IP Telephony ETS: IP Telephony Requirements for Emergency Telecommunication Service (ETS)
A short wish list for what IP phone systems must support so emergency calls can be recognised and given better service.
Carlberg, K. and R. Atkinson, "IP Telephony Requirements for Emergency Telecommunication Service (ETS)", RFC 3690, DOI 10.17487/RFC3690, February 2004, <https://www.rfc-editor.org/info/rfc3690>.
@misc{rfc3690,
series = {Request for Comments},
number = 3690,
howpublished = {RFC 3690},
publisher = {RFC Editor},
doi = {10.17487/RFC3690},
url = {https://www.rfc-editor.org/info/rfc3690},
author = {K. Carlberg and R. Atkinson},
title = {{IP Telephony Requirements for Emergency Telecommunication Service (ETS)}},
pagetotal = 7,
year = 2004,
month = feb,
} RFC text
Contents
Network Working Group K. Carlberg
Request for Comments: 3690 UCL
Category: Informational R. Atkinson
Extreme Networks
February 2004
IP Telephony Requirements for
Emergency Telecommunication Service (ETS)
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 (2004). All Rights Reserved.
Abstract
This document presents a list of requirements in support of Emergency
Telecommunications Service (ETS) within the context of IP telephony.
It is an extension to the general requirements presented in RFC 3689.
Solutions to these requirements are not presented in this document.
1. Introduction
Effective telecommunications capabilities can be imperative to
facilitate immediate recovery operations for serious disaster events,
such as, hurricanes, floods, earthquakes, and terrorist attacks.
Disasters can happen unexpectedly, at any time or place. Quick
response for recovery operations requires immediate access to any
public telecommunications capabilities at hand. These capabilities
include: conventional telephone, cellular phones, and Internet
access via online terminals, IP telephones, and wireless Personal
Digital Assistants (PDAs). The commercial telecommunications
infrastructure is rapidly evolving to Internet-based technology.
Therefore, the Internet community needs to consider how it can best
support emergency management and recovery operations.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
1.1. Problem
Standards have been developed by other standards bodies concerning
emergency communications. As discussed in [3], some of these
standards, such as T1.631 [5], define specific indicators or labels
for emergency communications in Signaling System 7 (SS7) networks.
Certain requirements must be defined in order to achieve peering
across hybrid networks (networks that communicate between IP and
other types of networks, such as that realized by the Public Switched
Telephone Network) in order to achieve an interworking of services.
2. Scope
[3] has defined a set of general system requirements to support
Emergency Telecommunications Service (ETS). This document defines an
additional set of system requirements to achieve support for ETS
within the specific context of IP telephony (note that this document
views IP telephony within the context of an end-to-end application
layer service). Solutions to requirements are not defined. The
document does not specify protocol enhancements or specifications.
Note that [4], Requirements for Resource Priority Mechanisms for the
Session Initiation Protocol (SIP), is an RFC that shares some overlap
with this document. However, [4] only applies to SIP and is not
meant to be applied to a more general perspective of IP telephony as
it relates to ETS.
2.1. Out of Scope
An item that is not in scope of this document is mandating acceptance
and support of the requirements presented in this document. The IETF
does not mandate requirements or capabilities to independent networks
that comprise the Internet. As an example, Internet Service
Providers (ISP) may choose not to operate any telephony-related
gateways or services. The IETF cannot and does not mandate that an
ISP deploy either telephony-related gateways or telephony-related
services. There is an expectation that business contracts, for
example Service Level Agreements (SLA), will be used to satisfy those
following requirements that apply to service providers. Absence of
an SLA implies best effort service is provided.
It is assumed that some ISPs will choose to offer ETS services and
that other carriers will choose not to offer ETS services. These
requirements do not apply to ISPs that have chosen not to offer ETS
services.
3. IP Telephony Requirements
The requirements in this section relate only to Telephony Signaling
as used in Internet-based telephony services. They are an extension
to the general requirements specified in [3]. The following
requirements explicitly do not relate to IP-layer mechanisms, such as
Differentiated Services or Integrated Services.
1) Telephony signaling applications used with Internet-based
telephony MUST be able to carry labels.
2) The ability to carry labels MUST be extensible to support various
types and numbers of labels. A single binary value will not be
sufficient given the various labeling standards in existence
today.
3) Telephony signaling labels SHOULD have a mapping with the various
emergency related labels/markings used in other telephony based
networks, such as the Public Switched Telephone Network (PSTN).
This ensures that a telephone call placed over a hybrid
infrastructure (traditional PSTN over some portion(s) of the path,
Internet telephony over some other portion(s) of the path) can
carry the labels end-to-end with appropriate translation at
PSTN/Internet boundaries. Absence of a mapping means that the
signaling reverts to a default service (presumably one attributed
to the general public).
4) Application layer IP telephony capabilities MUST NOT preclude the
ability to do application layer accounting.
Accounting is a useful feature in support of billing and tracking
down abuse of service. If specific solutions or protocols in
support of ETS require accounting, then this will be articulated
in future document(s).
5) Application layer mechanisms in gateways and stateful proxies that
are specifically in place to recognize ETS type labels MUST be
able to support "best available" service (this will probably be
realized as better than best effort). These labels MAY exist in
the application layer headers of data (i.e., bearer) traffic or
signaling traffic used for call completion.
The support for best available service SHOULD focus on probability
of forwarding packets. Probability MAY reach 100% depending on
the local policy associated with the label. Local policy MUST
also be used to determine if better than best effort is to be
applied to a specific label (or related set of labels).
Additional comments on this topic are presented below in item 2 of
section 4.
The above paragraphs MUST be taken in their entirety. The ability
to support best available service does not mean that the
application layer mechanism is expected to be activated. Further,
we do not define the means by which best available service is
realized. Application layer mechanisms that do not recognize ETS
type labels are not subject to this requirement.
4. Issues
This section presents issues that arise in considering solutions for
the telephony requirements that have been defined for ETS. This
section does not specify solutions, nor is it to be confused with
requirements. Subsequent documents that articulate a more specific
set of requirements for a particular service may make a statement
about the following issues.
1) Alternate paths
Experience with The Government Emergency Telecommunications
Service (GETS) over the PSTN has shown the utility of alternate
paths to a destination to help facilitate emergency-related
communications. From the perspective of the Internet, this
utility may be difficult to achieve and have a more limited
benefit. Unlike the PSTN, which creates a fixed path during call
setup phase, the Internet uses dynamic routing for IP packets.
This dynamic routing capability automatically causes IP packets to
travel the best current path. The Internet network infrastructure
does not have the concept of a "call" or the concept of "call
setup", though IP telephony applications might have application
layer awareness of calls or the call setup concept.
2) Application of Best Available Service
In item 5 of section 3 above, we discuss the requirement of
supporting best available service. We note that in this document,
the scope of that support is constrained to the application layer
and flows that traverse that layer. This may involve direct
support for the flow containing the ETS type label, or may involve
indirect support (e.g., ETS labels in signaling messages that
cause an effect on corresponding data or bearer flows).
It is critical to understand that how the support for best
available service can be realized is outside the scope of this
document. In addition, the perceived effectiveness of a given
approach or implementation is also outside the scope of this
document.
5. Security
Only authorized users or operators SHOULD be able to create non-
ordinary Labels (i.e., labels that may alter the default best effort
service). Labels SHOULD be associated with mechanisms to provide
strong end-to-end integrity during their transmission through the
telephony systems. Finally, in cases where labels are expected to be
acted upon by operators, these operators SHOULD have the capability
of authenticating the label on a received message or transmission in
order to prevent theft of service and reduce risk of denial of
service (e.g., by unauthorized users consuming any limited
resources).
Security is also discussed in the general requirements of [3], which
applies to section 3 above.
6. References
6.1. Normative Reference
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[2] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[3] Carlberg, K. and R. Atkinson, "General System Requirements for
Emergency Telecommunications Service", RFC 3689, February 2004.
[4] Schulzrinne, H., "Requirements for Resource Priority Mechanisms
for the Session Initiation Protocol (SIP)", RFC 3487, February
2003.
[5] ANSI, "Signaling System No. 7(SS7): High Probability of
Completion (HPC) Network Capability", ANSI T1.631, 1993.
7. Authors' Addresses
Ken Carlberg
University College London
Department of Computer Science
Gower Street
London, WC1E 6BT
United Kingdom
EMail: k.carlberg@cs.ucl.ac.uk
Ran Atkinson
Extreme Networks
3585 Monroe Street
Santa Clara, CA
95051 USA
EMail: rja@extremenetworks.com
8. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Why Emergency Calls Need Requirements
After disasters such as hurricanes, floods, earthquakes and terrorist attacks, recovery work needs immediate access to whatever public telecommunications are at hand. The RFC lists conventional telephones, cellular phones, and Internet access through online terminals, IP telephones and wireless PDAs. Because commercial telecommunications is moving to Internet-based technology, the Internet community needs to think about how to support emergency management.
Other standards bodies have already defined emergency indicators. For example, T1.631 defines specific indicators or labels for emergency communications in Signaling System 7 (SS7) networks. To work across hybrid networks, which connect IP to other networks such as the Public Switched Telephone Network, requirements must be defined so that services can interwork.
Emergency calls already have special markings on the old phone network. The Internet needs rules so those markings are not lost when a call moves between the two.
§2Scope and What Is Not Required
This document extends the general Emergency Telecommunications Service (ETS) requirements in RFC 3689 to the specific case of IP telephony, viewed as an end-to-end application layer service. It does not define solutions, protocol enhancements or specifications. It also notes that RFC 3487, the resource priority requirements for SIP, overlaps with it but applies only to SIP.
The RFC does not mandate that anyone accept the requirements. The IETF does not mandate capabilities for the independent networks that make up the Internet, so an ISP may choose not to run telephony gateways or services at all. The RFC expects that business contracts such as Service Level Agreements will be used for requirements that apply to service providers, and that without an SLA only best effort service is provided. The requirements do not apply to ISPs that choose not to offer ETS.
The list is advice, not a law. A company that does not offer emergency phone service does not have to follow it.
§3The Five IP Telephony Requirements
The requirements cover only telephony signaling, and explicitly not IP-layer mechanisms such as Differentiated Services or Integrated Services.
- Signaling applications MUST be able to carry labels.
- The ability to carry labels MUST be extensible to support various types and numbers of labels, because a single binary value is not enough given the labeling standards that exist.
- Signaling labels SHOULD map to the emergency labels used in other telephony networks such as the PSTN, so a call over a hybrid path can carry its labels end to end with translation at the boundaries. With no mapping, signaling reverts to a default service, presumably the one for the general public.
- Application layer capabilities MUST NOT preclude application layer accounting, which is useful for billing and for tracking down abuse.
- Gateways and stateful proxies that recognise ETS labels MUST be able to support a 'best available' service, probably better than best effort.
For best available service, the focus SHOULD be on the probability of forwarding packets. That probability may reach 100% depending on local policy for the label, and local policy MUST decide whether better than best effort applies to a given label. Being able to support it does not mean the mechanism is expected to be activated, and the RFC does not define how it is realised.
The phone system must be able to attach a flag to an urgent call, understand different kinds of flags, and give flagged calls a better chance of getting through. How exactly is left for someone else to work out.
§4Open Issues: Alternate Paths and Best Available Service
Section 4 lists issues for anyone designing solutions. It is not a list of requirements. The first is alternate paths. Experience with the Government Emergency Telecommunications Service (GETS) over the PSTN showed that alternate paths to a destination help emergency calls. On the Internet this may be harder and of more limited benefit: the PSTN fixes a path during call setup, while the Internet uses dynamic routing and has no concept of a call or call setup, even though IP telephony applications may be aware of calls.
The second is how far best available service reaches. Here its scope is limited to the application layer and the flows that pass through it. Support may be direct for the flow carrying the label, or indirect, for example when labels in signaling messages affect the matching data flows. How support is realised, and how effective it is, are outside the scope of the document.
On the old phone network a call gets one fixed road, so you can plan a backup road. On the Internet every little packet picks its own road, which makes that idea trickier.
§5Security Expectations for Labels
Only authorized users or operators SHOULD be able to create non-ordinary labels, meaning labels that may alter the default best effort service. Labels SHOULD come with mechanisms for strong end-to-end integrity while they travel through telephony systems.
Where operators are expected to act on labels, they SHOULD be able to authenticate the label on a received message. This is meant to prevent theft of service and reduce the risk of denial of service, for example by unauthorized users consuming limited resources. The general security discussion in RFC 3689 also applies.
If anyone could stamp URGENT on their call, the stamp would be useless. So only the right people should be able to use it, and the stamp must not be forgeable on the way.