292 lines
11 KiB
ReStructuredText
292 lines
11 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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================================
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The UDP-Lite protocol (RFC 3828)
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================================
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UDP-Lite is a Standards-Track IETF transport protocol whose characteristic
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is a variable-length checksum. This has advantages for transport of multimedia
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(video, VoIP) over wireless networks, as partly damaged packets can still be
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fed into the codec instead of being discarded due to a failed checksum test.
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This file briefly describes the existing kernel support and the socket API.
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For in-depth information, you can consult:
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- The UDP-Lite Homepage:
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http://web.archive.org/web/%2E/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
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From here you can also download some example application source code.
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- The UDP-Lite HOWTO on
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http://web.archive.org/web/%2E/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/files/UDP-Lite-HOWTO.txt
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- The Wireshark UDP-Lite WiKi (with capture files):
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https://wiki.wireshark.org/Lightweight_User_Datagram_Protocol
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- The Protocol Spec, RFC 3828, http://www.ietf.org/rfc/rfc3828.txt
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1. Applications
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===============
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Several applications have been ported successfully to UDP-Lite. Ethereal
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(now called wireshark) has UDP-Litev4/v6 support by default.
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Porting applications to UDP-Lite is straightforward: only socket level and
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IPPROTO need to be changed; senders additionally set the checksum coverage
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length (default = header length = 8). Details are in the next section.
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2. Programming API
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==================
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UDP-Lite provides a connectionless, unreliable datagram service and hence
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uses the same socket type as UDP. In fact, porting from UDP to UDP-Lite is
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very easy: simply add ``IPPROTO_UDPLITE`` as the last argument of the
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socket(2) call so that the statement looks like::
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s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDPLITE);
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or, respectively,
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::
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s = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDPLITE);
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With just the above change you are able to run UDP-Lite services or connect
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to UDP-Lite servers. The kernel will assume that you are not interested in
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using partial checksum coverage and so emulate UDP mode (full coverage).
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To make use of the partial checksum coverage facilities requires setting a
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single socket option, which takes an integer specifying the coverage length:
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* Sender checksum coverage: UDPLITE_SEND_CSCOV
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For example::
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int val = 20;
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setsockopt(s, SOL_UDPLITE, UDPLITE_SEND_CSCOV, &val, sizeof(int));
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sets the checksum coverage length to 20 bytes (12b data + 8b header).
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Of each packet only the first 20 bytes (plus the pseudo-header) will be
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checksummed. This is useful for RTP applications which have a 12-byte
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base header.
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* Receiver checksum coverage: UDPLITE_RECV_CSCOV
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This option is the receiver-side analogue. It is truly optional, i.e. not
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required to enable traffic with partial checksum coverage. Its function is
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that of a traffic filter: when enabled, it instructs the kernel to drop
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all packets which have a coverage _less_ than this value. For example, if
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RTP and UDP headers are to be protected, a receiver can enforce that only
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packets with a minimum coverage of 20 are admitted::
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int min = 20;
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setsockopt(s, SOL_UDPLITE, UDPLITE_RECV_CSCOV, &min, sizeof(int));
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The calls to getsockopt(2) are analogous. Being an extension and not a stand-
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alone protocol, all socket options known from UDP can be used in exactly the
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same manner as before, e.g. UDP_CORK or UDP_ENCAP.
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A detailed discussion of UDP-Lite checksum coverage options is in section IV.
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3. Header Files
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===============
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The socket API requires support through header files in /usr/include:
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* /usr/include/netinet/in.h
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to define IPPROTO_UDPLITE
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* /usr/include/netinet/udplite.h
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for UDP-Lite header fields and protocol constants
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For testing purposes, the following can serve as a ``mini`` header file::
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#define IPPROTO_UDPLITE 136
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#define SOL_UDPLITE 136
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#define UDPLITE_SEND_CSCOV 10
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#define UDPLITE_RECV_CSCOV 11
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Ready-made header files for various distros are in the UDP-Lite tarball.
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4. Kernel Behaviour with Regards to the Various Socket Options
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==============================================================
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To enable debugging messages, the log level need to be set to 8, as most
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messages use the KERN_DEBUG level (7).
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1) Sender Socket Options
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If the sender specifies a value of 0 as coverage length, the module
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assumes full coverage, transmits a packet with coverage length of 0
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and according checksum. If the sender specifies a coverage < 8 and
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different from 0, the kernel assumes 8 as default value. Finally,
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if the specified coverage length exceeds the packet length, the packet
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length is used instead as coverage length.
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2) Receiver Socket Options
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The receiver specifies the minimum value of the coverage length it
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is willing to accept. A value of 0 here indicates that the receiver
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always wants the whole of the packet covered. In this case, all
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partially covered packets are dropped and an error is logged.
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It is not possible to specify illegal values (<0 and <8); in these
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cases the default of 8 is assumed.
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All packets arriving with a coverage value less than the specified
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threshold are discarded, these events are also logged.
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3) Disabling the Checksum Computation
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On both sender and receiver, checksumming will always be performed
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and cannot be disabled using SO_NO_CHECK. Thus::
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setsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, ... );
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will always will be ignored, while the value of::
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getsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, &value, ...);
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is meaningless (as in TCP). Packets with a zero checksum field are
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illegal (cf. RFC 3828, sec. 3.1) and will be silently discarded.
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4) Fragmentation
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The checksum computation respects both buffersize and MTU. The size
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of UDP-Lite packets is determined by the size of the send buffer. The
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minimum size of the send buffer is 2048 (defined as SOCK_MIN_SNDBUF
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in include/net/sock.h), the default value is configurable as
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net.core.wmem_default or via setting the SO_SNDBUF socket(7)
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option. The maximum upper bound for the send buffer is determined
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by net.core.wmem_max.
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Given a payload size larger than the send buffer size, UDP-Lite will
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split the payload into several individual packets, filling up the
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send buffer size in each case.
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The precise value also depends on the interface MTU. The interface MTU,
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in turn, may trigger IP fragmentation. In this case, the generated
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UDP-Lite packet is split into several IP packets, of which only the
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first one contains the L4 header.
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The send buffer size has implications on the checksum coverage length.
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Consider the following example::
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Payload: 1536 bytes Send Buffer: 1024 bytes
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MTU: 1500 bytes Coverage Length: 856 bytes
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UDP-Lite will ship the 1536 bytes in two separate packets::
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Packet 1: 1024 payload + 8 byte header + 20 byte IP header = 1052 bytes
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Packet 2: 512 payload + 8 byte header + 20 byte IP header = 540 bytes
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The coverage packet covers the UDP-Lite header and 848 bytes of the
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payload in the first packet, the second packet is fully covered. Note
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that for the second packet, the coverage length exceeds the packet
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length. The kernel always re-adjusts the coverage length to the packet
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length in such cases.
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As an example of what happens when one UDP-Lite packet is split into
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several tiny fragments, consider the following example::
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Payload: 1024 bytes Send buffer size: 1024 bytes
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MTU: 300 bytes Coverage length: 575 bytes
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+-+-----------+--------------+--------------+--------------+
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|8| 272 | 280 | 280 | 280 |
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+-+-----------+--------------+--------------+--------------+
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280 560 840 1032
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^
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*****checksum coverage*************
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The UDP-Lite module generates one 1032 byte packet (1024 + 8 byte
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header). According to the interface MTU, these are split into 4 IP
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packets (280 byte IP payload + 20 byte IP header). The kernel module
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sums the contents of the entire first two packets, plus 15 bytes of
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the last packet before releasing the fragments to the IP module.
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To see the analogous case for IPv6 fragmentation, consider a link
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MTU of 1280 bytes and a write buffer of 3356 bytes. If the checksum
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coverage is less than 1232 bytes (MTU minus IPv6/fragment header
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lengths), only the first fragment needs to be considered. When using
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larger checksum coverage lengths, each eligible fragment needs to be
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checksummed. Suppose we have a checksum coverage of 3062. The buffer
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of 3356 bytes will be split into the following fragments::
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Fragment 1: 1280 bytes carrying 1232 bytes of UDP-Lite data
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Fragment 2: 1280 bytes carrying 1232 bytes of UDP-Lite data
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Fragment 3: 948 bytes carrying 900 bytes of UDP-Lite data
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The first two fragments have to be checksummed in full, of the last
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fragment only 598 (= 3062 - 2*1232) bytes are checksummed.
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While it is important that such cases are dealt with correctly, they
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are (annoyingly) rare: UDP-Lite is designed for optimising multimedia
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performance over wireless (or generally noisy) links and thus smaller
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coverage lengths are likely to be expected.
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5. UDP-Lite Runtime Statistics and their Meaning
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================================================
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Exceptional and error conditions are logged to syslog at the KERN_DEBUG
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level. Live statistics about UDP-Lite are available in /proc/net/snmp
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and can (with newer versions of netstat) be viewed using::
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netstat -svu
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This displays UDP-Lite statistics variables, whose meaning is as follows.
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============ =====================================================
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InDatagrams The total number of datagrams delivered to users.
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NoPorts Number of packets received to an unknown port.
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These cases are counted separately (not as InErrors).
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InErrors Number of erroneous UDP-Lite packets. Errors include:
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* internal socket queue receive errors
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* packet too short (less than 8 bytes or stated
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coverage length exceeds received length)
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* xfrm4_policy_check() returned with error
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* application has specified larger min. coverage
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length than that of incoming packet
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* checksum coverage violated
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* bad checksum
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OutDatagrams Total number of sent datagrams.
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============ =====================================================
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These statistics derive from the UDP MIB (RFC 2013).
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6. IPtables
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===========
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There is packet match support for UDP-Lite as well as support for the LOG target.
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If you copy and paste the following line into /etc/protocols::
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udplite 136 UDP-Lite # UDP-Lite [RFC 3828]
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then::
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iptables -A INPUT -p udplite -j LOG
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will produce logging output to syslog. Dropping and rejecting packets also works.
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7. Maintainer Address
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=====================
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The UDP-Lite patch was developed at
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University of Aberdeen
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Electronics Research Group
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Department of Engineering
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Fraser Noble Building
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Aberdeen AB24 3UE; UK
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The current maintainer is Gerrit Renker, <gerrit@erg.abdn.ac.uk>. Initial
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code was developed by William Stanislaus, <william@erg.abdn.ac.uk>.
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