Put the links and the crypto they use in link/, a part of their own

libpure-link.a holds them; pure-serve and the M5 harness link it instead
of naming its source files.

7 files changed, 2578 insertions, 0 deletions

diff --git a/link/crypto-openssl.c b/link/crypto-openssl.c
new file mode 100644
index 0000000..6bdf6fb
--- /dev/null
+++ b/link/crypto-openssl.c
@@ -0,0 +1,284 @@
+#define _GNU_SOURCE
+/* pure-crypto.so: what pure-serve asks of OpenSSL (link/crypto.h), mapped
+ * only when a server seals, opens a seal or reaches an https receiver. It
+ * exports one table, pure_crypto. */
+#include "crypto.h"
+#include <arpa/inet.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <openssl/crypto.h>
+#include <openssl/evp.h>
+#include <openssl/core_names.h>
+#include <openssl/hpke.h>
+#include <openssl/kdf.h>
+#include <openssl/ssl.h>
+#include <openssl/x509v3.h>
+#include <limits.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+/* ---- TLS: one client context, verifying the receiver's certificate against
+ * the system's CAs or the file given, and its name. */
+static SSL_CTX *tls_context;
+
+static bool tls_start(const char *ca_file) {
+    SSL_CTX *context = SSL_CTX_new(TLS_client_method());
+    if (!context) return false;
+    if (!SSL_CTX_set_min_proto_version(context, TLS1_2_VERSION) ||
+        !(ca_file ? SSL_CTX_load_verify_locations(context, ca_file, NULL) : SSL_CTX_set_default_verify_paths(context))) {
+        SSL_CTX_free(context);
+        return false;
+    }
+    SSL_CTX_set_verify(context, SSL_VERIFY_PEER, NULL);
+    SSL_CTX_set_mode(context, SSL_MODE_AUTO_RETRY);
+    tls_context = context;
+    return true;
+}
+
+/* TLS over a connected socket: the receiver's certificate must name the host
+ * (a DNS name, or an IP address written as one). */
+static PureTls *tls_open(const char *host, int fd) {
+    SSL *ssl = SSL_new(tls_context);
+    if (!ssl) return NULL;
+    unsigned char address[16];
+    bool literal = inet_pton(AF_INET, host, address) == 1 || inet_pton(AF_INET6, host, address) == 1;
+    bool named = literal ? X509_VERIFY_PARAM_set1_ip_asc(SSL_get0_param(ssl), host) == 1 :
+                           SSL_set_tlsext_host_name(ssl, host) == 1 && SSL_set1_host(ssl, host) == 1;
+    if (!named || SSL_set_fd(ssl, fd) != 1 || SSL_connect(ssl) != 1 || SSL_get_verify_result(ssl) != X509_V_OK) {
+        SSL_free(ssl);
+        return NULL;
+    }
+    return (PureTls *)ssl;
+}
+
+static int tls_read(PureTls *tls, void *bytes, int size) { return SSL_read((SSL *)tls, bytes, size); }
+static int tls_write(PureTls *tls, const void *bytes, int size) { return SSL_write((SSL *)tls, bytes, size); }
+static void tls_close(PureTls *tls) { SSL_free((SSL *)tls); }
+
+/* ---- Sealing: HPKE (RFC 9180), X25519 and HKDF-SHA256, with the AEAD the
+ * suite names; auth mode when the sender seals with a key of its own. A
+ * session is one encapsulation, its seals numbered by the context. */
+struct PureSealKey { EVP_PKEY *key; };
+struct PureSealer { OSSL_HPKE_CTX *context; };
+struct PureOpener { OSSL_HPKE_CTX *context; };
+
+static const unsigned char info[] = "pure sealed send 2";
+
+static bool suite_named(const char *name, OSSL_HPKE_SUITE *out) {
+    OSSL_HPKE_SUITE suite = {OSSL_HPKE_KEM_ID_X25519, OSSL_HPKE_KDF_ID_HKDF_SHA256, 0};
+    if (!strcmp(name, "x25519-sha256-aes128gcm")) suite.aead_id = OSSL_HPKE_AEAD_ID_AES_GCM_128;
+    else if (!strcmp(name, "x25519-sha256-chacha20poly1305")) suite.aead_id = OSSL_HPKE_AEAD_ID_CHACHA_POLY1305;
+    else return false;
+    *out = suite;
+    return true;
+}
+
+static pthread_once_t secure_once = PTHREAD_ONCE_INIT;
+static void secure_heap(void) { CRYPTO_secure_malloc_init(16384, 32); }
+
+static PureSealKey *seal_key(const char *path, char public_hex[2 * PURE_SEAL_KEY + 1], char *error, size_t size) {
+    pthread_once(&secure_once, secure_heap);
+    unsigned char *raw = OPENSSL_secure_malloc(PURE_SEAL_KEY), public_key[PURE_SEAL_KEY];
+    PureSealKey *made = calloc(1, sizeof(*made));
+    EVP_PKEY *key = NULL;
+    size_t length = PURE_SEAL_KEY;
+    if (!raw || !made) { snprintf(error, size, "out of memory"); goto fail; }
+    int fd = open(path, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
+    struct stat info_;
+    if (fd >= 0) {
+        bool ok = !fstat(fd, &info_) && S_ISREG(info_.st_mode) && !(info_.st_mode & 077) &&
+                  read(fd, raw, PURE_SEAL_KEY) == PURE_SEAL_KEY && info_.st_size == PURE_SEAL_KEY;
+        close(fd);
+        if (!ok) { snprintf(error, size, "%s: a key is 32 bytes that only its owner may read (0600)", path); goto fail; }
+        key = EVP_PKEY_new_raw_private_key(EVP_PKEY_X25519, NULL, raw, PURE_SEAL_KEY);
+    } else if (errno == ENOENT) {
+        /* A new key: made, written whole and synced before it is used. */
+        key = EVP_PKEY_Q_keygen(NULL, NULL, "X25519");
+        fd = open(path, O_WRONLY | O_CREAT | O_EXCL | O_NOFOLLOW | O_CLOEXEC, 0600);
+        bool ok = key && fd >= 0 && EVP_PKEY_get_raw_private_key(key, raw, &length) == 1 && length == PURE_SEAL_KEY &&
+                  write(fd, raw, PURE_SEAL_KEY) == PURE_SEAL_KEY && !fsync(fd);
+        if (fd >= 0 && close(fd)) ok = false;
+        if (!ok) { snprintf(error, size, "%s: cannot make a key there", path); unlink(path); goto fail; }
+    } else { snprintf(error, size, "%s: %s", path, strerror(errno)); goto fail; }
+    length = PURE_SEAL_KEY;
+    if (!key || EVP_PKEY_get_raw_public_key(key, public_key, &length) != 1 || length != PURE_SEAL_KEY) {
+        snprintf(error, size, "%s: not an X25519 key", path);
+        goto fail;
+    }
+    for (size_t i = 0; i < PURE_SEAL_KEY; ++i) snprintf(public_hex + 2 * i, 3, "%02x", public_key[i]);
+    OPENSSL_secure_clear_free(raw, PURE_SEAL_KEY);
+    made->key = key;
+    return made;
+fail:
+    EVP_PKEY_free(key);
+    if (raw) OPENSSL_secure_clear_free(raw, PURE_SEAL_KEY);
+    free(made);
+    return NULL;
+}
+
+static void seal_key_free(PureSealKey *key) {
+    if (!key) return;
+    EVP_PKEY_free(key->key);
+    free(key);
+}
+
+static PureSealer *sealer(const char *suite_name, const unsigned char peer[PURE_SEAL_KEY], const PureSealKey *own,
+                          unsigned char enc[PURE_SEAL_KEY]) {
+    OSSL_HPKE_SUITE suite;
+    PureSealer *made = suite_named(suite_name, &suite) ? calloc(1, sizeof(*made)) : NULL;
+    if (!made) return NULL;
+    made->context = OSSL_HPKE_CTX_new(own ? OSSL_HPKE_MODE_AUTH : OSSL_HPKE_MODE_BASE, suite, OSSL_HPKE_ROLE_SENDER,
+                                      NULL, NULL);
+    size_t length = PURE_SEAL_KEY;
+    if (!made->context || (own && OSSL_HPKE_CTX_set1_authpriv(made->context, own->key) != 1) ||
+        OSSL_HPKE_encap(made->context, enc, &length, peer, PURE_SEAL_KEY, info, sizeof(info) - 1) != 1 ||
+        length != PURE_SEAL_KEY) {
+        OSSL_HPKE_CTX_free(made->context);
+        free(made);
+        return NULL;
+    }
+    return made;
+}
+
+static bool seal(PureSealer *s, const char *aad, const unsigned char *plain, size_t size, unsigned char *out,
+                 uint64_t *n) {
+    size_t sealed = size + PURE_SEAL_OVERHEAD;
+    return OSSL_HPKE_CTX_get_seq(s->context, n) == 1 &&
+           OSSL_HPKE_seal(s->context, out, &sealed, (const unsigned char *)aad, strlen(aad), plain, size) == 1 &&
+           sealed == size + PURE_SEAL_OVERHEAD;
+}
+
+static void sealer_free(PureSealer *s) {
+    if (!s) return;
+    OSSL_HPKE_CTX_free(s->context);
+    free(s);
+}
+
+static PureOpener *opener(const PureSealKey *key, const char *suite_name, const unsigned char enc[PURE_SEAL_KEY],
+                          const unsigned char *from) {
+    OSSL_HPKE_SUITE suite;
+    PureOpener *made = key && suite_named(suite_name, &suite) ? calloc(1, sizeof(*made)) : NULL;
+    if (!made) return NULL;
+    made->context = OSSL_HPKE_CTX_new(from ? OSSL_HPKE_MODE_AUTH : OSSL_HPKE_MODE_BASE, suite, OSSL_HPKE_ROLE_RECEIVER,
+                                      NULL, NULL);
+    if (!made->context || (from && OSSL_HPKE_CTX_set1_authpub(made->context, from, PURE_SEAL_KEY) != 1) ||
+        OSSL_HPKE_decap(made->context, enc, PURE_SEAL_KEY, key->key, info, sizeof(info) - 1) != 1) {
+        OSSL_HPKE_CTX_free(made->context);
+        free(made);
+        return NULL;
+    }
+    return made;
+}
+
+/* A receiver may set the number (a sender may not: it would reuse a nonce). */
+static bool unseal(PureOpener *o, uint64_t n, const char *aad, const unsigned char *sealed, size_t size,
+                   unsigned char *plain) {
+    size_t opened = size - PURE_SEAL_OVERHEAD;
+    return size >= PURE_SEAL_OVERHEAD && OSSL_HPKE_CTX_set_seq(o->context, n) == 1 &&
+           OSSL_HPKE_open(o->context, plain, &opened, (const unsigned char *)aad, strlen(aad), sealed, size) == 1 &&
+           opened == size - PURE_SEAL_OVERHEAD;
+}
+
+static void opener_free(PureOpener *o) {
+    if (!o) return;
+    OSSL_HPKE_CTX_free(o->context);
+    free(o);
+}
+
+/* ---- Links: X25519 between two nodes' keys, HKDF-SHA256, AES-128-GCM with
+ * the caller's nonce (one context sealing and one opening, each keeping its
+ * key's schedule). */
+struct PureAead { EVP_CIPHER_CTX *seal, *open; };
+
+static bool link_secret(const PureSealKey *own, const unsigned char peer[PURE_SEAL_KEY], unsigned char out[32]) {
+    EVP_PKEY *theirs = EVP_PKEY_new_raw_public_key(EVP_PKEY_X25519, NULL, peer, PURE_SEAL_KEY);
+    EVP_PKEY_CTX *context = theirs && own ? EVP_PKEY_CTX_new(own->key, NULL) : NULL;
+    size_t length = 32;
+    bool ok = context && EVP_PKEY_derive_init(context) == 1 && EVP_PKEY_derive_set_peer(context, theirs) == 1 &&
+              EVP_PKEY_derive(context, out, &length) == 1 && length == 32;
+    EVP_PKEY_CTX_free(context);
+    EVP_PKEY_free(theirs);
+    unsigned char any = 0; /* a small-order key makes zeros: no secret */
+    for (size_t i = 0; ok && i < 32; ++i) any |= out[i];
+    return ok && any;
+}
+
+/* HKDF, fetched once. */
+static EVP_KDF *hkdf(void) {
+    static EVP_KDF *kept;
+    EVP_KDF *kdf = __atomic_load_n(&kept, __ATOMIC_ACQUIRE), *none = NULL;
+    if (kdf) return kdf;
+    if ((kdf = EVP_KDF_fetch(NULL, "HKDF", NULL)) &&
+        !__atomic_compare_exchange_n(&kept, &none, kdf, false, __ATOMIC_ACQ_REL, __ATOMIC_ACQUIRE)) {
+        EVP_KDF_free(kdf); /* another thread's is kept */
+        kdf = none;
+    }
+    return kdf;
+}
+
+static bool link_derive(const unsigned char secret[32], const unsigned char *info, size_t info_size, unsigned char *out,
+                        size_t out_size) {
+    static const char salt[] = "pure link 0";
+    EVP_KDF *kdf = hkdf();
+    EVP_KDF_CTX *context = kdf ? EVP_KDF_CTX_new(kdf) : NULL;
+    OSSL_PARAM params[] = {OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST, (char *)"SHA256", 0),
+                           OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY, (void *)secret, 32),
+                           OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT, (void *)salt, sizeof(salt) - 1),
+                           OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO, (void *)info, info_size),
+                           OSSL_PARAM_construct_end()};
+    bool ok = context && EVP_KDF_derive(context, out, out_size, params) == 1;
+    EVP_KDF_CTX_free(context);
+    return ok;
+}
+
+static void aead_free(PureAead *a) {
+    if (!a) return;
+    EVP_CIPHER_CTX_free(a->seal);
+    EVP_CIPHER_CTX_free(a->open);
+    free(a);
+}
+
+static PureAead *aead(const unsigned char key[PURE_AEAD_KEY]) {
+    PureAead *a = calloc(1, sizeof(*a));
+    if (!a) return NULL;
+    a->seal = EVP_CIPHER_CTX_new();
+    a->open = EVP_CIPHER_CTX_new();
+    if (!a->seal || !a->open || EVP_EncryptInit_ex(a->seal, EVP_aes_128_gcm(), NULL, key, NULL) != 1 ||
+        EVP_DecryptInit_ex(a->open, EVP_aes_128_gcm(), NULL, key, NULL) != 1) {
+        aead_free(a);
+        return NULL;
+    }
+    return a;
+}
+
+static bool aead_seal(PureAead *a, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                      const unsigned char *aad, size_t aad_size, const unsigned char *plain, size_t size,
+                      unsigned char *out) {
+    int n = 0, last = 0;
+    return size <= INT_MAX && EVP_EncryptInit_ex(a->seal, NULL, NULL, key, nonce) == 1 &&
+           (!aad_size || EVP_EncryptUpdate(a->seal, NULL, &n, aad, (int)aad_size) == 1) &&
+           (!size || EVP_EncryptUpdate(a->seal, out, &n, plain, (int)size) == 1) &&
+           EVP_EncryptFinal_ex(a->seal, out + size, &last) == 1 &&
+           EVP_CIPHER_CTX_ctrl(a->seal, EVP_CTRL_GCM_GET_TAG, PURE_AEAD_TAG, out + size) == 1;
+}
+
+static bool aead_open(PureAead *a, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                      const unsigned char *aad, size_t aad_size, const unsigned char *sealed, size_t size,
+                      unsigned char *plain) {
+    int n = 0, last = 0;
+    if (size < PURE_AEAD_TAG || size - PURE_AEAD_TAG > INT_MAX) return false;
+    size_t body = size - PURE_AEAD_TAG;
+    return EVP_DecryptInit_ex(a->open, NULL, NULL, key, nonce) == 1 &&
+           (!aad_size || EVP_DecryptUpdate(a->open, NULL, &n, aad, (int)aad_size) == 1) &&
+           (!body || EVP_DecryptUpdate(a->open, plain, &n, sealed, (int)body) == 1) &&
+           EVP_CIPHER_CTX_ctrl(a->open, EVP_CTRL_GCM_SET_TAG, PURE_AEAD_TAG, (void *)(sealed + body)) == 1 &&
+           EVP_DecryptFinal_ex(a->open, plain + body, &last) == 1;
+}
+
+const PureCrypto pure_crypto = {PURE_CRYPTO_VERSION, tls_start, tls_open, tls_read, tls_write, tls_close,
+                                seal_key, seal_key_free, sealer, seal, sealer_free, opener, unseal, opener_free,
+                                link_secret, link_derive, aead, aead_seal, aead_open, aead_free};
diff --git a/link/crypto.c b/link/crypto.c
new file mode 100644
index 0000000..2fa7d44
--- /dev/null
+++ b/link/crypto.c
@@ -0,0 +1,167 @@
+#define _GNU_SOURCE
+/* Sealing and TLS for pure-serve (link/seal.h, link/tls.h), by way of
+ * pure-crypto.so (link/crypto.h): mapped, and OpenSSL with it, the first
+ * time a server needs it. What needs no OpenSSL (a key's hex, the suite
+ * this machine seals with) is here. */
+#include "crypto.h"
+#include <dlfcn.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#if defined(__aarch64__) && defined(__linux__)
+#include <asm/hwcap.h>
+#include <sys/auxv.h>
+#endif
+
+static pthread_once_t once = PTHREAD_ONCE_INIT;
+static const PureCrypto *crypto;
+static char why[512];
+
+/* pure-crypto.so beside the executable, or where PURE_CRYPTO names it. */
+static void load(void) {
+    char path[4096];
+    const char *named = getenv("PURE_CRYPTO");
+    if (named) snprintf(path, sizeof(path), "%s", named);
+    else {
+        ssize_t length = readlink("/proc/self/exe", path, sizeof(path) - 32);
+        char *slash = length > 0 ? memrchr(path, '/', (size_t)length) : NULL;
+        if (!slash) { snprintf(why, sizeof(why), "pure-crypto.so: cannot find pure-serve's directory"); return; }
+        strcpy(slash + 1, "pure-crypto.so");
+    }
+    void *library = dlopen(path, RTLD_NOW | RTLD_LOCAL);
+    const PureCrypto *table = library ? dlsym(library, "pure_crypto") : NULL;
+    if (!table || table->version != PURE_CRYPTO_VERSION) {
+        snprintf(why, sizeof(why), "%s", !library ? dlerror() : "pure-crypto.so is not this pure-serve's");
+        return;
+    }
+    crypto = table;
+}
+
+static const PureCrypto *loaded(char *error, size_t size) {
+    pthread_once(&once, load);
+    if (!crypto && error) snprintf(error, size, "%s", why);
+    return crypto;
+}
+
+/* ---- TLS */
+
+bool pure_tls_start(const char *ca_file, char *error, size_t size) {
+    static int started; /* 1 made, -1 failed; by the thread that opens wires */
+    const PureCrypto *c = loaded(error, size);
+    if (!c) return false;
+    if (!started) started = c->tls_start(ca_file) ? 1 : -1;
+    if (started < 0) snprintf(error, size, "no TLS context: %s cannot be read", ca_file ? ca_file : "the system's CAs");
+    return started > 0;
+}
+
+/* A session exists only once the table is loaded. */
+PureTls *pure_tls_open(const char *host, int fd) { return crypto ? crypto->tls_open(host, fd) : NULL; }
+int pure_tls_read(PureTls *tls, void *bytes, int size) { return crypto->tls_read(tls, bytes, size); }
+int pure_tls_write(PureTls *tls, const void *bytes, int size) { return crypto->tls_write(tls, bytes, size); }
+void pure_tls_close(PureTls *tls) { if (tls) crypto->tls_close(tls); }
+
+/* ---- Sealing */
+
+static const char aes_name[] = "x25519-sha256-aes128gcm", chacha_name[] = "x25519-sha256-chacha20poly1305";
+
+/* AES-GCM where AES is in the CPU (two to three times ChaCha20-Poly1305 from
+ * a kilobyte up on a Cortex-A76, research 086); ChaCha20-Poly1305, fast in
+ * plain code, elsewhere. */
+static bool has_aes(void) {
+#if defined(__aarch64__) && defined(__linux__)
+    return (getauxval(AT_HWCAP) & HWCAP_AES) && (getauxval(AT_HWCAP) & HWCAP_PMULL);
+#elif defined(__x86_64__) && (defined(__GNUC__) || defined(__clang__))
+    return __builtin_cpu_supports("aes") && __builtin_cpu_supports("pclmul");
+#else
+    return false;
+#endif
+}
+
+const char *pure_seal_suite(void) { return has_aes() ? aes_name : chacha_name; }
+
+static bool hex_value(char c, unsigned *v) {
+    if (c >= '0' && c <= '9') *v = (unsigned)(c - '0');
+    else if (c >= 'a' && c <= 'f') *v = (unsigned)(c - 'a' + 10);
+    else if (c >= 'A' && c <= 'F') *v = (unsigned)(c - 'A' + 10);
+    else return false;
+    return true;
+}
+
+bool pure_seal_peer(const char *hex, unsigned char out[PURE_SEAL_KEY]) {
+    if (strlen(hex) != 2 * PURE_SEAL_KEY) return false;
+    for (size_t i = 0; i < PURE_SEAL_KEY; ++i) {
+        unsigned high, low;
+        if (!hex_value(hex[2 * i], &high) || !hex_value(hex[2 * i + 1], &low)) return false;
+        out[i] = (unsigned char)(high << 4 | low);
+    }
+    return true;
+}
+
+bool pure_seal_ready(char *error, size_t size) { return loaded(error, size) != NULL; }
+
+PureSealKey *pure_seal_key(const char *path, char public_hex[2 * PURE_SEAL_KEY + 1], char *error, size_t size) {
+    const PureCrypto *c = loaded(error, size);
+    return c ? c->seal_key(path, public_hex, error, size) : NULL;
+}
+
+/* A key or a session exists only once the table is loaded. */
+void pure_seal_key_free(PureSealKey *key) { if (key) crypto->seal_key_free(key); }
+
+PureSealer *pure_sealer(const unsigned char peer[PURE_SEAL_KEY], const PureSealKey *own,
+                        unsigned char enc[PURE_SEAL_KEY]) {
+    const PureCrypto *c = loaded(NULL, 0);
+    return c ? c->sealer(pure_seal_suite(), peer, own, enc) : NULL;
+}
+
+bool pure_seal(PureSealer *s, const char *aad, const unsigned char *plain, size_t size, unsigned char *out,
+               uint64_t *n) {
+    return crypto->seal(s, aad, plain, size, out, n);
+}
+
+void pure_sealer_free(PureSealer *s) { if (s) crypto->sealer_free(s); }
+
+PureOpener *pure_opener(const PureSealKey *key, const char *suite, const unsigned char enc[PURE_SEAL_KEY],
+                        const unsigned char *from) {
+    return key ? crypto->opener(key, suite, enc, from) : NULL;
+}
+
+bool pure_unseal(PureOpener *o, uint64_t n, const char *aad, const unsigned char *sealed, size_t size,
+                 unsigned char *plain) {
+    return crypto->unseal(o, n, aad, sealed, size, plain);
+}
+
+void pure_opener_free(PureOpener *o) { if (o) crypto->opener_free(o); }
+
+/* ---- Links */
+
+bool pure_link_secret(const PureSealKey *own, const unsigned char peer[PURE_SEAL_KEY], unsigned char out[32]) {
+    const PureCrypto *c = loaded(NULL, 0);
+    return c && own && c->link_secret(own, peer, out);
+}
+
+bool pure_link_derive(const unsigned char secret[32], const unsigned char *info, size_t info_size, unsigned char *out,
+                      size_t out_size) {
+    const PureCrypto *c = loaded(NULL, 0);
+    return c && c->link_derive(secret, info, info_size, out, out_size);
+}
+
+PureAead *pure_aead(const unsigned char key[PURE_AEAD_KEY]) {
+    const PureCrypto *c = loaded(NULL, 0);
+    return c ? c->aead(key) : NULL;
+}
+
+bool pure_aead_seal(PureAead *a, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                    const unsigned char *aad, size_t aad_size, const unsigned char *plain, size_t size,
+                    unsigned char *out) {
+    return crypto->aead_seal(a, key, nonce, aad, aad_size, plain, size, out);
+}
+
+bool pure_aead_open(PureAead *a, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                    const unsigned char *aad, size_t aad_size, const unsigned char *sealed, size_t size,
+                    unsigned char *plain) {
+    return crypto->aead_open(a, key, nonce, aad, aad_size, sealed, size, plain);
+}
+
+void pure_aead_free(PureAead *a) { if (a) crypto->aead_free(a); }
diff --git a/link/crypto.h b/link/crypto.h
new file mode 100644
index 0000000..53ac785
--- /dev/null
+++ b/link/crypto.h
@@ -0,0 +1,49 @@
+#ifndef PURE_CRYPTO_H
+#define PURE_CRYPTO_H
+#include "seal.h"
+#include "tls.h"
+
+/* What pure-serve asks of OpenSSL, as the table pure-crypto.so exports
+ * (link/crypto-openssl.c). pure-serve maps it, and OpenSSL with it, the
+ * first time a server seals, opens a seal or connects to an https receiver
+ * (link/crypto.c): one that does none of these maps no OpenSSL, 2.3 MiB
+ * less resident and 0.6 MiB less of its own. */
+enum { PURE_CRYPTO_VERSION = 4 };
+
+typedef struct {
+    unsigned version;
+    /* TLS client: the context (verifying against ca_file, or the system's
+     * CAs when NULL), sessions over connected sockets, their reads and
+     * writes (as SSL_read and SSL_write: bytes, or <= 0), and their end. */
+    bool (*tls_start)(const char *ca_file);
+    PureTls *(*tls_open)(const char *host, int fd);
+    int (*tls_read)(PureTls *, void *bytes, int size);
+    int (*tls_write)(PureTls *, const void *bytes, int size);
+    void (*tls_close)(PureTls *);
+    /* Sealing, as link/seal.h says; a sender's sessions in the suite named. */
+    PureSealKey *(*seal_key)(const char *path, char public_hex[2 * PURE_SEAL_KEY + 1], char *error, size_t size);
+    void (*seal_key_free)(PureSealKey *);
+    PureSealer *(*sealer)(const char *suite, const unsigned char peer[PURE_SEAL_KEY], const PureSealKey *own,
+                          unsigned char enc[PURE_SEAL_KEY]);
+    bool (*seal)(PureSealer *, const char *aad, const unsigned char *plain, size_t size, unsigned char *out,
+                 uint64_t *n);
+    void (*sealer_free)(PureSealer *);
+    PureOpener *(*opener)(const PureSealKey *, const char *suite, const unsigned char enc[PURE_SEAL_KEY],
+                          const unsigned char *from);
+    bool (*unseal)(PureOpener *, uint64_t n, const char *aad, const unsigned char *sealed, size_t size,
+                   unsigned char *plain);
+    void (*opener_free)(PureOpener *);
+    /* Links, as link/seal.h says. */
+    bool (*link_secret)(const PureSealKey *, const unsigned char peer[PURE_SEAL_KEY], unsigned char out[32]);
+    bool (*link_derive)(const unsigned char secret[32], const unsigned char *info, size_t info_size, unsigned char *out,
+                        size_t out_size);
+    PureAead *(*aead)(const unsigned char key[PURE_AEAD_KEY]);
+    bool (*aead_seal)(PureAead *, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                      const unsigned char *aad, size_t aad_size, const unsigned char *plain, size_t size,
+                      unsigned char *out);
+    bool (*aead_open)(PureAead *, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                      const unsigned char *aad, size_t aad_size, const unsigned char *sealed, size_t size,
+                      unsigned char *plain);
+    void (*aead_free)(PureAead *);
+} PureCrypto;
+#endif
diff --git a/link/link.c b/link/link.c
new file mode 100644
index 0000000..9c6794b
--- /dev/null
+++ b/link/link.c
@@ -0,0 +1,1862 @@
+#define _GNU_SOURCE
+/* Links without connections (link/link.h, docs/design/011-pure-transport.md). */
+#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>
+#include <poll.h>
+#include <pthread.h>
+#include <stdio.h>
+#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 */
+/* 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 };
+#ifndef UDP_SEGMENT
+#define UDP_SEGMENT 103
+#endif
+#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;
+    clock_gettime(CLOCK_MONOTONIC, &t);
+    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); }
+
+/* What a message of input looks like: its fields' names, types and places. */
+static uint32_t shape_of(const PureModule *m, uint32_t input) {
+    const PureInputShape *s = &m->input[input];
+    uint32_t h = fnv(2166136261u, &s->prefix, sizeof(s->prefix));
+    for (uint32_t f = 0; f < s->field_count; ++f) {
+        const PureField *field = &m->field[s->field_start + f];
+        unsigned char fixed[7] = {field->type, (unsigned char)field->rows, (unsigned char)(field->rows >> 8)};
+        put32(fixed + 3, field->offset);
+        h = fnv(fnv(h, field->name, strlen(field->name) + 1), fixed, sizeof(fixed));
+    }
+    return h;
+}
+
+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_SOURCE + 1], char input[PURE_MODULE_NAME]) {
+    if (size < 1 || !p[0] || p[0] > PURE_SOURCE || size < 2u + p[0]) return 0;
+    size_t a = p[0], b = p[1 + a];
+    if (!b || b >= PURE_MODULE_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 {
+    unsigned char bytes[TAKE][SLOT];
+    struct sockaddr_storage from[TAKE];
+    struct iovec parts[TAKE];
+    struct mmsghdr messages[TAKE];
+} Taking;
+
+/* With UDP GRO the kernel hands a listener the datagrams of one sender that
+ * came one after another as one buffer, each the segment's size but the
+ * last (a burst sent with GSO comes whole, not cut up): up to GATHER such
+ * buffers at once. */
+enum { GATHER = 8, GATHERED = 65536 };
+typedef struct {
+    unsigned char bytes[GATHER][GATHERED];
+    struct sockaddr_storage from[GATHER];
+    struct iovec parts[GATHER];
+    struct mmsghdr messages[GATHER];
+    _Alignas(8) unsigned char control[GATHER][64]; /* a cmsghdr's alignment */
+} Gathering;
+
+static int gather(int fd, Gathering *g) {
+    for (unsigned k = 0; k < GATHER; ++k) {
+        g->parts[k] = (struct iovec){g->bytes[k], GATHERED};
+        g->messages[k] = (struct mmsghdr){.msg_hdr = {.msg_name = &g->from[k], .msg_namelen = sizeof(g->from[k]),
+                                                      .msg_iov = &g->parts[k], .msg_iovlen = 1,
+                                                      .msg_control = g->control[k],
+                                                      .msg_controllen = sizeof(g->control[k])}};
+    }
+    int n = recvmmsg(fd, g->messages, GATHER, MSG_DONTWAIT, NULL);
+    return n > 0 ? n : 0;
+}
+
+/* The size of each datagram in a buffer gathered (all of it when one). */
+static size_t segment_of(struct msghdr *m, size_t length) {
+    for (struct cmsghdr *c = CMSG_FIRSTHDR(m); c; c = CMSG_NXTHDR(m, c))
+        if (c->cmsg_level == SOL_UDP && c->cmsg_type == UDP_GRO) {
+            int size;
+            memcpy(&size, CMSG_DATA(c), sizeof(size));
+            if (size > 0) return (size_t)size;
+        }
+    return length ? length : 1;
+}
+
+/* Up to TAKE datagrams from fd: how many (0 when none waits). */
+static int take_datagrams(int fd, Taking *t) {
+    for (unsigned k = 0; k < TAKE; ++k) {
+        t->parts[k] = (struct iovec){t->bytes[k], SLOT};
+        t->messages[k] = (struct mmsghdr){.msg_hdr = {.msg_name = &t->from[k], .msg_namelen = sizeof(t->from[k]),
+                                                      .msg_iov = &t->parts[k], .msg_iovlen = 1}};
+    }
+    int n = recvmmsg(fd, t->messages, TAKE, MSG_DONTWAIT, NULL);
+    return n > 0 ? n : 0;
+}
+
+/* ---- 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 {
+    const PureModule *module;
+    uint32_t output, shape, own_id, peer_id;
+    char host[256], port[8], input[PURE_MODULE_NAME], source[PURE_SOURCE + 1];
+    const PureSealKey *own;
+    unsigned char own_public[PURE_SEAL_KEY], peer[PURE_SEAL_KEY], secret[32];
+    bool peered;
+    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;
+    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 */
+    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 PureModule *m, const char *output, const char *url, const char *source,
+                         const PureSealKey *own, const unsigned char own_public[PURE_SEAL_KEY], char *error,
+                         size_t size) {
+    int index = pure_module_input(m, output);
+    if (index < 0 || m->input[index].kind != PURE_KIND_OUTPUT) {
+        snprintf(error, size, "no output '%s'", output);
+        return NULL;
+    }
+    if (!own) {
+        snprintf(error, size, "a pure:// link needs the node's --key");
+        return NULL;
+    }
+    size_t length = strlen(source);
+    const char *host = url + 7, *slash = strchr(host, '/'), *colon = slash ? memchr(host, ':', (size_t)(slash - host)) : NULL;
+    if (!length || length > PURE_SOURCE || strncmp(url, "pure://", 7) || !slash || !colon || colon == host ||
+        (size_t)(colon - host) >= 256 || slash - colon - 1 < 1 || slash - colon - 1 > 5 || !slash[1] ||
+        strlen(slash + 1) >= PURE_MODULE_NAME) {
+        snprintf(error, size, "a link is pure://host:port/INPUT");
+        return NULL;
+    }
+    PureLink *l = calloc(1, sizeof(*l));
+    if (!l || pthread_mutex_init(&l->lock, NULL)) {
+        free(l);
+        snprintf(error, size, "out of memory");
+        return NULL;
+    }
+    l->module = m;
+    l->output = (uint32_t)index;
+    l->shape = shape_of(m, (uint32_t)index);
+    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;
+    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;
+}
+
+uint32_t pure_link_output(const PureLink *l) { return l->output; }
+
+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;
+}
+
+static void wake(int fd) {
+    uint64_t one = 1;
+    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, PureSpan payload) {
+    Record r = {.seq = seq, .size = (uint32_t)payload.size};
+    r.pieces = payload.size ? (uint32_t)((payload.size + PIECE - 1) / PIECE) : 1;
+    if (payload.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, payload.size) : SIZE_MAX;
+    if (ok && at == SIZE_MAX) ok = ring_grow(l, payload.size) && (at = ring_place(l, payload.size)) != SIZE_MAX;
+    if (ok) {
+        r.at = at;
+        if (payload.size) memcpy(l->ring + at, payload.data, payload.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;
+    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;
+}
+
+uint64_t pure_link_queued(PureLink *l) {
+    pthread_mutex_lock(&l->lock);
+    uint64_t queued = l->queued;
+    pthread_mutex_unlock(&l->lock);
+    return queued;
+}
+
+uint64_t pure_link_delivered(PureLink *l) {
+    pthread_mutex_lock(&l->lock);
+    uint64_t stored = l->stored;
+    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_SOURCE + 1], input[PURE_MODULE_NAME];
+    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_SOURCE + 1], input[PURE_MODULE_NAME];
+            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);
+    }
+    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; }
+
+/* The packets of the burst to the socket (each but the last a whole
+ * datagram): with GSO in one call, else one by one. */
+static void flush(PureLink *l) {
+    unsigned n = l->burst_count, sent = 0;
+    bool whole = n > 1 && l->gso;
+    if (whole && send(l->fd, l->burst, l->burst_size, 0) >= 0) sent = n;
+    else if (whole && !transient()) l->gso = false, whole = false; /* no GSO here: one by one from now on */
+    for (size_t at = 0; !whole && sent < n; ++sent) {
+        size_t size = sent + 1 < n ? DATAGRAM : l->burst_size - at;
+        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]);
+    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;
+}
+
+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;
+    }
+    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);
+        bool stopping = l->stopping, more = l->queued_new; /* queued while it sent: no sleep */
+        l->sleeping = !more;
+        pthread_mutex_unlock(&l->lock);
+        if (stopping) break;
+        int timeout = more ? 0 : due ? (int)((due > now ? due - now : 0) / MS) + 1 : -1;
+        struct pollfd waits[2] = {{l->fd, POLLIN, 0}, {l->wake, POLLIN, 0}};
+        if (poll(waits, 2, timeout) < 0 && errno != EINTR) break;
+        if (waits[1].revents & POLLIN) {
+            uint64_t signals;
+            if (read(l->wake, &signals, sizeof(signals)) < 0) { /* cleared already */ }
+        }
+        pthread_mutex_lock(&l->lock);
+        l->sleeping = l->signaled = false;
+        now = now_ns();
+        if (waits[0].revents) take_answers(l, now);
+        run_timers(l, now);
+        send_packets(l, now);
+        bool told = l->stored != l->told;
+        l->told = l->stored;
+        pthread_mutex_unlock(&l->lock);
+        if (told && l->notify) l->notify(l->context);
+    }
+    return NULL;
+}
+
+bool pure_link_start(PureLink *l, void (*notify)(void *), void *context, char *error, size_t size) {
+    if (!l->peered) {
+        snprintf(error, size, "pure://%s:%s/%s: no receiver's key (--seal)", l->host, l->port, l->input);
+        return false;
+    }
+    struct addrinfo hints = {.ai_family = AF_UNSPEC, .ai_socktype = SOCK_DGRAM}, *found = NULL;
+    int resolved = getaddrinfo(l->host, l->port, &hints, &found);
+    if (resolved) {
+        snprintf(error, size, "%s: %s", l->host, gai_strerror(resolved));
+        return false;
+    }
+    l->fd = socket(found->ai_family, SOCK_DGRAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+    bool ok = l->fd >= 0 && !connect(l->fd, found->ai_addr, found->ai_addrlen);
+    freeaddrinfo(found);
+    int buffer = 4 << 20;
+    if (ok) {
+        setsockopt(l->fd, SOL_SOCKET, SO_SNDBUF, &buffer, sizeof(buffer));
+        setsockopt(l->fd, SOL_SOCKET, SO_RCVBUF, &buffer, sizeof(buffer));
+    }
+    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)) &&
+         (l->wake = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC)) >= 0;
+    l->notify = notify;
+    l->context = context;
+    if (!ok || pthread_create(&l->thread, NULL, run_link, l)) {
+        snprintf(error, size, "pure://%s:%s/%s: cannot start (a socket, the keys)", l->host, l->port, l->input);
+        return false;
+    }
+    pthread_setname_np(l->thread, "pure-link"); /* a link's CPU, seen apart */
+    l->started = true;
+    return true;
+}
+
+void pure_link_counts(PureLink *l, PureLinkCounts *out) {
+    pthread_mutex_lock(&l->lock);
+    *out = l->counts;
+    out->stored = l->stored;
+    pthread_mutex_unlock(&l->lock);
+}
+
+void pure_link_close(PureLink *l) {
+    if (!l) return;
+    if (l->started) {
+        pthread_mutex_lock(&l->lock);
+        l->stopping = true;
+        pthread_mutex_unlock(&l->lock);
+        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);
+    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_SOURCE + 1];
+    uint32_t input;
+    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 };
+
+struct PureLinkListener {
+    const PureModule *module;
+    const PureSealKey *own;
+    unsigned char own_public[PURE_SEAL_KEY];
+    uint32_t own_id;
+    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;
+    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;
+    bool (*deliver)(void *, const PureLinkDelivery *);
+    void *context;
+    unsigned char *batch;
+    size_t batch_capacity;
+    PureLinkCounts counts;
+};
+
+PureLinkListener *pure_link_listen(const PureModule *m, unsigned port, const PureSealKey *own,
+                                   const unsigned char own_public[PURE_SEAL_KEY],
+                                   bool (*deliver)(void *, const PureLinkDelivery *), void *context, char *error,
+                                   size_t size) {
+    PureLinkListener *s = own ? calloc(1, sizeof(*s)) : NULL;
+    if (!s || pthread_mutex_init(&s->lock, NULL)) {
+        free(s);
+        snprintf(error, size, own ? "out of memory" : "--link-port needs the node's --key");
+        return NULL;
+    }
+    s->module = m;
+    s->own = own;
+    memcpy(s->own_public, own_public, PURE_SEAL_KEY);
+    s->own_id = key_id(own_public);
+    s->port = port;
+    s->idle = IDLE_FIRST;
+    s->deliver = deliver;
+    s->context = context;
+    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_SOURCE) {
+        snprintf(error, size, "a source's name is 1 to %d bytes", PURE_SOURCE);
+        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 input takes no such messages. */
+static Stream *stream_of(PureLinkListener *s, Active *a, const char *source, const char *input, uint32_t shape) {
+    int index = pure_module_input(s->module, input);
+    if (index < 0) return NULL;
+    for (unsigned k = 0; k < a->stream_count; ++k) {
+        Stream *st = a->streams[k];
+        if (st->input == (uint32_t)index && !strcmp(st->source, source)) return st;
+    }
+    const PureInputShape *in = &s->module->input[index];
+    if (!allowed(s, &s->peers[a->peer], source) || in->kind != PURE_KIND_MESSAGE || in->keyed ||
+        shape_of(s->module, (uint32_t)index) != shape)
+        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;
+    snprintf(st->source, sizeof(st->source), "%s", source);
+    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->deliver(s->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->deliver(s->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_SOURCE + 1], input[PURE_MODULE_NAME];
+            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 + 2 + PURE_SOURCE + PURE_MODULE_NAME + 14 > BODY) break;
+            body[at++] = F_STORED;
+            at += put_names(body + at, st->source, s->module->input_names[st->input]);
+            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 void *run_listener(void *context) {
+    PureLinkListener *s = context;
+    Gathering *gathering = malloc(sizeof(*gathering));
+    if (!gathering) return NULL;
+    for (;;) {
+        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;
+        pthread_mutex_unlock(&s->lock);
+        if (stopping) break;
+        if (waits[1].revents & POLLIN) {
+            uint64_t signals;
+            if (read(s->wake, &signals, sizeof(signals)) < 0) { /* cleared already */ }
+        }
+        uint64_t 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);
+            }
+        }
+        take_done(s);
+        hand_over_ready(s);
+        answer(s, now);
+        sweep(s, now);
+    }
+    free(gathering);
+    return NULL;
+}
+
+bool pure_link_serve(PureLinkListener *s, char *error, size_t size) {
+    struct sockaddr_in6 six = {.sin6_family = AF_INET6, .sin6_port = htons((uint16_t)s->port), .sin6_addr = in6addr_any};
+    int off = 0, buffer = 4 << 20;
+    s->fd = socket(AF_INET6, SOCK_DGRAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+    bool bound = s->fd >= 0 && !setsockopt(s->fd, IPPROTO_IPV6, IPV6_V6ONLY, &off, sizeof(off)) &&
+                 !bind(s->fd, (struct sockaddr *)&six, sizeof(six));
+    if (!bound) { /* no IPv6 here */
+        if (s->fd >= 0) close(s->fd);
+        struct sockaddr_in four = {.sin_family = AF_INET, .sin_port = htons((uint16_t)s->port), .sin_addr = {INADDR_ANY}};
+        s->fd = socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+        bound = s->fd >= 0 && !bind(s->fd, (struct sockaddr *)&four, sizeof(four));
+    }
+    if (!bound) {
+        snprintf(error, size, "--link-port %u: %s", s->port, strerror(errno));
+        return false;
+    }
+    setsockopt(s->fd, SOL_SOCKET, SO_RCVBUF, &buffer, sizeof(buffer));
+    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)) {
+        snprintf(error, size, "--link-port %u: cannot start", s->port);
+        return false;
+    }
+    pthread_setname_np(s->thread, "pure-listen");
+    s->started = true;
+    return true;
+}
+
+void pure_link_done(PureLinkListener *s, void *token, uint64_t heard, bool stored, bool gap) {
+    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));
+        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};
+    pthread_mutex_unlock(&s->lock);
+    wake(s->wake);
+}
+
+void pure_link_listener_close(PureLinkListener *s, PureLinkCounts *counts) {
+    if (!s) return;
+    if (s->started) {
+        pthread_mutex_lock(&s->lock);
+        s->stopping = true;
+        pthread_mutex_unlock(&s->lock);
+        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);
+    free(s->done);
+    if (s->fd >= 0) close(s->fd);
+    if (s->wake >= 0) close(s->wake);
+    pthread_mutex_destroy(&s->lock);
+    free(s);
+}
diff --git a/link/link.h b/link/link.h
new file mode 100644
index 0000000..83cc645
--- /dev/null
+++ b/link/link.h
@@ -0,0 +1,118 @@
+#ifndef PURE_LINK_H
+#define PURE_LINK_H
+#include "module.h"
+#include "seal.h"
+
+/* Links without connections (docs/design/011-pure-transport.md): one node's
+ * saved sends to another node's input over UDP, sealed with a key the two
+ * nodes' keys share, with no handshake: the first packet carries records.
+ * A link's lasting state is the apps' own: the sender's outbox (what to send
+ * and send again) and the receiver's heard numbers (what it applied, saved
+ * with the events). What either side keeps in memory is a cache, so either
+ * may restart and the other goes on with the next packet.
+ *
+ * A packet is a header in the clear, then frames sealed with AES-128-GCM
+ * (the header as associated data):
+ *   u8 0x70 (version 0), u32le the key id of the node that sent it (FNV-1a
+ *   of its public key; two keys may share an id, and a receiver tries the
+ *   keys it allows with it), u64le that node's run (random, new each time it
+ *   starts, so no nonce repeats under a key across restarts), u64le the
+ *   packet's number in its run.
+ * Keys come from the secret the two nodes' keys share (X25519) by
+ * HKDF-SHA256, with both public keys and the sending node's run: a sender's
+ * packets use its own run; a receiver's answers add the run they answer,
+ * so an answer opens only in that run. The nonce is the key's base with the
+ * packet's number in its last eight bytes (XOR, big-endian, as QUIC does).
+ *
+ * Frames (numbers little-endian):
+ *   0x01 STREAM  source (u8 length, bytes), input (u8 length, bytes), u32 the
+ *                shape of the messages (the sender's output and the
+ *                receiver's input must have the same): the stream the
+ *                records after it are of
+ *   0x02 RECORD  u64 number, u32 size, u32 offset, u16 length, bytes: a piece
+ *                of a saved send (pieces are 1000 bytes, the last shorter)
+ *   0x03 ACK     u64 largest number, u32 delay (µs), u8 ranges, each as u64
+ *                first and u64 last, newest first
+ *   0x04 STORED  source, input (as STREAM), u64 the last number heard
+ *                (applied and saved), u8 flags (1: a record after a gap was
+ *                not taken: send what follows it again), u32 credit (records
+ *                it takes past that)
+ *   0x05 PING    to be acknowledged
+ *   0x06 RECORDS u64 the first one's number, u8 count, then whole records one
+ *                after another (numbers following on), each its size (a
+ *                varint: LEB128) and its bytes: a send that fits the packet
+ *                whole, with 1 to 3 bytes of framing, not the 19 of RECORD
+ *   0x00 PADDING to the end of the packet: a full packet sent in a burst is
+ *                a whole datagram
+ * Losses are detected as RFC 9002 says; congestion control is its NewReno
+ * (RFC 9002 §7). */
+
+/* The sending side: one output's saved sends to url, pure://host:port/INPUT,
+ * as source (a Pure receiver's name for it), from own (this node's key,
+ * whose public half is own_public) to the receiver's public key, given by
+ * pure_link_peer before pure_link_start. The owner queues and trims as it
+ * does a wire's (serve/deliver.h): a send counts as delivered once the
+ * receiver says it saved it. */
+typedef struct PureLink PureLink;
+PureLink *pure_link_open(const PureModule *m, const char *output, const char *url, const char *source,
+                         const PureSealKey *own, const unsigned char own_public[PURE_SEAL_KEY], char *error,
+                         size_t size);
+void pure_link_peer(PureLink *, const unsigned char peer[PURE_SEAL_KEY]);
+uint32_t pure_link_output(const PureLink *);
+bool pure_link_queue(PureLink *, uint64_t seq, PureSpan payload);
+uint64_t pure_link_queued(PureLink *);
+/* The last number the receiver saved; any thread. */
+uint64_t pure_link_delivered(PureLink *);
+/* Send on a thread of the link's own, calling notify when more was saved. */
+bool pure_link_start(PureLink *, void (*notify)(void *), void *context, char *error, size_t size);
+void pure_link_close(PureLink *);
+
+/* The receiving side: packets on a UDP port, from the senders allowed (a
+ * source name and the key it seals with, a few bytes each however many
+ * there are: see pure_link_idle). Whole records of a stream, in
+ * order, go to deliver (on the listener's thread) as one delivery; the host
+ * applies them from the one after the source's last heard number, stops at
+ * a gap, saves, and then calls pure_link_done (any thread) with the last
+ * number heard. Records wait here until then, and only the saved ones are
+ * acknowledged as stored. */
+typedef struct {
+    uint32_t input;
+    const char *source;
+    uint64_t first;
+    unsigned count;
+    const unsigned char *records; /* count of: u32le size, bytes */
+    size_t size;
+    void *token; /* for pure_link_done */
+} PureLinkDelivery;
+typedef struct PureLinkListener PureLinkListener;
+PureLinkListener *pure_link_listen(const PureModule *m, unsigned port, const PureSealKey *own,
+                                   const unsigned char own_public[PURE_SEAL_KEY],
+                                   bool (*deliver)(void *, const PureLinkDelivery *), void *context, char *error,
+                                   size_t size);
+bool pure_link_allow(PureLinkListener *, const char *source, const unsigned char key[PURE_SEAL_KEY], char *error,
+                     size_t size);
+/* A sender quiet for ms (30 s at first) with nothing of it waiting lets its
+ * keys, addresses and streams go: an idle link costs the listener only its
+ * allowing, and the sender's next packet makes them again (one X25519). */
+void pure_link_idle(PureLinkListener *, uint64_t ms);
+/* Remember the secret each sender's key shares with the node's in a file
+ * (path, 0600; begun again for another node's key): a place a sender in
+ * the order allowed, mapped, so a sender at rest takes no memory for it.
+ * A node that starts again then makes no X25519 for a sender it heard from
+ * before. After the senders are allowed, before serving. */
+bool pure_link_remember(PureLinkListener *, const char *path, char *error, size_t size);
+bool pure_link_serve(PureLinkListener *, char *error, size_t size);
+/* stored: the delivery's save succeeded; gap: it stopped at a gap. */
+void pure_link_done(PureLinkListener *, void *token, uint64_t heard, bool stored, bool gap);
+
+/* Counts for a host's report: packets and records sent, records sent again,
+ * packets lost and acknowledged, packets received, deliveries handed over,
+ * the last number stored; a listener's senders woken (their active state
+ * made), of them with a secret remembered (no X25519), let go when idle,
+ * and active when it closed. A listener's are given when it closes. */
+typedef struct {
+    uint64_t packets, records, resent, lost, acknowledged, received, deliveries, stored, woken, remembered, swept, active;
+} PureLinkCounts;
+void pure_link_counts(PureLink *, PureLinkCounts *);
+void pure_link_listener_close(PureLinkListener *, PureLinkCounts *counts);
+#endif
diff --git a/link/seal.h b/link/seal.h
new file mode 100644
index 0000000..9245f25
--- /dev/null
+++ b/link/seal.h
@@ -0,0 +1,80 @@
+#ifndef PURE_SEAL_H
+#define PURE_SEAL_H
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* Sends sealed end to end (docs/research/086-sealed-sends.md, 088): a
+ * receiving node's key opens what a sender sealed to its public key, and
+ * nothing between them (a relay, a proxy, a mailbox) can read or change it.
+ * HPKE (RFC 9180): X25519, HKDF-SHA256, and AES-128-GCM where the CPU has AES
+ * instructions, else ChaCha20-Poly1305; auth mode when the sender has a key
+ * of its own, so the receiver knows who sealed. A sender keeps one session
+ * (one encapsulation) and numbers its seals in it; a receiver opens a
+ * session once and then each seal by its number. A private key lives in
+ * OpenSSL's secure heap: locked in memory, out of core dumps, cleared when
+ * freed. OpenSSL comes with pure-crypto.so (link/crypto.c), mapped the
+ * first time a key is read or a session made. */
+enum { PURE_SEAL_KEY = 32, PURE_SEAL_OVERHEAD = 16 }; /* a public key; a seal's tag */
+
+typedef struct PureSealKey PureSealKey;
+typedef struct PureSealer PureSealer; /* a sender's session */
+typedef struct PureOpener PureOpener; /* a receiver's session */
+
+/* Whether this machine can seal (pure-crypto.so maps), with why not in error. */
+bool pure_seal_ready(char *error, size_t size);
+/* A node's key: read from path, or made and written there (0600) when there
+ * is none; its public half in hex. NULL with why in error. */
+PureSealKey *pure_seal_key(const char *path, char public_hex[2 * PURE_SEAL_KEY + 1], char *error, size_t size);
+void pure_seal_key_free(PureSealKey *);
+/* A public key from its hex. */
+bool pure_seal_peer(const char *hex, unsigned char out[PURE_SEAL_KEY]);
+/* The suite this machine seals with, as the X-Pure-Seal header names it. */
+const char *pure_seal_suite(void);
+
+/* A session sealing to peer, authenticated by own when given: its
+ * encapsulated key into enc (what a receiver opens the session with). */
+PureSealer *pure_sealer(const unsigned char peer[PURE_SEAL_KEY], const PureSealKey *own,
+                        unsigned char enc[PURE_SEAL_KEY]);
+/* plain sealed as the session's next seal, bound to aad: size +
+ * PURE_SEAL_OVERHEAD bytes into out, and its number into n. */
+bool pure_seal(PureSealer *, const char *aad, const unsigned char *plain, size_t size, unsigned char *out,
+               uint64_t *n);
+void pure_sealer_free(PureSealer *);
+
+/* A session a sender made to key's public half, in suite (this machine's or
+ * the other known one), from its enc; from is the sender's public key when
+ * it sealed with its own (NULL otherwise). NULL when it does not open. */
+PureOpener *pure_opener(const PureSealKey *key, const char *suite, const unsigned char enc[PURE_SEAL_KEY],
+                        const unsigned char *from);
+/* The session's seal number n: size - PURE_SEAL_OVERHEAD bytes into plain;
+ * false when it does not open (another session, another aad, or changed on
+ * the way). */
+bool pure_unseal(PureOpener *, uint64_t n, const char *aad, const unsigned char *sealed, size_t size,
+                 unsigned char *plain);
+void pure_opener_free(PureOpener *);
+
+/* Links (link/link.h): the secret two nodes share (X25519 of this node's
+ * private key and the other's public key: the same on both sides, and
+ * never all zeros), keys from it (HKDF-SHA256 with the info given), and
+ * AES-128-GCM under such a key with the nonce the caller gives (sealing may
+ * be in place: out may be plain). A context is used by one thread at a time;
+ * sealing or opening with a key given uses that key from then on, so one
+ * context serves many links (a key costs about as much as a small packet),
+ * and NULL keeps the key it has. */
+typedef struct PureAead PureAead;
+enum { PURE_AEAD_KEY = 16, PURE_AEAD_NONCE = 12, PURE_AEAD_TAG = 16 };
+bool pure_link_secret(const PureSealKey *own, const unsigned char peer[PURE_SEAL_KEY], unsigned char out[32]);
+bool pure_link_derive(const unsigned char secret[32], const unsigned char *info, size_t info_size, unsigned char *out,
+                      size_t out_size);
+PureAead *pure_aead(const unsigned char key[PURE_AEAD_KEY]);
+/* size + PURE_AEAD_TAG bytes into out. */
+bool pure_aead_seal(PureAead *, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                    const unsigned char *aad, size_t aad_size, const unsigned char *plain, size_t size,
+                    unsigned char *out);
+/* size - PURE_AEAD_TAG bytes into plain; false when it does not open. */
+bool pure_aead_open(PureAead *, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                    const unsigned char *aad, size_t aad_size, const unsigned char *sealed, size_t size,
+                    unsigned char *plain);
+void pure_aead_free(PureAead *);
+#endif
diff --git a/link/tls.h b/link/tls.h
new file mode 100644
index 0000000..3939f21
--- /dev/null
+++ b/link/tls.h
@@ -0,0 +1,18 @@
+#ifndef PURE_TLS_H
+#define PURE_TLS_H
+#include <stdbool.h>
+#include <stddef.h>
+
+/* TLS to https receivers (link/crypto.c): one client context, verifying a
+ * receiver's certificate against the system's CAs or ca_file, and its name. */
+typedef struct PureTls PureTls;
+/* The context, made once; false with why in error when it cannot be. */
+bool pure_tls_start(const char *ca_file, char *error, size_t size);
+/* A session over a connected socket to host (a DNS name, or an IP address
+ * written as one), its certificate verified; NULL when it fails. */
+PureTls *pure_tls_open(const char *host, int fd);
+/* As SSL_read and SSL_write: the bytes moved, or <= 0. */
+int pure_tls_read(PureTls *, void *bytes, int size);
+int pure_tls_write(PureTls *, const void *bytes, int size);
+void pure_tls_close(PureTls *);
+#endif