Request for Comments: 4454 M. Townsley
Category: Standards Track C. Pignataro
Cisco Systems
May 2006
Asynchronous Transfer Mode (ATM) over
Layer 2 Tunneling Protocol Version 3 (L2TPv3)
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
1. Introduction ....................................................2
1.1. Abbreviations ..............................................3
1.2. Specification of Requirements ..............................3
2. Control Connection Establishment ................................3
3. Session Establishment and ATM Circuit Status Notification .......4
3.1. L2TPv3 Session Establishment ...............................4
3.2. L2TPv3 Session Teardown ....................................6
3.3. L2TPv3 Session Maintenance .................................6
4. Encapsulation ...................................................6
4.1. ATM-Specific Sublayer ......................................7
4.2. Sequencing .................................................9
5. ATM Transport ...................................................9
5.1. ATM AAL5-SDU Mode .........................................10
5.2. ATM Cell Mode .............................................10
5.2.1. ATM VCC Cell Relay Service .........................11
5.2.2. ATM VPC Cell Relay Service .........................12
5.2.3. ATM Port Cell Relay Service ........................12
5.3. OAM Cell Support ..........................................12
5.3.1. VCC Switching ......................................12
5.3.2. VPC Switching ......................................13
6. ATM Maximum Concatenated Cells AVP .............................13
7. OAM Emulation Required AVP .....................................14
8. ATM Defects Mapping and Status Notification ....................14
8.1. ATM Alarm Status AVP ......................................14
9. Applicability Statement ........................................15
9.1. ATM AAL5-SDU Mode .........................................16
9.2. ATM Cell Relay Mode .......................................18
10. Congestion Control ............................................20
11. Security Considerations .......................................21
12. IANA Considerations ...........................................21
12.1. L2-Specific Sublayer Type ................................21
12.2. Control Message Attribute Value Pairs (AVPs) .............21
12.3. Result Code AVP Values ...................................22
12.4. ATM Alarm Status AVP Values ..............................22
12.5. ATM-Specific Sublayer Bits ...............................23
13. Acknowledgements ..............................................23
14. References ....................................................23
14.1. Normative References .....................................23
14.2. Informative References ...................................24
1. Introduction
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 [RFC3931].
1.1. Abbreviations
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 [RFC3931])
LCCE L2TP Control Connection Endpoint (see [RFC3931])
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
1.2. Specification of Requirements
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 [RFC2119].
2. Control Connection Establishment
To emulate ATM Pseudowires using L2TP, an L2TP Control Connection as
described in Section 3.3 of [RFC3931] MUST be established.
The Start-Control-Connection-Request (SCCRQ) and corresponding
Start-Control-Connection-Reply (SCCRP) MUST include the supported ATM
Pseudowire types (see Section 3.1), in the Pseudowire Capabilities
List as defined in Section 5.4.3 of [RFC3931]. This identifies the
Control Connection as able to establish L2TP sessions in support of
the ATM Pseudowires.
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 [RFC3931], though the
unstructured Hostname AVP [RFC3931] MAY be used to identify LCCEs as
well.
3. Session Establishment and ATM Circuit Status Notification
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.
3.1. L2TPv3 Session Establishment
ATM circuit (e.g., an AAL5 PVC) creation triggers establishment of an
L2TP session using three-way handshake described in Section 3.4.1 of
[RFC3931]. 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 Section 8) 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
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 [RFC3931].
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 Section 6.
b. Remote End ID: Each PW is associated with a Remote End ID akin to
the VC-ID in [PWE3ATM]. Two LCCEs of a PW would have the same
Remote End ID, and its format is described in Section 5.4.4 of
[RFC3931].
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 [RFC3931] 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
Section 8).
The ICCN is the final stage in the session establishment. It
confirms the receipt of the ICRP with acceptable parameters to allow
bidirectional traffic.
3.2. L2TPv3 Session Teardown
When an ATM circuit is unprovisioned (deleted) at either LCCE, the
associated L2TP session MUST be torn down via the CDN message defined
in Section 3.4.3 of [RFC3931].
3.3. L2TPv3 Session Maintenance
All sessions established by a given Control Connection utilize the
L2TP Hello facility defined in Section 4.4 of [RFC3931] 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 [RFC3931] 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 Section
8.
4. Encapsulation
This section describes the general encapsulation format for ATM
services over L2TP.
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 Section 4.1 for more details.
4.1. ATM-Specific Sublayer
This section defines a new ATM-Specific Sublayer, an alternative to
the Default L2-Specific Sublayer as mentioned in Section 4.6 of
[RFC3931]. Four new flag bits (T, G, C, and U) are defined that
concur with Section 8.2 of [PWE3ATM].
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 Section 4.6 of [RFC3931].
* B and E bits
Definitions of these bits are as per Section 5.5 of [L2TPFRAG].
If these bits are not used as per [L2TPFRAG], 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 Section 5.2.
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 Section 9.1).
* 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.
* U (Command/Response) Bit
When FRF.8.1 Frame Relay / ATM PVC Service Interworking (see
[FRF8.1]) 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
Section 4.2.
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 Section 4.6
of [RFC3931]) is used.
2 - The ATM-Specific Sublayer is used.
The first two values are already defined in the L2TPv3 base
specification [RFC3931].
4.2. Sequencing
Data Packet Sequencing MAY be enabled for ATMPWs. The sequencing
mechanisms described in [RFC3931] 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.
5. ATM Transport
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.
5.1. ATM AAL5-SDU Mode
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
Section 4.1.
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 [L2TPFRAG] 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 [I610-1],
[I610-2], [I610-3] and security OAM cells as specified in [ATMSEC],
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 Section 5.4.4 of [RFC3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM AAL5 SDU VCC transport PW Type of 0x0002.
5.2. ATM Cell Mode
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.
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|