Trailor Encapsulation

Trailor Encapsulation
This is described in RFC 1122 and RFC 892, but this scheme is not used very often today. The trailer
protocol [LINK:1] is a link-layer encapsulation method that rearranges the data contents of packets sent
on the physical network. It may be used but only after it is verified that both the sending and receiving
hosts support trailers. The verification is done for each host that is communicated with.
RFC 1122 states: "Only packets with specific size attributes are encapsulated using trailers, and typically
only a small fraction of the packets being exchanged have these attributes. Thus, if a system using trailers
exchanges packets with a system that does not, some packets disappear into a black hole while others are
delivered successfully."

Trailer negotiation is performed when ARP is used to discover the media access control (MAC) address
of the destination host. RFC 1122 states: "a host that wants to speak trailers will send an additional
"trailer ARP reply" packet, i.e., an ARP reply that specifies the trailer encapsulation protocol type but
otherwise has the format of a normal ARP reply. If a host configured to use trailers receives a trailer ARP
reply message from a remote machine, it can add that machine to the list of machines that understand
trailers, e.g., by marking the corresponding entry in the ARP cache."