TWAMP Light Session-Reflector

The Session-Reflector receives and processes TWAMP Light test packets.

Use the following context to configure the Session-Reflector functions for base router reflection.

configure router twamp-light

Use the following context to configure Session-Reflector functions for per VPRN reflection.

configure service vprn twamp-light

The TWAMP Light Session-Reflector function is configured per context and must be activated before reflection can occur; the function is not enabled by default for any context. The Session-Reflector requires the user to define the TWAMP Light UDP listening port that identifies the TWAMP Light protocol. All the prefixes that the reflector accepts as valid sources for a TWAMP Light request must also be configured. If the source IP address in the TWAMP Light test packet arriving on the server does not match the configured prefixes, the packet is dropped. Multiple prefix entries may be configured per context on the server. Configured prefixes can be modified without shutting down the reflector function.

Note:

The TWAMP Light Session-Reflector udp-port udp-port-number range configured as part of the config>service>twamp-light and router>twamp-light create commands implement a restricted, reserved UDP port range that must adhere to a range of 862, 64364 to 64373 before an upgrade or reboot operation. Configurations outside this range result in a failure of the TWAMP Light Session-Reflector or prevent the upgrade operation. If an In-Service Software Upgrade (ISSU) function is invoked when the udp-port udp-port-number is outside the allowable range and the TWAMP Light Session-Reflector is in a no shutdown state, the ISSU operation cannot proceed. The user must, at a minimum, disable the TWAMP Light Session-Reflector to allow the ISSU to proceed; however, the TWAMP Light Session-Reflector is not allowed to enabled until the allowable range is met. A non-ISSU upgrade can proceed regardless of the state (enabled or disabled) of the TWAMP Light Session-Reflector. The configuration can load, however, the TWAMP Light Session-Reflector remains inactive following the reload when the allowable range is not met. When the udp-port udp-port-number for a TWAMP Light Session-Reflector is modified, all tests using the services of that reflector must update the dest-udp-port udp-port-number configuration parameter to match the new reflector listening port.

The TWAMP-Light Session-Reflector is stateful and supports unidirectional synthetic loss detection. An inactivity timeout under the config>oam-test>twamp>twamp-light command hierarchy defines the amount of time the TWAMP-Light Session-Reflector maintains individual test session in the absence of the arrival of test packets.

The TWAMP-Light Session-Reflector responds using the timestamp format that is indicated in the test packet from the Session-Sender. The Error Estimate Field is a two-byte field that includes an optional Z bit to indicate the format of timestamp. The TWAMP-Light Session-Reflector checks this field and replies using the same format for timestamp two (T2) and timestamp three (T3). The TWAMP-Light Session-Reflector does not interrogate or change the other bits in the Error Estimate field. Except for the processing of the Z bit, the received Error Estimate is reflected back to the Session-Sender.

Configurations that require an IPv6 UDP checksum of zero are increasing. In some cases, hardware timestamping functions that occur in the UDP header occur after the computation of the UDP checksum. Typically, packets that arrive with an IPv6 UDP checksum of zero are discarded. However, an optional configuration command allow-ipv6-udp-checksum-zero allows those packets to be accepted and processed for the configured UDP port of the TWAMP Light Session-Reflector.

Multiple tests sessions between peers are allowed. These test sessions are unique entities and may have different properties. Each test generates TWAMP test packets specific to their configuration. The TWAMP Light Session-Reflector includes the SSID defined by RFC 8972 as a fifth element augmenting the source IP, destination IP, source UDP port, and destination UDP port when maintaining the test state.

As TWAMP Light evolved, the TWAMP Light Session-Reflector required a method to determine processing of the arriving Session-Sender packets. The default processing behavior is type twamp-light. This treats all additional bytes beyond the base TWAMP Light packet as padding. The type stamp attempts to locate STAMP TLVs defined by RFC 8972 for processing.

See Link measurement and OAM performance monitoring for more information about the integration of TWAMP Light in those applications.