One-Way Link Delay Measurement via Network Tap Timestamping
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Solution Overview
Problem
Existing methods for measuring one-way link delay in asymmetric networks are inaccurate, especially in networks without specialized equipment capable of port role reversal or timestamp generation for outgoing packets.
Innovation Solution
A method involving network taps to intercept and copy packets, allowing for the use of ingress timestamps as origin timestamps, and synchronizing devices with a common time source to calculate one-way link delay by transmitting and receiving packets with embedded timestamps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If specialized equipment with port role reversal capability is used, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
A network tap is introduced as an intermediary device that passively intercepts packets and creates copies. The tap redirects one copy to the originating device for timestamping and another copy to the destination device for receive timestamping, enabling one-way delay measurement without requiring specialized equipment at the network devices.
Solution Approach 2:
The network tap creates a copy of the original packet and sends it back to the transmitting device. This copied packet is used to generate the origin timestamp, while the original packet continues to the destination for receive timestamping. The copying mechanism enables timestamp generation without requiring the destination device to have egress timestamping capability.
2Ease of operation
If round-trip time divided by two is used in asymmetric networks, then measurement simplicity is maintained, but measurement precision deteriorates
Solution Approach 1:
The system implements a feedback mechanism where the destination device sends back the receive timestamp in a response packet. The originating device receives this feedback and combines it with the locally recorded origin timestamp to calculate the one-way link delay, providing accurate measurement without requiring complex port role reversal operations.
3Measurement precision
If egress packet timestamping is implemented, then measurement precision is improved, but device functionality requirements increase
Solution Approach 1:
Instead of requiring the destination device to timestamp outgoing packets (egress timestamping), the system inverts the approach by having the originating device timestamp incoming copies of its own outgoing packets (ingress timestamping). This inversion allows standard network devices to provide accurate timestamps without requiring egress timestamping capability.
Data Source
AI summary
A method for measuring one-way link delay includes transmitting a first packet from a first network device. The first packet is passively intercepted, a first copy of the first packet is transmitted to the first network device, and a second copy of the first packet is transmitted to a second network device. A time of receipt of the first copy of the first packet is recorded as an origin timestamp at the first network device. A time of receipt of the second copy of the first packet is recorded as a receive timestamp at the second network device. A second packet including the origin timestamp is transmitted from the first network device. The second packet is passively intercepted, and a first copy of the second packet is transmitted to the second network device. The first copy of the second packet is received at the second network device, and the origin timestamp is extracted from the first copy of the second packet. Link delay from the first network device to the second network device is calculated using the origin timestamp and the receive timestamp.