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+ ===========================================================================
+ The UDP-Lite protocol (RFC 3828)
+ ===========================================================================
+
+
+ UDP-Lite is a Standards-Track IETF transport protocol whose characteristic
+ is a variable-length checksum. This has advantages for transport of multimedia
+ (video, VoIP) over wireless networks, as partly damaged packets can still be
+ fed into the codec instead of being discarded due to a failed checksum test.
+
+ This file briefly describes the existing kernel support and the socket API.
+ For in-depth information, you can consult:
+
+ o The UDP-Lite Homepage:
+ http://web.archive.org/web/*/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
+ From here you can also download some example application source code.
+
+ o The UDP-Lite HOWTO on
+ http://web.archive.org/web/*/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
+ files/UDP-Lite-HOWTO.txt
+
+ o The Wireshark UDP-Lite WiKi (with capture files):
+ http://wiki.wireshark.org/Lightweight_User_Datagram_Protocol
+
+ o The Protocol Spec, RFC 3828, http://www.ietf.org/rfc/rfc3828.txt
+
+
+ I) APPLICATIONS
+
+ Several applications have been ported successfully to UDP-Lite. Ethereal
+ (now called wireshark) has UDP-Litev4/v6 support by default.
+ Porting applications to UDP-Lite is straightforward: only socket level and
+ IPPROTO need to be changed; senders additionally set the checksum coverage
+ length (default = header length = 8). Details are in the next section.
+
+
+ II) PROGRAMMING API
+
+ UDP-Lite provides a connectionless, unreliable datagram service and hence
+ uses the same socket type as UDP. In fact, porting from UDP to UDP-Lite is
+ very easy: simply add `IPPROTO_UDPLITE' as the last argument of the socket(2)
+ call so that the statement looks like:
+
+ s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDPLITE);
+
+ or, respectively,
+
+ s = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDPLITE);
+
+ With just the above change you are able to run UDP-Lite services or connect
+ to UDP-Lite servers. The kernel will assume that you are not interested in
+ using partial checksum coverage and so emulate UDP mode (full coverage).
+
+ To make use of the partial checksum coverage facilities requires setting a
+ single socket option, which takes an integer specifying the coverage length:
+
+ * Sender checksum coverage: UDPLITE_SEND_CSCOV
+
+ For example,
+
+ int val = 20;
+ setsockopt(s, SOL_UDPLITE, UDPLITE_SEND_CSCOV, &val, sizeof(int));
+
+ sets the checksum coverage length to 20 bytes (12b data + 8b header).
+ Of each packet only the first 20 bytes (plus the pseudo-header) will be
+ checksummed. This is useful for RTP applications which have a 12-byte
+ base header.
+
+
+ * Receiver checksum coverage: UDPLITE_RECV_CSCOV
+
+ This option is the receiver-side analogue. It is truly optional, i.e. 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 _less_ than this value. For example, if
+ RTP and UDP headers are to be protected, a receiver can enforce that only
+ packets with a minimum coverage of 20 are admitted:
+
+ int min = 20;
+ setsockopt(s, SOL_UDPLITE, UDPLITE_RECV_CSCOV, &min, sizeof(int));
+
+ The calls to getsockopt(2) are analogous. Being an extension and not a stand-
+ alone protocol, all socket options known from UDP can be used in exactly the
+ same manner as before, e.g. UDP_CORK or UDP_ENCAP.
+
+ A detailed discussion of UDP-Lite checksum coverage options is in section IV.
+
+
+ III) HEADER FILES
+
+ The socket API requires support through header files in /usr/include:
+
+ * /usr/include/netinet/in.h
+ to define IPPROTO_UDPLITE
+
+ * /usr/include/netinet/udplite.h
+ for UDP-Lite header fields and protocol constants
+
+ For testing purposes, the following can serve as a `mini' header file:
+
+ #define IPPROTO_UDPLITE 136
+ #define SOL_UDPLITE 136
+ #define UDPLITE_SEND_CSCOV 10
+ #define UDPLITE_RECV_CSCOV 11
+
+ Ready-made header files for various distros are in the UDP-Lite tarball.
+
+
+ IV) KERNEL BEHAVIOUR WITH REGARD TO THE VARIOUS SOCKET OPTIONS
+
+ To enable debugging messages, the log level need to be set to 8, as most
+ messages use the KERN_DEBUG level (7).
+
+ 1) Sender Socket Options
+
+ If the sender specifies a value of 0 as coverage length, the module
+ assumes full coverage, transmits a packet with coverage length of 0
+ and according checksum. If the sender specifies a coverage < 8 and
+ different from 0, the kernel assumes 8 as default value. Finally,
+ if the specified coverage length exceeds the packet length, the packet
+ length is used instead as coverage length.
+
+ 2) Receiver Socket Options
+
+ The receiver specifies the minimum value of the coverage length it
+ is willing to accept. A value of 0 here indicates that the receiver
+ always wants the whole of the packet covered. In this case, all
+ partially covered packets are dropped and an error is logged.
+
+ It is not possible to specify illegal values (<0 and <8); in these
+ cases the default of 8 is assumed.
+
+ All packets arriving with a coverage value less than the specified
+ threshold are discarded, these events are also logged.
+
+ 3) Disabling the Checksum Computation
+
+ On both sender and receiver, checksumming will always be performed
+ and cannot be disabled using SO_NO_CHECK. Thus
+
+ setsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, ... );
+
+ will always will be ignored, while the value of
+
+ getsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, &value, ...);
+
+ is meaningless (as in TCP). Packets with a zero checksum field are
+ illegal (cf. RFC 3828, sec. 3.1) and will be silently discarded.
+
+ 4) Fragmentation
+
+ The checksum computation respects both buffersize and MTU. The size
+ of UDP-Lite packets is determined by the size of the send buffer. The
+ minimum size of the send buffer is 2048 (defined as SOCK_MIN_SNDBUF
+ in include/net/sock.h), the default value is configurable as
+ net.core.wmem_default or via setting the SO_SNDBUF socket(7)
+ option. The maximum upper bound for the send buffer is determined
+ by net.core.wmem_max.
+
+ Given a payload size larger than the send buffer size, UDP-Lite will
+ split the payload into several individual packets, filling up the
+ send buffer size in each case.
+
+ The precise value also depends on the interface MTU. The interface MTU,
+ in turn, may trigger IP fragmentation. In this case, the generated
+ UDP-Lite packet is split into several IP packets, of which only the
+ first one contains the L4 header.
+
+ The send buffer size has implications on the checksum coverage length.
+ Consider the following example:
+
+ Payload: 1536 bytes Send Buffer: 1024 bytes
+ MTU: 1500 bytes Coverage Length: 856 bytes
+
+ UDP-Lite will ship the 1536 bytes in two separate packets:
+
+ Packet 1: 1024 payload + 8 byte header + 20 byte IP header = 1052 bytes
+ Packet 2: 512 payload + 8 byte header + 20 byte IP header = 540 bytes
+
+ The coverage packet covers the UDP-Lite header and 848 bytes of the
+ payload in the first packet, the second packet is fully covered. Note
+ that for the second packet, the coverage length exceeds the packet
+ length. The kernel always re-adjusts the coverage length to the packet
+ length in such cases.
+
+ As an example of what happens when one UDP-Lite packet is split into
+ several tiny fragments, consider the following example.
+
+ Payload: 1024 bytes Send buffer size: 1024 bytes
+ MTU: 300 bytes Coverage length: 575 bytes
+
+ +-+-----------+--------------+--------------+--------------+
+ |8| 272 | 280 | 280 | 280 |
+ +-+-----------+--------------+--------------+--------------+
+ 280 560 840 1032
+ ^
+ *****checksum coverage*************
+
+ The UDP-Lite module generates one 1032 byte packet (1024 + 8 byte
+ header). According to the interface MTU, these are split into 4 IP
+ packets (280 byte IP payload + 20 byte IP header). The kernel module
+ sums the contents of the entire first two packets, plus 15 bytes of
+ the last packet before releasing the fragments to the IP module.
+
+ To see the analogous case for IPv6 fragmentation, consider a link
+ MTU of 1280 bytes and a write buffer of 3356 bytes. If the checksum
+ coverage is less than 1232 bytes (MTU minus IPv6/fragment header
+ lengths), only the first fragment needs to be considered. When using
+ larger checksum coverage lengths, each eligible fragment needs to be
+ checksummed. Suppose we have a checksum coverage of 3062. The buffer
+ of 3356 bytes will be split into the following fragments:
+
+ Fragment 1: 1280 bytes carrying 1232 bytes of UDP-Lite data
+ Fragment 2: 1280 bytes carrying 1232 bytes of UDP-Lite data
+ Fragment 3: 948 bytes carrying 900 bytes of UDP-Lite data
+
+ The first two fragments have to be checksummed in full, of the last
+ fragment only 598 (= 3062 - 2*1232) bytes are checksummed.
+
+ While it is important that such cases are dealt with correctly, they
+ are (annoyingly) rare: UDP-Lite is designed for optimising multimedia
+ performance over wireless (or generally noisy) links and thus smaller
+ coverage lengths are likely to be expected.
+
+
+ V) UDP-LITE RUNTIME STATISTICS AND THEIR MEANING
+
+ Exceptional and error conditions are logged to syslog at the KERN_DEBUG
+ level. Live statistics about UDP-Lite are available in /proc/net/snmp
+ and can (with newer versions of netstat) be viewed using
+
+ netstat -svu
+
+ This displays UDP-Lite statistics variables, whose meaning is as follows.
+
+ InDatagrams: The total number of datagrams delivered to users.
+
+ NoPorts: Number of packets received to an unknown port.
+ These cases are counted separately (not as InErrors).
+
+ InErrors: Number of erroneous UDP-Lite packets. Errors include:
+ * internal socket queue receive errors
+ * packet too short (less than 8 bytes or stated
+ coverage length exceeds received length)
+ * xfrm4_policy_check() returned with error
+ * application has specified larger min. coverage
+ length than that of incoming packet
+ * checksum coverage violated
+ * bad checksum
+
+ OutDatagrams: Total number of sent datagrams.
+
+ These statistics derive from the UDP MIB (RFC 2013).
+
+
+ VI) IPTABLES
+
+ There is packet match support for UDP-Lite as well as support for the LOG target.
+ If you copy and paste the following line into /etc/protocols,
+
+ udplite 136 UDP-Lite # UDP-Lite [RFC 3828]
+
+ then
+ iptables -A INPUT -p udplite -j LOG
+
+ will produce logging output to syslog. Dropping and rejecting packets also works.
+
+
+ VII) MAINTAINER ADDRESS
+
+ The UDP-Lite patch was developed at
+ University of Aberdeen
+ Electronics Research Group
+ Department of Engineering
+ Fraser Noble Building
+ Aberdeen AB24 3UE; UK
+ The current maintainer is Gerrit Renker, <gerrit@erg.abdn.ac.uk>. Initial
+ code was developed by William Stanislaus, <william@erg.abdn.ac.uk>.