Network Working Group Luca Martini
Internet Draft Eric C. Rosen
Expiration Date: October 2004 Cisco Systems, Inc.
Jeremy Brayley Matthew Bocci
Laurel Networks, Inc. Alcatel
Ghassem Koleyni
Nortel Networks.
April 2004
Encapsulation Methods for Transport of ATM Over IP and MPLS Networks
draft-ietf-pwe3-atm-encap-05.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.
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Table of Contents
1 Specification of Requirements .......................... 3
2 Introduction ........................................... 3
3 Terminology ............................................ 4
4 General encapsulation method ........................... 6
4.1 MPLS Shim S Bit Value .................................. 6
4.2 MPLS Shim TTL Values ................................... 6
4.3 The Control Word ....................................... 7
4.3.1 The Generic Control Word ............................... 7
4.3.2 The Preferred Control Word ............................. 9
4.3.3 Setting the sequence number field in the control word .. 9
4.3.4 Sequence number field processing in the control word ... 10
4.4 MTU Requirements ....................................... 11
5 Applicability .......................................... 11
5.1 ATM N to 1 Cell Mode ................................... 12
5.2 ATM One-to-One Cell Encapsulation ...................... 14
5.3 AAL5 SDU Frame Encapsulation ........................... 14
5.4 AAL5 PDU Frame Encapsulation ........................... 15
6 ATM OAM Cell Support ................................... 16
6.1 VCC Case ............................................... 16
6.2 VPC Case ............................................... 16
6.3 Defect Handling ........................................ 17
7 ATM N-to-one Cell Mode ................................. 18
7.1 ATM N-to-one Service Encapsulation ..................... 18
8 ATM One-to-one Cell Mode ............................... 21
8.1 ATM One-to-one Service Encapsulation ................... 21
8.2 Sequence Number ........................................ 22
8.3 ATM VCC Cell Transport Service ......................... 22
8.4 ATM VPC Services ....................................... 24
8.4.1 ATM VPC Cell Transport Services ........................ 24
9 ATM AAL5 CPCS-SDU Mode ................................. 25
9.1 Transparent AAL5 SDU Frame Encapsulation ............... 26
10 AAL5 PDU frame mode .................................... 27
10.1 Transparent AAL5 PDU Frame Encapsulation ............... 27
10.2 Fragmentation .......................................... 29
10.2.1 Procedures in the ATM-to-PSN Direction ................. 29
10.2.2 Procedures in the PSN-to-ATM Direction ................. 30
11 Mapping of ATM and PSN Classes of Service .............. 30
12 Security Considerations ................................ 31
13 Intellectual Property Disclaimer ....................... 31
14 References ............................................. 31
15 Author Information ..................................... 32
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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
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 document describes a method to carry ATM services over 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 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 Pseudo Wire.
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 Pseudo Wire.
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 a
service originates and/or terminates. The CE is not aware that it is
using an emulated service rather than a native 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 Transmission Unit
OAM - Operations And Maintenance.
PVC - Permanent Virtual Connection. An ATM connection that is
provisioned via a network management interface. The connection is
not signaled.
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.
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4. General encapsulation method
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 2: 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
the encapsulated ATM information through the packet switched core.
The Pseudo Wire Header identifies a particular ATM service on a
tunnel. In case of MPLS the Pseudo Wire Header is the MPLS label at
the bottom of the MPLS label stack. In the Case of L2TP the Pseudo
Wire Header is the L2TP header.
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.
4.1. MPLS Shim S Bit Value
The ingress LSR, PE1, MUST set the S bit of the PW label to a value
of 1 to denote that the VC label is at the bottom of the stack.
4.2. MPLS Shim TTL Values
The setting of the TTL value in the PW label is application
dependent, however in a strict point to point application the TTL
SHOULD be appropriately set to 2.
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4.3. The Control Word
There are four requirements that may need to be satisfied when
transporting layer 2 protocols over an IP or MPLS backbone [8]:
-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.
-iv. To allow accurate packet inspection in an MPLS PSN, and/or
to operate correctly over MPLS PSNs that have deployed
equal-cost multiple-path load-balancing, a PW packet MUST
NOT alias an IP packet.
The PWE3 architecture document describes a generic control word and a
preferred control word. This document makes use of both of these
control words depending on the encapsulation mode. Both of these
control words addresses all of the above requirements.
For some encapsulation modes, the control word is REQUIRED, and for
others OPTIONAL. 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 pseudo wire. This may
be achieved by configuration of the routers, or by signaling, for
example as defined in [1].
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
control word must include a length field set to the PDU length. A
mechanism is required for the egress PE to detect and remove such
padding.
4.3.1. The Generic Control Word
This control word is used in the following encapsulation modes:
- ATM 1 to 1 Cell Mode
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- AAL5 PDU Frame Mode
The PWE3 architecture document [8] provides the following structure
for the generic control word:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Specified by PW Encapsulation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The detailed structure for the ATM 1 to 1 Cell Mode and for the AAL5
PDU Frame Mode is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram the first 4 bits MUST be set to 0 to indicate PW
data. They MUST be ignored by the receiving PE.
The next four bits are reserved and MUST be set to 0 upon
transmission and ignored upon reception.
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 that the sequence number
check alghorithm is not used.
The last 8 bits provide space for carrying ATM specific flags. These
are defined in the protocol-specific details below.
There is no requirement for a length field for the One-to-one cell
and PDU Frame modes because the PSN PDU is always greater than 64
bytes and so no padding is applied in Ethernet links in the PSN.
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4.3.2. The Preferred Control Word
This control word is used in the following encapsulation modes:
- ATM N to 1 Cell Mode
- AAL5 SDU Frame Mode
It 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Flags |Res| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram the first 4 bits MUST be set to 0 to indicate PW
data. They MUST be ignored by the receiving PE.
The next 4 bits provide space for carrying protocol specific flags.
These are defined in the protocol-specific details below.
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. Note that the length field is
not used in the N-to-1 mode , and MUST be set to 0.
The last 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 that the sequence number
check alghorithm is not used.
4.3.3. Setting the sequence number field in the control word
This section applies to the sequence number field of both the Generic
and Preferred Control Words.
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:
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- 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.
4.3.4. Sequence number field processing in the control word
This section applies to the sequence number field of both the Generic
and Preferred Control Words.
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 is in order then, it can be delivered immediately. If the
packet is in order, then the expected sequence number MUST 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.
A simple extension of the above processing algorithm can be used to
detect lost packets.
If a router PE2 does not support receive sequence number processing,
then the sequence number field MAY be ignored.
4.4. 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. Applicability
This Draft defines two methods for encapsulation of ATM cells,
namely, One-to-one mode and N-to-one mode.
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. This is the only REQUIRED mode. 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.
The One-to-one mode specifies an encapsulation method that maps one
ATM VCC or one ATM VPC to one Pseudo-Wire. For VCCs, the VPI/VCI is
not included. For VPCs, the VPI is not included. Cells from one VCC
or one VPC may be concatenated. This mode is OPTIONAL.
Furthermore different OPTIONAL encapsulations are supported for ATM
AAL5 transport: one for ATM AAL5 SDUs, and another for ATM AAL5 PDUs.
Three deployment models are supported by the encapsulations described
in this document:
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-i. Single ATM Connection: A PW carries the cells of only one
ATM VCC or VPC. This supports both the transport of
multiservice ATM and L2VPN service over a PSN for all AAL
types.
-ii. Multiple ATM Connections: A PW carries the cells of multiple
ATM VCCs and / or VPCs . This also supports both the
transport of multiservice ATM and L2VPN service over a PSN
for all AAL type.
-iii. AAL5: PW carries the AAL5 frames of only one ATM VCC. A
large proportion of the data carried on ATM networks is
frame based and therefore uses AAL5. The AAL5 mapping takes
advantage of the delineation of higher layer frames in the
ATM layer to provide increased bandwidth efficiency compared
with the basic cell mapping. The nature of the service, as
defined by the ATM service category [5] or the ATM transfer
capability [6] should be preserved.
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.
5.1. ATM N to 1 Cell Mode
This encapsulation supports both the Single and Multiple ATM
Connection deployment models. This encapsulation is REQUIRED.
The encapsulation allows multiple VCCs/VPCs to be carried within a
single pseudo wire. However, a service provider may wish to provision
a single VCC to a pseudo wire 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.
In the simplest case, this encapsulation can be used to transmit a
single ATM cell per PSN PDU. However, in order to provide better PSN
bandwidth efficiency, several ATM cells may optionally be
encapsulated in a single PSN PDU. This process is called cell
concatenation.
The encapsulation has the following attributes:
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-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. Cell concatenation adds latency
and delay variation to a cell relay service.
-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.
-v. If the Single ATM connection deployment model is used, then
it is simpler to provide an ATM layer service. The nature of
the service, as defined by the ATM service category [5] or
ATM transfer capability [6], should be preserved.
The limitations of the ATM N-to-one cell encapsulation are:
-vi. 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.
-vii. The ATM cell header checksum can detect a 2-bit error or
detect and 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.
-viii. Cells can be concatenated from multiple VCCs or VPCs
belonging to different service cathegories and qos
requirements. In this case the PSN packet must receive
treatment by the PSN to support the highest QoS of the ATM
VCCs/VPCs carried.
-ix. Cell encapsulation only supports point-to-point LSPs.
Multipoint-to-point and point-to-multi-point are for further
study (FFS).
-x. The number of concatenated ATM cells is limited by the MTU
size and the cell transfer delay (CTD) and cell delay
variation (CDV) objectives of multiple ATM connections that
are multiplexed into a single PW.
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5.2. ATM One-to-One Cell Encapsulation
This OPTIONAL encapsulation supports the Single ATM Connection
deployment model.
Like the N to one cell encapsulation mode, the One-to-one mode
supports cell concatenation. The advantage of this encapsulation is
that it utilizes less bandwidth that the N-to-one encapsulation, for
a given number of concatenated cells. Since only one ATM VCC or VPC
is carried on a PW, the VCI and/or VPI of the ATM VCC or VPC can be
derived from the context of the PW using the PW label. These fields
therefore do not need to be encapsulated for a VCC, and only the VCI
needs to be encapsulated for a VPC. This encapsulation thus allows
service providers to achieve a higher bandwidth efficiency on PSN
links than the N-to-one encapsulation for a given number of
concatenated cells.
The limitations vi,vii,ix,x of N to one mode apply.
5.3. AAL5 SDU Frame Encapsulation
This OPTIONAL encapsulation supports the AAL5 model.
The AAL5 SDU encapsulation is more efficient for small AAL5 SDUs than
the VCC cell encapsulations. 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.
The limitations of the AAL5 SDU encapsulation are:
-i. If an ATM OAM cell is received at the ingress PE, it is sent
before the cells of the surrounding AAL5 frame. Therefore,
OAM cell reordering may occur, which may cause certain ATM
OAM performance monitoring and ATM security applications to
operate incorrectly.
-ii. If the ALL5 PDU is scrambled using ATM security standards, a
PE will not be able to exctract the ALL5 SDU and therefore
the whole PDU will be dropped.
-iii. The AAL5 PDU CRC is not transported across the PSN. The CRC
must therefore be regenerated at the egress PE. Since the
CRC has end-to-end significance in ATM security. This means
that the AAL5 CRC may not be used to accurately check for
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errors on the end-to-end ATM VCC.
-iv. The 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.
-v. 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 [5].
-vi. 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.
5.4. AAL5 PDU Frame Encapsulation
This OPTIONAL encapsulation supports the AAL5 model.
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 main advantage of this AAL5
mode is that it is transparent to ATM OAM and ATM security
applications.
One important consideration is to allow OAM information to be treated
as in the original network. This encapsulation mode allows this
transparency while performing AAL5 frame encapsulation. This mode
supports fragmentation, which 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.
Fragmentation provides a means for the PE to set the size of the PW
packet to a different value than that of the original AAL5 PDU. This
means that the PE has control on the delay and jitter provided to the
ATM cells.
The whole AAL5-PDU is encapsulated. In this case all necessary
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 PDU.
Reassembly is not required at the egress PE for the PSN-to-ATM
direction.
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The limitations v and vi of the AAL5 SDU mode apply to this mode as
well.
6. ATM OAM Cell Support
6.1. VCC Case
In general when configured for ATM VCC 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)
- F5 loopback (segment and end-to-end)
- Resource Management
- Performance Management
- Continuity Check
- Security
F4 OAM cells are inserted or extracted at the VP link termination.
These OAM cells are not seen at the VC link termination and are
therefore not sent across the PSN.
6.2. VPC Case
When configured for a VPC cell relay service, both PEs SHOULD act as
a VP cross-connect in accordance with the OAM procedures defined in
[7].
The PEs SHOULD be able to process and pass the following OAM cells
transparently according to [7]:
- F4 AIS (segment and end-to-end)
- F4 RDI (segment and end-to-end)
- F4 loopback (segment and end-to-end)
F5 OAM are not inserted or extracted here. 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
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- Security
The OAM cell MAY be encapsulated together with other user data cells
if multiple cell encapsulation is used.
6.3. Defect Handling
Figure 3 illustrates four possible locations for defects on the PWE3
service:
- (a) On the ATM connection from CE to PE
- (b) On the ATM side of the PW
- (c) On the PSN side of the PE
- (d) In the PSN
+----+ +----+
+----+ | PE1|==================| PE2| +----+
| |---a------|b..c........PW1...d.........|----------| |
| CE1| | | | | |CE2 |
| |----------|............PW2.............|----------| |
+----+ | |==================| | +----+
^ +----+ +----+ ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 3: Defect Locations
For failures at (a) or (b) in the VPC case the ingress PE MUST be
able to generate an F4 AIS upon reception of a lower layer defect
(such as LOS). In the VCC case, 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). These messages are sent across the PSN.
For failures at (c) or (d), in the VCC case 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). In the VPC case, 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 such as the PW status message defined in [1].
Alternatively, 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
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(for VPC) or F5 AIS (for VCC).
If the ingress PE cannot support the generation of OAM cells, it MAY
notify the egress PE using a Pseudo Wire specific maintenance
mechanism such as the PW status message defined in [1].
Alternatively, 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.
If the PW in one direction fails, then the complete bidirectional
service is considered to have failed.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Flags |Res| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: 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
the encapsulated ATM information through the packet switched core.
The Pseudo Wire Header identifies a particular ATM service on a
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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.
The encapsulation of a single ATM cell is the only REQUIRED
encapsulation for ATM. The encapsulation of more than one ATM cell in
a PSN frame is OPTIONAL.
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, and length 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 5: Multiple Cell ATM Encapsulation
* When multiple VCCs or VPCs are transported in one pseudo-wire
VPI/VCI values MUST be unique. When the multiple VCCs or VPCs,
are from different a physical transmission path it may be
necessary to assign unique VPI/VCI values to the ATM connections.
If they are from the same physical transmission path, the VPI/VCI
values are unique.
* 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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8. 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.
8.1. 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 6
provides a general format for encapsulation of ATM cells into
packets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Optional Sequence Number | ATM Specific |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: 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 generic control word is inserted after the Pseudo Wire Header.
The presence of the control word is MANDATORY.
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
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being encapsulated.
8.2. Sequence Number
The sequence number is not required for all services.
Treatment of the sequence number is according to previous sections
"Setting the sequence number", and "Processing the sequence number".
8.3. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Optional Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload ( 48 bytes ) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: 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.
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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
specific 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Optional Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload ( 48 bytes ) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|V|Res| PTI |C| |
+-+-+-+-+-+-+-+-+ |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 8: Multiple ATM VCC Cell Encapsulation
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8.4. ATM VPC Services
The VPC service is defined by mapping a single VPC (VPI) to a Pseudo
Wire. As such it emulates as Virtual Path cross-connect across the
PSN. All VCCs belonging to the VPC are carried transparently by the
VPC service.
The egress PE may choose to apply a different VPI other than the one
that arrived at the ingress PE. The egress PE MUST choose the
outgoing VPI based solely upon the Pseudo Wire header. As a VPC
service, the egress PE MUST NOT change the VCI field.
8.4.1. ATM VPC Cell Transport Services
The ATM VPC cell transport service is OPTIONAL.
This service MUST use the following cell mode encapsulation:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Optional Sequence Number |M|V|Res| PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCI | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| ATM Cell Payload ( 48 bytes ) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: 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
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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 specific byte MUST be repeated for each cell. This means that 51
bytes are used to encapsulate each 53 byte ATM cell.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Optional 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 10: Multiple Cell VPC Encapsulation
9. 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
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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. All OAM cells,
regardless of their type, that arrive during the reassembly of a
single AAL5 CPCS-PDU are sent immediately on the Pseudo Wire using
N-to-one cell encapsulation, 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).
9.1. 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 11: 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
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
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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.
10. AAL5 PDU frame mode
The AAL5 payload PDU service is OPTIONAL.
10.1. 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
service is intended to be more efficient than the VCC cell transport
service. However, the AAL5 transparent VCC service carries the entire
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Resvd | Optional Sequence Number |M|V| Res |U|E|C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| " |
| AAL5 CPCS-PDU |
| (n * 48 bytes) |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: AAL5 transparent service encapsulation
The generic control word is inserted after the Pseudo Wire Header.
The presence of the control word is MANDATORY.
The M, V, Res, and C bits are as defined earlier for VCC One-to-one
cell mode.
* U Bit
This field indicates whether this frame contains the last cell of
an AAL5 PDU and represents the value of the ATM User-to-User bit
for the last ATM cell of the PSN frame. Note: The ATM User-to-
User bit is the least significant bit of the PTI field in the ATM
header. This field is used to support the fragmentation
functionality described later in this section.
* 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.
10.2. 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.
When fragmentation occurs, the procedures described in the following
subsections shall be followed.
10.2.1. Procedures in the ATM-to-PSN Direction
The following procedures shall apply while fragmenting AAL5 PDUs:
- Fragmentation shall always occur at cell boundaries within the
AAL5 PDU.
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- Set the UU bit to the value of the ATM User-to-User bit in the
cell header of the most recently received ATM cell.
- The E and C bits of the fragment shall be set as defined earlier
in section 9.
- If the arriving cell is an OAM or an RM cell, send the current
PSN frame and then send the OAM or RM cell using One-to-one
single cell encapsulation (VCC).
10.2.2. Procedures in the PSN-to-ATM Direction
The following procedures shall apply:
- The 3-bit PTI field of each ATM cell header is constructed as
follows:
-i. The most significant bit is set to 0, indicating a user
data cell.
-ii. The middle bit is set to the E bit value of the
fragment.
-iii. The least significant bit for the last ATM cell in the
PSN frame is set to the value of the UU bit of Figure
12.
-iv. The least significant PTI bit is set to 0 for all other
cells in the PSN frame.
- The CLP bit of each ATM cell header is set to the value of the C
bit of the control byte in Figure 12.
- When a fragment is received, each constituent ATM cell is sent in
correct order.
11. Mapping of ATM and PSN Classes of Service
This section is informational.
When ATM PW service is configured over a PSN, the ATM service
category of a connection 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, a class of service based on the EF PHB is a good
candidate.
Furthermore, ATM service categories have support for multiple
conformance definitions [5]. Some are CLP blind, e.g., CBR, meaning
that the QoS objectives apply to the aggregate CLP0+1 conforming cell
flow. Some are CLP significant, e.g., VBR.3, meaning that the QoS
objectives apply to the CLP0 conforming cell flow only.
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When the PSN is MPLS based, a mapping between the CLP bit and the EXP
field can be performed to provide visibility of the cell loss
priority in the MPLS network. The actual value to be marked in the
EXP field depends on the ATM service category, the ATM conformance
definition, and the type of tunnel LSP used (E-LSP or L-LSP). The
details of this mapping are outside the scope of this document.
Operators have the flexibility to design a specific mapping which
satisfies their own requirements.
In both the ATM-to-PSN and PSN-to-ATM directions, the method used to
transfer the CLP and EFCI information of the individual cells into
the ATM specific field, or flags, of the PW packet is described in
details in sections 6 through 9 for each encapsulation mode.
12. Security Considerations
This document specifies only encapsulations, and not the protocols
used to carry the encapsulated packets across the PSN. Each such
protocol may have its own set of security issues, but those issues
are not affected by the encapsulations specified herein. Note that
the security of the transported ATM service will only be as good as
the security of the PSN. This level of security might be less
rigorous then a native ATM service.
13. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
14. References
[1] "Transport of Layer 2 Frames Over MPLS", draft-ietf-pwe3-
control-protocol-06.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-08.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.
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[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.
[8] "PWE3 Architecture", draft-ietf-pwe3-arch-07.txt. (work in
progress)
15. Author Information
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
e-mail: lmartini@cisco.com
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
e-mail: giles.heron@tellabs.com
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
e-mail: d@mazunetworks.com
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Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
e-mail: tappan@cisco.com
Jayakumar Jayakumar,
Cisco Systems Inc.
170, W.Tasman,
San Jose , CA, 95134
e-mail: jjayakum@cisco.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
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
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John Shirron
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: jshirron@laurelnetworks.com
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
e-mail: Andy.Malis@tellabs.com
Vinai Sirkay
Reliance Infocomm
Dhirubai Ambani Knowledge City
Navi Mumbai 400 709
India
e-mail: vinai@sirkay.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
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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
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
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