Differentiated Service Code Point and Explicit Congestion Notification Monitoring in the Two-Way Active Measurement Protocol (TWAMP)
updates
- rfc-5357 — A Two-Way Active Measurement Protocol (TWAMP)
Extracted elements (17)
If the provisioned DSCP value is not known (e.g., in TWAMP Light mode), the Session-Reflector MAY copy the DSCP from the received packet or MAY set it to CS0 (zero), leaving the choice implementation-specific because TWAMP Light lacks a TWAMP-Control synchronization mechanism.
The Session-Reflector in DSCP and ECN Monitoring mode does not analyze or act on the ECN value of received test packets, ignoring congestion indications. This is justified because TWAMP deployment experience has shown sending rates are low enough that ignoring ECN will not significantly contribute to network congestion.
Adding the S-DSCP-ECN field increases the unauthenticated Session-Reflector packet by 4 octets; implementations combining DSCP and ECN Monitoring with Symmetrical Size or Reflects Octets modes (RFC 6038) must account for this size increase in Padding Length calculations.
TWAMP Light mode (Appendix I of RFC 5357) does not explicitly state how the Type-P Descriptor is synchronized between Session-Sender and Session-Reflector; this extension assumes that both sides are configured by some means (e.g., CLI) to use the same DSCP value and to support DSCP and ECN Monitoring.
If the provisioned ECN value is not known, ECN SHOULD be set to the Not-ECT codepoint value; otherwise the provisioned ECN value for the session SHALL be used.
The Control-Client MUST set the DSCP and ECN Monitoring flag in the Modes field of the Setup Response message if it agrees to monitor DSCP and ECN on some or all test sessions invoked with the control connection.
The Session-Reflector MUST extract the DSCP and ECN values from the received packet and MUST use them to populate the S-DSCP-ECN field of the corresponding reflected packet.
The Session-Reflector MUST transmit each reflected test packet with the DSCP set to the provisioned value, not to the received DSCP value.
The six least-significant bits of the Differentiated Service field MUST be copied from the received Session-Sender test packet into the Sender DSCP (S-DSCP) field, and the two ECN bits MUST be copied into the Sender ECN (S-ECN) field.
When DSCP and ECN Monitoring and Symmetrical Size and/or Reflects Octets modes are negotiated in unauthenticated mode, the Padding Length value SHOULD be greater than or equal to 28 octets to allow for the truncation process recommended in RFC 5357.
The DSCP and ECN Monitoring feature uses a new flag (bit position 8) in the TWAMP-Modes field, negotiated during the Server Greeting and Setup Response exchange, to indicate capability and willingness to monitor DSCP and ECN values.
IANA has allocated bit position 8 in the TWAMP-Modes registry (defined in RFC 5618) for the 'DSCP and ECN Monitoring Capability', with semantics defined in Section 2 of RFC 7750.
Monitoring of DSCP and ECN does not introduce additional security threats beyond those already present in TWAMP (RFC 5357) and its existing extensions (RFC 6038). The general security considerations for active measurement of live networks, as discussed in RFC 4656 and RFC 5357, apply.
In authenticated and encrypted modes, the Session-Reflector test packet includes the S-DSCP-ECN field (1 octet) placed after the Sender TTL field, followed by 14 octets MBZ and a 16-octet HMAC, as shown in Figure 3.
In unauthenticated mode, the Session-Reflector test packet includes the S-DSCP-ECN field (1 octet) placed after the Sender TTL field, followed by MBZ padding, as shown in Figure 2.
The Sender DSCP and ECN (S-DSCP-ECN) field is an 8-bit field: bits 0-5 carry the S-DSCP (six least-significant bits of the DS field), and bits 6-7 carry the S-ECN (two ECN bits), as defined in Figure 1.
When DSCP and ECN Monitoring is combined with Symmetrical Size mode in unauthenticated mode, the Session-Sender test packet format uses 28 octets of MBZ (Figure 4) to accommodate the 4-octet increase from the S-DSCP-ECN field in the reflector packet.