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// Copyright 2019 The GoPacket Authors. 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
// This file implements the ASF-RMCP header specified in section of
import (
// RMCPClass is the class of a RMCP layer's payload, e.g. ASF or IPMI. This is a
// 4-bit unsigned int on the wire; all but 6 (ASF), 7 (IPMI) and 8 (OEM-defined)
// are currently reserved.
type RMCPClass uint8
// LayerType returns the payload layer type corresponding to a RMCP class.
func (c RMCPClass) LayerType() gopacket.LayerType {
if lt := rmcpClassLayerTypes[uint8(c)]; lt != 0 {
return lt
return gopacket.LayerTypePayload
func (c RMCPClass) String() string {
return fmt.Sprintf("%v(%v)", uint8(c), c.LayerType())
const (
// RMCPVersion1 identifies RMCP v1.0 in the Version header field. Lower
// values are considered legacy, while higher values are reserved by the
// specification.
RMCPVersion1 uint8 = 0x06
// RMCPNormal indicates a "normal" message, i.e. not an acknowledgement.
RMCPNormal uint8 = 0
// RMCPAck indicates a message is acknowledging a received normal message.
RMCPAck uint8 = 1 << 7
// RMCPClassASF identifies an RMCP message as containing an ASF-RMCP
// payload.
RMCPClassASF RMCPClass = 0x06
// RMCPClassIPMI identifies an RMCP message as containing an IPMI payload.
RMCPClassIPMI RMCPClass = 0x07
// RMCPClassOEM identifies an RMCP message as containing an OEM-defined
// payload.
RMCPClassOEM RMCPClass = 0x08
var (
rmcpClassLayerTypes = [16]gopacket.LayerType{
RMCPClassASF: LayerTypeASF,
// RMCPClassIPMI is to implement; RMCPClassOEM is deliberately not
// implemented, so we return LayerTypePayload
// RegisterRMCPLayerType allows specifying that the payload of a RMCP packet of
// a certain class should processed by the provided layer type. This overrides
// any existing registrations, including defaults.
func RegisterRMCPLayerType(c RMCPClass, l gopacket.LayerType) {
rmcpClassLayerTypes[c] = l
// RMCP describes the format of an RMCP header, which forms a UDP payload. See
// section
type RMCP struct {
// Version identifies the version of the RMCP header. 0x06 indicates RMCP
// v1.0; lower values are legacy, higher values are reserved.
Version uint8
// Sequence is the sequence number assicated with the message. Note that
// this rolls over to 0 after 254, not 255. Seq num 255 indicates the
// receiver must not send an ACK.
Sequence uint8
// Ack indicates whether this packet is an acknowledgement. If it is, the
// payload will be empty.
Ack bool
// Class idicates the structure of the payload. There are only 2^4 valid
// values, however there is no uint4 data type. N.B. the Ack bit has been
// split off into another field. The most significant 4 bits of this field
// will always be 0.
Class RMCPClass
// LayerType returns LayerTypeRMCP. It partially satisfies Layer and
// SerializableLayer.
func (*RMCP) LayerType() gopacket.LayerType {
return LayerTypeRMCP
// CanDecode returns LayerTypeRMCP. It partially satisfies DecodingLayer.
func (r *RMCP) CanDecode() gopacket.LayerClass {
return r.LayerType()
// DecodeFromBytes makes the layer represent the provided bytes. It partially
// satisfies DecodingLayer.
func (r *RMCP) DecodeFromBytes(data []byte, df gopacket.DecodeFeedback) error {
if len(data) < 4 {
return fmt.Errorf("invalid RMCP header, length %v less than 4",
r.BaseLayer.Contents = data[:4]
r.BaseLayer.Payload = data[4:]
r.Version = uint8(data[0])
// 1 byte reserved
r.Sequence = uint8(data[2])
r.Ack = data[3]&RMCPAck != 0
r.Class = RMCPClass(data[3] & 0xF)
return nil
// NextLayerType returns the data layer of this RMCP layer. This partially
// satisfies DecodingLayer.
func (r *RMCP) NextLayerType() gopacket.LayerType {
return r.Class.LayerType()
// Payload returns the data layer. It partially satisfies ApplicationLayer.
func (r *RMCP) Payload() []byte {
return r.BaseLayer.Payload
// SerializeTo writes the serialized fom of this layer into the SerializeBuffer,
// partially satisfying SerializableLayer.
func (r *RMCP) SerializeTo(b gopacket.SerializeBuffer, _ gopacket.SerializeOptions) error {
// The IPMI v1.5 spec contains a pad byte for frame sizes of certain lengths
// to work around issues in LAN chips. This is no longer necessary as of
// IPMI v2.0 (renamed to "legacy pad") so we do not attempt to add it. The
// same approach is taken by FreeIPMI:
bytes, err := b.PrependBytes(4)
if err != nil {
return err
bytes[0] = r.Version
bytes[1] = 0x00
bytes[2] = r.Sequence
bytes[3] = bool2uint8(r.Ack)<<7 | uint8(r.Class) // thanks, BFD layer
return nil
// decodeRMCP decodes the byte slice into an RMCP type, and sets the application
// layer to it.
func decodeRMCP(data []byte, p gopacket.PacketBuilder) error {
rmcp := &RMCP{}
err := rmcp.DecodeFromBytes(data, p)
if err != nil {
return err
return p.NextDecoder(rmcp.NextLayerType())
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