207 lines
9.3 KiB
Plaintext
207 lines
9.3 KiB
Plaintext
DCCP protocol
|
|
=============
|
|
|
|
|
|
Contents
|
|
========
|
|
- Introduction
|
|
- Missing features
|
|
- Socket options
|
|
- Sysctl variables
|
|
- IOCTLs
|
|
- Other tunables
|
|
- Notes
|
|
|
|
|
|
Introduction
|
|
============
|
|
Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
|
|
oriented protocol designed to solve issues present in UDP and TCP, particularly
|
|
for real-time and multimedia (streaming) traffic.
|
|
It divides into a base protocol (RFC 4340) and plugable congestion control
|
|
modules called CCIDs. Like plugable TCP congestion control, at least one CCID
|
|
needs to be enabled in order for the protocol to function properly. In the Linux
|
|
implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
|
|
the TCP-friendly CCID3 (RFC 4342), are optional.
|
|
For a brief introduction to CCIDs and suggestions for choosing a CCID to match
|
|
given applications, see section 10 of RFC 4340.
|
|
|
|
It has a base protocol and pluggable congestion control IDs (CCIDs).
|
|
|
|
DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
|
|
is at http://www.ietf.org/html.charters/dccp-charter.html
|
|
|
|
|
|
Missing features
|
|
================
|
|
The Linux DCCP implementation does not currently support all the features that are
|
|
specified in RFCs 4340...42.
|
|
|
|
The known bugs are at:
|
|
http://linux-net.osdl.org/index.php/TODO#DCCP
|
|
|
|
For more up-to-date versions of the DCCP implementation, please consider using
|
|
the experimental DCCP test tree; instructions for checking this out are on:
|
|
http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
|
|
|
|
|
|
Socket options
|
|
==============
|
|
DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
|
|
a policy ID as argument and can only be set before the connection (i.e. changes
|
|
during an established connection are not supported). Currently, two policies are
|
|
defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
|
|
and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
|
|
u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
|
|
a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
|
|
be formatted using a cmsg(3) message header filled in as follows:
|
|
cmsg->cmsg_level = SOL_DCCP;
|
|
cmsg->cmsg_type = DCCP_SCM_PRIORITY;
|
|
cmsg->cmsg_len = CMSG_LEN(sizeof(uint32_t)); /* or CMSG_LEN(4) */
|
|
|
|
DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
|
|
value is always interpreted as unbounded queue length. If different from zero,
|
|
the interpretation of this parameter depends on the current dequeuing policy
|
|
(see above): the "simple" policy will enforce a fixed queue size by returning
|
|
EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
|
|
lowest-priority packet first. The default value for this parameter is
|
|
initialised from /proc/sys/net/dccp/default/tx_qlen.
|
|
|
|
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_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
|
|
supported by the endpoint. The option value is an array of type uint8_t whose
|
|
size is passed as option length. The minimum array size is 4 elements, the
|
|
value returned in the optlen argument always reflects the true number of
|
|
built-in CCIDs.
|
|
|
|
DCCP_SOCKOPT_CCID is write-only and sets both the TX and RX CCIDs at the same
|
|
time, combining the operation of the next two socket options. This option is
|
|
preferrable over the latter two, since often applications will use the same
|
|
type of CCID for both directions; and mixed use of CCIDs is not currently well
|
|
understood. This socket option takes as argument at least one uint8_t value, or
|
|
an array of uint8_t values, which must match available CCIDS (see above). CCIDs
|
|
must be registered on the socket before calling connect() or listen().
|
|
|
|
DCCP_SOCKOPT_TX_CCID is read/write. It returns the current CCID (if set) or sets
|
|
the preference list for the TX CCID, using the same format as DCCP_SOCKOPT_CCID.
|
|
Please note that the getsockopt argument type here is `int', not uint8_t.
|
|
|
|
DCCP_SOCKOPT_RX_CCID is analogous to DCCP_SOCKOPT_TX_CCID, but for the RX CCID.
|
|
|
|
DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
|
|
timewait state when closing the connection (RFC 4340, 8.3). The usual case is
|
|
that the closing server sends a CloseReq, whereupon the client holds timewait
|
|
state. When this boolean socket option is on, the server sends a Close instead
|
|
and will enter TIMEWAIT. This option must be set after accept() returns.
|
|
|
|
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 <linux/tfrc.h>) 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).
|
|
|
|
On unidirectional connections it is useful to close the unused half-connection
|
|
via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
|
|
|
|
|
|
Sysctl variables
|
|
================
|
|
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.
|
|
|
|
tx_ccid = 2
|
|
Default CCID for the sender-receiver half-connection. Depending on the
|
|
choice of CCID, the Send Ack Vector feature is enabled automatically.
|
|
|
|
rx_ccid = 2
|
|
Default CCID for the receiver-sender half-connection; see tx_ccid.
|
|
|
|
seq_window = 100
|
|
The initial sequence window (sec. 7.5.2) of the sender. This influences
|
|
the local ackno validity and the remote seqno validity windows (7.5.1).
|
|
|
|
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.
|
|
|
|
|
|
IOCTLS
|
|
======
|
|
FIONREAD
|
|
Works as in udp(7): returns in the `int' argument pointer the size of
|
|
the next pending datagram in bytes, or 0 when no datagram is pending.
|
|
|
|
|
|
Other tunables
|
|
==============
|
|
Per-route rto_min support
|
|
CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
|
|
of the RTO timer. This setting can be modified via the 'rto_min' option
|
|
of iproute2; for example:
|
|
> ip route change 10.0.0.0/24 rto_min 250j dev wlan0
|
|
> ip route add 10.0.0.254/32 rto_min 800j dev wlan0
|
|
> ip route show dev wlan0
|
|
CCID-3 also supports the rto_min setting: it is used to define the lower
|
|
bound for the expiry of the nofeedback timer. This can be useful on LANs
|
|
with very low RTTs (e.g., loopback, Gbit ethernet).
|
|
|
|
|
|
Notes
|
|
=====
|
|
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.
|