Internet DRAFT - draft-ietf-pals-ethernet-cw
draft-ietf-pals-ethernet-cw
PALS Working Group S. Bryant
Internet-Draft A. Malis
Updates: 4448 (if approved) Huawei
Intended status: Standards Track I. Bagdonas
Expires: January 3, 2019 Equinix
July 02, 2018
Use of Ethernet Control Word RECOMMENDED
draft-ietf-pals-ethernet-cw-07
Abstract
The pseudowire (PW) encapsulation of Ethernet, as defined in RFC
4448, specifies that the use of the control word (CW) is optional.
In the absence of the CW an Ethernet pseudowire packet can be
misidentified as an IP packet by a label switching router (LSR).
This in turn may lead to the selection of the wrong equal-cost-multi-
path (ECMP) path for the packet, leading in turn to the misordering
of packets. This problem has become more serious due to the
deployment of equipment with Ethernet MAC addresses that start with
0x4 or 0x6. The use of the Ethernet PW CW addresses this problem.
This document recommends the use of the Ethernet pseudowire control
word in all but exceptional circumstances.
This document updates RFC 4448.
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
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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 January 3, 2019.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Specification of Requirements . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 5
5. Equal Cost Multi-path (ECMP) . . . . . . . . . . . . . . . . 5
6. Mitigations . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Operational Considerations . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The pseudowire(PW) encapsulation of Ethernet, as defined in
[RFC4448], specifies that the use of the control word (CW) is
optional. It is common for label switching routers (LSRs) to search
past the end of the label stack to determine whether the payload is
an IP packet, and if the payload is an IP packet, to select the next
hop based on the so called "five-tuple" (IP source address, IP
destination address, protocol/next-header, transport layer source
port and transport layer destination port). In the absence of a PW
CW an Ethernet pseudowire packet can be misidentified as an IP packet
by a label switching router (LSR) selecting the equal-cost-multi-path
(ECMP) path based on the five-tuple. This in turn may lead to the
selection of the wrong ECMP path for the packet, leading in turn to
the misordering of packets. Further discussion of this topic is
published in [RFC4928].
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Flow misordering can also happen in a single path scenario when
traffic classification and differential forwarding treatment
mechanisms are in use. These errors occur when a forwarder
incorrectly assumes that the packet is IP and applies forwarding
policy based on fields in the PW payload.
IPv4 and IPv6 packets respectively start with the values 0x4 and 0x6.
Misidentification can arise if an Ethernet PW packet without a CW is
carrying an Ethernet packet with a destination address that starts
either of these values.
This problem has recently become more serious for a number of
reasons. Firstly, due to the deployment of equipment with Ethernet
MAC addresses that start with 0x4 or 0x6 assigned by the IEEE
Registration Authority Committee (RAC). Secondly, concerns over
privacy have led to the use of MAC address randomization which
assigns local MAC addresses randomly for privacy. Random assignment
results in addresses starting with one of these two values one time
in eight.
The use of the Ethernet PW CW addresses this problem.
This document recommends the use of the Ethernet pseudowire control
word in all but exceptional circumstances.
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Background
Ethernet pseudowire encapsulation is specified in [RFC4448]. In
particular the reader is drawn to section 4.6, part of which is
quoted below for the convenience of the reader:
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"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) [RFC2992], and
Operations and Management (OAM) mechanisms including VCCV
[RFC5085].
"[RFC4385] 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 labeled 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."
At the time when pseudowires were first deployed, some equipment of
commercial significance was unable to process the Ethernet Control
Word. In addition, at that time it was considered that no Ethernet
MAC address had been issued by the IEEE Registration Authority
Committee (RAC) that starts with 0x4 or 0x6, and thus it was thought
to be safe to deploy Ethernet PWs without the CW.
Since that time the RAC has issued Ethernet MAC addresses start with
0x4 or 0x6 and thus the assumption that in practical networks there
would be no confusion between an Ethernet PW packet without the CW
and an IP packet is no longer correct.
Possibly through the use of unauthorized Ethernet MAC addresses, this
assumption has been unsafe for a while, leading some equipment
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vendors to implement more complex, proprietary, methods to
discriminate between Ethernet PW packets and IP packets. Such
mechanisms rely on the heuristics of examining the transit packets in
trying to find out the exact payload type of the packet and cannot be
reliable due to the random nature of the payload carried within such
packets.
A posting on the NANOG email list highlighted this problem:
https://mailman.nanog.org/pipermail/nanog/2016-December/089395.html
RFC EDITOR Please delete this paragraph.
Kramdown does not include references when they are only found in
literal text so I include them here: [RFC4385] [RFC2992] [RFC5085] as
a fixup.
4. Recommendation
The ambiguity between an MPLS payload that is an Ethernet PW and one
that is an IP packet is resolved when the Ethernet PW control word is
used. This document updates [RFC4448] to state that where both the
ingress PE and the egress PE support the Ethernet pseudowire control
word, then the CW MUST be used.
Where the application of ECMP to an Ethernet PW traffic is required,
and where both the ingress and the egress PEs support [RFC6790]
(Entropy Label Indicator/Entropy Label (ELI/EL)) or both the ingress
and the egress PEs support [RFC6391] (FAT PW), then either method may
be used. The use of both methods on the same PW is not normally
necessary and should be avoided unless circumstances require it. In
the case of multi-segment PWs, if ELI/EL is used then it SHOULD be
used on every segment of the PW. The method by which usage of ELI/EL
on every segment is guaranteed is out of scope of this document.
5. Equal Cost Multi-path (ECMP)
Where the volume of traffic on an Ethernet PW is such that ECMP is
required then one of two methods may be used:
o Flow-Aware Transport (FAT) of Pseudowires over an MPLS Packet
Switched Network specified in [RFC6391], or
o LSP entropy labels specified in [RFC6790]
RFC6391 works by increasing the entropy of the bottom of stack label.
It requires that both the ingress and egress provider edge (PE)s
support this feature. It also requires that sufficient LSRs on the
LSP between the ingress and egress PE be able to select an
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ECMP path on an MPLS packet with the resultant stack depth.
RFC6790 works by including an entropy value in the LSP part of the
label stack. This requires that the Ingress and Egress PEs support
the insertion and removal of the EL and the entropy label indicator,
and that sufficient LSRs on the LSP are able to preform ECMP based on
the EL.
In both cases there are considerations in getting Operations,
Administration, and Maintenance (OAM) packets to follow the same path
as a data packet. This is described in detail section 7 of
[RFC6391], and section 6 of RFC6790. However in both cases the
situation is improved compared to the ECMP behavior in the case where
the Ethernet PW CW was not used, since there is currently no known
method of getting a PW OAM packet to follow the same path as a PW
data packet subjected to ECMP based on the five tuple of the IP
payload.
The PW label is pushed before the LSP label. As the EL/ELI labels
are part of the LSP layer rather than part of the PW layer, they are
pushed after the PW label has been pushed.
6. Mitigations
Where it is not possible to use the Ethernet PW CW, the effects of
ECMP can be disabled by carrying the PW over a traffic engineered
path that does not subject the payload to load balancing (for example
[RFC3209]). However such paths may be subjected to link bundle load
balancing and of course the single LSP has to carry the full PW load.
7. Operational Considerations
In some cases, the inclusion of a CW in the PW is determined by
equipment configuration. Furthermore, it is possible that the
default configuration in such cases is to disable use of the CW.
Care needs to be taken to ensure that software that implements this
recommendation does not depend on existing configuration settings
that prevents the use of control word. It is recommended that
platform software emits a rate limited message indicating that CW can
be used but is disabled due to existing configuration.
Instead of including a payload type in the packet, MPLS relies on the
control plane to signal the payload type that follows the bottom of
the label stack. Some LSRs attempt to deduce the packet type by MPLS
payload inspection, in some cases looking past the PW CW. If the
payload appears to be IP or IP carried in an Ethernet header they
perform an ECMP calculation based on what they assume to be the five
tuple fields. However deduction of the payload type in this way is
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not an exact science, and where a packet that is not IP is mistaken
for an IP packet the result can be packets delivered out of order.
Misordering of this type can be difficult for an operator to
diagnose. Operators should be aware when enabling capability that
allows information gleaned from packet inspection past the PW CW to
be used in any ECMP calculation, that this may cause Ethernet frames
to be delivered out of order despite the presence of the CW.
8. Security Considerations
This document expresses a preference for one existing and widely
deployed Ethernet PW encapsulation over another. These methods have
identical security considerations, which are discussed in [RFC4448].
This document introduces no additional security issues.
9. IANA Considerations
This document makes no IANA requests.
10. Acknowledgments
The authors thank Job Snijders for drawing attention to this problem.
The authors also thank Pat Thaler for clarifying the matter of local
MAC address assignment. We thank Sasha Vainshtein for his valuable
review comments.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
February 2006, <https://www.rfc-editor.org/info/rfc4385>.
[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
<https://www.rfc-editor.org/info/rfc4448>.
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[RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
Cost Multipath Treatment in MPLS Networks", BCP 128,
RFC 4928, DOI 10.17487/RFC4928, June 2007,
<https://www.rfc-editor.org/info/rfc4928>.
[RFC6391] Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
Regan, J., and S. Amante, "Flow-Aware Transport of
Pseudowires over an MPLS Packet Switched Network",
RFC 6391, DOI 10.17487/RFC6391, November 2011,
<https://www.rfc-editor.org/info/rfc6391>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
<https://www.rfc-editor.org/info/rfc2992>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC5085] Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control
Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
December 2007, <https://www.rfc-editor.org/info/rfc5085>.
Authors' Addresses
Stewart Bryant
Huawei
Email: stewart.bryant@gmail.com
Andrew G Malis
Huawei
Email: agmalis@gmail.com
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Ignas Bagdonas
Equinix
Email: ibagdona.ietf@gmail.com>
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