fips/docs/design/fips-links.md

FIPS Transport Protocols

FIPS nodes peer with each other over a variety of transport types. This document explores the requirements and characteristics of different transport protocols that FIPS can operate over.

Terminology

• Transport: A physical or logical interface over which FIPS communicates (e.g., a UDP socket, Ethernet NIC, or Tor client)
• Link: A connection instance to a specific peer over a transport

This document describes transport-level characteristics. See fips-architecture.md for the Transport trait definition.

Design Principles

FIPS is a Layer 3 (network) protocol. It exposes an IPv6 interface to local applications, with an address deterministically derived from the node's npub. This means existing UDP, TCP, and other IP-based applications work unmodified over FIPS.

However, this IPv6-over-transport architecture requires care to avoid classic encapsulation pitfalls. In particular, running TCP over a reliable transport (like TCP/IP overlay) creates "TCP-over-TCP" where retransmission and congestion control mechanisms at both layers interact adversely. FIPS prefers unreliable transports for this reason.

FIPS treats underlying connectivity as abstract transports, regardless of whether those transports are:

• True L2 protocols (Ethernet, Bluetooth)
• L4-over-L3 tunnels (UDP/IP) used as transport substrate for NAT traversal
• Application-layer overlays (Tor, I2P)

Each transport driver presents a uniform interface to the FIPS routing layer: send/receive datagrams to/from a transport-layer peer address.

Transport Characteristics

Overlay Transports (L3/L4 Substrate)

These transports tunnel FIPS over an existing network layer, typically for internet connectivity or anonymity. Overlay transports are expected to be the majority in early deployments, but all depend on existing IP/Internet infrastructure that FIPS is ultimately designed to replace.

Transport	Encapsulation	Addressing	MTU	Latency	Reliability	Bandwidth	Discovery
UDP/IP	UDP datagram	IP:port	1280-1472	1-500ms	Unreliable	High	DNS-SD, Nostr
TCP/IP	Framed stream	IP:port	Stream	10-500ms	Reliable	High	DNS-SD, Nostr
WebSocket	WS frames	URL	Stream	10-500ms	Reliable	High	Nostr
Tor	TCP stream	.onion	Stream	500ms-5s	Reliable	Low-Med	Static, Nostr
I2P	I2P datagram	Destination	~32K	1-10s	Unreliable	Low	I2P directory

Shared Medium Transports

These transports operate over broadcast or multicast-capable media where multiple endpoints share the same physical or logical channel.

Transport	Encapsulation	Addressing	MTU	Latency	Reliability	Bandwidth	Discovery
Ethernet	EtherType frame	MAC	1500	<1ms	Unreliable	High	Multicast
WiFi Direct	802.11 frame	MAC	1500	1-10ms	Unreliable	High	Service discovery
DOCSIS (Cable)	DOCSIS frame	MAC	1500	10-50ms	Unreliable	1M-1G	N/A (uses IP)
Bluetooth Classic	L2CAP	BD_ADDR	672-64K	10-100ms	Reliable	2-3 Mbps	Inquiry + SDP
BLE	L2CAP CoC/GATT	BD_ADDR	23-517	10-30ms	Reliable	125K-2M	GATT advertising
Zigbee	802.15.4 frame	16/64-bit	~100	15-30ms	Reliable	250 kbps	Network scan
LoRa	Raw packet	Device addr	51-222	100ms-10s	Unreliable	0.3-50 kbps	Beacons

Point-to-Point Transports

These transports connect exactly two endpoints with no shared medium or addressing.

Transport	Encapsulation	Addressing	MTU	Latency	Reliability	Bandwidth	Discovery
Serial	SLIP/COBS frame	None (P2P)	256-1500	1-100ms	Reliable	9.6K-1M	Configured
Dialup	PPP frame	None (P2P)	1500	100-200ms	Reliable	33.6-56K	Configured

Notes

MTU: Minimum/maximum or typical range. FIPS must handle heterogeneous MTUs across the mesh; the IPv6 minimum (1280) is a safe baseline for the FIPS packet format.

Latency: Typical range from best-case to worst-case. Affects spanning tree convergence and keepalive timing.

Reliability: Whether the link provides delivery guarantees. Unreliable links may drop, reorder, or duplicate packets. FIPS must tolerate this at the routing layer.

Bandwidth: Order of magnitude. Affects flow control and congestion decisions, but FIPS routing itself is low-bandwidth (control plane only).

UDP/IP as Primary Internet Transport

For internet-connected nodes, UDP/IP is the recommended transport:

• NAT traversal: UDP hole punching enables peer connections through NAT
• Firewall compatibility: UDP outbound rarely blocked; stateful firewalls pass return traffic
• No connection state: Matches FIPS datagram model
• Low overhead: 8-byte UDP header is negligible
• Avoids TCP-over-TCP: As noted in Design Principles, unreliable transports avoid adverse interactions with application-layer TCP

Raw IP with a custom protocol number would be cleaner but is blocked by most NAT devices and firewalls, limiting deployment to networks without NAT.

Transport Driver Interface

Note

: The definitive Transport trait is defined in fips-architecture.md. This section provides a simplified conceptual view.

Each transport driver provides:

• send(addr, data) — Send a FIPS packet to a transport-layer address
• recv() — Receive a FIPS packet from any peer
• mtu() — Maximum FIPS packet size for this transport
• discover() — Find potential peers (transport-specific mechanism)

Transport drivers handle any necessary framing, fragmentation, or encryption at the transport layer. The FIPS routing layer sees only FIPS packets.

Topics for Further Design

• Framing protocols for stream-based transports (TCP, WebSocket)
• Transport-layer encryption requirements vs FIPS-layer encryption
• Congestion control and flow control per transport type
• Multi-path: using multiple transports to same peer
• Transport quality metrics for parent selection