One document matched: draft-frost-mpls-tp-loss-delay-00.xml
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<rfc category="std" docName="draft-frost-mpls-tp-loss-delay-00"
ipr="trust200902">
<front>
<title abbrev="MPLS-TP Loss and Delay Measurement">Packet Loss and Delay
Measurement for the MPLS Transport Profile</title>
<author fullname="Dan Frost" initials="D" role="editor" surname="Frost">
<organization>Cisco Systems</organization>
<address>
<email>danfrost@cisco.com</email>
</address>
</author>
<author fullname="Stewart Bryant" initials="S" role="editor"
surname="Bryant">
<organization>Cisco Systems</organization>
<address>
<email>stbryant@cisco.com</email>
</address>
</author>
<date year="2009" />
<area>Routing</area>
<workgroup>MPLS</workgroup>
<keyword>MPLS</keyword>
<keyword>Internet-Draft</keyword>
<abstract>
<t>An essential Operations, Administration and Maintenance requirement
of the MPLS Transport Profile (MPLS-TP) is the ability to monitor
performance metrics for packet loss and one-way and two-way delay for
MPLS-TP pseudowires, Label Switched Paths, and Sections. This document
specifies protocol mechanisms to facilitate the efficient and accurate
measurement of these performance metrics.</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>The MPLS Transport Profile (MPLS-TP) <xref
target="I-D.ietf-mpls-tp-framework"></xref> comprises the set of
protocol functions that meet the requirements <xref
target="RFC5654"></xref> for the application of MPLS to transport
networks.</t>
<t>The document <xref target="I-D.ietf-mpls-tp-oam-requirements"></xref>
specifies Operations, Administration and Maintenance (OAM) definitions
and requirements for the measurement of packet loss and one-way and
two-way delay for MPLS-TP pseudowires (PWs), Label Switched Paths
(LSPs), and Sections. For convenience these definitions and requirements
are summarized in the following subsections.</t>
<section title="Review of Requirements">
<section title="Requirements for Packet Loss Measurement">
<t>The MPLS-TP OAM tool-set MUST provide a function to enable the
quantification of packet loss ratio over a PW, LSP or Section.</t>
<t>Packet loss ratio is the ratio of the user packets not delivered
to the total number of user packets transmitted during a defined
time interval. The number of user packets not delivered is the
difference between the number of user packets transmitted by an End
Point and the number of user packets received at an End Point.</t>
<t>This function MAY either be performed pro-actively or on-demand.
It SHOULD be performed between End Points of PWs, LSPs and Sections.
It SHOULD be possible to rely on user traffic to perform that
functionality.</t>
<t>The protocol solution(s) developed to perform this function MUST
apply to point-to-point bidirectional (associated and co-routed)
LSPs, point-to-point unidirectional LSPs and point-to-multipoint
LSPs.</t>
</section>
<section title="Requirements for Delay Measurement">
<t>The MPLS-TP OAM tool-set MUST provide a function to enable the
quantification of the one-way, and if appropriate, the two-way,
delay of a PW, LSP or Section. <list style="symbols">
<t>One-way delay is the time elapsed from the start of
transmission of the first bit of a packet by an End Point until
the reception of the last bit of that packet by the other End
Point.</t>
<t>Two-way delay is the time elapsed from the start of
transmission of the first bit of a packet by a End Point until
the reception of the last bit of that packet by the same End
Point, when loop-back is performed at the other End Point.</t>
</list></t>
<t>This function SHOULD be performed on-demand and MAY be performed
pro-actively. It SHOULD be performed between End Points of PWs, LSPs
and Sections.</t>
<t>In addition to co-routed bidirectional LSPs, the protocol
solution(s) developed to perform this function MUST also apply to
point-to-point associated bidirectional LSPs, point-to-point
unidirectional LSPs and point-to-multipoint LSPs but only to enable
the quantification of the one-way delay.</t>
</section>
</section>
<section title="Terminology">
<texttable align="left" style="headers">
<ttcol>Term</ttcol>
<ttcol>Definition</ttcol>
<c>ACH</c>
<c>Associated Channel Header</c>
<c>DM</c>
<c>Delay Measurement</c>
<c>G-ACh</c>
<c>Generic Associated Channel</c>
<c>LM</c>
<c>Loss Measurement</c>
<c>LSP</c>
<c>Label Switched Path</c>
<c>LSR</c>
<c>Label Switching Router</c>
<c>MPLS-TP</c>
<c>MPLS Transport Profile</c>
<c>OAM</c>
<c>Operations, Administration and Maintenance</c>
<c>PW</c>
<c>Pseudowire</c>
</texttable>
</section>
</section>
<section title="Overview">
<t>The basic procedures for measuring loss and delay over a
bidirectional connection are conceptually simple. The following figure
shows the reference scenario.</t>
<figure align="center" anchor="ov_fig">
<artwork><![CDATA[
T1 T2
+-------+/ Query \+-------+
| | - - - - - - - - ->| |
| A |===================| B |
| |<- - - - - - - - - | |
+-------+\ Response /+-------+
T4 T3
]]></artwork>
</figure>
<t>The figure shows a bidirectional connection between two LSRs, A and
B, and illustrates the temporal reference points T1-T4 associated with a
measurement operation that takes place at A. The operation consists of A
sending a query message to B, and B sending back a response. Each
reference point indicates the point in time at which either the query or
the response message is transmitted or received over the connection.</t>
<t>In this situation, A can arrange to measure the packet loss over the
connection 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 connection 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 connection from A (B_RxP), and the count of packets transmitted
prior to time T3 over the connection 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 connection from B (A_RxP).</t>
<t>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.</t>
<t>To measure at A the delay over the connection 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 connection. 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.</t>
<t>In the case of a unidirectional connection (i.e. a
unidirectional point-to-point or point-to-multipoint MPLS-TP LSP) rooted
at A, the first half of each of the above procedures can be carried out
to measure the forward one-way loss and delay associated with the LSP.
At this point the measurement can either take place at the terminal
node(s) of the connection rather than at A, or an out-of-band connection
can be used, if available, to communicate the data back to A.</t>
<t>LM and DM messages flow over the Generic Associated Channel (G-ACh)
<xref target="RFC5586"></xref> of an MPLS-TP connection (pseudowire, LSP
or Section).</t>
<t><cref anchor="N1" source="DF">The term "connection" is used in this
document to mean an MPLS-TP PW, LSP, or Section. Either this or another
term will be defined in the Framework for this purpose.</cref></t>
<section title="Implementation Considerations">
<t>The challenge in carrying out the above procedures lies with the
implementation. For accurate loss measurement 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. For accurate delay measurement,
timestamps must be recorded in DM messages at a point in time as close
as possible to when the message is actually transmitted or received
over the connection.</t>
<t>These accuracy requirements imply that a hardware-based forwarding
implementation may require hardware support for the processing of LM
and DM messages. An important consideration of the LM/DM protocol and
message format is therefore support for efficient hardware
processing.</t>
<t>In situations where such accuracy is not required, or the necessary
level of support is not available, an implementation MAY still
generate and respond to LM and DM messages but SHOULD make its
accuracy limitations clear to the user. In general the DM procedures
described in this document remain viable under these conditions, but
the procedures for LM may be inadequate. An alternate approach to LM
in such situations is to assemble an approximate view of connection
quality through sustained invasive generation of test messages
alongside client traffic. Such alternative procedures are outside the
scope of this document.</t>
</section>
<section anchor="ov_loss" title="Packet Loss Measurement">
<t>Suppose a bidirectional connection such as an MPLS-TP pseudowire,
bidirectional LSP, or Section exists between the LSRs A and B. The
objective is to measure at A the following two quantities associated
with the connection: <list style="empty">
<t>A_TxLoss (transmit loss): the number of packets transmitted by
A over the connection but not received at B;</t>
<t>A_RxLoss (receive loss): the number of packets transmitted by B
over the connection but not received at A.</t>
</list></t>
<t>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 connection at a specified
rate, such as one every 100 milliseconds. Each message LM[n] contains
the following value: <list style="empty">
<t>A_TxP[n]: the total count of packets transmitted by A over the
connection prior to the time this message is transmitted.</t>
</list></t>
<t>When such a message is received at B, the following value is
recorded in the message: <list style="empty">
<t>B_RxP[n]: the total count of packets received by B over the
connection at the time this message is received (excluding the
message itself).</t>
</list></t>
<t>At this point, B inserts an appropriate response code into the
message and transmits it back to A, recording within it the following
value: <list style="empty">
<t>B_TxP[n]: the total count of packets transmitted by B over the
connection prior to the time this response is transmitted.</t>
</list></t>
<t>When the message response is received back at A, the following
value is recorded in the message: <list style="empty">
<t>A_RxP[n]: the total count of packets received by A over the
connection at the time this response is received (excluding the
message itself).</t>
</list></t>
<t>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:</t>
<t>A_TxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] - B_RxP[n-1])
<vspace /> A_RxLoss[n-1,n] = (B_TxP[n] - B_TxP[n-1]) - (A_RxP[n] -
A_RxP[n-1])</t>
<t>where the arithmetic is modulo the counter size.</t>
<t>The derived values <list style="empty">
<t>A_TxLoss = A_TxLoss[1,2] + A_TxLoss[2,3] + ...</t>
<t>A_RxLoss = A_RxLoss[1,2] + A_RxLoss[2,3] + ...</t>
</list> are updated each time a response to an LM message is
received and processed, and represent the total transmit and receive
loss over the connection since the LM operation was initiated.</t>
<t>When computing the values A_TxLoss[n-1,n] and A_RxLoss[n-1,n] the
possibility of counter wrap must be taken into account. Consider for
example the values of the A_TxP counter at times 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 connection, 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.</t>
</section>
<section anchor="ov_delay" title="Delay Measurement">
<t>Suppose a bidirectional connection such as an MPLS-TP pseudowire,
bidirectional LSP, or Section exists between the LSRs A and B. The
objective is to measure at A one or more of the following quantities
associated with the connection: <list style="symbols">
<t>The one-way delay associated with the forward (A to B)
direction of the connection;</t>
<t>The one-way delay associated with the reverse (B to A)
direction of the connection;</t>
<t>The two-way delay (A to B to A) associated with the
connection.</t>
</list></t>
<t>Of course, if the first two quantities are known then the third is
immediate, being just their sum. Measurement of the one-way delay
quantities, however, requires that the clocks of A and B be
synchronized, whereas the two-way delay can be measured directly even
when this is not the case (provided A and B have stable clocks).</t>
<t>The measurement is accomplished by sending a Delay Measurement (DM)
query message over the connection to B which contains the following
timestamp: <list style="empty">
<t>T1: the time the DM query message is transmitted from A.</t>
</list></t>
<t>When the message arrives at B, the following timestamp is recorded
in the message: <list style="empty">
<t>T2: the time the DM query message is received at B.</t>
</list></t>
<t>At this point B inserts an appropriate response code into the
message and transmits it back to A, recording within it the following
timestamp: <list style="empty">
<t>T3: the time the DM response message is transmitted from B.</t>
</list></t>
<t>When the message arrives back at A, the following timestamp is
recorded in the message: <list style="empty">
<t>T4: the time the DM response message is received back at A.</t>
</list></t>
<t>At this point, A can compute the two-way delay associated with the
connection as <list style="empty">
<t>two-way delay = (T4 - T1) - (T3 - T2).</t>
</list></t>
<t>If the clocks of A and B are known at A to be synchronized, then
all three delay values can be computed at A as <list style="empty">
<t>forward one-way delay = T2 - T1</t>
<t>reverse one-way delay = T4 - T3</t>
<t>two-way delay = forward delay + reverse delay.</t>
</list></t>
<section title="Timestamp Format">
<t>There are at least two significant timestamp formats in common
use: the timestamp format of the Internet standard Network Time
Protocol (NTP), described in <xref target="RFC1305"></xref> and
<xref target="RFC2030"></xref>, and the timestamp format used in the
IEEE 1588 Precision Time Protocol (PTP) <xref
target="IEEE1588"></xref>.</t>
<t><cref anchor="N2" source="DF">There are actually two PTP
timestamp formats: the 1588v1 format consists of a 32-bit seconds
field and a 32-bit nanoseconds field; in 1588v2 the seconds field
was extended to 48 bits.</cref></t>
<t>The NTP format has the advantages of wide use and long deployment
in the Internet, and was specifically designed to make the
computation of timestamp differences as simple and efficient as
possible. On the other hand, there is also now a significant
deployment of equipment designed to support the PTP format.</t>
<t>The approach taken in this document is therefore to include in DM
messages fields which identify the timestamp formats used by the two
devices involved in a DM operation. This implies that an LSR
attempting to carry out a DM operation may be faced with the problem
of computing with and possibly reconciling different timestamp
formats. Support for multiple timestamp formats is OPTIONAL. An
implementation SHOULD, however, make clear which timestamp formats
it supports and the extent of its support for computation with and
reconciliation of different formats for purposes of delay
measurement.</t>
<t>In accordance with Internet standards for network time, the NTP
timestamp format is the default format used in DM messages. This
format MUST be supported.</t>
</section>
</section>
<section title="Delay Variation Measurement">
<t>Packet Delay Variation <xref target="RFC3393"></xref> is another
performance metric important in some applications. The PDV of a pair
of packets within a stream of packets is defined for a selected pair
of packets in the stream going from measurement point MP1 to
measurement point MP2. The PDV is the difference between the one-way
delay of the selected packets.</t>
<t>A PDV measurement can therefore be derived from successive delay
measurements obtained through the procedures in <xref
target="ov_delay"></xref>. An important point regarding PDV
measurement, however, is that it can be carried out based on one-way
delay measurements even when the clocks of the two systems involved in
those measurements are not synchronized.</t>
</section>
<section title="Unidirectional Connections">
<t>In the case that the connection from A to (B1, ..., Bk) is
unidirectional, i.e. is a unidirectional LSP, LM and DM
measurements can be carried out at B1, ..., Bk instead of at A.</t>
<t>For LM this is accomplished by initiating an LM operation at A and
carrying out the same procedures as for bidirectional connections,
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
connection in essentially the same way as described previously,
i.e.</t>
<t>B_RxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] -
B_RxP[n-1])</t>
<t>For DM, of course, only the forward one-way delay can be measured
and the clock synchronization requirement applies.</t>
<t>Alternatively, if an out-of-band connection from a terminal node B
back to A is available, the LM and DM message responses can be
communicated to A via this connection so that the measurements can be
carried out at A.</t>
</section>
</section>
<section title="Packet Format">
<t>Loss Measurement and Delay Measurement messages flow over the Generic
Associated Channel (G-ACh) <xref target="RFC5586"></xref> of an MPLS-TP
connection (pseudowire, LSP or Section).</t>
<t><cref anchor="N3" source="DF">The question of ACH TLV usage and the
manner of supporting metadata such as authentication keys and node
identifiers is deliberately omitted. These issues will be addressed in a
future version of the document.</cref></t>
<section anchor="pf_lm" title="Loss Measurement Message Format">
<t>The format of a Loss Measurement message, beginning with the
Associated Channel Header (ACH), is as follows:</t>
<figure anchor="pf_lm_f" title="Loss Measurement Message Format">
<artwork><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | 0xHH (MPLS-TP Loss) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Flags | Control Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Querier Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter 1 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter 4 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<texttable align="left" style="headers">
<preamble>The meanings of the fields following the ACH are
summarized in the following table.</preamble>
<ttcol width="30%">Field</ttcol>
<ttcol>Meaning</ttcol>
<c>Version</c>
<c>Protocol version</c>
<c>Flags</c>
<c>Message control flags</c>
<c>Control Code</c>
<c>Code identifying the query or response type</c>
<c>Reserved</c>
<c>Reserved for future specification</c>
<c>Querier Context</c>
<c>Set arbitrarily by the querier</c>
<c>Counter 1-4</c>
<c>64-bit packet counter values in network byte order</c>
</texttable>
<t>The possible values for these fields are as follows.</t>
<t>Version: Currently set to 0.</t>
<t>Flags: Each bit represents a message control flag. The flags,
listed in left-to-right (most- to least-significant-bit) order, are:
<list style="empty">
<t>Q/R: Set to 0 for a Query and 1 for a Response.</t>
<t>Remaining bits: Reserved for future specification and set to
0.</t>
</list></t>
<t>Control Code: Set as follows according to whether the message is a
Query or a Response as identified by the Q/R flag. <list style="empty">
<t>For a Query: <list style="empty">
<t>0x0: Query (in-band response requested). Indicates that
this query has been sent over a bidirectional connection and
the response is expected over the same connection.</t>
<t>0x1: Query (out-of-band response requested). Indicates that
the response should be sent via an out-of-band channel.</t>
<t>0x2: Query (no response requested). Indicates that no
response to the query should be sent.</t>
</list></t>
<t>For a Response: <list style="empty">
<t>0x1: Success. Indicates that the operation was
successful.</t>
<t>0x8: 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.</t>
<t>0x10: Error - Unspecified Error. Indicates that the
operation failed for an unspecified reason.</t>
<t>0x11: Error - Unsupported Version. Indicates that the
operation failed because the protocol version supplied in the
query message is not supported.</t>
<t>0x12: Error - Unsupported Control Code. Indicates that the
operation failed because the Control Code requested an
operation that is not available for this connection.</t>
<t>0x13: Error - Authentication Failure. Indicates that the
operation failed because the authentication data supplied in
the query was missing or incorrect.</t>
<t>0x14: Error - Invalid Source Node Identifier. Indicates
that the operation failed because the Source Node Identifier
supplied in the query is not expected.</t>
<t>0x15: Error - Invalid Destination Node Identifier.
Indicates that the operation failed because the Destination
Node Identifier supplied in the query is not the identifier of
this node.</t>
<t>0x16: Error - Connection Mismatch. Indicates that the
operation failed because the connection identifier supplied in
the query did not match the connection over which the query
was received.</t>
<t>0x17: Error - Query Rate Exceeded. Indicates that the
operation failed because the rate of query messages exceeded
the configured threshold.</t>
<t>0x18: Error - Administrative Block. Indicates that the
operation failed because it has been administratively
disallowed.</t>
<t>0x19: Error - Temporary Resource Exhaustion. Indicates that
the operation failed because node resources were not
available.</t>
</list></t>
</list></t>
<t>Reserved: Currently set to 0.</t>
<t>Querier Context: Set arbitrarily in a query and copied in the
response.</t>
<t>Counter 1-4: Referring to <xref target="ov_loss"></xref>, 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. All counter values MUST
be in network byte order.</t>
</section>
<section anchor="pf_dm" title="Delay Measurement Message Format">
<t>The format of a Delay Measurement message, beginning with the
Associated Channel Header (ACH), is as follows:</t>
<figure anchor="pf_dm_f" title="Delay Measurement Message Format">
<artwork><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | 0xHH (MPLS-TP Delay) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Flags | Control Code | QTF | RTF | RPTF | Resv |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Querier Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Timestamp 1 |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Timestamp 4 |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<texttable align="left" style="headers">
<preamble>The meanings of the fields following the ACH are
summarized in the following table.</preamble>
<ttcol width="30%">Field</ttcol>
<ttcol>Meaning</ttcol>
<c>Version</c>
<c>Protocol version</c>
<c>Flags</c>
<c>Message control flags</c>
<c>Control Code</c>
<c>Code identifying the query or response type</c>
<c>QTF</c>
<c>Querier timestamp format</c>
<c>RTF</c>
<c>Responder timestamp format</c>
<c>RPTF</c>
<c>Responder's preferred timestamp format</c>
<c>Resv (Reserved)</c>
<c>Reserved for future specification</c>
<c>Querier Context</c>
<c>Set arbitrarily by the querier</c>
<c>Timestamp 1-4</c>
<c>128-bit timestamp values</c>
<c>Padding</c>
<c>Optional padding</c>
</texttable>
<t>The possible values for these fields are as follows.</t>
<t>Version: Currently set to 0.</t>
<t>Flags: As specified in <xref target="pf_lm"></xref>.</t>
<t>Control Code: As specified in <xref target="pf_lm"></xref>.</t>
<t>Querier Timestamp Format: The format of the timestamp values
written by the querier, as specified in <xref
target="pf_tsf"></xref>.</t>
<t>Responder Timestamp Format: The format of the timestamp values
written by the responder, as specified in <xref
target="pf_tsf"></xref>.</t>
<t>Responder's Preferred Timestamp Format: The timestamp format
preferred by the responder, as specified in <xref
target="pf_tsf"></xref>.</t>
<t>Resv (Reserved): Currently set to 0.</t>
<t>Querier Context: Set arbitrarily in a query and copied in the
response.</t>
<t>Timestamp 1-4: Referring to <xref target="ov_delay"></xref>, 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.</t>
<t>Padding: One or more octets of padding may optionally follow the
Timestamp 4 field in a query, in order to allow for delay measurement
based on packets of a particular size. The values of the pad octets,
if present, are arbitrary, and if any are present they will be copied
in the response.</t>
<t>The next version of this document will describe a mechanism to
allow the querier to specify whether the responder should include
padding in the response.</t>
</section>
<section anchor="pf_tsf" title="Timestamp Field Formats">
<t>The following timestamp format field values are specified in this
document: <list style="empty">
<t>0x0: Network Time Protocol version 4 timestamp format <xref
target="RFC2030"></xref>. 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.</t>
<t>0x2: IEEE 1588-2008 Precision Time Protocol timestamp format
<xref target="IEEE1588"></xref>. This format consists of a 48-bit
seconds field followed by a 32-bit nanoseconds field.</t>
</list></t>
<t>In accordance with Internet standards for network time, the NTP
timestamp format is the default format used in Delay Measurement
messages. This format MUST be supported. Support for other timestamp
formats is OPTIONAL.</t>
<t>Timestamp formats of n < 128 bits in size SHALL be encoded in
the 128-bit timestamp fields specified in this document using the n
high-order bits of the field. The remaining 128 - n low-order bits in
the field SHOULD be set to 0 and MUST be ignored when reading the
field.</t>
</section>
</section>
<section title="Operation">
<section title="Loss Measurement Procedures">
<section title="Initiating a Loss Measurement Operation">
<t>An LM operation for a particular MPLS-TP connection consists of
sending a sequence (LM[1], LM[2], ...) of LM query messages over the
connection at a specific rate and processing the responses received,
if any. As described in <xref target="ov_loss"></xref>, the packet
loss associated with the connection 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
connection. The query message transmission rate MUST be sufficiently
high, given the 64-bit LM message counter size and the speed and
minimum packet size of the underlying connection, that the ambiguity
condition noted in <xref target="ov_loss"></xref> cannot arise.</t>
</section>
<section title="Transmitting a Loss Measurement Query">
<t>When transmitting an LM Query over an MPLS-TP connection, the
Version and Reserved fields MUST be set to 0. The Q/R flag MUST be
set to 0 and the remaining flag bits MUST be set to 0.</t>
<t>The Control Code field MUST be set to one of the values for Query
messages listed in <xref target="pf_lm"></xref>; if the connection
is unidirectional, this field MUST NOT be set to 0x0 (Query: in-band
response requested).</t>
<t>The Querier Context field can be set arbitrarily.</t>
<t>The Counter 1 field SHOULD be set to the total count of packets
transmitted over the connection prior to this LM Query. The
remaining Counter fields MUST be set to 0.</t>
</section>
<section title="Receiving a Loss Measurement Query">
<t>Upon receipt of an LM Query message, the Counter 2 field SHOULD
be set to the total count of packets received over the connection
prior to this LM Query.</t>
<t>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.</t>
</section>
<section title="Transmitting a Loss Measurement Response">
<t>When constructing a Response to an LM Query, the Version and
Reserved fields MUST be set to 0. The Q/R flag MUST be set to 1 and
the remaining flag bits MUST be set to 0.</t>
<t>The Querier Context field 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.</t>
<t>The Control Code field MUST be set to one of the values for
Response messages listed in <xref target="pf_lm"></xref>. The value
0x10 (Unspecified Error) SHOULD NOT be used if one of the other more
specific error codes is applicable.</t>
<t>If the response is transmitted in-band, the Counter 1 field
SHOULD be set to the total count of packets transmitted over the
connection 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.</t>
</section>
<section title="Receiving a Loss Measurement Response">
<t>Upon in-band receipt of an LM Response message, the Counter 2
field SHOULD be set to the total count of packets received over the
connection prior to this LM Response.</t>
<t>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.</t>
<t>If the Control Code in an LM Response is anything other than 0x1
(Success), the counter values in the response MUST NOT be used for
purposes of loss measurement. When the Control Code indicates an
error condition, the LM operation SHOULD be suspended and an
appropriate notification to the user generated. If a temporary error
condition is indicated, the LM operation MAY be restarted
automatically.</t>
</section>
<section title="Scope of Packet Loss Counters">
<t>By default the packet counts appearing in LM messages on a
connection MUST include packets transmitted and received over the
Generic Associated Channel (G-ACh) associated with the connection.
An implementation MAY provide the means to change the scope of the
LM counters to exclude some or all G-ACh messages. Care must be
taken in this case to ensure that the scopes of the counters at both
ends of a connection agree.</t>
</section>
<section title="Message Loss and Packet Misorder Conditions">
<t>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.</t>
<t>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 <xref
target="ov_loss"></xref>. 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 connection parameters.</t>
<t>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.</t>
<t>Finally, LM is subject to the possibility that data packets are
misordered relative to LM messages. This condition can result, for
example, in a transmit count of 100 and a corresponding receive
count of 101. The effect here is that the A_TxLoss[n-1,n] value (for
example) for a given measurement interval will appear to be
extremely (if not impossibly) large. The other case, where an LM
message arrives earlier than some of the packets, simply results in
those packets being counted as lost, which is usually what is
desired.</t>
<t>Perhaps the simplest way to detect and handle the case of lost or
out-of-order LM messages is to incorporate a sequence number in each
message. Such a sequence number can be inserted within the bounds of
the Querier Context field provided for implementation-specific use.
An implementation adopting this approach can now take the following
actions:</t>
<t><cref anchor="N4" source="DF">Text to be added here about
handling the above conditions with sequence numbers and
thresholds.</cref></t>
</section>
</section>
<section title="Delay Measurement Procedures">
<section title="Transmitting a Delay Measurement Query">
<t>When transmitting a DM Query over an MPLS-TP connection, the
Version and Reserved fields MUST be set to 0. The Q/R flag MUST be
set to 0 and the remaining flag bits MUST be set to 0.</t>
<t>The Control Code field MUST be set to one of the values for Query
messages listed in <xref target="pf_lm"></xref>; if the connection
is unidirectional, this field MUST NOT be set to 0x0 (Query: in-band
response requested).</t>
<t>The Querier Context field can be set arbitrarily.</t>
<t>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 <xref
target="pf_tsf"></xref>. The Responder Timestamp Format and
Responder's Preferred Timestamp Format fields MUST be set to 0.</t>
<t>The Timestamp 1 field SHOULD be set to the time at which this DM
Query is transmitted, in the format indicated by the Querier
Timestamp Format field. The other timestamp fields MUST be set to
0.</t>
<t>One or more pad octets with arbitrary values MAY follow the
Timestamp 4 field.</t>
</section>
<section title="Receiving a Delay Measurement Query">
<t>Upon receipt of a DM Query message, the Timestamp 2 field SHOULD
be set to the time at which this DM Query is received.</t>
<t>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.</t>
</section>
<section title="Transmitting a Delay Measurement Response">
<t>When constructing a Response to a DM Query, the Version and
Reserved fields MUST be set to 0. The Q/R flag MUST be set to 1 and
the remaining flag bits MUST be set to 0.</t>
<t>The Querier Context 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.</t>
<t>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 <xref
target="pf_tsf"></xref>. Furthermore, the RTF field MUST be set
equal either to the QTF or the RPTF field. See <xref
target="op_dm_tsfn"></xref> for guidelines on selection of the value
for this field.</t>
<t>The Responder's Preferred Timestamp Format (RPTF) field MUST be
set to one of the values listed in <xref target="pf_tsf"></xref> and
SHOULD be set to indicate the timestamp format with which the
responder can provide the best accuracy for purposes of delay
measurement.</t>
<t>The Control Code field MUST be set to one of the values for
Response messages listed in <xref target="pf_lm"></xref>. The value
0x10 (Unspecified Error) SHOULD NOT be used if one of the other more
specific error codes is applicable.</t>
<t>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.</t>
<t>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.</t>
<t>Padding SHALL be included in the response if, and only if,
padding was present in the DM Query, in which case the response
padding MUST be identical to the query padding.</t>
</section>
<section title="Receiving a Delay Measurement Response">
<t>Upon in-band receipt of a DM Response message, the Timestamp 2
field SHOULD be set to the time at which this DM Response is
received.</t>
<t>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.</t>
<t>If the Control Code in a DM Response is anything other than 0x1
(Success), the timestamp values in the response MUST NOT be used for
purposes of delay measurement. When the Control Code indicates an
error condition, an appropriate notification to the user SHOULD be
generated.</t>
</section>
<section anchor="op_dm_tsfn" title="Timestamp Format Negotiation">
<t>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 connection. The procedures for making this
determination SHALL be as follows.</t>
<t>Upon sending an initial DM Query over a connection, the querier
sets the Querier Timestamp Format (QTF) field to its preferred
timestamp format.</t>
<t>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.</t>
<t>The process of changing from one timestamp format to another at
the responder may result in the Timestamp 1 and Timestamp 4 fields
in an in-band DM Response having different formats. If this is the
case, the Control Code in the response MUST NOT be set to 0x1
(Success). Unless an error condition has occurred, the Control Code
MUST be set to 0x2 (Notification - Data Format Invalid).</t>
<t>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.</t>
</section>
</section>
</section>
<section title="A Uni-format Implementation">
<t>Editor's note. This text on the execution of the protocol on simple
hardware need further thought and will be updated in the next version of
this document.</t>
<t>A simple implementation of this protocol that only understands one
time format MAY discard all Query messages with a QTF type that it does
not support. Similarly a simple implementation may discard all Response
messages with an RTF type that it does not support. Sunch an
implementation would only successfully execute a delay measurement if
both the query and response systems were configured to use identical
formats.</t>
</section>
<section anchor="con_con" title="Congestion Considerations">
<t>An MPLS-TP 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.</t>
<t>The proactive generation of Loss Measurement and Delay Measurement
messages for purposes of monitoring the performance of an MPLS-TP
connection naturally results in a degree of additional load placed on
both the network and the terminal nodes of the connection. 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 <xref target="ov_loss"></xref>.</t>
<t>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 connection 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.</t>
<t>When carrying out a sustained measurement operation such as an LM
operation or continuous pro-active DM operation, the querier SHOULD take
note of the number of lost measurement messages (queries for which a
response is never received) and set a corresponding Measurement Message
Loss Threshold. If this threshold is exceeded, the measurement operation
SHOULD be suspended so as not to exacerbate the possible congestion
condition. This suspension SHOULD be accompanied by an appropriate
notification to the user so that the condition can be investigated and
corrected.</t>
<t>From the receiver perspective, the main consideration is the
possibility of receiving an excessive quantity of measurement messages.
An implementation SHOULD employ a mechanism such as rate-limiting to
guard against the effects of this case. Authentication procedures can
also be used to ensure that only queries from authorized devices are
processed.</t>
</section>
<section title="Security Considerations">
<t>There are two main types of security considerations associated with
the exchange of performance monitoring messages such as those described
in this document: the possibility of a malicious or misconfigured device
generating an excessive quantity of messages, causing service
impairment; and the possibility of an unauthorized device learning the
data contained in or implied by such messages.</t>
<t>The first consideration is discussed in <xref
target="con_con"></xref>. If reception of performance-related data by
unauthorized devices is an operational concern, message authentication
procedures such as those described in [xref] should be used to ensure
that only queries from authorized devices are processed.</t>
</section>
<section title="IANA Considerations">
<t>A future version of this document will detail IANA considerations
for: <list style="symbols">
<t>ACH Channel Types for LM and DM messages</t>
<t>Timestamp format registry</t>
<t>LM and DM Control Codes</t>
</list></t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.2119'?>
<?rfc include='reference.RFC.5654'?>
<?rfc include='reference.I-D.ietf-mpls-tp-oam-requirements'?>
<?rfc include='reference.RFC.5586'?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.ietf-mpls-tp-framework'?>
<?rfc include='reference.RFC.1305'?>
<?rfc include='reference.RFC.2030'?>
<?rfc include='reference.RFC.3393'?>
<reference anchor="IEEE1588">
<front>
<title>1588-2008 IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems</title>
<author surname="IEEE">
<organization abbrev="IEEE">IEEE</organization>
</author>
<date month="March" year="2008" />
</front>
</reference>
</references>
</back>
</rfc>
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