Always send an ACK back when we receive data from the peer.

A previous ACK could get lost, causing the peer to resend data. If the
retransmission doesn't contain any new data, we would consider this data
to be outside our receive window, and then not resend our ACK.

diff --git a/utcp.c b/utcp.c
index 7e28f9e27e3aef2a7f0937597eabbd5bd3c36f7a..d0e56d2c41e6dd278d1791828ff59375fc5b6391 100644 (file)
--- a/utcp.c
+++ b/utcp.c
@@ -1011,6 +1011,8 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
                ptr += 2;
        }
 
+       bool has_data = len;
+
        // Try to match the packet to an existing connection
 
        struct utcp_connection *c = find_connection(utcp, hdr.dst, hdr.src);
@@ -1105,8 +1107,6 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
        // It is for an existing connection.
 
-       uint32_t prevrcvnxt = c->rcv.nxt;
-
        // 1. Drop invalid packets.
 
        // 1a. Drop packets that should not happen in our current state.
@@ -1130,7 +1130,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
                break;
        }
 
-       // 1b. Drop packets with a sequence number not in our receive window.
+       // 1b. Discard data that is not in our receive window.
 
        if(is_reliable(c)) {
                bool acceptable;
@@ -1282,7 +1282,6 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
        // 3. Advance snd.una
 
        advanced = seqdiff(hdr.ack, c->snd.una);
-       prevrcvnxt = c->rcv.nxt;
 
        if(advanced) {
                // RTT measurement
@@ -1536,13 +1535,13 @@ skip_ack:
        }
 
        // Now we send something back if:
-       // - we advanced rcv.nxt (ie, we got some data that needs to be ACKed)
+       // - we received data, so we have to send back an ACK
        //   -> sendatleastone = true
        // - or we got an ack, so we should maybe send a bit more data
        //   -> sendatleastone = false
 
        if(is_reliable(c) || hdr.ctl & SYN || hdr.ctl & FIN) {
-               ack(c, len || prevrcvnxt != c->rcv.nxt);
+               ack(c, has_data);
        }
 
        return 0;