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A host communicates with a distant web server through a site-to-site VPN tunnel. TCP connections can be established and small pages load, but large file downloads stall partway. The MTU inside the tunnel is 1400 bytes, and the server sends 1500-byte packets with fragmentation prohibited (the DF bit set to 1). The firewall in front of the server is also set to discard all ICMP. Which is the most likely cause?

  1. ADNS name resolution is failing
  2. BThe TCP receive window is too small, so not enough data can be sent at once
  3. CThe ICMP messages saying that fragmentation is needed, sent by the router that discards the oversized packets, do not reach the server, so the server cannot make its packets smaller
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Answer: C. The ICMP messages saying that fragmentation is needed, sent by the router that discards the oversized packets, do not reach the server, so the server cannot make its packets smaller

In path MTU discovery, the source sends packets with the DF bit set to 1, and a router that discards a packet larger than its MTU reports the size that can pass in an ICMP message (Fragmentation Needed in IPv4, Packet Too Big in IPv6). If the firewall discards these messages, the server keeps sending large packets that are discarded every time, so only large data transfers stall (a path MTU black hole). Connection setup and small responses use packets smaller than 1400 bytes, so they get through without problems. Fixes are to stop discarding these ICMP messages, or to have the router at the tunnel entrance rewrite the TCP MSS to a smaller value. Taking the IPv4 and TCP headers as 20 bytes each, an MTU of 1400 bytes calls for an MSS of 1400 − 20 − 20 = 1360 bytes.

  • A. If name resolution failed, connections could not be established and even small pages would not load.
  • B. A small receive window slows transfers but does not make them stall, and it has nothing to do with packet size.
  • C. Correct. Without the ICMP messages, path MTU discovery cannot work, and only the large packets keep being discarded.
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