7: Ppp; Lcp; Packet Sizes; Header Compression - Lantronix SCS Reference Manual

7: PPP

The SCS can use PPP, the Point-to-Point Protocol, to transmit high layer protocols over a serial link, ISDN
connection, or other point-to-point based connection. Unlike SLIP (the Serial Line Internet Protocol), which
can also be used with the SCS, PPP supports authentication, escape sequences for flow control characters,
loopback detection, and per-packet checksums.
Two major components of PPP are discussed in the following sections:
LCP on page 7-1 discusses the Link Control Protocol (LCP).
NCP on page 7-3 discusses Network Control Protocols (NCPs).
The final section discusses Starting PPP on page 7-3. PPP is also discussed in Chapter 4, Basic Remote
Networking, and PPP authentication (PAP and CHAP) are described in PPP Logins on page 11-3.

7.1 LCP

The Link Control Protocol (LCP) is used by PPP to negotiate basic characteristics of the connection. These
characteristics include packet size, header compression, control character escaping, and authentication
mechanisms.
LCP is documented in RFCs 1661 and 1662.
Note:

7.1.1 Packet Sizes

Both sides of a connection negotiate the size of the packets each can receive. Packet size is also known as
Maximum Receive Unit (MRU). The MRU need not be the same in each direction. The SCS MRU is 1522
bytes.
To configure the maximum packet size that can be received from a remote node, set the Maximum
Transmission Unit (MTU), or maximum packet size, with the Define Site MTU command.

7.1.2 Header Compression

PPP frames each packet with certain data fields, some of which may be omitted or compressed (see Define
Ports PPP on page 12-81 for details). PPP header compression is enabled by default on all SCS ports. To
disable header compression, use the following command.
Local>> DEFINE PORT 2 PPP HEADERCOMPRESSION DISABLED

7.1.3 Character Escaping

PPP can be configured to substitute a two byte sequence of characters for specific characters. The
substituted characters are sent instead and the recipient translates them back into the original characters.
This substitution is called character escaping.
Figure 7-1: Disabling PPP Header Compression
7-1