Network Working Group L. Martini, Ed.

Network Working Group L. Martini, Ed.
Request for Comments: 4906 E. Rosen, Ed.
Category: Historic Cisco Systems, Inc.
N. El-Aawar, Ed.
Level 3 Communications, LLC
June 2007

Transport of Layer 2 Frames Over MPLS

Status of This Memo

This memo defines a Historic Document for the Internet community. It
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2007).

Abstract

This document describes methods for transporting the Protocol Data
Units (PDUs) of layer 2 protocols such as Frame Relay, Asynchronous
Transfer Mode (ATM) Adaption Layer 5 (AAL5), and Ethernet, and for
providing a Synchronized Optical Network (SONET) circuit emulation
service across an MPLS network. This document describes the so-
called "draft-martini" protocol, which has since been superseded by
the Pseudowire Emulation Edge to Edge Working Group specifications
described in RFC 4447 and related documents.

Martini, et al. Historic [Page 1]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

Table of Contents

1. Introduction ....................................................3
2. Specification of Requirements ...................................3
3. Special Note ....................................................3
4. Tunnel Labels and Virtual Circuit (VC) Labels ...................4
5. Protocol-Specific Details .......................................5
5.1. Frame Relay ................................................5
5.2. ATM ........................................................6
5.2.1. ATM AAL5 VCC Transport ..............................6
5.2.2. ATM Transparent Cell Transport ......................6
5.2.3. ATM VCC and VPC Cell Transport ......................6
5.2.4. OAM Cell Support ....................................6
5.2.5. ILMI Support ........................................7
5.3. Ethernet VLAN ..............................................7
5.4. Ethernet ...................................................8
5.5. HDLC .......................................................8
5.6. PPP ........................................................8
6. LDP .............................................................8
6.1. Interface Parameters Field ................................10
6.2. C Bit Handling Procedures .................................12
6.2.1. VC Types for Which the Control Word is REQUIRED ....12
6.2.2. VC Types for Which the Control Word is NOT
Mandatory ..........................................12
6.2.3. Status Codes .......................................15
6.3. LDP Label Withdrawal Procedures ...........................15
6.4. Sequencing Considerations .................................15
6.4.1. Label Mapping Advertisements .......................15
6.4.2. Label Mapping Release ..............................16
7. IANA Considerations ............................................16
8. Security Considerations ........................................16
9. Normative References ...........................................17
10. Informative References ........................................18
11. Co-Authors ....................................................18

Martini, et al. Historic [Page 2]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

1. 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
[RFC4905]. We restrict discussion to the case of point-to-point
transport. Quality of service (QoS)-related issues are not discussed
in this document. This document describes methods for transporting a
number of protocols; in some cases, transporting a particular
protocol may have several modes of operation. Each of these
protocols and/or modes may be implemented independently.

An accompanying document [CEM] 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) [ANSI.T1.105] / Synchronous
Digital Hierarchy (SDH) [ITU.G.707]. 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).

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. Special Note

This document describes the so-called "draft-martini" protocol, which
is used in many deployed implementations. This document and its
contents have since been superseded by the Pseudowire Emulation Edge
to Edge Working Group specifications: [RFC4447], [RFC4385],
[RFC4448], [RFC4717], [RFC4618], [RFC4619], [RFC4553], [RFC4842], and
related documents. This document serves as a documentation of
current implementations, and MUST NOT be used for new
implementations. The PWE3 Label Distribution Protocol (LDP) control
document [RFC4447], which is backward compatible with this document,
MUST be used for all new implementations of this protocol.

Martini, et al. Historic [Page 3]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

4. Tunnel Labels and Virtual Circuit (VC) Labels

Suppose it is desired to transport layer 2 PDUs from ingress Label
Switching Router (LSR) R1 to egress LSR R2, across an intervening
MPLS network. We assume that there is a Label Switched Path (LSP)
from R1 to R2. That is, we assume that R1 can cause a packet to be
delivered to R2 by pushing some label onto the packet and sending the
result to one of its adjacencies. Call this label the "tunnel
label", and the corresponding LSP the "tunnel LSP".

The tunnel LSP merely gets packets from R1 to R2; the corresponding
label doesn't tell R2 what to do with the payload. In fact, if
penultimate hop popping is used, R2 may never even see the
corresponding label. (If R1 itself is the penultimate hop, a 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 R2, that
tells R2 how to treat the received packet. Call this label the "VC
label".

So when R1 sends a layer 2 PDU to R2, it first pushes a VC label on
its label stack, and then (if R1 is not adjacent to R2) pushes on a
tunnel label. The tunnel label gets the MPLS packet from R1 to R2;
the VC label is not visible until the MPLS packet reaches R2. R2's
disposition of the packet is based on the VC label.

Note that the tunnel could be a Generic Routing Encapsulation (GRE)-
encapsulated MPLS tunnel between R1 and R2. In this case, R1 would
be adjacent to R2, and only the VC 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 R2.
That is, R2 needs to be able to infer from the VC label the outgoing
interface and the VPI/VCI (Virtual Path Identifier / Virtual Circuit
Identifier) value for the AAL5 PDU. If the payload is a Frame Relay
PDU, then R2 needs to be able to infer from the VC label the outgoing
interface and the DLCI (Data Link Connection Identifier) value. If
the payload is an Ethernet frame, then R2 needs to be able to infer
from the VC label the outgoing interface, and perhaps the VLAN
identifier. This process is unidirectional, and will be repeated
independently for bidirectional operation. It is REQUIRED to assign
the same VC ID, and VC 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.

Martini, et al. Historic [Page 4]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

Note that the VC label must always be at the bottom of the label
stack, and the tunnel label, if present, must be immediately above
the VC 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 R1 itself may push on additional labels above
the tunnel label. If R1 and R2 are directly adjacent LSRs, then it
may not be necessary to use a tunnel label at all.

This document does not specify a method for distributing the tunnel
label or any other labels that may appear above the VC label on the
stack. Any acceptable method of MPLS label distribution will do.

This document does specify a method for assigning and distributing
the VC label. Static label assignment MAY be used, and
implementations SHOULD provide support for this. When signaling is
used, the VC label MUST be distributed from R2 to R1 using LDP in the
downstream unsolicited mode; this requires that an LDP session be
created between R1 and R2. It should be noted that this LDP session
is not necessarily transported along the same path as the Layer 2
PDUs [RFC3036]. In addition, when using LDP to distribute the VC
label, liberal label retention mode SHOULD be used. However, as
required in [RFC3036], the label request operation (mainly used by
conservative label retention mode) MUST be implemented. VC labels
MUST be allocated from the per-platform label space.

Note that this technique allows an unbounded number of layer 2 "VCs"
to be carried together in a single "tunnel". Thus, it scales quite
well in the network backbone.

While this document currently defines the emulation of Frame Relay
and ATM Permanent Virtual Circuit (PVC) services, it specifically
does not preclude future enhancements to support switched service
(Switched Virtual Circuit (SVC) and Switched Permanent Virtual
Circuit (SPVC)) emulation.

5. Protocol-Specific Details

5.1. Frame Relay

The Frame Relay PDUs are encapsulated according to the procedures
defined in [RFC4905]. 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 [Q.933] Annex
A.5 cited in Implementation Agreement FRF.1.1, 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
[Q.933] on the egress Frame relay VC.

Martini, et al. Historic [Page 5]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

5.2. ATM

5.2.1. ATM AAL5 VCC Transport

ATM AAL5 Common Part Convergence Sublayer - Service Data Units
(CPCS-SDUs) are encapsulated according to [RFC4905] 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.

5.2.2. 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, R1, is
encapsulated according to [RFC4905], ATM cell mode, and sent across
the LSP to the egress LSR, R2. This mode allows an ATM port to be
connected to only one other ATM port. [RFC4905] allows for grouping
of multiple cells into a single MPLS frame. Grouping of ATM cells is
OPTIONAL for transmission at the ingress LSR, R1. If the Egress LSR
R2 supports cell concatenation, the ingress LSR, R1, should only
concatenate cells up to the "Maximum Number of concatenated ATM
cells" parameter received as part of the FEC element.

5.2.3. ATM VCC and VPC Cell Transport

This mode is similar to the ATM AAL5 Virtual Channel Connection (VCC)
transport except that cells are transported. Every cell that is
received on a pre-defined ATM PVC or ATM Permanent Virtual Path
(PVP), at the ingress ATM port on the ingress LSR, R1, is
encapsulated according to [RFC4905], ATM cell mode, and sent across
the LSP to the egress LSR R2. Grouping of ATM cells is OPTIONAL for
transmission at the ingress LSR, R1. If the egress LSR R2 supports
cell concatenation, the ingress LSR, R1, MUST only concatenate cells
up to the "Maximum Number of concatenated ATM cells in a frame"
parameter received as part of the FEC element.

5.2.4. OAM Cell Support

Operations and Management (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 [RFC4905]. When operating in AAL5 SDU transport mode,
an LSR that supports transport of OAM cells using the T bit defined
in [RFC4905], or an LSR operating in any of the three cell transport
modes, MUST follow the procedures outlined in [FAST] Section 8 for
mode 0 only, in addition to the applicable procedures specified in
[ITU.G.707].

Martini, et al. Historic [Page 6]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

5.2.4.1. 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 bidirectional ATM VC by two
unidirectional 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, R1, 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, R1, receives an AIS (Alarm Indication Signal) F5
OAM cell, or R1 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 VC label mapping is withdrawn, the egress LSR, R2, 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 time.

5.2.5. ILMI Support

An MPLS edge LSR MAY provide an ATM Integrated Local Management
Interface (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 VC label
mapping is withdrawn, the egress LSR SHOULD notify its client of this
failure by deleting the VC using ILMI.

5.3. Ethernet VLAN

The Ethernet frame will be encapsulated according to the procedures
in [RFC4905]. 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

Martini, et al. Historic [Page 7]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

port, or the port is administratively disabled, it MUST withdraw the
label mappings for all VCs associated with the port.

5.4. Ethernet

The Ethernet frame will be encapsulated according to the procedures
in [RFC4905]. If the LSR detects a failure on the Ethernet physical
port, or the port is administratively disabled, the corresponding VC
label mapping MUST be withdrawn.

5.5. HDLC

HDLC (High-Level Data Link Control) frames are encapsulated according
to the procedures in [RFC4905]. 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 link.

5.6. PPP

PPP frames are encapsulated according to the procedures in [RFC4905].
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 PPP link.

6. LDP

The VC label bindings are distributed using the LDP downstream
unsolicited mode described in [RFC3036]. The LSRs will establish an
LDP session using the Extended Discovery mechanism described in
sections 2.4.2 and 2.5 of [RFC3036]; for this purpose, a new type of
FEC element is defined. The FEC element type is 128. Only a single
VC FEC element MUST be advertised per LDP VC label. The Virtual
Circuit 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| VC Type |VC info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VC ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface parameters |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Martini, et al. Historic [Page 8]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

- VC Type

A 15-bit quantity containing a value that represents the type of
VC. Assigned values are:

VC 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 [CEM]
0x0009 ATM VCC cell transport
0x000A ATM VPC cell transport

- Control word bit (C)

The highest order bit (C) of the VC type is used to flag the
presence of a control word (defined in [RFC4905]) as follows:

bit 15 = 1 control word present on this VC.
bit 15 = 0 no control word present on this VC.

Please see Section 6.2, "C Bit Handling Procedures", for further
explanation.

- VC information length

Length of the VC ID field and the interface parameters field in
octets. If this value is 0, then it references all VCs using
the specified group ID, and there is no VC ID present, nor any
interface parameters.

- Group ID

An arbitrary 32-bit value, which represents a group of VCs 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 VC 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.

Martini, et al. Historic [Page 9]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

- VC ID

A non-zero 32-bit connection ID that, together with the VC type,
identifies a particular VC.

- Interface parameters

This variable length field is used to provide interface-specific
parameters, such as interface MTU.

6.1. 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:

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 [CEM] Payload Bytes.
0x05 4 CEM options.

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

Martini, et al. Historic [Page 10]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

parameter is applicable only to VC types 1, 2, 4, 5, 6, and 7,
and is REQUIRED for these VC types. If this parameter does not
match in both directions of a specific VC, that VC 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 VC 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.

- Optional Interface Description string

This arbitrary, OPTIONAL interface description string can be
used to send an administrative description text string to the
remote LSR. This parameter is OPTIONAL, and is applicable to
all VC types. The interface description parameter string length
is variable, and can be from 0 to 80 octets.

- 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 Synchronous Payload Envelope
(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 [CEM] for the
definition of the bit values.

Martini, et al. Historic [Page 11]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

6.2. C Bit Handling Procedures

6.2.1. VC Types for Which the Control Word is REQUIRED

The Label Mapping messages which are sent in order to set up these
VCs 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.

6.2.2. VC Types for Which the Control Word is NOT Mandatory

If a system is capable of sending and receiving the control word on
VC types for which the control word is not mandatory, then each such
VC endpoint MUST be configurable with a parameter that specifies
whether the use of the control word is PREFERRED or NOT PREFERRED.
For each VC, 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 exactly as if it were configured for the use of the control
word to be NOT PREFERRED.

If a Label Mapping message for the VC has already been received, but
no Label Mapping message for the VC 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 VC 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 VC 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 VC had been received (i.e.,
proceed to the next paragraph).

If a Label Mapping message for the VC 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. (That is, set c=1

Martini, et al. Historic [Page 12]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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 VC. The possibilities are:

-i. A Label Mapping message with the same c bit value as specified
in the Label Mapping message that was sent. VC setup is now
complete, and the control word is used if c=1 but not used if
c=0.

-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 VC.

-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. VC 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 VC.

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.

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 following diagram illustrates the above procedures:

Martini, et al. Historic [Page 13]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

------------------
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, |
| VC setup is complete. If not: |
----------------------------------
| | | |
------------------- -----------
| Receive | | Receive |
| C=1 | | C=0 |
------------------- -----------
| |
Wait for the Send
next message Wrong C-Bit
|
Send Label Mapping
Message with C=0

Martini, et al. Historic [Page 14]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

6.2.3. 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. The following new status codes are defined:

0x20000001 "Illegal C-Bit"
0x20000002 "Wrong C-Bit"

6.3. LDP Label Withdrawal Procedures

As mentioned above, the Group ID field can be used to withdraw all VC
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 VC information length field is set to 0,
the VC 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 VC
label withdrawal message or release message. A wild card release
message MUST include only the group ID. A Label Release message
initiated from the imposition router must always include the VC ID.

6.4. 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.

6.4.1. 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 (reuse) 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

Martini, et al. Historic [Page 15]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

imposition router, the disposition router SHOULD be expecting the
next packet to arrive with sequence number of 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 are for the disposition router to always advertise a
different VC label, or for the disposition router to wait for a
sufficient time before attempting to re-advertise a recently released
label. This is only an issue when sequence number processing at the
disposition router is enabled.

6.4.2. Label Mapping Release

In situations where the imposition router wants to restart forwarding
of packets with sequence number 1, the router shall 1) send a label
mapping release to the disposition router, and 2) send a label
mapping request to the disposition router. When sequencing is
supported, advertisement of a VC label in response to a label mapping
request MUST also consider the issues discussed in Section 6.4.1.

7. IANA Considerations

As specified in this document, a Virtual Circuit FEC element contains
the VC Type field. VC Type value 0 is reserved. VC Type values 1
through 10 are defined in this document. VC Type values 11 through
63 are to be assigned by IANA using the "IETF Consensus" policy
defined in RFC 2434. VC Type values 64 through 127 are to be
assigned by IANA, using the "First Come First Served" policy defined
in RFC 2434. VC 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 Virtual Circuit 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 5 are defined in this document. Parameter ID values 6
through 63 are to be assigned by IANA using the "IETF Consensus"
policy defined in RFC 2434. Parameter ID values 64 through 127 are
to be assigned by IANA, using the "First Come First Served" policy
defined in RFC 2434. Parameter ID values 128 through 255 are
vendor-specific, and values in this range are not to be assigned by
IANA.

8. Security Considerations

This document does not affect the underlying security issues of MPLS,
described in [RFC3032]. More detailed security considerations are
also described in Section 8 of [RFC4447].

Martini, et al. Historic [Page 16]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

9. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
and G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447, 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.

[RFC4842] Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
"Synchronous Optical Network/Synchronous Digital
Hierarchy (SONET/SDH) Circuit Emulation over Packet
(CEP)", RFC 4842, April 2007.

[RFC4553] Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
Agnostic Time Division Multiplexing (TDM) over Packet
(SAToP)", RFC 4553, June 2006.

[RFC4619] Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
"Encapsulation Methods for Transport of Frame Relay
over Multiprotocol Label Switching (MPLS) Networks",
RFC 4619, September 2006.

[RFC4717] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
Brayley, J., and G. Koleyni, "Encapsulation Methods for
Transport of Asynchronous Transfer Mode (ATM) over MPLS
Networks", RFC 4717, December 2006.

[RFC4618] Martini, L., Rosen, E., Heron, G., and A. Malis,
"Encapsulation Methods for Transport of PPP/High-Level
Data Link Control (HDLC) over MPLS Networks", RFC 4618,
September 2006.

[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G.
Heron, "Encapsulation Methods for Transport of Ethernet
over MPLS Networks", RFC 4448, April 2006.

[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
and B. Thomas, "LDP Specification", RFC 3036, January
2001.

[Q.933] ITU-T Recommendation Q.933, and Q.922 Specification for
Frame Mode Basic call control, ITU Geneva 1995.

Martini, et al. Historic [Page 17]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.

[ANSI.T1.105] American National Standards Institute, "Synchronous
Optical Network Formats," ANSI T1.105-1995.

[ITU.G.707] ITU Recommendation G.707, "Network Node Interface For
The Synchronous Digital Hierarchy", 1996.

[RFC4905] Martini, L., Ed., Rosen, E., Ed., and N. El-Aawar, Ed.,
"Encapsulation Methods for Transport of Layer 2 Frames
over MPLS Networks", RFC 4905, June 2007.

10. Informative References

[CEM] Malis, A., Brayley, J., Vogelsang., S., Shirron, J.,
and L. Martini, "SONET/SDH Circuit Emulation Service
Over MPLS (CEM) Encapsulation", Work in Progress, June
2007.

[FAST] ATM Forum, "Frame Based ATM over SONET/SDH Transport
(FAST)", af-fbatm-0151.000, July 2000.

11. Co-Authors

Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
EMail: [email protected]

Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
EMail: [email protected]

Martini, et al. Historic [Page 18]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
EMail: [email protected]

Jayakumar Jayakumar,
Cisco Systems Inc.
225, E.Tasman, MS-SJ3/3,
San Jose, CA 95134
EMail: [email protected]

Alex Hamilton,
Cisco Systems Inc.
285 W. Tasman, MS-SJCI/3/4,
San Jose, CA 95134
EMail: [email protected]

Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: [email protected]

John Shirron
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: [email protected]

Toby Smith
Network Appliance, Inc.
800 Cranberry Woods Drive
Suite 300
Cranberry Township, PA 16066
EMail: [email protected]

Martini, et al. Historic [Page 19]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
EMail: [email protected]

Vinai Sirkay
Reliance Infocomm
Dhirubai Ambani Knowledge City
Navi Mumbai 400 709
India
EMail: [email protected]

Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
EMail: [email protected]

Chris Liljenstolpe
Alcatel
11600 Sallie Mae Dr.
9th Floor
Reston, VA 20193
EMail: [email protected]

Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
EMail: [email protected]

Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: [email protected]

Martini, et al. Historic [Page 20]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

Authors' Addresses

Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
EMail: [email protected]

Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO 80021
EMail: [email protected]

Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
EMail: [email protected]

Martini, et al. Historic [Page 21]

RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

Full Copyright Statement

Copyright (C) The IETF Trust (2007).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
[email protected].

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Martini, et al. Historic [Page 22]