Implementation.

Socket option IPV6_FLOWLABEL_MGR allows to request flow label manager to allocate new flow label, to reuse already allocated one or to delete old flow label. Its argument is struct in6_flowlabel_req:

struct in6_flowlabel_req
{
        struct in6_addr flr_dst;
        __u32           flr_label;
        __u8            flr_action;
        __u8            flr_share;
        __u16           flr_flags;
        __u16           flr_expires;
        __u16           flr_linger;
        __u32         __flr_reserved;
        /* Options in format of IPV6_PKTOPTIONS */
};

• dst is IPv6 destination address associated with the label.

• label is flow label value in network byte order. If it is zero, kernel will allocate new pseudo-random number. Otherwise, kernel will try to lease flow label ordered by user. In this case, it is user task to provide necessary flow label randomness.

• action is requested operation. Currently, only three operations are defined:

  #define IPV6_FL_A_GET   0   /* Get flow label */
  #define IPV6_FL_A_PUT   1   /* Release flow label */
  #define IPV6_FL_A_RENEW 2   /* Update expire time */
  

• flags are optional modifiers. Currently only IPV6_FL_A_GET has modifiers:

  #define IPV6_FL_F_CREATE 1   /* Allowed to create new label */
  #define IPV6_FL_F_EXCL   2   /* Do not create new label */
  

• share defines who is allowed to reuse the same flow label.

  #define IPV6_FL_S_NONE    0   /* Not defined */
  #define IPV6_FL_S_EXCL    1   /* Label is private */
  #define IPV6_FL_S_PROCESS 2   /* May be reused by this process */
  #define IPV6_FL_S_USER    3   /* May be reused by this user */
  #define IPV6_FL_S_ANY     255 /* Anyone may reuse it */
  

• linger is time in seconds. After the last user releases flow label, it will not be reused with different destination and options at least during this time. If share is not IPV6_FL_S_EXCL the label still can be shared by another sockets. Current implementation does not allow unprivileged user to set linger longer than 60 sec.

• expires is time in seconds. Flow label will be kept at least for this time, but it will not be destroyed before user released it explicitly or closed all the sockets using it. Current implementation does not allow unprivileged user to set timeout longer than 60 sec. Proviledged applications MAY set longer lifetimes, but in this case they MUST save allocated labels at stable storage and restore them back after reboot before the first application allocates new flow.

This structure is followed by optional extension headers associated with this flow label in format of IPV6_PKTOPTIONS. Only IPV6_HOPOPTS, IPV6_RTHDR and, if IPV6_RTHDR presents, IPV6_DSTOPTS are allowed.