Internet Engineering Task Force Ken Carlberg
INTERNET DRAFT Ian Brown
Jun 24, 2002 UCL
Framework for Supporting IEPS in IP Telephony
<draft-ietf-ieprep-framework-02.txt>
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Abstract
This document presents a framework for supporting authorized
emergency related communication within the context of IP telephony.
We present a series of objectives that reflect a general view of how
authorized emergency service, in line with the International
Emergency Preparedness Scheme (IEPS), should be realized within
today's IP architecture and service models. From these objectives,
we present a corresponding set of functional requirements, which
provide a more specific set of recommendations regarding existing
IETF protocols. Finally, we present two scenarios that act as
guiding models for the objectives and functions listed in this
document. These, models, coupled with an example of an existing
service in the PSTN, contribute to a constrained solution space.
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1. Introduction
The Internet has become the primary target for worldwide communica-
tions. This is in terms of recreation, business, and various ima-
ginative reasons for information distribution. A constant fixture in
the evolution of the Internet has been the support of Best Effort as
the default service model. Best Effort, in general terms, infers
that the network will attempt to forward traffic to the destination
as best as it can with no guarantees being made, nor any resources
reserved, to support specific measures of Quality of Service (QoS).
An underlying goal is to be "fair" to all the traffic in terms of the
resources used to forward it to the destination.
In an attempt to go beyond best effort service, [2] presented an
overview of Integrated Services (int-serv) and its inclusion into the
Internet architecture. This was followed by [3], which specified the
RSVP signaling protocol used to convey QoS requirements. With the
addition of [4] and [5], specifying control load (bandwidth bounds)
and guaranteed service (bandwidth & delay bounds) respectively, a
design existed to achieve specific measures of QoS for an end-to-end
flow of traffic traversing an IP network. In this case, our refer-
ence to a flow is one that is granular in definition and applying to
specific application sessions.
From a deployment perspective (as of the date of this document),
int-serv has been predominantly constrained to stub intra-domain
paths, at best resembling isolated "island" reservations for specific
types of traffic (e.g., audio and video) by stub domains. [6] and
[7] will probably contribute to additional deployment of int-serv to
Internet Service Providers (ISP) and possibly some inter-domain
paths, but it seems unlikely that the original vision of end-to-end
int-serv between hosts in source and destination stub domains will
become a reality in the near future (the mid- to far-term is a sub-
ject for others to contemplate).
In 1998, the IETF produced [8], which presented an architecture for
Differentiated Services (diff-serv). This effort focused on a more
aggregated perspective and classification of packets than that of
[2]. This is accomplished with the recent specification of the
diff-serv field in the IP header (in the case of IPv4, it replaced
the old ToS field). This new field is used for code points esta-
blished by IANA, or set aside as experimental. It can be expected
that sets of microflows, a granular identification of a set of pack-
ets, will correspond to a given code point, thereby achieving an
aggregated treatment of data.
One constant in the introduction of new service models has been the
designation of Best Effort as the default service model. If traffic
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is not, or cannot be, associated as diff-serv or int-serv, then it is
treated as Best Effort and uses what resources are made available to
it.
Beyond the introduction of new services, the continued pace of addi-
tional traffic load experienced by ISPs over the years has continued
to place a high importance for intra-domain traffic engineering. The
explosion of IETF contributions, in the form of drafts and RFCs pro-
duced in the area of Multi Protocol Label Switching (MPLS), exempli-
fies the interest in versatile and manageable mechanisms for intra-
domain traffic engineering. One interesting observation is the work
involved in supporting QoS related traffic engineering. Specifically,
we refer to MPLS support of differentiated services [9], and the on-
going work in the inclusion of fault tolerance [10]. This latter
item can be viewed as an example that is similar to "crank-back" (or
"auto-rerouting"), a term used to describe the means by which the
Public Switched Telephone Network (PSTN) routes around congested
switches.
1.1. Emergency Related Data
The evolution of the IP service model architecture has traditionally
centered on the type of application protocols used over a network.
By this we mean that the distinction, and possible bounds on QoS,
usually centers on the type of application (e.g., audio video tools)
that is being referred to.
While protocols like SMTP [11] and SIP [12] have embedded fields
denoting "priority", there has not been a previous IETF standards
based effort to state or define what this distinction means with
respect to the underlying network and how it should be supported.
Given the emergence of IP telephony, a natural inclusion of it as
part of a telco carrier's backbone network, or into the Internet as a
whole, implies the ability to support existing emergency related ser-
vices. Typically, one associates emergency calls with "911" tele-
phone service in the U.S., or "999" in the U.K. -- both of which are
attributed to national boundaries and accessible by the general pub-
lic. Outside of this exists emergency telephone services that
involved authorized usage, as described in the following subsection.
1.1.1. Government Emergency Telecommunications Service (GETS)
GETS is an emergency telecommunications service available in the U.S.
and overseen by the National Communications System (NCS) -- an office
established by the White House under an executive order [30]. Unlike
"911", it is only accessible by authorized individuals. The majority
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of these individuals are from various government agencies like the
Department of Transportation, NASA, the Department of Defense, and
the Federal Emergency Management Agency (to name but a few). In
addition, a select set of individuals from private industry (telecom-
munications companies, utilities, etc.) that are involved in criti-
cial infrastructure recovery operations are also provided access to
GETS.
The purpose of GETS is to increase the probability that phone service
will be available to selected government agency personnel in times of
emergencies, such as hurricanes, earthquakes, and other disasters
that may produce a burden in the form of call blocking (i.e., conges-
tion) on the U.S. Public Switched Telephone Network by the general
public.
The key aspect of GETS is that it supports a probabilistic approach
to call completion through priority, as opposed to guaranteed
approach through preemption. This distinction is important because
emergency services like GETS are not allowed to tear down existing
calls (i.e., seize resources) in order to establish a GETS call.
Instead, GETS increases the probability of call completion by provid-
ing an additional label used in the contention for assignment of lim-
ited resources required for the call. Thus, the GETS features focus
on increasing the probability that a particular telephone call will
be established, but cannot guarantee call completion.
GETS is supported by Signaling System 7 (SS7) via the T1.631 protocol
on High Probability of Completion (HPC) network capability [13].
This document describes the specification of a National Security and
Emergency Preparedness (NS/EP) Calling Party Category (CPC) code
point used for SS7 ISDN User Part (ISUP) Initial Address Message
(IAM). In the presence of this code point, when a GETS call
encounters a restrictive network management control that has been
activated to reduce traffic overload to a congested route, the Local
Exchange Carriers (LECs) will provide the GETS call priority by
exempting the call from this restriction. After receiving the exemp-
tion, if the GETS call finds all circuits busy in the route, the LEC
will provide further priority by queuing the call for the next avail-
able circuit.
The procedure for a user (i.e., a person) establishing a GETS call is
as follows:
1) Dial a non-geographical area code number: 710-XXX-XXXX
2) Dial a PIN used to authenticate the call
3) Dial the actual destination number to be reached
In conjunction with the above, the source LEC (where the call
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originated) attempts to establish the call through an IXC. This is
done even if the destination number is within the LEC itself. If the
IXC cannot forward the call to the destination LEC, then the source
LEC attempts to route the call through an alternate IXC. If alter-
nate IXCs cannot help establish the call, then a busy signal is
finally returned to the user. Otherwise, the call is completed and
retains the same quality of service as all other telephone calls.
The HPC component of GETS is not ubiquitously supported by the U.S.
PSTN. The only expectation is that the 710 area code is recognized
by all carriers. Additional support is conditional and dependent
upon the equivalent Service Level Agreements (SLA) established
between the U.S. Government and various telco carriers to support
GETS. Thus, the default end-to-end service for establishing a GETS
call can be roughly viewed as best effort and associated with the
same priority as calls from the general public.
It should be noted from the above description that GETS is separate
and unrelated to other emergency services like "911".
1.1.2. International Emergency Preparedness Scheme (IEPS)
[18] is a recent ITU standard that describes emergency related com-
munications over international telephone service (Note, this document
has also been published as a draft-RFC in [28]). While systems like
GETS are national in scope, IEPS acts as an extension to local or
national authorized emergency call establishment and provides a
building block for a global service.
As in the case of GETS, IEPS promotes mechanisms like extended queu-
ing, alternate routing, and exemption from restrictive management
controls in order to increase the probability that international
emergency calls will be established. The specifics of how this is to
be accomplished are to be defined in future ITU document(s).
1.2. Scope of this Document
The scope of this document centers on the near and mid-term support
of IEPS within the context of IP telephony, though not necessarily
Voice over IP. We make a distinction between these two by treating
IP telephony as a subset of VoIP, where in the former case we assume
some form of application layer signaling is used to explicitly estab-
lish and maintain voice data traffic. This explicit signaling capa-
bility provides the hooks from which VoIP traffic can be bridged to
the PSTN.
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An example of this distinction is when the Robust Audio Tool (RAT)
[14] begins sending VoIP packets to a unicast (or multicast) destina-
tion. RAT does not use explicit signaling like SIP to establish an
end-to-end call between two users. It simply sends data packets to
the target destination. On the other hand, "SIP phones" are host
devices that use a signaling protocol to establish a call signal
before sending data towards the destination.
One other aspect we should probably assume exists with IP Telephony
is an association of a target level of QoS per session or flow. [31]
makes an arguement that there is a maximum packet loss and delay for
VoIP traffic, and both are interdependent. For delays of ~200ms, a
corresponding drop rate of 5% is deemed acceptable. When delay is
lower, a 15-20% drop rate can be experienced and still considered
acceptable. [32] discusses the same topic and makes an arguement
that packet size plays a significant role in what users tolerate as
"intelligible" VoIP. The larger the packet, correlating to longer
sampling rate, the lower the acceptable rate of loss.
Regardless of a definitive drop rate, it would seem that interactive
voice has a lower threshold of loss than other elastic applications.
This places a higher burden on the problem space of supporting VoIP
over the Internet. This problem is further compounded when toll-
quality service is expected because it assumes a default service
model that is better than best effort. This in turn can increase the
probability that a form of call-blocking can occur with VoIP or IP
telephony traffic.
Beyond this, part of our motivation in writing this document is to
provide a framework for ISPs and carriers so that they have an under-
standing of objectives and accompanying functional requirements used
to support IEPS related IP telephony traffic. In addition, we also
wish to provide a reference point for potential customers (users of
IEPS) in order to constrain their expectations. In particular, we
wish to avoid any temptation of trying to replicate the exact capa-
bilities of existing emergency voice service currently available in
the PSTN to that of IP and the Internet. If nothing else, intrinsic
differences between the two communications architectures precludes
this from happening. Note, this does not prevent us from borrowing
design concepts or objectives from existing systems.
Section 2 presents several primary objectives that articulate what is
considered important in supporting IEPS related IP telephony traffic.
These objectives represent a generic set of goals and capabilities
attributed to supporting IEPS based IP telephony. Section 3 presents
additional value added objectives. These are capabilities that are
viewed as useful, but not critical in support of IEPS. Section 4
presents a series of functional requirements that stem from the
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objectives articulated in section 2. Finally, Section 5 presents two
scenarios in IEPS that currently exist or are being deployed in the
near term over IP networks. These are not all-inclusive scenarios,
nor are they the only ones that can be articulated. However, they do
show cases where some of the functional requirements apply, and where
some do not.
Finally, we need to state that this document focuses its attention on
the IP layer and above. Specific operational procedures pertaining
to Network Operation Centers (NOC) or Network Information Centers
(NIC) are outside the scope of this document. This includes the
"bits" below IP, other specific technologies, and service level
agreements between ISPs and carriers with regard to dedicated links.
2. Objective
The support of IEPS within IP telephony can be realized in the form
of several primary objectives. These objectives define the generic
functions or capabilities associated with IEPS, and the scope of the
support needed to achieve these capabilities. From this generic set
of objectives, we present specific functional requirements of exist-
ing IP protocols (presented below in section 3).
There are two underlying goals in the selection of these objectives.
One goal is to produce a design that maximizes the use of existing IP
protocols and minimizes the set of additional specifications needed
to support IP-telephony based IEPS. Thus, with the inclusion of
these minimal augmentations, the bulk of the work in achieving IEPS
over an IP network that is connected or unconnected to the Internet
involves operational issues. Examples of this would be the estab-
lishment of Service Level Agreements (SLA) with ISPs, and/or the pro-
visioning of traffic engineered paths for IEPS-related telephony
traffic.
A second underlying goal in selecting the following objectives is to
take into account experiences from an existing emergency-type commun-
ication system (as described in section 1.1.1) as well as the exist-
ing restrictions and constraints placed by some countries. In the
former case, we do not attempt to mimic the system, but rather
extract information as a reference model. With respect to con-
straints based on laws or agency regulations, this would normally be
considered outside of the scope of any IETF document. However, these
constraints act as a means of determining the lowest common denomina-
tor in specifying technical functional requirements. If such con-
straints do not exist, then additional functions can be added to the
baseline set of functions. This last item will be expanded upon in
the description of Objective #3 below.
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The following list of objectives are termed primary because they per-
tain to that which defines the underlying goals of IEPS. However,
the primary objectives are not meant to dictate major overhauls of
existing IP protocols, nor do they require completely new protocols
to be developed.
Primary Objectives in support of authorized emergency calls:
1) High Probability of Call Completion
2) Interaction with PSTN
3) Distinction of IEPS data traffic
4) Non-preemptive action
5) Non-ubiquitous support
6) Authenticated service
The first objective is the crux of our work because it defines our
expectations for both data and call signaling for IP telephony. As
stated, our objective is achieving a high probability that emergency
related calls (both data and signaling) will be forwarded through an
IP network. Specifically, we envision the relevance of this objec-
tive during times of congestion, the context of which we describe
further below in this section. The critical word in this objective
is "probability", as opposed to assurance or guarantee -- the latter
two placing a higher burden on the network. It stands to reason,
though, that the word "probability" is a less tangible description
that cannot be easily quantified. It is relative in relation to
other traffic transiting the same network. Objectives 4 and 5 listed
above help us to qualify the term probability in the context of other
objectives.
The second objective involves the interaction of IP telephony signal-
ing with existing PSTN support for emergency related voice communica-
tions. As mentioned above in Section 1.2, standard T1.631 [26]
specifies emergency code points for SS7. Specifically, the National
Security and Emergency Preparedness (NS/EP) Calling Party Category
code point is defined for ISUP IAM messages used by SS7 [26]. Hence,
our objective in the interaction between the PSTN and IP telephony
with respect to IEPS (and national indicators) is a direct mapping
between related code points.
The third objective focuses on the ability to distinguish IEPS data
packets from other types of VoIP packets. With such an ability,
transit providers can more easily ensure that service level agree-
ments relating to IEPS are adhered to. Note that we do not assume
that the actions taken to distinguish IEPS type packets is easy.
Nor, in this section, do we state the form of this distinction. We
simply present the objective of identifying flows that relate to IEPS
versus others that traverse a transit network.
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At an abstract level, the fourth objective pertains to the actions
taken when an IP telephony call, via a signaling protocol such as
SIP, cannot be forwarded because the network is experiencing a form
of congestion. We state this in general terms because of two rea-
sons: a) there may exist applications other than SIP, like H.248,
used for call establishment, and b) congestion may come in several
forms. For example, congestion may exist at the IP packet layer with
respect to queues being filled to their configured limit. Congestion
may also arise from resource allocation (i.e., QoS) attributed per
call or aggregated sets of calls. In this latter case, while there
may exist resources to forward the packets, a signaling server may
have reached its limit as to how many telephony calls it will support
while retaining toll-quality service per call. Typically, one terms
this form of congestion as call blocking. Note that we do not
address the case when congestion occurs at the bit level below that
of IP, due to the position that it is outside the scope of IP and the
IETF.
So, given the existence of congestion in its various forms, our
objective is to support IEPS-related IP telephony call signaling and
data traffic via non-preemptive actions taken by the network. More
specifically, we associate this objective in the context of IP
telephony acting as part of the Public Telephone Network (PTN).
This, as opposed to the use of IP telephony within a private or stub
network. In section 5 below, we expand on this through the descrip-
tion of two distinct scenarios of IP telephony and its operation with
IEPS and the PSTN.
It is important to mention that this is a default objective influ-
enced by existing laws & regulations. Those countries not bound by
these restrictions can remove this objective and make provisions to
enforce preemptive action. In this case, it would probably be advan-
tageous to deploy a signaling system similar to that proposed in
[15], wherein multiple levels of priority are defined and preemption
via admission control from SIP servers is enforced.
The fifth objective stipulates that we do not advocate the need or
expectation for ubiquitous support of IEPS across all administrative
domains of the Internet. While it would be desirable to have ubiqui-
tous support, we feel the reliance of such a requirement would doom
even the contemplation of supporting IEPS by the IETF and the
expected entities (e.g., ISPs and vendors) involved in its deploy-
ment.
We use the existing GETS service in the U.S. as an existing example
in which emergency related communications does not need to be ubiqui-
tous. As mentioned previously, the measure and amount of support
provided by the U.S. PSTN for GETS does not exist for all U.S. IXCs
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nor LECs. Given the fact that GETS still works within this context,
it is our objective to follow this deployment model such that we can
accomplish the first objective listed above -- a higher probability
of call completion than that of normal IP telephony call traffic.
Our final objective is that only authorized users may use the ser-
vices outlined in this framework. GETS users are authenticated using
a PIN provided to the telecommunications carrier, which signals
authentication to subsequent networks via the HPC class mark. In an
IP network, the authentication center will need to securely signal
back to the IP ingress point that a given user is authorized to send
IEPS related flows. Similarly, transit networks with IEPS SLAs must
securely interchange authorized IEPS traffic. In both cases, IPSec
authentication transforms may be used to protect this traffic. This
is entirely separate from end-to-end IPSec protection of user
traffic, which will be configured by users. IP-PSTN gateways must
also be able to securely signal IEPS authorization for a given flow.
As these gateways are likely to act as SIP servers, we further con-
sider the use of SIP's security functions to aid this objective.
3. Value Added Objective
This objective is viewed as being helpful in achieving a high proba-
bility of call completion. Its realization within an IP network
would be in the form of new protocols or enhancements to existing
ones. Thus, objectives listed in this section are treated as value
added -- an expectation that their existence would be beneficial, and
yet not viewed as critical to support IEPS related IP telephony
traffic.
3.1. Alternate Path Routing
This objective involves the ability to discover and use a different
path to route IP telephony traffic around congestion points and thus
avoid them. Ideally, the discovery process would be accomplished in
an expedient manner (possibly even a priori to the need of its
existence). At this level, we make no requirements as to how the
alternate path is accomplished, or even at which layer it is achieved
-- e.g., the network versus the application layer. But this kind of
capability, at least in a minimal form, would help contribute to
increasing the probability of call completion of IEPS traffic by mak-
ing use of noncongested alternate paths. We use the term "minimal
form" to emphasize the fact that care must be taken in how the system
provides alternate paths so it does not significantly contribute to
the congestion that is to be avoided (e.g., via excess
control/discovery messages).
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At the time that this document was written, we can identify two
work-in-progress areas in the IETF that can be helpful in providing
alternate paths for call signaling. The first is [10], which is
focused on network layer routing and describes enhancements to the
LDP specification of MPLS to help achieve fault tolerance. This in
itself does not provide alternate path routing, but rather helps
minimize loss in intradomain connectivity when MPLS is used within a
domain.
The second effort comes from the IP Telephony working group and
involves Telephony Routing over IP (TRIP). To date, a framework
document [19] has been published as an RFC which describes the
discovery and exchange of IP telephony gateway routing tables between
providers. The TRIP protocol [22] specifies application level
telephony routing regardless of the signaling protocol being used
(e.g., SIP or H.323). TRIP is modeled after BGP-4 and advertises
reachability and attributes of destinations. In its current form,
several attributes have already been defined, such as LocalPreference
and MultiExitDisc. Additional attributes can be registered with
IANA. Initially, we would recommend two attributes that would relate
to emergency related flows. These being:
EmergencyMultiExitDisc
The EmergencyMultiExitDisc attribute is similar to the
MultiExitDisc in that it is an inter-domain attribute used
to express a preference of one or more links over others
between domains. Unlike the MultiExitDisc, this attribute
specifically identifies links that are preferred for emergency
related calls that span domains.
EmergencyLocalPreference
The EmergencyLocalPreference attribute is similar to the
LocalPreference in that it is an intra-domain attribute used
to inform other LSs of the local LSs preference for a given
route. The difference between the two types attributes is
that the preferred route specifically relates to emergency-type
calls (e.g., 911). This attribute has no significance between
domains. Local policy determines if there is an association
between the EmergencyLocalPreference and the
EmergencyMultiExitDisc attribute.
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3.2. End-to-End Fault Tolerance
This topic involves the work that has been done in trying to compen-
sate for lossy networks providing best effort service. In particu-
lar, we focus on the use of a) Forward Error Correction (FEC), and b)
redundant transmissions that can be used to compensate for lost data
packets. (Note that our aim is fault tolerance, as opposed to an
expectation of always achieving it).
In the former case, additional FEC data packets are constructed from
a set of original data packets and inserted into the end-to-end
stream. Depending on the algorithm used, these FEC packets can
reconstruct one or more of the original set that were lost by the
network. An example may be in the form of a 10:3 ratio, in which 10
original packets are used to generate three additional FEC packets.
Thus, if the network loses 30% or less number of packets, then the
FEC scheme will be able to compensate for that loss. The drawback to
this approach is that to compensate for the loss, a steady state
increase in offered load has been injected into the network. This
makes an arguement that the act of protection against loss has con-
tributed to additional pressures leading to congestion, which in turn
helps trigger packet loss. In addition, in using a ratio of 10:3,
the source (or some proxy) must "hold" all 10 packets in order to
construct the three FEC packets. This contributes to the end-to-end
delay of the packets as well as minor bursts of load in addition to
changes in jitter.
The other form of fault tolerance we discuss involves the use of
redundant transmissions. By this we mean the case in which an origi-
nal data packet is followed by one or more redundant packets. At
first glance, this would appear to be even less friendly to the net-
work than that of adding FEC packets. However, the encodings of the
redundant packets can be of a different type (or even transcoded into
a lower quality) that produce redundant data packets that are signi-
ficantly smaller than the original packet.
Two RFCs [24, 25] have been produced that define RTP payloads for FEC
and redundant audio data. An implementation example of a redundant
audio application can be found in [14]. We note that both FEC and
redundant transmissions can be viewed as rather specific and to a
degree tangential solutions regarding packet loss and emergency com-
munications. Hence, these topics are placed under the category of
value added objectives.
4. Functional Requirements
In this section, we take the objectives presented above and specify a
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corresponding set of functional requirements to achieve them. Given
that the objectives are predominantly atomic in nature, the
corresponding functional requirements are to be viewed separately
with no specific dependency upon each other as a whole. They may be
complimentary with each other, but there is no need for all to exist
given different scenarios of operation, and that IEPS support is not
viewed as a ubiquitously available service. We divide the functional
requirements into 4 areas:
1) Signaling
2) Policy
3) Traffic Engineering
4) Security
4.1. Signaling
Signaling is used to convey various information to either intermedi-
ate nodes or end nodes. It can be out-of-band of a data flow, and
thus in a separate flow of its own, such as SIP messages. It can be
in-band and part of the state information in a datagram containing
the voice data. This latter example could be realized in the form of
diff-serv code points in the IP packet.
In the following subsections, we discuss potential augmentations to
different types of signaling and state information to help support
the distinction of emergency related communications in general, and
IEPS specifically.
4.1.1. SIP
With respect to application level signaling for IP telephony, we
focus our attention to the Session Initiation Protocol (SIP).
Currently, SIP has an existing "priority" field in the Request-
Header-Field that distinguishes different types of sessions. The
five currently defined values are: "emergency", "urgent", "normal",
"non-urgent", "other-priority". These values are meant to convey
importance to the end-user and have no additional sematics associated
with them.
[15] is a work in progress example that defines a new header field
for SIP known as the Resource Priority Header. This new header field
is meant to provide an additional measure of distinction that can
influence the behavior of gateways and SIP proxies. The structure of
the field is in the form of a NameSpace.Priority. The "NameSpace"
provides a reference point by which the "Priority" values correspond
to.
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Additional namespaces and value(s) would be registered with IANA. It
would be our intention to follow the approach taken in [15] and
define a label for a new optional field in SIP to distinguish IEPS
calls from other calls. Assuming [15] becomes the accepted practice,
we would also register a new namespace attributed to IEPS. We would
define a single value (e.g., "authorized-emergency") that would
correspond to the single NS/EP code point of SS7. This will help
facilitate a seamless interaction between the PSTN and an IP network
acting as either an internal backbone or as a peering ISP.
This document does not specify the exact actions taken by a SIP node
upon receipt of the label corresponding to IEPS (authorized-
emergency) calls. This is a function of policy and to be articulated
in a separate document. We can speculate that in following the
objective of increased probability of call completion, there will be
a separate queue for IEPS-labeled calls. If a threshold for number
of calls exists within the SIP server one action may simply be "hold-
ing" the IEPS labeled request for an additional period in case a new
call can be placed. However, we stress this is only an example to
consider.
Note #1: Previous work that has been put forth by Polk in [34], which
describes an architecture for MLPP over IP networks, is similar to
the subject of IEPS in the sense that both aim at distinguishing cer-
tain VoIP flows from others. However, MLPP and IEPS are not the same
efforts. One critical difference is that MLPP involves the use of
preemption, while the default model for IEPS is simply an increase in
the probability of call completion.
Note #2: The term "Priority" has been a subject of strong debate. In
this document, we reference the term based on the terminology inher-
ited from other drafts and documents, such as can be found in [15],
and the Megaco RFC [23]. However, our focus is aimed at using the
"priority" value as simply a label by which we distinguish one set of
flows from another.
4.1.2. Diff-Serv
In accordance with [16], the differentiated services code point
(DSCP) field is divided into three sets of values. The first set is
assigned by IANA. Within this set, there are currently, three types
of Per Hop Behaviors that have been specified: Default (correlating
to best effort forwarding), Assured Forwarding, and Expedited For-
warding. The second set of DSCP values are set aside for local or
experimental use. The third set of DSCP values are also set aside
for local or experimental use, but may later be reassigned to IANA in
case the first set has been completely assigned.
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One candidate recomendation involves the specification of a new type
of Per-Hop Behavior (PHB). This would provide a specific means of
distinguishing emergency related traffic (signaling and user data)
from other traffic. The existence of this PHB then provides a base-
line by which specific code points may be defined related to various
emergency related traffic: authorized emergency sessions (e.g.,
IEPS), general public emergency calls (e.g., "911"), MLPP. Aggre-
gates would still exist with respect to the bundling of applications
per code point. Further, one would associate a forwarding paradigm
aimed at a low loss rate reflective of the code point selected. The
new PHB could be in the form of a one or more code points that dupli-
cate EF-type traffic characteristics. Policies would determine IF a
measure of importance exists per EF-type code-point.
A potential issue that could be addressed by a new PHB involves merge
points of flows within a diff-serv domain. With EF, one can expect
admission control being performed at the edges of the domain.
Presumably, careful traffic engineering would be applied to avoid
congestion of EF queues at internal/core merge points stemming from
flows originating from different ingress nodes of the diff-serv
domain. However, traffic engineering may not be able to compensate
for congestion of EF-type traffic at the domain's core routers.
Hence, a new PHB that has more than one code point to identify EF-
type traffic may address congestion by associating a drop precedence
for certain types of EF-type datagrams. Note that local policy and
SLAs would define which EF-type of traffic, if any, would be associ-
ated with a specific drop precedence.
Another candidate recomendation would be to define a new or fifth
class for the existing AF PHB. Unlike the other currently defined
classes, this new one would be based on five levels of drop pre-
cedence. This increase in the number of levels would conveniently
correlate to the the levels of MLPP, which has five types of priori-
ties. The five levels would also correlate to a recent effort in the
Study Group 11 of the ITU to define 5 levels for Emergency Telecom-
munications Service (ETS). Beyond these other standardization
efforts, the 5 levels would provide a higher level of variance that
could be used to supercede the existing 3 levels used in the other
classes. Hence, if other non-emergency aggregate traffic were
assigned to the new class, the highest drop precedence they are
assigned to is (3) -- corresponding to the other four currently
defined classes. Emergency traffic would be set to (4) or (5),
depending on the SLA tht has been defined.
It is important to note that as of the time that this document was
written, the IETF is taking a conservative approach in specifying new
PHBs. This is because the number of code points that can be defined
is relatively small, and thus understandably considered a scarce
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resource. Therefore, the possibility of a new PHB being defined for
emergency related traffic is at best a long term project that may or
may not be accepted by the IETF. In the near term, we would ini-
tially recommend using the Assured Forwarding (AF) PHB [20] for dis-
tinguishing emergency traffic from other types of flows. At a
minimum, AF would be used for the different SIP call signaling mes-
sages. If EF was also supported by the domain, then it would be used
for IP telephony data packets. Otherwise, another AF class would be
used for those data flows.
It is critical to note that one cannot specify an exact code point
used for emergency related data flows because the relevance of a code
point is local to the given diff-serv domain (i.e., they are not glo-
bally unique per micro-flow or aggregate of flows). In addition, we
can expect that the existence of a codepoint for emergency related
flows is based on the service level agreements established with a
given diff-serv domain.
4.1.3. RTP
The Real-Time Transport Protocol (RTP) provides end-to-end delivery
services for data with real-time characteristics. The type of data
is generally in the form of audio or video type applications, and are
frequently interactive in nature. RTP is typically run over UDP and
has been designed with a fixed header that identifies a specific type
of payload -- typically representing a specific form of application
media. The designers of RTP also assumed an underlying network pro-
viding best effort service. As such, RTP does not provide any
mechanism to ensure timely delivery or provide other QoS guarantees.
However, the emergence of applications like IP telephony, as well as
new service models, presents new environments where RTP traffic may
be forwarded over networks that support better than best effort ser-
vice. Hence, the original scope and target environment for RTP has
expanded to include networks providing services other than best
effort.
In 4.1.2, we discussed one means of marking a data packet for emer-
gencies under the context of the diff-serv architecture. However, we
also pointed out that diff-serv markings for specific PHBs are not
globally unique, and may be arbitrarily removed or even changed by
intermediary nodes or domains. Hence, with respect to emergency
related data packets, we are still missing an in-band marking in a
data packet that stays constant on an end-to-end basis.
There are have three choices in defining a persistent marking of data
packets and thus avoid the transitory marking of diff-serv code
points. We can propose a new PHB dedicated for emergency type
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traffic as discussed in 4.1.2. We can propose a specification of a
new shim layer protocol at some location above IP. Or, we can add a
new specification to an existing application layer protocol. The
first two cases are probably the "cleanest" architecturally, but they
are long term efforts that may not come to pass because of a limited
amount of diff-serv code points and the contention that yet another
shim layer will make the IP stack too large. The third case, placing
a marking in an application layer packet, also has drawbacks; the key
weakness being the specification of a marking on a per-application
basis.
Discussions have been held in the Audio/Visual Transport (AVT) work-
ing group of augmenting RTP so that it can carry a marking that dis-
tinguishes emergency-related traffic from that which is not. Specif-
ically, one would define a new extention that contains a "classifier"
field indicating the condition associated with the packet (e.g.,
authorized-emergency, emergency, normal) [29]. The rationale behind
this idea was that focusing on RTP would allow one to rely on a point
of aggregation that would apply to all payloads that it encapsulates.
However, the AVT group has expressed a rough consensus that placing
additional classifier state in the RTP header to denote the impor-
tance of one flow over another is not an approach that they wish to
advance. Objections ranging from relying on SIP to convey importance
of a flow, as well as the possibility of adversely affecting header
compression, were expressed. There was also the general feeling that
the extension header for RTP should not be used for RTP packet of a
flow.
Author's note: There was some debate as to whether to keep the above
subsection concerning RTP in this document. We have decided to
retain it because it is felt that information concerning directions
that should NOT be taken to support IEPS is important to the commun-
ity at large.
4.1.4. MEGACO/H.248
The Media Gateway Control protocol (MEGACO) [23] defines the interac-
tion between a media gateway and a media gateway controller. [23] is
viewed as common text with ITU-T Recommendation H.248 and is a result
of applying the changes of RFC 2886 (Megaco Errata) to the text of
RFC 2885 (Megaco Protocol version 0.8).
In [23], the protocol specifies a Priority and Emergency field for a
context attribute and descriptor. The Emergency is an optional
boolean (True or False) condition. The Priority value, which ranges
from 0 through 15, specifies the precedence handling for a context.
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The protocol does not specify individual values for priority. We
also do not recommend the definition of a well known value for the
MEGAGO priority. Any values set should be a function of any SLAs
that have been established regarding the handling of emergency
traffic. In addition, given that priority values denote precedence
(according to the Megaco protocol), then by default the IEPS data
flows should probably receive the same priority as other non-
emergency calls. This approach follows the objective of not relying
on preemption as the default treatment of emergency-related.
4.2. Policy
One of the objectives listed in section 3 above is to treat IEPS-
signaling, and related data traffic, as non-preemptive in nature.
Further, that this treatment is to be the default mode of operation
or service. This is in recognition that existing regulations or laws
of certain countries governing the establishment of SLAs may not
allow preemptive actions (e.g., dropping existing telephony flows).
On the other hand, the laws and regulations of other countries
influencing the specification of SLA(s) may allow preemption, or even
require its existence. Given this disparity, we rely on local policy
to determine the degree by which emergency related traffic affects
existing traffic load of a given network or ISP. Important note: we
reiterate our earlier comment that laws and regulations are generally
outside the scope of the IETF and its specification of designs and
protocols. However, these constraints can be used as a guide in pro-
ducing a baseline function to be supported; in our case, a default
policy for non-preemptive call establishment of IEPS signaling and
data.
Policy can be in the form of static information embedded in various
components (e.g., SIP servers or bandwidth brokers), or it can be
realized and supported via COPS with respect to allocation of a
domain's resources [17]. There is no requirement as to how policy is
accomplished. Instead, if a domain follows actions outside of the
default non-preemptive action of IEPS-related communication, then we
stipulate a functional requirement that some type of policy mechanism
is in place to satisfy the local policies of an SLA established for
IEPS type traffic.
4.3. Traffic Engineering
In those cases where a network operates under the constraints of
SLAs, one or more of which pertains to IEPS based traffic, it can be
expected that some form of traffic engineering is applied to the
operation of the network. We make no requirements as to which type
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of traffic engineering mechanism is used, but that such a system
exists and can distinguish and support IEPS signaling and data
traffic. We recommend a review by clients and providers of IEPS ser-
vice of [36], which gives an overview and a set of principles of
Internet traffic engineering.
MPLS is generally the first protocol that comes to mind when the sub-
ject of traffic engineering is brought up. This notion is hightened
concerning the subject of IP telephony because of MPLS's ability to
to permit a quasi circuit switching capability to be superimposed on
the current Internet routing model [33].
However, having cited MPLS, we need to stress that it is an intra-
domain protocol, and so may or may not exist within a given ISP.
Other forms of traffic engineering, such as weighted OSPF, may be the
mechanism of choice by an ISP.
Note: As a point of reference, existing SLAs established by the NCS
for GETS service tend to focus on a maximum allocation of (e.g., 1%)
of calls allowed to be established through a given LEC using HPC.
Once this limit is reached, all other GETS calls experience the same
probably of call completion as the general public. It is expected,
and encouraged, that IEPS related SLAs will have a limit with respect
to the amount of traffic distinguished as being emergency related,
and initiated by an authorized user.
4.4. Security
As IEPS support moves from intra-domain PSTN and IP networks to dif-
fuse inter-domain pure IP, authenticated service becomes more complex
to provide. Where an IEPS call is carried from PSTN to PSTN via one
carrier's backbone IP network, very little IP-specific security sup-
port is required. The user authenticates herself as usual to the
network using a PIN. The gateway from her PSTN connection into the
backbone IP network must be able to signal that the flow has IEPS
priority. Conversely, the gateway back into the PSTN must similarly
signal the call's higher priority. A secure link between the gateways
may be set up using IPSec or SIP security functionality. If the end-
point is an IP device on the carrier's network, the link may be set
up securely from the ingress gateway to the end device.
As flows traverse more than one IP network, domains whose peering
agreements include IEPS support must have means to securely signal a
given flow's IEPS status. They may choose to use physical link secu-
rity and/or IPSec authentication, combined with traffic conditioning
measures to limit the amount of IEPS traffic that may pass between
the two domains. The inter-domain agreement may require the
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originating network to take responsibility for ensuring only author-
ized traffic is marked with IEPS priority; the downstream domain may
still perform redundant conditioning to prevent the propagation of
theft and denial of service attacks. Security may be provided
between ingress and egress gateways or IP endpoints using IPSec or
SIP security functions.
When a call originates from an IP device, the ingress network may
authorize IEPS traffic over that link as part of its user authentica-
tion procedures without necessarily communicating with a central IEPS
authentication center as happens with POTS-originated calls. These
authentication procedures may occur at the link or network layers,
but are entirely at the discretion of the ingress network. That net-
work must decide how often it should update its list of authorized
IEPS users based on the bounds it is prepared to accept on traffic
from recently-revoked users.
5. Key Scenarios
There are various scenarios in which IP telephony can be realized,
each of which can infer a unique set of functional requirements that
may include just a subset of those listed above. We acknowledge that
a scenario may exist whose functional requirements are not listed
above. Our intention is not to consider every possible scenario by
which support for emergency related IP telephony can be realized.
Rather, we narrow our scope using a single guideline; we assume there
is a signaling & data interaction between the PSTN and the IP network
with respect to supporting emergency-related telephony traffic. We
stress that this does not preclude an IP-only end-to-end model, but
rather the inclusion of the PSTN expands the problem space and
includes the current dominant form of voice communication.
There are two scenarios that we use as a model for determining our
objectives and subsequent functional requirements. These are:
Single IP Administrative Domain
-------------------------------
This scenario is a direct reflection of the evolution of the PSTN.
Specifically, we refer to the case in which data networks have
emerged in various degrees as a backbone infrastructure connecting
PSTN switches at its edges. This represents a single isolated IP
administrative domain that has no directly adjacent IP domains con-
nected to it. We show an example of this scenario below in Figure 1.
In this example, we show two types of carriers. One is the legacy
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carrier, whose infrastructure retains the classic switching architec-
ture attributed to the PSTN. The other is the next generation car-
rier, which uses a data network (e.g., IP) as its core infrastruc-
ture, and Signaling Gateways at its edges. These gateways "speak"
SS7 externally with peering carriers, and another protocol (e.g.,
SIP) internally, which rides on top of the IP infrastructure.
Legacy Next Generation Next Generation
Carrier Carrier Carrier
******* *************** **************
* * * * ISUP * *
SW<--->SW <-----> SG <---IP---> SG <--IAM--> SG <---IP---> SG
* * (SS7) * (SIP) * (SS7) * (SIP) *
******* *************** **************
SW - Telco Switch
SG - Signaling Gateway
Figure 1
The significant aspect of this scenario is that all the resources of
each IP "island" fall within a given administrative authority.
Hence, there is not a problem of retaining toll quality Grade of Ser-
vice as the voice traffic (data and signaling) exits the IP network
because of the existing SS7 provisioned service between carriers.
Thus, the need for support of mechanisms like diff-serv, and an
expansion of the defined set of Per-Hop Behaviors is reduced (if not
eliminated) under this scenario.
Another function that has little or no importance within the closed
IP environment of Figure 1 is that of IP security. The fact that
each administrative domain peers with each other as part of the PSTN,
means that existing security, in the form of Personal Identification
Number (PIN) authentication (under the context of telephony infras-
tructure protection), is the default scope of security. We do not
claim that the reliance on a PIN based security system is highly
secure or even desirable. But, we use this system as a default
mechanism in order to avoid placing additional requirements on exist-
ing authorized emergency telephony systems.
Multiple IP Administrative Domains
----------------------------------
We view the scenario of multiple IP administrative domains as a
superset of the previous scenario. Specifically, we retain the
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notion that the IP telephony system peers with the existing PSTN. In
addition, segments (i.e., portions of the Internet) may exchange sig-
naling with other IP administrative domains via non-PSTN signaling
protocols like SIP.
Legacy Next Generation Next Generation
Carrier Carrier Carrier
******* *************** **************
* * * * * *
SW<--->SW <-----> SG <---IP---> SG <--IP--> SG <---IP---> SG
* * (SS7) * (SIP) * (SIP) * (SIP) *
******* *************** **************
SW - Telco Switch
SG - Signaling Gateway
Figure 2
Given multiple IP domains, and the presumption that SLAs relating to
IEPS traffic may exist between them, the need for something like
diff-serv grows with respect to being able to distinguish the emer-
gency related traffic from other types of traffic. In addition, IP
security becomes more important between domains in order to ensure
that the act of distinguishing IEPS-type traffic is indeed valid for
the given source.
We conclude this section by mentioning a complimentary work in pro-
gress in providing ISUP transparency across SS7-SIP interworking
[37]. The objective of this effort is to access services in the SIP
network and yet maintain transparency of end-to-end PSTN services.
Not all services are mapped (as per the design goals of [37], so we
anticipate the need for an additional document to specify the mapping
between new SIP labels and existing PSTN code points like NS/EP and
MLPP.
6. Security Considerations
Information on this topic is presented in sections 2 and 4.
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7. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Braden, R., et. al., "Integrated Services in the Internet
Architecture: An Overview", Informational, RFC 1633, June 1994.
3 Braden, R., et. al., "Resource Reservation Protocol (RSVP)
Version 1, Functional Specification", Proposed Standard, RFC
2205, Sept. 1997.
4 Shenker, S., et. al., "Specification of Guaranteed Quality of
Service", Proposed Standard, RFC 2212, Sept 1997.
5 Wroclawski, J., "Specification for Controlled-Load Network
Service Element", Proposed Standard, RFC 2211, Sept 1997.
6 Baker, F., et. al., "Aggregation of RSVP for IPv4 and IPv6
Reservations", Proposed Standard, RFC 3175, September 2001.
7 Berger, L, et. al., "RSVP Refresh Overhead Reduction Extensions",
Proposed Standard, RFC 2961, April, 2001.
8 Blake, S., et. al., "An Architecture for Differentiated
Service", Proposed Standard, RFC 2475, Dec. 1998.
9 Faucheur, F., et. al., "MPLS Support of Differentiated Services",
Standards Track, RFC 3270, May 2002.
10 Farrel, A., et. al., "Fault Tolerance for LDP and CR-LDP",
Internet Draft, Work In Progress, October 2001.
11 Postel, J., "Simple Mail Transfer Protocol", Standard, RFC 821,
August 1982.
12 Handley, M., et. al., "SIP: Session Initiation Protocol",
Proposed Standard, RFC 2543, March 1999.
13 ANSI, "Signaling System No. 7(SS7) _ High Probability of
Completion (HPC) Network Capability_, ANSI T1.631, 1993.
14 Robust Audio Tool (RAT):
http://www-mice.cs.ucl.ac.uk/multimedia/software/rat
15 Polk, J., Schulzrinne, H, "SIP Communications Resource Priority
Header", Internet Draft, Work In Progress, December, 2001.
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16 Nichols, K., et. al.,"Definition of the Differentiated Services
Field (DS Field) in the Ipv4 and Ipv6 Headers", Proposed
Standard, RFC 2474, December 1998.
17 Durham, D., "The COPS (Common Open Policy Service) Protocol",
Proposed Standard, RFC 2748, Jan 2000.
18 ITU, "International Emergency Preparedness Scheme", ITU
Recommendation, E.106, March 2000.
19 Rosenburg, J., Schulzrinne, H., "A Framework for Telephony Routing
Over IP", Informational, RFC 2871, June 2000
20 Heinanen. et. al, "Assured Forwarding PHB Group", Proposed
Standard, RFC 2597, June 1999
21 ITU, "Multi-Level Precedence and Preemption Service, ITU,
Recomendation, I.255.3, July, 1990.
22 Rosenburg, J, et. al, "Telephony Routing over IP (TRIP)",
Standards Track, RFC 3219, January 2002.
23 Cuervo, F., et. al, "Megaco Protocol Version 1.0", Standards
Track, RFC 3015, November 2000
24 Perkins, C., et al., "RTP Payload for Redundant Audio Data",
Standards Track, RFC 2198, September, 1997
25 Rosenburg, J., Schulzrinne, H., "An RTP Payload Format for
Generic Forward Error Correction", Standards Track, RFC 2733,
December, 1999.
26 ANSI, "Signaling System No. 7, ISDN User Part", ANSI T1.113-2000,
2000.
27 Brown, I., "Securing IEPS over IP", White Paper,
http://iepscheme.net/docs/secure_IEPS.doc
28 "Description of an International Emergency Preference
Scheme (IEPS)", ITU-T Recommendation E.106 March, 2002
29 Carlberg, K., "The Classifier Extension Header for RTP", Internet
Draft, Work In Progress, October 2001.
30 National Communications System: http://www.ncs.gov
31 Bansal, R., Ravikanth, R., "Performance Measures for Voice on IP",
http://www.ietf.org/proceedings/97aug/slides/tsv/ippm-voiceip/,
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IETF Presentation: IPPM-Voiceip, Aug, 1997
32 Hardman, V., et al, "Reliable Audio for Use over the Internet",
Proceedings, INET'95, Aug, 1995.
33 Awduche, D, et al, "Requirements for Traffic Engineering Over
MPLS", Informational, RFC 2702, September, 1999.
34 Polk, J., "An Architecture for Multi-Level Precedence and
Preemption over IP", Internet Draft, Work In Progress,
November, 2001.
35 "Service Class Designations for H.323 Calls", ITU Draft
Recommendation H.GEF.4, September, 2001
36 Awduche, D., et. al., "Overview and Principles of Internet Traffic
Engineering", Informational, RFC 3272, May 2002.
37 Vemuri, A., Peterson, J., "SIP for Telephones (SIP-T): Context and
Architectures", work in progress, Internet-Draft, June, 2002.
8. Appendix A: Government Telephone Preference Scheme (GTPS)
This framework document uses the T1.631 and ITU IEPS standard as a
target model for defining a framework for supporting authorized emer-
gency related communication within the context of IP telephony. We
also use GETS as a helpful model to draw experience from. We take
this position because of the various areas that must be considered;
from the application layer to the (inter)network layer, in addition
to policy, security (authorized access), and traffic engineering.
The U.K. has a different type of authorized use of telephony services
referred to as the Government Telephone Preference Scheme (GTPS).
This service was introduced in the 1950's at a time when loss of
power to the PSTN due to war or natural disaster was of prime con-
cern. If a loss of power did occur, it was felt that the critical
issue was to take action to limit phone usage by the general public
so that power would be conserved for use by critical personnel
involved in an emergency.
The design and implementation of GTPS focused on the ability of the
U.K. PSTN to withdraw outgoing telephone service from the majority
of the general public. Inbound calls can still be received, but the
net effect of the action is that power for the phone line service is
conserved. While power loss is still fo concern, a more important
issue to the U.K. government is is the volume of switched traffic
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during emergencies. And in fact the Cabinet Office is investigating
an evolutionary change to GTPS so that it reflects current needs and
requirements for supporting emergency communications through the U.K.
PSTN -- such as congestion, and the ability to provide roaming
authorized access like that of GETS.
At present, GTPS only applies to local loop lines of BT, Kingston,
and Cable & Wireless within the UK. The lines are divided into
Categories 1, 2, and 3. The first two categories involve authorized
personnel that are involved in emergencies such as natural disasters.
Catergory 3 identifes the general public.
Unlike the roaming ability of GETS users, GTPS associates preference
with an originating line. This simplifies the process of determining
who is allowed outbound phone service, but it is also quite restric-
tive in its usage. Hence, individuals that need preferential service
must use the phone that has been designated as Category 1 or 2.
Note: for the general public, pay phones have been designated as
Category 2 so that 999 (calls the emergency services operator) can be
made.
An updated version of GTPS has been made available following the
deregulation of the U.K. phone system. In this new scheme, local
exchanges retained the three category system while inter-exchange
calls use call-gaping. Priority marks, via C7/NUP, would bypass the
call-gaping. Exchanges belonging to other licensed operators (i.e.,
not BT, Kingston, or C&W) are not so equiped and will only tran-
sparently pass on "priority" marks, without affecting their own use
of call-gapping. The authority to activate GTPS has been extended to
either a central or delegated authority.
8.1. GTPS and the Framework Document
The design of the current GTPS, with its designation of preference
based on physical static devices, precludes the need for several
aspects presented in this document. However, one component that can
have a direct correlation is the labeling capability of the proposed
Resource Priority extension to SIP. In the case of GTPS, one simply
needs to define a new NameSpace that will define values for each of
its three Categories of users. These new labels will then allow a
more transparant interoperation between IP telephony using SIP and
the U.K. PSTN that supports GTPS. However, a strong possibility
exists that the IETF will discourage the registration of NameSpaces
attributed to specific organizations or geo-political boundaries. In
this case, the private definition of a NameSpace (one that is not
registered with IANA) may need to be used by systems like GTPS.
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Restricting outbound call establishment within the context of IP
telephony and SIP servers is a policy issue. Service Level Agree-
ments, presumably under the guidance or direction of local laws and
regulations would determine the characteristics of the policy.
9. Appendix B: Related Standards Work
The process of defining various labels to distinguish calls has been,
and continues to be, pursued in other standards groups. As mentioned
in section 1.1.1, the ANSI T1S1 group has previously defined a label
SS7 ISUP Initial Address Message. This single label or value is
referred to as the National Security and Emergency Preparedness
(NS/EP) indicator and is part of the T1.631 standard. The following
subsections presents a snap shot of parallel on-going efforts in
various standards groups.
It is important to note that the recent activity in other groups have
gravitated to defining 5 labels or levels of priority. The impact of
this approach is minimal in relation to this IEPS framework document
because it simply generates a requirement to define and register with
IANA a new NameSpace in the Resource-Priority header of SIP.
9.1. Study Group 16 (ITU)
Study Group 16 (SG16) of the ITU is responsible for studies relating
to multimedia service definition and multimedia systems, including
protocols and signal processing.
A draft contribution [35] has been introduced into this group that
would add a Priority Class parameter to the call establishment mes-
sages of H.323. This class is further divided into two parts; one
for Priority Value and the other is a Priority Extension for indicat-
ing subclasses. It is this former part that roughly corresponds to
the labels transported via the Resource Priority field for SIP [15].
The draft recommendation advocates defining PriorityClass information
that would be carried in the GenericData parameter in the H323-UU-PDU
or RAS messages. The GenericData parameter contains Priori-
tyClassGenericData. The PriorityClassInfo of the PriorityClassGener-
icData contains the Priority and Priority Extension fields.
At present, 5 levels have been defined for the Priority Value part of
the Priority Class parameter: Low, Normal, High, Emergency-Public,
Emergency-Authorized. An additional 8-bit priority extension has been
defined to provide for subclasses of service at each priority.
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The suggested ASN.1 definition of the service class is the following:
ServiceClassInfo ::= SEQUENCE
{
priority CHOICE
{
emergencyAuthorized NULL,
emergencyPublic NULL,
high NULL,
normal NULL,
low NULL
}
priorityExtension INTEGER (0..255) OPTIONAL;
requiredClass NULL OPTIONAL
tokens SEQUENCE OF ClearToken OPTIONAL
cryptoTokens SEQUENCE OF CryptoH323Token OPTIONAL
}
The advantage in using the GenericData parameter is that an existing
parameter is used, as opposed to defining a new parameter and causing
subsequent changes in existing H.323/H.225 documents.
10. Acknowledgments
The authors would like to acknowledge the helpful comments, opinions,
and clarifications of Stu Goldman, James Polk, Dennis Berg, as well
as those comments received from the IEPS and IEPREP mailing lists.
Additional thanks to Peter Walker of Oftel for private discussions on
the operation of GTPS, and Gary Thom on clarifications of the SG16
draft contribution.
11. Author's Addresses
Ken Carlberg Ian Brown
University College London University College London
Department of Computer Science Department of Computer Science
Gower Street Gower Street
London, WC1E 6BT London, WC1E 6BT
United Kingdom United Kingdom
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Internet Draft IEPS Framework June 24, 2002
Table of Contents
1. Introduction ................................................... 2
1.1 Emergency Related Data ....................................... 3
1.1.1 Government Emergency Telecommunications Service (GETS) ..... 3
1.1.2 International Emergency Preparedness Scheme (IEPS) ......... 5
1.2 Scope of this Document ....................................... 5
2. Objective ..................................................... 7
3. Value Added Objective ......................................... 10
3.1 Alternate Path Routing ....................................... 10
3.2 End-to-End Fault Tolerance ................................... 12
4. Functional Requirements ....................................... 12
4.1 Signaling & State Information ................................ 13
4.1.1 SIP ........................................................ 13
4.1.2 Diff-Serv .................................................. 14
4.1.3 RTP ........................................................ 16
4.1.4 MEGACO/H.248 ............................................... 17
4.2 Policy ....................................................... 18
4.3 Traffic Engineering .......................................... 18
4.4 Security ..................................................... 19
5. Key Scenarios ................................................. 20
6. Security Considerations ....................................... 22
7. References .................................................... 23
8. Appendix A: Government Telephone Preference Scheme (GTPS) ..... 25
8.1 GTPS and the Framework Document .............................. 26
9. Appendix B: Related Standards Work ............................ 27
9.1 Study Group 16 (ITU) ......................................... 27
10. Acknowledgments .............................................. 28
11. Author's Addresses ........................................... 28
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Internet Draft IEPS Framework June 24, 2002
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