1.1.e [i] IPv4 and IPv6 PMTU
TCP Maximum Segment Size (MSS) defines the maximum amount of data that a host is willing to accept in a single TCP/ IP datagram, it takes care of fragmentation at the two endpoints of a TCP connection, however it doesn’t handle the case where there is a smaller MTU link in the middle between these two endpoints. PMTUD was developed to avoid fragmentation in the path between the endpoints. It is used to dynamically determine the lowest MTU along the path from a packet’s source to its destination. PMTUD is only supported by TCP. If PMTUD is enabled on a host, and it almost always is, all TCP/ IP packets from the host will have the DF bit set.
When a host sends a full MSS data packet with the DF bit set, PMTUD works by reducing the send MSS value for the connection if it receives information that the packet would require fragmentation. A host usually “remembers” the MTU value for a destination by creating a “host” (/ 32) entry in its routing table with this MTU value.
If a router tries to forward an IP datagram, with the DF bit set, onto a link that has a lower MTU than the size of the packet, the router will drop the packet and return an Internet Control Message Protocol (ICMP) “Destination Unreachable” message to the source of this IP datagram, with the code indicating “fragmentation needed and DF set” (type 3, code 4). When the source station receives the ICMP message, it will lower the send MSS, and when TCP retransmits the segment, it will use the smaller segment size.
There are three things that can break PMTUD, two of which are uncommon and one of which is common.
● A router can drop a packet and not send an ICMP message.
● A router can generate and send an ICMP message but the ICMP message gets blocked by a router or firewall between this router and the sender. (Common)
● A router can generate and send an ICMP message, but the sender ignores the message. When path MTU discovery fails, it results in application slowdowns and timeouts since intermediary devices may be doing both fragmentation and reassembly.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1109-1121). . Kindle Edition.