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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Window scale option<br />

This option is not m<strong>and</strong>atory. Both sides must send<br />

the Window scale option in their SYN segments to<br />

enable windows scaling in their direction. The Window<br />

scale exp<strong>and</strong>s the definition of the <strong>TCP</strong> window to 32<br />

bits. It defines the 32-bit window size by using scale<br />

factor in the SYN segment over st<strong>and</strong>ard 16-bit<br />

window size. The receiver rebuilds the 32-bit window<br />

size by using the 16-bit window size <strong>and</strong> scale factor.<br />

This option is determined while h<strong>and</strong>shaking. There is<br />

no way to change it after the connection has been<br />

established. See Figure 4-14 for more details.<br />

3 3 shift.cnt<br />

Figure 4-14 <strong>TCP</strong>: Window scale option<br />

SACK-permitted option<br />

This option is set when selective acknowledgment is<br />

used in that <strong>TCP</strong> connection. See Figure 4-15 for<br />

details.<br />

4 2<br />

Figure 4-15 <strong>TCP</strong>: SACK-permitted option<br />

Chapter 4. Transport layer protocols 159

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