Type Indicates the particular type of Extension. The length does NOT include the Type and Length bytes. The particular data associated with this Extension. This field may be zero or more bytes in length. The format and length of the data field is determined by the type and length fields.
Extensions allow variable amounts of information to be carried within each datagram.
The end of the list of Extensions is indicated by the total length of the IP datagram. When an Extension numbered in either of these sets within the range 0 through 127 is encountered but not recognized, the message containing that Extension MUST be silently discarded. When an Extension numbered in the range 128 through 255 is encountered which is not recognized, that particular Extension is ignored, but the rest of the Extensions and message data MUST still be processed. The Length field of the Extension is used to skip the Data field in searching for the next Extension. Some IP protocol implementations insist upon padding ICMP messages to an even number of bytes.