未知行为：在使用P2P类软件（I2P、Fopnu、Bittorrent、ZeroNet等等）后，会被暂停一段时间对海外访问。/ Undocumented Filtering: P2P traffic (I2P, Fopnu, BitTorrent, etc.) triggers temporary international connectivity blackout. #583
Description
运营商：河南安阳-中国联通（5G移动网络和宽带），但我确定该行为在其他地区和运营商可复现。
具体问题：
在开启P2P类软件后，如果你的P2P软件依赖于TCP进行连接（例如：I2P-NTCP、Bittorrent-TCP、ZeroNet），则在运行一段时间后你就会发现本来能正常打开的海外网站被封锁（TCP连接握手被丢弃，而非RESET；不只是影响网站，是一切基于TCP协议的服务，此时仅有极少数海外域名可以访问，例如https://ajax.aspnetcdn.com/ ）。
但此时你使用基于UDP协议的VPN/代理软件则不受影响。
反过来，如果你的P2P软件基于UDP（例如：I2P-SSU、Fopnu、IPFS、Bittorrent-DHT/uTP），则触发反过来的审查现象——所有出国UDP包被丢弃，但基于TCP的代理/VPN还可以正常运行（例如Xray-Reality、Shadowsocks over HTTP ）。
在其他省份的复现方法为：使用4/5G移动网络，打开P2P类软件，即可迅速触发（比我在老家用中国联通宽带还要快，江苏省真有你的）。
此行为无法用封锁PCDN来解释，因为如果是为了封锁所谓的“PCDN”，则应该是断掉在国内的P2P连接；然而，在这个情况中，到国内的P2P连接完全未受影响，只有出国方向被破坏了。
此情况还有一个方法可以稳定触发：直接连接赛风3或者自由门的代理（不要设置前置代理），在江苏省还可以通过使用Snowflake/obfs4网桥触发。
恢复方法：重新启动光猫和路由器，如果你使用PPPoE拨号，则立即进行宽带重新拨号而无需重启设备。
附注：
PCDN（基于点对点的内容分发网络）是一个中国运营商的术语，指代一切点对点传输行为。
在中国有大量公司进行所谓的PCDN业务，但长话短说都是为了利用用户的网络和设备进行盈利（并降低自身的支出成本）。
在最初运营商是不在乎这类行为，但在后来由工信部主导下发了关于跨省传输结算的内容后，他们开始在乎起来P2P传输了。
如果你有兴趣可以访问 https://www.cww.net.cn/article?id=591786 ，请注意：这篇文章为简体中文。
ISP/Location: China Unicom in Anyang, Henan (both 5G mobile networks and broadband). However, I am certain this behavior is reproducible in other regions and with other ISPs.
Specific Issue/Behavior:
After launching P2P software, if your P2P connections rely on TCP (e.g., I2P-NTCP, BitTorrent-TCP, ZeroNet), you will notice after a period of running that previously accessible overseas websites become blocked.
Specifically, TCP handshakes are dropped rather than reset (no TCP RST). This affects not just websites, but all TCP-based services. During this time, only a tiny fraction of overseas domains (e.g., https://ajax.aspnetcdn.com/) remain accessible.
However, during this exact same period, UDP-based VPNs/proxies remain completely unaffected.
Conversely, if your P2P software is UDP-based (e.g., I2P-SSU, Fopnu, IPFS, BitTorrent-DHT/uTP), it triggers the inverse censorship behavior: all outbound (international) UDP packets are dropped, but TCP-based proxies/VPNs (such as Xray-Reality or Shadowsocks over HTTP) continue to function normally.
How to Reproduce (in other provinces):
Use a 4G/5G mobile network, launch any P2P software, and the filtering is triggered almost instantly (even faster than using my home China Unicom broadband. Jiangsu province is exceptionally aggressive in this regard).
Analysis:
This behavior cannot be explained away as an effort to block "PCDN" (P2P CDN) operations. If the ISP's goal were to block PCDN traffic, domestic P2P connections should be the ones being throttled or disconnected. In this scenario, however, domestic P2P connections remain completely intact—only outbound international traffic is disrupted.
Alternative Triggers:
There is another method to reliably trigger this phenomenon: connect directly to Psiphon 3 or Freegate (without setting up a front proxy). Furthermore, in Jiangsu province, using Snowflake or obfs4 bridges will also trigger this block.
Recovery Method:
Restart your ONT (modem) and router. If you are using a PPPoE connection, performing a simple PPPoE redial immediately restores connectivity without the need to reboot the devices.
Note / Context on PCDN:
"PCDN" (P2P Content Delivery Network) is a term widely used by Chinese ISPs to loosely refer to any peer-to-peer transmission activity.
In China, numerous companies operate so-called "PCDN businesses." Long story short, these are essentially schemes to exploit regular users' home networks and devices to generate profit (and significantly reduce the companies' own bandwidth overhead).
Initially, ISPs turned a blind eye to this behavior. However, after the MIIT (Ministry of Industry and Information Technology) introduced new regulations regarding inter-provincial network traffic settlement (cross-province bandwidth billing), ISPs suddenly became deeply invested in cracking down on P2P transmissions.
If you are interested in diving deeper, you can read this article: https://www.cww.net.cn/article?id=591786 (Please note: The article is in Simplified Chinese).