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Delay-Tolerant Networking Research Group                     S. Burleigh
Internet-Draft                                Jet Propulsion Laboratory,
Intended status: Experimental                   California Institute of
Expires: July 8, 2007                                         Technology
                                                         January 4, 2007


                Compressed Bundle Header Encoding (CBHE)
                         draft-burleigh-cbhe-00

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2007).

Abstract

   This document describes a convention for representing Delay-Tolerant
   Networking (DTN) Bundle Protocol (BP) endpoint identifiers in a
   compressed manner within the primary blocks of bundles.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Compression convention  . . . . . . . . . . . . . . . . . . . . 3
     2.1.  Constraints . . . . . . . . . . . . . . . . . . . . . . . . 4
     2.2.  Method  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . . . 6
     3.1.  Transmission  . . . . . . . . . . . . . . . . . . . . . . . 6
     3.2.  Reception . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

































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1.  Introduction

   This document describes a convention for representing Delay-Tolerant
   Networking (DTN) Bundle Protocol (BP) endpoint identifiers in a
   compressed manner within the primary blocks of bundles.

   Each DTN bundle's primary block contains four BP endpoint identifiers
   (EIDs), of which any two, any three, or even all four may be
   lexically identical: the endpoint identifiers of the source, the
   destination, the report-to endpoint, and the current custodian.  Each
   EID is a Uniform Record Identifier (URI), an ASCII string of the form
   "scheme_name:scheme_specific_part".

   A degree of block compression is provided by the design of the
   primary block: the scheme names and scheme-specific parts of the four
   endpoints' IDs - up to eight NULL-terminated strings - are
   concatenated at the end of the block in a variable-length character
   array called a "dictionary", enabling each EID to be represented by a
   pair of integers indicating the offsets (within the dictionary) of
   the EID's scheme name and scheme-specific part.  Duplicate strings
   may be omitted from the dictionary, so the actual number of
   concatenated NULL-terminated strings in the dictionary may be less
   than eight and two or more of the scheme name or scheme-specific part
   offsets in the block may have the same value.  Moreover, the eight
   offsets in the primary block are encoded as self-delimiting numeric
   values (SDNVs), which shrink to fit the encoded values; when the
   total length of the dictionary is less than 127 bytes, all eight
   offsets can be encoded into just eight bytes.

   However, these strategems do not prevent the scheme names and scheme-
   specific parts themselves from being arbitrarily lengthy strings of
   ASCII text.  It is therefore still possible for the length of a
   bundle's primary header to be a very large fraction of the total
   length of the bundle when the bundle's payload is relatively small,
   as is anticipated for a number of DTN applications such as space
   flight operations.

   The Compressed Bundle Header Encoding (CBHE) convention was developed
   to improve DTN transmission efficiency for such applications by
   further reducing the number of bytes used to express EIDs in the
   primary blocks of bundles.


2.  Compression convention







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2.1.  Constraints

   Compressed Bundle Header Encoding (CBHE) is possible only when the
   scheme names of all non-null endpoint IDs in the primary block of a
   given bundle (that is, all endpoint IDs in the primary block that are
   not "dtn:none") identify a single scheme that is understood
   universally within the network to be "CBHE-conformant".

   All CBHE-conformant schemes are numbered and are identifiable by
   their numbers as well as their names.  A one-to-one correspondence
   between scheme numbers and the names of CBHE-conformant schemes is
   assumed to be well-known throughout the network (either by management
   or, as the scope of the network grows, by means of a protocol yet to
   be developed).

   The scheme-specific part of each entity-identifying URI whose scheme
   name identifies a CBHE-conformant scheme must be of the form
   "element_number.service_number".

   By convention, element number is a non-negative integer that can be
   thought of as a BP node identifier, such as a spacecraft identifier.
   Element number zero is used to indicate the null endpoint; any EID
   whose scheme name identifies a CBHE-conformant scheme and whose
   element number is zero, regardless of service number, is interpreted
   within the CBHE convention as an alternative representation of the
   standard null endpoint ID "dtn:none".

   By convention, service number is a non-negative integer that
   functions as a de-multiplexing token.  When the protocol encapsulated
   within BP has its own de-multiplexing identifiers, the service number
   may function in a manner similar to that of the protocol number in an
   IP packet, characterizing the bundle payload; alternatively, the
   service number may function in a manner similar to that of the port
   number in a UDP datagram.  Service numbers enable inbound bundles'
   application data units to be de-multiplexed to elements of
   application functionality that are designed to process them, so that
   effective communication relationships can be developed between bundle
   producers and consumers.

   Service number zero is used exclusively to identify BP custody signal
   traffic.

   Conversion of an EID in URI form - where the URI's scheme is CBHE-
   conformant - to and from a tuple of three integers is therefore
   straightforward.  This ease of conversion enables an array of
   integers to serve the same function as a dictionary of EID ASCII
   strings.




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   To summarize, CBHE representation of EIDs in a given bundle's primary
   block is constrained as follows:

   o  The scheme names of all non-null endpoint IDs must be the same.

   o  The scheme name common to all non-null endpoint IDs must identify
      a scheme that is well-known to be CBHE-conformant, and that scheme
      must be uniquely identified by a well-known scheme number.

   o  The scheme-specific parts of all non-null endpoint IDs must be in
      "element_number.service_number" form, where both element_number
      and service_number are non-negative integers.

   o  If the bundle's Current Custodian endpoint ID is not null, then
      the service number for that endpoint ID must be zero.

2.2.  Method

   When the constraints summarized above are met, the CBHE block
   compression method can be applied.  In a CBHE-compressed primary
   block, the eight SDNVs that normally contain EIDs' offsets within the
   dictionary are instead used to contain the eight integer values
   listed below, in the order shown:

   1.  The scheme number that is common to all non-null endpoint IDs.

   2.  The element number of the destination endpoint ID, or zero if the
       destination endpoint is the null endpoint.

   3.  The service number of the destination endpoint ID, or zero if the
       destination endpoint is the null endpoint.

   4.  The element number of the source endpoint ID, or zero if the
       source endpoint is the null endpoint.

   5.  The service number of the source endpoint ID, or zero if the
       source endpoint is the null endpoint.

   6.  The element number of the report-to endpoint ID, or zero if the
       report-to endpoint is the null endpoint.

   7.  The service number of the report-to endpoint ID, or zero if the
       report-to endpoint is the null endpoint.

   8.  The element number of the current custodian endpoint ID, or zero
       if the current custodian endpoint is the null endpoint.

   Further, the dictionary is omitted from the primary block and the



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   primary block's dictionary length is set to zero.

   This compression method is applied at the convergence layer: the
   transmitting convergence-layer adaptation compresses the primary
   block as shown above, and upon reception the receiving convergence-
   layer adaptation de-compresses the block by simply reversing the
   process.


3.  Specification

   CBHE compression is a convergence-layer adaptation.  It is opaque to
   bundle processing.  It therefore has no impact on the
   interoperability of different Bundle Protocol implementations, but
   instead affects only the interoperability of different convergence
   layer adaptation implementations.

   Bundle Protocol convergence-layer adapters that conform to the CBHE
   specification must implement the following procedures.

3.1.  Transmission

   When and only when required by the bundle protocol agent to transmit
   to some CBHE-conformant convergence-layer adapter a bundle whose
   primary block's endpoint IDs satisfy the constraints summarized in
   section 2.1 above and whose extension blocks (if any) contain no
   citations of endpoint IDs that are contained in the primary block's
   dictionary, the convergence layer adapter may encode the primary
   block of the bundle in accordance with the CBHE compression
   convention described in section 2.2 above.

3.2.  Reception

   Upon receiving a bundle whose dictionary length is zero (and only in
   this circumstance), the convergence layer adapter must decode the
   primary block of the bundle in accordance with the CBHE compression
   convention described in section 2.2 above before delivering it to the
   bundle protocol agent.


4.  IANA Considerations

   A new registry of CBHE-conformant URI schemes and corresponding
   scheme numbers may be needed.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.




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5.  Security Considerations

   CBHE introduces no new security considerations beyond those discussed
   in the DTN Bundle Protocol and Bundle Security Protocol
   specifications.


6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.


Author's Address

   Scott Burleigh
   Jet Propulsion Laboratory, California Institute of Technology
   4800 Oak Grove Drive, m/s 301-490
   Pasadena, CA  91109
   USA































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Full Copyright Statement

   Copyright (C) The Internet Society (2007).

   This document is subject to the rights, licenses and restrictions
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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