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>. 279
