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PPP General Frame Format
(Page 2 of 4)
Protocol Field Ranges
The Protocol field is the
main frame type indicator for the device receiving the frame.
For data frames this is normally the network-layer protocol that created
the datagram, and for control frames, the PPP protocol that created
the control message. In the case of protocols that modify data such
as when compression
(CCP) or encryption
(ECP) are used, this field identifies
the data as being either compressed or encrypted, and the original Protocol
value is extracted after the Information field is decompressed/decrypted.
There are dozens of network-layer
protocols and PPP control protocols, and a correspondingly large number
of Protocol values The main PPP standard defines four ranges
for organizing these values, as shown in Table 33.
Table 33: PPP Protocol Field Ranges
Field Range (hexadecimal)
0000 - 3FFF
Encapsulated network layer datagrams
that have an associated Network
Control Protocol (NCP). In this case,
control frames from the corresponding NCP use a Protocol field
value that is computed by adding 8 to the first octet of
the network layer Protocol value. For example, for IP the Protocol
value is 0021, and control frames from the IP Control Protocol (IPCP)
use Protocol value 8021.
This range also includes several values used for specially-processed
encapsulated datagrams, such as when compression or encryption are employed.
datagrams from low-volume protocols. These are protocols
that do not have an associated NCP.
8000 - BFFF
Control Protocol (NCP) control frames
that correspond to the network layer Protocol values in the 0000-3FFF
used by LCP
and LCP support protocols such as PAP
and CHAP. Some miscellaneous protocol
values are included here as well.
The standard also specifies
that the Protocol value must be assigned so that the first octet
is even, and the second octet is odd. So, for example, 0x0021 is a valid
value but 0x0121 and 0x0120 are not. (The reason for this will become
apparent shortly). There are also certain blocks that are reserved and
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Version 3.0 - Version Date: September 20, 2005
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