Network Working Group Simon Delord, Alcatel-Lucent
Internet Draft Raymond Key, Huawei
Category: Standard Track Frederic Jounay, Orange France Telecom
Expires: April 2012 Wim Henderickx, Alcatel-Lucent
Lucy Yong, Huawei
Lizhong Jin, ZTE
October 16, 2011
Control Word Reserved bit for use in E-Tree
draft-delord-pwe3-cw-bit-etree-06
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Abstract
The extension described in this document allows a pair of PEs
connected via an Ethernet Pseudowire (PW) to signal via the use of
the Control Word (CW) whether the Ethernet frame encapsulated is
coming from a Root AC or a Leaf AC. This allows a PE receiving this
frame to decide whether it can be forwarded to a Leaf AC or not.
Such forward or drop decision is an additional filtering action
after the MAC-based forwarding decision. This applies to both P2P PW
and P2MP PW.
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Table of Contents
1. Introduction....................................................3
2. Terminology.....................................................3
3. Scope and Applicability.........................................4
3.1. Scope & Problem...............................................4
3.2. Proposed Approach.............................................5
3.3. Applicability.................................................5
4. Control Word Extensions.........................................6
4.1. Ethernet Control Word Reserved Bit............................6
5. Procedures......................................................7
5.1. Detailed PE PW Setup Procedures...............................7
5.2. Detailed PE Forwarding Procedures.............................7
5.2.1. Forwarding PE...............................................7
5.2.2. Receiving PE................................................7
6. Security Considerations.........................................8
7. IANA Considerations.............................................8
8. Acknowledgments.................................................8
9. References......................................................8
9.1. Normative References..........................................8
9.2. Informative References........................................8
Authors' Addresses.................................................9
Intellectual Property and Copyright Statements.....................9
Conventions used in this document
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 [RFC2119].
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1. Introduction
This document proposes to use a specific bit within the Control Word
(CW) present on top of an Ethernet PW for a PE to indicate to a
remote PE connected via a Pseudowire (PW) whether the Ethernet frame
carried comes from a Root AC or a Leaf AC in an E-Tree construct
using VPLS. This applies to both P2P PW and P2MP PW.
[RFC4385] describes the preferred design of a Pseudowire Emulation
Edge-to-Edge (PWE3) Control Word to be used over an MPLS packet
switched network.
[RFC4447] specifies extensions to Label Distribution Protocol (LDP)
for PEs to setup PWs. It also describes the procedures for PEs to
signal to each other whether the CW is to be used or not.
[RFC4448] specifies the encapsulation of Ethernet/802.3 PDUs within
a PW.
[Draft VPLS ETree Req] provides the functional requirements for MEF
E-Tree support in VPLS.
2. Terminology
E-Tree
An Ethernet VPN in which each Root AC can communicate with every
other AC, whereas Leaf ACs can only communicate with Root ACs. Each
AC on an E-Tree construct is designated as either a Root AC or a
Leaf AC. There can be multiple Root ACs and Leaf ACs per E-Tree
construct.
Root AC
An ingress frame at a Root AC can be delivered to one or more of
any of the other ACs in the E-Tree. Please note that this AC is
bidirectional.
Leaf AC
Ingress frame at a Leaf AC can only be delivered to one or more Root
ACs in the E-Tree. Ingress frame at a Leaf AC MUST NOT be delivered
to any Leaf ACs in the E-Tree. Please note that this AC is
bidirectional.
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3. Scope and Applicability
3.1. Scope & Problem
One important application for carriers is the deployment of E-Tree
services over an MPLS backbone using VPLS.
+---------+ +---------+
| PE1 | | PE2 |
| | | |
+---+ | +---+ | | +---+ | +---+
|CE1+-----AC1----+--+ | | | | +--+----AC3-----+CE3|
+---+ (Root AC) | | V | | | | V | | (Root AC) +---+
| | | | Ethernet | | | |
| | S +--+----PW----+--+ S | |
+---+ | | | | | | | | +---+
|CE2+-----AC2----+--+ I | | | | I +--+----AC4-----+CE4|
+---+ (Leaf AC) | | | | | | | | (Leaf AC) +---+
| +---+ | | +---+ |
| | | |
+---------+ +---------+
<---------- E-Tree ---------->
Figure 1. E-Tree over MPLS using VPLS
Figure 1 describes a scenario where PE1 and PE2 need to establish an
E-Tree construct between different Ethernet endpoints. Each PE has 2
ACs connected to a VSI. These VSIs are then linked together via an
Ethernet PW.
AC1 is a Root AC on PE1 and AC2 is a Leaf AC on PE1.
AC3 is a Root AC on PE2 and AC4 is a Leaf AC on PE2.
With an E-Tree construct:
- AC1 can receive from and transmit to AC2, AC3 and AC4
- AC3 can receive from and transmit to AC1, AC2 and AC4
- AC2 and AC4 can receive from and transmit to AC1 and AC3
- AC2 and AC4 cannot receive from or transmit to each other
When an Ethernet Frame is received on PE1 via AC1, the frame can be
transmitted to AC2 and via the Ethernet PW connecting PE1 and PE2 to
AC3 and AC4.
However when an Ethernet Frame is received on PE1 via AC2, the frame
can be transmitted to AC3 via the Ethernet PW but not to AC4.
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When PE2 receives an Ethernet frame from PE1 via the Ethernet PW,
there is no indication whether it is from a Leaf AC or a Root AC, PE2
is therefore unable to decide whether it can be forwarded to AC4
(Leaf AC) as per the E-Tree construct.
3.2. Proposed Approach
A simple fix to this problem is to carry one additional bit of
information (0 or 1) for each Ethernet payload frame on the Ethernet
PW.
This per-payload signaling approach indicates whether the payload
frame is from a Leaf AC or a Root AC on the ingress PE.
Based on this per-payload extra-information, the egress PE can decide
whether the payload frame can be forwarded to a Leaf AC or not.
[RFC4385] defines in section 3 the Preferred PW MPLS Control Word.
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| Flags |FRG| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags (bits 4 to 7):
These bits MAY be used by for per-payload signaling. Their
semantics MUST be defined in the PW specification.
[RFC4448] defines in section 4.6 the Control Word for Ethernet PW.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Flags (bits 4 to 7 of the Control Word) are "reserved for future
use" bits in the current Ethernet PW standard that can be used for
such per-payload signaling.
3.3. Applicability
The proposed approach in this document will allow the egress PE to
make a decision for each Ethernet frame arriving on the PW from the
ingress PE whether this frame can be forwarded towards a Leaf AC.
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Obviously, each PE is also responsible for locally identifying which
ACs are Root ACs or Leaf ACs and for not forwarding Ethernet frames
between Leaf ACs. The most common procedure for doing so is to apply
a split horizon rule between Leaf ACs. However this is out of scope
of this document.
4. Control Word Extensions
4.1. Ethernet Control Word Reserved Bit
[RFC4448] defines in section 4.6 the CW format for an Ethernet PW.
The Control Word defined in this section is based on the Generic PW
MPLS Control Word as defined in [RFC4385]. It provides the ability
to sequence individual frames on the PW, avoidance of equal-cost
multiple-path load-balancing (ECMP), and Operations and Management
(OAM) mechanisms including VCCV.
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|L| Reserved | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram, the first 4 bits MUST be set to 0 to indicate
PW data.
Leaf bit (L).
The L-bit is used to flag the presence of an Ethernet frame from
a Leaf AC as follows:
L = 1 The Ethernet Frame comes from a Leaf AC
L = 0 The Ethernet Frame comes from a Root AC
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 [RFC4385].
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5. Procedures
[RFC4447] defines the procedures by which PEs maintain and exchange
PW information via LDP.
This document does not change any of the procedures defined in
[RFC4447]. The rules for negotiating the use of the CW are defined
in section 6.
5.1. Detailed PE PW Setup Procedures
In order for a PEs to signal if an Ethernet frame travelling on an
Ethernet PW comes from a Root AC or a Leaf AC, it MUST use the
extension defined in this document.
The procedure MUST follow the standard procedures described in
[RFC4447].
The Label Mapping messages that are sent in order to set up these
PWs MUST have c=1. When a Label Mapping message for a PW of one
of these types is received and c=0, a Label Release message MUST
be sent, with an "Illegal C-bit" status code. In this case, the
PW will not be enabled.
5.2. Detailed PE Forwarding Procedures
5.2.1. Forwarding PE
The L-bit MUST only be used when the first 4 bits of the CW are
equal to 0000.
The L-bit MUST be set to 0 when the first 4 bits of the CW are equal
to 0001.
If the Ethernet frame that has to be sent across an Ethernet PW (or
set of Ethernet PWs) comes from a Root AC, the L bit MUST be set to
0.
If the Ethernet frame that has to be sent across an Ethernet PW (or
set of Ethernet PWs) comes from a Leaf AC, the L bit MUST be set to
1.
5.2.2 Receiving PE
Upon reception of an Ethernet Frame where the L bit in the CW is set
to 0, this frame can be forwarded to any AC belonging to this VPN
instance.
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Upon reception of an Ethernet Frame where the L bit in the CW is set
to 1, this frame can be forwarded to any Root AC belonging to this
VPN instance but it MUST NOT be forwarded to any Leaf AC. This
forward or drop decision is an additional filtering action after the
MAC-based forwarding decision.
6. Security Considerations
This will be added in later version of this document.
7. IANA Considerations
There are no specific IANA considerations in this document.
8. Acknowledgments
The authors would like to thank Yuji Kamite for his valuable
comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., Key words for use in RFCs to Indicate
Requirement Levels, BCP 14, RFC 2119, March 1997.
[RFC4385] Bryant,S., Swallow, G., and Al, Pseudowire Emulation
Edge-to-Edge (PWE3) Control Word for Use over an MPLS
PSN, February 2006.
[RFC4447] Martini, L., and al, Pseudowire Setup and Maintenance
Using the Label Distribution Protocol (LDP), April 2006
[RFC4448] Martini, L., and al, Encapsulation Methods for
Transport of Ethernet over MPLS Networks, April 2006
9.2. Informative References
[Draft VPLS ETree Req]
Key, et al., Requirements for MEF E-Tree Support in
VPLS, draft-ietf-l2vpn-etree-reqt-00 (work in progress),
October 2011
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Authors' Addresses
Simon Delord
Alcatel-Lucent
Email: simon.delord@gmail.com
Raymond Key
Huawei
Email: raymond.key@ieee.org
Frederic Jounay
Orange France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex, France
Email: frederic.jounay@orange.com
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
2018 Antwerp, Belgium
Email: wim.henderickx@alcatel-lucent.com
Lucy Yong
Huawei USA
1700 Alma Dr. Suite 500
Plano, TX 75075, USA
Email: lucy.yong@huawei.com
Lizhong Jin
ZTE Corporation
889, Bibo Road
Shanghai, 201203, China
Email: lizhong.jin@zte.com.cn
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