The first octet of each DLPDU is a frame control (FC) octet which specifies the format of the DLPDU, including the length of all address fields in the DLPDU, and the number and specific roles of the address fields explicitly present in the DLPDU. The FC octet also specifies, where appropriate, the DLPDU’s priority.
The encoding of the FC octet is largely orthogonal, as is shown in figure C.1. The exact function of each FC code point is shown in tables C.1 and C.2. In this figure and these tables, the high-order four bits of the FC octet are shown in the row heading on the left side of the figure or table, and the low-order four bits are shown in the column heading at the top of the figure or table.
The blocks shown in figure C.1 indicate the major functions of that section of the FC code point assignment matrix; a small number of unrelated FC code points are also included in each block.
FC = x\y 00_ _ 01_ _ 10_ _ 11_ _
00_ _ DLPDUs with
DL-addresses NODE spare DLPDUs with short DL-addresses
DLPDUs with long DL-addresses 01_ _
10_ _
11_ _
priority
PP PP PP PP
token release
retain token return token retain token return token DLPDU
address
scope short addresses long addresses
Figure C.1 – Gross structure of FC code points
In Figure C.1, the PP bits take only the values 01, 10 and 11. The PP bits specify the priority of the DLPDU.
Table C.1 shows the generic assignment of the FC code points to the different DLPDUs. It also shows
a) which address fields are explicitly present in the DLPDU (d for destination, s for source, and dss for the three addresses of a connection establishment DLPDU), and which are implicitly present ( – for implicit source address);
b) the common length of each of those d and s address fields, either long (four-octet DL-addresses), or short (one-octet NODE DL-addresses in the upper one-quarter of the
table; two-octet DL-addresses in the lower three-quarters of the table, all implicitly on the local link).
Table C.1 – Generic assignment of FC code points
FC=x\y yyyy 00_ _ 01_ _ 10_ _ 11_ _
xxxx 00 01 10 11 00 01 10 11 00 01 10 11 00 01 10 11 00_ _ 00 WK
d- CL
-s don’t use don’t use don’t
use TL d- 01 CT TD -s IDLE SR [d]s CT TD
-s IDLE SR [d]s 10 RI don’t use don’t use don’t use PN
d- PR -[s]
11 don’t use PT d- RT PT d- 01_ _ 00
01 DT
[s] DT s DT
[s] DT s don’t use DT s don’t use DT s 10 DC
s RC s DC
s RC s DC
s RC s DC
s RC s 11 DC
ds RC ds DC
ds RC ds DC
ds RC ds DC
ds RC ds 10_ _ 00 ES
d- ED d- ES
d- ED d- ES
d- ED d- ES
d- ED d- 01 DT
[d]- DT d- DT
[d]- DT d- don’t use DT d- don’t use DT d-
10 CA d- CA d- CA d- CA d-
11 CD d- CD d- CD d- CD d-
11_ _ 00 RQ
ds ED ds RQ
ds ED ds don’t use ED ds don’t use ED ds 01 RR
ds DT ds RR
ds DT ds don’t use DT ds don’t use DT ds 10 EC
ss CA ds EC
ss CA ds EC
ss CA ds EC
ss CA ds 11 EC
dss CD ds EC
dss CD ds EC
dss CD ds EC
dss CD ds
C.5.2 DLPDU code-point assignment rationale
The structure of the frame-control code points is quite regular.
For example
a) FC3 is the L subfield, specifying the length of all addresses in the DLPDU:
when FC3 = 0 all addresses are SHORT (2 octets when FC7654≥ 0100; 1 octet otherwise);
when FC3 = 1 all addresses are LONG (4 octets when FC7654≥ 0100).
b) FC2 is the F subfield, specifying the location of the token at the end of the current transaction: In general, when FC2 = 0 the token stays with the current token holder;
when FC2 = 1 the token returns to the LAS, or transfers to a new LAS, at the end of the current transaction.
c) When FC7654 ≥ 0011 and FC10≠ 00, then FC10 is the PP subfield.
Table C.2 shows the individual assignment of the FC code points to the different DLPDUs.
Table C.2 – Individual assignment of FC code points
FC=x\y yyyy 00_ _ 01_ _ 10_ _ 11_ _
xxxx 00 01 10 11 00 01 10 11 00 01 10 11 00 01 10 11 00_ _ 00 WK CL don’t use spare don’t
use don’t
use TL spare spare spare spare spare spare spare spare spare
01 CT TD IDLE SR CT TD IDLE SR spare spare spare spare spare spare spare spare
10 RI spare don’t use don’t
use don’t
use spare PN PR spare spare spare spare spare spare spare spare
11 don’t use PT PT PT RT PT PT PT spare spare spare spare spare spare spare spare
01_ _ 00 spare spare spare spare spare spare spare spare spare spare spare spare spare spare spare spare
01 DT DT DT DT DT DT DT DT don’t use DT DT DT don’t use DT DT DT 10 DC RC RC RC DC RC RC RC DC RC RC RC DC RC RC RC 11 DC RC RC RC DC RC RC RC DC RC RC RC DC RC RC RC 10_ _ 00 ES ED ED ED ES ED ED ED ES ED ED ED ES ED ED ED 01 DT DT DT DT DT DT DT DT don’t use DT DT DT don’t use DT DT DT
10 spare CA CA CA spare CA CA CA spare CA CA CA spare CA CA CA
11 spare CD CD CD spare CD CD CD spare CD CD CD spare CD CD CD
11_ _ 00 RQ ED ED ED RQ ED ED ED don’t use ED ED ED don’t use ED ED ED 01 RR DT DT DT RR DT DT DT don’t use DT DT DT don’t use DT DT DT 10 EC CA CA CA EC CA CA CA EC CA CA CA EC CA CA CA 11 EC CD CD CD EC CD CD CD EC CD CD CD EC CD CD CD