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Network Working Group J. Loughney
Internet-Draft Nokia Research Center
Expires: April 28, 2003 M. Tuexen
Siemens AG
J. Pastor-Balbas
Ericsson
October 28, 2002
Security Considerations for SIGTRAN Protocols
draft-ietf-sigtran-security-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on April 28, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This documents discusses how TLS and IPSec can be used to secure the
communication which is based on SIGTRAN protocols.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security in telephony networks . . . . . . . . . . . . . . . . 5
3. Threats . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Protecting Confidentiality . . . . . . . . . . . . . . . . . . 7
5. IPSec Usage . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. TLS Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Peer-to-Peer Considerations . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 18
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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 IUA
o service provider equipment only. This is the case for M2UA, M2PA,
M3UA and SUA. 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 Terminology
This document uses the following terms:
TBD: TDB.
1.3 Abbreviations
This document uses the following abbreviations:
CA: Certificate Authority.
DOI: Domain Of Interpretation.
ESP: Encapsulating Security Payload.
FQDN: Full-Qualified Domain Names.
IPSec: IP Security Protocol.
IKE: Internet Key Exchange Protocol.
IUA: ISDN Q.921 User Adaptation Layer.
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M2PA: SS7 MTP2 Peer-to-Peer User Adaptation Layer.
M2UA: SS7 MTP2 User Adaptation Layer.
M3UA: SS7 MTP3 User Adaptation Layer.
SA: Security Association.
SCTP: Stream Control Transmission Protocol.
SS7: Signaling System No. 7.
SUA: SS7 SCCP User Adaptation Layer.
TLS: Transport Layer Security.
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2. Security in telephony networks
The security in telephony networks is mainly based on the trusted
network principle. There are two totally different protocol used:
The ISDN access protocol is used for signaling in the access network
and the SS7 protocol stack in the core network.
As SS7 networks are often physically remoter and/or inacessable, it
is assumed that they are protected from malicious users. Often,
equipment is 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 ISDN access protocol is the separate protocol stack for end-user
signaling. End-user signaling protocols are translated to SS7 based
protocols by telephone switches run by network operators.
Often Regulatory Authorities require SS7 switches with connections to
different SS7 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 but also to multiple telephony networks being connected and
operated by different operators.
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3. Threats
There is no quick fix, one-size-fits-all solution for security. All
SIGTRAN protocols have the following security objectives:
o Availability of reliable and timely user data transport.
o Authentication of peers.
o Integrity of user data transport.
o Confidentiality of user data.
All SIGTRAN protocols use the Stream Control Transmission Protocol
(SCTP) being defined in [7] and [9] as its transport protocol. SCTP
provides certain transport related security features, such as:
o Blind Denial of Service Attacks
o Flooding
o Masquerade
o Improper Monopolization of Services
When SIGTRAN protocols are running in professionally managed
corporate or service provider network, it is reasonable to expect
that this network include an appropriate security policy framework.
The "Site Security Handbook" [1] should be consulted for guidance.
When the network in which SIGTRAN protocols are used 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 is used to ensure confidentiality of user payload. Consult
[3] for more information on configuring IPSec services.
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4. Protecting Confidentiality
If SIGTRAN information has to be protected either IPSec ESP in
transport mode or TLS can be used. In both cases the IP header
information is neither encrypted nor protected. If IPSec ESP is
chosen the SCTP control information is encrypted and protected
whereas if the TLS based solution the SCTP control information is not
encrypted and only protected by SCTP procedures.
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5. IPSec Usage
This section is relevant only for SIGTRAN nodes using IPSec to secure
communication between SIGTRAN node.
All SIGTRAN nodes using IPSec MUST support IPsec ESP [4] in transport
mode with non-null encryption and authentication algorithms to
provide per-packet authentication, integrity protection and
confidentiality, and MUST support the replay protection mechanisms of
IPSec.
These 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 both IKE Main Mode and
Aggressive 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 digital signatures are used to achieve authentication, an IKE
negotiator SHOULD use IKE Certificate Request Payload(s) to specify
the certificate authority (or authorities) that are 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.
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, and if additional traffic is sent on it, to bring up another IKE
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Phase 2 SA to protect it. This avoids the potential for 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
[14]. So IPSec implementations used by SIGTRAN nodes SHOULD support
the IPSec feature described in [14].
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6. TLS Usage
This section is relevant only 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.
[13] 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 and the 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. See [13] for more details.
The SIGTRAN protocols use separate SCTP port numbers and payload
protocol identifiers when run over TLS. These numbers are given in
Section 9. A SIGTRAN session MUST be aborted if the port number or
payload protocol identifier indicates the use of TLS and it is not
used.
As an alternative to a separate port number, a session upgrade
procedure can be used. This needs an extension for all adaptation
layers allowing the SIGTRAN protocols to use the same port number in
the case where TLS is used or not. This needs further discussions.
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7. 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 root certificate authorities
trusted by the peer. These root CAs are likely to be unique to
SIGTRAN usage and distinct from the root CAs that might be trusted
for other purposes such as Web browsing. In general, it is expected
that those root CAs 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 typically not be configured
to allow connectivity with any arbitrary peer. When certificate
authentication peers may not be known beforehand, and therefore peer
discovery may be required.
Note that IPsec is considerably less flexible than TLS when it comes
to configuring root CAs. Since use of Port identifiers is prohibited
within IKE Phase 1, within IPsec it is not possible to uniquely
configure trusted root CAs for each application individually; the
same policy must be used for all applications. This implies, for
example, that a root CA trusted for use with a SIGTRAN protocol must
also be trusted to protect SNMP. These restrictions can be awkward
at best. Since TLS supports application-level granularity in
certificate policy, TLS SHOULD be used to protect SIGTRAN sessions
between administrative domains. IPsec is most appropriate for intra-
domain usage when pre-shared keys are used as a security mechanism.
When pre-shared key authentication is used with IPSec to protect
SIGTRAN based communication, unique pre-shared keys are configured
with peers, who 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
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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 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.
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8. Security Considerations
This documents discusses the usage of IPSec and TLS for securing
SIGTRAN traffic.
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9. IANA Considerations
SCTP port numbers and SCTP payload protocol identifiers have to be
registered for:
o IUA over TLS
o M2UA over TLS
o M2PA over TLS
o M3UA over TLS
o SUA over TLS
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10. Acknowledgements
The authors would like to thank K. Morneau and many others for their
invaluable comments and suggestions.
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References
[1] Fraser, B., "Site Security Handbook", RFC 2196, September 1997.
[2] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "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] 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.
[8] Morneault, K., Rengasami, S., Kalla, M. and G. Sidebottom,
"ISDN Q.921-User Adaptation Layer", RFC 3057, February 2001.
[9] Stone, J., Stewart, R. and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002.
[10] 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.
[11] Sidebottom, G., Morneault, K. and J. Pastor-Balbas, "Signaling
System 7 (SS7) Message Transfer Part 3 (MTP3) - User Adaptation
Layer (M3UA)", RFC 3332, September 2002.
[12] George, T., "SS7 MTP2-User Peer-to-Peer Adaptation Layer",
draft-ietf-sigtran-m2pa-06 (work in progress), August 2002.
[13] Rescorla, E., Tuexen, M. and A. Jungmaier, "TLS over SCTP",
draft-ietf-tsvwg-tls-over-sctp-00 (work in progress), November
2001.
[14] Bellovin, S., "On the Use of SCTP with IPsec", draft-ietf-
ipsec-sctp-04 (work in progress), October 2002.
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Authors' Addresses
John Loughney
Nokia Research Center
PO Box 407
FIN-00045 Nokia Group
Finland
EMail: [email protected]
Michael Tuexen
Siemens AG
Hofmannstr. 51
81359 Munich
Germany
EMail: [email protected]
Javier Pastor-Balbas
Ericsson
?
Madrid
Spain
EMail: [email protected]
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