Update of the Simple Two-way Active Measurement Protocol Class-of-Service Extension

Class of Service TLV

TLV Definition The STAMP Session-Sender MAY include a CoS TLV in the STAMP test packet. The format of the CoS TLV is presented in .

Class of Service TLV Where the fields are defined as follows.

STAMP TLV Flags: eight-bit field; format presented in .
CoS (Class of Service) Type: one-octet field; the value MUST be set to 4.
Length: two-octet field; set equal to the value 4 (octets).
DSCP1: DSCP value intended by the Session-Sender to be used as the DSCP value of the reflected test packet.
DSCP2: received value in the DSCP field at the ingress of the Session-Reflector.
EC2: received value in the ECN field at the ingress of the Session-Reflector.
RPD (reverse path DSCP): two-bit field indicating whether the Session-Reflector used DSCP1 or DSCP2 as the DSCP value of the reflected test packet; a Session-Sender MUST set the value of the RPD field to 0b00 on transmission.
EC1: ECN value intended by the Session-Sender to be used as the ECN value of the reflected test packet.
RPE (reverse path ECN): two-bit field indicating whether the Session-Reflector used EC1 as the ECN value of the reflected test packet; a Session-Sender MUST set the value of the RPE field to 0b00 on transmission.
Reserved: twelve-bit field; MUST be zeroed on transmission and ignored on receipt.

Session-Reflector Behavior A STAMP Session-Reflector that receives a test packet with the CoS TLV MUST include the CoS TLV in the reflected test packet. The Session-Reflector MUST copy the value of the DSCP and ECN fields of the IP header of the received STAMP test packet into the DSCP2 and EC2 fields, respectively, in the CoS TLV in the reflected test packet. A Session-Reflector might have a local policy configuration that limits the DSCP values that it can use for reflected test packets. This local policy could be (for example) a system default policy, a global policy for the Session-Reflector, or a policy that is configured for specific destination addresses or networks. The Session-Reflector MUST verify whether the use of the value of the DSCP1 field is permitted in the reflected test packet. If it is, the Session-Reflector MUST set the DSCP field's value in the IP header of the reflected test packet equal to the value of the DSCP1 field of the received test packet. Otherwise, the Session-Reflector MUST use the DSCP value of the received STAMP packet and set the value of the RPD field to 0b01. Upon receiving the reflected packet, if the value of the RPD field is 0b00, the Session-Sender will save the DSCP value for analysis of the CoS in the reverse direction. If the value of the RPD field in the received reflected packet is 0b01, only CoS in the forward direction can be analyzed. A Session-Reflector might have a local policy configuration that limits the ECN values that it can use for reflected test packets. This local policy could be (for example) a system default policy, a global policy for the Session-Reflector, or a policy that is configured for specific destination addresses or networks. Additionally, a Session-Reflector could have platform limitations that prevent it from setting the ECN value in reflected test packets. The Session-Reflector MUST set the ECN value in the IP header of the reflected STAMP test packet to the value of the EC1 field, if it is permitted and capable to do so. If the Session-Reflector is able to set the ECN value in the IP header of the reflected STAMP test packet to the EC1 value, it MUST then set the RPE field in the CoS TLV in the reflected test packet to the value 0b11. If the Session-Reflector is unable to set the ECN value in the IP header of the reflected STAMP test packet to the EC1 value, it MUST instead set the RPE field in the CoS TLV in the reflected test packet to the value 0b10. As a result, the Session-Sender can detect whether the EC1 value was used by inspecting the value of the RPE field in the received reflected test packet.

Interoperability with RFC 8972 The extended CoS TLV defined in this draft is backward compatible with the specification in . The handling of the DSCP1, DSCP2, EC2 and RPD fields defined here is identical to the handling defined for the equivalent fields in . Consider a case when an implementation that supports this specification performs as Session-Sender, and the intended Session-Reflector's support of the CoS TLV is according to . If the operator requires monitoring ECN in the reverse direction, the value of the EC1 field will be set to a non-zero value. Because the Session-Reflector would treat the EC1 field as part of the Reserved field, it would ignore its value as per . Further, the Session-Reflector would treat the RPE field as part of the Reserved field and thus it would send the value 0b00 in the reflected STAMP packet, rather than sending the value 0b10 or 0b11. Consequently, the Session-Sender will determine that the Session-Reflector does not implement the current version of the CoS TLV, and that the ECN value in the IP header of the reflected test packet was not set as requested. Also, this specification supports the case when the Session-Reflector supports the extended CoS TLV as defined in this specification and the Session-Sender supports the CoS TLV according to . In that scenario, the Session-Sender will set its Reserved field to zeros. The Session-Reflector will interpret the first two bits of that field as the EC1 field, as shown in and thus will set the value of the ECN field in the IP header of the reflected packet to 0b00. Further the Session-Reflector will set the RPE field to 0b11. The Session-Sender will treat the RPE field as part of the Reserved field and will ignore its value.

Congestion Response and mandate that applications that send packets marked with the ECT0 or ECT1 codepoints implement a response to congestion notifications from the network. While the STAMP protocol doesn't include the concept of a connection between a Session-Sender and a Session-Reflector, there are situations in which the Session-Sender may send multiple STAMP packets to the same Session-Reflector within the round-trip time between them, and there are situations in which the Session-Sender may elicit multiple STAMP packets from a Session-Reflector in response to a single sent STAMP packet. As such, the Session-Sender is generally responsible for dictating both its sending rate, and the sending rate of the packets sent in response by an individual Session-Reflector, and so is not exempt from this mandate. When using this CoS TLV with either an ECT0 or an ECT1 value in the ECN field of the IP header of the STAMP packet, the Session-Sender gains the ability (via observing a CE reflected value in the EC2 field) to detect that congestion was present on the path from the Session-Sender to the Session-Reflector. Similarly, when using this CoS TLV with either an ECT0 or an ECT1 value in the EC1 field, the Session-Sender gains the ability (via observing a CE in the ECN field of the IP header of the reflected packet) to detect that congestion was present on the path from the Session-Reflector to the Session-Sender. If a Session-Sender sends multiple STAMP packets with the CoS TLV to a Session-Reflector within an RTT with either the ECT0 or ECT1 value in the ECN field of the IP header, it MUST observe the reflected EC2 field and reduce its sending rate upon observation of a CE value. If a Session-Sender sends multiple STAMP packets with the CoS TLV to a Session-Reflector within an RTT with either the ECT0 or ECT1 value in the EC1 field, it MUST observe the ECN value in the IP header of the reflected packets and reduce its sending rate upon observation of a CE value. If a Session-Sender sends a STAMP packet containing both the CoS TLV and the Reflected Test Packet Control TLV () to a Session-Reflector, and specifies either the ECT0 or ECT1 value in the EC1 field, it MUST observe the ECN value in the IP header of the reflected packets and adjust the Reflected Test Packet Control parameters in any future STAMP packet sent to the same Session-Reflector based on the observation of CE values.