|
|
|
|
|
|
IETF RFC 6374
Last modified on Sunday, September 18th, 2011
 Permanent link to RFC 6374
 Search GitHub Wiki for RFC 6374
 Show other RFCs mentioning RFC 6374
Internet Engineering Task Force (IETF) D. Frost
Request for Comments: 6374 S. Bryant
Category: Standards Track Cisco Systems
ISSN: 2070-1721 September 2011
Packet Loss and Delay Measurement for MPLS Networks
 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 measurement capability
also provides operators with greater visibility into the performance
characteristics of their networks, thereby facilitating planning,
troubleshooting, and network performance evaluation. This document
specifies protocol mechanisms to enable the efficient and accurate
measurement of these performance metrics in MPLS networks.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6374.
Copyright Notice
Copyright (c) 2011 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.
Frost & Bryant Standards Track PAGE 1 
RFC 6374 MPLS Loss and Delay Measurement September 2011
Table of Contents
1. Introduction ....................................................3
1.1. Applicability and Scope ....................................5
1.2. Terminology ................................................6
1.3. Requirements Language ......................................6
2. Overview ........................................................6
2.1. Basic Bidirectional Measurement ............................6
2.2. Packet Loss Measurement ....................................7
2.3. Throughput Measurement ....................................10
2.4. Delay Measurement .........................................10
2.5. Delay Variation Measurement ...............................12
2.6. Unidirectional Measurement ................................12
2.7. Dyadic Measurement ........................................13
2.8. Loopback Measurement ......................................13
2.9. Measurement Considerations ................................14
2.9.1. Types of Channels ..................................14
2.9.2. Quality of Service .................................14
2.9.3. Measurement Point Location .........................14
2.9.4. Equal Cost Multipath ...............................15
2.9.5. Intermediate Nodes .................................15
2.9.6. Different Transmit and Receive Interfaces ..........16
2.9.7. External Post-Processing ...........................16
2.9.8. Loss Measurement Modes .............................16
2.9.9. Loss Measurement Scope .............................18
2.9.10. Delay Measurement Accuracy ........................18
2.9.11. Delay Measurement Timestamp Format ................18
3. Message Formats ................................................19
3.1. Loss Measurement Message Format ...........................19
3.2. Delay Measurement Message Format ..........................25
3.3. Combined Loss/Delay Measurement Message Format ............27
3.4. Timestamp Field Formats ...................................28
3.5. TLV Objects ...............................................29
3.5.1. Padding ............................................30
3.5.2. Addressing .........................................31
3.5.3. Loopback Request ...................................31
3.5.4. Session Query Interval .............................32
4. Operation ......................................................33
4.1. Operational Overview ......................................33
4.2. Loss Measurement Procedures ...............................34
4.2.1. Initiating a Loss Measurement Operation ............34
4.2.2. Transmitting a Loss Measurement Query ..............34
4.2.3. Receiving a Loss Measurement Query .................35
4.2.4. Transmitting a Loss Measurement Response ...........35
4.2.5. Receiving a Loss Measurement Response ..............36
4.2.6. Loss Calculation ...................................36
4.2.7. Quality of Service .................................37
4.2.8. G-ACh Packets ......................................37
Frost & Bryant Standards Track PAGE 2
RFC 6374 MPLS Loss and Delay Measurement September 2011
4.2.9. Test Messages ......................................37
4.2.10. Message Loss and Packet Misorder Conditions .......38
4.3. Delay Measurement Procedures ..............................39
4.3.1. Transmitting a Delay Measurement Query .............39
4.3.2. Receiving a Delay Measurement Query ................39
4.3.3. Transmitting a Delay Measurement Response ..........40
4.3.4. Receiving a Delay Measurement Response .............41
4.3.5. Timestamp Format Negotiation .......................41
4.3.5.1. Single-Format Procedures ..................42
4.3.6. Quality of Service .................................42
4.4. Combined Loss/Delay Measurement Procedures ................42
5. Implementation Disclosure Requirements .........................42
6. Congestion Considerations ......................................44
7. Manageability Considerations ...................................44
8. Security Considerations ........................................45
9. IANA Considerations ............................................46
9.1. Allocation of PW Associated Channel Types .................47
9.2. Creation of Measurement Timestamp Type Registry ...........47
9.3. Creation of MPLS Loss/Delay Measurement Control
Code Registry .............................................47
9.4. Creation of MPLS Loss/Delay Measurement TLV Object
Registry ..................................................49
10. Acknowledgments ...............................................49
11. References ....................................................49
11.1. Normative References .....................................49
11.2. Informative References ...................................50
Appendix A. Default Timestamp Format Rationale ....................52
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 measurement capability
also provides operators with greater visibility into the performance
characteristics of their networks, thereby facilitating planning,
troubleshooting, and network performance 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:
o The LM and DM protocols are intended to be simple and to support
efficient hardware processing.
Frost & Bryant Standards Track PAGE 3
RFC 6374 MPLS Loss and Delay Measurement September 2011
o The LM and DM protocols operate over the MPLS Generic Associated
Channel (G-ACh) [RFC 5586] and support measurement of loss, delay,
and related metrics over Label Switched Paths (LSPs), pseudowires,
and MPLS sections (links).
o 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 [RFC 5921], and
MPLS-TP LSPs, pseudowires, and sections are discussed in detail in
[RFC 5960]. A profile describing the minimal functional subset of
the LM and DM protocols in the MPLS-TP context is provided in
[RFC 6375].
o The LM and DM protocols can be used both for continuous/proactive
and selective/on-demand measurement.
o 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.
o The LM and DM protocols use query messages for unidirectional loss
and delay measurement. The measurement can be carried out either
at the downstream node(s) or at the querier if an out-of-band
return path is available.
o The LM and DM protocols do not require that the transmit and
receive interfaces be the same when performing bidirectional
measurement.
o The DM protocol is stateless.
o 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.
o The LM protocol can perform two distinct kinds of loss
measurement: it can measure the loss of specially generated test
messages 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.
Frost & Bryant Standards Track PAGE 4
RFC 6374 MPLS Loss and Delay Measurement September 2011
The direct LM method is also known as "frame-based" in the context
of Ethernet transport networks [Y.1731]. Inferred LM is a
generalization of the "synthetic" measurement approach currently
in development for Ethernet networks, in the sense that it allows
test messages to be decoupled from measurement messages.
o The LM protocol supports measurement in terms of both packet
counts and octet counts.
o The LM protocol supports both 32-bit and 64-bit counters.
o The LM protocol can be used to measure channel throughput as well
as packet loss.
o 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.
o The DM protocol supports varying the measurement message size in
order to measure delays associated with different packet sizes.
The One-Way Active Measurement Protocol (OWAMP) [RFC 4656] and Two-Way
Active Measurement Protocol (TWAMP) [RFC 5357] provide capabilities
for the measurement of various performance metrics in IP networks.
These protocols are not streamlined for hardware processing and rely
on IP and TCP, as well as elements of the Network Time Protocol
(NTP), which may not be available or optimized in some network
environments; they also lack support for IEEE 1588 timestamps and
direct-mode LM, which may be required in some environments. The
protocols defined in this document thus are similar in some respects
to, but also differ from, these IP-based protocols.
1.1. Applicability and Scope
This document specifies measurement procedures and protocol messages
that are intended to be applicable in a wide variety of circumstances
and amenable to implementation by a wide range of hardware- and
software-based measurement systems. As such, it does not attempt to
mandate measurement quality levels or analyze specific end-user
applications.
Frost & Bryant Standards Track PAGE 5
RFC 6374 MPLS Loss and Delay Measurement September 2011
1.2. Terminology
Term Definition
----- -------------------------------------------
ACH Associated Channel Header
DM Delay Measurement
ECMP Equal Cost Multipath
G-ACh Generic Associated Channel
LM Loss Measurement
LSE Label Stack Entry
LSP Label Switched Path
NTP Network Time Protocol
OAM Operations, Administration, and Maintenance
PTP Precision Time Protocol
TC Traffic Class
1.3. 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 [RFC 2119].
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 is discussed that may come into play to
inform or modify these simple procedures. This section is limited to
theoretical discussion; for protocol specifics, the reader is
referred to Sections 3 and 4.
2.1. Basic Bidirectional Measurement
The following figure shows the reference scenario.
T1 T2
+-------+/ Query \+-------+
| | - - - - - - - - ->| |
| A |===================| B |
| |<- - - - - - - - - | |
+-------+\ Response /+-------+
T4 T3
This 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.
Frost & Bryant Standards Track PAGE 6
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
2.2. 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.
Frost & Bryant Standards Track PAGE 7
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 transmits the message 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.
(Strictly speaking, it is not necessary that the fourth count,
A_RxP[n], actually be written in the message, but this is convenient
for some implementations and useful if the message is to be forwarded
on to an external measurement system.)
Frost & Bryant Standards Track PAGE 8
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 they 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. For example,
consider 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 accordingly.
In addition to absolute aggregate loss counts, the individual loss
counts yield other metrics, such as the average loss rate over any
multiple of the measurement interval. An accurate loss rate can be
determined over time even in the presence of anomalies affecting
individual measurements, such as those due to packet misordering
(Section 4.2.10).
Frost & Bryant Standards Track PAGE 9
RFC 6374 MPLS Loss and Delay Measurement September 2011
Note that an approach for conducting packet loss measurement in IP
networks is documented in [RFC 2680]. This approach differs from the
one described here, for example by requiring clock synchronization
between the measurement points and lacking support for direct-mode
LM.
2.3. 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 measurements of the throughput offered and delivered
over the channel during the interval in terms of the counted units.
For a bidirectional channel, for example, given any two LM response
messages (separated in time by not more than the MaxLMInterval), the
difference between the counter values tells the querier the number of
units successfully transmitted and received in the interval between
the timestamps. Absolute offered throughput is the number of data
units transmitted and absolute delivered throughput is the number of
data units received. Throughput rate is the number of data units
sent or received per unit time.
Just as for loss measurement, the interval counts can be accumulated
to arrive at the absolute throughput of the channel since the start
of the measurement operation or be used to derive related metrics
such as the throughput rate. This procedure also enables out-of-
service throughput testing when combined with a simple packet
generator.
2.4. 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:
o The one-way delay associated with the forward (A to B) direction
of the channel;
o The one-way delay associated with the reverse (B to A) direction
of the channel;
o The two-way delay (A to B to A) associated with the channel.
The one-way delay metric for packet networks is described in
[RFC 2679]. In the case of two-way delay, there are actually two
possible metrics of interest. The "two-way channel delay" is the sum
of the one-way delays in each direction and reflects the delay of the
channel itself, irrespective of processing delays within the remote
Frost & Bryant Standards Track PAGE 10
RFC 6374 MPLS Loss and Delay Measurement September 2011
endpoint B. The "round-trip delay" is described in [RFC 2681] and
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 metrics can be
measured directly even when this is not the case (provided A and B
have stable clocks).
A measurement is accomplished by sending a Delay Measurement (DM)
query message over the channel to B that 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 transmits the message 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.
(Strictly speaking, it is not necessary that the fourth timestamp,
T4, actually be written in the message, but this is convenient for
some implementations and useful if the message is to be forwarded on
to an external measurement system.)
At this point, A can compute the two-way channel delay associated
with the channel as
two-way channel delay = (T4 - T1) - (T3 - T2)
and the round-trip delay as
round-trip delay = T4 - T1.
Frost & Bryant Standards Track PAGE 11
RFC 6374 MPLS Loss and Delay Measurement September 2011
If the clocks of A and B are known at A to be synchronized, then both
one-way delay values, as well as the two-way channel delay, can be
computed at A as
forward one-way delay = T2 - T1
reverse one-way delay = T4 - T3
two-way channel delay = forward delay + reverse delay.
Note that this formula for the two-way channel delay reduces to the
one previously given, and clock synchronization is not required to
compute this metric.
2.5. Delay Variation Measurement
Inter-Packet Delay Variation (IPDV) and Packet Delay Variation (PDV)
[RFC 5481] are performance metrics derived from one-way delay
measurement and are important in some applications. IPDV represents
the difference between the one-way delays of successive packets in a
stream. PDV, given a measurement test interval, represents the
difference between the one-way delay of a packet in the interval and
that of the packet in the interval with the minimum delay.
IPDV and PDV measurements can therefore be derived from delay
measurements obtained through the procedures in Section 2.4. An
important point regarding delay variation 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 with one another.
2.6. Unidirectional Measurement
In the case that the channel from A to (B1, ..., Bk) (where B2, ...,
Bk refers to the point-to-multipoint case) 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 used 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, that
is:
B_RxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] - B_RxP[n-1])
Frost & Bryant Standards Track PAGE 12
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
2.7. Dyadic Measurement
The basic procedures for bidirectional measurement assume that the
measurement process is conducted by and for the querier node A.
Instead, it is possible, with only minor variation of these
procedures, to conduct a dyadic or "dual-ended" measurement process
in which both nodes A and B perform loss or delay measurement based
on the same message flow. This is achieved by stipulating that A
copy the third and fourth counter or timestamp values from a response
message into the third and fourth slots of the next query, which are
otherwise unused, thereby providing B with equivalent information to
that learned by A.
The dyadic procedure has the advantage of halving the number of
messages required for both A and B to perform a given kind of
measurement, but comes at the expense of each node's ability to
control its own measurement process independently, and introduces
additional operational complexity into the measurement protocols.
The quantity of measurement traffic is also expected to be low
relative to that of user traffic, particularly when 64-bit counters
are used for LM. Consequently, this document does not specify a
dyadic operational mode. However, it is still possible, and may be
useful, for A to perform the extra copy, thereby providing additional
information to B even when its participation in the measurement
process is passive.
2.8. Loopback Measurement
Some bidirectional channels may be placed into a loopback state such
that messages are looped back to the sender without modification. In
this situation, LM and DM procedures can be used to carry out
measurements associated with the circular path. This is done by
generating "queries" with the Response flag set to 1.
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])
Frost & Bryant Standards Track PAGE 13
RFC 6374 MPLS Loss and Delay Measurement September 2011
For DM, the round-trip 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.
2.9. Measurement Considerations
A number of additional considerations apply in practice to the
measurement methods summarized above.
2.9.1. 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 [RFC 5586].
Broadly, a channel in an MPLS network may be either a link, a Label
Switched Path (LSP) [RFC 3031], or a pseudowire [RFC 3985]. Links are
bidirectional and are also referred to as MPLS sections; see
[RFC 5586] and [RFC 5960]. 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.
2.9.2. Quality of Service
Quality of Service (QoS) capabilities, in the form of the
Differentiated Services architecture, apply to MPLS as specified in
[RFC 3270] and [RFC 5462]. Different classes of traffic are
distinguished by the three-bit Traffic Class (TC) field of an MPLS
Label Stack Entry (LSE). Delay measurement 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.
2.9.3. Measurement Point Location
The location of the measurement points for loss and delay within the
sending and receiving nodes is implementation dependent but directly
affects the nature of the measurements. 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-
Frost & Bryant Standards Track PAGE 14
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 determines what,
precisely, is being measured. The principal consideration here is
that the behavior of an implementation in these respects MUST be made
clear to the user.
2.9.4. 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 [RFC 4928]. The typical
result of ECMP being performed on an LSP that 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.
2.9.5. 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. The motivation for conducting
measurements to intermediate nodes is often 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.
Frost & Bryant Standards Track PAGE 15
RFC 6374 MPLS Loss and Delay Measurement September 2011
2.9.6. Different Transmit and Receive Interfaces
The overview of the bidirectional measurement process presented in
Section 2 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:
o If different clocks are associated with transmit and receive
processing, these clocks must be synchronized in order to compute
the two-way delay.
o 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.
o 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.
2.9.7. External Post-Processing
In some circumstances, it may be desirable to carry out the final
measurement computation at an external post-processing device
dedicated to the purpose. This can be achieved in supporting
implementations by, for example, configuring the querier, in the case
of a bidirectional measurement session, to forward each response it
receives to the post-processor via any convenient protocol. The
unidirectional case can be handled similarly through configuration of
the receiver or by including an instruction in query messages for the
receiver to respond out-of-band to the appropriate return address.
Post-processing devices may have the ability to store measurement
data for an extended period and to generate a variety of useful
statistics from them. External post-processing also allows the
measurement process to be completely stateless at the querier and
responder.
2.9.8. Loss Measurement Modes
The summary of loss measurement at the beginning of Section 2 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".
Frost & Bryant Standards Track PAGE 16
RFC 6374 MPLS Loss and Delay Measurement September 2011
Direct LM has the advantage of being able to provide perfect loss
accounting when it is available. There are, however, several
constraints 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.
A 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) [RFC 5036] as
opposed to, for example, those based on Traffic Engineering
extensions to the Resource Reservation Protocol (RSVP-TE) [RFC 3209].
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. Direct LM is also vulnerable to
disruption in the event that the ingress or egress interface
associated with an LSP changes during the LSP's lifetime.
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) [RFC 5884] 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.
Frost & Bryant Standards Track PAGE 17
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
2.9.9. 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
MUST make their supported LM scopes clear to the user, and care must
be taken to ensure that the scopes of the channel endpoints agree.
2.9.10. 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
measurement accuracy will be determined by how closely the transmit
and receive timestamps correspond to actual packet departure and
arrival times.
As noted in Section 2.4, measurement of one-way delay requires clock
synchronization between the devices involved, while two-way delay
measurement does not involve direct comparison between non-local
timestamps and thus has no synchronization requirement. The
measurement accuracy will be limited by the quality of the local
clock and, in the case of one-way delay measurement, by the quality
of the synchronization.
2.9.11. Delay Measurement Timestamp Format
There are two significant timestamp formats in common use: the
timestamp format of the Network Time Protocol (NTP), described in
[RFC 5905], and the timestamp format used in the IEEE 1588 Precision
Time Protocol (PTP) [IEEE1588].
The NTP format has the advantages of wide use and long deployment in
the Internet, and it was specifically designed to make the
computation of timestamp differences as simple and efficient as
possible. On the other hand, there is now also a significant
deployment of equipment designed to support the PTP format.
Frost & Bryant Standards Track PAGE 18
RFC 6374 MPLS Loss and Delay Measurement September 2011
The approach taken in this document is therefore to include in DM
messages fields that 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. To ensure interoperability, it is necessary that support of
at least one timestamp format is mandatory. This specification
requires the support of the IEEE 1588 PTP format. Timestamp format
support requirements are discussed in detail in Section 3.4.
3. Message Formats
Loss Measurement and Delay Measurement messages flow over the MPLS
Generic Associated Channel (G-ACh) [RFC 5586]. 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 Loss Measurement (DLM)
MPLS Inferred Loss Measurement (ILM)
MPLS Delay Measurement (DM)
MPLS Direct Loss and Delay Measurement (DLM+DM)
MPLS Inferred Loss and Delay Measurement (ILM+DM)
The message formats for direct and inferred LM are identical. The
formats of the DLM+DM and ILM+DM messages are also identical.
For these channel types, the ACH SHALL NOT be followed by the ACH TLV
Header defined in [RFC 5586].
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.
All integer values for fields defined in this document SHALL be
encoded in network byte order.
3.1. Loss Measurement Message Format
The format of a Loss Measurement message, which follows the
Associated Channel Header (ACH), is as follows:
Frost & Bryant Standards Track PAGE 19
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 | DS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Origin Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter 1 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter 4 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TLV Block ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Loss Measurement Message Format
Reserved fields MUST be set to 0 and ignored upon receipt. The
possible values for the remaining fields are as follows.
Frost & Bryant Standards Track PAGE 20
RFC 6374 MPLS Loss and Delay Measurement September 2011
Field Meaning
--------------------- -----------------------------------------------
Version Protocol version
Flags Message control flags
Control Code Code identifying the query or response type
Message Length Total length of this message in bytes
Data Format Flags Flags specifying the format of message data
(DFlags)
Origin Timestamp Format of the Origin Timestamp field
Format (OTF)
Reserved Reserved for future specification
Session Identifier Set arbitrarily by the querier
Differentiated Differentiated Services Code Point (DSCP) being
Services (DS) Field measured
Origin Timestamp 64-bit field for query message transmission
timestamp
Counter 1-4 64-bit fields for LM counter values
TLV Block Optional block of Type-Length-Value fields
The possible values for these fields are as follows.
Version: Currently set to 0.
Flags: The format of the Flags field is shown below.
+-+-+-+-+
|R|T|0|0|
+-+-+-+-+
Loss Measurement Message Flags
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 (DSCP value), and 0 otherwise. When set to 1, the
DS 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.
Frost & Bryant Standards Track PAGE 21
RFC 6374 MPLS Loss and Delay Measurement September 2011
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. This mode can be used, for example, if
all nodes involved are being controlled by a Network Management
System.
For a Response:
Codes 0x0-0xF are reserved for non-error responses. Error
response codes imply that the response does not contain valid
measurement data.
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 that may
render the data in this response inconsistent relative to
earlier responses.
0x5: Notification - Resource Temporarily Unavailable.
Indicates that the query was processed, but resources were
unavailable to complete the requested measurement and that,
consequently, this response does not contain valid measurement
data.
0x10: Error - Unspecified Error. Indicates that the operation
failed for an unspecified reason.
Frost & Bryant Standards Track PAGE 22
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 - Unsupported Query Interval. 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 - Resource Unavailable. Indicates that the
operation failed because node resources were not available.
0x1B: Error - Resource Released. Indicates that the operation
failed because node resources for this measurement session were
administratively released.
0x1C: Error - Invalid Message. Indicates that the operation
failed because the received query message was malformed.
Frost & Bryant Standards Track PAGE 23
RFC 6374 MPLS Loss and Delay Measurement September 2011
0x1D: Error - Protocol Error. Indicates that the operation
failed because a protocol error was found in the received query
message.
Message Length: Set to the total length of this message in bytes,
including the Version, Flags, Control Code, and Message Length fields
as well as the TLV Block, if any.
DFlags: The format of the DFlags field is shown below.
+-+-+-+-+
|X|B|0|0|
+-+-+-+-+
Data Format Flags
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. The octet count applies to all
packets within the LM scope (Section 2.9.9), and the octet count
of a packet sent or received over a channel includes the total
length of that packet (but excludes headers, labels, or framing of
the channel itself). 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.
Session Identifier: Set arbitrarily in a query and copied in the
response, if any. This field uniquely identifies a measurement
operation (also called a session) that consists of a sequence of
messages. All messages in the sequence have the same Session
Identifier.
DS: When the T flag is set to 1, this field is set to the DSCP value
[RFC 3260] that corresponds to the traffic class being measured. For
MPLS, where the traffic class of a channel is identified by the
three-bit Traffic Class in the channel's LSE [RFC 5462], this field
Frost & Bryant Standards Track PAGE 24
RFC 6374 MPLS Loss and Delay Measurement September 2011
SHOULD be set to the Class Selector Codepoint [RFC 2474] that
corresponds to that Traffic Class. 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 2.2, 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 1-4 fields
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 may be important for hardware processing.
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:
Frost & Bryant Standards Track PAGE 25
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 | DS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp 1 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp 4 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TLV Block ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Delay Measurement Message Format
The meanings of the fields are summarized in the following table.
Field Meaning
--------------------- -----------------------------------------------
Version Protocol version
Flags Message control flags
Control Code Code identifying the query or response type
Message Length Total length of this message in bytes
QTF Querier timestamp format
RTF Responder timestamp format
RPTF Responder's preferred timestamp format
Reserved Reserved for future specification
Session Identifier Set arbitrarily by the querier
Differentiated Differentiated Services Code Point (DSCP) being
Services (DS) Field measured
Timestamp 1-4 64-bit timestamp values
TLV Block Optional block of Type-Length-Value fields
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.
Frost & Bryant Standards Track PAGE 26
RFC 6374 MPLS Loss and Delay Measurement September 2011
Flags: As specified in Section 3.1. The T flag in a DM message is
set to 1.
Control Code: As specified in Section 3.1.
Message Length: Set to the total length of this message in bytes,
including the Version, Flags, Control Code, and Message Length fields
as well as the TLV Block, if any.
Querier Timestamp Format: The format of the timestamp values written
by the querier, as specified in Section 3.4.
Responder Timestamp Format: The format of the timestamp values
written by the responder, as specified in Section 3.4.
Responder's Preferred Timestamp Format: The timestamp format
preferred by the responder, as specified in Section 3.4.
Session Identifier: As specified in Section 3.1.
DS: As specified in Section 3.1.
Timestamp 1-4: Referring to Section 2.4, 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 1-4 fields 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:
Frost & Bryant Standards Track PAGE 27
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 | DS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp 1 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp 4 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter 1 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter 4 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TLV Block ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Loss/Delay Measurement Message Format
The fields of this message have the same meanings as the
corresponding fields in the LM and DM message formats, except that
the roles of the OTF and Origin Timestamp fields for LM are here
played by the QTF and Timestamp 1 fields, respectively.
3.4. Timestamp Field Formats
The following timestamp format field values are specified in this
document:
0: Null timestamp format. This value is a placeholder indicating
that the timestamp field does not contain a meaningful timestamp.
Frost & Bryant Standards Track PAGE 28
RFC 6374 MPLS Loss and Delay Measurement September 2011
1: Sequence number. This value indicates that the timestamp field
is to be viewed as a simple 64-bit sequence number. This provides
a simple solution for applications that do not require a real
absolute timestamp, but only an indication of message ordering; an
example is LM exception detection.
2: Network Time Protocol version 4 64-bit timestamp format
[RFC 5905]. 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.
3: Low-order 64 bits of the IEEE 1588-2008 (1588v2) Precision Time
Protocol timestamp format [IEEE1588]. This truncated format
consists of a 32-bit seconds field followed by a 32-bit
nanoseconds field, and is the same as the IEEE 1588v1 timestamp
format.
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.
To ensure that it is possible to find an interoperable mode between
implementations, it is necessary to select one timestamp format as
the default. The timestamp format chosen as the default is the
truncated IEEE 1588 PTP format (format code 3 in the list above);
this format MUST be supported. The rationale for this choice is
discussed in Appendix A. Implementations SHOULD also be capable of
reading timestamps written in NTPv4 64-bit format and reconciling
them internally with PTP timestamps for measurement purposes.
Support for other timestamp formats is OPTIONAL.
The implementation MUST make clear which timestamp formats it
supports and the extent of its support for computation with and
reconciliation of different formats for measurement purposes.
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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Format
Frost & Bryant Standards Track PAGE 29
RFC 6374 MPLS Loss and Delay Measurement September 2011
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:
Type Range Semantics
-------------- ---------
0-127 Mandatory
128-255 Optional
Upon receipt of a query message including an unrecognized mandatory
TLV object, the recipient MUST respond with an Unsupported Mandatory
TLV Object error code.
The types defined are as follows:
Type Definition
-------------- ---------------------------------
Mandatory
0 Padding - copy in response
1 Return Address
2 Session Query Interval
3 Loopback Request
4-126 Unallocated
127 Experimental use
Optional
128 Padding - do not copy in response
129 Destination Address
130 Source Address
131-254 Unallocated
255 Experimental use
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. Asymmetrical padding may be
useful when responses are delivered out-of-band or when different
maximum transmission unit sizes apply to the two components of a
bidirectional channel.
More than one padding object MAY be present, in which case they MUST
be contiguous. The Value field of a padding object is arbitrary.
Frost & Bryant Standards Track PAGE 30
RFC 6374 MPLS Loss and Delay Measurement September 2011
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 | Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Addressing Object Format
The Address Family field indicates the type of the address, and it
SHALL be set to one of the assigned values in the "IANA Address
Family Numbers" registry.
The Source and Destination Address objects indicate the addresses of
the sender and the intended recipient of the message, respectively.
The Source Address of a query message SHOULD be used as the
destination for an out-of-band response 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. The Return Address object MUST NOT appear in
a response.
3.5.3. Loopback Request
The Loopback Request object, when included in a query, indicates a
request that the query message be returned to the sender unmodified.
This object has a Length of 0.
Upon receiving the reflected query message back from the responder,
the querier MUST NOT retransmit the message. Information that
uniquely identifies the original query source, such as a Source
Address object, can be included to enable the querier to
differentiate one of its own loopback queries from a loopback query
initiated by the far end.
This object may be useful, for example, when the querier is
interested only in the round-trip delay metric. In this case, no
support for delay measurement is required at the responder at all,
other than the ability to recognize a DM query that includes this
object and return it unmodified.
Frost & Bryant Standards Track PAGE 31
RFC 6374 MPLS Loss and Delay Measurement September 2011
3.5.4. Session Query Interval
The Value field of the Session Query Interval object is a 32-bit
unsigned integer that specifies a time interval in milliseconds.
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 | Session Query >
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
< Interval (ms) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Session Query Interval Object Format
This time interval indicates the interval between successive query
messages in a specific measurement session. The purpose of the
Session Query Interval (SQI) object is to enable the querier and
responder of a measurement session to agree on a query rate. The
procedures for handling this object SHALL be as follows:
1. The querier notifies the responder that it wishes to be informed
of the responder's minimum query interval for this session by
including the SQI object in its query messages, with a Value of
0.
2. When the responder receives a query that includes an SQI object
with a Value of 0, the responder includes an SQI object in the
response with the Value set to the minimum query interval it
supports for this session.
3. When the querier receives a response that includes an SQI object,
it selects a query interval for the session that is greater than
or equal to the Value specified in the SQI object and adjusts its
query transmission rate accordingly, including in each subsequent
query an SQI object with a Value equal to the selected query
interval. Once a response to one of these subsequent queries has
been received, the querier infers that the responder has been
apprised of the selected query interval and MAY then stop
including the SQI object in queries associated with this session.
Similar procedures allow the query rate to be changed during the
course of the session by either the querier or the responder. For
example, to inform the querier of a change in the minimum supported
query interval, the responder begins including a corresponding SQI
object in its responses, and the querier adjusts its query rate if
necessary and includes a corresponding SQI object in its queries
until a response is received.
Frost & Bryant Standards Track PAGE 32
RFC 6374 MPLS Loss and Delay Measurement September 2011
Shorter query intervals (i.e., higher query rates) provide finer
measurement granularity at the expense of additional load on
measurement endpoints and the network; see Section 6 for further
discussion.
4. Operation
4.1. Operational Overview
A loss or delay measurement operation, also called a session, is
controlled by the querier and consists of a sequence of query
messages associated with a particular channel and a common set of
measurement parameters. If the session parameters include a response
request, then the receiving node or nodes will (under normal
conditions) generate a response message for each query message
received, and these responses are also considered part of the
session. All query and response messages in a session carry a common
session identifier.
Measurement sessions are initiated at the discretion of the network
operator and are terminated either at the operator's request or as
the result of an error condition. A session may be as brief as a
single message exchange, for example when a DM query is used by the
operator to "ping" a remote node, or it may extend throughout the
lifetime of the channel.
When a session is initiated for which responses are requested, the
querier SHOULD initialize a timer, called the SessionResponseTimeout,
that indicates how long the querier will wait for a response before
abandoning the session and notifying the user that a timeout has
occurred. This timer persists for the lifetime of the session and is
reset each time a response message for the session is received.
When a query message is received that requests a response, a variety
of exceptional conditions may arise that prevent the responder from
generating a response that contains valid measurement data. Such
conditions fall broadly into two classes: transient exceptions from
which recovery is possible and fatal exceptions that require
termination of the session. When an exception arises, the responder
SHOULD generate a response with an appropriate Notification or Error
control code according to whether the exception is, respectively,
transient or fatal. When the querier receives an Error response, the
session MUST be terminated and the user informed.
A common example of a transient exception occurs when a new session
is initiated and the responder requires a period of time to become
ready before it can begin providing useful responses. The response
control code corresponding to this situation is Notification -
Frost & Bryant Standards Track PAGE 33
RFC 6374 MPLS Loss and Delay Measurement September 2011
Initialization in Progress. Typical examples of fatal exceptions are
cases where the querier has requested a type of measurement that the
responder does not support or where a query message is malformed.
When initiating a session, the querier SHOULD employ the Session
Query Interval mechanism (Section 3.5.4) to establish a mutually
agreeable query rate with the responder. Responders SHOULD employ
rate-limiting mechanisms to guard against the possibility of
receiving an excessive quantity of query messages.
4.2. Loss Measurement Procedures
4.2.1. 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 2.2, 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 or be used to derive related
metrics such as the average loss rate.
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 2.2 cannot arise. In evaluating
this rate, the implementation SHOULD assume 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.
4.2.2. Transmitting a Loss Measurement Query
When transmitting an LM Query, 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 DS 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.
Frost & Bryant Standards Track PAGE 34
RFC 6374 MPLS Loss and Delay Measurement September 2011
The Control Code field MUST be set to one of the values for Query
messages listed in Section 3.1; 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 SHALL 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.
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, or to 0 if this is the beginning of a
measurement session for which counter data is not yet available. The
Counter 2 field MUST be set to 0. If a response was previously
received in this measurement session, the Counter 1 and Counter 2
fields of the most recent such response MAY be copied to the Counter
3 and Counter 4 fields, respectively, of this query; otherwise, the
Counter 3 and Counter 4 fields MUST be set to 0.
4.2.3. 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.
In the case of a fatal exception that prevents the requested
measurement from being made, the error SHOULD be reported, via either
a response, if one was requested, or else as a notification to the
user.
4.2.4. 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.
Frost & Bryant Standards Track PAGE 35
RFC 6374 MPLS Loss and Delay Measurement September 2011
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. 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.
4.2.5. Receiving a Loss Measurement Response
Upon in-band receipt of an LM Response message, the Counter 2 field
is 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 is set to 0. (Since the life of the LM message
in the network has ended at this point, it is up to the receiver
whether these final modifications are made to the packet. If the
message is to be forwarded on for external post-processing
(Section 2.9.7), then these modifications MUST be made.)
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. If the Control Code indicates an error
condition, or if the response message is invalid, the LM operation
MUST be terminated and an appropriate notification to the user
generated.
4.2.6. Loss Calculation
Calculation of packet loss is carried out according to the procedures
in Section 2.2. 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
Frost & Bryant Standards Track PAGE 36
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
4.2.7. 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.
4.2.8. 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.
4.2.9. 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, but some guidelines are
discussed below.
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 DS fields SHOULD be used for this
purpose when possible. That is, test messages in this case will
include a 32-bit field that can carry the value of the combined
Session Identifier + DS field present in LM messages. When TC-
specific LM is conducted, the DS 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 correspond to the DS value in the associated
LM messages.
A separate test message protocol SHOULD include a timeout value in
its messages that informs the responder when to discard any state
associated with a specific test.
Frost & Bryant Standards Track PAGE 37
RFC 6374 MPLS Loss and Delay Measurement September 2011
4.2.10. 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. Note that in the absence of ECMP, packet misordering
within a traffic class is a relatively rare event.
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 2.2. 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.
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 2.2)
indicates the maximum amount of time that can elapse before the LM
state is discarded. If some messages are lost, but a message is
Frost & Bryant Standards Track PAGE 38
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
4.3. Delay Measurement Procedures
4.3.1. Transmitting a Delay Measurement Query
When transmitting a DM Query, 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; 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. 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 DS 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 Timestamp 2 field MUST be set to 0. If
a response was previously received in this measurement session, the
Timestamp 1 and Timestamp 2 fields of the most recent such response
MAY be copied to the Timestamp 3 and Timestamp 4 fields,
respectively, of this query; otherwise, the Timestamp 3 and Timestamp
4 fields MUST be set to 0.
4.3.2. 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 was received.
Frost & Bryant Standards Track PAGE 39
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
In the case of a fatal exception that prevents the requested
measurement from being made, the error SHOULD be reported, via either
a response, if one was requested, or else as a notification to the
user.
4.3.3. 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 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.
Furthermore, the RTF field MUST be set equal to either the QTF or the
RPTF field. See Section 4.3.5 for guidelines on the 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 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. 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.
Frost & Bryant Standards Track PAGE 40
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
4.3.4. Receiving a Delay Measurement Response
Upon in-band receipt of a DM Response message, the Timestamp 2 field
is set to the time at which this DM Response was received. (Since
the life of the DM message in the network has ended at this point, it
is up to the receiver whether this final modification is made to the
packet. If the message is to be forwarded on for external post-
processing (Section 2.9.7), then these modifications MUST be made.)
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. If the Control Code indicates an
error condition, or if the response message is invalid, the DM
operation MUST be terminated and an appropriate notification to the
user generated.
4.3.5. 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,
Frost & Bryant Standards Track PAGE 41
RFC 6374 MPLS Loss and Delay Measurement September 2011
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.
4.3.5.1. Single-Format Procedures
When an implementation supports only one timestamp format, the
procedures above reduce to the following simple behavior:
o All DM Queries are transmitted with the same QTF;
o All DM Responses are transmitted with the same RTF, and the RPTF
is always set equal to the RTF;
o All DM Responses received with RTF not equal to QTF are discarded;
o On a unidirectional channel, all DM Queries received with QTF not
equal to the supported format are discarded.
4.3.6. Quality of Service
The TC field of the LSE corresponding to the channel (e.g., LSP)
being measured MUST be set to the value that corresponds to the DS
field in the DM message.
4.4. Combined Loss/Delay Measurement Procedures
The combined LM/DM message defined in Section 3.3 allows loss and
delay measurement to be carried out simultaneously. This message
SHOULD be treated as an LM message that happens to carry additional
timestamp data, with the timestamp fields processed as per delay
measurement procedures.
5. Implementation Disclosure Requirements
This section summarizes the requirements placed on implementations
for capabilities disclosure. The purpose of these requirements is to
ensure that end users have a clear understanding of implementation
Frost & Bryant Standards Track PAGE 42
RFC 6374 MPLS Loss and Delay Measurement September 2011
capabilities and characteristics that have a direct impact on how
loss and delay measurement mechanisms function in specific
situations. Implementations are REQUIRED to state:
o METRICS: Which of the following metrics are supported: packet
loss, packet throughput, octet loss, octet throughput, average
loss rate, one-way delay, round-trip delay, two-way channel delay,
packet delay variation.
o MP-LOCATION: The location of loss and delay measurement points
with respect to other stages of packet processing, such as
queuing.
o CHANNEL-TYPES: The types of channels for which LM and DM are
supported, including LSP types, pseudowires, and sections (links).
o QUERY-RATE: The minimum supported query intervals for LM and DM
sessions, both in the querier and responder roles.
o LOOP: Whether loopback measurement (Section 2.8) is supported.
o LM-TYPES: Whether direct or inferred LM is supported, and for the
latter, which test protocols or proxy message types are supported.
o LM-COUNTERS: Whether 64-bit counters are supported.
o LM-ACCURACY: The expected measurement accuracy levels for the
supported forms of LM, and the expected impact of exception
conditions such as lost and misordered messages.
o LM-SYNC: The implementation's behavior in regard to the
synchronization conditions discussed in Section 2.9.8.
o LM-SCOPE: The supported LM scopes (Sections 2.9.9 and 4.2.8).
o DM-ACCURACY: The expected measurement accuracy levels for the
supported forms of DM.
o DM-TS-FORMATS: The supported timestamp formats and the extent of
support for computation with and reconciliation of different
formats.
Frost & Bryant Standards Track PAGE 43
RFC 6374 MPLS Loss and Delay Measurement September 2011
6. 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 2.2.
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 proactive 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.
7. Manageability Considerations
The measurement protocols described in this document are intended to
serve as infrastructure to support a wide range of higher-level
monitoring and diagnostic applications, from simple command-line
Frost & Bryant Standards Track PAGE 44
RFC 6374 MPLS Loss and Delay Measurement September 2011
diagnostic tools to comprehensive network performance monitoring and
analysis packages. The specific mechanisms and considerations for
protocol configuration, initialization, and reporting thus depend on
the nature of the application.
In the case of on-demand diagnostics, the diagnostic application may
provide parameters such as the measurement type, the channel, the
query rate, and the test duration when initiating the diagnostic;
results and exception conditions are then reported directly to the
application. The system may discard the statistics accumulated
during the test after the results have been reported or retain them
to provide a historical measurement record.
Alternatively, measurement configuration may be supplied as part of
the channel configuration itself in order to support continuous
monitoring of the channel's performance characteristics. In this
case, the configuration will typically include quality thresholds
depending on the service level agreement, the crossing of which will
trigger warnings or alarms, and result reporting and exception
notification will be integrated into the system-wide network
management and reporting framework.
8. Security Considerations
This document describes procedures for the measurement of performance
metrics over a pre-existing MPLS path (a pseudowire, LSP, or
section). As such, it assumes that a node involved in a measurement
operation has previously verified the integrity of the path and the
identity of the far end using existing MPLS mechanisms such as
Bidirectional Forwarding Detection (BFD) [RFC 5884]; tools,
techniques, and considerations for securing MPLS paths are discussed
in detail in [RFC 5920].
When such mechanisms are not available, and where security of the
measurement operation is a concern, reception of Generic Associated
Channel messages with the Channel Types specified in this document
SHOULD be disabled. Implementations MUST provide the ability to
disable these protocols on a per-Channel-Type basis.
Even when the identity of the far end has been verified, the
measurement protocols remain vulnerable to injection and man-in-the-
middle attacks. The impact of such an attack would be to compromise
the quality of performance measurements on the affected path. An
attacker positioned to disrupt these measurements is, however,
capable of causing much greater damage by disrupting far more
critical elements of the network such as the network control plane or
user traffic flows. At worst, a disruption of the measurement
protocols would interfere with the monitoring of the performance
Frost & Bryant Standards Track PAGE 45
RFC 6374 MPLS Loss and Delay Measurement September 2011
aspects of the service level agreement associated with the path; the
existence of such a disruption would imply that a serious breach of
basic path integrity had already occurred.
If desired, such attacks can be mitigated by performing basic
validation and sanity checks, at the querier, of the counter or
timestamp fields in received measurement response messages. The
minimal state associated with these protocols also limits the extent
of measurement disruption that can be caused by a corrupt or invalid
message to a single query/response cycle.
Cryptographic mechanisms capable of signing or encrypting the
contents of the measurement packets without degrading the measurement
performance are not currently available. In light of the preceding
discussion, the absence of such cryptographic mechanisms does not
raise significant security issues.
Users concerned with the security of out-of-band responses over IP
networks SHOULD employ suitable security mechanisms such as IPsec
[RFC 4301] to protect the integrity of the return path.
9. IANA Considerations
Per this document, IANA has completed the following actions:
o Allocation of Channel Types in the "PW Associated Channel Type"
registry
o Creation of a "Measurement Timestamp Type" registry
o Creation of an "MPLS Loss/Delay Measurement Control Code" registry
o Creation of an "MPLS Loss/Delay Measurement Type-Length-Value
(TLV) Object" registry
Frost & Bryant Standards Track PAGE 46
RFC 6374 MPLS Loss and Delay Measurement September 2011
9.1. Allocation of PW Associated Channel Types
As per the IANA considerations in [RFC 5586], IANA has allocated the
following Channel Types in the "PW Associated Channel Type" registry:
Value Description TLV Follows Reference
------ ---------------------------------------- ----------- ---------
0x000A MPLS Direct Loss Measurement (DLM) No RFC 6374
0x000B MPLS Inferred Loss Measurement (ILM) No RFC 6374
0x000C MPLS Delay Measurement (DM) No RFC 6374
0x000D MPLS Direct Loss and Delay Measurement No RFC 6374
(DLM+DM)
0x000E MPLS Inferred Loss and Delay Measurement No RFC 6374
(ILM+DM)
9.2. Creation of Measurement Timestamp Type Registry
IANA has created a new "Measurement Timestamp Type" registry, with
format and initial allocations as follows:
Type Description Size in Bits Reference
---- ----------------------------------------- ------------ ---------
0 Null Timestamp 64 RFC 6374
1 Sequence Number 64 RFC 6374
2 Network Time Protocol version 4 64-bit 64 RFC 6374
Timestamp
3 Truncated IEEE 1588v2 PTP Timestamp 64 RFC 6374
The range of the Type field is 0-15.
The allocation policy for this registry is IETF Review.
9.3. Creation of MPLS Loss/Delay Measurement Control Code Registry
IANA has created a new "MPLS Loss/Delay Measurement Control Code"
registry. This registry is divided into two separate parts, one for
Query Codes and the other for Response Codes, with formats and
initial allocations as follows:
Query Codes
Code Description Reference
---- ------------------------------ ---------
0x0 In-band Response Requested RFC 6374
0x1 Out-of-band Response Requested RFC 6374
0x2 No Response Requested RFC 6374
Frost & Bryant Standards Track PAGE 47
RFC 6374 MPLS Loss and Delay Measurement September 2011
Response Codes
Code Description Reference
---- ----------------------------------- ---------
0x0 Reserved RFC 6374
0x1 Success RFC 6374
0x2 Data Format Invalid RFC 6374
0x3 Initialization in Progress RFC 6374
0x4 Data Reset Occurred RFC 6374
0x5 Resource Temporarily Unavailable RFC 6374
0x10 Unspecified Error RFC 6374
0x11 Unsupported Version RFC 6374
0x12 Unsupported Control Code RFC 6374
0x13 Unsupported Data Format RFC 6374
0x14 Authentication Failure RFC 6374
0x15 Invalid Destination Node Identifier RFC 6374
0x16 Connection Mismatch RFC 6374
0x17 Unsupported Mandatory TLV Object RFC 6374
0x18 Unsupported Query Interval RFC 6374
0x19 Administrative Block RFC 6374
0x1A Resource Unavailable RFC 6374
0x1B Resource Released RFC 6374
0x1C Invalid Message RFC 6374
0x1D Protocol Error RFC 6374
IANA has indicated that the values 0x0 - 0xF in the Response Code
section are reserved for non-error response codes.
The range of the Code field is 0 - 255.
The allocation policy for this registry is IETF Review.
Frost & Bryant Standards Track PAGE 48
RFC 6374 MPLS Loss and Delay Measurement September 2011
9.4. Creation of MPLS Loss/Delay Measurement TLV Object Registry
IANA has created a new "MPLS Loss/Delay Measurement TLV Object"
registry, with format and initial allocations as follows:
Type Description Reference
---- --------------------------------- ---------
0 Padding - copy in response RFC 6374
1 Return Address RFC 6374
2 Session Query Interval RFC 6374
3 Loopback Request RFC 6374
127 Experimental use RFC 6374
128 Padding - do not copy in response RFC 6374
129 Destination Address RFC 6374
130 Source Address RFC 6374
255 Experimental use RFC 6374
IANA has indicated that Types 0-127 are classified as Mandatory, and
that Types 128-255 are classified as Optional.
The range of the Type field is 0 - 255.
The allocation policy for this registry is IETF Review.
10. 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, to Linda Dunbar for the idea of using LM messages for
throughput measurement, and to Ben Niven-Jenkins, Marc Lasserre, and
Ben Mack-Crane for their valuable comments.
11. References
11.1. Normative References
[IEEE1588] IEEE, "1588-2008 IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", March 2008.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Frost & Bryant Standards Track PAGE 49
RFC 6374 MPLS Loss and Delay Measurement September 2011
[RFC 2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC 3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC 3270] 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", RFC 3270, May 2002.
[RFC 5462] Andersson, L. and R. Asati, "Multiprotocol Label
Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
to "Traffic Class" Field", RFC 5462, February 2009.
[RFC 5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC 5905] Mills, D., Martin, J., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
11.2. Informative References
[RFC 2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Delay Metric for IPPM", RFC 2679, September 1999.
[RFC 2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999.
[RFC 2681] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
Delay Metric for IPPM", RFC 2681, September 1999.
[RFC 3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC 3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[RFC 3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC 4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
Frost & Bryant Standards Track PAGE 50
RFC 6374 MPLS Loss and Delay Measurement September 2011
[RFC 4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
[RFC 4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding
Equal Cost Multipath Treatment in MPLS Networks",
BCP 128, RFC 4928, June 2007.
[RFC 5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC 5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, October 2008.
[RFC 5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[RFC 5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
[RFC 5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[RFC 5921] Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
Berger, "A Framework for MPLS in Transport Networks",
RFC 5921, July 2010.
[RFC 5960] Frost, D., Bryant, S., and M. Bocci, "MPLS Transport
Profile Data Plane Architecture", RFC 5960, August 2010.
[RFC 6375] Frost, D., Ed. and S. Bryant, Ed., "A Packet Loss and
Delay Measurement Profile for MPLS-Based Transport
Networks", RFC 6375, September 2011.
[Y.1731] ITU-T Recommendation Y.1731, "OAM Functions and
Mechanisms for Ethernet based Networks", February 2008.
Frost & Bryant Standards Track PAGE 51
RFC 6374 MPLS Loss and Delay Measurement September 2011
Appendix A. Default Timestamp Format Rationale
This document initially proposed the Network Time Protocol (NTP)
timestamp format as the mandatory default, as this is the normal
default timestamp in IETF protocols and thus would seem the "natural"
choice. However, a number of considerations have led instead to the
specification of the truncated IEEE 1588 Precision Time Protocol
(PTP) timestamp as the default. NTP has not gained traction in
industry as the protocol of choice for high-quality timing
infrastructure, whilst IEEE 1588 PTP has become the de facto time
transfer protocol in networks that are specially engineered to
provide high-accuracy time distribution service. The PTP timestamp
format is also the ITU-T format of choice for packet transport
networks, which may rely on MPLS protocols. Applications such as
one-way delay measurement need the best time service available, and
converting between the NTP and PTP timestamp formats is not a trivial
transformation, particularly when it is required that this be done in
real time without loss of accuracy.
The truncated IEEE 1588 PTP format specified in this document is
considered to provide a more than adequate wrap time and greater time
resolution than it is expected will be needed for the operational
lifetime of this protocol. By truncating the timestamp at both the
high and low order bits, the protocol achieves a worthwhile reduction
in system resources.
Authors' Addresses
Dan Frost
Cisco Systems
EMail: danfrost@cisco.com
Stewart Bryant
Cisco Systems
EMail: stbryant@cisco.com
Frost & Bryant Standards Track PAGE 52
RFC TOTAL SIZE: 123462 bytes
PUBLICATION DATE: Sunday, September 18th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
|
