Transport Working Group Ghassem Koleyni
Khalid Ahmad
Mina Azad
Internet Draft Nortel Networks
Expiration Date: November 2002 John Fischer
Matthew Bocci
Mustapha Aissaoui
Alcatel
May 2002
Applicability Statement for ATM Cell Encapsulation over PSN (the basic
mode)
< draft-koleyni-pwe3-app-cell-over-psn-01.txt >
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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1. Abstract
This draft provides an applicability statement for the basic cell
mode encapsulation in draft-fischer [18].
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Draft-fischer describes methods to carry ATM services over IP, L2TP
or MPLS. The PSN (e.g., MPLS) is used to transport ATM layer
services such as those defined by ITU-T as ATM transfer capabilities
[17] and ATM Forum as ATM service categories [15]. The basic
requirement is to transparently transport the ATM VCC or VPC service
related information (e.g., traffic parameters, QoS, OAM, etc.) over
the Pseudo Wire (PW), over the packet network.
Table of contents
1. Abstract 1
2 Conventions used in this document 2
3 Introduction 3
4 Terminology and abbreviations 3
5 Applicability Statement 4
5.1 Applicability 4
5.2. Implementation and deployment considerations 6
5.3. Limitations 6
6 ATM Service Encapsulation 6
6.1 Length and Sequence Number 7
6.1.1 Setting the length field 8
6.1.2 Processing the length field 8
6.1.3 Setting the sequence number 8
6.1.4 Processing the sequence number 8
7 ATM VCC Services 9
7.1 ATM VCC Cell Transport Service 9
7.1.1 ATM OAM Cell Support 11
8 ATM VPC Services 12
8.1 ATM VPC Cell Transport Services 12
8.1.1 OAM Cell Support 13
9 ILMI support 14
10 QoS considerations14
11 ATM Pseudo-Wire over MPLS specific requirements 16
11.1 MPLS Transport Label 17
11.2 MPLS Pseudo Wire Label 17
12 ATM Pseudo-Wire over L2TP specific requirements 17
12.1 L2TP Session ID 18
12.2 Cookie 18
13 ATM Pseudo-Wire over IP specific requirements 19
13.1 C, K, and S bits19
13.2 Protocol Type field 20
13.3 Key Field 20
13.4 GRE Sequence Number Field 20
14 Security Considerations 20
15 Intellectual Property Disclaimer 20
16 References20
17 Acknowledgments 21
18 Authors' Addresses21
2 Conventions used in this document
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
3 Introduction
This draft provides an applicability statement for the basic cell
mode encapsulation in draft-fischer [18].
Draft-fischer describes methods to carry ATM services over a IP,
L2TP or MPLS based PSN. The PSN is used to transport ATM layer
services such as those defined by ITU-T as ATM transfer capabilities
[17] and by the ATM Forum as ATM service categories [15]. The basic
requirement is to transparently transport the ATM VCC or VPC service
related information (e.g., traffic parameters, QoS, OAM, etc.) over
the Pseudo Wire (PW), over the packet network.
Section 5 of this draft presents the applicability statement.
Sections 6 to 13 are taken from draft-fischer [18]. They provide the
details of the encapsulation methods as well as the OAM, ILMI, and
QoS procedures for the basic cell mode.
4 Terminology and abbreviations
Packet Switched Network - A Packet Switched Network (PSN) is a
network using IP, MPLS or L2TP as the unit of switching.
Pseudo Wire Emulation Edge to Edge - Pseudo Wire Emulation Edge to
Edge (PWE3) is a mechanism that emulates the essential attributes of
a service (such as a T1 leased line or Frame Relay) over a PSN.
Customer Edge - A Customer Edge (CE) is a device where one end of an
emulated service originates and terminates. The CE is not aware that
it is using an emulated service rather than a "real" service.
Provider Edge - A Provider Edge (PE) is a device that provides PWE3
to a CE.
Pseudo Wire - A Pseudo Wire (PW) is a connection between two PEs
carried over a PSN. The PE provides the adaptation between the CE
and the PW.
Pseudo Wire PDU - A Pseudo Wire PDU is a PDU sent on the PW that
contains all of the data and control information necessary to provide
the desired service.
PSN Tunnel - A PSN Tunnel is a tunnel inside which multiple PWs can
be nested so that they are transparent to core network devices.
Ingress - The point where the ATM service is encapsulated into a
Pseudo Wire PDU (ATM to PSN direction.)
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Egress - The point where the ATM service is de-capsulated from a
Pseudo Wire PDU (PSN to ATM direction.)
CTD Cell Transfer Delay
MTU Maximum Transfer Unit
OAM Operations, Administration, and Maintenance.
PVC Permanent Virtual Connection. An ATM connection that is
provisioning via a network management interface. The
connection is not signalled.
VCC Virtual Circuit Connection. An ATM connection that is
switched based on the cell header's VCI.
VPC Virtual Path Connection. An ATM connection that is switched
based on the cell header's VPI.
5 Applicability Statement
5.1 Applicability
The primary application of ATM cell encapsulation over PSN is the
transparent carriage of ATM layer services over a PSN. An ATM layer
service is the transfer of ATM cells over a VCC or a VPC between
communicating upper layer entities. The nature of the service, as
defined by the ATM service category [15] or ATM transfer capability
[17], should be preserved. To provide this, the basic requirement of
the ATM-PSN interworking function is to map the ATM cells belonging
to either VCC or VPC, together with any related OAM and protocol
control information into a PW.
Two network applications that utilize the cell mode encapsulation
described in draft-fischer [18] are:
a. The transport of multiservice ATM over a packet core network.
Many service providers have multiple service networks and the
Operational Support System capabilities needed to support these
existing service offerings. Packet Switched Networks (PSNs)
have the potential to reduce the complexity of a service
provider's infrastructure by allowing virtually any existing
digital service to be supported over a single networking
infrastructure.
The benefits of this model to a service provider are threefold:
i. Leveraging of the existing systems and services to
provide increased capacity from a packet switched core.
ii. Preserving existing network operational processes and
procedures used to maintain the legacy services.
iii. Using the common packet switched network infrastructure
to support both the core capacity requirements of
existing services and the requirements of new services
supported natively over the packet switched network.
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b. L2 VPN service over a PSN infrastructure. In this case, VPN
sites are connected through ATM VCCs or VPCs, as in today's L2
VPNs. The basic cell encapsulation allows the VPN service
provider to transparently extend this L2 connectivity over its
PSN while still providing the contracted SLS with the VPN
customer. The advantage is for the service provider to combine
L2 and L3 services over the same PSN.
Figure 1 shows the reference model for carrying ATM services over a
PSN. This model is adapted from [6].
|<------- Pseudo Wire ------>|
| |
| |<-- PSN Tunnel -->| |
V V V V
ATM Service+----+ +----+ ATM Service
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ | | |==================| | | +-----+
^ | +----+ +----+ | ^
| | Provider Provider | |
| | Edge 1 Edge 2 | |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 1: ATM-over-PSN Service Reference Model
An ATM VCC or VPC is carried over a PW. The PW corresponding to any
VCC or VPC may be further tunneled in a transport PSN tunnel to
achieve multiplexing gain and bandwidth efficiency.
When the QoS considerations in Section 10 are respected, this ATM
over PSN service provides end users with the same quality of service
on any given VPC or VCC as per the QoS commitments in the ATM
service traffic contract.
One important consideration to make when interworking is to allow
OAM information to be treated as in the original network. The
interworking function allows this transparency while performing cell
encapsulation.
Resource management cells are used extensively in certain service
categories like ABR. Encapsulating ATM cells to PW allows this
capability to be kept.
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Cell Loss priority (CLP) is used to provide discard information and
Payload Type Indicator (PTI) provides information regarding the
payload being transported. Information on both of these is obtained
from the ATM cell header. CLP and PTI are both part of the service
specific information fields.
Concatenation of ATM cells belonging to a VCC or a VPC provides
added bandwidth efficiency while preserving the specific information
(CLP/PTI) of each cell.
5.2. Implementation and deployment considerations
Although the Single ATM cell encapsulation provides the simplest way
for encapsulating ATM cells within a single MPLS packet, it lacks
bandwidth efficiency. This can be improved substantially by the use
of the procedures enabling cells from any given VCC or VPC to be
concatenated within the corresponding ATM PW.
5.3. Limitations
Cell encapsulation only supports point-to-point LSPs. Multi- point-
to-point and point-to-multi-point are for further study (FFS).
When PSN is MPLS network, to have bi-directional connectivity, as
required in ATM, two LSPs should be configured, one for each
direction (ATM-to-MPLS and MPLS-to-ATM) of the ATM connection.
The number of concatenated ATM cells is limited by the MTU (Maximum
Transfer Unit) size and the cell transfer delay (CTD) and cell delay
variation (CDV) objectives.
6 ATM Service Encapsulation
This section describes the general encapsulation format for ATM over
PSN pseudo wires, such as IP, L2TP, or MPLS. The specifics pertaining
to each packet technology are covered in later sections. All
encapsulation formats and procedures contained in the following
sections are from draft-fischer [18].
Figure 2 provides a general format for encapsulation of ATM cells
into packets.
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 (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
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| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: General format for ATM encapsulation over PSNs
The PSN Transport Header depends on the packet technology: IP, L2TP
or MPLS. This header is used to transport the encapsulated ATM
information through the packet switched core. This header is always
present if the Pseudo Wire is MPLS.
The Pseudo Wire Header depends on the packet technology: IP, L2TP or
MPLS. It identifies a particular ATM service within the PSN tunnel.
The Length and Sequence Number is inserted after the Pseudo Wire
Header. This field is optional.
The ATM Specific Header is inserted before the ATM service payload.
The ATM Specific Header contains control bits needed to carry the
service. These are defined in the ATM service descriptions below.
The length of ATM specific header may not always be one octet. It
depends on the service type.
The ATM payload octet group is the payload of the service that is
being encapsulated.
6.1 Length and Sequence Number
The length and sequence number are not required for all services.
Length and sequence number are to satisfy these requirements.
- Sequentiality may need to be preserved.
- Small packets may need to be padded in order to be transmitted on a
medium where the minimum transport unit is larger than the actual
packet size.
The one-octet Length indicates length of the packet payload that
includes, length of the length field, Sequence number length, the ATM
specific header length and the payload length (i.e., Pseudo Wire
PDU). The Length field is set to 0 by the ingress PE if not used and
is ignored by the egress PE.
If the Pseudo Wire traverses a network link that requires a minimum
frame size such as Ethernet as a practical example, with a minimum
frame size of 64 octets, then such links will apply padding to the
Pseudo Wire PDU to reach its minimum frame size. In this case the
length field MUST be set to the PDU length. A mechanism is required
for the egress PE to detect and remove such padding.
The Sequence Number is a 2-octet field that may be used to track
packet order delivery. This field is set to 0 by the ingress PE if
not used and is ignored by the egress PE. The sequence number space
is a 16-bit, unsigned circular space. Processing of the sequence
number field is OPTIONAL.
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In all cases the egress PE MUST be aware of whether the ingress PE
will send the length and sequence number over a specific Pseudo Wire.
This may be achieved using static configuration or using Pseudo Wire
specific signaling.
Length field is not required for the cell mode.
6.1.1 Setting the length field
The length field is required to enable the egress PE to remove any
padding that may have resulted if the pseudo-wire traversed a network
link that enforces a minimum frame size (e.g. Ethernet). Ethernet
applies padding to frames that are smaller than 64 bytes, where this
includes a minimum of 18 bytes for the Ethernet header and trailer.
The length field represents the size of the packet in bytes including
the length, sequence number, ATM specific header and ATM service
payload. If the size of the packet is larger than can be
represented by the length field, the field MUST be set to 0. In
addition, the length field MAY be set to 0 if the size of the packet
prevents any padding from being applied.
The AAL5 SDU Frame service is the only service that can generate
packets smaller than the Ethernet minimum and MUST set the length
field accordingly. The length field MUST either be set to the size
of the packet if the size is less than 46 bytes or to 0.
All cell transport services MUST always set the length field to 0 to
indicate to the remote PE that no padding was applied.
6.1.2 Processing the length field
Since length field is not used for cell mode, no processing is
required.
6.1.3 Setting the sequence number
The following procedures SHOULD be used by the ingress PE if
sequencing is desired for a given ATM service:
- the initial PDU transmitted on the Pseudo Wire MUST use sequence
number 1
- the PE MUST increment the sequence number by one for each
subsequent PDU
- when the transmit sequence number reaches the maximum 16 bit value
(65535) the sequence number MUST wrap to 1
If the ingress PE does not support sequence number processing, then
the sequence number field in the control word MUST be set to 0.
6.1.4 Processing the sequence number
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If the egress PE supports receive sequence number processing, then
the following procedures SHOULD be used:
When an ATM service is initially created, the "expected sequence
number" associated with it MUST be initialized to 1.
When a PDU is received on the Pseudo Wire associated with the ATM
service, the sequence number SHOULD be processed as follows:
- if the sequence number on the packet is 0, then the PDU passes the
sequence number check
- otherwise if the PDU sequence number >= the expected sequence
number and the PDU sequence number - the expected sequence number <
32768, then the PDU is in order.
- otherwise if the PDU sequence number < the expected sequence
number and the expected sequence number - the PDU sequence number >=
32768, then the PDU is in order.
- otherwise the PDU is out of order.
If a PDU passes the sequence number check, or is in order then, it
can be delivered immediately. If the PDU is in order, then the
expected sequence number SHOULD be set using the algorithm:
expected_sequence_number := PDU_sequence_number + 1 mod 2**16
if (expected_sequence_number = 0)
then expected_sequence_number := 1;
Pseudo Wire PDUs that are received out of order MAY be dropped or
reordered at the discretion of the egress PE.
If the egress PE does not support receive sequence number processing,
then the sequence number field MAY be ignored.
7 ATM VCC Services
This section defines ATM cell VCC services that may be supported over
the PSN.
7.1 ATM VCC Cell Transport Service
The VCC cell transport service is characterized by the mapping of a
single ATM VCC (VPI/VCI) to a Pseudo Wire. This service is fully
transparent to the ATM Adaptation Layer. The VCC single cell
transport service is MANDATORY.
This service MUST use the following encapsulation 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload ( 48 bytes ) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Single ATM VCC Cell Encapsulation
* M (transport mode) bit
Bit (M) of the control byte indicates whether the packet contains an
ATM cell or a frame payload. If set to 0, the packet contains an ATM
cell. If set to 1, the PDU contains an AAL5 payload.
* V (VCI present) bit
Bit (V) of the control byte indicates whether the VCI field is
present in the packet. If set to 1, the VCI field is present for the
cell. If set to 0, no VCI field is present. In the case of a VCC,
the VCI field is not required. For VPC, the VCI field is required
and is transmitted with each cell.
* Reserved bits
The reserved bits should be set to 0 at the transmitter and ignored
upon reception.
* PTI Bits
The 3-bit Payload Type Identifier (PTI) incorporates ATM Layer PTI
coding of the cell. These bits are set to the value of the PTI of
the encapsulated ATM cell.
* C (CLP) Bit
The Cell Loss Priority (CLP) field indicates CLP value of the
encapsulated cell.
For increased transport efficiency, the ingress PE SHOULD be able to
encapsulate multiple ATM cells into a Pseudo Wire PDU. The ingress
and egress PE SHOULD agree to a maximum number of cells in a single
Pseudo Wire PDU. This agreement may be accomplished via a Pseudo
Wire specific signaling mechanism or via static configuration.
When multiple cells are encapsulated in the same PSN packet, the ATM
control byte MUST be repeated for each cell. This means that 49
bytes are used to encapsulate each 53 byte ATM cell.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload ( 48 bytes ) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|V|Res| PTI |C| |
+-+-+-+-+-+-+-+-+ |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 4: Multiple ATM VCC Cell Encapsulation
7.1.1 ATM OAM Cell Support
When configured for a VCC cell relay service, both PE's SHOULD act as
a VC switch in accordance with the OAM procedures defined in [5].
The PEs MUST be able to pass the following OAM cells transparently:
- F5 AIS (segment and end-to-end)
- F5 RDI (segment and end-to-end)
- F5 loopback (segment and end-to-end)
- Resource Management
- Performance Management
- Continuity Check
- Security
The PEs SHALL use the ATM VCC cell mode encapsulation (Section 6.1)
when passing an OAM cell. The OAM cell MAY be encapsulated together
with other user data cells if multiple cell encapsulation is used.
The ingress PE SHOULD be able to generate an F5 AIS upon reception of
a corresponding F4 AIS or lower layer defect (such as LOS).
The egress PE SHOULD be able to generate an F5 AIS based on a PSN
failure (such as a PSN tunnel failure or LOS on the PSN port).
If the ingress PE cannot support the generation of OAM cells, it MAY
notify the egress PE using a Pseudo Wire specific maintenance
mechanism (to be defined). For example, the ingress PE MAY withdraw
the Pseudo Wire (VC label) associated with the service. Upon
receiving such a notification, the egress PE SHOULD generate the
appropriate F5 AIS.
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8 ATM VPC Services
The VPC service is defined by mapping a single VPC (VPI) to a Pseudo
Wire. As such it emulates as Virtual Path cross-connect across the
PSN. All VCCs belonging to the VPC are carried transparently by the
VPC service.
The egress PE may choose to apply a different VPI other than the one
that arrived at the ingress PE. The egress PE MUST choose the
outgoing VPI based solely upon the Pseudo Wire header. As a VPC
service, the egress PE MUST NOT change the VCI field.
8.1 ATM VPC Cell Transport Services
The ATM VPC cell transport service is OPTIONAL.
This service MUST use the following cell mode encapsulation:
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 (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Single Cell VPC Encapsulation
The ATM control byte contains the same information as in the VCC
encapsulation except for the VCI field.
* VCI Bits
The 16-bit Virtual Circuit Identifier (VCI) incorporates ATM Layer
VCI value of the cell.
For increased transport efficiency, the ingress PE SHOULD be able to
encapsulate multiple ATM cells into a Pseudo Wire PDU. The ingress
and egress PE SHOULD agree to a maximum number of cells in a single
Pseudo Wire PDU. This agreement may be accomplished via a Pseudo
Wire specific signaling mechanism or via static configuration.
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When multiple ATM cells are encapsulated in the same PSN packet, the
ATM control byte MUST be repeated for each cell. This means that 51
bytes are used to encapsulate each 53 byte ATM cell.
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 (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| ATM Cell Payload (48 bytes) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |M|V|Res| PTI |C| VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+ |
| ATM Cell Payload (48 bytes) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 6: Multiple Cell VPC Encapsulation
8.1.1 OAM Cell Support
When configured for a VPC cell relay service, both PE's SHOULD act as
a VP cross-connect in accordance with the OAM procedures defined in
[5].
The PEs MUST be able to pass the following OAM cells transparently:
- F4 AIS (segment and end-to-end)
- F4 RDI (segment and end-to-end)
- F4 loopback (segment and end-to-end)
- F5 AIS (segment and end-to-end)
- F5 RDI (segment and end-to-end)
- F5 loopback (segment and end-to-end)
- Resource Management
- Performance Management
- Continuity Check
- Security
The PEs SHALL use the ATM VPC cell encapsulation (Section 7.1) when
passing an OAM cell. The OAM cell MAY be encapsulated together with
other user data cells if multiple cell encapsulation is used.
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The ingress PE MUST be able to generate an F4 AIS upon reception of a
lower layer defect (such as LOS).
The egress PE SHOULD be able to generate an F4 AIS based on a PSN
failure (such as a PSN tunnel failure or LOS on the PSN port).
If the ingress PE cannot support the generation of OAM cells, it MAY
notify the egress PE using a Pseudo Wire specific maintenance
mechanism to be defined. For example, the ingress PE MAY withdraw
the Pseudo Wire (VC label) associated with the service. Upon
receiving such a notification, the egress PE SHOULD generate the
appropriate F4 AIS.
9 ILMI support
Integrated Local Management Interface (ILMI) typically is used in ATM
networks for neighbor resource availability detection, address
registration, auto-configuration, and loss of connectivity detection.
ILMI messages are sent as SNMP PDU's within ATM AAL5 cells.
A PE MAY provide an ATM ILMI to its attached CE. If the ingress PE
receives an ILMI message indicating that the ATM service (VCC or VPC)
is down, it MAY use a Pseudo Wire specific mechanism to notify the
egress PE of the ATM service status. For example, a PE using an MPLS
based Pseudo Wire may withdraw its advertised VC label.
When receiving such a notification, the egress PE MAY use ILMI to
signal the ATM service status to its attached CE.
10 QoS considerations
This section provides guidelines for the provision of QoS for the
individual ATM PWs over the PSN. The objective is to provide the
ability to support the traffic contracts and the QoS commitments made
to the ATM connections [8]. This section is informational and the
provided guidelines SHOULD be treated as good practices and the
mappings as examples only.
When ATM PW service is configured over a PSN, each ATM service
category SHOULD be mapped to a compatible class of service in the PSN
network. A compatible class of service maintains the integrity of
the service end to end. For example, the CBR service category SHOULD
be mapped to a class of service with stringent loss and delay
objectives. If the PSN implements the IP Diff-Serv framework to
provide QoS classes, a class of service based on the EF PHB is a good
candidate.
Furthermore, ATM service categories have support for multiple
conformance definitions. A conformance definition specifies the
conformance of cells of a connection at an interface, e.g., public
UNI, in relation to the conformance algorithm and corresponding
parameters specified in the connection traffic descriptor [15]. For
example, the conformance definition specifies if the requested QoS
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parameters, e.g., CLR, apply to the aggregate CLP0+1 conforming cell
flow or to the CLP0 conforming flow only. Thus, the conformance
definition SHOULD be respected in the selected PSN class of service.
For example, a connection CLP1 cell flow in a VBR.3 conformance
definition is treated as excess traffic in the ATM network and thus
has no QoS guarantees associated with it. This flow SHOULD be
provided a treatment no better than the treatment of the CLP0 cell
flow in the PSN. This does not mean however that the PSN network
should mirror the various conformance definitions of the ATM service
categories.
In the remainder of this section, it is assumed that the PSN
implements IP Diff-Serv framework to provide QoS.
All ATM traffic management functions specified in [15] are applicable
for the ATM part of the ATM PW in the ingress PE and egress PE. In
the ATM-to-PSN direction, each ATM connection MAY be policed and/or
shaped to conform to its traffic descriptor in the ATM endpoint of
the ATM PW in the PE. Whenever allowed by the conformance
definition, a non-conforming CLP0 cell may be turned into a CLP1 cell
and becomes conforming. Connection admission SHOULD be applied to
make sure sufficient resources are available to carry the ATM PW over
the transport LSP. The mapping of ATM service category and
conformance definition to the Diff-Serv PHB determines the outgoing
PHB. This is the PHB to be applied to the cells or packets of the
ATM PW in the ingress PE and egress PE and inside the PSN. The PSN
transport header of the outgoing PSN packet SHOULD be marked to
identify the selected PHB. This consists of marking the DS field in
the IP header in the case of IP PSN, or the EXP field in the
transport shim header in the case of a MPLS PSN.
Figure 9 provides an example of mapping ATM service category and
conformance definition to a Diff-Serv PHB.
ATM Service Conformance CLP Diff-Serv DSCP
Category Definition Setting PHB Marking
----------------------------------------------------------------
CBR CBR.1 0/1 EF 101110
rt-VBR VBR.1 0/1 EF 101110
VBR.2/VBR.3 0 AF11 001010
1 AF12 001100
nrt-VBR VBR.1 0/1 AF21 010010
VBR.2/VBR.3 0 AF21 010010
1 AF22 010100
ABR ABR 0 AF31 011010
UBR+MDCR UBR.1/UBR.2 0/1 AF31 011010
GFR GFR.1/GFR.2 0 AF31 011010
1 AF32 011100
UBR UBR.1/UBR.2 0/1 DF 000000
Figure 9: Example of ATM Service Category to PHB Mapping
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Note that an alternative mapping may not distinguish between the
conformance definitions in a given ATM service category. In this
case, the CLP0 and CLP1 flows of a ATM connection are provided with
the same QoS in the PSN. As an example, all conformance definitions
of the nrt-VBR service category MAY be mapped to the AF21 PHB in
Figure 9.
When the PSN is MPLS based, the selected PHB for the ATM PW is
conveyed in different ways depending if the transport LSP is an L-
LSP or an E-LSP [16]. In the case of an L-LSP, the PHB Scheduling
Class is signaled at the transport LSP establishment and is therefore
inferred from the transport label value. The Drop Precedence of the
individual PW packets is conveyed in the EXP field of the transport
LSP shim header. In the case of an E-LSP, the PHB is conveyed in the
EXP field of the transport LSP shim header. The actual values to be
marked in the EXP field to reflect the example in Figure 9 is outside
the scope of this document.
In the presence of congestion, the PE MAY mark the EFCI bit and MAY
perform selective cell discard based on CLP setting, if allowed by
the conformance definition, and in accordance with [15]. The method
used to transfer the CLP and EFCI information of the individual cells
into the ATM specific field of the PW packet is described in details
in sections 7 and 8.
In the PSN-to-ATM direction, the ATM service category and conformance
definition is obtained from the context accessed via the pseudo wire
label of the ATM PW. The information needed to reconstruct the ATM
header, including the setting of the CLP and EFCI fields, for the
individual cells is contained in the ATM specific information field
of the PW packet. The method used to determine the CLP and EFCI
information of the individual cells from the ATM specific information
field of the PW packet is described in details in sections 6 and 7.
11 ATM Pseudo-Wire over MPLS specific requirements
Figure 7 provides a general format for interworking between ATM and
MPLS. The ATM information is encapsulated inside two MPLS labels as
defined in [9].
The Pseudo Wire Endpoint uses a unique MPLS label, the pseudo wire
label, to identify each direction of an ATM connection. This label
allows the PWE to access context information for the connection. As
an example, the context may contain the information regarding
connection type such as VCC or VPC or VPI/VCI value that need to be
inserted into the ATM cell header in the MPLS-to-ATM direction.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPLS Transport Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| MPLS Pseudo Wire Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format for ATM PW over a MPLS PSN
11.1 MPLS Transport Label
The 4-octet MPLS transport label identifies an LSP used to transport
traffic between two ATM-MPLS pseudo wire endpoints. This label is
used to switch the transport LSP between core LSRs. Since an MPLS LSP
is unidirectional, for the case of bi-directional traffic there will
be two different pseudo wire LSPs, one for each direction of the
connection. These may have different label values. Setting of the
EXP and TTL is for further study. The S bit is set to 0 since this
is not the last label in the MPLS label stack.
11.2 MPLS Pseudo Wire Label
The 4-octet interworking label uniquely identifies one pseudo wire
LSP, carried inside a MPLS transport LSP. The pseudo wire label has
the structure of a standard MPLS shim header. More than one pseudo
wire LSP may be supported by one MPLS transport LSP. The pseudo wire
endpoint provides the association between the ATM connection or the
ATM port and MPLS LSP by means of the 20-bit label field of the
pseudo wire LSP. In this association, in the ATM-to-MPLS direction a
mapping of the VCI/VPI of the ATM connection or the Port to the 20-
bit label is performed, while in the MPLS-to-ATM direction the 20-bit
label is mapped to a VPI/VCI of the ATM connection or to a Port. This
association is signalled or provisioned between the two pseudo-wire
endpoints.
Since an MPLS LSP is unidirectional, for the case of bi-directional
ATM VCCs there will be two different pseudo wire LSPs, one for each
direction of the connection. These may have different label values.
Setting of the EXP and TTL is for further study. The S bit is set to
1 since this is the last label in the bottom of the MPLS stack. The
pseudo wire label is not visible to the LSRs within the MPLS core
network.
12 ATM Pseudo-Wire over L2TP specific requirements
Figure 8 provides a general format for interworking between ATM and
L2TP. This draft refers to L2TPv3, or L2TP base, as described in
[11]. L2TP base extends the original L2TP [12] to operate directly
over an IP PSN and to further generalize the control procedures to
carry any tunneled Layer 2 protocol other than PPP. Any further
reference to L2TP in this document assumes L2TPv3 or L2TP base as
specified in [11].
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The ATM information is encapsulated inside a L2TP tunnel packet. The
L2TP tunnel is setup over an IPv4 PSN. The IPv4 protocol in the IPv4
header is set to 115 to indicate that the L2TP packet is encapsulated
in an IPv4 packet [11]. Furthermore, L2TP can operate over IP or over
UDP. The use of either mode is outside the scope of this document.
The encapsulation format shown in Figure 11 represents the common
headers for carrying L2TP packet over UDP and IP. If UDP is used,
additional header information is required and is defined in [11].
The Pseudo Wire Endpoint uses a unique L2TP session ID to identify
each direction of an ATM connection. This session ID is local to each
PE and allows the PWE to identify each ATM PW in the L2TP tunnel in
order to access context information for the ATM connection. As an
example, the context may contain the information regarding connection
type such as VCC or VPC or VPI/VCI value that need to be inserted
into the ATM cell header in the L2TP-to-ATM direction. Multiple PWs
with locally unique Session IDs at each PE can be multiplexed into a
L2TP tunnel.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie (optional, up to 64 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Format for ATM PW over a L2TP PSN
12.1 L2TP Session ID
This is 32-bit field containing a non-zero identifier for a session,
or a PW in this case. L2TP sessions are named by identifiers that
have local significance only at each PE [11].
Each PE for the life of the session will give different Session IDs
the same PW. Multiple PWs with locally unique Session IDs at each PE
can be multiplexed into a L2TP tunnel. When the L2TP control
connection is used for session establishment, Session IDs are
selected and exchanged as Local Session ID Attribute Value Pairs
(AVPs) during the creation of a PW. A session ID of zero is reserved
for the carriage of L2TP control messages [11].
12.2 Cookie
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The optional Cookie field contains a variable length (maximum 64
bits), long word-aligned value used to check the association of a
received packet with the PW identified by the Session ID. The Cookie
MUST be configured with a random value utilizing all bits in the
field [11]. The Cookie provides an additional level of guarantee,
beyond the Session ID lookup, that a packet has been directed to the
proper PW identified by the Session ID.
When the L2TP control connection is used for PW session
establishment, random Cookie values are selected and exchanged as
Assigned Cookie AVPs during the creation of a PW. The maximum size
of the Cookie field is 64 bits.
13 ATM Pseudo-Wire over IP specific requirements
Figure 9 provides a general format for carrying an ATM PW over an IP
PSN. This is an alternative encapsulation to the one using L2TP, as
described in Section 11. The GRE encapsulation is used as specified
in [13] and [14]. The IPv4 protocol in the IPv4 header is set to 47
to indicate that the GRE packet is encapsulated in an IPv4 packet
[13].
The ATM information is encapsulated inside a GRE/IP packet. The
Pseudo Wire Endpoint uses a unique GRE Key to identify each direction
of an ATM connection. As an example, the context may contain the
information regarding connection type such as VCC or VPC or VPI/VCI
value that need to be inserted into the ATM cell header in the IP-to-
ATM direction. Multiple PWs with unique GRE Keys can be multiplexed
into a GRE/IP tunnel.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Header (N words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| |K|S| Reserved0 | Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Sequence Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format for ATM PW over an IP PSN
13.1 C, K, and S bits
The Checksum field in the GRE header is not required for carrying
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ATM PW over IP PSN. Therefore the C bit is set to zero.
The Key field in the GRE header is always used (see Section 12.3).
Therefore, the K bit is always set to 1.
If the GRE Sequence Number field is used, then the value of the K
bit is set to 1. Otherwise, its value is set to zero.
13.2 Protocol Type field
The Protocol Type field is set to a number to be allocated by IEEE
for the purpose of encapsulating ATM PW over GRE.
13.3 Key Field
The Key field contains a four-octet number that is inserted by the
transmitting PE. The Key field is used by the receiving PE for
identifying an individual ATM PW within a GRE/IP tunnel. Multiple PWs
with unique GRE Keys can be multiplexed into a GRE/IP tunnel. The
method by which the Key field value is negotiated between the
transmitting PE and a receiving PE is further study.
13.4 GRE Sequence Number Field
If the Sequence Number Present bit is set to 1, then it indicates
that the Sequence Number field is present in the GRE header.
Otherwise, the Sequence Number field is not present in the GRE
header. The use of the Sequence Number field MUST comply to [14].
A specific ATM PWE network may choose to rely on the GRE protocol to
track in-order delivery of ATM PW packets. Another way of tracking
in-order delivery is to use the PW Sequence number field as explained
in Section 5.1.
14 Security Considerations
This draft does not introduce any new security considerations to IP,
L2TP or MPLS.
15 Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
16 References
[1] IETF BCP 9, RFC 2026 (1996), The Internet Standards Process --
Revision 3.
[2] IETF BCP 14, RFC 2119 (1997), Key words for use in RFCs to
Indicate Requirement Levels.
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[3] Frame Relay Forum FRF.8.1 (2000), Frame Relay / ATM PVC Service
Interworking Implementation Agreement.
[4] ATM Forum af-fb-atm- 0151.000 (2000), Frame Based ATM over
SONET/SDH Transport (FAST).
[5] ITU-T Recommendation I.610, (1999), B-ISDN operation and
maintenance principles and functions.
[6] IETF draft-ietf-pwe3-requirements-02.txt (November 2001, work in
progress,), Requirements for pseudo Wire Emulation Edge-to-Edge
(PWE3).
[7] IETF draft-martini-l2circuit-encap-mpls-04.txt Martini (November
2001, Work in Progress), Encapsulation Methods for Transport of Layer
2 Frames Over IP and MPLS Networks.
[8] IETF draft-koleyni-pwe3-atm-over-mpls-04.txt (January 2002, Work
in Progress), Requirements and framework for ATM network interworking
over MPLS
[9] IETF RFC 3032 (2001), MPLS Label Stack Encoding.
[10] ATM Forum af-aic-0178.000 (2001), ATM-MPLS Network Interworking.
[11] IETF draft-ietf-l2tpext-l2tp-base-01.txt (October 2001, Work in
Progress), Layer Two Tunneling Protocol (L2TP).
[12] IETF RFC 2661 (1999), Layer Two Tunneling Layer Two Tunneling
Protocol (L2TP).
[13] IETF RFC 2784(2000), Generic Routing Encapsulation (GRE).
[14] IETF RFC 2890(2000), Key and Sequence Number Extensions to GRE.
[15] ATM Forum Specification af-tm-0121.000 (1999), Traffic
Management Specification Version 4.1.
[16] IETF draft-ietf-mpls-diff-ext-09.txt (April 2001, Work in
Progress), MPLS Support of Differentiated Services.
.
[17] ITU-T Recommendation I.371 (2000), Traffic control and
congestion control in B-ISDN.
[18] IETF draft-fischer-pwe3-atm-service-03.txt(March 2002, work in
progress), PWE3: ATM service description
17 Acknowledgments
The authors like to acknowledge the contribution to this work by TBD.
18 Authors' Addresses
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Ghassem Koleyni
Nortel Networks
P O Box 3511, Station C Ottawa, Ontario,
K1Y 4H7 Canada
Email: ghassem@nortelnetworks.com
Khalid Ahmad
Nortel Networks
P O Box 3511, Station C Ottawa, Ontario,
K1Y 4H7 Canada
Email: kmad@nortelnetworks.com
Mina Azad
Nortel Networks
P O Box 3511, Station C
Ottawa, ON, Canada K1Y 4H7
Email: mazad@nortelnetworks.com
John Fischer
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: john.fischer@alcatel.com
Matthew Bocci
Alcatel
Voyager Place, Shoppenhangers Rd
Maidenhead, Berks, UK SL6 2PJ
Email: matthew.bocci@alcatel.co.uk
Mustapha Aissaoui
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: mustapha.aissaoui@alcatel.com
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