+/* Populates 'b' with an Ethernet LLC+SNAP packet headed with the given
+ * 'eth_dst', 'eth_src', 'snap_org', and 'snap_type'. A payload of 'size'
+ * bytes is allocated in 'b' and returned. This payload may be populated with
+ * appropriate information by the caller. */
+void *
+snap_compose(struct ofpbuf *b, const uint8_t eth_dst[ETH_ADDR_LEN],
+ const uint8_t eth_src[ETH_ADDR_LEN],
+ unsigned int oui, uint16_t snap_type, size_t size)
+{
+ struct eth_header *eth;
+ struct llc_snap_header *llc_snap;
+ void *payload;
+
+ /* Compose basic packet structure. (We need the payload size to stick into
+ * the 802.2 header.) */
+ ofpbuf_clear(b);
+ ofpbuf_prealloc_tailroom(b, ETH_HEADER_LEN + LLC_SNAP_HEADER_LEN + size);
+ eth = ofpbuf_put_zeros(b, ETH_HEADER_LEN);
+ llc_snap = ofpbuf_put_zeros(b, LLC_SNAP_HEADER_LEN);
+ payload = ofpbuf_put_uninit(b, size);
+
+ /* Compose 802.2 header. */
+ memcpy(eth->eth_dst, eth_dst, ETH_ADDR_LEN);
+ memcpy(eth->eth_src, eth_src, ETH_ADDR_LEN);
+ eth->eth_type = htons(b->size - ETH_HEADER_LEN);
+
+ /* Compose LLC, SNAP headers. */
+ llc_snap->llc.llc_dsap = LLC_DSAP_SNAP;
+ llc_snap->llc.llc_ssap = LLC_SSAP_SNAP;
+ llc_snap->llc.llc_cntl = LLC_CNTL_SNAP;
+ llc_snap->snap.snap_org[0] = oui >> 16;
+ llc_snap->snap.snap_org[1] = oui >> 8;
+ llc_snap->snap.snap_org[2] = oui;
+ llc_snap->snap.snap_type = htons(snap_type);
+
+ return payload;
+}
+