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Versions: 00 01                                                         
Network Working Group                                          M.T. Rose
Internet-Draft                                    Invisible Worlds, Inc.
Expires: January 11, 2001                                  July 13, 2000


                  Mapping the BXXP Framework onto TCP
                     draft-mrose-bxxp-tcpmapping-01

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on January 11, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

   This memo describes how a BXXP session is mapped onto a single TCP
   connection.













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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Session Management . . . . . . . . . . . . . . . . . . . . .  4
   3.    Data Exchange  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.1   Flow Control . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.1.1 Channel Creation . . . . . . . . . . . . . . . . . . . . . .  6
   3.1.2 Sending REQ or RSP Messages  . . . . . . . . . . . . . . . .  7
   3.1.3 Processing SEQ Frames  . . . . . . . . . . . . . . . . . . .  8
   3.1.4 Use of Flow Control  . . . . . . . . . . . . . . . . . . . .  9
         References . . . . . . . . . . . . . . . . . . . . . . . . . 10
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
   A.    Changes from draft-mrose-bxxp-tcpmapping-00  . . . . . . . . 11
   B.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 13




































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1. Introduction

   This memo describes how a BXXP[1] session is mapped onto a single
   TCP[2] connection. Refer to Section 2.5 of [1] for an explanation of
   the mapping requirements.














































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2. Session Management

   The mapping of BXXP session management onto the TCP service is
   straight-forward.

   A BXXP session is established when a TCP connection is established
   between two BXXP peers:

   o  the BXXP peer that issues a passive OPEN call is termed the
      listener; and,

   o  the BXXP peer that issues an active OPEN call is termed the
      initiator.

   A BXXP session is released when either peer issues the CLOSE call,
   and the TCP connection is subsequently closed.

   A BXXP session is terminated when either peer issues the ABORT call,
   and the TCP connection is subsequently aborted.
































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3. Data Exchange

   The mapping of BXXP data exchange onto the TCP service is less
   straight-forward.

   Messages are reliably sent and received using the SEND and RECEIVE
   calls. (This also provides ordered delivery of messages on the same
   channel.)

   Although TCP imposes flow control on a per-connection basis, if
   multiple channels are simultaneously in use on a BXXP session, BXXP
   must provide a mechanism to avoid starvation and deadlock. To
   achieve this, BXXP re-introduces mechanisms used by the TCP:
   sequence numbers and window-based flow control -- each channel has a
   sliding window that indicates the number of payload octets that a
   peer may transmit before receiving further permission.



































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3.1 Flow Control

   Recall from Section 2.2.1.2 of [1] that every payload octet sent in
   each direction on a channel has an associated sequence number.
   Numbering of payload octets within a frame is such that the first
   payload octet is the lowest numbered, and the following payload
   octets are numbered consecutively.

   The actual sequence number space is finite, though very large,
   ranging from 0..4294967295 (2**32 - 1). Since the space is finite,
   all arithmetic dealing with sequence numbers is performed modulo
   2**32. This unsigned arithmetic preserves the relationship of
   sequence numbers as they cycle from 2**32 - 1 to 0 again.

3.1.1 Channel Creation

   When a channel is created, the sequence number associated with the
   first payload octet of the first frame is 0, and the initial window
   size for that channel is 4096 octets. After channel creation, a BXXP
   peer may update the window size by sending a "SEQ" frame (Section
   3.1.3).

   If a BXXP peer is requested to create a channel and it is unable to
   allocate at least 4096 octets for that channel, it must decline
   creation of the channel, as specified in Section 2.3.1.3 of [1].
   Similarly, during establishment of the BXXP session, if the BXXP
   peer acting in the listening role is unable to allocate at least
   4096 octets for channel 0, then it must return a negative response,
   as specified in Section 2.4 of [1]. instead of a greeting.






















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3.1.2 Sending REQ or RSP Messages

   Before a message is sent, the sending BXXP peer must ensure that the
   size of the payload is within the window advertised by the receiving
   BXXP peer. If not, it has three choices:

   o  if the window would allow for at least one payload octet to be
      sent, the BXXP peer may segment the message and start by sending
      a smaller frame (up to the size of the remaining window);

   o  the BXXP peer may delay sending the message until the window
      becomes larger; or,

   o  the BXXP peer may signal to its application that it is unable to
      send the message, allowing the application to try again at a
      later time (or perhaps signaling its application when a larger
      window is available.)

   The choice is implementation-dependent, although it is recommended
   that the application using BXXP be given a mechanism for influencing
   the decision.






























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3.1.3 Processing SEQ Frames

   As an application accepts responsibility for incoming frames, its
   BXXP peer should send "SEQ" frames to advertise a new window.

   The ABNF for a "SEQ" frame is:

       seq        = "SEQ" SP channel SP ackno SP window CR LF

       ackno      = seqno

       window     = size

       ; channel, seqno, and size are defined in Section 2.2.1 of [1].

   The "SEQ" frame has three parameters:

   o  a channel number;

   o  an acknowledgement number, that indicates the value of the next
      sequence number that the sender is expecting to receive on this
      channel; and,

   o  a window size, that indicates the number of payload octets
      beginning with the one indicated by the acknowledgement number
      that the sender is expecting to receive on this channel.

   A single space character (decimal code 32, " ") separates each
   component. The "SEQ" frame is terminated with a CRLF pair.

   When a "SEQ" frame is received, if any of the channel number,
   acknowledgement number, or window size cannot be determined or is
   invalid, then the BXXP session is terminated without generating a
   response, and it is recommended that a diagnostic entry be logged.

















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3.1.4 Use of Flow Control

   The key to successful use of flow control within BXXP is to balance
   performance and fairness:

   o  large messages should be segmented into multiple frames (e.g.,
      the ideal BXXP segment size should be no larger than TCP's
      negotiated maximum segment size minus some small constant);

   o  frames for different channels with traffic ready to send should
      be sent in a round-robin fashion; and,

   o  each time a "REQ" or "RSP" message is received, a "SEQ" frame
      should be sent whenever the window size is at least one half of
      the available buffer space (if the transport service presents
      multiple messages to a BXXP peer simultaneously, then a single
      consolidating "SEQ" frame may be sent).

   In order to avoid pathological interactions with the transport
   service, it is important that a BXXP peer advertise windows based on
   available buffer space, to allow data to be read from the transport
   service as soon as available. Further, "SEQ" frames for a channel
   should have higher priority than "REQ" or "RSP" messages for that
   channel.

   Finally, implementations may wish to provide queue management
   facilities to the application using BXXP, e.g., channel priorities,
   (relative) buffer allocations, and so on. In particular,
   implementations should not allow a given channel to monopolize the
   underlying transport window (e.g., slow readers should get small
   windows).




















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References

   [1]  Rose, M.T., "The Blocks eXtensible eXchange Protocol
        Framework", draft-mrose-bxxp-framework-01 (work in progress),
        July 2000.

   [2]  Postel, J., "Transmission Control Protocol", RFC 793, STD 7,
        Sep 1981.


Author's Address

   Marshall T. Rose
   Invisible Worlds, Inc.
   1179 North McDowell Boulevard
   Petaluma, CA  94954-6559
   US

   Phone: +1 707 789 3700
   EMail: mrose@invisible.net
   URI:   http://invisible.net/






























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Appendix A. Changes from draft-mrose-bxxp-tcpmapping-00

   o  The IPR notice is changed to be in full compliance of Section 10
      of RFC 2026.

   o  SEQ messages are now (correcty) called SEQ frames.

   o  In Section 3.1.4, the explanation of when to send a SEQ frame is
      clarified.

   o  In Section 3.1.4, the illustration of queue management facilities
      is expanded.







































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Appendix B. Acknowledgements

   Dave Crocker provided helpful suggestions on the nature of flow
   control in the mapping.















































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Full Copyright Statement

   Copyright (C) The Internet Society (2000). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
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   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

   Funding for the RFC editor function is currently provided by the
   Internet Society.



















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