text: // Copyright 2018 The gVisor Authors.
  //
  // 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 header
  
  const (
  	typeHLen   = 0
  	encapProto = 1
  )
  
  // GUEFields contains the fields of a GUE packet. It is used to describe the
  // fields of a packet that needs to be encoded.
  type GUEFields struct {
  	// Type is the "type" field of the GUE header.
  	Type uint8
  
  	// Control is the "control" field of the GUE header.
  	Control bool
  
  	// HeaderLength is the "header length" field of the GUE header. It must
  	// be at least 4 octets, and a multiple of 4 as well.
  	HeaderLength uint8
  
  	// Protocol is the "protocol" field of the GUE header. This is one of
  	// the IPPROTO_* values.
  	Protocol uint8
  }
  
  // GUE represents a Generic UDP Encapsulation header stored in a byte array, the
  // fields are described in https://tools.ietf.org/html/draft-ietf-nvo3-gue-01.
  type GUE []byte
  
  const (
  	// GUEMinimumSize is the minimum size of a valid GUE packet.
  	GUEMinimumSize = 4
  )
  
  // TypeAndControl returns the GUE packet type (top 3 bits of the first byte,
  // which includes the control bit).
  func (b GUE) TypeAndControl() uint8 {
  	return b[typeHLen] >> 5
  }
  
  // HeaderLength returns the total length of the GUE header.
  func (b GUE) HeaderLength() uint8 {
  	return 4 + 4*(b[typeHLen]&0x1f)
  }
  
  // Protocol returns the protocol field of the GUE header.
  func (b GUE) Protocol() uint8 {
  	return b[encapProto]
  }
  
  // Encode encodes all the fields of the GUE header.
  func (b GUE) Encode(i *GUEFields) {
  	ctl := uint8(0)
  	if i.Control {
  		ctl = 1 << 5
  	}
  	b[typeHLen] = ctl | i.Type<<6 | (i.HeaderLength-4)/4
  	b[encapProto] = i.Protocol
  }
  