2. cnp3-ebook
3. English
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201 ../../protocols/ipv6.rst:719
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — To be able to exchange IPv6 packets with ``hostB``, ``hostA`` needs to know the MAC address of the interface of ``hostB`` on the LAN. This is the `address resolution` problem. In IPv6, this problem is solved by using the Neighbor Discovery Protocol (NDP). NDP is specified in :rfc:`4861`. This protocol is part of ICMPv6 and uses the multicast datalink layer service. 367/10000 · 367
202 ../../glossary.rst:255
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The eXtensible Markup Language (XML) is a flexible text format derived from SGML. It was originally designed for the electronic publishing industry but is now used by a wide variety of applications that need to exchange structured data. The XML specifications are maintained by `several working groups <https://www.w3.org/XML/>`_ of the :term:`W3C` 348/10000 · 348
202 ../../protocols/email.rst:389
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The transfer of an email message is performed in three phases. During the first phase, the client opens a transport connection with the server. Once the connection has been established, the client and the server exchange greetings messages (`EHLO` command). Most servers insist on receiving valid greeting messages and some of them drop the underlying transport connection if they do not receive a valid greeting. Once the greetings have been exchanged, the email transfer phase can start. During this phase, the client transfers one or more email messages by indicating the email address of the sender (`MAIL FROM:` command), the email address of the recipient (`RCPT TO:` command) followed by the headers and the body of the email message (`DATA` command). Once the client has finished sending all its queued email messages to the SMTP server, it terminates the SMTP association (`QUIT` command). 898/10000 · 898
202 ../../protocols/ipv6.rst:727
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — NDP allows a host to discover the MAC address used by any other host attached to the same LAN. NDP operates in two steps. First, the querier sends a multicast ICMPv6 Neighbor Solicitation message that contains as parameter the queried IPv6 address. This multicast ICMPv6 NS is placed inside a multicast frame [#fndpmulti]_. The queried node receives the frame, parses it and replies with a unicast ICMPv6 Neighbor Advertisement that provides its own IPv6 and MAC addresses. Upon reception of the Neighbor Advertisement message, the querier stores the mapping between the IPv6 and the MAC address inside its NDP table. This table is a data structure that maintains a cache of the recently received Neighbor Advertisement. Thanks to this cache, a host only needs to send a Neighbor Solicitation message for the first packet that it sends to a given host. After this initial packet, the NDP table can provide the mapping between the destination IPv6 address and the corresponding MAC address. 989/10000 · 989
203 ../../protocols/email.rst:391
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A successful transfer of an email message is shown below 56/10000 · 56
203 ../../protocols/ipv6.rst:738
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The NS message can also be used to verify the reachability of a host in the local subnet. For this usage, NS messages can be sent in unicast since other nodes on the subnet do not need to process the message. 208/10000 · 208
204 ../../protocols/email.rst:418
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — In the example above, the MTA running on `mta.example.org` opens a TCP connection to the SMTP server on host `smtp.example.com`. The lines prefixed with `S:` (resp. `C:`) are the responses sent by the server (resp. the commands sent by the client). The server sends its greetings as soon as the TCP connection has been established. The client then sends the `EHLO` command with its fully qualified domain name. The server replies with reply-code `250` and sends its greetings. The SMTP association can now be used to exchange an email. 535/10000 · 535
204 ../../protocols/ipv6.rst:740
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — When an entry in the NDP table times out on a host, it may either be deleted or the host may try to validate it by sending the NS message again. 144/10000 · 144
205 ../../protocols/email.rst:420
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — To send an email, the client must first provide the address of the recipient with `RCPT TO:`. Then it uses the `MAIL FROM:` with the address of the sender. Both the recipient and the sender are accepted by the server. The client can now issue the `DATA` command to start the transfer of the email message. After having received the `354` reply code, the client sends the headers and the body of its email message. The client indicates the end of the message by sending a line containing only the `.` (dot) character [#fdot]_. The server confirms that the email message has been queued for delivery or transmission with a reply code of `250`. The client issues the `QUIT` command to close the session and the server confirms with reply-code `221`, before closing the TCP connection. 781/10000 · 781
205 ../../protocols/ipv6.rst:747
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — This is not the only usage of the Neighbor Solicitation and Neighbor Advertisement messages. They are also used to detect the utilization of duplicate addresses. In the network above, consider what happens when a new host is connected to the LAN. If this host is configured by mistake with the same address as ``hostA`` (i.e. ``2001:db8:1234:5678::AA``), problems could occur. Indeed, if two hosts have the same IPv6 address on the LAN, but different MAC addresses, it will be difficult to correctly reach them. IPv6 anticipated this problem and includes a `Duplicate Address Detection` Algorithm (DAD). When an IPv6 address [#flinklocal]_ is configured on a host, by any means, the host must verify the uniqueness of this address on the LAN. For this, it multicasts an ICMPv6 Neighbor Solicitation that queries the network for its newly configured address. The IPv6 source address of this NS is set to ``::`` (i.e. the reserved unassigned address) if the host does not already have an IPv6 address on this subnet). If the NS does not receive any answer, the new address is considered to be unique and can safely be used. Otherwise, the new address is refused and an error message should be returned to the system administrator or a new IPv6 address should be generated. The `Duplicate Address Detection` Algorithm can prevent various operational problems that are often difficult to debug. 1390/10000 · 1390
206 ../../protocols/email.rst:423
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Open SMTP relays and spam 25/10000 · 25
206 ../../protocols/ipv6.rst:753
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Few users manually configure the IPv6 addresses on their hosts. They prefer to rely on protocols that can automatically configure their IPv6 addresses. IPv6 supports two such protocols : DHCPv6 and the Stateless Address Autoconfiguration (SLAAC). 246/10000 · 246
207 ../../protocols/email.rst:425
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Since its creation in 1971, email has been a very useful tool that is used by many users to exchange lots of information. In the early days, all SMTP servers were open and anyone could use them to forward emails towards their final destination. Unfortunately, over the years, some unscrupulous users have found ways to use email for marketing purposes or to send malware. The first documented abuse of email for marketing purposes occurred in 1978 when a marketer who worked for a computer vendor sent a `marketing email <http://www.templetons.com/brad/spamreact.html#msg>`_ to many ARPANET users. At that time, the ARPANET could only be used for research purposes and this was an abuse of the acceptable use policy. Unfortunately, given the extremely low cost of sending emails, the problem of unsolicited emails has not stopped. Unsolicited emails are now called spam and a `study <http://www.enisa.europa.eu/act/res/other-areas/anti-spam-measures>`_ carried out by ENISA_ in 2009 reveals that 95% of email was spam and this number seems to continue to grow. This places a burden on the email infrastructure of Internet Service Providers and large companies that need to process many useless messages. 1203/10000 · 1203
207 ../../protocols/ipv6.rst:764
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Stateless Address Autoconfiguration (SLAAC) mechanism defined in :rfc:`4862` enables hosts to automatically configure their addresses without maintaining any state. When a host boots, it derives its identifier from its datalink layer address [#fprivacy]_ as explained earlier and concatenates this 64 bits identifier to the `FE80::/64` prefix to obtain its link-local IPv6 address. It then multicasts a Neighbor Solicitation with its link-local address as a target to verify whether another host is using the same link-local address on this subnet. If it receives a Neighbor Advertisement indicating that the link-local address is used by another host, it generates another 64 bits identifier and sends again a Neighbor Solicitation. If there is no answer, the host considers its link-local address to be valid. This address will be used as the source address for all NDP messages sent on the subnet. 904/10000 · 904
208 ../../protocols/email.rst:427
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Given the amount of spam messages, SMTP servers are no longer open :rfc:`5068`. Several extensions to SMTP have been developed in recent years to deal with this problem. For example, the SMTP authentication scheme defined in :rfc:`4954` can be used by an SMTP server to authenticate a client. Several techniques have also been proposed to allow SMTP servers to `authenticate` the messages sent by their users :rfc:`4870` :rfc:`4871` . 434/10000 · 434
208 ../../protocols/ipv6.rst:766
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — To automatically configure its global IPv6 address, the host must know the globally routable IPv6 prefix that is used on the local subnet. IPv6 routers regularly multicast ICMPv6 Router Advertisement messages that indicate the IPv6 prefix assigned to the subnet. The Router Advertisement message contains several interesting fields. 332/10000 · 332
209 ../../protocols/email.rst:433
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Post Office Protocol 24/10000 · 24
209 ../../protocols/ipv6.rst:772
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Format of the ICMPv6 Router Advertisement message 49/10000 · 49
210 ../../protocols/email.rst:435
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — When the first versions of SMTP were designed, the Internet was composed of minicomputers that were used by an entire university department or research lab. These minicomputers were used by many users at the same time. Email was mainly used to send messages from a user on a given host to another user on a remote host. At that time, SMTP was the only protocol involved in the delivery of the emails as all hosts attached to the network were running an SMTP server. On such hosts, an email destined to local users was delivered by placing the email in a special directory or file owned by the user. However, the introduction of personal computers in the 1980s changed this environment. Initially, users of these personal computers used applications such as :term:`telnet` to open a remote session on the local :term:`minicomputer` to read their email. This was not user-friendly. A better solution appeared with the development of user friendly email client applications on personal computers. Several protocols were designed to allow these client applications to retrieve the email messages destined to a user from his/her server. Two of these protocols became popular and are still used today. The Post Office Protocol (POP), defined in :rfc:`1939`, is the simplest one. It allows a client to download all the messages destined to a given user from his/her email server. We describe POP briefly in this section. The second protocol is the Internet Message Access Protocol (IMAP), defined in :rfc:`3501`. IMAP is more powerful, but also more complex than POP. IMAP was designed to allow client applications to efficiently access, in real-time, to messages stored in various folders on servers. IMAP assumes that all the messages of a given user are stored on a server and provides the functions that are necessary to search, download, delete or filter messages. 1862/10000 · 1862
210 ../../protocols/ipv6.rst:774
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — This message is sent from the link-local address of the router on the subnet. Its destination is the IPv6 multicast address that targets all IPv6 enabled hosts (i.e. ``ff02::1``). The `Cur Hop Limit` field, if different from zero, allows specifying the default `Hop Limit` that hosts should use when sending IPv6 packets from this subnet. ``64`` is a frequently used value. The `M` and `O` bits are used to indicate that some information can be obtained from DHCPv6. The `Router Lifetime` parameter provides the expected lifetime (in seconds) of the sending router acting as a default router. This lifetime enables planning the replacement of a router by another one in the same subnet. The `Reachable Time` and the `Retrans Timer` parameter are used to configure the utilization of the NDP protocol on the hosts attached to the subnet. 836/10000 · 836