One document matched: draft-irtf-dtnrg-cbhe-01.txt
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Network Working Group S. Burleigh
Internet-Draft Jet Propulsion Laboratory,
Intended status: Experimental California Institute of
Expires: September 6, 2009 Technology
March 5, 2009
Compressed Bundle Header Encoding (CBHE)
draft-irtf-dtnrg-cbhe-01
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Abstract
This document describes a convention for representing Delay-Tolerant
Networking (DTN) Bundle Protocol (BP) endpoint identifiers in a
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compressed manner within the primary blocks of bundles.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Method . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Transmission . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Reception . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
This document describes a convention for representing Delay-Tolerant
Networking (DTN) Bundle Protocol (BP) [RFC5050] 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) as defined by [RFC3986].
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 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
2.1. Constraints
Compressed Bundle Header Encoding (CBHE) is possible only when all
endpoint IDs in the primary block of a given bundle are "CBHE-
conformant". Two forms of endpoint ID are CBHE-conformant: (a) the
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null endpoint ID "dtn:none" and (b) any endpoint ID whose scheme name
is "dtn" and whose authority component is "cbhe.ccsds.org".
The specification for URIs formed within the "dtn" scheme requires
that the path component of each such URI that is characterized by
authority component "cbhe.ccsds.org" be a path-absolute of the form
/NODE_NUMBER.SERVICE_NUMBER.
By convention, node number is a natural number that identifies a BP
node. In a spacecraft flight operations context, for example,
spacecraft identifier might be used as node number.
Node number must be an integer in the range 1 to (2^64 - 1). Node
number zero is reserved for representation of the null endpoint ID in
the compressed form described later in this specification;
decompressing a compressed null EID must always yield the standard
null endpoint ID URI "dtn:none". Negative integers and integers
larger than (2^64 - 1) cannot be compressed into the SDNVs that are
used for representation of endpoint ID references in the primary
blocks of bundles and therefore could not be compressed as described
later.
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 instances 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 reserved for BP administrative traffic, i.e.,
custody signals and bundle status reports. Service number must be
zero whenever a CBHE-conformant EID is used as the source or
destination EID of a custody signal. Service number may be zero
whenever a CBHE-conformant EID is used as the source or destination
EID of a bundle status report. Service number must not be negative
or exceed (2^64 - 1) for the same reason that node number must not do
so.
For example, "dtn://cbhe.ccsds.org/9.37" would be a CBHE-conformant
endpoint ID.
Conversion of a CBHE-conformant EID to and from a tuple of two
integers is therefore straightforward. This ease of conversion
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enables an array of integers to serve the same function as a
dictionary of EID ASCII strings.
Note, however, that CBHE decompression cannot faithfully recreate the
dictionary of a compressed primary block from an array of integers
unless the order of the endpoint ID reference strings in the
dictionary of the original, uncompressed block is known. (The bundle
protocol specification does not require that the strings in the
dictionary appear in any particular order and does not require that
redundant strings be omitted from the dictionary.) Therefore, a
further precondition to CBHE block compression is that the endpoint
ID reference strings in the dictionary of the bundle to be compressed
must be as follows:
1. The scheme name of the destination endpoint ID.
2. The scheme-specific part of the destination endpoint ID.
3. The scheme name of the source endpoint ID, if and only if
different from any prior string in the dictionary.
4. The scheme-specific part of the source endpoint ID, if and only
if different from any prior string in the dictionary.
5. The scheme name of the report-to endpoint ID, if and only if
different from any prior string in the dictionary.
6. The scheme-specific part of the report-to endpoint ID, if and
only if different from any prior string in the dictionary.
7. The scheme name of the current custodian endpoint ID, if and only
if different from any prior string in the dictionary.
8. The scheme-specific part of the current custodian endpoint ID, if
and only if different from any prior string in the dictionary.
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 node number of the destination endpoint ID, or zero if the
destination endpoint is the null endpoint.
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2. The service number of the destination endpoint ID, or zero if the
destination endpoint is the null endpoint.
3. The node number of the source endpoint ID, or zero if the source
endpoint is the null endpoint.
4. The service number of the source endpoint ID, or zero if the
source endpoint is the null endpoint.
5. The node number of the report-to endpoint ID, or zero if the
report-to endpoint is the null endpoint.
6. The service number of the report-to endpoint ID, or zero if the
report-to endpoint is the null endpoint.
7. The node number of the current custodian endpoint ID, or zero if
the current custodian endpoint is the null endpoint.
8. The constant BP administration service number zero.
Further, the dictionary is omitted from the primary block and the
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. 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 identified 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
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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
This document has no IANA considerations.
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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, November 2007.
Author's Address
Scott Burleigh
Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive, m/s 301-490
Pasadena, CA 91109
USA
Phone: +1 818 393 3353
Email: Scott.C.Burleigh@jpl.nasa.gov
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