Liaison statement
IETF PALS Working Group on the Control Word for Ethernet Pseudowires

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,
pals@ietf.org .

Regards,
Andrew G. Malis and Stewart Bryant