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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 © The Internet Society (2001). All Rights Reserved.
This memo describes how a BEEP (Blocks Extensible Exchange Protocol) session is mapped onto a single TCP (Transmission Control Protocol) connection.
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2. Session Management
3. Message Exchange
3.1. Flow Control
3.1.1. Channel Creation
3.1.2. Sending Messages
3.1.3. Processing SEQ Frames
3.1.4. Use of Flow Control
4. Security Considerations
Appendix A. Acknowledgements
§ Author's Address
§ Intellectual Property and Copyright Statements
This memo describes how a BEEP (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.) [BEEP‑CORE] session is mapped onto a single TCP (Defense Advanced Research Projects Agency (DARPA), Information Processing Techniques Office and University of Southern California (USC)/Information Sciences Institute, “Transmission Control Protocol,” September 1981.) [RFC0793] connection. Refer to Section 2.5 of [BEEP‑CORE] (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.) for an explanation of the mapping requirements.
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:
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., [BEEP‑CORE] (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.)'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.
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.
Recall from Section 18.104.22.168 of [BEEP‑CORE] (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.) 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 [RFC1982] (Elz, R. and R. Bush, “Serial Number Arithmetic,” August 1996.) for a discussion of the arithmetic properties of sequence numbers.
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 (Processing SEQ Frames).
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 22.214.171.124 of [BEEP‑CORE] (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.). 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 [BEEP‑CORE] (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.), instead of a greeting.
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:
The choice is implementation-dependent, although it is recommended that the application using BEEP be given a mechanism for influencing the decision.
As an application accepts responsibility for incoming data frames, its BEEP peer should send SEQ frames to advertise a new window.
The ABNF (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” November 1997.) [RFC2234] 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 .
The SEQ frame has three parameters:
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.
The key to successful use of flow control within BEEP is to balance performance and fairness:
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 (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) [RFC1122] 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 126.96.36.199 of RFC1122 (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) [RFC1122] 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.
Consult Section [BEEP‑CORE] (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.)'s Section 9 for a discussion of security issues.
|[BEEP-CORE]||Rose, M., “The Blocks Extensible Exchange Protocol Core,” RFC 3080, March 2001.|
|[RFC0793]||Defense Advanced Research Projects Agency (DARPA), Information Processing Techniques Office and University of Southern California (USC)/Information Sciences Institute, “Transmission Control Protocol,” STD 7, RFC 793, September 1981.|
|[RFC1982]||Elz, R. and R. Bush, “Serial Number Arithmetic,” RFC 1982, August 1996.|
|[RFC2234]||Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” RFC 2234, November 1997.|
|[RFC1122]||Braden, R., “Requirements for Internet Hosts - Communication Layers,” STD 3, RFC 1122, October 1989.|
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.
|Marshall T. Rose|
|Invisible Worlds, Inc.|
|131 Stony Circle|
|Santa Rosa, CA 95401|
|Phone:||+1 707 578 2350|
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