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.
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
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.
|<-------------- 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
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.
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
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.
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.
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.
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