diff --git a/src/frame/data.rs b/src/frame/data.rs
index cfe5516..8945595 100644
--- a/src/frame/data.rs
+++ b/src/frame/data.rs
@@ -1,4 +1,3 @@
-use FrameSize;
 use frame::{util, Frame, Head, Error, StreamId, Kind};
 use bytes::{BufMut, Bytes, Buf};
 
@@ -55,16 +54,16 @@ impl<T> Data<T> {
         Head::new(Kind::Data, self.flags.into(), self.stream_id)
     }
 
+    pub fn payload(&self) -> &T {
+        &self.data
+    }
+
     pub fn into_payload(self) -> T {
         self.data
     }
 }
 
 impl<T: Buf> Data<T> {
-    pub fn len(&self) -> usize {
-        self.data.remaining()
-    }
-
     pub fn from_buf(stream_id: StreamId, data: T, eos: bool) -> Self {
         let mut flags = DataFlag::default();
         if eos {
@@ -80,7 +79,7 @@ impl<T: Buf> Data<T> {
     }
 
     pub fn encode_chunk<U: BufMut>(&mut self, dst: &mut U) {
-        let len = self.len() as usize;
+        let len = self.data.remaining() as usize;
         if len > dst.remaining_mut() {
             unimplemented!();
         }
diff --git a/src/lib.rs b/src/lib.rs
index 83352bf..00956cf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -53,8 +53,6 @@ pub enum Frame<T, B = Bytes> {
     Data {
         id: StreamId,
         data: B,
-        /// TODO figure out how to make this a requirement on `B`
-        //data_len: FrameSize,
         end_of_stream: bool,
     },
     Trailers {
diff --git a/src/proto/connection.rs b/src/proto/connection.rs
index 8d9fef1..92f09ba 100644
--- a/src/proto/connection.rs
+++ b/src/proto/connection.rs
@@ -1,4 +1,4 @@
-use {ConnectionError, Frame, FrameSize};
+use {ConnectionError, Frame};
 use client::Client;
 use error;
 use frame::{self, SettingSet, StreamId};
@@ -91,7 +91,6 @@ impl<T, P> Connection<T, P, Bytes>
     {
         self.send(Frame::Data {
             id,
-            data_len: data.len() as FrameSize,
             data,
             end_of_stream,
         })
@@ -176,7 +175,6 @@ impl<T, P, B> Stream for Connection<T, P, B>
                 Some(Data(v)) => Frame::Data {
                     id: v.stream_id(),
                     end_of_stream: v.is_end_stream(),
-                    //data_len: v.len(),
                     data: v.into_payload(),
                 },
 
diff --git a/src/proto/flow_control.rs b/src/proto/flow_control.rs
index f8f89f1..cb4f20e 100644
--- a/src/proto/flow_control.rs
+++ b/src/proto/flow_control.rs
@@ -1,5 +1,4 @@
-use ConnectionError;
-use error;
+use {error, ConnectionError, FrameSize};
 use frame::{self, Frame};
 use proto::*;
 
@@ -8,8 +7,8 @@ use std::collections::VecDeque;
 #[derive(Debug)]
 pub struct FlowControl<T>  {
     inner: T,
-    initial_local_window_size: u32,
-    initial_remote_window_size: u32,
+    initial_local_window_size: WindowSize,
+    initial_remote_window_size: WindowSize,
 
     /// Tracks the connection-level flow control window for receiving data from the
     /// remote.
@@ -37,8 +36,8 @@ impl<T, U> FlowControl<T>
           T: Sink<SinkItem = Frame<U>, SinkError = ConnectionError>,
           T: ControlStreams
 {
-    pub fn new(initial_local_window_size: u32,
-               initial_remote_window_size: u32,
+    pub fn new(initial_local_window_size: WindowSize,
+               initial_remote_window_size: WindowSize,
                inner: T)
         -> FlowControl<T>
     {
@@ -250,11 +249,12 @@ impl<T> Stream for FlowControl<T>
                 }
 
                 Some(Data(v)) => {
-                    if self.connection_local_flow_controller.claim_window(v.len()).is_err() {
+                    let sz = v.payload().len() as FrameSize;
+                    if self.connection_local_flow_controller.claim_window(sz).is_err() {
                         return Err(error::Reason::FlowControlError.into())
                     }
                     if let Some(fc) = self.local_flow_controller(v.stream_id()) {
-                        if fc.claim_window(v.len()).is_err() {
+                        if fc.claim_window(sz).is_err() {
                             return Err(error::Reason::FlowControlError.into())
                         }
                     }
@@ -271,6 +271,7 @@ impl<T, U> Sink for FlowControl<T>
     where T: Sink<SinkItem = Frame<U>, SinkError = ConnectionError>,
           T: ReadySink,
           T: ControlStreams,
+          U: Buf,
  {
     type SinkItem = T::SinkItem;
     type SinkError = T::SinkError;
@@ -289,11 +290,12 @@ impl<T, U> Sink for FlowControl<T>
                     return Ok(AsyncSink::NotReady(Data(v)));
                 }
 
-                if self.connection_remote_flow_controller.claim_window(v.len()).is_err() {
+                let sz = v.payload().remaining() as FrameSize;
+                if self.connection_remote_flow_controller.claim_window(sz).is_err() {
                     return Err(error::User::FlowControlViolation.into());
                 }
                 if let Some(fc) = self.remote_flow_controller(v.stream_id()) {
-                    if fc.claim_window(v.len()).is_err() {
+                    if fc.claim_window(sz).is_err() {
                         return Err(error::User::FlowControlViolation.into())
                     }
                 }
@@ -318,6 +320,7 @@ impl<T, U> ReadySink for FlowControl<T>
           T: Sink<SinkItem = Frame<U>, SinkError = ConnectionError>,
           T: ReadySink,
           T: ControlStreams,
+          U: Buf,
 {
     fn poll_ready(&mut self) -> Poll<(), ConnectionError> {
         try_ready!(self.inner.poll_ready());
diff --git a/src/proto/framed_write.rs b/src/proto/framed_write.rs
index ace33ae..fa0bc4d 100644
--- a/src/proto/framed_write.rs
+++ b/src/proto/framed_write.rs
@@ -74,7 +74,7 @@ impl<T, B> FramedWrite<T, B>
     }
 
     fn frame_len(&self, data: &frame::Data<B>) -> usize {
-        cmp::min(self.max_frame_size, data.len()) as usize
+        cmp::min(self.max_frame_size as usize, data.payload().remaining())
     }
 }
 
@@ -104,7 +104,7 @@ impl<T, B> Sink for FramedWrite<T, B>
 
         match item {
             Frame::Data(mut v) => {
-                if v.len() >= (CHAIN_THRESHOLD as FrameSize) {
+                if v.payload().remaining() >= CHAIN_THRESHOLD {
                     let head = v.head();
                     let len = self.frame_len(&v);
 
@@ -121,7 +121,7 @@ impl<T, B> Sink for FramedWrite<T, B>
 
                     // The chunk has been fully encoded, so there is no need to
                     // keep it around
-                    assert_eq!(v.len(), 0, "chunk not fully encoded");
+                    assert_eq!(v.payload().remaining(), 0, "chunk not fully encoded");
                 }
             }
             Frame::Headers(v) => {