|Title||Pseudowire Emulation Edge-to-Edge (PWE3) Asynchronous Transfer Mode
(ATM) Transparent Cell Transport Service
|Author||A. Malis, L. Martini, J.
Brayley, T. Walsh
Network Working Group A. Malis
Request for Comments: 4816 Verizon
Category: Standards Track L. Martini
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 (C) The IETF Trust (2007).
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
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.
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 .
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 , and MUST be used with pseudowires
of type 0x0003, "ATM transparent cell transport" .
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 , 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 , 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 ) 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
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. Security Considerations
This document does not introduce any new security considerations
beyond those in  and . This document defines an application
that utilizes the encapsulation specified in , 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 , the
congestion control considerations are identical to those discussed in
section 15 of . 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
5. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 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,
 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.
 Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
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.
Andrew G. Malis
40 Sylvan Road
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
1194 N Mathilda Ave
Sunnyvale, CA 94089
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