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.
Malis, et al. Standards Track [Page 1]
RFC 4816 PWE3 ATM Transparent Cell Transport Service February 2007
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
Malis, et al. Standards Track [Page 2]