SkyF2F tunnels censored traffic through Skype's encrypted overlay network, forcing the censor into an all-or-nothing dilemma: blocking SkyF2F requires blocking Skype entirely, which causes actual economic damage to businesses and users who depend on it. Because Skype users are identified by pseudonym and all messages are routed to overlay addresses rather than Internet addresses, IP-based blocking, DNS filtering, port blocking, and keyword filtering are all rendered ineffective.

Website fingerprinting attacks that match file sizes and access patterns against a database of known sites remain applicable to SkyF2F, but are limited to the granularity of 512-byte fixed-size stream cells, since streams are multiplexed within a single tunnel circuit. The authors note this is less effective than against SafeWeb, where full request/response sizes are directly observable.

§V.A detection

Because Skype relies on a central login server, it is technically possible for a censor to block Skype, but the paper observes that blocking widely-deployed services like Skype or Google inflicts real economic harm, making it a credible deterrent. Additionally, Skype's proprietary, closed-source protocol and P2P architecture make it harder to characterize and selectively filter than open protocols.

§V.C defense

SkyF2F's friend-to-friend service model, where a server publishes its appid only to trusted contacts rather than publicly, provides significant resistance to both sybil attacks (malicious censor-controlled servers) and DoS exhaustion attacks. A censor posing as a client can establish many tunnels to exhaust a public server's resources; restricting service to a trusted friend list eliminates most of this attack surface.

§V.A defense

A censor hosting Skype supernodes can perform passive traffic-flow analysis on relayed streams even without breaking encryption, since supernode-relayed conversations expose traffic metadata. However, with thousands of supernodes in the Skype network, the probability that any censor-controlled supernode relays a specific SkyF2F tunnel is low, making large-scale correlation high-cost.

§V.A detection

Table 1 of the survey documents that by 2013–2014 censors were deploying simultaneous blocking across BGP, DNS, IP/port filtering, TCP disruption, TLS, and application-layer keyword filtering. No single detection tool in the survey covers all six layers; the most comprehensive, OONI (2012), covers DNS, IP/port, TCP, TLS, keyword, and HTTP but notes only partial BGP coverage.

2015-aceto-internet Table 1 bgp-hijackdns-poisoningport-blockingip-blockingrst-injectiontls-fingerprintkeyword-filteringdpi

The June 2025 Iran shutdown—carried out during the Iran-Israel war beginning ~June 19—did not use BGP route withdrawals as in 2019. Instead, authorities applied service-level restrictions at the national border: DNS poisoning of foreign destinations, protocol whitelisting permitting only pre-approved domestic services, and DPI to block circumvention-tool traffic. Iran's international traffic fell roughly 90% while the country's BGP routes remained advertised, making the shutdown invisible to BGP-based monitoring systems. OONI measurement volume, which totalled 121,333 in June 2025, collapsed to under 200 submissions on June 19-20.

2025-iran-shutdown-measurement Executive Summary / §2 dpidns-poisoningport-blockingip-blocking

IRBlock discovered that 1.7M of 3.3M blocked apex domains (52%) were attributed to blanket suffix-level blocking rules rather than individual domain listings. Examples include regex patterns targeting all Israeli domains (.il TLD), adult content (.porn), and country-coded suffixes (.com.mx, .my.id). Of 87K Tranco-ranked apex domains analyzed, 37% fell into adult content, with entertainment and gambling following. Approximately 1.27M apex domains were jointly censored by both DNS and HTTP filters, while the two filters maintained operationally independent blocklists for a significant fraction of domains.

2025-tai-irblock §5.3 dns-poisoningip-blockingkeyword-filtering

GFWeb tested 1.02 billion domains against the GFW over 20 months and discovered 943,000 pay-level domains blocked by HTTP filters and 55,000 by HTTPS filters — the largest GFW blocklist dataset ever published. The HTTP-to-HTTPS ratio (17:1) confirms that the GFW's HTTPS keyword-based and SNI-based blocking covers far fewer domains than its HTTP host-header blocking, likely because HTTPS blocks carry higher collateral-damage risk.

2024-hoang-gfweb Abstract, §5.1 dpidns-poisoningip-blockingkeyword-filteringsni-blocking

The largest measurement study of Turkmenistan censorship to date tested 15.5 million domains and found more than 122,000 domains censored using separate blocklists for DNS, HTTP, and HTTPS. Reverse-engineering the blocking rules revealed approximately 6,000 over-blocking rules that cause incidental filtering of more than 5.4 million additional domains — a 44x collateral damage ratio relative to intentionally blocked domains.

2023-nourin-measuring §Results dns-poisoningdpikeyword-filteringip-blocking

The COVID-19 Wuhan lockdown caused geolocating Twitter users in China to increase 1.4-fold immediately, remaining 10% above pre-crisis baseline long-term; approximately 320,000 new Chinese users joined Twitter due to the crisis, and the available VPN application's ranking on the Chinese iPhone App Store jumped significantly around 23 January 2020 and maintained that elevated rank.

2022-chang-covid-19 Crisis Increased Censorship Circumvention ip-blockingdns-poisoningkeyword-filtering