Packet Loss and Delay Measurement for MPLS Networks
draft-ietf-mpls-loss-delay-00

Abstract

Many service provider service level agreements (SLAs) depend on the ability to measure and monitor performance metrics for packet loss and one-way and two-way delay, as well as related metrics such as delay variation and channel throughput. This capability, in addition, provides operators with greater visibility into the performance characteristics of their networks, thereby facilitating planning, troubleshooting, and evaluation. This document specifies protocol mechanisms to enable the efficient and accurate measurement of these performance metrics in MPLS networks.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

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 working documents as Internet-Drafts. The list of current Internet-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 June 26, 2011.

Copyright Notice

Copyright (c) 2010 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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1.  Introduction
    1.1.  Terminology
2.  Overview
    2.1.  Packet Loss Measurement
    2.2.  Throughput Measurement
    2.3.  Delay Measurement
    2.4.  Delay Variation Measurement
    2.5.  Unidirectional Measurement
    2.6.  Loopback Measurement
    2.7.  Measurement Considerations
        2.7.1.  Types of Channels
        2.7.2.  Quality of Service
        2.7.3.  Equal Cost Multipath
        2.7.4.  Intermediate Nodes
        2.7.5.  Distributed Systems
        2.7.6.  Loss Measurement Modes
        2.7.7.  Loss Measurement Scope
        2.7.8.  Delay Measurement Accuracy
        2.7.9.  Delay Measurement Timestamp Format
3.  Message Formats
    3.1.  Loss Measurement Message Format
    3.2.  Delay Measurement Message Format
    3.3.  Combined Loss/Delay Measurement Message Format
    3.4.  Timestamp Field Formats
    3.5.  TLV Objects
        3.5.1.  Padding
        3.5.2.  Addressing
4.  Operation
    4.1.  Loss Measurement Procedures
        4.1.1.  Initiating a Loss Measurement Operation
        4.1.2.  Transmitting a Loss Measurement Query
        4.1.3.  Receiving a Loss Measurement Query
        4.1.4.  Transmitting a Loss Measurement Response
        4.1.5.  Receiving a Loss Measurement Response
        4.1.6.  Loss Calculation
        4.1.7.  Quality of Service
        4.1.8.  G-ACh Packets
        4.1.9.  Test Messages
        4.1.10.  Message Loss and Packet Misorder Conditions
    4.2.  Delay Measurement Procedures
        4.2.1.  Transmitting a Delay Measurement Query
        4.2.2.  Receiving a Delay Measurement Query
        4.2.3.  Transmitting a Delay Measurement Response
        4.2.4.  Receiving a Delay Measurement Response
        4.2.5.  Timestamp Format Negotiation
        4.2.6.  Quality of Service
    4.3.  Combined Loss/Delay Measurement Procedures
5.  Congestion Considerations
6.  Security Considerations
7.  IANA Considerations
8.  Acknowledgments
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Authors' Addresses

1.  Introduction

Many service provider service level agreements (SLAs) depend on the ability to measure and monitor performance metrics for packet loss and one-way and two-way delay, as well as related metrics such as delay variation and channel throughput. This capability, in addition, provides operators with greater visibility into the performance characteristics of their networks, thereby facilitating planning, troubleshooting, and evaluation. This document specifies protocol mechanisms to enable the efficient and accurate measurement of these performance metrics in MPLS networks.

This document specifies two closely-related protocols, one for packet loss measurement (LM) and one for packet delay measurement (DM). These protocols have the following characteristics and capabilities:

• The LM and DM protocols are intended to be simple and to support efficient hardware processing.
• The LM and DM protocols operate over the MPLS Generic Associated Channel (G-ACh) [RFC5586] (Bocci, M., Vigoureux, M., and S. Bryant, “MPLS Generic Associated Channel,” June 2009.) and support measurement of loss and delay over Label Switched Paths (LSPs), pseudowires, and MPLS sections (links).
• The LM and DM protocols are applicable to the LSPs, pseudowires, and sections of networks based on the MPLS Transport Profile (MPLS-TP), because the MPLS-TP is based on a standard MPLS data plane. The MPLS-TP is defined and described in [RFC5921] (Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, “A Framework for MPLS in Transport Networks,” July 2010.), and MPLS-TP LSPs, pseudowires, and sections are discussed in detail in [RFC5960] (Frost, D., Bryant, S., and M. Bocci, “MPLS Transport Profile Data Plane Architecture,” August 2010.).
• The LM and DM protocols can be used for both continuous/proactive and selective/on-demand measurement.
• The LM and DM protocols use a simple query/response model for bidirectional measurement that allows a single node - the querier - to measure the loss or delay in both directions.
• The LM and DM protocols use query messages for unidirectional loss and delay measurement. The measurement can either be carried out at the downstream node(s) or at the querier if an out-of-band return path is available.
• The LM and DM protocols do not require that the transmit and receive interfaces be the same when performing bidirectional measurement.
• The DM protocol is stateless.
• The LM protocol is "almost" stateless: loss is computed as a delta between successive messages, and thus the data associated with the last message received must be retained.
• The LM protocol can perform two distinct kinds of loss measurement: it can measure the loss of specially generated test packets in order to infer the approximate data-plane loss level (inferred measurement); or it can directly measure data-plane packet loss (direct measurement). Direct measurement provides perfect loss accounting, but may require specialized hardware support and is only applicable to some LSP types. Inferred measurement provides only approximate loss accounting but is generally applicable.
• The LM protocol supports both 32-bit and 64-bit packet counters.
• The LM protocol supports measurement in terms of both packet counts and octet counts.
• The LM protocol can be used to measure channel throughput as well as packet loss.
• The DM protocol supports multiple timestamp formats, and provides a simple means for the two endpoints of a bidirectional connection to agree on a preferred format. This procedure reduces to a triviality for implementations supporting only a single timestamp format.
• The DM protocol supports varying the measurement message size in order to measure delays associated with different packet sizes.

1.1.  Terminology

2.  Overview

This section begins with a summary of the basic methods used for the bidirectional measurement of packet loss and delay. These measurement methods are then described in detail. Finally a list of practical considerations are discussed that may come into play to inform or modify these simple procedures.

The following figure shows the reference scenario.

          T1              T2
+-------+/     Query       \+-------+
|       | - - - - - - - - ->|       |
|   A   |===================|   B   |
|       |<- - - - - - - - - |       |
+-------+\     Response    /+-------+
          T4              T3

The figure shows a bidirectional channel between two nodes, A and B, and illustrates the temporal reference points T1-T4 associated with a measurement operation that takes place at A. The operation consists of A sending a query message to B, and B sending back a response. Each reference point indicates the point in time at which either the query or the response message is transmitted or received over the channel.

In this situation, A can arrange to measure the packet loss over the channel in the forward and reverse directions by sending Loss Measurement (LM) query messages to B each of which contains the count of packets transmitted prior to time T1 over the channel to B (A_TxP). When the message reaches B, it appends two values and reflects the message back to A: the count of packets received prior to time T2 over the channel from A (B_RxP), and the count of packets transmitted prior to time T3 over the channel to A (B_TxP). When the response reaches A, it appends a fourth value, the count of packets received prior to time T4 over the channel from B (A_RxP).

These four counter values enable A to compute the desired loss statistics. Because the transmit count at A and the receive count at B (and vice versa) may not be synchronized at the time of the first message, and to limit the effects of counter wrap, the loss is computed in the form of a delta between messages.

To measure at A the delay over the channel to B, a Delay Measurement (DM) query message is sent from A to B containing a timestamp recording the instant at which it is transmitted, i.e. T1. When the message reaches B, a timestamp is added recording the instant at which it is received (T2). The message can now be reflected from B to A, with B adding its transmit timestamp (T3) and A adding its receive timestamp (T4). These four timestamps enable A to compute the one-way delay in each direction, as well as the two-way delay for the channel. The one-way delay computations require that the clocks of A and B be synchronized; mechanisms for clock synchronization are outside the scope of this document.

2.1.  Packet Loss Measurement

Suppose a bidirectional channel exists between the nodes A and B. The objective is to measure at A the following two quantities associated with the channel:

A_TxLoss (transmit loss): the number of packets transmitted by A over the channel but not received at B;

A_RxLoss (receive loss): the number of packets transmitted by B over the channel but not received at A.

This is accomplished by initiating a Loss Measurement (LM) operation at A, which consists of transmission of a sequence of LM query messages (LM[1], LM[2], ...) over the channel at a specified rate, such as one every 100 milliseconds. Each message LM[n] contains the following value:

A_TxP[n]: the total count of packets transmitted by A over the channel prior to the time this message is transmitted.

When such a message is received at B, the following value is recorded in the message:

B_RxP[n]: the total count of packets received by B over the channel at the time this message is received (excluding the message itself).

At this point, B inserts an appropriate response code into the message and transmits it back to A, recording within it the following value:

B_TxP[n]: the total count of packets transmitted by B over the channel prior to the time this response is transmitted.

When the message response is received back at A, the following value is recorded in the message:

A_RxP[n]: the total count of packets received by A over the channel at the time this response is received (excluding the message itself).

The transmit loss A_TxLoss[n-1,n] and receive loss A_RxLoss[n-1,n] within the measurement interval marked by the messages LM[n-1] and LM[n] are computed by A as follows:

A_TxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] - B_RxP[n-1])
A_RxLoss[n-1,n] = (B_TxP[n] - B_TxP[n-1]) - (A_RxP[n] - A_RxP[n-1])

where the arithmetic is modulo the counter size.

The derived values

A_TxLoss = A_TxLoss[1,2] + A_TxLoss[2,3] + ...

A_RxLoss = A_RxLoss[1,2] + A_RxLoss[2,3] + ...

are updated each time a response to an LM message is received and processed, and represent the total transmit and receive loss over the channel since the LM operation was initiated.

When computing the values A_TxLoss[n-1,n] and A_RxLoss[n-1,n] the possibility of counter wrap must be taken into account. Consider for example the values of the A_TxP counter at sequence numbers n-1 and n. Clearly if A_TxP[n] is allowed to wrap to 0 and then beyond to a value equal to or greater than A_TxP[n-1], the computation of an unambiguous A_TxLoss[n-1,n] value will be impossible. Therefore the LM message rate MUST be sufficiently high, given the counter size and the speed and minimum packet size of the underlying channel, that this condition cannot arise. For example, a 32-bit counter for a 100 Gbps link with a minimum packet size of 64 bytes can wrap in 2^32 / (10^11/(64*8)) = ~22 seconds, which is therefore an upper bound on the LM message interval under such conditions. This bound will be referred to as the MaxLMInterval of the channel. It is clear that the MaxLMInterval will be a more restrictive constraint in the case of direct LM and for smaller counter sizes.

The loss measurement approach described in this section has the characteristic of being stateless at B and "almost" stateless at A. Specifically, A must retain the data associated with the last LM response received, in order to use it to compute loss when the next response arrives. This data MAY be discarded, and MUST NOT be used as a basis for measurement, if MaxLMInterval elapses before the next response arrives, because in this case an unambiguous measurement cannot be made.

The foregoing discussion has assumed the counted objects are packets, but this need not be the case. In particular, octets may be counted instead. This will, of course, reduce the MaxLMInterval proportionately.

2.2.  Throughput Measurement

If LM query messages contain a timestamp recording their time of transmission, this data can be combined with the packet or octet counts to yield a measurement of the throughput sustained over the channel during the interval. This metric can be called the delivered throughput. As for loss measurement, the interval counts can be accumulated to arrive at the delivered throughput of the channel since the start of the measurement operation. This procedure also enables out-of-service throughput testing when combined with a simple packet generator.

2.3.  Delay Measurement

Suppose a bidirectional channel exists between the nodes A and B. The objective is to measure at A one or more of the following quantities associated with the channel:

• The one-way delay associated with the forward (A to B) direction of the channel;
• The one-way delay associated with the reverse (B to A) direction of the channel;
• The two-way delay (A to B to A) associated with the channel.

In the case of two-way delay, there are actually two possible metrics of interest. The "strict" two-way delay is the sum of the one-way delays in each direction and reflects the two-way delay of the channel itself, irrespective of processing delays within the remote endpoint B. The "loose" two-way delay includes in addition any delay associated with remote endpoint processing.

Measurement of the one-way delay quantities requires that the clocks of A and B be synchronized, whereas the two-way delay can be measured directly even when this is not the case (provided A and B have stable clocks).

The measurement is accomplished by sending a Delay Measurement (DM) query message over the channel to B which contains the following timestamp:

T1: the time the DM query message is transmitted from A.

When the message arrives at B, the following timestamp is recorded in the message:

T2: the time the DM query message is received at B.

At this point B inserts an appropriate response code into the message and transmits it back to A, recording within it the following timestamp:

T3: the time the DM response message is transmitted from B.

When the message arrives back at A, the following timestamp is recorded in the message:

T4: the time the DM response message is received back at A.

At this point, A can compute the strict two-way delay associated with the channel as

strict two-way delay = (T4 - T1) - (T3 - T2)

and the loose two-way delay as

loose two-way delay = T4 - T1.

If the clocks of A and B are known at A to be synchronized, then both one-way delay values, as well as the strict two-way delay, can be computed at A as

forward one-way delay = T2 - T1

reverse one-way delay = T4 - T3

strict two-way delay = forward delay + reverse delay.

2.4.  Delay Variation Measurement

Packet Delay Variation (PDV) [RFC3393] (Demichelis, C. and P. Chimento, “IP Packet Delay Variation Metric for IP Performance Metrics (IPPM),” November 2002.) is another performance metric important in some applications. The PDV of a pair of packets within a stream of packets is defined for a selected pair of packets in the stream going from measurement point 1 to measurement point 2. The PDV is the difference between the one-way delay of the selected packets.

A PDV measurement can therefore be derived from successive delay measurements obtained through the procedures in Section 2.3 (Delay Measurement). An important point regarding PDV measurement, however, is that it can be carried out based on one-way delay measurements even when the clocks of the two systems involved in those measurements are not synchronized.

2.5.  Unidirectional Measurement

In the case that the channel from A to (B1, ..., Bk) is unidirectional, i.e. is a unidirectional LSP, LM and DM measurements can be carried out at B1, ..., Bk instead of at A.

For LM this is accomplished by initiating an LM operation at A and carrying out the same procedures as for bidirectional channels, except that no responses from B1, ..., Bk to A are generated. Instead, each terminal node B uses the A_TxP and B_RxP values in the LM messages it receives to compute the receive loss associated with the channel in essentially the same way as described previously, i.e.

B_RxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] - B_RxP[n-1])

For DM, of course, only the forward one-way delay can be measured and the clock synchronization requirement applies.

Alternatively, if an out-of-band channel from a terminal node B back to A is available, the LM and DM message responses can be communicated to A via this channel so that the measurements can be carried out at A.

2.6.  Loopback Measurement

Some bidirectional channels may be placed into a loopback state such that query messages are looped back to the querier without modification. In this situation, LM and DM procedures can be used to carry out measurements associated with the circular path.

For LM, the loss computation in this case is:

A_Loss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (A_RxP[n] - A_RxP[n-1])

For DM, the loose two-way delay is computed. In this case, however, the remote endpoint processing time component reflects only the time required to loop the message from channel input to channel output.

Query messages must include some form of source identifier in order for looped-back queries to be differentiated from queries initiated by the far end.

2.7.  Measurement Considerations

A number of additional considerations apply in practice to the measurement methods summarized above.

2.7.1.  Types of Channels

There are several types of channels in MPLS networks over which loss and delay measurement may be conducted. The channel type may restrict the kinds of measurement that can be performed. In all cases, LM and DM messages flow over the MPLS Generic Associated Channel (G-ACh), which is described in detail in [RFC5586] (Bocci, M., Vigoureux, M., and S. Bryant, “MPLS Generic Associated Channel,” June 2009.).

Broadly, a channel in an MPLS network may be either a link, a Label Switched Path (LSP) [RFC3031] (Rosen, E., Viswanathan, A., and R. Callon, “Multiprotocol Label Switching Architecture,” January 2001.), or a pseudowire [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.). Links are bidirectional and are also referred to as MPLS sections; see [RFC5586] (Bocci, M., Vigoureux, M., and S. Bryant, “MPLS Generic Associated Channel,” June 2009.) and [RFC5960] (Frost, D., Bryant, S., and M. Bocci, “MPLS Transport Profile Data Plane Architecture,” August 2010.). Pseudowires are bidirectional. Label Switched Paths may be either unidirectional or bidirectional.

The LM and DM protocols discussed in this document are initiated from a single node, the querier. A query message may be received either by a single node or by multiple nodes, depending on the nature of the channel. In the latter case these protocols provide point-to-multipoint measurement capabilities.

2.7.2.  Quality of Service

Quality of Service (QoS) capabilities, in the form of the Differentiated Services architecture, apply to MPLS as specified in [RFC3270] (Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, “Multi-Protocol Label Switching (MPLS) Support of Differentiated Services,” May 2002.) and [RFC5462] (Andersson, L. and R. Asati, “Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field,” February 2009.). Different classes of traffic are distinguished by the three-bit Traffic Class (TC) field of an MPLS Label Stack Entry (LSE). Delay measurement therefore applies on a per-traffic-class basis, and the TC values of LSEs above the G-ACh Label (GAL) that precedes a DM message are significant. Packet loss can be measured with respect either to the channel as a whole or to a specific traffic class.

Another aspect of packet processing which often arises in the context of QoS concerns the location of the measurement points for loss and delay within the sending and receiving nodes, which is implementation-dependent. For example, a sending implementation may or may not consider a packet to be "lost", for LM purposes, that was discarded prior to transmission for queuing-related reasons; conversely, a receiving implementation may or may not consider a packet to be "lost", for LM purposes, if it was physically received but discarded during receive-path processing. The location of delay measurement points similarly impacts what, precisely, is being measured. The principal consideration here is that the behavior of an implementation in these respects SHOULD be made clear to the user.

2.7.3.  Equal Cost Multipath

Equal Cost Multipath (ECMP) is the behavior of distributing packets across multiple alternate paths toward a destination. The use of ECMP in MPLS networks is described in BCP 128 [RFC4928] (Swallow, G., Bryant, S., and L. Andersson, “Avoiding Equal Cost Multipath Treatment in MPLS Networks,” June 2007.). The typical result of ECMP being performed on an LSP which is subject to delay measurement will be that only the delay of one of the available paths is and can be measured.

The effects of ECMP on loss measurement will depend on the LM mode. In the case of direct LM, the measurement will account for any packets lost between the sender and the receiver, regardless of how many paths exist between them. However, the presence of ECMP increases the likelihood of misordering both of LM messages relative to data packets, and of the LM messages themselves. Such misorderings tend to create unmeasurable intervals and thus degrade the accuracy of loss measurement. The effects of ECMP are similar for inferred LM, with the additional caveat that, unless the test packets are specially constructed so as to probe all available paths, the loss characteristics of one or more of the alternate paths cannot be accounted for.

2.7.4.  Intermediate Nodes

In the case of an LSP, it may be desirable to measure the loss or delay to or from an intermediate node as well as between LSP endpoints. This can be done in principle by setting the Time to Live (TTL) field in the outer LSE appropriately when targeting a measurement message to an intermediate node. This procedure may fail, however, if hardware-assisted measurement is in use, because the processing of the packet by the intermediate node occurs only as the result of TTL expiry, and the handling of TTL expiry may occur at a later processing stage in the implementation than the hardware-assisted measurement function. Often the motivation for conducting measurements to intermediate nodes is an attempt to localize a problem that has been detected on the LSP. In this case, if intermediate nodes are not capable of performing hardware-assisted measurement, a less accurate - but usually sufficient - software-based measurement can be conducted instead.

2.7.5.  Distributed Systems

The overview of the bidirectional measurement process presented in Section 2 (Overview) is also applicable when the transmit and receive interfaces at A or B differ from one another. Some additional considerations, however, do apply in this case:

• If different clocks are associated with transmit and receive processing, these clocks must be synchronized in order to compute the two-way delay.
• The DM protocol specified in this document requires that the timestamp formats used by the interfaces that receive a DM query and transmit a DM response agree.
• The LM protocol specified in this document supports both 32-bit and 64-bit counter sizes, but the use of 32-bit counters at any of the up to four interfaces involved in an LM operation will result in 32-bit LM calculations for both directions of the channel.

[Editor's note: The last two restrictions could be relaxed if desired, at the expense of some additional protocol complexity.]

2.7.6.  Loss Measurement Modes

The summary of loss measurement at the beginning of Section 2 (Overview) above made reference to the "count of packets" transmitted and received over a channel. If the counted packets are the packets flowing over the channel in the data plane, the loss measurement is said to operate in "direct mode". If, on the other hand, the counted packets are selected control packets from which the approximate loss characteristics of the channel are being inferred, the loss measurement is said to operate in "inferred mode".

Direct LM has the advantage of being able to provide perfect loss accounting when it is available. There are, however, several limitations associated with direct LM.

For accurate direct LM to occur, packets must not be sent between the time the transmit count for an outbound LM message is determined and the time the message is actually transmitted. Similarly, packets must not be received and processed between the time an LM message is received and the time the receive count for the message is determined. If these "synchronization conditions" do not hold, the LM message counters will not reflect the true state of the data plane, with the result that, for example, the receive count of B may be greater than the transmit count of A, and attempts to compute loss by taking the difference will yield an invalid result. This requirement for synchronization between LM message counters and the data plane may require special support from hardware-based forwarding implementations.

Another limitation of direct LM is that it may be difficult or impossible to apply in cases where the channel is an LSP and the LSP label at the receiver is either nonexistent or fails to identify a unique sending node. The first case happens when Penultimate Hop Popping (PHP) is used on the LSP, and the second case generally holds for LSPs based on the Label Distribution Protocol (LDP) [RFC5036] (Andersson, L., Minei, I., and B. Thomas, “LDP Specification,” October 2007.) as opposed to, for example, those based on Traffic Engineering extensions to the Resource Reservation Protocol (RSVP-TE) [RFC3209] (Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G. Swallow, “RSVP-TE: Extensions to RSVP for LSP Tunnels,” December 2001.). These conditions may make it infeasible for the receiver to identify the data-plane packets associated with a particular source and LSP in order to count them, or to infer the source and LSP context associated with an LM message.

Inferred LM works in the same manner as direct LM except that the counted packets are special control packets, called test messages, generated by the sender. Test messages may be either packets explicitly constructed and used for LM or packets with a different primary purpose, such as those associated with a Bidirectional Forwarding Detection (BFD) [RFC5884] (Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, “Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs),” June 2010.) session.

The synchronization conditions discussed above for direct LM also apply to inferred LM, the only difference being that the required synchronization is now between the LM counters and the test message generation process. Protocol and application designers MUST take these synchronization requirements into account when developing tools for inferred LM, and make their behavior in this regard clear to the user.

Inferred LM provides only an approximate view of the loss level associated with a channel, but is typically applicable even in cases where direct LM is not.

2.7.7.  Loss Measurement Scope

In the case of direct LM, where data-plane packets are counted, there are different possibilities for which kinds of packets are included in the count and which are excluded. The set of packets counted for LM is called the loss measurement scope. As noted above, one factor affecting the LM scope is whether all data packets are counted or only those belonging to a particular traffic class. Another is whether various "auxiliary" flows associated with a data channel are counted, such as packets flowing over the G-ACh. Implementations SHOULD make their supported LM scopes clear to the user, and care must be taken to ensure that the scopes of the channel endpoints agree.

2.7.8.  Delay Measurement Accuracy

The delay measurement procedures described in this document are designed to facilitate hardware-assisted measurement and to function in the same way whether or not such hardware assistance is used. The main difference in the two cases is one of measurement accuracy. Implementations SHOULD make their delay measurement accuracy levels clear to the user.

2.7.9.  Delay Measurement Timestamp Format

There are two significant timestamp formats in common use: the timestamp format of the Internet standard Network Time Protocol (NTP), described in [RFC5905] (Mills, D., Martin, J., Burbank, J., and W. Kasch, “Network Time Protocol Version 4: Protocol and Algorithms Specification,” June 2010.), and the timestamp format used in the IEEE 1588 Precision Time Protocol (PTP) [IEEE1588] (IEEE, “1588-2008 IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” March 2008.).

The NTP format has the advantages of wide use and long deployment in the Internet, and was specifically designed to make the computation of timestamp differences as simple and efficient as possible. On the other hand, there is also now a significant deployment of equipment designed to support the PTP format.

The approach taken in this document is therefore to include in DM messages fields which identify the timestamp formats used by the two devices involved in a DM operation. This implies that a node attempting to carry out a DM operation may be faced with the problem of computing with and possibly reconciling different timestamp formats. Support for multiple timestamp formats is OPTIONAL. An implementation SHOULD, however, make clear which timestamp formats it supports and the extent of its support for computation with and reconciliation of different formats for purposes of delay measurement.

In recognition of the wide deployment, particularly in hardware-based timing implementations, of IEEE 1588 PTP, the PTP timestamp format is the default format used in DM messages. This format MUST be supported.

3.  Message Formats

Loss Measurement and Delay Measurement messages flow over the MPLS Generic Associated Channel (G-ACh) [RFC5586] (Bocci, M., Vigoureux, M., and S. Bryant, “MPLS Generic Associated Channel,” June 2009.). Thus, a packet containing an LM or DM message contains an MPLS label stack, with the G-ACh Label (GAL) at the bottom of the stack. The GAL is followed by an Associated Channel Header (ACH) which identifies the message type, and the message body follows the ACH.

This document defines the following ACH Channel Types:

MPLS Direct Packet Loss Measurement (DLM)
MPLS Inferred Packet Loss Measurement (ILM)
MPLS Packet Delay Measurement (DM)
MPLS Direct Packet Loss and Delay Measurement (DLM+DM)
MPLS Inferred Packet Loss and Delay Measurement (ILM+DM)

The message formats for direct and inferred LM are identical, as are the formats for the DLM+DM and ILM+DM messages.

For these channel types, the ACH SHALL NOT be followed by the ACH TLV Header defined in [RFC5586] (Bocci, M., Vigoureux, M., and S. Bryant, “MPLS Generic Associated Channel,” June 2009.).

The fixed-format portion of a message MAY be followed by a block of Type-Length-Value (TLV) fields. The TLV block provides an extensible way of attaching subsidiary information to LM and DM messages. Several such TLV fields are defined below.

3.1.  Loss Measurement Message Format

The format of a Loss Measurement message, which follows the Associated Channel Header (ACH), is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version| Flags |  Control Code |        Message Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | DFlags|  OTF  |                   Reserved                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Session Identifier                | TC  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Origin Timestamp                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Counter 1                           |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Counter 4                           |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                           TLV Block                           ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved fields MUST be set to 0 and ignored upon receipt. The possible values for the remaining fields are as follows.

The possible values for these fields are as follows.

Version: Currently set to 0.

Flags: The format of the Flags field is shown below.

                            +-+-+-+-+
                            | Flags |
                            +-+-+-+-+

                            +-+-+-+-+
                            |R|T|0|0|
                            +-+-+-+-+

The meanings of the flag bits are:

R: Query/Response indicator. Set to 0 for a Query and 1 for a Response.

T: Traffic-class-specific measurement indicator. Set to 1 when the measurement operation is scoped to packets of a particular traffic class, and 0 otherwise. When set to 1, the TC field of the message indicates the measured traffic class.

0: Set to 0.

Control Code: Set as follows according to whether the message is a Query or a Response as identified by the R flag.

For a Query:

0x0: In-band Response Requested. Indicates that this query has been sent over a bidirectional channel and the response is expected over the same channel.

0x1: Out-of-band Response Requested. Indicates that the response should be sent via an out-of-band channel.

0x2: No Response Requested. Indicates that no response to the query should be sent.

For a Response:

Codes 0x0-0xF are reserved for non-error responses.

0x1: Success. Indicates that the operation was successful.

0x2: Notification - Data Format Invalid. Indicates that the query was processed but the format of the data fields in this response may be inconsistent. Consequently these data fields MUST NOT be used for measurement.

0x3: Notification - Initialization In Progress. Indicates that the query was processed but this response does not contain valid measurement data because the responder's initialization process has not completed.

0x4: Notification - Data Reset Occurred. Indicates that the query was processed but a reset has recently occurred which may render the data in this response inconsistent relative to earlier responses.

0x10: Error - Unspecified Error. Indicates that the operation failed for an unspecified reason.

0x11: Error - Unsupported Version. Indicates that the operation failed because the protocol version supplied in the query message is not supported.

0x12: Error - Unsupported Control Code. Indicates that the operation failed because the Control Code requested an operation that is not available for this channel.

0x13: Error - Unsupported Data Format. Indicates that the operation failed because the data format specified in the query is not supported.

0x14: Error - Authentication Failure. Indicates that the operation failed because the authentication data supplied in the query was missing or incorrect.

0x15: Error - Invalid Destination Node Identifier. Indicates that the operation failed because the Destination Node Identifier supplied in the query is not an identifier of this node.

0x16: Error - Connection Mismatch. Indicates that the operation failed because the channel identifier supplied in the query did not match the channel over which the query was received.

0x17: Error - Unsupported Mandatory TLV Object. Indicates that the operation failed because a TLV Object received in the query and marked as mandatory is not supported.

0x18: Error - Query Rate Exceeded. Indicates that the operation failed because the query message rate exceeded the configured threshold.

0x19: Error - Administrative Block. Indicates that the operation failed because it has been administratively disallowed.

0x1A: Error - Temporary Resource Exhaustion. Indicates that the operation failed because node resources were not available.

Message Length: Set to the total length of this message in bytes.

DFlags: The format of the DFlags field is shown below.

                            +-+-+-+-+
                            | DFlags|
                            +-+-+-+-+

                            +-+-+-+-+
                            |X|B|0|0|
                            +-+-+-+-+

The meanings of the DFlags bits are:

X: Extended counter format indicator. Indicates the use of extended (64-bit) counter values. Initialized to 1 upon creation (and prior to transmission) of an LM Query and copied from an LM Query to an LM response. Set to 0 when the LM message is transmitted or received over an interface that writes 32-bit counter values.

B: Octet (byte) count. When set to 1, indicates that the Counter 1-4 fields represent octet counts. When set to 0, indicates that the Counter 1-4 fields represent packet counts.

0: Set to 0.

Origin Timestamp Format: The format of the Origin Timestamp field, as specified in Section 3.4 (Timestamp Field Formats).

Session Identifier: Set arbitrarily in a query and copied in the response, if any.

TC: When the T flag is set to 1, this field is set to the TC being measured. When the T flag is set to 0, the value of this field is arbitrary, and the field can be considered part of the Session Identifier.

Origin Timestamp: Timestamp recording the transmit time of the query message.

Counter 1-4: Referring to Section 2.1 (Packet Loss Measurement), when a query is sent from A, Counter 1 is set to A_TxP and the other counter fields are set to 0. When the query is received at B, Counter 2 is set to B_RxP. At this point, B copies Counter 1 to Counter 3 and Counter 2 to Counter 4, and re-initializes Counter 1 and Counter 2 to 0. When B transmits the response, Counter 1 is set to B_TxP. When the response is received at A, Counter 2 is set to A_RxP.

The mapping of counter types such as A_TxP to the counter fields 1-4 is designed to ensure that transmit counter values are always written at the same fixed offset in the packet, and likewise for receive counters. This property is important for hardware processing.

All counter values MUST be in network byte order. When a 32-bit counter value is written to one of the counter fields, that value SHALL be written to the low-order 32 bits of the field; the high-order 32 bits of the field MUST, in this case, be set to 0.

TLV Block: Zero or more TLV fields.

3.2.  Delay Measurement Message Format

The format of a Delay Measurement message, which follows the Associated Channel Header (ACH), is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version| Flags |  Control Code |        Message Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  QTF  |  RTF  | RPTF  |              Reserved                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Session Identifier                | TC  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Timestamp 1                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Timestamp 4                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                           TLV Block                           ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The meanings of the fields are summarized in the following table.

Reserved fields MUST be set to 0 and ignored upon receipt. The possible values for the remaining fields are as follows.

Version: Currently set to 0.

Flags: As specified in Section 3.1 (Loss Measurement Message Format), except for the X flag, which is set to 0, and the T flag, which is set to 1.

Control Code: As specified in Section 3.1 (Loss Measurement Message Format).

Message Length: Set to the total length of this message in bytes.

Querier Timestamp Format: The format of the timestamp values written by the querier, as specified in Section 3.4 (Timestamp Field Formats).

Responder Timestamp Format: The format of the timestamp values written by the responder, as specified in Section 3.4 (Timestamp Field Formats).

Responder's Preferred Timestamp Format: The timestamp format preferred by the responder, as specified in Section 3.4 (Timestamp Field Formats).

Session Identifier: As specified in Section 3.1 (Loss Measurement Message Format).

TC: Set to the TC being measured.

Timestamp 1-4: Referring to Section 2.3 (Delay Measurement), when a query is sent from A, Timestamp 1 is set to T1 and the other timestamp fields are set to 0. When the query is received at B, Timestamp 2 is set to T2. At this point, B copies Timestamp 1 to Timestamp 3 and Timestamp 2 to Timestamp 4, and re-initializes Timestamp 1 and Timestamp 2 to 0. When B transmits the response, Timestamp 1 is set to T3. When the response is received at A, Timestamp 2 is set to T4. The actual formats of the timestamp fields written by A and B are indicated by the Querier Timestamp Format and Responder Timestamp Format fields respectively.

The mapping of timestamps to the timestamp fields 1-4 is designed to ensure that transmit timestamps are always written at the same fixed offset in the packet, and likewise for receive timestamps. This property is important for hardware processing.

TLV Block: Zero or more TLV fields.

3.3.  Combined Loss/Delay Measurement Message Format

The format of a combined Loss and Delay Measurement message, which follows the Associated Channel Header (ACH), is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version| Flags |  Control Code |        Message Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | DFlags|  QTF  |  RTF  | RPTF  |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Session Identifier                | TC  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Timestamp 1                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Timestamp 4                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Counter 1                           |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Counter 4                           |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                           TLV Block                           ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The LM/DM message fields have the same meanings as the corresponding fields in the LM and DM message formats.

3.4.  Timestamp Field Formats

The following timestamp format field values are specified in this document:

0x0: Null timestamp format. This value is a placeholder indicating that the timestamp field does not contain a meaningful timestamp.

0x1: Sequence number. This value indicates that the timestamp field is to be viewed as a simple 64-bit sequence number.

0x2: Network Time Protocol version 4 64-bit timestamp format [RFC5905] (Mills, D., Martin, J., Burbank, J., and W. Kasch, “Network Time Protocol Version 4: Protocol and Algorithms Specification,” June 2010.). This format consists of a 32-bit seconds field followed by a 32-bit fractional seconds field, so that it can be regarded as a fixed-point 64-bit quantity.

0x3: IEEE 1588-2002 (1588v1) Precision Time Protocol timestamp format [IEEE1588] (IEEE, “1588-2008 IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” March 2008.). This format consists of a 32-bit seconds field followed by a 32-bit nanoseconds field.

In recognition of the wide deployment, particularly in hardware-based timing implementations, of IEEE 1588 PTP, the PTP timestamp format is the default format used in Delay Measurement messages. This format MUST be supported. Support for other timestamp formats is OPTIONAL.

Timestamp formats of n < 64 bits in size SHALL be encoded in the 64-bit timestamp fields specified in this document using the n high-order bits of the field. The remaining 64 - n low-order bits in the field SHOULD be set to 0 and MUST be ignored when reading the field.

3.5.  TLV Objects

The TLV Block in LM and DM messages consists of zero or more objects with the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     |        Value                  ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Type and Length fields are each 8 bits long, and the Length field indicates the size in bytes of the Value field, which can therefore be up to 255 bytes long.

The Type space is divided into Mandatory and Optional subspaces:

Upon receipt of a query message including an unrecognized mandatory TLV object, the recipient MUST discard the message or respond with an appropriate error code.

The types defined are as follows:

3.5.1.  Padding

The two padding objects permit the augmentation of packet size; this is mainly useful for delay measurement. The type of padding indicates whether the padding supplied by the querier is to be copied to, or omitted from, the response. More than one padding object MAY be present, in which case they SHOULD be continguous. Padding objects SHOULD occur at the end of the TLV Block. The Value field of a padding object is arbitrary.

3.5.2.  Addressing

The addressing objects have the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     | AType |      Reserved         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                           Address                             ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The AType (Address Type) field indicates the type of the address. Address types defined are:

The Source and Destination address objects indicate the addresses of the sender and the intended recipient of the message, respectively. The Source Address SHOULD be used as the destination for out-of-band responses unless some other out-of-band response mechanism has been configured, and unless a Return Address object is present, in which case the Return Address specifies the target of the response.

4.  Operation

4.1.  Loss Measurement Procedures

4.1.1.  Initiating a Loss Measurement Operation

An LM operation for a particular channel consists of sending a sequence (LM[1], LM[2], ...) of LM query messages over the channel at a specific rate and processing the responses received, if any. As described in Section 2.1 (Packet Loss Measurement), the packet loss associated with the channel during the operation is computed as a delta between successive messages; these deltas can be accumulated to obtain a running total of the packet loss for the channel.

The query message transmission rate MUST be sufficiently high, given the LM message counter size (which can be either 32 or 64 bits) and the speed and minimum packet size of the underlying channel, that the ambiguity condition noted in Section 2.1 (Packet Loss Measurement) cannot arise. The implementation SHOULD assume, in evaluating this rate, that the counter size is 32 bits unless explicitly configured otherwise, or unless (in the case of a bidirectional channel) all local and remote interfaces involved in the LM operation are known to be 64-bit-capable, which can be inferred from the value of the X flag in an LM response.

When initiating an LM operation, the far end may require a period of time to become ready for the requested measurement operation. During this period, LM queries MAY simply be discarded, and the querier expecting a response SHOULD be prepared for this situation, for example by setting a timer to differentiate between an acceptable initialization delay and a permanent unavailability condition at the far end. Alternatively, the receiver MAY respond, possibly in a rate-limited manner, to queries received during this period with an appropriate notification code.

4.1.2.  Transmitting a Loss Measurement Query

When transmitting an LM Query over a channel, the Version field MUST be set to 0. The R flag MUST be set to 0. The T flag SHALL be set to 1 if, and only if, the measurement is specific to a particular traffic class, in which case the TC field SHALL identify that traffic class.

The X flag MUST be set to 1 if the transmitting interface writes 64-bit LM counters, and otherwise MUST be set to 0 to indicate that 32-bit counters are written. The B flag SHALL be set to 1 to indicate that the counter fields contain octet counts, or to 0 to indicate packet counts.

The Control Code field MUST be set to one of the values for Query messages listed in Section 3.1 (Loss Measurement Message Format); if the channel is unidirectional, this field MUST NOT be set to 0x0 (Query: in-band response requested).

The Session Identifier field can be set arbitrarily.

The Origin Timestamp field SHOULD be set to the time at which this message is transmitted, and the Origin Timestamp Format field MUST be set to indicate its format, according to Section 3.4 (Timestamp Field Formats).

The Counter 1 field SHOULD be set to the total count of units (packets or octets, according to the B flag) transmitted over the channel prior to this LM Query. The remaining Counter fields MUST be set to 0.

4.1.3.  Receiving a Loss Measurement Query

Upon receipt of an LM Query message, the Counter 2 field SHOULD be set to the total count of units (packets or octets, according to the B flag) received over the channel prior to this LM Query. If the receiving interface writes 32-bit LM counters, the X flag MUST be set to 0.

At this point the LM Query message must be inspected. If the Control Code field is set to 0x2 (no response requested), an LM Response message MUST NOT be transmitted. If the Control Code field is set to 0x0 (in-band response requested) or 0x1 (out-of-band response requested), then an in-band or out-of-band response, respectively, SHOULD be transmitted unless this has been prevented by an administrative, security or congestion control mechanism.

4.1.4.  Transmitting a Loss Measurement Response

When constructing a Response to an LM Query, the Version field MUST be set to 0. The R flag MUST be set to 1. The value of the T flag MUST be copied from the LM Query, and if the value of the T flag is 1, the value of the TC field MUST also be copied.

The X flag MUST be set to 0 if the transmitting interface writes 32-bit LM counters; otherwise its value MUST be copied from the LM Query. The B flag MUST be copied from the LM Query.

The Session Identifier, Origin Timestamp, and Origin Timestamp Format fields MUST be copied from the LM Query. The Counter 1 and Counter 2 fields from the LM Query MUST be copied to the Counter 3 and Counter 4 fields, respectively, of the LM Response.

The Control Code field MUST be set to one of the values for Response messages listed in Section 3.1 (Loss Measurement Message Format). The value 0x10 (Unspecified Error) SHOULD NOT be used if one of the other more specific error codes is applicable.

If the response is transmitted in-band, the Counter 1 field SHOULD be set to the total count of units transmitted over the channel prior to this LM Response. If the response is transmitted out-of-band, the Counter 1 field MUST be set to 0. In either case, the Counter 2 field MUST be set to 0.

4.1.5.  Receiving a Loss Measurement Response

Upon in-band receipt of an LM Response message, the Counter 2 field SHOULD be set to the total count of units received over the channel prior to this LM Response. If the receiving interface writes 32-bit LM counters, the X flag MUST be set to 0.

Upon out-of-band receipt of an LM Response message, the Counter 1 and Counter 2 fields MUST NOT be used for purposes of loss measurement.

If the Control Code in an LM Response is anything other than 0x1 (Success), the counter values in the response MUST NOT be used for purposes of loss measurement. When the Control Code indicates an error condition, the LM operation SHOULD be suspended and an appropriate notification to the user generated. If a temporary error condition is indicated, the LM operation MAY be restarted automatically.

4.1.6.  Loss Calculation

Calculation of packet loss is carried out according to the procedures in Section 2.1 (Packet Loss Measurement). The X flag in an LM message informs the device performing the calculation whether to perform 32-bit or 64-bit arithmetic. If the flag value is equal to 1, all interfaces involved in the LM operation have written 64-bit counter values, and 64-bit arithmetic can be used. If the flag value is equal to 0, at least one interface involved in the operation has written a 32-bit counter value, and 32-bit arithmetic is carried out using the low-order 32 bits of each counter value.

Note that the semantics of the X flag allow all devices to interoperate regardless of their counter size support. Thus, an implementation MUST NOT generate an error response based on the value of this flag.

4.1.7.  Quality of Service

The TC field of the LSE corresponding to the channel (e.g. LSP) being measured SHOULD be set to a traffic class equal to or better than the best TC within the measurement scope to minimize the chance of out-of-order conditions.

4.1.8.  G-ACh Packets

By default, direct LM MUST exclude packets transmitted and received over the Generic Associated Channel (G-ACh). An implementation MAY provide the means to alter the direct LM scope to include some or all G-ACh messages. Care must be taken when altering the LM scope to ensure that both endpoints are in agreement.

4.1.9.  Test Messages

In the case of inferred LM, the packets counted for LM consist of test messages generated for this purpose, or of some other class of packets deemed to provide a good proxy for data packets flowing over the channel. The specification of test protocols and proxy packets is outside the scope of this document.

An identifier common to both the test or proxy messages and the LM messages may be required to make correlation possible. The combined value of the Session Identifier and TC fields SHOULD be used for this purpose when possible. That is, test messages in this case will include a 32-bit field which can carry the value of the combined Session Identifier + TC field present in LM messages. When TC-specific LM is conducted, the TC field of the LSE in the label stack of a test message corresponding to the channel (e.g. LSP) over which the message is sent MUST equal the TC value in the associated LM messages.

4.1.10.  Message Loss and Packet Misorder Conditions

Because an LM operation consists of a message sequence with state maintained from one message to the next, LM is subject to the effects of lost messages and misordered packets in a way that DM is not. Because this state exists only on the querier, the handling of these conditions is, strictly speaking, a local matter. This section, however, presents recommended procedures for handling such conditions.

The first kind of anomaly that may occur is that one or more LM messages may be lost in transit. The effect of such loss is that when an LM Response is next received at the querier, an unambiguous interpretation of the counter values it contains may be impossible, for the reasons described at the end of Section 2.1 (Packet Loss Measurement). Whether this is so depends on the number of messages lost and the other variables mentioned in that section, such as the LM message rate and the channel parameters.

Another possibility is that LM messages are misordered in transit, so that for instance the response to LM[n] is received prior to the response to LM[n-1]. A typical implementation will discard the late response to LM[n-1], so that the effect is the same as the case of a lost message.

Finally, LM is subject to the possibility that data packets are misordered relative to LM messages. This condition can result, for example, in a transmit count of 100 and a corresponding receive count of 101. The effect here is that the A_TxLoss[n-1,n] value (for example) for a given measurement interval will appear to be extremely (if not impossibly) large. The other case, where an LM message arrives earlier than some of the packets, simply results in those packets being counted as lost, which is usually what is desired.

An implementation SHOULD identify a threshold value that indicates the upper bound of lost packets measured in a single computation beyond which the interval is considered unmeasurable. This is called the MaxLMIntervalLoss threshold. It is clear that this threshold should be no higher than the maximum number of packets (or bytes) the channel is capable of transmitting over the interval, but it may be lower. Upon encountering an unmeasurable interval, the LM state (i.e. data values from the last LM message received) SHOULD be discarded.

With regard to lost LM messages, the MaxLMInterval (see Section 2.1 (Packet Loss Measurement)) indicates the maximum amount of time that can elapse before the LM state is discarded. If some messages are lost, but a message is subsequently received within MaxLMInterval, its timestamp or sequence number will quantify the loss, and it MAY still be used for measurement, although the measurement interval will in this case be longer than usual.

If an LM message is received that has a timestamp less than or equal to the timestamp of the last LM message received, this indicates that an exception has occurred, and the current interval SHOULD be considered unmeasurable unless the implementation has some other way of handling this condition.

4.2.  Delay Measurement Procedures

4.2.1.  Transmitting a Delay Measurement Query

When transmitting a DM Query over a channel, the Version and Reserved fields MUST be set to 0. The R flag MUST be set to 0, the T flag MUST be set to 1, and the remaining flag bits MUST be set to 0.

The Control Code field MUST be set to one of the values for Query messages listed in Section 3.1 (Loss Measurement Message Format); if the channel is unidirectional, this field MUST NOT be set to 0x0 (Query: in-band response requested).

The Querier Timestamp Format field MUST be set to the timestamp format used by the querier when writing timestamp fields in this message; the possible values for this field are listed in Section 3.4 (Timestamp Field Formats). The Responder Timestamp Format and Responder's Preferred Timestamp Format fields MUST be set to 0.

The Session Identifier field can be set arbitrarily. The TC field MUST be set to the traffic class being measured.

The Timestamp 1 field SHOULD be set to the time at which this DM Query is transmitted, in the format indicated by the Querier Timestamp Format field. The other timestamp fields MUST be set to 0.

4.2.2.  Receiving a Delay Measurement Query

Upon receipt of a DM Query message, the Timestamp 2 field SHOULD be set to the time at which this DM Query is received.

At this point the DM Query message must be inspected. If the Control Code field is set to 0x2 (no response requested), a DM Response message MUST NOT be transmitted. If the Control Code field is set to 0x0 (in-band response requested) or 0x1 (out-of-band response requested), then an in-band or out-of-band response, respectively, SHOULD be transmitted unless this has been prevented by an administrative, security or congestion control mechanism.

4.2.3.  Transmitting a Delay Measurement Response

When constructing a Response to a DM Query, the Version and Reserved fields MUST be set to 0. The R flag MUST be set to 1, the T flag MUST be set to 1, and the remaining flag bits MUST be set to 0.

The Session Identifier, TC, and Querier Timestamp Format (QTF) fields MUST be copied from the DM Query. The Timestamp 1 and Timestamp 2 fields from the DM Query MUST be copied to the Timestamp 3 and Timestamp 4 fields, respectively, of the DM Response.

The Responder Timestamp Format (RTF) field MUST be set to the timestamp format used by the responder when writing timestamp fields in this message, i.e. Timestamp 4 and (if applicable) Timestamp 1; the possible values for this field are listed in Section 3.4 (Timestamp Field Formats). Furthermore, the RTF field MUST be set equal either to the QTF or the RPTF field. See Section 4.2.5 (Timestamp Format Negotiation) for guidelines on selection of the value for this field.

The Responder's Preferred Timestamp Format (RPTF) field MUST be set to one of the values listed in Section 3.4 (Timestamp Field Formats) and SHOULD be set to indicate the timestamp format with which the responder can provide the best accuracy for purposes of delay measurement.

The Control Code field MUST be set to one of the values for Response messages listed in Section 3.1 (Loss Measurement Message Format). The value 0x10 (Unspecified Error) SHOULD NOT be used if one of the other more specific error codes is applicable.

If the response is transmitted in-band, the Timestamp 1 field SHOULD be set to the time at which this DM Response is transmitted. If the response is transmitted out-of-band, the Timestamp 1 field MUST be set to 0. In either case, the Timestamp 2 field MUST be set to 0.

If the response is transmitted in-band and the Control Code in the message is 0x1 (Success), then the Timestamp 1 and Timestamp 4 fields MUST have the same format, which will be the format indicated in the Responder Timestamp Format field.

4.2.4.  Receiving a Delay Measurement Response

Upon in-band receipt of a DM Response message, the Timestamp 2 field SHOULD be set to the time at which this DM Response is received.

Upon out-of-band receipt of a DM Response message, the Timestamp 1 and Timestamp 2 fields MUST NOT be used for purposes of delay measurement.

If the Control Code in a DM Response is anything other than 0x1 (Success), the timestamp values in the response MUST NOT be used for purposes of delay measurement. When the Control Code indicates an error condition, an appropriate notification to the user SHOULD be generated.

4.2.5.  Timestamp Format Negotiation

In case either the querier or the responder in a DM transaction is capable of supporting multiple timestamp formats, it is desirable to determine the optimal format for purposes of delay measurement on a particular channel. The procedures for making this determination SHALL be as follows.

Upon sending an initial DM Query over a channel, the querier sets the Querier Timestamp Format (QTF) field to its preferred timestamp format.

Upon receiving any DM Query message, the responder determines whether it is capable of writing timestamps in the format specified by the QTF field. If so, the Responder Timestamp Format (RTF) field is set equal to the QTF field. If not, the RTF field is set equal to the Responder's Preferred Timestamp Format (RPTF) field.

The process of changing from one timestamp format to another at the responder may result in the Timestamp 1 and Timestamp 4 fields in an in-band DM Response having different formats. If this is the case, the Control Code in the response MUST NOT be set to 0x1 (Success). Unless an error condition has occurred, the Control Code MUST be set to 0x2 (Notification - Data Format Invalid).

Upon receiving a DM Response, the querier knows from the RTF field in the message whether the responder is capable of supporting its preferred timestamp format: if it is, the RTF will be equal to the QTF. The querier also knows the responder's preferred timestamp format from the RPTF field. The querier can then decide whether to retain its current QTF or to change it and repeat the negotiation procedures.

4.2.5.1.  Single-Format Procedures

When an implementation supports only one timestamp format, the procedures above reduce to the following simple behavior:

• All DM Queries are transmitted with the same QTF;
• All DM Responses are transmitted with the same RTF, and the RPTF is always set equal to the RTF;
• All DM Responses received with RTF not equal to QTF are discarded;
• On a unidirectional channel, all DM Queries received with QTF not equal to the supported format are discarded.

4.2.6.  Quality of Service

The TC field of the LSE corresponding to the channel (e.g. LSP) being measured MUST be set equal to the value of the TC field in the DM message.

4.3.  Combined Loss/Delay Measurement Procedures

The combined LM/DM message defined in Section 3.3 (Combined Loss/Delay Measurement Message Format) allows loss and delay measurement to be carried out simultaneously. This message SHOULD be treated as an LM message which happens to carry additional timestamp data, with the timestamp fields processed as per delay measurement procedures.

5.  Congestion Considerations

An MPLS network may be traffic-engineered in such a way that the bandwidth required both for client traffic and for control, management and OAM traffic is always available. The following congestion considerations therefore apply only when this is not the case.

The proactive generation of Loss Measurement and Delay Measurement messages for purposes of monitoring the performance of an MPLS channel naturally results in a degree of additional load placed on both the network and the terminal nodes of the channel. When configuring such monitoring, operators should be mindful of the overhead involved and should choose transmit rates that do not stress network resources unduly; such choices must be informed by the deployment context. In case of slower links or lower-speed devices, for example, lower Loss Measurement message rates can be chosen, up to the limits noted at the end of Section 2.1 (Packet Loss Measurement).

In general, lower measurement message rates place less load on the network at the expense of reduced granularity. For delay measurement this reduced granularity translates to a greater possibility that the delay associated with a channel temporarily exceeds the expected threshold without detection. For loss measurement, it translates to a larger gap in loss information in case of exceptional circumstances such as lost LM messages or misordered packets.

When carrying out a sustained measurement operation such as an LM operation or continuous pro-active DM operation, the querier SHOULD take note of the number of lost measurement messages (queries for which a response is never received) and set a corresponding Measurement Message Loss Threshold. If this threshold is exceeded, the measurement operation SHOULD be suspended so as not to exacerbate the possible congestion condition. This suspension SHOULD be accompanied by an appropriate notification to the user so that the condition can be investigated and corrected.

From the receiver perspective, the main consideration is the possibility of receiving an excessive quantity of measurement messages. An implementation SHOULD employ a mechanism such as rate-limiting to guard against the effects of this case. Authentication procedures can also be used to ensure that only queries from authorized devices are processed.

6.  Security Considerations

There are three main types of security considerations associated with the exchange of performance monitoring messages such as those described in this document: the possibility of a malicious or misconfigured device generating an excessive quantity of messages, causing service impairment; the possibility of unauthorized alteration of messages in transit; and the possibility of an unauthorized device learning the data contained in or implied by such messages.

The first consideration is discussed in Section 5 (Congestion Considerations). If reception or alteration of performance-related data by unauthorized devices is an operational concern, authentication and/or encryption procedures should be used to ensure message integrity and confidentiality. Such procedures are outside the scope of this document, but have general applicability to OAM protocols in MPLS-TP networks.

7.  IANA Considerations

A future version of this document will detail IANA considerations for:

• ACH Channel Types for LM and DM messages
• Timestamp format registry
• LM and DM Control Codes
• TLV Objects

8.  Acknowledgments

The authors wish to thank the many participants of the MPLS working group who provided detailed review and feedback on this document. The authors offer special thanks to Alexander Vainshtein, Loa Andersson, and Hiroyuki Takagi for many helpful thoughts and discussions, and to Linda Dunbar for the idea of using LM messages for throughput measurement.

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses