Compressed Bundle Header Encoding (CBHE)

Abstract

This document describes a convention by which Delay-Tolerant Networking (DTN) Bundle Protocol (BP) "convergence-layer" adapters may represent endpoint identifiers in a compressed form within the primary blocks of bundles, provided those endpoint identifiers conform to the structure prescribed by this convention.

Compressed Bundle Header Encoding (CBHE) compression is a convergence-layer adaptation. It is opaque to bundle processing. Therefore, it has no impact on the interoperability of different Bundle Protocol implementations, but instead affects only the interoperability of different convergence-layer adaptation implementations.

This document is a product of the Delay-Tolerant Networking Research Group and has been reviewed by that group. No objections to its publication as an RFC were raised.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.

This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internet-related research and development activities. These results might not be suitable for deployment. This RFC represents the consensus of the Delay-Tolerant Networking Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6260.

Copyright Notice

Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. Compression Convention ..........................................3
      2.1. Constraints ................................................3
      2.2. Method .....................................................6
   3. Specification ...................................................7
      3.1. Transmission ...............................................7
      3.2. Reception ..................................................7
   4. IANA Considerations .............................................8
   5. Security Considerations ........................................10
   6. References .....................................................11
      6.1. Normative References ......................................11
      6.2. Informative References ....................................11
   Appendix A. Acknowledgments .......................................12

1. Introduction

This document describes a convention by which Delay-Tolerant Networking (DTN) Bundle Protocol (BP) [RFC5050] "convergence-layer" adapters may represent endpoint identifiers (EIDs) in a compressed form within the primary blocks of bundles, provided those endpoint identifiers conform to the structure prescribed by this convention.

Each DTN bundle's primary block contains the following four BP endpoint identifiers, of which any two, any three, or even all four may be lexically identical: the endpoint identifiers of the bundle's source, destination, report-to endpoint, and current custodian. Each EID is a Uniform Record Identifier (URI) as defined by [RFC3986]. More specifically, each BP EID is a URI consisting of a "scheme name" followed by ":", followed by a sequence of characters -- historically, termed the "scheme-specific part" (SSP) in DTN specifications -- conforming to URI syntax as defined by RFC 3986.

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 especially the 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 (and as is in any case true of bundles carrying BP administrative records).

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 by convergence-layer adapters to represent EIDs in the primary blocks of bundles.

1.1. 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].

2. Compression Convention

2.1. Constraints

The only valid scheme name for BP EIDs identified to date is "dtn". Although no specification of valid SSP syntax for URIs composed within the "dtn" scheme has yet been formally defined, the syntax on which rough agreement has been reached in practice is unsuitable for CBHE's compression procedures. For the purposes of CBHE, then, this document defines an additional URI scheme named "ipn". As noted in Section 4, IANA has registered this new URI scheme.

Compressed Bundle Header Encoding (CBHE) is possible only when all endpoint IDs in the primary block of a given bundle are "CBHE conformant". The following two forms of endpoint ID are CBHE conformant: (a) the null endpoint ID "dtn:none" and (b) any endpoint ID formed within the "ipn" scheme.

The SSP of every URI formed within the "ipn" scheme must comprise:

1. A sequence of ASCII numeric digits representing an integer in the range 1 to (2^64 - 1), termed the "node number" of the URI.

2. An ASCII period ('.') character.

3. A sequence of ASCII numeric digits representing an integer in the range 0 to (2^64 - 1), termed the "service number" of the URI.

The node number notionally identifies a BP node. However, since CBHE is not used universally in Delay-Tolerant Networking, it must not be assumed that all BP nodes are identified by node numbers.

Negative integers and integers larger than (2^64 - 1) cannot be used as node numbers because they cannot be encoded into the SDNVs that are used for representation of scheme name and SSP offsets in the primary blocks of bundles and therefore could not be compressed as described later in this specification. Node number zero is reserved for representation of the null endpoint ID in the compressed form described later; decompressing a compressed null EID must always yield the standard null endpoint ID URI "dtn:none".

The service number notionally functions as a de-multiplexing token. When the bundle payload is a protocol data unit of some protocol that 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 encapsulated protocol; 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.

A service number must not be negative or exceed (2^64 - 1) for the same reason that a node number must not do so.

For example, "ipn: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: all characters in the EID other than the node number and service number are constant (as defined by the "ipn" scheme definition), and the node number and service number are taken as the two integers of the tuple. This ease of conversion enables an array of pairs of integers to serve the same function as a dictionary of ASCII string EIDs.

Note, however, that CBHE decompression cannot faithfully recreate the dictionary of a compressed primary block from an array of integer pairs unless the order of the scheme names and scheme-specific part 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 compression is that the strings in the dictionary of the bundle to be compressed must be exactly as follows, in this order and without addition:

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.

Note: this constraint implies that a bundle that includes any extension blocks containing EID-references to endpoint IDs other than the block's destination, source, report-to, and current custodian cannot be CBHE compressed since such compression would result in a dictionary that would deviate from this structure.

2.2. Method

When the constraints enumerated above are met, the CBHE block compression method can be applied by the convergence-layer adapter (CLA) at the time the bundle is transmitted via a convergence-layer protocol. In a CBHE-compressed primary block, the eight SDNVs that normally contain EIDs' scheme name and SSP 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.

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 service 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 primary block's dictionary length is set to zero.

Upon reception, the receiving convergence-layer adaptation de- compresses the block by simply reversing the process so that the bundle presented to the bundle protocol agent has the standard form (i.e., the dictionary is reconstituted).

3. Specification

CBHE compression is a convergence-layer adaptation. It is opaque to bundle processing. Therefore, it 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 a bundle whose primary block's endpoint IDs satisfy the constraints identified in Section 2.1, the CLA MAY encode the primary block of the bundle in accordance with the CBHE compression convention described in Section 2.2 UNLESS the CLA to which the bundle is to be transmitted is known not to be CBHE conformant. Note that CBHE compression may be applied only if the receiving CLA is known or presumed to be CBHE conformant, i.e., able to decode the encoded primary block. Knowledge as to whether or not a receiving CLA is (or might be) CBHE conformant may be asserted by node administration and/or may be inferred from reception of a CBHE-compressed bundle as noted in Section 3.2.

3.2. Reception

Upon receiving a bundle whose dictionary length is zero (and only in this circumstance), a CBHE-conformant convergence-layer adapter:

1. MAY infer that the CLA from which the bundle was received is CBHE conformant.

2. MUST decode the primary block of the bundle in accordance with the CBHE compression convention described in Section 2.2 before delivering it to the bundle protocol agent.

Note that when a CLA that is not CBHE conformant receives a bundle whose dictionary length is zero, it has no choice but to pass it to the bundle agent without modification. In this case, the bundle protocol agent will be unable to dispatch the received bundle, because it will be unable to determine the destination endpoint; the bundle will be judged to be malformed. The behavior of the bundle protocol agent in this circumstance is an implementation matter.

4. IANA Considerations

IANA has registered a provisional registration (per [RFC4395]) for a URI scheme for CBHE, with the string "ipn" as the scheme name, as follows:

URI scheme name: "ipn"

Status:

           provisional

URI scheme syntax:

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], including the core ABNF syntax rule for DIGIT defined by that specification.

   ipn-uri = "ipn:" ipn-hier-part
   ipn-hier-part = node-nbr nbr-delim service-nbr ; a path-rootless
   node-nbr = 1*DIGIT
   nbr-delim = "."
   service-nbr = 1*DIGIT

None of the reserved characters defined in the generic URI syntax are used as delimiters within URIs of the IPN scheme.

URI scheme semantics: URIs of the IPN scheme are used as endpoint identifiers in the Delay-Tolerant Networking (DTN) Bundle Protocol (BP) [RFC5050] as described in Section 2.1.

Encoding considerations: URIs of the IPN scheme are encoded exclusively in US-ASCII characters.

Applications and/or protocols that use this URI scheme name: the Delay-Tolerant Networking (DTN) Bundle Protocol (BP) [RFC5050].

Interoperability considerations: as noted above, URIs of the IPN scheme are encoded exclusively in US-ASCII characters.

Security considerations:

• Reliability and consistency: none of the BP endpoints identified by the URIs of the IPN scheme are guaranteed to be reachable at any time, and the identity of the processing entities operating on those endpoints is never guaranteed by the Bundle Protocol itself. Bundle authentication as defined by the Bundle Security Protocol is required for this purpose.

• Malicious construction: malicious construction of a conformant IPN-scheme URI is limited to the malicious selection of node numbers and the malicious selection of service numbers. That is, a maliciously constructed IPN-scheme URI could be used to direct a bundle to an endpoint that might be damaged by the arrival of that bundle or, alternatively, to declare a false source for a bundle and thereby cause incorrect processing at a node that receives the bundle. In both cases (and indeed in all bundle processing), the node that receives a bundle should verify its authenticity and validity before operating on it in any way.

• Back-end transcoding: the limited expressiveness of URIs of the IPN scheme effectively eliminates the possibility of threat due to errors in back-end transcoding.

• Rare IP address formats: not relevant, as IP addresses do not appear anywhere in conformant IPN-scheme URIs.

• Sensitive information: because IPN-scheme URIs are used only to represent the identities of Bundle Protocol endpoints, the risk of disclosure of sensitive information due to interception of these URIs is minimal. Examination of IPN-scheme URIs could be used to support traffic analysis; where traffic analysis is a plausible danger, bundles should be conveyed by secure convergence-layer protocols that do not expose endpoint IDs.

• Semantic attacks: the simplicity of IPN-scheme URI syntax minimizes the possibility of misinterpretation of a URI by a human user.

Contact:

      Scott Burleigh
      Jet Propulsion Laboratory,
      California Institute of Technology
      scott.c.burleigh@jpl.nasa.gov
      +1 (800) 393-3353

Author/Change controller:

      Scott Burleigh
      Jet Propulsion Laboratory,
      California Institute of Technology
      scott.c.burleigh@jpl.nasa.gov
      +1 (800) 393-3353
   
   References: S. Burleigh, "Compressed Bundle Header Encoding (CBHE)",
   RFC 6260, May 2011.

5. Security Considerations

The Bundle Security Protocol (BSP) may, under some conditions, insert additional endpoint ID strings into the dictionary of a bundle and reference those strings in BSP extension blocks. Because a bundle that includes any extension blocks containing EID-references to endpoint IDs other than the block's destination, source, report-to, and current custodian cannot be CBHE compressed, bundles cannot be compressed under those conditions.

Specifically, the specification detailed above implies that a bundle cannot be CBHE compressed when the security-source endpoint for the Bundle Authentication Block (BAB) is noted in the dictionary (typically, because there is no other way for the receiving bundle protocol agent to determine the security-source endpoint); when the security-destination endpoint for the BAB is noted in the dictionary (in the rare case that the receiving endpoint is not the security- destination endpoint); when the security-source endpoint for the Payload Integrity Block (PIB), Payload Confidentiality Block (PCB), or Extension Security Block (ESB) is not the source endpoint; or when the security-destination endpoint for the PIB, PCB, or ESB is not the destination endpoint.

Also, the CBHE-conformance inference mechanism identified in Section 3.2 introduces a possible denial-of-service attack. Malicious code could issue a CHBE-compressed bundle whose source EID falsely identifies the bundle origin as some node whose CLA is not CBHE conformant; a CBHE-conformant CLA receiving this bundle might incorrectly infer that the CLA at the purported source node was CBHE conformant and might then begin CBHE compressing all bundles that it sends to that node, thus preventing those bundles from being dispatched by the node's bundle protocol agent. Nodes can defend against such an attack by requiring Bundle Authentication Blocks and discarding any inference of CBHE conformance for the CLAs at nodes from which inauthentic bundles are received.

These caveats aside, CBHE introduces no new security considerations beyond those discussed in the DTN Bundle Protocol RFC 5050 [RFC5050] and Bundle Security Protocol RFC 6257 [RFC6257] Specifications.

6. References

6.1. 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.
   
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.
   
   [RFC6257]  Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", RFC 6257,
              May 2011.

6.2. Informative References

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

Appendix A. Acknowledgments

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged.

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