skbuff.h: Improve the checksum related comments

 * A. IP checksum related features

 *

 * Drivers advertise checksum offload capabilities in the features of a device.

 * From the stack's point of view these are capabilities offered by the driver,

 * a driver typically only advertises features that it is capable of offloading

 * From the stack's point of view these are capabilities offered by the driver.

 * A driver typically only advertises features that it is capable of offloading

 * to its device.

 *

 * The checksum related features are:

 *			  TCP or UDP packets over IPv4. These are specifically

 *			  unencapsulated packets of the form IPv4|TCP or

 *			  IPv4|UDP where the Protocol field in the IPv4 header

 *			  is TCP or UDP. The IPv4 header may contain IP options

 *			  is TCP or UDP. The IPv4 header may contain IP options.

 *			  This feature cannot be set in features for a device

 *			  with NETIF_F_HW_CSUM also set. This feature is being

 *			  DEPRECATED (see below).

 *			  DEPRECATED (see below).

 *

 *	NETIF_F_RXCSUM - Driver (device) performs receive checksum offload.

 *			 This flag is used only used to disable the RX checksum

 *			 This flag is only used to disable the RX checksum

 *			 feature for a device. The stack will accept receive

 *			 checksum indication in packets received on a device

 *			 regardless of whether NETIF_F_RXCSUM is set.

 *

 * B. Checksumming of received packets by device. Indication of checksum

 *    verification is in set skb->ip_summed. Possible values are:

 *    verification is set in skb->ip_summed. Possible values are:

 *

 * CHECKSUM_NONE:

 *

 *   the packet minus one that have been verified as CHECKSUM_UNNECESSARY.

 *   For instance if a device receives an IPv6->UDP->GRE->IPv4->TCP packet

 *   and a device is able to verify the checksums for UDP (possibly zero),

 *   GRE (checksum flag is set), and TCP-- skb->csum_level would be set to

 *   GRE (checksum flag is set) and TCP, skb->csum_level would be set to

 *   two. If the device were only able to verify the UDP checksum and not

 *   GRE, either because it doesn't support GRE checksum of because GRE

 *   GRE, either because it doesn't support GRE checksum or because GRE

 *   checksum is bad, skb->csum_level would be set to zero (TCP checksum is

 *   not considered in this case).

 *

 * CHECKSUM_COMPLETE:

 *

 *   This is the most generic way. The device supplied checksum of the _whole_

 *   packet as seen by netif_rx() and fills out in skb->csum. Meaning, the

 *   packet as seen by netif_rx() and fills in skb->csum. This means the

 *   hardware doesn't need to parse L3/L4 headers to implement this.

 *

 *   Notes:

 *   from skb->csum_start up to the end, and to record/write the checksum at

 *   offset skb->csum_start + skb->csum_offset. A driver may verify that the

 *   csum_start and csum_offset values are valid values given the length and

 *   offset of the packet, however they should not attempt to validate that the

 *   offset of the packet, but it should not attempt to validate that the

 *   checksum refers to a legitimate transport layer checksum -- it is the

 *   purview of the stack to validate that csum_start and csum_offset are set

 *   correctly.

 *

 * CHECKSUM_UNNECESSARY:

 *

 *   This has the same meaning on as CHECKSUM_NONE for checksum offload on

 *   This has the same meaning as CHECKSUM_NONE for checksum offload on

 *   output.

 *

 * CHECKSUM_COMPLETE:

 *   Not used in checksum output. If a driver observes a packet with this value

 *   set in skbuff, if should treat as CHECKSUM_NONE being set.

 *   set in skbuff, it should treat the packet as if CHECKSUM_NONE were set.

 *

 * D. Non-IP checksum (CRC) offloads

 *

 *   NETIF_F_SCTP_CRC - This feature indicates that a device is capable of

 *     offloading the SCTP CRC in a packet. To perform this offload the stack

 *     will set set csum_start and csum_offset accordingly, set ip_summed to

 *     will set csum_start and csum_offset accordingly, set ip_summed to

 *     CHECKSUM_PARTIAL and set csum_not_inet to 1, to provide an indication in

 *     the skbuff that the CHECKSUM_PARTIAL refers to CRC32c.

 *     A driver that supports both IP checksum offload and SCTP CRC32c offload

 *   NETIF_F_FCOE_CRC - This feature indicates that a device is capable of

 *     offloading the FCOE CRC in a packet. To perform this offload the stack

 *     will set ip_summed to CHECKSUM_PARTIAL and set csum_start and csum_offset

 *     accordingly. Note the there is no indication in the skbuff that the

 *     CHECKSUM_PARTIAL refers to an FCOE checksum, a driver that supports

 *     accordingly. Note that there is no indication in the skbuff that the

 *     CHECKSUM_PARTIAL refers to an FCOE checksum, so a driver that supports

 *     both IP checksum offload and FCOE CRC offload must verify which offload

 *     is configured for a packet presumably by inspecting packet headers.

 *     is configured for a packet, presumably by inspecting packet headers.

 *

 * E. Checksumming on output with GSO.

 *

 * is implied by the SKB_GSO_* flags in gso_type. Most obviously, if the

 * gso_type is SKB_GSO_TCPV4 or SKB_GSO_TCPV6, TCP checksum offload as

 * part of the GSO operation is implied. If a checksum is being offloaded

 * with GSO then ip_summed is CHECKSUM_PARTIAL, csum_start and csum_offset

 * are set to refer to the outermost checksum being offload (two offloaded

 * checksums are possible with UDP encapsulation).

 * with GSO then ip_summed is CHECKSUM_PARTIAL, and both csum_start and

 * csum_offset are set to refer to the outermost checksum being offloaded

 * (two offloaded checksums are possible with UDP encapsulation).

 */

/* Don't change this without changing skb_csum_unnecessary! */