Generic Routing Encapsulation

Exemplo do empilhamento do protocolo

Com base nos princípios de camadas de protocolo do Modelo OSI, o encapsulamento de protocolo, não especificamente GRE, quebra a ordem de estratificação. Pode ser visto como um separador entre duas pilhas de protocolos diferentes, um atuando como portadora pra outra.

Cabeçalho do pacote GRE padrão

A estrutura do cabeçalho GRE padrão, como definido em RFC 2784 e RFC 2890, é representada no diagrama abaixo.

C

Checksum bit. Set to 1 if a checksum is present.

K

Key bit. Set to 1 if a key is present.

S

Sequence number bit. Set to 1 if a sequence number is present.

Reserved0

Reserved bits; set to 0.

Version

GRE Version number; set to 0.

Protocol Type

Indicates the ether protocol type of the encapsulated payload. (For IPv4, this would be hex 0800.)

Checksum

Present if the C bit is set; contains the checksum for the GRE header and payload.

Reserved1

Present if the C bit is set; is set to 0.

Key

Present if the K bit is set; contains an application-specific key value.

Sequence Number

Present if the S bit is set; contains a sequence number for the GRE packet.

Cabeçalho de pacote PPTP GRE

O Point-to-Point Tunneling Protocol (PPTP), definido na RFC 2637, usa uma variante da estrutura GRE como cabeçalho, representada abaixo. PPTP cria um túnel GRE através do qual os pacotes PPTP GRE são enviadas.

C

Checksum bit. For PPTP GRE packets, this is set to 0.

R

Routing bit. For PPTP GRE packets, this is set to 0.

K

Key bit. For PPTP GRE packets, this is set to 1. (All PPTP GRE packets carry a key.)

S

Sequence number bit. Set to 1 if a sequence number is supplied, indicating a PPTP GRE data packet.

s

Strict source route bit. For PPTP GRE packets, this is set to 0.

Recur

Recursion control bits. For PPTP GRE packets, these are set to 0.

A

Acknowledgement number present. Set to 1 if an acknowledgement number is supplied, indicating a PPTP GRE acknowledgement packet.

Flags

Flag bits. For PPTP GRE packets, these are set to 0.

Version

GRE Version number. For PPTP GRE packets, this is set to 1.

Protocol Type

For PPTP GRE packets, this is set to hex 880B.

Key Payload Length

Contains the size of the payload, not including the GRE header.

Key Call ID

Contains the Peer's Call ID for the session to which the packet belongs.

Sequence Number

Present if the S bit is set; contains the GRE payload sequence number.

Acknowledgement Number

Present if the A bit is set; contains the sequence number of the highest GRE payload packet received by the sender.