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, 68 insertions, 1446 deletions

diff --git a/link/link.c b/link/link.c
index 103dcf5..0588633 100644
--- a/link/link.c
+++ b/link/link.c
@@ -1,10 +1,8 @@
 #define _GNU_SOURCE
-/* Links without connections (link/link.h, docs/design/011-pure-transport.md). */
+/* Links over UDP (link/link.h): link/link-protocol.h's two sides with
+ * sockets and threads. */
 #include "link.h"
-#include <arpa/inet.h>
 #include <errno.h>
-#include <fcntl.h>
-#include <malloc.h>
 #include <netdb.h>
 #include <netinet/in.h>
 #include <netinet/udp.h>
@@ -14,16 +12,11 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/eventfd.h>
-#include <sys/mman.h>
-#include <sys/random.h>
 #include <sys/socket.h>
-#include <sys/stat.h>
 #include <time.h>
 #include <unistd.h>
 
-enum { VERSION = 0x70, HEADER = 21, DATAGRAM = 1200, 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 { DATAGRAM = PURE_LINK_DATAGRAM };
 /* Packets sent at once with UDP GSO: each but the last is a whole datagram
  * (a full one padded), and 54 of them fit a UDP datagram's 65,507 bytes. */
 enum { BURST = 54 };
@@ -33,47 +26,7 @@ enum { BURST = 54 };
 #ifndef UDP_GRO
 #define UDP_GRO 104
 #endif
-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 uint64_t now_ns(void) {
     struct timespec t;
@@ -81,88 +34,6 @@ static uint64_t now_ns(void) {
     return (uint64_t)t.tv_sec * 1000000000u + (uint64_t)t.tv_nsec;
 }
 
-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;
-}
-
 /* Datagrams taken from a socket at once (recvmmsg). */
 enum { TAKE = 64, SLOT = DATAGRAM + 64 };
 typedef struct {
@@ -221,63 +92,27 @@ static int take_datagrams(int fd, Taking *t) {
 
 /* ---- 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 PureLink {
-    uint32_t shape, own_id, peer_id;
-    char host[256], port[8], 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];
-    bool peered;
+    PureLinkSender *sender; /* under the lock */
+    char host[256], port[8], input[PURE_LINK_NAME + 1];
+    bool peered, keyed; /* given the receiver's key; the keys made from it */
     pthread_mutex_t lock;
-    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, told; /* the last queued; saved by the receiver; told the owner */
     bool stopping, started;
     /* The thread sleeps (in poll), and has been woken since; records were
      * queued since it last sent. A queue wakes it only when it sleeps, once. */
     bool sleeping, signaled, queued_new;
+    uint64_t told; /* the last stored the owner was told of */
     int fd, wake;
     pthread_t thread;
     void (*notify)(void *);
     void *context;
-    /* The thread's own, under the lock. */
-    uint64_t run, next_pn, in_run;
-    PureAead *out, *in;
-    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 */
+    /* The thread's own. */
     Taking *taking;       /* answers, received at once */
     unsigned char *burst; /* packets sealed and not yet sent (BURST datagrams) */
     size_t burst_size;
     unsigned burst_count;
     uint64_t burst_pn[BURST];
     bool gso;
-    unsigned pto_count, probes;
-    PureLinkCounts counts;
 };
 
 PureLink *pure_link_open(const char *url, const char *source, uint32_t shape, const PureSealKey *own,
@@ -300,41 +135,22 @@ PureLink *pure_link_open(const char *url, const char *source, uint32_t shape, co
         snprintf(error, size, "out of memory");
         return NULL;
     }
-    l->shape = shape;
     memcpy(l->host, host, (size_t)(colon - host));
     memcpy(l->port, colon + 1, (size_t)(slash - colon - 1));
     strcpy(l->input, slash + 1);
-    strcpy(l->source, source);
-    l->own = own;
-    memcpy(l->own_public, own_public, PURE_SEAL_KEY);
-    l->own_id = key_id(own_public);
     l->fd = l->wake = -1;
+    if (!(l->sender = pure_link_sender(source, l->input, shape, own, own_public))) {
+        pthread_mutex_destroy(&l->lock);
+        free(l);
+        snprintf(error, size, "out of memory");
+        return NULL;
+    }
     return l;
 }
 
 void pure_link_peer(PureLink *l, const unsigned char peer[PURE_SEAL_KEY]) {
-    memcpy(l->peer, peer, PURE_SEAL_KEY);
-    l->peer_id = key_id(peer);
     l->peered = true;
-}
-
-static Record *record_at(PureLink *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(PureLink *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(PureLink *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;
+    l->keyed = pure_link_sender_peer(l->sender, peer);
 }
 
 static void wake(int fd) {
@@ -342,372 +158,39 @@ static void wake(int fd) {
     if (fd >= 0 && write(fd, &one, sizeof(one)) != sizeof(one)) { /* full: it is readable already */ }
 }
 
-/* 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(PureLink *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(PureLink *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 : 64 << 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_queue(PureLink *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;
     pthread_mutex_lock(&l->lock);
-    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 : 64;
-        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;
-    }
-    bool signal = false;
+    bool ok = pure_link_sender_queue(l->sender, seq, bytes, size), signal = false;
     if (ok) {
-        *record_at(l, l->count++) = r;
-        l->queued = seq;
         l->queued_new = true;
         signal = l->sleeping && !l->signaled;
         l->signaled = l->signaled || signal;
     }
     pthread_mutex_unlock(&l->lock);
-    if (!ok) {
-        free(r.more);
-        return false;
-    }
     if (signal) wake(l->wake);
-    return true;
+    return ok;
 }
 
 uint64_t pure_link_queued(PureLink *l) {
     pthread_mutex_lock(&l->lock);
-    uint64_t queued = l->queued;
+    uint64_t queued = pure_link_sender_queued(l->sender);
     pthread_mutex_unlock(&l->lock);
     return queued;
 }
 
 uint64_t pure_link_delivered(PureLink *l) {
     pthread_mutex_lock(&l->lock);
-    uint64_t stored = l->stored;
+    uint64_t stored = pure_link_sender_stored(l->sender);
     pthread_mutex_unlock(&l->lock);
     return stored;
 }
 
-static Flight *flight_of(PureLink *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];
-}
-
-static bool push_flight(PureLink *l, const Flight *f) {
-    if (l->flight_count == l->flight_capacity) {
-        size_t capacity = l->flight_capacity ? 2 * l->flight_capacity : 256;
-        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;
-    }
-    l->flights[(l->flight_head + l->flight_count++) % l->flight_capacity] = *f;
-    return true;
-}
-
-static void settle(PureLink *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(PureLink *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(PureLink *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(PureLink *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(PureLink *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(PureLink *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;
-}
-
-static void take_answer(PureLink *l, unsigned char *packet, ssize_t n, uint64_t now);
-
 static void take_answers(PureLink *l, uint64_t now) {
     for (unsigned rounds = 0, n = TAKE; rounds < 4 && n == TAKE; ++rounds) {
         n = (unsigned)take_datagrams(l->fd, l->taking);
-        for (unsigned k = 0; k < n; ++k) take_answer(l, l->taking->bytes[k], l->taking->messages[k].msg_len, now);
-    }
-}
-
-static void take_answer(PureLink *l, unsigned char *packet, ssize_t n, uint64_t now) {
-    unsigned char plain[DATAGRAM], nonce[PURE_AEAD_NONCE];
-    if (n < HEADER + PURE_AEAD_TAG || (size_t)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 = (size_t)n - HEADER - PURE_AEAD_TAG;
-    if (!pure_aead_open(l->in, NULL, nonce, packet, HEADER, packet + HEADER, (size_t)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(PureLink *l, unsigned char *body, Flight *f) {
-    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 (l->gso && f->count) {
-                memset(body + at, F_PADDING, BODY - at);
-                at = BODY;
-            }
-            break;
-        }
-    }
-    return f->count ? at : 0;
-}
-
-/* Seal and send f with body; its number is spent even when the socket
- * refuses it (what it carried is then marked unsent again). */
-/* A packet the socket refused: what it carried is unsent again, and it
- * counts as never sent. Its number is spent all the same. */
-static void refused(PureLink *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);
+        for (unsigned k = 0; k < n; ++k)
+            pure_link_sender_take(l->sender, l->taking->bytes[k], l->taking->messages[k].msg_len, now);
     }
-    settle(l, f);
-    f->eliciting = false;
-    f->count = 0;
-    --l->counts.packets;
 }
 
 static bool transient(void) { return errno == EAGAIN || errno == EWOULDBLOCK || errno == ENOBUFS || errno == EINTR; }
@@ -724,119 +207,32 @@ static void flush(PureLink *l) {
         if (send(l->fd, l->burst + at, size, 0) < 0 && transient()) break;
         at += size;
     }
-    for (unsigned k = sent; k < n; ++k) refused(l, l->burst_pn[k]);
+    for (unsigned k = sent; k < n; ++k) pure_link_sender_refused(l->sender, l->burst_pn[k]);
     l->burst_count = 0;
     l->burst_size = 0;
 }
 
-/* Seal f with body into the burst (a whole datagram each but the last), and
- * count it sent; its number is spent even when the socket then refuses it. */
-static bool transmit(PureLink *l, Flight *f, const unsigned char *body, size_t size, uint64_t now) {
-    if (l->burst_count && l->burst_size != (size_t)l->burst_count * DATAGRAM) flush(l); /* the last was short */
-    size_t total = seal_packet(l->out, NULL, l->out_base, l->own_id, l->run, f->pn, body, size, l->burst + l->burst_size);
-    ++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 false;
-    }
-    if (!push_flight(l, f)) return false;
-    l->burst_pn[l->burst_count++] = f->pn;
-    l->burst_size += total;
-    ++l->counts.packets;
-    if (f->eliciting) {
-        ++l->eliciting;
-        l->in_flight += f->bytes;
-        l->last_eliciting = now;
-    }
-    if (l->burst_count == BURST) flush(l);
-    return true;
-}
-
+/* Packets while the sender has them (up to 128 a turn), into bursts: a
+ * short packet ends one. */
 static void send_packets(PureLink *l, uint64_t now) {
-    unsigned char body[BODY];
     l->queued_new = false; /* all queued is looked at now */
     for (unsigned n = 0; n < 128; ++n) {
-        bool probe = l->probes > 0;
-        if (!probe && l->in_flight + DATAGRAM > l->window) break;
-        Flight f = {.pn = l->next_pn, .eliciting = true};
-        size_t size = fill(l, body, &f);
-        if (!size) {
-            if (!probe) break;
-            body[0] = F_PING; /* nothing to send again: a probe all the same */
-            size = 1;
-        }
-        if (!transmit(l, &f, body, size, now)) break;
-        if (probe) --l->probes;
+        if (l->burst_count && l->burst_size != (size_t)l->burst_count * DATAGRAM) flush(l); /* the last was short */
+        uint64_t pn;
+        size_t size = pure_link_sender_packet(l->sender, l->burst + l->burst_size, l->gso, now, &pn);
+        if (!size) break;
+        l->burst_pn[l->burst_count++] = pn;
+        l->burst_size += size;
+        if (l->burst_count == BURST) flush(l);
     }
     flush(l);
 }
 
-static uint64_t pto_period(const PureLink *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(PureLink *l) { return l->received > 0; }
-
-/* The next time a timer is due (ns), or 0. */
-static uint64_t next_timer(PureLink *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;
-}
-
-static void run_timers(PureLink *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;
-    }
-}
-
 static void *run_link(void *context) {
     PureLink *l = context;
     for (;;) {
         pthread_mutex_lock(&l->lock);
-        uint64_t now = now_ns(), due = next_timer(l);
+        uint64_t now = now_ns(), due = pure_link_sender_due(l->sender);
         bool stopping = l->stopping, more = l->queued_new; /* queued while it sent: no sleep */
         l->sleeping = !more;
         pthread_mutex_unlock(&l->lock);
@@ -852,10 +248,11 @@ static void *run_link(void *context) {
         l->sleeping = l->signaled = false;
         now = now_ns();
         if (waits[0].revents) take_answers(l, now);
-        run_timers(l, now);
+        pure_link_sender_tick(l->sender, now);
         send_packets(l, now);
-        bool told = l->stored != l->told;
-        l->told = l->stored;
+        uint64_t stored = pure_link_sender_stored(l->sender);
+        bool told = stored != l->told;
+        l->told = stored;
         pthread_mutex_unlock(&l->lock);
         if (told && l->notify) l->notify(l->context);
     }
@@ -883,13 +280,7 @@ bool pure_link_start(PureLink *l, void (*notify)(void *), void *context, char *e
     }
     int segment = DATAGRAM;
     l->gso = ok && !setsockopt(l->fd, SOL_UDP, UDP_SEGMENT, &segment, sizeof(segment));
-    ok = ok && (l->burst = malloc((size_t)BURST * DATAGRAM)) && (l->taking = malloc(sizeof(*l->taking)));
-    l->run = random64();
-    l->window = WINDOW_FIRST;
-    l->threshold = UINT64_MAX;
-    l->credit = CREDIT;
-    ok = ok && 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)) &&
+    ok = ok && l->keyed && (l->burst = malloc((size_t)BURST * DATAGRAM)) && (l->taking = malloc(sizeof(*l->taking))) &&
          (l->wake = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC)) >= 0;
     l->notify = notify;
     l->context = context;
@@ -904,8 +295,7 @@ bool pure_link_start(PureLink *l, void (*notify)(void *), void *context, char *e
 
 void pure_link_counts(PureLink *l, PureLinkCounts *out) {
     pthread_mutex_lock(&l->lock);
-    *out = l->counts;
-    out->stored = l->stored;
+    pure_link_sender_counts(l->sender, out);
     pthread_mutex_unlock(&l->lock);
 }
 
@@ -918,803 +308,46 @@ void pure_link_close(PureLink *l) {
         wake(l->wake);
         pthread_join(l->thread, NULL);
     }
-    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_link_sender_free(l->sender);
     free(l->burst);
     free(l->taking);
-    pure_aead_free(l->out);
-    pure_aead_free(l->in);
-    explicit_bzero(l->secret, sizeof(l->secret));
     if (l->fd >= 0) close(l->fd);
     if (l->wake >= 0) close(l->wake);
     pthread_mutex_destroy(&l->lock);
     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;
-    struct sockaddr_in6 address; /* or an IPv4 one */
-    socklen_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 */
-typedef struct { Stream *stream; uint64_t heard; bool stored, gap; } Done;
-
-/* 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 };
+/* ---- The receiving side */
 
 struct PureLinkListener {
-    PureLinkHost host;
-    const PureSealKey *own;
-    unsigned char own_public[PURE_SEAL_KEY];
-    uint32_t own_id;
+    PureLinkReceiver *receiver; /* the thread's */
     unsigned port;
-    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;
     int fd, wake;
     pthread_t thread;
     bool started;
     pthread_mutex_t lock; /* the done list and stopping */
-    Done *done;
+    PureLinkDone *done;
     size_t done_count, done_capacity;
     bool stopping;
-    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;
 };
 
-PureLinkListener *pure_link_listen(unsigned port, const PureSealKey *own, const unsigned char own_public[PURE_SEAL_KEY],
-                                   const PureLinkHost *host, char *error, size_t size) {
-    PureLinkListener *s = own ? calloc(1, sizeof(*s)) : NULL;
+PureLinkListener *pure_link_listen(PureLinkReceiver *r, unsigned port, char *error, size_t size) {
+    PureLinkListener *s = calloc(1, sizeof(*s));
     if (!s || pthread_mutex_init(&s->lock, NULL)) {
         free(s);
-        snprintf(error, size, own ? "out of memory" : "--link-port needs the node's --key");
+        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->receiver = r;
     s->port = port;
-    s->idle = IDLE_FIRST;
     s->fd = s->wake = -1;
     return s;
 }
 
-void pure_link_idle(PureLinkListener *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(PureLinkListener *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 PureLinkListener *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(PureLinkListener *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(PureLinkListener *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(PureLinkListener *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(PureLinkListener *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(PureLinkListener *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(PureLinkListener *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(PureLinkListener *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 PureLinkListener *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(PureLinkListener *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(PureLinkListener *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(PureLinkListener *s, Peer *peer, unsigned char *packet, size_t n,
-                      const struct sockaddr_storage *from, socklen_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;
-}
-
-static void take_packet(PureLinkListener *s, unsigned char *packet, size_t n, const struct sockaddr_storage *from,
-                        socklen_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(PureLinkListener *s, uint64_t now) {
-    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 && sendto(s->fd, packet, total, 0, (const struct sockaddr *)&r->address, r->address_size) >= 0)
-            ++s->counts.packets;
-    }
-    s->due_count = 0;
-}
-
-static void take_done(PureLinkListener *s) {
-    pthread_mutex_lock(&s->lock);
-    Done *done = s->done;
-    size_t count = s->done_count;
-    s->done = NULL;
-    s->done_count = s->done_capacity = 0;
-    pthread_mutex_unlock(&s->lock);
-    for (size_t k = 0; k < count; ++k) {
-        Stream *st = done[k].stream;
-        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;
-    }
-    free(done);
-}
-
-/* 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(PureLinkListener *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;
+static bool send_answer(void *context, const unsigned char *packet, size_t size, const unsigned char *to,
+                        size_t to_size) {
+    const PureLinkListener *s = context;
+    return sendto(s->fd, packet, size, 0, (const struct sockaddr *)(const void *)to, (socklen_t)to_size) >= 0;
 }
 
 static void *run_listener(void *context) {
@@ -1722,8 +355,9 @@ static void *run_listener(void *context) {
     Gathering *gathering = malloc(sizeof(*gathering));
     if (!gathering) return NULL;
     for (;;) {
+        uint64_t now = now_ns(), due = pure_link_receiver_due(s->receiver);
+        int timeout = due ? (int)((due > now ? due - now : 0) / MS) + 1 : -1;
         struct pollfd waits[2] = {{s->fd, POLLIN, 0}, {s->wake, POLLIN, 0}};
-        int timeout = s->active_count || s->ready_count ? (int)(SWEEP_EVERY / MS) : -1;
         if (poll(waits, 2, timeout) < 0 && errno != EINTR) break;
         pthread_mutex_lock(&s->lock);
         bool stopping = s->stopping;
@@ -1733,21 +367,25 @@ static void *run_listener(void *context) {
             uint64_t signals;
             if (read(s->wake, &signals, sizeof(signals)) < 0) { /* cleared already */ }
         }
-        uint64_t now = now_ns();
+        now = now_ns();
         for (unsigned rounds = 0, n = GATHER; rounds < 8 && n == GATHER; ++rounds) {
             n = (unsigned)gather(s->fd, gathering);
             for (unsigned k = 0; k < n; ++k) {
                 struct mmsghdr *m = &gathering->messages[k];
                 size_t length = m->msg_len, step = segment_of(&m->msg_hdr, length);
                 for (size_t at = 0; at < length; at += step)
-                    take_packet(s, gathering->bytes[k] + at, least(step, length - at), &gathering->from[k],
-                                m->msg_hdr.msg_namelen, now);
+                    pure_link_receiver_take(s->receiver, gathering->bytes[k] + at, step < length - at ? step : length - at,
+                                            &gathering->from[k], m->msg_hdr.msg_namelen, now);
             }
         }
-        take_done(s);
-        hand_over_ready(s);
-        answer(s, now);
-        sweep(s, now);
+        pthread_mutex_lock(&s->lock);
+        PureLinkDone *done = s->done;
+        size_t count = s->done_count;
+        s->done = NULL;
+        s->done_count = s->done_capacity = 0;
+        pthread_mutex_unlock(&s->lock);
+        pure_link_receiver_turn(s->receiver, done, count, now, send_answer, s);
+        free(done);
     }
     free(gathering);
     return NULL;
@@ -1773,11 +411,8 @@ bool pure_link_serve(PureLinkListener *s, char *error, size_t size) {
     setsockopt(s->fd, SOL_SOCKET, SO_SNDBUF, &buffer, sizeof(buffer));
     int one = 1; /* without GRO, a buffer is a datagram */
     setsockopt(s->fd, SOL_UDP, UDP_GRO, &one, sizeof(one));
-    s->run = random64();
-    s->swept = s->trimmed = now_ns();
     s->wake = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
-    unsigned char key[PURE_AEAD_KEY] = {0}; /* each packet brings its run's */
-    if (s->wake < 0 || !(s->aead = pure_aead(key)) || pthread_create(&s->thread, NULL, run_listener, s)) {
+    if (s->wake < 0 || pthread_create(&s->thread, NULL, run_listener, s)) {
         snprintf(error, size, "--link-port %u: cannot start", s->port);
         return false;
     }
@@ -1790,12 +425,12 @@ void pure_link_done(PureLinkListener *s, void *token, uint64_t heard, bool store
     pthread_mutex_lock(&s->lock);
     if (s->done_count == s->done_capacity) {
         size_t capacity = s->done_capacity ? 2 * s->done_capacity : 16;
-        Done *grown = realloc(s->done, capacity * sizeof(*grown));
+        PureLinkDone *grown = realloc(s->done, capacity * sizeof(*grown));
         if (!grown) abort(); /* a delivery's word cannot be lost */
         s->done = grown;
         s->done_capacity = capacity;
     }
-    s->done[s->done_count++] = (Done){token, heard, stored, gap};
+    s->done[s->done_count++] = (PureLinkDone){token, heard, stored, gap};
     pthread_mutex_unlock(&s->lock);
     wake(s->wake);
 }
@@ -1809,21 +444,8 @@ void pure_link_listener_close(PureLinkListener *s, PureLinkCounts *counts) {
         wake(s->wake);
         pthread_join(s->thread, NULL);
     }
-    if (counts) {
-        *counts = s->counts;
-        counts->active = s->active_count;
-    }
-    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);
+    if (counts) pure_link_receiver_counts(s->receiver, counts);
+    pure_link_receiver_free(s->receiver);
     free(s->done);
     if (s->fd >= 0) close(s->fd);
     if (s->wake >= 0) close(s->wake);