ld primarily be used between the end points of a connection (hosts). It encapsulates and encrypts only the data portion (payload) of each IP packet, but leaves the header untouched. The transport mode is less secure than the tunnel mode, since it does not conceal or encapsulate the IP control information and does not provide traffic flow confidentiality by hiding the source and destination of the IP packet. Understanding Changing Telecommunications - Building a Successful Telecom Business Transport Application New header IPsec Payload data Figure 6.25 The transport mode IPsec uses two protocols to provide traffic security: Authentication header (AH) Encapsulating security payload (ESP) AH and ESP supply partly the same functionality, but ESP is broader and includes encryption methods for IP payload and (in the tunnel mode) for a part of the IP header. AH is described in RFC 2402 and ESP is described in RFC 2406. The AH header is 24 bytes and includes a security parameter index (SPI) field, a sequence number field and an authentication data with variable length. The SPI iden- tifies a security association (SA) while the sequence number field is counter for anti- replay service. AH can be used in the transport mode and the tunnel mode. 6.13 CONFIDENTIALITY AND INTEGRITY FOR MAIL BY S/MIME E-mail has been used within the Internet community for a very long time. From the beginning, all e-mails were text-based, and text was the only transferred information. Later on, the mail standards were extended to embrace enhanced text, graphics and audio in the body part of the mail. The new standard was multipurpose Internet mail extensions (MIME). The Internet standard secure MIME or S/MIME was developed to add security by providing authentication, integrity, confidentiality and non-repudiation for Internet e-mail. S/MIME is also attractive for HTTP information retrieval and for corporate mails, now that more and more mobile users need to communicate securely with colleagues on corporate intranets. See Figure 6.26. S/MIME reli