TCP option 12, CC New

Description Glossary RFCs Publications Obsolete RFCs


Protocol suite: TCP/IP.
Protocol type:Transport layer protocol.
Option length:6 bytes.
Host implementation:
Router implementation:
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 12 Data

TCP Option 12:

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15
Kind Length
Connection Count

Kind. 8 bits. Set to 12.

Length. 8 bits. Set to 6.

Connection Count (SEG.CC). 4 bytes.
This option may be sent instead of a CC option in an initial <SYN> segment (i.e., SYN but not ACK bit), to indicate that the SEG.CC value may not be larger than the previous value. Its SEG.CC value is the TCB.CCsend value from the sender's TCB.



[RFC 1644] T/TCP -- TCP Extensions for Transactions Functional Specification.


Obsolete RFCs:

Description Glossary RFCs Publications Obsolete RFCs