Network Working Group Luca Martini
Internet Draft Nasser El-Aawar
Expiration Date: February 2003 Level 3 Communications, LLC.
Giles Heron Eric C. Rosen
PacketExchange Ltd. Cisco Systems, Inc.
August 2002
Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks
draft-ietf-pwe3-ethernet-encap-00.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 PW allows Ethernet/802.3 Protocol Data Units (PDUs) to be
carried over Packet Switched Networks (PSNs) using MPLS transport.
This enables Service Providers to leverage their existing PSN to
offer Ethernet services.
This document describes methods for encapsulating Ethernet/802.3 PDUs
for transport over an MPLS or IP network.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 3
3 Requirements for Ethernet Pseudo-Wire Emulation ........ 5
3.1 Packet Processing ...................................... 6
3.1.1 Encapsulation .......................................... 6
3.1.2 MTU Management ......................................... 6
3.1.3 Frame Ordering ......................................... 6
3.1.4 Frame Error Processing ................................. 6
3.1.5 IEEE 802.3x Flow Control Interworking .................. 7
3.2 Maintenance ............................................ 7
3.3 Management ............................................. 7
3.4 QoS Considerations ..................................... 8
3.5 Security Considerations ................................ 8
4 General encapsulation method ........................... 9
4.1 The Control Word ....................................... 9
4.1.1 Setting the sequence number ............................ 10
4.1.2 Processing the sequence number ......................... 10
4.2 MTU Requirements ....................................... 11
4.3 Tagged Mode ............................................ 12
4.4 Raw Mode ............................................... 12
5 Using an MPLS Label as the Demultiplexer Field ......... 12
5.1 MPLS Shim EXP Bit Values ............................... 12
5.2 MPLS Shim S Bit Value .................................. 12
5.3 MPLS Shim TTL Values ................................... 13
6 Security Considerations ................................ 13
7 Intellectual Property Disclaimer ....................... 13
8 References ............................................. 13
9 Author Information ..................................... 14
Appendix A - Interoperability Guidelines ............... 17
Appendix B - QoS Details ............................... 19
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119
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2. Introduction
In an MPLS or IP network, it is possible to use control protocols
such as those specified in [PWE3-CRTL] to set up "Pseudo Wires" (PW)
that carry the the Protocol Data Units of layer 2 protocols across
the network. A number of these PWs may be carried in a single
tunnel. This requires of course that the layer 2 Packet data Units
(PDUs) be encapsulated. We can Three layers of this encapsulation
can be distinguished:
- the "tunnel header", which contains the information needed to
transport the PDU across the IP or MPLS network; this is header
belongs to the tunneling protocol, e.g., MPLS, GRE, L2TP.
- the "demultiplexer field", which is used to distinguish
individual emulated virtual circuits within a single tunnel; this
field must be understood by the tunneling protocol as well; it
may be, e.g., an MPLS label or a GRE key field.
- the "PW encapsulation", which contains the information about the
enclosed layer 2 PDU which is necessary in order to properly
emulate the corresponding layer 2 protocol.
This document specifies the PW encapsulation for the ethernet
protocols. Although different layer 2 protocols require different
information to be carried in this encapsulation, an attempt has been
made to make the encapsulation as common as possible for all layer 2
protocols. Other layer 2 protocols are described in separate
documents. [PWE3-ATM] [PWE3-FRAME]
This document also specifies the way in which the demultiplexer field
is added to the PW encapsulation when an MPLS label is used as the
demultiplexer field.
The scope of this document also includes:
- Pseudo-wire (PW) requirements for emulating Ethernet trunking and
switching behavior.
- PE-bound and CE-bound packet processing of Ethernet PDUs
- QoS and security considerations
- Inter-domain transport considerations for Ethernet PE
The following two figures describe the reference models which are
derived from [PWE3-FRAME] [PWE3-REQ] to support the Ethernet PW
emulated services.
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Native |<----- Pseudo Wire ---->| Native
Ethernet | | Ethernet
or | |<-- PSN Tunnel -->| | or
VLAN V V V V VLAN
Service +----+ +----+ Service
+----+ | | PE1|==================| PE2| | +----+
| |----------|............PW1.............|----------| |
| CE1| | | | | | | |CE2 |
| |----------|............PW2.............|----------| |
+----+ | | |==================| | | +----+
^ +----+ +----+ | ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Figure 1: PWE3 Ethernet/VLAN Interface Reference Configuration
+-------------+ +-------------+
| 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 |
+-----+-------+ +-----+-------+
| |
| MPLS or IP Network |
| ____ ___ ____ |
| _/ ___/ _/ __ |
| / __/ _ |
| / |
+========/ |===+
/
/
___ ___ __ _/
_/ ____/ ___/ ____/
Figure 2: Ethernet PWE3 Protocol Stack Reference Model
For the purpose of this document R1 will be defined as the ingress
router, and R2 as the egress router. A layer 2 PDU will be received
at R1, encapsulated at R1, transported, decapsulated at R2, and
transmitted out of R2.
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3. Requirements for Ethernet Pseudo-Wire Emulation
An Ethernet PW emulates a single Ethernet link between exactly two
endpoints. 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
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 packet 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 packet processing needed to translate the
Ethernet packets that arrive at the CE-PE interface to/from the
Ethernet packets that are applied 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 packets according to the PSN type in use.
This document defines these operations, and the services offered and
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required at points A and B.
"PSN Tunnel" denotes the PSN tunneling technology that is being used:
MPLS or GRE/IP.
A pseudo wire can be one of the two types: raw or tagged. This is a
property of the emulated Ethernet link and indicates whether the
pseudo wire MUST contain an 802.1Q VLAN tag (i.e. tagged mode) or MAY
contain a tag (i.e. raw mode).
3.1. Packet Processing
3.1.1. Encapsulation
The entire Ethernet frame without any preamble or FCS is transported
as a single packet. A VC label is prepended to this and the packet
is forwarded through a PSN tunnel (either MPLS or GRE/IP).
3.1.2. MTU Management
Ingress and egress PWESs MUST agree on their maximum MTU size to be
transported over the PSN.
3.1.3. 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 are delivered in sequence.
Moreover, the PSN cannot (in the general case) be assumed to provide
or to guarantee frame ordering. Therefore if strict frame ordering
is required, the control word defined below MUST be utilized and its
sequence number processing enabled.
3.1.4. Frame Error Processing
An encapsulated Ethernet frame traversing a psuedo-wire may be
dropped, corrupted or delivered out-of-order. Per [PWE3-REQ],
packet-loss, corruption, and out-of-order delivery is considered to
be a "generalized bit error" of the psuedo-wire. Therefore, the
native Ethernet frame error processing mechanisms MUST be extended to
the corresponding psuedo-wire service. 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 this processing is part of the NSP function and is
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outside the scope of this draft.
3.1.5. 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.
3.2. Maintenance
It is desirable to have a signaling mechanism for establishing
Ethernet PWs and for detecting failure of an Ethernet PW. It is
recommended that the procedures defined in [PWE3-CRTL] be used for
this purpose.
3.3. Management
The PW management model of Ethernet PW follows the general management
guidelines for PW management as appear in [PW-MIB] and defined in
[PWE3-REQ], [PWE3-FRAME]. It is composed of 3 components. [PW-MIB]
defines the parameters common to all types of PW and PSNs, for
example common counters, error handling, some maintenance protocol
parameters etc. For each type of PSN there is a separate module that
defines the association of the PW to the PSN tunnel, see example in
[PW-MPLS-MIB] for the MPLS PSN. For Ethernet PW, an additional MIB
module [PW-ENET-MIB] defines the Ethernet specific parameters
required to be configured or monitored.
The above modules enable both manual configuration and the use of
maintenance procedures to set up the Ethernet PW and monitor PW state
where applicable.
As specified in [PWE3-REQ] and [PWE3-FRAME], an implementation SHOULD
support the relevant 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. 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 the
Quality of Service 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 Quality of Service field of the encapsulating protocol
when queuing the packet for CE-bound.
A PE MUST support the ability to carry the Ethernet PW as a best
effort service over the PSN. Transparency of PRI bits (if sent from
CE to PE) between CE devices, regardless of the COS support of the
PSN. Where the 802.1Q VLAN field is added at the PE, a default PRI
setting of zero MUST be supported, a configured default value is
recommended.
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.
Consideration of Ethernet flow control was discussed above.
3.5. Security Considerations
This document specifies the security consideration regarding the
encapsulation for the PW. In terms of encapsulation, security of the
encapsulated packets depends on the nature of the protocol that is
carried by these packets, while the encapsulation itself shall not
affect the related security issues.
Nevertheless, the security limitations of the PE and/or the PW MUST
not restrict the security implementation choices of the user of the
PWE3 (i.e. users should be able to implement IPSEC or any other
appropriate security mechanism in addition to the security inherent
in the PW)".
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It is required that PEs will have user separation between different
PW and different virtual ports that the PWs are connected to. For
example: if two PWs are connected to the same physical port and
associated to different virtual ports (i.e. VLANs), it is required
that packets from one VC will not be forwarded to the VLAN that is
associated to the second VCs.
A received packet is associated with a PW by means of the VC label.
However this mechanism provides no guarantee that the packet was sent
by the peer PE. Further checks may be useful to protect against
mis-configuration and connection hijacking.
The PE must be able to be protected from malformed, or maliciously
altered, customer traffic. This includes, but is not limited to,
illegal VLAN use, short packets, long packets, etc.
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.
In the case of a PW crossing from one autonomous system to another,
through a private interconnection, security considerations are much
the same as in the intra-domain case. However in some cases the PW
may travel through a third-party autonomous system, or across a
public interconnection point. In these cases there may be a
requirement to encrypt the user data using a method appropriate to
the PSN tunneling mechanism.
4. General encapsulation method
4.1. 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:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram 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 packet 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 an unsequenced packet.
4.1.1. Setting the sequence number
For a given emulated VC, and a pair of routers R1 and R2, if R1
supports packet sequencing then the following procedures should be
used:
- the initial packet transmitted on the emulated VC MUST use
sequence number 1
- subsequent packets MUST increment the sequence number by one for
each packet
- when the transmit sequence number reaches the maximum 16 bit
value (65535) the sequence number MUST wrap to 1
If the transmitting router R1 does not support sequence number
processing, then the sequence number field in the control word MUST
be set to 0.
4.1.2. Processing the sequence number
If a router R2 supports receive sequence number processing, then the
following procedures should be used:
When an emulated VC is initially set up, the "expected sequence
number" associated with it MUST be initialized to 1.
When a packet is received on that emulated VC, the sequence number
should be processed as follows:
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- if the sequence number on the packet is 0, then the packet passes
the sequence number check
- otherwise if the packet sequence number >= the expected sequence
number and the packet sequence number - the expected sequence
number < 32768, then the packet is in order.
- otherwise if the packet sequence number < the expected sequence
number and the expected sequence number - the packet sequence
number >= 32768, then the packet is in order.
- otherwise the packet is out of order.
If a packet passes the sequence number check, or is in order then, it
can be delivered immediately. If the packet is in order, then the
expected sequence number should be set using the algorithm:
expected_sequence_number := packet_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 R2 does not support receive sequence number processing,
then the sequence number field MAY be ignored.
4.2. MTU Requirements
The network MUST be configured with an MTU that is sufficient to
transport the largest encapsulation frames. 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.
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4.3. Tagged Mode
In this mode each frame MUST include an 802.1Q field. All frames in
a PW MUST have the same 802.1Q tag value. Note that the tag may be
overwritten by the NSP function at ingress or at egress.
Note that when using the signaling procedures defined in [PWE3-CRTL],
such a PW should be signaled as being of type "Ethernet VLAN".
4.4. Raw Mode
In this mode each frame SHOULD NOT include an 802.1Q field.
Note that when using the signaling procedures defined in [PWE3-CRTL],
such a PW should be signaled as being of type "Ethernet".
5. Using an MPLS Label as the Demultiplexer Field
To use an MPLS label as the demultiplexer field, a 32-bit label stack
entry [MPLS-LABEL] is simply prepended to the emulated VC
encapsulation, and hence will appear as the bottom label of an MPLS
label stack. This label may be called the "VC label". The
particular emulated VC identified by a particular label value must be
agreed by the ingress and egress LSRs, either by signaling (e.g, via
the methods of [PWE3-CRTL]) or by configuration. Other fields of the
label stack entry are set as follows.
5.1. MPLS Shim EXP Bit Values
If it is desired to carry Quality of Service information, the Quality
of Service information SHOULD be represented in the EXP field of the
VC label. If more than one MPLS label is imposed by the ingress LSR,
the EXP field of any labels higher in the stack SHOULD also carry the
same value.
5.2. MPLS Shim S Bit Value
The ingress LSR, R1, MUST set the S bit of the VC label to a value of
1 to denote that the VC label is at the bottom of the stack.
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5.3. MPLS Shim TTL Values
The ingress LSR, R1, SHOULD set the TTL field of the VC label to a
value of 255.
6. Security Considerations
This document specifies only encapsulations, and not the protocols
used to carry the encapsulated packets across the network. Each such
protocol may have its own set of security issues, but those issues
are not affected by the encapsulations specified herein.
Specific security issues related to encapsulation are addressed in
the requirements section above.
7. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
8. References
[PWE3-CRTL] "Transport of Layer 2 Frames Over MPLS",
Martini, L., et al., draft-ietf-pwe3-control-protocol-00.txt,
( work in progress ), February 2003.
[PWE3-ATM] "Encapsulation Methods for Transport of ATM Cells/Frame
Over IP and MPLS Networks", Martini L., et al.,
draft-ietf-pwe3-atm-encap-00.txt, ( work in progress ),
February 2003.
[PWE3-FRAME] "Encapsulation Methods for Transport of Frame-Relay
Over IP and MPLS Networks", Kawa, C., et al.,
draft-ietf-pwe3-frame-encap-00.txt, ( work in progress ),
June 2002.
[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-pwe3-requirements-03.txt, ( work in progress ), June 2002.
[PWE3-FRAME] "Framework for Pseudo Wire Emulation Edge-to-Edge
(PWE3)", Pate, P., Xiao, X., So, T., Malis, A., Nadeau, T.,
White, C., Kompella, K., Johnson, T., Bryant, S.,
draft-pate-pwe3-framework-03.txt, ( work in progress ),
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June 2002.
[PW-MIB] "Pseudo Wire (PW) Management Information Base using SMIv2",
Zelig, D., Mantin, S., Nadeau, T., Danenberg, D.,
draft-zelig-pw-mib-02.txt, ( work in progress), February 2002.
[PW-MPLS-MIB] "Pseudo Wire (PW) over MPLS PSN Management Information
Base", Zelig D., Mantin, S., Nadeau, T., Danenberg, D.,
Malis, A., draft-zelig-pw-mpls-mib-01.txt, ( work in progress ),
February 2002.
[PW-ENET-MIB] "Ethernet Pseudo Wire (PW) Management Information
Base", Zelig, D., Nadeau, T., draft-zelig-pw-enet-mib-00.txt,
( work in progress ) February 2002.
[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
-- 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.
[MPLS-LABEL] "MPLS Label Stack Encoding", Rosen, E., Rekhter, E.,
Tappan, D., Fedorkow, G., Farinacci, D., Li, T., Conta, A.,
RFC 3032.
9. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
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Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
e-mail: giles@packetexchange.net
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: tappan@cisco.com
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: sjv@laurelnetworks.com
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
e-mail: Andy.Malis@vivacenetworks.com
Vinai Sirkay
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
e-mail: sirkay@technologist.com
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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
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
Redback Networks
300 Holger Way,
San Jose, CA 95134
e-mail: xipeng@redback.com
Chris Flores
Austin, Texas
e-mail: chris_flores@hotmail.com
David Zelig
Corrigent Systems
126, Yigal Alon St.
Tel Aviv, ISRAEL
e-mail: davidz@corrigent.com
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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
Sunil Khandekar
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043
email: sunil@timetra.com
Loa Andersson
Utfors
P.O. Box 525,
SE-169 29 Solna, Sweden
e-mail: loa.andersson@utfors.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:
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--------------|---------------|---------------|------------------
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 physical port (A).
All other combinations are allowed, except that conflicting VLANs on
(A) are not allowed.
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.
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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:
- 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 packets 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 service over an L-LSP capable
network.
The specific value to be assigned at the PSN for various COS is
not specified and is application specific.
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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 QOS
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.
----------------------------------
|#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
packets based on the operator policy and required network behavior.
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This functionality is not discussed further here.
PSN COS labels interaction with VC label COS marking
Marking of COS bits at the VC level is not required if the PSN tunnel
is PE to PE based, since only the PSN COS marking is visible to the
PSN network. In cases where the VC multiplexing field is carried
without an external tunnel (for example directly connected PEs with
PHP, or PEs connected using GRE/IP), the rules stated above for
tunnel COS marking apply also for the VC level.
In summary, the rules for COS marking shall be as follows:
- If there is only a VC label then, it shall contain the
appropriate CoS value (e.g. MPLS between PEs which are directly
adjacent to each other).
- If the VC label and PSN tunnel labels are both being used, then
the CoS marking on the PSN header shall be marked with the
correct CoS value.
- If the PSN marking is stripped at a node before the PE, the PSN
marking MUST be copied to the VC label. An example is MPLS PSN
with the use of PHP.
PSN QOS support and signaling of QOS is out of scope of this
document.
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