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// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.
package layers
import (
"encoding/binary"
"errors"
"github.com/google/gopacket"
)
// LLC is the layer used for 802.2 Logical Link Control headers.
// See http://standards.ieee.org/getieee802/download/802.2-1998.pdf
type LLC struct {
BaseLayer
DSAP uint8
IG bool // true means group, false means individual
SSAP uint8
CR bool // true means response, false means command
Control uint16
}
// LayerType returns gopacket.LayerTypeLLC.
func (l *LLC) LayerType() gopacket.LayerType { return LayerTypeLLC }
// SNAP is used inside LLC. See
// http://standards.ieee.org/getieee802/download/802-2001.pdf.
// From http://en.wikipedia.org/wiki/Subnetwork_Access_Protocol:
// "[T]he Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing,
// on networks using IEEE 802.2 LLC, more protocols than can be distinguished
// by the 8-bit 802.2 Service Access Point (SAP) fields."
type SNAP struct {
BaseLayer
OrganizationalCode []byte
Type EthernetType
}
// LayerType returns gopacket.LayerTypeSNAP.
func (s *SNAP) LayerType() gopacket.LayerType { return LayerTypeSNAP }
func decodeLLC(data []byte, p gopacket.PacketBuilder) error {
l := &LLC{
DSAP: data[0] & 0xFE,
IG: data[0]&0x1 != 0,
SSAP: data[1] & 0xFE,
CR: data[1]&0x1 != 0,
Control: uint16(data[2]),
}
if l.Control&0x1 == 0 || l.Control&0x3 == 0x1 {
l.Control = l.Control<<8 | uint16(data[3])
l.Contents = data[:4]
l.Payload = data[4:]
} else {
l.Contents = data[:3]
l.Payload = data[3:]
}
p.AddLayer(l)
switch {
case l.DSAP == 0xAA && l.SSAP == 0xAA:
return p.NextDecoder(LayerTypeSNAP)
case l.DSAP == 0x42 && l.SSAP == 0x42:
return p.NextDecoder(LayerTypeSTP)
}
return p.NextDecoder(gopacket.DecodeUnknown)
}
func decodeSNAP(data []byte, p gopacket.PacketBuilder) error {
s := &SNAP{
OrganizationalCode: data[:3],
Type: EthernetType(binary.BigEndian.Uint16(data[3:5])),
BaseLayer: BaseLayer{data[:5], data[5:]},
}
p.AddLayer(s)
// BUG(gconnell): When decoding SNAP, we treat the SNAP type as an Ethernet
// type. This may not actually be an ethernet type in all cases,
// depending on the organizational code. Right now, we don't check.
return p.NextDecoder(s.Type)
}
// SerializeTo writes the serialized form of this layer into the
// SerializationBuffer, implementing gopacket.SerializableLayer.
// See the docs for gopacket.SerializableLayer for more info.
func (l *LLC) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
var ig_flag, cr_flag byte
var length int
if l.Control&0xFF00 != 0 {
length = 4
} else {
length = 3
}
if l.DSAP&0x1 != 0 {
return errors.New("DSAP value invalid, should not include IG flag bit")
}
if l.SSAP&0x1 != 0 {
return errors.New("SSAP value invalid, should not include CR flag bit")
}
if buf, err := b.PrependBytes(length); err != nil {
return err
} else {
ig_flag = 0
if l.IG {
ig_flag = 0x1
}
cr_flag = 0
if l.CR {
cr_flag = 0x1
}
buf[0] = l.DSAP + ig_flag
buf[1] = l.SSAP + cr_flag
if length == 4 {
buf[2] = uint8(l.Control >> 8)
buf[3] = uint8(l.Control)
} else {
buf[2] = uint8(l.Control)
}
}
return nil
}
// SerializeTo writes the serialized form of this layer into the
// SerializationBuffer, implementing gopacket.SerializableLayer.
// See the docs for gopacket.SerializableLayer for more info.
func (s *SNAP) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
if buf, err := b.PrependBytes(5); err != nil {
return err
} else {
buf[0] = s.OrganizationalCode[0]
buf[1] = s.OrganizationalCode[1]
buf[2] = s.OrganizationalCode[2]
binary.BigEndian.PutUint16(buf[3:5], uint16(s.Type))
}
return nil
}
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