diff --git a/Cargo.lock b/Cargo.lock
index 2880c79..752e980 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -12,6 +12,12 @@ version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cf1de2fe8c75bc145a2f577add951f8134889b4795d47466a54a5c846d691693"
 
+[[package]]
+name = "byteorder"
+version = "1.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "08c48aae112d48ed9f069b33538ea9e3e90aa263cfa3d1c24309612b1f7472de"
+
 [[package]]
 name = "bytes"
 version = "0.5.4"
@@ -55,6 +61,7 @@ checksum = "3dcaa9ae7725d12cdb85b3ad99a434db70b468c09ded17e012d86b5c1010f7a7"
 name = "hptp"
 version = "0.1.0"
 dependencies = [
+ "byteorder",
  "chrono",
  "thiserror",
  "tokio",
diff --git a/hptp-recv/src/main.rs b/hptp-recv/src/main.rs
index c12ee10..54250c4 100644
--- a/hptp-recv/src/main.rs
+++ b/hptp-recv/src/main.rs
@@ -7,6 +7,7 @@ extern crate thiserror;
 use hptp::logger::Logger;
 use hptp::msg::{DownMsg, UpMsg};
 use hptp::peer::{self, DownPeer, Peer};
+use std::collections::HashMap;
 use tokio::io::AsyncWrite;
 
 #[derive(Error, Debug)]
@@ -55,22 +56,34 @@ async fn download<OUT>(log: &mut Logger, peer: &mut DownPeer, out: &mut OUT) ->
 where
     OUT: AsyncWrite + Unpin,
 {
-    use tokio::io::AsyncWriteExt;
-    let mut pos = 0;
+    let mut segs = HashMap::new();
+    let mut flush_seg_idx = 0;
     loop {
         match peer.recv().await {
-            Ok(UpMsg::Data { payload, .. }) => {
+            Ok(UpMsg::Data { payload, seg_idx }) => {
                 let len = payload.len();
-                out.write_all(&payload).await?;
-                out.flush().await?;
-                log.recv_data_accepted(pos, len, hptp::logger::InOrder)
+                segs.entry(seg_idx).or_insert(Some(payload));
+                let ack = DownMsg::Ack {
+                    idxs: segs.keys().cloned().collect(),
+                };
+                log.debug_msg(format!("sent ack: {:?}",
+                                      segs.keys().collect::<Vec<_>>())).await;
+                send(peer, ack).await?;
+                log.recv_data_accepted(seg_idx as usize, len, hptp::logger::OutOfOrder)
                     .await;
-                send(peer, DownMsg::Ack {}).await?;
-                pos += len;
             }
 
             Err(peer::RecvError::InvalidMessage { .. }) => log.recv_corrupt().await,
             Err(peer::RecvError::Io { source }) => return Err(source.into()),
         }
+
+        while let Some(v) = segs.get_mut(&flush_seg_idx) {
+            if let Some(payload) = v.take() {
+                use tokio::io::AsyncWriteExt;
+                out.write_all(&payload).await?;
+                out.flush().await?;
+            }
+            flush_seg_idx += 1;
+        }
     }
 }
diff --git a/hptp-send/src/main.rs b/hptp-send/src/main.rs
index 76dc5a1..81d8754 100644
--- a/hptp-send/src/main.rs
+++ b/hptp-send/src/main.rs
@@ -5,7 +5,7 @@ extern crate tokio;
 extern crate thiserror;
 
 use hptp::logger::Logger;
-use hptp::msg::{DownMsg, UpMsg};
+use hptp::msg::UpMsg;
 use hptp::peer::{self, Peer, UpPeer};
 use std::net::SocketAddr;
 use tokio::io::AsyncRead;
@@ -91,30 +91,29 @@ async fn upload<IN>(log: &mut Logger, peer: &mut UpPeer, inp: &mut IN) -> Result
 where
     IN: AsyncRead + Unpin,
 {
-    let mut pos = 0;
-    let mut next_ack_len: Option<usize> = None;
+    let mut seg_idx = 0;
+    // TODO: async recieve acks
     loop {
-        if let Some(ack_len) = next_ack_len {
+        /*        if let Some(ack_len) = next_ack_len {
             match peer.recv().await {
-                Ok(DownMsg::Ack { .. }) => {
-                    log.recv_ack(pos).await;
-                    pos += ack_len;
+                Ok(DownMsg::Ack { idxs }) => {
+                    // log.recv_ack(pos).await;
                     next_ack_len = None;
                 }
-
                 Err(peer::RecvError::InvalidMessage { .. }) => log.recv_corrupt().await,
                 Err(peer::RecvError::Io { source }) => return Err(source.into()),
             }
-        } else {
-            match read_segment(inp).await? {
-                Some(data) => {
-                    next_ack_len = Some(data.len());
-                    log.send_data(pos, data.len()).await;
-                    send(peer, UpMsg::Data { payload: data }).await?;
-                }
-                None => break,
+        } else {*/
+        match read_segment(inp).await? {
+            Some(payload) => {
+                log.send_data(seg_idx as usize, payload.len()).await;
+                let data = UpMsg::Data { payload, seg_idx };
+                send(peer, data).await?;
+                seg_idx += 1;
             }
+            None => break,
         }
+        // }
     }
     log.completed().await;
     Ok(())
diff --git a/hptp/Cargo.toml b/hptp/Cargo.toml
index 3d53c90..b34975f 100644
--- a/hptp/Cargo.toml
+++ b/hptp/Cargo.toml
@@ -10,4 +10,5 @@ edition = "2018"
 [dependencies]
 tokio = {version = "0.2.*", features = ["io-std", "io-util", "udp"]}
 thiserror = "*"
-chrono = "0.4.*"
\ No newline at end of file
+chrono = "0.4.*"
+byteorder = "1.3.*"
\ No newline at end of file
diff --git a/hptp/src/lib.rs b/hptp/src/lib.rs
index ed3c22f..eec3778 100644
--- a/hptp/src/lib.rs
+++ b/hptp/src/lib.rs
@@ -1,8 +1,9 @@
 #[macro_use]
 extern crate thiserror;
+extern crate byteorder;
 extern crate chrono;
 
 pub mod logger;
-pub mod seg;
 pub mod msg;
 pub mod peer;
+pub mod seg;
diff --git a/hptp/src/msg.rs b/hptp/src/msg.rs
index e70e5a8..99cfa38 100644
--- a/hptp/src/msg.rs
+++ b/hptp/src/msg.rs
@@ -1,18 +1,20 @@
-pub use super::seg::{MAX_TOTAL_PACKET_SIZE, UP_HEADER_SIZE, DOWN_HEADER_SIZE};
+use super::seg::SegIdx;
+pub use super::seg::{DOWN_HEADER_SIZE, MAX_TOTAL_PACKET_SIZE, UP_HEADER_SIZE};
+use byteorder::ByteOrder;
+use std::collections::HashSet;
 
 #[derive(Clone, Debug)]
 pub enum UpMsg {
     Data {
         payload: Vec<u8>,
-        // seg_idx: SegIdx,
-        // encoding: SegmentEncoding,
+        seg_idx: SegIdx,
         // is_final_packet: bool,
     },
 }
 
 #[derive(Clone, Debug)]
 pub enum DownMsg {
-    Ack {}, // ackd: SegmentSet
+    Ack { idxs: HashSet<SegIdx> },
 }
 
 #[derive(Error, Debug)]
@@ -26,30 +28,47 @@ pub trait SerDes: Sized {
     fn ser_to(&self, buf: &mut [u8]) -> usize;
 }
 
+type BO = byteorder::LE;
+
 impl SerDes for UpMsg {
     fn des(buf: &[u8]) -> Result<Self, DesError> {
-        Ok(UpMsg::Data {
-            payload: buf.into(),
-        })
+        if buf.len() < UP_HEADER_SIZE {
+            Err(DesError)
+        } else {
+            Ok(UpMsg::Data {
+                seg_idx: BO::read_u32(&buf[0..4]),
+                payload: buf[4..].into(),
+            })
+        }
     }
 
     fn ser_to(&self, buf: &mut [u8]) -> usize {
         match self {
-            UpMsg::Data { payload, .. } => {
+            UpMsg::Data { payload, seg_idx } => {
                 let len = payload.len();
-                buf[..len].copy_from_slice(&payload[..]);
-                len
+                BO::write_u32(&mut buf[0..4], *seg_idx);
+                buf[4..4 + len].copy_from_slice(&payload[..]);
+                4 + len
             }
         }
     }
 }
 
 impl SerDes for DownMsg {
-    fn des(_buf: &[u8]) -> Result<Self, DesError> {
-        Ok(DownMsg::Ack {})
+    fn des(buf: &[u8]) -> Result<Self, DesError> {
+        let mut idxs = HashSet::new();
+        for (i, b) in buf.iter().cloned().enumerate() {
+            for j in 0..8 {
+                if b & (1 << j) != 0 {
+                    idxs.insert((i * 8 + j) as SegIdx);
+                }
+            }
+        }
+        Ok(DownMsg::Ack { idxs })
     }
 
     fn ser_to(&self, _buf: &mut [u8]) -> usize {
-        1
+        // TODO: implement this!
+        0
     }
 }
diff --git a/hptp/src/seg.rs b/hptp/src/seg.rs
index 50e2304..040a4ed 100644
--- a/hptp/src/seg.rs
+++ b/hptp/src/seg.rs
@@ -2,7 +2,7 @@
 pub const MAX_TOTAL_PACKET_SIZE: usize = 1472;
 
 // TODO: change these based off the decoders
-pub const UP_HEADER_SIZE: usize = 0;
+pub const UP_HEADER_SIZE: usize = 4;
 pub const DOWN_HEADER_SIZE: usize = 1;
 
 /// This is the maximum amount of segment data we can fit into a packet.