RFC 2140 (rfc2140) - Page 3 of 11

TCP Control Block Interdependence

RFC 2140           TCP Control Block Interdependence          April 1997


TCB Interdependence

   The observation that some TCB state is host-pair specific rather than
   application-pair dependent is not new, and is a common engineering
   decision in layered protocol implementations. A discussion of sharing
   RTT information among protocols layered over IP, including UDP and
   TCP, occurred in [8]. T/TCP uses caches to maintain TCB information
   across instances, e.g., smoothed RTT, RTT variance, congestion
   avoidance threshold, and MSS [3].  These values are in addition to
   connection counts used by T/TCP to accelerate data delivery prior to
   the full three-way handshake during an OPEN. The goal is to aggregate
   TCB components where they reflect one association - that of the
   host-pair, rather than artificially separating those components by
   connection.

   At least one current T/TCP implementation saves the MSS and
   aggregates the RTT parameters across multiple connections, but omits
   caching the congestion window information [4], as originally
   specified in [2]. There may be other values that may be cached, such
   as current window size, to permit new connections full access to
   accumulated channel resources.

   We observe that there are two cases of TCB interdependence. Temporal
   sharing occurs when the TCB of an earlier (now CLOSED) connection to
   a host is used to initialize some parameters of a new connection to
   that same host. Ensemble sharing occurs when a currently active
   connection to a host is used to initialize another (concurrent)
   connection to that host. T/TCP documents considered the temporal
   case; we consider both.

An Example of Temporal Sharing

   Temporal sharing of cached TCB data has been implemented in the SunOS
   4.1.3 T/TCP extensions [4] and the FreeBSD port of same [7]. As
   mentioned before, only the MSS and RTT parameters are cached, as
   originally specified in [2]. Later discussion of T/TCP suggested
   including congestion control parameters in this cache [3].

   The cache is accessed in two ways: it is read to initialize new TCBs,
   and written when more current per-host state is available. New TCBs
   are initialized as follows; snd_cwnd reuse is not yet implemented,
   although discussed in the T/TCP concepts [2]:









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