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88 lines
2.9 KiB
88 lines
2.9 KiB
7 years ago
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// Copyright 2012 Google, Inc. All rights reserved.
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//
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// Use of this source code is governed by a BSD-style license
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// that can be found in the LICENSE file in the root of the source
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// tree.
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package layers
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import (
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"encoding/binary"
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"errors"
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"github.com/google/gopacket"
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)
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// MPLS is the MPLS packet header.
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type MPLS struct {
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BaseLayer
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Label uint32
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TrafficClass uint8
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StackBottom bool
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TTL uint8
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}
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// LayerType returns gopacket.LayerTypeMPLS.
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func (m *MPLS) LayerType() gopacket.LayerType { return LayerTypeMPLS }
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// ProtocolGuessingDecoder attempts to guess the protocol of the bytes it's
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// given, then decode the packet accordingly. Its algorithm for guessing is:
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// If the packet starts with byte 0x45-0x4F: IPv4
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// If the packet starts with byte 0x60-0x6F: IPv6
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// Otherwise: Error
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// See draft-hsmit-isis-aal5mux-00.txt for more detail on this approach.
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type ProtocolGuessingDecoder struct{}
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func (ProtocolGuessingDecoder) Decode(data []byte, p gopacket.PacketBuilder) error {
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switch data[0] {
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// 0x40 | header_len, where header_len is at least 5.
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case 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f:
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return decodeIPv4(data, p)
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// IPv6 can start with any byte whose first 4 bits are 0x6.
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case 0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f:
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return decodeIPv6(data, p)
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}
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return errors.New("Unable to guess protocol of packet data")
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}
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// MPLSPayloadDecoder is the decoder used to data encapsulated by each MPLS
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// layer. MPLS contains no type information, so we have to explicitly decide
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// which decoder to use. This is initially set to ProtocolGuessingDecoder, our
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// simple attempt at guessing protocols based on the first few bytes of data
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// available to us. However, if you know that in your environment MPLS always
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// encapsulates a specific protocol, you may reset this.
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var MPLSPayloadDecoder gopacket.Decoder = ProtocolGuessingDecoder{}
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func decodeMPLS(data []byte, p gopacket.PacketBuilder) error {
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decoded := binary.BigEndian.Uint32(data[:4])
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mpls := &MPLS{
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Label: decoded >> 12,
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TrafficClass: uint8(decoded>>9) & 0x7,
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StackBottom: decoded&0x100 != 0,
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TTL: uint8(decoded),
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BaseLayer: BaseLayer{data[:4], data[4:]},
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}
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p.AddLayer(mpls)
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if mpls.StackBottom {
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return p.NextDecoder(MPLSPayloadDecoder)
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}
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return p.NextDecoder(gopacket.DecodeFunc(decodeMPLS))
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}
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// SerializeTo writes the serialized form of this layer into the
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// SerializationBuffer, implementing gopacket.SerializableLayer.
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// See the docs for gopacket.SerializableLayer for more info.
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func (m *MPLS) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
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bytes, err := b.PrependBytes(4)
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if err != nil {
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return err
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}
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encoded := m.Label << 12
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encoded |= uint32(m.TrafficClass) << 9
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encoded |= uint32(m.TTL)
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if m.StackBottom {
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encoded |= 0x100
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}
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binary.BigEndian.PutUint32(bytes, encoded)
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return nil
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}
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