NAT and firewalls make volunteer forwarders (JAPR) unreachable for inbound connections by default, removing the incentive for volunteers to reconfigure their systems for no personal benefit. The design response is to reverse the connection direction — JAPR initiates contact with JAPB — shifting the NAT/firewall configuration burden to the motivated blockee who gains direct benefit from solving it.

The paper presents a systematic taxonomy of blocking criteria across ISO/OSI layers: circumstance-based (addresses including sender/receiver/kind/physical location; timing including send time, receive time, duration, frequency; data-transfer properties; services including protocols, names, addresses) and content-based (file type/MIME, statistical detection of encrypted or compressed data, pattern matching for keywords or phrases, and website fingerprinting via request-count/byte-volume signatures).

§4, Appendix A defense

The paper proposes using CAPTCHAs (hard AI problems) to gate forwarder-list access, forcing the blocker to expend human resources solving every puzzle while each blockee solves only one. However, a 'stealing cycles from humans' attack allows a censor to relay CAPTCHAs to unwitting third parties (e.g., visitors to an attacker-operated website) who solve them on the censor's behalf.

§5.3 defense

For a secure steganographic system the embedding ratio is at least 1:10, meaning 1 MB of web content requires 10 MB of transmitted cover data; for a system robust against active attacks (e.g., StirMark bilinear distortions) the ratio is probably 1:100. A censor need not break the steganographic algorithm with high accuracy — suspicion alone is sufficient, since the censor can probe suspected nodes directly by acting as a blockee.

§5.1 evaluation

The protocol between blockee and volunteer forwarder is designed to be transport-layer independent from the outset, allowing substitution of plain TCP with SSL tunnels, SMTP, or steganographic channels as the censor escalates detection. The system is intentionally deployed in a weak initial form to observe how quickly and in what manner the censor adapts, then hardened iteratively based on measured censor behavior.

§6.2 defense

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

IP and port blocking dropped from 30% of countries historically to only 9% during the study period (six countries), with the decline attributed to difficulty maintaining ephemeral blocklists, CDN collateral damage, and IPv6 expansion. Iran is a significant exception: it has implemented port allowlisting — permitting only ports 80, 443, and 53 — on multiple occasions, blocking all other ports entirely.

2023-master-worldwide §4.3, Table 2 ip-blockingport-blocking

Camoufler's blocking-resistance relies on collateral-damage economics: IM platforms had ~2.5 billion active users as of January 2019 (projected >3 billion by 2022) and are embedded in essential business and commercial operations (airline e-tickets, professional collaboration tools). Blocking all IM to disrupt Camoufler would require the censor to harm its own economy; the threat model requires only that the censor permits at least one IM platform, in which case Camoufler remains operational.

2021-sharma-camoufler §3.1, §7 ip-blockingport-blocking

Protozoa uses the economic and social indispensability of popular WebRTC conferencing services as a censorship deterrent: blocking all WebRTC traffic imposes prohibitive collateral damage on legitimate commerce and communication. This 'parasitism' strategy means the circumvention tool inherits the blocking immunity of the carrier without requiring any protocol mimicry at the network level. Protozoa requires only one reachable WebRTC service to function, and Table 3 confirms at least five services remained unblocked in China during testing.

2020-barradas-poking §3, §7.2 ip-blockingport-blocking

Of 229 Thai Internet users surveyed, 63% (n=144) had attempted to circumvent censorship, and of those, roughly 90% (n=132) reported success using VPNs (32.64%), proxies (32.64%), or Tor (23.61%). Failures were isolated to proxies (n=2), VPNs (n=2), and alternative searches (n=3), indicating that existing circumvention tools were technically adequate but that availability and comprehensibility—not raw capability—were the binding constraints on user success.

2017-gebhart-internet §5.2.1, Table 2 ip-blockingport-blocking

GFW blocking was keyed on both IP address and port number, not IP address alone. Bridges with port 22 (SSH) open had that port remain reachable even as other ports on the same IP were blocked, confirming per-(IP, port) tuple granularity in the GFW blocklist.

2016-fifield-censors §4 Results ip-blockingport-blocking