diff --git a/artiq/runtime.rs/Cargo.lock b/artiq/runtime.rs/Cargo.lock
index 1f5913d15..eb4659fcb 100644
--- a/artiq/runtime.rs/Cargo.lock
+++ b/artiq/runtime.rs/Cargo.lock
@@ -5,7 +5,7 @@ dependencies = [
  "byteorder 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "fringe 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
- "log_buffer 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "log_buffer 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "lwip 0.0.0",
  "std_artiq 0.0.0",
  "walkdir 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -49,7 +49,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "log_buffer"
-version = "1.0.0"
+version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
@@ -96,7 +96,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum kernel32-sys 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "7507624b29483431c0ba2d82aece8ca6cdba9382bff4ddd0f7490560c056098d"
 "checksum libc 0.2.15 (registry+https://github.com/rust-lang/crates.io-index)" = "23e3757828fa702a20072c37ff47938e9dd331b92fac6e223d26d4b7a55f7ee2"
 "checksum log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)" = "ab83497bf8bf4ed2a74259c1c802351fcd67a65baa86394b6ba73c36f4838054"
-"checksum log_buffer 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "f8beb5ba24eca52f9958874445c4de5e086a7e82a1ec6b7ab81e5fcfb134f25a"
+"checksum log_buffer 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ec57723b84bbe7bdf76aa93169c9b59e67473317c6de3a83cb2a0f8ccb2aa493"
 "checksum walkdir 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)" = "c66c0b9792f0a765345452775f3adbd28dde9d33f30d13e5dcc5ae17cf6f3780"
 "checksum winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
 "checksum winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"
diff --git a/artiq/runtime.rs/Cargo.toml b/artiq/runtime.rs/Cargo.toml
index aba3d1aae..725da7533 100644
--- a/artiq/runtime.rs/Cargo.toml
+++ b/artiq/runtime.rs/Cargo.toml
@@ -14,7 +14,7 @@ path = "src/lib.rs"
 
 [dependencies]
 std_artiq = { path = "libstd_artiq" }
-lwip = { path = "liblwip" }
+lwip = { path = "liblwip", default-features = false }
 fringe = { version = "= 1.1.0", default-features = false, features = ["alloc"] }
 log = { version = "0.3", default-features = false }
 log_buffer = { version = "1.0" }
diff --git a/artiq/runtime.rs/liblwip/Cargo.toml b/artiq/runtime.rs/liblwip/Cargo.toml
index cd76fe969..886f69415 100644
--- a/artiq/runtime.rs/liblwip/Cargo.toml
+++ b/artiq/runtime.rs/liblwip/Cargo.toml
@@ -10,3 +10,7 @@ path = "lib.rs"
 [dependencies]
 lwip-sys = { path = "../liblwip-sys" }
 std_artiq = { path = "../libstd_artiq" }
+
+[features]
+default = ["preemption"]
+preemption = []
diff --git a/artiq/runtime.rs/liblwip/lib.rs b/artiq/runtime.rs/liblwip/lib.rs
index f8938653b..c996074f0 100644
--- a/artiq/runtime.rs/liblwip/lib.rs
+++ b/artiq/runtime.rs/liblwip/lib.rs
@@ -175,6 +175,9 @@ pub struct Pbuf<'payload> {
     phantom: PhantomData<&'payload [u8]>
 }
 
+#[cfg(not(feature = "preemption"))]
+unsafe impl<'payload> Send for Pbuf<'payload> {}
+
 impl<'payload> Pbuf<'payload> {
     unsafe fn from_raw(raw: *mut lwip_sys::pbuf) -> Pbuf<'payload> {
         Pbuf { raw: raw, phantom: PhantomData }
@@ -259,6 +262,9 @@ pub struct UdpSocket {
     state: Box<RefCell<UdpSocketState>>
 }
 
+#[cfg(not(feature = "preemption"))]
+unsafe impl Send for UdpSocket {}
+
 impl UdpSocket {
     pub fn new() -> Result<UdpSocket> {
         extern fn recv(arg: *mut c_void, _pcb: *mut lwip_sys::udp_pcb,
@@ -347,6 +353,9 @@ pub struct TcpListener {
     state: Box<RefCell<TcpListenerState>>
 }
 
+#[cfg(not(feature = "preemption"))]
+unsafe impl Send for TcpListener {}
+
 impl TcpListener {
     pub fn bind(addr: SocketAddr) -> Result<TcpListener> {
         extern fn accept(arg: *mut c_void, newpcb: *mut lwip_sys::tcp_pcb,
@@ -428,6 +437,9 @@ pub struct TcpStream {
     state: Box<RefCell<TcpStreamState>>
 }
 
+#[cfg(not(feature = "preemption"))]
+unsafe impl Send for TcpStream {}
+
 impl TcpStream {
     fn from_raw(raw: *mut lwip_sys::tcp_pcb) -> TcpStream {
         extern fn recv(arg: *mut c_void, _raw: *mut lwip_sys::tcp_pcb,
diff --git a/artiq/runtime.rs/src/kernel.rs b/artiq/runtime.rs/src/kernel.rs
index 4967ec597..050b57ec4 100644
--- a/artiq/runtime.rs/src/kernel.rs
+++ b/artiq/runtime.rs/src/kernel.rs
@@ -15,14 +15,14 @@ pub unsafe fn start() {
     stop();
 
     extern {
-        static _binary_ksupport_elf_start: ();
-        static _binary_ksupport_elf_end: ();
+        static _binary_ksupport_elf_start: u8;
+        static _binary_ksupport_elf_end: u8;
     }
-    let ksupport_start = &_binary_ksupport_elf_start as *const _ as usize;
-    let ksupport_end   = &_binary_ksupport_elf_end as *const _ as usize;
-    ptr::copy_nonoverlapping(ksupport_start as *const u8,
+    let ksupport_start = &_binary_ksupport_elf_start as *const _;
+    let ksupport_end   = &_binary_ksupport_elf_end as *const _;
+    ptr::copy_nonoverlapping(ksupport_start,
                              (KERNELCPU_EXEC_ADDRESS - KSUPPORT_HEADER_SIZE) as *mut u8,
-                             ksupport_end - ksupport_start);
+                             ksupport_end as usize - ksupport_start as usize);
 
     csr::kernel_cpu::reset_write(0);
 }
diff --git a/artiq/runtime.rs/src/lib.rs b/artiq/runtime.rs/src/lib.rs
index 8070552fc..9b913c9dd 100644
--- a/artiq/runtime.rs/src/lib.rs
+++ b/artiq/runtime.rs/src/lib.rs
@@ -1,5 +1,5 @@
 #![no_std]
-#![feature(libc)]
+#![feature(libc, borrow_state, const_fn)]
 
 #[macro_use]
 extern crate std_artiq as std;
@@ -8,16 +8,19 @@ extern crate libc;
 extern crate log;
 extern crate log_buffer;
 extern crate byteorder;
+extern crate fringe;
+extern crate lwip;
 
 use logger::BufferLogger;
 
 mod board;
-mod sched;
 mod config;
 mod clock;
 mod rtio_crg;
 mod mailbox;
 
+mod urc;
+mod sched;
 mod logger;
 mod cache;
 
@@ -36,9 +39,9 @@ include!(concat!(env!("OUT_DIR"), "/git_info.rs"));
 
 #[no_mangle]
 pub unsafe extern fn rust_main() {
-    static mut log_buffer: [u8; 4096] = [0; 4096];
-    BufferLogger::new(&mut log_buffer[..])
-                 .register(move |logger| {
+    static mut LOG_BUFFER: [u8; 4096] = [0; 4096];
+    BufferLogger::new(&mut LOG_BUFFER[..])
+                 .register(move || {
         info!("booting ARTIQ runtime ({})", GIT_COMMIT);
 
         clock::init();
@@ -46,13 +49,11 @@ pub unsafe extern fn rust_main() {
         network_init();
 
         let mut scheduler = sched::Scheduler::new();
-        scheduler.spawn(8192, move |waiter| {
-            session::handler(waiter, logger)
-        });
+        scheduler.spawner().spawn(8192, session::handler);
 
         loop {
+            scheduler.run();
             lwip_service();
-            scheduler.run()
         }
     })
 }
diff --git a/artiq/runtime.rs/src/logger.rs b/artiq/runtime.rs/src/logger.rs
index 377470a2e..d09995bf7 100644
--- a/artiq/runtime.rs/src/logger.rs
+++ b/artiq/runtime.rs/src/logger.rs
@@ -1,3 +1,4 @@
+use core::{mem, ptr};
 use core::cell::RefCell;
 use log::{self, Log, LogMetadata, LogRecord, LogLevelFilter};
 use log_buffer::LogBuffer;
@@ -6,9 +7,10 @@ pub struct BufferLogger {
     buffer: RefCell<LogBuffer<&'static mut [u8]>>
 }
 
-// We can never preempt from within the logger, so there can be no data races.
 unsafe impl Sync for BufferLogger {}
 
+static mut LOGGER: *const BufferLogger = ptr::null();
+
 impl BufferLogger {
     pub fn new(buffer: &'static mut [u8]) -> BufferLogger {
         BufferLogger {
@@ -16,7 +18,7 @@ impl BufferLogger {
         }
     }
 
-    pub fn register<F: FnOnce(&BufferLogger)>(&self, f: F) {
+    pub fn register<F: FnOnce()>(&self, f: F) {
         // log::set_logger_raw captures a pointer to ourselves, so we must prevent
         // ourselves from being moved or dropped after that function is called (and
         // before log::shutdown_logger_raw is called).
@@ -25,9 +27,17 @@ impl BufferLogger {
                 max_log_level.set(LogLevelFilter::Trace);
                 self as *const Log
             }).expect("global logger can only be initialized once");
+            LOGGER = self;
         }
-        f(self);
+        f();
         log::shutdown_logger_raw().unwrap();
+        unsafe {
+            LOGGER = ptr::null();
+        }
+    }
+
+    pub fn with_instance<R, F: FnOnce(&BufferLogger) -> R>(f: F) -> R {
+        f(unsafe { mem::transmute::<*const BufferLogger, &BufferLogger>(LOGGER) })
     }
 
     pub fn clear(&self) {
diff --git a/artiq/runtime.rs/src/sched.rs b/artiq/runtime.rs/src/sched.rs
index 1e7b58a5a..124af9410 100644
--- a/artiq/runtime.rs/src/sched.rs
+++ b/artiq/runtime.rs/src/sched.rs
@@ -1,14 +1,14 @@
 #![allow(dead_code)]
 
-extern crate fringe;
-extern crate lwip;
-
-use std::cell::RefCell;
+use std::cell::{RefCell, BorrowState};
 use std::vec::Vec;
+use std::rc::Rc;
 use std::time::{Instant, Duration};
 use std::io::{Read, Write, Result, Error, ErrorKind};
-use self::fringe::OwnedStack;
-use self::fringe::generator::{Generator, Yielder};
+use fringe::OwnedStack;
+use fringe::generator::{Generator, Yielder, State as GeneratorState};
+use lwip;
+use urc::Urc;
 
 #[derive(Debug)]
 struct WaitRequest {
@@ -25,38 +25,87 @@ enum WaitResult {
 
 #[derive(Debug)]
 struct Thread {
-    generator:   Generator<WaitResult, WaitRequest, OwnedStack>,
+    generator: Generator<WaitResult, WaitRequest, OwnedStack>,
     waiting_for: WaitRequest,
     interrupted: bool
 }
 
-#[derive(Debug)]
-pub struct Scheduler {
-    threads: Vec<Thread>,
-    index:   usize
-}
-
-impl Scheduler {
-    pub fn new() -> Scheduler {
-        Scheduler { threads: Vec::new(), index: 0 }
-    }
-
-    pub unsafe fn spawn<F: FnOnce(Waiter) + Send>(&mut self, stack_size: usize, f: F) {
+impl Thread {
+    unsafe fn new<F>(spawner: Spawner, stack_size: usize, f: F) -> ThreadHandle
+            where F: 'static + FnOnce(Waiter, Spawner) + Send {
         let stack = OwnedStack::new(stack_size);
-        let thread = Thread {
-            generator:   Generator::unsafe_new(stack, move |yielder, _| {
-                f(Waiter(yielder))
+        ThreadHandle::new(Thread {
+            generator: Generator::unsafe_new(stack, |yielder, _| {
+                f(Waiter(yielder), spawner)
             }),
             waiting_for: WaitRequest {
                 timeout: None,
                 event:   None
             },
             interrupted: false
-        };
-        self.threads.push(thread)
+        })
+    }
+
+    pub fn terminated(&self) -> bool {
+        // FIXME: https://github.com/nathan7/libfringe/pull/56
+        match self.generator.state() {
+            GeneratorState::Unavailable => true,
+            GeneratorState::Runnable => false
+        }
+    }
+
+    pub fn interrupt(&mut self) {
+        self.interrupted = true
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ThreadHandle(Urc<RefCell<Thread>>);
+
+impl ThreadHandle {
+    fn new(thread: Thread) -> ThreadHandle {
+        ThreadHandle(Urc::new(RefCell::new(thread)))
+    }
+
+    pub fn terminated(&self) -> bool {
+        match self.0.borrow_state() {
+            BorrowState::Unused => self.0.borrow().terminated(),
+            _ => false // the running thread hasn't terminated
+        }
+    }
+
+    pub fn interrupt(&self) {
+        // FIXME: use try_borrow() instead once it's available
+        match self.0.borrow_state() {
+            BorrowState::Unused => self.0.borrow_mut().interrupt(),
+            _ => panic!("cannot interrupt the running thread")
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Scheduler {
+    threads: Vec<ThreadHandle>,
+    index: usize,
+    spawner: Spawner
+}
+
+impl Scheduler {
+    pub fn new() -> Scheduler {
+        Scheduler {
+            threads: Vec::new(),
+            index: 0,
+            spawner: Spawner::new()
+        }
+    }
+
+    pub fn spawner(&self) -> &Spawner {
+        &self.spawner
     }
 
     pub fn run(&mut self) {
+        self.threads.append(&mut *self.spawner.queue.borrow_mut());
+
         if self.threads.len() == 0 { return }
 
         let now = Instant::now();
@@ -66,7 +115,7 @@ impl Scheduler {
             self.index = (self.index + 1) % self.threads.len();
 
             let result = {
-                let thread = &mut self.threads[self.index];
+                let thread = &mut *self.threads[self.index].0.borrow_mut();
                 match thread.waiting_for {
                     _ if thread.interrupted => {
                         thread.interrupted = false;
@@ -97,7 +146,8 @@ impl Scheduler {
                 },
                 Some(wait_request) => {
                     // The thread has suspended itself.
-                    self.threads[self.index].waiting_for = wait_request
+                    let thread = &mut *self.threads[self.index].0.borrow_mut();
+                    thread.waiting_for = wait_request
                 }
             }
 
@@ -106,8 +156,27 @@ impl Scheduler {
     }
 }
 
+#[derive(Debug, Clone)]
+pub struct Spawner {
+    queue: Urc<RefCell<Vec<ThreadHandle>>>
+}
+
+impl Spawner {
+    fn new() -> Spawner {
+        Spawner { queue: Urc::new(RefCell::new(Vec::new())) }
+    }
+
+    pub fn spawn<F>(&self, stack_size: usize, f: F) -> ThreadHandle
+            where F: 'static + FnOnce(Waiter, Spawner) + Send {
+        let handle = unsafe { Thread::new(self.clone(), stack_size, f) };
+        self.queue.borrow_mut().push(handle.clone());
+        handle
+    }
+}
+
 enum WaitEvent {
     Completion(*const (Fn() -> bool + 'static)),
+    Termination(*const RefCell<Thread>),
     UdpReadable(*const RefCell<lwip::UdpSocketState>),
     TcpAcceptable(*const RefCell<lwip::TcpListenerState>),
     TcpWriteable(*const RefCell<lwip::TcpStreamState>),
@@ -119,6 +188,8 @@ impl WaitEvent {
         match *self {
             WaitEvent::Completion(f) =>
                 unsafe { (*f)() },
+            WaitEvent::Termination(thread) =>
+                unsafe { (*thread).borrow().terminated() },
             WaitEvent::UdpReadable(state) =>
                 unsafe { (*state).borrow().readable() },
             WaitEvent::TcpAcceptable(state) =>
@@ -173,6 +244,13 @@ impl<'a> Waiter<'a> {
         }
     }
 
+    pub fn join(&self, thread: ThreadHandle) -> Result<()> {
+        self.suspend(WaitRequest {
+            timeout: None,
+            event:   Some(WaitEvent::Termination(&*thread.0))
+        })
+    }
+
     pub fn until<F: Fn() -> bool + 'static>(&self, f: F) -> Result<()> {
         self.suspend(WaitRequest {
             timeout: None,
@@ -211,7 +289,7 @@ impl<'a> Waiter<'a> {
 
 // Wrappers around lwip
 
-pub use self::lwip::{IpAddr, IP4_ANY, IP6_ANY, IP_ANY, SocketAddr};
+pub use lwip::{IpAddr, IP4_ANY, IP6_ANY, IP_ANY, SocketAddr};
 
 #[derive(Debug)]
 pub struct UdpSocket<'a> {
@@ -227,6 +305,14 @@ impl<'a> UdpSocket<'a> {
         })
     }
 
+    pub fn into_lower(self) -> lwip::UdpSocket {
+        self.lower
+    }
+
+    pub fn from_lower(waiter: Waiter<'a>, inner: lwip::UdpSocket) -> UdpSocket {
+        UdpSocket { waiter: waiter, lower: inner }
+    }
+
     pub fn bind(&self, addr: SocketAddr) -> Result<()> {
         Ok(try!(self.lower.bind(addr)))
     }
@@ -285,6 +371,14 @@ impl<'a> TcpListener<'a> {
         })
     }
 
+    pub fn into_lower(self) -> lwip::TcpListener {
+        self.lower
+    }
+
+    pub fn from_lower(waiter: Waiter<'a>, inner: lwip::TcpListener) -> TcpListener {
+        TcpListener { waiter: waiter, lower: inner }
+    }
+
     pub fn accept(&self) -> Result<(TcpStream, SocketAddr)> {
         try!(self.waiter.tcp_acceptable(&self.lower));
         let stream_lower = self.lower.try_accept().unwrap();
@@ -301,7 +395,9 @@ impl<'a> TcpListener<'a> {
     }
 }
 
-pub use self::lwip::Shutdown;
+pub use lwip::Shutdown;
+
+pub struct TcpStreamInner(lwip::TcpStream, Option<(lwip::Pbuf<'static>, usize)>);
 
 #[derive(Debug)]
 pub struct TcpStream<'a> {
@@ -311,6 +407,14 @@ pub struct TcpStream<'a> {
 }
 
 impl<'a> TcpStream<'a> {
+    pub fn into_lower(self) -> TcpStreamInner {
+        TcpStreamInner(self.lower, self.buffer)
+    }
+
+    pub fn from_lower(waiter: Waiter<'a>, inner: TcpStreamInner) -> TcpStream {
+        TcpStream { waiter: waiter, lower: inner.0, buffer: inner.1 }
+    }
+
     pub fn shutdown(&self, how: Shutdown) -> Result<()> {
         Ok(try!(self.lower.shutdown(how)))
     }
diff --git a/artiq/runtime.rs/src/session.rs b/artiq/runtime.rs/src/session.rs
index 56d7d81f6..3b280498c 100644
--- a/artiq/runtime.rs/src/session.rs
+++ b/artiq/runtime.rs/src/session.rs
@@ -1,11 +1,13 @@
 use std::prelude::v1::*;
-use std::mem;
-use std::str;
+use std::{mem, str};
+use std::cell::RefCell;
 use std::io::{self, Read};
 use {config, rtio_crg, clock, mailbox, kernel};
 use logger::BufferLogger;
 use cache::Cache;
-use sched::{Waiter, TcpListener, TcpStream, SocketAddr, IP_ANY};
+use urc::Urc;
+use sched::{ThreadHandle, Waiter, Spawner};
+use sched::{TcpListener, TcpStream, SocketAddr, IP_ANY};
 
 use session_proto as host;
 use kernel_proto as kern;
@@ -49,14 +51,16 @@ enum KernelState {
 
 // Per-connection state
 #[derive(Debug)]
-struct Session {
+struct Session<'a> {
+    congress: &'a mut Congress,
     kernel_state: KernelState,
     watchdog_set: clock::WatchdogSet
 }
 
-impl Session {
-    fn new() -> Session {
+impl<'a> Session<'a> {
+    fn new(congress: &mut Congress) -> Session {
         Session {
+            congress: congress,
             kernel_state: KernelState::Absent,
             watchdog_set: clock::WatchdogSet::new()
         }
@@ -70,7 +74,7 @@ impl Session {
     }
 }
 
-impl Drop for Session {
+impl<'a> Drop for Session<'a> {
     fn drop(&mut self) {
         kernel::stop()
     }
@@ -123,10 +127,9 @@ fn kern_acknowledge() -> io::Result<()> {
     Ok(())
 }
 
-fn comm_handle(logger: &BufferLogger,
-               waiter: Waiter,
-               stream: &mut TcpStream,
-               session: &mut Session) -> io::Result<()> {
+fn process_host_message(waiter: Waiter,
+                        stream: &mut TcpStream,
+                        session: &mut Session) -> io::Result<()> {
     match try!(host_read(stream)) {
         host::Request::Ident =>
             host_write(stream, host::Reply::Ident(::board::ident(&mut [0; 64]))),
@@ -135,13 +138,15 @@ fn comm_handle(logger: &BufferLogger,
         host::Request::Log => {
             // Logging the packet with the log is inadvisable
             trace!("comm->host Log(...)");
-            logger.extract(move |log| {
-                host::Reply::Log(log).write_to(stream)
+            BufferLogger::with_instance(|logger| {
+                logger.extract(|log| {
+                    host::Reply::Log(log).write_to(stream)
+                })
             })
         }
 
         host::Request::LogClear => {
-            logger.clear();
+            BufferLogger::with_instance(|logger| logger.clear());
             host_write(stream, host::Reply::Log(""))
         }
 
@@ -221,9 +226,8 @@ fn comm_handle(logger: &BufferLogger,
     }
 }
 
-fn kern_handle(waiter: Waiter,
-               congress: &mut Congress,
-               session: &mut Session) -> io::Result<()> {
+fn process_kern_message(waiter: Waiter,
+                        session: &mut Session) -> io::Result<()> {
     kern::Message::wait_and_receive(waiter, |request| {
         match (&request, session.kernel_state) {
             (&kern::LoadReply { .. }, KernelState::Loaded) |
@@ -246,10 +250,10 @@ fn kern_handle(waiter: Waiter,
             }
 
             kern::NowInitRequest =>
-                kern_send(waiter, kern::NowInitReply(congress.now)),
+                kern_send(waiter, kern::NowInitReply(session.congress.now)),
 
             kern::NowSave(now) => {
-                congress.now = now;
+                session.congress.now = now;
                 kern_acknowledge()
             }
 
@@ -265,14 +269,14 @@ fn kern_handle(waiter: Waiter,
             }
 
             kern::CacheGetRequest { key } => {
-                let value = congress.cache.get(key);
+                let value = session.congress.cache.get(key);
                 kern_send(waiter, kern::CacheGetReply {
                     value: unsafe { mem::transmute::<*const [u32], &'static [u32]>(value) }
                 })
             }
 
             kern::CachePutRequest { key, value } => {
-                let succeeded = congress.cache.put(key, value).is_ok();
+                let succeeded = session.congress.cache.put(key, value).is_ok();
                 kern_send(waiter, kern::CachePutReply { succeeded: succeeded })
             }
 
@@ -281,20 +285,17 @@ fn kern_handle(waiter: Waiter,
     })
 }
 
-fn handle(logger: &BufferLogger,
-          waiter: Waiter,
-          stream: &mut TcpStream,
-          congress: &mut Congress) -> io::Result<()> {
-    try!(check_magic(stream));
-
-    let mut session = Session::new();
+fn host_kernel_worker(waiter: Waiter,
+                      stream: &mut TcpStream,
+                      congress: &mut Congress) -> io::Result<()> {
+    let mut session = Session::new(congress);
     loop {
         if stream.readable() {
-            try!(comm_handle(logger, waiter, stream, &mut session))
+            try!(process_host_message(waiter, stream, &mut session));
         }
 
         if mailbox::receive() != 0 {
-            try!(kern_handle(waiter, congress, &mut session))
+            try!(process_kern_message(waiter, &mut session))
         }
 
         if session.kernel_state == KernelState::Running {
@@ -313,27 +314,79 @@ fn handle(logger: &BufferLogger,
     }
 }
 
-pub fn handler(waiter: Waiter,
-               logger: &BufferLogger) {
-    let mut congress = Congress::new();
-
-    let addr = SocketAddr::new(IP_ANY, 1381);
-    let listener = TcpListener::bind(waiter, addr).unwrap();
-    info!("accepting network sessions in Rust");
-
+fn flash_kernel_worker(waiter: Waiter,
+                       congress: &mut Congress) -> io::Result<()> {
+    let mut session = Session::new(congress);
     loop {
-        let (mut stream, addr) = listener.accept().unwrap();
-        info!("new connection from {:?}", addr);
-
-        match handle(logger, waiter, &mut stream, &mut congress) {
-            Ok(()) => (),
-            Err(err) => {
-                if err.kind() == io::ErrorKind::UnexpectedEof {
-                    info!("connection closed");
-                } else {
-                    error!("session aborted: {:?}", err);
-                }
-            }
-        }
+        try!(process_kern_message(waiter, &mut session))
+    }
+}
+
+fn respawn<F>(spawner: Spawner, waiter: Waiter,
+              handle: &mut Option<ThreadHandle>,
+              f: F) where F: 'static + FnOnce(Waiter, Spawner) + Send {
+    match handle.take() {
+        None => (),
+        Some(handle) => {
+            info!("terminating running kernel");
+            handle.interrupt();
+            waiter.join(handle).expect("cannot join interrupt thread")
+        }
+    }
+
+    *handle = Some(spawner.spawn(8192, f))
+}
+
+pub fn handler(waiter: Waiter, spawner: Spawner) {
+    let congress = Urc::new(RefCell::new(Congress::new()));
+
+    let addr = SocketAddr::new(IP_ANY, 1381);
+    let listener = TcpListener::bind(waiter, addr).expect("cannot bind socket");
+    info!("accepting network sessions in Rust");
+
+    let mut kernel_thread = None;
+    loop {
+        if listener.acceptable() {
+            let (mut stream, addr) = listener.accept().expect("cannot accept client");
+            match check_magic(&mut stream) {
+                Ok(()) => (),
+                Err(_) => continue
+            }
+            info!("new connection from {:?}", addr);
+
+            let stream = stream.into_lower();
+            let congress = congress.clone();
+            respawn(spawner.clone(), waiter, &mut kernel_thread, move |waiter, _spawner| {
+                let mut stream = TcpStream::from_lower(waiter, stream);
+                let mut congress = congress.borrow_mut();
+                match host_kernel_worker(waiter, &mut stream, &mut congress) {
+                    Ok(()) => (),
+                    Err(err) => {
+                        if err.kind() == io::ErrorKind::UnexpectedEof {
+                            info!("connection closed");
+                        } else {
+                            error!("session aborted: {:?}", err);
+                        }
+                    }
+                }
+            })
+        }
+
+        if kernel_thread.is_none() {
+            info!("no connection, starting idle kernel");
+            let congress = congress.clone();
+            respawn(spawner.clone(), waiter, &mut kernel_thread, move |waiter, _spawner| {
+                let mut congress = congress.borrow_mut();
+                match flash_kernel_worker(waiter, &mut congress) {
+                    Ok(()) =>
+                        info!("idle kernel finished, standing by"),
+                    Err(err) => {
+                        error!("idle kernel aborted: {:?}", err);
+                    }
+                }
+            })
+        }
+
+        waiter.relinquish()
     }
 }
diff --git a/artiq/runtime.rs/src/urc.rs b/artiq/runtime.rs/src/urc.rs
new file mode 100644
index 000000000..3ca67f9ec
--- /dev/null
+++ b/artiq/runtime.rs/src/urc.rs
@@ -0,0 +1,31 @@
+use std::rc::Rc;
+use std::ops::{Deref, DerefMut};
+use std::fmt;
+
+pub struct Urc<T: ?Sized>(Rc<T>);
+
+impl<T> Urc<T> {
+    pub fn new(value: T) -> Urc<T> { Urc(Rc::new(value)) }
+}
+
+unsafe impl<T: ?Sized> Send for Urc<T> {}
+
+unsafe impl<T: ?Sized> Sync for Urc<T> {}
+
+impl<T: ?Sized> Deref for Urc<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target { self.0.deref() }
+}
+
+impl<T: ?Sized> Clone for Urc<T> {
+    fn clone(&self) -> Urc<T> {
+        Urc(self.0.clone())
+    }
+}
+
+impl<T: ?Sized + fmt::Debug> fmt::Debug for Urc<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}