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<pre>Network Working Group S. Singh
Request for Comments: 4454 M. Townsley
Category: Standards Track C. Pignataro
Cisco Systems
May 2006
<span class="h1">Asynchronous Transfer Mode (ATM) over</span>
<span class="h1">Layer 2 Tunneling Protocol Version 3 (L2TPv3)</span>
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 Internet Society (2006).
Abstract
The Layer 2 Tunneling Protocol, Version 3 (L2TPv3) defines an
extensible tunneling protocol to transport layer 2 services over IP
networks. This document describes the specifics of how to use the
L2TP control plane for Asynchronous Transfer Mode (ATM) Pseudowires
and provides guidelines for transporting various ATM services over an
IP network.
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
<a href="#section-1.1">1.1</a>. Abbreviations ..............................................<a href="#page-3">3</a>
<a href="#section-1.2">1.2</a>. Specification of Requirements ..............................<a href="#page-3">3</a>
<a href="#section-2">2</a>. Control Connection Establishment ................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Session Establishment and ATM Circuit Status Notification .......<a href="#page-4">4</a>
<a href="#section-3.1">3.1</a>. L2TPv3 Session Establishment ...............................<a href="#page-4">4</a>
<a href="#section-3.2">3.2</a>. L2TPv3 Session Teardown ....................................<a href="#page-6">6</a>
<a href="#section-3.3">3.3</a>. L2TPv3 Session Maintenance .................................<a href="#page-6">6</a>
<a href="#section-4">4</a>. Encapsulation ...................................................<a href="#page-6">6</a>
<a href="#section-4.1">4.1</a>. ATM-Specific Sublayer ......................................<a href="#page-7">7</a>
<a href="#section-4.2">4.2</a>. Sequencing .................................................<a href="#page-9">9</a>
<a href="#section-5">5</a>. ATM Transport ...................................................<a href="#page-9">9</a>
<a href="#section-5.1">5.1</a>. ATM AAL5-SDU Mode .........................................<a href="#page-10">10</a>
<a href="#section-5.2">5.2</a>. ATM Cell Mode .............................................<a href="#page-10">10</a>
<span class="grey">Singh, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
<a href="#section-5.2.1">5.2.1</a>. ATM VCC Cell Relay Service .........................<a href="#page-11">11</a>
<a href="#section-5.2.2">5.2.2</a>. ATM VPC Cell Relay Service .........................<a href="#page-12">12</a>
<a href="#section-5.2.3">5.2.3</a>. ATM Port Cell Relay Service ........................<a href="#page-12">12</a>
<a href="#section-5.3">5.3</a>. OAM Cell Support ..........................................<a href="#page-12">12</a>
<a href="#section-5.3.1">5.3.1</a>. VCC Switching ......................................<a href="#page-12">12</a>
<a href="#section-5.3.2">5.3.2</a>. VPC Switching ......................................<a href="#page-13">13</a>
<a href="#section-6">6</a>. ATM Maximum Concatenated Cells AVP .............................<a href="#page-13">13</a>
<a href="#section-7">7</a>. OAM Emulation Required AVP .....................................<a href="#page-14">14</a>
<a href="#section-8">8</a>. ATM Defects Mapping and Status Notification ....................<a href="#page-14">14</a>
<a href="#section-8.1">8.1</a>. ATM Alarm Status AVP ......................................<a href="#page-14">14</a>
<a href="#section-9">9</a>. Applicability Statement ........................................<a href="#page-15">15</a>
<a href="#section-9.1">9.1</a>. ATM AAL5-SDU Mode .........................................<a href="#page-16">16</a>
<a href="#section-9.2">9.2</a>. ATM Cell Relay Mode .......................................<a href="#page-18">18</a>
<a href="#section-10">10</a>. Congestion Control ............................................<a href="#page-20">20</a>
<a href="#section-11">11</a>. Security Considerations .......................................<a href="#page-21">21</a>
<a href="#section-12">12</a>. IANA Considerations ...........................................<a href="#page-21">21</a>
<a href="#section-12.1">12.1</a>. L2-Specific Sublayer Type ................................<a href="#page-21">21</a>
<a href="#section-12.2">12.2</a>. Control Message Attribute Value Pairs (AVPs) .............<a href="#page-21">21</a>
<a href="#section-12.3">12.3</a>. Result Code AVP Values ...................................<a href="#page-22">22</a>
<a href="#section-12.4">12.4</a>. ATM Alarm Status AVP Values ..............................<a href="#page-22">22</a>
<a href="#section-12.5">12.5</a>. ATM-Specific Sublayer Bits ...............................<a href="#page-23">23</a>
<a href="#section-13">13</a>. Acknowledgements ..............................................<a href="#page-23">23</a>
<a href="#section-14">14</a>. References ....................................................<a href="#page-23">23</a>
<a href="#section-14.1">14.1</a>. Normative References .....................................<a href="#page-23">23</a>
<a href="#section-14.2">14.2</a>. Informative References ...................................<a href="#page-24">24</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
This document describes the specifics of how to use the Layer 2
Tunneling Protocol (L2TP) for Asynchronous Transfer Mode (ATM)
Pseudowires, including encapsulation, carrying various ATM services,
such as AAL5 SDU, ATM VCC/VPC/Port cell relay over L2TP, and mapping
ATM defects to L2TP Set-Link-Info (SLI) messages to notify the peer
L2TP Control Connection Endpoint (LCCE).
Any ATM-specific AVPs or other L2TP constructs for ATM Pseudowire
(ATMPW) support are defined here as well. Support for ATM Switched
Virtual Path/Connection (SVP/SVC) and Soft Permanent Virtual
Path/Connection (SPVP/SPVC) are outside the scope of this document.
The reader is expected to be very familiar with the terminology and
protocol constructs defined in [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>].
<span class="grey">Singh, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Abbreviations</span>
AIS Alarm Indication Signal
ATMPW ATM Pseudowire
AVP Attribute Value Pair
CC Continuity Check OAM Cell
CE Customer Edge
HEC Header Error Checksum
LAC L2TP Access Concentrator (see [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>])
LCCE L2TP Control Connection Endpoint (see [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>])
MSB Most Significant Byte
OAM Operation, Administration, and Maintenance
PE Provider Edge
PSN Packet Switched Network
PWE3 Pseudowire Emulation Edge to Edge
RDI Remote Defect Indicator
SAR Segmentation and Reassembly
SDU Service Data Unit
SLI Set-Link-Info, an L2TP control message
SVC Switched Virtual Connection
SVP Switched Virtual Path
SPVC Soft Permanent Virtual Connection
SPVP Soft Permanent Virtual Path
VC Virtual Circuit
VCC Virtual Channel Connection
VCI Virtual Channel Identifier
VPC Virtual Path Connection
VPI Virtual Path Identifier
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Specification of Requirements</span>
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Control Connection Establishment</span>
To emulate ATM Pseudowires using L2TP, an L2TP Control Connection as
described in <a href="./rfc3931#section-3.3">Section 3.3 of [RFC3931]</a> MUST be established.
The Start-Control-Connection-Request (SCCRQ) and corresponding
Start-Control-Connection-Reply (SCCRP) MUST include the supported ATM
Pseudowire types (see <a href="#section-3.1">Section 3.1</a>), in the Pseudowire Capabilities
List as defined in <a href="./rfc3931#section-5.4.3">Section 5.4.3 of [RFC3931]</a>. This identifies the
Control Connection as able to establish L2TP sessions in support of
the ATM Pseudowires.
<span class="grey">Singh, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
An LCCE MUST be able to uniquely identify itself in the SCCRQ and
SCCRP messages via a globally unique value. By default, this is
advertised via the structured Router ID AVP [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>], though the
unstructured Hostname AVP [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>] MAY be used to identify LCCEs as
well.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Session Establishment and ATM Circuit Status Notification</span>
This section describes how L2TP ATMPWs or sessions are established
between two LCCEs. This includes what will happen when an ATM
circuit (e.g., AAL5 PVC) is created, deleted, or changes state when
circuit state is in alarm.
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. L2TPv3 Session Establishment</span>
ATM circuit (e.g., an AAL5 PVC) creation triggers establishment of an
L2TP session using three-way handshake described in <a href="./rfc3931#section-3.4.1">Section 3.4.1 of
[RFC3931]</a>. An LCCE MAY initiate the session immediately upon ATM
circuit creation, or wait until the circuit state transitions to
ACTIVE before attempting to establish a session for the ATM circuit.
It MAY be preferred to wait until circuit status transitions to
ACTIVE in order to delay the allocation of resources until absolutely
necessary.
The Circuit Status AVP (see <a href="#section-8">Section 8</a>) MUST be present in the
Incoming-Call-Request (ICRQ) and Incoming-Call-Reply (ICRP) messages,
and MAY be present in the SLI message for ATMPWs.
The following figure shows how L2TP messages are exchanged to set up
an ATMPW after the ATM circuit (e.g., an AAL5 PVC) becomes ACTIVE.
LCCE (LAC) A LCCE (LAC) B
------------------ --------------------
ATM Ckt Provisioned
ATM Ckt Provisioned
ATM Ckt ACTIVE
ICRQ (status = 0x03) ---->
ATM Ckt ACTIVE
<----- ICRP (status = 0x03)
L2TP session established
OK to send data into PW
ICCN ----->
L2TP session established
OK to send data into PW
<span class="grey">Singh, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
The following signaling elements are required for the ATMPW
establishment.
a. Pseudowire Type: One of the supported ATM-related PW types should
be present in the Pseudowire Type AVP of [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>].
0x0002 ATM AAL5 SDU VCC transport
0x0003 ATM Cell transport Port Mode
0x0009 ATM Cell transport VCC Mode
0x000A ATM Cell transport VPC Mode
The above cell relay modes can also signal the ATM Maximum
Concatenated Cells AVP as described in <a href="#section-6">Section 6</a>.
b. Remote End ID: Each PW is associated with a Remote End ID akin to
the VC-ID in [<a href="#ref-PWE3ATM" title=""Encapsulation Methods for Transport of ATM Over MPLS Networks"">PWE3ATM</a>]. Two LCCEs of a PW would have the same
Remote End ID, and its format is described in <a href="./rfc3931#section-5.4.4">Section 5.4.4 of
[RFC3931]</a>.
This Remote End ID AVP MUST be present in the ICRQ in order for
the remote LCCE to associate the session to the ATM circuit. The
Remote End Identifier AVP defined in [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>] is of opaque form,
though ATMPW implementations MAY simply use a 4-octet value
that is known to both LCCEs (either by direct configuration or
some other means). The exact method of how this value is
configured, retrieved, discovered, or otherwise determined at
each LCCE is outside the scope of this document.
As with the ICRQ, the ICRP is sent only after the ATM circuit
transitions to ACTIVE. If LCCE B had not been provisioned yet for
the ATM circuit identified in the ICRQ, a Call-Disconnect-Notify
(CDN) would have been immediately returned indicating that the
circuit either was not provisioned or was not available at this LCCE.
LCCE A SHOULD then exhibit a periodic retry mechanism. If so, the
period and maximum number of retries MUST be configurable.
An implementation MAY send an ICRQ or ICRP before a PVC is ACTIVE, as
long as the Circuit Status AVP reflects that the ATM circuit is
INACTIVE and an SLI is sent when the ATM circuit becomes ACTIVE (see
<a href="#section-8">Section 8</a>).
The ICCN is the final stage in the session establishment. It
confirms the receipt of the ICRP with acceptable parameters to allow
bidirectional traffic.
<span class="grey">Singh, et al. Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. L2TPv3 Session Teardown</span>
When an ATM circuit is unprovisioned (deleted) at either LCCE, the
associated L2TP session MUST be torn down via the CDN message defined
in <a href="./rfc3931#section-3.4.3">Section 3.4.3 of [RFC3931]</a>.
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. L2TPv3 Session Maintenance</span>
All sessions established by a given Control Connection utilize the
L2TP Hello facility defined in <a href="./rfc3931#section-4.4">Section 4.4 of [RFC3931]</a> for session
keepalive. This gives all sessions basic dead peer and path
detection between LCCEs.
If the control channel utilizing the Hello message is not in-band
with data traffic over the PSN, then other method MAY be used to
detect the session failure, and it is left for further study.
ATMPWs over L2TP use the Set-Link-Info (SLI) control message as
defined in [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>] to signal ATM circuit status between LCCEs after
initial session establishment. This includes ACTIVE or INACTIVE
notifications of the ATM circuit, or any other parameters that may
need to be shared between the LCCEs in order to provide proper PW
emulation.
The SLI message MUST be sent whenever there is a status change that
may be reported by any values identified in the Circuit Status AVP.
The only exceptions to this are the initial ICRQ, ICRP, and CDN
messages, which establish and tear down the L2TP session itself when
the ATM circuit is created or deleted. The SLI message may be sent
from either LCCE at any time after the first ICRQ is sent (and
perhaps before an ICRP is received, requiring the peer to perform a
reverse Session ID lookup).
The other application of the SLI message is to map the ATM OAM or
physical layer alarms into Circuit Status AVP as described in <a href="#section-8">Section</a>
<a href="#section-8">8</a>.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Encapsulation</span>
This section describes the general encapsulation format for ATM
services over L2TP.
<span class="grey">Singh, et al. Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM-Specific Sublayer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: General Format for ATM Encapsulation over L2TPv3 over IP
The PSN Transport header is specific to IP and its underlying
transport header. This header is used to transport the encapsulated
ATM payload through the IP network.
The Session Header is a non-zero 32-bit Session ID with an optional
Cookie up to 64-bits. This Session ID is exchanged during session
setup.
The ATM-Specific Sublayer is REQUIRED for AAL5 SDU Mode and OPTIONAL
for ATM Cell Mode. Please refer to <a href="#section-4.1">Section 4.1</a> for more details.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. ATM-Specific Sublayer</span>
This section defines a new ATM-Specific Sublayer, an alternative to
the Default L2-Specific Sublayer as mentioned in <a href="./rfc3931#section-4.6">Section 4.6 of
[RFC3931]</a>. Four new flag bits (T, G, C, and U) are defined that
concur with Section 8.2 of [<a href="#ref-PWE3ATM" title=""Encapsulation Methods for Transport of ATM Over MPLS Networks"">PWE3ATM</a>].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|B|E|T|G|C|U| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ATM-Specific Sublayer Format
The meaning of the fields of the ATM-Specific Sublayer is as follows:
* S bit
Definition of this bit is as per <a href="./rfc3931#section-4.6">Section 4.6 of [RFC3931]</a>.
<span class="grey">Singh, et al. Standards Track [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
* B and E bits
Definitions of these bits are as per Section 5.5 of [<a href="#ref-L2TPFRAG" title=""PWE3 Fragmentation and Reassembly"">L2TPFRAG</a>].
If these bits are not used as per [<a href="#ref-L2TPFRAG" title=""PWE3 Fragmentation and Reassembly"">L2TPFRAG</a>], they MUST be set to
0 upon transmission and ignored upon reception.
* T (Transport type) bit
Bit (T) of the ATM-Specific Sublayer indicates whether the packet
contains an ATM admin cell or an AAL5 payload. If T = 1, the
packet contains an ATM admin cell, encapsulated according to the
VCC cell relay encapsulation of <a href="#section-5.2">Section 5.2</a>.
If not set, the PDU contains an AAL5 payload. The ability to
transport an ATM cell in the AAL5 SDU Mode is intended to provide
a means of enabling administrative functionality over the AAL5 VCC
(though it does not endeavor to preserve user-cell and admin-cell
arrival/transport ordering, as described in <a href="#section-9.1">Section 9.1</a>).
* G (EFCI) Bit
The ingress LCCE device SHOULD set this bit to 1 if the Explicit
Forward Congestion Indication (EFCI) bit of the final cell of the
incoming AAL5 payload is set to 1, or if the EFCI bit of the
single ATM cell to be transported in the packet is set to 1.
Otherwise, this bit SHOULD be set to 0. The egress LCCE device
SHOULD set the EFCI bit of all the outgoing cells that transport
the AAL5 payload to the value contained in this field.
* C (CLP) Bit
The ingress LCCE device SHOULD set this bit to 1 if the Cell Loss
Priority (CLP) bit of any of the incoming ATM cells of the AAL5
payload is set to 1, or if the CLP bit of the single ATM cell that
is to be transported in the packet is set to 1. Otherwise this
bit SHOULD be set to 0. The egress LCCE device SHOULD set the CLP
bit of all outgoing cells that transport the AAL5 CPCS-PDU to the
value contained in this field.
<span class="grey">Singh, et al. Standards Track [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
* U (Command/Response) Bit
When FRF.8.1 Frame Relay / ATM PVC Service Interworking (see
[<a href="#ref-FRF8.1" title=""Frame Relay / ATM PVC Service Interworking Implementation Agreement (FRF 8.1)"">FRF8.1</a>]) traffic is being transported, the CPCS-UU Least
Significant Bit (LSB) of the AAL5 CPCS-PDU may contain the Frame
Relay C/R bit. The ingress LCCE device SHOULD copy this bit to
the U bit of the ATM-Specific Sublayer. The egress LCCE device
SHOULD copy the U bit to the CPCS-UU Least Significant Bit (LSB)
of the AAL5 payload.
The Sequence Number field is used in sequencing, as described in
<a href="#section-4.2">Section 4.2</a>.
In case of a reassembly timeout, the encapsulating LCCE should
discard all component cells of the AAL5 frame.
An additional enumeration is added to the L2-Specific Sublayer AVP to
identify the ATM-Specific Sublayer:
0 - There is no L2-Specific Sublayer present.
1 - The Default L2-Specific Sublayer (defined in <a href="./rfc3931#section-4.6">Section 4.6
of [RFC3931]</a>) is used.
2 - The ATM-Specific Sublayer is used.
The first two values are already defined in the L2TPv3 base
specification [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>].
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Sequencing</span>
Data Packet Sequencing MAY be enabled for ATMPWs. The sequencing
mechanisms described in [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>] MUST be used to signal sequencing
support. ATMPWs over L2TPv3 MUST request the presence of the ATM-
Specific Sublayer when sequencing is enabled, and MAY request its
presence at all times.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. ATM Transport</span>
There are two encapsulations supported for ATM transport as described
below.
The ATM-Specific Sublayer is prepended to the AAL5-SDU. The other
cell mode encapsulation consists of the OPTIONAL ATM-Specific
Sublayer, followed by a 4-byte ATM cell header and a 48-byte ATM
cell-payload.
<span class="grey">Singh, et al. Standards Track [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. ATM AAL5-SDU Mode</span>
In this mode, each AAL5 VC is mapped to an L2TP session. The Ingress
LCCE reassembles the AAL5 CPCS-SDU without the AAL5 trailer and any
padding bytes. Incoming EFCI, CLP, and C/R (if present) are carried
in an ATM-Specific Sublayer across ATMPWs to the egress LCCE. The
processing of these bits on ingress and egress LCCEs is defined in
<a href="#section-4.1">Section 4.1</a>.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|T|G|C|U| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| AAL5 CPCS-SDU |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ATM AAL5-SDU Mode Encapsulation
If the ingress LCCE determines that an encapsulated AAL5 SDU exceeds
the MTU size of the L2TPv3 session, then AAL5 SDU may be fragmented
as per [<a href="#ref-L2TPFRAG" title=""PWE3 Fragmentation and Reassembly"">L2TPFRAG</a>] or underneath the transport layer (IP, etc.). F5
OAM cells that arrive during the reassembly of an AAL5 SDU are sent
immediately on the PW followed by the AAL5 SDU payload. In this
case, OAM cells' relative order with respect to user data cells is
not maintained.
Performance Monitoring OAM, as specified in ITU-T 610 [<a href="#ref-I610-1">I610-1</a>],
[<a href="#ref-I610-2">I610-2</a>], [<a href="#ref-I610-3">I610-3</a>] and security OAM cells as specified in [<a href="#ref-ATMSEC">ATMSEC</a>],
should not be used in combination with AAL5 SDU Mode. These cells
MAY be dropped at the ingress LCCE because cell sequence integrity is
not maintained.
The Pseudowire Type AVP defined in <a href="./rfc3931#section-5.4.4">Section 5.4.4 of [RFC3931]</a>,
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM AAL5 SDU VCC transport PW Type of 0x0002.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. ATM Cell Mode</span>
In this mode, ATM cells skip the reassembly process at the ingress
LCCE. These cells are transported over an L2TP session, either as a
single cell or as concatenated cells, into a single packet. Each ATM
cell consists of a 4-byte ATM cell header and a 48-byte ATM cell-
payload; the HEC is not included.
<span class="grey">Singh, et al. Standards Track [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
In ATM Cell Mode encapsulation, the ATM-Specific Sublayer is
OPTIONAL. It can be included, if sequencing support is required. It
is left to the implementation to choose to signal the Default L2-
Specific Sublayer or the ATM-Specific Sublayer.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|x|x|x|x| Sequence Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"
"
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ATM Cell Mode Encapsulation
In the simplest case, this encapsulation can be used to transmit a
single ATM cell per Pseudowire PDU. However, in order to provide
better Pseudowire bandwidth efficiency, several ATM cells may be
optionally encapsulated into a single Pseudowire PDU.
The maximum number of concatenated cells in a packet is limited by
the MTU size of the session and also by the ability of the egress
LCCE to process them. For more details about ATM Maximum
Concatenated Cells, please refer to <a href="#section-6">Section 6</a>.
<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>. ATM VCC Cell Relay Service</span>
A VCC cell relay service may be provided by mapping an ATM Virtual
Channel Connection to a single Pseudowire using cell mode
encapsulation as defined in <a href="#section-5.2">Section 5.2</a>.
An LCCE may map one or more VCCs to a single PW. However, a service
provider may wish to provision a single VCC to a PW in order to
satisfy QOS or restoration requirements.
<span class="grey">Singh, et al. Standards Track [Page 11]</span></pre>
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<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
The Pseudowire Type AVP defined in <a href="./rfc3931#section-5.4.4">Section 5.4.4 of [RFC3931]</a>,
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM cell transport VCC Mode PW Type of 0x0009.
<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>. ATM VPC Cell Relay Service</span>
A Virtual Path Connection cell relay service may be provided by
mapping an ATM Virtual Path Connection to a single Pseudowire using
cell mode encapsulation as defined in <a href="#section-5.2">Section 5.2</a>.
An LCCE may map one or more VPCs to a single Pseudowire.
The Pseudowire Type AVP defined in <a href="./rfc3931#section-5.4.4">Section 5.4.4 of [RFC3931]</a>,
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM cell transport VPC Mode PW Type of 0x000A.
<span class="h4"><a class="selflink" id="section-5.2.3" href="#section-5.2.3">5.2.3</a>. ATM Port Cell Relay Service</span>
ATM port cell relay service allows an ATM port to be connected to
another ATM port. All ATM cells that are received at the ingress ATM
port on the LCCE are encapsulated as per <a href="#section-5.2">Section 5.2</a>, into Pseudowire
PDU and sent to peer LCCE.
Each LCCE MUST discard any idle/unassigned cells received on an ATM
port associated with ATMPWs.
The Pseudowire Type AVP defined in <a href="./rfc3931#section-5.4.4">Section 5.4.4 of [RFC3931]</a>,
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM Cell transport Port Mode PW Type of 0x0003.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. OAM Cell Support</span>
The OAM cells are defined in [<a href="#ref-I610-1">I610-1</a>], [<a href="#ref-I610-2">I610-2</a>], [<a href="#ref-I610-3">I610-3</a>] and
[<a href="#ref-ATMSEC">ATMSEC</a>] can be categorized as follows:
a. Fault Management
b. Performance monitoring and reporting
c. Activation/deactivation
d. System Management (e.g., security OAM cells)
OAM Cells are always encapsulated using cell mode encapsulation,
regardless of the encapsulation format used for user data.
<span class="h4"><a class="selflink" id="section-5.3.1" href="#section-5.3.1">5.3.1</a>. VCC Switching</span>
The LCCEs SHOULD be able to pass the F5 segment and end-to-end Fault
Management, Resource Management (RM cells), Performance Management,
Activation/deactivation, and System Management OAM cells.
<span class="grey">Singh, et al. Standards Track [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
F4 OAM cells are inserted or extracted at the VP link termination.
These OAM cells are not seen at the VC link termination and are
therefore not sent across the PW.
<span class="h4"><a class="selflink" id="section-5.3.2" href="#section-5.3.2">5.3.2</a>. VPC Switching</span>
The LCCEs MUST be able to pass the F4 segment and end-to-end Fault
Management, Resource Management (RM cells), Performance Management,
Activation/deactivation, and System Management OAM cells
transparently according to [<a href="#ref-I610-1">I610-1</a>].
F5 OAM cells are not inserted or extracted at the VP cross-connect.
The LCCEs MUST be able to pass the F5 OAM cells transparently across
the PW.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. ATM Maximum Concatenated Cells AVP</span>
The "ATM Maximum Concatenated Cells AVP", Attribute Type 86,
indicates that the egress LCCE node can process a single PDU with
concatenated cells up to a specified number of cells. An LCCE node
transmitting concatenated cells on this PW MUST NOT exceed the
maximum number of cells as specified in this AVP. This AVP is
applicable only to ATM Cell Relay PW Types (VCC, VPC, Port Cell
Relay). This Attribute value may not be same in both directions of
the specific PW.
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Maximum Concatenated Cells|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP MAY be set to 0, but MAY vary (see <a href="./rfc3931#section-5.2">Section 5.2 of [RFC3931]</a>).
The length (before hiding) of this AVP is 8.
This AVP is sent in an ICRQ, ICRP during session negotiation or via
SLI control messages when LCCE changes the maximum number of
concatenated cells configuration for a given ATM cell relay circuit.
This AVP is OPTIONAL. If the egress LCCE is configured with a
maximum number of cells to be concatenated by the ingress LCCE, it
SHOULD signal this value to the ingress LCCE.
<span class="grey">Singh, et al. Standards Track [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. OAM Emulation Required AVP</span>
An "OAM Emulation Required AVP", Attribute Type 87, MAY be needed to
signal OAM emulation in AAL5 SDU Mode, if an LCCE cannot support the
transport of OAM cells across L2TP sessions. If OAM cell emulation
is configured or detected via some other means on one side, the other
LCCE MUST support OAM cell emulation as well.
This AVP is exchanged during session negotiation (in ICRQ and ICRP)
or during the life of the session via SLI control messages. If the
other LCCE cannot support the OAM cell emulation, the associated L2TP
session MUST be torn down via CDN message with result code 22.
OAM Emulation AVP is a boolean AVP, having no Attribute Value. Its
absence is FALSE and its presence is TRUE. This AVP MAY be hidden
(the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to
0, but MAY vary (see <a href="./rfc3931#section-5.2">Section 5.2 of [RFC3931]</a>). The Length (before
hiding) of this AVP is 6.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. ATM Defects Mapping and Status Notification</span>
ATM OAM alarms or circuit status is indicated via the Circuit Status
AVP as defined in <a href="./rfc3931#section-5.4.5">Section 5.4.5 of [RFC3931]</a>. For reference, usage
of this AVP is shown below.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Value is a 16-bit mask with the two least significant bits
defined, and the remaining bits are reserved for future use.
Reserved bits MUST be set to 0 when sending and ignored upon receipt.
The A (Active) bit indicates whether the ATM circuit is ACTIVE (1) or
INACTIVE (0).
The N (New) bit indicates whether the ATM circuit status indication
is for a new ATM circuit (1) or an existing ATM circuit (0).
<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. ATM Alarm Status AVP</span>
An "ATM Alarm Status AVP", Attribute Type 88, indicates the reason
for the ATM circuit status and specific alarm type, if any, to its
peer LCCE node. This OPTIONAL AVP MAY be present in the SLI message
with the Circuit Status AVP.
<span class="grey">Singh, et al. Standards Track [Page 14]</span></pre>
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The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Circuit Status Reason | Alarm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Circuit Status Reason is a 2-octet unsigned integer, and the
Alarm Type is also a 2-octet unsigned integer.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP SHOULD be set to 0, but MAY vary (see <a href="./rfc3931#section-5.2">Section 5.2 of [RFC3931]</a>).
The Length (before hiding) of this AVP is 10 octets.
This AVP is sent in the SLI message to indicate additional
information about the ATM circuit status.
Circuit Status Reason values for the SLI message are as follows:
0 - Reserved
1 - No alarm or alarm cleared (default for Active Status)
2 - Unspecified or unknown Alarm Received (default for
Inactive Status)
3 - ATM Circuit received F1 Alarm on ingress LCCE
4 - ATM Circuit received F2 Alarm on ingress LCCE
5 - ATM Circuit received F3 Alarm on ingress LCCE
6 - ATM Circuit received F4 Alarm on ingress LCCE
7 - ATM Circuit received F5 Alarm on ingress LCCE
8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE
9 - ATM Circuit down due to loop-back timeout on ingress LCCE
The general ATM Alarm failures are encoded as below:
0 - Reserved
1 - No Alarm type specified (default)
2 - Alarm Indication Signal (AIS)
3 - Remote Defect Indicator (RDI)
4 - Loss of Signal (LOS)
5 - Loss of Pointer (LOP)
6 - Loss of Framer (LOF)
7 - Loopback cells (LB)
8 - Continuity Check (CC)
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Applicability Statement</span>
The ATM Pseudowire emulation described in this document allows for
carrying various ATM services across an IP packet switched network
<span class="grey">Singh, et al. Standards Track [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
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(PSN). These ATM services can be PVC-based, PVP-based, or port-
based. In all cases, ATMPWs operate in a point-to-point deployment
model.
ATMPWs support two modes of encapsulation: ATM AAL5-SDU Mode and ATM
Cell Relay Mode. The following sections list their respective
characteristics in relationship to the native service.
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. ATM AAL5-SDU Mode</span>
ATMPWs operating in AAL5-SDU Mode only support the transport of PVC-
based services. In this mode, the AAL5 CPCS-PDU from a single VCC is
reassembled at the ingress LCCE, and the AAL5 CPCS-SDU (i.e., the
AAL5 CPCS-PDU without CPCS-PDU Trailer or PAD octets, also referred
to as AAL5 CPCS-PDU Payload) is transported over the Pseudowire.
Therefore, Segmentation and Reassembly (SAR) functions are required
at the LCCEs. There is a one-to-one mapping between an ATM PVC and
an ATMPW operating in AAL5-SDU Mode, supporting bidirectional
transport of variable length frames. With the exception of
optionally transporting OAM cells, only ATM Adaptation Layer (AAL)
Type 5 frames are carried in this mode, including multiprotocol over
AAL5 packets [<a href="./rfc2684" title=""Multiprotocol Encapsulation over ATM Adaptation Layer 5"">RFC2684</a>].
The following considerations stem from ATM AAL5-SDU Mode Pseudowires
not transporting the ATM cell headers and AAL5 CPCS-PDU Trailer (see
<a href="#section-5.1">Section 5.1</a>):
o An ATMPW operating in AAL5-SDU Mode conveys EFCI and CLP
information using the G and C bits in the ATM-Specific Sublayer.
In consequence, the EFCI and CLP values of individual ATM cells
that constitute the AAL5 frame may be lost across the ATMPW, and
CLP and EFCI transparency may not be maintained. The AAL5-SDU
Mode does not preserve EFCI and CLP values for every ATM cell
within the AAL5 PDU. The processing of these bits on ingress
and egress is defined in <a href="#section-4.1">Section 4.1</a>.
o Only the least significant bit (LSB) from the CPCS-UU (User-to-
User indication) field in the CPCS-PDU Trailer is transported
using the ATM-Specific Sublayer (see <a href="#section-4.1">Section 4.1</a>). This bit
contains the Frame Relay C/R bit when FRF.8.1 Frame Relay / ATM
PVC Service Interworking [<a href="#ref-FRF8.1" title=""Frame Relay / ATM PVC Service Interworking Implementation Agreement (FRF 8.1)"">FRF8.1</a>] is used. The CPCS-UU field is
not used in multiprotocol over AAL5 [<a href="./rfc2684" title=""Multiprotocol Encapsulation over ATM Adaptation Layer 5"">RFC2684</a>]. However,
applications that transfer user to user information using the
CPCS-UU octet would fail to operate.
<span class="grey">Singh, et al. Standards Track [Page 16]</span></pre>
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o The CPI (Common Part Indicator) field in the CPCS-PDU Trailer is
also not transported across the ATMPW. This does not affect
multiprotocol over AAL5 applications since the field is used for
alignment and MUST be coded as 0x00 [<a href="./rfc2684" title=""Multiprotocol Encapsulation over ATM Adaptation Layer 5"">RFC2684</a>].
o The trailing CRC field in the CPCS-PDU is stripped at the
ingress LCCE and not transported over the ATMPW operating in
AAL5-SDU Mode. It is in turn regenerated at the egress LCCE.
Since the CRC has end-to-end significance, this means that
errors introduced in the ATMPW payload during encapsulation or
transit across the packet switched network may not be detected.
To allow for payload integrity checking transparency on ATMPWs
operating in AAL5-SDU Mode using L2TP over IP or L2TP over
UDP/IP, the L2TPv3 session can utilize IPsec as specified in
<a href="./rfc3931#section-4.1.3">Section 4.1.3 of [RFC3931]</a>.
Some additional characteristics of the AAL5-SDU Mode are the
following:
o The status of the ATM PVC is signaled between LCCEs using the
Circuit Status AVP. More granular cause values for the ATM
circuit status and specific ATM alarm types are signaled using
the ATM Alarm Status AVP (see <a href="#section-8.1">Section 8.1</a>). Additionally, loss
of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see <a href="./rfc3931#section-4.4">Section 4.4 in [RFC3931]</a>).
o F5 OAM cells' relative order with respect to user data cells may
not be maintained. F5 OAM cells that arrive during the
reassembly of an AAL5 SDU are sent immediately over the PW and
before the AAL5 SDU payload. At egress, these OAM cells are
sent before the cells that comprise the AAL5-SDU. Therefore,
applications that rely on cell sequence integrity between OAM
and user data cells may not work. This includes Performance
Monitoring and Security OAM cells (see <a href="#section-5.1">Section 5.1</a>). In
addition, the AAL5-SDU service allows for OAM emulation in which
OAM cells are not transported over the ATMPW (see <a href="#section-7">Section 7</a>).
This is advantageous for AAL5-SDU Mode ATMPW implementations
that do not support cell transport using the T-bit.
o Fragmentation and Reassembly procedures MAY be used for managing
mismatched MTUs, as specified in Section 5 of [<a href="#ref-L2TPFRAG" title=""PWE3 Fragmentation and Reassembly"">L2TPFRAG</a>] or in
the underlying PSN (IP, etc.) between tunnel endpoints as
discussed in <a href="./rfc3931#section-4.1.4">Section 4.1.4 of [RFC3931]</a>. Only one of these
methods SHOULD be used for a given AAL5-SDU Mode ATMPW. The
procedures described in [<a href="#ref-L2TPFRAG" title=""PWE3 Fragmentation and Reassembly"">L2TPFRAG</a>] can be used to support the
maximum size of an AAL5 SDU, 2 ^ 16 - 1 (65535) octets.
However, relying on fragmentation on the L2TP/IPv4 packet
between tunnel endpoints limits the maximum size of the AAL5 SDU
<span class="grey">Singh, et al. Standards Track [Page 17]</span></pre>
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that can be transported, because the maximum total length of an
IPv4 datagram is already 65535 octets. In this case, the
maximum AAL5 SDU that can be transported is limited to 65535
minus the encapsulating headers, 24-36 octets for L2TP-over-IPv4
or 36-48 octets for L2TP-over-UDP/IPv4. When the AAL5 payload
is IPv4, an additional option is to fragment IP packets before
tunnel encapsulation with L2TP/IP (see <a href="./rfc3931#section-4.1.4">Section 4.1.4 of
[RFC3931]</a>).
o Sequencing may be enabled on the ATMPW using the ATM-Specific
Sublayer Sequence Number field, to detect lost, duplicate, or
out-of-order frames on a per-session basis (see <a href="#section-4.2">Section 4.2</a>).
o Quality of Service characteristics such as throughput (cell
rates), burst sizes and delay variation can be provided by
leveraging Quality of Service features of the LCCEs and the
underlying PSN, increasing the faithfulness of ATMPWs. This
includes mapping ATM service categories to a compatible PSN
class of service.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. ATM Cell Relay Mode</span>
In this mode, no reassembly takes place at the ingress LCCE. There
are no SAR requirements for LCCEs. Instead, ATM-layer cells are
transported over the ATMPW. Consequently, all AAL types can be
transported over ATMPWs operating in Cell Relay Mode. ATM Cell Relay
Pseudowires can operate in three different modes (see <a href="#section-5.2">Section 5.2</a>):
ATM VCC, ATM VPC, and ATM Port Cell Relay Services. The following
are some of their characteristics:
o The ATM cells transported over Cell Relay Mode ATMPWs consist of
a 4-byte ATM cell header and a 48-byte ATM cell-payload (see
<a href="#section-5.2">Section 5.2</a>). The ATM Service Payload of a Cell Relay Mode
ATMPW is a multiple of 52 bytes. The Header Error Checksum
(HEC) in the ATM cell header containing a Cyclic Redundancy
Check (CRC) calculated over the first 4 bytes of the ATM cell
header is not transported. Accordingly, the HEC field may not
accurately reflect errors on an end-to-end basis; errors or
corruption in the 4-byte ATM cell header introduced in the ATMPW
payload during encapsulation or transit across the PSN may not
be detected. To allow for payload integrity checking
transparency on ATMPWs operating in Cell Relay Mode using L2TP
over IP or L2TP over UDP/IP, the L2TPv3 session can utilize
IPsec as specified in <a href="./rfc3931#section-4.1.3">Section 4.1.3 of [RFC3931]</a>.
<span class="grey">Singh, et al. Standards Track [Page 18]</span></pre>
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o ATM PWs operating in Cell Relay Mode can transport a single ATM
cell or multiple concatenated cells (see <a href="#section-6">Section 6</a>). Cell
concatenation improves the bandwidth efficiency of the ATMPW (by
decreasing the overhead) but introduces latency and delay
variation.
o The status of the ATM PVC is signaled between LCCEs using the
Circuit Status AVP. More granular cause values for the ATM
circuit status and specific ATM alarm types are signaled using
the ATM Alarm Status AVP (see <a href="#section-8.1">Section 8.1</a>). Additionally, loss
of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see <a href="./rfc3931#section-4.4">Section 4.4 in [RFC3931]</a>).
o ATM OAM cells are transported in the same fashion as user cells,
and in the same order as they are received. Therefore,
applications that rely on cell sequence integrity between OAM
and user data cells are not adversely affected. This includes
performance management and security applications that utilize
OAM cells (see <a href="#section-5.3">Section 5.3</a>).
o The maximum number of concatenated cells is limited by the MTU
size of the session (see <a href="#section-5.2">Section 5.2</a> and <a href="#section-6">Section 6</a>). Therefore,
Fragmentation and Reassembly procedures are not used for Cell
Relay ATMPWs. Concatenating cells to then fragment the
resulting packet defeats the purpose of cell concatenation.
Concatenation of cells and fragmentation act as inverse
functions, with additional processing but null net effect, and
should not be used together.
o Sequencing may be enabled on the ATMPW to detect lost,
duplicate, or out-of-order packets on a per-session basis (see
<a href="#section-4.2">Section 4.2</a>).
o Quality of Service characteristics such as throughput (cell
rates), burst sizes, and delay variation can be provided by
leveraging Quality of Service features of the LCCEs and the
underlying PSN, increasing the faithfulness of ATMPWs. This
includes mapping ATM service categories to a compatible PSN
class of service, and mapping CLP and EFCI bits to PSN classes
of service. For example, mapping a Constant Bit Rate (CBR) PVC
to a class of service with tight loss and delay characteristics,
such as an Expedited Forwarding (EF) Per-Hop Behavior (PHB) if
the PSN is an IP DiffServ-enabled domain. The following
characteristics of ATMPWs operating in Cell Relay Mode include
additional QoS considerations:
- ATM Cell transport VCC Pseudowires allow for mapping
multiple ATM VCCs to a single ATMPW. However, a user may
<span class="grey">Singh, et al. Standards Track [Page 19]</span></pre>
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wish to map a single ATM VCC per ATMPW to satisfy QoS
requirements (see <a href="#section-5.2.1">Section 5.2.1</a>).
- Cell Relay ATMPWs allow for concatenating multiple cells in
a single Pseudowire PDU to improve bandwidth efficiency,
but may introduce latency and delay variation.
<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Congestion Control</span>
As explained in [<a href="./rfc3985" title=""Pseudo Wire Emulation Edge-to- Edge (PWE3) Architecture"">RFC3985</a>], the PSN carrying the PW may be subject to
congestion, with congestion characteristics depending on PSN type,
network architecture, configuration, and loading. During congestion
the PSN may exhibit packet loss and packet delay variation (PDV) that
will impact the timing and data integrity of the ATMPW. During
intervals of acute congestion, some Cell Relay ATMPWs may not be able
to maintain service. The inelastic nature of some ATM services
reduces the risk of congestion because the rates will not expand to
consume all available bandwidth, but on the other hand, those ATM
services cannot arbitrarily reduce their load on the network to
eliminate congestion when it occurs.
Whenever possible, Cell Relay ATMPWs should be run over traffic-
engineered PSNs providing bandwidth allocation and admission control
mechanisms. IntServ-enabled domains providing the Guaranteed Service
(GS) or DiffServ-enabled domains using Expedited Forwarding (EF) are
examples of traffic-engineered PSNs. Such PSNs will minimize loss
and delay while providing some degree of isolation of the Cell Relay
ATMPW's effects from neighboring streams.
If the PSN is providing a best-effort service, then the following
best-effort service congestion avoidance considerations apply: Those
ATMPWs that carry constant bit rate (CBR) and variable bit rate-real
time (VBR-rt) services across the PSN will most probably not behave
in a TCP-friendly manner prescribed by [<a href="./rfc2914" title=""Congestion Control Principles"">RFC2914</a>]. In the presence of
services that reduce transmission rate, ATMPWs carrying CBR and VBR-
rt traffic SHOULD be halted when acute congestion is detected, in
order to allow for other traffic or the network infrastructure itself
to continue. ATMPWs carrying unspecified bit rate (UBR) traffic,
which are equivalent to best-effort IP service, need not be halted
during acute congestion and MAY have cells delayed or dropped by the
ingress PE if necessary. ATMPWs carrying variable bit rate-non real
time (VBR-nrt) services may or may not behave in a TCP-friendly
manner, depending on the end user application, but are most likely
safe to continue operating, since the end-user application is
expected to be delay-insensitive and may also be somewhat loss-
insensitive.
<span class="grey">Singh, et al. Standards Track [Page 20]</span></pre>
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LCCEs SHOULD monitor for congestion (for example, by measuring packet
loss or as specified in <a href="./rfc3985#section-6.5">Section 6.5 of [RFC3985]</a>) in order to ensure
that the ATM service may be maintained. When severe congestion is
detected (for example, when enabling sequencing and detecting that
the packet loss is higher than a threshold), the ATM service SHOULD
be terminated by tearing down the L2TP session via a CDN message.
The PW may be restarted by manual intervention, or by automatic means
after an appropriate waiting time.
<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>. Security Considerations</span>
ATM over L2TPv3 is subject to the security considerations defined in
[<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>]. There are no additional considerations specific to
carrying ATM that are not present carrying other data link types.
<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>. IANA Considerations</span>
The signaling mechanisms defined in this document rely upon the
allocation of the following ATM Pseudowire Types (see Pseudowire
Capabilities List as defined in 5.4.3 of [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>] and L2TPv3
Pseudowire Types in 10.6 of [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>]) by the IANA (number space
created as part of publication of [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>]):
Pseudowire Types
----------------
0x0002 ATM AAL5 SDU VCC transport
0x0003 ATM Cell transparent Port Mode
0x0009 ATM Cell transport VCC Mode
0x000A ATM Cell transport VPC Mode
<span class="h3"><a class="selflink" id="section-12.1" href="#section-12.1">12.1</a>. L2-Specific Sublayer Type</span>
This number space is created and maintained per [<a href="./rfc3931" title=""Layer Two Tunneling Protocol - Version 3 (L2TPv3)"">RFC3931</a>].
L2-Specific Sublayer Type
-------------------------
2 - ATM L2-Specific Sublayer present
<span class="h3"><a class="selflink" id="section-12.2" href="#section-12.2">12.2</a>. Control Message Attribute Value Pairs (AVPs)</span>
This number space is managed by IANA as per [<a href="#ref-BCP0068" title=""Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update"">BCP0068</a>].
A summary of the three new AVPs follows:
Control Message Attribute Value Pairs
<span class="grey">Singh, et al. Standards Track [Page 21]</span></pre>
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Attribute
Type Description
--------- ----------------------------------
86 ATM Maximum Concatenated Cells AVP
87 OAM Emulation Required AVP
88 ATM Alarm Status AVP
<span class="h3"><a class="selflink" id="section-12.3" href="#section-12.3">12.3</a>. Result Code AVP Values</span>
This number space is managed by IANA as per [<a href="#ref-BCP0068" title=""Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update"">BCP0068</a>].
A new Result Code value for the CDN message is defined in <a href="#section-7">Section 7</a>.
Following is a summary:
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
General Error Codes
22 - Session not established due to other LCCE
cannot support the OAM Cell Emulation
<span class="h3"><a class="selflink" id="section-12.4" href="#section-12.4">12.4</a>. ATM Alarm Status AVP Values</span>
This is a new registry for IANA to maintain.
New Attribute values for the ATM Alarm Status AVP in the SLI message
are defined in <a href="#section-8.1">Section 8.1</a>. Additional values may be assigned by
Expert Review [<a href="./rfc2434" title="">RFC2434</a>]. Following is a summary:
ATM Alarm Status AVP (Attribute Type 88) Values
-----------------------------------------------
Circuit Status Reason values for the SLI message are as follows:
0 - Reserved
1 - No alarm or alarm cleared (default for Active Status)
2 - Unspecified or unknown Alarm Received (default for
Inactive Status)
3 - ATM Circuit received F1 Alarm on ingress LCCE
4 - ATM Circuit received F2 Alarm on ingress LCCE
5 - ATM Circuit received F3 Alarm on ingress LCCE
6 - ATM Circuit received F4 Alarm on ingress LCCE
7 - ATM Circuit received F5 Alarm on ingress LCCE
8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE
9 - ATM Circuit down due to loop-back timeout on ingress LCCE
<span class="grey">Singh, et al. Standards Track [Page 22]</span></pre>
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The general ATM Alarm failures are encoded as below:
0 - Reserved
1 - No Alarm type specified (default)
2 - Alarm Indication Signal (AIS)
3 - Remote Defect Indicator (RDI)
4 - Loss of Signal (LOS)
5 - Loss of Pointer (LOP)
6 - Loss of Framer (LOF)
7 - Loopback cells (LB)
8 - Continuity Check (CC)
<span class="h3"><a class="selflink" id="section-12.5" href="#section-12.5">12.5</a>. ATM-Specific Sublayer Bits</span>
This is a new registry for IANA to maintain.
The ATM-Specific Sublayer contains 8 bits in the low-order portion of
the header. Reserved bits may be assigned by IETF Consensus
[<a href="./rfc2434" title="">RFC2434</a>].
Bit 0 - Reserved
Bit 1 - S (Sequence) bit
Bit 2 - B (Fragmentation) bit
Bit 3 - E (Fragmentation) bit
Bit 4 - T (Transport type) bit
Bit 5 - G (EFCI) bit
Bit 6 - C (CLP) bit
Bit 7 - U (Command/Response) bit
<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>. Acknowledgements</span>
Thanks for the contributions from Jed Lau, Pony Zhu, Prasad Yaditi,
Durai, and Jaya Kumar.
Many thanks to Srinivas Kotamraju for editorial review.
Thanks to Shoou Yiu and Fred Shu for giving their valuable time to
review this document.
<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>. References</span>
<span class="h3"><a class="selflink" id="section-14.1" href="#section-14.1">14.1</a>. Normative References</span>
[<a id="ref-RFC3931">RFC3931</a>] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", <a href="./rfc3931">RFC 3931</a>, March 2005.
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
<span class="grey">Singh, et al. Standards Track [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
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<span class="h3"><a class="selflink" id="section-14.2" href="#section-14.2">14.2</a>. Informative References</span>
[<a id="ref-PWE3ATM">PWE3ATM</a>] Martini, L., "Encapsulation Methods for Transport of ATM
Over MPLS Networks", Work in Progress, September 2005.
[<a id="ref-L2TPFRAG">L2TPFRAG</a>] Malis, A. and M. Townsley, "PWE3 Fragmentation and
Reassembly", Work in Progress, November 2005.
[<a id="ref-FRF8.1">FRF8.1</a>] "Frame Relay / ATM PVC Service Interworking Implementation
Agreement (FRF 8.1)", Frame Relay Forum 2000.
[<a id="ref-BCP0068">BCP0068</a>] Townsley, W., "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA) Considerations
Update", <a href="https://www.rfc-editor.org/bcp/bcp68">BCP 68</a>, <a href="./rfc3438">RFC 3438</a>, December 2002.
[<a id="ref-RFC2434">RFC2434</a>] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc2434">RFC 2434</a>,
October 1998.
[<a id="ref-I610-1">I610-1</a>] ITU-T Recommendation I.610 (1999): B-ISDN operation and
maintenance principles and functions
[<a id="ref-I610-2">I610-2</a>] ITU-T Recommendation I.610, Corrigendum 1 (2000): B-ISDN
operation and maintenance principles and functions
(corrigendum 1)
[<a id="ref-I610-3">I610-3</a>] ITU-T Recommendation I.610, Amendment 1 (2000): B-ISDN
operation and maintenance principles and functions
(Amendment 1)
[<a id="ref-ATMSEC">ATMSEC</a>] ATM Forum Specification, af-sec-0100.002 (2001): ATM
Security Specification version 1.1
[<a id="ref-RFC2684">RFC2684</a>] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation
over ATM Adaptation Layer 5", <a href="./rfc2684">RFC 2684</a>, September 1999.
[<a id="ref-RFC3985">RFC3985</a>] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", <a href="./rfc3985">RFC 3985</a>, March 2005.
[<a id="ref-RFC2914">RFC2914</a>] Floyd, S., "Congestion Control Principles", <a href="https://www.rfc-editor.org/bcp/bcp41">BCP 41</a>, <a href="./rfc2914">RFC</a>
<a href="./rfc2914">2914</a>, September 2000.
<span class="grey">Singh, et al. Standards Track [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
Authors' Addresses
Sanjeev Singh
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
EMail: sanjeevs@cisco.com
W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
EMail: mark@townsley.net
Carlos Pignataro
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
EMail: cpignata@cisco.com
<span class="grey">Singh, et al. Standards Track [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc4454">RFC 4454</a> ATM over L2TPv3 May 2006</span>
Full Copyright Statement
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Singh, et al. Standards Track [Page 26]
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