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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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<strong>IP</strong> can provide fragmentation <strong>and</strong> reassembly of datagrams. The maximum<br />

length of an <strong>IP</strong> datagram is 65,535 octets. All <strong>IP</strong> hosts must support 576 octets<br />

datagrams without fragmentation.<br />

Fragments of a datagram each have a header. The header is copied from the<br />

original datagram. A fragment is treated as a normal <strong>IP</strong> datagrams while being<br />

transported to their destination. However, if one of the fragments gets lost, the<br />

complete datagram is considered lost. Because <strong>IP</strong> does not provide any<br />

acknowledgment mechanism, the remaining fragments are discarded by the<br />

destination host.<br />

<strong>IP</strong> datagram format<br />

The <strong>IP</strong> datagram header has a minimum length of 20 octets, as illustrated in<br />

Figure 3-17.<br />

0<br />

VERS<br />

TTL<br />

4<br />

HLEN<br />

ID<br />

<strong>IP</strong> Options<br />

Service Type<br />

Protocol<br />

FLG<br />

Source <strong>IP</strong> Address<br />

Destination <strong>IP</strong> Address<br />

Data …<br />

…<br />

…<br />

Figure 3-17 <strong>IP</strong>: Format of an <strong>IP</strong> datagram header<br />

1<br />

8 6<br />

Total Length<br />

Fragment Offset<br />

Header Checksum<br />

Padding<br />

Where:<br />

► VERS: The field contains the <strong>IP</strong> protocol version. The current version is 4.<br />

Version 5 is an experimental version. Version 6 is the version for <strong>IP</strong>v6 (see<br />

9.2, “The <strong>IP</strong>v6 header format” on page 330).<br />

► HLEN: The length of the <strong>IP</strong> header counted in 32-bit quantities. This does not<br />

include the data field.<br />

1<br />

9<br />

2<br />

4<br />

Chapter 3. Internetworking protocols 99<br />

3<br />

1

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