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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 22-36 shows in detail how this combination is realized. Consider that host<br />

H1 in Figure 22-35 on page 827 sends an <strong>IP</strong> packet to host H2. Here is what<br />

happens:<br />

1. Host H1 constructs the <strong>IP</strong> packet <strong>and</strong> applies ESP transport to it. H1 then<br />

sends the datagram to gateway G1, the destination address being H2.<br />

2. Gateway G1 realizes that this packet should be routed to G2. Upon consulting<br />

its <strong>IP</strong>Sec databases (SPD <strong>and</strong> SAD), G1 concludes that AH in tunnel mode<br />

must be applied before sending the packet out. It does the required<br />

encapsulation. Now the <strong>IP</strong> packet has the address of G2 as its destination,<br />

the ultimate destination H2 being encapsulated.<br />

3. Gateway G2 receives the AH-tunneled packet. It is destined to itself, so it<br />

authenticates the datagram <strong>and</strong> strips off the outer header. G2 sees that the<br />

payload is yet another <strong>IP</strong> packet (that one sent by H1) with destination H2, so<br />

it forwards to H2. G2 does not care that this packet has an ESP header.<br />

4. Finally H2 receives the packet. Because this is the destination, ESP-transport<br />

processing is applied <strong>and</strong> the original payload retrieved.<br />

<strong>IP</strong> Hdr<br />

Src:H1<br />

Dest:H2<br />

<strong>IP</strong> Hdr<br />

Src:H1<br />

Dest:H2<br />

New <strong>IP</strong> hdr<br />

Src:G1<br />

Dest:G2<br />

Payload<br />

ESP<br />

Hdr<br />

AH<br />

Payload<br />

<strong>IP</strong> Hdr<br />

Src:H1<br />

Dest:H2<br />

<strong>IP</strong> Hdr<br />

Src:H1<br />

Dest:H2<br />

<strong>IP</strong> Hdr<br />

Src:H1<br />

Dest:H2<br />

Figure 22-36 Nesting of <strong>IP</strong>Sec protocols<br />

828 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

ESP<br />

Hdr<br />

ESP<br />

Hdr<br />

Payload<br />

ESP<br />

Trl<br />

Payload<br />

Payload<br />

ESP<br />

Auth<br />

ESP<br />

Trl<br />

ESP<br />

Trl<br />

ESP<br />

Auth<br />

ESP<br />

Auth

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