Documentation/networking/udplite.txt

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