diff --git a/Cargo.lock b/Cargo.lock
index 1b531f0..7261445 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1,19 +1,320 @@
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diff --git a/hptp/Cargo.toml b/hptp/Cargo.toml
index 957b676..60a7352 100644
--- a/hptp/Cargo.toml
+++ b/hptp/Cargo.toml
@@ -8,3 +8,5 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+tokio = {version = "0.2.*", features = ["io-std", "io-util"]}
+failure = "0.1.7"
\ No newline at end of file
diff --git a/hptp/src/lib.rs b/hptp/src/lib.rs
index 9b711df..6e5efe4 100644
--- a/hptp/src/lib.rs
+++ b/hptp/src/lib.rs
@@ -1,2 +1,4 @@
 pub const SEND_GREETING: &'static str = "*pat*";
 pub const RECV_GREETING: &'static str = "owo";
+
+pub mod log;
diff --git a/hptp/src/log.rs b/hptp/src/log.rs
new file mode 100644
index 0000000..cec02fc
--- /dev/null
+++ b/hptp/src/log.rs
@@ -0,0 +1,113 @@
+use failure::Error;
+use std::fmt::Display;
+use std::time::SystemTime;
+use tokio::io::{AsyncWriteExt, Stderr};
+
+type Result = std::result::Result<(), Error>;
+
+pub struct Logger {
+    out: Stderr,
+}
+
+impl Logger {
+    pub fn new() -> Self {
+        Logger {
+            out: tokio::io::stderr(),
+        }
+    }
+
+    async fn log_message(&mut self, msg: LogMessage) -> Result {
+        self.out.write_all(format!("{}\n", msg).as_bytes()).await?;
+        self.out.flush().await?;
+        Ok(())
+    }
+
+    async fn log_payload(&mut self, payload: LogPayload) -> Result {
+        self.log_message(LogMessage {
+            timestamp: SystemTime::now(),
+            payload,
+        })
+        .await
+    }
+
+    // -----------------------------------------------------------------------------------
+
+    pub async fn debug_msg(&mut self, what: String) -> Result {
+        self.log_payload(LogPayload::Debug { what }).await
+    }
+
+    pub async fn bound(&mut self, port: u16) -> Result {
+        self.log_payload(LogPayload::Bound { port }).await
+    }
+
+    pub async fn recv_data_accepted(
+        &mut self,
+        start: usize,
+        len: usize,
+        order: AcceptedOrder,
+    ) -> Result {
+        self.log_payload(LogPayload::RecvDataAccepted { start, len, order })
+            .await
+    }
+
+    pub async fn recv_data_ignored(&mut self) -> Result {
+        self.log_payload(LogPayload::RecvDataIgnored).await
+    }
+}
+
+pub struct LogMessage {
+    pub timestamp: SystemTime,
+    pub payload: LogPayload,
+}
+
+pub enum LogPayload {
+    Debug {
+        what: String,
+    },
+    Bound {
+        port: u16,
+    },
+    RecvDataAccepted {
+        start: usize,
+        len: usize,
+        order: AcceptedOrder,
+    },
+    RecvDataIgnored,
+}
+
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub enum AcceptedOrder {
+    InOrder,
+    OutOfOrder,
+}
+
+pub use AcceptedOrder::*;
+
+impl Display for LogMessage {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        // TODO: write timestamp
+        self.payload.fmt(f)
+    }
+}
+
+impl Display for LogPayload {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            LogPayload::Debug { what } => write!(f, "[debug] {}", what),
+            LogPayload::Bound { port } => write!(f, "[bound] {}", port),
+            LogPayload::RecvDataAccepted { start, len, order } => {
+                write!(f, "[recv data] {} ({}) ACCEPTED ({})", start, len, order)
+            }
+            LogPayload::RecvDataIgnored => write!(f, "[recv data] IGNORED"),
+        }
+    }
+}
+
+impl Display for AcceptedOrder {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        f.write_str(match self {
+            InOrder => "in-order",
+            OutOfOrder => "out-of-order",
+        })
+    }
+}