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../../protocols/email.rst:60
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a `header` that plays the same role as the letterhead in regular mail. It contains metadata about the message.
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../../protocols/ethernet.rst:72
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The transport and datalink layers usually chose different strategies to place their CRCs or checksums. Transport layer protocols usually place their CRCs or checksums in the segment header. Datalink layer protocols sometimes place their CRC in the frame header, but often in a trailer at the end of the frame. This choice reflects implementation assumptions, but also influences performance :rfc:`893`. When the CRC is placed in the trailer, as in Ethernet, the datalink layer can compute it while transmitting the frame and insert it at the end of the transmission. All Ethernet interfaces use this optimization today. When the checksum is placed in the header, as in a TCP segment, it is impossible for the network interface to compute it while transmitting the segment. Some network interfaces provide hardware assistance to compute the TCP checksum, but this is more complex than if the TCP checksum were placed in the trailer [#ftso]_.
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../../protocols/http.rst:50
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A standardized protocol to efficiently retrieve the documents stored on a server
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../../protocols/http2.rst:62
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A second way to improve the web performance is to reduce the time required to retrieve web objects. While the first web servers returned an HTML documents with possibly a few images, today's rich web servers return one HTML document with associated style sheets, javascript code, images, fonts, ... Some of these web objects come from the original server while others are hosted on different servers. Today, a typical web page contains almost 2 MBytes of data on average. The size of the web pages continues to grow according to statistics collected by `httparchive.org`. Web pages targeted to mobile devices are slightly smaller.
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../../protocols/ipv6.rst:79
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In the late 1980s and early 1990s the growth of the Internet was causing several operational problems on routers. Many of these routers had a single CPU and up to 1 MByte of RAM to store their operating system, packet buffers and routing tables. Given the rate of allocation of IPv4 prefixes to companies and universities willing to join the Internet, the routing tables where growing very quickly and some feared that all IPv4 prefixes would quickly be allocated. In 1987, a study cited in :rfc:`1752`, estimated that there would be 100,000 networks in the near future. In August 1990, estimates indicated that the class B space would be exhausted by March 1994. Two types of solution were developed to solve this problem. The first short term solution was the introduction of Classless Inter Domain Routing (:term:`CIDR`). A second short term solution was the Network Address Translation (:term:`NAT`) mechanism, defined in :rfc:`1631`. NAT allowed multiple hosts to share a single public IPv4 address.
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../../protocols/ppp.rst:27
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PPP frame format
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../../protocols/ppp.rst:34
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Footnotes
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../../protocols/rpc.rst:25
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Encoding data
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../../protocols/ssh.rst:86
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Message Authentication Code (MAC)
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../../protocols/tcp.rst:32
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TCP header format
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../../protocols/tls.rst:43
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995
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../../protocols/transport-service.rst:23
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`IP version 6` addresses that are 128 bits wide
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../../protocols/udp.rst:53
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a 16 bits source port
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../../protocols/wifi.rst:22
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802.11
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../../bibliography.rst:39
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Calvert, K., Donahoo, M., `TCP/IP Sockets in Java : Practical Guide for Programmers <http://books.google.com/books?id=lfHo7uMk7r4C>`_, Morgan Kaufman, 2008
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../../glossary.rst:48
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ARPANET
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../../exercises/ex-sharing.rst:231
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With CSMA, hosts need to listen to the communication channel before starting their transmission. Consider again a 2 kilometers long cable where hosts send frames at 100 Mbps. Show in the figure below the correct transmission of frames with CSMA.
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../../principles/security.rst:155
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Eavesdropping and pervasive monitoring are not the only possible attacks against a network. Another type of attacker is the active attacker. In the literature, these attacks are often called `Man in the middle` or `MITM` attacks. Such attacks occur when one user, let us call him `Mallory`, has managed to configure the network so that he can both capture and modify the packets exchanged by two users. The simplest scenario is when Mallory controls a router that is on the path used by both Alice and Bob. For example, Alice could be connected to a WiFi access router controlled by Mallory and Bob would be a regular server on the Internet.
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../../exercises/dns.rst:41
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What are the IP addresses of the resolvers that the `dig` implementation you are using relies on [#fdig]_ ?
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../../exercises/email.rst:27
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On Unix hosts, a subset of the port assignments is often placed in `/etc/services`.
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