[Pals] Liaison from IETF PALS Working Group on the Control Word for Ethernet Pseudowires

IETF PALS Working Group liaison from:
Andrew G. Malis <[email protected]>
Stewart Bryant <[email protected]>
Co-chairs, IETF PALS Working Group

To:
Broadband Forum (for action)
ITU-T SG15 WP3 (for action)
MEF (for action)
IEEE 802.1 (for information)
IEEE Registration Authority (for information)

The pseudowire (PW) encapsulation of Ethernet, as defined in IETF RFC 4448,
specifies that the use of the Control Word (CW) is optional. When the CW is
not used in an Ethernet PW, it has been found to be operationally common
for deployed MPLS label switching routers (LSRs) to search past the end of
the label stack to determine whether the payload is an IP packet, and if it
thinks that the payload is an IP packet based on the first four bits of the
payload being equal to 0x4 or 0x6, to use equal-cost-multi-path (ECMP)
algorithms to select the next hop based of the so called IP "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 LSR selecting the ECMP path based on the
presumed (but incorrect) IP five-tuple. 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 payload Ethernet frames. Further
discussion of this topic is published in IETF RFC 4928.

Frame misordering can also happen in a single path scenario when traffic
classification and differential forwarding treatment mechanisms are in use.
This occurs when a forwarder incorrectly assumes that the packet is IP and
applies forwarding policy based on fields in the PW payload.

This problem has recently become more serious in operational networks for
several reasons. The first is due to the more common deployment of
equipment with Ethernet MAC addresses that start with 0x4 or 0x6 as
assigned by the IEEE Registration Authority (RA). Secondly, concerns over
privacy have led to the use of MAC address randomization which assigns
local MAC addresses randomly for privacy. Random assignment produces
addresses starting with one of the two values about 1/8 of the time.

The use of the Ethernet PW CW addresses this problem.

The IETF PALS Working Group has written a draft RFC that recommends the use
of the Ethernet pseudowire Control Word in all but exceptional
circumstances. This draft is close to completion in the PALS Working Group,
and may be found online at
https://tools.ietf.org/html/draft-ietf-pals-ethernet-cw . Organizations
that write specifications that include the use of Ethernet PW technology
are recommended to be familiar with this draft and take any actions with
their respective specifications they think are needed to address the draft.

We anticipate its publication as an IETF RFC in the coming months.
Interested individuals are invited to provide comments to the PALS email
list, [email protected] .

Regards,
Andrew G. Malis and Stewart Bryant

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