Performance Measurement (PM) with Marking Method in Bit Index Explicit Replication (BIER) Layer

Theory of Operation The marking method can be used in the multicast environment supported by BIER layer. Without limiting any generality consider multicast network presented in . Any combination of markings can be applied to a multicast flow by the Bit Forwarding Ingress Router (BFIR) at either ingress or egress point to perform node, link, segment or end-to-end measurement to detect performance degradation defect and localize it efficiently.

Multicast network Using the marking method, a BFIR creates distinct sub-flows in the particular multicast traffic over BIER layer. Each sub-flow consists of consecutive blocks of identically marked packets. For example, a block of N packets, with each packet being marked as X, is followed by the block of M packets with each packet being marked as Y. These blocks are unambiguously recognizable by a monitoring point at any Bit Forwarding Router (BFR) and can be measured to calculate packet loss and/or packet delay metrics. The marking method can be used on multiple flows concurently. Demultiplexing of monitored flows might be achived using n-tuple, for example, two-tuple as combination of the values in the Entropy and BFIR-id fields . Also, that can be achieved by using an explicit Flow Identifiier. The definition of the Flow Identifier is outside the scope of this specification. It is expected that the marking values be set and cleared at the edge of BIER domain. Thus for the scenario presented in if the operator initially monitors the A-C-G and A-B-D segments he may enable measurements on segments C-F and B-E at any time.

Single-Marking Enabled Measurement As explained in , marking can be applied to delineate blocks of packets based either on the equal number of packets in a block or based on the equal time interval. The latter method offers better control as it allows a better account for capabilities of downstream nodes to report statistics related to batches of packets and, at the same time, time resolution that affects defect detection interval. If the Single-Marking measurement is used to measure packet loss, then the D flag MUST be set to zero on transmit and ignored by the monitoring point. The S flag is used to create sub-flows to measure the packet loss by switching the value of the S flag every N-th packet or at certain time intervals. Delay metrics MAY be calculated with the sub-flow using any of the following methods:

First/Last Packet Delay calculation: whenever the marking, i.e., the value of S flag changes, a BFR can store the timestamp of the first/last packet of the block. The timestamp can be compared with the timestamp of the packet that arrived in the same order through a monitoring point at a downstream BFR to compute packet delay. Because timestamps collected based on the order of arrival this method is sensitive to packet loss and re-ordering of packets (see for more details).
Average Packet Delay calculation: an average delay is calculated by considering the average arrival time of the packets within a single block. A BFR may collect timestamps for each packet received within a single block. Average of the timestamp is the sum of all the timestamps divided by the total number of packets received. Then the difference between the average packet arrival time calculated for the downstream monitoring point and the same metric but calculated at the upstream monitoring point is the average packet delay on the segment between these two points. This method is robust to out of order packets and also to packet loss on the segment between the measurement points (packet loss may cause a minor loss of accuracy in the calculated metric because the number of packets used is different at each measurement point). This method only provides a single metric for the duration of the block, and it doesn't give the minimum and maximum delay values. This limitation of producing only the single metric could be overcome by reducing the duration of the block. As a result, the calculated value of the average delay will better reflect the minimum and maximum delay values of the block's duration time.

Double-Marking Enabled Measurement Double-Marking method allows measurement of minimum and maximum delays for the monitored flow, but it requires more nodal and network resources. If the Double-Marking method used, then the S flag is used to create the sub-flow, i.e., mark blocks of packets. The D flag is used to mark single packets within a block to measure delay and jitter. The first marking (S flag alternation) is needed for packet loss and also for average delay measurement. The second marking (D flag is put to one) creates a new set of marked packets that are fully identified over the BIER network, so that a BFR can store the timestamps of these packets; these timestamps can be compared with the timestamps of the same packets on a second BFR to compute packet delay values for each packet. The number of measurements can be easily increased by changing the frequency of the second marking. On the other hand, the higher frequency of the second marking will cause a higher volume of the measurement data being transported through the BIER domain. An operator should consider and balance both effects. This method is useful to measure not only the average delay but also the minimum and maximum delay values and, in wider terms, to know more about the statistic distribution of delay values.

Operational Considerations For the ease of operational procedures, the initial marking of a multicast flow is performed at BFIR. and cleared, by way of removing BIER encapsulation form a payload packet, at the edge of the BIER domain by BFERs. Since at the time of writing this specification, there are no proposals to using auto-discovery or signaling mechanism to inform downstream nodes what methodology is used each monitoring point MUST be configured beforehand. Section 5 provides a detailed analysis of how packet re-ordering and the duration of the block in the Single-Marking mode of the marking method impact the accuracy of the packet loss measurement. Re-ordering of packets in the Single-Marking mode will be noticeable only at the edge of a block of packets (re-ordering within the block cannot be detected in the Single-Marking mode). If the extra delay for some packets is much smaller than half of the duration of a block, then it should be easier to attribute re-ordered packets to the proper block and thus maintain the accuracy of the packet loss measurement. Selection of a time interval to switch the marking of a batch of packets should be based on the service requirements. In the course of the regular operation, reports, including performance metrics like packet loss ratio, packet delay, and inter-packet delay variation, are logged every 15 minutes. Thus, it is reasonable to maintain the duration of the measurement interval at 5 minutes with 100 measurements per each interval. To support these measurements, marking of the packet batch is switched every 3 seconds. In case when performance metrics are required in near-real-time, the duration interval of a single batch of identically marked packets will be in the range of tens of milliseconds.