rfc7200
Internet Engineering Task Force (IETF) C. Shen
Request for Comments: 7200 H. Schulzrinne
Category: Standards Track Columbia U.
ISSN: 2070-1721 A. Koike
NTT
April 2014
A Session Initiation Protocol (SIP) Load-Control Event Package
Abstract
This specification defines a load-control event package for the
Session Initiation Protocol (SIP). It allows SIP entities to
distribute load-filtering policies to other SIP entities in the
network. The load-filtering policies contain rules to throttle calls
from a specific user or based on their source or destination domain,
telephone number prefix. The mechanism helps to prevent signaling
overload and complements feedback-based SIP overload control efforts.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7200.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................3
2. Conventions .....................................................3
3. SIP Load-Filtering Overview .....................................4
3.1. Load-Filtering Policy Format ...............................4
3.2. Load-Filtering Policy Computation ..........................4
3.3. Load-Filtering Policy Distribution .........................4
3.4. Applicable Network Domains .................................8
4. Load-Control Event Package ......................................9
4.1. Event Package Name .........................................9
4.2. Event Package Parameters ...................................9
4.3. SUBSCRIBE Bodies ...........................................9
4.4. SUBSCRIBE Duration .........................................9
4.5. NOTIFY Bodies .............................................10
4.6. Notifier Processing of SUBSCRIBE Requests .................10
4.7. Notifier Generation of NOTIFY Requests ....................10
4.8. Subscriber Processing of NOTIFY Requests ..................10
4.9. Handling of Forked Requests ...............................12
4.10. Rate of Notifications ....................................12
4.11. State Delta ..............................................12
5. Load-Control Document ..........................................13
5.1. Format ....................................................13
5.2. Namespace .................................................13
5.3. Conditions ................................................14
5.3.1. Call Identity ......................................14
5.3.2. Method .............................................16
5.3.3. Target SIP Entity ..................................17
5.3.4. Validity ...........................................18
5.4. Actions ...................................................18
6. XML Schema Definition for Load Control .........................20
7. Security Considerations ........................................23
8. IANA Considerations ............................................24
8.1. Load-Control Event Package Registration ...................24
8.2. application/load-control+xml Media Type Registration ......24
8.3. URN Sub-Namespace Registration ............................25
8.4. Load-Control Schema Registration ..........................26
9. Acknowledgements ...............................................27
10. References ....................................................27
10.1. Normative References .....................................27
10.2. Informative References ...................................28
Appendix A. Definitions ...........................................30
Appendix B. Design Requirements ...................................30
Appendix C. Discussion of How This Specification Meets the
Requirements of RFC 5390 ..............................31
Appendix D. Complete Examples .....................................36
D.1. Load-Control Document Examples ............................36
D.2. Message Flow Examples .....................................40
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Appendix E. Related Work .........................................41
E.1. Relationship to Load Filtering in PSTN ....................41
E.2. Relationship with Other IETF SIP Overload Control Efforts .42
1. Introduction
SIP load-control mechanisms are needed to prevent congestion collapse
[RFC6357] in cases of SIP server overload [RFC5390]. There are two
types of load-control approaches. In the first approach, feedback
control, SIP servers provide load limits to upstream servers, to
reduce the incoming rate of all SIP requests [SIP-OVERLOAD]. These
upstream servers then drop or delay incoming SIP requests. Feedback
control is reactive and affects signaling messages that have already
been issued by user agent clients. This approach works well when SIP
proxy servers in the core networks (core proxy servers) or
destination-specific SIP proxy servers in the edge networks (edge
proxy servers) are overloaded. By their nature, they need to
distribute rate, drop, or window information to all upstream SIP
proxy servers and normally affect all calls equally, regardless of
destination.
This specification proposes an additional, complementary load-control
mechanism, called "load filtering". It is most applicable for
situations where a traffic surge and its source/destination
distribution can be predicted in advance. In those cases, network
operators create load-filtering policies that indicate calls to
specific destinations or from specific sources should be rate-limited
or randomly dropped. These load-filtering policies are then
distributed to SIP servers and possibly SIP user agents that are
likely to generate calls to the affected destinations or from the
affected sources. Load filtering works best if it prevents calls as
close to the originating user agent clients as possible. The
applicability of SIP load filtering can also be extended beyond
overload control, e.g., to implement service level agreement
commitments.
2. Conventions
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 [RFC2119].
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3. SIP Load-Filtering Overview
3.1. Load-Filtering Policy Format
Load-filtering policies are specified by sets of rules. Each rule
contains both load-filtering conditions and actions. The load-
filtering conditions define identities of the targets to be filtered
(Section 5.3.1). For example, there are two typical resource limits
in a possible overload situation, i.e., human destination limits
(number of call takers) and node capacity limits. The load-filtering
targets in these two cases can be the specific callee numbers or the
destination domain corresponding to the overload. Load-filtering
conditions also indicate the specific message type to be matched
(Section 5.3.2), with which target SIP entity the filtering policy is
associated (Section 5.3.3), and the period of time when the filtering
policy should be activated and deactivated (Section 5.3.4). Load-
filtering actions describe the desired control functions such as
keeping the request rate below a specified level (Section 5.4).
3.2. Load-Filtering Policy Computation
When computing the load-filtering policies, one needs to take into
consideration information such as overload time, scope and network
topology, as well as service policies. It is also important to make
sure that there is no resource allocation loop and that server
capacity is allocated in a way that both prevents overload and
maximizes effective throughput (commonly called goodput). In some
cases, in order to better utilize system resources, it may be
preferable to employ an algorithm that dynamically computes the load-
filtering policies based on currently observed server load status,
rather than using a purely static filtering policy assignment. The
computation algorithm for load-filtering policies is beyond the scope
of this specification.
3.3. Load-Filtering Policy Distribution
For distributing load-filtering policies, this specification defines
the SIP event package for load control, which is an "instantiation"
of the generic SIP event notification framework [RFC6665]. This
specification also defines the XML schema of a load-control document
(Section 5), which is used to encode load-filtering policies.
In order for load-filtering policies to be properly distributed, each
capable SIP entity in the network subscribes to the SIP load-control
event package of each SIP entity to which it sends signaling
requests. A SIP entity that accepts subscription requests is called
a "notifier" (Section 4.6). Subscription is initiated and maintained
during normal server operation. The subscription of neighboring SIP
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entities needs to be persistent, as described in Sections 4.1 and 4.2
of [RFC6665]. The refresh procedure is described in Section 4.7
below. Subscribers may terminate the subscription if they have not
received notifications for an extended time period, and can
resubscribe if they determine that signaling with the notifier
becomes active again.
An example architecture is shown in Figure 1 to illustrate SIP load-
filtering policy distribution. This scenario consists of two
networks belonging to Service Provider A and Service Provider B,
respectively. Each provider's network is made up of two SIP core
proxy servers and four SIP edge proxy servers. The core proxy
servers and edge proxy servers of Service Provider A are denoted as
CPa1 to CPa2 and EPa1 to EPa4; the core proxy servers and edge proxy
servers of Service Provider B are denoted as CPb1 to CPb2 and EPb1 to
EPb4.
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+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
| EPa1 | | EPa2 | | EPa3 | | EPa4 |
| | | | | | | |
+-----------+ +-----------+ +-----------+ +-----------+
\ / \ /
\ / \ /
\ / \ /
+-----------+ +-----------+
| | | |
| CPa1 |------------------| CPa2 |
| | | |
+-----------+ +-----------+
| |
Service | |
Provider A | |
| |
=================================================================
| |
Service | |
Provider B | |
| |
+-----------+ +-----------+
| | | |
| CPb1 |------------------| CPb2 |
| | | |
+-----------+ +-----------+
/ \ / \
/ \ / \
/ \ / \
+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
| EPb1 | | EPb2 | | EPb3 | | EPb4 |
| | | | | | | |
+-----------+ +-----------+ +-----------+ +-----------+
Figure 1: Example Network Scenario Using SIP Load-Control Event
Package Mechanism
During the initialization stage, the proxy servers first identify all
their outgoing signaling neighbors and subscribe to them. Service
providers can provision neighbors, or the proxy servers can
incrementally learn who their neighbors are by inspecting signaling
messages that they send and receive. Assuming all signaling
relationships in Figure 1 are bidirectional, after this
initialization stage, each proxy server will be subscribed to all its
neighbors.
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Case I: EPa1 serves a TV program hotline and decides to limit the
total number of incoming calls to the hotline to prevent an overload.
To do so, EPa1 sends a notification to CPa1 with the specific hotline
number, time of activation, and total acceptable call rate.
Depending on the load-filtering policy computation algorithm, CPa1
may allocate the received total acceptable call rate among its
neighbors, namely, EPa2, CPa2, and CPb1, and notify them about the
resulting allocation along with the hotline number and the activation
time. CPa2 and CPb1 may perform further allocation among their own
neighbors and notify the corresponding proxy servers. This process
continues until all edge proxy servers in the network have been
informed about the event and have proper load-filtering policies
configured.
In the above case, the network entity where load-filtering policy is
first introduced is the SIP server providing access to the resource
that creates the overload situation. In other cases, the network
entry point of introducing load-filtering policy could also be an
entity that hosts this resource. For example, an operator may host
an application server that performs toll-free-number ("800 number")
translation services. The application server itself may be a SIP
proxy server or a SIP Back-to-Back User Agent (B2BUA). If one of the
toll-free numbers hosted at the application server creates the
overload condition, the load-filtering policies can be introduced
from the application server and then propagated to other SIP proxy
servers in the network.
Case II: A hurricane affects the region covered by CPb2, EPb3, and
EPb4. All three of these SIP proxy servers are overloaded. The
rescue team determines that outbound calls are more valuable than
inbound calls in this specific situation. Therefore, EPb3 and EPb4
are configured with load-filtering policies to accept more outbound
calls than inbound calls. CPb2 may be configured the same way or
receive dynamically computed load-filtering policies from EPb3 and
EPb4. Depending on the load-filtering policy computation algorithm,
CPb2 may also send out notifications to its outside neighbors, namely
CPb1 and CPa2, specifying a limit on the acceptable rate of inbound
calls to CPb2's responsible domain. CPb1 and CPa2 may subsequently
notify their neighbors about limiting the calls to CPb2's area. The
same process could continue until all edge proxy servers are notified
and have load-filtering policies configured.
Note that this specification does not define the provisioning
interface between the party who determines the load-filtering policy
and the network entry point where the policy is introduced. One of
the options for the provisioning interface is the Extensible Markup
Language (XML) Configuration Access Protocol (XCAP) [RFC4825].
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3.4. Applicable Network Domains
This specification MUST be applied inside a "Trust Domain". The
concept of a Trust Domain is similar to that defined in [RFC3324] and
[RFC3325]. A Trust Domain, for the purpose of SIP load filtering, is
a set of SIP entities such as SIP proxy servers that are trusted to
exchange load-filtering policies defined in this specification. In
the simplest case, a Trust Domain is a network of SIP entities
belonging to a single service provider who deploys it and accurately
knows the behavior of those SIP entities. Such simple Trust Domains
may be joined to form larger Trust Domains by bilateral agreements
between the service providers of the SIP entities.
The key requirement of a Trust Domain for the purpose of SIP load
filtering is that the behavior of all SIP entities within a given
Trust Domain is known to comply to the following set of
specifications.
o SIP entities in the Trust Domain agree on the mechanisms used to
secure the communication among SIP entities within the Trust
Domain.
o SIP entities in the Trust Domain agree on the manner used to
determine which SIP entities are part of the Trust Domain.
o SIP entities in the Trust Domain are compliant to SIP [RFC3261].
o SIP entities in the Trust Domain are compliant to SIP-Specific
Event Notification[RFC6665].
o SIP entities in the Trust Domain are compliant to this
specification.
o SIP entities in the Trust Domain agree on what types of calls can
be affected by this SIP load-filtering mechanism. For example,
<call-identity> condition elements (Section 5.3.1) <one> and
<many> might be limited to describe within certain prefixes.
o SIP entities in the Trust Domain agree on the destinations to
which calls may be redirected when the "redirect" action
(Section 5.4) is used. For example, the URI might have to match a
given set of domains.
SIP load filtering is only effective if all neighbors that are
possible signaling sources participate and enforce the designated
load-filtering policies. Otherwise, a single non-conforming neighbor
could make all filtering efforts useless by pumping in excessive
traffic to overload the server. Therefore, the SIP server that
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distributes load-filtering policies needs to take countermeasures
towards any non-conforming neighbors. A simple method is to reject
excessive requests with 503 "Service Unavailable" response messages
as if they were obeying the rate. Considering the rejection costs, a
more complicated but fairer method would be to allocate at the
overloaded server the same amount of processing to the combination of
both normal processing and rejection as the overloaded server would
devote to processing requests for a conforming upstream SIP server.
These approaches work as long as the total rejection cost does not
overwhelm the entire server resources. In addition, SIP servers need
to handle message prioritization properly while performing load
filtering, which is described in Section 4.8.
4. Load-Control Event Package
The SIP load-filtering mechanism defines a load-control event package
for SIP based on [RFC6665].
4.1. Event Package Name
The name of this event package is "load-control". This name is
carried in the Event and Allow-Events header, as specified in
[RFC6665].
4.2. Event Package Parameters
No package-specific event header field parameters are defined for
this event package.
4.3. SUBSCRIBE Bodies
This specification does not define the content of SUBSCRIBE bodies.
Future specifications could define bodies for SUBSCRIBE messages, for
example, to request specific types of load-control event
notifications.
A SUBSCRIBE request sent without a body implies the default
subscription behavior as specified in Section 4.7.
4.4. SUBSCRIBE Duration
The default expiration time for a subscription to load-filtering
policy is one hour. Since the desired expiration time may vary
significantly for subscriptions among SIP entities with different
signaling relationships, the subscribers and notifiers are
RECOMMENDED to explicitly negotiate appropriate subscription duration
when knowledge about the mutual signaling relationship is available.
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4.5. NOTIFY Bodies
The body of a NOTIFY request in this event package contains load-
filtering policies. The format of the NOTIFY request body MUST be in
one of the formats defined in the Accept header field of the
SUBSCRIBE request or be the default format, as specified in
[RFC6665]. The default data format for the NOTIFY request body of
this event package is "application/load-control+xml" (defined in
Section 5). This means that when a NOTIFY request body exists but no
Accept header field is specified in a SUBSCRIBE request, the NOTIFY
request body MUST contain content conforming to the "application/
load-control+xml" format.
4.6. Notifier Processing of SUBSCRIBE Requests
The notifier accepts a new subscription or updates an existing
subscription upon receiving a valid SUBSCRIBE request.
If the identity of the subscriber sending the SUBSCRIBE request is
not allowed to receive load-filtering policies, the notifier MUST
return a 403 "Forbidden" response.
If none of the media types specified in the Accept header of the
SUBSCRIBE request are supported, the notifier SHOULD return a 406
"Not Acceptable" response.
4.7. Notifier Generation of NOTIFY Requests
A notifier MUST send a NOTIFY request with its current load-filtering
policy to the subscriber upon successfully accepting or refreshing a
subscription. If no load-filtering policy needs to be distributed
when the subscription is received, the notifier SHOULD sent a NOTIFY
request without a body to the subscriber. The content-type header
field of this NOTIFY request MUST indicate the correct body format as
if the body were present (e.g., "application/load-control+xml").
Notifiers are likely to send NOTIFY requests without a body when a
subscription is initiated for the first time, e.g., when a SIP entity
is just introduced, because there may be no planned events that
require load filtering at that time. A notifier SHOULD generate
NOTIFY requests each time the load-filtering policy changes, with the
maximum notification rate not exceeding values defined in
Section 4.10.
4.8. Subscriber Processing of NOTIFY Requests
The subscriber is the load-filtering server that enforces load-
filtering policies received from the notifier. The way subscribers
process NOTIFY requests depends on the load-filtering policies
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conveyed in the notifications. Typically, load-filtering policies
consist of rules specifying actions to be applied to requests
matching certain conditions. A subscriber receiving a notification
first installs these rules and then enforces corresponding actions on
requests matching those conditions, for example, limiting the sending
rate of call requests destined for a specific callee.
In the case when load-filtering policies specify a future validity,
it is possible that when the validity time arrives, the subscription
to the specific notifier that conveyed the rules has expired. In
this case, it is RECOMMENDED that the subscriber re-activate its
subscription with the corresponding notifier. Regardless of whether
or not this re-activation of subscription is successful, when the
validity time is reached, the subscriber SHOULD enforce the
corresponding rules.
Upon receipt of a NOTIFY request with a Subscription-State header
field containing the value "terminated", the subscription status with
the particular notifier will be terminated. Meanwhile, subscribers
MUST also terminate previously received load-filtering policies from
that notifier.
The subscriber MUST discard unknown bodies. If the NOTIFY request
contains several bodies, none of them being supported, it SHOULD
unsubscribe unless it has knowledge that it will possibly receive
NOTIFY requests with supported bodies from that notifier. A NOTIFY
request without a body indicates that no load-filtering policies need
to be updated.
When the subscriber enforces load-filtering policies, it needs to
prioritize requests and select those requests that need to be
rejected or redirected. This selection is largely a matter of local
policy. It is expected that the subscriber will follow local policy
as long as the result in reduction of traffic is consistent with the
overload algorithm in effect at that node. Accordingly, the
normative behavior described in the next three paragraphs should be
interpreted with the understanding that the subscriber will aim to
preserve local policy to the fullest extent possible.
o The subscriber SHOULD honor the local policy for prioritizing SIP
requests such as policies based on message type, e.g., INVITEs
versus requests associated with existing sessions.
o The subscriber SHOULD honor the local policy for prioritizing SIP
requests based on the content of the Resource-Priority header
(RPH, [RFC4412]). Specific (namespace.value) RPH contents may
indicate high-priority requests that should be preserved as much
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as possible during overload. The RPH contents can also indicate a
low-priority request that is eligible to be dropped during times
of overload.
o The subscriber SHOULD honor the local policy for prioritizing SIP
requests relating to emergency calls as identified by the sos URN
[RFC5031] indicating an emergency request.
A local policy can be expected to combine both the SIP request type
and the prioritization markings and SHOULD be honored when overload
conditions prevail.
4.9. Handling of Forked Requests
Forking is not applicable when this load-control event package
mechanism is used within a single-hop distance between neighboring
SIP entities. If communication scope of the load-control event
package mechanism is among multiple hops, forking is also not
expected to happen because the subscription request is addressed to a
clearly defined SIP entity. However, in the unlikely case when
forking does happen, the load-control event package only allows the
first potential dialog-establishing message to create a dialog, as
specified in Section 5.4.9 of [RFC6665].
4.10. Rate of Notifications
The rate of notifications is unlikely to be of concern for this local
control event package mechanism when it is used in a non-real-time
mode for relatively static load-filtering policies. Nevertheless, if
a situation does arise in which a rather frequently used load
filtering policy update is needed, it is RECOMMENDED that the
notifier not generate notifications at a rate higher than once per
second in all cases, in order to avoid the NOTIFY request itself
overloading the system.
4.11. State Delta
It is likely that updates to specific load-filtering policies are
made by changing only part of the policy parameters (e.g., acceptable
request rate or percentage, but not matching identities). This will
typically be because the utilization of a resource subject to
overload depends upon dynamic unknowns such as holding time and the
relative distribution of offered loads over subscribing SIP entities.
The updates could originate manually or be determined automatically
by an algorithm that dynamically computes the load-filtering policies
(Section 3.2). Another factor that is usually not known precisely or
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needs to be computed automatically is the duration of the event
requiring load filtering. Therefore, it would also be common for the
validity to change frequently.
This event package allows the use of state delta as in [RFC6665] to
accommodate frequent updates of partial policy parameters. For each
NOTIFY transaction in a subscription, a version number that increases
by exactly one MUST be included in the NOTIFY request body when the
body is present. When the subscriber receives a state delta, it
associates the partial updates to the particular policy by matching
the appropriate rule id (Appendix D). If the subscriber receives a
NOTIFY request with a version number that is increased by more than
one, it knows that it has missed a state delta and needs to ask for a
full state snapshot. Therefore, the subscriber ignores that NOTIFY
request containing the state delta, and resends a SUBSCRIBE request
to force a NOTIFY request containing a complete state snapshot.
5. Load-Control Document
5.1. Format
A load-control document is an XML document that describes the load-
filtering policies. It inherits and enhances the common policy
document defined in [RFC4745]. A common policy document contains a
set of rules. Each rule consists of three parts: conditions,
actions, and transformations. The conditions part is a set of
expressions containing attributes such as identity, domain, and
validity time information. Each expression evaluates to TRUE or
FALSE. Conditions are matched on "equality" or "greater than" style
comparison. There is no regular expression matching. Conditions are
evaluated on receipt of an initial SIP request for a dialog or
standalone transaction. If a request matches all conditions in a
rule set, the action part and the transformation part are consulted
to determine the "permission" on how to handle the request. Each
action or transformation specifies a positive grant to the policy
server to perform the resulting actions. Well-defined mechanism are
available for combining actions and transformations obtained from
more than one sources.
5.2. Namespace
The namespace URI for elements defined by this specification is a
Uniform Resource Namespace (URN) ([RFC2141]), using the namespace
identifier "ietf" defined by [RFC2648] and extended by [RFC3688].
The URN is as follows:
urn:ietf:params:xml:ns:load-control
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5.3. Conditions
[RFC4745] defines three condition elements: <identity>, <sphere>, and
<validity>. This specification defines new condition elements and
reuses the <validity> element. The <sphere> element is not used.
5.3.1. Call Identity
Since the problem space of this specification is different from that
of [RFC4745], the [RFC4745] <identity> element is not sufficient for
use with load filtering. First, load filtering may be applied to
different identities contained in a request, including identities of
both the receiving entity and the sending entity. Second, the
importance of authentication varies when different identities of a
request are concerned. This specification defines new identity
conditions that can accommodate the granularity of specific SIP
identity header fields. The requirement for authentication depends
on which field is to be matched.
The identity condition for load filtering is specified by the
<call-identity> element and its sub-element <sip>. The <sip> element
itself contains sub-elements representing SIP sending and receiving
identity header fields: <from>, <to>, <request-uri>, and
<p-asserted-identity>. All those sub-elements are of an extended
form of the [RFC4745] <identity> element. In addition to the sub-
elements including <one>, <except>, and <many> in the <identity>
element from [RFC4745], the extended form adds two new sub-elements,
namely, <many-tel> and <except-tel>, which will be explained later in
this section.
The [RFC4745] <one> and <except> elements may contain an "id"
attribute, which is the URI of a single entity to be included or
excluded in the condition. When used in the <from>, <to>,
<request-uri>, and <p-asserted-identity> elements, this "id" value is
the URI contained in the corresponding SIP header field, i.e., From,
To, Request-URI, and P-Asserted-Identity.
When the <call-identity> element contains multiple <sip> sub-
elements, the result is combined using logical OR. When the <from>,
<to>, <request-uri>, and <p-asserted-identity> elements contain
multiple <one>, <many>, or <many-tel> sub-elements, the result is
also combined using logical OR. When the <many> sub-element further
contains one or more <except> sub-elements, or when the <many-tel>
sub-element further contains one or more <except-tel> sub-elements,
the result of each <except> or <except-tel> sub-element is combined
using a logical OR, similar to that of the [RFC4745] <identity>
element. However, when the <sip> element contains multiple <from>,
<to>, <request-uri>, and <p-asserted-identity> sub-elements, the
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result is combined using logical AND. This allows the call identity
to be specified by multiple fields of a SIP request simultaneously,
e.g., both the From and the To header fields.
The following shows an example of the <call-identity> element, which
matches call requests whose To header field contains the SIP URI
"sip:alice@hotline.example.com" or the 'tel' URI
"tel:+1-212-555-1234".
<call-identity>
<sip>
<to>
<one id="sip:alice@hotline.example.com"/>
<one id="tel:+1-212-555-1234"/>
</to>
</sip>
</call-identity>
Before evaluating <call-identity> conditions, the subscriber shall
convert URIs received in SIP header fields in canonical form as per
[RFC3261], except that the "phone-context" parameter shall not be
removed, if present.
The [RFC4745] <many> and <except> elements may take a "domain"
attribute. The "domain" attribute specifies a domain name to be
matched by the domain part of the candidate identity. Thus, it
allows matching a large and possibly unknown number of entities
within a domain. The "domain" attribute works well for SIP URIs.
A URI identifying a SIP user, however, can also be a 'tel' URI.
Therefore, a similar way to match a group of 'tel' URIs is needed.
There are two forms of 'tel' URIs: for global numbers and local
numbers. According to [RFC3966], "All phone numbers MUST use the
global form unless they cannot be represented as such...Local numbers
MUST be tagged with a 'phone-context'". The global number 'tel' URIs
start with a "+". The "phone-context" parameter of local numbers may
be labeled as a global number or any number of its leading digits or
a domain name. Both forms of the 'tel' URI make the resulting URI
globally unique.
'tel' URIs of global numbers can be grouped by prefixes consisting of
any number of common leading digits. For example, a prefix formed by
a country code or both the country and area code identifies telephone
numbers within a country or an area. Since the length of the country
and area code for different regions are different, the length of the
number prefix also varies. This allows further flexibility such as
Shen, et al. Standards Track [Page 15]
RFC 7200 SIP Load-Control Event Package April 2014
grouping the numbers into sub-areas within the same area code. 'tel'
URIs of local numbers can be grouped by the value of the
"phone-context" parameter.
The <many> and <except> sub-elements in the <identity> element of
[RFC4745] do not allow additional attributes to be added directly.
Redefining behavior of their existing "domain" attribute creates
backward-compatibility issues. Therefore, this specification defines
the <many-tel> and <except-tel> sub-elements that extend the
[RFC4745] <identity> element. Both of them have a "prefix" attribute
for grouping 'tel' URIs, similar to the "domain" attribute for
grouping SIP URIs in existing <many> and <except> sub-elements. For
global numbers, the "prefix" attribute value holds any number of
common leading digits, for example, "+1-212" for US phone numbers
within area code "212" or "+1-212-854" for the organization with US
area code "212" and local prefix "854". For local numbers, the
"prefix" attribute value contains the "phone-context" parameter
value. It should be noted that visual separators (such as the "-"
sign) in 'tel' URIs are not used for URI comparison as per [RFC3966].
The following example shows the use of the "prefix" attribute along
with the "domain" attribute. It matches those requests calling to
the number "+1-202-999-1234" but are not calling from a "+1-212"
prefix or a SIP From URI domain of "manhattan.example.com".
<call-identity>
<sip>
<from>
<many>
<except domain="manhattan.example.com"/>
</many>
<many-tel>
<except-tel prefix="+1-212"/>
</many-tel>
</from>
<to>
<one id="tel:+1-202-999-1234"/>
</to>
</sip>
</call-identity>
5.3.2. Method
The load created on a SIP server depends on the type of initial SIP
requests for dialogs or standalone transactions. The <method>
element specifies the SIP method to which the load-filtering action
applies. When this element is not included, the load-filtering
actions are applicable to all applicable initial requests. These
Shen, et al. Standards Track [Page 16]
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requests include INVITE, MESSAGE, REGISTER, SUBSCRIBE, OPTIONS, and
PUBLISH. Non-initial requests, such as ACK, BYE, and CANCEL MUST NOT
be subjected to load filtering. In addition, SUBSCRIBE requests are
not filtered if the event-type header field indicates the event
package defined in this specification.
The following example shows the use of the <method> element in the
case the filtering actions should be applied to INVITE requests.
<method>INVITE</method>
5.3.3. Target SIP Entity
A SIP server that performs load-filtering may have multiple paths to
route call requests matching the same set of call identity elements.
In those situations, the SIP load-filtering server may desire to take
advantage of alternative paths and only apply load-filtering actions
to matching requests for the next-hop SIP entity that originated the
corresponding load-filtering policy. To achieve that, the SIP load-
filtering server needs to associate every load-filtering policy with
its originating SIP entity. The <target-sip-entity> element is
defined for that purpose, and it contains the URI of the entity that
initiated the load-filtering policy, which is generally the
corresponding notifier. A notifier MAY include this element as part
of the condition of its filtering policy being sent to the
subscriber, as below.
<target-sip-entity>sip:biloxi.example.com</target-sip-entity>
When a SIP load-filtering server receives a policy with a
<target-sip-entity> element, it SHOULD record it and take it into
consideration when making load-filtering decisions. If the load-
filtering server receives a load-filtering policy that does not
contain a <target-sip-entity> element, it MAY still record the URI of
the load-filtering policy's originator as the <target-sip-entity>
information and consider it when making load-filtering decisions.
The following are two examples of using the <target-sip-entity>
element.
Use case I: The network has user A connected to SIP Proxy 1 (SP1),
user B connected to SIP Proxy 3 (SP3), SP1 and SP3 connected via
SIP Proxy 2 (SP2), and SP2 connected to an Application Server
(AS). Under normal load conditions, a call from A to B is routed
along the following path: A-SP1-SP2-AS-SP3-B. The AS provides a
nonessential service and can be bypassed in case of overload. Now
let's assume that AS is overloaded and sends to SP2 a load-
filtering policy requesting that 50% of all INVITE requests be
Shen, et al. Standards Track [Page 17]
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dropped. SP2 can maintain AS as the <target-sip-entity> for that
policy so that it knows the 50% drop action is only applicable to
call requests that must go through AS, without affecting those
calls directly routed through SP3 to B.
Use case II: A translation service for toll-free numbers is
installed on two Application Servers, AS1 and AS2. User A is
connected to SP1 and calls 800-1234-4529, which is translated by
AS1 and AS2 into a regular E.164 number depending on, e.g., the
caller's location. SP1 forwards INVITE requests with Request-URI
= "800 number" to AS1 or AS2 based on a load-balancing strategy.
As calls to 800-1234-4529 create a pre-overload condition in AS1,
AS1 sends to SP1 a load-filtering policy requesting that 50% of
calls towards 800-1234-4529 be rejected. In this case, SP1 can
maintain AS1 as the <target-sip-entity> for the rule, and only
apply the load-filtering policy on incoming requests that are
intended to be sent to AS1. Those requests that are sent to AS2,
although matching the <call-identity> of the filter, will not be
affected.
5.3.4. Validity
A filtering policy is usually associated with a validity period
condition. This specification reuses the <validity> element of
[RFC4745], which specifies a period of validity time by pairs of
<from> and <until> sub-elements. When multiple time periods are
defined, the validity condition is evaluated to TRUE if the current
time falls into any of the specified time periods. That is, it
represents a logical OR operation across all validity time periods.
The following example shows a <validity> element specifying a valid
period from 12:00 to 15:00 US Eastern Standard Time on 2008-05-31.
<validity>
<from>2008-05-31T12:00:00-05:00</from>
<until>2008-05-31T15:00:00-05:00</until>
</validity>
5.4. Actions
The actions a load-filtering server takes on loads matching the load-
filtering conditions are defined by the <accept> element in the load-
filtering policy, which includes any one of the three sub-elements
<rate>, <percent>, and <win>. The <rate> element denotes an absolute
value of the maximum acceptable request rate in requests per second;
the <percent> element specifies the relative percentage of incoming
requests that should be accepted; the <win> element describes the
acceptable window size supplied by the receiver, which is applicable
Shen, et al. Standards Track [Page 18]
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in window-based load-filtering. In static load-filtering policy
configuration scenarios, using the <rate> sub-element is RECOMMENDED
because it is hard to enforce the percentage rate or window-based
load filtering when incoming load from upstream or reactions from
downstream are uncertain. (See [SIP-OVERLOAD] and [RFC6357] for more
details on rate-based, loss-based, and window-based load control.)
In addition, the <accept> element takes an optional "alt-action"
attribute that can be used to explicitly specify the desired action
in case a request cannot be processed. The "alt-action" can take one
of the following three values: "reject", "redirect", or "drop".
o The "reject" action is the default value for "alt-action". It
means that the load-filtering server will reject the request with
a 503 "Service Unavailable" response message.
o The "redirect" action means redirecting the request to another
target. When it is used, an "alt-target" attribute MUST be
defined. The "alt-target" specifies one URI or a list of URIs
where the request should be redirected. The server sends out the
redirect URIs in a 300-class response message.
o The "drop" action means simply ignoring the request without doing
anything, which can, in certain cases, help save processing
capability during overload. For example, when SIP is running over
a reliable transport such as TCP, the "drop" action does not send
out the rejection response, neither does it close the transport
connection. However, when running SIP over an unreliable
transport such as UDP, using the "drop" action will create message
retransmissions that further worsen the possible overload
situation. Therefore, any "drop" action applied to an unreliable
transport MUST be treated as if it were "reject".
The above "alt-action" processing can also be illustrated through the
following pseudocode.
SWITCH "alt-action"
"redirect": "redirect"
"drop":
IF unreliable-transport
THEN treat as "reject"
ELSE
"drop"
"reject": "reject"
default: "reject"
END
Shen, et al. Standards Track [Page 19]
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In the following <actions> element example, the server accepts
maximum of 100 call requests per second. The remaining calls are
redirected to an answering machine.
<actions>
<accept alt-action="redirect" alt-target=
"sip:answer-machine@example.com">
<rate>100</rate>
</accept>
</actions>
6. XML Schema Definition for Load Control
This section defines the XML schema for the load-control document.
It extends the Common Policy schema in [RFC4745] in two ways.
Firstly, it defines two mandatory attributes for the <ruleset>
element: "version" and "state". The "version" attribute allows the
recipient of the notification to properly order them. Versions start
at zero and increase by one for each new document sent to a
subscriber within the same subscription. Versions MUST be
representable using a non-negative 32-bit integer. The "state"
attribute indicates whether the document contains a full load-
filtering policy update or only state delta as partial update.
Secondly, it defines new members of the <conditions> and <actions>
elements.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="urn:ietf:params:xml:ns:load-control"
xmlns:lc="urn:ietf:params:xml:ns:load-control"
xmlns:cp="urn:ietf:params:xml:ns:common-policy"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
attributedFormDefault="unqualified">
<xs:import namespace="urn:ietf:params:xml:ns:common-policy"/>
<!-- RULESET -->
<xs:element name="ruleset">
<xs:complexType>
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:element name="rule" type="cp:ruleType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
Shen, et al. Standards Track [Page 20]
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<xs:attribute name="version" type="xs:integer" use="required"/>
<xs:attribute name="state" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="full"/>
<xs:enumeration value="partial"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
<!-- CONDITIONS -->
<!-- CALL IDENTITY -->
<xs:element name="call-identity" type="lc:call-identity-type"/>
<!-- CALL IDENTITY TYPE -->
<xs:complexType name="call-identity-type">
<xs:choice>
<xs:element name="sip" type="lc:sip-id-type"/>
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:choice>
<anyAtrribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!-- SIP ID TYPE -->
<xs:complexType name="sip-id-type">
<xs:sequence>
<element name="from" type="lc:identityType" minOccurs="0"/>
<element name="to" type="lc:identityType" minOccurs="0"/>
<element name="request-uri" type="lc:identityType"
minOccurs="0"/>
<element name="p-asserted-identity" type="lc:identityType"
minOccurs="0"/>
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:sequence>
<anyAtrribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!-- IDENTITY TYPE -->
<xs:complexType name="identityType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="1" maxOccurs="unbounded">
<xs:element name="one" type="cp:oneType"/>
Shen, et al. Standards Track [Page 21]
RFC 7200 SIP Load-Control Event Package April 2014
<xs:element name="many" type="lc:manyType"/>
<xs:element name="many-tel" type="lc:manyTelType"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:choice>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- MANY-TEL TYPE -->
<xs:complexType name="manyTelType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="except-tel" type="lc:exceptTelType"/>
<xs:any namespace="##other"
minOccurs="0" processContents="lax"/>
</xs:choice>
<xs:attribute name="prefix"
use="optional" type="xs:string"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- EXCEPT-TEL TYPE -->
<xs:complexType name="exceptTelType">
<xs:attribute name="prefix" type="xs:string" use="optional"/>
<xs:attribute name="id" type="xs:anyURI" use="optional"/>
</xs:complexType>
<!-- METHOD -->
<xs:element name="method" type="lc:method-type"/>
<!-- METHOD TYPE -->
<xs:simpleType name="method-type">
<xs:restriction base="xs:string">
<xs:enumeration value="INVITE"/>
<xs:enumeration value="MESSAGE"/>
<xs:enumeration value="REGISTER"/>
<xs:enumeration value="SUBSCRIBE"/>
<xs:enumeration value="OPTIONS"/>
<xs:enumeration value="PUBLISH"/>
</xs:restriction>
</xs:simpleType>
<!-- TARGET SIP ENTITY -->
<xs:element name="target-sip-entity" type="xs:anyURI" minOccurs="0"/>
<!-- ACTIONS -->
Shen, et al. Standards Track [Page 22]
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<xs:element name="accept">
<xs:choice>
<element name="rate" type="xs:decimal" minOccurs="0"/>
<element name="win" type="xs:integer" minOccurs="0"/>
<element name="percent" type="xs:decimal" minOccurs="0"/>
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:choice>
<xs:attribute name="alt-action" type="xs:string" default="reject"/>
<xs:attribute name="alt-target" type="lc:alt-target-type"
use="optional"/>
<anyAtrribute namespace="##other" processContents="lax"/>
</xs:element>
<!-- ALT TARGET TYPE -->
<xs:simpleType name="alt-target-type">
<xs:list itemType="xs:anyURI"/>
</xs:simpleType>
</xs:schema>
7. Security Considerations
Two primary security considerations arise from this specification.
One is the distribution mechanism for the load filtering policy that
is based on the SIP event notification framework, and the other is
the enforcement mechanism for the load-filtering policy.
Security considerations for SIP event package mechanisms are covered
in Section 6 of [RFC6665]. A particularly relevant security concern
for this event package is that if the notifiers can be spoofed,
attackers can send fake notifications asking subscribers to throttle
all traffic, leading to denial-of-service (DoS) attacks. Therefore,
this SIP load-filtering mechanism MUST be used in a Trust Domain
(Section 3.4). But if a legitimate notifier in the Trust Domain is
itself compromised, additional mechanisms will be needed to detect
the attack.
Security considerations for load-filtering policy enforcement depends
very much on the contents of the policy. This specification defines
a possible match of the following SIP header fields in a load-
filtering policy: <from>, <to>, <request-uri>, and
<p-asserted-identity>. The exact requirement to authenticate and
authorize these fields is up to the service provider. In general, if
the identity field represents the source of the request, it SHOULD be
authenticated and authorized; if the identity field represents the
destination of the request, the authentication and authorization is
optional.
Shen, et al. Standards Track [Page 23]
RFC 7200 SIP Load-Control Event Package April 2014
In addition, the "redirect" action (Section 5.4) could facilitate a
reflection denial-of-service attack. If a number of SIP proxy
servers in a Trust Domain are using UDP and configured to get their
policies from a central server. An attacker spoofs the central
server's address to send a number of NOTIFY bodies telling the proxy
servers to redirect all calls to victim@outside-of-trust-domain.com.
The proxy servers then redirect all calls to the victim, who then
becomes a victim of Denial of Service attack and becomes
inaccessiable from the Internet. To address this type of threat,
this specification requires that a Trust Domain agrees on what types
of calls can be affected as well as on the destinations to which
calls may be redirected, as in Section 3.4.
8. IANA Considerations
This specification registers a SIP event package, a new media type, a
new XML namespace, and a new XML schema.
8.1. Load-Control Event Package Registration
This section registers an event package based on the registration
procedures defined in [RFC6665].
Package name: load-control
Type: package
Published specification: This specification
Person to contact: Charles Shen, charles@cs.columbia.edu
8.2. application/load-control+xml Media Type Registration
This section registers a new media type based on the procedures
defined in [RFC6838] and guidelines in [RFC3023].
Type name: application
Subtype name: load-control+xml
Required parameters: none
Optional parameters: Same as charset parameter of application/xml as
specified in [RFC3023].
Encoding considerations: Same as encoding considerations of
application/xml as specified in [RFC3023].
Shen, et al. Standards Track [Page 24]
RFC 7200 SIP Load-Control Event Package April 2014
Security considerations: See Section 10 of [RFC3023] and Section 7 of
this specification.
Interoperability considerations: none
Published specification: This specification
Applications that use this media type: Applications that perform load
control of SIP entities.
Fragment identifier considerations: Same as fragment identifier
considerations of application/xml as specified in [RFC3023].
Additional Information:
Deprecated alias names for this type: none
Magic Number(s): none
File Extension(s): .xml
Macintosh file type code(s): "TEXT"
Person and email address for further information: Charles Shen,
charles@cs.columbia.edu
Intended usage: COMMON
Restrictions on usage: none
Author: Charles Shen, Henning Schulzrinne, Arata Koike
Change controller: IESG
Provisional registration? (standards tree only): no
8.3. URN Sub-Namespace Registration
This section registers a new XML namespace, as per the guidelines in
[RFC3688]
URI: The URI for this namespace is
urn:ietf:params:xml:ns:load-control
Registrant Contact: IETF SOC Working Group <sip-overload@ietf.org>,
as designated by the IESG <iesg@ietf.org>
Shen, et al. Standards Track [Page 25]
RFC 7200 SIP Load-Control Event Package April 2014
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=iso-8859-1"/>
<title>SIP Load-Control Namespace</title>
</head>
<body>
<h1>Namespace for SIP Load Control</h1>
<h2>urn:ietf:params:xml:ns:load-control</h2>
<p>See <a href="http://www.rfc-editor.org/rfc/rfc7200.txt">
RFC 7200</a>.</p>
</body>
</html>
END
8.4. Load-Control Schema Registration
URI: urn:ietf:params:xml:schema:load-control
Registrant Contact: IETF SOC working group, Charles Shen
(charles@cs.columbia.edu).
XML: the XML schema contained in Section 6 has been registered.
Its first line is
<?xml version="1.0" encoding="UTF-8"?>
and its last line is
</xs:schema>
Shen, et al. Standards Track [Page 26]
RFC 7200 SIP Load-Control Event Package April 2014
9. Acknowledgements
The authors would like to thank Jari Arkko, Richard Barnes, Stewart
Bryant, Gonzalo Camarillo, Bruno Chatras, Benoit Claise, Spencer
Dawkins, Martin Dolly, Keith Drage, Ashutosh Dutta, Donald Eastlake,
Adrian Farrel, Stephen Farrell, Janet Gunn, Vijay Gurbani, Brian
Haberman, Volker Hilt, Geoff Hunt, Carolyn Johnson, Hadriel Kaplan,
Paul Kyzivat, Barry Leiba, Pearl Liang, Salvatore Loreto, Timothy
Moran, Eric Noel, Parthasarathi R, Pete Resnick, Adam Roach, Dan
Romascanu, Shida Schubert, Robert Sparks, Martin Stiemerling, Sean
Turner, Phil Williams, and other members of the SOC and SIPPING
working groups for many helpful comments. In particular, Bruno
Chatras proposed the <method> and <target-sip-entity> condition
elements along with many other text improvements. Janet Gunn
provided detailed text suggestions including Appendix C. Eric Noel
suggested clarification on load-filtering policy distribution
initialization process. Shida Schubert made many suggestions such as
terminology usage. Phil Williams suggested adding support for delta
updates. Ashutosh Dutta gave pointers to Public Switched Telephone
Network (PSTN) references. Adam Roach suggested improvements related
to RFC 6665 and offered other helpful clarifications. Richard Barnes
made many suggestions such as referencing the Trust Domain concept of
RFCs 3324 and 3325, the use of a separate element for 'tel' URI
grouping, and addressing the "redirect" action security threat.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
Shen, et al. Standards Track [Page 27]
RFC 7200 SIP Load-Control Event Package April 2014
[RFC4745] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J.,
Polk, J., and J. Rosenberg, "Common Policy: A Document
Format for Expressing Privacy Preferences", RFC 4745,
February 2007.
[RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665,
July 2012.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, January 2013.
10.2. Informative References
[E.300SerSup3]
ITU-T, "North American Precise Audible Tone Plan",
Recommendation E.300 Series Supplement 3, November 1988.
[E.412] ITU-T, "Network Management Controls", Recommendation
E.412-2003, January 2003.
[Q.1248.2] ITU-T, "Interface Recommendation for Intelligent Network
Capability Set4:SCF-SSF interface", Recommendation
Q.1248.2, July 2001.
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
August 1999.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)", RFC
4412, February 2006.
[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
January 2008.
Shen, et al. Standards Track [Page 28]
RFC 7200 SIP Load-Control Event Package April 2014
[RFC5390] Rosenberg, J., "Requirements for Management of Overload in
the Session Initiation Protocol", RFC 5390, December 2008.
[RFC6357] Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
Considerations for Session Initiation Protocol (SIP)
Overload Control", RFC 6357, August 2011.
[SIP-OVERLOAD]
Gurbani, V., Ed., Hilt, V., and H. Schulzrinne, "Session
Initiation Protocol (SIP) Overload Control", Work in
Progress, March 2014.
Shen, et al. Standards Track [Page 29]
RFC 7200 SIP Load-Control Event Package April 2014
Appendix A. Definitions
This specification reuses the definitions for "Event Package",
"Notification", "Notifier", "Subscriber", and "Subscription" as in
[RFC6665]. The following additional definitions are also used.
Load Filtering: A load-control mechanism that applies specific
actions to selected loads (e.g., SIP requests) matching specific
conditions.
Load-Filtering Policy: A set of zero or more load-filtering rules,
also known as load-filtering rule set.
Load-Filtering Rule: Conditions and actions to be applied for load
filtering.
Load-Filtering Condition: Elements that describe how to select loads
to apply load-filtering actions. This specification defines the
<call-identity>, <method>, <target-sip-identity>, and <validity>
condition elements (Section 5.3).
Load-Filtering Action: An operation to be taken by a load-filtering
server on loads that match the load-filtering conditions. This
specification allows actions such as accept, reject, and redirect
of loads (Section 5.4).
Load-Filtering Server: A server that performs load filtering. In
the context of this specification, the load-filtering server is
the subscriber, which receives load-filtering policies from the
notifier and enforces those policies during load filtering.
Load-Control Document: An XML document that describes the load-
filtering policies (Section 5). It inherits and enhances the
common policy document defined in [RFC4745].
Appendix B. Design Requirements
The SIP load-filtering mechanism needs to satisfy the following
requirements:
o For simplicity, the solution should focus on a method for
controlling SIP load, rather than a generic application-layer
mechanism.
o The load-filtering policy needs to be distributed efficiently to
possibly a large subset of all SIP elements.
Shen, et al. Standards Track [Page 30]
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o The solution should reuse existing SIP protocol mechanisms to
reduce implementation and deployment complexity.
o For predictable overload situations, such as holidays and mass
calling events, the load-filtering policy should specify during
what time it is to be applied, so that the information can be
distributed ahead of time.
o For destination-specific overload situations, the load-filtering
policy should be able to describe the destination domain or the
callee.
o To address accidental and intentional high-volume call generators,
the load-filtering policy should be able to specify the caller.
o Caller and callee need to be specified as both SIP URIs and 'tel'
URIs [RFC3966] in load-filtering policies.
o It should be possible to specify particular information in the SIP
headers (e.g., prefixes in telephone numbers) that allow load
filtering over limited regionally focused overloads.
o The solution should draw upon experiences from related PSTN
mechanisms [Q.1248.2] [E.412] [E.300SerSup3] where applicable.
o The solution should be extensible to meet future needs.
Appendix C. Discussion of How This Specification Meets the Requirements
of RFC 5390
This section evaluates whether the load-control event package
mechanism defined in this specification satisfies various SIP
overload control requirements set forth by [RFC5390]. As mentioned
in Section 1, this specification complements other efforts in the
overall SIP load-control solution space. Therefore, not all RFC 5390
requirements are found applicable to this specification. This
specification categorizes the assessment results into Yes (the
requirement is met), P/A (Partially Applicable), No (must be used in
conjunction with another mechanism to meet the requirement), and N/A
(Not Applicable).
REQ 1: The overload mechanism shall strive to maintain the overall
useful throughput (taking into consideration the quality-of-
service needs of the using applications) of a SIP server at
reasonable levels, even when the incoming load on the network is
far in excess of its capacity. The overall throughput under load
is the ultimate measure of the value of an overload control
mechanism.
Shen, et al. Standards Track [Page 31]
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P/A. The goal of load filtering is to prevent overload or maintain
overall goodput during the time of overload, but it is dependent on
the predictions of the load and the computations as well as
distribution of the filtering policies. If the load predictions or
filtering policy computations are incorrect, or the filtering policy
is not properly distributed, the mechanism will be less effective.
On the other hand, if the load can be accurately predicted and
filtering policies be computed and distributed appropriately, this
requirement can be met.
REQ 2: When a single network element fails, goes into overload, or
suffers from reduced processing capacity, the mechanism should
strive to limit the impact of this on other elements in the
network. This helps to prevent a small-scale failure from
becoming a widespread outage.
N/A if load-filtering policies are installed in advance and do not
change during the potential overload period, P/A if load-filtering
policies are dynamically adjusted. The algorithm to dynamically
compute load-filtering policies is outside the scope of this
specification, while the distribution of the updated filtering
policies uses the event package mechanism of this specification.
REQ 3: The mechanism should seek to minimize the amount of
configuration required in order to work. For example, it is
better to avoid needing to configure a server with its SIP message
throughput, as these kinds of quantities are hard to determine.
No. This mechanism is entirely dependent on advance configuration,
based on advance knowledge. In order to satisfy REQ 3, it should be
used in conjunction with other mechanisms that are not based on
advance configuration.
REQ 4: The mechanism must be capable of dealing with elements that
do not support it, so that a network can consist of a mix of
elements that do and don't support it. In other words, the
mechanism should not work only in environments where all elements
support it. It is reasonable to assume that it works better in
such environments, of course. Ideally, there should be
incremental improvements in overall network throughput as
increasing numbers of elements in the network support the
mechanism.
No. This mechanism is entirely dependent on the participation of all
possible neighbors. In order to satisfy REQ 4, it should be used in
conjunction with other mechanisms, some of which are described in
Section 3.4.
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REQ 5: The mechanism should not assume that it will only be
deployed in environments with completely trusted elements. It
should seek to operate as effectively as possible in environments
where other elements are malicious; this includes preventing
malicious elements from obtaining more than a fair share of
service.
No. This mechanism is entirely dependent on the non-malicious
participation of all possible neighbors. In order to satisfy REQ 5,
it should be used in conjunction with other mechanisms, some of which
are described in Section 3.4.
REQ 6: When overload is signaled by means of a specific message,
the message must clearly indicate that it is being sent because of
overload, as opposed to other, non overload-based failure
conditions. This requirement is meant to avoid some of the
problems that have arisen from the reuse of the 503 response code
for multiple purposes. Of course, overload is also signaled by
lack of response to requests. This requirement applies only to
explicit overload signals.
N/A. This mechanism signals anticipated overload, not actual
overload. However, the signals in this mechanism are not used for
any other purpose.
REQ 7: The mechanism shall provide a way for an element to
throttle the amount of traffic it receives from an upstream
element. This throttling shall be graded so that it is not all-
or-nothing as with the current 503 mechanism. This recognizes the
fact that "overload" is not a binary state and that there are
degrees of overload.
Yes. This event package allows rate-/loss-/window-based overload
control options as discussed in Section 5.4.
REQ 8: The mechanism shall ensure that, when a request was not
processed successfully due to overload (or failure) of a
downstream element, the request will not be retried on another
element that is also overloaded or whose status is unknown. This
requirement derives from REQ 1.
N/A to the load-control event package mechanism itself.
REQ 9: That a request has been rejected from an overloaded element
shall not unduly restrict the ability of that request to be
submitted to and processed by an element that is not overloaded.
This requirement derives from REQ 1.
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Yes. For example, load-filtering policy (Section 3.1) can include
alternative forwarding destinations for rejected requests.
REQ 10: The mechanism should support servers that receive requests
from a large number of different upstream elements, where the set
of upstream elements is not enumerable.
No. Because this mechanism requires advance configuration of
specifically identified neighbors, it does not support environments
where the number and identity of the upstream neighbors are not known
in advance. In order to satisfy REQ 10, it should be used in
conjunction with other mechanisms.
REQ 11: The mechanism should support servers that receive requests
from a finite set of upstream elements, where the set of upstream
elements is enumerable.
Yes. See also answer to REQ 10.
REQ 12: The mechanism should work between servers in different
domains.
Yes. The load-control event package mechanism is not limited by
domain boundaries. However, it is likely more applicable in intra-
domain scenarios than in inter-domain scenarios due to security and
other concerns (see also Section 3.4).
REQ 13: The mechanism must not dictate a specific algorithm for
prioritizing the processing of work within a proxy during times of
overload. It must permit a proxy to prioritize requests based on
any local policy, so that certain ones (such as a call for
emergency services or a call with a specific value of the
Resource-Priority header field [RFC4412]) are given preferential
treatment, such as not being dropped, being given additional
retransmission, or being processed ahead of others.
P/A. This mechanism does not specifically address the prioritizing
of work during times of overload. But it does not preclude any
particular local policy.
REQ 14: The mechanism should provide unambiguous directions to
clients on when they should retry a request and when they should
not. This especially applies to TCP connection establishment and
SIP registrations, in order to mitigate against avalanche restart.
N/A to the load-control event package mechanism itself.
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REQ 15: In cases where a network element fails, is so overloaded
that it cannot process messages, or cannot communicate due to a
network failure or network partition, it will not be able to
provide explicit indications of the nature of the failure or its
levels of congestion. The mechanism must properly function in
these cases.
P/A. Because the load-filtering policies are provisioned in advance,
they are not affected by the overload or failure of other network
elements. On the other hand, they may not, in those cases, be able
to protect the overloaded network elements (see REQ 1).
REQ 16: The mechanism should attempt to minimize the overhead of
the overload control messaging.
Yes. The standardized SIP event package mechanism [RFC6665] is used.
REQ 17: The overload mechanism must not provide an avenue for
malicious attack, including DoS and DDoS attacks.
P/A. This mechanism does provide a potential avenue for malicious
attacks. Therefore, the security mechanisms for SIP event packages,
in general, [RFC6665] and Section 7 of this specification should be
used.
REQ 18: The overload mechanism should be unambiguous about whether
a load indication applies to a specific IP address, host, or URI,
so that an upstream element can determine the load of the entity
to which a request is to be sent.
Yes. The identity of load indication is covered in the load-
filtering policy format definition in Section 3.1.
REQ 19: The specification for the overload mechanism should give
guidance on which message types might be desirable to process over
others during times of overload, based on SIP-specific
considerations. For example, it may be more beneficial to process
a SUBSCRIBE refresh with Expires of zero than a SUBSCRIBE refresh
with a non-zero expiration (since the former reduces the overall
amount of load on the element), or to process re-INVITEs over new
INVITEs.
N/A to the load-control event package mechanism itself.
REQ 20: In a mixed environment of elements that do and do not
implement the overload mechanism, no disproportionate benefit
shall accrue to the users or operators of the elements that do not
implement the mechanism.
Shen, et al. Standards Track [Page 35]
RFC 7200 SIP Load-Control Event Package April 2014
No. This mechanism is entirely dependent on the participation of all
possible neighbors. In order to satisfy REQ 20, it should be used in
conjunction with other mechanisms, some of which are described in
Section 3.4.
REQ 21: The overload mechanism should ensure that the system
remains stable. When the offered load drops from above the
overall capacity of the network to below the overall capacity, the
throughput should stabilize and become equal to the offered load.
N/A to the load-control event package mechanism itself.
REQ 22: It must be possible to disable the reporting of load
information towards upstream targets based on the identity of
those targets. This allows a domain administrator who considers
the load of their elements to be sensitive information, to
restrict access to that information. Of course, in such cases,
there is no expectation that the overload mechanism itself will
help prevent overload from that upstream target.
N/A to the load-control event package mechanism itself.
REQ 23: It must be possible for the overload mechanism to work in
cases where there is a load balancer in front of a farm of
proxies.
Yes. The load-control event package mechanism does not preclude its
use in a scenario with server farms.
Appendix D. Complete Examples
D.1. Load-Control Document Examples
This section presents two complete examples of load-control documents
valid with respect to the XML schema defined in Section 6.
The first example assumes that a set of hotlines are set up at
"sip:alice@hotline.example.com" and "tel:+1-212-555-1234". The
hotlines are activated from 12:00 to 15:00 US Eastern Standard Time
on 2008-05-31. The goal is to limit the incoming calls to the
hotlines to 100 requests per second. Calls that exceed the rate
limit are explicitly rejected.
Shen, et al. Standards Track [Page 36]
RFC 7200 SIP Load-Control Event Package April 2014
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:lc="urn:ietf:params:xml:ns:load-control"
version="0" state="full">
<rule id="f3g44k1">
<conditions>
<lc:call-identity>
<lc:sip>
<lc:to>
<one id="sip:alice@hotline.example.com"/>
<one id="tel:+1-212-555-1234"/>
</lc:to>
</lc:sip>
</lc:call-identity>
<method>INVITE</method>
<validity>
<from>2008-05-31T12:00:00-05:00</from>
<until>2008-05-31T15:00:00-05:00</until>
</validity>
</conditions>
<actions>
<lc:accept alt-action="reject">
<lc:rate>100</lc:rate>
</lc:accept>
</actions>
</rule>
</ruleset>
The second example optimizes the usage of server resources during the
three-day period following a hurricane. Incoming calls to the domain
"sandy.example.com" or to call destinations with prefix "+1-212" will
be limited to a rate of 100 requests per second, except for those
calls originating from a particular rescue team domain
"rescue.example.com". Outgoing calls from the hurricane domain or
calls within the local domain are never limited. All calls that are
throttled due to the rate limit will be forwarded to an answering
machine with updated hurricane rescue information.
Shen, et al. Standards Track [Page 37]
RFC 7200 SIP Load-Control Event Package April 2014
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:lc="urn:ietf:params:xml:ns:load-control"
version="1" state="full">
<rule id="f3g44k2">
<conditions>
<lc:call-identity>
<lc:sip>
<lc:to>
<many domain="sandy.example.com"/>
<many-tel prefix="+1-212"/>
</lc:to>
<lc:from>
<many>
<except domain="sandy.example.com"/>
<except domain="rescue.example.com"/>
</many>
</lc:from>
</lc:sip>
</lc:call-identity>
<method>INVITE</method>
<validity>
<from>2012-10-25T09:00:00+01:00</from>
<until>2012-10-28T09:00:00+01:00</until>
</validity>
</conditions>
<actions>
<lc:accept alt-action="redirect" alt-target=
"sip:sandy@update.example.com">
<lc:rate>100</lc:rate>
</lc:accept>
</actions>
</rule>
</ruleset>
Sometimes it may occur that multiple rules in a ruleset define
actions that match the same methods, call identity and validity. In
those cases, the "first-match-wins" principle is used. For example,
in the following ruleset, the first rule requires all calls from the
"example.com" domain to be rejected. Even though the rule following
that one specifies that calls from "sip:alice@example.com" be
redirected to a specific target "sip:eve@example.com", the calls from
"sip:alice@example.com" will still be rejected because they have
already been matched by the earlier rule.
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<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
xmlns:lc="urn:ietf:params:xml:ns:load-control"
version="1" state="full">
<rule id="f3g44k3">
<conditions>
<lc:call-identity>
<lc:sip>
<lc:from>
<many domain="example.com"/>
</lc:from>
</lc:sip>
</lc:call-identity>
<method>INVITE</method>
<validity>
<from>2013-7-2T09:00:00+01:00</from>
<until>2013-7-3T09:00:00+01:00</until>
</validity>
</conditions>
<actions>
<lc:accept alt-action="reject">
<lc:rate>0</lc:rate>
</lc:accept>
</actions>
</rule>
<rule id="f3g44k4">
<conditions>
<lc:call-identity>
<lc:sip>
<lc:from>
<one id="sip:alice@example.com"/>
</lc:from>
</lc:sip>
</lc:call-identity>
<method>INVITE</method>
<validity>
<from>2013-7-2T09:00:00+01:00</from>
<until>2013-7-3T09:00:00+01:00</until>
</validity>
</conditions>
<actions>
<lc:accept alt-action="redirect" alt-target=
"sip:eve@example.com">
<lc:rate>0</lc:rate>
</lc:accept>
</actions>
Shen, et al. Standards Track [Page 39]
RFC 7200 SIP Load-Control Event Package April 2014
</rule>
</ruleset>
D.2. Message Flow Examples
This section presents an example message flow of using the load-
control event package mechanism defined in this specification.
atlanta biloxi
| F1 SUBSCRIBE |
|------------------>|
| F2 200 OK |
|<------------------|
| F3 NOTIFY |
|<------------------|
| F4 200 OK |
|------------------>|
F1 SUBSCRIBE atlanta.example.com -> biloxi.example.com
SUBSCRIBE sip:biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy7cjbu3
From: sip:atlanta.example.com;tag=162ab5
To: sip:biloxi.example.com
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 2012 SUBSCRIBE
Contact: sip:atlanta.example.com
Event: load-control
Max-Forwards: 70
Accept: application/load-control+xml
Expires: 3600
Content-Length: 0
F2 200 OK biloxi.example.com -> atlanta.example.com
SIP/2.0 200 OK
Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy7cjbu3
;received=192.0.2.1
To: <sip:biloxi.example.com>;tag=331dc8
From: <sip:atlanta.example.com>;tag=162ab5
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 2012 SUBSCRIBE
Expires: 3600
Contact: sip:biloxi.example.com
Content-Length: 0
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RFC 7200 SIP Load-Control Event Package April 2014
F3 NOTIFY biloxi.example.com -> atlanta.example.com
NOTIFY sip:atlanta.example.com SIP/2.0
Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy71g2ks
From: <sip:biloxi.example.com>;tag=331dc8
To: <sip:atlanta.example.com>;tag=162ab5
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
Event: load-control
Subscription-State: active;expires=3599
Max-Forwards: 70
CSeq: 1775 NOTIFY
Contact: sip:biloxi.example.com
Content-Type: application/load-control+xml
Content-Length: ...
[Load-Control Document]
F4 200 OK atlanta.example.com -> biloxi.example.com
SIP/2.0 200 OK
Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy71g2ks
;received=192.0.2.2
From: <sip:biloxi.example.com>;tag=331dc8
To: <sip:atlanta.example.com>;tag=162ab5
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 1775 NOTIFY
Content-Length: 0
Appendix E. Related Work
E.1. Relationship to Load Filtering in PSTN
It is known that an existing PSTN network also uses a load-filtering
mechanism to prevent overload and the filtering policy configuration
is done manually except in specific cases when the Intelligent
Network architecture is used [Q.1248.2][E.412]. This specification
defines a load-filtering mechanism based on the SIP event
notification framework that allows automated filtering policy
distribution in suitable environments.
PSTN overload control uses messages that specify an outgoing control
list, call gap duration, and control duration [Q.1248.2][E.412].
These items correspond roughly to the identity, action, and time
fields of the SIP load-filtering policy defined in this
specification. However, the load-filtering policy defined in this
specification is much more generic and flexible as opposed to its
PSTN counterpart.
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RFC 7200 SIP Load-Control Event Package April 2014
Firstly, PSTN load filtering only applies to telephone numbers. The
identity element of SIP load-filtering policy allows both SIP URI and
telephone numbers (through 'tel' URI) to be specified. These
identities can be arbitrarily grouped by SIP domains or any number of
leading prefixes of the telephone numbers.
Secondly, the PSTN load-filtering action is usually limited to call
gapping. The action field in SIP load-filtering policy allows more
flexible possibilities such as rate throttle and others.
Thirdly, the duration field in PSTN load filtering specifies a value
in seconds for the load-filtering duration only, and the allowed
values are mapped into a value set. The time field in SIP load-
filtering policy may specify not only a duration, but also a future
activation time that could be especially useful for automating load
filtering for predictable overloads.
PSTN load filtering can be performed in both edge switches and
transit switches; the SIP load filtering can also be applied in both
edge proxy servers and core proxy servers, and even in capable user
agents.
PSTN load filtering also has special accommodation for High
Probability of Completion (HPC) calls, which would be similar to
calls designated by the SIP Resource Priority Headers [RFC4412]. The
SIP load-filtering mechanism also allows prioritizing the treatment
of these calls by specifying favorable actions for them.
PSTN load filtering also provides an administrative option for
routing failed call attempts to either a reorder tone [E.300SerSup3]
indicating overload conditions or a special recorded announcement. A
similar capability can be provided in the SIP load-filtering
mechanism by specifying appropriate "alt-action" attribute in the SIP
load-filtering action field.
E.2. Relationship with Other IETF SIP Overload Control Efforts
The load-filtering policies in this specification consist of
identity, action, and time. The identity can range from a single
specific user to an arbitrary user aggregate, domains, or areas. The
user can be identified by either the source or the destination. When
the user is identified by the source and a favorable action is
specified, the result is, to some extent, similar to identifying a
priority user based on authorized Resource Priority Headers [RFC4412]
in the requests. Specifying a source user identity with an
unfavorable action would cause an effect to some extent similar to an
inverse SIP resource priority mechanism.
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RFC 7200 SIP Load-Control Event Package April 2014
The load-filtering policy defined in this specification is generic
and expected to be applicable not only to the load-filtering
mechanism but also to the feedback overload control mechanism in
[SIP-OVERLOAD]. In particular, both mechanisms could use specific or
wildcard identities for load control and could share well-known load-
control actions. The time duration field in the load-filtering
policy could also be used in both mechanisms. As mentioned in
Section 1, the load-filtering policy distribution mechanism and the
feedback overload control mechanism address complementary areas in
the overload control problem space. Load filtering is more proactive
and focuses on distributing filtering policies towards the source of
the traffic; the hop-by-hop feedback-based approach is reactive and
reduces traffic already accepted by the network. Therefore, they
could also make different use of the generic load-filtering policy
components. For example, the load-filtering mechanism may use the
time field in the filtering policy to specify not only a control
duration but also a future activation time to accommodate a
predicable overload such as the one caused by Mother's Day greetings
or a viewer-voting program; the feedback-based control might not need
to use the time field or might use the time field to specify an
immediate load-control duration.
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Authors' Addresses
Charles Shen
Columbia University
Department of Computer Science
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
Phone: +1 212 854 3109
EMail: charles@cs.columbia.edu
Henning Schulzrinne
Columbia University
Department of Computer Science
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
Phone: +1 212 939 7004
EMail: schulzrinne@cs.columbia.edu
Arata Koike
NTT Network Technology Labs
3-9-11 Midori-cho Musashino-shi
Tokyo 180-8585
Japan
Phone: +81 422 59 6099
EMail: koike.arata@lab.ntt.co.jp
Shen, et al. Standards Track [Page 44]
ERRATA
