Fragmentation The path from one node to another always includes at least one hop, and through the Internet usually many more. Each one of these hops, or links, is able to transport packets at some maximum size, called the MTU, as illustrated in Figure 3.10. A node must adjust the size of the packets it sends to accommodate the hop that has the smallest MTU in the packet's path from the node to its peer [40]. If a node sends too large a packet, it will be dropped by the router prior to the link whose MTU is not large enough to transport the packet. When a packet is dropped, an ICMPv6 Packet Too Big (see Section 3.3.1) Introduction to IPv6 Peer A Router 1 Router 2 Router 3 Peer B Link 1 Link 2 Link 3 Link 4 Figure 3.10 Illustration of links with different MTUs. Path MTU would be 1300 bytes - the smallest of MTUs on the path. message is generated. IPv6 mandates 1280 byte minimum MTU that must be provided by every link on the Internet [21]. If a host intents to use larger than a 1280 byte packet size, it must perform a process called Path MTU Discovery [40]. The MTU of the first-hop link can have a default value depending on the link type, such as commonly 1500 bytes for Ethernet links, but it should also be dynamically learned via MTU Option on Router Advertisement [41]. In the hypothetical example scenario presented in Figure 3.10 two peers, A and B, are communicating over a route consisting of three routers and four links with different MTUs. When peer A sends a packet towards peer B, it initially uses a MTU of 1400 bytes, because that is the MTU of the first-hop link in this illustration. However, when a packet sent by peer A towards peer B reaches router 2, a problem occurs: the packet cannot be sent over link 3, which has an MTU of 1300 bytes - 100 bytes less than packet sent by peer A. The router 2 will send ICMPv6 Packet Too Big message containing the MTU of the next-hop link, in this case 1300 byes, to peer A. Peer A, having received this message, learns what is the MTU of the link that could not transport p