Documented Internet shutdown events grew from 75 in 2016 to 213 in 2019 across 33 countries, with individual shutdowns lasting from hours to 472 days (Chad). These shutdowns completely sever IP connectivity, rendering all existing circumvention tools (Tor, VPNs, Shadowsocks, etc.) non-functional since they require at least partial Internet access to operate.

At 64 bps FSK encoding over cellular voice, Dolphin achieves a bit error rate below 2% across all tested data sizes (100–5000 bytes), cellular providers, and geographic distances up to ~3600 miles. Rates of 128 bps and above cause BER to jump to 5–22%, making transmission too unreliable for practical use.

§5.1, Table 1 evaluation

A 280-character tweet via Dolphin takes under 1 minute end-to-end; a 500-character email takes approximately 2.7 minutes (∼1 minute for ECDH secure-channel setup plus ∼1.7 minutes for data transmission). Performance was confirmed during a real Internet shutdown in Delhi, India, where a 300-character email transferred reliably in about 1 minute.

§5.2, §5.4, Figs. 8–10 evaluation

FSK-encoded Dolphin audio is distinguishable from normal human speech via offline amplitude analysis: Dolphin's mean signal amplitude is 0.4 (std 224) versus 205 (std 1590) for natural speech — approximately an order of magnitude lower — enabling classification by a telecom operator who records calls. The paper also notes that standard CRC checksums appearing periodically every chunk provide a unique detectable signature if the adversary attempts to decode the audio.

§6.5, Figs. 11–12 detection

An adversary introducing audio perturbations every 2.5 seconds (sufficient to corrupt each 20-byte chunk at 64 bps) degrades PESQ call quality to 1.6, below the 2.0 'unusable' threshold, making the attack self-defeating. However, targeting only acknowledgment windows (every ~12.5 s under Dolphin's default batch-of-5 configuration) achieves PESQ 3.6 — acceptable to human callers — while fully disrupting Dolphin data transfer.

§6.2 defense

Brussee measures a systematic pattern of Chinese government websites actively blocking access from outside China (the "reverse Great Firewall"), publishing a CSV dataset of affected domains (available at zenodo.org/records/18172145). The paper frames this outbound geo-blocking as a cybersecurity-motivated practice — Chinese authorities classify foreign access to domestic government infrastructure as an attack surface — distinct from the inbound information control goal of the GFW.

2026-brussee-reverse-great-firewall §3, §5 ip-blockingasn-blackholing

Brussee develops a conceptual framework distinguishing two logics of government geo-blocking: (1) information control (blocking inbound foreign content from domestic users) and (2) data sovereignty / attack-surface reduction (blocking outbound access by foreign actors to domestic systems). Chinese government site blocking of external IPs is motivated primarily by the second logic, creating an asymmetric internet topology where CN citizens cannot reach the outside world, and outside actors cannot probe CN government infrastructure.

2026-brussee-reverse-great-firewall §2, §4 ip-blockingasn-blackholing

NetShuffle targets edge networks — small autonomous systems and entities that obtain IP address blocks from upstream providers — as a new class of support base for circumvention infrastructure. This class has received scant attention from prior work, which has focused on cloud providers and volunteer desktop machines. Edge networks represent a large pool of diverse IP space that is harder to block via ASN blackholing compared to a small number of major cloud providers.

2024-kon-netshuffle §1, §2 ip-blockingasn-blackholing

27 of 80 tested VPN providers had servers within a single AS (AS 9009, M247 Ltd), and VPNalyzer identified 14 providers sharing 4 specific IP blocks within that AS; 2 additional providers shared an IP block in AS 60068 (Datacamp). Such infrastructure concentration enables censors to block multiple VPN products simultaneously with a single IP-range or AS-level rule.

2022-ramesh-vpnalyzer §VI-D ip-blockingasn-blackholing

Seven years of Roskomnadzor blocklist history (Nov 2012–April 2019) show the list grew to 132,798 unique domains and 324,695 unique IPs, with a dramatic spike in 2018 when Russia blocked Telegram by adding subnets covering approximately 16 million IP addresses—producing major collateral damage to co-hosted Google and Amazon services and illustrating that subnet-level blocking is the blunt instrument of last resort for CDN-hosted targets.

2020-ramesh-decentralized §VI-A, §II-B ip-blockingasn-blackholing

Cloud-based onion routing confronts censors with a collateral-damage dilemma: blocking a cloud provider's IP prefixes requires blocking all co-hosted services (Amazon EC2 hosted over 1 million instances sharing common IP prefixes in 2010), while allowing the traffic means circumvention succeeds. Rotating IP addresses—by retiring and spinning up new VM instances or via DHCP/gratuitous ARPs—reduces the window a blocked address remains in service, forcing censors into a perpetual cat-and-mouse game across all major cloud providers simultaneously.

2011-jones-hiding §2.3 ip-blockingasn-blackholing