diff --git a/hptp-send/src/main.rs b/hptp-send/src/main.rs
index e358b32..9033742 100644
--- a/hptp-send/src/main.rs
+++ b/hptp-send/src/main.rs
@@ -10,7 +10,6 @@ use hptp::msg::{DownMsg, UpMsg};
 use hptp::peer::{self, Peer, UpPeer};
 use hptp::seg::{SegData, SegIdx, MAX_SEG_SIZE};
 use std::cmp;
-use std::collections::HashMap;
 use std::net::SocketAddr;
 use tokio::io::AsyncRead;
 // use tokio::net::UdpSocket;
@@ -75,8 +74,9 @@ struct SegmentSource<'i, IN> {
     read_input: bool,
     is_meow: bool,
     is_cut: bool,
-    unacked_segs: HashMap<SegIdx, UnAcked>,
-    unacked_upper_bound: u32,
+    // ordered by seg idx
+    unacked_segs: Vec<(SegIdx, UnAcked)>,
+    next_seg_idx: u32,
     eof: bool,
 }
 
@@ -87,6 +87,15 @@ struct UnAcked {
 
 const IN_FLIGHT: usize = 30;
 
+impl UnAcked {
+    fn nack(self) -> Self {
+        Self {
+            nacks: self.nacks + 1,
+            payload: self.payload,
+        }
+    }
+}
+
 impl<'i, IN> SegmentSource<'i, IN>
 where
     IN: AsyncRead + Unpin,
@@ -98,20 +107,12 @@ where
             read_input: false,
             is_meow: false,
             is_cut: false,
-            unacked_segs: HashMap::new(),
-            unacked_upper_bound: 0,
+            unacked_segs: Vec::new(),
+            next_seg_idx: 0,
             eof: false,
         }
     }
 
-    fn retrans_seg_idx(&self) -> Option<SegIdx> {
-        self.unacked_segs.keys().skip(IN_FLIGHT).next().cloned()
-    }
-
-    fn any_unacked_seg_idx(&self) -> Option<SegIdx> {
-        self.unacked_segs.keys().next().cloned()
-    }
-
     async fn read_next(&mut self) -> SegData {
         use tokio::io::AsyncReadExt;
         if !self.read_input {
@@ -147,43 +148,53 @@ where
         }
     }
 
+    fn retrans_seg_idx(&self) -> Option<SegIdx> {
+        self.unacked_segs.iter().map(|(si, _)| *si).next()
+    }
+
     async fn fresh_seg_idx(&mut self) -> Option<SegIdx> {
         if self.eof {
             None
         } else {
-            let seg_idx = self.unacked_upper_bound;
-            let payload = self.read_next().await;
-            self.eof = payload.is_last_segment;
-            self.unacked_upper_bound += 1;
-            self.unacked_segs
-                .insert(seg_idx, UnAcked { payload, nacks: 0 });
+            let seg_idx = self.next_seg_idx;
+            let unack = UnAcked {
+                payload: self.read_next().await,
+                nacks: 0,
+            };
+            self.next_seg_idx += 1;
+            self.eof = unack.payload.is_last_segment;
+            self.unacked_segs.push((seg_idx, unack));
             Some(seg_idx)
         }
     }
 
     async fn get_segment(&mut self) -> Option<(SegIdx, &SegData)> {
         let seg_idx = {
-            if let Some(si) = self.retrans_seg_idx() {
+            if let Some(si) = self.fresh_seg_idx().await {
                 si
-            } else if let Some(si) = self.fresh_seg_idx().await {
-                si
-            } else if let Some(si) = self.any_unacked_seg_idx() {
+            } else if let Some(si) = self.retrans_seg_idx() {
                 si
             } else {
                 // early exit cus there's nothing left
                 return None;
             }
         };
-        self.unacked_segs
-            .get(&seg_idx)
-            .map(|u| (seg_idx, &u.payload))
+        let i = self
+            .unacked_segs
+            .binary_search_by_key(&seg_idx, |x| x.0)
+            .expect("seg doesn't exist!");
+        Some((seg_idx, &self.unacked_segs[i].1.payload))
     }
 
     /// `seg_idxs` should be distinct and in increasing order.
-    fn ack(&mut self, seg_idxs: &[SegIdx]) {
-        for seg_idx in seg_idxs {
-            self.unacked_segs.remove(seg_idx);
+    fn ack(&mut self, ack_seg_idxs: &[SegIdx]) {
+        let mut new_unacked_segs = Vec::new();
+        for (seg_idx, unack) in self.unacked_segs.drain(..) {
+            if ack_seg_idxs.binary_search(&seg_idx).is_err() {
+                new_unacked_segs.push((seg_idx, unack.nack()))
+            }
         }
+        self.unacked_segs = new_unacked_segs;
     }
 }