Network Working Group Luca Martini
Internet Draft Level 3 Communications, LLC.
Expiration Date: April 2003
Jeremy Brayley Matthew Bocci
Laurel Networks, Inc. Alcatel
Eric C. Rosen Ghassem Koleyni
Cisco Systems, Inc. Nortel Networks.
October 2002
Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks
draft-ietf-pwe3-atm-encap-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
A framework for providing various Layer 1 and Layer 2 services over a
Packet Switched Network has been described in [3]. This draft
provides encapsulation formats and guidelines for transporting a
variety of ATM services over a PSN. This draft merges three internet
drafts previously submitted to the PWE3 working group. These are:
draft-martini-atm-encap-mpls-01.txt; draft-koleyni-pwe3-app-cell-
over-psn-01.txt; and draft-bocci-pwe3-app-frame-over-psn-00.txt. This
draft describes two methods of ATM cell encapsulation, one-to-one
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mode based on draft-koleyni-pwe3-app-cell-over-psn-01.txt and n-to-
one mode (n=>1) based on draft-martini-atm-encap-mpls-01.txt. This
draft describes two methods of AAL5 encapsulation, PDU mode based on
draft-bocci-pwe3-app-frame-over-psn-00.txt and SDU mode based on
draft-martini-atm-encap-mpls-01.txt. It includes an applicability
statement for each service along with ATM OAM handling and QoS
guidelines. Please note that this section will change in the next
revisions.
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Table of Contents
1 Specification of Requirements .......................... 4
2 Introduction ........................................... 4
3 Terminology ............................................ 5
4 General encapsulation method ........................... 6
4.1 The Control Word ....................................... 6
4.1.1 Setting the sequence number ............................ 8
4.1.2 Processing the sequence number ......................... 8
4.2 MTU Requirements ....................................... 9
5 ATM .................................................... 9
6 ATM one-to-one Cell Mode ............................... 10
6.1 Applicability .......................................... 10
6.2 Implementation and deployment considerations ........... 11
6.3 Limitations ............................................ 11
6.4 ATM one-to-one Service Encapsulation ................... 11
6.5 Length and Sequence Number ............................. 12
6.5.1 Setting the length field ............................... 13
6.5.2 Processing the length field ............................ 13
6.6 ATM VCC Services ....................................... 13
6.7 ATM VCC Cell Transport Service ......................... 14
6.7.1 ATM OAM Cell Support ................................... 15
6.8 ATM VPC Services ....................................... 16
6.8.1 ATM VPC Cell Transport Services ........................ 16
6.8.2 OAM Cell Support ....................................... 18
7 ATM n-to-one Cell Mode ................................. 19
7.1 ATM n-to-one Service Encapsulation ..................... 19
7.2 ATM OAM Cell Support ................................... 22
7.3 CLP bit to Quality of Service mapping .................. 22
7.4 Applicability Statement for n-to-one mode .............. 22
7.5 Review of header information ........................... 23
7.6 MPLS Shim S Bit Value .................................. 24
7.7 MPLS Shim TTL Values ................................... 24
8 ATM AAL5 CPCS-SDU Mode ................................. 24
8.1 Applicability Statement ................................ 24
8.2 Transparent AAL5 SDU Frame Encapsulation ............... 25
8.3 ATM OAM Cell Support ................................... 26
9 AAL5 PDU frame mode .................................... 27
9.1 Applicability .......................................... 27
9.1.1 Implementation and deployment considerations ........... 29
9.1.2 Limitations ............................................ 29
9.2 Transparent AAL5 PDU Frame encapsulation ............... 29
9.3 ATM OAM Cell Support ................................... 31
9.4 Fragmentation .......................................... 32
9.4.1 Procedures in the ATM-to-MPLS Direction ................ 32
9.4.2 Procedures in the MPLS-to-ATM Direction ................ 32
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10 Security Considerations ................................ 33
11 Intellectual Property Disclaimer ....................... 33
12 References ............................................. 33
13 Author Information ..................................... 34
1. Specification of Requirements
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
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 providers
infrastructure by allowing virtually any existing digital service to
be supported over a single networking infrastructure. The benefit of
this model to a service provider is threefold:
- Leveraging of the existing systems and services to provide
increased capacity from a packet switched core.
- Preserving existing network operational processes and procedures
used to maintain the legacy services.
- 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 [3].
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|<----- Pseudo Wire ---->|
| |
| |<-- PSN Tunnel -->| |
ATM Service V V V V ATM Service
| +----+ +----+ |
+----+ | | PE1|==================| PE2| | +----+
| |----------|............PW1.............|----------| |
| CE1| | | | | | | |CE2 |
| |----------|............PW2.............|----------| |
+----+ | | |==================| | | +----+
^ +----+ +----+ | ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 1: ATM Service Reference Model
QoS related issues are not discussed in this draft. This draft
describes two methods of ATM cell encapsulation, one-to-one mode
and n-to-one mode. This draft describes two methods of AAL5
encapsulation, PDU mode and SDU mode.
3. Terminology
One-to-one mode: The One-to-one mode specifies an encapsulation
method which maps one ATM VCC (or one ATM VPC) to one PSN Tunnel.
N-to-one mode (N >= 1): The N-to-one mode specifies an encapsulation
method which maps one or more ATM VCCs (or one or more ATM VPCs) to
one PSN tunnel.
Packet Switched Network - A Packet Switched Network (PSN) is an IP or
MPLS network.
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.
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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 PSN devices.
PSN Bound - The traffic direction where information from a CE is
adapted to a PW, and PW-PDUs are sent into the PSN. CE Bound - The
traffic direction where PW-PDUs are received on a PW from the PSN,
re-converted back in the emulated service, and sent out to a CE.
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.)
CTD Cell Transfer Delay MTU Maximum Transfer Unit OAM
Operations, Administration, and Maintenance. PVC Permanent
Virtual Connection. An ATM connection that is
provisioned 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.
4. General encapsulation method
4.1. The Control Word
There are three requirements that may need to be satisfied when
transporting layer 2 protocols over an IP or MPLS backbone:
-i. Sequentiality may need to be preserved.
-ii. 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.
-iii. Control bits carried in the header of the layer 2 frame may
need to be transported.
The control word defined here addresses all three of these
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requirements. For some protocols this word is REQUIRED, and for
others OPTIONAL. For protocols where the control word is OPTIONAL
implementations MUST support sending no control word, and MAY support
sending a control word.
In all cases the egress router must be aware of whether the ingress
router will send a control word over a specific virtual circuit.
This may be achieved by configuration of the routers, or by
signaling, for example as defined in [1].
The control word is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsvd | Flags |Res| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram the first 4 bits are reserved for future use.
They MUST be set to 0 when transmitting, and MUST be ignored upon
receipt.
The next 4 bits provide space for carrying protocol specific flags.
These are defined in the protocol-specific details below.
These bits are reserved and MUST be set to 0 upon transmission and
ignored upon reception.
The next 6 bits provide a length field, which is used as follows: If
the packet's length (defined as the length of the layer 2 payload
plus the length of the control word) is less than 64 bytes, the
length field MUST be set to the packet's length. Otherwise the length
field MUST be set to zero. The value of the length field, if non-
zero, can be used to remove any padding. When the packet reaches the
service provider's egress router, it may be desirable to remove the
padding before forwarding the packet.
The next 16 bits provide a sequence number that can be used to
guarantee ordered packet delivery. The processing of the sequence
number field is OPTIONAL.
The sequence number space is a 16 bit, unsigned circular space. The
sequence number value 0 is used to indicate an unsequenced packet.
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4.1.1. Setting the sequence number
For a given emulated VC, and a pair of routers PE1 and PE2, if PE1
supports packet sequencing then the following procedures should be
used:
- the initial packet transmitted on the emulated VC MUST use
sequence number 1
- subsequent packets MUST increment the sequence number by one for
each packet
- when the transmit sequence number reaches the maximum 16 bit
value (65535) the sequence number MUST wrap to 1
If the transmitting router PE1 does not support sequence number
processing, then the sequence number field in the control word MUST
be set to 0.
4.1.2. Processing the sequence number
If a router PE2 supports receive sequence number processing, then the
following procedures should be used:
When an emulated VC is initially set up, the "expected sequence
number" associated with it MUST be initialized to 1.
When a packet is received on that emulated VC, the sequence number
should be processed as follows:
- if the sequence number on the packet is 0, then the packet passes
the sequence number check
- otherwise if the packet sequence number >= the expected sequence
number and the packet sequence number - the expected sequence
number < 32768, then the packet is in order.
- otherwise if the packet sequence number < the expected sequence
number and the expected sequence number - the packet sequence
number >= 32768, then the packet is in order.
- otherwise the packet is out of order.
If a packet passes the sequence number check, or is in order then, it
can be delivered immediately. If the packet is in order, then the
expected sequence number should be set using the algorithm:
expected_sequence_number := packet_sequence_number + 1 mod 2**16
if (expected_sequence_number = 0) then expected_sequence_number := 1;
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Packets which are received out of order MAY be dropped or reordered
at the discretion of the receiver.
If a router PE2 does not support receive sequence number processing,
then the sequence number field MAY be ignored.
4.2. MTU Requirements
The network MUST be configured with an MTU that is sufficient to
transport the largest encapsulation frames. If MPLS is used as the
tunneling protocol, for example, this is likely to be 12 or more
bytes greater than the largest frame size. Other tunneling protocols
may have longer headers and require larger MTUs. If the ingress
router determines that an encapsulated layer 2 PDU exceeds the MTU of
the tunnel through which it must be sent, the PDU MUST be dropped. If
an egress router receives an encapsulated layer 2 PDU whose payload
length (i.e., the length of the PDU itself without any of the
encapsulation headers), exceeds the MTU of the destination layer 2
interface, the PDU MUST be dropped.
5. ATM
This Draft defines two methods for encapsulation of ATM cells,
namely, One-to-one mode and N-to-one mode.
The One-to-one mode specifies an encapsulation method that maps one
ATM VCC or one ATM VPC to one Pseudo-Wire. Two formats are specified:
one for transporting a VCC on a tunnel and another for transporting a
VPC on a tunnel. In the VCC format, the VPI/VCI is not included. In
the VPC format, the VPI is not included. Cells from one VCC or one
VPC may be concatenated.
The N-to-one mode (N >= 1) specifies an encapsulation method that
maps one or more ATM VCCs (or one or more ATM VPCs) to one Pseudo-
Wire. One format is used for both the VCC or VPC mapping to the
tunnel. The 4-octet ATM header is unaltered in the encapsulation,
thus the VPI/VCI is always present. Cells from one or more VCCs (or
one or more VPCs) may be concatenated.
Furthermore different encapsulations are supported for ATM AAL5
transport: one for ATM AAL5 SDUs, and and another for ATM AAL5 PDUs.
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6. ATM one-to-one Cell Mode
The One-to-one mode described in this Draft allows a service provider
to offer an ATM PVC or SVC based service across a network. The
encapsulation allows one ATM VCC or VPC to be carried within a single
Pseudo-Wire.
6.1. Applicability
The primary application of one-to-one 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 [5] or ATM transfer
capability [6], 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
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.
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 SLA with the VPN
customer. The advantage is for the service provider to combine
L2 and L3 services over the same PSN.
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Figure 1 shows the reference model for carrying ATM services over a
PSN. 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.
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.
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.
6.2. 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.
6.3. 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.
6.4. ATM one-to-one 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 into
packets.
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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 | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: General format for one-to-one mode 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.
6.5. 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
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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.
Treatment of the sequence number is according to previous sections
"Setting the sequence number", and "Processing the sequence number".
Length field is not required for the cell mode.
6.5.1. Setting the length field
All cell transport services MUST always set the length field to 0 to
indicate to the remote PE that no padding was applied.
6.5.2. Processing the length field
Since length field is not used for cell mode, no processing is
required.
6.6. ATM VCC Services
This section defines ATM cell VCC services that may be supported over
the PSN.
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6.7. 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 OPTIONAL. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
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* 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
6.7.1. 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 [7].
The PEs MUST be able to pass the following OAM cells transparently:
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- 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 one-to-one cell mode encapsulation 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.
6.8. 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.
6.8.1. 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 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.
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 6: Multiple Cell VPC Encapsulation
6.8.2. 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
[7].
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 one-to-one cell encapsulation when
passing an OAM cell. The OAM cell MAY be encapsulated together with
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other user data cells if multiple cell encapsulation is used.
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.
7. ATM n-to-one Cell Mode
The N-to-one mode (N >= 1) described in this Draft allows a service
provider to offer an ATM PVC or SVC based service across a network.
The encapsulation allows multiple ATM VCCs or VPCs to be carried
within a single PSN tunnel. A service provider may also use N-to-one
mode to provision either one VCC or one VPC on a tunnel. This section
defines the VCC and VPC cell relay services over a PSN and their
applicability.
7.1. ATM n-to-one Service Encapsulation
This section describes the general encapsulation format for ATM over
PSN pseudo wires.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Control Word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: General format for ATM encapsulation over PSNs
The PSN Transport Header depends on the particular tunneling
technology in use (L2TP or MPLS). This header is used to transport
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the encapsulated ATM information through the packet switched core.
The Pseudo Wire Header identifies a particular ATM service on a
tunnel. Non-ATM services may also be carried on the PSN tunnel.
The ATM Control Word is inserted before the ATM service payload. It
may contain a length and sequence number in addition to certain
control bits needed to carry the service.
The ATM Service Payload is specific to the service being offered via
the Pseudo Wire. It is defined in the following sections.
In this encapsulation mode ATM cells are transported individually
without a SAR process. This is the only REQUIRED encapsulation for
ATM. The ATM cell encapsulation consists of an OPTIONAL control
word, and one or more ATM cells - each consisting of a 4 byte ATM
cell header and the 48 byte ATM cell payload. This ATM cell header is
defined as in the FAST encapsulation [4] section 3.1.1, but without
the trailer byte. The length of each frame, without the encapsulation
headers, is a multiple of 52 bytes long. The maximum number of ATM
cells that can be fitted in a frame, in this fashion, is limited only
by the network MTU and by the ability of the egress router to process
them. The ingress router MUST NOT send more cells than the egress
router is willing to receive. The number of cells that the egress
router is willing to receive may either be configured in the ingress
router or may be signaled, for example using the methods described in
[1]. The number of cells encapsulated in a particular frame can be
inferred by the frame length. The control word is OPTIONAL. If the
control word is used then the flag bits in the control word are not
used, and MUST be set to 0 when transmitting, and MUST be ignored
upon receipt.
The EFCI and CLP bits are carried across the network in the ATM cell
header. The edge routers that implement this document MAY, when
either adding or removing the encapsulation described herein, change
the EFCI bit from zero to one in order to reflect congestion in the
network that is known to the edge router, and the CLP bit from zero
to one to reflect marking from edge policing of the ATM Sustained
Cell Rate. The EFCI and CLP bits SHOULD NOT be changed from one to
zero.
This diagram illustrates an encapsulation of two ATM cells:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control word ( Optional ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI | PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Payload ( 48 bytes ) |
| " |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI | PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Payload ( 48 bytes ) |
| " |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Multiple Cell ATM Encapsulation
* VPI
The ingress router MUST copy the VPI field from the incoming cell
into this field. For particular emulated VCs, the egress router
MAY generate a new VPI and ignore the VPI contained in this
field.
* VCI
The ingress router MUST copy the VCI field from the incoming ATM
cell header into this field. For particular emulated VCs, the
egress router MAY generate a new VCI.
* PTI & CLP ( C bit )
The PTI and CLP fields are the PTI and CLP fields of the incoming
ATM cells. The cell headers of the cells within the packet are
the ATM headers (without HEC) of the incoming cell.
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7.2. ATM OAM Cell Support
OAM cells MAY be transported on the VC LSP. An egress router that
does not support transport of OAM cells MUST discard frames that
contain an ATM cell with the high-order bit of the PTI field set to
1. A router that supports transport of OAM cells MUST follow the
procedures outlined in [4] section 8 for mode 0 only, in addition to
the applicable procedures specified in [1].
7.3. CLP bit to Quality of Service mapping
The ingress router MAY consider the CLP bit when determining the
value to be placed in the Quality of Service fields (e.g. the EXP
fields of the MPLS label stack) of the encapsulating protocol. This
gives the network visibility of the CLP bit. Note however that cells
from the same VC MUST NOT be reordered.
7.4. Applicability Statement for n-to-one mode
The N-to-one cell relay encapsulation described in this document
allows a service provider to offer a PVC/PVP or SVC/SVP based VCC/VPC
cell relay service across an IP or MPLS PSN.
The encapsulation allows multiple VCCs/VPCs to be carried within a
single PSN tunnel. This does not preclude the possibility that a
service provider may wish to provision a single VCC to a PSN tunnel
in order to satisfy QoS or restoration requirements.
The encapsulation also supports the binding of multiple VCCs/VPCs to
a single Pseudo Wire. This capability is useful in order to make
more efficient use of the PW demultiplexing header space as well as
to ease provisioning of the VCC/VPC services.
The VCC/VPC cell relay service has the following attributes:
-i. Supports all ATM Adaptation Layers Types.
-ii. Non-terminating OAM/Admin cells are transported among the
user cells in the same order as they are received. This
requirement enables the use of various performance
management and security applications.
-iii. In order to gain transport efficiency on the PSN, multiple
cells may be encapsulated in a single PW PDU. This process
is called cell concatenation . How many cells to insert or
how long to wait for cell arrival before sending a PW PDU is
an implementation decision. Like any SAR scheme, cell
concatenation introduces latency to a cell relay service.
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-iv. The CLP bit from each cell may be mapped to a corresponding
marking on the PW PDU. This allows the drop precedence to be
preserved across the PSN.
The VCC cell relay service encapsulation has the following drawbacks:
-i. There is no currently defined method to translate the
forward congestion indication (EFCI) to a corresponding
function in the PSN. Nor is there a way to translate PSN
congestion to the EFCI upon transmission by the egress PE.
-ii. The ATM cell header checksum can correct a single bit error
in the cell header. Analogous functionality does not exist
in most PSNs. A single bit error in a PW PDU will most
likely cause the packet to be dropped due to a L2 FCS
failure.
-iii. There is no currently defined method to support EPD/PPD on
the PSN.
-iv. There are currently no OAM mechanisms defined for the PSN
like those defined for ATM. Therefore the methods for the
detection/consequent-actions of failures in the PSN are not
specified. This also means that QoS/availability metrics
cannot be specified for the PSN.
7.5. Review of header information
The review of the ATM header at PE devices is OPTIONAL. While
information carried in the cell encapsulation is carried
transparently through the PSN, and does not require a SAR function,
inspection of the header information provides a mechanism to map
characteristics of the transported information to the PSN. Each cell
is inspected at the PE device and service requirements are mapped
accordingly in the packet based network.
It is through this examination that control mechanisms such as
congestion management can be translated for transport in the PSN.
This capability could also be used to support the mapping of ATM QoS
to CoS.
Similar mechanisms can be used to map ATM header information to other
type of PSN tunnel PDU headers.
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7.6. MPLS Shim S Bit Value
The ingress LSR, PE1, MUST set the S bit of the VC label to a value
of 1 to denote that the VC label is at the bottom of the stack.
7.7. MPLS Shim TTL Values
The ingress LSR, PE1, SHOULD set the TTL field of the VC label to a
value of 2.
8. ATM AAL5 CPCS-SDU Mode
The AAL5 payload VCC service defines a mapping between the payload of
an AAL5 VCC and a single Pseudo Wire. The AAL5 payload VCC service
requires ATM segmentation and reassembly support on the PE.
The AAL5 payload CPCS-SDU service is OPTIONAL.
Even the smallest TCP packet requires two ATM cells when sent over
AAL5 on a native ATM device. It is desirable to avoid this padding on
the Pseudo Wire. Therefore, once the ingress PE reassembles the AAL5
CPCS-PDU, the PE discards the PAD and CPCS-PDU trailer then inserts
the resulting payload into a Pseudo Wire PDU.
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 will not be suitable for ATM
applications that require strict ordering of OAM cells (such as
performance monitoring and security applications).
8.1. Applicability Statement
It is possible to carry any ATM service using the VCC and VPC cell
relay encapsulations defined in the previous section. After all, ATM
is inherently a cell-based technology. However, a vast majority of
the data carried on ATM networks is frame based and therefore uses
AAL5. For example, most Frame Relay services are provided on an ATM
backbone using AAL5 and of course AAL5 is used to carry IP PDUs
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between ATM attached routers.
The AAL5-SDU service is designed with this reality in mind. The
encapsulation defined below is more efficient for small AAL5 SDUs
than the VCC cell relay service. In turn it presents a more efficient
alternative to the cell relay service when carrying RFC 2684
encapsulated IP PDUs across a PSN.
The AAL5-SDU encapsulation requires Segmentation and Reassembly on
the PE-CE ATM interface. This SAR function is provided by common
off-the-shelf hardware components. Once reassembled, the AAL5-SDU is
carried via a Pseudo Wire to the egress PE. Herein lies another
advantage of the AAL5-SDU encapsulation. Using the AAL5-SDU mode the
egress PE does not have to perform reassembly itself on the PSN
facing interface when converting to a frame based medium. For
example, the AAL5-SDU mode allows easier extraction of an IP PDU for
processing, or conversion to a different frame technology such as
Frame Relay or Ethernet. When using the cell relay service to provide
this same functionality, the egress PE must reassemble cells arriving
over a PSN tunnel.
8.2. Transparent AAL5 SDU Frame Encapsulation
The AAL5 CPCS-SDU is prepended by the following header:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res |T|E|C|U|Res| Length | Sequence Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| " |
| ATM cell or AAL5 CPCS-SDU |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: AAL5 CPCS-SDU Encapsulation
The AAL5 payload service encapsulation requires the ATM control word.
The Flag bits are described below.
* Res (Reserved) These bits are reserved and MUST be set to 0 upon
transmission and ignored upon reception.
* T (transport type) bit
Bit (T) of the control word indicates whether the packet contains
an ATM admin cell or an AAL5 payload. If T = 1, the packet
contains an ATM admin cell, encapsulated according to the VCC
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cell relay encapsulation, figure 8. If not set, the PDU contains
an AAL5 payload. The ability to transport an ATM cell in the AAL5
SDU mode is intended to provide a means of enabling
administrative functionality over the AAL5 VCC (though it does
not endeavor to preserve user-cell and admin-cell
arrival/transport ordering).
* E ( EFCI ) Bit
The ingress router, PE1, SHOULD set this bit to 1 if the EFCI bit
of the final cell of those that transported the AAL5 CPCS-SDU is
set to 1, or if the EFCI bit of the single ATM cell to be
transported in the packet is set to 1. Otherwise this bit
SHOULD be set to 0. The egress router, PE2, SHOULD set the EFCI
bit of all cells that transport the AAL5 CPCS-SDU to the value
contained in this field.
* C ( CLP ) Bit
The ingress router, PE1, SHOULD set this bit to 1 if the CLP bit
of any of the ATM cells that transported the AAL5 CPCS-SDU is set
to 1, or if the CLP bit of the single ATM cell to be transported
in the packet is set to 1. Otherwise this bit SHOULD be set to
0. The egress router, PE2, SHOULD set the CLP bit of all cells
that transport the AAL5 CPCS-SDU to the value contained in this
field.
* U ( Command / Response Field ) Bit
When FRF.8.1 Frame Relay / ATM PVC Service Interworking [3]
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 router, PE1, SHOULD copy this bit to the U bit of the
control word. The egress router, PE2, SHOULD copy the U bit to
the CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS PDU.
8.3. ATM 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 [7].
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)
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- F5 loopback (segment and end-to-end)
- Resource Management
- Continuity Check (segment and end-to-end)
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
- Security
The ingress PE SHOULD be able to generate an F5 AIS upon reception of
a corresponding F4 AIS from the CE or due to a lower layer defect
(such as LOS) on the ingress PE port.
The egress PE SHOULD be able to generate an F5 AIS for the VCC due to
a PSN failure. A method to reliably detect a PSN tunnel failure is
required but not specified in this draft.
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.
9. AAL5 PDU frame mode
The AAL5 payload PDU service is OPTIONAL.
9.1. Applicability
The primary application supported by AAL5 PDU frame encapsulation
over PSN is the transparent carriage of ATM layer services that use
AAL5 to carry higher layer frames. The PDU frame mode takes advantage
of the delineation of higher layer frames in the ATM layer to provide
increased bandwidth efficiency compared with the basic cell
encapsulation mode. The nature of the service, as defined by the ATM
service category or the ATM transfer capability should be preserved.
To provide this, the basic requirement of the ATM-PSN interworking
function is to map the AAL5 PDU frames belonging to a VCC, together
with any related OAM and protocol control information, into a PW.
Two network applications that utilize the PDU frame mode
encapsulation are:
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a The transport of multi-service ATM over a packet core network
where AAL5 is used as the adaptation layer. 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 e.g. ATM
OAM and ATM security.
-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.
b L2 VPN service over a PSN infrastructure. In this case, VPN
sites are connected through ATM VCCs, as in today's L2 VPNs.
The transparent PDU frame mode encapsulation allows the VPN
service provider to transparently extend this L2 connectivity
over its PSN while achieving bandwidth efficiency gains over
the basic cell mode and supporting ATM layer applications of
the VPN customer, such as ATM security. The advantage is for
the service provider to combine L2 and L3 services over the
same PSN.
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
AAL5 frame encapsulation. Fragmentation may be performed in order
to maintain the position of the OAM cells with respect to the
user cells.
Fragmentation may also be performed to maintain the size of the
packet carrying the AAL5 PDU within the MTU of the link.
Cell Loss priority (CLP) field conveys the priority of the cell
in the connection. The Explicit Forward Congestion Indicator
(EFCI) field conveys the congestion state of ATM network.
Information on both of these fields is obtained from the ATM cell
header. CLP and EFCI fields are both part of the ATM service
specific information header.
The whole AAL5-PDU is encapsulated. In this case all necessary
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parameters such as CPCS-UU (CPCS User-to-User indicator), CPI
(Common Part Indicator), Length (Length of the CPCS-SDU) and CRC
(Cyclic Redundancy Check) are transported as part of the payload.
Note that carrying of the full PDU also allows the simplification
of the fragmentation operation since it is performed at cell
boundaries and the CRC in the trailer of the AAL5 PDU can be used
to check the integrity of the reassembled fragments.
9.1.1. Implementation and deployment considerations
AAL5 transparent mode is only applicable to services that use AAL5 to
carry higher layer frames over ATM VCCs.
9.1.2. Limitations
AAL5 frame encapsulation only supports point-to-point LSPs. Multi-
point-to-point and point-to-multi-point are for further study (FFS).
Length of AAL5 frame may exceed the MTU of the PSN. This requires
fragmentation, which may not be available to all nodes at the PW
endpoint.
The maximum number of cells of an AAL5 PDU that may be reassembled
before transport across the PSN may be limited by the cell transfers
delay (CTD) and cell delay variation (CDV) objectives of the
connection.
This mode does not preserve the value of the CLP bit for every ATM
cell within an AAL5 PDU. Therefore, transparency of the CLP setting
may be violated. Additionally, tagging of some cells may occur when
tagging is not allowed by the conformance definition.
This mode does not preserve the EFCI state for every ATM cell within
an AAL5 PDU. Therefore, transparency of the EFCI state may be
violated.
9.2. Transparent AAL5 PDU Frame encapsulation
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
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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 10: 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)
- 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
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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.
9.3. ATM 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 [7].
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
The PEs SHALL use the single ATM VCC cell mode encapsulation 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).
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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.
9.4. 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.
9.4.1. 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 7.
9.4.2. 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
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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.
10. Security Considerations
This document specifies only encapsulations, and not the protocols
used to carry the encapsulated packets across the network. Each such
protocol may have its own set of security issues, but those issues
are not affected by the encapsulations specified herein.
11. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
12. References
[1] "Transport of Layer 2 Frames Over MPLS", draft-ietf-pwe3-
control-protocol-00.txt. ( work in progress )
[2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032
[3] "Requirements for Peudo Wire Emulation Edge-to-Edge (PWE3",
draft-ietf-pwe3-requirements-03.txt. ( work in Progress )
[4] ATM Forum Specification fb-fbatm-0151.000 (2000) ,Frame Based ATM
over SONET/SDH Transport (FAST)
[5] ATM Forum Specification af-tm-0121.000 (1999), Traffic Management
Specification Version 4.1.
[6] ITU-T Recommendation I.371 (2000), Traffic control and congestion
control in B-ISDN.
[7] ITU-T Recommendation I.610, (1999), B-ISDN operation and
maintenance principles and functions.
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13. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
e-mail: giles@packetexchange.net
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
e-mail: d@mazunetworks.com
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: tappan@cisco.com
Jayakumar Jayakumar,
Cisco Systems Inc.
225, E.Tasman, MS-SJ3/3,
San Jose , CA, 95134
e-mail: jjayakum@cisco.com
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Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: sjv@laurelnetworks.com
Jeremy Brayley
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: jbrayley@laurelnetworks.com
Gerald de Grace
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: gdegrace@laurelnetworks.com
John Shirron
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: jshirron@laurelnetworks.com
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
e-mail: Andy.Malis@vivacenetworks.com
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Vinai Sirkay
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
e-mail: sirkay@technologist.com
Chris Liljenstolpe
Cable & Wireless
11700 Plaza America Drive
Reston, VA 20190
e-mail: chris@cw.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
e-mail: kireeti@juniper.net
Ghassem Koleyni
Nortel Networks
P O Box 3511, Station C Ottawa, Ontario,
K1Y 4H7 Canada
e-mail: ghassem@nortelnetworks.com
John Fischer
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
e-mail: john.fischer@alcatel.com
Matthew Bocci
Alcatel
Grove House, Waltham Road Rd
White Waltham, Berks, UK. SL6 3TN
e-mail: matthew.bocci@alcatel.co.uk
Mustapha Aissaoui
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
e-mail: mustapha.aissaoui@alcatel.com
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Tom Walsh
Lucent Technologies
1 Robbins Road
Westford, MA 01886 USA
e-mail: tdwalsh@lucent.com
John Rutemiller
Marconi Networks
1000 Marconi Drive
Warrendale, PA 15086
e-mail: John.Rutemiller@marconi.com
Rick Wilder
Masergy Communications
2901 Telestar Ct.
Falls Church, VA 22042
e-mail: rwilder@masergy.com
Laura Dominik
Qwest Communications, Inc.
600 Stinson Blvd.
Minneapolis, MN, 55413
Email: ldomini@qwest.com
Martini, et al. [Page 37]
