Link two nodes without a connection over UDP (093, design 011's first stage)

pure-serve --send OUTPUT=pure://host:port/INPUT sends an output's saved
sends to another node's --link-port with no handshake: the first packet
carries records. Keys come from the two nodes' keys (X25519, HKDF-SHA256,
AES-128-GCM), with a random run in each packet's header so no nonce
repeats across restarts; nothing is stored for them. What lasts is the
apps' own: the sender's outbox and the receiver's heard numbers
(pure_app_heard, new). The sender detects losses as RFC 9002 says, with
NewReno, and trims a send once the receiver says it stored it; the
receiver hands the owner whole records in order, which it applies from
the one after the source's last heard number, stopping at a gap, and
answers how far it stored with every acknowledgment. Stored bytes are
the bytes on the wire: an output and an input of one shape need no JSON.

server_links_udp kills the receiver, the sender, then both (-9) while
four clients buy: every saved order is applied once, with no handshake
(five runs, 10k-11k orders each). server_links_udp_lossy does it again in
a namespace of its own with the link's packets delayed, lost (3 %) and
reordered (10 %) by netem (three runs, 11k-12k orders).

1 files changed, 98 insertions, 0 deletions

diff --git a/serve/link.h b/serve/link.h
new file mode 100644
index 0000000..bbda882
--- /dev/null
+++ b/serve/link.h
@@ -0,0 +1,98 @@
+#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), 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 saved
+ *                send as stored, or a piece of one
+ *   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
+ * 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). 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);
+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 are given when it closes. */
+typedef struct {
+    uint64_t packets, records, resent, lost, acknowledged, received, deliveries, stored;
+} PureLinkCounts;
+void pure_link_counts(PureLink *, PureLinkCounts *);
+void pure_link_listener_close(PureLinkListener *, PureLinkCounts *counts);
+#endif