RFC 2890 (rfc2890) - Page 3 of 7

Key and Sequence Number Extensions to GRE

RFC 2890       Key and Sequence Number Extensions to GRE  September 2000


2.1. Key Field (4 octets)

   The Key field contains a four octet number which was inserted by the
   encapsulator. The actual method by which this Key is obtained is
   beyond the scope of the document. The Key field is intended to be
   used for identifying an individual traffic flow within a tunnel. For
   example, packets may need to be routed based on context information
   not present in the encapsulated data.  The Key field provides this
   context and defines a logical traffic flow between encapsulator and
   decapsulator.  Packets belonging to a traffic flow are encapsulated
   using the same Key value and the decapsulating tunnel endpoint
   identifies packets belonging to a traffic flow based on the Key Field
   value.

2.2. Sequence Number (4 octets)

   The Sequence Number field is a four byte field and is inserted by the
   encapsulator when Sequence Number Present Bit is set. The Sequence
   Number MUST be used by the receiver to establish the order in which
   packets have been transmitted from the encapsulator to the receiver.
   The intended use of the Sequence Field is to provide unreliable but
   in-order delivery. If the Key present bit (bit 2) is set, the
   sequence number is specific to the traffic flow identified by the Key
   field. Note that packets without the sequence bit set can be
   interleaved with packets with the sequence bit set.

   The sequence number value ranges from 0 to (2**32)-1. The first
   datagram is sent with a sequence number of 0. The sequence number is
   thus a free running counter represented modulo 2**32.  The receiver
   maintains the sequence number value of the last successfully
   decapsulated packet. Upon establishment of the GRE tunnel, this value
   should be set to (2**32)-1.

   When the decapsulator receives an out-of sequence packet it SHOULD be
   silently discarded. A packet is considered an out-of-sequence packet
   if the sequence number of the received packet is less than or equal
   to the sequence number of last successfully decapsulated packet. The
   sequence number of a received message is considered less than or
   equal to the last successfully received sequence number if its value
   lies in the range of the last received sequence number and the
   preceding 2**31-1 values, inclusive.

   If the received packet is an in-sequence packet, it is successfully
   decapsulated. An in-sequence packet is one with a sequence number
   exactly 1 greater than (modulo 2**32) the last successfully
   decapsulated packet, or one in which the sequence number field is not
   present (S bit not set).




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