Network Working Group Sanjeev Singh
Internet-Draft W. Mark Townsley
Category: Standards Track Carlos Pignataro
Expiration Date: July 2006 Cisco Systems
January 2006
ATM over L2TPv3
draft-ietf-l2tpext-pwe3-atm-04.txt
Status of this Memo
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Abstract
The Layer 2 Tunneling Protocol, Version 3, (L2TPv3) defines an
extensible tunneling protocol to transport layer 2 services over IP
network. 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.
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Contents
Status of this Memo.......................................... 1
1. Introduction.............................................. 3
1.1 Abbreviations......................................... 3
2. Control Connection Establishment.......................... 4
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............................................. 7
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
5.3.1 VPC switching.................................... 13
6. ATM Maximum Concatenated Cells AVP........................ 13
7. OAM Emulation Required AVP................................ 13
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....................................... 19
11. Security Considerations.................................. 20
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............................... 21
12.4 ATM Alarm Status AVP Values.......................... 22
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12.5 ATM-Specific Sublayer bits........................... 22
13. Acknowledgments.......................................... 23
14. References............................................... 23
14.1 Normative References................................. 23
14.2 Informative References............................... 23
15. Authors' Addresses....................................... 25
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].
1. Introduction
This document describes the specifics of how to use the L2TP for 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) message to notify the peer
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 Control
LAC L2TP Access Concentrator (See [RFC3931])
LCCE L2TP control connection endpoint (See [RFC3931])
MSB Most Significant Byte
OAM Operation, Administration, and Management
PE Provider Edge
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PSN Packet Service Network
PWE3 Pseudowire Edge-to-edge emulation
RDI Remote Defect Indicator
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
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 SCCRQ and corresponding 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 via
this value.
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 a
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 avoid wasting L2TP resources.
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The Circuit Status AVP (see Section 8) MUST be present in the ICRQ
and ICRP messages, and MAY be present in the SLI message for ATMPWs.
The following figure shows how L2TP messages are exchanged to setup
an ATMPWs after 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 Cell Concatenation
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 four-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
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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 CDN would have been immediately
returned indicating that the circuit was either not provisioned or is
not available at this LCCE. LCCE A should then exhibit a periodic
retry mechanism. 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 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 which
may be reported by any values identified in the Circuit Status AVP.
The only exception to this are the initial ICRQ, ICRP and CDN
messages which establish and teardown the L2TP session itself when
ATM circuit is created or deleted. The SLI message may be sent from
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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.
Figure 1: General format for ATM encapsulation over L2TPv3 over IP
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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 optional
cookies 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 as, an alternative
to default L2-Specific Sublayer as mentioned in Section 4.6 of
[RFC3931]. Four new flag bits (T,G,C,U) are defined which concur
with Section 8.2 of [PWE3ATM].
Figure 2: ATM-Specific Sublayer Format
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Definition of these four bits are as per Section 8.2 of [PWE3ATM] and
also included here for reference.
* S bit
Definition of this bit is as per Section 4.6 of [RFC3931].
* B and E bits
Definition of these bits as per Section 5.5 of [L2TPFRAG]
These bits are reserved and MUST be set to 0 upon transmission
and ignored upon reception, unless otherwise, these bits are
used as per [L2TPFRAG].
* T (Transport type) bit
Bit (T) of the control word 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).
* G (EFCI) Bit
The ingress LCCE device SHOULD set this bit to 1 if the 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 CLP bit
of any of the incoming ATM cells of the AAL5 payload are set
to 1, or if the CLP bit of the single ATM cell that is to be
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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 control word. The egress LCCE device SHOULD copy the
U bit to the CPCS-UU Least Significant Bit (LSB) of the AAL5
payload.
The Sequence Number fields are described in Section 4.3
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.
ATM Specific Sublayer is prepended to AAL5-SDU. The other Cell-mode
encapsulation consists of the OPTIONAL ATM-Specific Sublayer and 4-
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byte ATM Cell Header and 48-byte ATM Cell-payload.
5.1 ATM AAL5-SDU Mode
In this mode each AAL5 VC is mapped to an L2TP session. Ingress LCCE
reassembles AAL5 CPCS-SDU without AAL5 trailer and any padding bytes.
Incoming EFCI, CLP and C/R (if present) are carried in ATM Specific
sublayer across ATMPWs to 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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If 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 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 cell's
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 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 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 4 byte ATM cell header and 48-byte ATM Cell-payload, HEC
is not included.
In ATM Cell Mode encapsulation, ATM-Specific Sublayer is OPTIONAL.
It can be included, if sequencing support is required. It is left to
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the implementation to choose to signal Default L2-Specific Sublayer
or 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) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 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 egress
LCCE to process them. For more details about ATM Maximum
Concatenated cells, please refer to Section 6.
5.2.1 ATM VCC Cell-Relay Service
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 Section 5.2.
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
satify QOS or restoration requirement.
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 Cell transport VCC mode PW Type of 0x0009.
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5.2.2 ATM VPC Cell-Relay Service
A Virtual Path Connection cell relay service may be provided by
mapping an ATM Virtual Path Connection to single Pseudowire using
cell mode encapsulation as defined in Section 5.2.
An LCCE may map one or more VPCs to a single Pseudowire.
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 Cell transport VPC mode PW Type of 0x000A.
5.2.3 ATM Port Cell-Relay Service
ATM port cell relay service allows an ATM port to be connected to
only another ATM port. All ATM cells that are received at the
ingress ATM port on the LCCE, are encapsulated as per Section 5.2,
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 Section 5.4.4 of [RFC3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM Cell transport Port mode PW Type of 0x0003.
5.3 OAM Cell Support
The OAM cells are defined in [I610-1], [I610-2], [I610-3] and
[ATMSEC] can be categorized as:
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.
5.3.1 VCC switching
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.
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.
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5.3.1 VPC switching
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 [I610-1].
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.
6. ATM Maximum Concatenated Cells AVP
The "ATM Maximum Cells Concatenated AVP", Attribute type 86,
indicates that the egress LCCE node can process a single PDU with
concatenated cells upto 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 Section 5.2 of [RFC3931]).
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 egress LCCE is configured with maximum
number of cells to be concatenated by ingress LCEE, it should signal
to ingress LCCE.
7. OAM Emulation Required AVP
An "OAM Emulation Required AVP", Attribute type 87, MAY be needed to
signal OAM Emulation in AAL5 SDU mode, if LCCE can not support
transport of OAM cells across L2TP session. If OAM Cell Emulation is
configured or detected via some other means on one side, the other
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LCCE MUST support OAM Cell Emulation as well.
This AVP is exchanged during session negotiation (in ICRQ, ICRP) or
during life of the session via SLI control message. If the other LCCE
can not 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 Section 5.2 of [RFC3931]). The Length (before
hiding) of this AVP is 6.
8. ATM defects mapping and status notification
ATM OAM alarms or circuit status is indicated via Circuit Status AVP
as defined in Section 5.4.5 of [RFC3931]. 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 Circuit (1) or an existing ATM Circuit (0).
8.1 ATM Alarm Status AVP
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 SLI message with
Circuit Status AVP.
The Attribute Value field for this AVP has the following format:
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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-octets unsigned integer and Alarm
Type is also a 2-octets 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 Section 5.2 of [RFC3931]).
The Length (before hiding) of this AVP is 10 octets.
This AVP is sent in SLI message to indicate the 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)
9. Applicability Statement
The ATM Pseudowire emulation described in this document allows for
carrying various ATM services across an IP packet switched network
(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.
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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.
9.1 ATM AAL5-SDU Mode
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 bi-directional
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
[RFC2684].
The following considerations stem from ATM AAL5-SDU Mode Pseudowires
not transporting the ATM cell headers and AAL5 CPCS-PDU Trailer (see
Section 5.1):
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 value of individual ATM cells
that consititute 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 value for every ATM cell
within the AAL5 PDU. The processing of these bits on ingress and
egress is defined in Section 4.1.
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 Section 4.1). This bit
contains the Frame Relay C/R bit when FRF.8.1 Frame Relay / ATM
PVC Service Interworking [FRF8.1] is used. The CPCS-UU field is
not used in Multiprotocol Over AAL5 [RFC2684]. However,
applications that transfer user to user information using the
CPCS-UU octet would fail to operate.
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 [RFC2684].
o The trailing CRC field in the CPCS-PDU is stripped at the
ingress LCCE and not transported over the ATMPW operating in
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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
Section 4.1.3 of [RFC3931].
Some additional characteristics of the AAL5-SDU Mode are:
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 Section 8.1). Additionally, loss
of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see Section 4.4 in [RFC3931]).
o F5 OAM cell's 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 Section 5.1). In
addition, the AAL5-SDU service allows for OAM Emulation in which
OAM cells are not transported over the ATMPW (see Section 7).
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, both as
specified in Section 5 of [L2TPFRAG] or in the underlying PSN
(i.e., IP, etc) between tunnel endpoints as discussed in Section
4.1.4 of [RFC3931]. The procedures described in [L2TPFRAG] 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 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 Section 4.1.4 of [RFC3931]).
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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 Section 4.2).
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.
9.2 ATM Cell-Relay Mode
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 Section 5.2):
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
Section 5.2). 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 CRC (Cyclic Redundancy
Check) 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 Section 4.1.3 of [RFC3931].
o ATM PWs operating in Cell-Relay mode can transport a single ATM
cell or multiple concatenated cells (see Section 6). 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 Section 8.1). Additionally, loss
of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see Section 4.4 in [RFC3931]).
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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 Section 5.3).
o The maximum number of concatenated cells is limited by the MTU
size of the session (see Section 5.2 and Section 6). 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
Section 4.2).
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 CBR PVC to a class of
service with tight loss and delay characteristics, such as an EF
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
wish to map a single ATM VCC per ATMPW to satisfy QoS
requirements (see Section 5.2.1).
- Cell-Relay ATMPWs allow for concatenating multiple cells in
a single Pseudowire PDU to improve bandwidth efficiency,
but may introduce latency and delay variation.
10. Congestion Control
As explained in [RFC3985], 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 PDV that will impact the timing
and data integrity of the ATMPW. During intervals of acute
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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 its 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 VBR-rt (Variable Bit
Rate-real time) services across the PSN will most probably not behave
in a TCP-friendly manner prescribed by [RFC2914]. 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 UBR (Unspecified Bit Rate) 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 VBR-nrt (Variable Bit
Rate-non real time) 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.
LCCEs SHOULD monitor for congestion (for example by measuring packet
loss or as specified in Section 6.5 of [RFC3985]) 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.
11. Security Considerations
ATM over L2TPv3 is subject to the security considerations defined in
[RFC3931]. There are no additional considerations specific to
carrying ATM that are not present carrying other data link types.
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12. IANA Considerations
The signaling mechanisms defined in this document rely upon the
allocation of following ATM Pseudowire Types (see Pseudowire
Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3
Pseudowire Types in 10.6 of [RFC3931]) by the IANA (number space
created as part of publication of [RFC3931]):
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
12.1 L2-Specific Sublayer Type
This number space is created and maintained per [RFC3931].
L2-Specific Sublayer Type
-------------------------
2 - ATM L2-Specific Sublayer present
12.2 Control Message Attribute Value Pairs (AVPs)
This number space is managed by IANA as per [BCP0068].
A summary of the three new AVPs follows:
Control Message Attribute Value Pairs
Attribute
Type Description
--------- ----------------------------------
86 ATM Maximum Concatenated Cells AVP
87 OAM Emulation Required AVP
88 ATM Alarm Status AVP
12.3 Result Code AVP Values
This number space is managed by IANA as per [BCP0068].
New Result Code value for the CDN message is defined in Section 7.
Following is a summary:
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Result Code AVP (Attribute Type 1) Values
-----------------------------------------
General Error Codes
22 - Session not established due to other LCCE
can not support the OAM Cell Emulation,
12.4 ATM Alarm Status AVP Values
This is a new registry for IANA to maintain.
New Attribute values for the SLI message is defined in Section 8.
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
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)
12.5 ATM-Specific Sublayer bits
This is a new registry for IANA to maintain.
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The ATM-Specific Sublayer contains 8 bits in the low-order portion of
the header. Reserved bits may be assigned by IETF Consensus
[RFC2434].
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
13. Acknowledgments
Thanks for the contribution 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 their valuable time to review
this document.
14. References
14.1 Normative References
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
14.2 Informative References
[PWE3ATM] Martini, L., "Encapsulation Methods for Transport of ATM
Over MPLS Networks", draft-ietf-pwe3-atm-encap-10 (work in
progress), September 2005.
[L2TPFRAG] Malis, A. and M. Townsley, "PWE3 Fragmentation and
Reassembly", draft-ietf-pwe3-fragmentation-10 (work in
progress), November 2005.
[FRF8.1] "Frame Relay / ATM PVC Service Interworking
Implementation Agreement (FRF 8.1)", Frame Relay
Forum 2000.
Sanjeev, et al. Standards Track [Page 23]
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[BCP0068] Townsley, W., "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA) Considerations
Update", BCP 68, RFC 3438, December 2002.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[I610-1] ITU-T Recommendation I.610 (1999): B-ISDN operation and
maintenance principles and functions
[I610-2] ITU-T Recommendation I.610, Corrigendum 1 (2000):
B-ISDN operation and maintenance principles and
functions (corrigendum 1)
[I610-3] ITU-T Recommendation I.610, Amendment 1 (2000): B-ISDN
operation and maintenance principles and functions
(Amendment 1)
[ATMSEC] ATM Forum Specification, af-sec-0100.002 (2001): ATM
Security Specification version 1.1
[RFC2684] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation
over ATM Adaptation Layer 5", RFC 2684, September 1999.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.
Sanjeev, et al. Standards Track [Page 24]
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15. Authors' Addresses
Sanjeev Singh
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
sanjeevs@cisco.com
W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
mark@townsley.net
Carlos Pignataro
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
cpignata@cisco.com
Sanjeev, et al. Standards Track [Page 25]
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Sanjeev, et al. Standards Track [Page 26]
