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RFC 6374

 
 
 

Packet Loss and Delay Measurement for MPLS Networks

Part 2 of 3, p. 19 to 42
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3.  Message Formats

   Loss Measurement and Delay Measurement messages flow over the MPLS
   Generic Associated Channel (G-ACh) [RFC5586].  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 [RFC5586].

   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:

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        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.

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   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.

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      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.

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         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.

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         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
   [RFC3260] 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 [RFC5462], this field

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   SHOULD be set to the Class Selector Codepoint [RFC2474] 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:

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        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.

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   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:

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        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.

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      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
      [RFC5905].  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

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   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.

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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.

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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.

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   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 -

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   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.

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   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.

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   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

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   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.

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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

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   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.

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   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.

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   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,

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   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.



(page 42 continued on part 3)

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