|TCP option 13, CC Echo|
|Protocol type:||Transport layer protocol.|
|Option length:||6 bytes.|
|Links:||IANA: TCP option numbers.|
RFC 1644, pg. 1:
An experimental TCP extension for efficient transaction-oriented (request/response) service.
RFC 1644, pg. 4:
T/TCP introduces a 32-bit incarnation number, called a "connection count" (CC), that is carried in a TCP option in each segment. A distinct CC value is assigned to each direction of an open connection. A T/TCP implementation assigns monotonically increasing CC values to successive connections that it opens actively or passively.
T/TCP uses the monotonic property of CC values in initial <SYN> segments to bypass the Three-Way Handshake (3WHS), using a mechanism that we call TCP Accelerated Open (TAO). Under the TAO mechanism, a host caches a small amount of state per remote host. Specifically, a T/TCP host that is acting as a server keeps a cache containing the last valid CC value that it has received from each different client host.
T/TCP defines three new TCP options, each of which carries one 32-bit CC value. These options are named, Connection Count CC, CC.NEW, and CC.ECHO.
|MAC header||IP header||TCP header||TCP option 13||Data|
TCP Option 13:
Kind. 8 bits. Set to 13.
Length. 8 bits. Set to 6.
Connection Count (SEG.CC).
This option must be sent (in addition to a CC option) in a segment containing both a SYN and an ACK bit, if the initial SYN segment contained a CC or CC.NEW option. Its SEG.CC value is the SEG.CC value from the initial SYN. A CC.ECHO option should be sent only in a <SYN,ACK> segment and should be ignored if it is received in any other segment.
[RFC 1644] T/TCP -- TCP Extensions for Transactions Functional Specification.