找不到函数Marshal（）

I am writing small code for generating the ICMP packet. For that I am following the code from golang net package.

I identified the following code to use for ICMP packet generation:

   109          c, err := Dial(tt.net, tt.raddr)
   110          if err != nil {
   111              t.Fatalf("Dial failed: %v", err)
   112          }
   113          c.SetDeadline(time.Now().Add(100 * time.Millisecond))
   114          defer c.Close()
   115  
   116          typ := icmpv4EchoRequest
   117          if net == "ip6" {
   118              typ = icmpv6EchoRequest
   119          }
   120          xid, xseq := os.Getpid()&0xffff, i+1
   121          wb, err := (&icmpMessage{
   122              Type: typ, Code: 0,
   123              Body: &icmpEcho{
   124                  ID: xid, Seq: xseq,
   125                  Data: bytes.Repeat([]byte("Go Go Gadget Ping!!!"), 3),
   126              },
   127          }).Marshal()
   128          if err != nil {
   129              t.Fatalf("icmpMessage.Marshal failed: %v", err)
   130          }
   131          if _, err := c.Write(wb); err != nil {
   132              t.Fatalf("Conn.Write failed: %v", err)
   133          }

I am not able to understand followings in this code :

  1. Marshal() is not included in the net package documentation, please let me know how to find this function and why it is not included in the net package.

  2. please explain me line #121 (icmpMessage struct and function calling as these are not defined in this file).