tun+ip: clarify what happens where

include/llarp/ip.hpp

@@ -153,9 +153,13 @@ namespace llarp
         Header()->daddr = htonl(ip);
       }
 
-      // update ip packet checksum
+      // update ip packet checksum (after packet gets out of network)
       void
-      UpdateChecksum();
+      UpdateChecksumsOnDst();
+
+      // update ip packet checksum (before packet gets inserted into network)
+      void
+      UpdateChecksumsOnSrc();
     };
 
   }  // namespace net

llarp/handlers/tun.cpp

@@ -329,7 +329,7 @@ namespace llarp
                memcpy(pkt.buf, buf.base, pkt.sz);
                pkt.src(themIP);
                pkt.dst(usIP);
-               pkt.UpdateChecksum();
+               pkt.UpdateChecksumsOnDst();
                return true;
              }))
 
@@ -440,11 +440,12 @@ namespace llarp
       // called in the isolated network thread
       TunEndpoint *self = static_cast< TunEndpoint * >(tun->user);
       self->m_NetworkToUserPktQueue.Process([self, tun](net::IPv4Packet &pkt) {
+        // prepare packet for insertion into network
+        pkg.UpdateChecksumsOnSrc();
         // clear addresses
         pkt.src(0);
         pkt.dst(0);
-        // clear checksum
-        pkt.Header()->check = 0;
+
         if(!llarp_ev_tun_async_write(tun, pkt.buf, pkt.sz))
           llarp::LogWarn("packet dropped");
       });

llarp/ip.cpp

@@ -78,12 +78,15 @@ namespace llarp
                *check = ipchksum(pktbuf, 12 + pktsz);
              }}};
     void
-    IPv4Packet::UpdateChecksum()
+    IPv4Packet::UpdateChecksumsOnDst()
     {
+      // IPv4 checksum
       auto hdr   = Header();
       hdr->check = 0;
       auto len   = hdr->ihl * 4;
       hdr->check = ipchksum(buf, len);
+
+      // L4 checksum
       auto proto = hdr->protocol;
       auto itr   = protoCheckSummer.find(proto);
       if(itr != protoCheckSummer.end())
@@ -91,5 +94,12 @@ namespace llarp
         itr->second(hdr, buf, sz);
       }
     }
+
+    void
+    IPv4Packet::UpdateChecksumsOnSrc()
+    {
+      // IPv4
+      Header()->check = 0;
+    }
   }  // namespace net
 }  // namespace llarp