Keep a link's two sides as state alone, the sockets and threads apart

link/link-protocol.h is the protocol with no socket, thread or clock of
its own: packets and time in, packets out. link/link.h carries it over
UDP as before. A sender starts small (8 flights, 16 records, a 4 KiB
ring) so one thread can hold many. The transport's CPU a record is the
same within 2% (five rounds each way, 61 B to 16 KiB).

test-link runs both sides through a network of its own: loss, packets
held back and doubled, failed saves, an idle sweep, and both sides
starting again, every record applied once as sent.

1 files changed, 1494 insertions, 0 deletions

diff --git a/link/link-protocol.c b/link/link-protocol.c
new file mode 100644
index 0000000..94a965b
--- /dev/null
+++ b/link/link-protocol.c
@@ -0,0 +1,1494 @@
+#define _GNU_SOURCE
+/* Links without connections as state alone (link/link-protocol.h,
+ * docs/design/011-pure-transport.md). */
+#include "link-protocol.h"
+#include <fcntl.h>
+#include <malloc.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/random.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+enum { VERSION = 0x70, HEADER = 21, DATAGRAM = PURE_LINK_DATAGRAM, BODY = DATAGRAM - HEADER - PURE_AEAD_TAG, PIECE = 1000 };
+enum { F_PADDING = 0, F_STREAM = 1, F_RECORD = 2, F_ACK = 3, F_STORED = 4, F_PING = 5, F_RECORDS = 6, STORED_GAP = 1 };
+enum { RECORDS_HEAD = 10 }; /* type, the first number, the count */
+enum { RANGES = 32, RUNS = 2, UNITS = 64, CREDIT = 1024, BATCH = 256, RECORD_HEAD = 19 };
+enum { WINDOW_FIRST = 10 * DATAGRAM, WINDOW_LEAST = 2 * DATAGRAM };
+#define MS UINT64_C(1000000)
+/* RFC 9002's values, but a first RTT of 100 ms (a LAN's, not the 333 ms of
+ * the Internet at large); a probe at least every second while a receiver is
+ * away; a record received but not stored after a second is sent again. */
+static const uint64_t FIRST_RTT = 100 * MS, ACK_DELAY_MOST = 25 * MS, GRANULARITY = 1 * MS, PTO_MOST = 1000 * MS,
+                      STORED_WAIT = 1000 * MS, RECORD_MOST = UINT64_C(64) << 20, HELD_MOST = UINT64_C(16) << 20;
+
+static void put16(unsigned char *p, uint32_t v) { p[0] = (unsigned char)v, p[1] = (unsigned char)(v >> 8); }
+static void put32(unsigned char *p, uint32_t v) { for (int i = 0; i < 4; ++i) p[i] = (unsigned char)(v >> 8 * i); }
+static void put64(unsigned char *p, uint64_t v) { for (int i = 0; i < 8; ++i) p[i] = (unsigned char)(v >> 8 * i); }
+static uint32_t get16(const unsigned char *p) { return (uint32_t)p[0] | (uint32_t)p[1] << 8; }
+static uint32_t get32(const unsigned char *p) {
+    return (uint32_t)p[0] | (uint32_t)p[1] << 8 | (uint32_t)p[2] << 16 | (uint32_t)p[3] << 24;
+}
+static uint64_t get64(const unsigned char *p) { return (uint64_t)get32(p) | (uint64_t)get32(p + 4) << 32; }
+/* LEB128: seven bits a byte, low first. */
+static size_t put_varint(unsigned char *p, uint64_t v) {
+    size_t n = 0;
+    for (; v >= 0x80; v >>= 7) p[n++] = (unsigned char)(v | 0x80);
+    p[n++] = (unsigned char)v;
+    return n;
+}
+static size_t varint_size(uint64_t v) {
+    size_t n = 1;
+    for (; v >= 0x80; v >>= 7) ++n;
+    return n;
+}
+/* A varint from p (size bytes): how many bytes it took, or 0. */
+static size_t get_varint(const unsigned char *p, size_t size, uint64_t *v) {
+    *v = 0;
+    for (size_t n = 0; n < size && n < 10; ++n) {
+        *v |= (uint64_t)(p[n] & 0x7f) << (7 * n);
+        if (!(p[n] & 0x80)) return n + 1;
+    }
+    return 0;
+}
+
+static uint64_t least(uint64_t a, uint64_t b) { return a < b ? a : b; }
+static uint64_t most(uint64_t a, uint64_t b) { return a > b ? a : b; }
+
+static uint32_t fnv(uint32_t h, const void *bytes, size_t size) {
+    const unsigned char *p = bytes;
+    for (size_t i = 0; i < size; ++i) h = (h ^ p[i]) * 16777619u;
+    return h;
+}
+
+static uint32_t key_id(const unsigned char key[PURE_SEAL_KEY]) { return fnv(2166136261u, key, PURE_SEAL_KEY); }
+
+static uint64_t random64(void) {
+    uint64_t value = 0;
+    while (getrandom(&value, sizeof(value), 0) != sizeof(value) || !value) {}
+    return value;
+}
+
+/* The key and nonce base of packets from `from` (in its run from_run) to
+ * `to`, answering to_run (0 for none). */
+static bool derive(const unsigned char secret[32], const unsigned char from[PURE_SEAL_KEY],
+                   const unsigned char to[PURE_SEAL_KEY], uint64_t from_run, uint64_t to_run,
+                   unsigned char key[PURE_AEAD_KEY], unsigned char base[PURE_AEAD_NONCE]) {
+    unsigned char info[16 + 2 * PURE_SEAL_KEY + 16], out[PURE_AEAD_KEY + PURE_AEAD_NONCE];
+    memcpy(info, "pure link 0 keys", 16);
+    memcpy(info + 16, from, PURE_SEAL_KEY);
+    memcpy(info + 16 + PURE_SEAL_KEY, to, PURE_SEAL_KEY);
+    put64(info + 16 + 2 * PURE_SEAL_KEY, from_run);
+    put64(info + 24 + 2 * PURE_SEAL_KEY, to_run);
+    if (!pure_link_derive(secret, info, sizeof(info), out, sizeof(out))) return false;
+    memcpy(key, out, PURE_AEAD_KEY);
+    memcpy(base, out + PURE_AEAD_KEY, PURE_AEAD_NONCE);
+    explicit_bzero(out, sizeof(out));
+    return true;
+}
+
+/* The same as a context of its own, for a sender's one link. */
+static PureAead *aead_of(const unsigned char secret[32], const unsigned char from[PURE_SEAL_KEY],
+                         const unsigned char to[PURE_SEAL_KEY], uint64_t from_run, uint64_t to_run,
+                         unsigned char base[PURE_AEAD_NONCE]) {
+    unsigned char key[PURE_AEAD_KEY];
+    PureAead *aead = derive(secret, from, to, from_run, to_run, key, base) ? pure_aead(key) : NULL;
+    explicit_bzero(key, sizeof(key));
+    return aead;
+}
+
+static void nonce_of(unsigned char out[PURE_AEAD_NONCE], const unsigned char base[PURE_AEAD_NONCE], uint64_t pn) {
+    memcpy(out, base, PURE_AEAD_NONCE);
+    for (int i = 0; i < 8; ++i) out[4 + i] ^= (unsigned char)(pn >> (56 - 8 * i));
+}
+
+/* A packet: the header, then body sealed (with key, or the context's own
+ * when NULL); its size, or 0. */
+static size_t seal_packet(PureAead *aead, const unsigned char *key, const unsigned char base[PURE_AEAD_NONCE], uint32_t id,
+                          uint64_t run, uint64_t pn, const unsigned char *body, size_t size, unsigned char *out) {
+    unsigned char nonce[PURE_AEAD_NONCE];
+    out[0] = VERSION;
+    put32(out + 1, id);
+    put64(out + 5, run);
+    put64(out + 13, pn);
+    nonce_of(nonce, base, pn);
+    return pure_aead_seal(aead, key, nonce, out, HEADER, body, size, out + HEADER) ? HEADER + size + PURE_AEAD_TAG : 0;
+}
+
+static size_t put_names(unsigned char *p, const char *source, const char *input) {
+    size_t a = strlen(source), b = strlen(input);
+    p[0] = (unsigned char)a;
+    memcpy(p + 1, source, a);
+    p[1 + a] = (unsigned char)b;
+    memcpy(p + 2 + a, input, b);
+    return 2 + a + b;
+}
+
+/* A source and an input's names from p (size bytes); how many it takes, or 0. */
+static size_t get_names(const unsigned char *p, size_t size, char source[PURE_LINK_NAME + 1],
+                        char input[PURE_LINK_NAME + 1]) {
+    if (size < 1 || !p[0] || p[0] > PURE_LINK_NAME || size < 2u + p[0]) return 0;
+    size_t a = p[0], b = p[1 + a];
+    if (!b || b > PURE_LINK_NAME || size < 2 + a + b) return 0;
+    memcpy(source, p + 1, a);
+    source[a] = 0;
+    memcpy(input, p + 2 + a, b);
+    input[b] = 0;
+    return 2 + a + b;
+}
+
+/* ---- The sending side */
+
+enum { UNSENT, FLYING, RECEIVED };
+typedef struct { uint64_t pn; uint8_t state; } Piece;
+/* A record queued: its bytes at `at` in the link's byte ring, its pieces'
+ * states (one kept here, more beside). */
+typedef struct { uint64_t seq; size_t at; uint32_t size, pieces; Piece *more, one; } Record;
+static Piece *piece_of(Record *r, uint32_t i) { return r->pieces == 1 ? &r->one : &r->more[i]; }
+typedef struct { uint64_t seq; uint32_t piece; } Unit;
+typedef struct {
+    uint64_t pn, sent; /* its number; when it went (ns) */
+    uint32_t bytes;
+    bool eliciting, done; /* acknowledged or lost */
+    uint16_t count;
+    Unit units[UNITS];
+} Flight;
+
+struct PureLinkSender {
+    uint32_t shape, own_id, peer_id;
+    char input[PURE_LINK_NAME + 1], source[PURE_LINK_NAME + 1];
+    const PureSealKey *own;
+    unsigned char own_public[PURE_SEAL_KEY], peer[PURE_SEAL_KEY], secret[32];
+    Record *records; /* the sends not yet stored, oldest first (a ring of consecutive numbers) */
+    size_t head, count, capacity;
+    /* Their bytes, one after another as queued (a record's never cut by the
+     * ring's end): taken back in order as they are stored. */
+    unsigned char *ring;
+    size_t ring_capacity;
+    uint64_t queued, stored; /* the last queued; saved by the receiver */
+    uint64_t run, next_pn, in_run;
+    PureAead *out, *in; /* made once the receiver's key is known */
+    unsigned char out_base[PURE_AEAD_NONCE], in_base[PURE_AEAD_NONCE];
+    Flight *flights; /* one for each number sent, oldest first (a ring) */
+    size_t flight_head, flight_count, flight_capacity;
+    uint64_t in_flight, window, threshold, recovery_start, eliciting;
+    uint64_t largest_acked, latest_rtt, smoothed_rtt, rtt_var, min_rtt;
+    bool acked_any, sampled;
+    uint64_t loss_time, last_eliciting, credit, stored_changed;
+    uint64_t received;    /* pieces received and not yet stored */
+    uint64_t unsent_from; /* no record before this number has a piece unsent */
+    unsigned pto_count, probes;
+    PureLinkCounts counts;
+};
+
+static Record *record_at(PureLinkSender *l, size_t k) { return &l->records[(l->head + k) % l->capacity]; }
+
+/* The queued record numbered seq, or NULL (stored already, or not queued). */
+static Record *record_of(PureLinkSender *l, uint64_t seq) {
+    if (!l->count || seq < record_at(l, 0)->seq || seq - record_at(l, 0)->seq >= l->count) return NULL;
+    return record_at(l, (size_t)(seq - record_at(l, 0)->seq));
+}
+
+/* Every change of a piece's state goes through here: the link knows at once
+ * how many pieces wait to be stored, and where the unsent ones begin. */
+static void set_piece(PureLinkSender *l, Record *r, uint32_t i, uint8_t state) {
+    uint8_t was = (*piece_of(r, i)).state;
+    if (was == state) return;
+    if (was == RECEIVED) --l->received;
+    if (state == RECEIVED) ++l->received;
+    if (state == UNSENT && r->seq < l->unsent_from) l->unsent_from = r->seq;
+    (*piece_of(r, i)).state = state;
+}
+
+/* Where size bytes go at the end of the byte ring (after the newest record,
+ * or from its start when that is free and the end is not), or SIZE_MAX. */
+static size_t ring_place(PureLinkSender *l, size_t size) {
+    if (!l->count) return size <= l->ring_capacity ? 0 : SIZE_MAX;
+    Record *oldest = record_at(l, 0), *newest = record_at(l, l->count - 1);
+    size_t head = oldest->at, tail = newest->at + newest->size;
+    if (newest->at >= head) /* the bytes are [head, tail): after them, or before them from the start */
+        return l->ring_capacity - tail >= size ? tail : size <= head ? 0 : SIZE_MAX;
+    return head - tail >= size ? tail : SIZE_MAX; /* around the end: between the newest and the oldest */
+}
+
+/* A ring room for size bytes more: twice as large (or more), the records
+ * moved into it one after another. */
+static bool ring_grow(PureLinkSender *l, size_t size) {
+    size_t used = 0;
+    for (size_t k = 0; k < l->count; ++k) used += record_at(l, k)->size;
+    size_t capacity = l->ring_capacity ? 2 * l->ring_capacity : 4 << 10;
+    while (capacity < used + size + 1) capacity *= 2;
+    unsigned char *ring = malloc(capacity);
+    if (!ring) return false;
+    for (size_t k = 0, at = 0; k < l->count; ++k) {
+        Record *r = record_at(l, k);
+        if (r->size) memcpy(ring + at, l->ring + r->at, r->size);
+        r->at = at;
+        at += r->size;
+    }
+    free(l->ring);
+    l->ring = ring;
+    l->ring_capacity = capacity;
+    return true;
+}
+
+bool pure_link_sender_queue(PureLinkSender *l, uint64_t seq, const unsigned char *bytes, size_t size) {
+    Record r = {.seq = seq, .size = (uint32_t)size};
+    r.pieces = size ? (uint32_t)((size + PIECE - 1) / PIECE) : 1;
+    if (size > RECORD_MOST || (r.pieces > 1 && !(r.more = calloc(r.pieces, sizeof(Piece))))) return false;
+    bool ok = seq > l->queued && (!l->count || seq == l->queued + 1);
+    size_t at = ok ? ring_place(l, size) : SIZE_MAX;
+    if (ok && at == SIZE_MAX) ok = ring_grow(l, size) && (at = ring_place(l, size)) != SIZE_MAX;
+    if (ok) {
+        r.at = at;
+        if (size) memcpy(l->ring + at, bytes, size);
+    }
+    if (ok && l->count == l->capacity) {
+        size_t capacity = l->capacity ? 2 * l->capacity : 16;
+        Record *grown = malloc(capacity * sizeof(*grown));
+        if (grown) {
+            for (size_t k = 0; k < l->count; ++k) grown[k] = *record_at(l, k);
+            free(l->records);
+            l->records = grown;
+            l->capacity = capacity;
+            l->head = 0;
+        } else
+            ok = false;
+    }
+    if (!ok) {
+        free(r.more);
+        return false;
+    }
+    *record_at(l, l->count++) = r;
+    l->queued = seq;
+    return true;
+}
+
+static Flight *flight_of(PureLinkSender *l, uint64_t pn) {
+    if (!l->flight_count) return NULL;
+    uint64_t first = l->flights[l->flight_head].pn;
+    if (pn < first || pn - first >= l->flight_count) return NULL;
+    return &l->flights[(l->flight_head + (size_t)(pn - first)) % l->flight_capacity];
+}
+
+/* Room for one more flight: made before a packet is, so every number sent
+ * has its flight (they are found by number). */
+static bool flight_room(PureLinkSender *l) {
+    if (l->flight_count < l->flight_capacity) return true;
+    size_t capacity = l->flight_capacity ? 2 * l->flight_capacity : 8;
+    Flight *grown = malloc(capacity * sizeof(*grown));
+    if (!grown) return false;
+    for (size_t k = 0; k < l->flight_count; ++k) grown[k] = l->flights[(l->flight_head + k) % l->flight_capacity];
+    free(l->flights);
+    l->flights = grown;
+    l->flight_capacity = capacity;
+    l->flight_head = 0;
+    return true;
+}
+
+static void push_flight(PureLinkSender *l, const Flight *f) {
+    l->flights[(l->flight_head + l->flight_count++) % l->flight_capacity] = *f;
+}
+
+static void settle(PureLinkSender *l, Flight *f) {
+    f->done = true;
+    if (f->eliciting) {
+        --l->eliciting;
+        l->in_flight -= f->bytes;
+    }
+}
+
+/* A packet lost: what it carried and was not received otherwise goes again. */
+static void lose(PureLinkSender *l, Flight *f, uint64_t now) {
+    settle(l, f);
+    ++l->counts.lost;
+    for (uint16_t u = 0; u < f->count; ++u) {
+        Record *r = record_of(l, f->units[u].seq);
+        Piece *p = r ? &(*piece_of(r, f->units[u].piece)) : NULL;
+        if (p && p->state == FLYING && p->pn == f->pn) {
+            set_piece(l, r, f->units[u].piece, UNSENT);
+            ++l->counts.resent;
+        }
+    }
+    if (f->sent > l->recovery_start) { /* one congestion event per round trip */
+        l->recovery_start = now;
+        l->window = most(l->window / 2, WINDOW_LEAST);
+        l->threshold = l->window;
+    }
+}
+
+static void detect_lost(PureLinkSender *l, uint64_t now) {
+    l->loss_time = 0;
+    if (!l->acked_any) return;
+    uint64_t rtt = most(l->latest_rtt, l->sampled ? l->smoothed_rtt : FIRST_RTT);
+    uint64_t delay = most(rtt * 9 / 8, GRANULARITY);
+    for (size_t k = 0; k < l->flight_count; ++k) {
+        Flight *f = &l->flights[(l->flight_head + k) % l->flight_capacity];
+        if (f->pn >= l->largest_acked) break;
+        if (f->done) continue;
+        if (f->sent + delay <= now || l->largest_acked >= f->pn + 3) lose(l, f, now);
+        else if (!l->loss_time || f->sent + delay < l->loss_time) l->loss_time = f->sent + delay;
+    }
+}
+
+static void sample_rtt(PureLinkSender *l, uint64_t latest, uint64_t ack_delay) {
+    l->latest_rtt = latest;
+    if (!l->sampled) {
+        l->min_rtt = l->smoothed_rtt = latest;
+        l->rtt_var = latest / 2;
+        l->sampled = true;
+        return;
+    }
+    l->min_rtt = least(l->min_rtt, latest);
+    ack_delay = least(ack_delay, ACK_DELAY_MOST);
+    uint64_t adjusted = latest >= l->min_rtt + ack_delay ? latest - ack_delay : latest;
+    uint64_t difference = l->smoothed_rtt > adjusted ? l->smoothed_rtt - adjusted : adjusted - l->smoothed_rtt;
+    l->rtt_var = (3 * l->rtt_var + difference) / 4;
+    l->smoothed_rtt = (7 * l->smoothed_rtt + adjusted) / 8;
+}
+
+static void on_ack(PureLinkSender *l, const unsigned char *p, size_t size, uint64_t now) {
+    if (size < 13 || size < 13 + 16u * p[12]) return;
+    uint64_t largest = get64(p), delay = (uint64_t)get32(p + 8) * 1000;
+    Flight *top = NULL;
+    for (unsigned k = 0; k < p[12]; ++k) {
+        uint64_t first = get64(p + 13 + 16 * k), last = get64(p + 21 + 16 * k);
+        if (first > last || !l->flight_count) continue;
+        uint64_t oldest = l->flights[l->flight_head].pn, newest = oldest + l->flight_count - 1;
+        for (uint64_t pn = most(first, oldest); pn <= least(last, newest); ++pn) {
+            Flight *f = flight_of(l, pn);
+            if (!f || f->done) continue;
+            settle(l, f);
+            ++l->counts.acknowledged;
+            for (uint16_t u = 0; u < f->count; ++u) {
+                Record *r = record_of(l, f->units[u].seq);
+                if (r && (*piece_of(r, f->units[u].piece)).state != RECEIVED) {
+                    set_piece(l, r, f->units[u].piece, RECEIVED);
+                    l->stored_changed = now; /* waits for its save from now */
+                }
+            }
+            if (f->eliciting && f->sent > l->recovery_start)
+                l->window += l->window < l->threshold ? f->bytes : (uint64_t)DATAGRAM * f->bytes / l->window;
+            if (pn == largest) top = f;
+        }
+    }
+    if (!l->acked_any || largest > l->largest_acked) l->largest_acked = largest;
+    l->acked_any = true;
+    if (top && top->eliciting) sample_rtt(l, now - top->sent, delay);
+    l->pto_count = 0;
+    detect_lost(l, now);
+    while (l->flight_count && l->flights[l->flight_head].done) {
+        l->flight_head = (l->flight_head + 1) % l->flight_capacity;
+        --l->flight_count;
+    }
+}
+
+static void on_stored(PureLinkSender *l, const unsigned char *p, size_t size, uint64_t now) {
+    char source[PURE_LINK_NAME + 1], input[PURE_LINK_NAME + 1];
+    size_t names = get_names(p, size, source, input);
+    if (!names || size < names + 13 || strcmp(source, l->source) || strcmp(input, l->input)) return;
+    uint64_t heard = get64(p + names);
+    unsigned flags = p[names + 8];
+    uint32_t credit = get32(p + names + 9);
+    if (heard > l->stored) {
+        while (l->count && record_at(l, 0)->seq <= heard) {
+            Record *r = record_at(l, 0);
+            for (uint32_t i = 0; i < r->pieces; ++i) l->received -= (*piece_of(r, i)).state == RECEIVED;
+            free(r->more); /* its bytes go back with the ring's head */
+            l->head = (l->head + 1) % l->capacity;
+            --l->count;
+        }
+        l->stored = heard;
+        l->stored_changed = now;
+    }
+    if (flags & STORED_GAP) /* it did not keep what came after a gap: send it again */
+        for (size_t k = 0; k < l->count && l->received; ++k) {
+            Record *r = record_at(l, k);
+            for (uint32_t i = 0; i < r->pieces; ++i)
+                if ((*piece_of(r, i)).state == RECEIVED) set_piece(l, r, i, UNSENT);
+        }
+    l->credit = credit ? least(credit, 65536) : 1;
+}
+
+void pure_link_sender_take(PureLinkSender *l, const unsigned char *packet, size_t n, uint64_t now) {
+    unsigned char plain[DATAGRAM], nonce[PURE_AEAD_NONCE];
+    if (!l->out || n < HEADER + PURE_AEAD_TAG || n > DATAGRAM || packet[0] != VERSION || get32(packet + 1) != l->peer_id)
+        return; /* not ours, or an error from an earlier send (nobody listening) */
+    uint64_t run = get64(packet + 5), pn = get64(packet + 13);
+    if (!l->in || l->in_run != run) {
+        pure_aead_free(l->in);
+        l->in = aead_of(l->secret, l->peer, l->own_public, run, l->run, l->in_base);
+        l->in_run = run;
+        if (!l->in) return;
+    }
+    nonce_of(nonce, l->in_base, pn);
+    size_t size = n - HEADER - PURE_AEAD_TAG;
+    if (!pure_aead_open(l->in, NULL, nonce, packet, HEADER, packet + HEADER, n - HEADER, plain)) return;
+    ++l->counts.received;
+    for (size_t at = 0; at < size;) {
+        unsigned type = plain[at++];
+        if (type == F_ACK && at + 13 <= size && at + 13 + 16u * plain[at + 12] <= size) {
+            on_ack(l, plain + at, size - at, now);
+            at += 13 + 16u * plain[at + 12];
+        } else if (type == F_STORED) {
+            char source[PURE_LINK_NAME + 1], input[PURE_LINK_NAME + 1];
+            size_t names = get_names(plain + at, size - at, source, input);
+            if (!names || at + names + 13 > size) break;
+            on_stored(l, plain + at, size - at, now);
+            at += names + 13;
+        } else if (type != F_PING)
+            break; /* padding, or what this side does not know: the rest */
+    }
+}
+
+/* A packet of pieces not sent yet, in order, within the receiver's credit:
+ * its body's size, or 0 when there is none. */
+static size_t fill(PureLinkSender *l, unsigned char *body, Flight *f, bool pad) {
+    size_t at = 0;
+    body[at++] = F_STREAM;
+    at += put_names(body + at, l->source, l->input);
+    put32(body + at, l->shape);
+    at += 4;
+    /* The credit counts from the oldest record not stored: after a restart
+     * the sender does not know yet how far the receiver stored. */
+    uint64_t base = l->count ? record_at(l, 0)->seq - 1 : l->stored;
+    /* From the first record that may have an unsent piece: the ones before
+     * it are sent, and every one found sent moves that mark on. */
+    size_t k = l->count && l->unsent_from > base + 1 ? (size_t)(l->unsent_from - base - 1) : 0;
+    bool before = true;
+    /* A record none of whose pieces went yet goes whole when it fits what is
+     * left of the packet: whole records whose numbers follow on share a
+     * RECORDS frame (its count at run). Else its unsent pieces go, each in a
+     * RECORD frame. */
+    size_t run = 0;
+    uint64_t next = 0;
+    for (; k < l->count && f->count < UNITS; ++k) {
+        Record *r = record_at(l, k);
+        if (r->seq > base + l->credit) break;
+        bool unsent = false, fresh = true, joins = run && r->seq == next;
+        for (uint32_t i = 0; i < r->pieces && fresh; ++i) fresh = piece_of(r, i)->state == UNSENT;
+        size_t whole = (joins ? 0 : RECORDS_HEAD) + varint_size(r->size) + r->size;
+        if (fresh && f->count + r->pieces <= UNITS && at + whole <= BODY) {
+            if (!joins) {
+                body[at] = F_RECORDS;
+                put64(body + at + 1, r->seq);
+                body[at + 9] = 0;
+                run = at + 9;
+                at += RECORDS_HEAD;
+            }
+            at += put_varint(body + at, r->size);
+            if (r->size) memcpy(body + at, l->ring + r->at, r->size);
+            at += r->size;
+            ++body[run];
+            next = r->seq + 1;
+            for (uint32_t i = 0; i < r->pieces; ++i) {
+                set_piece(l, r, i, FLYING);
+                piece_of(r, i)->pn = f->pn;
+                f->units[f->count++] = (Unit){r->seq, i};
+            }
+            ++l->counts.records;
+        } else if (fresh && r->pieces == 1)
+            unsent = true; /* too large for what is left: the next packet */
+        else
+            for (uint32_t i = 0; i < r->pieces; ++i) {
+                if (piece_of(r, i)->state != UNSENT) continue;
+                uint32_t offset = i * PIECE, length = (uint32_t)least(PIECE, r->size - offset);
+                if (f->count == UNITS || at + RECORD_HEAD + length > BODY) {
+                    unsent = true;
+                    break;
+                }
+                body[at] = F_RECORD;
+                put64(body + at + 1, r->seq);
+                put32(body + at + 9, r->size);
+                put32(body + at + 13, offset);
+                put16(body + at + 17, length);
+                if (length) memcpy(body + at + RECORD_HEAD, l->ring + r->at + offset, length);
+                at += RECORD_HEAD + length;
+                run = 0;
+                set_piece(l, r, i, FLYING);
+                piece_of(r, i)->pn = f->pn;
+                f->units[f->count++] = (Unit){r->seq, i};
+                ++l->counts.records;
+            }
+        if (before && !unsent) l->unsent_from = r->seq + 1;
+        before = before && !unsent;
+        if (unsent) { /* the packet is full: whole, so it can go in a burst */
+            if (pad && f->count) {
+                memset(body + at, F_PADDING, BODY - at);
+                at = BODY;
+            }
+            break;
+        }
+    }
+    return f->count ? at : 0;
+}
+
+/* Its number is spent all the same. */
+void pure_link_sender_refused(PureLinkSender *l, uint64_t pn) {
+    Flight *f = flight_of(l, pn);
+    if (!f || f->done) return;
+    for (uint16_t u = 0; u < f->count; ++u) {
+        Record *r = record_of(l, f->units[u].seq);
+        if (r && (*piece_of(r, f->units[u].piece)).pn == pn) set_piece(l, r, f->units[u].piece, UNSENT);
+    }
+    settle(l, f);
+    f->eliciting = false;
+    f->count = 0;
+    --l->counts.packets;
+}
+
+/* Seal f with body into out, and count it sent: its size, or 0 (what it
+ * carried is then unsent again). Its number is spent either way. */
+static size_t transmit(PureLinkSender *l, Flight *f, const unsigned char *body, size_t size, unsigned char *out,
+                       uint64_t now) {
+    size_t total = seal_packet(l->out, NULL, l->out_base, l->own_id, l->run, f->pn, body, size, out);
+    ++l->next_pn;
+    f->sent = now;
+    f->bytes = (uint32_t)total;
+    if (!total) { /* not sealed: what it carried is unsent again */
+        for (uint16_t u = 0; u < f->count; ++u) {
+            Record *r = record_of(l, f->units[u].seq);
+            if (r && (*piece_of(r, f->units[u].piece)).pn == f->pn) set_piece(l, r, f->units[u].piece, UNSENT);
+        }
+        f->done = true;
+        f->eliciting = false;
+        f->count = 0;
+        push_flight(l, f);
+        return 0;
+    }
+    push_flight(l, f);
+    ++l->counts.packets;
+    if (f->eliciting) {
+        ++l->eliciting;
+        l->in_flight += f->bytes;
+        l->last_eliciting = now;
+    }
+    return total;
+}
+
+size_t pure_link_sender_packet(PureLinkSender *l, unsigned char out[PURE_LINK_DATAGRAM], bool whole, uint64_t now,
+                               uint64_t *number) {
+    bool probe = l->probes > 0;
+    if (!l->out || (!probe && l->in_flight + DATAGRAM > l->window) || !flight_room(l)) return 0;
+    unsigned char body[BODY];
+    Flight f = {.pn = l->next_pn, .eliciting = true};
+    size_t size = fill(l, body, &f, whole);
+    if (!size) {
+        if (!probe) return 0;
+        body[0] = F_PING; /* nothing to send again: a probe all the same */
+        size = 1;
+    }
+    size_t total = transmit(l, &f, body, size, out, now);
+    if (!total) return 0;
+    if (probe) --l->probes;
+    *number = f.pn;
+    return total;
+}
+
+static uint64_t pto_period(const PureLinkSender *l) {
+    uint64_t smoothed = l->sampled ? l->smoothed_rtt : FIRST_RTT, var = l->sampled ? l->rtt_var : FIRST_RTT / 2;
+    uint64_t period = smoothed + most(4 * var, GRANULARITY) + ACK_DELAY_MOST;
+    for (unsigned i = 0; i < l->pto_count && period < PTO_MOST; ++i) period *= 2;
+    return least(period, PTO_MOST);
+}
+
+static bool waiting_for_save(const PureLinkSender *l) { return l->received > 0; }
+
+uint64_t pure_link_sender_due(const PureLinkSender *l) {
+    uint64_t due = l->loss_time;
+    if (l->eliciting) {
+        uint64_t pto = l->last_eliciting + pto_period(l);
+        if (!due || pto < due) due = pto;
+    }
+    if (waiting_for_save(l)) {
+        uint64_t stored = l->stored_changed + STORED_WAIT;
+        if (!due || stored < due) due = stored;
+    }
+    return due;
+}
+
+void pure_link_sender_tick(PureLinkSender *l, uint64_t now) {
+    if (l->loss_time && l->loss_time <= now) detect_lost(l, now);
+    else if (l->eliciting && l->last_eliciting + pto_period(l) <= now) {
+        /* Nothing heard for a while: a probe, what flies longest sent again. */
+        ++l->pto_count;
+        l->probes = 1;
+        for (size_t k = 0; k < l->count; ++k) {
+            Record *r = record_at(l, k);
+            uint32_t i = 0;
+            while (i < r->pieces && (*piece_of(r, i)).state != FLYING) ++i;
+            if (i < r->pieces) {
+                set_piece(l, r, i, UNSENT);
+                ++l->counts.resent;
+                break;
+            }
+        }
+    }
+    if (waiting_for_save(l) && l->stored_changed + STORED_WAIT <= now) {
+        /* Received long ago and still not stored: the receiver lost them (it
+         * restarted) or its word got lost. Sent again: it takes a number once. */
+        for (size_t k = 0; k < l->count && l->received; ++k) {
+            Record *r = record_at(l, k);
+            for (uint32_t i = 0; i < r->pieces; ++i)
+                if ((*piece_of(r, i)).state == RECEIVED) set_piece(l, r, i, UNSENT), ++l->counts.resent;
+        }
+        l->stored_changed = now;
+    }
+}
+
+PureLinkSender *pure_link_sender(const char *source, const char *input, uint32_t shape, const PureSealKey *own,
+                                 const unsigned char own_public[PURE_SEAL_KEY]) {
+    size_t a = strlen(source), b = strlen(input);
+    PureLinkSender *l = a && a <= PURE_LINK_NAME && b && b <= PURE_LINK_NAME ? calloc(1, sizeof(*l)) : NULL;
+    if (!l) return NULL;
+    memcpy(l->source, source, a);
+    memcpy(l->input, input, b);
+    l->shape = shape;
+    l->own = own;
+    memcpy(l->own_public, own_public, PURE_SEAL_KEY);
+    l->own_id = key_id(own_public);
+    l->run = random64();
+    l->window = WINDOW_FIRST;
+    l->threshold = UINT64_MAX;
+    l->credit = CREDIT;
+    return l;
+}
+
+bool pure_link_sender_peer(PureLinkSender *l, const unsigned char peer[PURE_SEAL_KEY]) {
+    memcpy(l->peer, peer, PURE_SEAL_KEY);
+    l->peer_id = key_id(peer);
+    pure_aead_free(l->out);
+    pure_aead_free(l->in);
+    l->out = l->in = NULL;
+    return l->own && pure_link_secret(l->own, l->peer, l->secret) &&
+           (l->out = aead_of(l->secret, l->own_public, l->peer, l->run, 0, l->out_base));
+}
+
+uint64_t pure_link_sender_queued(const PureLinkSender *l) { return l->queued; }
+
+uint64_t pure_link_sender_stored(const PureLinkSender *l) { return l->stored; }
+
+void pure_link_sender_counts(const PureLinkSender *l, PureLinkCounts *out) {
+    *out = l->counts;
+    out->stored = l->stored;
+}
+
+void pure_link_sender_free(PureLinkSender *l) {
+    if (!l) return;
+    for (size_t k = 0; k < l->count; ++k) free(record_at(l, k)->more);
+    free(l->records);
+    free(l->ring);
+    free(l->flights);
+    pure_aead_free(l->out);
+    pure_aead_free(l->in);
+    explicit_bzero(l->secret, sizeof(l->secret));
+    free(l);
+}
+
+/* ---- The receiving side
+ *
+ * A sender allowed is a peer: its key and the sources it may send as, a few
+ * bytes each, found by its key's id. What a sending peer needs besides (the
+ * secret its key shares with the node's, its runs' keys and addresses, its
+ * streams) is its active state: made at its first packet (one X25519), let
+ * go once the peer has been quiet for the idle period with nothing of it
+ * waiting here. So a link at rest costs the node its allowing, and the next
+ * packet goes on without a handshake. */
+
+typedef struct { uint64_t first, last; } Range;
+typedef struct Active Active;
+typedef struct {
+    Active *active;
+    uint64_t run, seen; /* the sender's run; when it last sent */
+    bool used, ack_due, due;
+    unsigned char in_key[PURE_AEAD_KEY], out_key[PURE_AEAD_KEY], in_base[PURE_AEAD_NONCE], out_base[PURE_AEAD_NONCE];
+    uint64_t next_pn, largest_at;
+    _Alignas(8) unsigned char address[PURE_LINK_ADDRESS]; /* where it sends from: a socket's address */
+    size_t address_size;
+    Range got[RANGES]; /* the numbers received, newest first */
+    unsigned got_count;
+} Run;
+typedef struct {
+    uint64_t seq;
+    unsigned char *bytes; /* its size, then a byte a piece: had */
+    uint32_t size, pieces, got;
+} Held;
+typedef struct Stream {
+    Active *active;
+    char source[PURE_LINK_NAME + 1], input_name[PURE_LINK_NAME + 1];
+    uint32_t input, shape; /* the host's number for the input; the messages' shape */
+    uint64_t heard;              /* the last number heard (stored), once known */
+    bool known, out, stored_due, gap, ready;
+    /* The line: once the last heard is known, whole records from the one
+     * after it on, in order, as a delivery carries them (u32 size, bytes),
+     * from line_head: handed over as they are, let go by moving the head.
+     * Records that come early or in pieces, or before the last heard is
+     * known, wait in held until the line reaches them. */
+    unsigned char *line;
+    size_t line_head, line_size, line_capacity;
+    uint64_t line_first;
+    unsigned line_count;
+    Held *held;                  /* records not yet stored, by number: held_count from held_first */
+    size_t held_first, held_count, held_capacity, held_bytes;
+    Run *run;                    /* the run that sent to it last: where STORED goes */
+    uint64_t run_id;
+} Stream;
+struct Active {
+    uint32_t peer;               /* its place among the peers */
+    unsigned char secret[32];
+    uint64_t seen;
+    size_t place;                /* among the active */
+    Run runs[RUNS];
+    Stream **streams;
+    unsigned stream_count, stream_capacity;
+};
+typedef struct {
+    unsigned char key[PURE_SEAL_KEY];
+    uint32_t id, allowed; /* allowed: the place + 1 of its first source (0: none) */
+    Active *active;
+} Peer;
+typedef struct { uint32_t name, next; } Allowed; /* name: where in names (u8 length, bytes); next: a place + 1 */
+
+/* A peer quiet this long lets its active state go; the active are looked at
+ * that often; what they held goes back to the system once TRIM_AFTER have
+ * gone, or TRIM_WAIT after any went. */
+static const uint64_t IDLE_FIRST = 30000 * MS, SWEEP_EVERY = 250 * MS, TRIM_WAIT = 10000 * MS;
+enum { TRIM_AFTER = 1024 };
+
+struct PureLinkReceiver {
+    PureLinkHost host;
+    const PureSealKey *own;
+    unsigned char own_public[PURE_SEAL_KEY];
+    uint32_t own_id;
+    uint64_t run, idle, swept, trimmed, untrimmed;
+    PureAead *aead; /* keyed by each packet's run */
+    unsigned char open_key[PURE_AEAD_KEY], seal_key[PURE_AEAD_KEY]; /* the keys it has now */
+    bool open_keyed, seal_keyed;
+    unsigned char *memo; /* the secrets remembered (pure_link_remember), mapped */
+    size_t memo_size;
+    Peer *peers;
+    size_t peer_count, peer_capacity;
+    uint32_t *index; /* the peers by key id: open addressing, a place + 1 (0: empty) */
+    size_t index_size;
+    Allowed *allowed;
+    size_t allowed_count, allowed_capacity;
+    unsigned char *names;
+    size_t names_size, names_capacity;
+    Active **actives;
+    size_t active_count, active_capacity;
+    Stream **ready; /* streams that may have whole records to hand over */
+    size_t ready_count, ready_capacity;
+    Run **due; /* runs to answer */
+    size_t due_count, due_capacity;
+    unsigned char *batch;
+    size_t batch_capacity;
+    PureLinkCounts counts;
+};
+
+PureLinkReceiver *pure_link_receiver(const PureSealKey *own, const unsigned char own_public[PURE_SEAL_KEY],
+                                     const PureLinkHost *host, char *error, size_t size) {
+    if (!own) {
+        snprintf(error, size, "a receiver needs the node's key");
+        return NULL;
+    }
+    if (!pure_seal_ready(error, size)) return NULL;
+    PureLinkReceiver *s = calloc(1, sizeof(*s));
+    unsigned char key[PURE_AEAD_KEY] = {0}; /* each packet brings its run's */
+    if (!s || !(s->aead = pure_aead(key))) {
+        free(s);
+        snprintf(error, size, "out of memory");
+        return NULL;
+    }
+    s->host = *host;
+    s->own = own;
+    memcpy(s->own_public, own_public, PURE_SEAL_KEY);
+    s->own_id = key_id(own_public);
+    s->run = random64();
+    s->idle = IDLE_FIRST;
+    return s;
+}
+
+void pure_link_idle(PureLinkReceiver *s, uint64_t ms) { s->idle = ms * MS; }
+
+/* items (count used of capacity) with room for one more: the same or grown,
+ * or NULL when memory runs out (items stay as they were). */
+static void *room(void *items, size_t *capacity, size_t count, size_t item_size, size_t first) {
+    if (count < *capacity) return items;
+    size_t more = *capacity ? 2 * *capacity : first;
+    void *grown = realloc(items, more * item_size);
+    if (grown) *capacity = more;
+    return grown;
+}
+
+/* The peers by key id again, in an index twice as large. */
+static bool grow_index(PureLinkReceiver *s) {
+    size_t size = s->index_size ? 2 * s->index_size : 1024;
+    uint32_t *index = calloc(size, sizeof(*index));
+    if (!index) return false;
+    for (size_t p = 0; p < s->peer_count; ++p) {
+        size_t slot = s->peers[p].id & (size - 1);
+        while (index[slot]) slot = (slot + 1) & (size - 1);
+        index[slot] = (uint32_t)p + 1;
+    }
+    free(s->index);
+    s->index = index;
+    s->index_size = size;
+    return true;
+}
+
+static bool same_name(const PureLinkReceiver *s, uint32_t name, const char *source, size_t length) {
+    return s->names[name] == length && !memcmp(s->names + name + 1, source, length);
+}
+
+/* Keys are found by their ids, which are 32 bits: two keys may share one. */
+bool pure_link_allow(PureLinkReceiver *s, const char *source, const unsigned char key[PURE_SEAL_KEY], char *error,
+                     size_t size) {
+    size_t length = strlen(source), p = SIZE_MAX;
+    uint32_t id = key_id(key);
+    if (!length || length > PURE_LINK_NAME) {
+        snprintf(error, size, "a source's name is 1 to %d bytes", PURE_LINK_NAME);
+        return false;
+    }
+    for (size_t slot = s->index_size ? id & (s->index_size - 1) : 0; s->index_size && s->index[slot];
+         slot = (slot + 1) & (s->index_size - 1)) {
+        const Peer *peer = &s->peers[s->index[slot] - 1];
+        if (peer->id == id && !memcmp(peer->key, key, PURE_SEAL_KEY)) {
+            p = s->index[slot] - 1;
+            break;
+        }
+    }
+    if (p == SIZE_MAX) {
+        Peer *peers = s->peer_count < UINT32_MAX - 1 && ((s->peer_count + 1) * 2 <= s->index_size || grow_index(s)) ?
+                      room(s->peers, &s->peer_capacity, s->peer_count, sizeof(Peer), 64) : NULL;
+        if (!peers) goto full;
+        s->peers = peers;
+        p = s->peer_count++;
+        s->peers[p] = (Peer){.id = id};
+        memcpy(s->peers[p].key, key, PURE_SEAL_KEY);
+        size_t slot = id & (s->index_size - 1);
+        while (s->index[slot]) slot = (slot + 1) & (s->index_size - 1);
+        s->index[slot] = (uint32_t)p + 1;
+    }
+    for (uint32_t a = s->peers[p].allowed; a; a = s->allowed[a - 1].next)
+        if (same_name(s, s->allowed[a - 1].name, source, length)) return true;
+    Allowed *kept = s->allowed_count < UINT32_MAX - 1 && s->names_size + 1 + length <= UINT32_MAX ?
+                    room(s->allowed, &s->allowed_capacity, s->allowed_count, sizeof(Allowed), 64) : NULL;
+    if (!kept) goto full;
+    s->allowed = kept;
+    if (s->names_size + 1 + length > s->names_capacity) {
+        size_t more = s->names_capacity ? 2 * s->names_capacity : 4096;
+        unsigned char *names = realloc(s->names, more);
+        if (!names) goto full;
+        s->names = names;
+        s->names_capacity = more;
+    }
+    s->names[s->names_size] = (unsigned char)length;
+    memcpy(s->names + s->names_size + 1, source, length);
+    s->allowed[s->allowed_count] = (Allowed){(uint32_t)s->names_size, s->peers[p].allowed};
+    s->peers[p].allowed = (uint32_t)++s->allowed_count;
+    s->names_size += 1 + length;
+    return true;
+full:
+    snprintf(error, size, "out of memory");
+    return false;
+}
+
+static Held *held_at(Stream *st, size_t k) { return &st->held[st->held_first + k]; }
+
+static uint64_t line_end(const Stream *st) { return st->line_first + st->line_count; }
+
+/* Room for more bytes at the line's end: its front let go given back
+ * first when that is at least half of it (so no byte moves twice in a
+ * doubling), else a longer line. */
+static bool line_room(Stream *st, size_t more) {
+    if (st->line_size + more <= st->line_capacity) return true;
+    if (st->line_head && st->line_head >= st->line_capacity / 2) {
+        memmove(st->line, st->line + st->line_head, st->line_size - st->line_head);
+        st->line_size -= st->line_head;
+        st->line_head = 0;
+        if (st->line_size + more <= st->line_capacity) return true;
+    }
+    size_t capacity = st->line_capacity ? st->line_capacity : 4096;
+    while (capacity < st->line_size + more) capacity *= 2;
+    unsigned char *grown = realloc(st->line, capacity);
+    if (!grown) return false;
+    st->line = grown;
+    st->line_capacity = capacity;
+    return true;
+}
+
+static bool line_append(Stream *st, const unsigned char *bytes, uint32_t size) {
+    if (!line_room(st, 4 + (size_t)size)) return false;
+    put32(st->line + st->line_size, size);
+    if (size) memcpy(st->line + st->line_size + 4, bytes, size);
+    st->line_size += 4 + (size_t)size;
+    ++st->line_count;
+    return true;
+}
+
+/* The line's records up to heard let go (all of them, when heard is past it). */
+static void line_drop(Stream *st, uint64_t heard) {
+    if (heard < st->line_first) return;
+    if (heard >= line_end(st)) {
+        st->line_head = st->line_size = 0;
+        st->line_count = 0;
+        st->line_first = heard + 1;
+        return;
+    }
+    for (uint64_t n = heard - st->line_first + 1; n; --n, --st->line_count, ++st->line_first)
+        st->line_head += 4 + (size_t)get32(st->line + st->line_head);
+}
+
+/* Records held that the line reached, moved onto it; ones behind it let go. */
+static void drop_first(Stream *st);
+static void line_pull(Stream *st) {
+    while (st->held_count) {
+        Held *h = held_at(st, 0);
+        if (h->seq < line_end(st)) drop_first(st);
+        else if (h->seq == line_end(st) && h->got == h->pieces && line_append(st, h->bytes, h->size)) drop_first(st);
+        else break;
+    }
+}
+
+/* The first record held let go: the ones after it stay where they are. */
+static void drop_first(Stream *st) {
+    st->held_bytes -= held_at(st, 0)->size;
+    free(held_at(st, 0)->bytes);
+    ++st->held_first;
+    if (!--st->held_count) st->held_first = 0;
+}
+
+/* The memo of secrets (pure_link_remember): a header, then a place a peer
+ * in the order allowed, each the first 28 bytes of its key, a check (FNV-1a
+ * of its whole key and the secret) and the secret. A place whose check
+ * fails (never made, another key, cut short by a crash) is made again. */
+enum { MEMO_HEAD = 64, MEMO_PLACE = 64 };
+static const unsigned char memo_magic[8] = {'P', 'L', 'M', '1', 0, 0, 0, 0};
+
+static uint32_t memo_check(const unsigned char key[PURE_SEAL_KEY], const unsigned char secret[32]) {
+    return fnv(fnv(2166136261u, key, PURE_SEAL_KEY), secret, 32);
+}
+
+bool pure_link_remember(PureLinkReceiver *s, const char *path, char *error, size_t size) {
+    int fd = open(path, O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC, 0600);
+    struct stat info;
+    unsigned char head[MEMO_HEAD];
+    size_t needed = MEMO_HEAD + (s->peer_count ? s->peer_count : 1) * MEMO_PLACE;
+    bool ok = fd >= 0 && !fstat(fd, &info) && S_ISREG(info.st_mode) && !(info.st_mode & 077);
+    /* Another node's key, or none yet: begun again. */
+    bool ours = ok && pread(fd, head, MEMO_HEAD, 0) == MEMO_HEAD && !memcmp(head, memo_magic, 8) &&
+                !memcmp(head + 8, s->own_public, PURE_SEAL_KEY);
+    if (ok && !ours) {
+        memset(head, 0, MEMO_HEAD);
+        memcpy(head, memo_magic, 8);
+        memcpy(head + 8, s->own_public, PURE_SEAL_KEY);
+        ok = !ftruncate(fd, 0) && pwrite(fd, head, MEMO_HEAD, 0) == MEMO_HEAD;
+    }
+    ok = ok && (ours && (size_t)info.st_size >= needed ? true : !ftruncate(fd, (off_t)needed));
+    void *map = ok ? mmap(NULL, needed, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0) : MAP_FAILED;
+    if (fd >= 0) close(fd);
+    if (map == MAP_FAILED) {
+        snprintf(error, size, "%s: cannot keep secrets there (a file only its owner may read)", path);
+        return false;
+    }
+    s->memo = map;
+    s->memo_size = needed;
+    return true;
+}
+
+/* A peer's active state, made at its first packet: the secret its key
+ * shares with ours, remembered or made (one X25519, then remembered: the
+ * secret first, the key that makes the place its own last). */
+static Active *activate(PureLinkReceiver *s, Peer *peer, uint64_t now) {
+    Active *a = calloc(1, sizeof(*a)), **actives = room(s->actives, &s->active_capacity, s->active_count, sizeof(*actives), 64);
+    if (actives) s->actives = actives;
+    if (!a || !actives) {
+        free(a);
+        return NULL;
+    }
+    size_t index = (size_t)(peer - s->peers);
+    unsigned char *place = s->memo && MEMO_HEAD + (index + 1) * MEMO_PLACE <= s->memo_size ?
+                           s->memo + MEMO_HEAD + index * MEMO_PLACE : NULL;
+    if (place && !memcmp(place, peer->key, 28) && get32(place + 28) == memo_check(peer->key, place + 32)) {
+        memcpy(a->secret, place + 32, 32);
+        ++s->counts.remembered;
+    } else if (pure_link_secret(s->own, peer->key, a->secret)) {
+        if (place) {
+            memset(place, 0, 28);
+            __atomic_thread_fence(__ATOMIC_RELEASE);
+            memcpy(place + 32, a->secret, 32);
+            put32(place + 28, memo_check(peer->key, a->secret));
+            __atomic_thread_fence(__ATOMIC_RELEASE);
+            memcpy(place, peer->key, 28);
+        }
+    } else {
+        explicit_bzero(a, sizeof(*a));
+        free(a);
+        return NULL;
+    }
+    a->peer = (uint32_t)(peer - s->peers);
+    a->seen = now;
+    a->place = s->active_count;
+    s->actives[s->active_count++] = a;
+    peer->active = a;
+    return a;
+}
+
+static void let_go(PureLinkReceiver *s, Active *a) {
+    for (unsigned k = 0; k < a->stream_count; ++k) {
+        while (a->streams[k]->held_count) drop_first(a->streams[k]);
+        free(a->streams[k]->held);
+        free(a->streams[k]->line);
+        free(a->streams[k]);
+    }
+    free(a->streams);
+    s->peers[a->peer].active = NULL;
+    s->actives[a->place] = s->actives[--s->active_count];
+    s->actives[a->place]->place = a->place;
+    explicit_bzero(a, sizeof(*a));
+    free(a);
+}
+
+/* Whether any of a peer's records or answers waits here. */
+static bool busy(const Active *a) {
+    for (unsigned k = 0; k < a->stream_count; ++k)
+        if (a->streams[k]->out || a->streams[k]->held_count || a->streams[k]->line_count || a->streams[k]->ready) return true;
+    for (unsigned i = 0; i < RUNS; ++i)
+        if (a->runs[i].due) return true;
+    return false;
+}
+
+/* Let go the active state of peers quiet for the idle period; what idle
+ * links held goes back to the system. */
+static void sweep(PureLinkReceiver *s, uint64_t now) {
+    if (now - s->swept < SWEEP_EVERY) return;
+    s->swept = now;
+    size_t freed = 0;
+    for (size_t k = 0; k < s->active_count;) {
+        Active *a = s->actives[k];
+        if (now - a->seen < s->idle || busy(a)) ++k;
+        else let_go(s, a), ++freed;
+    }
+    s->counts.swept += freed;
+    s->untrimmed += freed;
+    if (s->untrimmed >= TRIM_AFTER || (s->untrimmed && now - s->trimmed >= TRIM_WAIT)) {
+        malloc_trim(0);
+        s->untrimmed = 0;
+        s->trimmed = now;
+    }
+}
+
+static void mark_ready(PureLinkReceiver *s, Stream *st) {
+    Stream **ready = st->ready ? NULL : room(s->ready, &s->ready_capacity, s->ready_count, sizeof(*ready), 64);
+    if (!ready) return;
+    s->ready = ready;
+    s->ready[s->ready_count++] = st;
+    st->ready = true;
+}
+
+static void mark_due(PureLinkReceiver *s, Run *r) {
+    Run **due = r->due ? NULL : room(s->due, &s->due_capacity, s->due_count, sizeof(*due), 64);
+    if (!due) return;
+    s->due = due;
+    s->due[s->due_count++] = r;
+    r->due = true;
+}
+
+static Run *run_found(Active *a, uint64_t run) {
+    for (unsigned i = 0; i < RUNS; ++i)
+        if (a->runs[i].used && a->runs[i].run == run) return &a->runs[i];
+    return NULL;
+}
+
+/* A new run's keys, in the slot unused longest: only once its first packet
+ * opened with them, so a forged one displaces nothing. */
+static Run *install_run(Active *a, const Run *made) {
+    Run *slot = &a->runs[0];
+    for (unsigned i = 0; i < RUNS; ++i) {
+        Run *r = &a->runs[i];
+        if (!r->used || (slot->used && r->seen < slot->seen)) slot = r;
+    }
+    bool due = slot->due; /* the answer list may hold the slot */
+    *slot = *made;
+    slot->due = due;
+    return slot;
+}
+
+static bool had(const Run *r, uint64_t pn) {
+    for (unsigned i = 0; i < r->got_count; ++i)
+        if (pn >= r->got[i].first && pn <= r->got[i].last) return true;
+    return false;
+}
+
+static void note(Run *r, uint64_t pn, uint64_t now) {
+    unsigned i = 0, n = r->got_count;
+    while (i < n && r->got[i].first > pn + 1) ++i;
+    if (i < n && r->got[i].first == pn + 1) { /* joins the range above it */
+        r->got[i].first = pn;
+        if (i + 1 < n && r->got[i + 1].last + 1 == pn) {
+            r->got[i].first = r->got[i + 1].first;
+            memmove(&r->got[i + 1], &r->got[i + 2], (n - i - 2) * sizeof(Range));
+            --r->got_count;
+        }
+    } else if (i < n && r->got[i].last + 1 == pn) {
+        r->got[i].last = pn;
+    } else if (i < RANGES) {
+        if (n == RANGES) --n; /* the oldest range is forgotten */
+        memmove(&r->got[i + 1], &r->got[i], (n - i) * sizeof(Range));
+        r->got[i] = (Range){pn, pn};
+        r->got_count = n + 1;
+    }
+    if (r->got_count && r->got[0].last == pn) r->largest_at = now;
+}
+
+static bool allowed(const PureLinkReceiver *s, const Peer *peer, const char *source) {
+    for (uint32_t a = peer->allowed; a; a = s->allowed[a - 1].next)
+        if (same_name(s, s->allowed[a - 1].name, source, strlen(source))) return true;
+    return false;
+}
+
+/* The stream of a peer's source to input, made the first time: NULL when the
+ * source is not allowed from the peer, or the host has no input that takes
+ * such messages. */
+static Stream *stream_of(PureLinkReceiver *s, Active *a, const char *source, const char *input, uint32_t shape) {
+    for (unsigned k = 0; k < a->stream_count; ++k) {
+        Stream *st = a->streams[k];
+        if (!strcmp(st->input_name, input) && !strcmp(st->source, source)) return st->shape == shape ? st : NULL;
+    }
+    int index = allowed(s, &s->peers[a->peer], source) ? s->host.input(s->host.context, input, shape) : -1;
+    if (index < 0) return NULL;
+    size_t capacity = a->stream_capacity;
+    Stream **streams = room(a->streams, &capacity, a->stream_count, sizeof(*streams), 2), *st;
+    if (!streams) return NULL;
+    a->streams = streams;
+    a->stream_capacity = (unsigned)capacity;
+    if (!(st = calloc(1, sizeof(*st)))) return NULL;
+    st->active = a;
+    st->input = (uint32_t)index;
+    st->shape = shape;
+    snprintf(st->source, sizeof(st->source), "%s", source);
+    snprintf(st->input_name, sizeof(st->input_name), "%s", input);
+    a->streams[a->stream_count++] = st;
+    return st;
+}
+
+/* A record's piece, kept until the record is whole and handed over. */
+static void hold(Stream *st, uint64_t seq, uint32_t size, uint32_t offset, const unsigned char *bytes, uint32_t length) {
+    if (st->known && seq <= st->heard) {
+        st->stored_due = true; /* sent again: say again how far it is stored */
+        return;
+    }
+    if ((st->known && seq > st->heard + CREDIT) || size > RECORD_MOST) return;
+    if (st->known && seq < line_end(st)) return; /* on the line already */
+    if (st->known && seq == line_end(st) && !offset && length == size) { /* the next, whole */
+        if (st->held_bytes + (st->line_size - st->line_head) + size <= HELD_MOST && line_append(st, bytes, size))
+            line_pull(st);
+        return;
+    }
+    /* In order it goes last; out of order, its place is found by halving. */
+    size_t k = st->held_count;
+    if (k && held_at(st, k - 1)->seq >= seq) {
+        size_t low = 0;
+        while (low < k) {
+            size_t middle = low + (k - low) / 2;
+            if (held_at(st, middle)->seq < seq) low = middle + 1;
+            else k = middle;
+        }
+    }
+    if (k == st->held_count || held_at(st, k)->seq != seq) {
+        if (st->held_bytes + size > HELD_MOST) return;
+        if (st->held_first && st->held_first + st->held_count == st->held_capacity) { /* the front is free: used again */
+            memmove(st->held, held_at(st, 0), st->held_count * sizeof(Held));
+            st->held_first = 0;
+        }
+        Held *kept = room(st->held, &st->held_capacity, st->held_first + st->held_count, sizeof(Held), 16);
+        if (!kept) return;
+        st->held = kept;
+        Held h = {.seq = seq, .size = size, .pieces = size ? (size + PIECE - 1) / PIECE : 1};
+        if (!(h.bytes = malloc(size + h.pieces))) return;
+        memset(h.bytes + size, 0, h.pieces);
+        memmove(held_at(st, k + 1), held_at(st, k), (st->held_count - k) * sizeof(Held));
+        *held_at(st, k) = h;
+        ++st->held_count;
+        st->held_bytes += size;
+    }
+    Held *h = held_at(st, k);
+    if (h->size != size) return;
+    if (!offset && length == size) { /* whole: every piece had */
+        if (size) memcpy(h->bytes, bytes, size);
+        memset(h->bytes + size, 1, h->pieces);
+        h->got = h->pieces;
+        return;
+    }
+    uint32_t piece = offset / PIECE;
+    if (offset % PIECE || piece >= h->pieces || length != least(PIECE, size - offset) || h->bytes[size + piece]) return;
+    if (length) memcpy(h->bytes + offset, bytes, length);
+    h->bytes[size + piece] = 1;
+    ++h->got;
+}
+
+/* Whole records from the one after the last heard, in order, to the host:
+ * one delivery a stream at a time. Before the last heard is known (after a
+ * restart) they go from the first held: the host says how far it heard.
+ * False when the host could not take them now. */
+static bool hand_over(PureLinkReceiver *s, Stream *st) {
+    if (st->out) return true;
+    if (st->known) { /* the line as it is, up to BATCH records */
+        line_pull(st);
+        if (!st->line_count) return true;
+        unsigned n = 0;
+        size_t bytes = 0;
+        for (; n < st->line_count && n < BATCH; ++n) bytes += 4 + (size_t)get32(st->line + st->line_head + bytes);
+        PureLinkDelivery d = {st->input, st->source, st->line_first, n, st->line + st->line_head, bytes, st};
+        if (!s->host.deliver(s->host.context, &d)) return false;
+        st->out = true;
+        ++s->counts.deliveries;
+        return true;
+    }
+    if (!st->held_count || (st->known && held_at(st, 0)->seq != st->heard + 1)) return true;
+    size_t n = 0, bytes = 0;
+    while (n < st->held_count && n < BATCH && held_at(st, n)->got == held_at(st, n)->pieces &&
+           held_at(st, n)->seq == held_at(st, 0)->seq + n)
+        bytes += 4 + held_at(st, n++)->size;
+    if (!n) return true;
+    if (bytes > s->batch_capacity) {
+        unsigned char *grown = realloc(s->batch, bytes);
+        if (!grown) return false;
+        s->batch = grown;
+        s->batch_capacity = bytes;
+    }
+    size_t at = 0;
+    for (size_t k = 0; k < n; ++k) {
+        put32(s->batch + at, held_at(st, k)->size);
+        memcpy(s->batch + at + 4, held_at(st, k)->bytes, held_at(st, k)->size);
+        at += 4 + held_at(st, k)->size;
+    }
+    PureLinkDelivery d = {st->input, st->source, held_at(st, 0)->seq, (unsigned)n, s->batch, bytes, st};
+    if (!s->host.deliver(s->host.context, &d)) return false;
+    st->out = true;
+    ++s->counts.deliveries;
+    return true;
+}
+
+/* A packet from peer, if it opens with the keys of peer's run (made for a
+ * run not seen yet, and for a peer at rest its active state): its frames. */
+static bool take_from(PureLinkReceiver *s, Peer *peer, const unsigned char *packet, size_t n, const void *from,
+                      size_t from_size, uint64_t now) {
+    uint64_t pn = get64(packet + 13), run = get64(packet + 5);
+    Active *a = peer->active;
+    Run *r = a ? run_found(a, run) : NULL;
+    if (r && had(r, pn)) {
+        r->ack_due = true; /* its acknowledgment was lost, perhaps */
+        mark_due(s, r);
+        return true;
+    }
+    bool woke = !a;
+    if (woke && !(a = activate(s, peer, now))) return false;
+    Run made = {.active = a, .run = run, .seen = now, .used = true};
+    bool keyed = r || (derive(a->secret, peer->key, s->own_public, run, 0, made.in_key, made.in_base) &&
+                       derive(a->secret, s->own_public, peer->key, s->run, run, made.out_key, made.out_base));
+    unsigned char plain[DATAGRAM], nonce[PURE_AEAD_NONCE];
+    nonce_of(nonce, r ? r->in_base : made.in_base, pn);
+    size_t size = n - HEADER - PURE_AEAD_TAG;
+    const unsigned char *key = r ? r->in_key : made.in_key, *give = key;
+    if (s->open_keyed && !memcmp(s->open_key, key, PURE_AEAD_KEY)) give = NULL; /* the key it has */
+    else if (keyed) memcpy(s->open_key, key, PURE_AEAD_KEY), s->open_keyed = true;
+    if (!keyed || !pure_aead_open(s->aead, give, nonce, packet, HEADER, packet + HEADER, n - HEADER, plain)) {
+        explicit_bzero(&made, sizeof(made));
+        if (woke) let_go(s, a);
+        return false;
+    }
+    if (woke) ++s->counts.woken;
+    if (!r) r = install_run(a, &made);
+    a->seen = r->seen = now;
+    note(r, pn, now);
+    r->address_size = from_size <= sizeof(r->address) ? from_size : sizeof(r->address);
+    memcpy(r->address, from, r->address_size); /* where it is now: answers go there */
+    ++s->counts.received;
+    Stream *st = NULL;
+    bool eliciting = false;
+    for (size_t at = 0; at < size;) {
+        unsigned type = plain[at++];
+        if (type == F_STREAM) {
+            char source[PURE_LINK_NAME + 1], input[PURE_LINK_NAME + 1];
+            size_t names = get_names(plain + at, size - at, source, input);
+            if (!names || at + names + 4 > size) break;
+            st = stream_of(s, a, source, input, get32(plain + at + names));
+            if (st) {
+                st->run = r;
+                st->run_id = r->run;
+            }
+            at += names + 4;
+            eliciting = true;
+        } else if (type == F_RECORD) {
+            if (at + RECORD_HEAD - 1 > size) break;
+            uint32_t length = get16(plain + at + 16);
+            if (at + RECORD_HEAD - 1 + length > size) break;
+            if (st) {
+                hold(st, get64(plain + at), get32(plain + at + 8), get32(plain + at + 12), plain + at + 18, length);
+                mark_ready(s, st);
+            }
+            at += RECORD_HEAD - 1 + length;
+            eliciting = true;
+        } else if (type == F_RECORDS) {
+            if (at + RECORDS_HEAD - 1 > size) break;
+            uint64_t first = get64(plain + at), length = 0;
+            unsigned count = plain[at + 8];
+            at += RECORDS_HEAD - 1;
+            unsigned i = 0;
+            for (size_t used; i < count && (used = get_varint(plain + at, size - at, &length)) &&
+                              length <= size - at - used; ++i) {
+                if (st) hold(st, first + i, (uint32_t)length, 0, plain + at + used, (uint32_t)length);
+                at += used + length;
+            }
+            if (st) mark_ready(s, st);
+            eliciting = true;
+            if (i < count) break; /* cut short: the rest is not read */
+        } else if (type == F_PING) {
+            eliciting = true;
+        } else
+            break; /* padding, or what this side does not know: the rest */
+    }
+    if (eliciting) {
+        r->ack_due = true;
+        mark_due(s, r);
+    }
+    return true;
+}
+
+void pure_link_receiver_take(PureLinkReceiver *s, const unsigned char *packet, size_t n, const void *from,
+                             size_t from_size, uint64_t now) {
+    if (n < HEADER + PURE_AEAD_TAG || n > DATAGRAM || packet[0] != VERSION || !s->index_size) return;
+    uint32_t id = get32(packet + 1);
+    size_t mask = s->index_size - 1;
+    uint64_t run = get64(packet + 5);
+    /* The peers with the packet's key id: first one whose run is known (no
+     * key to make), then any. */
+    for (size_t slot = id & mask; s->index[slot]; slot = (slot + 1) & mask) {
+        Peer *peer = &s->peers[s->index[slot] - 1];
+        if (peer->id == id && peer->active && run_found(peer->active, run) &&
+            take_from(s, peer, packet, n, from, from_size, now))
+            return;
+    }
+    for (size_t slot = id & mask; s->index[slot]; slot = (slot + 1) & mask) {
+        Peer *peer = &s->peers[s->index[slot] - 1];
+        if (peer->id == id && !(peer->active && run_found(peer->active, run)) &&
+            take_from(s, peer, packet, n, from, from_size, now))
+            return;
+    }
+}
+
+/* Acknowledgments and how far each stream is stored, a packet to each run
+ * that has any. How far a stream is stored goes with every acknowledgment
+ * to its run, so a lost word is said again at the next packet. */
+static void answer(PureLinkReceiver *s, uint64_t now, PureLinkSend *send, void *context) {
+    unsigned char body[BODY], packet[DATAGRAM];
+    for (size_t k = 0; k < s->due_count; ++k) {
+        Run *r = s->due[k];
+        Active *a = r->active;
+        r->due = false;
+        if (!r->used) continue;
+        size_t at = 0;
+        if (r->ack_due && r->got_count) {
+            body[at++] = F_ACK;
+            put64(body + at, r->got[0].last);
+            put32(body + at + 8, (uint32_t)least((now - r->largest_at) / 1000, UINT32_MAX));
+            body[at + 12] = (unsigned char)r->got_count;
+            at += 13;
+            for (unsigned i = 0; i < r->got_count; ++i, at += 16) {
+                put64(body + at, r->got[i].first);
+                put64(body + at + 8, r->got[i].last);
+            }
+        }
+        for (unsigned j = 0; j < a->stream_count; ++j) {
+            Stream *st = a->streams[j];
+            if (!(st->stored_due || r->ack_due) || st->run != r || st->run_id != r->run || !st->known) continue;
+            if (at + strlen(st->source) + strlen(st->input_name) + 16 > BODY) break; /* type, names, 13 */
+            body[at++] = F_STORED;
+            at += put_names(body + at, st->source, st->input_name);
+            put64(body + at, st->heard);
+            body[at + 8] = st->gap ? STORED_GAP : 0;
+            size_t room = st->held_bytes + (st->line_size - st->line_head) < HELD_MOST ? CREDIT : 1;
+            put32(body + at + 9, (uint32_t)room);
+            at += 13;
+            st->stored_due = st->gap = false;
+        }
+        r->ack_due = false;
+        if (!at) continue;
+        const unsigned char *give = r->out_key;
+        if (s->seal_keyed && !memcmp(s->seal_key, r->out_key, PURE_AEAD_KEY)) give = NULL; /* the key it has */
+        else memcpy(s->seal_key, r->out_key, PURE_AEAD_KEY), s->seal_keyed = true;
+        size_t total = seal_packet(s->aead, give, r->out_base, s->own_id, s->run, r->next_pn++, body, at, packet);
+        if (total && send(context, packet, total, r->address, r->address_size)) ++s->counts.packets;
+    }
+    s->due_count = 0;
+}
+
+/* What the host said of its deliveries: the saved records let go, and how
+ * far they are stored said. */
+static void apply_done(PureLinkReceiver *s, const PureLinkDone *done, size_t count) {
+    for (size_t k = 0; k < count; ++k) {
+        Stream *st = done[k].token;
+        st->out = false;
+        mark_ready(s, st);
+        if (!done[k].stored) continue; /* not saved: handed over again */
+        if (!st->known || done[k].heard > st->heard) st->heard = done[k].heard;
+        if (!st->known) st->line_first = st->heard + 1; /* the line begins */
+        line_drop(st, st->heard);
+        while (st->held_count && held_at(st, 0)->seq <= st->heard) drop_first(st);
+        st->known = true;
+        st->gap = done[k].gap;
+        st->stored_due = true;
+        if (st->run && st->run->used && st->run_id == st->run->run) mark_due(s, st->run);
+        s->counts.stored = st->heard;
+    }
+}
+
+/* The streams ready, each handed over; one the host could not take now is
+ * tried again at the next turn. */
+static void hand_over_ready(PureLinkReceiver *s) {
+    size_t count = s->ready_count;
+    for (size_t k = 0; k < count; ++k) {
+        Stream *st = s->ready[k];
+        st->ready = false;
+        if (!hand_over(s, st)) mark_ready(s, st);
+    }
+    memmove(s->ready, s->ready + count, (s->ready_count - count) * sizeof(*s->ready));
+    s->ready_count -= count;
+}
+
+void pure_link_receiver_turn(PureLinkReceiver *s, const PureLinkDone *done, size_t count, uint64_t now,
+                             PureLinkSend *send, void *context) {
+    if (!s->swept) s->swept = s->trimmed = now;
+    apply_done(s, done, count);
+    hand_over_ready(s);
+    answer(s, now, send, context);
+    sweep(s, now);
+}
+
+uint64_t pure_link_receiver_due(const PureLinkReceiver *s) {
+    return s->active_count || s->ready_count ? s->swept + SWEEP_EVERY : 0;
+}
+
+void pure_link_receiver_counts(const PureLinkReceiver *s, PureLinkCounts *counts) {
+    *counts = s->counts;
+    counts->active = s->active_count;
+}
+
+void pure_link_receiver_free(PureLinkReceiver *s) {
+    if (!s) return;
+    while (s->active_count) let_go(s, s->actives[0]);
+    pure_aead_free(s->aead);
+    if (s->memo) munmap(s->memo, s->memo_size);
+    free(s->actives);
+    free(s->peers);
+    free(s->index);
+    free(s->allowed);
+    free(s->names);
+    free(s->ready);
+    free(s->due);
+    free(s->batch);
+    free(s);
+}