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     ng_pptpgre — PPTP GRE protocol netgraph node type


     #include <sys/types.h>
     #include <netgraph/ng_pptpgre.h>


     The pptpgre node type performs Generic Routing Encapsulation (GRE) over IP for the PPTP
     protocol as specified by RFC 2637.  This involves packet encapsulation, sequencing,
     acknowledgement, and an adaptive timeout sliding window mechanism.  This node type does not
     handle any of the TCP control protocol or call negotiation defined by PPTP.

     This node type expects to receive complete IP packets, including the IP header, on the
     “lower” hook, but it transmits outgoing frames without any IP header.  The typical use for
     this node type would be to connect the “upper” hook to one of the link hooks of a ng_ppp(4)
     node, and the “lower” hook to the “inet/raw/gre” hook of a ng_ksocket(4) node.


     This node type supports the following hooks:

     session_hhhh  Session 0xhhhh data packets to the upper protocol layers

     upper         Same as session_hhhh, but for single session with configurable cid (legacy)

     lower         Connection to the lower protocol layers


     This node type supports the generic control messages, plus the following:

     NGM_PPTPGRE_SET_CONFIG (setconfig)
          This command resets and configures hook for a session.  If corresponding session_hhhh
          hook is not connected, upper hook will be configured.  This command takes a struct
          ng_pptpgre_conf as an argument:

          /* Configuration for a session */
          struct ng_pptpgre_conf {
              u_char      enabled;          /* enables traffic flow */
              u_char      enableDelayedAck; /* enables delayed acks */
              u_char      enableAlwaysAck;  /* always include ack with data */
              u_char      enableWindowing;  /* enable windowing algorithm */
              uint16_t    cid;              /* my call id */
              uint16_t    peerCid;          /* peer call id */
              uint16_t    recvWin;          /* peer recv window size */
              uint16_t    peerPpd;          /* peer packet processing delay
                                               (in 1/10 of a second) */

          The enabled field enables traffic flow through the node.  The enableDelayedAck field
          enables delayed acknowledgement (maximum 250 milliseconds), which is a useful
          optimization and should generally be turned on.  enableAlwaysAck field enables sending
          acknowledgements with every data packet, which is probably helpful as well.

          enableWindowing enables the PPTP packet windowing mechanism specified by the protocol.
          Disabling this will cause the node to violate the protocol, possibly confusing other
          PPTP peers, but often results in better performance.  The windowing mechanism is a
          design error in the PPTP protocol; L2TP, the successor to PPTP, removes it.

          The remaining fields are as supplied by the PPTP virtual call setup process.

     NGM_PPTPGRE_GET_CONFIG (getconfig)
          Takes two byte argument as cid and returns the current configuration as a struct

     NGM_PPTPGRE_GET_STATS (getstats)
          This command returns a struct ng_pptpgre_stats containing various node statistics.

     NGM_PPTPGRE_CLR_STATS (clrstats)
          This command resets the node statistics.

     NGM_PPTPGRE_GETCLR_STATS (getclrstats)
          This command atomically gets and resets the node statistics, returning a struct


     This node shuts down upon receipt of a NGM_SHUTDOWN control message, or when both hooks have
     been disconnected.


     A set of sysctl(8) variables controls ability of this node to deal with some amount of
     packet reorder that sometimes happens in transit.  Packet reorder results in packet drops
     (unless the order is restored) as PPP protocol can not deliver reordered data.  These
     variables are shown below together with their default value and meaning:

     net.graph.pptpgre.reorder_max: 1
             Defines maximum length of node's private reorder queue used to keep data waiting for
             late packets.  Zero value disables reordering.  Default value allows the node to
             restore the order for two packets swapped in transit.  Greater values allow the node
             to deliver packets being late after more packets in sequence at cost of increased
             kernel memory usage.

     net.graph.pptpgre.reorder_timeout: 1
             Defines time value in miliseconds used to wait for late packets.


     netgraph(4), ng_ksocket(4), ng_ppp(4), ngctl(8), sysctl(8)

     K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, and G. Zorn, Point-to-Point Tunneling
     Protocol (PPTP), RFC 2637.

     S. Hanks, T. Li, D. Farinacci, and P. Traina, Generic Routing Encapsulation over IPv4
     networks, RFC 1702.


     The ng_pptpgre node type was implemented in FreeBSD 4.0.


     Archie Cobbs <>


     The node should not expect incoming GRE packets to have an IP header.  This behavior is
     inherited from the (converse) behavior of raw IP sockets.  An intermediate node that strips
     IP headers in one direction should be used instead.