diff --git a/src/crdt.rs b/src/crdt.rs
index 4c9e294870..58cb8db1fc 100644
--- a/src/crdt.rs
+++ b/src/crdt.rs
@@ -32,6 +32,9 @@ use std::sync::{Arc, RwLock};
 use std::thread::{sleep, Builder, JoinHandle};
 use std::time::Duration;
 use streamer::{BlobReceiver, BlobSender, Window};
+use timing::timestamp;
+
+const GOSSIP_SLEEP_MILLIS: u64 = 100;
 
 pub fn parse_port_or_addr(optstr: Option<String>) -> SocketAddr {
     let daddr: SocketAddr = "0.0.0.0:8000".parse().expect("default socket address");
@@ -180,6 +183,7 @@ pub struct Crdt {
     /// The value of the remote update index that I have last seen
     /// This Node will ask external nodes for updates since the value in this list
     pub remote: HashMap<PublicKey, u64>,
+    pub alive: HashMap<PublicKey, u64>,
     pub update_index: u64,
     pub me: PublicKey,
 }
@@ -204,6 +208,7 @@ impl Crdt {
             table: HashMap::new(),
             local: HashMap::new(),
             remote: HashMap::new(),
+            alive: HashMap::new(),
             me: me.id,
             update_index: 1,
         };
@@ -248,6 +253,35 @@ impl Crdt {
                 self.table[&v.id].version
             );
         }
+        //update the liveness table
+        let now = timestamp();
+        *self.alive.entry(v.id).or_insert(now) = now;
+    }
+
+    /// purge old validators
+    /// TODO: we need a robust membership protocol
+    /// http://asc.di.fct.unl.pt/~jleitao/pdf/dsn07-leitao.pdf
+    /// challenging part is that we are on a permissionless network
+    pub fn purge(&mut self, now: u64) {
+        //wait for 4x as long as it would randomly take to reach our node
+        //assuming everyone is waiting the same amount of time as this node
+        let limit = self.table.len() as u64 * GOSSIP_SLEEP_MILLIS * 4;
+        let purge_set: Vec<PublicKey> = self.alive
+            .iter()
+            .filter_map(|(k, v)| {
+                if *k != self.me && (now - v) > limit {
+                    Some((*k).clone())
+                } else {
+                    None
+                }
+            })
+            .collect();
+        for p in purge_set.iter() {
+            self.alive.remove(p);
+            self.table.remove(p);
+            self.remote.remove(p);
+            self.local.remove(p);
+        }
     }
 
     pub fn index_blobs(
@@ -540,7 +574,7 @@ impl Crdt {
                 }
                 //TODO: possibly tune this parameter
                 //we saw a deadlock passing an obj.read().unwrap().timeout into sleep
-                sleep(Duration::from_millis(100));
+                sleep(Duration::from_millis(GOSSIP_SLEEP_MILLIS));
             })
             .unwrap()
     }
@@ -585,9 +619,9 @@ impl Crdt {
     ) -> Option<SharedBlob> {
         match deserialize(&blob.data[..blob.meta.size]) {
             // TODO sigverify these
-            Ok(Protocol::RequestUpdates(v, reqdata)) => {
+            Ok(Protocol::RequestUpdates(v, from_rd)) => {
                 trace!("RequestUpdates {}", v);
-                let addr = reqdata.gossip_addr;
+                let addr = from_rd.gossip_addr;
                 // only lock for this call, dont lock during IO `sock.send_to` or `sock.recv_from`
                 let (from, ups, data) = obj.read()
                     .expect("'obj' read lock in RequestUpdates")
@@ -595,7 +629,7 @@ impl Crdt {
                 trace!("get updates since response {} {}", v, data.len());
                 let len = data.len();
                 let rsp = Protocol::ReceiveUpdates(from, ups, data);
-                obj.write().unwrap().insert(&reqdata);
+                obj.write().unwrap().insert(&from_rd);
                 if len < 1 {
                     let me = obj.read().unwrap();
                     trace!(
@@ -610,7 +644,7 @@ impl Crdt {
                         "sending updates me {:?} len {} to {:?} {}",
                         &obj.read().unwrap().me[..4],
                         len,
-                        &reqdata.id[..4],
+                        &from_rd.id[..4],
                         addr,
                     );
                     Some(r)
@@ -759,7 +793,7 @@ impl TestNode {
 
 #[cfg(test)]
 mod tests {
-    use crdt::{parse_port_or_addr, Crdt, ReplicatedData};
+    use crdt::{parse_port_or_addr, Crdt, ReplicatedData, GOSSIP_SLEEP_MILLIS};
     use packet::BlobRecycler;
     use result::Error;
     use signature::{KeyPair, KeyPairUtil};
@@ -998,6 +1032,28 @@ mod tests {
         assert!(one && two);
     }
 
+    #[test]
+    fn purge_test() {
+        let me = ReplicatedData::new_entry_point("127.0.0.1:1234".parse().unwrap());
+        let mut crdt = Crdt::new(me.clone());
+        let nxt = ReplicatedData::new_entry_point("127.0.0.2:1234".parse().unwrap());
+        crdt.insert(&nxt);
+        let rv = crdt.gossip_request().unwrap();
+        assert_eq!(rv.0, nxt.gossip_addr);
+        let now = crdt.alive[&nxt.id];
+        crdt.purge(now);
+        let rv = crdt.gossip_request().unwrap();
+        assert_eq!(rv.0, nxt.gossip_addr);
+
+        crdt.purge(now + GOSSIP_SLEEP_MILLIS * 4);
+        let rv = crdt.gossip_request().unwrap();
+        assert_eq!(rv.0, nxt.gossip_addr);
+
+        crdt.purge(now + GOSSIP_SLEEP_MILLIS * 4 + 1);
+        let rv = crdt.gossip_request();
+        assert_matches!(rv, Err(Error::CrdtTooSmall));
+    }
+
     /// test window requests respond with the right blob, and do not overrun
     #[test]
     fn run_window_request() {