Re: Request for a new LINKTYPE_/DLT_ type.

[Guy Harris <gharris () sonic net>]

On Nov 26, 2018, at 12:43 PM, Dave Barach (dbarach) <dbarach () cisco com> wrote:

> On November 26, 2018, at 3:01 PM, Guy Harris <guy () alum mit edu> wrote:
>
> > So which of those structures describes the primary metadata?
>
> vlib_buffer_t. The key fields are flags, current_data, and current_length. 

So that's:

> /* VLIB buffer representation. */
> typedef struct
> {
>  /* Offset within data[] that we are currently processing.
>     If negative current header points into predata area. */
>  i16 current_data;  /**< signed offset in data[], pre_data[]
>                        that we are currently processing.
>                        If negative current header points into predata area.
>                     */
>  u16 current_length;  /**< Nbytes between current data and
>                          the end of this buffer.
>                       */
>  u32 flags; /**< buffer flags */
>  u32 flow_id; /**< Generic flow identifier */
>
>
>  u32 next_buffer;   /**< Next buffer for this linked-list of buffers.
>                        Only valid if VLIB_BUFFER_NEXT_PRESENT flag is set.
>                     */
>
>  u32 current_config_index; /**< Used by feature subgraph arcs to
>                               visit enabled feature nodes
>                            */
>  u16 error;   /**< Error code for buffers to be enqueued
>                           to error handler.
>                        */
>  u8 n_add_refs; /**< Number of additional references to this buffer. */
>
>  u8 buffer_pool_index;        /**< index of buffer pool this buffer belongs. */
>
>  u32 opaque[10]; /**< Opaque data used by sub-graphs for their own purposes.
>                       See above */
>  u32 trace_index; /**< Specifies index into trace buffer
>                      if VLIB_PACKET_IS_TRACED flag is set.
>                   */
>  u32 recycle_count; /**< Used by L2 path recycle code */
>
>  u32 total_length_not_including_first_buffer;
>  /**< Only valid for first buffer in chain. Current length plus
>     total length given here give total number of bytes in buffer chain.
>  */
>  u8 free_list_index; /** < only used if
>                                                  VLIB_BUFFER_NON_DEFAULT_FREELIST
>                                                  flag is set */
>  u8 align_pad[3]; /**< available */
>  u32 opaque2[12];  /**< More opaque data, see ../vnet/vnet/buffer.h */
>
>  /***** end of second cache line */
>  u8 pre_data[VLIB_BUFFER_PRE_DATA_SIZE];  /**< Space for inserting data
>                                               before buffer start.
>                                               Packet rewrite string will be
>                                               rewritten backwards and may extend
>                                               back before buffer->data[0].
>                                               Must come directly before packet data.
>                                            */
>
>  u8 data[0]; /**< Packet data. Hardware DMA here */
> } vlib_buffer_t;              /* Must be a multiple of 64B. */

which is 128 bytes followed by VLIB_BUFFER_PRE_DATA_SIZE bytes of data.

Which of those 64 of those 128 bytes are the primary metadata?
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