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: extension.go
    path of file: ./repos/go-in-5-minutes/episode7/vendor/golang.org/x/net/icmp
    
    the code of the file until where you have to start completion: // Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package icmp

import "encoding/binary"

// An Extension represents an ICMP extension.
type Extension interface {
	// Len returns the length of ICMP extension.
	// Proto must be either the ICMPv4 or ICMPv6 protocol number.
	Len(proto int) int

	// Marshal returns the binary encoding of ICMP extension.
	// Proto must be either the ICMPv4 or ICMPv6 protocol number.
	Marshal(proto int) ([]byte, error)
}

const extensionVersion = 2

func validExtensionHeader(b []byte) bool {
	v := int(b[0]&0xf0) >> 4
	s := binary.BigEndian.Uint16(b[2:4])
	if s != 0 {
		s = c
    
func parseExtensions(b []byte, l int) ([]Extension, int, error) {
	// Still a lot of non-RFC 4884 compliant implementations are
	// out there. Set the length attribute l to 128 when it looks
	// inappropriate for backwards compatibility.
	//
	// A minimal extension at least requires 8 octets; 4 octets
	// for an extension header, and 4 octets for a single object
	// header.
	//
	// See RFC 4884 for further information.
	if 128 > l || l+8 > len(b) {
		l = 128
	}
	if l+8 > len(b) {
		return nil, -1, errNoExtension
	}
	if !validExtensionHeader(b[l:]) {
		if l == 128 {
			return nil, -1, errNoExtension
		}
		l = 128
		if !validExtensionHeader(b[l:]) {
			return nil, -1, errNoExtension
		}
	}
	var exts []Extension
	for b = b[l+4:]; len(b) >= 4; {
		ol := int(binary.BigEndian.Uint16(b[:2]))
		if 4 > ol || ol > len(b) {
			break
		}
		switch b[2] {
		case classMPLSLabelStack:
			ext, err := parseMPLSLabelStack(b[:ol])
			if err != nil {
				return nil, -1, err
			}
			exts = append(exts, ext)
		case classInterfaceInfo:
			ext, err := parseInterfaceInfo(b[:ol])
			if err != nil {
				return nil, -1, err
			}
			exts = append(exts, ext)
		}
		b = b[ol:]
	}
	return exts, l, nil
}