--- /dev/null
+Buffer Metadata
+===============
+
+Each vlib_buffer_t (packet buffer) carries buffer metadata which
+describes the current packet-processing state. The underlying
+techniques have been used for decades, across multiple packet
+processing environments.
+
+We will examine vpp buffer metadata in some detail, but folks who need
+to manipulate and/or extend the scheme should expect to do a certain
+level of code inspection.
+
+Vlib (Vector library) primary buffer metatdata
+----------------------------------------------
+
+The first 64 octets of each vlib_buffer_t carries the primary buffer
+metadata. See .../src/vlib/buffer.h for full details.
+
+Important fields:
+
+* i16 current_data: the signed offset in data[], pre_data[] that we
+are currently processing. If negative current header points into
+the pre-data (rewrite space) area.
+* u16 current_length: nBytes between current_data and the end of this buffer.
+* u32 flags: Buffer flag bits. Heavily used, not many bits left
+ * src/vlib/buffer.h flag bits
+ * VLIB_BUFFER_IS_TRACED: buffer is traced
+ * VLIB_BUFFER_NEXT_PRESENT: buffer has multiple chunks
+ * VLIB_BUFFER_TOTAL_LENGTH_VALID: total_length_not_including_first_buffer is valid (see below)
+ * src/vnet/buffer.h flag bits
+ * VNET_BUFFER_F_L4_CHECKSUM_COMPUTED: tcp/udp checksum has been computed
+ * VNET_BUFFER_F_L4_CHECKSUM_CORRECT: tcp/udp checksum is correct
+ * VNET_BUFFER_F_VLAN_2_DEEP: two vlan tags present
+ * VNET_BUFFER_F_VLAN_1_DEEP: one vlag tag present
+ * VNET_BUFFER_F_SPAN_CLONE: packet has already been cloned (span feature)
+ * VNET_BUFFER_F_LOOP_COUNTER_VALID: packet look-up loop count valid
+ * VNET_BUFFER_F_LOCALLY_ORIGINATED: packet built by vpp
+ * VNET_BUFFER_F_IS_IP4: packet is ipv4, for checksum offload
+ * VNET_BUFFER_F_IS_IP6: packet is ipv6, for checksum offload
+ * VNET_BUFFER_F_OFFLOAD_IP_CKSUM: hardware ip checksum offload requested
+ * VNET_BUFFER_F_OFFLOAD_TCP_CKSUM: hardware tcp checksum offload requested
+ * VNET_BUFFER_F_OFFLOAD_UDP_CKSUM: hardware udp checksum offload requested
+ * VNET_BUFFER_F_IS_NATED: natted packet, skip input checks
+ * VNET_BUFFER_F_L2_HDR_OFFSET_VALID: L2 header offset valid
+ * VNET_BUFFER_F_L3_HDR_OFFSET_VALID: L3 header offset valid
+ * VNET_BUFFER_F_L4_HDR_OFFSET_VALID: L4 header offset valid
+ * VNET_BUFFER_F_FLOW_REPORT: packet is an ipfix packet
+ * VNET_BUFFER_F_IS_DVR: packet to be reinjected into the l2 output path
+ * VNET_BUFFER_F_QOS_DATA_VALID: QoS data valid in vnet_buffer_opaque2
+ * VNET_BUFFER_F_GSO: generic segmentation offload requested
+ * VNET_BUFFER_F_AVAIL1: avaliable bit
+ * VNET_BUFFER_F_AVAIL2: avaliable bit
+ * VNET_BUFFER_F_AVAIL3: avaliable bit
+ * VNET_BUFFER_F_AVAIL4: avaliable bit
+ * VNET_BUFFER_F_AVAIL5: avaliable bit
+ * VNET_BUFFER_F_AVAIL6: avaliable bit
+ * VNET_BUFFER_F_AVAIL7: avaliable bit
+* u32 flow_id: generic flow identifier
+* u8 ref_count: buffer reference / clone count (e.g. for span replication)
+* u8 buffer_pool_index: buffer pool index which owns this buffer
+* vlib_error_t (u16) error: error code for buffers enqueued to error handler
+* u32 next_buffer: buffer index of next buffer in chain. Only valid if VLIB_BUFFER_NEXT_PRESENT is set
+* union
+ * u32 current_config_index: current index on feature arc
+ * u32 punt_reason: reason code once packet punted. Mutually exclusive with current_config_index
+* u32 opaque[10]: primary vnet-layer opaque data (see below)
+* END of first cache line / data initialized by the buffer allocator
+* u32 trace_index: buffer's index in the packet trace subsystem
+* u32 total_length_not_including_first_buffer: see VLIB_BUFFER_TOTAL_LENGTH_VALID above
+* u32 opaque2[14]: secondary vnet-layer opaque data (see below)
+* u8 pre_data[VLIB_BUFFER_PRE_DATA_SIZE]: rewrite space, often used to prepend tunnel encapsulations
+* u8 data[0]: buffer data received from the wire. Ordinarily, hardware devices use b->data[0] as the DMA target but there are exceptions. Do not write code which blindly assumes that packet data starts in b->data[0]. Use vlib_buffer_get_current(...).
+
+Vnet (network stack) primary buffer metadata
+--------------------------------------------
+
+Vnet primary buffer metadata occupies space reserved in the vlib
+opaque field shown above, and has the type name
+vnet_buffer_opaque_t. Ordinarily accessed using the vnet_buffer(b)
+macro. See ../src/vnet/buffer.h for full details.
+
+Important fields:
+
+* u32 sw_if_index[2]: RX and TX interface handles. At the ip lookup
+ stage, vnet_buffer(b)->sw_if_index[VLIB_TX] is interpreted as a FIB
+ index.
+* i16 l2_hdr_offset: offset from b->data[0] of the packet L2 header.
+ Valid only if b->flags & VNET_BUFFER_F_L2_HDR_OFFSET_VALID is set
+* i16 l3_hdr_offset: offset from b->data[0] of the packet L3 header.
+ Valid only if b->flags & VNET_BUFFER_F_L3_HDR_OFFSET_VALID is set
+* i16 l4_hdr_offset: offset from b->data[0] of the packet L4 header.
+ Valid only if b->flags & VNET_BUFFER_F_L4_HDR_OFFSET_VALID is set
+* u8 feature_arc_index: feature arc that the packet is currently traversing
+* union
+ * ip
+ * u32 adj_index[2]: adjacency from dest IP lookup in [VLIB_TX], adjacency
+ from source ip lookup in [VLIB_RX], set to ~0 until source lookup done
+ * union
+ * generic fields
+ * ICMP fields
+ * reassembly fields
+ * mpls fields
+ * l2 bridging fields, only valid in the L2 path
+ * l2tpv3 fields
+ * l2 classify fields
+ * vnet policer fields
+ * MAP fields
+ * MAP-T fields
+ * ip fragmentation fields
+ * COP (whitelist/blacklist filter) fields
+ * LISP fields
+ * TCP fields
+ * connection index
+ * sequence numbers
+ * header and data offsets
+ * data length
+ * flags
+ * SCTP fields
+ * NAT fields
+ * u32 unused[6]
+
+Vnet (network stack) secondary buffer metatdata
+-----------------------------------------------
+
+Vnet primary buffer metadata occupies space reserved in the vlib
+opaque2 field shown above, and has the type name
+vnet_buffer_opaque2_t. Ordinarily accessed using the vnet_buffer2(b)
+macro. See ../src/vnet/buffer.h for full details.
+
+Important fields:
+
+* qos fields
+ * u8 bits
+ * u8 source
+* u8 loop_counter: used to detect and report internal forwarding loops
+* group-based policy fields
+ * u8 flags
+ * u16 sclass: the packet's source class
+* u16 gso_size: L4 payload size, persists all the way to
+ interface-output in case GSO is not enabled
+* u16 gso_l4_hdr_sz: size of the L4 protocol header
+* union
+ * packet trajectory tracer (largely deprecated)
+ * u16 *trajectory_trace; only #if VLIB_BUFFER_TRACE_TRAJECTORY > 0
+ * packet generator
+ * u64 pg_replay_timestamp: timestamp for replayed pcap trace packets
+ * u32 unused[8]
+
+Buffer Metadata Extensions
+==========================
+
+Plugin developers may wish to extend either the primary or secondary
+vnet buffer opaque unions. Please perform a
+manual live variable analysis, otherwise nodes which use shared buffer metadata space may break things.
+
+It's not OK to add plugin or proprietary metadata to the core vpp
+engine header files named above. Instead, proceed as follows. The
+example concerns the vnet primary buffer opaque union
+vlib_buffer_opaque_t. It's a very simple variation to use the vnet
+secondary buffer opaque union vlib_buffer_opaque2_t.
+
+In a plugin header file:
+
+```
+ /* Add arbitrary buffer metadata */
+ #include <vnet/buffer.h>
+
+ typedef struct
+ {
+ u32 my_stuff[6];
+ } my_buffer_opaque_t;
+
+ STATIC_ASSERT (sizeof (my_buffer_opaque_t) <=
+ STRUCT_SIZE_OF (vnet_buffer_opaque_t, unused),
+ "Custom meta-data too large for vnet_buffer_opaque_t");
+
+ #define my_buffer_opaque(b) \
+ ((my_buffer_opaque_t *)((u8 *)((b)->opaque) + STRUCT_OFFSET_OF (vnet_buffer_opaque_t, unused)))
+```
+To set data in the custom buffer opaque type given a vlib_buffer_t *b:
+
+```
+ my_buffer_opaque (b)->my_stuff[2] = 123;
+```
+
+To read data from the custom buffer opaque type:
+
+```
+ stuff0 = my_buffer_opaque (b)->my_stuff[2];
+```
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