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../../principles/transport.rst:60
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Application-layer protocols exchange two types of messages. Some protocols such as those used to support electronic mail exchange messages expressed as strings or lines of characters. As the transport layer allows hosts to exchange bytes, they need to agree on a common representation of the characters. The first and simplest method to encode characters is to use the :term:`ASCII` table. :rfc:`20` provides the ASCII table that is used by many protocols on the Internet. For example, the table defines the following binary representations :
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../../principles/sharing.rst:139
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Congestion collapse on the Internet
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../../principles/security.rst:319
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DES, the Data Encryption Standard that became a standard in 1977 and has been widely used by industry. It uses 56 bits keys that are not considered sufficiently secure nowadays since attackers can launch brute-force attacks by testing all possible keys. Triple DES combines three 56 bits keys, making the brute force attacks more difficult.
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../../exercises/dns.rst:77
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sends DNS requests containing always the same identifier
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../../exercises/ipv6.rst:230
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router ``R4`` uses address ``2001:db8:1341:34::4`` on its West interface, address ``2001:db8:1341:24::4`` on its North interface, address ``2001:db8:1341:14::4`` on its North-West interface and address ``2001:db8:1341:4::4`` on its East interface
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../../exercises/lan.rst:310
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Thanks to the traces collected by tcpdump_, we can easily analyze the messages exchanged by the switches. Here is the fist message sent by switch ``s3``.
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../../exercises/routing-protocols.rst:357
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We can observe the RIPng messages that are exchanged over the network. :rfc:`2080` defines two types of RIPng messages:
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../../exercises/tcp-2.rst:145
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Another interesting features of packetdrill_ is that it is possible to inspect the state maintained by the Linux kernel for the underlying connection using the ``TCP_INFO`` socket option. This makes it possible to retrieve the value of some variables of the TCP control block.
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../../protocols/bgp.rst:84
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A domain's import and export filters can be defined by using the Route Policy Specification Language (RPSL) specified in :rfc:`2622` [GAVE1999]_ . Some Internet Service Providers, notably in Europe, use RPSL to document [#fripedb]_ their import and export policies. Several tools help to easily convert a RPSL policy into router commands.
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../../protocols/congestion.rst:146
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To solve this problem, :rfc:`3168` uses an additional bit in the TCP header : the `Congestion Window Reduced` (CWR) bit.
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../../protocols/dnssec.rst:288
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Given the above security risks, DNSSEC opted for a different approach that allows the negative replies to be authenticated by using offline signatures. The `NSEC` record exploits the lexicographical ordering of all the domain names. To understand its usage, consider a simple domain that contains three names (the associated `AAAA` and other records that are not shown) :
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../../protocols/email.rst:128
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the `Received:` header line is used when an email message is processed by several servers before reaching its destination. Each intermediate email server adds a `Received:` header line. These header lines are useful to debug problems in delivering email messages.
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../../protocols/ethernet.rst:169
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Standard
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../../protocols/http.rst:116
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a `method`, that indicates the type of request, a URI, and the version of the HTTP protocol used by the client
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../../protocols/http2.rst:167
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Consider now that the first object is 12500 bytes long and the second 125 bytes long. With a 1 Mbps connection, this object will use the underlying connection during 100 milliseconds. The client will thus need to wait 100 milliseconds to retrieve the second object. This is the `Head of Line` (HoL) blocking problem that affects the performance of many web services. If the short web object is a javascript code that requests other web objects, its retrieval may be critical to display the retrieved web page.
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../../protocols/ipv6.rst:111
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It is sometimes necessary to write IPv6 addresses in text format, e.g. when manually configuring addresses or for documentation purposes. The preferred format for writing IPv6 addresses is ``x:x:x:x:x:x:x:x``, where the ``x`` 's are hexadecimal digits representing the eight 16-bit parts of the address. Here are a few examples of IPv6 addresses :
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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/routing.rst:84
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OSPF areas
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../../protocols/rpc.rst:104
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`params`: a structure that contains the values of the parameters that are passed to the method
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../../protocols/ssh.rst:211
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It is common practice among designers of security protocols to never use the same key for different purposes. For example, allowing the client and the server to use the same key to encrypt data could enable an attacker to launch a replay attack by sending to the client data that it has itself encrypted.
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