Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

DCCP protocol

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Contents

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- Introduction

- Missing features

- Socket options

- Notes

Introduction

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Datagram Congestion Control Protocol (DCCP) is an unreliable, connection

based protocol designed to solve issues present in UDP and TCP particularly

for real time and multimedia traffic.

It has a base protocol and pluggable congestion control IDs (CCIDs).

It is at proposed standard RFC status and the homepage for DCCP as a protocol

is at:

        http://www.read.cs.ucla.edu/dccp/

Missing features

================

The DCCP implementation does not currently have all the features that are in

the RFC.

The known bugs are at:

        http://linux-net.osdl.org/index.php/TODO#DCCP

Socket options

==============

DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of

service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,

the socket will fall back to 0 (which means that no meaningful service code

is present). On active sockets this is set before connect(); specifying more

than one code has no effect (all subsequent service codes are ignored). The

case is different for passive sockets, where multiple service codes (up to 32)

can be set before calling bind().

DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet

size (application payload size) in bytes, see RFC 4340, section 14.

DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the

partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums

always cover the entire packet and that only fully covered application data is

accepted by the receiver. Hence, when using this feature on the sender, it must

be enabled at the receiver, too with suitable choice of CsCov.

DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the

        range 0..15 are acceptable. The default setting is 0 (full coverage),

        values between 1..15 indicate partial coverage.

DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it

        sets a threshold, where again values 0..15 are acceptable. The default

        of 0 means that all packets with a partial coverage will be discarded.

        Values in the range 1..15 indicate that packets with minimally such a

        coverage value are also acceptable. The higher the number, the more

        restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage

        settings are inherited to the child socket after accept().

The following two options apply to CCID 3 exclusively and are getsockopt()-only.

In either case, a TFRC info struct (defined in ) is returned.

DCCP_SOCKOPT_CCID_RX_INFO

        Returns a `struct tfrc_rx_info' in optval; the buffer for optval and

        optlen must be set to at least sizeof(struct tfrc_rx_info).

DCCP_SOCKOPT_CCID_TX_INFO

        Returns a `struct tfrc_tx_info' in optval; the buffer for optval and

        optlen must be set to at least sizeof(struct tfrc_tx_info).

Sysctl variables

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Several DCCP default parameters can be managed by the following sysctls

(sysctl net.dccp.default or /proc/sys/net/dccp/default):

request_retries

        The number of active connection initiation retries (the number of

        Requests minus one) before timing out. In addition, it also governs

        the behaviour of the other, passive side: this variable also sets

        the number of times DCCP repeats sending a Response when the initial

        handshake does not progress from RESPOND to OPEN (i.e. when no Ack

        is received after the initial Request).  This value should be greater

        than 0, suggested is less than 10. Analogue of tcp_syn_retries.

retries1

        How often a DCCP Response is retransmitted until the listening DCCP

        side considers its connecting peer dead. Analogue of tcp_retries1.

retries2

        The number of times a general DCCP packet is retransmitted. This has

        importance for retransmitted acknowledgments and feature negotiation,

        data packets are never retransmitted. Analogue of tcp_retries2.

send_ndp = 1

        Whether or not to send NDP count options (sec. 7.7.2).

send_ackvec = 1

        Whether or not to send Ack Vector options (sec. 11.5).

ack_ratio = 2

        The default Ack Ratio (sec. 11.3) to use.

tx_ccid = 2

        Default CCID for the sender-receiver half-connection.

rx_ccid = 2

        Default CCID for the receiver-sender half-connection.

seq_window = 100

        The initial sequence window (sec. 7.5.2).

tx_qlen = 5

        The size of the transmit buffer in packets. A value of 0 corresponds

        to an unbounded transmit buffer.

sync_ratelimit = 125 ms

        The timeout between subsequent DCCP-Sync packets sent in response to

        sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit

        of this parameter is milliseconds; a value of 0 disables rate-limiting.

Notes

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DCCP does not travel through NAT successfully at present on many boxes. This is

because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT

support for DCCP has been added.