TCP option 13, CC Echo

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 13 Data

TCP Option 13:

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 13.

Length. 8 bits. Set to 6.

Connection Count (SEG.CC). 4 bytes.
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.


Obsolete RFCs:

Description Glossary RFCs Publications Obsolete RFCs