Internet Engineering Task Force M. Pierce
Internet Draft Artel
draft-pierce-ieprep-assured-service-arch-00.txt Don Choi
October 2002 DISA
Expires April 2003
Architecture for Assured Service Capabilities in Voice over IP
draft-pierce-ieprep-assured-service-arch-00.txt
Status of this memo
This document is an Internet-Draft and is in full conformance with
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Copyright
Copyright (C) Internet Society 2002. All rights reserved.
Reproduction or translation of the complete document, but not of
extracts, including this notice, is freely permitted.
Abstract
Assured Service refers to the set of capabilities used to ensure
that mission critical communications are setup and remain connected.
This memo describes the architecture required to meet the
requirements detailed in [Pierce1].
Table of Contents
0. History...........................................................2
1. Introduction......................................................2
2. Architectures.....................................................3
2.1.End-to-end Architecture..........................................3
2.2. Service Provider Network Architecture..........................3
3. Required Architecture.............................................3
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4. Required Procedures...............................................5
4.1. Authentication.................................................5
4.2. Function of Proxy..............................................6
4.3. Function of the Access Router..................................6
4.4. Session Control................................................7
5. Security Considerations...........................................7
6. References........................................................7
7. Authors' Addresses................................................7
0. History
This draft was originally submitted under SIPPING. This revision is
being submitted under IEPREP to be included in the discussions for
related services such as IEPS.
(SIPPING) -00: Original
(IEPREP) -00: Added Access Router to architecture required to
support Assured Service.
1. Introduction
The requirements for Assured Service are given in [Pierce1]. Many
other drafts and RFCs have addressed the assumed architecture for
the provision of SIP-based services. A lot of consideration has been
given to continued reliance on the pure peer-to-peer model on which
the Internet (and especially HTTP) has been based vs. migration to
centralized control models in which dedicated proxies perform
specific functions for the control of telephony services. This would
include, possibly, full knowledge of the state of each call.
While there is an wide-spread desire expressed in various IETF
discussions to maintain (or return to) the pure peer-to-peer
architecture, there has been increasing admissions in various drafts
that centralized control or intelligent "middleboxes" are required
in many cases. This list of examples will likely continue to grow.
Some examples are:
1. RFC 3261 defines the notion of a "Call Stateful proxy", which
"retains state for a dialog from the initiating INVITE to the
terminating BYE request", i.e., for the duration of a call. However,
no use of this state has been included in the current version of SIP
[SIP-3261].
2. Draft-ietf-sipping-cc-framework-01 includes the concept of a
"central control" signaling model (although its reference to 3pcc
indicates that the actual concept is not "centralized" but rather a
specialized end-user performing control for other users.)
3. The abstract for draft-ietf-sipping-service-examples-02
recognizes that "some [services] require the assistance of a SIP
Proxy", but that "most ... shown in this document are implemented in
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the SIP User Agents". However, it then states that the flows shown
assume "a network of proxies, registrars, PSTN gateways, and other
SIP servers that have a pre-established trust relationship with each
other... User agents wishing to use the services in this network are
required to authenticate themselves with an edge proxy..."
4. The draft for identity and privacy [SIP-IDENTITY] states that, in
order for an originating device to achieve privacy concerning its
identity related information, one must "assume an architecture where
the caller initiates a session to the callee via a trusted entity in
its network. The callee in turn receives the session initiation via
a trusted entity". It further states that the "trusted entity ...
belongs to and is controlled by the Network".
5. Several current individual drafts related to privacy recognize
the need for centralized proxies. Draft-peterson-sip-privacy-
longterm-00 (expired) described the notion of an intermediary
providing the privacy function for services which "user agents
cannot satisfy themselves".
2. Architectures
Various discussions and memos have identified two potential network
architectures for the provision of SIP services. They are briefly:
2.1. End-to-end Architecture
All service provision is between and under control of the calling
and called party, referred to as "User Agent Client (UAC)" and "User
Agent Server (UAS)", respectively. This terminology of "client" and
"server" are based on the HTTP model from which this model is
derived and have no real significance to this model. Either end can
initiate a transaction. There is no device in between which provides
service support, only routers for packets.
2.2. Service Provider Network Architecture
A Service Provider maintains and controls network elements which
play an active role in the provision of services to end users. These
network elements may be referred to as back-to-back user agents
(B2BUA), proxies, servers, middleboxes, or intermediaries but they
all have the common characteristic of being provided by a Service
Provider and they provide a logical function between the end users.
These elements terminate SIP messages, perform service control, and
send new or modified SIP messages to other network elements or to
the other user. The result is that no SIP message goes direct from
one UA to the other (unless specifically authorized by the control
element).
3. Required Architecture
In order to provide the security and feature control required for
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Assured Service, it is necessary to utilize the Service Provider
Network Architecture in which proxies are used to support call
origination and termination for each user involved in the service.
The architecture is the "trapezoid" described in SIP [SIP-3261] and
[SIP-IDENTITY] as follows (actually copied from draft-ietf-sip-srv-
06):
......................... ..........................
. . . .
. +-------+ . . +-------+ .
. | | . (2) . | | .
. | Proxy |----------------- | Proxy | .
. | 1 | . . | 2 | .
. | | . . | | .
. +-------+ . . +-------+ .
. / . . \ .
. (1) / . . \ (3) .
. / . . \ .
. / . . \ .
. +-------+ . . +-------+ .
. | | . (4) . | | .
. | UA 1 |------------------------------------| UA 2 | .
. | | . . | | .
. +-------+ . . +-------+ .
. Domain A . . Domain B .
......................... ..........................
Interfaces:
(1) Originating UA 1 to Proxy 1: Authentication and all SIP messages
to/from UA 1
(2) Proxy 1 to Proxy 2 (and to other devices such as policy
servers): SIP messages and policy actions
(3) Proxy 2 to terminating UA 2: Authentication and all SIP messages
to/from U 2
(4) Originating UA 1 to terminating UA 2: Voice packets
However, the above architecture requires the addition of another
component to provide control of the user's data packets (voice) in
the Assured Service case. This is important since the packets
themselves need to be marked for preferential treatment, including
the ability to get "priority" over the packet transfer of another
user.
There must be an access router, generally at the boundary between
the local network and the core network. This may be between the
Ethernet LAN and the IP "cloud" or it may be between the locally
controlled IP network and the global IP network. In any case, its
function is to regulate the transport of priority marked packets
into the core.
The following figure depicts this architecture:
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............................. ..............................
. . . .
. +-------+ . . +-------+ .
. | | . (2) . | | .
. | Proxy |------------------------ | Proxy | .
. | 1 | . . | 2 | .
. | | . . | | .
. +-------+ . . +-------+ .
. / \ . . / \ .
. (1) / \ (1a) . . (3a) / \ (3) .
. / \ . . / \ .
. / \ . . / \ .
. +-------+ +----+ . . +----+ +-------+ .
. | | (4a) | AR | . (4b) . | AR | (4c) | | .
. | UA 1 |------>| 1 |---------------->| 2 |------>| UA 2 | .
. | | | | . . | | | | .
. +-------+ +----+ . . +----+ +-------+ .
. Domain A . . Domain B .
............................. ..............................
Interfaces:
(1) as above
(1a and 3a) Proxy to AR: instructions to allow voice packet
transport
(2) as above
(3) as above
(4a) Originating UA 1 to AR 1: attempted voice packets
(4b) AR 1 to AR 2: authorized voice packets
(4c) AR 2 to UA 2: authorized voice packets
4. Required Procedures
4.1. Authentication
Each UA which might use the Assured Service capability must
authenticate with a designated proxy before any service activation
is attempted. Normally, this would be at the time the device is
powered on, connected to the network, or is initialized, or it might
be done at pre-determined time intervals. Whether or not this
authentication requires a user interaction (human entry of a
password, retina scan, etc.) is not important and depends on the
application. Such an authentication may be very time consuming, with
password verification and policy data-base look-ups. After this
authentication, this proxy must handle all session establishments,
both to and from this UA.
This authentication function may be performed when the user attempts
the first session setup, for example, when an individual is allowed
to use a common device by first "logging on" with their identity and
password. In fact, this is still an "authentication" function
performed before the session setup is attempted. However, in this
case, it must be understood that there may be an additional delay
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due to the authentication process before a call can be placed.
This authentication process is not unique to the provision of the
Assured Service capability. It is also required for many other
services which are to be provided by the service provider's proxy
based on pre-established authorizations.
4.2. Function of Proxy
Besides the processing of the authentication, each proxy is
responsible for a number of functions important to the provision of
Assured Service (as well as other services) and the handling of
interactions, where required, between different services. This
includes (for Assured Service):
- maintaining state of all existing sessions, including their
priority, which exist on all UAs under its control (both
proxies).
- maintaining knowledge of other services being used by the UA
which might need to be taken into consideration when applying
the Assured Service capabilities (both proxies).
- verifying that the originating UA is allowed to establish the
session at the precedence level requested (originating proxy).
- establish permission at the access router for it to handle the
precedence marked packets from the UA (both proxies).
- performing the timing function to control the diversion service
(terminating proxy).
- deciding when to preempt the end user and sending the
appropriate preempt messages to the other party (both proxies).
- maintaining records of the use of the service, whether for
accounting or auditing purposes (both proxies).
4.3. Function of the Access Router
The access router, under control of the proxy, decides which packets
are to be transported between networks or domains. If authorization
has not been granted for the transport of a specific packet flow at
the precedence level indicated in the packets, the access router
must discard the packets.
Additionally, there may be cases in which a currently transported
packet stream must be stopped. Since the Assured Service may not be
able to rely on the UA to stop the flow, it may be necessary for the
access router, again under control of the proxy, to stop
transporting a particular flow.
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4.4. Session Control
Session establishment and release should follow the same message
sequence as defined in SIP and its extensions for non-Assured
Service calls. There should not be any additional messages. The only
additional requirements are the inclusion of:
- the priority level as defined in [Resource-priority] in the
INVITE
- security related information in every message which might
consist of an authentication header (AH) using cryptographic
techniques to allow the receiving end (user or proxy) to
validate the authenticity of the message before acting on it.
(This requirement is not unique to Assured Service, but is also
required to secure other capabilities.)
5. Security Considerations
This memo mostly deals with the architecture required to support the
necessary security. While it does not attempt to define the actual
security mechanisms used for authentication and authorization, it
establishes the service architecture required.
6. References
[SIP-CALL-AUTH] draft-ietf-sip-call-auth-04, "SIP Extension for
Media Authorization", February 2002.
[SIP-3261] "SIP: Session Initiation Protocol", June 2002.
[SIP-IDENTITY] draft-ietf-sip-privacy-04, "SIP extensions for
Network-asserted Caller Identity and Privacy within Trusted
Networks", February 2002.
[Baker] draft-baker-ieprep-requirements-00, "IEPS Requirement
Statement", February 2002.
[Pierce1] draft-pierce-ieprep-assured-service-req-00, "Requirements
for Assured Service Capabilities in Voice over IP", October 2002.
[Pierce2] draft-pierce-ieprep-pref-treat-examples-00, "Examples for
Provision of Preferential Treatment in Voice over IP", October 2002.
7. Authors' Addresses
Michael Pierce
Artel
1893 Preston White Drive
Reston, VA 20191
Phone: +1 410.817.4795
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Email: pierce1m@ncr.disa.mil
Don Choi
DISA
5600 Columbia Pike
Falls Church, VA 22041-2717
Phone: +1 703.681.2312
Email: choid@ncr.disa.mil
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