Path MTU Discovery

Path MTU Discovery
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Path MTU Discovery is a technique in computer networking for determining the maximum transmission unit (MTU) size on the network path between two IP hosts, usually with the goal of avoiding IP fragmentation.

Overview

Path MTU Discovery works by sending ICMP packets with the Don't Fragment (DF) flag set. If a packet is too large to be forwarded by a router along the path, the router will drop the packet and send back an ICMP "Fragmentation Needed" message, indicating the maximum packet size that the router can forward. The source host can then adjust its packet size accordingly.

Importance

The primary purpose of Path MTU Discovery is to optimize network performance by ensuring that packets are transmitted without fragmentation. Fragmentation can lead to increased latency and reduced throughput, as well as increased packet loss if any fragment is lost.

Implementation

Path MTU Discovery is implemented in most modern operating systems and is typically enabled by default. It is an important feature for IPv6, where fragmentation is handled only by the source node, unlike IPv4, where routers can fragment packets.

Challenges

One of the challenges with Path MTU Discovery is that some network devices incorrectly block ICMP messages, which can prevent the discovery process from working correctly. This can lead to a situation known as a "black hole," where packets are silently dropped.

Solutions

To mitigate issues with ICMP blocking, some systems implement Packetization Layer Path MTU Discovery (PLPMTUD), which does not rely on ICMP messages. Instead, it uses TCP or other transport protocols to probe the path and determine the MTU.

See also

• Maximum Transmission Unit
• Internet Protocol
• IPv4
• IPv6
• ICMP
• Packetization Layer Path MTU Discovery

References

• RFC 1191 - Path MTU Discovery for IPv4
• RFC 8201 - Path MTU Discovery for IPv6

External links

• RFC 1191 - Path MTU Discovery for IPv4
• RFC 8201 - Path MTU Discovery for IPv6