mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-25 03:55:09 +08:00
b07d496177
This patch fix URL (http to https) for wiki.wireshark.org. Signed-off-by: Masanari Iida <standby24x7@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
279 lines
11 KiB
Plaintext
279 lines
11 KiB
Plaintext
===========================================================================
|
|
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:
|
|
|
|
o The UDP-Lite Homepage:
|
|
http://web.archive.org/web/*/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
|
|
From here you can also download some example application source code.
|
|
|
|
o The UDP-Lite HOWTO on
|
|
http://web.archive.org/web/*/http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
|
|
files/UDP-Lite-HOWTO.txt
|
|
|
|
o The Wireshark UDP-Lite WiKi (with capture files):
|
|
https://wiki.wireshark.org/Lightweight_User_Datagram_Protocol
|
|
|
|
o The Protocol Spec, RFC 3828, http://www.ietf.org/rfc/rfc3828.txt
|
|
|
|
|
|
I) 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.
|
|
|
|
|
|
II) 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.
|
|
|
|
|
|
III) 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.
|
|
|
|
|
|
IV) KERNEL BEHAVIOUR WITH REGARD 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.
|
|
|
|
|
|
V) 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).
|
|
|
|
|
|
VI) 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.
|
|
|
|
|
|
VII) 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>.
|