Network Working Group Luca Martini
Internet Draft Eric C. Rosen
Expiration Date: October 2004 Cisco Systems, Inc.
Nasser El-Aawar Giles Heron
Level 3 Communications, LLC. Tellabs
April 2004
Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks
draft-ietf-pwe3-ethernet-encap-06.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
An Ethernet Pseudowire (PW) is used to carry Ethernet/802.3 Protocol
Data Units over an IP or MPLS network. This enables service providers
to offer "emulated" ethernet services over existing IP or 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 Specification of Requirements .......................... 2
2 Introduction ........................................... 2
3 Requirements for Ethernet PWs Emulating P2P Ethernet Links .4
3.1 Frame Processing at the PW Endpoints ................... 6
3.1.1 Generic Procedures ..................................... 6
3.1.2 Raw Mode vs. Tagged Mode ............................... 6
3.1.3 MTU Management on the PE/CE Links ...................... 7
3.1.4 Frame Ordering ......................................... 7
3.1.5 Frame Error Processing ................................. 8
3.1.6 IEEE 802.3x Flow Control Interworking .................. 8
3.2 PW Setup and Maintenance ............................... 8
3.3 Management ............................................. 8
3.4 The Control Word ....................................... 9
3.4.1 Setting the sequence number ............................ 9
3.4.2 Processing the sequence number ......................... 10
3.5 QoS Considerations ..................................... 11
3.6 Security Considerations ................................ 11
3.7 PSN MTU Requirements ................................... 12
4 Intellectual Property Disclaimer ....................... 12
5 References ............................................. 12
6 Author Information ..................................... 13
Appendix A - Interoperability Guidelines ............... 16
Appendix B - QoS Details ............................... 17
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119
2. Introduction
An Ethernet Pseudowire (PW) allows Ethernet/802.3 Protocol Data Units
(PDUs) to be carried over an IP network or an MPLS network. In
addressing the issues associated with carrying an Ethernet PDU over a
PSN, this document assumes that a Pseudowire (PW) has been set up by
some means outside the scope of this document. This may be via manual
configuration, or a signaling protocol such as that defined in
[PWE3-CTRL] or [L2TPv3]. As described in [PWE3-ARCH], this PW may be
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tunneled through an MPLS, IPv4 or IPv6 PSN.
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 CEs with
an emulated (point-to-point) ethernet service, including the
procedures for the processing of PE-bound and CE-bound ethernet
PDUs. [PWE3-ARCH]
- Ethernet-specific QoS and security considerations
- Inter-domain transport considerations for Ethernet PW
The following two figures describe the reference models which are
derived from [PWE3-ARCH] to support the Ethernet PW emulated
services.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- 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
| |
| |
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
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two CEs are out of scope of the current document.
+-------------+ +-------------+
| Emulated | | Emulated |
| Ethernet | | Ethernet |
| (including | Emulated Service | (including |
| VLAN) |<==============================>| VLAN) |
| Services | | Services |
+-------------+ Pseudo Wire +-------------+
|Demultiplexer|<==============================>|Demultiplexor|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
| MPLS or IP |<==============================>| MPLS or IP |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
Figure 2: Ethernet PWE3 Protocol Stack Reference Model
For the purpose of this document, PE1 will be defined as the ingress
router, and PE2 as the egress router. A layer 2 PDU will be received
at PE1, encapsulated at PE1, transported, decapsulated at PE2, and
transmitted out of PE2.
3. Requirements for Ethernet PWs Emulating P2P Ethernet Links
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 "Pseudo Wire" 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:
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+-----------------------------------+
| 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 A
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 "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.
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 according to the PSN type in use.
An ethernet PW can operate in one of two modes: "raw mode" or "tagged
mode". In tagged mode, each frame MUST contain an 802.1Q VLAN tag,
and the tag value is meaningful to the NSPs at the two PW endpoints.
That is, the two endpoints 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
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meaningful to the NSPs, and passes transparently through them.
3.1. Frame Processing at the PW Endpoints
3.1.1. Generic Procedures
When the NSP/Forwarder hands a frame to the PW endpoint:
- The preamble (if any) and FCS are stripped off.
- The control word as defined in the "The Control Word" section 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 demultiplexor 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
demultiplexor are chosen depends on the procedures that were used to
set up the pseudowire.
When a packet arrives over a PW, the tunnel encapsulation and PW
demultiplexor are stripped off. If the control word is present, any
processing required by control word is performed, and the control
word is stripped off. The resulting is then handed to the
Forwarder/NSP. Regeneration of the FCS is considered to be an NSP
responsibility.
3.1.2. Raw Mode vs. Tagged Mode
When the PE receives an ethernet frame from a CE, 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 provider-operated
equipment, and the tag is used by the provider to distinguish
the traffic. For example, LANs from different customers might
be attached to the same 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 the CE (or other piece of customer
equipment), and is not meaningful to 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 CE 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 CE 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.
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 to the CE;
however it MUST not rewrite or remove any tags which are already
present.
3.1.3. 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.
3.1.4. 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 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 which get misordered by the PSN will be dropped
by the receiving PW endpoint. If strict frame ordering is a
requirement for a particular PW, this option MUST be enabled.
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3.1.5. Frame Error Processing
An encapsulated Ethernet frame traversing a psuedo-wire may be
dropped, corrupted or delivered out-of-order. As described in [PWE3-
REQ], frame-loss, corruption, and out-of-order delivery is considered
to be a "generalized bit error" of the psuedo-wire. 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 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 draft.
3.1.6. IEEE 802.3x Flow Control Interworking
In a standard gigabit 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.
3.2. PW Setup and Maintenance
This document assumes that a mechanism exists to set up the ethernet
PW. Maintenance of the PW (e.g. keepalives, status updates, etc) is
generally tied closely to the PW Setup mechanisms. [PWE3-CTRL] and
[L2TPv3] define two mechanisms for setup and maintenance of Ethernet
PWs.
3.3. Management
The Ethernet PW management model follows the general management
defined in [PWE3-ARCH] 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].
As specified in [PWE3-ARCH], an implementation SHOULD support the
generic and specific PW MIB modules for PW set-up and monitoring.
Other mechanisms for PW set up (command line interface for example)
MAY be supported.
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3.4. The Control Word
When carrying Ethernet over an IP or MPLS backbone sequentiality may
need to be preserved. The OPTIONAL control word defined here
addresses this requirement. Implementations MUST support sending no
control word, and MAY support sending a control word.
In all cases the egress router must be aware of whether the ingress
router will send a control word over a specific virtual circuit.
This may be achieved by configuration of the routers, or by
signaling, for example as defined in [PWE3-CRTL].
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 alghorithm is not used.
3.4.1. Setting the sequence number
For a given PW, and a pair of routers PE1 and PE2, if PE1 supports
frame sequencing then the following procedures should be used:
- the initial frame transmitted on the PW MUST use sequence number
1
- subsequent frames MUST increment the sequence number by one for
each frame
- when the transmit sequence number reaches the maximum 16 bit
value (65535) the sequence number MUST wrap to 1
If the transmitting router PE1 does not support sequence number
processing, then the sequence number field in the control word MUST
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be set to 0.
3.4.2. Processing the sequence number
If a router PE2 supports receive sequence number processing, then the
following procedures should be used:
When a PW is initially set up, the "expected sequence number"
associated with it MUST be initialized to 1.
When a frame is received on that PW, the sequence number should be
processed as follows:
- if the sequence number on the frame is 0, then the frame passes
the sequence number check
- otherwise if the frame sequence number >= the expected sequence
number and the frame sequence number - the expected sequence
number < 32768, then the frame is in order.
- otherwise if the frame sequence number < the expected sequence
number and the expected sequence number - the frame sequence
number >= 32768, then the frame is in order.
- otherwise the frame is out of order.
If a frame passes the sequence number check, or is in order then, it
can be delivered immediately. If the frame is in order, then the
expected sequence number should be set using the algorithm:
expected_sequence_number := frame_sequence_number + 1 mod 2**16
if (expected_sequence_number = 0) then expected_sequence_number := 1;
Packets which are received out of order MAY be dropped or reordered
at the discretion of the receiver.
If a router PE2 does not support receive sequence number processing,
then the sequence number field MAY be ignored.
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3.5. 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 or the DSCP of an IP packet). In a similar way, the
egress PE MAY consider the QoS field of the PSN's encapsulating
protocol when queuing the frame for CE-bound.
A PE MUST support the ability to carry the Ethernet PW as a best
effort service over the 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 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.
Examples of the cases above and details of the service mapping
considerations are described in Appendix B.
The PW guaranteed rate at the PSN level is PW provider policy based
on agreement with the customer, and may be different from the
Ethernet physical port rate.
3.6. Security Considerations
The ethernet pseudowire type is subject to all of the general
security considerations discussed in [PWE3-ARCH].
Security achieved by access control of MAC addresses is out of 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 draft.
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3.7. PSN MTU Requirements
The PSN MUST be configured with an MTU that is large enough to
transport a maximum sized ethernet frame which has been encapsulated
with a control word, a pseudowire demultiplexor, and a tunnel
encapsulation. If MPLS is used as the tunneling protocol, for
example, this is likely to be 8 or more bytes greater than the
largest frame size. Other tunneling protocols may have longer headers
and require larger MTUs. If the ingress router determines that an
encapsulated layer 2 PDU exceeds the MTU of the tunnel through which
it must be sent, the PDU MUST be dropped. 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. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
5. References
[PWE3-CRTL] "Transport of Layer 2 Frames Over MPLS",
Martini, L., et al., draft-ietf-pwe3-control-protocol-05.txt,
( work in progress ), May 2003.
[PWE3-ARCH] "PWE3 Architecture"
Bryant, et al., draft-ietf-pwe3-arch-07.txt
( work in progress ), March 2003.
[PWE3-REQ] "Requirements for Pseudo Wire Emulation Edge-to-Edge
(PWE3)", Xiao, X., McPherson, D., Pate, P., White, C.,
Kompella, K., Gill, V., Nadeau, T.,
draft-ietf-pwe3-requirements-08.txt, ( work in progress ),
September
2003.
[PW-MIB] "Pseudo Wire (PW) Management Information Base using SMIv2",
Zelig, D., Mantin, S., Nadeau, T., Danenberg, D.,
draft-ietf-pwe3-pw-mib-04.txt, ( work in progress), February
2004.
[802.3] IEEE, ISO/IEC 8802-3: 2000 (E), "IEEE Standard for
Information technology -- Telecommunications and information
exchange between systems -- Local and metropolitan area networks
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-- Specific requirements -- Part 3: Carrier Sense Multiple
Access with Collision Detection (CSMA/CD) Access Method and
Physical Layer Specifications", 2000.
[802.1Q] ANSI/IEEE Standard 802.1Q, "IEEE Standards for Local and
Metropolitan Area Networks: Virtual Bridged Local Area
Networks", 1998.
[L2TPv3] J. Lau, M. Townsley, A. Valencia, G. Zorn, I. Goyret,
G. Pall, A. Rubens, B. Palter, Layer Two Tunneling Protocol
(Version 3) "L2TPv3", work in progress,
draft-ietf-l2tpext-l2tp-base-12.txt, March 2004.
6. Author Information
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
e-mail: lmartini@cisco.com
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
e-mail: giles.heron@tellabs.com
Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
e-mail: tappan@cisco.com
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Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
e-mail: erosen@cisco.com
Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: sjv@laurelnetworks.com
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
e-mail: Andy.Malis@tellabs.com
Vinai Sirkay
Reliance Infocomm
Dhirubai Ambani Knowledge City
Navi Mumbai 400 709
India
e-mail: vinai@sirkay.com
Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
e-mail: vasile@nortelnetworks.com
Chris Liljenstolpe
Cable & Wireless
11700 Plaza America Drive
Reston, VA 20190
e-mail: chris@cw.net
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Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
e-mail: kireeti@juniper.net
Tricci So
e-mail: tricciso@yahoo.ca
XiPeng Xiao
Riverstone Networks
5200 Great America Parkway
Santa Clara, CA 95054
e-mail: xxiao@riverstonenet.com
Christopher O. Flores
T-Systems
10700 Parkridge Boulevard
Reston, VA 20191
USA
e-mail: christopher.flores@usa.telekom.de
David Zelig
Corrigent Systems
126, Yigal Alon St.
Tel Aviv, ISRAEL
e-mail: davidz@corrigent.com
Raj Sharma
Luminous Netwokrs, Inc.
10460 Bubb Road
Cupertino, CA 95014
e-mail: raj@luminous.com
Nick Tingle
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043
e-mail: nick@timetra.com
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Sunil Khandekar
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043
email: sunil@timetra.com
Loa Andersson
TLA-group
e-mail: loa@pi.se
Appendix A - Interoperability Guidelines
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:
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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
application 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.
-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.
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 3.7 describes various modes for supporting PW QOS over the
PSN. Examples of the above for a point to point VLAN service are:
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- The classification to the PW is based on VLAN field only,
regardless of the user PRI bits. The PW is assigned a specific
COS (marking, scheduling, etc.) at the tunnel level.
- The classification to the PW is based on VLAN field, but the PRI
bits of the user is mapped to different COS marking (and network
behavior) at the PW level. Examples are DiffServ coding in case
of IP PSN, and E-LSP in MPLS PSN.
- 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 multiple L2TPv3 sessions [L2TPv3].
The specific value to be assigned at the PSN for various COS is
out of scope for this document.
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 set of PSN
COS based on user's PRI bits.
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----------------------------------
|#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 Service Mapping
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 is out of scope of this
document.
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