Bundle Protocol Version Demultiplexing

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Table of Contents

1. Introduction

With the publication of the Bundle Protocol version 6 (BPv6) in [RFC5050], many groups began experimenting and deploying Delay and Disruption Tolerant Networks (DTNs). These networks relied on Convergence Layer Adaptors (CLAs) to perform the hop-by-hop transport of bundles, and several specifications emerged defining how to encapsulate bundles in suitable wrappers for transport over existing space data-links. At the same time, new link-layer protocols were developed and standardized to provide the efficient transport of bundles between nodes of larger DTN networks.¶

The success of these networks drove the development of Bundle Protocol version 7 (BPv7), incorporating the lessons learned from BPv6, resulting in [RFC9171] some fifteen years later. Since the publication of BPv7 there has been impetus to deploy larger, more capable BPv7 DTNs, but there remains a need to update the existing BPv6 networks to support in some way coexistence with BPv6. Many of the CLAs and link-layer encapsulations were defined with a completely reasonable assumption that any future bundle protocol version would maintain a similar binary representation to BPv6, and hence determining the version in use would be a matter for the Bundle Processing Agent (BPA) and not a concern of the CLA or encapsulation protocol. Hence many of these existing specifications lack a field an implementation can use to distinguish the version of the bundle protocol used to format the bundle.¶

Unfortunately, Bundle Protocol version 6 and 7 fundamentally differ in binary representation: the former uses Self-Delimiting Numeric Values (SDNVs), see Section 4.1 of [RFC5050], and the latter Concise Binary Object Representation (CBOR), see Section 4.1 of [RFC9171]. Luckily differentiating between SDNVs and CBOR is simple: the first octet of a SDNV encoded BPv6 bundle is not a valid first octet of the CBOR representation of a BPv7 bundle. However, given CBOR is a general-purpose representation of binary encoded objects, differentiating between a CBOR-encoded BPv7 bundle and another data item that is not a bundle, but is binary encoded using CBOR, is less simple and can require more expensive parsing to determine the difference.¶

The purpose of this document is to provide two simple to implement mechanisms that can:¶

1. Avoid the need to upgrade every CLA and bundle encapsulation specification published in the era of BPv6 that lack an explicit version indicator, to support BPv7. This document can instead be normatively referenced as a standard demultiplexing mechanism in a simple update to an affected specification.¶

2. Avoid the need for a bundle processing agent to fully parse every received sequence of octets with an SDNV parser and a CBOR parser, both reasonably expensive operations, just to determine if the octets are even a binary encoding of a version 6 or 7 bundle.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Bundle version detection

In order to make the determination of the version of the Bundle Protocol used to encode a particular bundle, or even if a sequence of octets is an encoded bundle at all, the following demultiplexing logic is RECOMMENDED to implementations.¶

The table below (Table 1) describes the range of values of the initial octets of a sequence of octets and the likely data item that the octet stream represents. By matching the initial octets of a protocol data unit to the values in this table the likely format of the encoded bundle can be efficiently determined, avoiding repeated processor-intensive parsing. An implementation MAY match against the first octet only, or up to all four initial octets, depending on the accuracy of determination required.¶

The ability to make a distinction between the types and representations of data items by examining the initial octets makes the technique suitable for implementation in firmware, however this mechanism does not indicate the correctness of any encoding or data represented, and further parsing and validation MUST be performed.¶

Note: Octet ranges in the table above (Table 1) are inclusive, meaning that for every value V in X..Y, X <= V <= Y.¶

4. The BPv7 CBOR Tag type

The absence of any standard tag complicates the implementation of any transport protocol that must handle not only BPv7 bundles, but also other protocol data units that may be represented as CBOR indefinite-length arrays. In order to reduce the complexity of demultiplexing CBOR-encoded BPv7 bundles from other CBOR-encoded protocol data units, the presence of a IANA registered tag can provide additional assurance to a decoding implementation that a CBOR-encoded indefinite array is most likely a CBOR-representation of a BPv7 bundle. This document requests a new CBOR Tag Section 3.4 of [RFC8949] from IANA, see IANA Considerations (Section 6), to indicate that the array is indeed the CBOR representation of a BPv7 bundle.¶

The addition of this CBOR tag increases the overall size of the CBOR representation of a bundle by three octets, and in some resource constrained environments this may be unwanted overhead. However, in environments where speed of processing is more important than the size of encoded bundles, the improvement in accuracy of fast bundle validity and version detection gained by using the tag may be advantageous.¶

The addition of this tag updates Section 4.1 of [RFC9171] in the following way:¶

• An implementation MAY tag the CBOR representation of the outermost indefinite-length array of blocks of a BPv7 bundle with the tag defined in this document.¶

• Implementations that process bundles MAY use the presence of this tag to determine that the CBOR array encodes a Bundle Protocol version 7 bundle.¶

TBD: What about tag(55799)?¶

5. Security Considerations

This document does not impact or alter the existing security considerations described in Section 8 of [RFC9171], however it adds an additional consideration.¶

6. IANA Considerations

IANA is requested to allocate a new entry in the "CBOR Tags" sub-registry of the IANA "Concise Binary Object Representation (CBOR) Tags" registry, with the following values:¶

• Tag: 9171, to correspond with [RFC9171]¶

• Data Item: array¶

• Semantics: "Bundle Protocol version 7 bundle"¶

• Reference: This document¶

7. References

Are random tags allowed? (To be removed)

This behavior is already prohibited by Section 4.1-2 of [RFC9171], which states:¶

• To ensure that blocks are always in canonical representation when they are transmitted and received, the CBOR encodings of the values of all fields in all blocks MUST conform to the core deterministic encoding requirements as specified in [RFC8949], except that indefinite-length items are not prohibited.¶

Section 4.2.1 of [RFC8949] sets out the core deterministic encoding requirements, but is a little non-obvious on the correct behavior with respect to tags. Section 4.2.2-1 of [RFC8949] however clarifies the situation with the following statement:¶

• ... the deterministic format would not allow the presence of the tag, based on the "shortest form" principle.¶

Indicating that tags appearing in an encoding that are not explicitly permitted by the relevant protocol specification, that requires compliance with the core deterministic encoding requirements, means the encoded value is not Well-formed as defined in Section 1.2 of [RFC8949]. Therefore a compliant BPv7 implementation MUST consider any CBOR representation of a bundle that includes tags not specified in Appendix B of [RFC9171] or this document, as not Well-formed.¶

Acknowledgments

Thanks are owed to Brian Sipos for doing similar work on demultiplexing the initial octets of bundles for the UDP-CL update, and Erik Kline for his valuable discussion and early review of this document.¶

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