rfc9779v2.txt   rfc9779.txt 
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query messages. The responder that supports this TLV MUST return query messages. The responder that supports this TLV MUST return
Control Code 0x1 (Success) [RFC6374] if it is the intended Control Code 0x1 (Success) [RFC6374] if it is the intended
destination for the query. Otherwise, it MUST return Error 0x15: destination for the query. Otherwise, it MUST return Error 0x15:
Invalid Destination Node Identifier [RFC6374]. Invalid Destination Node Identifier [RFC6374].
4.2. Response Message for Links and SR-MPLS Policies 4.2. Response Message for Links and SR-MPLS Policies
4.2.1. One-Way Measurement Mode 4.2.1. One-Way Measurement Mode
In the one-way measurement mode defined in Section 2.4 of [RFC6374], In the one-way measurement mode defined in Section 2.4 of [RFC6374],
the querier can receive response messages with an IP/UDP header "out- the querier can receive a response to an Out-of-band Response
of-band" by properly setting the UDP Return Object (URO) TLV in the Requested message by properly setting the UDP Return Object (URO) TLV
query message. The URO TLV (Type 131) is defined in [RFC7876] and in the IP/UDP header. The URO TLV (Type 131) is defined in [RFC7876]
includes the UDP-Destination-Port and IP address. When the querier and includes the UDP-Destination-Port and IP address. When the
sets an IP address and a UDP port in the URO TLV, the response querier sets an IP address and a UDP port in the URO TLV, the
message MUST be sent to that IP address, with that IP address as the response message MUST be sent to that IP address, with that IP
destination address and the UDP port as the destination port. In address as the destination address and the UDP port as the
addition, the Control Code in the query message MUST be set to Out- destination port. In addition, the Control Code in the query message
of-band Response Requested [RFC6374]. MUST be set to Out-of-band Response Requested [RFC6374].
4.2.2. Two-Way Measurement Mode 4.2.2. Two-Way Measurement Mode
In the two-way measurement mode defined in Section 2.4 of [RFC6374], In the two-way measurement mode defined in Section 2.4 of [RFC6374],
the response messages SHOULD be sent back one of two ways: either the response messages SHOULD be sent back one of two ways: either
they are sent back in-band on the same link, or they are sent back on they are sent back in-band on the same link, or they are sent back on
the same end-to-end SR-MPLS path (i.e., the same set of links and the same end-to-end SR-MPLS path (i.e., the same set of links and
nodes) in the reverse direction to the querier. This is done in nodes) in the reverse direction to the querier. This is done in
order to perform accurate two- way delay measurement. order to perform accurate two- way delay measurement.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Example Query Message Header for an End-to-End SR-MPLS Figure 3: Example Query Message Header for an End-to-End SR-MPLS
Policy in the Loopback Measurement Mode Policy in the Loopback Measurement Mode
5. Delay and Loss Measurement 5. Delay and Loss Measurement
5.1. Delay Measurement Message 5.1. Delay Measurement Message
As defined in [RFC6374], MPLS Delay Measurement (DM) query and As defined in [RFC6374], MPLS Delay Measurement (DM) query and
response messages use the Associated Channel Header (ACH) (with the response messages use the Associated Channel Header (ACH) (with value
value 0x000C for delay measurement). This value identifies the 0x000C for delay measurement). The value identifies the message type
message type and message payload that follow the ACH, as defined in and message payload that follow the ACH, as defined in Section 3.2 of
Section 3.2 of [RFC6374]. For delay measurement, the same ACH value [RFC6374]. For delay measurement, the same ACH value is used for
is used for both links and end-to-end SR-MPLS Policies. both links and end-to-end SR-MPLS Policies.
5.2. Loss Measurement Message 5.2. Loss Measurement Message
The Loss Measurement (LM) protocol can perform two distinct kinds of The Loss Measurement (LM) protocol can perform two distinct kinds of
loss measurement as described in Section 2.9.8 of [RFC6374]. loss measurement as described in Section 2.9.8 of [RFC6374].
* In the inferred mode, LM will measure the loss of specially * In the inferred mode, LM will measure the loss of specially
generated test messages to infer the approximate data plane loss generated test messages to infer the approximate data plane loss
level. Inferred mode LM provides only approximate loss level. Inferred mode LM provides only approximate loss
accounting. accounting.
* In the direct mode, LM will directly measure data plane packet * In the direct mode, LM will directly measure data plane packet
loss. Direct mode LM provides perfect loss accounting but may loss. Direct mode LM provides perfect loss accounting but may
require hardware support. require hardware support.
As defined in [RFC6374], MPLS LM query and response messages use the As defined in [RFC6374], MPLS LM query and response messages use the
ACH (with the value 0x000A for direct loss measurement or value ACH (with value 0x000A for direct loss measurement or value 0x000B
0x000B for inferred loss measurement). This value identifies the for inferred loss measurement). This value identifies the message
message type and message payload that follow the ACH, as defined in type and message payload that follow the ACH, as defined in
Section 3.1 of [RFC6374]. For loss measurement, the same ACH value Section 3.1 of [RFC6374]. For loss measurement, the same ACH value
is used for both links and end-to-end SR-MPLS Policies. is used for both links and end-to-end SR-MPLS Policies.
5.3. Combined Loss/Delay Measurement Message 5.3. Combined Loss/Delay Measurement Message
As defined in [RFC6374], combined LM/DM query and response messages As defined in [RFC6374], combined LM/DM query and response messages
use the ACH (with the value 0x000D for direct loss and delay use the ACH (with value 0x000D for direct loss and delay measurement
measurement or the value 0x000E for inferred loss and delay or value 0x000E for inferred loss and delay measurement). The value
measurement). The value identies the message type and the message identifies the message type and the message payload that follows the
payload that follows the ACH, as defined in Section 3.3 of [RFC6374]. ACH, as defined in Section 3.3 of [RFC6374]. For combined loss and
For combined loss and delay measurement, the same ACH value is used delay measurement, the same ACH value is used for both links and end-
for both links and end-to-end SR-MPLS Policies. to-end SR-MPLS Policies.
5.4. Counters 5.4. Counters
The Path Segment Identifier (PSID) [RFC9545] MUST be carried in the The Path Segment Identifier (PSID) [RFC9545] MUST be carried in the
received data packet for the traffic flow under measurement, in order received data packet for the traffic flow under measurement, in order
to account for received traffic on the egress node of the SR-MPLS to account for received traffic on the egress node of the SR-MPLS
Policy. In the direct mode, the PSID in the received query message Policy. In the direct mode, the PSID in the received query message
(as shown in Figure 4) can be used to associate the received traffic (as shown in Figure 4) can be used to associate the received traffic
counter on the responder to detect the transmit packet loss for the counter on the responder to detect the transmit packet loss for the
end-to-end SR-MPLS Policy. end-to-end SR-MPLS Policy.
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