TTLmeting

TTLmeting is the practice of measuring the time-to-live (TTL) field in IP packets to characterize network paths and devices. The TTL is an 8-bit value set by the sender and decremented by each router it traverses; when the TTL reaches zero, the packet is discarded and an ICMP Time Exceeded message may be returned. Because TTL values are typically observed in responses, TTLmeting can reveal hop counts, routing behavior, and, to some extent, the operating system characteristics of intermediate devices.

Methods of TTLmeting can be active or passive. Active measurement uses traceroute-like techniques that elicit responses from routers or end hosts by sending probes with controlled or increasing TTL values, allowing estimation of the number of hops and the path taken. Tools such as traceroute, tracepath, and variations of ping implement these techniques. Passive measurement analyzes observed traffic to study the distribution of TTL values and their changes over time, which can help infer network structure, detect routing anomalies, or identify devices behind NAT.

Applications of TTLmeting include topology discovery, troubleshooting routing issues, estimating latency and path stability, and aiding in operating system fingerprinting where initial TTL values correlate with common OS defaults (though this is not definitive due to network devices and configurations that alter TTL). Limitations arise from firewalls that block ICMP Time Exceeded messages, NATs that hide internal TTLs, and configurations that modify TTLs, as well as the fact that TTL observations may not reflect the exact path due to load balancing and asymmetric routes. Ethical considerations and authorization are important when performing active TTL measurements on networks.