You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: conntrack_linux.go
    path of file: ./repos/moby/vendor/github.com/vishvananda/netlink
    
    the code of the file until where you have to start completion: package netlink

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"net"
	"time"

	"github.com/vishvananda/netlink/nl"
	"golang.org/x/sys/unix"
)

// ConntrackTableType Conntrack table for the netlink operation
type ConntrackTableType uint8

const (
	// ConntrackTable Conntrack table
	// https://github.com/torvalds/linux/blob/master/include/uapi/linux/netfi
    
func (f *ConntrackFilter) AddPort(tp ConntrackFilterType, port uint16) error {
	switch f.protoFilter {
	// TCP, UDP, DCCP, SCTP, UDPLite
	case 6, 17, 33, 132, 136:
	default:
		return fmt.Errorf("Filter attribute not available without a valid Layer 4 protocol: %d", f.protoFilter)
	}

	if f.portFilter == nil {
		f.portFilter = make(map[ConntrackFilterType]uint16)
	}
	if _, ok := f.portFilter[tp]; ok {
		return errors.New("Filter attribute already present")
	}
	f.portFilter[tp] = port
	return nil
}