| Rfc | 4717 |
|---|
| Title | Encapsulation Methods for Transport of Asynchronous Transfer Mode
(ATM) over MPLS Networks |
|---|
| Author | L. Martini, J. Jayakumar, M. Bocci, N.
El-Aawar, J. Brayley, G. Koleyni |
|---|
| Date | December 2006 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group L. Martini
Request for Comments: 4717 J. Jayakumar
Category: Standards Track Cisco Systems, Inc.
M. Bocci
Alcatel
N. El-Aawar
Level 3 Communications, LLC
J. Brayley
ECI Telecom Inc.
G. Koleyni
Nortel Networks
December 2006
Encapsulation Methods for Transport of
Asynchronous Transfer Mode (ATM) over MPLS Networks
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Abstract
An Asynchronous Transfer Mode (ATM) Pseudowire (PW) is used to carry
ATM cells over an MPLS network. This enables service providers to
offer "emulated" ATM services over existing MPLS networks. This
document specifies methods for the encapsulation of ATM cells within
a pseudowire. It also specifies the procedures for using a PW to
provide an ATM service.
Table of Contents
1. Introduction ....................................................3
2. Specification of Requirements ...................................4
3. Applicability Statement .........................................4
4. Terminology .....................................................4
5. General Encapsulation Method ....................................6
5.1. The Control Word ...........................................6
5.1.1. The Generic Control Word ............................7
5.1.2. The Preferred Control Word ..........................8
5.1.3. Setting the Sequence Number Field in the
Control Word ........................................9
5.2. MTU Requirements ...........................................9
5.3. MPLS Shim S Bit Value .....................................10
5.4. MPLS Shim TTL Values ......................................10
6. Encapsulation Mode Applicability ...............................10
6.1. ATM N-to-One Cell Mode ....................................11
6.2. ATM One-to-One Cell Encapsulation .........................13
6.3. AAL5 SDU Frame Encapsulation ..............................13
6.4. AAL5 PDU Frame Encapsulation ..............................14
7. ATM OAM Cell Support ...........................................15
7.1. VCC Case ..................................................15
7.2. VPC Case ..................................................16
7.3. SDU/PDU OAM Cell Emulation Mode ...........................16
7.4. Defect Handling ...........................................17
8. ATM N-to-One Cell Mode .........................................18
8.1. ATM N-to-One Service Encapsulation ........................19
9. ATM One-to-One Cell Mode .......................................21
9.1. ATM One-to-One Service Encapsulation ......................21
9.2. Sequence Number ...........................................22
9.3. ATM VCC Cell Transport Service ............................22
9.4. ATM VPC Services ..........................................24
9.4.1. ATM VPC Cell Transport Services ....................25
10. ATM AAL5 CPCS-SDU Mode ........................................26
10.1. Transparent AAL5 SDU Frame Encapsulation .................27
11. AAL5 PDU Frame Mode ...........................................28
11.1. Transparent AAL5 PDU Frame Encapsulation .................28
11.2. Fragmentation ............................................30
11.2.1. Procedures in the ATM-to-PSN Direction ............30
11.2.2. Procedures in the PSN-to-ATM Direction ............31
12. Mapping of ATM and PSN Classes of Service .....................31
13. ILMI Support ..................................................32
14. ATM-Specific Interface Parameter Sub-TLVs .....................32
15. Congestion Control ............................................32
16. Security Considerations .......................................33
17. Normative References ..........................................34
18. Informative References ........................................34
19. Significant Contributors ......................................36
1. Introduction
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 benefit of this model to a service
provider is 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.
This document describes a method to carry ATM services over MPLS. It
lists ATM-specific requirements and provides encapsulation formats
and semantics for connecting ATM edge networks through a packet-
switched network using MPLS.
Figure 1, below, displays the ATM services reference model. This
model is adapted from [RFC3985].
|<----- Pseudowire ----->|
| |
| |<-- 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
2. 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 [RFC2119].
3. Applicability Statement
The ATM over PW service is not intended to perfectly emulate a
traditional ATM service, but it can be used for applications that
need an ATM transport service.
The following are notable differences between traditional ATM service
and the protocol described in this document:
- ATM cell ordering can be preserved using the OPTIONAL sequence
field in the control word; however, implementations are not
required to support this feature. The use of this feature may
impact other ATM quality of service (QoS) commitments.
- The QoS model for traditional ATM can be emulated. However, the
detailed specification of ATM QoS emulation is outside the scope
of this document. The emulation must be able to provide the
required ATM QoS commitments for the end-user application.
- The ATM flow control mechanisms are transparent to the MPLS
network and cannot reflect the status of the MPLS network.
- Control plane support for ATM SVCs, SVPs, SPVCs, and SPVPs is
outside the scope of this document.
Note that the encapsulations described in this specification are
identical to those described in [Y.1411] and [Y.1412].
4. Terminology
One-to-one mode: specifies an encapsulation method that maps one ATM
Virtual Channel Connection (VCC) (or one ATM Virtual Path Connection
(VPC)) to one pseudowire.
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 pseudowire.
Packet-Switched Network (PSN): an IP or MPLS network.
Pseudowire Emulation Edge to Edge (PWE3): a mechanism that emulates
the essential attributes of a service (such as a T1 leased line or
Frame Relay) over a PSN.
Customer Edge (CE): 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 (PE): a device that provides PWE3 to a CE.
Pseudowire (PW): a connection between two PEs carried over a PSN.
The PE provides the adaptation between the CE and the PW.
Pseudowire PDU: a PDU sent on the PW that contains all of the data
and control information necessary to provide the desired service.
PSN Tunnel: 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
pseudowire PDU (ATM to PSN direction).
Egress: the point where the ATM service is decapsulated from a
pseudowire PDU (PSN to ATM direction).
CTD: Cell Transfer Delay.
MTU: Maximum Transmission Unit.
SDU: Service Data 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.
Additional terminology relevant to pseudowires and Layer 2 Virtual
Private Networking (L2VPN) in general may be found in [RFC4026].
5. General Encapsulation Method
This section describes the general encapsulation format for ATM over
PSN pseudowires.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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. This header is used to transport the encapsulated
ATM information through the packet-switched core.
The Pseudowire Header identifies a particular ATM service on a
tunnel. In case of MPLS, the pseudowire header is one or more MPLS
labels at the bottom of the MPLS label stack.
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.
5.1. The Control Word
The Control Words defined in this section are based on the Generic PW
MPLS Control Word as defined in [RFC4385]. They provide the ability
to sequence individual frames on the PW, avoidance of equal-cost
multiple-path load-balancing (ECMP) [RFC2992], and OAM mechanisms
including VCCV [VCCV].
[RFC4385] states, "If a PW is sensitive to packet misordering and is
being carried over an MPLS PSN that uses the contents of the MPLS
payload to select the ECMP path, it MUST employ a mechanism which
prevents packet misordering." This is necessary because ECMP
implementations may examine the first nibble after the MPLS label
stack to determine whether or not the labelled packet is IP. Thus,
if the VPI of an ATM connection carried over the PW using N-to-one
cell mode encapsulation, without a control word present, begins with
0x4 or 0x6, it could be mistaken for an IPv4 or IPv6 packet. This
could, depending on the configuration and topology of the MPLS
network, lead to a situation where all packets for a given PW do not
follow the same path. This may increase out-of-order frames on a
given PW, or cause OAM packets to follow a different path than actual
traffic (see section 4.4.3 on Frame Ordering).
The features that the control word provides may not be needed for a
given ATM PW. For example, ECMP may not be present or active on a
given MPLS network, strict frame sequencing may not be required, etc.
If this is the case, and the control word is not REQUIRED by the
encapsulation mode for other functions (such as length or the
transport of ATM protocol specific information), the control word
provides little value and is therefore OPTIONAL. Early ATM PW
implementations have been deployed that do not include a control word
or the ability to process one if present. To aid in backwards
compatibility, future implementations MUST be able to send and
receive frames without a control word present.
In all cases, the egress PE MUST be aware of whether the ingress PE
will send a control word over a specific PW. This may be achieved by
configuration of the PEs, or by signaling, as defined in [RFC4447].
If the pseudowire traverses a network link that requires a minimum
frame size (Ethernet is a practical example), with a minimum frame
size of 64 octets, then such links will apply padding to the
pseudowire 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.
5.1.1. The Generic Control Word
This control word is used in the following encapsulation modes:
- ATM One-to-one Cell Mode
- AAL5 PDU Frame Mode
The PWE3 control word document [RFC4385] 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 One-to-one 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 algorithm 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; therefore, no padding is applied in Ethernet links in the PSN.
5.1.2. The Preferred Control Word
This control word is used in the following encapsulation modes:
- ATM N-to-one 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-one 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 algorithm is not used.
5.1.3. 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 sequencing procedures defined in
[RFC4385] MUST be used.
Packets that are received out of order MAY be dropped or reordered at
the discretion of the receiver.
A simple extension of the processing algorithm in [RFC4385] MAY be
used to detect lost packets.
If a PE router negotiated not to use receive sequence number
processing, and it received a non-zero sequence number, then it
SHOULD send a PW status message indicating a receive fault and
disable the PW.
5.2. 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.
5.3. MPLS Shim S Bit Value
The ingress label switching router (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. For more information on setting the S Bit, see
[RFC3032].
5.4. MPLS Shim TTL Values
The setting of the TTL value in the PW label is application
dependent. In any case, [RFC3032] TTL processing procedure,
including handling of expired TTLs, MUST be followed.
6. Encapsulation Mode Applicability
This document 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
pseudowire. 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 pseudowire. 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:
-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
types.
-iii. AAL5: A 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 [TM4.0] or the ATM
transfer capability [I.371], should be preserved.
6.1. ATM N-to-One 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 pseudowire. However, a service provider may wish to provision
a single VCC to a pseudowire in order to satisfy QoS or restoration
requirements.
The encapsulation also supports the binding of multiple VCCs/VPCs to
a single pseudowire. 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:
-i. Supports all ATM Adaptation Layer 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
[TM4.0] or ATM transfer capability [I.371], 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 an L2 Frame Check Sequence (FCS)
failure.
-viii. Cells can be concatenated from multiple VCCs or VPCs
belonging to different service categories 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 Label
Switched Paths (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.
6.2. 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, and x of N-to-one mode apply.
6.3. AAL5 SDU Frame Encapsulation
This OPTIONAL encapsulation supports the AAL5 model. This mode
allows the transport of ATM AAL5 CSPS-SDUs traveling on a particular
ATM PVC across the network to another ATM PVC. This encapsulation is
used by a PW of type 0x0002 "ATM AAL5 SDU VCC transport" as allocated
in [RFC4446].
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 [RFC2684]-
encapsulated IP PDUs across a PSN.
The AAL5-SDU encapsulation requires Segmentation and Reassembly (SAR)
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 pseudowire 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 extract 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
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 [TM4.0].
-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.
6.4. 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.
The limitations v and vi of the AAL5 SDU mode apply to this mode as
well.
7. ATM OAM Cell Support
7.1. 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
[I.610].
The PEs SHOULD be able to pass the following OAM cells transparently:
- F5 Alarm Indication Signal (AIS) (segment and end-to-end)
- F5 Remote Defect Indicator (RDI) (segment and end-to-end)
- F5 loopback (segment and end-to-end)
- Resource Management
- Performance Management
- Continuity Check
- Security
However, if configured to be an administrative segment boundary, the
PE SHOULD terminate and process F5 segment OAM cells.
F4 OAM cells are inserted or extracted at the VP link termination.
These OAM cells are not seen at the VC link termination and are
therefore not sent across the PSN.
When the PE is operating in AAL5 CPCS-SDU transport mode if it does
not support transport of ATM cells, the PE MUST discard incoming MPLS
frames on an ATM PW that contain a PW label with the T bit set.
7.2. 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
[I.610].
The PEs SHOULD be able to process and pass the following OAM cells
transparently according to [I.610]:
- F4 AIS (segment and end-to-end)
- F4 RDI (segment and end-to-end)
- F4 loopback (segment and end-to-end)
However, if configured to be an administrative segment boundary, the
PE SHOULD terminate and process F4 segment OAM cells.
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
- Security
The OAM cell MAY be encapsulated together with other user data cells
if multiple cell encapsulation is used.
7.3. SDU/PDU OAM Cell Emulation Mode
A PE operating in ATM SDU or PDU transport mode that does not support
transport of OAM cells across a PW MAY provide OAM support on ATM
PVCs using the following procedures:
- Loopback cells response
If an F5 end-to-end OAM cell is received from an ATM VC, by
either PE that is transporting this ATM VC, with a loopback
indication value of 1, and the PE has a label mapping for the ATM
VC, then the PE MUST decrement the loopback indication value and
loop back the cell on the ATM VC. Otherwise, the loopback cell
MUST be discarded by the PE.
- AIS alarm
If an ingress PE, PE1, receives an AIS F4/F5 OAM cell, it MUST
notify the remote PE of the failure. The remote PE, PE2, MUST in
turn send F5 OAM AIS cells on the respective PVCs. Note that if
the PE supports forwarding of OAM cells, then the received OAM
AIS alarm cells MUST be forwarded along the PW as well.
- Interface failure
If the PE detects a physical interface failure, or the interface
is administratively disabled, the PE MUST notify the remote PE
for all VCs associated with the failure.
- PSN/PW failure detection
If the PE detects a failure in the PW, by receiving a label
withdraw for a specific PW ID, or the targeted Label Distribution
Protocol (LDP) session fails, or a PW status TLV notification is
received, then a proper AIS F5 OAM cell MUST be generated for all
the affected ATM PVCs. The AIS OAM alarm will be generated on
the ATM output port of the PE that detected the failure.
7.4. 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 pseudowire-specific maintenance
mechanism such as the PW status message defined in [RFC4447].
Alternatively, for example, the ingress PE MAY withdraw the
pseudowire (PW label) label associated with the service. Upon
receiving such a notification, the egress PE SHOULD generate the
appropriate F4 AIS (for VPC) or F5 AIS (for VCC).
If the PW in one direction fails, then the complete bidirectional
service is considered to have failed.
8. ATM N-to-One Cell Mode
The N-to-one mode (N >= 1) described in this document 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.
8.1. ATM N-to-One Service Encapsulation
This section describes the general encapsulation format for ATM over
PSN pseudowires.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| pseudowire 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. This header is used to transport the encapsulated
ATM information through the packet-switched core.
The Pseudowire Header identifies a particular ATM service on a
tunnel. Non-ATM services may also be carried on the PSN tunnel.
As shown above, in Figure 4, the ATM Control Word is inserted before
the ATM service payload. It may contain a length field and a
sequence number field in addition to certain control bits needed to
carry the service.
The ATM Service Payload is specific to the service being offered via
the pseudowire. 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 [FBATM] section 3.1.1, but without the
trailer byte. The length of each frame, without the encapsulation
headers, is a multiple of 52 bytes. 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 be signaled, for example using the methods described later
in this document and in [RFC4447]. 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. These bits 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 change the CLP bit from
zero to one in order 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:
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 pseudowire,
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 Header Error Check (HEC) field) of the
incoming cell.
9. ATM One-to-One Cell Mode
The One-to-one mode described in this document 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 pseudowire.
9.1. ATM One-to-One Service Encapsulation
This section describes the general encapsulation format for ATM over
pseudowires on an MPLS PSN. 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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 MPLS PSN Transport Header depends on how the MPLS network is
configured. The Pseudowire Header identifies a particular ATM
service within the PSN tunnel created by the PSN Transport Header.
This header is used to transport the encapsulated ATM information
through the packet-switched core.
The generic control word is inserted after the Pseudowire Header.
The presence of the control word is REQUIRED.
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.
9.2. Sequence Number
The sequence number is not required for all services.
Treatment of the sequence number is according to section 5.1.3.
9.3. ATM VCC Cell Transport Service
The VCC cell transport service is characterized by the mapping of a
single ATM VCC (VPI/VCI) to a pseudowire. 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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.
* 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 pseudowire PDU. The ingress
and egress PE MUST agree to a maximum number of cells in a single
pseudowire PDU. This agreement may be accomplished via a
pseudowire-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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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
9.4. ATM VPC Services
The VPC service is defined by mapping a single VPC (VPI) to a
pseudowire. As such, it emulates a 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 pseudowire header. As a VPC
service, the egress PE MUST NOT change the VCI field.
9.4.1. 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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
encapsulate multiple ATM cells into a pseudowire PDU. The ingress
and egress PE MUST agree to a maximum number of cells in a single
pseudowire PDU. This agreement may be accomplished via a
pseudowire-specific signaling mechanism or via static configuration.
If the Egress PE supports cell concatenation, the ingress PE MUST
only concatenate cells up to the "Maximum Number of concatenated ATM
cells in a frame" interface parameter sub-TLV as received as part of
the control protocol [RFC4447].
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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
10. ATM AAL5 CPCS-SDU Mode
The AAL5 payload VCC service defines a mapping between the payload of
an AAL5 VCC and a single pseudowire. 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 pseudowire. Therefore, once the ingress PE reassembles the
AAL5 CPCS-PDU, the PE discards the PAD and CPCS-PDU trailer, and then
the ingress PE inserts the resulting payload into a pseudowire 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 it
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 pseudowire 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).
10.1. 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 N-to-
one cell relay encapsulation, Figure 4. 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 [RFC3916]
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.
11. AAL5 PDU Frame Mode
The AAL5 payload PDU service is OPTIONAL.
11.1. 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 procedures described in the
"Fragmentation" section below, in order to maintain OAM cell
sequencing.
Like the ATM AAL5 payload VCC service, the AAL5 transparent VCC
service is intended to be more efficient than the VCC cell transport
service. However, the AAL5 transparent VCC service carries the
entire AAL5 CPCS-PDU, including the PAD and trailer. Note that the
AAL5 CPCS-PDU is not processed, i.e., an AAL5 frame with an invalid
CRC or length field will be transported. One reason for this is that
there may be a security agent that has scrambled the ATM cell
payloads that form the AAL5 CPCS-PDU.
This service supports all OAM cell flows by using a fragmentation
procedure that ensures that OAM cells are not repositioned in respect
to AAL5 composite cells.
The AAL5 transparent VCC service is OPTIONAL.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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 Pseudowire 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 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.
11.2. Fragmentation
The ingress PE may not always be able to reassemble a full AAL5
frame. This may be because the AAL5 PDU exceeds the pseudowire MTU
or because 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.
11.2.1. 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.
- 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 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).
11.2.2. 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.
12. Mapping of ATM and PSN Classes of Service
This section is provided for informational purposes, and for guidance
only. This section should not be considered part of the standard
proposed in this document.
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
Diffserv framework, a class of service based on the EF PHB is a good
candidate.
Furthermore, ATM service categories have support for multiple
conformance definitions [TM4.0]. 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.
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 that
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
detail in sections 6 through 9 for each encapsulation mode.
13. ILMI Support
An MPLS edge PE MAY provide an ATM Integrated Local Management
Interface (ILMI) to the ATM edge switch. If an ingress PE receives
an ILMI message indicating that the ATM edge switch has deleted a VC,
or if the physical interface goes down, it MUST send a PW status
notification message for all PWs associated with the failure. When a
PW label mapping is withdrawn, or PW status notification message is
received, the egress PE MUST notify its client of this failure by
deleting the VC using ILMI.
14. ATM-Specific Interface Parameter Sub-TLVs
The Interface parameter TLV is defined in [RFC4447], and the IANA
registry with initial values for interface parameter sub-TLV types is
defined in [RFC4446], but the ATM PW-specific interface parameter is
specified as follows:
- 0x02 Maximum Number of concatenated ATM cells.
A 2-octet value specifying the maximum number of concatenated ATM
cells that can be processed as a single PDU by the egress PE. An
ingress PE transmitting concatenated cells on this PW can
concatenate a number of cells up to the value of this parameter,
but MUST NOT exceed it. This parameter is applicable only to PW
types 3, 9, 0x0a, 0xc, [RFC4446], and 0xd and is REQUIRED for
these PWC types. This parameter does not need to match in both
directions of a specific PW.
15. Congestion Control
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion, with congestion characteristics depending on PSN type,
network architecture, configuration, and loading. During congestion
the PSN may exhibit packet loss that will impact the service carried
by the ATM PW. In addition, since ATM PWs carry a variety of
services across the PSN, including but not restricted to TCP/IP, they
may or may not behave in a TCP-friendly manner prescribed by
[RFC2914]. In the presence of services that reduce transmission
rate, ATM PWs may thus consume more than their fair share and in that
case SHOULD be halted.
Whenever possible, ATM PWs should be run over traffic-engineered PSNs
providing bandwidth allocation and admission control mechanisms.
IntServ-enabled domains providing the Guaranteed Service (GS) or
Diffserv-enabled domains using EF (expedited forwarding) are examples
of traffic-engineered PSNs. Such PSNs will minimize loss and delay
while providing some degree of isolation of the ATM PW's effects from
neighboring streams.
It should be noted that when transporting ATM, Diffserv-enabled
domains may use AF (Assured Forwarding) and/or DF (Default
Forwarding) instead of EF, in order to place less burden on the
network and gain additional statistical multiplexing advantage. In
particular, Table 1 of Appendix "V" in [ATM-MPLS] contains a detailed
mapping between ATM classes and Diffserv classes.
The PEs SHOULD monitor for congestion (by using explicit congestion
notification, [VCCV], or by measuring packet loss) in order to ensure
that the service using the ATM PW may be maintained. When a PE
detects significant congestion while receiving the PW PDUs, the PE
MAY use RM cells for ABR connections to notify the remote PE.
If the PW has been set up using the protocol defined in [RFC4447],
then procedures specified in [RFC4447] for status notification can be
used to disable packet transmission on the ingress PE from the egress
PE. The PW may be restarted by manual intervention, or by automatic
means after an appropriate waiting time.
16. Security Considerations
This document specifies only encapsulations, not the protocols used
to carry the encapsulated packets across the PSN. Each such protocol
may have its own set of security issues [RFC4447][RFC3985], 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 than a native ATM service.
17. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, February 2006.
18. Informative References
[FBATM] ATM Forum Specification af-fbatm-0151.000 (2000), "Frame
Based ATM over SONET/SDH Transport (FAST)"
[TM4.0] ATM Forum Specification af-tm-0121.000 (1999), "Traffic
Management Specification Version 4.1"
[I.371] ITU-T Recommendation I.371 (2000), "Traffic control and
congestion control in B-ISDN".
[I.610] ITU-T Recommendation I.610, (1999), "B-ISDN operation and
maintenance principles and functions".
[Y.1411] ITU-T Recommendation Y.1411 (2003), ATM-MPLS Network
Interworking - Cell Mode user Plane Interworking
[Y.1412] ITU-T Recommendation Y.1412 (2003), ATM-MPLS network
interworking - Frame mode user plane interworking
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC3916] Xiao, X., McPherson, D., and P. Pate, "Requirements for
Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
September 2004.
[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026, March 2005.
[VCCV] Nadeau, T., Pignataro, C., and R. Aggarwal, "Pseudowire
Virtual Circuit Connectivity Verification (VCCV)", Work in
Progress, June 2006.
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, November 2000.
[ATM-MPLS] ATM Forum Specification af-aic-0178.001, "ATM-MPLS Network
Interworking Version 2.0", August 2003.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC2684] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation
over ATM Adaptation Layer 5", RFC 2684, September 1999.
19. Significant Contributors
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
EMail: giles.heron@tellabs.com
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
EMail: d@mazunetworks.com
Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: tappan@cisco.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: erosen@cisco.com
Steve Vogelsang
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: stephen.vogelsang@ecitele.com
Gerald de Grace
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: gerald.degrace@ecitele.com
John Shirron
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: john.shirron@ecitele.com
Andrew G. Malis
Verizon Communications
40 Sylvan Road
Waltham, MA
EMail: andrew.g.malis@verizon.com
Phone: 781-466-2362
Vinai Sirkay
Redback Networks
300 Holger Way
San Jose, CA 95134
EMail: vsirkay@redback.com
Chris Liljenstolpe
Alcatel
11600 Sallie Mae Dr.
9th Floor
Reston, VA 20193
EMail: chris.liljenstolpe@alcatel.com
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: kireeti@juniper.net
John Fischer
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
EMail: john.fischer@alcatel.com
Mustapha Aissaoui
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
EMail: mustapha.aissaoui@alcatel.com
Tom Walsh
Lucent Technologies
1 Robbins Road
Westford, MA 01886 USA
EMail: tdwalsh@lucent.com
John Rutemiller
Marconi Networks
1000 Marconi Drive
Warrendale, PA 15086
EMail: John.Rutemiller@marconi.com
Rick Wilder
Alcatel
45195 Business Court
Loudoun Gateway II Suite 300
M/S STERV-SMAE
Sterling, VA 20166
EMail: Rick.Wilder@alcatel.com
Laura Dominik
Qwest Communications, Inc.
600 Stinson Blvd.
Minneapolis, MN 55413
Email: ldomini@qwest.com
Authors' Addresses
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
EMail: lmartini@cisco.com
Jayakumar Jayakumar
Cisco Systems, Inc.
225 E.Tasman, MS-SJ3/3
San Jose, CA 95134
EMail: jjayakum@cisco.com
Matthew Bocci
Alcatel
Grove House, Waltham Road Rd
White Waltham, Berks, UK. SL6 3TN
EMail: matthew.bocci@alcatel.co.uk
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO 80021
EMail: nna@level3.net
Jeremy Brayley
ECI Telecom Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: jeremy.brayley@ecitele.com
Ghassem Koleyni
Nortel Networks
P O Box 3511, Station C Ottawa, Ontario,
K1Y 4H7 Canada
EMail: ghassem@nortelnetworks.com
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