diff --git a/hptp-recv/src/main.rs b/hptp-recv/src/main.rs
index d9b0184..f0bc835 100644
--- a/hptp-recv/src/main.rs
+++ b/hptp-recv/src/main.rs
@@ -63,13 +63,13 @@ where
     let mut pos = 0;
     loop {
         match peer.recv().await {
-            Ok(UpMsg::Data(data)) => {
-                let len = data.len();
-                out.write_all(&data).await?;
+            Ok(UpMsg::Data { payload, .. }) => {
+                let len = payload.len();
+                out.write_all(&payload).await?;
                 out.flush().await?;
                 log.recv_data_accepted(pos, len, hptp::logger::InOrder)
                     .await;
-                send(peer, DownMsg::Ack).await?;
+                send(peer, DownMsg::Ack {}).await?;
                 pos += len;
             }
 
diff --git a/hptp-send/src/main.rs b/hptp-send/src/main.rs
index 9f8b9d6..7c0236e 100644
--- a/hptp-send/src/main.rs
+++ b/hptp-send/src/main.rs
@@ -74,7 +74,7 @@ where
     IN: AsyncRead + Unpin,
 {
     use tokio::io::AsyncReadExt;
-    let mut buf = [0u8; msg::MAX_DATA_SIZE];
+    let mut buf = [0u8; msg::MAX_SEG_SIZE];
     let len = inp.read(&mut buf).await?;
     Ok(if len > 0 {
         Some(buf[..len].into())
@@ -100,7 +100,7 @@ where
     loop {
         if let Some(ack_len) = next_ack_len {
             match peer.recv().await {
-                Ok(DownMsg::Ack) => {
+                Ok(DownMsg::Ack { .. }) => {
                     log.recv_ack(pos).await;
                     pos += ack_len;
                     next_ack_len = None;
@@ -114,7 +114,7 @@ where
                 Some(data) => {
                     next_ack_len = Some(data.len());
                     log.send_data(pos, data.len()).await;
-                    send(peer, UpMsg::Data(data)).await?;
+                    send(peer, UpMsg::Data { payload: data }).await?;
                 }
                 None => break,
             }
diff --git a/hptp/src/msg.rs b/hptp/src/msg.rs
index b18f326..2314ac9 100644
--- a/hptp/src/msg.rs
+++ b/hptp/src/msg.rs
@@ -1,37 +1,89 @@
-#[derive(Clone)]
+/// Per the assignment spec, `1472` is the maximum size packet we're allowed to send.
+pub const MAX_SERIALIZED_SIZE: usize = 1472;
+
+// TODO: change these based off the decoders
+pub const UP_HEADER_SIZE: usize = 0;
+pub const DOWN_HEADER_SIZE: usize = 0;
+
+/// This is the maximum amount of segment data we can fit into a packet.
+pub const MAX_SEG_SIZE: usize = MAX_SERIALIZED_SIZE - UP_HEADER_SIZE;
+
+// Note: we can only keep so much file data in RAM, so let's see what would be the
+// maximum amount of file data we keep in flux.
+//
+//   1456 B (MAX_SEG_SIZE) * 2.03% (compression factor for PSHex) = 2.96 KB/seg
+//   2.96 KB/seg * 11,648 seg (max possible SEGS_PER_CHUNK) = 34.5 MB
+//
+// 34 MB is actually much larger than the maximum test case size.
+
+/// This is calculated based on the max size we would need for a bit-field specifying
+/// which segments are present in a chunk.
+pub const SEG_PER_CHUNK: usize = (MAX_SERIALIZED_SIZE - DOWN_HEADER_SIZE) * 8;
+
+/// Is `u32` big enough to handle all file sizes?
+///
+///   1.46 KB/seg * 2^32 seg = 6.3 TB
+///
+/// If we used `u16` instead we would be just barely OK:
+///
+///   1.46 KB/seg * 2^16 seg = 95.4 MB
+///
+pub type SegIdx = u32;
+
+#[derive(Clone, Debug)]
 pub enum UpMsg {
-    Data(Vec<u8>),
+    Data {
+        payload: Vec<u8>,
+        // seg_idx: SegIdx,
+        // encoding: SegmentEncoding,
+        // is_final_packet: bool,
+    },
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub enum DownMsg {
-    Ack,
+    Ack {}, // ackd: SegmentSet
 }
 
-pub const MAX_DATA_SIZE: usize = 999;
-pub const MAX_SERIALIZED_SIZE: usize = 1 + MAX_DATA_SIZE;
+/*
+#[derive(Copy, Clone, Eq, PartialEq, Debug)]
+pub enum SegmentEncoding {
+    /// Un-encoded byte sequence.
+    Raw,
+    /// "PostScript style" Hex w/ linebreaks: ([0-9a-f]{60}\n)*.
+    PSHex,
+}
+
+#[derive(Clone, Debug)]
+pub struct SegmentSet {
+    pub latest_seg_idx: SegIdx,
+    pub other_segs: std::collections::HashSet<SegIdx>,
+}
+ */
 
 #[derive(Error, Debug)]
 #[error("deserialization failed; malformed packet")]
 pub struct DesError;
 
 pub trait SerDes: Sized {
-    fn des(data: &[u8]) -> Result<Self, DesError>;
+    // For both methods: `buf.len()` must be >= `MAX_SERIALIZED_SIZE`
 
-    // `buf.len()` must be >= `MAX_SERIALIZED_SIZE`
+    fn des(buf: &[u8]) -> Result<Self, DesError>;
     fn ser_to(&self, buf: &mut [u8]) -> usize;
 }
 
 impl SerDes for UpMsg {
-    fn des(data: &[u8]) -> Result<Self, DesError> {
-        Ok(UpMsg::Data(data.into()))
+    fn des(buf: &[u8]) -> Result<Self, DesError> {
+        Ok(UpMsg::Data {
+            payload: buf.into(),
+        })
     }
 
     fn ser_to(&self, buf: &mut [u8]) -> usize {
         match self {
-            UpMsg::Data(data) => {
-                let len = data.len();
-                buf[..len].copy_from_slice(&data[..]);
+            UpMsg::Data { payload, .. } => {
+                let len = payload.len();
+                buf[..len].copy_from_slice(&payload[..]);
                 len
             }
         }
@@ -39,15 +91,11 @@ impl SerDes for UpMsg {
 }
 
 impl SerDes for DownMsg {
-    fn des(data: &[u8]) -> Result<Self, DesError> {
-        match data.first() {
-            Some(0) => Ok(DownMsg::Ack),
-            _ => Err(DesError),
-        }
+    fn des(_buf: &[u8]) -> Result<Self, DesError> {
+        Ok(DownMsg::Ack {})
     }
 
-    fn ser_to(&self, buf: &mut [u8]) -> usize {
-        buf[0] = 0;
+    fn ser_to(&self, _buf: &mut [u8]) -> usize {
         1
     }
 }