Exit Zen
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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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../../protocols/ipv6.rst:675
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Appendix A of :rfc:`4291` provides the algorithm used to convert a 48 bits MAC address into a 64 bits host identifier. This algorithm builds upon the structure of the MAC addresses. A MAC address is represented as shown in the figure below.
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../../protocols/tcp.rst:640
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In practice, only the `SYN` segment do not have their `ACK` flag set.
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../../glossary.rst:92
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User Datagram Protocol is a protocol of the transport layer in the TCP/IP protocol suite that provides an unreliable connectionless service that includes a mechanism to detect corruption
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../../principles/linkstate.rst:98
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Static or dynamic link metrics ?
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../../principles/reliability.rst:788
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When a `selective repeat` receiver receives a data frame, it first verifies whether the frame is inside its receiving window. If yes, the frame is placed in the receive buffer. If not, the received frame is discarded and an acknowledgment containing `lastack` is sent to the sender. The receiver then removes all consecutive frames starting at `lastack` (if any) from the receive buffer. The payloads of these frames are delivered to the user, `lastack` and the receiving window are updated, and an acknowledgment acknowledging the last frame received in sequence is sent.
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../../protocols/email.rst:344
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The Simple Mail Transfer Protocol (:term:`SMTP`) defined in :rfc:`5321` is a client-server protocol. The SMTP specification distinguishes between five types of processes involved in the delivery of email messages. Email messages are composed on a Mail User Agent (MUA). The MUA is usually either an email client or a webmail. The MUA sends the email message to a Mail Submission Agent (MSA). The MSA processes the received email and forwards it to the Mail Transmission Agent (MTA). The MTA is responsible for the transmission of the email, directly or via intermediate MTAs to the MTA of the destination domain. This destination MTA will then forward the message to the Mail Delivery Agent (MDA) where it will be accessed by the recipient's MUA. SMTP is used for the interactions between MUA and MSA [#fsmtpauth]_, MSA-MTA and MTA-MTA.
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../../protocols/ipv6.rst:680
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A MAC address
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../../protocols/tcp.rst:642
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A complete TCP implementation contains additional information in its TCB, notably to support the `urgent` pointer. However, this part of TCP is not discussed in this book. Refer to :rfc:`793` and :rfc:`2140` for more details about the TCB.
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../../glossary.rst:18
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unicast
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../../principles/linkstate.rst:100
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As link state packets are flooded regularly, routers are able to measure the quality (e.g. delay or load) of their links and adjust the metric of each link according to its current quality. Such dynamic adjustments were included in the ARPANET routing protocol [MRR1979]_ . However, experience showed that it was difficult to tune the dynamic adjustments and ensure that no forwarding loops occur in the network [KZ1989]_. Today's link state routing protocols use metrics that are manually configured on the routers and are only changed by the network operators or network management tools [FRT2002]_.
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../../principles/reliability.rst:790
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The `selective repeat` sender maintains a sending buffer that can store up to `W` unacknowledged frames. These frames are sent as long as the sending buffer is not full. Several implementations of a `selective repeat` sender are possible. A simple implementation associates one retransmission timer to each frame. The timer is started when the frame is sent and canceled upon reception of an acknowledgment that covers this frame. When a retransmission timer expires, the corresponding frame is retransmitted and this retransmission timer is restarted. When an acknowledgment is received, all the frames that are covered by this acknowledgment are removed from the sending buffer and the sliding window is updated.
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