Internet Engineering Task Force James M. Polk Internet Draft Cisco Systems Expiration: Dec 30th, 2003 File: draft-polk-sipping-mlpp-reqs-01.txt Multilevel Precedence and Preemption in the Session Initiation Protocol June 30th, 2003 Status of this Memo 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. Abstract This document proposes considerations and requirements for the extension of the Session Initiation Protocol (SIP) [1] to support Multi-Level Precedence and Preemption (MLPP) functionality originally set forth in [2&3]. This document will be limited in scope to the requirements of the SIP Protocol; MLPP within the IETF, utilizing other IETF Protocols, is left to other documents, therefore is considered out of scope here - although there is a general explanation of how MLPP functions currently within private circuit switched networks to give the necessary operational background for these requirements. Polk Page [1] Internet Draft MLPP Requirements for SIP June 30th, 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1 Conventions used in this document . . . . . . . . . . . 3 2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 4 3. MLPP Operational Functionality . . . . . . . . . . . . . . . 5 3.1 MLPP Precedence . . . . . . . . . . . . . . . . . . . . 6 3.2 Operational Behavior for Preemption . . . . . . . . . . 7 3.2.1 Modes of Preemption in CSN Systems . . . . . . . . . . 7 3.3 Access Preemption Event . . . . . . . . . . . . . . . . 8 3.4 Network Preemption Event . . . . . . . . . . . . . . . 10 4. MLPP/IP Basic Model . . . . . . . . . . . . . . . . . . . . . 11 4.1 Components of the Basic Model . . . . . . . . . . . . . 11 5. SIP Multilevel Precedence and Preemption Requirements . . . . 12 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 10. Author Information . . . . . . . . . . . . . . . . . . . . . 16 11. Full Copyright Statement . . . . . . . . . . . . . . . . . . 16 1.0 Introduction This document proposes considerations and requirements for the extension of the Session Initiation Protocol (SIP) [1] to support an IP version of Multi-Level Precedence and Preemption (MLPP) functionality originally set forth in [2&3]. This document will be limited in scope to aspects having to do with the SIP Protocol. MLPP within the IETF utilizing other IETF Protocols is left to other documents, therefore is considered out of scope here. MLPP was originally written to create "a prioritized call handling service" in combination with ISDN supplementary services. MLPP has two very simple concepts for voice and video (Real-Time) communications: A) labeling or marking every call (at inception) with a Precedence level relative to other calls within a single administrative domain, or federation of domains; and B) during times of congestion at any point in the network, the device experiencing congestion will determine if preempting (seizing) the resources of any identifiable lower relative priority call(s) is required to properly set-up a newly signaled higher priority call Polk Page [2] Internet Draft MLPP Requirements for SIP June 30th, 2003 This administrative control and network functionality exists today in several large deployments. It is based, or founded, in US Government network requirements. MLPP [2, 3] is a supplementary service to ISDN [8, 9, 10]. Several other non-US networks have been enabled with this MLPP functionality in the past decade. Most of these networks are looking to incorporate IP signaling and transport of their voice and video services and require the MLPP functionality during the transition and progression/evolution of their networks in times of government or military emergencies when congestion causes critical systems communications to falter. The applications currently run by these networks are voice and video services only. In the future, Instant Messaging and email are targets for this capability as well, but will not be further discussed within this document. This document will focus on the considerations and requirements on SIP Proxies, Gateways and User Agents, concentrating on what needs to be addressed to enable MLPP functionality. Considerations need to be met and realized in the user experience of the existing MLPP service. Because of the existing size of these networks (one network has several million end-stations), the migration of their communications over to an IP based system cannot occur quickly. With this in mind, many considerations should be kept in mind that this is not a brand new installation. Further, all new implementations of IP endpoints with MLPP functionality will be replacing the old infrastructure (and endpoints). Most of the requirements here have been taken from [2&3]. Any remaining details and concepts attained from documents came from the certification materials which all products must be tested against to achieve MLPP compliance and interoperability status in [4&5]. There are a few concepts mentioned here that were attained from interviewing users and testers of MLPP for guidance of how this MLPP-concept might be enhanced with the additional capabilities that IP and IP-based services brings to offer. This document will cover new terminology used within MLPP infrastructures. Also included will be an overview of the current decision process that exists within the MLPP enabled network. This will be followed by the SIP(PING) requirements for enabling this functionality in this working group. 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 [6]. Polk Page [3] Internet Draft MLPP Requirements for SIP June 30th, 2003 2.0 Terms and Definitions The following is a list of definitions and conventions to used throughout this document. Note that some of the definitions are either MLPP *or* IP centric, and might not make sense to the other. Advice is taking these words in the context of the section of this document they are written in. Alternate Party is the party to which a precedence call will be diverted. Diversion will occur either when the response timer T-sub-k expires, when the called party is busy on a call of equal or higher precedence, or when the called party is busy with access resources non-preemptable. Alternate party diversion is an optional terminating feature that is subscribed to by the called party; thus, the alternate party to which a precedence call is diverted is specified by the called party at the time of subscription CSN Circuit Switched Network - Public or private infrastructure using circuit-switched technology, such as provided by TDM transmission facilities, rather than packet technologies; most often this will refer to the existing MLPP enabled closed network or within the same domain, and not the publicly available dial circuits Domain A network under one single administrative control entity End Office Node EN - see EOS End Office Switch EOS - An MLPP capable circuit switching system configured to interconnect user lines and trunks ISDN Integrated Services Digital Network Look ahead For Busy LFB - a feature of MLPP in which a phone can look ahead in the network to determine if a call it is about to place has available resources for call completion MLPP Multi-level Precedence and Preemption [2&3] - ANSI T1.619 and 619A Standards stipulating mechanisms for marking each voice communication with a Precedence level, and defining the requirement for the Preemption of existing lower Precedence sessions during congestion in favor of new higher Precedence session(s) Polk Page [4] Internet Draft MLPP Requirements for SIP June 30th, 2003 Multifunction Switch MFS - A combination of a End Office Switch (EOS) and Tandem Switch (TS); having trunking and CPE connection capabilities within one, more economical unit Precedence The relative priority level assigned to each call by the caller at inception Precedence Call Any call that has a Precedence level higher than Routine Preempt Notification The audible notification sent to all endstations who have been preempted for any reason Preempted Any caller who has had their existing call cleared Or disconnected Preempting Call A call with a Precedence level higher than others on a specified interface at a time of congestion Registrar Server SIP Server [1] that serves as a Registration point principally for mobility Response Timer T-sub-K Is started when the network notifies the Called device of a inbound precedence call; acceptance must occur by the Called device; the timer is specified in [2] at from 4-30 seconds Response Timer T-sub-L Is started when an LFB information unit is sent into the network to establish an open path between the Calling endstation and the intended called endstation; the timer is expected in [2] as from 5û20 seconds Tandem Switch TS - Only connects to EOS's and other TS's; is the primary backbone of a circuit-switched MLPP Network 3.0 MLPP Operational Functionality This section will provide the operational functionality of an MLPP infrastructure. The requirements section later in this document will be based on this section (and subsections) for its operational requirements in SIP(PING). The following subsections are from the core MLPP documents [2,3], as well as the documents involving the actual testing of any component for certification of MLPP compliance [4,5,7]. Polk Page [5] Internet Draft MLPP Requirements for SIP June 30th, 2003 The root specification [2] states that there are two conceptual parts to MLPP: Precedence and Preemption. 3.1 MLPP Precedence Precedence means Priority. It is the relative priority of a call to all other calls within that domain (or federation of domains if applicable) when traversing any interface, including an endstation. It is set or assigned by the calling party at the beginning of a call, on a per call basis. Once the precedence level is chosen by a caller, it cannot be changed for the duration of that call. The next call being independent of the first call, can be made at another authorized level, also chosen by the calling party. The table below from [2] specifies the precedence values as: Priority ISDN Text Level Sequence Sequence --- ---- -------- 1 "0000" = "Flash Override" (highest level) 2 "0001" = "Flash" 3 "0010" = "Immediate" 4 "0011" = "Priority" 5 "0100" = "Routine" (lowest level) "0101 - 1111" are unspecified The possible levels the call can be assigned, in CSN MLPP infrastructures, are bound to the allowable levels set on the switch (EOS) for that line. Each line in this infrastructure is configured to only allow certain levels to be chosen by anyone accessing that phone. Someone with personal access to higher levels than that of the phone they're in front of, needs to go to a phone with access to those higher precedence levels in order to make a higher precedence call. Conversely, a person with lower allowable privileges can access higher precedence levels by placing calls from a phone that has those levels authorized on that line. Because the precedence level chosen for a (or any) call is used solely in the determination of which call or calls are preempted (should congestion occur at any point or interface this call traverses) as explained in the next section, the user of that phone cannot use a level above what they are authorized to gain access to. Since UAs aren't bound by any physical connection to a switch, this restraint no longer will exist. Thus, another means will be required by SIP to restrict the unauthorized use of higher precedence calls by those that are not allowed to signal these precedence values in their INVITE messages. An important background note, the determination of who is granted Polk Page [6] Internet Draft MLPP Requirements for SIP June 30th, 2003 permission to make precedence calls (meaning any call with a precedence level higher than routine - the lowest level) is by job function in most MLPP networks, and not by who they are, or how long they've been with that organization. This is the case within the US "DSN" network. This means that if there is a job related rank to each person's employment, higher ranking employees don't necessarily dictate access to higher precedence call privileges, but in practice, this is generally close to alignment. 3.2 Operational Behavior for Preemption Preemption (in a CSN case) is the seizing of otherwise used resources of one or more calls in order to complete another call in a congestion situation. The nodes that determine preemption are EOSs or TNs in the CSN infrastructure. The decision is based on the precedence values assigned to each circuit with the trunk groups on those nodes. When a call is placed, the transiting node maintains state as to the precedence value of each call assigned to a inbound and outbound port on that node. When a new call is signaled (via SS7) into that CSN node, the node looks for available resources to route that call through. If the node determines that it has no more outbound (egress) resources available (for example on a T1 interface) for this new call, a comparison is performed of this new call's precedence value to that of all the other calls existing on that outbound interface. If this new call has a higher precedence value than any one of the other calls, one or more calls (in fact all that are necessary) are cleared to complete this new call. 3.2.1 Modes of Preemption in CSN Systems There are two modes of Preemption: preemption of the called device with another inbound higher precedence call (Access Preemption Event), and preemption at any point of congestion between non- endpoint nodes within the network (Network Preemption Event). MLPP is mandated in [2] as having call handling influence within a single domain based implementation only. The precedence value set in one MLPP Domain SHOULD NOT cross domain boundaries into another domain and have any preferential treatment applied to that call. In other words, no preemption of any resources shall occur within a domain as a result of a call into that domain from outside the domain, even if both domains are MLPP compliant networks. The MLPP Domain-Identifier [2] was included in the ISDN and SS7 in order to provide for a final check that the domains match before applying preemption. Polk Page [7] Internet Draft MLPP Requirements for SIP June 30th, 2003 Here are the three preemption conditions: a) A distinctive preemption notification (tone) shall be introduced into any connection(s) that is to be cleared due to either a Access or network Preemption event; (this is not a SIP protocol issue, but an implementation one, yet worth noting here) b) The called party MUST acknowledge an inbound precedence call before connection to that call; c) Upon determination of no available resources and calls existing on an interface of lower precedence, the lowest level call(s) MUST be cleared in order to complete the higher precedence call; A call can be preempted at any time after the precedence level has been determined to be lower than the new call and before call clearing has begun. However, no preemption of any resources shall occur within a domain as a result of a call into that domain from another domain, even if both domains are MLPP compliant networks. MLPP [2] also established the Alternative Party, and the Non- Preemptable Resources options. The Alternative Party option is pre- configured to a secondary phone to ring in the times where the original phone is being used. This can prevent a preemption event, even when that new inbound MLPP call is of higher precedence. The Alternative Party must answer before the Timer T sub K expires. Additionally, a party of a phone can preset their phone with the option of 'Non-Preemptable Resources'. This prevents Access Preemption events, but does not prevent Network Preemption events. The Alternative Party redirect MUST be to a valid domain address and is RECOMMENDED to a location which always answers the phone, such as a operator or ACD pool of personnel. A limit in [11] set the maximum number of call diversions to 5. An additional benefit to the Timer T sub K is that it limits the time of these diversions when it expires for a call. The example below give this mechanism more clarity. 3.3 Access Preemption Event The following is a CSN example from [2] of the MLPP mandated process for how Access-based Preemption events MUST occur, similar to a flow chart: Polk Page [8] Internet Draft MLPP Requirements for SIP June 30th, 2003 Scenario #1: Caller A and D are on an MLPP call when Caller C calls D IP Phone A \ \ EOS =====> IP Phone D / / IP Phone C Figure 1. Call A-D exists when C calls D If there is an existing call between two parties (A & D), and a third party (C) calls into D (provided there is no congestion between C & the EOS directly connected to D), the EOS (which is attached to D) first checks the Precedence of this new inbound call. If the Precedence value is equal to or less than that of the existing call between D & A, then C either is returned a Disconnect (user busy), or is diverted to an alternate party (another phone) if there is one specified; C is returned a Disconnect (Precedence Call Blocked indication) if an alternate party isn't specified. If the MLPP call from C has a greater Precedence value than the A to D call, then a determination is made for D (by the EOS connected to D) whether D is Preemptable. If D is not Preemptable, then an alternate party is looked for. If an alternate party is set up within the EOS for D, the call is diverted to this alternate party. If there is not one set up within the EOS for D, C is returned a Disconnect (Not Equipped for Preemption). If D is Preemptable, the user and device of D is notified. So is Device A. The device at D is offered Call Setup information, while also starting the T sub K timer (defined as being between 4-30 seconds). A Disconnect is sent to A now, releasing it from the A- to-D call. The device at D is waiting for the user at D to determine 1 of 3 possible paths to take: Path #1 is when nothing occurs until the T sub K timer expires. This results in a determination if an alternate party was specified by D. If there is, C is then connected to this alternate party. [11] stipulates a maximum number of 5 call diversions can occur. If no alternate party is specified, C's call is normally set-up into D. Path #2 is if there is a request from C to Clear the call. This results in C and D being released now (A has already been released). Path #3 is when D acknowledges the inbound Preemption by C, thereby accepting the call from C. This stops the T sub K timer. Polk Page [9] Internet Draft MLPP Requirements for SIP June 30th, 2003 The Call is set-up between C to D. 3.4 Network Preemption Event The following is a CSN example from [2] of the MLPP mandated process for how Network-based Preemption events MUST occur, similar to a flow chart: Scenario #2: Caller A and B are on an MLPP call when Caller C initiates a higher precedence call to Caller D (than the existing one between A and B) IP Phone A IP Phone B \ / EOS 1 EOS 2 \ / TS 1 <=========> TS 2 / \ EOS 3 EOS 4 / \ IP Phone C IP Phone D Figure 2. Call A-B exists when C calls D If there is an existing MLPP call between two parties (A & B), and a new MLPP call (C-D) is signaled through the MLPP network (shown between TS's 1 and 2 in Figure 2 above), the network first checks to see if there are available resources for that new call between the two new parties (C & D); if there is, everything works as if both calls were on the same Precedence level with no congestion. But if there is congestion at any common interface between the calls A-B this new call C-D, there is now a search at that congested interface for Preemptable resources (any call with a lower Precedence level than this new C-D call attempt). If there is not, a determination is made whether the Call from C is a Precedence call. If the call from C is not, C is returned from the network a "Disconnect: Network Resources Unavailable" indication. If the call from C is a Precedence Call, C is returned a "Disconnect: Precedence Call Blocked" indication. The call remains between A and B through both cases. If the congested interface is the trunk interface of TS 1 (connected to TS 2), there are common resources for both calls. In the case where TS 1 determines that the established call between A-B is of lower precedence value than the new call set-up between C-D, A and B are notified of preemption and TS 1 now releases (disconnects, clears) the amount of resources at that congested interface in order to have the C-D call be set-up normally. Phone A and B are not notified from where the preemption occurred from. Polk Page [10] Internet Draft MLPP Requirements for SIP June 30th, 2003 Under this Network Preemption scenario within MLPP, the amount of resources necessary for this call C-D, even if it requires more than one other call to be preempted, MUST be made to satisfy the higher precedence call completion. All necessary lower Precedence level resources MUST be cleared for any higher Precedence Call. 4.0 MLPP/IP Basic Model Figure 3 (below) is a basic model of an MLPP over IP (MLPP/IP) domain connected to both an MLPP CSN domain where the above requirements MUST be extended throughout, and to the public CSN where the above requirements cease at the edge of the MLPP/IP network at GW#1. Additionally, Phone A will start an MLPP/IP aware call at GW#1, likely with a minimum precedence value, just as is deployed today within existing MLPP networks where the entrance to an MLPP network is precedence marked "routine", with no possibility of upgrading or requesting higher precedence values for that call. GW#1-- Public CSN -- Phone / A / UA#1 ----- MLPP/IP Domain (or federation of domains) / | \ / | \ Proxy | GW#2-- MLPP CSN ---- Phone Server UA#2 B Figure 3. Generic MLPP-MLPP/IP-CSN Interworking model The MLPP/IP portion of the network above is part of the MLPP CSN network domain (via GW#2). The MLPP domain boundary is at GW#1. 4.1 Components of the Basic Model Figure 1 shows several components in the diagram. The generic scope of each is as follows: UA's 1 & 2 SIP user agents; Phone A MLPP-based phone that exists within and adheres to the MLPP Standards as written in [2&3] and those connected directly to EOS's; Phone B TDM-based phone which could be one from a corporate network, private network or residential Gateway#1 Seamless translator between the public CSN communications methods and the MLPP/IP domain Polk Page [11] Internet Draft MLPP Requirements for SIP June 30th, 2003 Gateway#2 Seamless translator between the MLPP communications methods specified in [2&3] and the MLPP/IP domain Proxy Server SIP-based Server serving the functions defined in [1] There is not mention of the details within each network-type (MLPP/IP or CSN) for the purposes of keeping this explanation a simple a possible; the burden should mostly fall on the Gateways to seamlessly translate the communications from one type of network to the adjacent type. 5.0 SIP Multilevel Precedence and Preemption Requirements Section 3 explained the operational conditions needed in an MLPP circuit-switched infrastructure. The following are the requirements SIP for the interoperating with existing MLPP CSN infrastructures, as well as on SIP for operating within a IP based domain or federation of domains with MLPP functionality: REQ#1 - There MUST be a means by which the user of a UAC can select a precedence level for a session. This requirement is calling for a mechanism of session resource priority that will influence behaviors of User Agent and gateway resources. [2] specifies the precedence values as: Priority ISDN Text Level Sequence Sequence --- ---- -------- 1 "0000" = "Flash Override" (highest level) 2 "0001" = "Flash" 3 "0010" = "Immediate" 4 "0011" = "Priority" 5 "0100" = "Routine" (lowest level) "0101 - 1111" are unspecified However SIP or SIPPING chooses to actually solve this binding between MLPP in ISDN and SIP message (Headers or something else), at least 5 distinct and relative precedence levels MUST be maintained to ensure interoperability with existing networks. Further, if more relative levels are chosen within SIP, a binding of these 5 ISDN levels to the higher precedence or priority levels MUST be maintained throughout a domain (or federation of domains) to ensure interoperability. Polk Page [12] Internet Draft MLPP Requirements for SIP June 30th, 2003 REQ#2 - This precedence choice by the UAC SHOULD be able to influence Proxy Server behavior. The choice of whether this function is utilized should be a matter of local policy. REQ#3 - The precedence levels available to the user of a SIP entity should be limited to only those levels granted that user within that domain (or federation of domains). Each MLPP/IP infrastructure SHOULD have an mechanism to authenticate and then authorize the use of precedence levels other than the "routine" (or default "normal" level for everyday voice communications). This might be mandatory in some domains, but that assignment is policy, and should be left for local administration (and not part of this document). REQ#4 - Once a precedence level has been chosen and the SIP Request transmitted, the precedence level (however signified within the message) MUST be maintained for the duration of that session. The UAS cannot alter this precedence level within the SIP response. REQ#5 - User migration from a CSN infrastructure to an IP infrastructure should not impact user behavior with reduced capabilities. SIP GWs MUST maintain the precedence level chosen that originate within a MLPP enabled CSN network. This configuration will be from a CSN to IP transition, and the users shouldn't expect a loss in preferential treatment. REQ#6 - SIP GWs SHOULD set all the (non-IP side) received calls to the minimum precedence setting, for there is no reasonable means of authenticating a CSN call is from a user authorized for higher precedence levels REQ#7 - Any session SHOULD be considered independent to the session initiated before it and the one after it from a precedence setting point of view. REQ#8 - There MUST be some means of identifying a domain of origin, or a domain for the use of this precedence level set within the SIP message. This is to ensure those SIP entities that are enabled for preferential treatment based on the precedence level present within the SIP message have a means of easily differentiating those requests that are from their domain and those that are not. REQ#9 - There SHOULD be a means in which a UAS can authenticate the included precedence level within a SIP Request. This should not burden the UAS to authenticate each and every UAC possible of sending SIP Request messages. It is only relevant to the UAS that an authorized precedence label is Polk Page [13] Internet Draft MLPP Requirements for SIP June 30th, 2003 included within an INVITE, and not the identity of the UAC sending the INVITE. This requirement is specifically to address Access Preemption Events in which local policy could mandate the preemption of an existing session in lieu of a higher precedence level in this new SIP Request. REQ#10 - The User of a UAC MAY be able to remain anonymous, therefore there MUST be a means by which an anonymous UAC can transmit a SIP Request that can be authenticated by the UAS receiving the request. This requirement should also apply to Proxies. REQ#11 - There SHOULD be a means by which a UAC can signal QOS, or that the UAC can react to an error which was sent by a UAS requiring QOS for that session, with the indication within that error of which QOS (perhaps a level within itself) to use. This requirement will address Network Preemption Events within IP infrastructures. REQ#12 - All SIP entities that do not recognize the means in which a SIP message indicates precedence, or which domain the precedence level is from, MUST ignore the indication but not fail the SIP Request based solely on that criteria. This applies to SIP UAs, SIP GWs and SIP servers. REQ#13 - There SHOULD be a mechanism in which any MLPP/IP domain can determine the functional and configuration capabilities for Registering UAs to ensure each can behave as the domain MIGHT mandate. REQ#14 - Call Detail Records SHOULD be kept within a SIP entity within an MLPP/IP infrastructure to ensure an administrative means for addressing various misuses of precedence calling. 6.0 IANA Considerations There are no IANA considerations requested with this document 7.0 Security Considerations This topic is chock full of security concerns. However, this document is not requesting capabilities that are to be implemented on the open Internet. The intention here is for SIP to extend itself to meet these requirements for interoperation and transition with Polk Page [14] Internet Draft MLPP Requirements for SIP June 30th, 2003 existing closed networks that are MLPP enabled; which are few, yet very large. Further, some requirements stated here call for the authentication abilities of the receiving UAS (or Proxy) of a SIP message with a precedence level indication to the UAC. If this authentication, or more accurately authenticated authorization doesnÆt pass, the precedence level request should be ignored. Existing MLPP enabled domains will likely fail the session for many reasons, this one being only one of them. User authentication to their networks will be mandated, and policed heavily. Properly built infrastructures with these capabilities should not influence the Internet or individual SIP Proxies that process non- MLPP transactions. Certain domains will likely mandate that all SIP entities conform to these functionalities in order to communicate, with appropriate challenges configured at each SIP entity to prevent unwanted or disallowed SIP communications. 8.0 Acknowledgements To Mike Pierce and Janet Gunn for their insightful comments in framing this document 9.0 References [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "Session Initiation Protocol", RFC 3261, June 2002 [2] ANSI T1.619-1992 (R1999) [3] ANSI T1.619a-1994 (R1999) [4] "Generic Switching Center Requirements" (GSCR), JIEO Technical Report 8249, March 2003 [5] Defense Switched Network "Generic Switching Test Plan" (GSTP), June 1999 [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [7] ITU-T Recommendation Q.735.3 (1993), "Description for Community of Interest Supplementary Services using SS7 - Multilevel precedence and preemption" Polk Page [15] Internet Draft MLPP Requirements for SIP June 30th, 2003 [8] ANSI T1.604-1990 "ISDN - Layer 3 Signaling Specification for Circuit-Switched Bearer service for Digital Subscriber System Number 1 (DSS1)" [9] T1.113-2000 "Signaling System Number 7 (SS7) - ISDN User Part" [10] ANSI T1.610-1990 (R2000) "DSS1 - Generic Procedures for the Control of ISDN Supplementary Services" [11] ITU-T Recommendation I.255.3 (1998), "Multilevel precedence and preemption service (MLPP)". 10.0 Author Information James M. Polk Cisco Systems 2200 East President George Bush Turnpike Richardson, Texas 75082 USA jmpolk@cisco.com 11. Full Copyright Statement "Copyright (C) The Internet Society (February 23rd, 2001). 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 languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Polk Page [16] Internet Draft MLPP Requirements for SIP June 30th, 2003 The Expiration date for this Internet Draft is: Dec 30th, 2003 Polk MLPP Requirements for SIP Page [17]