diff --git a/src/builder.rs b/src/builder.rs
index afcdb62..70b64e4 100644
--- a/src/builder.rs
+++ b/src/builder.rs
@@ -91,6 +91,16 @@ impl Builder {
         self
     }
 
+    pub fn tcp_capacity(&mut self, value: usize) -> &mut Self {
+        self.config.tcp_capacity = value;
+        self
+    }
+
+    pub fn udp_capacity(&mut self, value: usize) -> &mut Self {
+        self.config.udp_capacity = value;
+        self
+    }
+
     pub fn build<'a>(&self) -> Sim<'a> {
         self.build_with_rng(Box::new(rand::rngs::SmallRng::from_entropy()))
     }
diff --git a/src/config.rs b/src/config.rs
index 4573ced..5e3a247 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -11,6 +11,12 @@ pub(crate) struct Config {
 
     /// When the simulation starts
     pub(crate) epoch: SystemTime,
+
+    /// Max size of the tcp receive buffer
+    pub(crate) tcp_capacity: usize,
+
+    /// Max size of the udp receive buffer
+    pub(crate) udp_capacity: usize,
 }
 
 /// Configures link behavior.
@@ -52,6 +58,8 @@ impl Default for Config {
             duration: Duration::from_secs(10),
             tick: Duration::from_millis(1),
             epoch: SystemTime::now(),
+            tcp_capacity: 64,
+            udp_capacity: 64,
         }
     }
 }
diff --git a/src/host.rs b/src/host.rs
index ed0b0dc..2418367 100644
--- a/src/host.rs
+++ b/src/host.rs
@@ -40,11 +40,11 @@ pub(crate) struct Host {
 }
 
 impl Host {
-    pub(crate) fn new(addr: IpAddr) -> Host {
+    pub(crate) fn new(addr: IpAddr, tcp_capacity: usize, udp_capacity: usize) -> Host {
         Host {
             addr,
-            udp: Udp::new(),
-            tcp: Tcp::new(),
+            udp: Udp::new(udp_capacity),
+            tcp: Tcp::new(tcp_capacity),
             next_ephemeral_port: 49152,
             elapsed: Duration::ZERO,
             now: None,
@@ -138,11 +138,10 @@ struct UdpBind {
 }
 
 impl Udp {
-    fn new() -> Self {
+    fn new(capacity: usize) -> Self {
         Self {
             binds: IndexMap::new(),
-            // TODO: Make capacity configurable
-            capacity: 64,
+            capacity,
         }
     }
 
@@ -276,7 +275,7 @@ impl StreamSocket {
     // Buffer and re-order received segments by `seq` as the network may deliver
     // them out of order.
     fn buffer(&mut self, seq: u64, segment: SequencedSegment) -> Result<(), Protocol> {
-        use mpsc::error::TrySendError::Closed;
+        use mpsc::error::TrySendError::*;
 
         let exists = self.buf.insert(seq, segment);
 
@@ -288,7 +287,7 @@ impl StreamSocket {
             let segment = self.buf.remove(&self.recv_seq).unwrap();
             self.sender.try_send(segment).map_err(|e| match e {
                 Closed(_) => Protocol::Tcp(Segment::Rst),
-                _ => todo!("{} socket buffer full", self.local_addr),
+                Full(_) => panic!("{} socket buffer full", self.local_addr),
             })?;
         }
 
@@ -297,13 +296,12 @@ impl StreamSocket {
 }
 
 impl Tcp {
-    fn new() -> Self {
+    fn new(capacity: usize) -> Self {
         Self {
             binds: IndexMap::new(),
             sockets: IndexMap::new(),
-            // TODO: Make capacity configurable
-            server_socket_capacity: 64,
-            socket_capacity: 64,
+            server_socket_capacity: capacity,
+            socket_capacity: capacity,
         }
     }
 
@@ -364,7 +362,7 @@ impl Tcp {
                 // connection refused on the client.
                 if let Some(b) = self.binds.get_mut(&dst.port()) {
                     if b.deque.len() == self.server_socket_capacity {
-                        todo!("{} server socket buffer full", dst);
+                        panic!("{} server socket buffer full", dst);
                     }
 
                     if matches(b.bind_addr, dst) {
@@ -427,7 +425,7 @@ mod test {
 
     #[test]
     fn recycle_ports() -> Result {
-        let mut host = Host::new(std::net::Ipv4Addr::UNSPECIFIED.into());
+        let mut host = Host::new(std::net::Ipv4Addr::UNSPECIFIED.into(), 1, 1);
 
         host.udp.bind((host.addr, 65534).into())?;
         host.udp.bind((host.addr, 65535).into())?;
diff --git a/src/sim.rs b/src/sim.rs
index 92152a7..f8cae6b 100644
--- a/src/sim.rs
+++ b/src/sim.rs
@@ -70,7 +70,7 @@ impl<'a> Sim<'a> {
             let world = RefCell::get_mut(&mut self.world);
 
             // Register host state with the world
-            world.register(addr);
+            world.register(addr, &self.config);
         }
 
         let rt = World::enter(&self.world, || Rt::client(client));
@@ -95,7 +95,7 @@ impl<'a> Sim<'a> {
             let world = RefCell::get_mut(&mut self.world);
 
             // Register host state with the world
-            world.register(addr);
+            world.register(addr, &self.config);
         }
 
         let rt = World::enter(&self.world, || Rt::host(host));
diff --git a/src/world.rs b/src/world.rs
index 9c4343f..1fddd7d 100644
--- a/src/world.rs
+++ b/src/world.rs
@@ -1,3 +1,4 @@
+use crate::config::Config;
 use crate::envelope::Protocol;
 use crate::ip::IpVersionAddrIter;
 use crate::{config, Dns, Host, ToIpAddr, ToIpAddrs, Topology, TRACING_TARGET};
@@ -98,7 +99,7 @@ impl World {
     }
 
     /// Register a new host with the simulation.
-    pub(crate) fn register(&mut self, addr: IpAddr) {
+    pub(crate) fn register(&mut self, addr: IpAddr, config: &Config) {
         assert!(
             !self.hosts.contains_key(&addr),
             "already registered host for the given ip address"
@@ -112,7 +113,10 @@ impl World {
         }
 
         // Initialize host state
-        self.hosts.insert(addr, Host::new(addr));
+        self.hosts.insert(
+            addr,
+            Host::new(addr, config.tcp_capacity, config.udp_capacity),
+        );
     }
 
     /// Send `message` from `src` to `dst`. Delivery is asynchronous and not
diff --git a/tests/tcp.rs b/tests/tcp.rs
index 43b595f..f792ffc 100644
--- a/tests/tcp.rs
+++ b/tests/tcp.rs
@@ -910,3 +910,33 @@ fn remote_dropped() -> Result {
 
     sim.run()
 }
+
+#[test]
+#[should_panic(expected = "192.168.0.1:80 server socket buffer full")]
+fn socket_capacity() {
+    let mut sim = Builder::new()
+        .min_message_latency(Duration::from_millis(1))
+        .max_message_latency(Duration::from_millis(1))
+        .tcp_capacity(1)
+        .build();
+
+    sim.host("server", || async {
+        let l = TcpListener::bind(("0.0.0.0", 80)).await?;
+
+        loop {
+            _ = l.accept().await?;
+        }
+    });
+
+    sim.client("client1", async move {
+        _ = TcpStream::connect("server:80").await?;
+        Ok(())
+    });
+
+    sim.client("client2", async move {
+        _ = TcpStream::connect("server:80").await?;
+        Ok(())
+    });
+
+    _ = sim.run();
+}
diff --git a/tests/udp.rs b/tests/udp.rs
index 3a9360c..76beadd 100644
--- a/tests/udp.rs
+++ b/tests/udp.rs
@@ -559,3 +559,38 @@ fn localhost_ping_pong() -> Result {
     });
     sim.run()
 }
+
+#[test]
+fn socket_capacity() -> Result {
+    let mut sim = Builder::new()
+        .min_message_latency(Duration::from_millis(1))
+        .max_message_latency(Duration::from_millis(1))
+        .udp_capacity(1)
+        .build();
+
+    let (tx, rx) = oneshot::channel();
+
+    sim.client("server", async move {
+        let s = bind().await?;
+
+        _ = rx.await;
+        recv_ping(&s).await?;
+        assert!(timeout(Duration::from_secs(1), recv_ping(&s))
+            .await
+            .is_err());
+
+        Ok(())
+    });
+
+    sim.client("client", async move {
+        let s = bind().await?;
+
+        send_ping(&s).await?;
+        send_ping(&s).await?; // dropped
+        _ = tx.send(());
+
+        Ok(())
+    });
+
+    sim.run()
+}