Provided by: libxs-dev_1.2.0-2_amd64 
NAME
xs_tcp - Crossroads unicast transport using TCP
SYNOPSIS
TCP is an ubiquitous, reliable, unicast transport. When connecting distributed applications over a
network with Crossroads, using the TCP transport will likely be your first choice.
ADDRESSING
A Crossroads address string consists of two parts as follows: transport://[source address;]endpoint. The
transport part specifies the underlying transport protocol to use, and for the TCP transport shall be set
to tcp. source address is optional. The meaning of the endpoint part for the TCP transport is defined
below.
Assigning a local address to a socket
When assigning a local address to a socket using xs_bind() with the tcp transport, the endpoint shall be
interpreted as an interface followed by a colon and the TCP port number to use.
An interface may be specified by either of the following:
• The wild-card *, meaning all available interfaces.
• The primary IPv4 or IPv6 address assigned to the interface, in its numeric representation.
• The interface name as defined by the operating system.
Note
Interface names are not standardised in any way and should be assumed to be arbitrary and platform
dependent. On Win32 platforms no short interface names exist, thus only the primary IP address may be
used to specify an interface.
Connecting a socket
When connecting a socket to a peer address using xs_connect() with the tcp transport, the endpoint shall
be interpreted as a peer address followed by a colon and the TCP port number to use.
A peer address may be specified by either of the following:
• The DNS name of the peer.
• The IPv4 or IPv6 address of the peer, in it’s numeric representation.
WIRE FORMAT
Crossroads messages are transmitted over TCP in frames consisting of an encoded payload length, followed
by a flags field and the message body. The payload length is defined as the combined length in octets of
the message body and the flags field.
For frames with a payload length not exceeding 254 octets, the payload length shall be encoded as a
single octet. The minimum valid payload length of a frame is 1 octet, thus a payload length of 0 octets
is invalid and such frames SHOULD be ignored.
For frames with a payload length exceeding 254 octets, the payload length shall be encoded as a single
octet with the value 255 followed by the payload length represented as a 64-bit unsigned integer in
network byte order.
The flags field consists of a single octet containing various control flags:
Bit 0 (MORE): More message parts to follow. A value of 0 indicates that there are no more message parts
to follow; or that the message being sent is not a multi-part message. A value of 1 indicates that the
message being sent is a multi-part message and more message parts are to follow.
Bits 1-7: Reserved. Bits 1-7 are reserved for future expansion and MUST be set to zero.
The following ABNF grammar represents a single frame:
frame = (length flags data)
length = OCTET / (escape 8OCTET)
flags = OCTET
escape = %xFF
data = *OCTET
The following diagram illustrates the layout of a frame with a payload length not exceeding 254 octets:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload length| Flags | Message body ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message body ...
+-+-+-+-+-+-+- ...
The following diagram illustrates the layout of a frame with a payload length exceeding 254 octets:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xff | Payload length ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload length ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload length| Flags | Message body ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message body ...
+-+-+-+-+-+-+-+ ...
EXAMPLES
Assigning a local address to a socket.
/* TCP port 5555 on all available interfaces */
rc = xs_bind(socket, "tcp://*:5555");
assert (rc != -1);
/* TCP port 5555 on the local loop-back interface on all platforms */
rc = xs_bind(socket, "tcp://127.0.0.1:5555");
assert (rc != -1);
/* TCP port 5555 on the first Ethernet network interface on Linux */
rc = xs_bind(socket, "tcp://eth0:5555");
assert (rc != -1);
Connecting a socket.
/* Connecting using an IP address */
rc = xs_connect(socket, "tcp://192.168.1.1:5555");
assert (rc != -1);
/* Connecting using a DNS name */
rc = xs_connect(socket, "tcp://server1:5555");
assert (rc != -1);
SEE ALSO
xs_bind(3) xs_connect(3) xs_pgm(7) xs_ipc(7) xs_inproc(7) xs(7)
AUTHORS
The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and Martin Lucina <martin@lucina.net[2]>.
NOTES
1. sustrik@250bpm.com mailto:sustrik@250bpm.com 2. martin@lucina.net mailto:martin@lucina.net