Cool! I did something similar when I wanted to learn Go, but did my own parsers instead of using gopacket, I would recommend doing that yourself if you want to learn more low level stuff.

How I parsed IP for example:

  type Addr [4]uint8
  
  func (ip Addr) String() string {
   return fmt.Sprintf("%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3])
  }
  
  type Hdr struct {
   Version    uint8
   IHL        uint8
   DSCP       uint8
   ECN        uint8
   Length     uint16
   Id         uint16
   Flags      uint8
   Fragoffset uint16
   TTL        uint8
   Protocol   uint8
   Checksum   uint16
   Src        Addr
   Dst        Addr
  }
  
  func (hdr *Hdr) Parse(d []byte) error {
   hdr.Version = uint8(d[0] >> 4)
   hdr.IHL = uint8(d[0] & 0x0f)
   hdr.DSCP = uint8(d[1] >> 6)
   hdr.ECN = uint8(d[1] & 0x03)
   hdr.Length = uint16(binary.BigEndian.Uint16(d[2:4]))
   hdr.Id = uint16(binary.BigEndian.Uint16(d[4:6]))
   hdr.Flags = uint8(d[6] >> 5)
   hdr.Fragoffset = uint16(binary.BigEndian.Uint16(d[6:8])) & 0x1fff
   hdr.TTL = d[8]
   hdr.Protocol = d[9]
   hdr.Checksum = uint16(binary.BigEndian.Uint16(d[10:12]))
   hdr.Src = Addr{d[12], d[13], d[14], d[15]}
   hdr.Dst = Addr{d[16], d[17], d[18], d[19]}
  
   if hdr.IHL > 5 {
    fmt.Println("extra options detected") // TODO: support for extra options
   }
   return nil
  }