diff --git a/hptp-recv/src/main.rs b/hptp-recv/src/main.rs
index bc5f142..8a6878d 100644
--- a/hptp-recv/src/main.rs
+++ b/hptp-recv/src/main.rs
@@ -11,6 +11,7 @@ use hptp::peer::{self, DownPeer, Peer};
 use hptp::seg::{SegData, SegIdx};
 use std::collections::HashMap;
 use tokio::io::AsyncWrite;
+use std::cmp;
 
 #[derive(Error, Debug)]
 enum Error {
@@ -128,10 +129,12 @@ where
     }
 
     let mut sink = SegmentSink::new(out);
-
-    const DELAY_MS: u64 = 1000;
     let mut deadline = Instant::now();
 
+    let mut ack_buildup: usize = 0;
+    let mut seen_eof = false;
+    let mut max_seen_idx: SegIdx = 0;
+
     loop {
         let timer = tokio::time::delay_until(deadline);
         let evt = tokio::select!(
@@ -150,12 +153,15 @@ where
 
         match evt {
             Evt::Timer => {
-                deadline += Duration::from_millis(DELAY_MS);
+                deadline += Duration::from_millis(1000);
                 log.debug_msg("timeout").await;
+                ack_buildup += 1;
             }
 
             Evt::Recv(UpMsg::Data { payload, seg_idx }) => {
                 let len = payload.len();
+                let eof = payload.is_last_segment || seg_idx < max_seen_idx;
+                max_seen_idx = cmp::max(seg_idx + 1, max_seen_idx);
                 match sink.put(seg_idx, payload).await {
                     Put::Duplicate => {
                         log.recv_data_ignored(seg_idx as usize, len).await;
@@ -163,15 +169,29 @@ where
                     Put::Fresh => {
                         let ord = hptp::logger::OutOfOrder;
                         log.recv_data_accepted(seg_idx as usize, len, ord).await;
+                        ack_buildup += if eof { 10 } else { 2 };
                     }
                 }
+
+                if eof && !seen_eof {
+                    seen_eof = true;
+                    ack_buildup += 999;
+                }
             }
         };
 
-        let num_acks = if sink.is_file_complete() { 8 } else { 1 };
+        let num_acks = if sink.is_file_complete() {
+            5
+        } else if ack_buildup > 10 {
+            ack_buildup = 0;
+            2
+        } else {
+            0
+        };
+
         for _ in 0..num_acks {
             let idxs = sink.ack_idxs();
-            log.debug_msg(format!("send acks {:?}", idxs)).await;
+            log.debug_msg(format!("send {} acks", idxs.len())).await;
             let m = DownMsg::Ack { idxs };
             match peer.send(m).await {
                 Ok(()) => (),