omci.go - omci-lib-go - Gitiles

 /*
  * Copyright (c) 2018 - present.  Boling Consulting Solutions (bcsw.net)
  * Copyright 2020-present Open Networking Foundation

  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at

  * http://www.apache.org/licenses/LICENSE-2.0

  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 // Package omci provides a library of routines to create, manipulate, serialize, and
 // decode ITU-T G.988 OMCI messages/packets
 package omci

 import (
 	"encoding/binary"
 	"errors"
 	"fmt"
 	"github.com/aead/cmac/aes"
 	me "github.com/opencord/omci-lib-go/generated"
 	"github.com/google/gopacket"
 	"github.com/google/gopacket/layers"
 )

 // DeviceIdent identifies the OMCI message format. Currently either baseline or extended.
 type DeviceIdent byte

 // LayerTypeOmci provide a gopacket LayerType for OMCI messages
 var (
 	LayerTypeOMCI gopacket.LayerType
 )

 func init() {
 	LayerTypeOMCI = gopacket.RegisterLayerType(1000,
 		gopacket.LayerTypeMetadata{
 			Name:    "OMCI",
 			Decoder: gopacket.DecodeFunc(decodeOMCI),
 		})
 }

 const (
 	// Device Identifiers
 	_ = iota
 	// BaselineIdent message are composed of a fixed 40 octet packet + 8-octet trailer. All
 	// G-PON OLTs and ONUs support the baseline message set
 	BaselineIdent DeviceIdent = 0x0A

 	// ExtendedIdent messager are up to 1920 octets but may not be supported by all ONUs or OLTs.
 	ExtendedIdent DeviceIdent = 0x0B
 )

 var omciIK = []byte{0x18, 0x4b, 0x8a, 0xd4, 0xd1, 0xac, 0x4a, 0xf4,
 	0xdd, 0x4b, 0x33, 0x9e, 0xcc, 0x0d, 0x33, 0x70}

 func (di DeviceIdent) String() string {
 	switch di {
 	default:
 		return "Unknown"

 	case BaselineIdent:
 		return "Baseline"

 	case ExtendedIdent:
 		return "Extended"
 	}
 }

 // MaxBaselineLength is the maximum number of octets allowed in an OMCI Baseline
 // message.  Depending on the adapter, it may or may not include the
 const MaxBaselineLength = 48

 // MaxExtendedLength is the maximum number of octets allowed in an OMCI Extended
 // message (including header).
 const MaxExtendedLength = 1980

 // MaxAttributeMibUploadNextBaselineLength is the maximum payload size for attributes for
 // a Baseline MIB Upload Next message.29
 const MaxAttributeMibUploadNextBaselineLength = MaxBaselineLength - 14 - 8

 // MaxAttributeGetNextBaselineLength is the maximum payload size for attributes for
 // a Baseline MIB Get Next message. This is just the attribute portion of the
 // message contents and does not include the Result Code & Attribute Mask.
 const MaxAttributeGetNextBaselineLength = MaxBaselineLength - 11 - 8

 // MaxManagedEntityMibUploadNextExtendedLength is the maximum payload size for ME
 // entries for an Extended MIB Upload Next message. Extended messages differ from
 // the baseline as multiple MEs can be reported in a single frame, just not multiple
 // attributes.
 const MaxManagedEntityMibUploadNextExtendedLength = MaxExtendedLength - 10 - 4

 // MaxAttributeGetNextExtendedLength is the maximum payload size for attributes for
 // a Extended MIB Get Next message. This is just the attribute portion of the
 // message contents and does not include the Result Code & Attribute Mask.
 const MaxAttributeGetNextExtendedLength = MaxExtendedLength - 13 - 4

 // NullEntityID is often used as the Null/void Managed Entity ID for attributes
 // that are used to refer to other Managed Entities but are currently not provisioned.
 const NullEntityID = uint16(0xffff)

 // OMCI defines the common protocol. Extended will be added once
 // I can get basic working (and layered properly).  See ITU-T G.988 11/2017 section
 // A.3 for more information
 type OMCI struct {
 	layers.BaseLayer
 	TransactionID    uint16
 	MessageType      MessageType
 	DeviceIdentifier DeviceIdent
 	ResponseExpected bool // Significant for Download Section Request only
 	Payload          []byte
 	padding          []byte
 	Length           uint16
 	MIC              uint32
 }

 func (omci *OMCI) String() string {
 	//msgType := me.MsgType(byte(omci.MessageType) & me.MsgTypeMask)
 	//if me.IsAutonomousNotification(msgType) {
 	//	return fmt.Sprintf("OMCI: Type: %v:", msgType)
 	//} else if byte(omci.MessageType)&me.AK == me.AK {
 	//	return fmt.Sprintf("OMCI: Type: %v Response", msgType)
 	//}
 	return fmt.Sprintf("Type: %v, TID: %d (%#x), Ident: %v",
 		omci.MessageType, omci.TransactionID, omci.TransactionID, omci.DeviceIdentifier)
 }

 // LayerType returns LayerTypeOMCI
 func (omci *OMCI) LayerType() gopacket.LayerType {
 	return LayerTypeOMCI
 }

 // LayerContents returns the OMCI specific layer information
 func (omci *OMCI) LayerContents() []byte {
 	b := make([]byte, 8)
 	binary.BigEndian.PutUint16(b, omci.TransactionID)
 	b[2] = byte(omci.MessageType)
 	b[3] = byte(omci.DeviceIdentifier)
 	return b
 }

 // CanDecode returns the layers that this class can decode
 func (omci *OMCI) CanDecode() gopacket.LayerClass {
 	return LayerTypeOMCI
 }

 // NextLayerType returns the layer type contained by this DecodingLayer.
 func (omci *OMCI) NextLayerType() gopacket.LayerType {
 	return gopacket.LayerTypeZero
 }

 func decodeOMCI(data []byte, p gopacket.PacketBuilder) error {
 	// Allow baseline messages without Length & MIC, but no less
 	if len(data) < MaxBaselineLength-8 {
 		return errors.New("frame header too small")
 	}
 	switch DeviceIdent(data[3]) {
 	default:
 		return errors.New("unsupported message type")

 	case BaselineIdent:
 		//omci := &BaselineMessage{}
 		omci := &OMCI{}
 		return omci.DecodeFromBytes(data, p)

 	case ExtendedIdent:
 		//omci := &ExtendedMessage{}
 		omci := &OMCI{}
 		return omci.DecodeFromBytes(data, p)
 	}
 }

 func calculateMicAes128(data []byte) (uint32, error) {
 	// See if upstream or downstream
 	var downstreamCDir = [...]byte{0x01}
 	var upstreamCDir = [...]byte{0x02}

 	tid := binary.BigEndian.Uint16(data[0:2])
 	var sum []byte
 	var err error

 	if (data[2]&me.AK) == me.AK || tid == 0 {
 		sum, err = aes.Sum(append(upstreamCDir[:], data[:44]...), omciIK, 4)
 	} else {
 		sum, err = aes.Sum(append(downstreamCDir[:], data[:44]...), omciIK, 4)
 	}
 	if err != nil {
 		return 0, err
 	}
 	return binary.BigEndian.Uint32(sum), nil
 }

 /////////////////////////////////////////////////////////////////////////////
 //   Baseline Message encode / decode

 // DecodeFromBytes will decode the OMCI layer of a packet/message
 func (omci *OMCI) DecodeFromBytes(data []byte, p gopacket.PacketBuilder) error {
 	if len(data) < 10 {
 		p.SetTruncated()
 		return errors.New("frame too small")
 	}
 	omci.TransactionID = binary.BigEndian.Uint16(data[0:])
 	omci.MessageType = MessageType(data[2])
 	omci.DeviceIdentifier = DeviceIdent(data[3])
 	omci.ResponseExpected = byte(omci.MessageType)&me.AR == me.AR

 	isNotification := (int(omci.MessageType) & ^me.MsgTypeMask) == 0
 	if omci.TransactionID == 0 && !isNotification {
 		return errors.New("omci Transaction ID is zero for non-Notification type message")
 	}
 	// Decode length
 	var payloadOffset int
 	var micOffset int
 	if omci.DeviceIdentifier == BaselineIdent {
 		omci.Length = MaxBaselineLength - 8
 		payloadOffset = 8
 		micOffset = MaxBaselineLength - 4

 		if len(data) >= micOffset {
 			length := binary.BigEndian.Uint32(data[micOffset-4:])
 			if uint16(length) != omci.Length {
 				return me.NewProcessingError("invalid baseline message length")
 			}
 		}
 	} else {
 		payloadOffset = 10
 		omci.Length = binary.BigEndian.Uint16(data[8:10])
 		micOffset = int(omci.Length) + payloadOffset

 		if omci.Length > MaxExtendedLength {
 			return me.NewProcessingError("extended frame exceeds maximum allowed")
 		}
 		if int(omci.Length) != micOffset {
 			if int(omci.Length) < micOffset {
 				p.SetTruncated()
 			}
 			return me.NewProcessingError("extended frame too small")
 		}
 	}
 	// Extract MIC if present in the data
 	if len(data) >= micOffset+4 {
 		omci.MIC = binary.BigEndian.Uint32(data[micOffset:])
 		actual, _ := calculateMicAes128(data[:micOffset])
 		if omci.MIC != actual {
 			_ = fmt.Sprintf("invalid MIC, expected %#x, got %#x",
 				omci.MIC, actual)
 			//return errors.New(msg)
 		}
 	}
 	omci.BaseLayer = layers.BaseLayer{data[:4], data[4:]}
 	p.AddLayer(omci)
 	nextLayer, err := MsgTypeToNextLayer(omci.MessageType)
 	if err != nil {
 		return err
 	}
 	return p.NextDecoder(nextLayer)
 }

 // SerializeTo writes the serialized form of this layer into the
 // SerializationBuffer, implementing gopacket.SerializableLayer.
 // See the docs for gopacket.SerializableLayer for more info.
 func (omci *OMCI) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
 	// TODO: Hardcoded for baseline message format for now. Will eventually need to support
 	//       the extended message format.
 	bytes, err := b.PrependBytes(4)
 	if err != nil {
 		return err
 	}
 	// OMCI layer error checks
 	isNotification := (int(omci.MessageType) & ^me.MsgTypeMask) == 0
 	if omci.TransactionID == 0 && !isNotification {
 		return errors.New("omci Transaction ID is zero for non-Notification type message")
 	}
 	if omci.DeviceIdentifier == 0 {
 		omci.DeviceIdentifier = BaselineIdent // Allow uninitialized device identifier
 	}
 	if omci.DeviceIdentifier == BaselineIdent {
 		if omci.Length == 0 {
 			omci.Length = MaxBaselineLength - 8 // Allow uninitialized length
 		} else if omci.Length != MaxBaselineLength-8 {
 			msg := fmt.Sprintf("invalid Baseline message length: %v", omci.Length)
 			return errors.New(msg)
 		}
 	} else if omci.DeviceIdentifier == ExtendedIdent {
 		if omci.Length == 0 {
 			omci.Length = uint16(len(bytes) - 10) // Allow uninitialized length
 		}
 		if omci.Length > MaxExtendedLength {
 			msg := fmt.Sprintf("invalid Baseline message length: %v", omci.Length)
 			return errors.New(msg)
 		}
 	} else {
 		msg := fmt.Sprintf("invalid device identifier: %#x, Baseline or Extended expected",
 			omci.DeviceIdentifier)
 		return errors.New(msg)
 	}
 	binary.BigEndian.PutUint16(bytes, omci.TransactionID)
 	// Download section request can optionally have the AR bit set or cleared.  If user passes in this
 	// message type and sets download requested, fix up the message type for them.
 	if omci.MessageType == DownloadSectionRequestType && omci.ResponseExpected {
 		bytes[2] = byte(DownloadSectionRequestWithResponseType)
 	} else {
 		bytes[2] = byte(omci.MessageType)
 	}
 	bytes[3] = byte(omci.DeviceIdentifier)
 	b.PushLayer(LayerTypeOMCI)

 	bufLen := len(b.Bytes())
 	padSize := int(omci.Length) - bufLen + 4
 	if padSize < 0 {
 		msg := fmt.Sprintf("invalid OMCI Message Type length, exceeded allowed frame size by %d bytes",
 			-padSize)
 		return errors.New(msg)
 	}
 	padding, err := b.AppendBytes(padSize)
 	copy(padding, lotsOfZeros[:])

 	if omci.DeviceIdentifier == BaselineIdent {
 		// For baseline, always provide the length
 		binary.BigEndian.PutUint32(b.Bytes()[MaxBaselineLength-8:], 40)
 	}
 	if opts.ComputeChecksums {
 		micBytes, err := b.AppendBytes(4)
 		if err != nil {
 			return err
 		}
 		omci.MIC, _ = calculateMicAes128(bytes[:MaxBaselineLength-4])
 		binary.BigEndian.PutUint32(micBytes, omci.MIC)
 	}
 	return nil
 }

 // hacky way to zero out memory... there must be a better way?
 var lotsOfZeros [MaxExtendedLength]byte // Extended OMCI messages may be up to 1980 bytes long, including headers
	/*
	* Copyright (c) 2018 - present. Boling Consulting Solutions (bcsw.net)
	* Copyright 2020-present Open Networking Foundation

	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at

	* http://www.apache.org/licenses/LICENSE-2.0

	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	// Package omci provides a library of routines to create, manipulate, serialize, and
	// decode ITU-T G.988 OMCI messages/packets
	package omci

	import (
	"encoding/binary"
	"errors"
	"fmt"
	"github.com/aead/cmac/aes"
	me "github.com/opencord/omci-lib-go/generated"
	"github.com/google/gopacket"
	"github.com/google/gopacket/layers"
	)

	// DeviceIdent identifies the OMCI message format. Currently either baseline or extended.
	type DeviceIdent byte

	// LayerTypeOmci provide a gopacket LayerType for OMCI messages
	var (
	LayerTypeOMCI gopacket.LayerType
	)

	func init() {
	LayerTypeOMCI = gopacket.RegisterLayerType(1000,
	gopacket.LayerTypeMetadata{
	Name: "OMCI",
	Decoder: gopacket.DecodeFunc(decodeOMCI),
	})
	}

	const (
	// Device Identifiers
	_ = iota
	// BaselineIdent message are composed of a fixed 40 octet packet + 8-octet trailer. All
	// G-PON OLTs and ONUs support the baseline message set
	BaselineIdent DeviceIdent = 0x0A

	// ExtendedIdent messager are up to 1920 octets but may not be supported by all ONUs or OLTs.
	ExtendedIdent DeviceIdent = 0x0B
	)

	var omciIK = []byte{0x18, 0x4b, 0x8a, 0xd4, 0xd1, 0xac, 0x4a, 0xf4,
	0xdd, 0x4b, 0x33, 0x9e, 0xcc, 0x0d, 0x33, 0x70}

	func (di DeviceIdent) String() string {
	switch di {
	default:
	return "Unknown"

	case BaselineIdent:
	return "Baseline"

	case ExtendedIdent:
	return "Extended"
	}
	}

	// MaxBaselineLength is the maximum number of octets allowed in an OMCI Baseline
	// message. Depending on the adapter, it may or may not include the
	const MaxBaselineLength = 48

	// MaxExtendedLength is the maximum number of octets allowed in an OMCI Extended
	// message (including header).
	const MaxExtendedLength = 1980

	// MaxAttributeMibUploadNextBaselineLength is the maximum payload size for attributes for
	// a Baseline MIB Upload Next message.29
	const MaxAttributeMibUploadNextBaselineLength = MaxBaselineLength - 14 - 8

	// MaxAttributeGetNextBaselineLength is the maximum payload size for attributes for
	// a Baseline MIB Get Next message. This is just the attribute portion of the
	// message contents and does not include the Result Code & Attribute Mask.
	const MaxAttributeGetNextBaselineLength = MaxBaselineLength - 11 - 8

	// MaxManagedEntityMibUploadNextExtendedLength is the maximum payload size for ME
	// entries for an Extended MIB Upload Next message. Extended messages differ from
	// the baseline as multiple MEs can be reported in a single frame, just not multiple
	// attributes.
	const MaxManagedEntityMibUploadNextExtendedLength = MaxExtendedLength - 10 - 4

	// MaxAttributeGetNextExtendedLength is the maximum payload size for attributes for
	// a Extended MIB Get Next message. This is just the attribute portion of the
	// message contents and does not include the Result Code & Attribute Mask.
	const MaxAttributeGetNextExtendedLength = MaxExtendedLength - 13 - 4

	// NullEntityID is often used as the Null/void Managed Entity ID for attributes
	// that are used to refer to other Managed Entities but are currently not provisioned.
	const NullEntityID = uint16(0xffff)

	// OMCI defines the common protocol. Extended will be added once
	// I can get basic working (and layered properly). See ITU-T G.988 11/2017 section
	// A.3 for more information
	type OMCI struct {
	layers.BaseLayer
	TransactionID uint16
	MessageType MessageType
	DeviceIdentifier DeviceIdent
	ResponseExpected bool // Significant for Download Section Request only
	Payload []byte
	padding []byte
	Length uint16
	MIC uint32
	}

	func (omci *OMCI) String() string {
	//msgType := me.MsgType(byte(omci.MessageType) & me.MsgTypeMask)
	//if me.IsAutonomousNotification(msgType) {
	// return fmt.Sprintf("OMCI: Type: %v:", msgType)
	//} else if byte(omci.MessageType)&me.AK == me.AK {
	// return fmt.Sprintf("OMCI: Type: %v Response", msgType)
	//}
	return fmt.Sprintf("Type: %v, TID: %d (%#x), Ident: %v",
	omci.MessageType, omci.TransactionID, omci.TransactionID, omci.DeviceIdentifier)
	}

	// LayerType returns LayerTypeOMCI
	func (omci *OMCI) LayerType() gopacket.LayerType {
	return LayerTypeOMCI
	}

	// LayerContents returns the OMCI specific layer information
	func (omci *OMCI) LayerContents() []byte {
	b := make([]byte, 8)
	binary.BigEndian.PutUint16(b, omci.TransactionID)
	b[2] = byte(omci.MessageType)
	b[3] = byte(omci.DeviceIdentifier)
	return b
	}

	// CanDecode returns the layers that this class can decode
	func (omci *OMCI) CanDecode() gopacket.LayerClass {
	return LayerTypeOMCI
	}

	// NextLayerType returns the layer type contained by this DecodingLayer.
	func (omci *OMCI) NextLayerType() gopacket.LayerType {
	return gopacket.LayerTypeZero
	}

	func decodeOMCI(data []byte, p gopacket.PacketBuilder) error {
	// Allow baseline messages without Length & MIC, but no less
	if len(data) < MaxBaselineLength-8 {
	return errors.New("frame header too small")
	}
	switch DeviceIdent(data[3]) {
	default:
	return errors.New("unsupported message type")

	case BaselineIdent:
	//omci := &BaselineMessage{}
	omci := &OMCI{}
	return omci.DecodeFromBytes(data, p)

	case ExtendedIdent:
	//omci := &ExtendedMessage{}
	omci := &OMCI{}
	return omci.DecodeFromBytes(data, p)
	}
	}

	func calculateMicAes128(data []byte) (uint32, error) {
	// See if upstream or downstream
	var downstreamCDir = [...]byte{0x01}
	var upstreamCDir = [...]byte{0x02}

	tid := binary.BigEndian.Uint16(data[0:2])
	var sum []byte
	var err error

	if (data[2]&me.AK) == me.AK \|\| tid == 0 {
	sum, err = aes.Sum(append(upstreamCDir[:], data[:44]...), omciIK, 4)
	} else {
	sum, err = aes.Sum(append(downstreamCDir[:], data[:44]...), omciIK, 4)
	}
	if err != nil {
	return 0, err
	}
	return binary.BigEndian.Uint32(sum), nil
	}

	/////////////////////////////////////////////////////////////////////////////
	// Baseline Message encode / decode

	// DecodeFromBytes will decode the OMCI layer of a packet/message
	func (omci *OMCI) DecodeFromBytes(data []byte, p gopacket.PacketBuilder) error {
	if len(data) < 10 {
	p.SetTruncated()
	return errors.New("frame too small")
	}
	omci.TransactionID = binary.BigEndian.Uint16(data[0:])
	omci.MessageType = MessageType(data[2])
	omci.DeviceIdentifier = DeviceIdent(data[3])
	omci.ResponseExpected = byte(omci.MessageType)&me.AR == me.AR

	isNotification := (int(omci.MessageType) & ^me.MsgTypeMask) == 0
	if omci.TransactionID == 0 && !isNotification {
	return errors.New("omci Transaction ID is zero for non-Notification type message")
	}
	// Decode length
	var payloadOffset int
	var micOffset int
	if omci.DeviceIdentifier == BaselineIdent {
	omci.Length = MaxBaselineLength - 8
	payloadOffset = 8
	micOffset = MaxBaselineLength - 4

	if len(data) >= micOffset {
	length := binary.BigEndian.Uint32(data[micOffset-4:])
	if uint16(length) != omci.Length {
	return me.NewProcessingError("invalid baseline message length")
	}
	}
	} else {
	payloadOffset = 10
	omci.Length = binary.BigEndian.Uint16(data[8:10])
	micOffset = int(omci.Length) + payloadOffset

	if omci.Length > MaxExtendedLength {
	return me.NewProcessingError("extended frame exceeds maximum allowed")
	}
	if int(omci.Length) != micOffset {
	if int(omci.Length) < micOffset {
	p.SetTruncated()
	}
	return me.NewProcessingError("extended frame too small")
	}
	}
	// Extract MIC if present in the data
	if len(data) >= micOffset+4 {
	omci.MIC = binary.BigEndian.Uint32(data[micOffset:])
	actual, _ := calculateMicAes128(data[:micOffset])
	if omci.MIC != actual {
	_ = fmt.Sprintf("invalid MIC, expected %#x, got %#x",
	omci.MIC, actual)
	//return errors.New(msg)
	}
	}
	omci.BaseLayer = layers.BaseLayer{data[:4], data[4:]}
	p.AddLayer(omci)
	nextLayer, err := MsgTypeToNextLayer(omci.MessageType)
	if err != nil {
	return err
	}
	return p.NextDecoder(nextLayer)
	}

	// SerializeTo writes the serialized form of this layer into the
	// SerializationBuffer, implementing gopacket.SerializableLayer.
	// See the docs for gopacket.SerializableLayer for more info.
	func (omci *OMCI) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
	// TODO: Hardcoded for baseline message format for now. Will eventually need to support
	// the extended message format.
	bytes, err := b.PrependBytes(4)
	if err != nil {
	return err
	}
	// OMCI layer error checks
	isNotification := (int(omci.MessageType) & ^me.MsgTypeMask) == 0
	if omci.TransactionID == 0 && !isNotification {
	return errors.New("omci Transaction ID is zero for non-Notification type message")
	}
	if omci.DeviceIdentifier == 0 {
	omci.DeviceIdentifier = BaselineIdent // Allow uninitialized device identifier
	}
	if omci.DeviceIdentifier == BaselineIdent {
	if omci.Length == 0 {
	omci.Length = MaxBaselineLength - 8 // Allow uninitialized length
	} else if omci.Length != MaxBaselineLength-8 {
	msg := fmt.Sprintf("invalid Baseline message length: %v", omci.Length)
	return errors.New(msg)
	}
	} else if omci.DeviceIdentifier == ExtendedIdent {
	if omci.Length == 0 {
	omci.Length = uint16(len(bytes) - 10) // Allow uninitialized length
	}
	if omci.Length > MaxExtendedLength {
	msg := fmt.Sprintf("invalid Baseline message length: %v", omci.Length)
	return errors.New(msg)
	}
	} else {
	msg := fmt.Sprintf("invalid device identifier: %#x, Baseline or Extended expected",
	omci.DeviceIdentifier)
	return errors.New(msg)
	}
	binary.BigEndian.PutUint16(bytes, omci.TransactionID)
	// Download section request can optionally have the AR bit set or cleared. If user passes in this
	// message type and sets download requested, fix up the message type for them.
	if omci.MessageType == DownloadSectionRequestType && omci.ResponseExpected {
	bytes[2] = byte(DownloadSectionRequestWithResponseType)
	} else {
	bytes[2] = byte(omci.MessageType)
	}
	bytes[3] = byte(omci.DeviceIdentifier)
	b.PushLayer(LayerTypeOMCI)

	bufLen := len(b.Bytes())
	padSize := int(omci.Length) - bufLen + 4
	if padSize < 0 {
	msg := fmt.Sprintf("invalid OMCI Message Type length, exceeded allowed frame size by %d bytes",
	-padSize)
	return errors.New(msg)
	}
	padding, err := b.AppendBytes(padSize)
	copy(padding, lotsOfZeros[:])

	if omci.DeviceIdentifier == BaselineIdent {
	// For baseline, always provide the length
	binary.BigEndian.PutUint32(b.Bytes()[MaxBaselineLength-8:], 40)
	}
	if opts.ComputeChecksums {
	micBytes, err := b.AppendBytes(4)
	if err != nil {
	return err
	}
	omci.MIC, _ = calculateMicAes128(bytes[:MaxBaselineLength-4])
	binary.BigEndian.PutUint32(micBytes, omci.MIC)
	}
	return nil
	}

	// hacky way to zero out memory... there must be a better way?
	var lotsOfZeros [MaxExtendedLength]byte // Extended OMCI messages may be up to 1980 bytes long, including headers