hightower-stun 0.1.1

# RFC 8489 Implementation Status

This document tracks what has been implemented from RFC 8489 (STUN).

## STUN Methods

| Method | Code | Status | Notes |
|--------|------|--------|-------|
| Binding | 0x001 | ✅ Implemented | Request and response fully supported |

## STUN Message Types

| Type | Code | Status | Notes |
|------|------|--------|-------|
| Binding Request | 0x0001 | ✅ Implemented | Client can send, server can receive |
| Binding Response (Success) | 0x0101 | ✅ Implemented | Server can send, client can receive |
| Binding Error Response | 0x0111 | ❌ Not Implemented | Error responses not yet supported |
| Binding Indication | 0x0011 | ❌ Not Implemented | Indications not needed for basic usage |

## STUN Attributes

### Implemented

| Attribute | Code | Status | Notes |
|-----------|------|--------|-------|
| MAPPED-ADDRESS | 0x0001 | ✅ Implemented | Decode only, fallback for XOR-MAPPED-ADDRESS |
| XOR-MAPPED-ADDRESS | 0x0020 | ✅ Implemented | Full encode/decode, IPv4 and IPv6 |

### Not Implemented

| Attribute | Code | Status | Reason |
|-----------|------|--------|--------|
| USERNAME | 0x0006 | ❌ Not Implemented | Authentication not implemented |
| USERHASH | 0x001E | ❌ Not Implemented | Authentication not implemented |
| MESSAGE-INTEGRITY | 0x0008 | ❌ Not Implemented | Authentication not implemented |
| MESSAGE-INTEGRITY-SHA256 | 0x001C | ❌ Not Implemented | Authentication not implemented |
| FINGERPRINT | 0x8028 | ❌ Not Implemented | Optional integrity check |
| ERROR-CODE | 0x0009 | ❌ Not Implemented | Error responses not implemented |
| REALM | 0x0014 | ❌ Not Implemented | Authentication not implemented |
| NONCE | 0x0015 | ❌ Not Implemented | Authentication not implemented |
| PASSWORD-ALGORITHM | 0x001D | ❌ Not Implemented | Authentication not implemented |
| PASSWORD-ALGORITHMS | 0x8002 | ❌ Not Implemented | Authentication not implemented |
| UNKNOWN-ATTRIBUTES | 0x000A | ❌ Not Implemented | Error handling not implemented |
| SOFTWARE | 0x8022 | ❌ Not Implemented | Optional server identification |
| ALTERNATE-SERVER | 0x8023 | ❌ Not Implemented | Redirection not implemented |
| ALTERNATE-DOMAIN | 0x8003 | ❌ Not Implemented | Redirection not implemented |

## Core Protocol Features

| Feature | Status | Notes |
|---------|--------|-------|
| STUN Message Header | ✅ Implemented | 20-byte header with magic cookie |
| Transaction ID | ✅ Implemented | 96-bit transaction ID generation and matching |
| Attribute Encoding/Decoding | ✅ Implemented | TLV encoding with 32-bit padding |
| Magic Cookie | ✅ Implemented | 0x2112A442 validation |
| UDP Transport | ✅ Implemented | Client and server support |
| TCP Transport | ❌ Not Implemented | Not needed for basic usage |
| TLS Transport | ❌ Not Implemented | Not needed for basic usage |
| DTLS Transport | ❌ Not Implemented | Not needed for basic usage |

## Security Features

| Feature | Status | Notes |
|---------|--------|-------|
| Short-Term Credentials | ❌ Not Implemented | RFC 12 specifies basic servers SHOULD NOT use auth |
| Long-Term Credentials | ❌ Not Implemented | RFC 12 specifies basic servers SHOULD NOT use auth |
| Message Integrity (HMAC-SHA1) | ❌ Not Implemented | Not required for basic server |
| Message Integrity (HMAC-SHA256) | ❌ Not Implemented | Not required for basic server |
| Fingerprint (CRC-32) | ❌ Not Implemented | Optional, basic server MAY use |

## Server Behavior

| Feature | Status | Notes |
|---------|--------|-------|
| Basic STUN Server (RFC Section 12) | ✅ Implemented | Minimal compliant server |
| Binding Request Processing | ✅ Implemented | Receives and validates requests |
| XOR-MAPPED-ADDRESS Response | ✅ Implemented | Returns client's reflexive address |
| IPv4 Support | ✅ Implemented | Full support |
| IPv6 Support | ✅ Implemented | Full support |
| Authentication | ❌ Not Implemented | Per RFC 12, basic servers SHOULD NOT use |
| ALTERNATE-SERVER | ❌ Not Implemented | Per RFC 12, basic servers SHOULD NOT use |
| Error Responses | ❌ Not Implemented | Not essential for basic operation |

## Client Behavior

| Feature | Status | Notes |
|---------|--------|-------|
| Binding Request Generation | ✅ Implemented | Creates valid requests |
| DNS Resolution | ✅ Implemented | Supports hostnames and IP addresses |
| Transaction ID Matching | ✅ Implemented | Validates response matches request |
| XOR-MAPPED-ADDRESS Parsing | ✅ Implemented | Extracts reflexive address |
| IPv4 Support | ✅ Implemented | Full support |
| IPv6 Support | ✅ Implemented | Full support |
| Retransmission (RTO) | ❌ Not Implemented | No retry logic |
| Authentication | ❌ Not Implemented | Not needed for public STUN servers |

## What's Missing

The implementation focuses on **Basic STUN Server** functionality (RFC Section 12). According to the RFC:

- Basic STUN servers **SHOULD NOT** use authentication (short-term or long-term credentials)
- Basic STUN servers **SHOULD NOT** use ALTERNATE-SERVER
- Basic STUN servers **MAY** use FINGERPRINT but **MUST NOT** require it

### Not Critical for Basic Operation

- **Authentication mechanisms** - RFC explicitly states basic servers should not use
- **Error responses** - Server ignores invalid requests rather than responding with errors
- **TCP/TLS/DTLS** - UDP is sufficient for basic operation
- **Retransmission** - Client can be wrapped with retry logic if needed
- **FINGERPRINT** - Optional integrity checking

### Potential Future Additions

- Error response handling for better debugging
- FINGERPRINT support for integrity checking
- TCP transport for reliability
- Retransmission logic in client
- SOFTWARE attribute for server identification

## Compliance

This implementation provides a **compliant Basic STUN Server** as defined in RFC 8489, Section 12. It implements the minimal required functionality for NAT traversal and public IP discovery.