diff --git a/src/model/k2v/causality.rs b/src/model/k2v/causality.rs
index 9a692870..665b02b2 100644
--- a/src/model/k2v/causality.rs
+++ b/src/model/k2v/causality.rs
@@ -1,3 +1,13 @@
+//! Implements a CausalContext, which is a set of timestamps for each
+//! node -- a vector clock --, indicating that the versions with
+//! timestamps <= these numbers have been seen and can be
+//! overwritten by a subsequent write.
+//!
+//! The textual representation of a CausalContext, which we call a
+//! "causality token", is used in the API and must be sent along with
+//! each write or delete operation to indicate the previously seen
+//! versions that we want to overwrite or delete.
+
 use std::collections::BTreeMap;
 use std::convert::TryInto;
 
@@ -9,23 +19,36 @@ use garage_util::data::*;
 /// of full Garage node IDs which are 256-bit UUIDs.
 pub type K2VNodeId = u64;
 
+pub type VectorClock = BTreeMap<K2VNodeId, u64>;
+
 pub fn make_node_id(node_id: Uuid) -> K2VNodeId {
 	let mut tmp = [0u8; 8];
 	tmp.copy_from_slice(&node_id.as_slice()[..8]);
 	u64::from_be_bytes(tmp)
 }
 
-#[derive(PartialEq, Eq, Debug, Serialize, Deserialize)]
+pub fn vclock_gt(a: &VectorClock, b: &VectorClock) -> bool {
+	a.iter().any(|(n, ts)| ts > b.get(n).unwrap_or(&0))
+}
+
+pub fn vclock_max(a: &VectorClock, b: &VectorClock) -> VectorClock {
+	let mut ret = a.clone();
+	for (n, ts) in b.iter() {
+		let ent = ret.entry(*n).or_insert(0);
+		*ent = std::cmp::max(*ts, *ent);
+	}
+	ret
+}
+
+#[derive(PartialEq, Eq, Debug, Serialize, Deserialize, Default)]
 pub struct CausalContext {
-	pub vector_clock: BTreeMap<K2VNodeId, u64>,
+	pub vector_clock: VectorClock,
 }
 
 impl CausalContext {
 	/// Empty causality context
-	pub fn new_empty() -> Self {
-		Self {
-			vector_clock: BTreeMap::new(),
-		}
+	pub fn new() -> Self {
+		Self::default()
 	}
 	/// Make binary representation and encode in base64
 	pub fn serialize(&self) -> String {
@@ -72,9 +95,7 @@ impl CausalContext {
 	}
 	/// Check if this causal context contains newer items than another one
 	pub fn is_newer_than(&self, other: &Self) -> bool {
-		self.vector_clock
-			.iter()
-			.any(|(k, v)| v > other.vector_clock.get(k).unwrap_or(&0))
+		vclock_gt(&self.vector_clock, &other.vector_clock)
 	}
 }
 
diff --git a/src/model/k2v/item_table.rs b/src/model/k2v/item_table.rs
index 1ba9bb46..bc2b1aef 100644
--- a/src/model/k2v/item_table.rs
+++ b/src/model/k2v/item_table.rs
@@ -106,7 +106,7 @@ impl K2VItem {
 
 	/// Extract the causality context of a K2V Item
 	pub fn causal_context(&self) -> CausalContext {
-		let mut cc = CausalContext::new_empty();
+		let mut cc = CausalContext::new();
 		for (node, ent) in self.items.iter() {
 			cc.vector_clock.insert(*node, ent.max_time());
 		}
diff --git a/src/model/k2v/mod.rs b/src/model/k2v/mod.rs
index 83ad2512..c488b4c6 100644
--- a/src/model/k2v/mod.rs
+++ b/src/model/k2v/mod.rs
@@ -1,4 +1,5 @@
 pub mod causality;
+pub mod seen;
 
 pub mod item_table;
 
diff --git a/src/model/k2v/seen.rs b/src/model/k2v/seen.rs
new file mode 100644
index 00000000..b8f4ff27
--- /dev/null
+++ b/src/model/k2v/seen.rs
@@ -0,0 +1,85 @@
+//! Implements a RangeSeenMarker, a data type used in the PollRange API
+//! to indicate which items in the range have already been seen
+//! and which have not been seen yet.
+//!
+//! It consists of a vector clock that indicates that for each node,
+//! all items produced by that node with timestamps <= the value in the
+//! vector clock has been seen, as well as a set of causal contexts for
+//! individual items.
+
+use std::collections::BTreeMap;
+
+use serde::{Deserialize, Serialize};
+
+use garage_util::data::Uuid;
+use garage_util::encode::{nonversioned_decode, nonversioned_encode};
+use garage_util::error::{Error, OkOrMessage};
+
+use crate::k2v::causality::*;
+use crate::k2v::item_table::*;
+
+#[derive(Debug, Serialize, Deserialize, Default)]
+pub struct RangeSeenMarker {
+	vector_clock: VectorClock,
+	items: BTreeMap<String, VectorClock>,
+}
+
+impl RangeSeenMarker {
+	pub fn new() -> Self {
+		Self::default()
+	}
+
+	pub fn mark_seen_node_items<'a, I: IntoIterator<Item = &'a K2VItem>>(
+		&mut self,
+		node: Uuid,
+		items: I,
+	) {
+		let node = make_node_id(node);
+		for item in items.into_iter() {
+			let cc = item.causal_context();
+
+			if let Some(ts) = cc.vector_clock.get(&node) {
+				let ent = self.vector_clock.entry(node).or_insert(0);
+				*ent = std::cmp::max(*ent, *ts);
+			}
+
+			if vclock_gt(&cc.vector_clock, &self.vector_clock) {
+				match self.items.get_mut(&item.sort_key) {
+					None => {
+						self.items.insert(item.sort_key.clone(), cc.vector_clock);
+					}
+					Some(ent) => *ent = vclock_max(&ent, &cc.vector_clock),
+				}
+			}
+		}
+	}
+
+	pub fn canonicalize(&mut self) {
+		let self_vc = &self.vector_clock;
+		self.items.retain(|_sk, vc| vclock_gt(&vc, self_vc))
+	}
+
+	pub fn encode(&mut self) -> Result<String, Error> {
+		self.canonicalize();
+
+		let bytes = nonversioned_encode(&self)?;
+		let bytes = zstd::stream::encode_all(&mut &bytes[..], zstd::DEFAULT_COMPRESSION_LEVEL)?;
+		Ok(base64::encode(&bytes))
+	}
+
+	pub fn decode(s: &str) -> Result<Self, Error> {
+		let bytes = base64::decode(&s).ok_or_message("invalid base64")?;
+		let bytes = zstd::stream::decode_all(&mut &bytes[..])?;
+		Ok(nonversioned_decode(&bytes)?)
+	}
+
+	pub fn is_new_item(&self, item: &K2VItem) -> bool {
+		let cc = item.causal_context();
+		vclock_gt(&cc.vector_clock, &self.vector_clock)
+			&& self
+				.items
+				.get(&item.sort_key)
+				.map(|vc| vclock_gt(&cc.vector_clock, &vc))
+				.unwrap_or(true)
+	}
+}