





Network Working Group                                          A. Malis
Request for Comments: 4816                                      Verizon
Category: Standards Track                                    L. Martini
                                                          Cisco Systems
                                                             J. Brayley
                                                            ECI Telecom
                                                               T. Walsh
                                                       Juniper Networks
                                                          February 2007


               Pseudowire Emulation Edge-to-Edge (PWE3)
  Asynchronous Transfer Mode (ATM) Transparent Cell Transport Service

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The document describes a transparent cell transport service that
   makes use of the "N-to-one" cell relay mode for Pseudowire Emulation
   Edge-to-Edge (PWE3) Asynchronous Transfer-Mode (ATM) cell
   encapsulation.

1.  Introduction

   This transparent cell transport service allows migration of ATM
   services to a PSN without having to provision the ATM subscriber or
   customer edge (CE) devices.  The ATM CEs will view the ATM
   transparent cell transport service as if they were directly connected
   via a Time Division Multiplexer (TDM) leased line.  This service is
   most likely to be used as an internal function in an ATM service
   provider's network as a way to connect existing ATM switches via a
   higher-speed PSN, or to provide ATM "backhaul" services for remote
   access to existing ATM networks.







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1.1.  Specification of Requirements

   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].

2.  Transparent Cell Transport Definition

   The transparent port service is a natural application of the "N-to-
   one" Virtual Circuit Connection (VCC) cell transport mode for PWE3
   ATM encapsulation described in [2], and MUST be used with pseudowires
   of type 0x0003, "ATM transparent cell transport" [4].

   The ATM transparent port service emulates connectivity between two
   remote ATM ports.  This service is useful when one desires to connect
   two CEs without processing or switching at the Virtual Path
   Connection (VPC) or VCC layer.  The ingress PE discards any
   idle/unassigned cells received from the ingress ATM port, and maps
   all other received cells to a single pseudowire.

   The egress PE does not change the Virtual Path Identifier (VPI),
   Virtual Circuit Identifier (VCI), Payload Type Identifier (PTI), or
   Cell Loss Priority (CLP) bits when it sends these cells on the egress
   ATM port.  Therefore, the transparent port service appears to emulate
   an ATM transmission convergence layer connection between two ports.
   However, since the ingress PE discards idle/unassigned cells, this
   service benefits from statistical multiplexing bandwidth savings.

   In accordance with [2], cell concatenation MAY be used for
   transparent cell-relay transport in order to save the PSN bandwidth.
   If used, it MUST be agreed between the ingress and egress PEs.  In
   particular, if the Pseudo Wire has been set up using the PWE3 control
   protocol [3], the ingress PE MUST NOT exceed the value of the
   "Maximum Number of concatenated ATM cells" Pseudowire Interface
   Parameter Sub-TLV (Interface Parameter ID = 0x02 [4]) received in the
   Label Mapping message for the Pseudo Wire, and MUST NOT use cell
   concatenation if this parameter has been omitted by the egress PE.

   ATM Operations and Management (OAM) cells MUST be transported
   transparently, and the PEs do not act on them.  If the PEs detect a
   PSN or pseudowire failure between them, they do not generate any OAM
   cells, but rather bring down the ATM interfaces to the CEs (e.g.,
   generating LOS on the ATM port), just as if it were a transmission
   layer failure.







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RFC 4816      PWE3 ATM Transparent Cell Transport Service  February 2007


   Similarly, ATM Integrated Local Management Interface (ILMI) signaling
   from the CEs, if any, MUST be transported transparently, and the PEs
   do not act on it.  However, the PEs must act on physical interface
   failure by either withdrawing the PW labels or by using pseudowire
   status signaling to indicate the interface failure.  The procedures
   for both alternatives are described in [3].

3.  Security Considerations

   This document does not introduce any new security considerations
   beyond those in [2] and [3].  This document defines an application
   that utilizes the encapsulation specified in [2], and does not
   specify the protocols used to carry the encapsulated packets across
   the PSN.  Each such protocol may have its own set of security issues,
   but those issues are not affected by the application specified
   herein.  Note that the security of the transported ATM service will
   only be as good as the security of the PSN.  This level of security
   might be less rigorous than a native ATM service.

4.  Congestion Control

   Since this document discusses an application of the "N-to-one" VCC
   cell transport mode for PWE3 ATM encapsulation described in [2], the
   congestion control considerations are identical to those discussed in
   section 15 of [2].  The PWE3 Working Group is also undertaking
   additional work on ATM-related congestion issues, and implementers
   should anticipate that an RFC will be published describing additional
   congestion control techniques that should be applied to ATM emulation
   over pseudowires.

5.  Normative References

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

   [2] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N., Brayley, J.,
       and G. Koleyni, "Encapsulation Methods for Transport of
       Asynchronous Transfer Mode (ATM) over MPLS Networks", RFC 4717,
       December 2006.

   [3] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron,
       "Pseudowire Setup and Maintenance Using the Label Distribution
       Protocol (LDP)", RFC 4447, April 2006.

   [4] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
       Emulation (PWE3)", BCP 116, RFC 4446, April 2006.





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RFC 4816      PWE3 ATM Transparent Cell Transport Service  February 2007


Acknowledgments

   The authors would like to thank the members of the PWE3 working group
   for their assistance on this document, and Sasha Vainshtein of Axerra
   in particular for his comments and suggestions.

Author's Addresses

   Andrew G. Malis
   Verizon Communications
   40 Sylvan Road
   Waltham, MA

   EMail: andrew.g.malis@verizon.com


   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112

   EMail: lmartini@cisco.com


   Jeremy Brayley
   ECI Telecom
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205

   EMail: jeremy.brayley@ecitele.com


   Tom Walsh
   Juniper Networks
   1194 N Mathilda Ave
   Sunnyvale, CA 94089

   EMail: twalsh@juniper.net












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