AETHERIX
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Phase 1 · DTN & Routing Day 2 of 42 6 min read July 24, 2026

Bundle Protocol v7 — Structure, Encoding, Flags & Priority

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🎯Learning Objective

Master the internal structure of a BPv7 bundle — primary block, payload block, extension blocks — along with CBOR encoding, processing control flags, and the five-level priority system. This maps to LO1 and is one of the most frequently probed topics in the oral interview.

📖The Core Concept

The Three-Part Bundle

A Bundle Protocol Version 7 bundle (RFC 9171) is not an IP packet. It is a structured message designed to survive extended storage and intermittent transmission. Every bundle consists of three logical parts:

  1. Primary Block — the routing metadata. This is CBOR-encoded and contains the source EID (Endpoint ID), destination EID, creation timestamp, lifetime, hop-count limit, fragmentation info, and processing control flags. Think of it as the envelope: it tells every node where the bundle came from, where it's going, how long it's valid, and how it should be handled.

  2. Payload Block — the application data: telemetry, images, commands, science measurements. This is the letter inside the envelope. In AETHERIX, payloads range from 2 MB emergency telemetry to 8 GB panoramic imagery.

  3. Extension Blocks (optional) — additional metadata for security (BPSec), previous-node tracking, bundle age, and custody tracking. Extension blocks are extensible: new standards can add block types without changing the primary block format.

CBOR Encoding

BPv7 encodes the primary block using CBOR (Concise Binary Object Representation, RFC 8949) — a compact binary data format. Unlike JSON (text-based, verbose), CBOR produces minimal wire size, which matters enormously when every byte costs 12.5 minutes of light-time to transmit. CBOR is self-describing: a decoder can parse the structure without a separate schema.

Processing Control Flags

The primary block carries processing control flags — bit-fields that tell nodes how to handle the bundle. These are defined in RFC 9171 §4.2.3 and implemented in AETHERIX's BundleFlags enum. The critical flags for your exam:

Flag Hex Value Meaning
IS_FRAGMENT 0x01 This bundle is a fragment of a larger ADU
PAYLOAD_IS_ADMIN 0x02 Payload is an administrative record
NO_FRAGMENT 0x04 Bundle must not be fragmented
CUSTODY_REQUESTED 0x08 Source requests custodial retransmission
DEST_IS_SINGLETON 0x10 Destination is a single endpoint (not multicast)
ACK_REQUESTED 0x20 Administrative acknowledgement requested
STATUS_REQUESTED 0x40 Status report requested

The CUSTODY_REQUESTED flag (0x08) is the single most important flag for your exam. When set, it tells every intermediate node: "do not delete your copy until a downstream node has confirmed custody." This is the mechanism that enables reliable delivery over intermittent links.

The Five Priority Levels

AETHERIX defines five priority classes (P0–P4), each with distinct latency targets and handling rules. These are not arbitrary — they encode operational reality:

Priority Class Target Latency Custody LTP Mode
P0 Emergency (spacecraft health, safety) < 1 min Mandatory Red
P1 High Science (time-sensitive observations) < 30 min Yes Red
P2 Standard (regular telemetry, daily data) < 24 hr Yes Green
P3 Housekeeping (logs, status) < 7 days No Green
P4 Bulk (archives, software updates) < 30 days No Green

The RL routing agent uses bundle_priority (0–4) as a state variable. Dropping a P0 bundle incurs the full drop penalty (δ = 10.0) in the reward function; dropping a P4 bundle costs far less. The priority also determines whether custody transfer is used and whether LTP segments are red (reliable) or green (best-effort).

Endpoint IDs (EIDs)

Every bundle carries source and destination EIDs. AETHERIX follows the LNIS v5 format (CCSDS 142.0-B-2): scheme://node_id/service_id. Examples: - dtn://mars.surface.rover-01/science — a Mars rover's science service - dtn://earth.dsn.goldstone/ops — Goldstone DSN operations - dtn://transit.esl4.relay/forward — the ES-L4 Lagrange relay's forwarding service

The dtn:// scheme is the default; ipn:// is an alternative for numeric IPN addressing.

📊Diagrams & Data

BPv7 bundle structure (RFC 9171)
flowchart TD
    B["BPv7 Bundle"] --> P["Primary Block (exactly one)"]
    B --> C["Canonical Blocks (zero or more)"]
    P --> P1["Version = 7"]
    P --> P2["Source / Destination EID<br/>dtn://mars.surface.rover-01/science"]
    P --> P3["Creation timestamp"]
    P --> P4["Lifetime (default 7 days)"]
    P --> P5["Processing control flags<br/>priority, custody requested"]
    C --> C1["Payload Block (type 1)"]
    C --> C2["Previous Node Block"]
    C --> C3["Bundle Age Block"]
    C --> C4["Hop Count Block"]
    C --> C5["Security Blocks - BIB / BCB<br/>RFC 9172 BPSec"]
A bundle is self-describing: the primary block carries identity and lifetime, so it can sit in storage for days without a live session.

🔬In AETHERIX

The bundle structure is fully implemented in src/routing/bundle.py. The BundlePriority IntEnum defines all five levels:

class BundlePriority(IntEnum):
    EMERGENCY = 0       # Spacecraft health, safety alerts
    HIGH_SCIENCE = 1    # Time-sensitive observations
    STANDARD = 2        # Regular telemetry and data
    HOUSEKEEPING = 3    # Status updates, logs
    BULK = 4            # Archived datasets, software updates

The BundleFlags IntEnum mirrors RFC 9171 exactly: CUSTODY_REQUESTED = 0x08, IS_FRAGMENT = 0x01, DEST_IS_SINGLETON = 0x10, etc.

The Bundle dataclass stores all primary-block fields: bundle_id (auto-generated 8-char hex), source/destination (EndpointID objects), creation_time, lifetime_seconds (default 86400 * 7 = 7 days), priority, flags, payload, hops list, and custody_holders list. The __post_init__ method auto-calculates payload_size_bytes.

The accept_custody() method appends to custody_holders and records a CUSTODY_ACCEPTED hop. The release_custody() method removes the node from custody_holders. The is_expired property checks time.time() > (creation_time + lifetime_seconds).

The create_science_bundle() factory function generates standard bundles with 7-day lifetime, useful for demonstrations.

📐Key Numbers & Formulas

  • CUSTODY_REQUESTED flag: 0x08 (bit 3)
  • IS_FRAGMENT flag: 0x01 (bit 0)
  • DEST_IS_SINGLETON flag: 0x10 (bit 4)
  • Default bundle lifetime: 86400 × 7 = 604,800 seconds (7 days)
  • Priority levels: P0–P4 (EMERGENCY=0 through BULK=4)
  • Bundle ID format: 8-character uppercase hex (e.g., A3F2B1C9)
  • CBOR standard: RFC 8949
  • Bundle Protocol standard: RFC 9171

🔗Standards & References

💡How the Examiner Will Probe This

QExplain the structure of a BPv7 bundle and how priority classes work.

Three parts: primary block (CBOR-encoded routing metadata — source/dest EID, timestamp, lifetime, flags), payload block (application data), optional extension blocks (security, tracking). Five priority levels P0–P4 with distinct latency targets. The RL agent uses priority as a state variable to select routing actions and convergence layer modes — P0/P1 use LTP red segments with custody; P3/P4 use LTP green without custody.

QWhat flag would you set on an emergency bundle?

CUSTODY_REQUESTED (0x08) + DEST_IS_SINGLETON (0x10) + ACK_REQUESTED (0x20). These ensure guaranteed single-destination delivery with custody tracking and acknowledgement.

QWhy CBOR instead of JSON or Protobuf?

CBOR is compact binary (minimal wire size — critical when every byte costs 12+ minutes light-time), self-describing (no external schema needed), and standardised in RFC 8949. JSON is text-based and verbose; Protobuf requires a separate schema file.

✅What You Should Be Able to Answer

📁Deep Dive Resources

  • src/routing/bundle.py — Bundle, BundlePriority, BundleFlags, EndpointID classes
  • interview_prep/topic_summaries/dtn_fundamentals.md — priority table and custody detail
  • interview_prep/practice/mock_interview.md — Question 2 (Bundle Protocol)
  • Live demo: https://matx104.github.io/AETHERIX/