SPP(4)

NAME
spp - Xerox Sequenced Packet Protocol

SYNOPSIS
#include <sys/socket.h>
#include <netns/ns.h>
#include <netns/sp.h>
int
socket(AF_NS, SOCK_STREAM, 0);
int
socket(AF_NS, SOCK_SEQPACKET, 0);

DESCRIPTION
The SPP protocol provides reliable, flow-controlled, two-way transmission of data. It is a byte-stream protocol used to support the SOCK_STREAM abstraction. SPP uses the standard NS(tm) address formats.

Sockets utilizing the SPP protocol are either “active” or “passive”. Active sockets initiate connections to passive sockets. By default SPP sockets are created active; to create a passive socket the listen(2) system call must be used after binding the socket with the bind(2) system call. Only passive sockets may use the accept(2) call to accept incoming connections. Only active sockets may use the connect(2) call to initiate connections.

Passive sockets may “underspecify” their location to match incoming connection requests from multiple networks. This technique, termed “wildcard addressing”, allows a single server to provide service to clients on multiple networks. To create a socket which listens on all networks, the NS address of all zeroes must be bound. The SPP port may still be specified at this time; if the port is not specified the system will assign one. Once a connection has been established the socket's address is fixed by the peer entity's location. The address assigned the socket is the address associated with the network interface through which packets are being transmitted and received. Normally this address corresponds to the peer entity's network.

If the SOCK_SEQPACKET socket type is specified, each packet received has the actual 12 byte sequenced packet header left for the user to inspect:

    struct sphdr {
    	u_char		sp_cc;	/* connection control */
    #define	SP_EM	0x10		/* end of message */
    	u_char		sp_dt;	/* datastream type */
    	u_short		sp_sid;
    	u_short		sp_did;
    	u_short		sp_seq;
    	u_short		sp_ack;
    	u_short		sp_alo;
    };

This facilitates the implementation of higher level Xerox protocols which make use of the data stream type field and the end of message bit. Conversely, the user is required to supply a 12 byte header, the only part of which inspected is the data stream type and end of message fields.

For either socket type, packets received with the Attention bit sent are interpreted as out of band data. Data sent with “send(..., ..., ..., MSG_OOB”) cause the attention bit to be set.

DIAGNOSTICS
A socket operation may fail with one of the following errors returned:

[EISCONN]
    when trying to establish a connection on a socket which already has one;

[ENOBUFS]
    when the system runs out of memory for an internal data structure;

[ETIMEDOUT]
    when a connection was dropped due to excessive retransmissions;

[ECONNRESET]
    when the remote peer forces the connection to be closed;

[ECONNREFUSED]
    when the remote peer actively refuses connection establishment (usually because no process is listening to the port);

[EADDRINUSE]
    when an attempt is made to create a socket with a port which has already been allocated;

[EADDRNOTAVAIL]
    when an attempt is made to create a socket with a network address for which no network interface exists.

SOCKET OPTIONS

SO_DEFAULT_HEADERS
    when set, this determines the data stream type and whether the end of message bit is to be set on every ensuing packet.

SO_MTU
    This specifies the maximum amount of user data in a single packet. The default is 576 bytes - sizeof(struct spidp). This quantity affects windowing - increasing it without increasing the amount of buffering in the socket will lower the number of unread packets accepted. Anything larger than the default will not be forwarded by a bona fide XEROX product internetwork router. The data argument for the setsockopt call must be an unsigned short.

SEE ALSO
intro(4), ns(4)

HISTORY
The spp protocol appeared in 4.3BSD.

BUGS
There should be some way to reflect record boundaries in a stream. For stream mode, there should be an option to get the data stream type of the record the user process is about to receive.