ddn.net.dtls.pmtud

Path MTU Discovery for DTLS (RFC 8899 PLPMTUD).

Implements Packetization Layer Path MTU Discovery to dynamically discover the optimal path MTU for each peer. Uses probe packets of increasing size to detect the maximum transmission unit without relying on ICMP messages.

The PLPMTUD state machine (RFC 8899 §5):

DISABLED → BASE (start probing at MIN_PROBE_SIZE) BASE → SEARCHING (binary search between min and max) SEARCHING → COMPLETE (found optimal MTU) SEARCHING → BASE (probe lost, reduce MTU) COMPLETE → SEARCHING (periodic re-probe)

Thread safety: NOT thread-safe. All access must be serialized by the owning DtlsEngine.

enum PmtudState

struct PmtudPeerState PmtudProbeResult

class DtlsPmtudEngine

Types 4

enumPmtudState : ubyte

PLPMTUD state machine states (RFC 8899 §5).

DISABLED

BASE

SEARCHING

COMPLETE

ERROR

structPmtudPeerState

Per-peer PLPMTUD state.

Tracks the current MTU probe state for a single remote endpoint. The engine creates one instance per connected peer when PLPMTUD is enabled.

Fields

DatagramEndpoint peer

PmtudState state

size_t probeSize

size_t confirmedMtu

size_t minMtu

size_t maxMtu

size_t probeSuccessCount

size_t probeAttemptCount

MonoTime lastProbeSent

size_t searchMin

size_t searchMax

structPmtudProbeResult

Result of a PMTU probe check.

Fields

bool shouldProbe

size_t probeSize

Duration timeout

classDtlsPmtudEngine

PLPMTUD engine for DTLS.

Manages per-peer MTU discovery state and determines when to send probe packets and at what size. The engine calls checkProbe() for each peer periodically and sends probe packets when indicated.

Fields

bool enabled_

Duration probeTimeout_

size_t minProbeSize_

size_t maxProbeSize_

size_t requiredSuccesses_

size_t maxAttempts_

Methods

bool enabled() @property const @safe pure nothrow @nogcReturns: true if PLPMTUD is enabled.

void enable(bool on) @safe pure nothrow @nogcEnables or disables PLPMTUD.

PmtudPeerState createPeerState(DatagramEndpoint peer, size_t baseMtu) @safe pure nothrow @nogcCreates initial PMTU state for a new peer.

PmtudProbeResult checkProbe(ref PmtudPeerState state, MonoTime now) @safe pure nothrow @nogcChecks if a probe should be sent for the given peer.

bool recordProbeSuccess(ref PmtudPeerState state) @safe pure nothrow @nogcRecords a successful probe acknowledgment.

size_t recordProbeFailure(ref PmtudPeerState state) @safe pure nothrow @nogcRecords a probe failure (timeout).

size_t effectiveMtu(ref const PmtudPeerState state) const @safe pure nothrow @nogcReturns the effective MTU for a peer.

void resetPeerState(ref PmtudPeerState state, size_t baseMtu) @safe pure nothrow @nogcResets a peer's PMTU state (e.g., after path change).

Constructors

this(Duration probeTimeout =  dur!"msecs"(DtlsPmtud.PROBE_TIMEOUT_MS),
          size_t minProbeSize =  DtlsPmtud.MIN_PROBE_SIZE,
          size_t maxProbeSize =  DtlsPmtud.MAX_PROBE_SIZE,
          size_t requiredSuccesses =  DtlsPmtud.PROBE_COUNT,
          size_t maxAttempts =  DtlsPmtud.MAX_PROBE_ATTEMPTS)

Constructs a PLPMTUD engine with configurable parameters.