IBM SC30-3681-08 manual PPP Link Control Protocol LCP

Using PPP

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8 bits 8 bits 8 bits 16 bits variable 16 bits 8 bits

Figure 27. PPP Frame Structure

Flag Fields

The ¯ag ®eld begins and ends each frame with a unique pattern of 01111110. Generally a single ¯ag ends one frame and begins the next. The receiver attached to the link continuously search for the ¯ag sequence to synchronize the start of the next frame.

Address Field

The address ®eld is a single octet (8 bits) and contains the binary sequence 11111111 (0xff hexadecimal). This is known as the All-Station Address. PPP does not assign individual station addresses.

Control Field

The control ®eld is a single octet and contains the binary sequence 00000011 (0x03 hexadecimal). This sequence identi®es the Unnumbered Information (UI) command with the P/F bit set to zero.

Protocol Field

The protocol ®eld is de®ned by PPP. The ®eld is 2 octets (16 bits) and its value identi®es the protocol datagram encapsulated in the Information ®eld of the frame.

Protocol ®eld values in the range '0xC000'±'0xFFFF' indicate Layer 3 data (protocol datagrams) such as LCP, PAP, CHAP, SPAP, and CCP. Values in the range '8000'±'BFFF' indicate that the datagrams belong to the Network Control Protocols (NCP). Values in the range '0'±'3FFF' identify the network protocol of speci®c datagrams.

Information Field

The information ®eld contains the datagram for the protocol speci®ed in the protocol ®eld. This is zero or more octets.

When the protocol type is LCP, exactly one LCP packet is encapsulated in the information ®eld of PPP Data Link Layer frames.

Frame Check Sequence (FCS) Field

The frame check sequence ®eld is a 16-bit cyclic redundancy check (CRC).

PPPlinks can negotiate the use of various options which may modify the basic frame format; the description below applies to the frame format prior to any such modi®cations. PPP LCP packets are always sent in this format as well, regardless of negotiated options, so that LCP packets can be recognized even when there is a loss of synchronization on the line.

The router supports two such options: Address and Control Field Compression (ACFC) and Protocol Field Compression (PFC). These are described in detail in a later section.

The PPP Link Control Protocol (LCP)

PPP's Link Control Protocol (LCP) establishes, con®gures, maintains, and terminates the point-to-point link. This process is carried out in four phases:

Chapter 32. Using Point-to-Point Protocol Interfaces 451