Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 291 insertions, 0 deletions

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+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+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:
+
+   - The UDP-Lite Homepage:
+     http://web.archive.org/web/%2E/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
+
+     From here you can also download some example application source code.
+
+   - The UDP-Lite HOWTO on
+     http://web.archive.org/web/%2E/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/files/UDP-Lite-HOWTO.txt
+
+   - The Wireshark UDP-Lite WiKi (with capture files):
+     https://wiki.wireshark.org/Lightweight_User_Datagram_Protocol
+
+   - The Protocol Spec, RFC 3828, http://www.ietf.org/rfc/rfc3828.txt
+
+
+1. 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.
+
+2. 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.
+
+3. 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.
+
+4. Kernel Behaviour with Regards 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.
+
+5. 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).
+
+6. 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.
+
+7. 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>.