| Generic Routing Encapsulation |
Article Index for Generic |
Website Links For Generic |
Information AboutGeneric Routing Encapsulation |
| CATEGORIES ABOUT GENERIC ROUTING ENCAPSULATION | |
| tunneling protocols | |
|
GRE was developed by Cisco and was designed to be Stateless ; the tunnel end-points do not monitor the state or availability of other tunnel end-points. This feature helps service providers support IP tunnels for clients, who won't know the service provider's internal tunneling architecture; and it gives clients the flexibility of reconfiguring their IP architectures without worrying about connectivity. GRE creates a virtual point-to-point link with routers at remote points on an IP internetwork. GRE uses IP Protocol number 47. EXAMPLE USES
EXAMPLE PROTOCOL STACK USING GRE RADIUS UDP IPv6 (2) GRE IPv4 (1) Ethernet HEADER DETAILS 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 ┌─┬─┬─┬─┬─┬─────┬─────────┬─────┬───────────────────────────────┐ │C│R│K│S│s│Recur│ Flags │ Ver │ Protocol Type │ ├─┴─┴─┴─┴─┴─────┴─────────┴─────┼───────────────────────────────┤ │ Checksum (optional) │ Offset (optional) │ ├───────────────────────────────┴───────────────────────────────┤ │ Key (optional) │ ├───────────────────────────────────────────────────────────────┤ │ Sequence Number (optional) │ ├───────────────────────────────────────────────────────────────┤ │ Routing (optional) │ └───────────────────────────────────────────────────────────────┘ C, Checksum Present. 1 bit. The Checksum field is present and contains valid information if set. If either the Checksum Present bit or the Routing Present bit are set, the Checksum and Offset fields are both present. R, Routing Present. 1 bit. If set then the Offset field is present and contains valid information. If either the Checksum Present bit or the Routing Present bit are set, the Checksum and Offset fields are both present. K, Key Present. 1 bit. If set then the Key field is present and contains valid information. S, Sequence Number present. 1 bit. If set then the Sequence Number field is present and contains valid information. s, Strict Source Route. 1 bit. The meaning of this bit is defined in other documents. It is recommended that this bit only be set if all of the the Routing Information consists of Strict Source Routes. Recur, Recursion Control. 3 bits, unsigned integer. Contains the number of additional encapsulations which are permitted. 0 is the default. Flags. 5 bits. These bits are reserved and must be transmitted as 0. Version. 3 bits. GRE protocol version. Must be cleared to 0. Protocol. 16 bits. Contains the protocol type of the payload packet. In general, the value will be the Ethernet protocol type field for the packet. Additional values may be defined in other documents. Checksum. 16 bits. Optional. Contains the IP (one's complement) checksum of the GRE header and the payload packet. Offset. 16 bits. Optional. Indicates the byte offset from the start of the Routing field to the first byte of the active Source Route Entry to be examined. Key. 32 bits. Optional. Contains a number which was inserted by the encapsulator. It may be used by the receiver to authenticate the source of the packet. Sequence Number. 32 bits, unsigned integer. Optional. Contains a number which is inserted by the encapsulator. It may be used by the receiver to establish the order in which packets have been transmitted from the encapsulator to the receiver. Routing. Variable length. Optional. This field is a list of SREs. REFERENCES
|
|
|