Network Working Group M.T. Rose Internet-Draft Invisible Worlds, Inc. Expires: June 16, 2001 December 16, 2000 Mapping the BEEP Framework onto TCP draft-ietf-beep-tcpmapping-05 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 June 16, 2001. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract This memo describes how a BEEP session is mapped onto a single TCP connection. Rose Expires June 16, 2001 [Page 1] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Session Management . . . . . . . . . . . . . . . . . . . . . 4 3. Message Exchange . . . . . . . . . . . . . . . . . . . . . . 5 3.1 Flow Control . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.1 Channel Creation . . . . . . . . . . . . . . . . . . . . . . 6 3.1.2 Sending Messages . . . . . . . . . . . . . . . . . . . . . . 7 3.1.3 Processing SEQ Frames . . . . . . . . . . . . . . . . . . . 8 3.1.4 Use of Flow Control . . . . . . . . . . . . . . . . . . . . 9 References . . . . . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . 10 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 Full Copyright Statement . . . . . . . . . . . . . . . . . . 12 Rose Expires June 16, 2001 [Page 2] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 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 Expires June 16, 2001 [Page 3] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 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 OPEN call is termed the listener; and, o the BEEP peer that issues an active OPEN call is termed the initiator. A BEEP session is released when either peer issues the CLOSE call, and the TCP connection is subsequently closed. A BEEP session is terminated when either peer issues the ABORT call, and the TCP connection is subsequently aborted. Rose Expires June 16, 2001 [Page 4] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 3. Message Exchange The mapping of BEEP exchanges 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 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 Expires June 16, 2001 [Page 5] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 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. 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. Rose Expires June 16, 2001 [Page 6] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 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, 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. Rose Expires June 16, 2001 [Page 7] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 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[3] 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. Rose Expires June 16, 2001 [Page 8] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 3.1.4 Use of Flow Control The key to successful use of flow control within BEEP is to balance performance and fairness: o large messages should be segmented into multiple frames (e.g., the ideal BEEP 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 frame 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 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 should 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. Finally, implementors should follow the guidelines given in the relevant portions of RFC1122[4] 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[4] 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 Expires June 16, 2001 [Page 9] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 References [1] Rose, M.T., "The Blocks Extensible Exchange Protocol Framework", draft-ietf-beep-framework-09 (work in progress), December 2000. [2] Postel, J., "Transmission Control Protocol", RFC 793, STD 7, Sep 1981. [3] Crocker, D. H. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [4] Braden, R.T., "Requirements for Internet hosts - communication layers", RFC 1122, STD 3, Oct 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 Expires June 16, 2001 [Page 10] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 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 Expires June 16, 2001 [Page 11] Internet-Draft Mapping the BEEP Framework onto TCP December 2000 Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. 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