[manpages]: First stab at a udplite(7) manpage

[Gerrit Renker <gerrit-VsoRXqaLlyfQzY9nttDBhA@public.gmane.org>]

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Hi Michael (cc-ed to list),

as per earlier conversation, I have sat down and assembled a udplite.7
manpage. I have checked it against the conventions (in particular spelling)
mentioned in man-pages(7) and it seems ok.

Since I am rather a groff-newbie, I'd welcome suggestions to improve the
format, the content seems now stable (after a few iterations).

With best regards
Gerrit

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.TH UDPLITE  7 2008-06-23 "Linux" "Linux Programmer's Manual"

.SH NAME
udplite \- Lightweight User Datagram Protocol
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.\" FIXME: see #defines under `BUGS',
.\"        when glibc supports this, add 
.\"        #include <netinet/udplite.h>
.sp
.B s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDPLITE);
.SH DESCRIPTION
This is an implementation of the Lightweight User Datagram Protocol
(UDP-Lite), as described in RFC\ 3828.

UDP-Lite is an extension of UDP (RFC\ 768) to support variable-length
checksums. This has advantages for some types of multimedia transport,
as it allows to reuse partly damaged frames (with few bit errors).

The variable-length checksum coverage is set via a
.BR setsockopt (2)
option. If this option is not set, the only difference to UDP is
in using a different IP protocol identifier (IANA number 136).

The UDP-Lite implementation is a full extension of
.BR udp (7),
i.e.  it shares the same API and API behaviour, and in addition
offers two socket options to control the checksum coverage.
.SS "Address Format"
UDP-Litev4 uses the
.I sockaddr_in
address format described in
.BR ip (7).
UDP-Litev6 uses the
.I sockaddr_in6
address format described in
.BR ipv6 (7).
.SS "Socket Options"
To set or get a UDP-Lite socket option, call
.BR getsockopt (2)
to read or
.BR setsockopt (2)
to write the option with the option level argument set to
.BR IPPROTO_UDPLITE .
In addition, all 
.B IPPROTO_UDP
socket options are valid on a UDP-Lite socket. See
.BR udp (7)
for more information.

The following two options are specific to UDP-Lite.
.TP
.BR UDPLITE_SEND_CSCOV
This option sets the sender checksum coverage and takes an 
.B int
as argument, with a checksum coverage value in the range 0..2^16-1.

A value of 0 means that always the entire datagram is covered,
values from 1-7 are illegal (RFC\ 3828, 3.1) and are rounded up to
the minimum coverage of 8.

With regard to IPv6 jumbograms (RFC\ 2675), the UDP-Litev6 checksum
coverage is limited to the first 2^16-1 octets, as per RFC\ 3828, 3.5.
Higher values are therefore not supported.
.TP
.BR UDPLITE_RECV_CSCOV
This is the receiver-side analogue and uses uses the same argument format
and value range as
.BR UDPLITE_SEND_CSCOV .
This option is not required to enable traffic with partial checksum 
coverage. Its function is that of a traffic filter: when enabled, it
instructs the kernel to drop all packets which have a coverage
.I less
than the specified coverage value.
.\" FIXME: SO_NO_CHECK exists and is supported by UDPv4, but is
.\" commented out in socket(7), hence also here ???
.\".PP
.\"Since UDP-Lite mandates checksums, checksumming can not be disabled 
.\"via the
.\".B SO_NO_CHECK
.\"option from 
.\".BR socket (7).
.SH ERRORS
All errors documented for
.BR udp (7)
may be returned. UDP-Lite does not add further errors. When the value of
.B UDPLITE_RECV_CSCOV
exceeds the actual coverage, incoming packets are silently dropped,
but may generate a warning message in the system log.
.SH BUGS
.\" FIXME: remove this section once glibc supports UDP-Lite
Where glibc support is missing, the following defintions are needed:
.sp
.br
#define IPPROTO_UDPLITE     136
.br
.\" The following two are defined in the kernel in linux/net/udplite.h
#define UDPLITE_SEND_CSCOV  10
.br
#define UDPLITE_RECV_CSCOV  11
.SH FILES
.I /proc/net/snmp 
.br
.IR /proc/net/snmp6
.SH VERSIONS
UDP-Litev4/v6 first appeared in Linux 2.6.20.
.SH "SEE ALSO"
.BR udp (7),
.BR ip (7),
.BR ipv6 (7),
.BR socket (7)

RFC\ 3828 for the Lightweight User Datagram Protocol (UDP-Lite)
.br
.I Documentation/networking/udplite.txt