Network Working Group Luca Martini
Internet Draft Nasser El-Aawar
Expiration Date: December 2003 Level 3 Communications, LLC.
Toby Smith Eric C. Rosen
Laurel Networks, Inc. Cisco Systems, Inc.
Giles Heron
PacketExchange Ltd.
June 2003
Pseudowire Setup and Maintenance using LDP
draft-ietf-pwe3-control-protocol-03.txt
Status of this Memo
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Abstract
Layer 2 services (such as Frame Relay, ATM, ethernet) can be
"emulated" over an IP and/or MPLS backbone by encapsulating the layer
2 PDUs and then transmitting them over "pseudowires". It is also
possible to use pseudowires to provide SONET circuit emulation over
an IP and/or MPLS network. This document specifies a protocol for
establishing and maintaining the pseudowires, using extensions to
LDP. Procedures for encapsulating layer 2 PDUs are specified in a
set of companion documents.
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Table of Contents
1 Specification of Requirements .......................... 3
2 Introduction ........................................... 3
3 The Pseudowire Label ................................... 5
4 Details Specific to Particular Emulated Services ....... 6
4.1 Frame Relay ............................................ 6
4.2 ATM .................................................... 6
4.2.1 ATM AAL5 SDU VCC Transport ............................. 6
4.2.2 ATM Transparent Cell Transport ......................... 7
4.2.3 ATM n-to-one VCC and VPC Cell Transport ................ 7
4.2.4 OAM Cell Support ....................................... 7
4.2.5 ILMI Support ........................................... 8
4.2.6 ATM AAL5 PDU VCC Transport ............................. 9
4.2.7 ATM one-to-one VCC and VPC Cell Transport .............. 9
4.3 Ethernet VLAN .......................................... 9
4.4 Ethernet ............................................... 9
4.5 HDLC and PPP ........................................... 10
4.6 IP Layer2 Transport .................................... 10
5 LDP .................................................... 10
5.1 The PWid FEC Element ................................... 11
5.2 The Generalized ID FEC Element ......................... 12
5.2.1 Attachment Identifiers ................................. 13
5.2.2 Encoding the Generalized ID FEC Element ................ 14
5.2.3 Procedures ............................................. 15
5.3 Signaling of Pseudo Wire Status ........................ 16
5.3.1 Use of Label Mappings. ................................. 16
5.3.2 Signaling PW status. ................................... 16
5.4 Interface Parameters Field ............................. 17
5.4.1 PW types for which the control word is REQUIRED ........ 19
5.4.2 PW types for which the control word is NOT mandatory ... 20
5.4.3 Status codes ........................................... 21
5.5 LDP label Withdrawal procedures ........................ 21
5.6 Sequencing Considerations .............................. 22
5.6.1 Label Mapping Advertisements ........................... 22
5.6.2 Label Mapping Release .................................. 23
6 Security Considerations ................................ 23
7 References ............................................. 23
8 Author Information ..................................... 24
9 Additional Contributing Authors ........................ 25
10 Appendix A - C-bit Handling Procedures Diagram ......... 28
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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
In [7], [10], and [12] it is explained how to encapsulate a layer 2
Protocol Data Unit (PDU) for transmission over an IP and/or MPLS
network. Those specifications that a "pseudowire header", consisting
of a demultiplexor field, will be prepended to the encapsulated PDU.
The pseudowire demultiplexor field is put on before transmitting a
packet on a pseudowire. When the packet arrives at the remote
endpoint of the pseudowire, the demultiplexor is what enables the
receiver to identify the particular pseudowire on which the packet
has arrived. To actually transmit the packet from one pseudowire
endpoint to another, the packet may need to travel through a "PSN
tunnel"; this will require an additional header to be prepended to
the packet.
An accompanying document [8] also describes a method for transporting
time division multiplexed (TDM) digital signals (TDM circuit
emulation) over a packet-oriented MPLS network. The transmission
system for circuit-oriented TDM signals is the Synchronous Optical
Network (SONET)[5]/Synchronous Digital Hierarchy (SDH) [6]. To
support TDM traffic, which includes voice, data, and private leased
line service, the pseudowires must emulate the circuit
characteristics of SONET/SDH payloads. The TDM signals and payloads
are encapsulated for transmission over pseudowires. To this
encapsulation is prepended a pseudowire demultiplexor and a PSN
tunnel header.
In this document, we specify the use of the MPLS Label Distribution
Protocol, LDP [RFC3036], as a protocol a protocol for setting up and
maintaining the pseudowires. In particular, we define new TLVs for
LDP, which enable LDP to identify pseudowires and to signal
attributes of pseudowires. We specify how a pseudowire endpoint uses
these TLVs in LDP to bind a demultiplexor field value to a
pseudowire, and how it informs the remote endpoint of the binding.
We also specify procedures for reporting pseudowire status changes,
passing additional information about the pseudowire as needed, and
for releasing the bindings.
In the protocol specified herein, the pseudowire demultiplexor field
is an MPLS label. Thus the packets which are transmitted from one
end of the pseudowire to the other are MPLS packets. Unless the
pseudowire endpoints are immediately adjacent, these MPLS packets
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must be transmitted through a PSN tunnel. Any sort of PSN tunnel can
be used, as long as it is possible to transmit MPLS packets through
it. The PSN tunnel can itself be an LSP, but it could equally well
be an IP tunnel, a GRE tunnel, an IPsec tunnel, or any other sort of
tunnel which can carry MPLS packets. Procedures for setting up and
maintaining the PSN tunnels are outside the scope of this document.
This document deals only with the setup and maintenance of point-to-
point pseudowires. Neither point-to-multipoint nor multipoint-to-
point pseudowires are discussed.
QoS related issues are not discussed in this document.
The following two figures describe the reference models which are
derived from [13] to support the Ethernet PW emulated services.
Native |<----- Pseudo Wire ---->| Native
Layer2 | | Layer2
Service | |<-- PSN Tunnel -->| | Service
| V V V V |
| +----+ +----+ |
+----+ | | PE1|==================| PE2| | +----+
| |----------|............PW1.............|----------| |
| CE1| | | | | | | |CE2 |
| |----------|............PW2.............|----------| |
+----+ | | |==================| | | +----+
^ +----+ +----+ | ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Figure 1: PWE3 Reference Model
+-------------+ +-------------+
| Layer2 | | Layer2 |
| Emulated | | Emulated |
| Services | Emulated Service | Services |
| |<==============================>| |
+-------------+ Pseudo Wire +-------------+
|Demultiplexor|<==============================>|Demultiplexor|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
| MPLS |<==============================>| MPLS |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
Figure 2: PWE3 Protocol Stack Reference Model
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For the purpose of this document, PE1 will be defined as the ingress
router, and PE2 as the egress router. A layer 2 PDU will be received
at PE1, encapsulated at PE1, transported, decapsulated at PE2, and
transmitted out of PE2.
3. The Pseudowire Label
Suppose it is desired to transport layer 2 PDUs from ingress LSR PE1
to egress LSR PE2, across an intervening PSN. We assume that there is
a PSN tunnel from PE1 to PE2. That is, we assume that PE1 can cause a
packet to be delivered to PE2 by encapsulating the packet in a "PSN
tunnel header" and sending the result to one of its adjacencies. If
the PSN tunnel is an MPLS Label Switched Path (LSP), then putting on
a PSN tunnel encapsulation is a matter of pushing on an additional
MPLS label; if the PSN tunnel is, e.g., a GRE tunnel, then putting on
the tunnel encapsulation requires prepending an IP header and a GRE
header.
We presuppose that an arbitrary number of pseudowires can be carried
through a single PSN tunnel. Thus it is never necessary to maintain
state in the network core for individual pseudowires. We do not
presuppose that the PSN tunnels are point-to-point; although the
pseudowires are point-to-point, the PSN tunnels may be multipoint-
to-point. We do not presuppose that PE2 will even be able to
determine the PSN tunnel through which a received packet was
transmitted. (E.g., if the PSN tunnel is an LSP, and penultimate hop
popping is used, when the packet arrives at PE2 it will contain no
information identifying the tunnel.)
When PE2 receives a packet over a pseudowire, it must be able to
determine that the packet was in fact received over a pseudowire, and
it must be able to associate that packet with a particular
pseudowire. PE2 is able to do this by examining the MPLS label which
serves as the pseudowire demultiplexor field shown in Figure 2. Call
this label the "PW label".
So when PE1 sends a layer 2 PDU to PE2, it first pushes a PW label on
its label stack, thereby creating an MPLS packet. It then (if PE1 is
not adjacent to PE2) encapsulates that MPLS packet in a PSN tunnel
header. (If the PSN tunnel is an LSP, this is just a matter of
pushing on a second label.) The PW label is not visible again until
the MPLS packet reaches PE2. PE2's disposition of the packet is based
on the PW label.
Note that the PW label must always be at the bottom of the packet's
label stack and labels MUST be allocated from the per-platform label
space.
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This document specifies a protocol for assigning and distributing the
PW label. This protocol is LDP, extended as specified in the
remainder of this document. An LDP session must be set up between
the pseudowire endpoints. LDP MUST be used in its "downstream
unsolicited" mode. LDP's "liberal label retention" mode SHOULD be
used.
In addition to the protocol specified herein, static assignment of PW
labels MAY be used, and implementations of this protocol SHOULD
provide support for static assignment.
This document specifies all the procedures necessary to set up and
maintain the pseudowires needed to support "unswitched" point-to-
point services, where each endpoint of the pseudowire is provisioned
with the identify of the other endpoint. There are also protocol
mechanisms specified herein which can be used to support switched
services, and which can be used to support other provisioning models.
However, the use of the protocol mechanisms to support those other
models and services is not described in this document.
4. Details Specific to Particular Emulated Services
4.1. Frame Relay
When emulating a frame relay service, the Frame Relay PDUs are
encapsulated according to the procedures defined in [7]. The PE MUST
provide Frame Relay PVC status signaling to the Frame Relay network.
If the PE detects a service affecting condition for a particular
DLCI, as defined in [2] Q.933 Annex A.5 sited in IA FRF1.1, PE MUST
communicate to the remote PE the status of the PW corresponds to the
frame relay DLCI. The Egress PE SHOULD generate the corresponding
errors and alarms as defined in [2] on the egress Frame relay PVC.
4.2. ATM
4.2.1. ATM AAL5 SDU VCC Transport
ATM AAL5 CSPS-SDUs are encapsulated according to [10] ATM AAL5 CPCS-
SDU mode. This mode allows the transport of ATM AAL5 CSPS-SDUs
traveling on a particular ATM PVC across the network to another ATM
PVC.
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4.2.2. ATM Transparent Cell Transport
This mode is similar to the Ethernet port mode. Every cell that is
received at the ingress ATM port on the ingress PE, PE1, is
encapsulated according to [10], ATM cell mode n-to-one, and sent
across the PW to the egress PE, PE2. This mode allows an ATM port to
be connected to only one other ATM port. [10] ATM cell n-to-one mode
allows for concatenation ( grouping ) of multiple cells into a single
MPLS frame. Concatenation of ATM cells is OPTIONAL for transmission
at the ingress PE, PE1. If the Egress PE PE2 supports cell
concatenation the ingress PE, PE1, should only concatenate cells up
to the "Maximum Number of concatenated ATM cells" parameter received
as part of the FEC element.
4.2.3. ATM n-to-one VCC and VPC Cell Transport
This mode is similar to the ATM AAL5 VCC transport except that cells
are transported. Every cell that is received on a pre-defined ATM
PVC, or ATM PVP, at the ingress ATM port on the ingress PE, PE1, is
encapsulated according to [10], ATM n-to-one cell mode, and sent
across the LSP to the egress PE PE2. Grouping of ATM cells is
OPTIONAL for transmission at the ingress PE, PE1. If the Egress PE
PE2 supports cell concatenation the ingress PE, PE1, MUST only
concatenate cells up to the "Maximum Number of concatenated ATM cells
in a frame" parameter received as part of the FEC element.
4.2.4. OAM Cell Support
OAM cells MAY be transported on the VC LSP. 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 LSP
that contain a PW label with the T bit set [10]. When operating in
AAL5 SDU transport mode an PE that supports transport of OAM cells
using the T bit defined in [10], or an PE operating in any of the
cell transport modes MUST follow the procedures outlined in [9]
section 8 for mode 0 only, in addition to the applicable procedures
specified in [6].
4.2.4.1. 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 an LSP MAY provide OAM support
on ATM PVCs using the following procedures:
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- Loopback cells response
If an F5 end-to-end OAM cell is received from a 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 is
the PE supports forwarding of OAM cells, then the received OAM
AIS alarm cells MUTS 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 LDP session fails,
or a PW status TLV notification is received, then a propper 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.
4.2.5. ILMI Support
An MPLS edge PE MAY provide an ATM 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 SHOULD notify its
client of this failure by deleting the VC using ILMI.
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4.2.6. ATM AAL5 PDU VCC Transport
ATM AAL5 CSPS-PDUs are encapsulated according to [10] ATM AAL5 CPCS-
PDU mode. This mode allows the transport of ATM AAL5 CSPS-PDUs
traveling on a particular ATM PVC across the network to another ATM
PVC. This mode supports fragmentation of the ATM AAL5 CPCS-PDU 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.
4.2.7. ATM one-to-one VCC and VPC Cell Transport
This mode is similar to the ATM AAL5 n-to-one cell transport except
an encapsulation method that maps one ATM VCC or one ATM VPC to one
Pseudo-Wire is used. Every cell that is received on a pre-defined ATM
PVC, or ATM PVP, at the ingress ATM port on the ingress PE, PE1, is
encapsulated according to [10], ATM one-to-one cell mode, and sent
across the LSP to the egress PE PE2. Grouping of ATM cells is
OPTIONAL for transmission at the ingress PE, PE1. If the Egress PE
PE2 supports cell concatenation the ingress PE, PE1, MUST only
concatenate cells up to the "Maximum Number of concatenated ATM cells
in a frame" parameter received as part of the FEC element.
4.3. Ethernet VLAN
The Ethernet frame will be encapsulated according to the procedures
in [12] tagged mode. It should be noted that if the VLAN identifier
is modified by the egress PE, according to the procedures outlined
above, the Ethernet spanning tree protocol might fail to work
properly. If the PE detects a failure on the Ethernet physical port,
or the port is administratively disabled, it MUST send PW status
notification message for all PWs associated with the port. This mode
uses service-delimiting tags to map input ethernet frames to
respective PWs.
4.4. Ethernet
The Ethernet frame will be encapsulated according to the procedures
in [12] "ethernet raw mode". If the PE detects a failure on the
Ethernet input port, or the port is administratively disabled, the PE
MUST send a corresponding PW status notification message.
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4.5. HDLC and PPP
HDLC and PPP frames are encapsulated according to the procedures in
[11]. If the MPLS edge PE detects that the physical link has failed,
or the port is administratively disabled, it MUST send a PW status
notification message that corresponds to the HDLC or PPP PW.
4.6. IP Layer2 Transport
This mode switches IP packets into a Peudo-Wire. the encapsulation
used is according to [3]. IP interworking is implementation specific,
part of the NSP function [13], and is outside the scope of this
document.
5. LDP
The PW label bindings are distributed using the LDP downstream
unsolicited mode described in [1]. The PEs will establish an LDP
session using the Extended Discovery mechanism described in [1,
section 2.4.2 and 2.5].
An LDP Label Mapping message contains a FEC TLV, a Label TLV, and
zero or more optional parameter TLVs.
The FEC TLV is used to indicate the meaning of the label. In the
current context, the FEC TLV would be used to identify the particular
pseudowire that a particular label is bound to. In this
specification, we define two new FEC TLVs to be used for identifying
pseudowires. When setting up a particular pseudowire, only one of
these FEC TLVs is used. The one to be used will depend on the
particular service being emulated and on the particular provisioning
model being supported.
LDP allows each FEC TLV to consist of a set of FEC elements. For
setting up and maintaining pseudowires, however, each FEC TLV MUST
contain exactly one FEC element.
LDP has several kinds of label TLVs. For setting up and maintaining
pseudowires, the Generic Label TLV MUST be used.
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5.1. The PWid FEC Element
The PWid FEC element may be used whenever both pseudowire endpoints
have been provisioned with the same 32-bit identifier for the
pseudowire.
For this purpose a new type of FEC element is defined. The FEC
element type is 128 [note1], and 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW tlv |C| PW type |PW info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface parameters |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- PW type
A 15 bit quantity containing a value which represents the type of
PW. Assigned Values are specified in "IANA Allocations for pseudo
Wire Edge to Edge Emulation (PWE3)" [14].
- Control word bit (C)
The highest order bit (C) of the PW type is used to flag the
presence of a control word ( defined in [7] ) as follows:
bit 15 = 1 control word present on this VC.
bit 15 = 0 no control word present on this VC.
Please see the section "C-Bit Handling Procedures" for further
explanation.
- PW information length
Length of the PW ID field and the interface parameters field in
octets. If this value is 0, then it references all PWs using the
specified group ID and there is no PW ID present, nor any
interface parameters.
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- Group ID
An arbitrary 32 bit value which represents a group of PWs that is
used to create groups in the VC space. The group ID is intended
to be used as a port index, or a virtual tunnel index. To
simplify configuration a particular PW ID at ingress could be
part of the virtual tunnel for transport to the egress router.
The Group ID is very useful to send wild card label withdrawals,
or PW wild card status notification messages to remote PEs upon
physical port failure.
- PW ID
A non-zero 32-bit connection ID that together with the PW type,
identifies a particular PW. Note that the PW ID and the PW type
must be the same at both endpoints.
- Interface parameters
This variable length field is used to provide interface specific
parameters, such as CE-facing interface MTU.
Note that as the "interface parameters" are part of the FEC, the
rules of LDP make it impossible to change the interface
parameters once the pseudowire has been set up. Thus the
interface parameters field must not be used to pass information,
such as status information, which may change during the life of
the pseudowire. Optional parameter TLVs should be used for that
purpose.
Using the PWid FEC, each of the two pseudowire endpoints
independently initiates the set up of a unidirectional LSP. An
outgoing LSP and an incoming LSP are bound together into a single
pseudowire if they have the same PW ID and PW type.
5.2. The Generalized ID FEC Element
There are cases where the PWid FEC element cannot be used, because
both endpoints have not been provisioned with a common 32-bit PWid.
In such cases, the "Generalized ID FEC Element" is used instead.
This is FEC type 129 (provisionally, subject to assignment by IANA).
It differs from the PWid FEC element in that the PWid and the group
id are eliminated, and their place is taken by a generalized
identifier field as described below. The Generalized ID FEC element
includes a PW type field, a C bit, and an interface parameters field;
these three fields are identical to those in the PWid FEC, and are
used as discussed in the previous section.
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5.2.1. Attachment Identifiers
As discussed in [13], a pseudowire can be thought of as connecting
two "forwarders". The protocol used to setup a pseudowire must allow
the forwarder at one end of a pseudowire to identify the forwarder at
the other end. We use the term "attachment identifier", or "AI", to
refer to the field which the protocol uses to identify the
forwarders. In the PWid FEC, the PWid field serves as the AI. In
this section we specify a more general form of AI which is structured
and of variable length.
Every Forwarder in a PE must be associated with an Attachment
Identifier (AI), either through configuration or through some
algorithm. The Attachment Identifier must be unique in the context
of the PE router in which the Forwarder resides. The combination <PE
router, AI> must be globally unique.
It is frequently convenient to a set of Forwarders as being members
of a particular "group", where PWs may only be set up among members
of a group. In such cases, it is convenient to identify the
Forwarders relative to the group, so that an Attachment Identifier
would consist of an Attachment Group Identifier (AGI) plus an
Attachment Individual Identifier (AII).
An Attachment Group Identifier may be thought of as a VPN-id, or
a VLAN identifier, some attribute which is shared by all the
Attachment VCs (or pools thereof) which are allowed to be connected.
The details of how to construct the AGI and AII fields identifying
the pseudowire endpoints are outside the scope of this specification.
Different pseudowire application, and different provisioning models,
will require different sorts of AGI and AII fields. The
specification of each such application and/or model must include the
rules for constructing the AGI and AII fields.
As previously discussed, a (bidirectional) pseudowire consists of a
pair of unidirectional LSPs, one in each direction. If a particular
pseudowire connects PE1 with PE2, the LSP in the PE1-->PE2 direction
can be identified as:
<PE1, <AGI, AII1>, PE2, <AGI, AII2>>,
and the LSP in the PE2--PE1 direction can be identified by:
<PE2, <AGI, AII2>, PE1, <AGI, AII1>>.
Note that the AGI must be the same at both endpoints, but the AII
will in general be different at each endpoint. Thus from the
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perspective of a particular PE, each pseudowire has a local or
"Source AII", and a remote or "Target AII". The pseudowire setup
protocol can carry all three of these quantities:
- Attachment Group Identifier (AGI).
- Source Attachment Individual Identifier (SAII)
- Target Attachment Individual Identifier (TAII)
If the AGI is non-null, then the Source AI (SAI) consists of the AGI
together with the SAII, and the Target AI (TAI) consists of the TAII
together with the AGI. If the AGI is null, then the SAII and TAII
are the SAI and TAI respectively.
The interpretation of the SAI and TAI is a local matter at the
respective endpoint.
The association of two unidirectional LSPs into a single
bidirectional pseudowire depends on the SAI and the TAI. Each
application and/or provisioning model which uses the Generalized ID
FEC element must specify the rules for performing this association.
5.2.2. Encoding the Generalized ID FEC Element
FEC element type 129 is used. The FEC element is encoded 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 129 |C| PW Type |VC info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameters |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
additional Parameters are:
- SAII, encoded as a one byte length field followed by the SAII.
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- TAII, encoded as a one byte length field followed by the TAII.
- AGI, encoded as a one byte length field followed by the AGI.
The SAII, TAII, and AGI are simply carried as octet strings. The
length byte specifies the size of the field, excluding the length
byte itself. The null string can be sent by setting the length
byte to 0.
5.2.3. Procedures
In order for PE1 to begin signaling PE2, PE1 must know the address of
the remote PE2, and a TAI. This information may have been configured
at PE1, or it may have been learned dynamically via some
autodiscovery procedure.
To begin the signaling procedure, a PE (PE1) that has knowledge of
the other endpoint (PE2) initiates the setup of the LSP in the
incoming (PE2-->PE1) direction by sending a Label Mapping message
containing the FEC type 129. The FEC element includes the SAII, AGI,
and TAII.
What happens when PE2 receives such a Label Mapping message?
PE2 interprets the message as a request to set up a PW whose endpoint
(at PE2) is the Forwarder identified by the TAI. From the
perspective of the signaling protocol, exactly how PE2 maps AIs to
Forwarders is a local matter. In some VPWS provisioning models, the
TAI might, e.g., be a string which identifies a particular Attachment
Circuit, such as "ATM3VPI4VCI5", or it might, e.g., be a string such
as "Fred" which is associated by configuration with a particular
Attachment Circuit. In VPLS, the TAI would be a VPN-id, identifying
a particular VPLS instance.
If PE2 cannot map the TAI to one of its Forwarders, then PE2 sends a
Label Release message to PE1, with a Status Code meaning "invalid
TAI", and the processing of the Mapping message is complete.
If the Label Mapping Message has a valid TAI, PE2 must decide whether
to accept it or not. The procedures for so deciding will depend on
the particular type of Forwarder identified by the TAI. Of course,
the Label Mapping message may be rejected due to standard LDP error
conditions as detailed in [LDP].
If PE2 decides to accept the Label Mapping message, then it has to
make sure that an LSP is set up in the opposite (PE1-->PE2)
direction. If it has already signaled for the corresponding LSP in
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that direction, nothing more need be done. Otherwise, it must
initiate such signaling by sending a Label Mapping message to PE1.
This is very similar to the Label Mapping message PE2 received, but
with the SAI and TAI reversed.
5.3. Signaling of Pseudo Wire Status
5.3.1. Use of Label Mappings.
The PEs MUST send PW label mapping messages to their peers as soon as
the PW is configured and administratively enabled, regardless of the
CE-facing interface state. The PW label should not be withdrawn
unless the user administratively configures the CE-facing interface
down (or the PW configuration is deleted entirely). A simple label
withdraw method MAY also be supported as an alternative. In any case
if the Label mapping is not available the PW MUST be considered in
the down state.
5.3.2. Signaling PW status.
The PE devices use an LDP TLV to indicate status to their remote
peers. This PW Status TLV contains more information than the
alternative simple Label Withdraw message.
The format of the PW Status TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| PW Status (0x0???) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where status is a 4 octet bit field is specified in the PW IANA
Allocations document [14]
Each bit in the status code field can be set individually to indicate
more then a single failure at once. Each fault can be cleared by
sending an appropriate status message with the respective bit
cleared. The presence of the lowest bit (PW Not Forwarding) acts only
as a generic failure indication when there is a link-down event for
which none of the other bits apply.
The Status TLV is transported to the remote PW peer via the LDP
notification message. The format of the Notification Message is:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW FEC TLV or Generalized ID FEC Element |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PW FEC TLV SHOULD not include the interface parameters as they
are ignored in the context of this message. When a PE's CE-facing
interface encounters an error, use of the PW status message allows
the PE to send a single status message, using a PW FEC TLV with only
the group ID set, to denote this change in status for all affected PW
connections.
As mentioned above the Group ID field can be used to send a status
notification for all PWs associated with a particular group ID. This
procedure is OPTIONAL, and if it is implemented the LDP Notification
message should be as follows: the PW information length field is set
to 0, the PW ID field is not present, and the interface parameters
field is not present. For the purpose of this document this is called
the "wild card PW status notification procedure", and all PEs
implementing this design are REQUIRED to accept such a notification
message, but are not required to send it.
5.4. Interface Parameters Field
This field specifies interface specific parameters. When applicable,
it MUST be used to validate that the PEs, and the ingress and egress
ports at the edges of the circuit, have the necessary capabilities to
interoperate with each other. The field structure 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter ID | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The parameter ID Values are specified in "IANA Allocations for pseudo
Wire Edge to Edge Emulation (PWE3)" [14].
The Length field is defined as the length of the interface parameter
including the parameter id and length field itself. Processing of the
interface parameters should continue when encountering unknown
interface parameters and they MUST be silently ignored.
- Interface MTU
A 2 octet value indicating the MTU in octets. This is the Maximum
Transmission Unit, excluding encapsulation overhead, of the
egress packet interface that will be transmitting the
decapsulated PDU that is received from the MPLS network. This
parameter is applicable only to PW types 1, 2, 4, 5, 6, 7,14, and
15 and is REQUIRED for these PW types. If this parameter does not
match in both directions of a specific PW, that PW MUST NOT be
enabled.
- 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, and 0x0a, and is REQUIRED for these PWC types. This
parameter does not need to match in both directions of a specific
PW.
- Optional Interface Description string
This arbitrary, OPTIONAL, interface description string is used to
send a human-readable administrative string describing the
interface to the remote. This parameter is OPTIONAL, and is
applicable to all PW types. The interface description parameter
string length is variable, and can be from 0 to 80 octets.
Human-readable text MUST be provided in the UTF-8 charset using
the Default Language [RFC2277].
- Payload Bytes
A 2 octet value indicating the number of TDM payload octets
contained in all packets on the CEM stream, from 48 to 1,023
octets. All of the packets in a given CEM stream have the same
number of payload bytes. Note that there is a possibility that
the packet size may exceed the SPE size in the case of an STS-1
SPE, which could cause two pointers to be needed in the CEM
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header, since the payload may contain two J1 bytes for
consecutive SPEs. For this reason, the number of payload bytes
must be less than or equal to 783 for STS-1 SPEs.
- CEP Options.
An optional 16 Bit value of CEM Flags. See [8] for the definition
of the bit values.
- Requested VLAN ID.
An Optional 16 bit value indicating the requested VLAN ID. This
parameter MAY be used by an PE that is incapable of rewriting the
802.1Q ethernet VLAN tag on output. If the ingress PE receives
this request it MAY rewrite the VLAN ID tag in input to match the
requested VLAN ID. If this is not possible, and the VLAN ID does
not already match configured ingress VLAN ID the PW should not be
enabled.This parameter is applicable only to PW type 4.
- CEP/TDM bit rate.
This 32-bit integer is mandatory for CEP. For other PWs carrying
TDM traffic it is mandatory if the bit-rate cannot be directly
inferred from the service type. If present, it expresses the bit
rate of the attachment circuit as known to the advertizing PE in
"units" of 64 kbit/s. I.e., the value 26 must be used for CEP
carrying VT1.5 SPE, 35 - for CEP carrying a VT2 SPE, 99 - for VT6
SPE, 783 - for STS-1 SPE and n*783 - for STS-nc, n = 3, 12, 48,
192. Attempts to establish a PWC between a pair of TDM ports
with different bit-rates MUST be rejected with the appropriate
status code (see section "Status codes" below).
- Frame-Relay DLCI lenth.
An optional 16 bit value indicating the lenght of the frame-relay
DLCI field. This OPTIONAL interface paremeter can have value of 2
, or 4, with the default being equal to 2. If this interface
parameter is not present the default value of 2 is assumed.
5.4.1. PW types for which the control word is REQUIRED
The Label Mapping messages which are sent in order to set up these
PWs MUST have c=1. When a Label Mapping message for a PW of one of
these types is received, and c=0, a Label Release MUST be sent, with
an "Illegal C-bit" status code. In this case, the PW will not be
enabled.
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5.4.2. PW types for which the control word is NOT mandatory
If a system is capable of sending and receiving the control word on
PW types for which the control word is not mandatory, then each such
PW endpoint MUST be configurable with a parameter that specifies
whether the use of the control word is PREFERRED or NOT PREFERRED.
For each PW, there MUST be a default value of this parameter. This
specification does NOT state what the default value should be.
If a system is NOT capable of sending and receiving the control word
on PWC types for which the control word is not mandatory, then it
behaves as exactly as if it were configured for the use of the
control word to be NOT PREFERRED.
If a Label Mapping message for the PW has already been received, but
no Label Mapping message for the PW has yet been sent, then the
procedure is the following:
-i. If the received Label Mapping message has c=0, send a Label
Mapping message with c=0, and the control word is not used.
-ii. If the received Label Mapping message has c=1, and the PW is
locally configured such that the use of the control word is
preferred, then send a Label Mapping message with c=1, and
the control word is used.
-iii. If the received Label Mapping message has c=1, and the PW is
locally configured such that the use of the control word is
not preferred or the control word is not supported, then act
as if no Label Mapping message for the PW had been received
(i.e., proceed to the next paragraph).
If a Label Mapping message for the PW has not already been received
(or if the received Label Mapping message had c=1 and either local
configuration says that the use of the control word is not preferred
or the control word is not supported), then send a Label Mapping
message in which the c bit is set to correspond to the locally
configured preference for use of the control word. (I.e., set c=1 if
locally configured to prefer the control word, set c=0 if locally
configured to prefer not to use the control word or if the control
word is not supported).
The next action depends on what control message is next received for
that PW. The possibilities are:
-i. A Label Mapping message with the same c bit value as
specified in the Label Mapping message that was sent. PW
setup is now complete, and the control word is used if c=1
but not used if c=0.
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-ii. A Label Mapping message with c=1, but the Label Mapping
message that was sent has c=0. In this case, ignore the
received Label Mapping message, and continue to wait for the
next control message for the PW.
-iii. A Label Mapping message with c=0, but the Label Mapping
message that was sent has c=1. In this case, send a Label
Withdraw message with a "Wrong c-bit" status code, followed
by a Label Mapping message that has c=0. PW setup is now
complete, and the control word is not used.
-iv. A Label Withdraw message with the "Wrong c-bit" status code.
Treat as a normal Label Withdraw, but do not respond.
Continue to wait for the next control message for the PW.
If at any time after a Label Mapping message has been received, a
corresponding Label Withdraw or Release is received, the action taken
is the same as for any Label Withdraw or Release that might be
received at any time. Note that receiving a Label Withdraw should not
cause a corresponding Label Release to be sent.
If both endpoints prefer the use of the control word, this procedure
will cause it to be used. If either endpoint prefers not to use the
control word, or does not support the control word, this procedure
will cause it not to be used. If one endpoint prefers to use the
control word but the other does not, the one that prefers not to use
it is has no extra protocol to execute, it just waits for a Label
Mapping message that has c=0.
The diagram in Appendix A illustrates the above procedure.
5.4.3. Status codes
RFC 3036 has a range of Status Code values which are assigned by IANA
on a First Come, First Served basis. These additional status codes,
and assigned Values are specified in "IANA Allocations for pseudo
Wire Edge to Edge Emulation (PWE3)" [14].
5.5. LDP label Withdrawal procedures
As mentioned above the Group ID field can be used to withdraw all PW
labels associated with a particular group ID. This procedure is
OPTIONAL, and if it is implemented the LDP label withdraw message
should be as follows: the PW information length field is set to 0,
the PW ID field is not present, and the interface parameters field is
not present. For the purpose of this document this is called the
"wild card withdraw procedure", and all PEs implementing this design
are REQUIRED to accept such a withdraw message, but are not required
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to send it.
The interface parameters field MUST NOT be present in any LDP PW
label withdrawal message or release message. A wildcard release
message MUST include only the group ID. A Label Release message
initiated from the imposition router must always include the PW ID.
5.6. Sequencing Considerations
In the case where the router considers the sequence number field in
the control word, it is important to note the following when
advertising labels
5.6.1. Label Mapping Advertisements
After a label has been withdrawn by the disposition router and/or
released by the imposition router, care must be taken to not re-
advertise (re-use) the released label until the disposition router
can be reasonably certain that old packets containing the released
label no longer persist in the MPLS network.
This precaution is required to prevent the imposition router from
restarting packet forwarding with sequence number of 1 when it
receives the same label mapping if there are still older packets
persisting in the network with sequence number between 1 and 32768.
For example, if there is a packet with sequence number=n where n is
in the interval[1,32768] traveling through the network, it would be
possible for the disposition router to receive that packet after it
re-advertises the label. Since the label has been released by the
imposition router, the disposition router SHOULD be expecting the
next packet to arrive with sequence number to be 1. Receipt of a
packet with sequence number equal to n will result in n packets
potentially being rejected by the disposition router until the
imposition router imposes a sequence number of n+1 into a packet.
Possible methods to avoid this is for the disposition router to
always advertise a different PW label, or for the disposition router
to wait for a sufficient time before attempting to re-advertised a
recently released label. This is only an issue when sequence number
processing at the disposition router is enabled.
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5.6.2. Label Mapping Release
In situations where the imposition router wants to restart forwarding
of packets with sequence number 1, the router shall 1) Send to
disposition router a label mapping release, and 2) Send to
disposition router a label mapping request. When sequencing is
supported, advertisement of a PW label in response to a label mapping
request MUST also consider the issues discussed in the section on
Label Mapping Advertisements.
6. Security Considerations
This document does not affect the underlying security issues of MPLS.
7. References
[1] "LDP Specification." L. Andersson, P. Doolan, N. Feldman, A.
Fredette, B. Thomas. January 2001. RFC3036
[2] ITU-T Recommendation Q.933, and Q.922 Specification for Frame
Mode Basic call control, ITU Geneva 1995
[3] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032
[4] "IEEE 802.3ac-1998" IEEE standard specification.
[5] American National Standards Institute, "Synchronous Optical
Network Formats," ANSI T1.105-1995.
[6] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[7] "Frame Relay over Pseudo-Wires", draft-ietf-pwe3-frame-relay-
01.txt. ( work in progress )
[8] "SONET/SDH Circuit Emulation Service Over Packet (CEP)",
draft-ietf-pwe3-sonet-01.txt ( Work in progress )
[9] ATM Forum Specification fb-fbatm-0151.000 (2000) ,Frame Based ATM
over SONET/SDH Transport (FAST)
[10] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP
and MPLS Networks", draft-ietf-pwe3-atm-encap-02.txt ( work in
progress )
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[11] "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP
and MPLS Networks", draft-ietf-pwe3-hdlc-ppp-encap-00.txt. ( work in
progress )
[12] "Encapsulation Methods for Transport of Ethernet Frames Over
IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-01.txt. ( work in
progress )
[13] "PWE3 Architecture" Bryant, et al., draft-ietf-pwe3-arch-04.txt
( work in progress ), August 2003.
[14] "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)"
Martini, Townsley, draft-ietf-pwe3-iana-allocation-01.txt ( work in
progress ), February 2003
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations section in RFCs", BCP 26, RFC 2434, October 1998.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[note1] FEC element type 128 is pending IANA approval.
[note2] Status codes assigment is pending IANA approval.
8. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
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Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
e-mail: giles@packetexchange.net
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: tappan@cisco.com
9. Additional Contributing Authors
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
e-mail: d@mazunetworks.com
Jayakumar Jayakumar,
Cisco Systems Inc.
225, E.Tasman, MS-SJ3/3,
San Jose, CA, 95134
e-mail: jjayakum@cisco.com
Alex Hamilton,
Cisco Systems Inc.
285 W. Tasman, MS-SJCI/3/4,
San Jose, CA, 95134
e-mail: tahamilt@cisco.com
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Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: sjv@laurelnetworks.com
John Shirron
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
Laurel Networks, Inc.
e-mail: jshirron@laurelnetworks.com
Toby Smith
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
Laurel Networks, Inc.
e-mail: tob@laurelnetworks.com
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Phone: +1 408 383 7223
Email: Andy.Malis@vivacenetworks.com
Vinai Sirkay
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
e-mail: sirkay@technologist.com
Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
e-mail: vasile@nortelnetworks.com
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Chris Liljenstolpe
Cable & Wireless
11700 Plaza America Drive
Reston, VA 20190
e-mail: chris@cw.net
Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
e-mail: dcooper@gblx.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
e-mail: kireeti@juniper.net
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10. Appendix A - C-bit Handling Procedures Diagram
------------------
Y | Received Label | N
-------| Mapping Msg? |--------------
| ------------------ |
-------------- |
| | |
------- ------- |
| C=0 | | C=1 | |
------- ------- |
| | |
| ---------------- |
| | Control Word | N |
| | Capable? |----------- |
| ---------------- | |
| Y | | |
| | | |
| ---------------- | |
| | Control Word | N | |
| | Preferred? |---- | |
| ---------------- | | |
| Y | | | |
| | | | ----------------
| | | | | Control Word |
| | | | | Preferred? |
| | | | ----------------
| | | | N | Y |
| | | | | |
Send Send Send Send Send Send
C=0 C=1 C=0 C=0 C=0 C=1
| | | |
----------------------------------
| If receive the same as sent, |
| PW setup is complete. If not: |
----------------------------------
| | | |
------------------- -----------
| Receive | | Receive |
| C=1 | | C=0 |
------------------- -----------
| |
Wait for the Send
next message Wrong C-Bit
|
Send Label
Mapping Message
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