diff --git a/atalk/raw.go b/atalk/raw.go
index 9a1b5cb..01cea20 100644
--- a/atalk/raw.go
+++ b/atalk/raw.go
@@ -23,6 +23,8 @@ import (
 	"github.com/google/gopacket/pcap"
 )
 
+const DDPExtHeaderSize = 13
+
 // StartPcap opens an AppleTalk and AARP listening session on a network device.
 func StartPcap(device string) (*pcap.Handle, error) {
 	handle, err := pcap.OpenLive(device, 4096, true, 100*time.Millisecond)
diff --git a/rtmp.go b/rtmp.go
index db7cb51..86d8c66 100644
--- a/rtmp.go
+++ b/rtmp.go
@@ -22,6 +22,7 @@ import (
 	"log"
 	"time"
 
+	"gitea.drjosh.dev/josh/jrouter/atalk"
 	"gitea.drjosh.dev/josh/jrouter/atalk/rtmp"
 	"github.com/google/gopacket/pcap"
 	"github.com/sfiera/multitalk/pkg/aarp"
@@ -97,7 +98,7 @@ func (m *RTMPMachine) Run(ctx context.Context, incomingCh <-chan *ddp.ExtPacket)
 					}
 					ddpPkt := &ddp.ExtPacket{
 						ExtHeader: ddp.ExtHeader{
-							Size:      uint16(len(respPktRaw)),
+							Size:      uint16(len(respPktRaw)) + atalk.DDPExtHeaderSize,
 							Cksum:     0,
 							DstNet:    pkt.SrcNet,
 							DstNode:   pkt.SrcNode,
@@ -127,7 +128,7 @@ func (m *RTMPMachine) Run(ctx context.Context, incomingCh <-chan *ddp.ExtPacket)
 
 					ddpPkt := &ddp.ExtPacket{
 						ExtHeader: ddp.ExtHeader{
-							Size:      uint16(len(dataPktRaw)),
+							Size:      uint16(len(dataPktRaw)) + atalk.DDPExtHeaderSize,
 							Cksum:     0,
 							DstNet:    pkt.SrcNet,
 							DstNode:   pkt.SrcNode,
@@ -182,7 +183,7 @@ func (m *RTMPMachine) broadcastData(myAddr aarp.AddrPair) error {
 
 	ddpPkt := &ddp.ExtPacket{
 		ExtHeader: ddp.ExtHeader{
-			Size:      uint16(len(dataPktRaw)),
+			Size:      uint16(len(dataPktRaw)) + atalk.DDPExtHeaderSize,
 			Cksum:     0,
 			DstNet:    0,    // this network
 			DstNode:   0xff, // broadcast packet
diff --git a/zip.go b/zip.go
index 8166e38..d2c8172 100644
--- a/zip.go
+++ b/zip.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"log"
 
+	"gitea.drjosh.dev/josh/jrouter/atalk"
 	"gitea.drjosh.dev/josh/jrouter/atalk/atp"
 	"gitea.drjosh.dev/josh/jrouter/atalk/zip"
 	"github.com/google/gopacket/pcap"
@@ -68,7 +69,7 @@ func handleZIP(pcapHandle *pcap.Handle, srcHWAddr, myHWAddr ethernet.Addr, myAdd
 			}
 			respDDP := ddp.ExtPacket{
 				ExtHeader: ddp.ExtHeader{
-					Size:      uint16(len(ddpBody)),
+					Size:      uint16(len(ddpBody)) + atalk.DDPExtHeaderSize,
 					Cksum:     0,
 					DstNet:    ddpkt.SrcNet,
 					DstNode:   ddpkt.SrcNode,
@@ -158,7 +159,7 @@ func handleZIP(pcapHandle *pcap.Handle, srcHWAddr, myHWAddr ethernet.Addr, myAdd
 		// routers should respond with a broadcast."
 		outDDP := ddp.ExtPacket{
 			ExtHeader: ddp.ExtHeader{
-				Size:      uint16(len(respRaw)),
+				Size:      uint16(len(respRaw)) + atalk.DDPExtHeaderSize,
 				Cksum:     0,
 				DstNet:    ddpkt.SrcNet,
 				DstNode:   ddpkt.SrcNode,