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../../protocols/ipv6.rst:653
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IPv6 hosts and routers frequently interact with the datalink layer service. To understand the main interactions, it is useful to analyze all the packets that are exchanged when a simple network containing a few hosts and routers is built. Let us first start with a LAN containing two hosts [#fMAC]_.
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../../protocols/tcp.rst:634
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Of course, such a simultaneous TCP establishment can only occur if the source port chosen by the client is equal to the destination port chosen by the server. This may happen when a host can serve both as a client as a server or in peer-to-peer applications when the communicating hosts do not use ephemeral port numbers.
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../../glossary.rst:307
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TLS
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../../principles/linkstate.rst:87
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Flooding allows LSPs to be distributed to all routers inside the network without relying on routing tables. In the example above, the LSP sent by router `E` is likely to be sent twice on some links in the network. For example, routers `B` and `C` receive `E`'s LSP at almost the same time and forward it over the `B-C` link. To avoid sending the same LSP twice on each link, a possible solution is to slightly change the pseudo-code above so that a router waits for some random time before forwarding a LSP on each link. The drawback of this solution is that the delay to flood an LSP to all routers in the network increases. In practice, routers immediately flood the LSPs that contain new information (e.g. addition or removal of a link) and delay the flooding of refresh LSPs (i.e. LSPs that contain exactly the same information as the previous LSP originating from this router) [FFEB2005]_.
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../../principles/reliability.rst:778
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A `selective repeat` receiver maintains a sliding window of `W` frames and stores in a buffer the out-of-sequence frames that it receives. The figure below shows a five-frame receive window on a receiver that has already received frames `7` and `9`.
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../../principles/sharing.rst:1218
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When the connection starts, its congestion window is set to one segment. Segment `S0` is sent an acknowledgment at roughly :math:`t_{0}+4`. The congestion window is increased by one segment and `S1` and `S2` are transmitted at time :math:`t_{0}+4` and :math:`t_{0}+5`. The corresponding acknowledgments are received at times :math:`t_{0}+8` and :math:`t_{0}+10`. Upon reception of this last acknowledgment, the congestion window reaches `3` and segments can be sent (`S4` and `S5`). When segment `S6` reaches router `R1`, its buffers already contain `S5`. The packet containing `S6` is thus marked to inform the sender of the congestion. Note that the sender will only notice the congestion once it receives the corresponding acknowledgment at :math:`t_{0}+18`. In the meantime, the congestion window continues to increase. At :math:`t_{0}+16`, upon reception of the acknowledgment for `S5`, it reaches `4`. When congestion is detected, the congestion window is decreased down to `2`. This explains the idle time between the reception of the acknowledgment for `S*6` and the transmission of `S10`.
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../../protocols/email.rst:311
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F P s \=
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../../protocols/ipv6.rst:671
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Hosts ``A`` and ``B`` are attached to the same datalink layer network. They can thus exchange frames by using the MAC addresses shown in the figure above. To be able to use IPv6 to exchange packets, they need to have an IPv6 address. One possibility would be to manually configure an IPv6 address on each host. However, IPv6 provides a better solution thanks to the `link-local` IPv6 addresses. A `link-local` IPv6 address is an address that is composed by concatenating the ``fe80:://64`` prefix with the MAC address of the device. In the example above, host A would use IPv6 `link-local` address ``fe80::0223:45FF:FE67:89ab`` and host B ``fe80::0234:56FF:FE78:9abc``. With these two IPv6 addresses, the hosts can exchange IPv6 packets.
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../../protocols/tcp.rst:636
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Sending a packet with a different source IP address than the address allocated to the host is called sending a :term:`spoofed packet`.
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../../glossary.rst:309
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Transport Layer Security, defined in :rfc:`5246` is a cryptographic protocol that is used to provide communication security for Internet applications. This protocol is used on top of the transport service but a detailed description is outside the scope of this book.
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../../principles/linkstate.rst:89
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To ensure that all routers receive all LSPs, even when there are transmissions errors, link state routing protocols use `reliable flooding`. With `reliable flooding`, routers use acknowledgments and if necessary retransmissions to ensure that all link state packets are successfully transferred to each neighboring router. Thanks to reliable flooding, all routers store in their LSDB the most recent LSP sent by each router in the network. By combining the received LSPs with its own LSP, each router can build a graph that represents the entire network topology.
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../../principles/reliability.rst:784
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The receiving window with selective repeat
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../../principles/sharing.rst:1220
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In practice, a router is connected to multiple input links. The figure below shows an example with two hosts.
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../../protocols/email.rst:318
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Now that we have explained the format of the email messages, we can discuss how these messages can be exchanged through the Internet. The figure below illustrates the protocols that are used when `Alice` sends an email message to `Bob`. `Alice` prepares her email with an email client or on a webmail interface. To send her email to `Bob`, `Alice`'s client will use the Simple Mail Transfer Protocol (:term:`SMTP`) to deliver her message to her SMTP server. `Alice`'s email client is configured with the name of the default SMTP server for her domain. There is usually at least one SMTP server per domain. To deliver the message, `Alice`'s SMTP server must find the SMTP server that contains `Bob`'s mailbox. This can be done by using the Mail eXchange (MX) records of the DNS. A set of MX records can be associated to each domain. Each MX record contains a numerical preference and the fully qualified domain name of a SMTP server that is able to deliver email messages destined to all valid email addresses of this domain. The DNS can return several MX records for a given domain. In this case, the server with the lowest numerical preference is used first :rfc:`2821`. If this server is not reachable, the second most preferred server is used etc. `Bob`'s SMTP server will store the message sent by `Alice` until `Bob` retrieves it using a webmail interface or protocols such as the Post Office Protocol (:term:`POP`) or the Internet Message Access Protocol (:term:`IMAP`).
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../../protocols/ipv6.rst:673
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Converting MAC addresses in host identifiers
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../../protocols/tcp.rst:638
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The full list of all TCP options may be found at http://www.iana.org/assignments/tcp-parameters/
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../../glossary.rst:90
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UDP
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../../principles/reliability.rst:786
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A `selective repeat` receiver discards all frames having an invalid CRC, and maintains the variable `lastack` as the sequence number of the last in-sequence frame that it has received. The receiver always includes the value of `lastack` in the acknowledgments that it sends. Some protocols also allow the `selective repeat` receiver to acknowledge the out-of-sequence frames that it has received. This can be done for example by placing the list of the correctly received, but out-of-sequence frames in the acknowledgments together with the `lastack` value.
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../../principles/sharing.rst:1279
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In general, the links have a non-zero delay. This is illustrated in the figure below where a delay has been added on the link between `R` and `C`.
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../../protocols/email.rst:342
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The Simple Mail Transfer Protocol
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