mbuf.9: describe checksum offloading for SCTP

This describes the current status of the implementation.
While there, be a bit more precise on how long the checksum
computation is delayed.

Reviewed by:		Timo Völker, bcr
Differential Revision:	https://reviews.freebsd.org/D51590

(cherry picked from commit fe35f275ab0240cb5ed05484c943293a71aadb5f)

share/man/man9/mbuf.9

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd December 28, 2023
+.Dd July 29, 2025
 .Dt MBUF 9
 .Os
 .\"
@@ -1091,7 +1091,7 @@ network code, when data must be encrypted or otherwise
 altered prior to transmission.
 .El
 .Sh HARDWARE-ASSISTED CHECKSUM CALCULATION
-This section currently applies to TCP/IP only.
+This section currently applies to SCTP, TCP, and UDP over IP only.
 In order to save the host CPU resources, computing checksums is
 offloaded to the network interface hardware if possible.
 The
@@ -1117,7 +1117,7 @@ in the
 .Vt mbuf chain
 containing the packet.
 .Pp
-On output, checksum offloading is attempted after the outgoing
+On output, the computation of the checksum is delayed until the outgoing
 interface has been determined for a packet.
 The interface-specific field
 .Va ifnet.if_data.ifi_hwassist
@@ -1135,12 +1135,15 @@ such actions will never be requested through
 .Va csum_flags .
 .Pp
 The flags demanding a particular action from an interface are as follows:
-.Bl -tag -width ".Dv CSUM_TCP" -offset indent
+.Bl -tag -width ".Dv CSUM_SCTP" -offset indent
 .It Dv CSUM_IP
 The IP header checksum is to be computed and stored in the
 corresponding field of the packet.
 The hardware is expected to know the format of an IP header
 to determine the offset of the IP checksum field.
+.It Dv CSUM_SCTP
+The SCTP checksum is to be computed.
+(See below.)
 .It Dv CSUM_TCP
 The TCP checksum is to be computed.
 (See below.)
@@ -1149,14 +1152,16 @@ The UDP checksum is to be computed.
 (See below.)
 .El
 .Pp
-Should a TCP or UDP checksum be offloaded to the hardware,
+Should a SCTP, TCP, or UDP checksum be offloaded to the hardware,
 the field
 .Va csum_data
 will contain the byte offset of the checksum field relative to the
 end of the IP header.
-In this case, the checksum field will be initially
-set by the TCP/IP module to the checksum of the pseudo header
+In the case of TCP or UDP, the checksum field will be initially
+set by the TCP or UDP implementation to the checksum of the pseudo header
 defined by the TCP and UDP specifications.
+In the case of SCTP, the checksum field will be initially
+set by the SCTP implementation to 0.
 .Pp
 On input, an interface indicates the actions it has performed
 on a packet by setting one or more of the following flags in
@@ -1187,13 +1192,13 @@ to obtain the final checksum to be used for TCP or UDP validation purposes.
 .El
 .Pp
 If a particular network interface just indicates success or
-failure of TCP or UDP checksum validation without returning
+failure of SCTP, TCP, or UDP checksum validation without returning
 the exact value of the checksum to the host CPU, its driver can mark
 .Dv CSUM_DATA_VALID
-and
-.Dv CSUM_PSEUDO_HDR
 in
-.Va csum_flags ,
+.Va csum_flags
+as well as, for TCP and UDP,
+.Dv CSUM_PSEUDO_HDR
 and set
 .Va csum_data
 to
@@ -1203,6 +1208,13 @@ It is a peculiarity of the algorithm used that the Internet checksum
 calculated over any valid packet will be
 .Li 0xFFFF
 as long as the original checksum field is included.
+Note that for SCTP the value of
+.Va csum_data
+is not relevant and
+.Dv CSUM_PSEUDO_HDR
+in
+.Va csum_flags
+is not set, since SCTP does not use a pseudo header checksum.
 .Sh STRESS TESTING
 When running a kernel compiled with the option
 .Dv MBUF_STRESS_TEST ,