Loss and Delay Measurement in Transparent Interconnection of Lots of Links (TRILL)
Extracted elements (29)
Loss counters are based on differences (TXc-TXp) and (RXc-RXp) rather than absolute values, so the sender's TX counter and receiver's RX counter do not need to be synchronized to a common initial value.
Loss Measurement is performed using synthetic (OAM) packets rather than counting user data frames. The TRILL OAM framework [OAM-FRAMEWK] Section 5.1 documents the reasons why user-data-based packet loss computation is not defined.
Two-way delay computation (T4-T1)-(T3-T2) does not require the two MEPs to be time-synchronized, unlike one-way delay which requires synchronized clocks. When synchronized MEPs run two-way DM, they can additionally derive one-way delays.
PM OpCodes are reused from ITU-T G.8013/Y.1731 (1DM=45, DMR=46, DMM=47, 1SL=53, SLR=54, SLM=55). The packet formats in this document conform to Y.1731-2013.
The 64-bit timestamp format (32-bit seconds + 32-bit nanoseconds) is compatible with IEEE 1588v1, IEEE 1588v2, Y.1731-2013, and MPLS-LM-DM (RFC 6374), enabling cross-protocol consistency.
A MEP that implements TRILL PM MUST support two-way PM, and MUST support both the sender and the reflector PM functionality.
A MEP that implements TRILL PM SHOULD support one-way Performance Monitoring, and SHOULD support both the sender (RB1) and receiver (RB2) PM functionality.
In point-to-multipoint PM, reflectors MUST send response messages after a random delay in the range of 0 to 2 seconds to prevent reply storms from overwhelming the initiating RBridge.
Senders and receivers SHOULD NOT clear the TX and RX counters during a measurement interval, as doing so produces false measurement results per Equation (1).
The 1SL receiver MUST maintain a reception counter per peer MEP and probe instance, MUST verify the packet is destined to the current MEP and that the MD level matches, and MUST perform a wraparound check when computing packet loss.
The FirstTLVOffset field MUST be 16 in 1SL and SLM/SLR packets, and MUST be 32 in DMM and DMR packets.
The sender MUST maintain a packet counter for each peer MEP and probe instance (test ID), incrementing it on every 1SL transmission and integrating the value into the Counter TX field.
Two MEPs running one-way Delay Measurement MUST be time-synchronized, as the one-way delay calculation T2-T1 requires a common reference time.
Upon receiving an SLM packet with valid destination MEP and MD level, the reflector MUST increment its TRX counter and generate an SLR that is identical to the SLM except for updated OpCode, TRX counter, Reflector MEP ID, and transport headers.
A PM Session is defined by PM function, Start Time, Stop Time, Message Period, Measurement Interval, and Repetition Time. Loss Measurement sessions measure Frame Loss Ratio, availability, and resiliency; Delay Measurement sessions measure Frame Delay, Inter-Frame Delay Variation, Frame Delay Range, and Mean Frame Delay.
Performance Monitoring supports three granularity levels: Network-level (dedicated test VLAN/FGL), Service-level (actual user VLANs/FGLs), and Flow-level (per-flow using a Flow Entropy field at the beginning of the OAM packet header).
Six PM message types are defined: 1SL (One-way Synthetic Loss), SLM (Synthetic Loss Measurement Message), SLR (Synthetic Loss Measurement Reply), 1DM (One-way Delay Measurement), DMM (Delay Measurement Message), and DMR (Delay Measurement Reply).
The 1DM and DMM/DMR packet formats include a T (Type) flag in the Flags field: when set, indicates proactive operation; when cleared, indicates on-demand mode.
OpCode values 45 (1DM), 46 (DMR), 47 (DMM), 53 (1SL), 54 (SLR), and 55 (SLM) are used as defined in Y.1731-2013, from the range allocated by IEEE 802.1Q for control by ITU-T.
Attacks on PM protocols can falsely indicate nonexistent performance issues, prevent detection of real ones (DoS), or allow network reconnaissance via passive eavesdropping of PM messages. Synthetic PM messages can themselves be used as a DoS vector against RBridges.
TRILL PM OAM messages MAY include the OAM Authentication TLV, but the cryptographic overhead may affect measurement accuracy. Operators must weigh security benefits against potential measurement inaccuracy based on a system-specific threat assessment.
Loss Measurement packets use a 32-bit packet counter field; when incremented beyond 0xFFFFFFFF it wraps around to 0, and receivers must perform wraparound checks when computing loss.
The 1SL packet format includes: OAM header (MD-L, Ver=0, OpCode, Flags=0, FirstTLVOffset=16), Sender MEP ID (16 bits), Reserved (16 bits), Test ID (32 bits), Counter TX (32 bits), Reserved (32 bits), and TLVs.
The DMM packet format includes Timestamp T1 (64 bits), a reserved field for T2 (64 bits, filled by reflector), a reserved field for T3 (64 bits, filled in DMR), and a reserved field for T4 (64 bits, for DMR receiver), with FirstTLVOffset=32.
The DMR packet carries all four timestamps: T1 (DMM transmission time), T2 (DMM reception time at reflector), T3 (DMR transmission time), and a reserved field for T4 (DMR reception time at sender), each 64 bits.
The OAM Message Channel header common to all OpCodes is 4 octets: MD-L (3 bits), Version (5 bits), OpCode (8 bits), Flags (8 bits), FirstTLVOffset (8 bits), followed by OpCode-specific fields and TLVs ending with an End TLV.
The SLM packet reserves the Reflector MEP ID field (16 bits) and the Counter TRX field (32 bits) for use by the reflector in the corresponding SLR reply; the SLR populates both fields.
The TRILL OAM encapsulation consists of Link Header (variable), TRILL Header (6+ bytes), Flow Entropy (96 bytes), OAM Ethertype, OAM Message Channel (variable), and Link Trailer (variable).
Timestamps in Delay Measurement packets are 64 bits (32-bit seconds + 32-bit nanoseconds), using the 64 least significant bits of the IEEE 1588-2008 (PTPv2) format. This truncated format is also used in IEEE 1588v1, Y.1731-2013, and MPLS-LM-DM.