Network Working Group M. Watson
Request for Comments: 3324 Nortel Networks
Category: Informational November 2002
Short Term Requirements for Network Asserted Identity
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
A Network Asserted Identity is an identity initially derived by a
Session Initiation Protocol (SIP) network intermediary as a result of
an authentication process. This document describes short term
requirements for the exchange of Network Asserted Identities within
networks of securely interconnected trusted nodes and to User Agents
securely connected to such networks.
There is no requirement for identities asserted by a UA in a SIP
message to be anything other than the user's desired alias.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Identity . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Network Asserted Identity . . . . . . . . . . . . . . . . . . 3
2.3 Trust Domains . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4 Spec(T) . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Generation of Networks Asserted Identity . . . . . . . . . . . 7
4. Transport of Network Asserted Identity . . . . . . . . . . . . 7
4.1 Sending of Networks Asserted Identity within a Trust Domain . 7
4.2 Receiving of Network Asserted Identity within a Trust Domain . 7
4.3 Sending of Network Asserted Identity to entities outside a
Trust Domain . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4 Receiving of Network Asserted Identity by a node outside the
Trust Domain . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Parties with Network Asserted Identities . . . . . . . . . . . 8
6. Types of Network Asserted Identity . . . . . . . . . . . . . . 8
7. Privacy of Network Asserted Identity . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11
1. Introduction
SIP [1] allows users to assert their identity in a number of ways
e.g., using the From: header. However, there is no requirement for
these identities to be anything other than the users desired alias.
An authenticated identity of a user can be obtained using SIP Digest
Authentication (or by other means). However, UAs do not always have
the necessary key information to authenticate another UA.
A Network Asserted Identity is an identity initially derived by a SIP
network intermediary as a result of an authentication process. This
may or may not be based on SIP Digest authentication. This document
describes short term requirements for the exchange of Network
Asserted Identities within networks of securely interconnected
trusted nodes and also to User Agents with secure connections to such
networks.
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Such a network is described in this document as a Trust Domain and we
present a strict definition of trust and Trust Domain for the
purposes of this document. These short-term requirements provide
only for the exchange of Network Asserted Identity within a Trust
Domain and to an entity directly connected to the trust domain.
General requirements for transport of Network Asserted Identities on
the Internet are out of scope of this document.
2. Definitions
2.1 Identity
An Identity, for the purposes of this document, is a sip:, sips: or
tel: URI, and optionally a Display Name.
The URI MUST be meaningful to the domain identified in the URI (in
the case of sip: or sips: URIs) or the owner of the E.164 number (in
the case of tel: URIs), in the sense that when used as a SIP
Request-URI in a request sent to that domain/number range owner, it
would cause the request to be routed to the user/line that is
associated with the identity, or to be processed by service logic
running on that user's behalf.
If the URI is a sip: or sips: URI, then depending on the local policy
of the domain identified in the URI, the URI MAY identify some
specific entity, such as a person.
If the URI is a tel: URI, then depending on the local policy of the
owner of the number range within which the telephone number lies, the
number MAY identify some specific entity, such as a telephone line.
However, it should be noted that identifying the owner of the number
range is a less straightforward process than identifying the domain
which owns a sip: or sips: URI.
2.2 Network Asserted Identity
A Network Asserted Identity is an identity derived by a SIP network
entity as a result of an authentication process, which identifies the
authenticated entity in the sense defined in Section 2.1.
In the case of a sip: or sips: URI, the domain included in the URI
MUST be within the Trust Domain.
In the case of a tel: URI, the owner of the E.164 number in the URI
MUST be within the Trust Domain.
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The authentication process used, or at least it's reliability/
strength, is a known feature of the Trust Domain using the Network
Asserted Identity mechanism i.e., in the language of 2.3 below, it is
defined in Spec(T).
2.3 Trust Domains
A Trust Domain for the purposes of Network Asserted Identity is a set
of SIP nodes (UAC, UAS, proxies or other network intermediaries) that
are trusted to exchange Network Asserted Identity information in the
sense described below.
A node can be a member of a Trust Domain, T, only if the node is know
to be compliant to a certain set of specifications, Spec(T), which
characterize the handling of Network Asserted Identity within the
Trust Domain, T.
Trust Domains are constructed by human beings who know the properties
of the equipment they are using/deploying. In the simplest case, a
Trust Domain is a set of devices with a single owner/operator who can
accurately know the behaviour of those devices.
Such simple Trust Domains may be joined into larger Trust Domains by
bi-lateral agreements between the owners/operators of the devices.
We say a node is 'trusted' (with respect to a given Trust Domain) if
and only if it is a member of that domain.
We say that a node, A, in the domain is 'trusted by' a node, B, (or
'B trusts A') if and only if:
1. there is a secure connection between the nodes, AND
2. B has configuration information indicating that A is a member
of the Trust Domain.
Note that B may or may not be a member of the Trust Domain. For
example, B may be a UA which trusts a given network intermediary, A
(e.g., its home proxy).
A 'secure connection' in this context means that messages cannot be
read by third parties, cannot be modified by third parties without
detection and that B can be sure that the message really did come
from A. The level of security required is a feature of the Trust
Domain i.e., it is defined in Spec(T).
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Within this context, SIP signaling information received by one node
FROM a node that it trusts is known to have been generated and passed
through the network according to the procedures of the particular
specification set Spec(T), and therefore can be known to be valid, or
at least as valid as specified in the specifications Spec(T).
Equally, a node can be sure that signaling information passed TO a
node that it trusts will be handled according to the procedures of
Spec(T).
For these capabilities to be useful, Spec(T) must contain
requirements as to how the Network Asserted Identity is generated,
how its privacy is protected and how its integrity is maintained as
it is passed around the network. A reader of Spec(T) can then make
an informed judgement about the authenticity and reliability of
Network Asserted Information received from the Trust Domain T.
The term 'trusted' (with respect to a given Trust Domain) can be
applied to a given node in an absolute sense - it is just equivalent
to saying the node is a member of the Trust Domain. However, the
node itself does not know whether another arbitrary node is
'trusted', even within the Trust Domain. It does know about certain
nodes with which it has secure connections as described above.
With the definition above, statements such as 'A trusted node SHALL
...' are just shorthand for 'A node compliant to this specification
SHALL...'.
Statements such as 'When a node receives information from a trusted
node...' are NOT valid, because one node does not have complete
knowledge about all the other nodes in the trust domain.
Statements such as 'When a node receives information from another
node that it trusts...' ARE valid, and should be interpreted
according to the criteria (1) and (2) above.
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The above relationships are illustrated in the following figure:
+------+
| |
| F |
| |
+------+
x
..............................x.........
. x .
. +------+ +------+ . +------+
. | | | | . | |
. | A | | B |-----.----| E |
. | | | | . | |
. +------+ +------+ . +------+
. \\ / .
. \\ +------+ // .
. \\ | | // .
. \ | C |/ .
. | | .
. +------+ .
. | Trust Domain.
........................................
|
|
+------+
| |
| D |
| |
+------+
xxxxxx Insecure connection
------ Secure connection
......
. .All boxes within the dotted line
......are part of the same Trust Domain
o A, B and C are part of the same trust domain
o A trusts C, but A does not trust B
o since E knows that B is inside of the trust domain, E
o trusts B, but B does not trust E
o B does not trust F, F does not trust B
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2.4 Spec(T)
An aspect of the definition of a trust domain is that all the
elements in that domain are compliant to a set of configurations and
specifications generally referred to as Spec(T). Spec(T) is not a
specification in the sense of a written document; rather, its an
agreed upon set of information that all elements are aware of.
Proper processing of the asserted identities requires that the
elements know what is actually being asserted, how it was determined,
and what the privacy policies are. All of that information is
characterized by Spec(T).
3. Generation of Networks Asserted Identity
A Network Asserted Identity is generated by a network intermediary
following an Authentication process which authenticates the entity
(UA) to be identified.
The Authentication process(es) used are a characteristic feature of
the Trust Domain, and MUST be specified in Spec(T).
It shall be possible for a UA to provide a preferred identity to the
network intermediary, which MAY be used to inform the generation of
the Network Asserted Identity according to the policies of the Trust
Domain.
4. Transport of Network Asserted Identity
4.1 Sending of Networks Asserted Identity within a Trust Domain
It shall be possible for one node within a Trust Domain to securely
send a Network Asserted Identity to another node that it trusts.
4.2 Receiving of Network Asserted Identity within a Trust Domain
It shall be possible for one node within a Trust Domain to receive a
Network Asserted identity from another node that it trusts.
4.3 Sending of Network Asserted Identity to entities outside a Trust
Domain
If a node, A, within the Trust Domain, is trusted by a node, B,
outside the Trust Domain, then it shall be possible for A to securely
send a Network Asserted Identity to B, if allowed by the privacy
policies of the user that has been identified, and the trust domain.
This is most often used to pass a Network Asserted Identity directly
to a UA.
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4.4 Receiving of Network Asserted Identity by a node outside the Trust
Domain
It shall be possible for a node outside the Trust Domain to receive a
Network Asserted Identity from a node that it trusts.
Network Asserted Identity received in this way may be considered
valid, and used for display to the user, input data for services etc.
Network Asserted Identity information received by one node from a
node which it does not trust carries no guarantee of authenticity or
integrity because it is not known that the procedures of Spec(T) were
followed to generate and transport the information. Such information
MUST NOT be used. (i.e., it shall not be displayed to the user,
passed to other nodes, used as input data for services, etc.)
5. Parties with Network Asserted Identities
A Network Asserted Identity identifies the originator of the message
in which it was received.
For example,
a Network Asserted Identity received in an initial INVITE (outside
the context of any existing dialog) identifies the calling party.
a Network Asserted Identity received in a 180 Ringing response to
such an INVITE identifies the party who is ringing.
a Network Asserted Identity received in a 200 response to such an
INVITE identifies the party who has answered.
6. Types of Network Asserted Identity
It shall be possible to assert multiple identities associated with a
given party (in a given message), provided that these are of distinct
types.
The types of identity supported shall be sip:, sips: and tel: URIs,
all of which identify the user as described in Section 2.1. It is
not required to transport both a sip: and sips: URI.
It shall be possible for the capability to transport additional types
of identity associated with a single party to be introduced in
future.
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7. Privacy of Network Asserted Identity
The means by which any privacy requirements in respect of the Network
Asserted Identity are determined are outside the scope of this
document.
It shall be possible to indicate within a message containing a
Network Asserted Identity that this Network Asserted Identity is
subject to a privacy requirement which prevents it being passed to
other users. This indication should not carry any semantics as to
the reason for this privacy requirement.
It shall be possible to indicate that the user has requested that the
Network Asserted Identity be not passed to other users. This is
distinct from the above indication, in that it implies specific user
intent with respect to the Network Asserted Identity.
The mechanism shall support Trust Domain policies where the above two
indications are equivalent (i.e., the only possible reason for a
privacy requirement is a request from the user), and policies where
they are not.
In this case, the Network Asserted Identity specification shall
require that the mechanism of Section 4.3 SHALL NOT be used i.e., a
trusted node shall not pass the identity to a node it does not trust.
However, the mechanism of Section 4.3 MAY be used to transfer the
identity within the trusted network.
Note that 'anonymity' requests from users or subscribers may well
require functionality in addition to the above handling of Network
Asserted Identities. Such additional functionality is out of the
scope of this document.
8. Security Considerations
The requirements in this document are NOT intended to result in a
mechanism with general applicability between arbitrary hosts on the
Internet.
Rather, the intention is to state requirements for a mechanism to be
used within a community of devices which are known to obey the
specification of the mechanism (Spec(T)) and between which there are
secure connections. Such a community is known here as a Trust
Domain.
The requirements on the mechanisms used for security and to initially
derive the Network Asserted Identity must be part of the
specification Spec(T).
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The requirements also support the transfer of information from a node
within the Trust Domain, via a secure connection to a node outside
the Trust Domain.
Use of this mechanism in any other context has serious security
shortcomings, namely that there is absolutely no guarantee that the
information has not been modified, or was even correct in the first
place.
9. IANA Considerations
This document does not have any implications for IANA.
10. Acknowledgments
Thanks are due to Jon Peterson, Cullen Jennings, Allison Mankin and
Jonathan Rosenberg for comments on this document.
Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session
Initiation Protocol", RFC 3261, June 2002.
Author's Address
Mark Watson
Nortel Networks
Maidenhead Office Park
Westacott Way
Maidenhead, BERKS SL6 3QH
UK
EMail: mwatson@nortelnetworks.com
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