Network Working Group Luca Martini
Internet Draft Nasser El-Aawar
Expiration Date: May 2003 Level 3 Communications, LLC.
Toby Smith Eric C. Rosen
Laurel Networks, Inc. Cisco Systems, Inc.
Giles Heron
PacketExchange Ltd.
November 2002
Transport of Layer 2 Frames Over MPLS
draft-ietf-pwe3-control-protocol-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
This document describes methods for transporting the Protocol Data
Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,
Ethernet, and providing a SONET circuit emulation service across an
MPLS network.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 3
3 PSN Tunnel Labels and PW Labels ........................ 4
4 Protocol-Specific Details .............................. 6
4.1 Frame Relay ............................................ 6
4.2 ATM .................................................... 6
4.2.1 ATM AAL5 VCC Transport ................................. 6
4.2.2 ATM Transparent Cell Transport ......................... 6
4.2.3 ATM VCC and VPC Cell Transport ......................... 6
4.2.4 OAM Cell Support ....................................... 7
4.2.5 ILMI Support ........................................... 8
4.3 Ethernet VLAN .......................................... 8
4.4 Ethernet ............................................... 8
4.5 HDLC and PPP ........................................... 8
5 LDP .................................................... 9
5.1 Interface Parameters Field ............................. 10
5.1.1 PW types for which the control word is REQUIRED ........ 12
5.1.2 PW types for which the control word is NOT mandatory ... 13
5.1.3 Status codes ........................................... 14
5.2 LDP label Withdrawal procedures ........................ 15
5.3 Sequencing Considerations .............................. 15
5.3.1 Label Mapping Advertisements ........................... 15
5.3.2 Label Mapping Release .................................. 16
6 IANA Considerations .................................... 16
7 Security Considerations ................................ 16
8 References ............................................. 17
9 Author Information ..................................... 18
10 Appendix A - C-bit Handling Procedures Diagram ......... 21
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.
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2. Introduction
In an MPLS network, it is possible to carry the Protocol Data Units
(PDUs) of layer 2 protocols by prepending an MPLS label stack to
these PDUs. This document specifies the necessary label distribution
procedures for accomplishing this using the encapsulation methods in
[7],[10],[12]. We restrict discussion to the case of point-to-point
transport. QoS related issues are not discussed in this draft. This
document describes methods for transporting a number of protocols; in
some cases, transporting a particular protocol may have several modes
of operation, though a default mode of operation which MUST be
supported is identified for each. Each of these protocols and/or
modes may be implemented independently.
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 MPLS network must emulate the circuit
characteristics of SONET/SDH payloads. MPLS labels and a new circuit
emulation header are used to encapsulate TDM signals and provide the
Circuit Emulation Service over MPLS (CEM). The following two figures
describe the reference models which are derived from [PWE3-FRAME] 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 Ethernet/VLAN Interface Reference Configuration
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+-------------+ +-------------+
| Layer2 | | Layer2 |
| Emulated | | Emulated |
| Services | Emulated Service | Services |
| |<==============================>| |
+-------------+ Pseudo Wire +-------------+
|Demultiplexer|<==============================>|Demultiplexor|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
| MPLS |<==============================>| MPLS |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
Figure 2: Ethernet PWE3 Protocol Stack Reference Model
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. PSN Tunnel Labels and PW Labels
Suppose it is desired to transport layer 2 PDUs from ingress LSR PE1
to egress LSR PE2, across an intervening MPLS network. We assume that
there is an LSP ( PSN tunnel ) from PE1 to PE2. That is, we assume
that PE1 can cause a packet to be delivered to PE2 by pushing some
label onto the packet and sending the result to one of its
adjacencies. Call this label the "PSN tunnel label", and the
corresponding LSP the "PSN tunnel LSP".
The PSN tunnel LSP merely gets packets from PE1 to PE2, the
corresponding label doesn't tell PE2 what to do with the payload, and
in fact if penultimate hop popping is used, PE2 may never even see
the corresponding label. (If PE1 itself is the penultimate hop, a PSN
tunnel label may not even get pushed on.) Thus if the payload is not
an IP packet, there must be a label, which becomes visible to PE2,
that tells PE2 how to treat the received packet. 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, and then (if PE1 is not adjacent to PE2) pushes on a
PSN tunnel label. The PSN tunnel label gets the MPLS packet from PE1
to PE2; the PW label is not visible until the MPLS packet reaches
PE2. PE2's disposition of the packet is based on the PW label.
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Note that the tunnel could be a GRE encapsulated MPLS tunnel between
PE1 and PE2. In this case PE1 would be adjacent to PE2, and only the
PW label would be used, and the intervening network need only carry
IP packets.
If the payload of the MPLS packet is, for example, an ATM AAL5 PDU,
the VC label will generally correspond to a particular ATM VC at PE2.
That is, PE2 needs to be able to infer from the PW label the outgoing
interface and the VPI/VCI value for the AAL5 PDU. If the payload is a
Frame Relay PDU, then PE2 needs to be able to infer from the PW label
the outgoing interface and the DLCI value. If the payload is an
Ethernet frame, then PE2 needs to be able to infer from the PW label
the outgoing interface, and perhaps the VLAN identifier. This process
is uni-directional, and will be repeated independently for bi-
directional operation. It is REQUIRED to assign the same PW ID, and
PW type for a given circuit in both directions. The group ID (see
below) MUST NOT be required to match in both directions. The
transported frame MAY be modified when it reaches the egress router.
If the header of the transported layer 2 frame is modified, this MUST
be done at the egress LSR only.
Note that the PW label must always be at the bottom of the label
stack, and the PSN tunnel label, if present, must be immediately
above the PW label. Of course, as the packet is transported across
the MPLS network, additional labels may be pushed on (and then popped
off) as needed. Even PE1 itself may push on additional labels above
the PSN tunnel label. If PE1 and PE2 are directly adjacent LSRs, then
it may not be necessary to use a PSN tunnel label at all.
This document does not specify a method for distributing the PSN
tunnel label or any other labels that may appear above the PW label
on the stack. Any acceptable method of MPLS label distribution will
do.
This document does specify a method for assigning and distributing
the PW label. Static label assignment MAY be used, and
implementations SHOULD provide support for this. When signaling is
used, the PW label MUST be distributed from PE2 to PE1 using LDP in
the downstream unsolicited mode; this requires that an LDP session be
created between PE1 and PE2. It should be noted that this LDP session
is not necessarily transported along the same path as the Layer 2
PDUs. [1] In addition, when using LDP to distribute the PW label,
liberal label retention mode SHOULD be used.
Note that this technique allows an unbounded number of layer 2 "PWs"
to be carried together in a single "tunnel". Thus it scales quite
well in the network backbone.
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While this document currently defines the emulation of Frame Relay
and ATM PVC services, it specifically does not preclude future
enhancements to support switched service (SVC and SPVC) emulation.
4. Protocol-Specific Details
4.1. Frame Relay
The Frame Relay PDUs are encapsulated according to the procedures
defined in [7]. The MPLS edge LSR MUST provide Frame Relay PVC status
signaling to the Frame Relay network. If the MPLS edge LSR detects a
service affecting condition as defined in [2] Q.933 Annex A.5 sited
in IA FRF1.1, it MUST withdraw the label that corresponds to the
frame relay DLCI. The Egress LSR SHOULD generate the corresponding
errors and alarms as defined in [2] on the egress Frame relay VC.
4.2. ATM
4.2.1. ATM AAL5 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 MPLS network to another
ATM PVC.
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 LSR, PE1, is
encapsulated according to [10], ATM cell mode n-to-one, and sent
across the PW to the egress LSR, 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 grouping of multiple cells into a single MPLS frame.
Grouping of ATM cells is OPTIONAL for transmission at the ingress
LSR, PE1. If the Egress LSR PE2 supports cell concatenation the
ingress LSR, 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 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 LSR, PE1, is
encapsulated according to [10], ATM cell mode, and sent across the
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LSP to the egress LSR PE2. Grouping of ATM cells is OPTIONAL for
transmission at the ingress LSR, PE1. If the Egress LSR PE2 supports
cell concatenation the ingress LSR, 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 LSR is operating
in AAL5 CPCS-SDU transport mode if it does not support transport of
ATM cells, the LSR MUST discard incoming MPLS frames on an ATM VC LSP
that contain a VC label with the T bit set [10]. When operating in
AAL5 SDU transport mode an LSR that supports transport of OAM cells
using the T bit defined in [7], or an LSR operating in any of the
three 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. OAM Cell Emulation Mode
AN LSR that does not support transport of OAM cells across an LSP MAY
provide OAM support on ATM PVCs using the following procedures:
A pair of LSRs MAY emulate a bi-directional ATM VC by two uni-
directional LSPs. If an F5 end-to-end OAM cell is received from a
ATM VC, by either LSR that is transporting this ATM VC, with a
loopback indication value of 1, and the LSR has a label mapping for
the ATM VC, then the LSR MUST decrement the loopback indication value
and loop back the cell on the ATM VC. Otherwise the loopback cell
MUST be discarded by the LSR.
The ingress LSR, PE1, may also optionally be configured to
periodically generate F5 end-to-end loopback OAM cells on a VC. If
the LSR fails to receive a response to an F5 end-to-end loopback OAM
cell for a pre-defined period of time it MUST withdraw the label
mapping for the VC.
If an ingress LSR, PE1, receives an AIS F5 OAM cell, fails to receive
a pre-defined number of the End-to-End loop OAM cells, or a physical
interface goes down, it MUST withdraw the label mappings for all VCs
associated with the failure. When a PW label mapping is withdrawn,
the egress LSR, PE2, MUST generate AIS F5 OAM cells on the VC
associated with the withdrawn label mapping. In this mode it is very
useful to apply a unique group ID to each interface. In the case
where a physical interface goes down, a wild card label withdraw can
be sent to all LDP neighbors, greatly reducing the signaling response
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time.
4.2.5. ILMI Support
An MPLS edge LSR MAY provide an ATM ILMI to the ATM edge switch. If
an ingress LSR receives an ILMI message indicating that the ATM edge
switch has deleted a VC, or if the physical interface goes down, it
MUST withdraw the label mappings for all VCs associated with the
failure. When a PW label mapping is withdrawn, the egress LSR SHOULD
notify its client of this failure by deleting the VC using ILMI.
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 LSR, according to the procedures outlined
above, the Ethernet spanning tree protocol might fail to work
properly. If the LSR detects a failure on the Ethernet physical port,
or the port is administratively disabled, it MUST withdraw the label
mappings for all PWs associated with the port.
4.4. Ethernet
The Ethernet frame will be encapsulated according to the procedures
in [12]. If the LSR detects a failure on the Ethernet physical port,
or the port is administratively disabled, the corresponding PW label
mapping MUST be withdrawn.
4.5. HDLC and PPP
HDLC and PPP frames are encapsulated according to the procedures in
[11]. If the MPLS edge LSR detects that the physical link has failed,
or the port is administratively disabled, it MUST withdraw the label
mapping that corresponds to the HDLC or PPP link.
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5. LDP
The PW label bindings are distributed using the LDP downstream
unsolicited mode described in [1]. The LSRs will establish an LDP
session using the Extended Discovery mechanism described in [1,
section 2.4.2 and 2.5], for this purpose a new type of FEC element is
defined. The FEC element type is 128. [note1] Only a single PW FEC
element MUST be advertised per LDP PW label. The pseudo wire FEC
element, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VC tlv |C| PW type |VC info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface parameters |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- PW type
A 15 bit quantity containing a value which represents the type of
VC. Assigned Values are:
PW type Description
0x0001 Frame Relay DLCI
0x0002 ATM AAL5 VCC transport
0x0003 ATM transparent cell transport
0x0004 Ethernet VLAN
0x0005 Ethernet
0x0006 HDLC
0x0007 PPP
0x8008 CEM [8]
0x0009 ATM VCC cell transport
0x000A ATM VPC cell transport
- 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:
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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.
- VC 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.
- 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
to remote LSRs upon physical port failure.
- PW ID
A non-zero 32-bit connection ID that together with the PW type,
identifies a particular PW.
- Interface parameters
This variable length field is used to provide interface specific
parameters, such as interface MTU.
5.1. Interface Parameters Field
This field specifies interface specific parameters. When applicable,
it MUST be used to validate that the LSRs, 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:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter ID | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The parameter ID is defined as follows:
Parameter ID Length Description
0x01 4 Interface MTU in octets.
0x02 4 Maximum Number of concatenated ATM cells.
0x03 up to 82 Optional Interface Description string.
0x04 4 CEM [8] Payload Bytes.
0x05 2 CEM options.
0x06 2 Requested VLAN ID
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, and 7,
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 LSR. An
ingress LSR transmitting concatenated cells on this VC 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 VC types. This
parameter does not need to match in both directions of a specific
VC.
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- 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
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.
- CEM 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 LSR that is
incapable of rewriting the 802.1Q ethernet VLAN tag on output. If
the ingress LSR 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.
5.1.1. 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 VC 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 VC will not come
up.
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5.1.2. 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 VC 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.
5.1.3. Status codes
RFC 3036 has a range of Status Code values which are assigned by IANA
on a First Come, First Served basis. These are in the range
0x20000000-0x3effffff [note2]. The following new status codes are
defined:
0x20000001 "Illegal C-Bit"
0x20000002 "Wrong C-Bit"
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5.2. 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 LSRs implementing this design
are REQUIRED to accept such a withdraw message, but are not required
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.
5.3. 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
5.3.1. 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
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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.
5.3.2. 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. IANA Considerations
As specified in this document, a Virtual Circuit FEC element contains
the PW Type field. PW type value 0 is reserved. PW type values 1
through 10 are defined in this document. PW type values 11 through 63
are to be assigned by IANA using the "IETF Consensus" policy defined
in [RFC2434]. PW type values 64 through 127 are to be assigned by
IANA, using the "First Come First Served" policy defined in [RFC
2434]. PW type values 128 through 32767 are vendor-specific, and
values in this range are not to be assigned by IANA.
As specified in this document, a Pseudo Wire FEC element contains the
Interface Parameters field, which is a list of one or more
parameters, and each parameter is identified by the Parameter ID
field. Parameter ID value 0 is reserved. Parameter ID values 1
through 6 are defined in this document. Parameter ID values 7
through 63 are to be assigned by IANA using the "IETF Consensus"
policy defined in RFC2434. Parameter ID values 64 through 127 are to
be assigned by IANA, using the "First Come First Served" policy
defined in RFC2434. Parameter ID values 128 through 255 are vendor-
specific, and values in this range are not to be assigned by IANA.
7. Security Considerations
This document does not affect the underlying security issues of MPLS.
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8. 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] "Encapsulation Methods for Transport of Frame-Relay Over IP and
MPLS Networks", draft-ietf-pwe3-frame-encap-01.txt. ( work in
progress )
[8] "SONET/SDH Circuit Emulation Service Over MPLS (CEM)
Encapsulation", draft-malis-sonet-ces-mpls-05.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-00.txt ( work in
progress )
[11] "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP
and MPLS Networks", draft-martini-ppp-hdlc-encap-mpls-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 )
[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.
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[note1] FEC element type 128 is pending IANA approval.
[note2] Status codes assigment is pending IANA approval.
9. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
e-mail: giles@packetexchange.net
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
e-mail: d@mazunetworks.com
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: tappan@cisco.com
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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
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: sjv@laurelnetworks.com
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
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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
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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