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-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 metadata
-----------------------------------------------
-
-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 vlan 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: available bit
- * VNET_BUFFER_F_AVAIL2: available bit
- * VNET_BUFFER_F_AVAIL3: available bit
- * VNET_BUFFER_F_AVAIL4: available bit
- * VNET_BUFFER_F_AVAIL5: available bit
- * VNET_BUFFER_F_AVAIL6: available bit
- * VNET_BUFFER_F_AVAIL7: available 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 metadata
------------------------------------------------
-
-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];
-```