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 a drive 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 update every CLA and bundle encapsulation specification, published in the era of BPv6 that lack an explicit version indicator, to support BPv7.¶

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. The BPv7 CBOR Tag type

Section 4.1 of [RFC9171] defines the CBOR representation of a BPv7 bundle as an indefinite-length array, but does not require the use of CBOR tags with this encoding. The absence of any 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, 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 tag updates Section 4.1 of [RFC9171] in the following way:¶

• An implementation SHOULD prefix the CBOR representation 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.¶

4. 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. 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.¶

5. Security Considerations

This document does not impact or alter the existing security considerations described in Section 8 of [RFC9171].¶

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

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.¶

Author's Address