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What is Path MTU discovery?

A mechanism that finds the smallest MTU on the path from source to destination (the path MTU) and sends packets no larger than that. The source sends packets with fragmentation prohibited, and a router that discards a packet too large to pass reports the size that can pass with ICMP (Fragmentation Needed in IPv4, Packet Too Big in IPv6). If this ICMP is discarded along the way, only large packets fail to arrive (a path MTU black hole).

Japanese name: 経路MTU探索

Path MTU discovery appears in 1 practice question across 1 topic, including Network Design and Operations.

Field: Network Design and Operations

Try it: questions about Path MTU discovery

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
Show answer and explanation

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.

Path MTU discovery in the explanations

Questions that mention Path MTU discovery (1)

  1. 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?

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Related terms

VPN (virtual private network) TCP (Transmission Control Protocol) MTU (maximum transmission unit) Packet Firewall ICMP (Internet Control Message Protocol) Router IPv4 IPv6 ACL (access control list)

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