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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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connectionless, in that they operate on a per-packet basis. The set of desired<br />

services are selectable upon SA establishment. However, some restrictions<br />

apply:<br />

► Integrity check <strong>and</strong> authentication are used together.<br />

► Replay protection is selectable only in conjunction with integrity check <strong>and</strong><br />

authentication.<br />

► Replay protection can be selected only by the receiver.<br />

Encryption can be selected independently of other services. It is highly<br />

recommended that, if encryption is enabled, integrity check <strong>and</strong> authentication<br />

be turned on. If only encryption is used, intruders can forge packets in order to<br />

mount cryptanalytic attacks.<br />

Although both authentication (with integrity check) <strong>and</strong> encryption are optional, at<br />

least one of them is always selected; otherwise, you would not be using ESP.<br />

ESP is identified by protocol number 50, as assigned by the IANA. The protocol<br />

header (<strong>IP</strong>v4, <strong>IP</strong>v6, or extension) immediately preceding the AH header will<br />

contain this value in its protocol (<strong>IP</strong>v4) or the next header field (<strong>IP</strong>v6, extension).<br />

ESP processing is applied only to non-fragmented <strong>IP</strong> packets. However, an <strong>IP</strong><br />

packet with ESP applied can be fragmented by intermediate routers. In this case,<br />

the destination first reassembles the packet <strong>and</strong> then applies ESP processing to<br />

it. If an <strong>IP</strong> packet that appears to be a fragment is input to ESP processing (offset<br />

field is non-zero, or the More Fragments bit is set), it is discarded. This prevents<br />

the overlapping fragment attack mentioned in 22.4.2, “Authentication Header<br />

(AH)” on page 813.<br />

If both encryption <strong>and</strong> authentication with integrity check are selected, the<br />

receiver first authenticates the packet <strong>and</strong>, only if this step was successful,<br />

proceeds with decryption. This mode of operation saves computing resources<br />

<strong>and</strong> reduces the vulnerability to denial-of-service attacks.<br />

ESP packet format<br />

The current ESP packet format is described in RFC 2406. It contains important<br />

modifications compared to the previous ESP specification, RFC 1827. The<br />

information in this section is based on RFC 2406.<br />

The format of the ESP packet is more complicated than that of the AH packet.<br />

There is not only an ESP header, but also an ESP trailer <strong>and</strong> ESP authentication<br />

data (see Figure 22-28 on page 819). The payload is located (encapsulated)<br />

between the header <strong>and</strong> the trailer, thus the name of the protocol.<br />

818 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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