Network Working Group B. Foster

Network Working Group B. Foster
Request for Comments: 3991 F. Andreasen
Category: Informational Cisco Systems
February 2005

Media Gateway Control Protocol (MGCP) Redirect and Reset Package

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

IESG Note

This document is being published for the information of the
community. It describes a non-IETF protocol that is currently being
deployed in a number of products. Implementers should be aware of
RFC 3015, which was developed in the IETF Megaco Working Group and
the ITU-T SG16, and which is considered the standards-based
(including reviewed security considerations) way to meet the needs
that MGCP was designed to address by the IETF and the ITU-T.

Abstract

The base Media Gateway Control Protocol (MGCP) specification (RFC
3435) allows endpoints to be redirected one endpoint at a time. This
document provides extensions in the form of a new MGCP package that
provides mechanisms for redirecting and resetting a group of
endpoints. It also includes the ability to more accurately redirect
endpoints by allowing a list of Call Agents to be specified in a
preferred order.

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Table of Contents

1. Introduction.................................................. 2
1.1. Conventions Used in This Document....................... 3
2. Redirect and Reset Package.................................... 3
2.1. NotifiedEntityList Extension Parameter.................. 3
2.2. Endpoint Specifier...................................... 4
2.2.1. EndpointList and EndpointMap Extension
Parameters...................................... 4
2.2.2. Application to Out-of-Service Endpoints......... 6
2.3. Redirect................................................ 6
2.4. Reset Extension Parameter............................... 7
2.5. Return Codes............................................ 8
3. IANA Considerations........................................... 9
4. Security Considerations....................................... 9
5. Normative References.......................................... 9
Authors' Addresses................................................ 10
Full Copyright Statement.......................................... 11

1. Introduction

The base Media Gateway Control Protocol (MGCP) specification [2]
allows a Call Agent to specify a new NotifiedEntity parameter in
order to redirect one or more endpoints to a new Call Agent. This
must be done in a NotificationRequest or a connection handling
command. However, because these commands affect endpoint or
connection state, such a request cannot typically be sent to a group
of endpoints with a single command. This means that if a new Call
Agent takes over for a failed one, the new Call Agent must redirect
endpoints one at a time. If there is a large number of endpoints
(e.g., within a large trunking gateway), this could take considerable
time.

This document defines a new redirect and reset package for MGCP that
allows the Call Agent to redirect a group of endpoints without
affecting endpoint or connection state.

Also included is a new NotifiedEntityList parameter, which is similar
to the NotifiedEntity parameter but allows for multiple domain names
to be provided. This allows the Call Agent to more accurately direct
endpoints to a preferred ordered list of alternate Call Agents.

A third capability contained in this package is the ability to reset
and re-initialize one or more groups of endpoints efficiently. This
capability is useful in Call Agent failover situations.

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1.1. Conventions Used in This Document

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 BCP 14, RFC 2119 [1].

2. Redirect and Reset Package

Package Name: RED
Version: 0

This package does the following:

* Defines a new NotifiedEntityList extension parameter. This
works the same as the NotifiedEntity parameter in [2] but
allows more than one domain name to be specified.

* Allows a Call Agent to pass a new NotifiedEntity or
NotifiedEntityList to a collection of endpoints specified by an
"all of" wildcard. This is useful if a new Call Agent takes
over from a previous one and wants to redirect endpoint(s) to
send messages to it from now on.

* Allows a Call Agent to request one or more groups of endpoints
to do a reset, which can be useful following certain types of
failures.

2.1. NotifiedEntityList Extension Parameter

The NotifiedEntityList parameter is encoded as "NL" and is followed
by a colon and a comma-separated list of NotifiedEntity values as
defined in the MGCP specification [2], as follows:

RED/NL: [email protected], [email protected]

The NotifiedEntityList works in a way similar to the NotifiedEntity
parameter, except that it allows multiple domain names to be listed.
The NotifiedEntityList thus specifies a new "notified entity" for the
endpoint.

The NotifiedEntityList parameter is optional in any command or
response where the NotifiedEntity parameter is allowed. Following a
restart, the NotifiedEntityList is initially empty, unless
provisioned otherwise. In subsequent commands, it retains its
current value until explicitly changed. If both a NotifiedEntity
parameter and a non-empty NotifiedEntityList parameter have been set
(not necessarily at the same time), the NotifiedEntity parameter
value will be viewed as being implicitly added to the beginning of

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the NotifiedEntityList parameter. The NotifiedEntity parameter thus
always defines the first domain name to contact unless it has
explicitly been set to empty. In that case, the NotifiedEntityList
defines the "notified entity". If the NotifiedEntityList is also
empty, then the normal MGCP handling of an empty "notified entity"
applies. We will refer to the list of domain names that result from
the above rules as the "notified entity list".

When the "notified entity list" is non-empty, transmission is first
attempted with the first domain name in the list, as in the normal
MGCP retransmission procedures described in [2]. Each of the IP
addresses for this domain name MUST first be tried as specified in
[2], and if this is unsuccessful, each of the IP-addresses for the
second domain name MUST then be attempted, etc., following the normal
MGCP retransmission procedures, with "N" (the number of
retransmissions) set to zero for each domain name (see Section 4.3 in
[2]). Whenever retransmission to a new domain name is initiated, the
default retransmission timer value (RTO), etc., SHOULD be used. The
estimator (T-DELAY) and measurements (AAD and ADEV) used for the
transmission to the previous domain name are considered obsolete.
Note, however, that the maximum transaction lifetime considerations
apply as usual; therefore, retransmission to any of the IP addresses
for any of the domain names MUST NOT occur more than T-Max seconds
after the command is initially sent, irrespective of where it was
sent. The Max1 DNS query MAY be performed for each of the domain
names, or it MAY simply be performed for the first domain name. The
Max2 DNS query however MUST NOT be performed for any but the last
domain name. Also note that only the last IP-address for the last
domain name can reach Max2 retransmissions; therefore, retransmission
to all IP addresses other than the last IP address of the last domain
name in the list MUST end after Max1 retransmissions.

The current value of the NotifiedEntityList parameter can be audited
via AuditEndpoint; the value of the NotifiedEntity parameter will not
be included here and must be audited separately. Support for the
NotifiedEntityList in AuditConnection is permissible, but it is
neither required nor recommended.

2.2. Endpoint Specifier

2.2.1. EndpointList and EndpointMap Extension Parameters

A simple "all-of" wildcard, as defined in [2], may not be sufficient
to accurately specify endpoints of interest. An example of this is a
case where a Call Agent fails over, resulting in a state mismatch for
endpoints involved with transient calls. To re-synchronize, the Call
Agent can use the reset extension parameter described in section 2.4
of this document, to ensure that idle endpoints are in fact idle.

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However, these endpoints may be randomly distributed across the
available endpoints in a large trunk gateway.

To satisfy this requirement, the RED package introduces some new
parameters that may be used to specify the endpoints of interest for
the EndpointConfiguration Command. These are the EndpointList and
the EndpointMap extension parameters. These parameters MUST only be
used when a virtual endpoint corresponding to the gateway is
specified as the LocalEndpointName, such as:

EPCF 1200 [email protected] MGCP 1.0

where "MG" is the virtual endpoint name associated with the gateway.

The EndPointList parameters is a list of endpoint names that can
include one or more lines in the following format:

"RED/EL:" 0*WSP RangedLocalName 0*("," 0*WSP RangedLocalName)

RangedLocalName is a LocalEndpointName that may include the range
wildcard notation described in Appendix E (section E.5) of [2] as
well as an "all" wildcard, but the two forms MUST NOT be mixed in a
single command:

RangeWildcard = "*" / "[" NumericalRange *("," NumericalRange)"]"
NumericalRange = 1*(DIGIT) [ "-" 1*(DIGIT) ]

Example:

RED/EL: ds/ds1-1/[1-24], ds/ds1-2/[1-24], ds/ds1-3/[1-24]

Including an EndpointMap parameter with the following format can
further specify the endpoints:

"RED/MP:" 0*WSP TrueOrFalse 0*(TrueOrFalse)

TrueOrFalse = "T" / "F"

"T" indicates that the command should be applied to the corresponding
endpoint, and "F" indicates that it should not. This parameter can
be used in conjunction with the reset extension parameter described
in section 2.4 of this document to force arbitrarily distributed
endpoints into an idle state.

If the EndpointMap parameter is used, it MUST be immediately preceded
(i.e., on the previous line) by an EndPointList parameter to specify
the endpoints the EndpointMap is referring to (the EndPointList MUST
NOT contain the "all" wildcard). Several EndpointList and

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EndpointMap parameter lines can be provided. It is considered an
error if an EndpointMap parameter extends beyond the endpoints
specified in the preceding EndPointList parameter. In that case,
return code 800 MUST be used (see section 2.5).

The EndpointList and EndpointMap parameters MUST only be used with
the EndpointConfiguration command. The EndpointList parameter MAY be
provided without an EndpointMap parameter. However, as indicated
earlier, an EndpointMap parameter MUST be immediately preceded by an
EndpointList parameter. Neither of these parameters is auditable.

For an example of EndpointMap parameter usage, see Section 2.4.

2.2.2. Application to Out-of-Service Endpoints

Note that the EndpointConfiguration command is normally only valid
for in-service endpoints. If an EndpointConfiguration request is
sent to a wildcarded LocalEndpointName [2] and any of the endpoints
specified are out-of-service, the command will fail with return code
501 (endpoint not ready).

However, as long as the gateway is in service and able to respond to
MGCP commands, it can apply the endpoint configuration command to
endpoints specified by the EndpointList and/or EndpointMap parameters
(regardless of whether those endpoints are in-service). Of course,
the endpoint configuration information will not be maintained over
gateway restarts (as the Call Agent would have to reapply the
endpoint configuration after it receives an RSIP with the restart
method "restart"). For example, if a new "notified entity" was
provided, it would have no effect since the provisioned value would
be used upon restart.

EndpointList and/or EndpointMap parameters MUST only be used with a
virtual endpoint name corresponding to the gateway (as indicated
above). If it is used with any other endpoint name (whether wild-
carded or not), then error code 801 (section 2.5) MUST be returned.

2.3. Redirect

A new extension parameter for use with the EndpointConfiguration
command is defined. A new NotifiedEntity value can be included with
a "RED/N" parameter as follows:

EPCF 1200 *@gw1.whatever.net MGCP 1.0
RED/N: [email protected]

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This changes the "notified entity" for the endpoint(s) to the value
specified. If the "all of" wildcard convention is used, the
NotifiedEntity value replaces all of the existing "notified entities"
for those endpoints. If NotifiedEntity is omitted in a subsequent
EndpointConfiguration command, the "notified entity" remains
unchanged.

If the "notified entity" is a domain name that resolves to multiple
IP addresses, one of the resolved addresses MUST be selected. If one
of those IP addresses is the IP address of the Call Agent sending the
request, that IP address SHOULD be selected first.

The NotifiedEntityList parameter can also be specified in an endpoint
configuration command, such as follows:

EPCF 1200 *@gw1.whatever.net MGCP 1.0
RED/NL: [email protected], [email protected]

Note that this command will only succeed if all the endpoints on the
gateway are in-service.

As indicated in section 2.2, it can also apply this to the gateway
virtual endpoint:

EPCF 1200 [email protected] MGCP 1.0
RED/EL: *
RED/NL: [email protected], [email protected]

Note that the outcome of this command is not affected by the service
state of the endpoints on the gateway.

As indicated in section 2.1, the NotifiedEntityList ("RED/NL")
parameter may be used with any command for which a NotifiedEntity
parameter is allowed. However, the "RED/N" parameter SHOULD only be
used with the endpoint configuration command.

The "RED/N" parameter does not have a default value, and the auditing
behavior for auditing the "NotifiedEntity" is unchanged from that
specified in [2], regardless of how the "NotifiedEntity" was set
(i.e., there is no specific audit associated with the "RED/N"
parameter, and therefore the "RED/N" parameter cannot be audited).

2.4. Reset Extension Parameter

Another EndpointConfiguration parameter ("RED/R") allows the Call
Agent to reset one or more endpoints. The ABNF syntax for the
parameter line is as follows:

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"RED/R:" 0*WSP "reset"

This has the effect of resetting and re-initializing the specified
endpoints (i.e., any connections on the endpoint will be deleted, and
the endpoint will be returned to its clean default state without any
active signals).

Example:

EPCF 1200 [email protected] MGCP 1.0
RED/EL: ds/e1-3/[1-30]
RED/MP: TFTTTTTFFFTTTTTFFFFTFFTTFTTTFF
RED/EL: ds/e1-5/[1-30]
RED/MP: TFFFFFTFFFTTFTTFFFFTFFFTFTTTTT
RED/R: reset

In this case, the particular endpoints specified by "T" by the
EndpointMap parameter in the E1 spans ds/e1-3 and ds/e1-5 are reset.

The "RED/R" parameter MUST NOT be used with any command other than
the endpoint configuration command. There is no default value for
the parameter, and therefore it is unaffected when omitted. There is
no specific audit behavior associated with this parameter, i.e., it
cannot be audited.

2.5. Return Codes

The following package-specific return codes are defined for the "RED"
package:

Code Text Explanation

800 EndpointMap Either the EndpointMap parameters
Out of Range are outside the range specified
by the EndpointList parameter, or
the EndpointList Parameter was
not included when an EndpointMap
parameter was included.

801 Incorrect Usage Incorrect usage of parameters,
Of Parameters such as EndpointList parameter,
used where the endpoint name was
not the virtual endpoint name
corresponding to the gateway.

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3. IANA Considerations

The MGCP package title "Redirect and Reset" with the name "RED" and
version number 0 has been registered with IANA, as indicated in
Appendix C.1 in [2].

4. Security Considerations

Section 5 of the base MGCP specification [2] discusses security
requirements for the base protocol that apply equally to the package
defined in this document. Use of a security protocol that provides
per message authentication and integrity services, such as IPsec (RFC
2401 [3], RFC 2406 [4]), is required in order to ensure that requests
and responses are obtained from authenticated sources and that
messages have not been modified. Without these services, gateways
and Call Agents are open to attacks.

For example, an attacker could masquerade as a Call Agent and
initiate a denial of service attack by resetting endpoints that were
involved in valid calls. Another attack using the package described
in this document could involve redirecting endpoints to the attacker
so that it acts as the Call Agent for those endpoints.

5. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Andreasen, F. and B. Foster, "Media Gateway Control Protocol
(MGCP) Version 1.0", RFC 3435, January 2003.

[3] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.

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Authors' Addresses

Flemming Andreasen
Cisco Systems
499 Thornall Street, 8th Floor
Edison, NJ 08837

EMail: [email protected]

Bill Foster
Cisco Systems

EMail: [email protected]

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RFC 3991 MGCP Redirect and Reset Package February 2005

Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and at www.rfc-editor.org, and except as set
forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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