Thursday, October 28, 2010


On October 12nd and 13th the IESG (Internet Engineering Steering Group) and IAB (Internet Architecture Board), the two IETF management bodies, held a joint design session on OAM. I was a bit surprised that the IETF leadership would be interested in devoting a separate meeting (not coinciding with an IETF conference) to the subject of OAM; OAM has never been an area of IETF expertise. Indeed, when the meeting was first announced on the IETF main discussion email list several long-time IETF participants asked for the acronym OAM to be spelled out! Of course, the ICMP (ping) was defined in RFC 792 circa 1981, and BFD that runs between routers has its own BFD Working Group (WG) in the IETF, but the overall concept of OAM has never been central to the IETF world view.

However, just as the interests of the ITU-T have been migrating up from synchronous networks to ones based on Ethernet, IP and MPLS, so have the interests of the IETF been migrating down from applications, end-to-end transport, and routing to pure transport functionality. And OAM is a crucial element of transport networks.

The physical meeting was held at George Mason University in Fairfax Virginia, but was also WebEx’ed. Thus I managed to actively participate, and even present slides, without having to travel; but did find myself jet-lagged due to shifting my work day by 6 hours. Unfortunately, the Internet connectivity at the conference site was not completely solid, and the remote attendees frequently found themselves talking to themselves on how to alert those on-site that the connection had failed. Some connectivity OAM would definitely have been useful …

So where does the IETF want to use OAM ? The main interest is now MPLS-TP, but there are still open issues regarding PW OAM.

The IETF PWE3 WG standardized an associated channel that shares fate with the PW traffic, which is mostly employed for OAM. This OAM is misnamed VCCV for Virtual Channel (an old name for PW) Connectivity Verification (which should be Continuity Check). VCCV presently allows IP ping, LSP ping, and BFD protocols to run inside the associated channel in order to provide FM. Back at IETF-67 (November 2006) I proposed using Y.1731 inside the associated channel. This idea was later developed into draft-mohan-pwe3-vccv-eth, backed by Nortel, RAD, France Telecom, KDDI, Huawei, NTT and Sprint, but was rejected by the larger community due to confusion as to its use of Ethernet (it was never intended to be limited to MPLS over Ethernet, or Ethernet PWs).

As I am sure all my readers know, MPLS-TP is a transport technology, being jointly developed by the ITU-T and IETF. The ITU-T views MPLS-TP as yet another transport network, which needs the same OAM functionality as all the other transport networks developed to date (SDH, OTN, carrier-grade Ethernet). In particular, the generic research on OAM for packet-based networks, and the protocol development (in cooperation with IEEE 802.1) of Y.1731, is seen by the ITU-T community as directly relevant to MPLS-TP. Work in the ITU, and Internet Draft draft-bhh-mpls-tp-oam-y1731 submitted to the IETF, proposed maximizing re-use of Y.1731 formats. This approach is strongly advocated by Alcatel-Lucent and Huawei, and is being backed by many operators, including China Mobile, China Telecom, Telecom Italia, France Telecom, Deutche Telekom, Telstra, and KPN. The idea expands on my earlier proposal, solves both FM and PM with a single OAM protocol, and is expected to undergo major deployment in the near future.

IETF participants from Cisco, Juniper, and Ericsson produced an alternative OAM FM proposal, based on the IETF’s own BFD instead of the ITU-T’s Y.1731. The IETF MPLS WG could not reach consensus as to which mechanism to prefer (in an email poll the community was split about 50/50). The MPLS WG chairs decided to exercise their authority to break such ties, and elevated the BFD-based draft to WG status as draft-ietf-mpls-tp-cc-cv-rdi, thus effectively killing draft-bhh for FM. The issue of PM was open for a while, but a Cisco draft has recently been elevated to become draft-ietf-mpls-tp-loss-delay, thus blocking draft-bhh from the PM function as well. This draft is not fully fleshed out, but it uses the MPLS-TP G-ACh mechanism, and allows either NTP-style or 1588-style timestamps.

So we see that OAM has become a hot (and contentious) topic in the IETF.

After this long introduction, my next entry will delve into a few of the subjects discussed at the joint design session.