IETF SOC Working Group C. Shen
Internet-Draft H. Schulzrinne
Intended status: Standards Track Columbia U.
Expires: April 25, 2013 A. Koike
NTT
October 22, 2012
A Session Initiation Protocol (SIP) Load Control Event Package
draft-ietf-soc-load-control-event-package-05.txt
Abstract
We define a load control event package for the Session Initiation
Protocol (SIP). It allows SIP servers to distribute load filters to
other SIP servers in the network. The load filters contain rules to
throttle calls based on their source or destination domain, telephone
number prefix or for a specific user. The mechanism helps to prevent
signaling overload and complements feedback-based SIP overload
control efforts.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2013.
Copyright Notice
Copyright (c) 2012 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
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 6
3. Design Requirements . . . . . . . . . . . . . . . . . . . . . 6
4. SIP Load Filtering Overview . . . . . . . . . . . . . . . . . 6
4.1. Filter Format . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Filter Computation . . . . . . . . . . . . . . . . . . . . 7
4.3. Filter Distribution . . . . . . . . . . . . . . . . . . . 7
4.4. Applicability in Different Network Environments . . . . . 10
5. Load Control Event Package . . . . . . . . . . . . . . . . . . 11
5.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 11
5.2. Event Package Parameters . . . . . . . . . . . . . . . . . 11
5.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 11
5.4. SUBSCRIBE Duration . . . . . . . . . . . . . . . . . . . . 12
5.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 12
5.7. Notifier Generation of NOTIFY Requests . . . . . . . . . . 12
5.8. Subscriber Processing of NOTIFY Requests . . . . . . . . . 13
5.9. Handling of Forked Requests . . . . . . . . . . . . . . . 14
5.10. Rate of Notifications . . . . . . . . . . . . . . . . . . 14
5.11. State Delta . . . . . . . . . . . . . . . . . . . . . . . 14
6. Load Control Document . . . . . . . . . . . . . . . . . . . . 15
6.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Namespace . . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. Conditions . . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.1. Call Identity . . . . . . . . . . . . . . . . . . . . 15
6.3.2. Method . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3.3. Target SIP Entity . . . . . . . . . . . . . . . . . . 19
6.3.4. Validity . . . . . . . . . . . . . . . . . . . . . . . 20
6.4. Actions . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.5. Complete Examples . . . . . . . . . . . . . . . . . . . . 21
6.5.1. Load Control Document Examples . . . . . . . . . . . . 21
6.5.2. Message Flow Examples . . . . . . . . . . . . . . . . 23
7. XML Schema Definition for Load Control . . . . . . . . . . . . 25
8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Relationship with Load Filtering in PSTN . . . . . . . . . 27
8.2. Relationship with Other IETF SIP Load Control Efforts . . 28
9. Discussion of this specification meeting the requirements
of RFC5390 . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 34
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
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11.1. Load Control Event Package Registration . . . . . . . . . 34
11.2. application/load-control+xml MIME Registration . . . . . . 35
11.3. Load Control Schema Registration . . . . . . . . . . . . . 36
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
13.1. Normative References . . . . . . . . . . . . . . . . . . . 36
13.2. Informative References . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
Proper functioning of Session Initiation Protocol (SIP) [RFC3261]
signaling servers is critical in SIP-based communications networks.
The performance of SIP servers can be severely degraded when the
server is overloaded with excessive number of signaling requests.
Both legitimate and malicious traffic can overload SIP servers,
despite appropriate capacity planning.
There are three common examples of legitimate short-term increases in
call volumes. Viewer-voting TV shows or ticket giveaways may
generate millions of calls within a few minutes. Call volume may
also spike during special holidays such as New Year's Day and
Mother's Day. Finally, callers may want to reach friends and family
in natural disaster areas such as those affected by hurricanes. When
possible, only calls traversing overloaded servers should be
throttled under those conditions.
SIP load control mechanisms are needed to prevent congestion collapse
in these cases [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 [I-D.ietf-soc-overload-control]. 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. They work 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. However,
feedback control is usually ineffective for overload of more general
purpose SIP edge proxy servers. For example, in the ticket giveaway
case, almost all calls to the hotline will fail at the core proxy
servers; if the edge proxy servers leading to the core proxy servers
are also overloaded, calls to other destinations will also be
rejected or dropped.
Here, we propose an additional, complementary mechanism, called load
filtering. Network operators create load filters that indicate that
calls to specific destinations or from specific sources should be
rate-limited or randomly dropped. These load filters are then
distributed to SIP servers and possibly user agents 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 load filtering approach is most applicable for situations where a
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traffic surge and its source/destination distribution can be
predicted in advance. For instance, it is appropriate for a mass-
phone-voting event, Mother's Day, New Years, and even a hurricane.
However, it is less likely to be effective for the initial phase of
unpredicted/unpredictable mass calling events, such as earthquakes or
terrorist attacks. In these latter cases, the local traffic load may
peak by more than an order of magnitude in minutes, if not seconds.
This does not allow time to either effectively identify the filters
needed, nor distribute them to the appropriate servers soon enough to
prevent server congestion. Once other, more immediate, techniques
(such as the loss-based or rate-based feedback control methods) have
prevented the initial congestion collapse, the load filtering
approach can be used to effectively control the continuing overload.
Performing SIP load filtering requires three components: load filter
format, load filter computation method, and load filter distribution
mechanism. This specification addresses two of these three
components. The load filter format is defined in a SIP load control
event package, while the load filter distribution mechanism is built
upon the existing SIP event framework. The remaining component, load
filter computation method, depends heavily on the actual network
topology and service provider policies. Therefore it is out of scope
of this specification.
It is helpful to clarify two aspects regarding some terminology used
in this specification. Firstly, although the SIP load filtering
mechanism is motivated by the overload control problem, which is why
this specification refers extensively to other parallel SIP overload
control related efforts, the applicability of filtering extends
beyond the overload control purpose. For example, it can also be
used to implement quality of service or other service level agreement
commitments. Therefore, we use the term SIP "load control event
package", instead of a narrower term "overload control event
package". Secondly, since we are describing a specific control
mechanism based on filtering, the term "load control" in this
specification is used inter-changeably with the term "load filtering"
unless associated with other explicit context.
The rest of this specification is structured as follows: we begin by
listing the design requirements for this work in Section 3. We then
give an overview of load filtering operation in Section 4. The load
control event package for filter distribution is detailed in
Section 5. The load filter format is defined in the two sections
that follow, with Section 6 introducing the XML document for load
control and Section 7 listing the associated schema. Section 8
relates this work to corresponding mechanisms in PSTN and other IETF
efforts addressing SIP load control. Section 9 evaluates whether
this specification meets the SIP overload control requirements set
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forth by RFC5390 [RFC5390]. Finally, Section 10 presents security
considerations and Section 11 provides IANA considerations.
2. Requirements Notation
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].
3. Design Requirements
The SIP load filtering mechanism needs to satisfy the following
requirements:
o To simplify the solution, we focus on a method for controlling SIP
load, rather than a generic application-layer mechanism.
o The load filter needs to be distributed efficiently to possibly a
large subset of all SIP elements.
o The solution should re-use existing SIP protocol mechanisms to
reduce implementation and deployment complexity.
o For predictable overload situations, such as holidays and call-in
events, the load filter should specify during what time period it
is to be applied, so that the information can be distributed ahead
of time.
o For destination-specific overload situations, the load filter
should be able to describe the callee.
o To address accidental and intentional high-volume call generators,
the load filter should be able to specify the caller.
o Caller and callee need to be specified as both SIP URIs and 'Tel'
URIs[RFC3966].
o It should be possible to specify particular information in the SIP
headers (e.g., prefixes in telephone numbers) which allow control
over limited regionally-focused overloads.
o The solution should draw upon experiences from related PSTN
mechanisms where applicable.
o The solution should be extensible to meet future needs.
4. SIP Load Filtering Overview
4.1. Filter Format
A load filter contains both conditions and actions. Filter
conditions include the identities of the targets to be controlled
(Section 6.3.1). For example, there are two typical resource limits
in a possible overload situation, i.e., human destination limits (N
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number of call takers) and node capacity limits. The control targets
in these two cases can be the specific callee numbers or the
destination domains corresponding to the overload. Filter conditions
also indicate the specific message type to be controlled
(Section 6.3.2), with which target SIP entity the filter is
associated (Section 6.3.3) and the period of time when the filtering
should be activated (Section 6.3.4). Filter actions describe the
desired control functions such as limiting the request rate below a
certain level (Section 6.4).
4.2. Filter Computation
Load filter computation needs to take into consideration information
such as the 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 loads are allocated in a way which
both prevents overload and maximizes effective throughput (aka
goodput). In some cases, in order to better utilize system
resources, it may be preferable to employ a dynamic load computation
algorithm which adapts to current network status, rather than using a
purely static mechanism. The load filter computation algorithm is
out of scope of this specification.
4.3. Filter Distribution
For load filter distribution, this specification defines the SIP
event package for load control, which is an "instantiation" of the
generic SIP events framework [RFC6665]. The SIP events framework
provides an existing method for SIP entities to subscribe to and
receive notifications when certain events have occurred. Such a
framework forms a scalable event distribution architecture that suits
our needs. This specification also defines the XML schema of a load
control document (Section 6), which is used to encode load filtering
rules.
In order for load filters to be properly distributed, each SIP node
in the network SHOULD subscribe to the load control event package
from all its outgoing signaling neighbors, known as notifiers
(Section 5.6). Subscription is initiated and maintained during
normal server operation. Signaling neighbors are defined by sending
signaling messages. For instance, if A sends signaling requests to
B, B is an outgoing signaling neighbor of A. A needs to subscribe to
the load control event package of B in case B wants to curb requests
from A. On the other hand, if B also sends signaling requests to A,
then B also subscribes to A. Subscription of neighboring SIP entities
needs to be persistent so that they are in place independently of any
specific load filtering events. Key to this is the fact that
notification following initial subscription includes an empty message
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body if no events are configured (Section 5.7), and that the
subscription needs to be refreshed periodically to make it
persistent, as described in Section 4.1 and Section 4.2 of [RFC6665].
The notifier will send a notification with its current control policy
to its subscribers each time a new subscription or a subscription
refreshing is accepted (Section 5.7). The same subscription dialog
can also be used to convey policies in a set of rules for multiple
load filtering events. (Section 6.1). The subscribers MAY terminate
the subscription if it no longer considers the notifiers as its
signaling neighbor, e.g., after an extended period of absence of
signaling message exchange. However, if after un-subscription, the
subscriber determines that the signaling with the notifier is active
again, it MUST immediately subscribe to that notifier again.
We use the example architecture shown in Figure 1 to illustrate load
filter distribution based on the SIP load control event package.
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.
+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
| EPa1 | | EPa2 | | EPa3 | | EPa4 |
| | | | | | | |
+-----------+ +-----------+ +-----------+ +-----------+
\ / \ /
\ / \ /
\ / \ /
+-----------+ +-----------+
| | | |
| CPa1 |------------------| CPa2 |
| | | |
+-----------+ +-----------+
| |
Service | |
Provider A | |
| |
=================================================================
| |
Service | |
Provider B | |
| |
+-----------+ +-----------+
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| | | |
| CPb1 |------------------| CPb2 |
| | | |
+-----------+ +-----------+
/ \ / \
/ \ / \
/ \ / \
+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
| EPb1 | | EPb2 | | EPb3 | | EPb4 |
| | | | | | | |
+-----------+ +-----------+ +-----------+ +-----------+
Figure 1: Example Network Scenario Using SIP Load Control Event
Package Mechanism
At the initialization stage, the proxy servers first identify all
their outgoing signaling neighbors and subscribe to them. The
neighbor identification process can be performed by service providers
through direct provisioning, or by the proxy servers themselves via
progressively learning from the signaling messages sent and received.
Assuming all signaling relationships in Figure 1 are bi-directional,
after this initialization stage, each proxy server will be subscribed
to all its neighbors. That is, EPa1 subscribes to CPa1; CPa1
subscribes to EPa1, EPa2, CPa2 and CPb1, so on and so forth. The
following cases then show two examples of how load filter
distribution in this network works.
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 filter computation algorithm, CPa1 may allocate the received
total acceptable 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 filter 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 condition. In other cases, the network
entry point of load filtering policy could also be an entity that
hosts this resource. For example, an operator may host an
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application server that performs 800 number translation services.
The application server may itself be a SIP proxy or a SIP Back-to-
Back User Agent (B2BUA). If one of the 800 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 the three 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 filters to accept more outbound calls than inbound calls. CPb2
may be configured the same way or receive dynamically computed
filters from EPb3 and EPb4. Depending on the filter 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 filters configured.
Note that this specification does not define the provisioning
interface between the party who determines the load control 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].
4.4. Applicability in Different Network Environments
SIP load filtering is more effective when the filters can be pushed
to the proximity of signaling sources. But even if only part of the
signaling path towards the signaling source could be covered, use of
this mechanism can still be beneficial. In fact, due to possibly
sophisticated call routing and security concerns, trying to apply
automated load filter distribution in the entire inter-domain network
path could get extremely complicated and be unrealistic.
The scenarios where this mechanism could be most useful are
environments consisting of servers with secure and trust relationship
and with relatively straightforward routing configuration known to
the filter computation algorithm. These scenarios may include intra-
domain environments such as those inside a service provider or
enterprise domain; inter-domain environments such as where enterprise
connecting to a few service providers or between service providers
with manageable routing configurations.
Another important aspect that affects the applicability of SIP load
filtering is that all possible signaling source neighbors need to
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participate and enforce the designated filter. Otherwise, a single
non-conforming neighbor could make the whole control efforts useless
by pumping in excessive traffic to overload the server. Therefore,
the SIP server that initiates the filter needs to take counter-
measures towards any non-conforming neighbors. A simple policy is to
reject excessive requests with 500 responses as if they were obeying
the rate. Considering the rejection costs, a more complicated but
fairer policy 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, whatever the actual policy is, SIP
servers SHOULD honor the local policy for prioritizing SIP requests
such as policies based on the contents of the Resource-Priority
Header (RPH) [RFC4412]. The RPH contents may indicate high priority
requests that should be preserved as much as possible, or low
priority requests that could be dropped during overload. Other
indicators, such as the SOS Uniform Resource Name (URN) [RFC5031]
indicating an emergency request, may also be used for prioritization.
SIP request rejection and message prioritization at an overloaded
server are also discussed in Section 5.10 of
[I-D.ietf-soc-overload-control] and Section 12 of [RFC6357].
5. Load Control Event Package
The SIP load filtering mechanism uses the SIP event package for load
control. This section defines details of the SIP event package for
load control according to [RFC6665].
5.1. 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].
5.2. Event Package Parameters
No package specific event header field parameters are defined for
this event package.
5.3. SUBSCRIBE Bodies
The effectiveness of SIP load filtering relies on the scope of
distribution and installation of the control policies in the network.
Since wide distribution of control policies is desirable, subscribers
SHOULD try to subscribe to all those notifiers with which they have
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regular signaling exchanges, although not all such notifiers may
permit such a subscription.
A SUBSCRIBE request for the SIP load control event package MAY
contain a body to filter the requested load control event
notification. The details of the subscription filter specification
are not yet defined.
A SUBSCRIBE request sent without a body implies the default
subscription behavior as specified in Section 5.7.
5.4. SUBSCRIBE Duration
The default expiration time for a subscription to load control 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.
5.5. NOTIFY Bodies
The body of a NOTIFY request in this event package contains load
control policy. As specified in [RFC6665], the format of the NOTIFY
body MUST be in one of the formats defined in the Accept header field
of the SUBSCRIBE request or be the default format. The default data
format for the NOTIFY body of this event package is "application/
load-control+xml" (defined in Section 6). This means that if no
Accept header field is specified to a SUBSCRIBE request, the NOTIFY
request will contain a body in the "application/load-control+xml"
format. If the Accept header field is present, it MUST include
"application/load-control+xml" and MAY include any other types.
5.6. Notifier Processing of SUBSCRIBE Requests
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 control policy, the notifier MUST return
a 403 "Forbidden" response.
If none of MIME types specified in the Accept header of the SUBSCRIBE
request is supported, the Notifier SHOULD return 406 "Not Acceptable"
response.
5.7. Notifier Generation of NOTIFY Requests
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Following [RFC6665] specification, a notifier MUST send a NOTIFY with
its current load control policy to the subscriber upon successfully
accepting or refreshing a subscription. If no applicable restriction
is active when the subscription request is received, an empty message
body is attached to the NOTIFY request. This is often the case 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
configured at that time. A notifier SHOULD generate NOTIFY requests
each time the load control policy changes, with the maximum
notification rate not exceeding values defined in Section 5.10.
5.8. Subscriber Processing of NOTIFY Requests
The way subscribers process NOTIFY requests depends on the contents
of the notifications. Typically, a load control notification
consists of rules that should be applied to requests matching certain
identities. A subscriber receiving the notification first installs
these rules and then filter incoming requests to enforce actions on
appropriate requests, for example, limiting the sending rate of call
requests destined for a specific SIP entity.
In the case when load control rules specify a future validity time,
it is possible that when the validity time comes, 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
this re-activation of subscription is successful or not, when the
validity time is reached, the subscriber SHOULD enforce the
corresponding rules.
When enforcing actions on requests matching the filters, subscribers
SHOULD honor the local policy for prioritizing SIP requests such as
policies based on the content of RPH [RFC4412]). Specific
(namespace.value) RPH contents may indicate high priority requests
that should be preserved as much as possible during overload. The
RPH contents can also indicate a low-priority request that is
eligible to be dropped during times of overload. Other indicators,
such as the SOS URN [RFC5031] indicating an emergency request, may
also be used for prioritization.
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 control policies from
that notifier.
The subscriber SHALL discard unknown bodies. If the NOTIFY request
contains several bodies, none of them being supported, it SHOULD
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unsubscribe. A NOTIFY request that does not contain a body or
contains an empty body indicates no filtering rules need to be
updated.
5.9. Handling of Forked Requests
Forking is not applicable when the load control event package is used
within a single-hop distance between neighboring SIP entities. If
the communication scope of the load control event package is among
multiple hops, forking is not expected to happen either 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 5.9
of [RFC6665].
5.10. Rate of Notifications
Rate of notifications is likely not a concern for this event package
when it is used in a non-real-time mode for relatively static load
control policies. Nevertheless, if situation does arise that a
rather frequent load control policy update is needed, it is
RECOMMENDED that the notifier does 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.
5.11. State Delta
It is likely that updates to specific load control events are made by
changing the control restriction parameter information only (e.g.
rate, percent), but not other rule elements, such as call-identity.
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 a dynamic filter computation algorithm
(Section 4.2). Another factor that is usually not known precisely or
needs to be computed automatically is the validity duration of the
load control event. Therefore it would also be common for the
validity to change frequently.
This event package allows the use of state delta to accommodate
frequent updates of partial rule parameters. As in [RFC6665], a
version number that increases by exactly one is included in the
NOTIFY body for each NOTIFY transaction in a subscription. When the
subscriber receives a state delta, it associates the partial updates
to the particular rules by matching the appropriate rule id
(Section 6.5). If the subscriber receives a NOTIFY that has a
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version number that is increased by more than one, it knows that it
has missed a state delta. The subscriber then keeps the version
number, ignores the NOTIFY request containing the state delta, and
re-sends a SUBSCRIBE to force a NOTIFY containing a complete state
snapshot.
6. Load Control Document
6.1. Format
A load control document is an XML document that 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
ruleset, 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.
6.2. 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
6.3. Conditions
[RFC4745] defines three condition elements: <identity>, <sphere> and
<validity>. In this specification, we re-define an element for
identity, define a new element for method and reuse the <validity>
element. The <sphere> element is not used.
6.3.1. Call Identity
Since the problem space of this specification is different from that
of [RFC4745], the [RFC4745] <identity> element is not sufficient for
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use with load control. First, load control 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 control 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>, each is of the same type as the <identity> element in
[RFC4745]. Therefore, they also inherit the sub-elements of the
<identity> element, including <one>, <except>, and <many>.
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> or <many> sub-elements, the result is also combined
using logical OR. When the <many> sub-element further contains one
or more <except> sub-elements, the result of each <except> sub-
element is combined using a logical OR, similar to that of the
<identity> element in [RFC4745]. However, when the <sip> element
contains multiple of the <from>, <to>, <request-uri> and <p-asserted-
identity> sub-elements, the 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.
<call-identity>
<sip>
<to>
<one id="sip:alice@hotline.example.com"/>
<one id="tel:+1-212-555-1234"/>
</to>
</sip>
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</call-identity>
This example 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".
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. We
therefore need a similar way to match a group of 'tel' URIs.
According to [RFC3966], there are two forms of 'tel' URIs for global
numbers and local numbers, respectively. All phone numbers must be
expressed in global form when possible. The global number 'tel' URIs
start with a "+". The rest of the numbers are expressed as local
numbers, which must be qualified by a "phone-context" parameter. The
"phone-context" parameter may be labelled 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 is also variable. This allows further flexibility such
as 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.
To include the two forms of 'tel' URI grouping in the <many> and
<except> elements, one approach is to add a new attribute similar to
the "domain" attribute. In this specification, we decided on a
simpler approach. There are basically two types of grouping
attribute values for both SIP URIs and 'tel' URIs: domain name and
number prefix starting with "+". Both of them can be expressed as
strings. Therefore, we re-interpret the existing "domain" attribute
of the <many> and <except> elements to allow it to contain both types
of grouping attribute values. In particular, when the "domain"
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attribute value starts with "+", it denotes a number prefix,
otherwise, the value denotes a domain name. Note that the tradeoff
of this simpler approach is the overlap in matching different types
of URIs. Specifically, a domain name in the "domain" attribute could
be matched by both a SIP URI with that domain name and a local number
'tel' URI containing the same domain name in the "phone-context". On
the other hand, a number prefix in the "domain" attribute could be
matched by both global number 'tel' URIs starting with those leading
digits, and local number 'tel' URIs having the same prefix in the
"phone-context" parameter. However, when the "phone-context"
coincides with the SIP domain name or the global number prefix, in
many cases the related phone numbers indeed belong to the same domain
or the same area, which means the overlap is not inappropriate. It
should be noted that the method of grouping local numbers as defined
in this document does not support all cases. For example, if the
phone-context for short service numbers in a country is the country
code, this solution does not permit to define a filter that excludes
all E.164 numbers in that country but retain all short service
numbers. A complete solution for local number grouping requires a
separate method outside the scope of this document.
The following example shows the use of the re-interpreted "domain"
attribute.
<call-identity>
<sip>
<from>
<many>
<except domain="+1-212"/>
<except domain="manhattan.example.com"/>
</many>
</from>
<to>
<one id="tel:+1-202-999-1234"/>
</to>
</sip>
</call-identity>
This example 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".
6.3.2. Method
The load created on a SIP server depends on the type of an initial
SIP request for a dialog or standalone transaction. The <method>
element specifies the SIP method to which a particular action
applies. When this element is not included, the rule actions are
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applicable to all initial methods. The applicable initial methods
include INVITE, MESSAGE, REGISTER, SUBSCRIBE, OPTIONS, PUBLISH
requests. Non-initial requests, such as ACK, BYE and CANCEL requests
are not subjected to load filtering. In addition, SUBSCRIBE requests
are not filtered if the event-type header field indicates the event
package defined in this document.
The following example shows the use of the <method> element.
<method>INVITE</method>
6.3.3. Target SIP Entity
A SIP load filtering server may have multiple paths to route call
requests matching the same set of call identity elements. In those
situations, the filtering server may desire to take advantage of
alternative paths and only apply filter actions to matching outgoing
requests for the next hop SIP entity that originated the filter. To
achieve that, the filtering server needs to associate every filter
rule with its originating SIP entity. The <target-sip-entity>
element is defined for that purpose and it contains the URI of the
entity that initiates the filter which is generally the notifier
itself. A notifier MAY include this element as part of the condition
of the filter rules being sent to the subscriber, as below.
<target-sip-entity>sip:biloxi.example.com</target-sip-entity>
When a filtering server receives a rule with a <target-sip-entity>
element, it SHOULD record it and take it into consideration when
making filtering decisions for outgoing requests. If the filtering
server receives a filter that does not contain a <target-sip-entity>
element, it MAY still record the URI of the filter's originator as
the <target-sip-entity> information and consider it in the filtering
decisions.
The following are two examples of using the <target-sip-entity>
element. Usecase I: the network has user A connected to SIP Proxy
1 (SP1), user B connected to SP3, SP1 and SP3 connected via 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 non-essential service
and can be bypassed in case of overload. Now let's assume that AS
is overloaded and sends to SP2 a filter requesting that 50% of all
INVITE requests be dropped. SP2 can maintain AS as the <target-
sip-entity> for the rule 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. Usecase
II: An 800 translation service is installed on two Application
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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 E164
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
creates a pre-overload condition in AS1, AS1 sends to SP1 a filter
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 filter 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.
6.3.4. Validity
A rule 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. i.e., 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>
6.4. Actions
As [RFC4745] specified, conditions form the 'if'-part of rules, while
actions and transformations form the 'then'-part. Transformations
are not used in the load control document. The actions for load
control are defined by the <accept> element, which takes 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 in window-based load control. In
static load filter configuration scenarios, using the <rate> sub-
element is RECOMMENDED because it is hard to enforce the percentage
rate or window-based control when the incoming load from upstream or
the reactions from downstream are uncertain. (See
[I-D.ietf-soc-overload-control] [RFC6357] for more details on rate-
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based, loss-based and window-based load control)
In addition, the <accept> element takes an optional "alt-action"
attribute which can be used to explicitly specify the desired action
in case a request cannot be processed. The default "alt-action" is
value is "reject" where the server will reject the request with a 503
(Service Unavailable) response message. Other possible "alt-action"
values include "drop" for simple drop, and "redirect" for redirecting
the request to another target. It should be noted that when running
SIP over an unreliable transport such as UDP, using the "drop" action
will create message retransmissions that further worsen the overload
situation. Therefore, any "drop" action applied to an unreliable
transport MUST be treated as if it were "reject". When the "alt-
action" value is "redirect", an "alt-target" attribute MUST be
defined. The "alt-target" specifies one or a list of URIs where the
request should be redirected. The server sends out the redirection
URIs in a 300-class response message.
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>
6.5. Complete Examples
6.5.1. Load Control Document Examples
This section presents two complete examples of load control documents
valid with respect to the XML schema defined in Section 7.
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.
<?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">
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<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 considers optimizing server resource usage of a
three-day period during the aftermath of a hurricane. Incoming calls
to the hurricane domain "neworleans.example.com" 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.
<?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="katrina.example.com"/>
</lc:to>
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<lc:from>
<many>
<except domain="katrina.example.com"/>
<except domain="rescue.example.com"/>
</many>
</lc:from>
</lc:sip>
</lc:call-identity>
<method>INVITE</method>
<validity>
<from>2005-08-28T09:00:00+01:00</from>
<until>2005-08-31T09:00:00+01:00</until>
</validity>
</conditions>
<actions>
<lc:accept alt-action="redirect" alt-target=
"sip:katrina@update.example.com">
<lc:rate>100</lc:rate>
</lc:accept>
</actions>
</rule>
<ruleset>
6.5.2. Message Flow Examples
This section presents an example message flow of using the load
control event package mechanism defined in this document.
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
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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
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
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7. 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 0, 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 control policy update, or whether it
contains 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>
<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>
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<!-- 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="cp:identityType" minOccurs="0"/>
<element name="to" type="cp:identityType" minOccurs="0"/>
<element name="request-uri" type="cp:identityType" minOccurs="0"/>
<element name="p-asserted-identity" type="cp:identityType"
minOccurs="0"/>
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:sequence>
<anyAtrribute namespace="##other" processContents="lax"/>
</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 -->
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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>
8. Related Work
8.1. Relationship with Load Filtering in PSTN
It is known that the existing PSTN network also uses a load filtering
mechanism to prevent overload and the filter configuration is done
manually except in specific cases when the Intelligent Network
architecture is used [Q.1248.2][E.412]. This specification defines a
SIP events framework based distribution mechanism which allows
automated filter distribution in suitable environments.
There are control messages associated with PSTN overload control
which would specify an outgoing control list, call gap duration and
control duration [Q.1248.2][E.412]. These items could be roughly
correlated to the identity, action and time fields of the SIP load
filter defined in this specification. However, the filter defined in
this specification is much more generic and flexible as opposed to
its PSTN counterpart.
Firstly, PSTN load filtering only applies to telephone numbers. The
SIP filter identity allows both SIP URI and telephone numbers
(through Tel URI) to be specified. The identities can be arbitrarily
grouped by SIP domains or any number of leading prefix of the
telephone numbers.
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Secondly, the PSTN filtering action is usually limited to call
gapping. The action field in the SIP load filter allows more
flexible rate throttle and other possibilities.
Thirdly, the duration field in PSTN filtering specifies a value in
seconds for the control duration only, and the allowed values are
mapped into a value set. The time field in the SIP load filter may
specify not only a duration, but also a future activation time which
could be especially useful for automating load control for
predictable overloads.
PSTN filtering can be performed in both edge switches and transit
switches; SIP filtering can also be applied in both edge proxy
servers and core proxy servers, and even in capable user agents.
PSTN overload control also has special accommodation for High
Probability of Completion (HPC) calls, which would be similar to the
calls designated by the SIP Resource Priority Headers [RFC4412]. SIP
filtering mechanism can also prioritize the treatment of these calls
by specifying favorable actions for these calls.
PSTN filtering also provides administrative option for routing failed
call attempts to either a reorder tone [E.300SerSup3] indicating
overload conditions, or a special recorded announcement. Similar
capability can be provided in the SIP filtering mechanism by
specifying the appropriate "alt-action" attribute in the SIP
filtering action field.
8.2. Relationship with Other IETF SIP Load Control Efforts
The load filtering rules in this specification consists 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.
The load filter 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
[I-D.ietf-soc-overload-control]. In particular, both mechanisms
could use specific or wildcard filter identities for load control and
could share well-known load control actions. The time duration field
in the load filter could also be used in both mechanisms. As
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mentioned in Section 1, the load filter distribution mechanism and
the feedback overload control mechanism address complementary areas
in the load control problem space. Load filtering is more proactive
and focuses on distributing the filter towards the source of the
traffic; the hop-by-hop feedback based approach is reactive and
targets more at traffic already accepted in the network. Therefore,
they could also make different use of the generic filter components.
For example, the load filtering mechanism may use the time field in
the filter 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 control duration.
9. Discussion of this specification meeting the requirements of RFC5390
This section evaluates whether the load control event package defined
in this specification satisfies the various SIP overload control
requirements set forth by RFC5390 [RFC5390]. Not all RFC5390
requirements are found applicable due to the scope of this document.
Therefore, we categorize the assessment results into Yes (meet the
requirement), 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.
P/A. The goal of the load filtering is to prevent overload or
maintain overall goodput during the time of overload, but it is
dependent on the advance predictions of the load. If the predictions
are incorrect, in either direction, the effectiveness of the
mechanism will be affected.
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 filter values are installed in advance and do not change
during the potential overload period. P/A if filter values are
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dynamically adjusted due to the specific filter computation
algorithm. The dynamic filter computation algorithm is outside the
scope of this specification, while the distribution of the updated
filters 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 which 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 4.4.
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 4.4.
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
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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/windows-based overload
control options as discussed in Section 6.4.
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 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.
Yes. For example, the load filter [Section 4.1] allows the inclusion
of 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.
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REQ 12: The mechanism should work between servers in different
domains.
Yes. The load control event package 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 4.4).
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 itself.
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 filters are provisioned in advance, they are not
affected by the overload or failure of other nodes. But, on the
other hand, they may not, in those cases, be able to protect the
overloaded node (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.
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P/A. This mechanism does provide a potential avenue for malicious
attacks. Therefore the security mechanisms for SIP event packages in
general [RFC6665] and of section 10 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 filter format
definition in Section 4.1.
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 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.
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 4.4.
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 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.
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N/A to the load control event package 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 does not preclude its use in a
scenario with server farms.
10. Security Considerations
Two aspects of security considerations arise from this specification.
One is the SIP event framework based filter distribution mechanism,
the other is the filter enforcement mechanism.
Security considerations for SIP event framework based 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 attacks.
Therefore, all load control notification MUST be authenticated and
authorized before being accepted. Standard authentication and
authorization mechanisms recommended in [RFC3261] such as TLS
[RFC5246] and IPSec [RFC4301] may serve this purpose. On the other
hand, if a legitimate notifier is itself compromised, additional
mechanisms will be needed to detect the attack.
Security considerations for filter enforcement vary depending on the
filter itself. This specification defines possible filter match of
the following SIP header fields: <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.
11. IANA Considerations
This specification registers a SIP event package, a new MIME type, a
new XML namespace, and a new XML schema.
11.1. Load Control Event Package Registration
This section registers an event package based on the registration
procedures defined in [RFC6665].
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Package name: load-control
Type: package
Published specification: This specification
Person to contact: Charles Shen, charles@cs.columbia.edu
11.2. application/load-control+xml MIME Registration
This section registers a new MIME type based on the procedures
defined in [RFC4288] and guidelines in [RFC3023].
MIME media type name: application
MIME subtype name: load-control+xml
Mandatory parameters: none
Optional parameters: Same as charset parameter application/xml in
[RFC3023]
Encoding considerations: Same as encoding considerations of
application/xml in [RFC3023]
Security considerations: See Section 10 of [RFC3023] and Section 10
of this specification
Interoperability considerations: None
Published Specification: This specification
Applications which use this media type: load control of SIP entities
Additional information:
Magic number: None
File extension: .xml
Macintosh file type code: 'TEXT'
Personal and email address for further information:
Charles Shen, charles@cs.columbia.edu
Intended usage: COMMON
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Author/Change Controller: IETF SOC Working Group
<sip-overload@ietf.org>, as designated by the IESG <iesg@ietf.org>
11.3. 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 to be registered is contained in Section 7.
Its first line is
<?xml version="1.0" encoding="UTF-8"?>
and its last line is
</xs:schema>
12. Acknowledgements
The authors would like to thank Bruno Chatras, Martin Dolly, Keith
Drage, Ashutosh Dutta, Janet Gunn, Vijay Gurbani, Volker Hilt, Geoff
Hunt, Carolyn Johnson, Hadriel Kaplan, Paul Kyzivat, Salvatore
Loreto, Timothy Moran, Eric Noel, Parthasarathi R, Adam Roach, Shida
Schubert, Robert Sparks, Phil Williams and other members of the SOC
and SIPPING working group 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 for Section 9. Eric Noel
suggested clarification on filter distribution initialization
process. Shida Schubert made many suggestions about terminology
usage. Phil Williams suggested adding support for delta updates.
Ashutosh Dutta gave pointers to PSTN references. Adam Roach
suggested RFC6665-related and other helpful clarifications.
13. References
13.1. 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.
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[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.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[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.
13.2. Informative References
[E.300SerSup3]
ITU-T, "North American Precise Audible Tone Plan", E.300
Series Supplement 3 , November 1988.
[E.412] ITU-T, "Network Management Controls", E.412-2003 ,
January 2003.
[I-D.ietf-soc-overload-control]
Gurbani, V., Hilt, V., and H. Schulzrinne, "Session
Initiation Protocol (SIP) Overload Control",
draft-ietf-soc-overload-control-09 (work in progress),
July 2012.
[Q.1248.2]
ITU-T, "Interface Recommendation for Intelligent Network
Capability Set4:SCF-SSF interface", Q.1248.2 , July 2001.
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
August 1999.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
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Internet Protocol", RFC 4301, December 2005.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
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
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Arata Koike
NTT Service Integration Labs
3-9-11 Midori-cho Musashino-shi
Tokyo, 184-0013
Japan
Phone: +81 422 59 6099
Email: koike.arata@lab.ntt.co.jp
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