fips/docs/design/fips-ipv6-adapter.md

FIPS IPv6 Adapter

The IPv6 adapter sits above the FIPS Session Protocol (FSP) and adapts the FIPS datagram service for unmodified IPv6 applications. It presents each FIPS node as an IPv6 endpoint, so standard socket applications (SSH, HTTP, SCP) can communicate over the mesh without modification.

Role

The adapter bridges two worlds: IPv6 applications that address destinations by IP address, and the FIPS mesh that addresses destinations by public key (npub). The adapter handles the translation: DNS resolution from npub to fd00::/8 ULA (Unique Local Address) address, identity cache management so FIPS can route IPv6 packets, MTU enforcement so packets fit through the mesh, and the TUN interface that connects to the kernel's IPv6 stack.

Applications that are FIPS-aware can bypass the adapter entirely and use the native FIPS datagram API, addressing destinations directly by npub.

DNS Integration

The Problem

IPv6 addresses in the fd00::/8 range are derived from public keys via a one-way hash (SHA-256). Given only an IPv6 address, the public key cannot be recovered — and without the public key, FIPS cannot compute the node_addr needed for routing.

The identity cache must be populated before packets arrive at the TUN interface, or they cannot be routed.

DNS as Entry Point

DNS resolution serves as the "routing intent" signal. When an application resolves npub1xxx...xxx.fips, the FIPS DNS service:

1. Extracts the npub from the .fips domain name
2. Derives the fd00::/8 IPv6 address from the public key
3. Primes the identity cache with the mapping (IPv6 address prefix ↔ NodeAddr ↔ PublicKey)
4. Returns the IPv6 address to the application

When the application subsequently sends packets to that address, the identity cache already contains the mapping needed for routing.

DNS Name Format

npub1xxxxxx...xxxxx.fips

The FIPS DNS server recognizes names ending in .fips and extracts the npub for address derivation.

Traffic Without Prior DNS Lookup

A packet may arrive at the TUN for an fd00::/8 destination without a prior DNS lookup — cached address, manual configuration, etc. Since the address derivation is one-way, the npub cannot be recovered from the address alone.

FIPS returns ICMPv6 Destination Unreachable (Code 0: No route to destination) for packets to unknown addresses. The identity cache must be populated before traffic can be routed.

Known cache population mechanisms:

• DNS lookup: The primary path
• Inbound traffic: Authenticated sessions from other nodes populate the cache with their identity information

IPv6 Address Derivation

FIPS addresses use the IPv6 Unique Local Address (ULA) prefix fd00::/8:

Public Key (32 bytes)
    │
    ▼
SHA-256 → node_addr (16 bytes, truncated)
    │
    ▼
fd + node_addr[0..15] → IPv6 address (16 bytes)

The fd prefix places FIPS addresses in the IPv6 Unique Local Address (ULA) space defined by RFC 4193. ULAs are the IPv6 equivalent of RFC 1918 private addresses (10.x, 172.16.x, 192.168.x) — they are reserved for local use and are not routable over the public Internet. This means FIPS overlay addresses cannot conflict with native IPv6 traffic that may be present on the same host or network, and they will not leak beyond the local system even if routing is misconfigured. These are overlay identifiers — they appear in the TUN interface for application compatibility but have no meaning outside the FIPS mesh.

Identity Cache

The derivation from public key to NodeAddr and IPv6 address is one-way (SHA-256 truncation). Given a destination IPv6 address from an outbound packet on the TUN interface, the adapter cannot recover the public key or NodeAddr needed for FIPS routing. The identity cache provides the reverse lookup: it maps the FIPS address prefix (15 bytes — the IPv6 address minus the fd prefix) back to (NodeAddr, PublicKey), allowing the adapter to route IPv6 traffic into the mesh. This cache is needed only when using the IPv6 adapter; the native FIPS API provides the public key directly.

Eviction Policy

The mapping is deterministic (derived from the public key) and never becomes stale. The cache uses LRU-only eviction bounded by a configurable size (default 10K entries). There is no TTL — entries are evicted only when the cache is full and space is needed for a new entry. LRU-only eviction is necessary because there is no other way for the FIPS router to recover the routing identity from an IPv6 address, and IPv6 traffic for a destination may arrive an arbitrarily long time after the DNS resolution that populated the cache entry.

Relationship to DNS TTL

The identity cache timeout must be longer than the DNS TTL to ensure that while an application believes its DNS resolution is valid, the corresponding routing entry remains present. The DNS TTL (default 300s) governs when applications re-query; the identity cache (LRU, no TTL) is always available as long as the entry hasn't been evicted by memory pressure.

MTU Enforcement

FIPS does not provide fragmentation or reassembly at the session or mesh protocol layers — every datagram must fit in a single transport-layer packet. Some transports may perform fragmentation and reassembly internally (e.g., BLE L2CAP) and can advertise a larger virtual MTU than the physical medium supports, but this is transparent to FIPS. The mesh layer provides two facilities to manage MTU across heterogeneous paths: route discovery can constrain results to paths that support a required minimum MTU, and transit nodes that cannot forward an oversized datagram send an MtuExceeded error signal back to the source. The adapter must ensure that IPv6 packets from applications fit within the FIPS encapsulation budget after all layers of wrapping.

Encapsulation Overhead

Layer	Overhead	Purpose
Link encryption	37 bytes	16-byte outer header + 5-byte inner header (timestamp + msg_type) + 16-byte AEAD tag
SessionDatagram body	35 bytes	ttl + path_mtu + src_addr + dest_addr (msg_type counted in inner header)
FSP header	12 bytes	4-byte prefix + 8-byte counter (used as AEAD AAD)
FSP inner header	6 bytes	4-byte timestamp + 1-byte msg_type + 1-byte inner_flags (inside AEAD)
Session AEAD tag	16 bytes	ChaCha20-Poly1305 tag on session-encrypted payload
Data path total	106 bytes	FIPS_OVERHEAD constant

Coordinate piggybacking (CP flag) adds variable overhead: 2 + entries × 16 per coordinate, with both src and dst coords sent. The send path skips the CP flag if adding coords would exceed the transport MTU.

The FIPS_OVERHEAD constant (106 bytes) represents the fixed data path overhead and is used for MTU calculations.

Effective IPv6 MTU

The effective IPv6 MTU visible to applications is:

effective_ipv6_mtu = transport_mtu - FIPS_OVERHEAD

For typical deployments:

Transport MTU	Effective IPv6 MTU	Notes
1472 (UDP/Ethernet)	1366	Standard deployment
1280 (UDP minimum)	1174	Below IPv6 minimum

IPv6 mandates that every link support at least 1280 bytes. The minimum transport path MTU for the IPv6 adapter is therefore:

1280 + 106 = 1386 bytes

Transports with smaller MTUs (radio at ~250 bytes, serial at 256 bytes) cannot support the IPv6 adapter without some form of internal fragmentation and reassembly. Otherwise, applications on those transports must use the native FIPS datagram API.

ICMP Packet Too Big

When an outbound packet at the TUN exceeds the effective IPv6 MTU, the adapter generates an ICMPv6 Packet Too Big message and delivers it back to the application via the TUN. This triggers the kernel's Path MTU Discovery (PMTUD) mechanism, which adjusts TCP segment sizes for subsequent transmissions.

ICMP Packet Too Big generation is rate-limited per source address (100ms interval) to prevent storms from applications sending many oversized packets.

The ICMP response is delivered locally (back through the TUN to the kernel) — no network traversal is needed, so delivery is reliable.

TCP MSS Clamping

The adapter intercepts TCP SYN and SYN-ACK packets at the TUN interface and clamps the Maximum Segment Size (MSS) option:

clamped_mss = effective_ipv6_mtu - 40 (IPv6 header) - 20 (TCP header)

This prevents TCP connections from negotiating segment sizes that would exceed the FIPS path MTU. Clamping is applied in two places:

• TUN reader (outbound): Clamps MSS on outbound SYN packets
• TUN writer (inbound): Clamps MSS on inbound SYN-ACK packets

Together, these ensure both directions of a TCP connection use appropriately sized segments from the start, avoiding the initial oversized packet loss that would occur with ICMP Packet Too Big alone.

ICMP Rate Limiting

ICMPv6 error generation is rate-limited per source address using a token bucket (100ms interval). This matches the standard ICMP rate limiting approach and prevents amplification when an application sends a burst of oversized packets.

TUN Interface

The TUN device (fips0) is the mechanism that connects the adapter to the kernel's IPv6 stack. It is an implementation detail of the adapter, not its defining feature.

Architecture

Applications (sockets using fd00::/8 addresses)
       │
       ▼
Kernel IPv6 Stack (routing: fd00::/8 → fips0)
       │
       ▼
TUN Device (fips0)
    ├── Reader Thread (blocking I/O → packet processing)
    └── Writer Thread (mpsc queue → TUN writes)

Reader Thread

The TUN reader receives raw IPv6 packets from applications and processes them:

1. Validate IPv6 header
2. Extract destination fd00::/8 address
3. Look up identity cache — miss returns ICMPv6 Destination Unreachable
4. Retrieve NodeAddr and PublicKey from cache
5. Look up or establish FSP session
6. Encrypt payload with session keys
7. Route through FMP toward destination

Writer Thread

A single writer thread services an mpsc queue of outbound packets:

• Inbound mesh traffic (decrypted session payloads destined for local applications)
• ICMPv6 error responses (Packet Too Big, Destination Unreachable)
• TCP MSS-clamped SYN-ACK packets

The queue-based design eliminates contention on TUN writes and cleanly separates concerns. New packet sources can be added by cloning the sender handle.

Local Address Guarantee

The Linux kernel routing table processes rules in priority order:

1. Local table: Intercepts traffic to addresses assigned to this machine
2. Main table: Routes fd00::/8 to the TUN device

This means every packet arriving at the TUN reader is guaranteed to be for a remote FIPS destination. No "is this for me?" check is needed on the read path.

Configuration

tun:
  enabled: true
  name: fips0
  mtu: 1280

Privileges

TUN device creation requires CAP_NET_ADMIN. Options:

• Run as root
• Set capability: sudo setcap cap_net_admin+ep ./target/debug/fips
• Pre-created persistent TUN device

Implementation Status

Feature	Status
TUN device creation and configuration	Implemented
IPv6 address assignment (netlink)	Implemented
TUN reader/writer threads	Implemented
ICMPv6 Destination Unreachable	Implemented
ICMPv6 Packet Too Big	Implemented
ICMP rate limiting (per-source)	Implemented
TCP MSS clamping (SYN + SYN-ACK)	Implemented
DNS service (.fips domain)	Implemented
Per-destination route MTU (netlink)	Planned
Transit MTU error signal	Implemented
Path MTU tracking (SessionDatagram field)	Implemented
Path MTU notification (end-to-end echo)	Implemented
Endpoint fragmentation/reassembly	Transport drivers

Design Considerations

Path MTU Discovery

Two complementary mechanisms support full PMTUD:

1. Proactive: The path_mtu field (2 bytes) in the SessionDatagram envelope is implemented at the FMP level. The source sets it to its outbound link MTU minus overhead; each transit node applies min(current, own_outbound_mtu - overhead). The destination receives the forward-path minimum. PathMtuNotification is handled at the session layer; the destination sends the observed forward-path MTU back to the source, which applies it with decrease-immediate / increase-requires-3-consecutive hysteresis.

2. Reactive: When a transit node cannot forward a packet (MTU exceeded), it sends an error signal back to the source. This handles the in-flight gap between a path MTU decrease and the source learning via the echo.

Both are needed: proactive handles steady state; reactive handles the transient window when oversized packets hit a new bottleneck before the source adapts.

No Fragmentation

FIPS remains a pure datagram service with no fragmentation at transit nodes. Session-layer encryption is end-to-end — the AEAD tag authenticates the entire plaintext. Fragmenting encrypted datagrams would require either exposing plaintext structure to transit nodes (unacceptable) or reassembly before decryption (opens attack surface).

References

• fips-intro.md — Protocol overview and architecture
• fips-session-layer.md — FSP (below the adapter)
• fips-wire-formats.md — FSP and SessionDatagram wire formats
• fips-configuration.md — TUN configuration parameters