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--- mywiki:linux:checksum [2016/04/05 16:51] – created shaoguoh
+++ mywiki:linux:checksum [2019/09/15 18:55] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 Linux Checksum Calculation Logic
-API: skb_checksum_help
+^API | Comments |
+| skb_checksum_help |  |
+| ip_fast_csum | |
+| csum_fold | |
+| skb_checksum | |
+| csum_tcpudp_magic | |
+| nf_ip_checksum |
+| csum_tcpudp_nofold |
+| csum_replace4 |
+| csum_replace2 |
+| nf_nat_ipv4_csum_recalc |
+| csum_tcpudp_magic |
+skb->csum_start = skb_transport_header(skb) - skb->head;
+skb->csum_offset = offsetof(struct tcphdr, check);
+**TSO** is a hardware feature whereas **GSO** is a software feature.
+In the sense, TSO needs "the device" to break the frame into MTU sized segments.
+BUT, GSO is a strategy followed by kernel to avoid processing smaller packets throughout the stack to improve the porformance
+====== Linux Checksum offloading ======
+  - For bridging and non-udp/tcp packet, no so called offloading support in tx path
+  - For Fragmented packets, no checksum offloading support in tx path
+  - For UDP/TCP routing packet (incremental), no checksum offloading support in tx path. at the same time, no ip_summed flag change
+  - Only Linux local output UDP/TCP’s checksum can be offloaded in tx path
+  - For L2TP related tunnels, only inner UDP/TCP can be offloaded, outer UDP/TCP is done by linux itself in tx path.
+Below variables in skb is very important for checksum offloading:
+  - ip_summed (CHECKSUM_PARTIAL)
+  - encapsulation
+  - inner_network_header/network_header
+  - inner_transport_header/transport_header
+====== Linux Checksum ======
+<file>
+union {
+    __wsum                       csum;
+    struct {
+        __u16  csum_start;
+        __u16  csum_offset;
+   };
+};
+</file>
+| Variables/Direction | skb->ip_summed | skb->csum or csum_start/offset |
+^ Input | CHECKSUM_NONE 0  | not valid | SW need do L4 payload + pseudo header verification  |
+| | CHECKSUM_UNNECESSARY 1 | not valid | HW already do L4 payload + pseudo header verification  |
+| | CHECKSUM_COMPLETE 2 | csum valid (stored with L4 payload checksum) | SW need to do skb->csum + pseudo header verification |
+^ Output | CHECKSUM_NONE 0 | not valid | SW already do necessary L4 checksum calculation  |
+| | CHECKSUM_PARTIAL 3 | csum_start/offset valid | SW already do pseudo header and HW need to further calculate its payload based on   |