IBR-derived metrics γ (average SYN retransmits per flow) and η (inter-packet time between retransmits) can distinguish packet-loss-induced outages from packet-filtering censorship: during Libya's 2011 packet-filtering phase γC remained near pre-censorship values despite reduced source counts, whereas BGP route leaks caused measurable γ decreases and η increases. This difference exists because filtering reduces the host population but preserves per-flow OS retransmit behavior, while congestion causes routers to drop individual packets mid-flow.

In the August 2012 Bell-Dery BGP route leak, TTL analysis at per-prefix granularity revealed that two IP addresses within AS577 maintained constant TTLs and unaffected packet rates throughout the disruption, while 37 of 38 other active /16 prefixes experienced significant volume drops and TTL changes indicating rerouting through longer paths. This demonstrates that BGP route leaks can affect subnets within a single AS asymmetrically, and that TTL inspection can identify unaffected sub-AS paths.

§IV-B evaluation

During the February 2012 Dodo-Telstra BGP route leak, AS1221 (Telstra) exhibited a 20-minute congestion phase in which γC and γ3 both dropped while η rose from approximately 3 to 5 seconds, followed by a complete outage during which zero darknet sources were observed from the AS. The congestion phase produced measurable packet loss before the full blackout, providing an early-warning window of roughly 20 minutes.

§IV-A evaluation

Conficker-like traffic to TCP port 445 constitutes more than 40% of packets observed at the UCSD /8 Network Telescope and Windows XP/NT hosts consistently emit exactly 2-packet SYN flows; γC stayed within the narrow band 1.98–2.02 throughout an entire month (January 2012) with no large-scale outages. A second signal from default Windows 3-SYN flows (approximately 156 million flows/month from ~14K hosts/hour) provides a non-malware-specific validation stream with inter-packet times consistently between 3.09 and 3.37 seconds.

§II–III evaluation

Libya's 2011 Internet shutdown combined two distinct censor techniques across separate episodes: BGP-level route withdrawal and later packet filtering. During the packet-filtering episode, γC remained near its pre-censorship baseline (~2.0 packets/flow) even as the number of reachable Conficker sources dropped, confirming that the mechanism was per-subnet allowlisting rather than link saturation.

§IV-C detection

Article 19 documents that Iran's National Information Network (NIN / SHOMA) was designed with explicit reference to China's Great Firewall as a model, with institutional mirroring: Iran's Supreme Council of Cyberspace parallels China's Cyberspace Administration of China, and both governments share a "cyber sovereignty" doctrine used to justify domestic content controls and cross-border technology transfer. The report frames Iran's filtering infrastructure as deliberately architected to replicate GFW capabilities, not as an independently developed system.

2026-article19-tightening-the-net §2, §3 dpisni-blockingdns-poisoningip-blockingbgp-hijackthrottling

The automated probe list generation system discovered 45.79 potentially blocked domains per 1,000 domains crawled, compared to 4.11 for FilteredWeb — over 10× higher efficacy. It uncovered 1,490 potentially blocked domains in crawls of just 71,960 URLs, versus 1,255 blocked domains found by Hounsel et al. in crawls of 1,000,000 URLs, with 1,473 of the 1,490 domains not overlapping with prior work.

2024-tang-automatic §5.5 measurement-platformdns-poisoningip-blocking

VPS-based vantage points in Singapore and India detected censorship patterns similar to 'free' locations, failing to observe blocking known to be enforced by local ISPs following government directives. This occurred because ISP-level censorship is implemented per-carrier rather than centrally, and the VPS provider's ISP did not enforce those blocks — confirmed by re-testing from a residential IP that did observe the expected blocks.

2024-tang-automatic §6.2 measurement-platformip-blocking

The authors' blockpage-based methodology cannot detect transit censorship implemented via TCP RST injection or packet drops, because distinguishing these from transient network errors requires identifying their location on the routing path. As a result, the 8-country, 6-AS finding is explicitly characterized as a lower bound on the true extent of Russian transit censorship.

2023-ortwein-towards §4.1 Gathering Blockpages / §6 Conclusion rst-injectionip-blockingmeasurement-platform

29 of 80 VPN providers — including paid services — configure clients to resolve DNS through third-party public resolvers (Google Public DNS, Cloudflare, OpenDNS, Quad9) rather than provider-operated infrastructure. Three self-hosted solutions (Algo, Streisand, Outline) hardcode public DNS with no easy override, causing connection failures in regions where those services are blocked.

2022-ramesh-vpnalyzer §VI (Fig. 5) ip-blockingmeasurement-platform

ICLab's commercial VPN vantage points reside in data-center ('content') ASes for 41% of monitored networks, which may experience less aggressive censorship than residential ISPs, making VPN-based measurements a systematic lower bound on blocking rates faced by ordinary users. In countries where both VPN and volunteer-operated device (VOD) vantages coexist, identical block pages were observed from both AS types, indicating similar overt blocking policies, but covert IP-based or RST-injection blocking may still differ by AS class.

2020-niaki-iclab §III-B measurement-platformip-blocking