]> Ori Industries

rick.taylor@ori.co

JHU/APL

Edward.Birrane@jhuapl.edu

Transport Delay/Disruption Tolerant Networking DTN ipn BPv7 CBHE Bundle Protocol The 'ipn' URI scheme was first defined in as a format for endpoint identifiers with the Delay Tolerant Networking Bundle Protocol version 6 (BPv6) , when using Compressed Bundle Header Encoding (CBHE). requested IANA registries to control the allocation of the numeric identifiers used with the 'ipn' URI scheme. The Bundle Protocol version 7 (BPv7) specification repeats the definition of the 'ipn' URI scheme, for use with BPv7, reusing the format from . Because the specification of the 'ipn' URI scheme has been split over several documents, referencing different versions of the Bundle Protocol, some confusion has occurred amongst readers and implementers. This document pulls together the information contained in those previous documents and asserts the specification of the 'ipn' URI scheme for use with BPv7, acting as an update to those previous documents. A criticism of the existing 'ipn' URI scheme node number allocation strategy as defined in is that sub-ranges of a single number space are assigned for the use by individual organisations. This allocation strategy results in inefficient encoding of URIs with BPv7. This document extends the format of the 'ipn' URI scheme to include Numbering Authorities, allowing for a more flexible sub-allocation strategy, resulting in a more efficient encoding with BPv7. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Delay/Disruption Tolerant Networking Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction From the earliest days of experimentation with "store and forward" data transfer with the Bundle Protocol there has been a need to identify the source and destination of a bundle. Starting with the IRTF standardisation of the experimental Bundle Protocol version 6 (BPv6) , a logical source or destination of bundles is referred to as an Endpoint, with a corresponding Endpoint Identifier (EID). Rather than define yet another identifier format, the IRTF DTN Working Group decided to reuse the existing Universal Resource Identifier (URI) syntax defined in . However, given the particular on-the-wire encodings of both BPv6 and BPv7, a specific encoding for every supported URI scheme must be specified in the relevant protocol specification. Initially only encoding support for the textual 'dtn' URI scheme was specified in , however during implementations of BPv6, it became increasingly obvious that there was a desire for a simple way to name the endpoints in a Delay Tolerant Network (DTN) using short numeric identifiers. To address this, introduced the 'ipn' URI scheme which identifies a DTN endpoint using a concatenation of node and service identifiers, with a suitable encoding for use with BPv6. The acronym IPN was originally a contraction of the term "InterPlanetary Network" as the original aim of this scheme was to provide a compact namespace for an interoperable space-based DTN architecture, but beyond space-based applications, terrestrial nodes might also operate with limited power, bandwidth, and/or compute budget. In all cases, concisely encoded numeric identifiers for both nodes and services provides processing advantages over more verbose naming schemes. Therefore additional focus has been placed on the capabilities of the 'ipn' URI scheme for use beyond its historical purpose for space-based DTN architectures. The publication of the Bundle Protocol version 7 (BPv7) has resulted in operational deployments of BPv7 nodes for both terrestrial and non-terrestrial use cases. This includes BPv7 networks operating over the terrestrial Internet and BPv7 networks operating in self-contained environments behind a shared administrative domain. This growth in the number and scale of deployments of BPv7 DTNs has been accompanied by a growth in the usage of 'ipn' URI scheme endpoint identifiers, and this increased usage has highlighted shortcomings in the allocation strategy for such identifiers, as specified in . This document collates the information contained in previous specifications, clarifying and updating the usage of 'ipn' scheme URIs when used with BPv7, as well as extending the specification of 'ipn' scheme URIs in a backwards compatible way, to address shortcomings in the assignment of identifiers of specified in , concentrating on ensuring interoperable, scalable deployment of 'ipn' scheme URIs for use with BPv7 DTNs.

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 when, and only when, they appear in all capitals, as shown here.

The 'ipn' URI Scheme as defined in RFC9171 This section describes the specification of the 'ipn' URI scheme as defined in and is included as convenient reference for the remainder of this document. A bundle endpoint may have a multiplicity of membership, allowing some EIDs to refer to a group of bundle destinations, and others to refer to only a single bundle destination. The latter are referred to as Singleton endpoints, see . Both and specify that all 'ipn' scheme URIs identify Singleton endpoints. specifies the syntax of an 'ipn' scheme URI, with an identical format to the specification of the 'ipn' URI scheme syntax in , namely as a sequence of two unsigned integers. The first number representing the identifier of the node (node-nbr), and the second being the identifier of a particular service expected at that node (service-nbr).

Text Syntax As specified in , the schema-specific part of an 'ipn' scheme URI must comply with the following ABNF syntax, including the core ABNF syntax rule for DIGIT defined by that specification:

CBOR Encoding As specified in , a BPv7 endpoint identified by an 'ipn' scheme URI, when encoded in Concise Binary Object Representation (CBOR) , must comply with the following Concise Data Definition Language (CDDL) specification: Because the encoding of node-nbr and service-nbr (specified in the CDDL as nodenum and servicenum) are defined as CBOR uint types, both values are restricted by this encoding to a range of [0 .. 2^64-1].

Node and Service Numbers The formulation of 'ipn' scheme URIs dictates a hierarchy, whereby the URI for a particular service is composed of the well-known numeric identifier of the service, prepended with the numeric identifier of the node where the service is expected to exist. This strongly implies that all endpoints named with 'ipn' scheme URIs available on a single node must share a common node-nbr, but is not explicitly specified in nor . The use of predefined numeric identifiers for well-known services is based on familiarity with TCP and UDP well-known port numbers for services, first introduced in . Although this behaviour is not required for the correct naming of service endpoints, it has long been considered useful, and is implied in both and . The only service that is required to exist for BPv7, as described in , is the "administrative endpoint" of a node. This service is used as the destination for administrative control messaging needed for the correct functioning of BPv7, and is required to have a Singleton EID. A similar service is required for BPv6, and the service-nbr zero (0) is allocated for that service when using 'ipn' scheme URIs in . only recommends that the service-nbr zero (0) should be used with BPv7.

Allocation Ranges introduces the concept of the globally unique Node ID of a particular BPv7 node, specifically stating that it must be the Singleton EID of the administrative endpoint of the node. When using a 'ipn' scheme URI as an EID, the value of the service-nbr component of the URI may be reused between the EIDs of services on different nodes. This means that in order for an 'ipn' scheme URI to be used as a Node ID, the node-nbr component must be globally unique within a single interoperating DTN. This uniqueness constraint means that all the node-nbr values must be allocated from a single, global namespace. The reliance on such a namespace is not problematic when deploying a private, self-contained network: If there are few nodes that can ever intercommunicate, then those nodes can have node-nbr values allocated by the administrator of that network, and there will be no problem with uniqueness coming from a serialized, central authority. However, as the number of nodes and number of administrative authorities in a network scale, the administrative burden of assigning unique node-nbr values increases. A potential solution to this, as described in , is to pre-assign ranges of node-nbr values to different authorities, from which they can independently allocate values without risk of duplication. This division of the number space is an adequate solution for the uniqueness problem, but it introduces a new issue: The encoding-length of each node-nbr is no longer minimal, as the offset to the start of the range assigned to the allocating authority is included in each node-nbr value assigned by that authority. For example: allocates the range [2^14 .. 2^21-1] for node-nbr values for use with BPv6, and if CCSDS choses to continue to use this number range for BPv7, the CBOR encoding of every 'ipn' scheme URI will be at least 7 octets (including 2 octets for the outer array with uri scheme discriminator type code), even when interoperability is not required: Another side-effect of assigning ranges of a single number space to different sub-allocating authorities is to reduce the total availability of node-nbr values. Although the current allocation strategy defined in leaves approximately 2^42 numbers unallocated, the recommendation to IANA is that these numbers should be allocated in blocks of 2^14. The history of IPv4 address allocation, see , demonstrates that exhaustion of a 2^32 bit number space happens surprisingly quickly.

Updates to RFC9171 This section updates to clarify the construction and use of 'ipn' scheme URIs when used as EIDs with BPv7, to address the limitations described above.

'ipn' URI Scheme Node Numbers for BPv7 The following rules update the specification of node-nbr in :

1. The value of the node-nbr component of an 'ipn' scheme URI MUST be an unsigned integer greater than or equal to zero (0).
2. The URI "ipn:0.0" is assigned to the 'null' endpoint, see .
3. The value zero (0) for the node-nbr component of an 'ipn' scheme URI MUST NOT be used except as part of the URI "ipn:0.0".
4. Values greater than or equal to 2^64 for the node-nbr component of an 'ipn' scheme URI MUST NOT be used, to allow concise unsigned integer (type 0) CBOR encoding.
5. All 'ipn' scheme URIs for endpoints co-located on a single bundle processing node MUST share the same value for the node-nbr component.

'ipn' URI Scheme Service Numbers for BPv7 The following rules update the specification of service-nbr in :

1. The value of the service-nbr component of an 'ipn' scheme URI MUST be an unsigned integer greater than or equal to zero (0).
2. The value of the service-nbr component of an 'ipn' scheme URI of the EID of an administrative endpoint, as defined in , MUST be zero (0).
3. Values greater than or equal to 2^64 for the service-nbr component of an 'ipn' scheme URI MUST NOT be used, to allow concise unsigned integer (type 0) CBOR encoding.

To support this update, a new IANA "Bundle Protocol Version 7 'ipn' Scheme URI Service Numbers" registry is defined to control the allocation of values for the service-nbr component of an 'ipn' scheme URI when used as EIDs with BPv7, see IANA Considerations.

Updates to RFC7116 This section renames two of the registries defined in , without change, to clarify their use for the allocation of node and service numbers for BPv6 only.

'ipn' URI Scheme Node Numbers for BPv6 The "CBHE Node Numbers" registry specified in is renamed without change to the "Bundle Protocol Version 6 'ipn' Scheme URI Node Numbers" registry, to clarify that it is for use solely with BPv6, see IANA Considerations.

'ipn' URI Scheme Service Numbers for BPv6 The "CBHE Service Numbers" registry specified in is renamed without change to the "'Bundle Protocol Version 6 'ipn' Scheme URI Service Numbers" registry, to clarify that it is for use solely with BPv6, see IANA Considerations.

Update to the 'ipn' URI Scheme A consequence of there not being a central registry of allocated values for node-nbr components for 'ipn' scheme URIs, and instead leaving their allocation to the administrators of a given DTN deployment, is that if two or more deployments wish to interoperate, there must be agreement between the respective administrators around the policy for the allocation of numbers to avoid duplication. This situation is obviously untenable when building DTNs beyond a fairly small scale. Underlying the 'ipn' scheme URI node-nbr range assignment for BPv6, described in , is the desire to reduce the administrative burden on a single allocation authority by delegating the authority to assign numbers to a registered set of numbering authorities. Although the range-based mechanism of delegating this authority has been criticised above, the desire for delegation of numbering to a group of independent authorities in an interoperable way is still valid.

Numbering Authorities To address this, this document introduces the concept of Numbering Authorities. A Numbering Authority has a unique numeric identifier, and this identifier is used to discriminate between the values of node-nbr components of 'ipn' scheme URIs allocated by different Numbering Authorities. The use of this identify allows a Numbering Authority to allocate any value to the node-nbr component of an 'ipn' scheme URI, in the full 2^64 unsigned integer range, according to its own policies, as permitted by the rules in the 'ipn' URI Scheme Node Numbers for BPv7 section of this document. A new IANA "Bundle Protocol Version 7 'ipn' Scheme URI Authority Numbers" registry is defined for the registration of Authority Numbers, see IANA Considerations. Although the uniqueness of Numbering Authority identifiers is required for interoperable DTN operations, identifier ranges are explicitly reserved for experimentation.

Numbering Sub-authorities Some organisations that register as Numbering Authorities may be sufficiently large that acting as a single allocating authority for their desired number range becomes administratively untenable, for example government agencies. In this case, the ability to delegate number allocation to sub-authorities is desired. To address this, this specification introduces a Numbering Sub-authority numeric identifier, to be used when required, allocated from a registry controlled by each independent Numbering Authority. In order to avoid unbounded nesting of sub-sub-authorities, making processing in constrained devices overly onerous, only a single level of Numbering Sub-authority is permitted.

Prefix Encoding Fundamentally, 'ipn' scheme URIs are represented as a sequence of identifiers: In the text syntax, the numbers are separated with the '.' delimiter; in CBOR, encoded as an array of unsigned integers. Adding the numeric identifier of the numbering authority, and optional sub-authority, that allocated the node-nbr to the sequence of identifiers allows for a concise encoding. In the text syntax, the numeric identifiers for the numbering authority, and optional sub-authority, are prepended to the text, separated with the '.' delimiter. For the CBOR encoding, the dimension of the unsigned integer array is increased to include the required numbering authority and sub-authority identifiers. For example, the URI "ipn:2.1.0" uniquely identifies the administrative endpoint of the node allocated the node-nbr 1 by the numbering authority with identifier 2. This URI can be concisely encoded in CBOR as 6 octets, including 2 octets for the outer array with uri-code:

Text Syntax The text syntax of an 'ipn' scheme URI MUST comply with the following ABNF syntax, including the core ABNF syntax rule for DIGIT defined by that specification: Additional leading zeros ('0') MUST NOT appear in the textual representation of any component of an 'ipn' scheme URI.

CBOR Encoding A BPv7 endpoint identified by an 'ipn' scheme URI, when encoded in Concise Binary Object Representation (CBOR) , MUST comply with the following Concise Data Definition Language (CDDL) specification: Because the encoding of auth-nbr, sub-auth-nbr, node-nbr, and service-nbr are defined as CBOR uint types, all values are restricted by this encoding to a range of [0 .. 2^64-1].

Backwards Compatibility Although this update to the 'ipn' URI scheme introduces the ability to include Numbering Authority identifiers, it does not prohibit the use of 'ipn' scheme URIs without such an identifier. To maintain this useful capability, a new IANA "Bundle Protocol Version 7 'ipn' Scheme URI Null Authority Node Numbers" registry is defined to control the allocation of values for the node-nbr component of 'ipn' scheme URIs without a auth-nbr component, when used as EIDs with BPV7. This new registry inherits behaviours and existing assignments from the IANA "CBHE Node Numbers" registry, reserves the value zero (0), and assigns values in the range [1 .. 2^14-1] as "Private Use", as defined in . Use of an 'ipn' scheme URI with a "Private Use" value for the node-nbr component without an auth-nbr component as an EID with BPv7 removes any guarantee of uniqueness. Such EIDs must be considered 'non-routeable' to the extent that they MUST NOT be permitted to exit a single administrative domain. They can be considered to be equivalent to "Private Address Space" IPv4 addresses, as defined in .

Recommendations mandates the concept of "late binding" of an EID, where-by the address of the destination of a bundle is resolved from its identifier hop by hop as it transits a DTN. This per-hop binding of identifiers to addresses underlines the fact that EIDs are purely names, and should not carry any implicit or explicit information concerning the current location or reachability of an identified node and service. This removes the need to rename a node as its location changes. Because of this late binding concept, the authority components of an 'ipn' scheme URI SHOULD NOT be regarded as some kind of "type field", and used to derive additional information from the other components of the URI. An example of incorrect behaviour would be: "I know authority X allocates node-nbr values derived from the link-layer address of some device on each node, and so I can just send packets directly to that link-layer address". No matter the authority that controls the allocation of node-nbr values, they remain just numbers, without additional meaning.

Security Considerations TODO

IANA Considerations The following sections detail requests to IANA for new registries, and the renaming of existing registries.

Bundle Protocol Version 7 'ipn' Scheme URI Authority Numbers registry IANA is requested to create a new registry entitled "Bundle Protocol Version 7 'ipn' Scheme URI Authority Numbers" The registration policy for this registry is: Bundle Protocol Version 7 'ipn' Scheme URI Authority Numbers registration policies

Range	Registration Policy
0	Reserved
1 .. 2^32-1	First Come First Served
2^32 .. 2^64-1	Experimental Use
>= 2^64	Reserved

The initial values for the registry are: Bundle Protocol Version 7 'ipn' Scheme URI Authority Numbers initial values

Value	Description	Reference
1	Allocated to	To be defined - pre-allocated as a courtesy, as they have an existing allocation in

Bundle Protocol Version 7 'ipn' Scheme URI Null Authority Node Numbers registry IANA is requested to create a new registry entitled "Bundle Protocol Version 7 'ipn' Scheme URI Null Authority Node Numbers" The registration policy for this registry is: Bundle Protocol Version 7 'ipn' Scheme URI Null Authority Node Numbers registration policies

Range	Registration Policy
0	Reserved
1 .. 2^14-1	Private Use
2^14 .. 2^42-1	First Come First Served requests for up to 2^14 values
2^14 .. 2^42-1	Expert Review requests for more than 2^14 values
2^42 .. 2^64-1	Experimental Use
>= 2^64	Reserved

The initial values for the registry are: Bundle Protocol Version 7 'ipn' Scheme URI Null Authority Node Numbers initial values

Value	Description	Reference
16384-2097151	Allocated to the Space Assigned Numbers Authority (SANA) for use by Consultative Committee for Space Data Systems (CCSDS) missions.	Inherited from
268435456-268451839	Allocated to Spacely Packets, LLC to provide IPN/IP Gateway services to private sector stakeholders.	Scott Johnson - see existing allocation in CBHE Node Numbers registry.

Bundle Protocol Version 7 'ipn' Scheme URI Service Numbers registry IANA is requested to create a new registry entitled "Bundle Protocol Version 7 'ipn' Scheme URI Service Numbers" The registration policy for this registry is: Bundle Protocol Version 7 'ipn' Scheme URI Service Numbers registration policies

Range	Registration Policy
0 .. 23	RFC Required
24 .. 4095	Specification Required
4096 .. 2^32-1	Private Use
2^32 .. 2^64-1	Experimental Use
>= 2^64	Reserved

The initial values for the registry are: Bundle Protocol Version 7 'ipn' Scheme URI Service Numbers initial values

Value	Description	Reference
0	The administrative endpoint	This document

CBHE Node Numbers registry IANA is request to rename the "CBHE Node Numbers" registry defined in to the "Bundle Protocol Version 6 'ipn' Scheme URI Node Numbers" registry, with no change to its allocation rules or current allocations.

CBHE Service Numbers registry IANA is requested to rename the "CBHE Service Numbers" registry defined in to the "Bundle Protocol Version 6 'ipn' Scheme URI Service Numbers" registry, with no change to its allocation rules or current allocations.

References Normative References 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. This document defines an Experimental Protocol for the Internet community. The DTNRG Research Group has defined the experimental Licklider Transmission Protocol (LTP) and the Compressed Bundle Header Encoding (CBHE) mechanism for the InterPlanetary Network ('ipn' URI scheme). Moreover, RFC 5050 defines values for the Bundle Protocol administrative record type. All of these fields are subject to a registry. For the purpose of its research work, the group has created ad hoc registries. As the specifications are stable and have multiple interoperable implementations, the group would like to hand off the registries to IANA for official management. This document describes the necessary IANA actions. This document presents a specification for the Bundle Protocol, adapted from the experimental Bundle Protocol specification developed by the Delay-Tolerant Networking Research Group of the Internet Research Task Force and documented in RFC 5050. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Informative References This informational RFC discusses important directions for possible future evolution of the Internet architecture, and suggests steps towards the desired goals. This memo provides information for the Internet community. It does not specify an Internet standard. This document describes the end-to-end protocol, block formats, and abstract service description for the exchange of messages (bundles) in Delay Tolerant Networking (DTN). This document was produced within the IRTF's Delay Tolerant Networking Research Group (DTNRG) and represents the consensus of all of the active contributors to this group. See http://www.dtnrg.org for more information. This memo defines an Experimental Protocol for the Internet community. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This Network Working Group Request for Comments documents the currently assigned values from several series of numbers used in network protocol implementations. This memo is a status report on the parameters (i.e., numbers and keywords) used in protocols in the Internet community. This document describes address allocation for private internets. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.

Discussion Points (This whole section is to be removed prior to publication)

Why not just keep the number space sub-division as defined in RFC7116? See Allocation Ranges.

Why not a new 'ipn3' EID scheme? Is there really any difference in outcome between the following cases?:

1. An existing parser receives a bundle with an EID with a 3-ary ipn EID
2. An existing parser receives a bundle with an EID with an unrecognised scheme identifier

In the former case, the parser will recognised the scheme as 'ipn' but then fail as the dimension of the subsequent array is not 2. In the latter case the parser will fail one octet earlier when the scheme is not recognised. In both cases, the EID will not be recognised as valid, forwarding will be "contraindicated", and the process described in Step 2 of should be followed. It is believed that introducing a new EID scheme will just result in fragmentation of support. 'ipn' is popular because it is simple; let's not introduce another 'simple' EID scheme to compete with it, but rather add just enough support for universal interoperability. 'ipn' as defined in RFC9171 needs clarification, so why not just add the tweaks necessary as long as we don't break back-compatibility?

Why not use just the 'dtn' scheme for interoperability? Because the 'dtn' scheme definition in RFC9171 is intentionally left wide open for further work. That work has yet to happen and is a considered a much more complex task than a simple update to the 'ipn' scheme.

Won't these changes break BPv6 compatibility? Because of the difference in encoding between BPv6 and BPv7, there is no on-the-wire compatibility between the versions. Any 'dual-stack' gateway BPA is going to have to encapsulate BPv6 in BPv7 (or vice-versa), so the EID of the decapsulating endpoint will have to be used in the 'envelope' bundle. There is no way a BPv7 node can send a bundle to a BPv6 node directly using BPv7, so backwards compatibility of EIDs between protocol versions is not needed.

Why not have an unbounded number of sub-authorities? It's possible to encode, and has been considered by the authors, but the conclusion was: ipn:it.is.really.not.a.great.idea.to.have.unbounded.sequences.of.identifiers.when.it.comes.to.processing.EIDs.in.constrained.environments.0 An optional, single sub-authority seemed like a sensible idea, but feel free to argue on the list for more.

Acknowledgments TODO acknowledge.