rfc1644.txt

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Network Working Group                                          R. Braden
Request for Comments: 1644                                           ISI
Category: Experimental                                         July 1994

                T/TCP -- TCP Extensions for Transactions
                        Functional Specification

Status of this Memo

   This memo describes an Experimental Protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   It does not specify an Internet Standard.  Distribution is unlimited.

Abstract

   This memo specifies T/TCP, an experimental TCP extension for
   efficient transaction-oriented (request/response) service.  This
   backwards-compatible extension could fill the gap between the current
   connection-oriented TCP and the datagram-based UDP.

   This work was supported in part by the National Science Foundation
   under Grant Number NCR-8922231.

Table of Contents

 1. INTRODUCTION ..................................................  2
 2.  OVERVIEW .....................................................  3
    2.1  Bypassing the Three-Way Handshake ........................  4
    2.2  Transaction Sequences ....................................  6
    2.3  Protocol Correctness .....................................  8
    2.4  Truncating TIME-WAIT State ............................... 12
    2.5  Transition to Standard TCP Operation ..................... 14
 3.  FUNCTIONAL SPECIFICATION ..................................... 17
    3.1  Data Structures .......................................... 17
    3.2  New TCP Options .......................................... 17
    3.3  Connection States ........................................ 19
    3.4  T/TCP Processing Rules ................................... 25
    3.5  User Interface ........................................... 28
 4.  IMPLEMENTATION ISSUES ........................................ 30
    4.1  RFC-1323 Extensions ...................................... 30
    4.2  Minimal Packet Sequence .................................. 31
    4.3  RTT Measurement .......................................... 31
    4.4  Cache Implementation ..................................... 32
    4.5  CPU Performance .......................................... 32
    4.6  Pre-SYN Queue ............................................ 33
 6.  ACKNOWLEDGMENTS .............................................. 34
 7.  REFERENCES ................................................... 34
 APPENDIX A.  ALGORITHM SUMMARY ................................... 35



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 Security Considerations .......................................... 38
 Author's Address ................................................. 38

1. INTRODUCTION

   TCP was designed to around the virtual circuit model, to support
   streaming of data.  Another common mode of communication is a
   client-server interaction, a request message followed by a response
   message.  The request/response paradigm is used by application-layer
   protocols that implement transaction processing or remote procedure
   calls, as well as by a number of network control and management
   protocols (e.g., DNS and SNMP).  Currently, many Internet user
   programs that need request/response communication use UDP, and when
   they require transport protocol functions such as reliable delivery
   they must effectively build their own private transport protocol at
   the application layer.

   Request/response, or "transaction-oriented", communication has the
   following features:

   (a)  The fundamental interaction is a request followed by a response.

   (b)  An explicit open or close phase may impose excessive overhead.

   (c)  At-most-once semantics is required; that is, a transaction must
        not be "replayed" as the result of a duplicate request packet.

   (d)  The minimum transaction latency for a client should be RTT +
        SPT, where RTT is the round-trip time and SPT is the server
        processing time.

   (e)  In favorable circumstances, a reliable request/response
        handshake should be achievable with exactly one packet in each
        direction.

   This memo concerns T/TCP, an backwards-compatible extension of TCP to
   provide efficient transaction-oriented service in addition to
   virtual-circuit service.  T/TCP provides all the features listed
   above, except for (e); the minimum exchange for T/TCP is three
   segments.

   In this memo, we use the term "transaction" for an elementary
   request/response packet sequence.  This is not intended to imply any
   of the semantics often associated with application-layer transaction
   processing, like 3-phase commits.  It is expected that T/TCP can be
   used as the transport layer underlying such an application-layer
   service, but the semantics of T/TCP is limited to transport-layer
   services such as reliable, ordered delivery and at-most-once



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   operation.

   An earlier memo [RFC-1379] presented the concepts involved in T/TCP.
   However, the real-world usefulness of these ideas depends upon
   practical issues like implementation complexity and performance.  To
   help explore these issues, this memo presents a functional
   specification for a particular embodiment of the ideas presented in
   RFC-1379.  However, the specific algorithms in this memo represent a
   later evolution than RFC-1379.  In particular, Appendix A in RFC-1379
   explained the difficulties in truncating TIME-WAIT state.  However,
   experience with an implementation of the RFC-1379 algorithms in a
   workstation later showed that accumulation of TCB's in TIME-WAIT
   state is an intolerable problem; this necessity led to a simple
   solution for truncating TIME-WAIT state, described in this memo.

   Section 2 introduces the T/TCP extensions, and section 3 contains the
   complete specification of T/TCP.  Section 4 discusses some
   implementation issues, and Appendix A contains an algorithmic
   summary.  This document assumes familiarity with the standard TCP
   specification [STD-007].

2.  OVERVIEW

   The TCP protocol is highly symmetric between the two ends of a
   connection.  This symmetry is not lost in T/TCP; for example, T/TCP
   supports TCP's symmetric simultaneous open from both sides (Section
   2.3 below).  However, transaction sequences use T/TCP in a highly
   unsymmetrical manner.  It is convenient to use the terms "client
   host" and "server host" for the host that initiates a connection and
   the host that responds, respectively.

   The goal of T/TCP is to allow each transaction, i.e., each
   request/response sequence, to be efficiently performed as a single
   incarnation of a TCP connection.  Standard TCP imposes two
   performance problems for transaction-oriented communication.  First,
   a TCP connection is opened with a "3-way handshake", which must
   complete successfully before data can be transferred.  The 3-way
   handshake adds an extra RTT (round trip time) to the latency of a
   transaction.

   The second performance problem is that closing a TCP connection
   leaves one or both ends in TIME-WAIT state for a time 2*MSL, where
   MSL is the maximum segment lifetime (defined to be 120 seconds).
   TIME-WAIT state severely limits the rate of successive transactions
   between the same (host,port) pair, since a new incarnation of the
   connection cannot be opened until the TIME-WAIT delay expires.  RFC-
   1379 explained why the alternative approach, using a different user
   port for each transaction between a pair of hosts, also limits the



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   transaction rate: (1) the 16-bit port space limits the rate to
   2**16/240 transactions per second, and (2) more practically, an
   excessive amount of kernel space would be occupied by TCP state
   blocks in TIME-WAIT state [RFC-1379].

   T/TCP solves these two performance problems for transactions, by (1)
   bypassing the 3-way handshake (3WHS) and (2) shortening the delay in
   TIME-WAIT state.

   2.1  Bypassing the Three-Way Handshake

      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 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.  If
      an initial <SYN> segment (i.e., a segment containing a SYN bit but
      no ACK bit) from a particular client host carries a CC value
      larger than the corresponding cached value, the monotonic property
      of CC's ensures that the <SYN> segment must be new and can
      therefore be accepted immediately.  Otherwise, the server host
      does not know whether the <SYN> segment is an old duplicate or was
      simply delivered out of order; it therefore executes a normal 3WHS
      to validate the <SYN>.  Thus, the TAO mechanism provides an
      optimization, with the normal TCP mechanism as a fallback.

      The CC value carried in non-<SYN> segments is used to protect
      against old duplicate segments from earlier incarnations of the
      same connection (we call such segments 'antique duplicates' for
      short).  In the case of short connections (e.g., transactions),
      these CC values allow TIME-WAIT state delay to be safely discuss
      in Section 2.3.

      T/TCP defines three new TCP options, each of which carries one
      32-bit CC value.  These options are named CC, CC.NEW, and CC.ECHO.
      The CC option is normally used; CC.NEW and CC.ECHO have special
      functions, as follows.






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      (a)  CC.NEW

           Correctness of the TAO mechanism requires that clients
           generate monotonically increasing CC values for successive
           connection initiations.  These values can be generated using
           a simple global counter.  There are certain circumstances
           (discussed below in Section 2.2) when the client knows that
           monotonicity may be violated; in this case, it sends a CC.NEW
           rather than a CC option in the initial <SYN> segment.
           Receiving a CC.NEW causes the server to invalidate its cache
           entry and do a 3WHS.

      (b)  CC.ECHO

           When a server host sends a <SYN,ACK> segment, it echoes the
           connection count from the initial <SYN> in a CC.ECHO option,
           which is used by the client host to validate the <SYN,ACK>
           segment.

      Figure 1 illustrates the TAO mechanism bypassing a 3WHS.  The
      cached CC values, denoted by cache.CC[host], are shown on each
      side.  The server host compares the new CC value x in segment #1
      against x0, its cached value for client host A; this comparison is
      called the "TAO test".  Since x > x0, the <SYN> must be new and
      can be accepted immediately; the data in the segment can therefore
      be delivered to the user process B, and the cached value is
      updated.  If the TAO test failed (x <= x0), the server host would
      do a normal three-way handshake to validate the <SYN> segment, but
      the cache would not be updated.






















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          TCP A  (Client)                              TCP B (Server)
          _______________                              ______________

                                                          cache.CC[A]
                                                            V

                                                          [ x0 ]

        #1        -->  <SYN, data1, CC=x> -->  (TAO test OK (x > x0) =>
                                                     data1->user_B and
                                                     cache.CC[A]= x; )

                                                           [ x ]
        #2       <-- <SYN, ACK(data1), data2, CC=y, CC.ECHO=x> <--
            (data2->user_A;)