6f4e5fff1e
This patch does the following:
a) introduces variable-length checksums as specified in [RFC 4340, sec. 9.2]
b) provides necessary socket options and documentation as to how to use them
c) basic support and infrastructure for the Minimum Checksum Coverage feature
[RFC 4340, sec. 9.2.1]: acceptability tests, user notification and user
interface
In addition, it
(1) fixes two bugs in the DCCPv4 checksum computation:
* pseudo-header used checksum_len instead of skb->len
* incorrect checksum coverage calculation based on dccph_x
(2) removes dccp_v4_verify_checksum() since it reduplicates code of the
checksum computation; code calling this function is updated accordingly.
(3) now uses skb_checksum(), which is safer than checksum_partial() if the
sk_buff has is a non-linear buffer (has pages attached to it).
(4) fixes an outstanding TODO item:
* If P.CsCov is too large for the packet size, drop packet and return.
The code has been tested with applications, the latest version of tcpdump now
comes with support for partial DCCP checksums.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
75 lines
2.5 KiB
Plaintext
75 lines
2.5 KiB
Plaintext
DCCP protocol
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============
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Contents
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========
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- Introduction
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- Missing features
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- Socket options
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- Notes
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Introduction
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============
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Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
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based protocol designed to solve issues present in UDP and TCP particularly
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for real time and multimedia traffic.
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It has a base protocol and pluggable congestion control IDs (CCIDs).
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It is at draft RFC status and the homepage for DCCP as a protocol is at:
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http://www.icir.org/kohler/dcp/
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Missing features
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================
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The DCCP implementation does not currently have all the features that are in
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the draft RFC.
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In particular the following are missing:
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- CCID2 support
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- feature negotiation
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When testing against other implementations it appears that elapsed time
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options are not coded compliant to the specification.
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Socket options
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==============
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DCCP_SOCKOPT_PACKET_SIZE is used for CCID3 to set default packet size for
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calculations.
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DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
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service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
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the socket will fall back to 0 (which means that no meaningful service code
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is present). Connecting sockets set at most one service option; for
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listening sockets, multiple service codes can be specified.
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DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
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partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
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always cover the entire packet and that only fully covered application data is
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accepted by the receiver. Hence, when using this feature on the sender, it must
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be enabled at the receiver, too with suitable choice of CsCov.
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DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
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range 0..15 are acceptable. The default setting is 0 (full coverage),
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values between 1..15 indicate partial coverage.
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DCCP_SOCKOPT_SEND_CSCOV is for the receiver and has a different meaning: it
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sets a threshold, where again values 0..15 are acceptable. The default
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of 0 means that all packets with a partial coverage will be discarded.
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Values in the range 1..15 indicate that packets with minimally such a
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coverage value are also acceptable. The higher the number, the more
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restrictive this setting (see [RFC 4340, sec. 9.2.1]).
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Notes
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=====
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SELinux does not yet have support for DCCP. You will need to turn it off or
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else you will get EACCES.
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DCCP does not travel through NAT successfully at present. This is because
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the checksum covers the psuedo-header as per TCP and UDP. It should be
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relatively trivial to add Linux NAT support for DCCP.
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