Internet Engineering Task Force V.Bajaj Internet Draft S.Ranjhan Document: draft-ietf-megaco-r2package-01.txt P.Jain April 11, 2000 B.M.Babu Expires october 11, 2000 K.Laha (Hughes Software Systems) Megaco/H.248 R2 Package Status of this document This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [ ]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 1. Abstract This document is work in progress and defines the R2 package in association with the Megaco/H.248 Protocol that can be used to control a Media Gateway (MG) from an external controller, called a Media Gateway controller (MGC). It is intended to satisfy the requirements in section 12 of the Megaco/H.248 requirement document [ ]. 2. 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 RFC-2119 [ ]. Bajaj,Ranjhan,Jain,Babu,Laha [page 1] Internet Draft Megaco/H.248 R2 Package April 11, 2000 3. Document Organization The subsequent portion of this draft is organized into the following major sections: * Introduction to R2 signaling * Assumptions * Description of the R2 package * References * Author's Address 4. Introduction to R2 signaling Signaling System R2 is used as an international signaling system within international regions, for integrated international/national signaling and for both automatic and semiautomatic working. It allows for rapid call set-up and provides sufficient signals in both directions to permit the transmission of numerical and other information relating to the called and calling subscribers lines and to increase routing facilities. Line signaling uses an out-band low-level tone-on-idle signaling method for analogue working. For PCM operations, the tones are replaced by bit-codes. The inter-register signaling is a compelled multi-frequency code system. Inter-register signaling is performed end-to-end using a 2- out-of-6 in-band multi frequency code with forward and backward compelled signaling. The register signaling is faster as there is no signal regeneration in intermediate exchanges. Overlap operation of address signaling makes it faster than MF-R1 or decadic. The following operational features are to name a few which are provided using multi-frequency code: - Calling party number identification - Calling party category identification - Nature of the circuit - Reason for Call failure - Language digits for international calls - Forward and backward signals for control of echo suppressers 5. Assumptions The following draft assumes the support of ITU-T compliant [4] compelled digital R2 signaling only. However there are as many variations in R2 signaling in terms of signal to frequency mapping and interpretation of each of these signals, as there are countries. It is therefore assumed that the MG is provisioned with the actual MF R2 signaling frequencies along with their amplitude, tone Bajaj,Ranjhan,Jain,Babu,Laha [page 2] Internet Draft Megaco/H.248 R2 Package April 11, 2000 duration and cadence and their interpretation. The line signaling patterns generated at the physical layer, is also assumed to be provisioned at the MG. The MG is expected to perform the compelling action with the help of some pre-provisioned logic to keep the MEGACO interface transparent of country dependant implementations. As mentioned earlier, the compelled address signaling sequence is executed at the MG. This is in accordance to MEGACO philosophy that the MG collects digits according to digit map and reports them to the MGC en-bloc. If the compelled signaling is done under supervision of the MGC then the call control at the MGC will become very complex. As R2 defines strict constraints on inter-register signal timing issues, any inter-event timer specified in the digitmaps for collecting the destination number shall be over-ruled by the protocol specific timers. Country specific implementation of R2 signaling may also define new line and register signals not defined in the ITU-T version of R2 signaling to incorporate features such as re-answering, trunk offering, re-ring, operator break-in etc. Such features shall be realized as new signals and events in country specific R2 packages that shall extend this basic R2 package. This draft defines an ITU-T R2 specification [4] compliant "R2 package" which is used to set up calls on R2 signaling trunks at one or both ends of the VoIP network. As the VoIP Gateway forms the inter-working unit between the PSTN and packet network both in terms of signaling and media, The MG shall either originate or terminate R2 signaling depending on whether it emulates the originating or incoming end in the signal path. It cannot act as a tandem unit for an end-to-end R2 signaling path. 6. R2 Package Description 6.1 Package Package Name: R2 Package Package ID: r2 (To be allocated through IANA) Description: This package is used to handle signals and events on terminations supporting compelled R2-signaling (ITU-T) between the Media Gateway and the PSTN network. Version: 1 Extends: None 6.2 Properties Bajaj,Ranjhan,Jain,Babu,Laha [page 3] Internet Draft Megaco/H.248 R2 Package April 11, 2000 None defined by this package 6.3 Events 6.3.1 Event Name: Seizure EventID: sz (0x0002) Description: The Seizure event shall be reported when an incoming R2 "Seized" signal occurs on the termination and an events descriptor is activated that requests the Seizure event. The actual bit pattern against which the "Seized" signal is verified is provisioned in the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: none defined by this package 6.3.2 Event Name: Seizure Acknowledge EventID: sa (0x0003) Description: The Seizure Acknowledge event shall be reported when an incoming R2 line signal "Seizure Acknowledged" occurs on the termination in response to an outgoing "Seized" signal already sent and an events descriptor is activated that requests the Seizure Acknowledge event. The actual bit pattern against which the "Seizure Acknowledged" signal is verified is provisioned in the MG. EventsDescriptor Parameters: Parameter Name: Seize Acknowledgement timeout Parameter id: satim (0x0001) Description: specifies the maximum duration within which the "Seizure Acknowledged" line signal is expected in response to "Seizure" beyond which this event shall be dishonored by the Gateway. Gateway shall have default value for this timer. Type: integer, in milliseconds Possible values: 0 and up ObservedEventsDescriptor Parameters: none defined by this package 6.3.3 Event Name: Answer EventID: ans (0x0004) Description: The Answer event shall be reported when an incoming R2 line signal "Answered" occurs on the termination after receiving "Seizure Acknowledged" for a "Seized" signal already sent and an events descriptor is activated that requests the Answer event. The actual bit pattern against which the "Answered" signal is verified is provisioned in Bajaj,Ranjhan,Jain,Babu,Laha [page 4] Internet Draft Megaco/H.248 R2 Package April 11, 2000 the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: none defined by this package 6.3.4 Event Name: Clear EventID: clr (0x0005) Description: The Clear event shall be reported when an incoming R2 line signal "Clear-forward" or "Clear-back" occurs on the termination and the an events descriptor is activated that requests the Clear event. Whether "Clear-forward" or "Clear- back" shall depend on the end initiating the call clearing. The actual bit pattern against which the "Clear-forward" or "Clear-back" signal is verified is provisioned in the MG. EventsDescriptor Parameters: Parameter Name: Incoming release guard timeout Parameter id: icrgtim (0x0001) Description: specifies the duration for which the Gateway if behaving as an incoming end, maintains the release guard sequence before restoring to the "Idle" condition, when "Clear-forward" is initiated by the other end (outgoing end). Corresponds to the T2 timer as defined in ITU-T R2 specification [4]. Type: integer, in milliseconds Possible values: 0 and up Parameter Name: Outgoing release guard timeout Parameter id: ogrgtim (0x0002) Description: specifies the duration for which the Gateway if behaving as an outgoing end, maintains the release guard sequence before restoring to the "Idle" condition, when "Clear-back" is initiated by the other end (incoming end). Corresponds to the T1 timer as defined in ITU-T R2 specification [4]. Type: integer, in milliseconds Possible values: 0 and up ObservedEventsDescriptor Parameters: none defined by this package 6.3.5 Event Name: Block EventID: blk (0x0006) Description: The Block event shall be reported when an incoming R2 line signal "Blocked" occurs on an "Idle" termination to Bajaj,Ranjhan,Jain,Babu,Laha [page 5] Internet Draft Megaco/H.248 R2 Package April 11, 2000 render the termination engaged and an events descriptor is activated that requests the Block event. The actual bit pattern against which the "Blocked" signal is verified is provisioned in the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: none defined by this package 6.3.6 Event Name: Unblock EventID: ublk (0x0007) Description: The Unblock event shall be reported when an incoming R2 line signal "Idle"/"Released" occurs on a "Blocked" termination to remove the engaged condition and an events descriptor is activated that requests the Unblock event. The actual bit pattern against which the "Idle"/"Released" signal is verified is provisioned in the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: none defined by this package 6.3.7 Event Name: Address EventID: addr (0x0008) Description: Detects the complete R2 address as received through compelled register signaling. The event shall be reported after collecting the complete address digits collected by virtue of compelled register signaling at the Gateway and an events descriptor is activated requesting the Address event. It includes as optional parameters to this event the source and destination number digits, the subscriber category, echo suppression, country code and language digit information as defined in [ ], which shall be reported as part of the observed events. The destination number shall be collected as per a digitmap. The actual frequency, cadence, duration and amplitude values against which the multi-frequency tones that convey the address information are validated are provisioned in the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: Bajaj,Ranjhan,Jain,Babu,Laha [page 6] Internet Draft Megaco/H.248 R2 Package April 11, 2000 Parameter Name: Destination number ParameterID: di (0x0001) Description: The portion of the collected destination number collected through the compelled register signaling, which matched part or all of an alternative event sequence specified in the digit map. Type: string of digits (possibly empty) returned as a quoted string. Possible Values: a sequence of the characters "0" through "9" Parameter Name: Destination number termination method ParameterID: ditm (0x0002) Description: indicates the reason for the generation of the "Destination Number" parameter Type: enumeration Possible Values: "UM" (0x0001) Unambiguous Match "PM" (0x0002) Partial Match, completion by timer expiry or unmatched event "FM" (0x0003) Full Match, completion by timer expiry or unmatched event Parameter Name: Source number ParameterID: si (0x0003) Description: The collected source number digits collected through the compelled register signaling. Type: string of digits (possibly empty) returned as a quoted string. Possible Values: a sequence of the characters "0" through "9" Parameter Name: Subscriber Category ParameterID: sc (0x0004) Description: Calling Party's category as collected through compelled register signaling Type: enumeration Possible Values: "NP" (0x0001) Non-priority subscriber "PR" (0x0002) Priority subscriber "MNT" (0x0003) Call from maintenance equipment "OPNOFT" (0x0004) Call from operator without forward transfer facility "DT" (0x0005) Call setup for data transmission "OPFT" (0x0006) Call from operator with forward transfer facility "OPR" (0x0007) Operator call "NA" (0x0008) Information not available or applicable Parameter Name: Echo Suppression Information ParameterID: es (0x0005) Description: Information on the applicability of echo Bajaj,Ranjhan,Jain,Babu,Laha [page 7] Internet Draft Megaco/H.248 R2 Package April 11, 2000 suppressors Type: enumeration Possible Values: "RQ" (0x0001) Outgoing half-echo suppression required "NRQ" (0x0002) Echo suppressors not required "INS" (0x0003) Outgoing half-echo suppressors inserted "NA" (0x0004) Information not available or applicable Parameter Name: Country Code ParameterID: cc (0x0006) Description: Country code information collected as a part of the compelled register signaling. Type: string of digits (possibly empty) returned as a quoted string. Possible values: A sequence of the characters "0" through "9" Parameter Name: Language ParameterID: lang (0x0007) Description: Language information collected as a part of the compelled register signaling. Type: enumeration Possible Values: "FR" (0x0001) Language digit French "EN" (0x0002) Language digit English "GR" (0x0003) Language digit German "RU" (0x0004) Language digit Russian "SP" (0x0005) Language digit Spanish Parameter Name: Nature of Circuit ParameterID: nac (0x0008) Description: reports the nature of circuits involved in the connection so far viz. inclusion of satellite link. Type: Enumeration Possible Values: "SATINC" (0x0001) Satellite link included "SATNOINC" (0x0002) Satellite link not included 6.3.8 Event Name: Network congestion EventID: nwcng (0x0009) Description: Reports congestion information if obtained as a backward signal from the other end. The event shall be reported when an R2 backward signal signifying network congestion on national/international links or congestion in terminal selection or occurrence of timeout or abnormal release of R2 registers is received by the Gateway during the compelled signaling action and an events descriptor is activated requesting such an event. The actual frequency, cadence, duration and amplitude values against which the multi-frequency tone that conveys the congestion information Bajaj,Ranjhan,Jain,Babu,Laha [page 8] Internet Draft Megaco/H.248 R2 Package April 11, 2000 are validated are provisioned in the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: none defined by this package 6.3.9 Event Name: Subscriber Line Status EventID: sls (0x000a) Description: Reports the line status of the called subscriber. The event shall be reported if the line status information is available with the Gateway as a part of the compelled signaling action and an events descriptor is activated requesting the Subscriber Line Status. The actual frequency, cadence, duration and amplitude values against which the multi-frequency tones that convey the subscriber line status information are validated are provisioned in the MG. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: Parameter Name: Subscriber Line Condition ParameterID: lsts (0x0001) Description: Line conditions of the called subscriber Type: enumeration Possible Values: "UN" (0x0001) Unallocated number "SLB" (0x0002) Subscriber line busy "SLFC" (0x0003) Subscriber line free, charge "SLFNOC" (0x0004) Subscriber line free, no charge "SOO" (0x0005) Subscriber out of order "SIT" (0x0006) Send special information tone "NK" (0x0007) Subscriber status not known, set-up speech path 6.3.10 Event Name: Dual Seizure EventID: ds (0x000b) Description: Reports the detection of simultaneous seizure of a both-way R2 trunk by both the ends. This event shall be reported when "Seizure" has been sent and in response an incoming "Seizure" is detected instead of "Seizure Acknowledgement" and an events descriptor is activated requesting such an event. EventsDescriptor Parameters: Bajaj,Ranjhan,Jain,Babu,Laha [page 9] Internet Draft Megaco/H.248 R2 Package April 11, 2000 none defined by this package ObservedEventsDescriptor Parameters: none defined by this package 6.3.11 Event Name: Failure EventID: fail (0x000c) Description: Reports a failure condition in the working of the R2 line or compelled register signaling and an events descriptor is activated requesting the Failure event. EventsDescriptor Parameters: none defined by this package ObservedEventsDescriptor Parameters: Parameter Name: Error code ParameterID: ec (0x0001) Description: describes the failure reason Type: enumeration Possible Values: "ULS" (0x0001) Unexpected line signal "URS" (0x0002) Unexpected register signal "LTO" (0x0003) Line signal timeout "RTO" (0x0004) Register signal timeout "SME" (0x0005) Protocol State machine malfunction 6.4 Signals 6.4.1 Signal Name: Seizure SignalID: sz (0x0002) Description: Applies a "Seizure" signal on the R2 termination. The bit pattern that is actually sent on the physical termination is provisioned in the MG. Signal Type: (OO) on/off Duration: Not applicable Additional Parameters: None 6.4.2 Signal Name: Seizure Acknowledge SignalID: sz (0x0003) Description: Applies a "Seizure Acknowledged" signal on the R2 termination. The bit pattern that is actually sent on the physical termination is provisioned in the MG. Signal Type: (OO) on/off Duration: Not applicable Additional Parameters: None Bajaj,Ranjhan,Jain,Babu,Laha [page 10] Internet Draft Megaco/H.248 R2 Package April 11, 2000 6.4.3 Signal Name: Answer SignalID: ans (0x0004) Description: Applies an "Answered" signal on the R2 termination. The bit pattern that is actually sent on the physical termination is provisioned in the MG. Signal Type: (OO) on/off Duration: Not applicable Additional Parameters: None 6.4.4 Signal Name: Clear SignalID: clr (0x0005) Description: Applies a "Clear-forward" or a "Clear-back" signal depending on whether the Gateway represents the outgoing or incoming side for the call on the concerned physical termination (R2 trunk). The bit pattern that is actually sent on the physical termination is provisioned in the MG. Signal Type: (TO) Timeout Duration: Provisioned Additional Parameters: Parameter Name: Incoming release guard timeout Parameter id: icrgtim (0x0001) Description: specifies the duration for which the Gateway if behaving as an incoming end, maintains the release guard sequence before restoring to the "Idle" condition, after it generates a "Clear-back" signal. Corresponds to the T2 timer as defined in the ITU-T R2 specifications. Type: integer, in millisecond. Default value provisioned. Possible values: 0 and up Parameter Name: Outgoing release guard timeout Parameter id: ogrgtim (0x0002) Description: specifies the duration for which the Gateway if behaving as an outgoing end, maintains the release guard sequence before restoring to the "Idle" condition, after it generates a "Clear-forward" signal. Corresponds to the T2 timer as defined in the ITU-T R2 specifications. Type: integer, in milliseconds. Default value provisioned. Possible values: 0 and up 6.4.5 Signal Name: Block SignalID: blk (0x0006) Description: Applies a "Blocked" signal on the R2 termination. The bit pattern that is actually sent on the physical termination is provisioned in the MG. Signal Type: (OO) on/off Duration: Not applicable Additional Parameters: None Bajaj,Ranjhan,Jain,Babu,Laha [page 11] Internet Draft Megaco/H.248 R2 Package April 11, 2000 6.4.6 Signal Name: Unblock SignalID: ublk (0x0007) Description: Applies an "Idle"/"Released" signal on the R2 termination. The bit pattern that is actually sent on the physical termination is provisioned in the MG. Signal Type: (OO) on/off Duration: Not applicable Additional Parameters: None 6.4.7 Signal Name: Address SignalID: addr (0x0008) Description: Applies any one or multiple of destination number digits, source number digits, subscriber category, echo suppression information, country code, language digit information and nature of circuit information (whichever is applicable) encapsulated within a single signal to the Gateway. The Gateway applies the relevant information in the form of compelled R2 signaling on the R2 trunk. The actual frequency, cadence, duration and amplitude values for the multi-frequency tones that convey the address information are provisioned in the MG. Signal Type: (BR) Brief Duration: Provisioned Additional Parameters: Parameter Name: Destination number ParameterID: di (0x0001) Description: The called party number digits Type: string of digits Possible Values: a sequence of the characters "0" through "9" Parameter Name: Source number ParameterID: si (0x0003) Description: The calling party number digits Type: string of digits Possible Values: a sequence of the characters "0" through "9" Parameter Name: Subscriber Category ParameterID: sc (0x0004) Description: Calling Party's category Type: enumeration Possible Values: Possible Values: "NP" (0x0001) Non-priority subscriber "PR" (0x0002) Priority subscriber "MNT" (0x0003) Call from maintenance equipment "OPNOFT" (0x0004) Call from operator without forward transfer facility Bajaj,Ranjhan,Jain,Babu,Laha [page 12] Internet Draft Megaco/H.248 R2 Package April 11, 2000 "DT" (0x0005) Call setup for data transmission "OPFT" (0x0006) Call from operator with forward transfer facility "OPR" (0x0007) Operator call "NA" (0x0008) Information not available or applicable Parameter Name: Echo Suppression Information ParameterID: es (0x0005) Description: Information on the applicability of echo suppressors Type: enumeration Possible Values: "RQ" (0x0001) Outgoing half-echo suppression required "NRQ" (0x0002) Echo suppressors not required "INS" (0x0003) Outgoing half-echo suppressors inserted "NA" (0x0004) Information not available or applicable Parameter Name: Country Code ParameterID: cc (0x0006) Description: Country code information Type: string of digits Possible Values: a sequence of the characters "0" through "9" Parameter Name: Language ParameterID: lang (0x0007) Description: Language information Type: enumeration Possible Values: "FR" (0x0001) Language digit French "EN" (0x0002) Language digit English "GR" (0x0003) Language digit German "RU" (0x0004) Language digit Russian "SP" (0x0005) Language digit Spanish Parameter Name: Nature of Circuit ParameterID: nac (0x0008) Description: conveys the nature of circuits involved in the connection so far viz. inclusion of satellite link. Type: Enumeration Possible Values: "SATINC" (0x0001) Satellite link included "SATNOINC" (0x0002) Satellite link not included 6.4.8 Signal Name: Network congestion SignalID: nwcng (0x0009) Description: Applies the compelled R2 signal that translates to network congestion information. The congestion information can be network congestion on national/international links or congestion in terminal selection or occurrence of timeout or Bajaj,Ranjhan,Jain,Babu,Laha [page 13] Internet Draft Megaco/H.248 R2 Package April 11, 2000 abnormal release of R2 registers is received by the Gateway during the compelled signaling action. The actual frequency, cadence, duration and amplitude values for the multi- frequency tones that convey the congestion information are provisioned in the MG. Signal Type: (BR) Brief Duration: Provisioned Additional Parameters: None 6.4.9 Signal Name: Subscriber Line Status SignalID: sls (0x000a) Description: Applies the compelled R2 signal that translates to called subscriber line status information. The actual frequency, cadence, duration and amplitude values for the multi-frequency tones that convey this signal information are provisioned in the MG. Signal Type: (BR) Brief Duration: Provisioned Additional Parameters: Parameter Name: Subscriber Line Condition ParameterID: lsts (0x0001) Description: Line conditions of the called subscriber Type: enumeration Possible Values: "UN" (0x0001) Unallocated number "SLB" (0x0002) Subscriber line busy "SLFC" (0x0003) Subscriber line free, charge "SLFNOC" (0x0004) Subscriber line free, no charge "SOO" (0x0005) Subscriber out of order "SIT" (0x0006) Send special information tone "NK" (0x0007) Subscriber status not known, set-up speech path 6.5 Statistics 6.5.1 Statistics Name: Call Duration StatisticsID: cd (0x0001) Description: Provides cumulative duration of time the termination had been in active call context from the point of applying/receiving "Answered" signal till the point of release ("Clear-forward" or "Clear-back") initiation. Units: Double, in seconds. 6.6 Procedures None 7. References Bajaj,Ranjhan,Jain,Babu,Laha [page 14] Internet Draft Megaco/H.248 R2 Package April 11, 2000 8. Author's Addresses Vikas Bajaj Hughes Software Systems,Ltd. Gurgaon,Haryana,India. 122015. Ph: (91)-11-6346666.Ex-2804. Fax: (91)-124-6342810 or 811. mEmail: vbajaj@hss.hns.com. Sandip Ranjhan Hughes Software Systems,Ltd. Gurgaon,Haryana,India. 122015. Ph: (91)-11-6346666.Ex-2248 Fax: (91)-124-6342810 or 811. Email: sranjhan@hss.hns.com. Prashant Jain Hughes Software Systems,Ltd. Gurgaon,Haryana,India. 122015. Ph: (91)-11-6346666.Ex-2992 Email: prjain@hss.hns.com. Madhu Babu Hughes Software Systems,Ltd. Gurgaon,Haryana,India. 122015. Ph: (91)-11-6346666.Ex-2992 Email: bmbabu@hss.hns.com. Kushanava Laha Hughes Software Systems,Ltd. Gurgaon,Haryana,India. 122015. Ph: (91)-11-6346666.Ex-3083 Email: klaha@hss.hns.com. Full Copyright Statement "Copyright (C) The Internet Society (April,2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into 1. Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2. Draft Recommendation H.248 (February 2000), Temporary Document 37 (PLEN), Study Group 16, ITU-Telecommunication Standardization Sector (to be issued as draft-ietf-megaco-protocol-07.txt) Bajaj,Ranjhan,Jain,Babu,Laha [page 15] Internet Draft Megaco/H.248 R2 Package April 11, 2000 3. Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 4. Specifications of Signaling System R2, Q.400 to Q.490, Blue Book, CCITT