SIGTRAN security: Security Considerations for Signaling Transport (SIGTRAN) Protocols

Network Working Group                                        J. Loughney
Request for Comments: 3788                         Nokia Research Center
Category: Standards Track                                 M. Tuexen, Ed.
                                      Univ. of Applied Sciences Muenster
                                                        J. Pastor-Balbas
                                                    Ericsson Espana S.A.
                                                               June 2004


                      Security Considerations for
                Signaling Transport (SIGTRAN) Protocols

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document discusses how Transport Layer Security (TLS) and IPsec
   can be used to secure communication for SIGTRAN protocols.  The main
   goal is to recommend the minimum security means that a SIGTRAN node
   must implement in order to attain secured communication.  The support
   of IPsec is mandatory for all nodes running SIGTRAN protocols.  TLS
   support is optional.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.2.  Abbreviations  . . . . . . . . . . . . . . . . . . . . .  3
   2.  Convention . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Security in Telephony Networks . . . . . . . . . . . . . . . .  4
   4.  Threats and Goals  . . . . . . . . . . . . . . . . . . . . . .  4
   5.  IPsec Usage  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  TLS Usage  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Support of IPsec and TLS . . . . . . . . . . . . . . . . . . .  8
   8.  Peer-to-Peer Considerations  . . . . . . . . . . . . . . . . .  9
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       12.1. Normative References . . . . . . . . . . . . . . . . . . 11
       12.2. Informative References . . . . . . . . . . . . . . . . . 11
   13. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
   14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

1.  Introduction

1.1.  Overview

   The SIGTRAN protocols are designed to carry signaling messages for
   telephony services.  These protocols will be used between

   o  customer premise and service provider equipment in case of ISDN
      Q.921 User Adaptation Layer (IUA) [9].

   o  service provider equipment only.  This is the case for SS7 MTP2
      User Adaptation Layer (M2UA) [12], SS7 MTP2 Peer-to-Peer User
      Adaptation Layer (M2PA) [15], SS7 MTP3 User Adaptation Layer
      (M3UA) [13] and SS7 SCCP User Adaptation Layer (SUA) [16].  The
      carriers may be different and may use other transport network
      providers.

   The security requirements for these situations may be different.

   SIGTRAN protocols involve the security needs of several parties, the
   end-users of the services, the service providers and the applications
   involved.  Additional security requirements may come from local
   regulation.  While having some overlapping security needs, any
   security solution should fulfill all of the different parties' needs.

   The SIGTRAN protocols assume that messages are secured by using
   either IPsec or TLS.

1.2.  Abbreviations

   This document uses the following abbreviations:

   ASP: Application Server Process

   CA: Certification Authority

   DOI: Domain Of Interpretation

   ESP: Encapsulating Security Payload

   FQDN: Full-Qualified Domain Names

   IPsec: IP Security Protocol

   IKE: Internet Key Exchange Protocol

   ISDN: Integrated Services Digital Network

   IUA: ISDN Q.921 User Adaptation Layer

   M2PA: SS7 MTP2 Peer-to-Peer User Adaptation Layer

   M2UA: SS7 MTP2 User Adaptation Layer

   M3UA: SS7 MTP3 User Adaptation Layer

   PKI: Public Key Infrastructure

   SA: Security Association

   SCTP: Stream Control Transmission Protocol

   SS7: Signaling System No. 7

   SUA: SS7 SCCP User Adaptation Layer

   TLS: Transport Layer Security

2.  Convention

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   [1].

3.  Security in Telephony Networks

   The security in telephony networks is mainly based on the closed
   network principle.  There are two main protocols used: Access
   protocols (ISDN and others) are used for signaling in the access
   network and the SS7 protocol stack in the core network.

   As SS7 networks are often physically remote and/or inaccessible to
   the user, it is assumed that they are protected from malicious users.
   Equipment is often under lock and key.  At network boundaries between
   SS7 networks, packet filtering is sometimes used.  End-users are not
   directly connected to SS7 networks.

   The access protocols are used for end-user signaling.  End-user
   signaling protocols are translated to SS7 based protocols by
   telephone switches run by network operators.

   Regulatory Authorities often require SS7 switches with connections to
   different SS7 switches to be conformant to national and/or
   international test specifications.

   There are no standardized ways of using encryption technologies for
   providing confidentiality or using technologies for authentication.

   This description applies to telephony networks operated by a single
   operator, and also to multiple telephony networks being connected and
   operated by different operators.

4.  Threats and Goals

   The Internet threats can be divided into one of two main types.  The
   first one is called "passive attacks".  It happens whenever the
   attacker reads packets off the network but does not write them.
   Examples of such attacks include confidentiality violations, password
   sniffing and offline cryptographic attacks amongst others.

   The second kind of threat is called "active attacks".  In this case,
   the attacker also writes data to the network.  Examples for this
   attack include replay attacks, message insertion, message deletion,
   message modification or man-in-the-middle attacks, amongst others.

   In general, Internet protocols have the following security
   objectives:

   o  Communication Security:

      *  Authentication of peers

      *  Integrity of user data transport

      *  Confidentiality of user data

      *  Replay protection

   o  Non-repudiation

   o  System Security, avoidance of:

      *  Unauthorized use

      *  Inappropriate use

      *  Denial of Service

   Communication security is mandatory in some network scenarios to
   prevent malicious attacks.  The main goal of this document is to
   recommend the minimum security means that a SIGTRAN node must
   implement in order to attain secured communication.  To achieve this
   goal, we will explore the different existing security options
   regarding communication.

   All SIGTRAN protocols use the Stream Control Transmission Protocol
   (SCTP) defined in [8] and [11] as its transport protocol.  SCTP
   provides certain transport related security features, such as
   resistance against:

   o  Blind Denial of Service Attacks such as:

      *  Flooding.

      *  Masquerade.

      *  Improper Monopolization of Services.

   There is no quick fix, one-size-fits-all solution for security.

   When a network using SIGTRAN protocols involves more than one party,
   it may not be reasonable to expect that all parties have implemented
   security in a sufficient manner.  End-to-end security should be the
   goal; therefore, it is recommended that IPsec or TLS be used to
   ensure confidentiality of user payload.  Consult [3] for more
   information on configuring IPsec services.

5.  IPsec Usage

   This section is only relevant for SIGTRAN nodes using IPsec to secure
   communication between SIGTRAN nodes.

   All SIGTRAN nodes using IPsec MUST implement IPsec ESP [4] in
   transport mode with non-null encryption and authentication algorithms
   to provide per-packet authentication, integrity protection and
   confidentiality, and MUST implement the replay protection mechanisms
   of IPsec.  In those scenarios where IP layer protection is needed,
   ESP in tunnel mode SHOULD be used.  Non-null encryption should be
   used when using IPSec ESP.

   All SIGTRAN nodes MUST support IKE for peer authentication,
   negotiation of security associations, and key management, using the
   IPsec DOI [5].  The IPsec implementations MUST support peer
   authentication using a pre-shared key, and MAY support certificate-
   based peer authentication using digital signatures.  Peer
   authentication using the public key encryption methods outlined in
   IKE's sections 5.2 and 5.3 [6] SHOULD NOT be used.

   Conformant implementations MUST support IKEs Main Mode and Aggressive
   Mode.  For transport mode, when pre-shared keys are used for
   authentication, IKE Aggressive Mode SHOULD be used, and IKE Main Mode
   SHOULD NOT be used.  When digital signatures are used for
   authentication, either IKE Main Mode or IKE Aggressive Mode MAY be
   used.  When using ESP tunnel mode, IKE Main Mode MAY be used to
   create an ISAKMP association with identity protection during Phase 1.

   When digital signatures are used to achieve authentication, an IKE
   negotiator SHOULD use IKE Certificate Request Payload(s) to specify
   the certification authority (or authorities) that is trusted in
   accordance with its local policy.  IKE negotiators SHOULD use
   pertinent certificate revocation checks before accepting a PKI
   certificate for use in IKE's authentication procedures.  See [10] for
   certificate revocation and [7] for online-checking.

   The Phase 2 Quick Mode exchanges used to negotiate protection for
   SIGTRAN sessions MUST explicitly carry the Identity Payload fields
   (IDci and IDcr).  The DOI provides for several types of
   identification data.  However, when used in conformant
   implementations, each ID Payload MUST carry a single IP address and a
   single non-zero port number, and MUST NOT use the IP Subnet or IP
   Address Range formats.  This allows the Phase 2 security association
   to correspond to specific TCP and SCTP connections.

   Since IPsec acceleration hardware may only be able to handle a
   limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
   be sent for idle SAs as a means of keeping the number of active Phase
   2 SAs to a minimum.  The receipt of an IKE Phase 2 delete message
   SHOULD NOT be interpreted as a reason for tearing down a SIGTRAN
   session.  Rather, it is preferable to leave the connection up,
   whereby another IKE Phase 2 SA will be brought up to protect it if
   additional traffic is sent.  This avoids the potential of continually
   bringing connections up and down.

   It should be noted that SCTP supports multi-homed hosts and this
   results in the need for having multiple security associations for one
   SCTP association. This disadvantage of IPsec has been addressed by
   [17]. So IPsec implementations used by SIGTRAN nodes SHOULD support
   the IPsec feature described in [17].

6.  TLS Usage

   This section is only relevant for SIGTRAN nodes using TLS to secure
   the communication between SIGTRAN nodes.

   A SIGTRAN node that initiates a SCTP association to another SIGTRAN
   node acts as a TLS client according to [2], and a SIGTRAN node that
   accepts a connection acts as a TLS server.  SIGTRAN peers
   implementing TLS for security MUST mutually authenticate as part of
   TLS session establishment.  In order to ensure mutual authentication,
   the SIGTRAN node acting as TLS server must request a certificate from
   the SIGTRAN node acting as TLS client, and the SIGTRAN node acting as
   TLS client MUST be prepared to supply a certificate on request.

   [14] requires the support of the cipher suite
   TLS_RSA_WITH_AES_128_CBC_SHA.  SIGTRAN nodes MAY negotiate other TLS
   cipher suites.

   TLS MUST be used on all bi-directional streams.  Other uni-
   directional streams MUST NOT be used.

   It should also be noted that a SCTP implementation used for TLS over
   SCTP MUST support fragmentation of user data and might also need to
   support the partial delivery API.  This holds even if all SIGTRAN
   messages are small.  Furthermore, the 'unordered delivery' feature of
   SCTP can not be used in conjunction with TLS.  See [14] for more
   details.

   Because TLS only protects the payload, the SCTP header and all
   control chunks are not protected.  This can be used for DoS attacks.
   This is a general problem with security provided at the transport
   layer.

   The SIGTRAN protocols use the same SCTP port number and payload
   protocol identifier when run over TLS.  A session upgrade procedure
   has to be used to initiate the TLS based communication.

   The session upgrade procedure should be as follows:

   o  If an ASP has been configured to use TLS, it sends a STARTTLS
      message on stream 0 and starts a timer T_TLS.  This is the first
      message sent and the ASP sends no other adaptation layer messages
      until the TLS based communication has been established.

   o  If the peer does not support TLS, it sends back an ERROR message
      indicating an unsupported message type.  In this case, the SCTP
      association is terminated and it is reported to the management
      layer that the peer does not support TLS.

   o  If the peer does support TLS, it sends back a STARTTLS_ACK
      message.  The client then starts TLS based communication.

   o  If T_TLS expires without getting any of the above answers, the
      association is terminated and the failure is reported to the
      management layer.

   All SIGTRAN adaptation layers share a common message format.  The
   STARTTLS message consists of a common header only using the message
   class 10 and message type 1.  The STARTTLS_ACK message uses the same
   message class 10 and the message type 2.  Neither messages contain
   any parameters.

   Using this procedure, it is possible for a man-in-the-middle to do a
   denial of service attack by indicating that the peer does not support
   TLS.  But this kind of attack is always possible for a man-in-the-
   middle.

7.  Support of IPsec and TLS

   If content of SIGTRAN protocol messages is to be protected, either
   IPsec ESP or TLS can be used.  In both IPsec ESP Transport Mode and
   TLS cases, the IP header information is neither encrypted nor
   protected.  If IPsec ESP is chosen, the SCTP control information is
   encrypted and protected whereas in the TLS based solution, the SCTP
   control information is not encrypted and only protected by SCTP
   procedures.

   In general, both IPsec and TLS have enough mechanisms to secure the
   SIGTRAN communications.

   Therefore, in order to have a secured model working as soon as
   possible, the following recommendation is made: A SIGTRAN node MUST
   support IPsec and MAY support TLS.

8.  Peer-to-Peer Considerations

   M2PA, M3UA and SUA support the peer-to-peer model as a generalization
   to the client-server model which is supported by IUA and M2UA.  A
   SIGTRAN node running M2PA, M3UA or SUA and operating in the peer-to-
   peer mode is called a SIGTRAN peer.

   As with any peer-to-peer protocol, proper configuration of the trust
   model within a peer is essential to security.  When certificates are
   used, it is necessary to configure the trust anchors trusted by the
   peer.  These trust anchors are likely to be unique to SIGTRAN usage
   and distinct from the trust anchors that might be trusted for other
   purposes such as Web browsing.  In general, it is expected that those
   trust anchors will be configured so as to reflect the business
   relationships between the organization hosting the peer and other
   organizations.  As a result, a peer will not typically be configured
   to allow connectivity with any arbitrary peer.  When certificate
   authentication peers may not be known beforehand, peer discovery may
   be required.

   Note that IPsec is considerably less flexible than TLS when it comes
   to configuring trust anchors.  Since use of Port identifiers is
   prohibited within IKE Phase 1, it is not possible to uniquely
   configure trusted trust anchors for each application individually
   within IPsec; the same policy must be used for all applications.
   This implies, for example, that a trust anchor trusted for use with a
   SIGTRAN protocol must also be trusted to protect other protocols (for
   example SNMP).  These restrictions are awkward at best.

   When pre-shared key authentication is used with IPsec to protect
   SIGTRAN based communication, unique pre-shared keys are configured
   with peers that are identified by their IP address (Main Mode), or
   possibly their FQDN (AggressivenMode).  As a result, it is necessary
   for the set of peers to be known beforehand.  Therefore, peer
   discovery is typically not necessary.

   The following is intended to provide some guidance on the issue.

   It is recommended that SIGTRAN peers use the same security mechanism
   (IPsec or TLS) across all its sessions with other SIGTRAN peers.
   Inconsistent use of security mechanisms can result in redundant
   security mechanisms being used (e.g., TLS over IPsec) or worse,
   potential security vulnerabilities.  When IPsec is used with a
   SIGTRAN protocol, a typical security policy for outbound traffic is
   "Initiate IPsec, from me to any, destination port P"; for inbound
   traffic, the policy would be "Require IPsec, from any to me,
   destination port P".  Here, P denotes one of the registered port
   numbers for a SIGTRAN protocol.

   This policy causes IPsec to be used whenever a SIGTRAN peer initiates
   a session to another SIGTRAN peer, and to be required whenever an
   inbound SIGTRAN session occurs.  This policy is attractive, since it
   does not require policy to be set for each peer or dynamically
   modified each time a new SIGTRAN session is created; an IPsec SA is
   automatically created based on a simple static policy.  Since IPsec
   extensions are typically not available to the sockets API on most
   platforms, and IPsec policy functionality is implementation
   dependent, use of a simple static policy is the often the simplest
   route to IPsec-enabling a SIGTRAN peer.

   If IPsec is used to secure a SIGTRAN peer-to-peer session, IPsec
   policy SHOULD be set so as to require IPsec protection for inbound
   connections, and to initiate IPsec protection for outbound
   connections.  This can be accomplished via use of inbound and
   outbound filter policy.

9.  Security Considerations

   This document discusses the usage of IPsec and TLS for securing
   SIGTRAN traffic.

10.  IANA Considerations

   The message class 12 has been reserved in the Signaling User Adaption
   Layer Assignments Registry.  For this message class, message type 1
   has been reserved for the STARTTLS message, and message type 2 for
   the STARTTLS_ACK message.

11.  Acknowledgements

   The authors would like to thank B. Aboba, K. Morneault and many
   others for their invaluable comments and suggestions.

12.  References

12.1.  Normative References

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

12.2.  Informative References

   [2]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.

   [3]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [4]   Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406, November 1998.

   [5]   Piper, D., "The Internet IP Security Domain of Interpretation
         for ISAKMP", RFC 2407, November 1998.

   [6]   Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [7]   Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams,
         "X.509 Internet Public Key Infrastructure Online Certificate
         Status Protocol - OCSP", RFC 2560, June 1999.

   [8]   Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
         "Stream Control Transmission Protocol", RFC 2960, October 2000.

   [9]   Morneault, K., Rengasami, S., Kalla, M. and G. Sidebottom,
         "ISDN Q.921-User Adaptation Layer", RFC 3057, February 2001.

   [10]  Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [11]  Stone, J., Stewart, R. and D. Otis, "Stream Control
         Transmission Protocol (SCTP) Checksum Change", RFC 3309,
         September 2002.

   [12]  Morneault, K., Dantu, R., Sidebottom, G., Bidulock, B. and J.
         Heitz, "Signaling System 7 (SS7) Message Transfer Part 2 (MTP2)
         - User Adaptation Layer", RFC 3331, September 2002.

   [13]  Sidebottom, G., Ed., Morneault, K., Ed. and J. Pastor-Balbas,
         Ed., "Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) -
         User Adaptation Layer (M3UA)", RFC 3332, September 2002.

   [14]  Jungmaier, A., Rescorla, E. and M. Tuexen, "Transport Layer
         Security over Stream Control Transmission Protocol", RFC 3436,
         December 2002.

   [15]  George, T., "SS7 MTP2-User Peer-to-Peer Adaptation Layer", Work
         in Progress, February 2004.

   [16]  Loughney, J., "Signalling Connection Control Part User
         Adaptation  Layer (SUA)", Work in Progress, December 2003.

   [17]  Bellovin, S., Ioannidis, J., Keromytis, A. and R. Stewart, "On
         the Use of Stream Control Transmission Protocol (SCTP) with
         IPsec", RFC 3554, July 2003.

13.  Authors' Addresses

   John Loughney
   Nokia Research Center
   PO Box 407
   FIN-00045 Nokia Group
   Finland

   EMail: john.loughney@nokia.com


   Michael Tuexen (editor)
   Univ. of Applied Sciences Muenster
   Stegerwaldstr. 39
   48565 Steinfurt
   Germany

   EMail: tuexen@fh-muenster.de


   Javier Pastor-Balbas
   Ericsson Espana S.A.
   Via de los Poblados, 13
   28033 Madrid
   Spain

   EMail: j.javier.pastor@ericsson.com

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