forked from luck/tmp_suning_uos_patched
2e2e9e92bd
This adds 3 sysctls which govern the retransmission behaviour of DCCP control packets (3way handshake, feature negotiation). It removes 4 FIXMEs from the code. The close resemblance of sysctl variables to their TCP analogues is emphasised not only by their name, but also by giving them the same initial values. This is useful since there is not much practical experience with DCCP yet. Furthermore, with regard to the previous patch, it is now possible to limit the number of keepalive-Responses by setting net.dccp.default.request_retries (also a bit like in TCP). Lastly, added documentation of all existing DCCP sysctls. Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk> Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
116 lines
3.9 KiB
Plaintext
116 lines
3.9 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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Sysctl variables
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================
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Several DCCP default parameters can be managed by the following sysctls
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(sysctl net.dccp.default or /proc/sys/net/dccp/default):
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request_retries
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The number of active connection initiation retries (the number of
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Requests minus one) before timing out. In addition, it also governs
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the behaviour of the other, passive side: this variable also sets
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the number of times DCCP repeats sending a Response when the initial
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handshake does not progress from RESPOND to OPEN (i.e. when no Ack
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is received after the initial Request). This value should be greater
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than 0, suggested is less than 10. Analogue of tcp_syn_retries.
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retries1
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How often a DCCP Response is retransmitted until the listening DCCP
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side considers its connecting peer dead. Analogue of tcp_retries1.
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retries2
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The number of times a general DCCP packet is retransmitted. This has
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importance for retransmitted acknowledgments and feature negotiation,
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data packets are never retransmitted. Analogue of tcp_retries2.
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send_ndp = 1
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Whether or not to send NDP count options (sec. 7.7.2).
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send_ackvec = 1
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Whether or not to send Ack Vector options (sec. 11.5).
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ack_ratio = 2
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The default Ack Ratio (sec. 11.3) to use.
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tx_ccid = 2
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Default CCID for the sender-receiver half-connection.
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rx_ccid = 2
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Default CCID for the receiver-sender half-connection.
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seq_window = 100
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The initial sequence window (sec. 7.5.2).
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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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