27.7 IPv6 Sticky Options
We have described the use of ancillary data with
sendmsg and recvmsg to send and receive seven
different ancillary data objects:
-
IPv6 packet
information: the in6_pktinfo structure containing either
the destination address and outgoing interface index, or the source
address and the arriving interface index (Figure 22.21)
-
The outgoing
hop limit or received hop limit (Figure 22.21)
-
The next-hop
address (Figure 22.21)
-
The outgoing or
received traffic class (Figure 22.21)
-
Hop-by-hop
options (Figure 27.10)
-
Destination
options (Figure 27.10)
-
Routing header
(Figure 27.12)
We summarized the cmsg_level and
cmsg_type values for these objects, along with the values
for the other ancillary data object in Figure 14.11.
When the same value will be used for all packets
sent on a socket, instead of sending these options in every call to
sendmsg, we can set the corresponding socket options
instead. The socket options use the same constants as the ancillary
data, that is, the option level is always IPPROTO_IPV6 and
the option name is IPV6_DSTOPTS, IPV6_HOPLIMIT,
IPV6_HOPOPTS, IPV6_NEXTHOP,
IPV6_PKTINFO, IPV6_RTHDR, or
IPV6_TCLASS. But, these sticky options can be overridden
on a per-packet basis for a UDP socket or for a raw IPv6 socket by
specifying ancillary data in a call to sendmsg. If any
ancillary data is specified in a call to sendmsg, the
corresponding sticky options are not sent with that datagram.
The concept of sticky options can also be used
with TCP because ancillary data is never sent or received by
sendmsg or recvmsg on a TCP socket. Instead, a
TCP application can set the corresponding socket option and specify
any of the seven option types mentioned at the beginning of this
section. These objects then affect all packets sent on this socket.
However, retransmission of packets that were originally sent when
other (or no) sticky options were set may use either the original
or the new sticky options.
There is no way to retrieve options received via
TCP since there is no relationship between received packets and
user receive operations.
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