Module mtu 

VCL MTU Negotiation

Automatic MTU (Maximum Transmission Unit) discovery and negotiation for VCL connections.

Why MTU matters for VPN

Physical MTU:  1500 bytes (Ethernet)
IP header:       20 bytes
UDP header:       8 bytes
VCL header:      ~64 bytes
─────────────────────────
Usable payload: 1408 bytes  ← this is what fragment_size should be

If packets are larger than the path MTU they get fragmented by the OS or silently dropped — causing mysterious slowdowns in VPN tunnels.

Example

use vcl_protocol::mtu::{MtuConfig, MtuNegotiator, PathMtu};

let config = MtuConfig::default();
let mut negotiator = MtuNegotiator::new(config);

// Probe results come in as you send test packets
negotiator.record_probe(1400, true);
negotiator.record_probe(1450, false); // dropped — too large

let mtu = negotiator.current_mtu();
println!("Path MTU: {}", mtu);

let vcl_payload = negotiator.recommended_fragment_size();
println!("Use fragment_size: {}", vcl_payload);

Structs

MtuConfig

Configuration for MTU negotiation.

MtuNegotiator

Manages MTU discovery via binary search probing.

PathMtu

The result of MTU discovery for a network path.

Enums

MtuState

State of the MTU negotiation process.

Constants

ETHERNET_MTU

Standard Ethernet MTU.

IPV4_HEADER

IPv4 header size (minimum, no options).

IPV6_HEADER

IPv6 header size (fixed).

MAX_MTU

Maximum MTU we will ever probe.

MIN_MTU

Minimum safe MTU — guaranteed to work everywhere (RFC 791).

UDP_HEADER

UDP header size.

VCL_HEADER_OVERHEAD

Overhead added by VCL Protocol headers on top of IP+UDP. Ed25519 sig(64) + prev_hash(32) + nonce(24) + sequence(8) + version(1) + bincode overhead(~20)

Functions

fragment_size_for_mtu

Compute the recommended fragment size for a known MTU and transport.