PWE3 Working Group John Fischer
Internet Draft Matthew Bocci
Expiration Date: August 2002 Mustapha Aissaoui
Alcatel
A. Siddiqui
Cable & Wireless Mina Azad
Ghassem Koleyni
Anna Cui Nortel Networks
Advanced Fibre Communications
Jim Harford
Dave Paw AdvanceNet Systems
MCI WorldCom
Cheng C. Chen
Sat Sahota NEC America, Inc.
Telus Communications
Sushil Shelly
Eric Letourneau Avici Systems
Bell Canada
Phong Khuu
Dave King Turin Networks
Jeffery See
General Dynamics Aditya Sehgal
SBC
Sohel Q. Khan
Sprint
March 2002
PWE3: ATM service description
draft-fischer-pwe3-atm-service-03.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC 2026 [1].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Internet-Drafts are draft documents valid for a maximum of six
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
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The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
Generic requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3)
have been described in [6]. This draft lists ATM specific
requirements and provides encapsulation formats and semantics for
connecting ATM edge networks through a core packet network using IP,
L2TP or MPLS. This basic application of ATM interworking will allow
ATM service providers to take advantage of new technologies in the
core in order to provide ATM multi-services.
Table of Contents
1 Conventions used in this document................................3
2 Introduction.....................................................3
3 Terminology......................................................4
4 General Requirements.............................................5
5 ATM Service Encapsulation........................................5
5.1 Length and Sequence Number ....................................6
5.1.1 Setting the length field .................................7
5.1.2 Processing the length field ..............................7
5.1.3 Setting the sequence number ..............................8
5.1.4 Processing the sequence number ...........................8
6 ATM VCC Services.................................................9
6.1 ATM VCC Cell Transport Service ................................9
6.1.1 ATM OAM Cell Support ....................................11
6.2 ATM VCC Frame Transport Service ..............................11
6.2.1 Transparent AAL5 PDU Frame Service ......................12
6.2.1.1 OAM Cell Support ....................................13
6.2.1.2 Fragmentation .......................................14
6.2.1.2.1 Procedures in the ATM-to-MPLS Direction ........14
6.2.1.2.2 Procedures in the MPLS-to-ATM Direction ........15
6.2.2 AAL5 SDU Frame Service ..................................15
6.2.2.1 OAM Cell Support ....................................16
7 ATM VPC Services................................................17
7.1 ATM VPC Cell Transport Services ..............................17
7.1.1 OAM Cell Support ........................................19
8 ILMI support....................................................20
9 QoS considerations..............................................20
10 ATM Pseudo-Wire over MPLS specific requirements................22
10.1 MPLS Transport Label ........................................23
10.2 MPLS Pseudo Wire Label ......................................23
11 ATM Pseudo-Wire over L2TP specific requirements................24
11.1 L2TP Session ID .............................................25
11.2 Cookie ......................................................25
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12 ATM Pseudo-Wire over IP specific requirements..................25
12.1 C, K, and S bits ............................................26
12.2 Protocol Type field .........................................26
12.3 Key Field ...................................................26
12.4 GRE Sequence Number Field ...................................27
13 Security Considerations........................................27
14 Intellectual Property Disclaimer...............................27
15 References.....................................................27
16 Acknowledgments................................................28
17 Authors' Addresses.............................................28
1 Conventions used in this document
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].
2 Introduction
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:
1. Leveraging of the existing systems and services to provide
increased capacity from a packet switched core.
2. Preserving existing network operational processes and
procedures used to maintain the legacy services.
3. 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.
This draft describes a method to carry ATM services over IP, L2TP
and MPLS. It lists ATM specific requirements and provides
encapsulation formats and semantics for connecting ATM edge networks
through a core packet network using IP, L2TP or MPLS. The
techniques described in this draft will allow ATM service providers
to take advantage of new technologies in the core in order to
provide ATM multi-services.
Figure 1, below displays the ATM services reference model. This
model is adapted from [6].
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|<------- 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 Service Reference Model
3 Terminology
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.
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Ingress _ The point where the ATM service is encapsulated into a
Pseudo Wire PDU (ATM to PSN direction.)
Egress - The point where the ATM service is decapsulated from a
Pseudo Wire PDU (PSN to ATM direction.)
4 General Requirements
For transport of an ATM service across a PSN, the PSN SHOULD be able
to:
1. Carry all AAL types transparently.
2. Carry multiple ATM connections (VPCs and/or VCCs).
3. Support ATM OAM applications.
4. Transport Cell Loss Priority (CLP) and Payload Type Indicator
(PTI) information from the ATM cell header.
5. Provide a mechanism to detect mis-ordering of ATM cells over
the PSN.
6. Support traffic contracts and the QoS commitments made to the
ATM connections (through the use of existing IETF defined Diff-
Serv techniques).
5 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.
Figure 2 provides a general format for encapsulation of ATM cells
(or frames) 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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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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.
5.1 Length and Sequence Number
The length and sequence number are not required for all services.
The control word is designed 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 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.
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.
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5.1.1 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 other cell or frame transport services MUST either follow the
same procedure as the SDU frame service or always set the length
field to 0 to indicate to the remote PE that no padding was applied.
5.1.2 Processing the length field
When the length field is present the egress PE MUST follow these
procedures:
- If the length field of the packet is 0, then the packet does not
require padding to be stripped.
- Otherwise, the length field MUST be verified against the size of
the packet as follows.
- if the packet size is smaller than indicated by the length
field, the packet MUST be discarded
- otherwise, if the packet size is as indicated by the length
field then the packet does not require padding to be stripped
- otherwise, the packet is altered by removing the padding
bytes from the end of the packet to match the size indicated
by the length field.
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5.1.3 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.
5.1.4 Processing the sequence number
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.
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If the egress PE does not support receive sequence number
processing, then the sequence number field MAY be ignored.
6 ATM VCC Services
This section defines three types of ATM VCC services that may be
supported over the PSN: ATM cell, ATM AAL5 PDU, and ATM AAL5 SDU.
6.1 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 cell transport
service is MANDATORY.
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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| 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. The ability to transport an ATM cell in the AAL5
mode is intended to provide a means of enabling OAM
functionality over the AAL5 VCC.
* 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.
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* 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.
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
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6.1.1 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.
6.2 ATM VCC Frame Transport Service
The frame mode services were designed specifically for better
transport efficiency than the cell mode. Two modes of AAL5 frame
transport are available. The transparent AAL5 PDU mode is intended
to be more efficient than cell mode, yet still provide full ATM
transparency including the correct sequencing of OAM cells on an
AAL5 flow. The payload AAL5 SDU mode is intended to provide more
transport efficiency than the PDU mode while foregoing some ATM
transparency.
It is important that the PEs be able to efficiently switch between
the frame and cell modes in order to support ATM OAM functions.
The agreement to transport transparent AAL5 PDUs or payload AAL5
SDUs may be accomplished via a Pseudo Wire specific signaling
mechanism or via static configuration.
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6.2.1 Transparent AAL5 PDU Frame Service
In this mode, the ingress PE encapsulates the entire CPCS-PDU
including the PAD and trailer.
This mode MAY support fragmentation in order to maintain OAM cell
sequencing.
Like the ATM AAL5 payload VCC service, the AAL5 transparent VCC
service is intended to be more efficient than the VCC cell transport
service. However, the AAL5 transparent VCC service carries the
entire AAL5 CPCS-PDU, including the PAD and trailer. Note that the
AAL5 CPCS-PDU is not processed _ i.e. an AAL5 frame with an invalid
CRC or length field will be transported. One reason for this is
that there may be a security agent that has scrambled the ATM cell
payloads that form the AAL5 CPCS-PDU.
This service supports all OAM cell flows by using a fragmentation
procedure that ensures that OAM cells are not repositioned in
respect to AAL5 composite cells.
The AAL5 transparent VCC service is OPTIONAL.
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|Frg|E|C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ " |
| AAL5 CPCS-PDU |
| (n * 48 bytes) |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: AAL5 transparent service encapsulation
The first octet following the Pseudo Wire Header carries
control information. The M, V, Res, and C bits are as defined
earlier for VCC cell mode.
* Frg (Fragmentation) Bits
This field is used to support the fragmentation functionality
described later in this section.
- 11 Single Segment Message (Beginning and End of Message)
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- 10 Beginning of Message
- 00 Continuation of Message
- 01 End of Message
* E (EFCI) bit
This field is used to convey the EFCI state of the ATM cells.
The EFCI state is indicated in the middle bit of each ATM
cell's PTI field.
ATM-to-PSN direction (ingress): The EFCI field of the
control byte is set to the EFCI state of the last cell of
the AAL5 PDU or AAL5 fragment.
PSN-to-ATM direction (egress): The EFCI state of all
constituent cells of the AAL5 PDU or AAL5 fragment is set to
the value of the EFCI field in the control byte.
* C (CLP) bit
This field is used to convey the cell loss priority of the ATM
cells.
ATM-to-PSN direction (ingress): The CLP field of the
control byte is set to 1 if any of the constituent cells of
the AAL5 PDU or AAL5 fragment has its CLP bit set to 1;
otherwise this field is set to 0.
PSN-to-ATM direction (egress): The CLP bit of all
constituent cells for an AAL5 PDU or AAL5 fragment is set to
the value of the CLP field in the control byte.
The payload consists of the re-assembled AAL5 CPCS-PDU,
including the AAL5 padding and trailer or the AAL5 fragment.
6.2.1.1 OAM Cell Support
When configured for the AAL5 transparent VCC service, both PE's
SHOULD act as a VC switch, in accordance with the OAM procedures
defined in [5].
The PEs SHOULD 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
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The PEs SHALL use the single ATM VCC cell mode encapsulation
(Section 6.1) when passing an OAM cell.
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.
6.2.1.2 Fragmentation
The ingress PE may not always be able to reassemble a full AAL5
frame. This may be due to the AAL5 PDU exceeding the Pseudo Wire MTU
or when OAM cells arrive during reassembly of the AAL5 PDU. In these
cases, the AAL5 PDU shall be fragmented. In addition, fragmentation
may be desirable to bound ATM cell delay.
If no fragmentation occurs, then the fragmentation bits are set to
11 (SSM, Single Segment Message).
When fragmentation occurs, the procedures described in the following
subsections shall be followed.
6.2.1.2.1 Procedures in the ATM-to-MPLS Direction
The following procedures shall apply while fragmenting AAL5 PDUs:
- Fragmentation shall always occur at cell boundaries within the
AAL5 PDU.
- For the first fragment, the FRG bits shall be set to 10 (BOM,
Beginning Of Message).
- For the last fragment, the FRG bits shall be set to 01 (EOM,
End Of Message).
- For all other fragments, the FRG bits shall be set to 00 (COM,
Continuation Of Message).
- The E and C bits of the fragment shall be set as defined
earlier in section 6.
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6.2.1.2.2 Procedures in the MPLS-to-ATM Direction
The following procedures shall apply:
- The 3-bit PTI field of each ATM cell header is constructed as
follows:
+ The most significant bit is set to 0, indicating a user
data cell.
+ The middle bit is set to the E bit value of the
fragment.
+ The least significant bit is set to 1 for the last ATM
cell of a fragment where the FRG bits are 01 (EOM) or
11(SSM); otherwise this bit is set to 0.
- The C bit of each ATM cell header is set to the value of the C
bit of the control byte in Figure 5.
6.2.2 AAL5 SDU Frame Service
The AAL5 payload VCC service defines a mapping between the payload
of an AAL5 VCC and a single Pseudo Wire. This service is OPTIONAL.
The AAL5 payload VCC service requires ATM segmentation and
reassembly support on the PE. Once the ingress PE reassembles the
AAL5 CPCS-PDU, the PE discards the PAD and CPCS-PDU trailer and then
inserts the resulting payload into a Pseudo Wire PDU. Although any
AAL5 PDU may be transported using the VCC cell relay service and
cell mode encapsulation, the AAL5 payload VCC service is designed
for better transport efficiency.
The egress PE MUST regenerate the PAD and trailer before
transmitting the AAL5 frame on the egress ATM port.
This service does allow the transport of OAM and RM cells, but does
not attempt to maintain the relative order of these cells with
respect to the cells that comprise the AAL5 CPCS-PDU. OAM cells
that arrive during the reassembly of a single AAL5 CPCS-PDU are sent
immediately on the Pseudo Wire, followed by the AAL5 payload.
Therefore, the AAL5 payload VCC service may not be suitable for some
ATM applications that require strict ordering of OAM cells (such as
performance monitoring and security applications).
The AAL5 payload service encapsulation is shown below.
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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|R|U|Frg|E|C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| " |
| AAL5 SDU payload |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: AAL5 payload service encapsulation
The first octet following the Pseudo Wire Header carries
control information. The M, V, R (reserved), E and C bits are
as defined earlier for VCC cell mode. Since fragmentation is
not required, the fragmentation bits are set to 11 to indicate
a complete frame.
* U (UU Octet Command/Response) Bit
When FRF.8.1 Frame Relay / ATM PVC Service Interworking 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 PE device SHOULD copy this bit to the C bit of the
control byte. The egress PE device SHOULD copy the C bit to the
CPCS-UU Least Significant Bit (LSB) of the AAL5 payload.
6.2.2.1 OAM Cell Support
Similar to the VCC cell relay service, both PEs SHOULD act as a VC
switch in accordance with the OAM procedures defined in [5].
The PEs SHOULD 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
- Continuity Check
Because this service does not guarantee the original OAM cell
position within the AAL5 composite cells, the following cell types
are discarded by the ingress PE:
- Performance Management
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- Security
The PEs SHALL use the single ATM VCC cell mode encapsulation
(Section 6.1) when passing an OAM cell.
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.
7 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.
7.1 ATM VPC Cell Transport Services
The ATM VPC cell transport service is OPTIONAL.
This service MUST use the following cell mode encapsulation:
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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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: 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.
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.
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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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |M|V|Res| PTI |C| VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+ |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 8: Multiple Cell VPC Encapsulation
7.1.1 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.
8 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.
9 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
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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
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
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1 AF32 011100
UBR UBR.1/UBR.2 0/1 DF 000000
Figure 9: Example of ATM Service Category to PHB Mapping
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 6 and 7.
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.
10 ATM Pseudo-Wire over MPLS specific requirements
Figure 10 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
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPLS Pseudo Wire Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Length and Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Format for ATM PW over a MPLS PSN
10.1 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.
10.2 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
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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.
11 ATM Pseudo-Wire over L2TP specific requirements
Figure 11 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 a 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].
The ATM information is encapsulated inside a L2TP tunnel packet. The
L2TP tunnel is setup over a IPv4 PSN. The IPv4 protocol in the IPv4
header is set to 115 to indicate that the L2TP packet is
encapsulated in a 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 11: Format for ATM PW over a L2TP PSN
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11.1 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].
The same PW will be given different Session IDs by each PE for the
life of the session. 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].
11.2 Cookie
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.
12 ATM Pseudo-Wire over IP specific requirements
Figure 12 provides a general format for carrying a ATM PW over a 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 a 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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 12: Format for ATM PW over a IP PSN
12.1 C, K, and S bits
The Checksum field in the GRE header is not required for carrying
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.
12.2 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.
12.3 Key Field
The Key field contains a four octet number which 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.
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12.4 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.
13 Security Considerations
This draft does not introduce any new security considerations to IP,
L2TP or MPLS.
14 Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
15 References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[3] "Frame Relay / ATM PVC Service Interworking Implementation
Agreement (FRF.8.1)", Frame Relay Forum, 2000.
[4] "Frame Based ATM over SONET/SDH Transport (FAST)", af-fb-atm-
0151.000, ATM Forum 2000.
[5] _B-ISDN operation and maintenance principles and functions_,
ITU-T Recommendation I.610, February 1999.
[6] Xiao, X., et al., _Requirements for pseudo Wire Emulation Edge-
to-Edge (PWE3)_, IETF draft-ietf-pwe3-requirements-02.txt, work
in progress, November 2001.
[7] Martini, L., et al., "Encapsulation Methods for Transport of
Layer 2 Frames Over IP and MPLS Networks", draft-martini-
l2circuit-encap-mpls-04.txt, Work in Progress, November 2001.
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[8] Koleyni, G., et al., "Requirements and framework for ATM network
interworking over MPLS", draft-koleyni-pwe3-atm-over-mpls-
04.txt, Work in Progress, January 2002.
[9] Rosen, E., et al., "MPLS Label Stack Encoding", RFC 3032,
January 2001.
[10] _ATM-MPLS Network Interworking_, af-aic-0178.000, ATM Forum,
2001.
[11] Lau, J., et al., "Layer Two Tunneling Protocol "L2TP"", draft-
ietf-l2tpext-l2tp-base-01.txt, Work in Progress, October 2001.
[12] Townsley, W., et al., "Layer Two Tunneling Layer Two Tunneling
Protocol (L2TP)", RFC 2661, August 1999.
[13] Farinacci, D., et al., "Generic Routing Encapsulation (GRE)",
RFC 2784, March 2000.
[14] Dommety, G., et al., "Key and Sequence Number Extensions to
GRE", RFC 2890, September 2000.
[15] _Traffic Management Specification Version 4.1_, AF-TM-0121.000,
ATM Forum, March 1999.
[16] Le Faucheur, F., et al., "MPLS Support of Differentiated
Services", draft-ietf-mpls-diff-ext-09.txt, Work in Progress,
April 2001.
16 Acknowledgments
The authors like to acknowledge the contribution to this work by
Fred Kaudel and Dr. Khalid Ahmad.
17 Authors' Addresses
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
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Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: mustapha.aissaoui@alcatel.com
Mina Azad
Nortel Networks
P O Box 3511, Station C
Ottawa, ON, Canada K1Y 4H7
Email: mazad@nortelnetworks.com
Ghassem Koleyni
Nortel Networks
P O Box 3511, Station C Ottawa, Ontario,
K1Y 4H7 Canada
Email: ghassem@nortelnetworks.com
Adeel A. Siddiqui
Cable & Wireless
11700 Plaza America Drive
Reston, Virginia 20190, USA
Email: adeel.siddiqui@cwusa.com
Anna Cui
Advanced Fibre Communications
3350 S.W. 148th Ave. Suite 300
Miramar, FL 33027 USA
Email: anna.cui@afc.com
Jim Harford
AdvanceNet Systems
Research Triangle Park, NC
E-mail: harford@atmware.com
Dave Paw
MCI WorldCom
6929 N. Lakewood Ave.
Tulsa, OK 74117
Email: dave.paw@wcom.com
Cheng C. Chen
Network Systems Division,
NEC America, Inc.
6555 N. State Highway 161,
Irving, TX 75039
Email: cchen@necam.com
Sat Sahota
Telus Communications
10020 100 Street
Edmonton Alberta T5J 0N5
Canada
Fischer, et al. Expires August 2002 [Page 29]
Internet Draft draft-fischer-pwe3-atm-service-03.txt March 2002
Email: Sat.Sahota@telus.com
Sushil Shelly
Avici Systems
Email: sshelly@avici.com
Eric Letourneau
Bell Canada
700, De LaGauchetiere W.
Montreal, Quebec H3B 4L1
Email: eric.letourneau@bellnexxia.com
Phong Khuu
Turin Networks
1415 N McDowell Blvd
Petaluma, CA 94954 USA
Email: pkhuu@turinnetworks.com
Dave King
General Dynamics
Email: dave.king@gsc.gte.com
Jeffery See
General Dynamics
Email: Jeffery.See@GD-CS.COM
Aditya Sehgal
SBC
530 McCullough Rm 10 M 03
San Antonio, TX 78215
Email: sehgal@tri.sbc.com
Sohel Q. Khan
Sprint
7171 W 95th Street
Overland Park, KS 66212
Email: sohel.khan@mail.sprint.com