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Lectures notes for 2010 - KTH

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PPP frames<br />

FLAG ADDR CNTL<br />

7E FF protocol<br />

03<br />

1 1 1 2<br />

data<br />

CRC<br />

upto 1500 bytes 2<br />

FLAG<br />

7E<br />

1<br />

protocol<br />

0021<br />

IP datagram<br />

2<br />

protocol<br />

C021<br />

Link control data<br />

2<br />

protocol network<br />

8021 control data<br />

2<br />

Figure 12: Format of PPP frame (see Stevens, Volume 1, figure 2.3, pg. 26)<br />

• The protocol field behaves like the Ethernet TYPE field.<br />

• CRC can be used to detect errors in the frame.<br />

• Either character or bit stuffing is done depending on the link.<br />

• you can negotiate away the CNTL and ADDRESS fields, and reduce<br />

the protocol field to 1 byte ⇒ minimum overhead of 3 bytes<br />

• Van Jacobson header compression <strong>for</strong> IP and TCP<br />

Maguire PPP frames 1: 67 of 104<br />

maguire@kth.se <strong>2010</strong>.03.21 Internetworking/Internetteknik

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