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rfc:rfc3081

Network Working Group M. Rose Request for Comments: 3081 Invisible Worlds, Inc. Category: Standards Track March 2001

                   Mapping the BEEP Core onto TCP

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

 This memo describes how a BEEP (Blocks Extensible Exchange Protocol)
 session is mapped onto a single TCP (Transmission Control Protocol)
 connection.

Table of Contents

 1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1
 2.    Session Management . . . . . . . . . . . . . . . . . . . . . 2
 3.    Message Exchange . . . . . . . . . . . . . . . . . . . . . . 2
 3.1   Flow Control . . . . . . . . . . . . . . . . . . . . . . . . 3
 3.1.1 Channel Creation . . . . . . . . . . . . . . . . . . . . . . 3
 3.1.2 Sending Messages . . . . . . . . . . . . . . . . . . . . . . 3
 3.1.3 Processing SEQ Frames  . . . . . . . . . . . . . . . . . . . 4
 3.1.4 Use of Flow Control  . . . . . . . . . . . . . . . . . . . . 4
 4.    Security Considerations  . . . . . . . . . . . . . . . . . . 6
       References . . . . . . . . . . . . . . . . . . . . . . . . . 6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . 6
 A.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
       Full Copyright Statement . . . . . . . . . . . . . . . . . . 8

1. Introduction

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

Rose Standards Track [Page 1] RFC 3081 Mapping the BEEP Core onto TCP March 2001

2. Session Management

 The mapping of BEEP session management onto the TCP service is
 straight-forward.
 A BEEP session is established when a TCP connection is established
 between two BEEP peers:
 o  the BEEP peer that issues a passive TCP OPEN call is termed the
    listener; and,
 o  the BEEP peer that issues an active TCP OPEN call is termed the
    initiator.
 A simultaneous TCP OPEN would result in both BEEP peers believing
 they are the initiator and neither peer will be able to start any
 channels.  Because of this, services based on BEEP must be designed
 so that simultaneous TCP OPENs cannot occur.
 If both peers agree to release a BEEP session (c.f., [1]'s Section
 2.4), the peer sending the "ok" reply, immediately issues the TCP
 CLOSE call.  Upon receiving the reply, the other peer immediately
 issues the TCP CLOSE call.
 A BEEP session is terminated when either peer issues the TCP ABORT
 call, and the TCP connection is subsequently aborted.

3. Message Exchange

 The mapping of BEEP exchanges onto the TCP service is less straight-
 forward.
 Messages are reliably sent and received using TCP's 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 BEEP session, BEEP
 must provide a mechanism to avoid starvation and deadlock.  To
 achieve this, BEEP re-introduces a mechanism used by the TCP:
 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.

Rose Standards Track [Page 2] RFC 3081 Mapping the BEEP Core onto TCP March 2001

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 data 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.  Consult
 Sections 2 through 5 of [3] for a discussion of the arithmetic
 properties of sequence numbers.

3.1.1 Channel Creation

 When a channel is created, the sequence number associated with the
 first payload octet of the first data frame is 0, and the initial
 window size for that channel is 4096 octets.  After channel creation,
 a BEEP peer may update the window size by sending a SEQ frame
 (Section 3.1.3).
 If a BEEP peer is asked 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 2.3.1.2 of [1].
 Similarly, during establishment of the BEEP session, if the BEEP peer
 acting in the listening role is unable to allocate at least 4096
 octets for channel 0, then it must return a negative reply, as
 specified in Section 2.4 of [1], instead of a greeting.

3.1.2 Sending Messages

 Before a message is sent, the sending BEEP peer must ensure that the
 size of the payload is within the window advertised by the receiving
 BEEP peer.  If not, it has three choices:
 o  if the window would allow for at least one payload octet to be
    sent, the BEEP peer may segment the message and start by sending a
    smaller data frame (up to the size of the remaining window);
 o  the BEEP peer may delay sending the message until the window
    becomes larger; or,

Rose Standards Track [Page 3] RFC 3081 Mapping the BEEP Core onto TCP March 2001

 o  the BEEP 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 BEEP be given a mechanism for influencing
 the decision.

3.1.3 Processing SEQ Frames

 As an application accepts responsibility for incoming data frames,
 its BEEP peer should send SEQ frames to advertise a new window.
 The ABNF [4] 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 BEEP session is terminated without generating a
 response, and it is recommended that a diagnostic entry be logged.

3.1.4 Use of Flow Control

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

Rose Standards Track [Page 4] RFC 3081 Mapping the BEEP Core onto TCP March 2001

 o  large messages should be segmented into frames no larger than
    two-thirds of TCP's negotiated maximum segment size;
 o  frames for different channels with traffic ready to send should be
    sent in a round-robin fashion;
 o  each time a frame is received, a SEQ frame should be sent whenever
    the window size that will be sent is at least one half of the
    buffer space available to this channel; and,
 o  if the transport service presents multiple frames to a BEEP 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 BEEP 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 must
 have higher priority than messages for that channel.
 Implementations may wish to provide queue management facilities to
 the application using BEEP, 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).
 In addition, where possible, implementations should support transport
 layer APIs that convey congestion information.  These APIs allow an
 implementation to determine its share of the available bandwidth, and
 also be notified of changes in the estimated path bandwidth.  Note
 that when a BEEP session has multiple channels that are
 simultaneously exchanging large messages, implementations without
 access to this information may have uncertain fairness and progress
 properties during times of network congestion.
 Finally, implementors should follow the guidelines given in the
 relevant portions of RFC1122 [5] that deal with flow control (and
 bear in mind that issues such as retransmission, while they interact
 with flow control in TCP, are not applicable to this memo).  For
 example, Section 4.2.2.16 of RFC1122 [5] indicates that a "receiver
 SHOULD NOT shrink the window, i.e., move the right window edge to the
 left" and then discusses the impact of this rule on unacknowledged
 data.  In the context of mapping BEEP onto a single TCP connection,
 only the portions concerning flow control should be implemented.

Rose Standards Track [Page 5] RFC 3081 Mapping the BEEP Core onto TCP March 2001

4. Security Considerations

 Consult Section [1]'s Section 9 for a discussion of security issues.

References

 [1]  Rose, M., "The Blocks Extensible Exchange Protocol Core", RFC
      3080, March 2001.
 [2]  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
      September 1981.
 [3]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
      August 1996.
 [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.
 [5]  Braden, R., "Requirements for Internet Hosts -- Communication
      Layers", STD 3, RFC 1122, October 1989.

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/

Rose Standards Track [Page 6] RFC 3081 Mapping the BEEP Core onto TCP March 2001

Appendix A. Acknowledgements

 The author gratefully acknowledges the contributions of: Dave
 Crocker, Steve Harris, Eliot Lear, Keith McCloghrie, Craig Partridge,
 Vernon Schryver, and, Joe Touch.  In particular, Dave Crocker
 provided helpful suggestions on the nature of flow control in the
 mapping.

Rose Standards Track [Page 7] RFC 3081 Mapping the BEEP Core onto TCP March 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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
 and distributed, in whole or in part, without restriction of any
 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
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Rose Standards Track [Page 8]

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