One document matched: draft-bocci-bryant-pwe3-ms-pw-arch-00.txt


Network Working Group                                         M Bocci 
Internet Draft                                                Alcatel 
                                                                      
                                                             S.Bryant 
                                                         Cisco Systems 
 
Expires: January 2006                                     July 9, 2005 
                                   
 
                                      
    An Architecture for Multi-Segment Pseudo Wire Emulation Edge-to-Edge  


                                      
                 draft-bocci-bryant-pwe3-ms-pw-arch-00.txt 


 

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Copyright Notice 

   Copyright (C) The Internet Society (2005).  All Rights Reserved. 

Abstract 

   This document describes an architecture for extending pseudo wire 
   emulation across multiple packet switched network segments. Scenarios 
   are discussed where each segment of a given edge-to-edge emulated 
   service spans a different provider's PSN, and where the emulated 
   service originates and terminates on the same providers PSN, but may 
   pass through several PSN tunnel segments in that PSN. It presents an 
   architectural framework for such multi-segment pseudo wires, defines 
   terminology, and specifies the various protocol elements and their 
   functions.  

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 [1]. 

Table of Contents 

    
   1. Introduction................................................3 
      1.1. Motivation.............................................3 
      1.2. Non-Goals of this Document..............................6 
      1.3. Terminology............................................6 
   2. Applicability...............................................7 
   3. Protocol Layering model......................................7 
      3.1. Domain of Multi-Segment PWE3............................7 
      3.2. Payload Types..........................................8 
   4. Multi-Segment PWE3 Reference Model...........................8 
      4.1. Intra-Provider Architecture.............................9 
      4.2. Inter-Provider Architecture.............................9 
      4.3. PW Switching Models....................................10 
         4.3.1. Switching using ACs...............................10 
         4.3.2. Switching using PWs...............................10 
   5. PE Reference Model.........................................10 
      5.1. PWE3 Pre-processing....................................10 
         5.1.1. Forwarding........................................11 
         5.1.2. Native Service Processing.........................11 
   6. Protocol Stack reference Model..............................11 
   7. Maintenance Reference Model.................................12 
   8. PW Demultiplexer Layer and PSN Requirements.................12 
   9. Control Plane..............................................12 
 
 
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   10. Fragmentation.............................................13 
   11. Management and Monitoring..................................13 
   12. IANA Considerations........................................13 
   13. Security Considerations....................................13 
   14. Acknowledgments...........................................13 
   15. References................................................14 
      15.1. Normative References..................................14 
   Author's Addresses............................................14 
   Intellectual Property Statement................................14 
   Disclaimer of Validity........................................15 
   Copyright Statement...........................................15 
   Acknowledgment................................................15 
    
1. Introduction 

   RFC 3985 [2] defines the architecture for pseudo wires, where a 
   pseudo wire (PW) both originates and terminates on the edge of the 
   same packet switched network (PSN). The PW passes through a maximum 
   of one PSN tunnel between the originating and terminating PEs. 

   This document extends the architecture in RFC 3985 to enable pseudo 
   wires to be extended through multiple PSN tunnels. Use cases for 
   multi-segment pseudo wires, and the consequent requirements, are 
   defined in [3].  

1.1. Motivation 

   PWE3 aims to provide point-to-point connectivity between two edges of 
   a provider network. Requirements for Multi-Segment Pseudo-Wires for 
   this are specified in [3]. These requirements address three main 
   problems: 

   o How to scale PWE3 when the number of PEs grows to many hundreds or 
      thousands, while minimizing the complexity of the PEs and P 
      routers. 

   o How to provide PWE3 across multiple PSN routing domains or areas 
      in the same provider. 

   o How to provide PWE3 across multiple provider domains, and 
      different PSN types. 

   Consider a single PWE3 domain, such as that shown in Figure 1. There 
   are 4 PEs, and PWE3 must be provided from any PE to any other PE.  
   Traditionally, this would be achieved by establishing a full mesh of 
   PSN tunnels between the PEs. This would also require a full mesh of 
   LDP signaling adjacencies between the PEs. Pseudo wires could then be 
 
 
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   established between any PE and any other PE via a single, direct 
   tunnel. PEs must terminate all pseudo wires that are carried on PSN 
   tunnels that terminate on that PE according to the architecture of 
   RFC 3985. This solution is adequate for small numbers of PEs, but the 
   number of PEs and signaling adjacencies will grow in proportion to 
   the square of the number of PEs.  

   A more efficient solution for large numbers of PEs would be to 
   support a partial mesh of PSN tunnels between the PEs, as shown in 
   Figure 1. For example, consider a PWE3 service whose endpoints are 
   PE1 and PE4. Pseudo wires for this can take the path PE1->PE2->PE3, 
   and rather than terminating at PE2, be switched between ingress and 
   egress PSN tunnels on that PE. This requires a capability in PE2 that 
   can concatenate PW segments PE1-PE2 to PW segments PE2-PE3. The end-
   to-end PW is known as a multi-segment PW. 

                                ,,..--..,,_ 
                            .-``           `'., 
                    +-----+`                   '+-----+ 
                    | PE1 |---------------------| PE2 | 
                    |     |---------------------|     | 
                    +-----+      PSN Tunnel     +-----+ 
                    / ||                          || \ 
                   /  ||                          ||  \ 
                  |   ||                          ||   | 
                  |   ||         PSN              ||   | 
                  |   ||                          ||   | 
                   \  ||                          ||  / 
                    \ ||                          || / 
                     \||                          ||/ 
                    +-----+                     +-----+ 
                    | PE3 |---------------------| PE4 | 
                    |     |---------------------|     | 
                    +-----+`'.,_           ,.'` +-----+ 
                                `'''---''`` 
                     Figure 1 Single PSN PWE3 Scaling 

   Figure 1 shows a simple flat PSN topology. However, large provider 
   networks are typically not flat, consisting of many domains that are 
   connected together to provide edge-to-edge services. The elements in 
   each domain are specialized for a particular role.  

   An example application is shown in Figure 2. Here, the providers 
   network is divided into three domains: Two access domains and the 
   core domain. The access domains represent the edge of the provider's 
   network at which services are delivered. In the access domain, 
   simplicity is required in order to minimize the cost of the network. 
 
 
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   The core domain must support all of the aggregated services from the 
   access domains, and the design requirements here are for scalability, 
   performance, and information hiding (i.e. minimal state). The core 
   must not be exposed to the state associated with large numbers of 
   individual edge-to-edge flows. That is, the core must be simple and 
   fast.  

   In a traditional layer 2 network, the interconnection points between 
   the domains are where services in the access domains are aggregated 
   for transport across the core to other access domains. In an IP 
   network, the interconnection points would also represent interworking 
   points between different types of IP networks e.g. those with MPLS 
   and those without, and also points where network policies can be 
   applied. 

       <----------------Edge to Edge Emulated Services---------> 
                                                     
                                                            
                .-.,          ,..-..,            .-.,        
              ,'    .      ,-`       `',       ,'    .       
             /       \   .`             `,    /       \      
            /        \  /                 ,  /        \      
     AC  +----+     +----+               +----+       +----+    AC 
      ---| PE |=====| PE |===============| PE |=======| PE |--- 
         |  1 |     |  2 |               | 3  |       | 4  | 
         +----+     +----+               +----+       +----+ 
            \        /  \                 /  \        /      
             \       /  \      Core       `   \       /      
              `,    `    '.             ,`     `,    `       
                '-'`       `.,       _.`         '-'`        
             Access 1         `''-''`         Access 2       
    
                    Figure 2 Multi-Domain Network Model 

   This model can also be applied to inter-provider services, where they 
   also rely on a number of separate provider networks to be connected 
   together. 

   Consider the application of this model to PWE3. PWE3 uses tunneling 
   mechanisms such as MPLS to enable the underlying IP PSN to emulate 
   characteristics of the native service. One solution to the multi-
   domain network model above is to extend PSN tunnels edge-to-edge 
   between all of the PEs in access domain 1 and all of the PEs in 
   access domain 2, but this runs into the scaling issues described 
   above, and also exposes access and the core of the network to 
   undesirable complexity. An alternative is to constrain the complexity 
   to the network domain interconnection points (PE2 and PE3 in the 
 
 
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   example above). Pseudo-wires between PE1 and PE4 would then be 
   switched between PSN tunnels at the interconnection points, enabling 
   PWs from may PEs in the access domains to be aggregated across only a 
   few PSN tunnels in the core of the network. PEs in the access domains 
   would only need to maintain direct signaling sessions, and PSN 
   tunnels, with other PEs in their own domain, thus minimizing 
   complexity of the access domains. 

1.2. Non-Goals of this Document 

   The following are non-goals for this document: 

   o The on-the-wire specification of PW encapsulations 

   o Requirements on multi-segment pseudo-wires. 

   o The detailed specification of mechanisms for establishing and 
      maintaining multi-segment pseudo-wires. 

1.3. Terminology 

   The terminology specified in RFC 3985 applies. In addition, we define 
   the following terms: 

   o Ultimate PE (U-PE).  A PE where the customer-facing attachment 
      circuits (ACs) are bound to a PW forwarder. An ultimate PE is 
      present in the first and last segments of a MS-PW. 

   o Single-Segment PW (SS-PW). A PW setup directly between two U-PE 
      devices. Each PW in one direction of a SS-PW traverses one PSN 
      tunnel that connects the two U-PEs. 

   o Multi-Segment PW (MS-PW).  A static or dynamically configured set 
      of two or more contiguous PW segments that behave and function as 
      a single point-to-point PW. Each end of a MS-PW by definition MUST 
      terminate on a U-PE. 

   o PW Switching Provider Edge (S-PE).  A PE capable of switching the 
      control and data planes of the preceding and succeeding PW 
      segments in a MS-PW. It is therefore a PW switching point for a 
      MS-PW. A PW Switching Point is never the S-PE and the U-PE for the 
      same MS-PW. A PW switching point runs necessary protocols to setup 
      and manage PW segments with other PW switching points and ultimate 
      PEs. 

   o PW Segment. A part of a single-segment or multi-segment PW, which 
      is set up between two PE devices, U-PEs and/or S-PEs. 
 
 
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2. Applicability 

   A MS-PW is a single PW that for technical or administrative reasons    
   is segmented into a number of concatenated hops. From the   
   perspective of a U-PE, a MS-PW is indistinguishable from a SS-PW.    
   Thus, the following are equivalent from the perspective of the UPE 

       +----+                                                  +----+ 
       |UPE1+--------------------------------------------------+UPE2| 
       +----+                                                  +----+ 
    
            |<----------------------PW------------------------>| 
    
       +----+              +---+           +---+               +----+ 
       |UPE1+--------------+SPE+-----------+SPE+---------------+UPE2| 
       +----+              +---+           +---+               +----+ 
    

                      Figure 3     MS-PW Equivalence 

   Although a MS-PW may require services such as node discovery and path 
   signaling to construct the PW, it should not be confused with a L2VPN 
   system, which also requires these services. A VPWS connects its 
   endpoints via a set of PWs. MS-PW is a mechanism that abstracts the 
   construction of complex PWs from the construction of a L2VPN. Thus a 
   U-PE might be an edge device optimized for simplicity and an S-PE 
   might be an aggregation device designed to absorb the complexity of 
   continuing the PW across the core of one or more service provider 
   networks to another UPE located at the edge of the network. 

3. Protocol Layering model 

   The protocol-layering model specified in RFC 3985 applies to multi-
   segment PWE3 with the following clarification: the pseudo-wires may 
   be considered to be a separate layer to the PSN tunnel. That is, they 
   are independent of the PSN tunnel routing, operations, signaling and 
   maintenance. The design of PW routing domains should not imply that 
   the underlying PSN routing domains are the same. However, MS-PW will 
   reuse the protocols of the PSN. 

3.1. Domain of Multi-Segment PWE3 

   PWE3 defines the Encapsulation Layer, the method of carrying various   
   payload types, and the interface to the PW Demultiplexer Layer. It   
   is expected that other layers will provide the following: 

      . PSN tunnel setup, maintenance and routing 
 
 
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      . U-PE discovery 

   It is assumed that any node that is reachable via a PSN tunnel from 
   an S-PE or U-PE is a PE, a subset of which may be capable of behaving 
   as an S-PE. The selection of which S-PEs to use to reach a U-PE is 
   considered to be in the domain of PWE3.  

3.2. Payload Types 

   Multi-segment PWE3 is applicable to all PWE3 payload types. The same 
   encapsulations are used in both SS-PW and MH-PW. 

4. Multi-Segment PWE3 Reference Model 

   The PWE3 reference architecture for the single segment case is shown 
   in [2]. This architecture applies to the case where a PSN tunnel 
   extends between two edges of a single PSN domain to transport a PW 
   with endpoints at these edges. 

 

       Native   |<-----------Pseudo Wire----------->|  Native 
       Service  |                                   |  Service 
        (AC)    |    |<-PSN1-->|     |<-PSN2-->|    |   (AC) 
          |     V    V         V     V         V    V     | 
          |     +----+         +-----+         +----+      
   +----+ |     |UPE1|=========|SPE1 |=========|UPE2|     |    +----+ 
   |    |-------|....PW.Seg't1........PW Seg't3.....|----------|    | 
   | CE1| |     |    |         |     |         |    |     |    |CE2 | 
   |    |-------|....PW.Seg't2.......|PW Seg't4.....|----------|    | 
   +----+ |     |    |=========|     |=========|    |     |    +----+ 
     ^          +----+         +-----+         +----+          ^ 
     |      Provider Edge 1       ^        Provider Edge 2     | 
     |                            |                            | 
     |                            |                            | 
     |                    PW switching point                   | 
     |                                                         | 
     |<------------------- Emulated Service ------------------>| 
    
                   Figure 4 PW switching Reference Model 

   Figure 4 extends this architecture to show a multi-segment case. The 
   PEs that provide PWE3 to CE1 and CE2 are Ultimate-PE1 (U-PE1) and 
   Ultimate-PE2 (U-PE2) respectively. A PSN tunnel extends from U-PE1 to 
   switching-PE1 (S-PE1) across PSN1, and a second PSN tunnel extends 
   from S-PE1 to S-PE2 across PSN2. PWs are used to connect the 
   attachment circuits (ACs) attached to PE1 to the corresponding ACs 
 
 
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   attached to PE3. Each PW segment on the tunnel across PSN1 is 
   switched to a PW segment in the tunnel across PSN2 at S-PE1 to 
   complete the multi-segment PW (MS-PW) between U-PE1 and U-PE2. S-PE1 
   is therefore the PW switching point. PW segment 1 and PW segment 3 
   are segments of the same MS-PW while PW segment 2 and PW segment 4 
   are segments of another MS-PW. PW segments of the same MS-PW (e.g., 
   PW1 and PW3) MAY be of the same PW type or different type, and PSN 
   tunnels (e.g., PSN1 and PSN2) can be the same or different 
   technology. This document requires support for MS-PWs with segments 
   of the same type. An S-PE switches an MS-PW from one segment to 
   another based on the PW identifiers (e.g., PW label in case of MPLS 
   PWs). 

   Note that although Figure 4 only shows a single S-PE, a PW may 
   transit more one S-PE along its path. This architecture is applicable 
   when the S-PEs are statically chosen, or when they are chosen using a 
   dynamic path selection mechanism. 

4.1. Intra-Provider Architecture 

   There is a requirement to deploy PWs edge to edge in large           
   service provider networks [3]. Such networks typically encompass            
   hundreds or thousands of aggregation devices at the edge, each of 
   which would be a PE. These networks may be partitioned into separate 
   metro and core PWE3 domains, where the PEs are interconnected by a 
   sparse mesh of tunnels.  

   Whether or not the network is partitioned in to separate PWE3 
   domains, there is a also a requirement to support a partial mesh of 
   traffic engineered PSN tunnels. 

   The architecture shown in Figure 4 can be used to support such cases. 
   PSN1 and PSN2 may be in different administrative domains or access, 
   core or metro regions within the same providers network. 
   Alternatively, U-PE1, SPE1 and U-PE2 may reside at the edges of the 
   same PSN. 

4.2. Inter-Provider Architecture 

   Intra-provider PWs may need to be switched between PSN tunnels at the 
   provider boundary in order to minimize the number of tunnels required 
   to provide PWE3 services to CEs attached to each providers network. 
   In addition, AAA and security and mechanisms may need to be 
   implemented on a per-PW basis at the provider boundary.  



 
 
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4.3. PW Switching Models 

4.3.1. Switching using ACs. 

   In this model, the PW reverts to the native service at the provider 
   boundary PE. This AC is then connected to a separate PW at the peer 
   provider boundary PE. In this case, the reference models of RFC 3965 
   apply to each segment and to the PEs. The remaining PE architectural 
   considerations in this document do not apply to this case. 

4.3.2. Switching using PWs. 

   In this model, PW segments are switched between PSN tunnels in each 
   providers network, without reverting to the native service at the 
   boundary. For example, in Figure 4, PSN 1 and PSN 2 would be 
   different provider's networks. However, this would require that S-PE1 
   be a member of both provider networks.  

   An alternative architecture is shown in Figure 5. 

                   |<--------------Pseudo Wire----------->| 
                   |       Provider         Provider      | 
               AC  |    |<----1---->|     |<----2--->|    |  AC 
               |   V    V           V     V          V    V  | 
               |   +----+     +-----+     +----+     +----+  | 
      +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+ 
      |    |-------|.....PW.1........PW.2.......PW.3......|-------|    | 
      | CE1|   |   |    |     |     |     |    |     |    |  |    |CE2 | 
      +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+ 
           ^       +----+     +-----+     +----+     +----+       ^ 
           |         PE1        PE2        PE3         PE4        | 
           |                     ^          ^                     | 
           |                     |          |                     | 
           |                  PW switching points                 | 
           |                                                      | 
           |                                                      | 
           |<------------------- Emulated Service --------------->| 
    

                  Figure 5 Inter-Provider Reference Model 

5. PE Reference Model 

5.1. PWE3 Pre-processing 

   PWE3 preprocessing is applied in the U-PEs as specified in RFC 3985. 
   Processing at the S-PEs is specified in the following sections. 
 
 
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5.1.1. Forwarding 

   The forwarders in the S-PE forward packets from one or more PW 
   segments on the ingress PSN facing interface of the S-PE to one or 
   more PW segments on the egress PSN facing interface of the S-PE. 

   The forwarder selects the egress segment PW based on the ingress PW 
   label. The mapping of ingress to egress PW label may be statically or 
   dynamically configured. Figure 5 shows how a single forwarder is 
   associated with each PW segment at the S-PE.  

               +------------------------------------------+ 
               |                S-PE Device               | 
               +------------------------------------------+ 
     Ingress   |             |             |              |   Egress 
   PW instance |   Single    |             |    Single    | PW Instance 
   <==========>X PW Instance +  Forwarder  + PW Instance  X<==========> 
               |             |             |              | 
               +------------------------------------------+ 
    
                      Figure 6 Point-to-Point Service 

   Other mappings of PW to forwarder are for further study.                    
    
5.1.2. Native Service Processing 

   There is no native service processing in the S-PEs. 

6. Protocol Stack reference Model 

   Figure 7 illustrates the protocol stack reference model for multi-
   segment PWs. 














 
 
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+----------------+                                  +----------------+     
|Emulated Service|                                  |Emulated Service| 
|(e.g., TDM, ATM)|<======= Emulated Service =======>|(e.g., TDM, ATM)| 
+----------------+                                  +----------------+ 
|    Payload     |                                  |    Payload     | 
|  Encapsulation |<== Multi-segment Pseudo Wire ===>|  Encapsulation | 
+----------------+            +--------+            +----------------+ 
|PW Demultiplexer|<PW Segment>|PW Demux|<PW Segment>|PW Demultiplexer| 
+----------------+            +--------+            +----------------+ 
|   PSN Tunnel,  |<PSN Tunnel>|   PSN  |<PSN Tunnel>|  PSN Tunnel,   | 
| PSN & Physical |            |Physical|            | PSN & Physical | 
|     Layers     |            | Layers |            |    Layers      | 
+-------+--------+            +--------+            +----------------+ 
        |            ..........   |   ..........            |  
        |           /          \  |  /          \           |         
        +==========/     PSN    \===/    PSN     \==========+         
                   \  domain 1  /   \  domain 2  /                        
                    \__________/     \__________/                         
                     ``````````       `````````` 

                 Figure 7 Multi-Segment PW Protocol Stack 

   The MS-PW provides the CE with an emulated physical or virtual 
   connection to its peer at the far end. Native service PDUs from the   
   CE are passed through an Encapsulation Layer and a PW demultiplexer 
   is added at the sending U-PE. The PDU is sent over PSN domain 1. The 
   receiving S-PE removes the existing PW demultiplexer, adds a new 
   demultiplexer, and then sends the PDU over PSN2. Policies may also be 
   applied to the PW at this point. The receiving U-PE removes the PW 
   demultiplexer and restores the payload to its native format for 
   transmission to the destination CE. 

   Where the encapsulation format is different e.g. MPLS and L2TPv3, the 
   payload encapsulation may be transparently translated at the S-PE. 

7. Maintenance Reference Model 

   To be added in a future version.  

8. PW Demultiplexer Layer and PSN Requirements 

   To be added in a future version. 

9. Control Plane 

   For multi-segment pseudo wires, the intermediate PW switching points 
   may be statically provisioned, or they may be dynamically signaled. 
 
 
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   For the dynamic case, there are two options for selecting the path of 
   the PW: 

   o U-PEs determine the full path of the PW through intermediate 
      switching points. This may be either static or based on a dynamic 
      PW path selection mechanism.  

   o The each segment of the PW path is determined locally by each U-PE 
      or S-PE, either through static configuration or based on a dynamic 
      PW path selection mechanism. 

   Further details of the impact of these on the control plane 
   architecture will be provided in a future revision. 

10. Fragmentation 

An SPE is not required to make any attempt to reassemble a fragmented PW 
payload. An SPE may fragment a PW payload fragment. 

11. Management and Monitoring 

   To be added in a later version. 

12. IANA Considerations 

   To be added in a future version. 

13. Security Considerations 

   To be added in a later version. 

14. Acknowledgments 

   The authors gratefully acknowledge the input of Mustapha Aissaoui, 
   Dimitri Papadimitrou, and Luca Martini.  

    









 
 
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15. References 

15.1. Normative References 

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

   [2]  Bryant, S. and Pate, P. (Editors), "Pseudo Wire Emulation Edge-
         to-Edge (PWE3) Architecture", RFC 3985, March 2005 

   [3]  Martini, S. Bitar, N. and Bocci, M (Editors), "Requirements for 
         inter domain Pseudo-Wires", draft-martini-pwe3-mh-pw-
         requirements-01.txt, internet Draft, March 2005  

    

Author's Addresses 

   Matthew Bocci 
   Alcatel 
   Voyager Place,  
   Shoppenhangers Rd,  
   Maidenhead, Berks, UK    Email: matthew.bocci@alcatel.co.uk 
    

   Stewart Bryant 
   Cisco Systems, 
   250, Longwater, 
   Green Park, 
   Reading, RG2 6GB, 
   United Kingdom.             Email: stbryant@cisco.com 
    

Intellectual Property Statement 

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   Copies of IPR disclosures made to the IETF Secretariat and any 
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Acknowledgment 

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