server-IV as the IV.  The IVs themselves are transmitted in
   cleartext.  Encryption starts with the block immediately following
   that containing the IV.




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   The AES Session-key and HMAC Session-key are generated randomly by
   the client.  The HMAC Session-key is communicated along with the AES
   Session-key during O/TWAMP-Control connection setup.  The HMAC
   Session-key is derived independently of the AES Session-key.

3.2.  O/TWAMP-Test Security

   The O/TWAMP-Test protocol runs over UDP, using the sender and
   receiver IP and port numbers that were negotiated during the Request-
   Session exchange.  O/TWAMP-Test has the same three modes as with O/
   TWAMP-Control (unauthenticated, authenticated, and encrypted) and all
   O/TWAMP-Test sessions inherit the corresponding O/TWAMP-Control
   session mode.

   The O/TWAMP-Test packet format is the same in authenticated and
   encrypted modes.  The encryption and authentication operations are,
   however, different.  Similarly with the respective O/TWAMP-Control
   session, each O/TWAMP-Test session has two keys: an AES Session-key
   and an HMAC Session-key.  However, there is a difference in how the
   keys are obtained:

   O/TWAMP-Control:  the keys are generated by the client and
           communicated (as part of the Token) during connection
           establishment with the Set-Up-Response message.

   O/TWAMP-Test:  the keys are derived from the O/TWAMP-Control keys and
           the session identifier (SID), which serve as inputs of the
           key derivation function (KDF).  The O/TWAMP-Test AES Session-
           key is generated using the O/TWAMP-Control AES Session-key,
           with the 16-octet session identifier (SID), for encrypting
           and decrypting the packets of the particular O/TWAMP-Test
           session.  The O/TWAMP-Test HMAC Session-key is generated
           using the O/TWAMP-Control HMAC Session-key, with the 16-octet
           session identifier (SID), for authenticating the packets of
           the particular O/TWAMP-Test session.

3.3.  O/TWAMP Security Root

   As discussed above, the AES Session-key and HMAC Session-key used in
   the O/TWAMP-Test protocol are derived from the AES Session-key and
   HMAC Session-key which are used in O/TWAMP-Control protocol.  The AES
   Session-key and HMAC Session-key used in the O/TWAMP-Control protocol
   are generated randomly by the client, and encrypted with the shared
   secret associated with KeyID.  Therefore, the security root is the
   shared secret key.  Thus, key provision and management may become
   overly complicated.  Comparatively, a certificate-based approach
   using IKEv2/IPsec can automatically manage the security root and
   solve this problem, as we explain in Section 4.



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3.4.  O/TWAMP and IPsec

   According to RFC 4656 the "deployment paths of IPsec and OWAMP could
   be separate if OWAMP does not depend on IPsec."  However, the problem
   that arises in practice is that the security mechanism of O/TWAMP and
   IPsec cannot coexist at the same time without adding overhead or
   increasing complexity.

   IPsec provides confidentiality and data integrity to IP datagrams.
   Distinct protocols are provided: Authentication Header (AH),
   Encapsulating Security Payload (ESP) and Internet Key Exchange (IKE
   v1/v2).  AH provides only integrity protection, while ESP can also
   provide encryption.  IKE is used for dynamical key negotiation and
   automatic key management.

   When sender and receiver implement O/TWAMP over IPsec, they need to
   agree on a shared secret key during the IPsec tunnel establishment.
   Subsequently, all IP packets sent by the sender are protected.  If
   the AH protocol is used, IP packets are transmitted in plaintext.
   The authentication part covers the entire packet.  So all test
   information, such as UDP port number, and the test results will be
   visible to any attacker, which can intercept these test packets, and
   introduce errors or forge packets that may be injected during the
   transmission.  In order to avoid this attack, the receiver must
   validate the integrity of these packets with the negotiated secret
   key.  If ESP is used, IP packets are encrypted, and hence only the
   receiver can use the IPsec secret key to decrypt the IP packet, and
   obtain the test data in order to assess the IP network performance
   based on the measurements.  Both the sender and receiver must support
   IPsec to generate the security secret key of IPsec.

   Currently, after the test packets are received by the receiver, it
   cannot execute active measurement over IPsec.  That is because the
   receiver knows only the shared secret key but not the IPsec key,
   while the test packets are protected by the IPsec key ultimately.
   Therefore, it needs to be considered how to measure IP network
   performance in an IPsec tunnel with O/TWAMP.  Without this
   functionality, the use of OWAMP and TWAMP over IPsec is hindered.

   Of course, backward compatibility should be considered as well.  That
   is, the intrinsic security method based on shared key as specified in
   the O/TWAMP standards can also still be suitable for other network
   settings.  There should be no impact on the current security
   mechanisms defined in O/TWAMP for other use cases.  This document
   describes possible solutions to this problem which take advantage of
   the secret key derived by IPsec, in order to provision the key needed
   for active network measurements based on RFC 4656 and RFC 5357.




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4.  O/TWAMP for IPsec Networks

   This section presents a method of binding O/TWAMP and IKEv2 for
   network measurements between a sender and a receiver which both
   support IPsec.  In short, the shared key used for securing O/TWAMP
   traffic is derived using IKEv2 [RFC5996].

4.1.  Shared Key Derivation

   If the AH protocol is used, the IP packets are transmitted in
   plaintext, but all O/TWAMP traffic is integrity-protected by IPsec.
   Therefore, even if the peers choose to opt for the unauthenticated
   mode, IPsec integrity protection is extended to O/TWAMP.

   In the authenticated and encrypted modes, the shared secret can be
   derived from the IKEv2 Security Association (SA), or IPsec SA.  If
   the shared secret key is derived from the IKEv2 SA, SKEYSEED must be
   generated firstly.

   SKEYSEED and its derivatives are computed as per [RFC5996], where prf
   is a pseudorandom function:

      SKEYSEED = prf( Ni | Nr, g^ir )

   Ni and Nr are nonces negotiated during the initial exchange.  g^ir is
   the shared secret from the ephemeral Diffie-Hellman exchange and is
   represented as a string of octets.  Note that this SKEYSEED can be
   used as the O/TWAMP shared secret key directly.

   Alternatively, the shared secret key can be generated as follows:

      Shared secret key = PRF{ SKEYSEED, Session ID }

   wherein the Session ID is the O/TWAMP-Test SID.

   If the shared secret key is derived from the IPsec SA, the shared
   secret key can be equal to KEYMAT, wherein

      KEYMAT = prf+( SK_d, Ni | Nr )

   The term "prf+" stands for a function that outputs a pseudorandom
   stream based on the inputs to a prf, while SK_d is defined in
   [RFC5996] (Sections 2.13 and 1.2, respectively).  The shared secret
   key can alternatively be generated as follows:

      Shared secret key = PRF{ KEYMAT, Session ID }

   wherein the session ID is is the O/TWAMP-Test SID.



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   If rekeying for the IKE SA and IPsec SA occurs, the corresponding key
   of the SA is updated.  Generally, ESP and AH SAs always exist in
   pairs, with one SA in each direction.  If the SA is deleted, the key
   generated from the IKE SA or IPsec SA should also be updated.

   As discussed above, a binding association between the key generated
   from IPsec and the O/TWAMP shared secret key needs to be considered.
   The Security Association can be identified by the Security Parameter
   Index (SPI) and protocol uniquely for a given sender and receiver
   pair.  So these parameters should be agreed upon during the
   initiation of O/TWAMP.  At the stage that the sender and receiver
   negotiate the integrity key, the IPsec protocol and SPI SHOULD be
   checked.  Only if the two parameters are matched with the IPsec
   information, should the O/TWAMP connection be established.

   The SPI and protocol type are included in the Server Greeting of the
   O/TWAMP-Control protocol (Figure 1).  After the client receives the
   greeting, it MUST close the connection if it receives a greeting with
   an erroneous SPI and protocol value (Figure 2).  Otherwise, the
   client SHOULD respond with the following Set-Up-Response message and
   generates the shared secret key.

   +--------+                  +--------+
   | Client |                  | Server |
   +--------+                  +--------+
       |                           |
       |<---- TCP Connection ----->|
       |                           |
       |<---- Greeting message ----|
       |                           |
       |----- Set-Up-Response ---->|
       |                           |
       |<---- Server-Start --------|
       |                           |

                  Figure 1: Initiation of O/TWAMP-Control

   When using ESP, all IP packets are encrypted, and therefore only the
   receiver can use the IPsec key to decrypt the IP active measurement
   packets.  In this case, the IPsec tunnel between the sender and
   receiver provides additional security: even if the peers choose the
   unauthenticated mode, IPsec encryption and integrity protection is
   provided to O/TWAMP.  If the sender and receiver decide to use the
   authenticated or encrypted mode, the shared secret can also be
   derived from IKE SA or IPsec SA.  The method for key generation and
   binding association is the same discussed above for the AH protocol
   mode.




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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Protocol                            |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SPIi                                |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SPIr                                |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Mode                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Challenge (16 octets)                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        Salt (16 octets)                       |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Count (4 octets)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        MBZ (12 octets)                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: Server Greeting format

   There is an encryption-only configuration in ESP, though this is not
   recommended due to its limitations.  Since it does not produce
   integrity key in this case, either encryption-only ESP should be
   prohibited for O/TWAMP, or a decryption failure should be
   distinguished due to possible integrity attack.

4.2.  Optimizations

   The previous subsection described a method for deriving the shared
   key for O/TWAMP by capitalizing on IPsec.  We note, however, that the
   O/TWAMP protocol uses distinct encryption and integrity keys for O/
   TWAMP-Control and O/TWAMP-Test.  Consequently, four keys are
   generated to protect O/TWAMP-Control and O/TWAMP-Test messages.








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   In fact, once IPsec is employed, one key for encryption and another
   for authentication is sufficient for both the Control and Test
   protocols.  Therefore, in an IPsec environment we can reduce the
   operational complexity of O/TWAMP protocols in a straightforward
   manner, as discussed below.

   EDITOR'S NOTE:
         We expect that both optimization alternatives will be discussed
         in the IPPM working group and we are looking forward to
         community comments and feedback.

4.2.1.  Alternative 1

   If an IPsec SA is established between the server and the client, or
   both server and client support IPsec, the root key for O/TWAMP-based
   active network measurements can be derived from the IKE or IPsec SA.

   If the root key that will be used in O/TWAMP network performance
   measurements is derived from the IKE SA, SKEYSEED must be generated
   first.  SKEYSEED and its derivatives are computed as per [RFC5996].
   SKEYSEED can be used as the root key of O/TWAMP directly; then the
   root key of O/TWAMP is equal to SKEYSEED.

   If the root key of O/TWAMP is derived from the IPsec SA, the shared
   secret key can be equal to KEYMAT.  KEYMAT and its derivatives are
   computed as per usual [RFC5996].  Then, the session keys for
   encryption and authentication can be derived from the root key of O/
   TWAMP, wherein:

   Session key for enc = PRF{ root key of O/TWAMP, "O/TWAMP enc" }

   Session key for auth = PRF{ root key of O/TWAMP, "O/TWAMP auth" }

   The former can provide encryption protection for O/TWAMP-Control and
   O/TWAMP-Test messages, while the latter can provide integrity
   protection.

   Note that there are cases where rekeying the IKE SA and IPsec SA is
   necessary, and after which the corresponding key of SA is updated.
   If the SA is deleted, the O/TWAMP shared key generated from the IKE
   SA or IPsec SA should also be updated.

   In this optimization, the O/TWAMP-Control message exchange flow
   remains as per Figure 1.  However, the optimized Server Greeting
   (Figure 3) can do without the Salt and Count parameters (cf. Figure
   2) since the root key of O/TWAMP is derived from IKE SA or IPsec SA.
   O/TWAMP security can rely on IPsec and the SPI can uniquely identify
   the IPsec SA from which the root key was derived from.



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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Protocol                            |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SPIi                                |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SPIr                                |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Mode                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Challenge (16 octets)                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        MBZ (12 octets)                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Optimized Server Greeting format

   The format of the Set-Up-Response is illustrated in Figure 4.  The
   Token carried in the Set-Up-Response is calculated as follows:

      Token = Enc_root-key( Challenge )

   where Challenge is the value received earlier in the Server Greeting
   (Figure 3)

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Mode                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Token (16 octets)                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    Client-IV (12 octets)                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 4: Set-Up-Response in Alternative 1




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   If the server authenticates the token successfully, then the O/TWAMP-
   Control message exchange flow can continue.

4.2.2.  Alternative 2

   Another way for optimizing the shared key use is to set the O/TWAMP
   session keys equal to the keys of the IPsec SA directly, i.e:

   Session key for enc = encryption key of the IPsec SA

   Session key for auth = integrity key of the IPsec SA

   The former session key can provide encryption protection for O/TWAMP-
   Control and O/TWAMP-Test messages, while the latter can provide
   integrity protection.  The point made in the previous subsection
   about rekeying the IPsec SA applies here too.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Protocol                            |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SPIi                                |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SPIr                                |
   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Mode                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                        MBZ (12 octets)                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 5: Optimized Server Greeting format

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Mode                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    Client-IV (12 octets)                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 6: Set-Up-Response in Alternative 2





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   The O/TWAMP control message exchange flow is the same (Figure 1),
   while the Server Greeting format is illustrated in Figure 5.  The
   Salt, Count and Challenge parameters can be eliminated since the
   session keys of O/TWAMP are equal to keys of an IPsec SA directly.
   SPI can identify the IPsec SA where the session keys derived from.
   The Set-Up-Response is illustrated in Figure 6.

5.  Security Considerations

   As the shared secret key is derived from IPsec, the key derivation
   algorithm strength and limitations are as per [RFC5996].  The
   strength of a key derived from a Diffie-Hellman exchange using any of
   the groups defined here depends on the inherent strength of the
   group, the size of the exponent used, and the entropy provided by the
   random number generator employed.  The strength of all keys and
   implementation vulnerabilities, particularly Denial of Service (DoS)
   attacks are as defined in [RFC5996].

   EDITOR'S NOTE:
         The IPPM community may want to revisit the arguments listed in
         [RFC4656], Sec. 6.6.  Other widely-used Internet security
         mechanisms, such as TLS and DTLS, may also be considered for
         future use over and above of what is already specified in
         [RFC4656] [RFC5357].

6.  IANA Considerations

   IANA may need to allocate additional values for the options presented
   in this document.  The values of the protocol field needed to be
   assigned from the numbering space.

7.  Acknowledgments

   Emily Bi contributed to an earlier version of this document.

   We thank Eric Chen and Yakov Stein for their comments on this draft,
   and Al Morton for the discussion on related earlier work in IPPM WG.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, September 2006.



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   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, October 2008.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
              5996, September 2010.

8.2.  Informative References

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898, September 2000.

Authors' Addresses

   Kostas Pentikousis (editor)
   Huawei Technologies
   Carnotstrasse 4
   10587 Berlin
   Germany

   Email: k.pentikousis@huawei.com


   Yang Cui
   Huawei Technologies
   Huawei Building, Q20, No.156, Rd. BeiQing
   Haidian District , Beijing   100095
   P. R. China

   Email: cuiyang@huawei.com


   Emma Zhang
   Huawei Technologies
   Huawei Building, Q20, No.156, Rd. BeiQing
   Haidian District , Beijing   100095
   P. R. China

   Email: emma.zhanglijia@huawei.com











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