Network Working Group L. Martini, Ed.

Network Working Group L. Martini, Ed.
Request for Comments: 4448 E. Rosen
Category: Standards Track Cisco Systems, Inc.
N. El-Aawar
Level 3 Communications, LLC
G. Heron
Tellabs
April 2006

Encapsulation Methods for Transport of Ethernet over MPLS Networks

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

An Ethernet pseudowire (PW) is used to carry Ethernet/802.3 Protocol
Data Units (PDUs) over an MPLS network. This enables service
providers to offer "emulated" Ethernet services over existing MPLS
networks. This document specifies the encapsulation of
Ethernet/802.3 PDUs within a pseudowire. It also specifies the
procedures for using a PW to provide a "point-to-point Ethernet"
service.

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Table of Contents

1. Introduction ....................................................3
2. Specification of Requirements ...................................6
3. Applicability Statement .........................................6
4. Details Specific to Particular Emulated Services ................7
4.1. Ethernet Tagged Mode .......................................7
4.2. Ethernet Raw Mode ..........................................8
4.3. Ethernet-Specific Interface Parameter LDP Sub-TLV ..........8
4.4. Generic Procedures .........................................9
4.4.1. Raw Mode vs. Tagged Mode ............................9
4.4.2. MTU Management on the PE/CE Links ..................11
4.4.3. Frame Ordering .....................................11
4.4.4. Frame Error Processing .............................11
4.4.5. IEEE 802.3x Flow Control Interworking ..............11
4.5. Management ................................................12
4.6. The Control Word ..........................................12
4.7. QoS Considerations ........................................13
5. Security Considerations ........................................14
6. PSN MTU Requirements ...........................................14
7. Normative References ...........................................15
8. Informative References .........................................15
9. Significant Contributors .......................................17
Appendix A. Interoperability Guidelines ...........................20
A.1. Configuration Options .....................................20
A.2. IEEE 802.3x Flow Control Considerations ...................21
Appendix B. QoS Details ...........................................21
B.1. Adaptation of 802.1Q CoS to PSN CoS .......................22
B.2. Drop Precedence ...........................................23

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1. Introduction

An Ethernet pseudowire (PW) allows Ethernet/802.3 [802.3] Protocol
Data Units (PDUs) to be carried over a Multi-Protocol Label Switched
[MPLS-ARCH] network. In addressing the issues associated with
carrying an Ethernet PDU over a packet switched network (PSN), this
document assumes that a pseudowire (PW) has been set up by using a
control protocol such as the one as described in [PWE3-CTRL]. The
design of Ethernet pseudowire described in this document conforms to
the pseudowire architecture described in [RFC3985]. It is also
assumed in the remainder of this document that the reader is familiar
with RFC 3985.

The Pseudowire Emulation Edge-to-Edge (PWE3) Ethernet PDU consists of
the Destination Address, Source Address, Length/Type, MAC Client
Data, and padding extracted from a MAC frame as a concatenated octet
sequence in their original order [PDU].

In addition to the Ethernet PDU format used within the pseudowire,
this document discusses:

- Procedures for using a PW in order to provide a pair of Customer
Edge (CE) routers with an emulated (point-to-point) Ethernet
service, including the procedures for the processing of Provider
Edge (PE)-bound and CE-bound Ethernet PDUs [RFC3985]

- Ethernet-specific quality of service (QoS) and security
considerations

- Inter-domain transport considerations for Ethernet PW

The following two figures describe the reference models that are
derived from [RFC3985] to support the Ethernet PW emulated services.

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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
| | |<-- PSN Tunnel -->| | |
| PW End V V V V PW End |
V Service +----+ +----+ Service V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
| |
Attachment Circuit (AC) Attachment Circuit (AC)
native Ethernet service native Ethernet service

Figure 1: PWE3 Ethernet/VLAN Interface Reference Configuration

The "emulated service" shown in Figure 1 is, strictly speaking, a
bridged LAN; the PEs have MAC interfaces, consume MAC control frames,
etc. However, the procedures specified herein only support the case
in which there are two CEs on the "emulated LAN". Hence we refer to
this service as "emulated point-to-point Ethernet". Specification of
the procedures for using pseudowires to emulate LANs with more than
two CEs are out of the scope of the current document.

+-------------+ +-------------+
| Emulated | | Emulated |
| Ethernet | | Ethernet |
| (including | Emulated Service | (including |
| VLAN) |<==============================>| VLAN) |
| Services | | Services |
+-------------+ Pseudowire +-------------+
|Demultiplexer|<==============================>|Demultiplexer|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
| MPLS |<==============================>| MPLS |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+

Figure 2: Ethernet 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 on the attachment circuit of PE2.

An Ethernet PW emulates a single Ethernet link between exactly two
endpoints. The mechanisms described in this document are agnostic to
that which is beneath the "Pseudowire" level in Figure 2, concerning
itself only with the "Emulated Service" portion of the stack.

The following reference model describes the termination point of each
end of the PW within the PE:

+-----------------------------------+
| PE |
+---+ +-+ +-----+ +------+ +------+ +-+
| | |P| | | |PW ter| | PSN | |P|
| |<==|h|<=| NSP |<=|minati|<=|Tunnel|<=|h|<== From PSN
| | |y| | | |on | | | |y|
| C | +-+ +-----+ +------+ +------+ +-+
| E | | |
| | +-+ +-----+ +------+ +------+ +-+
| | |P| | | |PW ter| | PSN | |P|
| |==>|h|=>| NSP |=>|minati|=>|Tunnel|=>|h|==> To PSN
| | |y| | | |on | | | |y|
+---+ +-+ +-----+ +------+ +------+ +-+
| |
+-----------------------------------+
^ ^ ^
| | |
A B C

Figure 3: PW Reference Diagram

The PW terminates at a logical port within the PE, defined at point B
in the above diagram. This port provides an Ethernet MAC service
that will deliver each Ethernet frame that is received at point A,
unaltered, to the point A in the corresponding PE at the other end of
the PW.

The Native Service Processing (NSP) function includes frame
processing that is required for the Ethernet frames that are
forwarded to the PW termination point. Such functions may include
stripping, overwriting or adding VLAN tags, physical port
multiplexing and demultiplexing, PW-PW bridging, L2 encapsulation,
shaping, policing, etc. These functions are specific to the Ethernet
technology, and may not be required for the PW emulation service.

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The points to the left of A, including the physical layer between the
CE and PE, and any adaptation (NSP) functions between it and the PW
terminations, are outside of the scope of PWE3 and are not defined
here.

"PW Termination", between A and B, represents the operations for
setting up and maintaining the PW, and for encapsulating and
decapsulating the Ethernet frames as necessary to transmit them
across the MPLS network.

An Ethernet PW operates in one of two modes: "raw mode" or "tagged
mode". In tagged mode, each frame MUST contain at least one 802.1Q
[802.1Q] VLAN tag, and the tag value is meaningful to the NSPs at the
two PW termination points. That is, the two PW termination points
must have some agreement (signaled or manually configured) on how to
process the tag. On a raw mode PW, a frame MAY contain an 802.1Q
VLAN tag, but if it does, the tag is not meaningful to the NSPs, and
passes transparently through them.

Additional terminology relevant to pseudowires and Layer 2 Virtual
Private Networking may be found in [RFC4026].

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 RFC 2119 [RFC2119].

3. Applicability Statement

The Ethernet PW emulation allows a service provider to offer a "port
to port" Ethernet-based service across an MPLS packet switched
network (PSN) while the Ethernet VLAN PW emulation allows an
"Ethernet VLAN to VLAN" based service across an MPLS packet switched
network (PSN).

The Ethernet or Ethernet VLAN PW has the following characteristics in
relationship to the respective native service:

- An Ethernet PW connects two Ethernet ACs while an Ethernet VLAN
PW connects two Ethernet VLAN ACs, supporting bidirectional
transport of variable length Ethernet frames. The ingress
Native Service Processing (NSP) function strips the preamble and
frame check sequence (FCS) from the Ethernet frame and
transports the frame in its entirety across the PW. This is
done regardless of the presence of the 802.1Q tag in the frame.
The egress NSP function receives the Ethernet frame from the PW
and regenerates the preamble or FCS before forwarding the frame

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to the attachment circuit. Since the FCS is not transported
across either Ethernet or Ethernet VLAN PWs, payload integrity
transparency may be lost. The OPTIONAL method described in
[FCS] can be used to achieve payload integrity transparency on
Ethernet or Ethernet VLAN PWs.

- For an Ethernet VLAN PW, VLAN tag rewrite can be achieved by NSP
at the egress PE, which is outside the scope of this document.

- The Ethernet or Ethernet VLAN PW only supports homogeneous
Ethernet frame type across the PW; both ends of the PW must be
either tagged or untagged. Heterogeneous frame type support
achieved with NSP functionality is outside the scope of this
document.

- Ethernet port or Ethernet VLAN status notification is provided
using the PW Status TLV in the Label Distribution Protocol (LDP)
status notification message. Loss of connectivity between PEs
can be detected by the LDP session closing, or by using [VCCV]
mechanisms. The PE can convey these indications back to its
attached Remote System.

- The maximum frame size that can be supported is limited by the
PSN MTU minus the MPLS header size, unless fragmentation and
reassembly are used [FRAG].

- The packet switched network may reorder, duplicate, or silently
drop packets. Sequencing MAY be enabled in the Ethernet or
Ethernet VLAN PW to detect lost, duplicate, or out-of-order
packets on a per-PW basis.

- The faithfulness of an Ethernet or Ethernet VLAN PW may be
increased by leveraging Quality of Service features of the PEs
and the underlying PSN. (See Section 4.7, "QoS
Considerations".)

4. Details Specific to Particular Emulated Services

4.1. Ethernet Tagged Mode

The Ethernet frame will be encapsulated according to the procedures
defined later in this document for tagged mode. It should be noted
that if the VLAN identifier is modified by the egress PE, the
Ethernet spanning tree protocol might fail to work properly. If this
issue is of significance, the VLAN identifier MUST be selected in
such a way that it matches on the attachment circuits at both ends of
the PW.

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If the PE detects a failure on the Ethernet physical port, or the
port is administratively disabled, it MUST send a 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 and corresponds to PW type 0x0004 "Ethernet Tagged
Mode" [IANA].

4.2. Ethernet Raw Mode

The Ethernet frame will be encapsulated according to the procedures
defined later in this document for raw mode. If the PE detects a
failure on the Ethernet input port, or the port is administratively
disabled, the PE MUST send an appropriate PW status notification
message to the corresponding remote PE.

In this mode, all Ethernet frames received on the attachment circuit
of PE1 will be transmitted to PE2 on a single PW. This service
corresponds to PW type 0x0005 "Ethernet" [IANA].

4.3. Ethernet-Specific Interface Parameter LDP Sub-TLV

This LDP sub-Type Length Value [LDP] 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
Interface parameter TLV is defined in [PWE3-CTRL], the IANA registry
with initial values for interface parameter sub-TLV types is defined
in [IANA], but the Ethernet-specific interface parameters are
specified as follows:

- 0x06 Requested VLAN ID Sub-TLV

An Optional 16-bit value indicating the requested VLAN ID. This
parameter MUST be used by a PE that is incapable of rewriting
the 802.1Q Ethernet VLAN tag on output. If the ingress PE
receives this request, it MUST rewrite the VLAN ID contained
inside the VLAN Tag at the input to match the requested VLAN ID.
If this is not possible, and the VLAN ID does not already match
the configured ingress VLAN ID, the PW MUST not be enabled.
This parameter is applicable only to PW type 0x0004.

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4.4. Generic Procedures

When the NSP/Forwarder hands a frame to the PW termination function:

- The preamble (if any) and FCS are stripped off.

- The control word as defined in Section 4.6, "The Control Word",
is, if necessary, prepended to the resulting frame. The
conditions under which the control word is or is not used are
specified below.

- The proper pseudowire demultiplexer (PW Label) is prepended to
the resulting packet.

- The proper tunnel encapsulation is prepended to the resulting
packet.

- The packet is transmitted.

The way in which the proper tunnel encapsulation and pseudowire
demultiplexer is chosen depends on the procedures that were used to
set up the pseudowire.

The tunnel encapsulation depends on how the MPLS PSN is set up. This
can include no label, one label, or multiple labels. The proper
pseudowire demultiplexer is an MPLS label whose value is determined
by the PW setup and maintenance protocols.

When a packet arrives over a PW, the tunnel encapsulation and PW
demultiplexer are stripped off. If the control word is present, it
is processed and stripped off. The resulting frame is then handed to
the Forwarder/NSP. Regeneration of the FCS is considered to be an
NSP responsibility.

4.4.1. Raw Mode vs. Tagged Mode

When the PE receives an Ethernet frame, and the frame has a VLAN tag,
we can distinguish two cases:

1. The tag is service-delimiting. This means that the tag was
placed on the frame by some piece of service provider-operated
equipment, and the tag is used by the service provider to
distinguish the traffic. For example, LANs from different
customers might be attached to the same service provider
switch, which applies VLAN tags to distinguish one customer's
traffic from another's, and then forwards the frames to the PE.

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2. The tag is not service-delimiting. This means that the tag was
placed in the frame by a piece of customer equipment, and is
not meaningful to the PE.

Whether or not the tag is service-delimiting is determined by local
configuration on the PE.

If an Ethernet PW is operating in raw mode, service-delimiting tags
are NEVER sent over the PW. If a service-delimiting tag is present
when the frame is received from the attachment circuit by the PE, it
MUST be stripped (by the NSP) from the frame before the frame is sent
to the PW.

If an Ethernet PW is operating in tagged mode, every frame sent on
the PW MUST have a service-delimiting VLAN tag. If the frame as
received by the PE from the attachment circuit does not have a
service-delimiting VLAN tag, the PE must prepend the frame with a
dummy VLAN tag before sending the frame on the PW. This is the
default operating mode. This is the only REQUIRED mode.

In both modes, non-service-delimiting tags are passed transparently
across the PW as part of the payload. It should be noted that a
single Ethernet packet may contain more than one tag. At most, one
of these tags may be service-delimiting. In any case, the NSP
function may only inspect the outermost tag for the purpose of
adapting the Ethernet frame to the pseudowire.

In both modes, the service-delimiting tag values have only local
significance, i.e., are meaningful only at a particular PE-CE
interface. When tagged mode is used, the PE that receives a frame
from the PW may rewrite the tag value, or may strip the tag entirely,
or may leave the tag unchanged, depending on its configuration. When
raw mode is used, the PE that receives a frame may or may not need to
add a service-delimiting tag before transmitting the frame on the
attachment circuit; however, it MUST not rewrite or remove any tags
that are already present.

The following table illustrates the operations that might be
performed at input from the attachment circuit:

+-----------------------------------------------------------+
| Tag-> | service delimiting | non service delimiting|
|-------------+---------------------+-----------------------|
| Raw Mode | 1st VLAN Tag Removed| no operation performed|
|-------------+---------------------+-----------------------|
| Tagged Mode | NO OP or Tag Added | Tag Added |
+-----------------------------------------------------------+

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4.4.2. MTU Management on the PE/CE Links

The Ethernet PW MUST NOT be enabled unless it is known that the MTUs
of the CE-PE links are the same at both ends of the PW. If an egress
router receives an encapsulated layer 2 PDU whose payload length
(i.e., the length of the PDU itself without any of the encapsulation
headers) exceeds the MTU of the destination layer 2 interface, the
PDU MUST be dropped.

4.4.3. Frame Ordering

In general, applications running over Ethernet do not require strict
frame ordering. However, the IEEE definition of 802.3 [802.3]
requires that frames from the same conversation in the context of
link aggregation (clause 43) are delivered in sequence. Moreover,
the PSN cannot (in the general case) be assumed to provide or to
guarantee frame ordering. An Ethernet PW can, through use of the
control word, provide strict frame ordering. If this option is
enabled, any frames that get misordered by the PSN will be dropped or
reordered by the receiving PW endpoint. If strict frame ordering is
a requirement for a particular PW, this option MUST be enabled.

4.4.4. Frame Error Processing

An encapsulated Ethernet frame traversing a pseudowire may be
dropped, corrupted, or delivered out-of-order. As described in
[PWE3-REQ], frame loss, corruption, and out-of-order delivery are
considered to be a "generalized bit error" of the pseudowire. PW
frames that are corrupted will be detected at the PSN layer and
dropped.

At the ingress of the PW, the native Ethernet frame error processing
mechanisms MUST be enabled. Therefore, if a PE device receives an
Ethernet frame containing hardware-level Cyclic Redundancy Check
(CRC) errors, framing errors, or a runt condition, the frame MUST be
discarded on input. Note that defining this processing is part of
the NSP function and is outside the scope of this document.

4.4.5. IEEE 802.3x Flow Control Interworking

In a standard Ethernet network, the flow control mechanism is
optional and typically configured between the two nodes on a point-
to-point link (e.g., between the CE and the PE). IEEE 802.3x PAUSE
frames MUST NOT be carried across the PW. See Appendix A for notes
on CE-PE flow control.

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4.5. Management

The Ethernet PW management model follows the general PW management
model defined in [RFC3985] and [PWE3-MIB]. Many common PW management
facilities are provided here, with no additional Ethernet specifics
necessary. Ethernet-specific parameters are defined in an additional
MIB module, [PW-MIB].

4.6. The Control Word

The control word defined in this section is based on the Generic PW
MPLS Control Word as defined in [PWE3-CW]. It provides the ability
to sequence individual frames on the PW, avoidance of equal-cost
multiple-path load-balancing (ECMP) [RFC2992], and Operations and
Management (OAM) mechanisms including VCCV [VCCV].

[PWE3-CW] states, "If a PW is sensitive to packet misordering and is
being carried over an MPLS PSN that uses the contents of the MPLS
payload to select the ECMP path, it MUST employ a mechanism which
prevents packet misordering." This is necessary because ECMP
implementations may examine the first nibble after the MPLS label
stack to determine whether the labelled packet is IP or not. Thus,
if the source MAC address of an Ethernet frame carried over the PW
without a control word present begins with 0x4 or 0x6, it could be
mistaken for an IPv4 or IPv6 packet. This could, depending on the
configuration and topology of the MPLS network, lead to a situation
where all packets for a given PW do not follow the same path. This
may increase out-of-order frames on a given PW, or cause OAM packets
to follow a different path than actual traffic (see Section 4.4.3,
"Frame Ordering").

The features that the control word provides may not be needed for a
given Ethernet PW. For example, ECMP may not be present or active on
a given MPLS network, strict frame sequencing may not be required,
etc. If this is the case, the control word provides little value and
is therefore optional. Early Ethernet PW implementations have been
deployed that do not include a control word or the ability to process
one if present. To aid in backwards compatibility, future
implementations MUST be able to send and receive frames without the
control word present.

In all cases, the egress PE MUST be aware of whether the ingress PE
will send a control word over a specific PW. This may be achieved by
configuration of the PEs, or by signaling, as defined in [PWE3-CTRL].

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The control word is defined as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Reserved | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In the above diagram, the first 4 bits MUST be set to 0 to indicate
PW data. The rest of the first 16 bits are reserved for future use.
They MUST be set to 0 when transmitting, and MUST be ignored upon
receipt.

The next 16 bits provide a sequence number that can be used to
guarantee ordered frame delivery. The processing of the sequence
number field is OPTIONAL.

The sequence number space is a 16-bit, unsigned circular space. The
sequence number value 0 is used to indicate that the sequence number
check algorithm is not used. The sequence number processing
algorithm is found in [PWE3-CW].

4.7. QoS Considerations

The ingress PE MAY consider the user priority (PRI) field [802.1Q] of
the VLAN tag header when determining the value to be placed in a QoS
field of the encapsulating protocol (e.g., the EXP fields of the MPLS
label stack). In a similar way, the egress PE MAY consider the QoS
field of the encapsulating protocol (e.g., the EXP fields of the MPLS
label stack) when queuing the frame for transmission towards the CE.

A PE MUST support the ability to carry the Ethernet PW as a best-
effort service over the MPLS PSN. PRI bits are kept transparent
between PE devices, regardless of the QoS support of the PSN.

If an 802.1Q VLAN field is added at the PE, a default PRI setting of
zero MUST be supported, a configured default value is recommended, or
the value may be mapped from the QoS field of the PSN, as referred to
above.

A PE may support additional QoS support by means of one or more of
the following methods:

i. One class of service (CoS) per PW End Service (PWES), mapped
to a single CoS PW at the PSN.
ii. Multiple CoS per PWES mapped to a single PW with multiple
CoS at the PSN.
iii. Multiple CoS per PWES mapped to multiple PWs at the PSN.

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Examples of the cases above and details of the service mapping
considerations are described in Appendix B.

The PW guaranteed rate at the MPLS PSN level is PW service provider
policy based on agreement with the customer, and may be different
from the Ethernet physical port rate.

5. Security Considerations

The Ethernet pseudowire type is subject to all of the general
security considerations discussed in [RFC3985] and [PWE3-CTRL].

The Ethernet pseudowire is transported on an MPLS PSN; therefore, the
security of the pseudowire itself will only be as good as the
security of the MPLS PSN. The MPLS PSN can be secured by various
methods, as described in [MPLS-ARCH].

Security achieved by access control of MAC addresses is out of the
scope of this document. Additional security requirements related to
the use of PW in a switching (virtual bridging) environment are not
discussed here as they are not within the scope of this document.

6. PSN MTU Requirements

The MPLS PSN MUST be configured with an MTU that is large enough to
transport a maximum-sized Ethernet frame that has been encapsulated
with a control word, a pseudowire demultiplexer, and a tunnel
encapsulation. With MPLS used as the tunneling protocol, for
example, this is likely to be 8 or more bytes greater than the
largest frame size. The methodology described in [FRAG] MAY be used
to fragment encapsulated frames that exceed the PSN MTU. However, if
[FRAG] is not used and if the ingress router determines that an
encapsulated layer 2 PDU exceeds the MTU of the PSN tunnel through
which it must be sent, the PDU MUST be dropped.

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7. Normative References

[PWE3-CW] Bryant, S., Swallow, G., and D. McPherson, "Pseudowire
Emulation Edge-to-Edge (PWE3) Control Word for Use over
an MPLS PSN", RFC 4385, February 2006.

[IANA] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

[PWE3-CTRL] Martini, L., El-Aawar, N., Heron, G., Rosen, E., Tappan,
D., and T. Smith, "Pseudowire Setup and Maintenance
using the Label Distribution Protocol (LDP)", RFC 4447,
April 2006.

[MPLS-ARCH] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC 3031,
January 2001.

[802.3] IEEE802.3-2005, ISO/IEC 8802-3: 2000 (E), "IEEE Standard
for Information technology -- Telecommunications and
information exchange between systems -- Local and
metropolitan
area networks -- Specific requirements -- Part 3:
Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) Access Method and Physical Layer
Specifications", 2005.

[802.1Q] ANSI/IEEE Standard 802.1Q-2005, "IEEE Standards for
Local and Metropolitan Area Networks: Virtual Bridged
Local Area Networks", 2005.

[PDU] IEEE Std 802.3, 1998 Edition, "Part 3: Carrier sense
multiple access with collision detection (CSMA/CD)
access method and physical layer specifications" figure
3.1, 1998

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

8. Informative References

[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.

[PW-MIB] Zelig, D. and T. Nadeau, "Ethernet Pseudo Wire (PW)
Management Information Base", Work in Progress, February
2006.

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RFC 4448 Encapsulation of Ethernet over MPLS April 2006

[PWE3-REQ] Xiao, X., McPherson, D., and P. Pate, "Requirements for
Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
September 2004.

[PWE3-MIB] Zelig, D., Ed. and T. Nadeau, Ed., "Pseudo Wire (PW)
Management Information Base", Work in Progress, February
2006.

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

[FRAG] Malis, A. and W. Townsley, "PWE3 Fragmentation and
Reassembly", Work in Progress, February 2005.

[FCS] Malis, A., Allan, D., and N. Del Regno, "PWE3 Frame
Check Sequence Retention", Work in Progress, September
2005.

[VCCV] Nadeau, T., Ed. and R. Aggarwal, Ed., "Pseudo Wire
Virtual Circuit Connectivity Verification (VCCV)", Work
in Progress, August 2005.

[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, November 2000.

[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned
Virtual Private Network (VPN) Terminology", RFC 4026,
March 2005.

[L2TPv3] Lau, J., Townsley, M., and I. Goyret, "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
March 2005.

Martini, et al. Standards Track [Page 16]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

9. Significant Contributors

Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134

EMail: [email protected]

Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719

EMail: [email protected]

Steve Vogelsang
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205

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]

Martini, et al. Standards Track [Page 17]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Chris Liljenstolpe
Alcatel
11600 Sallie Mae Dr.
9th Floor
Reston, VA 20193

EMail: [email protected]

Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089

EMail: [email protected]

Tricci So
Nortel Networks 3500 Carling Ave.,
Nepean, Ontario,
Canada, K2H 8E9.

EMail: [email protected]

XiPeng Xiao
Riverstone Networks
5200 Great America Parkway
Santa Clara, CA 95054

EMail: [email protected]

Christopher O. Flores
T-Systems
10700 Parkridge Boulevard
Reston, VA 20191
USA

EMail: [email protected]

David Zelig
Corrigent Systems
126, Yigal Alon St.
Tel Aviv, ISRAEL

EMail: [email protected]

Martini, et al. Standards Track [Page 18]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Raj Sharma
Luminous Networks, Inc.
10460 Bubb Road
Cupertino, CA 95014

EMail: [email protected]

Nick Tingle
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043

EMail: [email protected]

Sunil Khandekar
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043

EMail: [email protected]

Loa Andersson
TLA-group

EMail: [email protected]

Martini, et al. Standards Track [Page 19]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Appendix A. Interoperability Guidelines

A.1. Configuration Options

The following is a list of the configuration options for a point-to-
point Ethernet PW based on the reference points of Figure 3:

--------------|---------------|---------------|------------------
Service and | Encap on C |Operation at B | Remarks
Encap on A | |ingress/egress |
--------------|---------------|---------------|------------------
1) Raw | Raw - Same as | |
| A | |
| | |
--------------|---------------|---------------|------------------
2) Tag1 | Tag2 |Optional change| VLAN can be
| |of VLAN value | 0-4095
| | | Change allowed in
| | | both directions
--------------|---------------|---------------|------------------
3) No Tag | Tag |Add/remove Tag | Tag can be
| |field | 0-4095
| | | (note i)
| | |
--------------|---------------|---------------|------------------
4) Tag | No Tag |Remove/add Tag | (note ii)
| |field |
| | |
| | |
--------------|---------------|---------------|------------------

Figure 4: Configuration Options

Allowed combinations:

Raw and other services are not allowed on the same NSP virtual port
(A). All other combinations are allowed, except that conflicting
VLANs on (A) are not allowed. Note that in most point-to-point PW
applications the NSP virtual port is the same entity as the physical
port.

Notes:

i. Mode #3 MAY be limited to adding VLAN NULL only, since
change of VLAN or association to specific VLAN can be done
at the PW CE-bound side.

Martini, et al. Standards Track [Page 20]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

ii. Mode #4 exists in layer 2 switches, but is not recommended
when operating with PW since it may not preserve the user's
PRI bits. If there is a need to remove the VLAN tag (for
TLS at the other end of the PW), it is recommended to use
mode #2 with tag2=0 (NULL VLAN) on the PW and use mode #3 at
the other end of the PW.

A.2. IEEE 802.3x Flow Control Considerations

If the receiving node becomes congested, it can send a special frame,
called the PAUSE frame, to the source node at the opposite end of the
connection. The implementation MUST provide a mechanism for
terminating PAUSE frames locally (i.e., at the local PE). It MUST
operate as follows: PAUSE frames received on a local Ethernet port
SHOULD cause the PE device to buffer, or to discard, further Ethernet
frames for that port until the PAUSE condition is cleared.
Optionally, the PE MAY simply discard PAUSE frames.

If the PE device wishes to pause data received on a local Ethernet
port (perhaps because its own buffers are filling up or because it
has received notification of congestion within the PSN), then it MAY
issue a PAUSE frame on the local Ethernet port, but MUST clear this
condition when willing to receive more data.

Appendix B. QoS Details

Section 4.7, "QoS Considerations", describes various modes for
supporting PW QOS over the PSN. Examples of the above for a point-
to-point VLAN service are:

- The classification to the PW is based on VLAN field, but the
user PRI bits are mapped to different CoS markings (and network
behavior) at the PW level. An example of this is a PW mapped to
an E-LSP in an MPLS network.

- The classification to the PW is based on VLAN field and the PRI
bits, and frames with different PRI bits are mapped to different
PWs. An example is to map a PWES to different L-LSPs in MPLS
PSN in order to support multiple CoS over an L-LSP-capable
network, or to map a PWES to multiple L2TPv3 sessions [L2TPv3].

The specific value to be assigned at the PSN for various CoS is
out of the scope of this document.

Martini, et al. Standards Track [Page 21]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

B.1. Adaptation of 802.1Q CoS to PSN CoS

It is not required that the PSN will have the same CoS definition of
CoS as defined in [802.1Q], and the mapping of 802.1Q CoS to PSN CoS
is application specific and depends on the agreement between the
customer and the PW provider. However, the following principles
adopted from 802.1Q, Table 8-2, MUST be met when applying the set of
PSN CoS based on user's PRI bits.

----------------------------------
|#of available classes of service|
-------------||---+---+---+---+---+---+---+---|
User || 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
Priority || | | | | | | | |
===============================================
0 Best Effort|| 0 | 0 | 0 | 1 | 1 | 1 | 1 | 2 |
(Default) || | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
1 Background || 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
|| | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
2 Spare || 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
|| | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
3 Excellent || 0 | 0 | 0 | 1 | 1 | 2 | 2 | 3 |
Effort || | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
4 Controlled || 0 | 1 | 1 | 2 | 2 | 3 | 3 | 4 |
Load || | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
5 Interactive|| 0 | 1 | 1 | 2 | 3 | 4 | 4 | 5 |
Multimedia || | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
6 Interactive|| 0 | 1 | 2 | 3 | 4 | 5 | 5 | 6 |
Voice || | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|
7 Network || 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
Control || | | | | | | | |
------------ ||---+---+---+---+---+---+---+---|

Figure 5: IEEE 802.1Q CoS Mapping

Martini, et al. Standards Track [Page 22]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

B.2. Drop Precedence

The 802.1P standard does not support drop precedence; therefore, from
the PW PE-bound point of view there is no mapping required. It is,
however, possible to mark different drop precedence for different PW
frames based on the operator policy and required network behavior.
This functionality is not discussed further here.

PSN QoS support and signaling of QoS are out of the scope of this
document.

Authors' Addresses

Luca Martini, Editor
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]

Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK

EMail: [email protected]

Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719

EMail: [email protected]

Martini, et al. Standards Track [Page 23]

RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Full Copyright Statement

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Martini, et al. Standards Track [Page 24]