The protocol parameters influence the format of the test packet, processing, and QoS handling.
TWAMP Light, also called Simple Two-way Activation Monitoring Protocol (STAMP), is the test packet used to gather IP link measurement delay. The link measurement request source is the session-sender.
TWAMP Light requires the explicit configuration of a reflector on the peer. For SR OS, TWAMP Light reflectors are configured in the config>router>twamp-light> reflector context. The reflector is referred to as the session-reflector, the responder to the request.
The session-sender and the session-reflector must agree on the UDP port used to identify TWAMP Light test packets. The SR OS configuration of the session-sender (configured using the dest-udp-port command) and session-reflector (configured using the udp-port command) must match.
The TWAMP Light test packet was first introduced to support the Network Time Protocol (NTP) encoding of the timestamp in the packet. Updates since the initial standardization of TWAMP Light supports the use of the truncated Precision Time Protocol (PTP) timestamp format. A bit in the TWAMP Light test packet header is repurposed to indicate the timestamp format encoded by the session-sender and the session-reflector. This change leads to some interoperability considerations. The timestamp format should be consistent with the session-sender and session-reflector behavior. The link measurement session-sender can be configured to encode NTP (default) or PTP and set the ‟z-bit” in the Error Estimate field accordingly. This bit indicates the timestamp format carried in the packet. If the session-reflector sets the ‟z-bit” in the Error Estimate field to indicate the timestamp format of the reply, the link measurement session-sender can perform the necessary conversion (format and epoch) to produce the correct results. However, if the session-reflector only reflects the original ‟z-bit” it received from the session-sender and uses a different timestamp format in the packet, the delay calculations are not reliable because of the misinterpretation of the returned timestamp format. SR OS session-reflectors running Release 21.5 and earlier always reflect the ‟z-bit” received from the session-sender. Regardless of the ‟z-bit”, these session-reflectors always encode an NTP timestamp format in the packet. When these session-reflectors receive a TWAMP Light test packet with the PTP timestamp format, there is a mismatch between the actual timestamp format and the timestamp it has encoded. There is no mechanism for the session-reflector to detect this mismatch and report the correct delay. To ensure accurate delay measurements, any session-sender sending TWAMP Light test packets to an SR OS TWAMP Light reflector that is running Release 21.5 and earlier, must use a timestamp format of NTP. Release 21.7 session-reflectors reply in kind for the timestamp format and properly set the timestamp format bit to match the timestamp encoded by the session-reflector.
IPv6 packets arriving with a UDP checksum of zero are discarded. However, recent work in the IETF is suggesting that selected protocols may register on the local router to accept and process IPv6 packets with a UDP checksum of zero. To provide interoperability, the allow-ipv6-udp-checksum-zero command allows the session-sender and the session-reflector to process IPv6 TWAMP Light test packets that arrive with a UDP checksum of zero. This is specific to the link measurement template session-sender and session-reflector and only for the specific UDP ports for TWAMP Light test packets.