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NAME
ltp - Licklider Transmission Protocol (LTP) communications library
SYNOPSIS
#include "ltp.h"
typedef enum
{
LtpNoNotice = 0,
LtpExportSessionStart,
LtpXmitComplete,
LtpExportSessionCanceled,
LtpExportSessionComplete,
LtpRecvGreenSegment,
LtpRecvRedPart,
LtpImportSessionCanceled
} LtpNoticeType;
[see description for available functions]
DESCRIPTION
The ltp library provides functions enabling application software to use LTP to send and receive
information reliably over a long-latency link. It conforms to the LTP specification as documented by the
Delay-Tolerant Networking Research Group of the Internet Research Task Force.
The LTP notion of engine ID corresponds closely to the Internet notion of a host, and in ION engine IDs
are normally indistinguishable from node numbers (and from the "element numbers" in Bundle Protocol
endpoint IDs conforming to the "ipn" scheme).
The LTP notion of client ID corresponds closely to the Internet notion of "protocol number" as used in
the Internet Protocol. It enables data from multiple applications -- clients -- to be multiplexed over a
single reliable link. However, for ION operations we normally use LTP exclusively for the transmission
of Bundle Protocol data, identified by client ID = 1.
int ltp_attach()
Attaches the application to LTP functionality on the lcoal computer. Returns 0 on success, -1 on any
error.
void ltp_detach()
Terminates all access to LTP functionality on the local computer.
int ltp_send(uvast destinationEngineId, unsigned int clientId, Object clientServiceData, unsigned int
redLength, LtpSessionId *sessionId)
Sends a client service data unit to the application that is waiting for data tagged with the
indicated clientId as received at the remote LTP engine identified by destinationEngineId.
clientServiceData must be a "zero-copy object" reference as returned by zco_create(). Note that LTP
will privately make and destroy its own reference to the client service data object; the application
is free to destroy its reference at any time.
redLength indicates the number of leading bytes of data in clientServiceData that are to be sent
reliably, i.e., with selective retransmission in response to explicit or implicit negative
acknowledgment as necessary. All remaining bytes of data in clientServiceData will be sent as
"green" data, i.e., unreliably. If redLength is zero, the entire client service data unit will be
sent unreliably. If the entire client service data unit is to be sent reliably, redLength may be
simply be set to LTP_ALL_RED (i.e., -1).
On success, the function populates *sessionId with the source engine ID and the "session number"
assigned to transmission of this client service data unit and returns zero. The session number may
be used to link future LTP processing events, such as transmission cancellation, to the affected
client service data. ltp_send() returns -1 on any error.
int ltp_open(unsigned int clientId)
Establishes the application's exclusive access to received service data units tagged with the
indicated client service data ID. At any time, only a single application task is permitted to
receive service data units for any single client service data ID.
Returns 0 on success, -1 on any error (e.g., the indicated client service is already being held open
by some other application task).
int ltp_get_notice(unsigned int clientId, LtpNoticeType *type, LtpSessionId *sessionId, unsigned char
*reasonCode, unsigned char *endOfBlock, unsigned int *dataOffset, unsigned int *dataLength, Object *data)
Receives notices of LTP processing events pertaining to the flow of service data units tagged with
the indicated client service ID. The nature of each event is indicated by *type. Additional
parameters characterizing the event are returned in *sessionId, *reasonCode, *endOfBlock,
*dataOffset, *dataLength, and *data as relevant.
The value returned in *data is always a zero-copy object; use the zco_* functions defined in "zco.h"
to retrieve the content of that object.
When the notice is an LtpRecvGreenSegment, the ZCO returned in *data contains the content of a single
LTP green segment. Reassembly of the green part of some block from these segments is the
responsibility of the application.
When the notice is an LtpRecvRedPart, the ZCO returned in *data contains the red part of a possibly
aggregated block. The ZCO's content may therefore comprise multiple service data objects.
Extraction of individual service data objects from the aggregated block is the responsibility of the
application. A simple way to do this is to prepend the length of the service data object to the
object itself (using zco_prepend_header) before calling ltp_send, so that the receiving application
can alternate extraction of object lengths and objects from the delivered block's red part.
The cancellation of an export session may result in delivery of multiple LtpExportSessionCanceled
notices, one for each service data unit in the export session's (potentially) aggregated block. The
ZCO returned in *data for each such notice is a service data unit ZCO that had previously been passed
to ltp_send().
ltp_get_notice() always blocks indefinitely until an LTP processing event is delivered.
Returns zero on success, -1 on any error.
void ltp_interrupt(unsigned int clientId)
Interrupts an ltp_get_notice() invocation. This function is designed to be called from a signal
handler; for this purpose, clientId may need to be obtained from a static variable.
void ltp_release_data(Object data)
Releases the resources allocated to hold data, a client service data ZCO.
void ltp_close(unsigned int clientId)
Terminates the application's exclusive access to received service data units tagged with the
indicated client service data ID.
SEE ALSO
ltpadmin(1), ltprc(5), zco(3)