In an IEEE 802.3 Ethernet frame, what is the significance of the DSAP field?

TCP and UDP port numbers reside in the Layer 4 transport header, not in the Layer 2 LLC DSAP field, which has no awareness of transport-layer protocols.

The DSAP field is a standard IEEE 802.2 LLC construct used in any compliant 802.3 network worldwide and has no DoD-specific restriction or designation.

The DSAP field belongs to IEEE 802.3 frames using 802.2 LLC encapsulation - Ethernet II (DIX) frames use a 2-byte EtherType field in its place and do not contain a DSAP field.

The DSAP is a 1-byte field in the IEEE 802.2 LLC header that functions as a service access point, directing the frame payload to the correct upper-layer network protocol such as IP or IPX at the receiving station. Unlike Ethernet II frames which use an EtherType field for the same purpose, 802.3 frames with LLC encapsulation rely on DSAP to identify the destination protocol. This allows multiplexing of multiple network-layer protocols over the same 802.3 physical medium.