RFC 1598 (rfc1598) - Page 3 of 7
PPP in X
Alternative Format: Original Text Document
RFC 1598 PPP in X.25 March 1994
3.1. Frame Format
Since both "PPP in HDLC Framing" [3] and X.25 use ISO 3309 as a basis
for framing, the X.25 header is easily substituted for the smaller
HDLC header. The fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+
| Flag (0x7e) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address | Control |D|Q| SVC# (hi) | SVC# (lo) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|p(r) |M|p(s) |0| PPP Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PPP Protocol field and the following Information and Padding
fields are described in the Point-to-Point Protocol Encapsulation
[1].
3.2. Modification of the Basic Frame
The Link Control Protocol can negotiate modifications to the basic
frame structure. However, modified frames will always be clearly
distinguishable from standard frames.
Address-and-Control-Field-Compression
Because the Address and Control field values are not constant, and
are modified as the frame is transported by the network switching
fabric, Address-and-Control-Field-Compression MUST NOT be
negotiated.
Protocol-Field-Compression
Note that unlike the HDLC framing, the X.25 framing does not align
the Information field on a 32-bit boundary. Alignment to a 16-bit
boundary occurs when the Protocol field is compressed to a single
octet. When this improves throughput, Protocol-Field-Compression
SHOULD be negotiated.
Simpson