Internet Engineering Task Force (IETF)
Request for Comments: 7717
Updates: 4656, 5357
Category: Standards Track
ISSN: 2070-1721
K. Pentikousis, Ed.
E. Zhang
Y. Cui
Huawei Technologies
December 2015

IKEv2-Derived Shared Secret Key for

the One-Way Active Measurement Protocol (OWAMP) and

Two-Way Active Measurement Protocol (TWAMP)


The One-Way Active Measurement Protocol (OWAMP) and Two-Way Active Measurement Protocol (TWAMP) security mechanisms require that both the client and server endpoints possess a shared secret. This document describes the use of keys derived from an IKEv2 security association (SA) as the shared key in OWAMP or TWAMP. If the shared key can be derived from the IKEv2 SA, OWAMP or TWAMP can support certificate-based key exchange; this would allow for more operational flexibility and efficiency. The key derivation presented in this document can also facilitate automatic key management.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2015 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 ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  O/TWAMP Security  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  O/TWAMP-Control Security  . . . . . . . . . . . . . . . .   5
     4.2.  O/TWAMP-Test Security . . . . . . . . . . . . . . . . . .   6
     4.3.  O/TWAMP Security Root . . . . . . . . . . . . . . . . . .   7
   5.  O/TWAMP for IPsec Networks  . . . . . . . . . . . . . . . . .   7
     5.1.  Shared Key Derivation . . . . . . . . . . . . . . . . . .   7
     5.2.  Server Greeting Message Update  . . . . . . . . . . . . .   8
     5.3.  Set-Up-Response Update  . . . . . . . . . . . . . . . . .   9
     5.4.  O/TWAMP over an IPsec Tunnel  . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   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.

According to [RFC4656] and [RFC5357], the OWAMP and TWAMP (O/TWAMP) security mechanisms require that endpoints (i.e., both the client and the server) possess a shared secret. In today's network deployments, however, the use of pre-shared keys is far from optimal. For example, in wireless infrastructure networks, certain network elements (which can be seen as the two endpoints from an O/TWAMP perspective) support certificate-based security. For instance, consider the case in which one wants to measure IP performance between an E-UTRAN Evolved Node B (eNB) and Security Gateway (SeGW), both of which are 3GPP Long Term Evolution (LTE) nodes and support certificate mode and the Internet Key Exchange Protocol version 2 (IKEv2).

The O/TWAMP security mechanism specified in [RFC4656] and [RFC5357] supports the pre-shared key (PSK) mode only, hindering large-scale deployment of O/TWAMP: provisioning and management of "shared secrets" for massive deployments consumes a tremendous amount of effort and is prone to human error. At the same time, recent trends point to wider IKEv2 deployment that, in turn, calls for mechanisms and methods that enable tunnel end-users, as well as operators, to measure one-way and two-way network performance in a standardized manner.

With IKEv2 widely deployed, employing shared keys derived from an IKEv2 security association (SA) can be considered a viable alternative through the method described in this document. If the shared key can be derived from the IKEv2 SA, O/TWAMP can support certificate-based key exchange and practically increase operational flexibility and efficiency. The use of IKEv2 also makes it easier to extend automatic key management.

In general, O/TWAMP measurement packets can be transmitted inside the IPsec tunnel, as typical user traffic is, or transmitted outside the IPsec tunnel. This may depend on the operator's policy and the performance evaluation goal, and it is orthogonal to the mechanism described in this document. When IPsec is deployed, protecting O/TWAMP traffic in unauthenticated mode using IPsec is one option. Another option is to protect O/TWAMP traffic using the O/TWAMP security established using the PSK derived from IKEv2 and bypassing the IPsec tunnel.

Protecting unauthenticated O/TWAMP control and/or test traffic via the Authentication Header (AH) [RFC4302] or Encapsulating Security Payload (ESP) [RFC4303] cannot provide various security options, e.g., it cannot authenticate part of an O/TWAMP packet as mentioned in [RFC4656]. For measuring latency, a timestamp is carried in O/ TWAMP test traffic. The sender has to fetch the timestamp, encrypt it, and send it. When the mechanism described in this document is used, partial authentication of O/TWAMP packets is possible and therefore the middle step can be skipped, potentially improving accuracy as the sequence number can be encrypted and authenticated before the timestamp is fetched. The receiver obtains the timestamp without the need for the corresponding decryption step. In such cases, protecting O/TWAMP traffic using O/TWAMP security but bypassing the IPsec tunnel has its advantages.

This document specifies a method for enabling network measurements between a TWAMP client and a TWAMP server. In short, the shared key used for securing TWAMP traffic is derived from IKEv2 [RFC7296]. TWAMP implementations signal the use of this method by setting IKEv2Derived (see Section 7). IKEv2-derived keys SHOULD be used instead of shared secrets when O/TWAMP is employed in a deployment using IKEv2. From an operations and management perspective [RFC5706], the mechanism described in this document requires that both the TWAMP Control-Client and Server support IPsec.

The remainder of this document is organized as follows. Section 4 summarizes O/TWAMP protocol operation with respect to security. Section 5 presents the method for binding TWAMP and IKEv2 for network measurements between the client and the server that both support IKEv2. Finally, Section 6 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].

3. Scope

This document specifies a method using keys derived from an IKEv2 SA as the shared key in O/TWAMP. O/TWAMP implementations signal the use of this method by setting IKEv2Derived (see Section 7).

4. O/TWAMP Security

Security for O/TWAMP-Control and O/TWAMP-Test are briefly reviewed in the following subsections.

4.1. O/TWAMP-Control Security

O/TWAMP uses a simple cryptographic protocol that relies on

  • AES-CBC for confidentiality
  • HMAC-SHA1 truncated to 128 bits for message authentication

Three modes of operation are supported in the OWAMP-Control protocol: unauthenticated, authenticated, and encrypted. In addition to these modes, the TWAMP-Control protocol also supports a mixed mode, i.e., the TWAMP-Control protocol operates in encrypted mode while TWAMP- Test protocol operates in unauthenticated mode. The authenticated, encrypted, and mixed 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, encrypted, and mixed modes, the security parameters are negotiated during the control connection establishment.

Figure 1 illustrates the initiation stage of the O/TWAMP-Control protocol between a Control-Client and a Server. In short, the Control-Client opens a TCP connection to the Server in order to be able to send O/TWAMP-Control commands. The Server responds with a Server Greeting, which contains the Modes, Challenge, Salt, Count, and MBZ ("MUST be zero") fields (see Section 3.1 of [RFC4656]). If the Control-Client preferred mode is available, the client responds with a Set-Up-Response message, wherein the selected Mode, as well as the KeyID, Token, and Client initialization vector (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.

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

Figure 1: Initiation of O/TWAMP-Control

Encryption uses a key derived from the shared secret associated with KeyID. In the authenticated, encrypted, and mixed modes, all further communication is encrypted using the AES Session-key and authenticated with the HMAC Session-key. After receiving the Set-Up- Response, the Server responds with a Server-Start message containing the Server-IV. The Control-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.

The AES Session-key and HMAC Session-key are generated randomly by the Control-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.

4.2. O/TWAMP-Test Security

The O/TWAMP-Test protocol runs over UDP, using the Session-Sender and Session-Reflector IP and port numbers that were negotiated during the Request-Session exchange. O/TWAMP-Test has the same mode with O/ TWAMP-Control and all O/TWAMP-Test sessions inherit the corresponding O/TWAMP-Control session mode except when operating in mixed 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 Control-Client and
           communicated to the Server during the control connection
           establishment with the Set-Up-Response message (as part of
           the Token).
   O/TWAMP-Test:  the keys are derived from the O/TWAMP-Control keys and
           the session identifier (SID), which serve as inputs to 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 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 SID, for authenticating
           the packets of the particular O/TWAMP-Test session.

4.3. O/TWAMP Security Root

As discussed above, the O/TWAMP-Test AES Session-key and HMAC Session-key are derived, respectively, from the O/TWAMP-Control AES Session-key and HMAC Session-key. The AES Session-key and HMAC Session-key used in the O/TWAMP-Control protocol are generated randomly by the Control-Client, and encrypted with the shared secret associated with KeyID. Therefore, the security root is the shared secret key. Thus, for large deployments, key provision and management may become overly complicated. Comparatively, a certificate-based approach using IKEv2 can automatically manage the security root and solve this problem, as we explain in Section 5.

5. O/TWAMP for IPsec Networks

This section presents a method of binding O/TWAMP and IKEv2 for network measurements between a client and a server that both support IPsec. In short, the shared key used for securing O/TWAMP traffic is derived using IKEv2 [RFC7296].

5.1. Shared Key Derivation

In the authenticated, encrypted, and mixed modes, the shared secret key MUST be derived from the IKEv2 SA. Note that we explicitly opt to derive the shared secret key from the IKEv2 SA, rather than the child SA, since it is possible that an IKEv2 SA is created without generating any child SA [RFC6023].

When the shared secret key is derived from the IKEv2 SA, SK_d must be generated first. SK_d must be computed as per [RFC7296].

The shared secret key MUST be generated as follows:

Shared secret key = prf( SK_d, "IPPM" )

Wherein the string "IPPM" is encoded in ASCII and "prf" is a pseudorandom function.

It is recommended that the shared secret key is derived in the IPsec layer so that IPsec keying material is not exposed to the O/TWAMP client. Note, however, that the interaction between the O/TWAMP and IPsec layers is host internal and implementation specific. Therefore, this is clearly outside the scope of this document, which focuses on the interaction between the O/TWAMP client and server. That said, one possible way could be the following: at the Control- Client side, the IPsec layer can perform a lookup in the Security Association Database (SAD) using the IP address of the Server and thus match the corresponding IKEv2 SA. At the Server side, the IPsec layer can look up the corresponding IKEv2 SA by using the Security Parameter Indexes (SPIs) sent by the Control-Client (see Section 5.3), and therefore extract the shared secret key.

If both the client and server do support IKEv2 but there is no current IKEv2 SA, two alternative ways could be considered. First, the O/TWAMP Control-Client initiates the establishment of the IKEv2 SA, logs this operation, and selects the mode that supports IKEv2. Alternatively, the O/TWAMP Control-Client does not initiate the establishment of the IKEv2 SA, logs an error for operational management purposes, and proceeds with the modes defined in [RFC4656], [RFC5357], and [RFC5618]. Again, although both alternatives are feasible, they are in fact implementation specific.

If rekeying for the IKEv2 SA or deletion of the IKEv2 SA occurs, the corresponding shared secret key generated from the SA MUST continue to be used until the O/TWAMP session terminates.

5.2. Server Greeting Message Update

To trigger a binding association between the key generated from IKEv2 and the O/TWAMP shared secret key, the Modes field in the Server Greeting Message (Figure 2) must support key derivation as discussed in Section 5.1. Support for deriving the shared key from the IKEv2 SA is indicated by setting IKEv2Derived (see Section 7). Therefore, when this method is used, the Modes value extension MUST be supported.

   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
   |                                                               |
   |                       Unused (12 octets)                      |
   |                                                               |
   |                           Modes                               |
   |                                                               |
   |                     Challenge (16 octets)                     |
   |                                                               |
   |                                                               |
   |                                                               |
   |                        Salt (16 octets)                       |
   |                                                               |
   |                                                               |
   |                        Count (4 octets)                       |
   |                                                               |
   |                        MBZ (12 octets)                        |
   |                                                               |

Figure 2: Server Greeting Format

The choice of this set of Modes values poses no backwards- compatibility problems to existing O/TWAMP clients. Robust legacy Control-Client implementations would disregard the fact that the IKEv2Derived Modes bit in the Server Greeting is set. On the other hand, a Control-Client implementing this method can identify that the O/TWAMP Server contacted does not support this specification. If the Server supports other Modes, as one could assume, the Control-Client would then decide which Mode to use and indicate such accordingly as per [RFC4656] and [RFC5357]. A Control-Client that is implementing this method and decides not to employ IKEv2 derivation can simply behave as a client that is purely compatible with [RFC4656] and [RFC5357].

5.3. Set-Up-Response Update

The Set-Up-Response message Figure 3 is updated as follows. When an O/TWAMP Control-Client implementing this method receives a Server Greeting indicating support for Mode IKEv2Derived, it SHOULD reply to the O/TWAMP Server with a Set-Up-Response that indicates so. For example, a compatible O/TWAMP Control-Client choosing the authenticated mode with IKEv2 shared secret key derivation should set the Mode bits as per Section 7.

   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                               |
   |                                                               |
   |                      KeyID (80 octets)                        |
   |                                                               |
   |                                                               |
   |                                                               |
   |                     Token (16 octets)                         |
   |                                                               |
   |                                                               |
   |                                                               |
   |                    Client-IV (12 octets)                      |
   |                                                               |

Figure 3: Set-Up-Response Message

The Security Parameter Index (SPI), as described in [RFC4301] and [RFC7296], uniquely identifies the SA. If the Control-Client supports shared secret key derivation for the IKEv2 SA, it will choose the corresponding Mode value and carry SPIi and SPIr in the KeyID field. SPIi and SPIr MUST be included in the KeyID field of the Set-Up-Response Message to indicate the IKEv2 SA from which the O/TWAMP shared secret key was derived. The length of SPI is 8 octets. Therefore, the first 8 octets of the KeyID field MUST carry SPIi, and the second 8 octets MUST carry SPIr. The remaining bits of the KeyID field MUST be set to zero.

An O/TWAMP Server implementation MUST obtain the SPIi and SPIr from the first 16 octets and ignore the remaining octets of the KeyID field. Then, the Control-Client and the Server can derive the shared secret key based on the Mode value and SPI. If the O/TWAMP Server cannot find the IKEv2 SA corresponding to the SPIi and SPIr received, it MUST log the event for operational management purposes. In addition, the O/TWAMP Server SHOULD set the Accept field of the Server-Start message to the value 6 to indicate that the Server is not willing to conduct further transactions in this O/TWAMP-Control session since it cannot find the corresponding IKEv2 SA.

5.4. O/TWAMP over an IPsec Tunnel

The IPsec Authentication Header (AH) [RFC4302] and Encapsulating Security Payload (ESP) [RFC4303] provide confidentiality and data integrity to IP datagrams. An IPsec tunnel can be used to provide the protection needed for O/TWAMP Control and Test packets, even if the peers choose the unauthenticated mode of operation. In order to ensure authenticity and security, O/TWAMP packets between two IKEv2 systems SHOULD be configured to use the corresponding IPsec tunnel running over an external network even when using the O/TWAMP unauthenticated mode.

6. Security Considerations

As the shared secret key is derived from the IKEv2 SA, the key derivation algorithm strength and limitations are as per [RFC7296]. 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 [RFC7296].

7. IANA Considerations

During the production of this document, the authors and reviewers noticed that the TWAMP-Modes registry should describe a field of single bit position flags, rather than the existing registry construction with assignment of integer values. In addition, the Semantics Definition column seemed to have spurious information in it. The registry has been reformatted to simplify future assignments. Thus, the contents of the TWAMP-Modes registry are as follows:

   Bit|Description                               |Semantics   |Reference
   Pos|                                          |Definition  |
   0   Unauthenticated                            Section 3.1  [RFC4656]
   1   Authenticated                              Section 3.1  [RFC4656]
   2   Encrypted                                  Section 3.1  [RFC4656]
   3   Unauth. TEST protocol, Encrypted CONTROL   Section 3.1  [RFC5618]
   4   Individual Session Control                              [RFC5938]
   5   Reflect Octets Capability                               [RFC6038]
   6   Symmetrical Size Sender Test Packet Format              [RFC6038]

Figure 4: TWAMP Modes

The new description and registry management instructions follow.

Registry Specification: TWAMP-Modes are specified in TWAMP Server Greeting messages and Set-Up-Response messages consistent with Section 3.1 of [RFC5357]. Modes are indicated by setting single bits in the 32-bit Modes field.

Registry Management: Because the "TWAMP-Modes" are based on only 32 bit positions with each position conveying a unique feature, and because TWAMP is an IETF protocol, this registry must be updated only by "IETF Review" as specified in [RFC5226]. IANA SHOULD allocate monotonically increasing bit positions when requested.

Experimental Numbers: No experimental bit positions are currently assigned in the Modes registry, as indicated in the initial contents above.

In addition, per this document, a new entry has been added to the TWAMP-Modes registry:

   Bit|Description                               |Semantics   |Reference
   Pos|                                          |Definition  |
   7   IKEv2Derived Mode Capability               Section 5    RFC 7717

Figure 5: TWAMP IKEv2-Derived Mode Capability

For the new OWAMP-Modes registry, see the IANA Considerations in [RFC7718].

8. References

8.1. 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,
   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
   [RFC5618]  Morton, A. and K. Hedayat, "Mixed Security Mode for the
              Two-Way Active Measurement Protocol (TWAMP)", RFC 5618,
              DOI 10.17487/RFC5618, August 2009,
   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <>.
   [RFC7718]  Morton, A., "Registries for the One-Way Active Measurement
              Protocol (OWAMP)", RFC 7718, DOI 10.17487/RFC7718,
              December 2015, <>.

8.2. Informative References

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898,
              DOI 10.17487/RFC2898, September 2000,
   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.
   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
   [RFC5938]  Morton, A. and M. Chiba, "Individual Session Control
              Feature for the Two-Way Active Measurement Protocol
              (TWAMP)", RFC 5938, DOI 10.17487/RFC5938, August 2010,
   [RFC6023]  Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
              Childless Initiation of the Internet Key Exchange Version
              2 (IKEv2) Security Association (SA)", RFC 6023,
              DOI 10.17487/RFC6023, October 2010,
   [RFC6038]  Morton, A. and L. Ciavattone, "Two-Way Active Measurement
              Protocol (TWAMP) Reflect Octets and Symmetrical Size
              Features", RFC 6038, DOI 10.17487/RFC6038, October 2010,


We thank Eric Chen, Yaakov Stein, Brian Trammell, Emily Bi, John Mattsson, Steve Baillargeon, Spencer Dawkins, Tero Kivinen, Fred Baker, Meral Shirazipour, Hannes Tschofenig, Ben Campbell, Stephen Farrell, Brian Haberman, and Barry Leiba for their reviews, comments, and text suggestions.

Al Morton deserves a special mention for his thorough reviews and text contributions to this document as well as the constructive discussions over several IPPM meetings.

Authors' Addresses

Kostas Pentikousis (editor)
EUREF-Campus Haus 13
Torgauer Strasse 12-15
10829 Berlin

   Emma Zhang
   Huawei Technologies
   Huawei Building, No.3, Rd. XinXi
   Haidian District, Beijing  100095

Yang Cui
Huawei Technologies
Otemachi First Square 1-5-1 Otemachi
Chiyoda-ku, Tokyo 100-0004