Although not specific to Mobile IP, it makes sense to discuss path maximum transmission unit (MTU), because it has implications on the tunnel soft state and encapsulation methods. This is especially significant in the metro mobility model, because the tunnel path can traverse multiple autonomous systems with varying MTU capabilities. Further discussion of path MTU discovery as it relates to Mobile IP is presented in Chapter 8, "Deployment Scalability and Management."
Usually, an upper limit exists on the size of a frame that can be sent over a link, and this limit is called the link MTU. An IP packet that is larger than the link MTU is fragmented (broken into smaller portions or fragments) and sent over the link. When considering an entire path to the destination, the path MTU is then the smallest link MTU among all links in the path. The path MTU represents the largest amount of data that can be sent along the entire path without having to be fragmented. To prevent fragmentation, devices usually determine the path MTU of the route between a source and destination by using the Path MTU Discovery mechanism, which works as follows:
When using tunnel encapsulation, the DF bit must be copied onto the outer tunnel header. This ensures that the path MTU discovery can still be used. However, the sender might have to adjust the size of the IP packet to account for the extra tunnel header in meeting path MTU requirements.
Path MTU discovery can be problematic in open Internet scenarios. Many firewalls are configured to discard all ICMP messages, and encountering a PTU MTU discovery black hole is therefore possible. That is the case, for example, when ICMP messages are being generated but not returned to the source. Some operating systems include black-hole discovery mechanisms that disable path MTU discovery after multiple retransmissions fail. Without black-hole discovery, users often see odd behavior on their computers, such as web pages that start to load but never finish.
The black-hole problem is more common when Mobile IP is used because the added header of the tunnel might be detectable in only one direction. Typically the smallest MTU seen in the Internet is 1500 bytes, so one common workaround is to lower the maximum MTU on the Mobile Nodes to account for the 20-byte tunnel header. Users often round down and choose an MTU of 1400 bytes.