Control and Monitoring Differentiated Service Code Point in Two-Way Active Measurement Protocol (TWAMP)

The One-Way Active Measurement Protocol (OWAMP) defines the Type-P Descriptor field and negotiation of its value in OWAMP-Control protocol. The Two-Way Active Measurement Protocol (TWAMP) states that only a Differentiated Services Code Point (DSCP) , , value can be defined by Type-P Descriptor and the negotiated value must be used by both Session-Sender and Session-Reflector. The TWAMP specification also states that the same DSCP value (found in the Session-Sender packet) MUST be used in the test packet reflected by the Session-Reflector. The introduced optional DSCP Monitoring mode that can be negotiated using TWAMP Control protocol and supported by TWAMP-Test protocol or by TWAMP Light mode. Still the TWAMP-Test protocol does not support discovery of how Differentiated Services policies configured along the IP path process various DSCP values in single test session. Hence method defined in can be characterized as per-session DSCP Monitoring. To provide higher efficiency and flexibility to monitoring how Differentiated Services policies being applied this document proposes ability to control DSCP value to be used by Session-Reflector for each TWAMP-Test packet. Such method can be characterized as per-packet DSCP monitoring with TWAMP. This document describes an OPTIONAL feature for TWAMP. It is called the DSCP and ECN Testing. It allows the Session-Sender to use set of DSCP values through single test session and to instruct the Session-Reflector on what DSCP value it must use for the reflected test packet. Furthermore this feature tracks the Explicit Congestion Notification (ECN) , , value received at the Session-Reflector. This is helpful to determine if ECN is actually operating or if an ECN-capable node has detected congestion in the forward direction.

DSCP: Differentiated Services Code Point ECN: Explicit Congestion Notification IPPM: IP Performance Metrics TWAMP: Two-Way Active Measurement Protocol OWAMP: One-Way Active Measurement Protocol

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

TWAMP connection establishment follows the procedure defined in Section 3.1 of and Section 3.1 of where the Modes field is used to identify and select specific communication capabilities. At the same time the Modes field been recognized and used as an extension mechanism . The new feature requires a new flag to identify the ability of a Session-Reflector to support the new Session-Sender packet format in the TWAMP-Test protocol and to use received DSCP and ECN values in the reflected to a Session-Sender test packet, See the for details on the assigned bit position.

The Server sets the DSCP and ECN Testing flag in the Modes field of the Server Greeting message to indicate its capabilities and willingness to monitor them. If the Control-Client agrees to test DSCP and ECN on some or all test sessions invoked with this control connection, it MUST set the DSCP and ECN Testing flag in the Modes field in the Setup Response message.

Testing of DSCP and ECN requires support by the Session-Sender and changes the test packet format in all the original (unauthenticated, authenticated and encrypted) modes. Testing of DSCP and ECN does not alter the Session-Reflector test packet format but certain considerations must be taken when and if this mode is accepted in combination with Symmetrical Size mode and/or with DSCP and ECN Monitoring mode .

When the Session-Sender supports DSCP and ECN Testing it constructs the Reflector DSCP and ECN (R-DSCP-ECN) field, presented in , for each test packet it sends to Session-Reflector according to the following procedure: value of the Reflector DSCP (R-DSCP) field MUST be set to the value that the Session-Reflector MUST use for the reflected test packet; value of the Reflector ECN (R-ECN) field MUST be set to the value that the Session-Reflector MAY use for the reflected test packet. When the Session-Reflector supports DSCP and ECN Testing mode it uses R-DSCP-ECN field of the received test packet to construct the reflected test packet according to the following procedure the R-DSCP field MUST be used as six (least-significant) bits of the Differentiated Service field of the reflected test packet; the R-ECN field MAY be used as the two bits of the ECN field of the reflected test packet.

Formats of the test packet transmitted by the Session-Sender in unauthenticated, authenticated and encrypted modes been defined in Section 4.1.2 . For the Session-Sender that supports DSCP and ECN Testing these formats are displayed in and . For unauthenticated mode:

Neither Session-Reflector, nor Session-Sender in the DSCP and ECN Testing mode analyze, nor act on ECN value of the received TWAMP test packet and therefore ignore congestion indications from the network. It is expected that sending rates are low enough, as TWAMP deployment experience had demonstrated since TWAMP base RFC 5357 publication in 2008, that ignoring these congestion indications will not significantly contribute to network congestion. For authenticated and encrypted modes:

defined DSCP and ECN Monitoring extension. Using testing and monitoring modes in the same test session allows test DSCP in forward and reverse directions because Session-Reflector returns received DSCP and ECN values in S-DSCP-ECN field in the reflected test packet.

defined two extensions to TWAMP. First, to ensure that Session-Sender and Session-Reflector exchange TWAMP-Test packets of equal size. Second, to specify number of octets to be reflected by Session-Reflector. If DSCP and ECN Testing and Symmetrical Size and/or Reflects Octets modes are being negotiated between Server and Control-Client in Unauthenticated mode, then, because R-DSCP-ECN field increases size of unauthenticated Session-Sender packet by 4 octets, the Padding Length value SHOULD be >= 26 octets to allow for the truncation process that TWAMP recommends in Section 4.2.1 of . If DSCP and ECN Testing mode to be used in combination with Symmetrical Size and DSCP and ECN Monitoring modes, then the Padding Length value SHOULD be >= 27 octets to allow the truncation process that TWAMP recommends in Section 4.2.1 of .

Appendix I of does not explicitly state how the value of the Type-P Descriptor is synchronized between Session-Sender and Session-Reflector and whether different values are considered as error condition and should be reported. In order to test DSCP over round-trip path between Session-Sender and Session-Reflector it is sufficient that Session-Reflector uses received DSCP value for the reflected test packet. If the Session-Reflector supports both testing and monitoring of DSCP, then TWAMP Light mode MAY be used to test DSCP in forward and reverse directions.

The TWAMP-Modes registry defined in . IANA is requested to reserve a new DSCP and ECN Testing Capability as follows: Bit Description Semantics Definition Reference TBA DSCP and ECN Testing Capability This document

Testing of DSCP and ECN does not appear to introduce any additional security threat to hosts that communicate with TWAMP as defined in , and existing extensions . Sections such as 3.2, 4., 4.1.2, 4.2, and 4.2.1 of discuss unauthenticated, authenticated, and encrypted modes in varying degrees of detail. The security considerations that apply to any active measurement of live networks are relevant here as well. See the Security Considerations sections in , , and .

TBD