Figure 4 shows the protocol stack at the switch. Arriving IP packets are interpreted by a veneer layer that recognizes the transport, classifies flows, and extracts frames. The Anypoint layer maintains connection-related state, active set membership, and ALRM bindings as described in the previous sections. The TCP-derived ACP receiver uses TCP byte sequence numbers for reordering, and ignores FSNs. The switch also translates byte sequence numbers and caches them in the frame ring. It uses this state to identify the frames covered by acks, which are encoded as byte sequence numbers rather than FSNs.
The switch prototype validates TCP checksums for incoming ACP segments and recomputes a fresh checksum from scratch for transformed outgoing segments, using network cards with TCP checksum offloading. Redirect patches are carried in each ACP segment as a monotonically increasing sequence number. This field, updated by the switch for inbound flows, indicates that all data with sequence numbers less than this number may be delivered to the application. The switch has limited support for dynamic changes to the active set; it can remove failed servers, but it cannot add servers.