Packet Loss Measurement

Packet loss measurement (LM), a performance monitoring function provided by MPLS-TP, is implemented on the two ends of a PW, LSP, or MPLS section to collect statistics about dropped packets. Packet loss measurement results contain near- and far-end packet loss values:

Near-end packet loss value: the number of dropped packets expected to arrive at the local MEP.
Far-end packet loss value: the number of dropped packets the local MEP has sent.

To collect packet loss statistics for both incoming and outgoing packets, each MEP must have both of the following counters enabled:

TxFCl: records the number of packets sent to the RMEP.
RxFCl: records the number of packets received by the local MEP.

Packet loss measurement can be performed in either single- or dual-ended mode. Table 1 describes the single- and dual-ended packet loss measurement.

**Table 1** Packet loss measurement functions
Function	Description	Usage Scenario
Dual-ended packet loss measurement	Collects packet loss statistics to assess the quality of the link between two MEPs that have connectivity fault management (CFM) continuity check (CC) enabled.	Dual-ended packet loss measurement provides more accurate results than the single-ended method. The interval between dual-ended packet loss measurements varies with the interval between CCM transmissions. The CCM transmission interval is shorter than the interval between LMM transmissions. Therefore, the dual-ended method allows for a shorter measurement interval than the single-ended method.
Single-ended packet loss measurement	Collects packet loss statistics to assess the quality of the link between two MEPs. This method is independent of CC.	Sending CCMs imposes a heavier burden on the network than sending LMMs and LMRs. To minimize the burden, single-ended packet loss measurement can be used.

Dual-ended Packet Loss Measurement

Figure 1 illustrates proactive dual-ended packet loss measurement. Dual-ended packet loss measurement can only be performed in proactive mode. Two MEPs on both ends of a link periodically exchange CCMs carrying the following information:

TxFCf: the local TxFCl value recorded when the local MEP sent a CCM.
RxFCb: the local RxFCl value recorded when the local MEP received a CCM.
TxFCb: the TxFCf value carried in a received CCM. This TxFCb value is the local TxFCl when the local MEP receives a CCM.

Figure 1 Proactive dual-ended packet loss measurement

After receiving CCMs carrying packet count information, both MEPs use the following formulas to measure near- and far-end packet loss values:

Near-end packet loss value = |TxFCf[tc] - TxFCb[tp]| - |RxFCl[tc] - RxFCl[tp]|

Far-end packet loss value = |TxFCb[tc] - TxFCb[tp]| - |RxFCb[tc] - RxFCb[tp]|

TxFCf[tc], RxFCb[tc], and TxFCb[tc] are the TxFCf, RxFCb, and TxFCb values, respectively, which are carried in the most recently received CCM. RxFCl[tc] is the local RxFCl value recorded when the local MEP received the CCM.
TxFCf[tp], RxFCb[tp], and TxFCb[tp] are the TxFCf, RxFCb, and TxFCb values, respectively, which are carried in the previously received CCM. RxFCl[tp] is the local RxFCl value recorded when the local MEP received the previous CCM.
tc is the time a current CCM was received.
tp is the time the previous CCM was received.

Single-ended Packet Loss Measurement

Single-ended packet loss measurement is performed in either proactive or on-demand mode. In proactive mode, a local MEP periodically sends loss measurement messages (LMMs) to an RMEP carrying the following information:

TxFCl: the local TxFCl value recorded when the LMM was sent.

After receiving an LMM, the RMEP responds to the local MEP with loss measurement replies (LMRs) carrying the following information:

TxFCf: equal to the TxFCf value carried in the LMM.
RxFCf: the local RxFCl value recorded when the LMM was received.
TxFCb: the local TxFCl value recorded when the LMR was sent.

Figure 2 illustrates proactive single-end packet loss measurement.

Figure 2 Proactive single-ended packet loss measurement

After receiving an LMR, the local MEP uses the following formulas to calculate near- and far-end packet loss values:

Near-end packet loss value = |TxFCb[tc] - TxFCb[tp]| - |RxFCl[tc] - RxFCl[tp]|

Far-end packet loss value = |TxFCf[tc] - TxFCf[tp]| - |RxFCf[tc] - RxFCf[tp]|

TxFCf[tc], RxFCf[tc], and TxFCb[tc] are the TxFCf, RxFCf, and TxFCb values, respectively, which are carried in the most recently received LMR. RxFCl[tc] is the local RxFCl value recorded when the most recent LMR arrives at the local MEP.
TxFCf[tp], RxFCf[tp], and TxFCb[tp] are the TxFCf, RxFCf, and TxFCb values, respectively, which are carried in the previously received LMR. RxFCl[tp] is the local RxFCl value recorded when the previous LMR arrived at the local MEP.
tc is the time a current LMR was received.
tp is the time the previous LMR was received.