Internet Protocol (IP) Frames

Internet Protocol (IP) frames contain encapsulated IP datagrams for transmission across PPP links. A single IP datagram is contained in the information field of a PPP frame. Large IP datagrams may be fragmented for transmission, if necessary. IP frames have the general form shown in Figure A-9.

Figure A-9 IP Frame Format

Address Field

The address field is one octet in length, and is part of the HDLC-like framing for PPP. It is always set to 0xff.

Control Field

The control field is one octet in length, and is part of the HDLC-like framing for PPP. It is always set to 0x03.

Protocol Id

The protocol id identifies the type of information contained in the information field of the frame. It can have the following values for IP frames:

0x0021 Internet protocol

0x002d Van Jacobson Compressed TCP/IP

0x002f Van Jacobson Uncompressed TCP/IP

Id Field

The id field is one octet in length, and carries an identifier that is used to match associated requests and replies.

Length Field

The length field is two octets in length, and indicates the total length of the IPCP frame including the code, id, length, and data fields. The length must not exceed the maximum receive unit (MRU).

Data Field

The data field is zero or more octets in length, as indicated by the length field. It contains the IP datagram for transmission over the PPP link.