RFC 6435, "MPLS Transport Profile Lock Instruct and Loopback Functions", November 2011Source of RFC: mpls (rtg)
See Also: RFC 6435 w/ inline errata
Errata ID: 3429
Publication Format(s) : TEXT
Reported By: David Ball
Date Reported: 2012-12-12
Verifier Name: Adrian Farrel
Date Verified: 2013-01-26
Section 4 says:
The whole of Section 4
It should say:
4. Loopback Function This section provides a description of the loopback function within an MPLS network. This function is achieved through management commands, so there is no protocol specification necessary. However, the loopback function is dependent on the lock function, so it is appropriate to describe it in this document. The loopback function is used to test the integrity of a transport path from a MEP to any other node along the same transport path. This is achieved by setting the target node into loopback mode for that transport path, and transmitting a pattern of test data from the MEP. The target node loops all data received on the transport path back towards the sending MEP, which extracts the test data and compares it with what it sent. Loopback is a function that enables a given node on a transport path to return traffic to the sending MEP for that transport path when in the loopback mode. This mode corresponds to the situation where, at a given node, a forwarding plane loop is configured, and the incoming direction of a transport path is cross-connected to the outgoing reverse direction. Therefore, except in the case of early TTL expiry, traffic sent by the source will be received by that source. Data-plane loopback is an out-of-service function, as required in Section 2.2.5 of RFC 5860 . This function loops back all traffic (including user data and OAM). The traffic can be originated from one internal point at the ingress of a transport path within an interface or inserted from an input port of an interface using external test equipment. The traffic is looped back unmodified (other than normal per-hop processing such as TTL decrement) in the direction of the point of origin by an interface at either an intermediate node or a terminating node. It should be noted that the data-plane loopback function for a given transport path can be applied to data-plane loopback points residing on interfaces where there may be no MEP or MIP for that transport path. For data-plane loopback at an intermediate point in a transport path, the loopback needs to be configured to occur at either the ingress or egress interface. This can be done using the management plane. The management plane can be used to configure the loopback function. The management plane must ensure that the MEPs at either end of a transport path are locked before it requests setting a given node of that transport path into loopback mode. The nature of test data and the use of loopback traffic to measure packet loss, delay, and delay variation are outside the scope of this document.
The changes here reflect a number of minor editorial clarifications.
The original Errata Report raised by David Ball has been extensively
debated by the working group resulting in these changes.
The motivation is that the published text has caused confusion about
exactly where the loopback function is applied. There was apparent
contradiction between paragraphs 2 and 7 which seemed to imply that
the loopback point must be a MEP or a MIP, while paragraph 5 seemded
to imply that loopback is performed at a point where there is no MEP
The working group agrees that the intent was to state that there may
or may not be a MEP or a MIP at the loopback point. I.e., that
loopback may be performed at any point along the transport path.
The new text updates paragraphs 2, 3, 5, and 7 to embody this
Additionally, paragraphs 6 and 7 have received a minor change to
show the role of the "management plane" rather than "management" in
the control of loopback.