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udplite.txt

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  1.   ===========================================================================
  2.                       The UDP-Lite protocol (RFC 3828)
  3.   ===========================================================================
  4.  
  5.  
  6.   UDP-Lite is a Standards-Track IETF transport protocol whose characteristic
  7.   is a variable-length checksum. This has advantages for transport of multimedia
  8.   (video, VoIP) over wireless networks, as partly damaged packets can still be
  9.   fed into the codec instead of being discarded due to a failed checksum test.
  10.  
  11.   This file briefly describes the existing kernel support and the socket API.
  12.   For in-depth information, you can consult:
  13.  
  14.    o The UDP-Lite Homepage: http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
  15.        From here you can also download some example application source code.
  16.  
  17.    o The UDP-Lite HOWTO on
  18.        http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/files/UDP-Lite-HOWTO.txt
  19.  
  20.    o The Wireshark UDP-Lite WiKi (with capture files):
  21.        http://wiki.wireshark.org/Lightweight_User_Datagram_Protocol
  22.  
  23.    o The Protocol Spec, RFC 3828, http://www.ietf.org/rfc/rfc3828.txt
  24.  
  25.  
  26.   I) APPLICATIONS
  27.  
  28.   Several applications have been ported successfully to UDP-Lite. Ethereal
  29.   (now called wireshark) has UDP-Litev4/v6 support by default. The tarball on
  30.  
  31.    http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/files/udplite_linux.tar.gz
  32.  
  33.   has source code for several v4/v6 client-server and network testing examples.
  34.  
  35.   Porting applications to UDP-Lite is straightforward: only socket level and
  36.   IPPROTO need to be changed; senders additionally set the checksum coverage
  37.   length (default = header length = 8). Details are in the next section.
  38.  
  39.  
  40.   II) PROGRAMMING API
  41.  
  42.   UDP-Lite provides a connectionless, unreliable datagram service and hence
  43.   uses the same socket type as UDP. In fact, porting from UDP to UDP-Lite is
  44.   very easy: simply add `IPPROTO_UDPLITE' as the last argument of the socket(2)
  45.   call so that the statement looks like:
  46.  
  47.       s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDPLITE);
  48.  
  49.                       or, respectively,
  50.  
  51.       s = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDPLITE);
  52.  
  53.   With just the above change you are able to run UDP-Lite services or connect
  54.   to UDP-Lite servers. The kernel will assume that you are not interested in
  55.   using partial checksum coverage and so emulate UDP mode (full coverage).
  56.  
  57.   To make use of the partial checksum coverage facilities requires setting a
  58.   single socket option, which takes an integer specifying the coverage length:
  59.  
  60.     * Sender checksum coverage: UDPLITE_SEND_CSCOV
  61.  
  62.       For example,
  63.  
  64.         int val = 20;
  65.         setsockopt(s, SOL_UDPLITE, UDPLITE_SEND_CSCOV, &val, sizeof(int));
  66.  
  67.       sets the checksum coverage length to 20 bytes (12b data + 8b header).
  68.       Of each packet only the first 20 bytes (plus the pseudo-header) will be
  69.       checksummed. This is useful for RTP applications which have a 12-byte
  70.       base header.
  71.  
  72.  
  73.     * Receiver checksum coverage: UDPLITE_RECV_CSCOV
  74.  
  75.       This option is the receiver-side analogue. It is truly optional, i.e. not
  76.       required to enable traffic with partial checksum coverage. Its function is
  77.       that of a traffic filter: when enabled, it instructs the kernel to drop
  78.       all packets which have a coverage _less_ than this value. For example, if
  79.       RTP and UDP headers are to be protected, a receiver can enforce that only
  80.       packets with a minimum coverage of 20 are admitted:
  81.  
  82.         int min = 20;
  83.         setsockopt(s, SOL_UDPLITE, UDPLITE_RECV_CSCOV, &min, sizeof(int));
  84.  
  85.   The calls to getsockopt(2) are analogous. Being an extension and not a stand-
  86.   alone protocol, all socket options known from UDP can be used in exactly the
  87.   same manner as before, e.g. UDP_CORK or UDP_ENCAP.
  88.  
  89.   A detailed discussion of UDP-Lite checksum coverage options is in section IV.
  90.  
  91.  
  92.   III) HEADER FILES
  93.  
  94.   The socket API requires support through header files in /usr/include:
  95.  
  96.     * /usr/include/netinet/in.h
  97.         to define IPPROTO_UDPLITE
  98.  
  99.     * /usr/include/netinet/udplite.h
  100.         for UDP-Lite header fields and protocol constants
  101.  
  102.   For testing purposes, the following can serve as a `mini' header file:
  103.  
  104.     #define IPPROTO_UDPLITE       136
  105.     #define SOL_UDPLITE           136
  106.     #define UDPLITE_SEND_CSCOV     10
  107.     #define UDPLITE_RECV_CSCOV     11
  108.  
  109.   Ready-made header files for various distros are in the UDP-Lite tarball.
  110.  
  111.  
  112.   IV) KERNEL BEHAVIOUR WITH REGARD TO THE VARIOUS SOCKET OPTIONS
  113.  
  114.   To enable debugging messages, the log level need to be set to 8, as most
  115.   messages use the KERN_DEBUG level (7).
  116.  
  117.   1) Sender Socket Options
  118.  
  119.   If the sender specifies a value of 0 as coverage length, the module
  120.   assumes full coverage, transmits a packet with coverage length of 0
  121.   and according checksum.  If the sender specifies a coverage < 8 and
  122.   different from 0, the kernel assumes 8 as default value.  Finally,
  123.   if the specified coverage length exceeds the packet length, the packet
  124.   length is used instead as coverage length.
  125.  
  126.   2) Receiver Socket Options
  127.  
  128.   The receiver specifies the minimum value of the coverage length it
  129.   is willing to accept.  A value of 0 here indicates that the receiver
  130.   always wants the whole of the packet covered. In this case, all
  131.   partially covered packets are dropped and an error is logged.
  132.  
  133.   It is not possible to specify illegal values (<0 and <8); in these
  134.   cases the default of 8 is assumed.
  135.  
  136.   All packets arriving with a coverage value less than the specified
  137.   threshold are discarded, these events are also logged.
  138.  
  139.   3) Disabling the Checksum Computation
  140.  
  141.   On both sender and receiver, checksumming will always be performed
  142.   and cannot be disabled using SO_NO_CHECK. Thus
  143.  
  144.         setsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK,  ... );
  145.  
  146.   will always will be ignored, while the value of
  147.  
  148.         getsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, &value, ...);
  149.  
  150.   is meaningless (as in TCP). Packets with a zero checksum field are
  151.   illegal (cf. RFC 3828, sec. 3.1) and will be silently discarded.
  152.  
  153.   4) Fragmentation
  154.  
  155.   The checksum computation respects both buffersize and MTU. The size
  156.   of UDP-Lite packets is determined by the size of the send buffer. The
  157.   minimum size of the send buffer is 2048 (defined as SOCK_MIN_SNDBUF
  158.   in include/net/sock.h), the default value is configurable as
  159.   net.core.wmem_default or via setting the SO_SNDBUF socket(7)
  160.   option. The maximum upper bound for the send buffer is determined
  161.   by net.core.wmem_max.
  162.  
  163.   Given a payload size larger than the send buffer size, UDP-Lite will
  164.   split the payload into several individual packets, filling up the
  165.   send buffer size in each case.
  166.  
  167.   The precise value also depends on the interface MTU. The interface MTU,
  168.   in turn, may trigger IP fragmentation. In this case, the generated
  169.   UDP-Lite packet is split into several IP packets, of which only the
  170.   first one contains the L4 header.
  171.  
  172.   The send buffer size has implications on the checksum coverage length.
  173.   Consider the following example:
  174.  
  175.   Payload: 1536 bytes          Send Buffer:     1024 bytes
  176.   MTU:     1500 bytes          Coverage Length:  856 bytes
  177.  
  178.   UDP-Lite will ship the 1536 bytes in two separate packets:
  179.  
  180.   Packet 1: 1024 payload + 8 byte header + 20 byte IP header = 1052 bytes
  181.   Packet 2:  512 payload + 8 byte header + 20 byte IP header =  540 bytes
  182.  
  183.   The coverage packet covers the UDP-Lite header and 848 bytes of the
  184.   payload in the first packet, the second packet is fully covered. Note
  185.   that for the second packet, the coverage length exceeds the packet
  186.   length. The kernel always re-adjusts the coverage length to the packet
  187.   length in such cases.
  188.  
  189.   As an example of what happens when one UDP-Lite packet is split into
  190.   several tiny fragments, consider the following example.
  191.  
  192.   Payload: 1024 bytes            Send buffer size: 1024 bytes
  193.   MTU:      300 bytes            Coverage length:   575 bytes
  194.  
  195.   +-+-----------+--------------+--------------+--------------+
  196.   |8|    272    |      280     |     280      |     280      |
  197.   +-+-----------+--------------+--------------+--------------+
  198.                280            560            840           1032
  199.                                     ^
  200.   *****checksum coverage*************
  201.  
  202.   The UDP-Lite module generates one 1032 byte packet (1024 + 8 byte
  203.   header). According to the interface MTU, these are split into 4 IP
  204.   packets (280 byte IP payload + 20 byte IP header). The kernel module
  205.   sums the contents of the entire first two packets, plus 15 bytes of
  206.   the last packet before releasing the fragments to the IP module.
  207.  
  208.   To see the analogous case for IPv6 fragmentation, consider a link
  209.   MTU of 1280 bytes and a write buffer of 3356 bytes. If the checksum
  210.   coverage is less than 1232 bytes (MTU minus IPv6/fragment header
  211.   lengths), only the first fragment needs to be considered. When using
  212.   larger checksum coverage lengths, each eligible fragment needs to be
  213.   checksummed. Suppose we have a checksum coverage of 3062. The buffer
  214.   of 3356 bytes will be split into the following fragments:
  215.  
  216.     Fragment 1: 1280 bytes carrying  1232 bytes of UDP-Lite data
  217.     Fragment 2: 1280 bytes carrying  1232 bytes of UDP-Lite data
  218.     Fragment 3:  948 bytes carrying   900 bytes of UDP-Lite data
  219.  
  220.   The first two fragments have to be checksummed in full, of the last
  221.   fragment only 598 (= 3062 - 2*1232) bytes are checksummed.
  222.  
  223.   While it is important that such cases are dealt with correctly, they
  224.   are (annoyingly) rare: UDP-Lite is designed for optimising multimedia
  225.   performance over wireless (or generally noisy) links and thus smaller
  226.   coverage lengths are likely to be expected.
  227.  
  228.  
  229.   V) UDP-LITE RUNTIME STATISTICS AND THEIR MEANING
  230.  
  231.   Exceptional and error conditions are logged to syslog at the KERN_DEBUG
  232.   level.  Live statistics about UDP-Lite are available in /proc/net/snmp
  233.   and can (with newer versions of netstat) be viewed using
  234.  
  235.                             netstat -svu
  236.  
  237.   This displays UDP-Lite statistics variables, whose meaning is as follows.
  238.  
  239.    InDatagrams:     The total number of datagrams delivered to users.
  240.  
  241.    NoPorts:         Number of packets received to an unknown port.
  242.                     These cases are counted separately (not as InErrors).
  243.  
  244.    InErrors:        Number of erroneous UDP-Lite packets. Errors include:
  245.                       * internal socket queue receive errors
  246.                       * packet too short (less than 8 bytes or stated
  247.                         coverage length exceeds received length)
  248.                       * xfrm4_policy_check() returned with error
  249.                       * application has specified larger min. coverage
  250.                         length than that of incoming packet
  251.                       * checksum coverage violated
  252.                       * bad checksum
  253.  
  254.    OutDatagrams:    Total number of sent datagrams.
  255.  
  256.    These statistics derive from the UDP MIB (RFC 2013).
  257.  
  258.  
  259.   VI) IPTABLES
  260.  
  261.   There is packet match support for UDP-Lite as well as support for the LOG target.
  262.   If you copy and paste the following line into /etc/protocols,
  263.  
  264.   udplite 136     UDP-Lite        # UDP-Lite [RFC 3828]
  265.  
  266.   then
  267.               iptables -A INPUT -p udplite -j LOG
  268.  
  269.   will produce logging output to syslog. Dropping and rejecting packets also works.
  270.  
  271.  
  272.   VII) MAINTAINER ADDRESS
  273.  
  274.   The UDP-Lite patch was developed at
  275.                     University of Aberdeen
  276.                     Electronics Research Group
  277.                     Department of Engineering
  278.                     Fraser Noble Building
  279.                     Aberdeen AB24 3UE; UK
  280.   The current maintainer is Gerrit Renker, <gerrit@erg.abdn.ac.uk>. Initial
  281.   code was developed by William  Stanislaus, <william@erg.abdn.ac.uk>.

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