Simple Two-Way Active Measurement Protocol Extensions for Performance Measurement on LAG

Link Aggregation Group (LAG), as defined in , provides mechanisms to combine multiple physical links into a single logical link. This logical link offers higher bandwidth and better resiliency, because if one of the physical member links fails, the aggregate logical link can continue to forward traffic over the remaining operational physical member links. Usually, when forwarding traffic over LAG, a hash-based mechanism is used to load balance the traffic across the LAG member links. The link delay might vary between member links because of different transport paths, especially when LAG is used in wide area network. To provide low latency service for time sensitive traffic, we need to explicitly steer the traffic across the LAG member links based on the link delay, loss and so on. That requires a solution to measure the performance metrics of each member link of a LAG. Hence, the measured performance metrics can work together with layer 2 bundle member link attributes advertisement for traffic steering. According to the classifications in , Simple Two-Way Active Measurement Protocol (STAMP) is an active measurement method, and it can complement passive and hybrid methods. It provides a mechanism to measure both one-way and round-trip performance metrics, like delay, delay variation, and packet loss. One STAMP test session over the LAG can measure the performance of a member link with fixed five tuples. Or it can measure an average of some/all member links of the LAG by varying the five tuples. However, without the knowledge of each member link, a STAMP test session cannot measure the performance of every physical member link. This document extends STAMP to implement performance measurement on every member link of a LAG. It can provide the same metrics as OWAMP and TWAMP can measure, such as delay, jitter, and packet loss.

This document addresses the scenario where a LAG directly connects two nodes. An example of this is in Figure 1, where the LAG consisting of four links connects nodes A and B. The goal is to measure the performance of each link of the LAG.

To measure the performance metrics of every member link of a LAG, multiple sessions (one session for each member link) need to be established between the two end points that are connected by the LAG. These sessions are called micro sessions in the remainder of this document. Although micro sessions are in fact STAMP sessions established on member links of a LAG, test packets of micro sessions MUST carry member link information for validation. All micro sessions of a LAG share the same Sender IP Address and Receiver IP Address of the LAG. As for the UDP Port, the micro sessions may share the same Sender Port and Receiver Port pair, or each micro session is configured with a different Sender Port and Receiver Port pair. But from the operational point of view, the former is simpler and is RECOMMENDED. Test packets of a micro session MUST carry the member link information for validation check. For example, when a micro STAMP Session-Sender receives a reflected test packet, it checks whether the test packet is from the expected member link. The member link information is encoded in the Micro-session ID TLV introduced in Section 3 of this document, and the detailed description about the member link validation is also in this section. A micro STAMP Session-Sender MAY include the Follow-Up Telemetry TLV to request information from the micro Session-Reflector. This timestamp might be important for the micro Session-Sender, as it improves the accuracy of network delay measurement by minimizing the impact of egress queuing delays on the measurement.

Test packets MUST carry member link information in Micro-session ID TLV introduced in this section for validation check. The micro Session-Sender verifies whether the test packet is received from the expected member link. It also verifies whether the packet is sent from the expected member link at the Reflector side. The micro Session-Reflector verifies whether the test packet is received from the expected member link.

STAMP TLV mechanism extends STAMP test packets with one or more optional TLVs. This document defines the TLV Type (value TBA1) for the Micro-session ID TLV that carries the micro STAMP Session-Sender member link identifier and Session-Reflector member link identifier in Sender Micro-session ID field and Reflector Micro-session ID field respectively. The format of the Micro-session ID TLV is shown as follows:

Type (one-octet in length): It is defined to indicate this TLV is a Micro-session ID TLV. Value TBA1 is allocated by IANA (Section 5). Length (2-octets in length): It is defined to carry the length of the Value field in octets. The Length field value MUST be 4. Sender Micro-session ID (2-octets in length): It is now defined to carry the LAG member link identifier of the Sender side. In the future, it may be used generically to cover use-cases beyond LAG. The value of this field MUST be unique within a STAMP session at the Session-Sender. Reflector Micro-session ID (2-octets in length): It is now defined to carry the LAG member link identifier of the Reflector side. In the future, it may be used generically to cover use-cases beyond LAG. The value of this field MUST be unique within a STAMP session at the Session-Reflector.

The micro STAMP-Test reuses the procedures as defined in Section 4 of STAMP with the following additions. The micro STAMP Session-Sender MUST send the micro STAMP-Test packets over the member link with which the session is associated. The mapping between a micro STAMP session and the Sender/Reflector member link identifiers can be configured by augmenting the STAMP YANG. The detailed augmentation is not in the scope of this document. When sending a test packet, the micro STAMP Session-Sender MUST set the Sender Micro-session ID field with the member link identifier associated with the micro STAMP session. If the Session-Sender knows the Reflector member link identifier, the Reflector Micro-session ID field MUST be set. Otherwise, the Reflector Micro-session ID field MUST be zero. The Reflector member link identifier can be obtained from pre-configuration or learned from data plane (e.g., the reflected test packet). This document does not specify the way to obtain the Reflector member link identifier. When the micro STAMP Session-Reflector receives a test packet, if the Reflector Micro-session ID is not zero, the micro STAMP Session-Reflector MUST use the Reflector member link identifier to check whether it is associated with the micro STAMP session. If the validation fails, the test packet MUST be discarded. If the Reflector Micro-session ID is zero, it will not be verified. If all validations passed, the Session-Reflector sends a reflected test packet to the Session-Sender. The micro STAMP Session-Reflector MUST put the Sender and Reflector member link identifiers that are associated with the micro STAMP session in the Sender Micro-session ID and Reflector Micro-session ID fields respectively. The Sender member link identifier is copied from the received test packet. When receiving a reflected test packet, the micro Session-Sender MUST use the Sender Micro-session ID to validate whether the reflected test packet is correctly received from the expected member link. If the validation fails, the test packet MUST be discarded. The micro Session-Sender MUST use the Reflector Micro-session ID to validate the Reflector's behavior. If the validation fails, the test packet MUST be discarded. Two modes of the STAMP Session-Reflector, stateless and stateful, characterize the expected behavior, as described in Section 4 of STAMP. The micro STAMP-Test also supports both stateless and stateful modes. However, the micro STAMP-Test does not introduce any additional state to STAMP, i.e, any procedure with regard to the Micro-session ID is stateless.

The micro STAMP Session-Sender sends micro Session-Sender packets with the Micro-session ID TLV. The micro Session-Reflector checks whether a test packet is received from the member link associated with the correct micro STAMP session, if the Reflector Micro-session ID field is set. When reflecting, the micro STAMP Session-Reflector copies the Sender Micro-session ID from the received micro Session-Sender packet to the micro Session-Reflector packet, and sets the Reflector Micro-session ID field with the member link identifier that is associated with the micro STAMP session. When receiving the micro Session-Reflector packet, the micro Session-Sender uses the Sender Micro-session ID to check whether the packet is received from the member link associated with the correct micro STAMP session. The micro Session-Sender also use the Reflector Micro-session ID to validate the Reflector's behavior.

In the "STAMP TLV Types" registry created for [RFC8972], a new STAMP TLV Type for Micro-session ID TLV is requested from IANA as follows:

The STAMP extension defined in this document is intended for deployment in LAG scenario where Session-Sender and Session-Reflector are directly connected. As such, it's assumed that a node involved in STAMP protocol operation has previously verified the integrity of the LAG connection and the identity of its one-hop-away peer node. This document does not introduce any additional security issues and the security mechanisms defined in and apply in this document.

The authors would like to thank Mach Chen, Min Xiao, Fang Xin, Marcus Ihlar, Richard Foote for the valuable comments to this work.