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 |
+-----------------------------------------------------------+
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.
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].
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.
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.
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.
[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.
9. Significant Contributors
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
EMail: Andy.Malis@tellabs.com
Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: tappan@cisco.com
Steve Vogelsang
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: stephen.vogelsang@ecitele.com
Vinai Sirkay
Reliance Infocomm
Dhirubai Ambani Knowledge City
Navi Mumbai 400 709
India
EMail: vinai@sirkay.com
Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
EMail: vasile@nortelnetworks.com
Chris Liljenstolpe
Alcatel
11600 Sallie Mae Dr.
9th Floor
Reston, VA 20193
EMail: chris.liljenstolpe@alcatel.com
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: kireeti@juniper.net
Tricci So
Nortel Networks 3500 Carling Ave.,
Nepean, Ontario,
Canada, K2H 8E9.
EMail: tso@nortelnetworks.com
XiPeng Xiao
Riverstone Networks
5200 Great America Parkway
Santa Clara, CA 95054
EMail: xxiao@riverstonenet.com
Christopher O. Flores
T-Systems
10700 Parkridge Boulevard
Reston, VA 20191
USA
EMail: christopher.flores@usa.telekom.de
David Zelig
Corrigent Systems
126, Yigal Alon St.
Tel Aviv, ISRAEL
EMail: davidz@corrigent.com
Raj Sharma
Luminous Networks, Inc.
10460 Bubb Road
Cupertino, CA 95014
EMail: raj@luminous.com
Nick Tingle
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043
EMail: nick@timetra.com
Sunil Khandekar
TiMetra Networks
274 Ferguson Drive
Mountain View, CA 94043
EMail: sunil@timetra.com
Loa Andersson
TLA-group
EMail: loa@pi.se
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.
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.
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
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.