RFC3991: Media Gateway Control Protocol (MGCP) Redirect and Reset Package

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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).


   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.


   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

   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",
   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

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: ca1@myca.whatever.net, ca2@mybackupca.whatever.net

   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

   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 MG@gw1.whatever.net 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) ]


      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: ca1@ca1234.whatever.net

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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

   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: ca1@myca.whatever.net, ca2@mybackupca.whatever.net

   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 MG@gw1.whatever.net MGCP 1.0
      RED/EL: *
      RED/NL: ca1@myca.whatever.net, ca2@mybackupca.whatever.net

   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).


      EPCF 1200 mg@gw1.whatever.net MGCP 1.0
      RED/EL: ds/e1-3/[1-30]
      RED/EL: ds/e1-5/[1-30]
      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"

      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: fandreas@cisco.com

   Bill Foster
   Cisco Systems

   EMail: bfoster@cisco.com

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