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rfc:rfc955

Network Working Group R. Braden Request for Comments: 955 UCLA OAC

                                                        September 1985
                  Towards a Transport Service for
                Transaction Processing Applications

STATUS OF THIS MEMO

 This RFC is concerned with the possible design of one or more new
 protocols for the ARPA-Internet, to support kinds of applications
 which are not well supported at present.  The RFC is intended to spur
 discussion in the Internet research community towards the development
 of new protocols and/or concepts, in order to meet these unmet
 application requirements.  It does not represent a standard, nor even
 a concrete protocol proposal.  Distribution of this memo is
 unlimited.

1. INTRODUCTION

 The DoD Internet protocol suite includes two alternative transport
 service [1] protocols, TCP and UDP, which provide virtual circuit and
 datagram service, respectively [RFC-793, RFC-768].  These two
 protocols represent points in the space of possible transport service
 attributes which are quite "far apart".  We want to examine an
 important class of applications, those which perform what is often
 called "transaction processing".  We will see that the communication
 needs for these applications fall into the gap "between" TCP and UDP
 -- neither protocol is very appropriate.
 We will then characterize the attributes of a possible new
 transport-level protocol, appropriate for these ill-served
 transaction-processing applications.
 In writing this memo, the author had in mind several assumptions
 about Internet protocol development.
  • Assumption 1: The members of the Internet research community

now understand a great deal about protocols, and given a list

       of consistent attributes we can probably generate a reasonable
       protocol to meet that specification.
       This is not to suggest that design of good protocols is easy.
       It does reflect an assumption (perhaps wrong) that the set of
       basic protocol techniques we have invented so far is sufficient
       to give a good solution for any point in the attribute space,
       and that we can forsee (at least in a general way) many of the
       consequences of particular protocol design choices.

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RFC 955 September 1985 Transaction Protocol

  • Assumption 2: We need to develop appropriate service

requirements for a "transaction processing protocol".

       The classifications "virtual circuit" and "datagram"
       immediately define in our minds the most important attributes
       of TCP and UDP.  We have no such immediate agreement about the
       services to be provided for transaction processing.  The
       existing and proposed transaction-oriented protocols show a
       number of alternative choices [e.g., Cour81, BiNe84, Coop84,
       Cher85, Crow85, Gurw85, Mill85].
 Many of the ideas discussed here are not new.  For example, Birrell
 and Nelson [BiNe84] and Watson [Wats81] have described
 transport-level protocols appropriate for transactions.  Our purpose
 here is to urge the solution of this problem within the Internet
 protocol family.

2. TRANSACTION PROCESSING COMMUNICATIONS

 We begin by listing the characteristics of the communication patterns
 typical in "transaction processing" applications.
  • Unsymmetrical Model
       The two end points of the communication typically take
       different roles, generally called "client" and "server".  This
       leads to an unsymmetrical communication pattern.
       For example, the client always initiates a communication
       sequence or "transaction".  Furthermore, an important subclass
       of applications uses only a simple exchange of messages, a
       "request" to the server followed by a "reply" to the client.
       Other applications may require a continuing exchange of
       messages, a dialog or "conversation".  For example, a request
       to read a file from a file server might result in a series of
       messages, one per file block, in reply. More complex patterns
       may occur.
  • Simplex Transfers
       Regardless of the pattern, it always consists of a series of
       SIMPLEX data transfers; at no time is it necessary to send data
       in both directions simultaneously.

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RFC 955 September 1985 Transaction Protocol

  • Short Duration
       Transaction communication sequences generally have short
       duration, typically 100's of milliseconds up to 10's of
       seconds, but never hours.
  • Low Delay
       Some applications require minimal communication delay.
  • Few Data Packets
       In many applications, the data to be sent can be compressed
       into one or a few IP packets.  Applications which have been
       designed with LAN's in mind are typically very careful to
       minimize the number of data packets for each request/reply
       sequence.
  • Message Orientation
       The natural unit of data which is passed in a transaction is an
       entire message ("record"), not a stream of bytes.

3. EXAMPLE: NAME SERVERS

 To focus our ideas, we will now discuss several particular types of
 distributed applications which are of pressing concern to members of
 the Internet research community, and which require
 transaction-oriented communication.
 First, consider the name server/name resolver system [RFC-882,
 RFC-883] which is currently being introduced into the (research)
 Internet.  Name servers must use TCP and/or UDP as their transport
 protocol.  TCP is appropriate for the bulk transfers needed to update
 a name server's data base.  For this case, reliability is essential,
 and virtual-circuit setup overhead is negligible compared to the data
 transfer itself.  However, the choice of a transport protocol for the
 transaction traffic -- queries and responses -- is problematic.
  • TCP would provide reliable and flow-controlled transfer of

arbitrary-sized queries and responses. However, TCP exacts a

       high cost as a result of its circuit setup and teardown phases.
  • UDP avoids the overhead of TCP connection setup. However, UDP

has two potentially-serious problems – (1) unreliable

       communication, so that packets may be lost, duplicated, and/or

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RFC 955 September 1985 Transaction Protocol

       reordered; and (2) the limitation of a data object
       (query/response) to the 548-byte maximum in a single UDP
       packet.
 At present, name servers are being operated using UDP for transaction
 communication.  Note that name server requests have a special
 property, idempotency; as a result, lost, duplicated, or reordered
 queries do not prevent the name-server system from working.  This
 would seem to favor the use of UDP.
 However, it seems quite likely that the defects of UDP will make it
 unusable for an increasing fraction of queries.
  • The average size of individual replies will certainly increase,

as the more esoteric mail lookup features are used, as the host

       population explodes (resulting in a logarithmic increase in
       domain name sizes), and as the number of alternate acceptable
       answers increases.  As a result, a single response will more
       often overflow a single UDP packet.
  • The average end-to-end reliability will decrease as some of the

flakier paths of the Internet are brought into use by name

       resolvers.
       This will lead to a serious problem of choosing an appropriate
       retransmission timeout.  A name resolver using UDP cannot
       distinguish packet loss in the Internet from queueing delay in
       the server.  As a result, name servers we have seen have chosen
       long fixed timeouts (e.g., 30 seconds or more).  This will
       result in long delays in name resolution when packets are lost.
       One might think that delays in name resolution might not be an
       issue since most name lookups are done by a mailer daemon.
       However, ARPANET experience with user mail interfaces has shown
       that it is always desirable to verify the correctness of each
       host name as the user enters the "To:" and "CC:" addresses for
       a message. Hence, delays due to lost UDP packets will be
       directly visible to users.
 More generally, the use of UDP violates sound communication system
 design in two important ways:
  • The name resolver/server applications have to provide timeouts

and retransmissions to protect against "errors" (losses) in the

       communication system.  This certainly violates network
       transparency, and requires the application to make decisions
       for which it is not well-equipped.

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RFC 955 September 1985 Transaction Protocol

       As a general design principle, it seems that (Inter-) network
       properties, especially bad properties, ought to a large extent
       to be hidden below the transport-service boundary [2].
  • The name resolver/server applications must know the maximum

size of a UDP datagram.

       It is clearly wrong for an application program to contain
       knowledge of the number 576 or 548!  This does not imply that
       there cannot be a limitation on the size of a message, but any
       such limitation should be imposed by the particular
       application-level protocol, not the transport or internetwork
       level.
 It seems that the TCP/UDP choice for name servers presents an ugly
 dilemma.  We suggest that the solution should be a new
 transaction-oriented transport protocol with the following features:
  • Reliable ("at-least-once") Delivery of Data;
  • No Explicit Connection Setup or Teardown Phases;
  • Fragmentation and Reassembly of Messages;
  • Minimal Idle State in both Client and Server.

4. ANOTHER EXAMPLE: DISTRIBUTED OPERATING SYSTEMS

 Distributed operating systems represent another potential application
 for a transaction-oriented transport service.  A number of examples
 of distributed operating systems have been built using high-speed
 local area networks (LAN's) for communication (e.g, Cronus, Locus,
 V-System).  Typically, these systems use private communication
 protocols above the network layer, and the private transport-level
 protocol is carefully designed to minimize latency across the LAN.
 They make use of the inherent reliability of the LAN and of simple
 transactions using single-packet exchanges.
 Recently there have been efforts to extend these systems to operate
 across the Internet [Cher85, Shel85].  Since these are not "open"
 systems, there is no requirement that they use a standard transport
 protocol. However, the availability of a suitable transport protocol
 for transactions could considerably simplify development of future
 distributed systems.
 The essential requirement here seems to be packet economy.  The same

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RFC 955 September 1985 Transaction Protocol

 minimal two-packet exchange used over the LAN should be possible
 across the Internet.  This leads to two requirements for supporting
 distributed operating systems:
  • No Explicit Connection Setup or Teardown Phases;
  • Implicit ("piggy-backed") Acknowledgments Whenever Possible.
       This implies that the response packet will serve as an implicit
       acknowledgment to the request packet (when timing makes this
       possible).  Similarly, a new request (for the same pair of
       addressable entities) would implicitly acknowledge the previous
       response, if it came soon enough.
 The nature of the application imposes two other requirements:
  • Reliable ("at-most-once"), Ordered Delivery
       However, it should be possible to relax the reliability to take
       advantage of special cases like an idempotent request.
  • Multicast Capability
       The transport service should mesh cleanly with the proposed
       Internet multicast facility, using host groups [ChDe85].

5. OBJECTIVES FOR A PROTOCOL

 We believe that it is possible to design a new transport protocol for
 the Internet which is suitable for a wide variety of
 transaction-oriented applications.  Such a transport protocol would
 have the following attributes:
  • Reliable Delivery
       Data will be delivered reliably, i.e., exactly once, or the
       sender will be informed.  The protocol must be able to handle
       loss, duplication, and reordering of request and response
       packets.  In particular, old duplicate request packets must not
       cause erroneous actions.
       It should also be possible for the application programs to
       request that the reliability be relaxed for particular
       transactions.  This would allow communication economies in the
       case of idempotent requests or of notification without reply.

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RFC 955 September 1985 Transaction Protocol

  • Minimum Number of Packets in Simple Cases
       In the simplest case (small messages, no packet losses, and the
       interval between requests and replies between the same pair of
       addressable entities shorter than applicable timeouts), a
       simple two-packet exchange should result.
  • No Explicit Connection Setup or Teardown Phases
       The protocol will not create virtual circuits, but will provide
       what is sometimes (confusingly) called "reliable datagram"
       service.
       However, in order to provide a minimum two-packet exchange,
       there must be some implicit state or "soft" virtual circuit
       between a pair of addressable entities. In recent discussions
       this has been dubbed a "conversation", to distinguish it from a
       connection.
  • Minimal Idle State
       When a server is not processing a transaction, there will be no
       state kept (except enough to recognize old duplicate packets
       and to suppress unneeded ACK packets).
  • Fragmentation/Reassembly of Large Messages
       There is a range of possible objectives here. The minimum
       requirement is that the application not have to know the number
       576, 548, etc. For example, each application might establish
       its own "natural" upper limit on the size of a message, and
       always provide a buffer of that size [3].
       At the other extreme, the protocol might allow very large
       messages (e.g., a megabyte or more).  In this case, the
       proposed protocol would, in the large-message limit, be
       performing the bulk data transfer function of TCP.  It would be
       interesting to know whether this is possible, although it is
       not necessarily a requirement.
       The introduction of multi-packet messages leads to the complex
       issues of window sizes and flow control.  The challenge is to
       handle these efficiently in the absence of connection setup.
  • Message Orientation

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RFC 955 September 1985 Transaction Protocol

       The basic unit of communication will be an entire message, not
       a stream of bytes.  If a message has to be segmented, it will
       be delivered in units of segments or buffers, not bytes.
  • Multicast Capability
 Based on this discussion, we can suggest some of the key issues and
 problems in design of this protocol.
  • Choice of Addressable Entity
       What will be the addressable entity?  It must be unique in
       space; must it be unique in time (even across system crashes) ?
  • Timeout Dynamics
       Timeouts must be the key to operation of this protocol.
       Experience with TCP has shown the need for dynamic selection of
       an appropriate timeout, since Internet delays range over four
       decimal orders of magnitude.
       However, the absence of connection setup and the
       typically-short duration of a single interaction seem to
       preclude the dynamic measurement of delays.
  • Multi-Packet Messages
       How can flow control be provided for multi-packet messages, to
       provide reasonable throughput over long-delay paths without
       overrun with short-delay paths, when there is no virtual
       circuit setup?
  • Implementation Efficiency
       The protocol should lend itself to efficient (which probably
       implies simple) implementations, so that hosts will be willing
       to use it over LAN's as well as for general Internet
       communication.
 We believe further study is needed on these questions.
 The reader may wonder: how is the proposed protocol related to an RPC
 (Remote Procedure Call) facility?  The intent is that RPC facilities
 and message-passing IPC facilities will be built on top of the
 proposed transport layer.  These higher-level mechanisms will need to
 address a number of additional issues, which are not relevant to the
 communication substrate:

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RFC 955 September 1985 Transaction Protocol

    1.  Application Interface
       This includes binding and stub generators.
    2.  Structured Data Encoding
    3.  Server Location and Binding
    4.  Authentication and Access Control

6. CONCLUSION

 Distributed processing and distributed data bases will underlie many
 of the future computer system research projects and applications
 based upon the Internet.  As a result, transaction-based
 communication will be an increasingly important activity on the
 Internet.  We claim that there is a pressing need for an appropriate
 transport protocol for transaction processing.  In this memo, we have
 given examples to support this claim, and have outlined the service
 which such a new transport protocol would provide.
 This memo is based upon discussions within the New End-to-End
 Protocols taskforce, and it is a pleasure to acknowledge the
 participation and sagacity of the members of that group.  I want to
 thank Dave Clark, an ex officio taskforce member, for his
 contribution to these discussions, and Robert Cole for very helpful
 suggestions.

NOTES:

 [1]  For the purposes of this RFC, in fact, the reader may consider
      "transport service" to be defined as that protocol layer which
      contains TCP and UDP, as in Figure 1 of RFC-791.  Alternatively,
      we may use the ISO definition -- the transport service is the
      lowest layer providing end-to-end service which is essentially
      independent of the characteristics of the particular (Inter-)
      network used to support the communication.
 [2]  This idea is implicit in the ISO definition of a transport
      service.
 [3]  It would be reasonable for the name server definition to specify
      an upper bound on the size of a single query or response; e.g.,
      2K bytes.  This would imply (large) limits on the number of RR's
      that could be returned per response. If that limit is exceeded,
      we are doing something wrong!

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RFC 955 September 1985 Transaction Protocol

REFERENCES

 BiNe84   Birrell, Andrew D. and Nelson, Bruce Jay, "Implementing
          Remote Procedure Calls". ACM TOCS, Vol. 2, No. 1, February
          1984.
 ChDe85   Cheriton, David R. and Deering, Steven, "Host Groups: a
          Multicast Extension for Datagram Networks".  To be presented
          to 9th Data Communication Symposium.
 Cher85   Cheriton, David R., "V Message Transaction Protocol".
          Private communication, July 1985.
 Cour81   Xerox Corp., "Courier: The Remote Procedure Call Protocol".
          XSIS 038112, Xerox Corp., Stamford, Conn., December 1981.
 Coop84   Cooper, Eric C., "Circus: a Replicated Procedure Call
          Facility".  Proc. 4th Symposium on Reliability in
          Distributed Software and Database Systems, October 1984.
 Crow85   Crowcroft, Jon, "A Sequential Exchange Protocol".  Internal
          Note 1688, Computer Science Department, University College
          London, June 1985.
 Gurw85   Gurwitz, Robert F., "Object Oriented Interprocess
          Communication in the Internet".  Private communication,
          April 1985.
 Mill85   Miller, Trudy, "Internet Reliable Transaction Protocol --
          Functional and Interface Specification".  RFC-938, February
          1985.
 Shel85   Sheltzer, Alan B. , "Network Transparency in an Internetwork
          Environment", PhD Thesis, UCLA Department of Computer
          Science, July 1985.
 Wats81   Watson, Richard W., "Timer-based Mechanisms in Reliable
          Transport Protocol Connection Management".  Computer
          Networks, Vol. 5, pp47-56, 1981 (also distributed as
          IEN-193).

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