Media Gateway Control Protocol (MGCP)

Lockstep State Reporting Mechanism

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 © 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

A Media Gateway Control Protocol (MGCP) endpoint that has encountered an adverse failure condition (such as being involved in a transient call when a Call Agent failover occurred) could be left in a lockstep state whereby events are quarantined but not notified. The MGCP package described in this document provides a mechanism for reporting these situations so that the new Call Agent can take the necessary fault recovery procedures.

1. Introduction

In the Media Gateway Control Protocol (MGCP) [2], when an endpoint operating in "step" mode generates a Notify, it will enter the notification state, where it waits for a response to the Notify. Furthermore, the endpoint must wait for a new NotificationRequest before it can resume event processing. As long as the endpoint is waiting for this NotificationRequest, we say that it is in the lockstep state.

An endpoint that is in the lockstep state cannot perform any event processing and therefore also cannot generate a new Notify. Endpoints should only be in the lockstep state for a very short time. However, in adverse conditions, an endpoint could potentially end in the lockstep state without the Call Agent realizing it. Clearly, this could have very negative consequences in terms of the service provided.

The Lockstep package defined in this document defines extensions to the EndpointConfiguration and RestartInProgress commands that allow a Call Agent to request an endpoint to inform it when the endpoint is in the lockstep state for a specified period of time.

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. Lockstep Package

   Package Name: LCK
   Version: 0

Package Description: The purpose of this package is to provide a mechanism for reporting a condition in which an endpoint has been in the "lockstep state" for a specified period of time.

There are two aspects of this package:

• The ability for a Call Agent to request endpoints to report if they are in lockstep state for a specified period of time. This is done with the EndpointConfiguration command, as described in section 2.1.

• The reporting mechanism itself, which is done with a new "lockstep" RestartMethod for the RSIP command as described in section 2.2.

2.1. Request to Report Lockstep State

The new "LCK/LST" EndpointConfiguration parameter is used by the Call Agent to request the reporting of "lockstep" state. It uses the following ABNF:

      "LCK/LST:" 0*WSP LSTIME
      
      LSTIME = 1*(4DIGIT)

where LSTIME is expressed in seconds, with a value ranging from 0 to 9999. A value greater than 2*T-HIST (refer to [2]) is RECOMMENDED.

LSTIME is the amount of time the endpoint is in the lockstep state before reporting. The timer starts when the endpoint enters the lockstep state and is canceled if the endpoint leaves the lockstep state before the timeout occurs. The value provided remains in effect until explicitly changed (or a restart occurs). If the endpoint is already in the lockstep state when a non-zero timer value is provided, the lockstep timer is simply started with the value provided; any existing lockstep timer is cancelled. The value zero is used to turn off reporting.

This parameter can be audited by using the AuditEndpoint command. The value returned is the last configured value, or the value zero when no value was configured.

2.2. Lockstep Restart Method

A new "lockstep" restart method is defined in the "LCK" package. A RestartInProgress (RSIP) will be sent with this RestartMethod if the endpoint has been configured with a non-zero value for LSTIME and that timer has expired. Note that once the lockstep timer has been set, it can fire only once per Notify command; however it is possible to set the timer more than once while an endpoint is in lockstep state (and hence rearm it for that particular Notify). The syntax of the restart method is as per [2]:

"RM" ":" 0*(WSP) "LCK/lockstep"

RestartDelay (see [2]) is not used with the "lockstep" RestartMethod. Also, the "lockstep" RestartMethod does not define a service-state, and thus it will never be returned when auditing the RestartMethod.

3. IANA Considerations

The MGCP package title "Lockstep" with the name "LCK" and version number zero 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 MGCP protocol that apply equally to the package defined in this document. Use of a security Protocol such as IPsec (RFC 2401, RFC 2406) that provides per message authentication and integrity services is required 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.

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.

Authors' Addresses

   Bill Foster
   
   Phone: +1 250 758 9418
   EMail: bfoster@cisco.com

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

EMail:

          fandreas@cisco.com

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