Network Working Group J. Arkko Internet-Draft V. Lehtovirta Obsoletes: 3310,4169 (if approved) Ericsson Intended status: Informational March 15, 2021 Expires: September 16, 2021 Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA) Version-2 draft-rfc4169-algupdate-00-pre4 Abstract HTTP Digest, as specified in RFC 7616, is known to be vulnerable to man-in-the-middle attacks if the client fails to authenticate the server in TLS, or if the same passwords are used for authentication in some other context without TLS. This is a general problem that exists not just with HTTP Digest, but also with other IETF protocols that use tunneled authentication. This document specifies version 2 of the HTTP Digest AKA algorithm. This algorithm can be implemented in a way that it is resistant to the man-in-the-middle attack, and uses updated hash algorithms. This document obsoletes both HTTP Digest AKAv1 (RFC 3110) and AKAv2 (RFC 4169). 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 16, 2021. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Arkko & Lehtovirta Expires September 16, 2021 [Page 1] Internet-Draft HTTP Digest AKAv2 March 2021 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 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. HTTP Digest AKAv2 . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Password Generation . . . . . . . . . . . . . . . . . . . 6 2.2. Session Keys . . . . . . . . . . . . . . . . . . . . . . 7 3. Example Digest AKAv2 Operation . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 4.1. Multiple Authentication Schemes and Algorithms . . . . . 8 4.2. Session Protection . . . . . . . . . . . . . . . . . . . 9 4.3. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 9 4.4. Entropy . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 5.1. Registration Information . . . . . . . . . . . . . . . . 11 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.1. Normative References . . . . . . . . . . . . . . . . . . 12 6.2. Informative References . . . . . . . . . . . . . . . . . 13 Appendix A. Changes from RFC 3310 and RFC 4169 . . . . . . . . 13 Appendix B. Contributors . . . . . . . . . . . . . . . . . . . . 14 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction The Hypertext Transfer Protocol (HTTP) Digest Authentication, described in [RFC7616], has been extended in [RFC3310] to support the Authentication and Key Agreement (AKA) mechanism [TS-3GPP.33.102]. The AKA mechanism performs authentication and session key agreement in Universal Mobile Telecommunications System (UMTS) networks. HTTP Digest AKA enables the usage of AKA as a one-time password generation mechanism for Digest authentication. HTTP Digest is known to be vulnerable to man-in-the-middle attacks, even when run inside TLS, if the same HTTP Digest authentication credentials are used in some other context without TLS. The attacker may initiate a TLS session with a server, and when the server challenges the attacker with HTTP Digest, the attacker masquerades Arkko & Lehtovirta Expires September 16, 2021 [Page 2] Internet-Draft HTTP Digest AKAv2 March 2021 the server to the victim. If the victim responds to the challenge, the attacker is able to use this response towards the server in HTTP Digest. Note that this attack is an instance of a general attack that affects a number of IETF protocols, such as PIC. The general problem is discussed in [Asokan] and [Puthenkulam]. Because of the vulnerability described above, the use of HTTP Digest "AKAv1" should be limited to the situations in which the client is able to demonstrate that, in addition to the AKA response, it possesses the AKA session keys. This is possible, for example, if the underlying security protocol uses the AKA-generated session keys to protect the authentication response. This is the case, for example, in the 3GPP IP Multimedia Core Network Subsystem (IMS), where HTTP Digest "AKAv1" is currently applied. However, HTTP Digest "AKAv1" should not be used with tunnelled security protocols that do not utilize the AKA session keys. For example, the use of HTTP Digest "AKAv1" is not necessarily secure with TLS if the server side is authenticated using certificates and the client side is authenticated using HTTP Digest AKA. There are at least four potential solutions to the problem: 1. The use of the authentication credentials is limited to one application only. In general, this approach is good and can be recommended from the security point of view. However, this will increase the total number of authentication credentials for an end-user, and may cause scalability problems in the server side. 2. The keys used in the underlying security protocols are somehow bound to the keys used in the tunneled authentication protocol. However, this would cause problems with the current implementations of underlying security protocols. For example, it is not possible to use the session keys from TLS at the application layer. Furthermore, this solution would only solve the problem when HTTP Digest is used over one hop, and would leave the problem of using HTTP Digest via multiple hops (e.g., via proxy servers) unsolved. 3. Authentication credentials are used in a cryptographically different way for each media and/or access network. However, it may be difficult to know which underlying media is used below the application. 4. Authentication credentials are used in a cryptographically different way for each application. This document specifies a new algorithm version for HTTP Digest AKA (i.e., "AKAv2"). "AKAv2" specifies a cryptographically different way Arkko & Lehtovirta Expires September 16, 2021 [Page 3] Internet-Draft HTTP Digest AKAv2 March 2021 to use AKA credentials in use cases that are based on either HTTP Digest authentication or UMTS authentication (cf. approach 4 above). The only difference to "AKAv1" is that, in addition to an AKA response RES, the AKA related session keys, IK and CK, are also used as the password for HTTP Digest. AKAv2 is immune to the man-in-the- middle attack described above. However, if AKAv2 is used in some environment, both with and without some underlying security, such as TLS, the problem still exists. New HTTP Digest AKA algorithm versions can be registered with IANA, based on Expert Review. Documentation of new algorithm versions is not mandated as RFCs. However, "AKAv2" is documented as an RFC because the use of different AKA algorithm versions includes security implications of which the implementors should be aware. The extension version and security implications are presented in this document. This new version of this specification incorporates the definition of the use of AKAv2 with the SHA-256 algorithm, aligning industry usage between IETF and 3GPP [TS-3GPP.33.203]. This version of the HTTP Digest AKAv2 specification obsoletes both the earlier AKAv2 specification (RFC 4169) as well as the AKAv1 specification (RFC 3110). Those existing RFCs continue to be usable by implementations that already conform to the existing RFCs, and other standards may still refer to those older RFCs. However, adopting this memo over RFC 3310 and RFC 4169 provides better security. TODO: Decide whether we should update 3310 algorithm separately (giving some further life to 3310), just provide this specififation as an "update" not replacement, or declare that this is the new specification and obseleting the old ones (as is done in the -pre3 version). 1.1. Terminology This chapter explains the terminology used in this document. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. AKA Authentication and Key Agreement. Arkko & Lehtovirta Expires September 16, 2021 [Page 4] Internet-Draft HTTP Digest AKAv2 March 2021 AKA is a challenge-response based mechanism that uses symmetric cryptography. AKA can be run in a UMTS IM Services Identity Module (ISIM) or in UMTS Subscriber Identity Module (USIM), which reside on a smart-card-like device that also provides tamper resistant storage of shared secrets. CK Cipher Key. An AKA session key for encryption. CK' Cipher Key. HTTP Digest AKAv2 session key for encryption. CK' is derived from CK using a pseudo-random function. IK Integrity Key. An AKA session key for integrity check. IK' Integrity Key. HTTP Digest AKAv2 session key for integrity check. IK' is derived from IK using a pseudo-random function. ISIM IP Multimedia Services Identity Module. Sometimes ISIM is implemented using USIM. RES Authentication Response. Generated by the ISIM. PRF Pseudo-random function that is used to construct the AKAv2 password and related session keys IK' and CK'. In this document, PRF is presented in the format KD(secret, data), denoting a keyed digest algorithm (KD) performed to the data ("data") with the secret ("secret"). SIM Subscriber Identity Module. GSM counter part for ISIM and USIM. UMTS Universal Mobile Telecommunications System. Arkko & Lehtovirta Expires September 16, 2021 [Page 5] Internet-Draft HTTP Digest AKAv2 March 2021 USIM UMTS Subscriber Identity Module. UMTS counter part for ISIM and SIM. XRES Expected Authentication Response. In a successful authentication, this is equal to RES. 2. HTTP Digest AKAv2 In general, the Digest AKAv2 operation is identical to the Digest AKAv1 operation described in [RFC3310]. This chapter specifies the parts in which Digest AKAv2 is different from Digest AKAv1 operation. The notation used in the Augmented BNF definitions for the new and modified syntax elements in this section is as used in SIP [RFC3261], and any elements not defined in this section are as defined in [RFC3310]. In order to direct the client into using AKAv2 for authentication instead of other AKA versions or other HTTP Digest algorithms, the AKA version directive of [RFC3310] shall have the following new value: aka-version = "AKAv2" The AKA version directive is used as a part of the algorithm field as defined in [RFC3310]. Example: algorithm=AKAv2-MD5 Another example: algorithm=AKAv2-SHA-256 2.1. Password Generation The client shall use base64 encoded [RFC2045] parameters PRF(RES||IK||CK, "http-digest-akav2-password") as a "password" when calculating the HTTP Digest response directive for AKAv2. The server shall use base64 encoded [RFC2045] parameters PRF(XRES||IK||CK, "http-digest-akav2-password") as a "password" when checking the HTTP Digest response or when calculating the "response- auth" of the "Authentication-Info" header. The pseudo-random function (PRF) used to construct the HTTP Digest password is equal to HMAC [RFC2104] using the hash algorithm that is used in producing the digest and the checksum. For example, if the Arkko & Lehtovirta Expires September 16, 2021 [Page 6] Internet-Draft HTTP Digest AKAv2 March 2021 algorithm is AKAv2-MD5, then the PRF is HMAC_MD5 [RFC1321] [RFC2104]. If the algorithm is AKAv2-SHA-256, then the PRF is HMAC_SHA256 [FIPS.180-4] [RFC2104]. Support for AKAv2-SHA-256 is REQUIRED for servers implementing 3GPP Release 17 or above specifications. Support for AKAv2-SHA-256 is RECOMMENDED for all clients, and REQUIRED for Release 17 clients. The string "http-digest-akav2-password" included in the key derivation is case sensitive. 2.2. Session Keys Even though the HTTP Digest AKA framework does not specify the use of the session keys IK and CK for confidentiality and integrity protection, the keys may be used for creating additional security within HTTP authentication or some other security mechanism. However, the original session keys IK and CK MUST NOT be directly re- used for such additional security in "AKAv2". Instead, session keys IK' and CK' are derived from the original keys IK and CK in the following way: IK' = PRF(IK, "http-digest-akav2-integritykey") CK' = PRF(CK, "http-digest-akav2-cipherkey") Any application using the HTTP authentication framework is allowed to use these masked session keys. The unmasked session keys MAY also be re-used in some other context if application-specific strings other than "http-digest-akav2-integritykey" or "http-digest- akav2-cipherkey" are used to mask the original session keys. The pseudo-random function (PRF) used to construct the HTTP Digest session keys is equal to HMAC [RFC2104] using the hash algorithm that is used in producing the digest and the checksum. For example, if the algorithm is AKAv2-MD5, then the PRF is HMAC_MD5. If the algorithm is AKAv2-SHA-256, then the PRF is HMAC_SHA256. The algorithms MUST be used in the HMAC format, as defined in [RFC2104]. Support for AKAv2-SHA-256 is again REQUIRED. The strings "http-digest-akav2-integritykey" and "http-digest-akav2- cipherkey" included in the key derivation are case sensitive. 3. Example Digest AKAv2 Operation This document does not introduce any changes to the operations of HTTP Digest or HTTP Digest AKA. Examples defined in [RFC3310] apply directly to AKAv2 with the following two exceptions: Arkko & Lehtovirta Expires September 16, 2021 [Page 7] Internet-Draft HTTP Digest AKAv2 March 2021 1. The algorithm directive has a prefix "AKAv2" instead of "AKAv1". 2. The HTTP Digest password is derived from base64 encoded PRF(RES|| IK||CK, "http-digest-akav2-password") or PRF(XRES||IK||CK, "http- digest-akav2-password") instead of (RES) or (XRES) respectively. 3. The optional session keys are derived from PRF(IK, "http-digest- akav2-integritykey") and PRF(CK, "http-digest-akav2-cipherkey") instead of IK and CK respectively. Note that the password in "AKAv1" is in binary format. The "AKAv2" password is base64 encoded [RFC2045]. 4. Security Considerations This specification provides two specific improvements over RFC 3310 and and one over RFC 4169: o Algorithm update, to prevent vulnerabilities associated with weaker hash algorithms. This is a change over both RFC 3310 and RFC 4169. o Key calculation update, to prevent vulnerabilities associated with man-in-the-middle attacks. This is a change over RFC 3310, while RFC 4169 already had protection for this. The next subsections discuss specific details of the properties of this specification. 4.1. Multiple Authentication Schemes and Algorithms The rules for a user agent for choosing among multiple authentication schemes and algorithms are as defined in [RFC3310], except that the user agent MUST choose "AKAv2" if both "AKAv1" and "AKAv2" are present. Similarly, the user agent MUST choose AKAv2-SHA-256 if both AKAv2-MD5 and AKAv2-SHA-256 is present and AKAv2-SHA-256 is supported by the user agent. TODO: Figure out if the negotiation actually works in the details, and in the HTTP protocol exchanges. Since HTTP Digest is known to be vulnerable for bidding-down attacks in environments where multiple authentication schemes and/or algorithms are used, the system implementors should pay special attention to scenarios in which both "AKAv1" and "AKAv2" are used. The use of both AKA algorithm versions should be avoided, especially if the AKA generated sessions keys or some other additional security Arkko & Lehtovirta Expires September 16, 2021 [Page 8] Internet-Draft HTTP Digest AKAv2 March 2021 measures to authenticate the clients (e.g., client certificates) are not used. 4.2. Session Protection Even though "AKAv2" uses the additional integrity (IK) and confidentiality (CK) keys as a part of the HTTP Digest AKA password, these session keys may still be used for creating additional security within HTTP authentication or some other security mechanism. This recommendation is based on the assumption that algorithms used in HTTP Digest, such as MD5, are sufficiently strong one-way functions, and, consequently, HTTP Digest responses leak no or very little computational information about IK and CK. Furthermore, the session keys are masked into IK' and CK' before they can be used for session protection. 4.3. Man-in-the-Middle Attacks Reference [Asokan] describes a "man-in-the-middle" attack related to tunnelled authentication protocols. The attack can occur in an EAP context or any similar contexts where tunnelled authentication is used and where the same authentication credentials are used without protection in some other context or the client fails to authenticate the server. For example, the use of TLS with HTTP Digest authentication (i.e., TLS for server authentication, and subsequent use of HTTP Digest for client authentication) is an instance of such scenario. HTTP challenges and responses can be fetched from and to different TLS tunnels without noticing their origin. The attack is especially easy to perform if the client fails to authenticate the server. If the same HTTP credentials are used with an unsecured connection, the attack is also easy to perform. This is how the "man-in-the-middle" attack works with HTTP Digest and TLS if the victim (i.e., the client) fails to authenticate the server: 1. The victim contacts the attacker using TLS. If the attacker has a valid server certificate, the client may continue talking to the attacker and use some HTTP authentication compatible protocol, such as the Session Initiation Protocol (SIP). 2. The attacker contacts a real proxy/server also using TLS and an HTTP-authentication-compatible protocol. The proxy/server responds to the attacker with the HTTP Authentication challenge. Arkko & Lehtovirta Expires September 16, 2021 [Page 9] Internet-Draft HTTP Digest AKAv2 March 2021 3. The attacker forwards the HTTP Authentication challenge from the proxy/server to the victim. If the victim is not careful, and does not check whether the identity in the server certificate in TLS matches the realm in the HTTP authentication challenge, it may send a new request that carries a valid response to the HTTP Authentication challenge. 4. The attacker may use the response with the victims HTTP Digest username and password to authenticate itself to the proxy/server. The man-in-the-middle attack is not possible if the client compares the identities in the TLS server certificate and the HTTP Digest authentication challenge. Note that with HTTP Basic, the client would send the password to the attacker. Another variant of the "man-in-the-middle" attack is the so-called "interleaving attack". This attack is possible if the HTTP Digest authentication credentials are used in several contexts, and in one of them without protection. This is how the attack could proceed: 1. The attacker establishes a TLS tunnel to the proxy/server using one-way server authentication. The attacker sends a request to the proxy/server. 2. The proxy/server challenges the attacker with the HTTP Digest challenge. 3. The attacker challenges the victim in some other context using the challenge carried in the HTTP Digest challenge. The HTTP Digest challenge needs to be modified to the format used in the protocol of this other context. 4. The victim responds with a response. 5. The attacker uses the response from the other context for authentication in HTTP Digest. 6. The proxy/server accepts the response, and delivers the service to the attacker. In some circumstances, HTTP Digest AKAv1 may be vulnerable for the interleaving attack. In particular, if ISIM is implemented using USIM, the HTTP Digest AKAv1 should not be used with tunneled security protocols unless the AKA-related session keys, IK and CK, are somehow used with the solution. Arkko & Lehtovirta Expires September 16, 2021 [Page 10] Internet-Draft HTTP Digest AKAv2 March 2021 HTTP Digest AKAv2 is not vulnerable to this interleaving attack, and it can be used with tunneled security protocols without using the related AKA session keys. 4.4. Entropy AKAv1 passwords should only be used as one-time passwords if the entropy of the used RES value is limited (e.g., only 32 bits). For this reason, the re-use of the same RES value in authenticating subsequent requests and responses is not recommended. Furthermore, algorithms such as "MD5-sess", which limit the amount of material hashed with a single key by producing a session key for authentication, should not be used with AKAv1. Passwords generated using AKAv2 can more securely be used for authenticating subsequent requests and responses because the concatenation of AKA credentials (i.e., RES||IK||CK) makes the passwords significantly longer, and the pseudo-random function heuristically provides an entropy equal to the length of this string, or the length of the PRF output, whichever is the shortest. The user agent does not need to assume that AKAv2 passwords are limited to one-time use only, and it may try to re-use the AKAv2 passwords with the server. However, note that AKAv2 passwords cannot be re-used with the HTTP Digest AKAv2 algorithm because such an authentication challenge will automatically generate a fresh password. AKAv2 passwords can be used with other HTTP Digest algorithms, such as "MD5". The underlying AKA protocol (e.g., UMTS AKA) has been designed to keep CK and IK confidential, but will typically send RES in the clear. We note that, even if (by some unfortunate misuse of AKA) RES values were revealed, the inclusion of RES in PRF(RES||IK||CK) is still beneficial, as it makes pre-calculated dictionaries of IK||CK values rather useless (though such dictionaries are infeasible for typical sizes of IK and CK). 5. IANA Considerations This document specifies a new aka-version, "AKAv2", to the aka- version namespace maintained by IANA. The procedure for allocation of new aka-versions is defined in [RFC3310]. 5.1. Registration Information To: ietf-digest-aka@iana.org Subject: Registration of a new AKA version Arkko & Lehtovirta Expires September 16, 2021 [Page 11] Internet-Draft HTTP Digest AKAv2 March 2021 Version identifier: "AKAv2" Contacts for further information: vesa.lehtovirta@ericsson.com o jari.arkko@ericsson.com. 6. References 6.1. Normative References [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, April 1992, . [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, . [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, . [RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI 10.17487/RFC7616, September 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [FIPS.180-4] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-4, August 2015, . Arkko & Lehtovirta Expires September 16, 2021 [Page 12] Internet-Draft HTTP Digest AKAv2 March 2021 6.2. Informative References [RFC3310] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA)", RFC 3310, DOI 10.17487/RFC3310, September 2002, . [RFC4169] Torvinen, V., Arkko, J., and M. Naslund, "Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA) Version-2", RFC 4169, DOI 10.17487/RFC4169, November 2005, . [TS-3GPP.33.102] 3GPP, "3rd Generation Partnership Project, "Security Architecture (Release 4)", 3GPP Technical Specification33.102, December 2001. [TS-3GPP.33.203] 3GPP, "3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; 3G security; Access security for IP-based services (Release 15)", 3GPP Technical Specification33.203, March 2018. [Asokan] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in Tunnelled Authentication Protocols", Cryptology ePrint Archive http://eprint.iacr.org Report 2002/163, October 2002. [Puthenkulam] Puthenkulam, J., Lortz, V., Palekar, A., and D. Simon, "The Compound Authentication Binding Problem", March 2003 (Work in Progress). Appendix A. Changes from RFC 3310 and RFC 4169 This version of the specification has changed the algorithms so that support for SHA256 is now available. User agent preferences to force the selecetion of the preferred algorithm have also changed. This version of the specification also adopts the key generation that is resistant to man-in-the-middle attacks, i.e., the use of AKAv2 instead of AKAv1, thereby requesting switching AKAv1 usage to AKAv2. Other changes in this document include some minor editorial improvements, and the update of references to their most current versions. Arkko & Lehtovirta Expires September 16, 2021 [Page 13] Internet-Draft HTTP Digest AKAv2 March 2021 Appendix B. Contributors This is a new version of a specification based on [RFC3310] and [RFC4169]. Much of the text in those RFCs was provided by Mats Naslund and Vesa Torvinen, and text largely remains in this version as well. Appendix C. Acknowledgements The authors would like to thank John Mattsson, Mohit Sethi, Pinar Comak, Ferhat Karakoc, and Christer Holmberg for discussions around this problem space. Authors' Addresses Jari Arkko Ericsson Jorvas 02420 Finland Email: jari.arkko@piuha.net Vesa Lehtovirta Ericsson Jorvas 02420 Finland Email: vesa.lehtovirta@ericsson.com Arkko & Lehtovirta Expires September 16, 2021 [Page 14]