TCP/IP Tutorial and Technical Overview - IBM Redbooks

2.4.1 Point-to-point encapsulation
A summary of the PPP encapsulation is shown in Figure 2-5.
Figure 2-5 PPP encapsulation frame
The encapsulation fields are defined as follows:
Protocol field The protocol field is one or two octets, and its value
identifies the datagram encapsulated in the Information
field of the packet. Up-to-date values of the Protocol field
are specified in RFC 3232.
Information field The Information field is zero or more octets. The
Information field contains the datagram for the protocol
specified in the Protocol field. The maximum length for the
information field, including padding, but not including the
Protocol field, is termed the Maximum Receive Unit
(MRU), which defaults to 1500 octets. By negotiation,
other values can be used for the MRU.
Padding On transmission, the information field can be padded with
an arbitrary number of octets up to the MRU. It is the
responsibility of each protocol to distinguish padding
octets from real information.
The IP Control Protocol (IPCP) is the NCP for IP and is responsible for
configuring, enabling, and disabling the IP protocol on both ends of the
point-to-point link. The IPCP options negotiation sequence is the same as for
LCP, thus allowing the possibility of reusing the code.
One important option used with IPCP is Van Jacobson Header Compression,
which is used to reduce the size of the combined IP and TCP headers from 40
bytes to approximately 3-4 by recording the states of a set of TCP connections at
each end of the link and replacing the full headers with encoded updates for the
normal case, where many of the fields are unchanged or are incremented by
small amounts between successive IP datagrams for a session. This
compression is described in RFC 1144.
Chapter 2. Network interfaces 37