diff --git a/util/arena.go b/util/arena.go
new file mode 100644
index 00000000..0f7328de
--- /dev/null
+++ b/util/arena.go
@@ -0,0 +1,190 @@
+package util
+
+import (
+	"bytes"
+	"encoding/binary"
+)
+
+// flowKey identifies a transport flow by {src, dst, sport, dport, family}.
+// Comparable, so map lookups and linear scans over the slot list stay tight.
+// Shared by the TCP and UDP coalescers; each coalescer keeps its own
+// openSlots map, so a TCP and UDP flow on the same 5-tuple-without-proto
+// never alias.
+type flowKey struct {
+	src, dst     [16]byte
+	sport, dport uint16
+	isV6         bool
+}
+
+// initialSlots is the starting capacity of the slot pool. One flow per
+// packet is the worst case so this matches a typical carrier-side
+// recvmmsg batch on the encrypted UDP socket.
+const initialSlots = 64
+
+// parsedIP is the IP-level result of parseIPPrologue. The caller layers
+// L4-specific parsing (TCP / UDP) on top.
+type parsedIP struct {
+	fk       flowKey
+	ipHdrLen int
+	// pkt is the original buffer trimmed to the IP-declared total length.
+	// Anything below the IP layer (transport parsers) should slice into
+	// pkt rather than the unbounded original.
+	pkt []byte
+}
+
+// parseIPPrologue extracts the IP-level fields the coalescers care about:
+// IHL/payload length, version, src/dst addresses, and the L4 protocol byte.
+// Returns ok=false for malformed input, IPv4 with options or fragmentation,
+// or IPv6 with extension headers (all rejected by both coalescers in
+// identical ways before this refactor).
+//
+// On success, p.pkt is len-trimmed to the IP-declared length so callers
+// don't have to repeat the trim. wantProto is the IANA protocol number to
+// require (6 for TCP, 17 for UDP); ok=false for any other value.
+func parseIPPrologue(pkt []byte, wantProto byte) (parsedIP, bool) {
+	var p parsedIP
+	if len(pkt) < 20 {
+		return p, false
+	}
+	v := pkt[0] >> 4
+	switch v {
+	case 4:
+		ihl := int(pkt[0]&0x0f) * 4
+		if ihl != 20 {
+			return p, false
+		}
+		if pkt[9] != wantProto {
+			return p, false
+		}
+		// Reject actual fragmentation (MF or non-zero frag offset).
+		if binary.BigEndian.Uint16(pkt[6:8])&0x3fff != 0 {
+			return p, false
+		}
+		totalLen := int(binary.BigEndian.Uint16(pkt[2:4]))
+		if totalLen > len(pkt) || totalLen < ihl {
+			return p, false
+		}
+		p.ipHdrLen = 20
+		p.fk.isV6 = false
+		copy(p.fk.src[:4], pkt[12:16])
+		copy(p.fk.dst[:4], pkt[16:20])
+		p.pkt = pkt[:totalLen]
+	case 6:
+		if len(pkt) < 40 {
+			return p, false
+		}
+		if pkt[6] != wantProto {
+			return p, false
+		}
+		payloadLen := int(binary.BigEndian.Uint16(pkt[4:6]))
+		if 40+payloadLen > len(pkt) {
+			return p, false
+		}
+		p.ipHdrLen = 40
+		p.fk.isV6 = true
+		copy(p.fk.src[:], pkt[8:24])
+		copy(p.fk.dst[:], pkt[24:40])
+		p.pkt = pkt[:40+payloadLen]
+	default:
+		return p, false
+	}
+	return p, true
+}
+
+// ipHeadersMatch compares the IP portion of two packet header prefixes for
+// byte-for-byte equality on every field that must be identical across
+// coalesced segments. Size/IPID/IPCsum and the 2-bit IP-level ECN field are
+// masked out — the appendPayload step merges CE into the seed.
+//
+// The transport (L4) portion of the header is checked separately by the
+// per-protocol matcher.
+func ipHeadersMatch(a, b []byte, isV6 bool) bool {
+	if isV6 {
+		// IPv6: byte 0 = version/TC[7:4], byte 1 = TC[3:0]/flow[19:16],
+		// bytes [2:4] = flow[15:0], [6:8] = next_hdr/hop, [8:40] = src+dst.
+		// ECN lives in TC[1:0] = byte 1 mask 0x30. Skip [4:6] payload_len.
+		if a[0] != b[0] {
+			return false
+		}
+		if a[1]&^0x30 != b[1]&^0x30 {
+			return false
+		}
+		if !bytes.Equal(a[2:4], b[2:4]) {
+			return false
+		}
+		if !bytes.Equal(a[6:40], b[6:40]) {
+			return false
+		}
+		return true
+	}
+	// IPv4: byte 0 = version/IHL, byte 1 = DSCP(6)|ECN(2),
+	// [6:10] flags/fragoff/TTL/proto, [12:20] src+dst.
+	// Skip [2:4] total len, [4:6] id, [10:12] csum.
+	if a[0] != b[0] {
+		return false
+	}
+	if a[1]&^0x03 != b[1]&^0x03 {
+		return false
+	}
+	if !bytes.Equal(a[6:10], b[6:10]) {
+		return false
+	}
+	if !bytes.Equal(a[12:20], b[12:20]) {
+		return false
+	}
+	return true
+}
+
+// mergeECNIntoSeed ORs the 2-bit IP-level ECN field of pkt's IP header
+// onto the seed's IP header, so a CE mark on any coalesced segment
+// propagates to the final superpacket. (CE is 0b11; ORing yields CE if
+// any segment carried it.) Used by both TCP and UDP coalescers, so the
+// invariant lives in one place.
+func mergeECNIntoSeed(seedHdr, pktHdr []byte, isV6 bool) {
+	if isV6 {
+		seedHdr[1] |= pktHdr[1] & 0x30
+	} else {
+		seedHdr[1] |= pktHdr[1] & 0x03
+	}
+}
+
+// Arena is an injectable byte-slab that hands out non-overlapping borrowed
+// slices via Reserve and releases them in bulk via Reset. Coalescers take
+// an *Arena at construction so the caller controls the slab lifetime and
+// can share one slab across multiple coalescers (MultiCoalescer hands the
+// same *Arena to every lane so the lanes don't carry their own backings).
+//
+// Arena is not safe for concurrent use.
+//
+// Reserve borrows; the slice is valid until the next Reset. The slab grows
+// (by allocating a fresh, larger backing array) if a Reserve doesn't fit;
+// pre-size the arena via NewArena to avoid that path on the hot path.
+type Arena struct {
+	buf []byte
+}
+
+// NewArena returns an Arena with a pre-allocated backing of the given capacity
+func NewArena(capacity int) *Arena {
+	return &Arena{buf: make([]byte, 0, capacity)}
+}
+
+// Reserve hands out a non-overlapping sz-byte slice from the arena. If the
+// request doesn't fit the current backing, a fresh, larger backing is
+// allocated; already-borrowed slices reference the old backing and remain
+// valid until Reset.
+func (a *Arena) Reserve(sz int) []byte {
+	if len(a.buf)+sz > cap(a.buf) {
+		newCap := max(cap(a.buf)*2, sz)
+		a.buf = make([]byte, 0, newCap)
+	}
+	start := len(a.buf)
+	a.buf = a.buf[:start+sz]
+	return a.buf[start : start+sz : start+sz]
+}
+
+// Reset releases every slice handed out since the last Reset. Callers must
+// not use any previously-borrowed slice after this returns. The underlying
+// backing array is retained so subsequent Reserves don't re-allocate.
+func (a *Arena) Reset() {
+	a.buf = a.buf[:0]
+}