XOT (X.25 Over TCP) is designed by Cisco to transport X.25 over IP internets. The X.25 Packet Level requires a reliable link level below it and normally uses LAPB. XOT is a method of sending X.25 packets over IP internets by encapsulating the X.25 Packet Level in TCP packets.

TCP provides a reliable byte stream. X.25 requires that the layer below it provide message semantics, in particular the boundary between packets. To provide this, a small (4-byte) XOT header is used between TCP and X.25. The primary content of this header is a length field, which is used to separate the X.25 packets within the TCP stream.

In general, the normal X.25 protocol packet formats and state transition rules apply to the X.25 layer in XOT. Exceptions to this are noted.

Protocol Structure

• Version - The version number of XOT. It must be 0. If a none zero number received, the TCP session must be closed.
• Length - The length of the XOT packet. Values must be legal X.25 packet lengths. If the length field has an illegal value, then the TCP connection MUST be closed.

Related Terms: IP, IPv6, TCP, X.25

Sponsor Source: XOT is a Cisco protocol and circulated in IETF (http://www.ietf.org) RFC 1613.

Reference Links: http://www.javvin.com/protocol/rfc1613.pdf: Cisco Systems X.25 over TCP (XOT)