Internet DRAFT - draft-ietf-pwe3-packet-pw
draft-ietf-pwe3-packet-pw
Network Working Group S. Bryant, Ed.
Internet-Draft L. Martini
Intended status: Standards Track G. Swallow
Expires: November 13, 2012 Cisco Systems
A. Malis
Verizon Communications
May 12, 2012
Packet Pseudowire Encapsulation over an MPLS PSN
draft-ietf-pwe3-packet-pw-04.txt
Abstract
This document describes a pseudowire mechanism that is used to
transport a packet service over an MPLS PSN in the case where the
client Label Switching Router (LSR) and the server Provider Edge
equipments are co-resident in the same equipment. This pseudowire
mechanism may be used to carry all of the required layer 2 and layer
3 protocols between the pair of client LSRs.
Requirements Language
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 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 13, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Network Reference Model . . . . . . . . . . . . . . . . . . . 4
3. Client Network Layer Model . . . . . . . . . . . . . . . . . . 4
4. Forwarding Model . . . . . . . . . . . . . . . . . . . . . . . 5
5. Packet PW Encapsulation . . . . . . . . . . . . . . . . . . . 6
6. Ethernet and IEEE 802.1 Functional Restrictions . . . . . . . 8
7. Congestion Considerations . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Encapsulation Approaches Considered . . . . . . . . . 10
A.1. A Protocol Identifier in the Control Word . . . . . . . . 11
A.2. PID Label . . . . . . . . . . . . . . . . . . . . . . . . 11
A.3. Parallel PWs . . . . . . . . . . . . . . . . . . . . . . 12
A.4. Virtual Ethernet . . . . . . . . . . . . . . . . . . . . 13
A.5. Recommended Encapsulation . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
There is a need to provide a method of carrying a packet service over
an MPLS PSN in a way that provides isolation between the two
networks. The server MPLS network may be an MPLS network or a
network conforming to the MPLS Transport Profile (MPLS-TP) [RFC5317].
The client may also be either an MPLS network or a network conforming
to the MPLS-TP. Considerations regarding the use of an MPLS network
as a server for an MPLS-TP network are outside the scope of this
document.
Where the client equipment is connected to the server equipment via a
physical interface, the same data-link type must be used to attach
the clients to the Provider Edge equipments (PE)s, and a pseudowire
(PW) of the same type as the data-link must be used [RFC3985]. The
reason that inter-working between different physical and data-link
attachment types is specifically disallowed in the pseudowire
architecture is because this is a complex task and not a simple bit-
mapping exercise. The inter-working is not limited to the physical
and data-link interfaces and the state-machines. It also requires a
compatible approach to the formation of the adjacencies between
attached client network equipment. As an example the reader should
consider the differences between router adjacency formation on a
point-to-point link compared to a multipoint-to-multipoint interface
(e.g. Ethernet).
A further consideration is that two adjacent MPLS Label Switching
Routers (LSRs) do not simply exchange MPLS packets. They exchange IP
packets for adjacency formation, control, routing, label exchange,
management and monitoring purposes. In addition they may exchange
data-link packets as part of routing (e.g. IS-IS Hellos and IS-IS
Link State Packets) and for Operations, Administration, and
Maintenance (OAM) purposes such as Link Layer Discovery protocol
[IEEE standard 802.1AB-2009]. Thus the two clients require an
attachment mechanism that can be used to multiplex a number of
protocols. In addition it is essential to the correct operation of
the network layer that all of these protocols fate share.
Where the client LSR and server PE is co-located in the same
equipment, the data-link layer can be simplified to a point-to-point
Ethernet used to multiplex the various data-link types onto a
pseudowire. This is the method that described in this document.
Appendix A provides information on alternative approaches to
providing a packet PW that were considered by PWE3 Working Group and
the reasons for using the method defined in this specification.
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2. Network Reference Model
The network reference model for the packet pseudowire operating in an
MPLS network is shown in Figure 1. This is an extension of Figure 3
"Pre-processing within the PWE3 Network Reference Model" from
[RFC3985].
PW PW
End Service End Service
| |
|<------- Pseudowire ------->|
| |
| Server |
| |<- PSN Tunnel ->| |
| V V |
------- +-----+-----+ +-----+-----+ -------
) | | |================| | | (
client ) | MPLS| PE1 | PW1 | PE2 | MPLS| ( Client
MPLS PSN )+ LSR1+............................+ LSR2+( MPLS PSN
) | | | | | | (
) | | |================| | | (
------- +-----+-----+ +-----+-----+ --------
^ ^
| |
| |
|<---- Emulated Service----->|
| |
Virtual physical Virtual physical
termination termination
Figure 1: Packet PW Network Reference Model
In this model LSRs, LSR1 and LSR2, are part of the client MPLS PSN.
The PEs, PE1 and PE2 are part of the server PSN, that is to be used
to provide connectivity between the client LSRs. The attachment
circuit that is used to connect the MPLS LSRs to the PEs is a virtual
interface within the equipment. A packet pseudowire is used to
provide connectivity between these virtual interfaces. This packet
pseudowire is used to transport all of the required layer 2 and layer
3 between protocols between LSR1 and LSR2.
3. Client Network Layer Model
The packet PW appears as a single point-to-point link to the client
layer. Network Layer adjacency formation and maintenance between the
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client equipments will the follow normal practice needed to support
the required relationship in the client layer. The assignment of
metrics for this point-to-point link is a matter for the client
layer. In a hop by hop routing network the metrics would normally be
assigned by appropriate configuration of the embedded client network
layer equipment (e.g. the embedded client LSR). Where the client was
using the packet PW as part of a traffic engineered path, it is up to
the operator of the client network to ensure that the server layer
operator provides the necessary service level agreement.
4. Forwarding Model
The packet PW forwarding model is illustrated in Figure 2. The
forwarding operation can be likened to a virtual private network
(VPN), in which a forwarding decision is first taken at the client
layer, an encapsulation is applied and then a second forwarding
decision is taken at the server layer.
+------------------------------------------------+
| |
| +--------+ +--------+ |
| | | Pkt +-----+ | | |
------+ +---------+ PW1 +--------+ +------
| | Client | AC +-----+ | Server | |
Client | | LSR | | LSR | | Server
Network | | | Pkt +-----+ | | | Network
------+ +---------+ PW2 +--------+ +------
| | | AC +-----+ | | |
| +--------+ +--------+ |
| |
+------------------------------------------------+
Figure 2: Packet PW Forwarding Model
A packet PW PE comprises three components, the client LSR, PW
processor and a server LSR. Note that [RFC3985] does not formally
indicate the presence of the server LSR because it does not concern
itself with the server layer. However it is useful in this document
to recognise that the server LSR exists.
It may be useful to first recall the operation of a layer 2 PW such
as an Ethernet PW [RFC4448] within this model. The client LSR is not
present and packets arrive directly on the attachment circuit (AC)
which is part of the client network. The PW function undertakes any
header processing, if configured to do so, it then optionally pushes
the PW control word (CW), and finally pushes the PW label. The PW
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function then passes the packet to the LSR function which pushes the
label needed to reach the egress PE and forwards the packet to the
next hop in the server network. At the egress PE, the packet
typically arrives with the PW label at top of stack, the packet is
thus directed to the correct PW instance. The PW instance performs
any required reconstruction using, if necessary, the CW and the
packet is sent directly to the attachment circuit.
Now let us consider the case client layer MPLS traffic being carried
over a packet PW. An LSR belonging to the client layer is embedded
within the PE equipment. This is a type of native service processing
element [RFC3985]. The client LSR determines the next hop in the
client layer, and pushes the label needed by the next hop in the
client layer. It then encapsulates the packet in an Ethernet header
setting the Ethertype to MPLS. The client LSR then passes the packet
to the correct PW instance. The PW instance then proceeds as defined
for an Ethernet PW [RFC4448] by optionally pushing the control word,
then pushing the PW label, and finally handing the packet to the
server layer LSR for delivery to the egress PE in the server layer.
At the egress PE in the server layer, the packet is first processed
by the server LSR which uses the PW label to pass the packet to the
correct PW instance. This PW instance processed the packet as
described in RFC4448. The resultant Ethernet encapsulated client
packet is then passed to the egress client LSR which then processes
the packet in the normal manner.
Note that although the description above is written in terms of the
behaviour of an MPLS LSR, the processing model would be similar for
an IP packet, or indeed any other protocol type.
Note that the semantics of the PW between the client LSRs is a point-
to-point link.
5. Packet PW Encapsulation
The client network work layer packet encapsulation into a packet PW
is shown in Figure 3.
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+-------------------------------+
| Client |
| Network Layer |
| packet | n octets
| |
+-------------------------------+
| |
| Ethernet | 14 octets
| Header |
| +---------------+
| |
+---------------+---------------+
| Optional Control Word | 4 octets
+-------------------------------+
| PW label | 4 octets
+-------------------------------+
| Server MPLS Tunnel Label(s) | n*4 octets (four octets per label)
+-------------------------------+
Figure 3: Packet PW Encapsulation
This conforms to the PW protocols stack as defined in [RFC4448]. The
protocol stack is unremarkable except to note that the stack does not
retain 32 bit alignment between the virtual Ethernet header and the
PW optional control word (or the PW label when the optional
components are not present in the PW header). This loss of 32 bit of
alignment is necessary to preserve backwards compatibility with the
Ethernet PW design [RFC4448]
Ethernet Raw Mode (PW type 5) MUST be used for the packet PW.
The PEs MAY use a local Ethernet address for the Ethernet header used
to encapsulate the client network layer packet, or MAY use the
special Ethernet addresses "PacketPWEthA" or "PacketPWEthB" as
described below.
IANA is requested to allocate [ed note: RFC Editor will change to
"has allocated"] two unicast Ethernet addresses [RFC5342] for use
with this protocol, referred to as "PacketPWEthA" and "PacketPWEthB".
Where [RFC4447] signalling is used to set up the PW, the LDP peers
numerically compare their IP addresses. The LDP PE with the higher
value IP address will use PacketPWEthA, whilst the LDP peer with the
lower value IP address uses PacketPWEthB.
Where no signalling PW protocol is used, suitable Ethernet addresses
MUST be configured at each PE.
Although this PW represents a point-to-point connection, the use of a
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multicast destination address in the Ethernet encapsulation is
REQUIRED by some client layer protocols. Peers MUST be prepared to
handle a multicast destination address in the Ethernet encapsulation.
6. Ethernet and IEEE 802.1 Functional Restrictions
The use of Ethernet as the encapsulation mechanism for traffic
between the server LSRs is a convenience based on the widespread
availability of existing hardware. In this application there is no
requirement for any Ethernet feature other than its protocol
multiplexing capability. Thus, for example, a server LSR is not
required to implement the Ethernet OAM.
The use and applicability of VLANs, IEEE 802.1p, and IEEE 802.1Q
tagging between PEs is not supported.
Point-to-multipoint and multipoint-to-multipoint operation of the
virtual Ethernet is not supported.
7. Congestion Considerations
A packet pseudowire is normally used to carry IP, MPLS and their
associated support protocols over an MPLS network. There are no
congestion considerations beyond those that ordinarily apply to an IP
or MPLS network. Where the packet protocol being carried is not IP
or MPLS and the traffic volumes are greater than that ordinarily
associated with the support protocols in an IP or MPLS network, the
congestion considerations developed for PWs apply [RFC3985],
[RFC5659].
8. Security Considerations
The virtual Ethernet approach to packet PW introduces no new security
risks. A more detailed discussion of pseudowire security is given in
[RFC3985], [RFC4447] and [RFC3916].
9. IANA Considerations
IANA are requested to allocate two Ethernet unicast addresses from
the IANA Ethernet Address Block - Unicast Use
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Dotted Decimal Description Reference
------------------- ---------------- ---------
000.00x.000 PacketPWEthA [This RFC]
000.00x.001 PacketPWEthB [This RFC]
The value of x is open for IANA to choose. A value of 3 is suggested.
10. Acknowledgements
The authors acknowledge the contribution make by Sami Boutros, Giles
Herron, Siva Sivabalan and David Ward to this document.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4448] Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, April 2006.
[RFC5342] Eastlake, D., "IANA Considerations and IETF Protocol Usage
for IEEE 802 Parameters", BCP 141, RFC 5342,
September 2008.
11.2. Informative References
[RFC3916] Xiao, X., McPherson, D., and P. Pate, "Requirements for
Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
September 2004.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC5317] Bryant, S. and L. Andersson, "Joint Working Team (JWT)
Report on MPLS Architectural Considerations for a
Transport Profile", RFC 5317, February 2009.
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[RFC5385] Touch, J., "Version 2.0 Microsoft Word Template for
Creating Internet Drafts and RFCs", RFC 5385,
February 2010.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
October 2009.
[RFC5921] Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
Berger, "A Framework for MPLS in Transport Networks",
RFC 5921, July 2010.
Appendix A. Encapsulation Approaches Considered
A number of approaches to the design of a packet pseudowire (PW) were
investigated by the PWE3 Working Group and were discussed in IETF
meetings and on the PWE3 list. This section describes the approaches
that were analysed and the technical issues that the authors took
into consideration in arriving at the approach described in the main
body of this document. This appendix is provided so that engineers
considering alternative optimizations can have access to the rational
for the selection of the approach described above.
In a typical network there are usually no more that four network
layer protocols that need to be supported: IPv4, IPv6, MPLS and CLNS
although any solution needs to be scalable to a larger number of
protocols. The approaches considered in this document all satisfy
this minimum requirement, but vary in their ability to support larger
numbers of network layer protocols.
Additionally it is beneficial if the complete set of protocols
carried over the network between in support of a set of CE peers fate
share. It is additionally beneficial if a single OAM session can be
used to monitor the behaviour of this complete set. During the
investigation various views were expressed as to where on the scale
from absolutely required to "nice to have" these benefits lay, but in
the end they were not a factor in reaching our conclusion.
There are four candidate approaches that have been analysed:
1. A protocol identifier (PID) in the PW Control Word (CW)
2. A PID label
3. Parallel PWs - one per protocol.
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4. Virtual Ethernet
A.1. A Protocol Identifier in the Control Word
This is the approach that we proposed in draft 0 of this document .
The proposal was that a Protocol Identifier (PID) would included in
the PW control word (CW), by appending it to the generic control word
[RFC5385] to make a 6 byte CW (the version 0 draft actually included
two reserved bytes to provide 32bit alignment, but let us assume that
was optimized out). A variant of this is just to use a 2 byte PID
without a control word.
This is a simple approach, and is basically a virtual PPP interface
without the PPP control protocol. This has a smaller MTU than for
example a virtual Ethernet would need, however in forwarding terms it
is not as simple as the PID label or multiple PW approaches described
next, and may not be deployable on a number of existing hardware
platforms.
A.2. PID Label
This is the approach that we described in Version 2 of this document.
The in this mechanism the PID is indicated by including a label after
the PW label that indicates the protocol type as shown in Figure 4.
+-------------------------------+
| Client |
| Network Layer |
| packet | n octets
| |
+-------------------------------+
| Optional Control Word | 4 octets
+-------------------------------+
| PID Label (S=1) | 4 Octets
+-------------------------------+
| PW label | 4 octets
+-------------------------------+
| Server MPLS Tunnel Label(s) | n*4 octets (four octets per label)
+-------------------------------+
Figure 4: Encapsulation of a pseudowire with a pseudowire load
balancing label
In the PID Label approach a new Label Distribution protocol (LDP)
Forwarding Equivalence Class (FEC) element is used to signal the
mapping between protocol type and the PID label. This approach
complies with RFC3031.
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A similar approach to PID label is described in Section 3.4.5 of
[RFC5921]. In this case when the client is a network layer packet
service such as IP or MPLS, a service label and demultiplexer label
(which may be combined) is used to provide the necessary
identifications needed to carry this traffic over an LSP.
The authors surveyed the hardware designs produced by a number of
companies across the industry and concluded that whilst the approach
complies with the MPLS architecture, it may conflict with a number of
designer's interpretation of the existing MPLS architecture. This
led to concerns that the approach may result in unexpected
difficulties in the future. Specifically there is an assumption in
many designs that a forwarding decision should be made on the basis
of a single label. Whilst the approach is attractive, it cannot be
supported by many commodity chip sets and this would require new
hardware which would increase the cost of deployment and delay the
introduction of a packet PW service.
A.3. Parallel PWs
In this approach one PW is constructed for each protocol type that
must be carried between the PEs. Thus a complete packet PW would
therefore consist of a bundle of PWs . This model would be very
simple and efficient from a forwarding point of view. The number of
parallel PWs required would normally be relatively small. In a
typical network there are usually no more that four network layer
protocols that need to be supported: IPv4, IPv6, MPLS and CLNS
although any solution needs to be scalable to a larger number of
protocols.
The are a number of serious downsides with this approach:
1. From an operational point of view the lack of fate sharing
between the protocol types can lead to complex faults which are
difficult to diagnose.
2. There is an undesirable trade off in the OAM related to the first
point. Either we would have to run an OAM on each PW and bind
them together which lead to significant protocol and software
complexity and does not scale well. Alternatively we would need
to run a single OAM session on one of the PWs as a proxy for the
others and the diagnose any more complex failure on a case by
case basis. To some extent the issue of fate sharing between
protocol in the bundle (for example the assumed fate sharing
between CLNS and IP in IS-IS) can be mitigated through the use of
BFD.
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3. The need to configure manage and synchronize the behaviour of a
group of PWs as if they were a single PW leads to an increase in
control plane complexity.
The Parallel PW mechanism is therefore an approach which simplifies
the forwarding plane, but only at a cost of a considerable increase
in other aspects of the design and in particular operation of the PW.
A.4. Virtual Ethernet
Using a virtual Ethernet to provide a packet PW would require PEs to
include a virtual (internal) Ethernet interface and then to use an
Ethernet PW [RFC4448] to carry the user traffic. This is
conceptually simple and can be implemented today without any further
standards action, although there are a number of applicability
considerations that it is useful to draw to the attention of the
community.
Conceptually this is a simple approach and some deployed equipments
can already do this. However the requirement to run a complete
Ethernet adjacency lead us to conclude that there was a need to
identify a simpler approach. The packets encapsulated in an Ethernet
header have a larger MTU than the other approaches, although this is
not considered to be an issue on the networks needing to carry packet
PWs.
The virtual Ethernet mechanism was the first approach that the
authors considered, before the merits of the other approaches
appeared to make them more attractive. As we shall see below
however, the other approaches were not without issues and it appears
that the virtual Ethernet is preferred approach to providing a packet
PW.
A.5. Recommended Encapsulation
The operational complexity and the breaking of fate sharing
assumptions associated with the parallel PW approach would suggest
that this is not an approach that should be further pursued.
The PID Label approach gives rise to the concerns that it will break
implicit behavioural and label stack size assumptions in many
implementations. Whilst those assumptions may be addressed with new
hardware this would delay the introduction of the technology to the
point where it was unlikely to gain acceptance in competition with an
approach that needed no new protocol design and is already
supportable on many existing hardware platforms.
The PID in the CW leads to the most compact protocol stack, is simple
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and requires minimal protocol work. However it is a new forwarding
design, and apart from the issue of the larger packet header and the
simpler adjacency formation offers no advantage over the virtual
Ethernet.
The above considerations bring us back to the virtual Ethernet, which
is a well known protocol stack, with a well known (internal) client
interface. It is already implemented in many hardware platforms and
is therefore readily deployable. The authors conclude that having
considered a number of initially promising alternatives, the
simplicity and existing hardware make the virtual Ethernet approach
to the packet PW the most attractive solution.
Authors' Addresses
Stewart Bryant (editor)
Cisco Systems
250, Longwater, Green Park,
Reading, Berks RG2 6GB
UK
Email: stbryant@cisco.com
Luca Martini
Cisco Systems
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
USA
Email: lmartini@cisco.com
George Swallow
Cisco Systems
1414 Massachusetts Ave
Boxborough, MA 01719
USA
Email: swallow@cisco.com
URI:
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Andy Malis
Verizon Communications
117 West St.
Waltham, MA 02451
USA
Email: andrew.g.malis@verizon.com
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