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4 ../../protocols/rpc.rst:12
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The caller places the values of the parameters at a location (register, stack, ...) where the callee can access them 116/10000 · 116
5 ../../protocols/ssh.rst:46
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The secure shell protocol was designed in the mid 1990s by T. Ylonen to counter the eavesdropping attacks against :term:`telnet` and similar protocols [Ylonen1996]_. :term:`ssh` became quickly popular and system administrators encouraged its usage. The original version of :term:`ssh` was freely available. After a few years, his author created a company to distribute it commercially, but other programmers continued to develop an open-source version of :term:`ssh` called `OpenSSH <http://www.openssh.com>`_. Over the years, :term:`ssh` evolved and became a flexible applicable whose usage extends beyond remote login to support features such as file transfers, protocol tunneling, ... In this section, we only discuss the basic features of :term:`ssh` and explain how it differs from :term:`telnet`. Entire books have been written to describe :term:`ssh` in details [BS2005]_. An overview of the protocol appeared in [Stallings2009]_. 937/10000 · 937
5 ../../protocols/tcp.rst:17
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — World wide web ( :term:`HTTP`, ...) 35/10000 · 35
6 ../../protocols/tcp.rst:18
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Most file transfer protocols ( :term:`ftp`, peer-to-peer file sharing applications , ...) 89/10000 · 89
7 ../../protocols/rpc.rst:15
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The callee places the return value(s) at a location (register, stack, ...) where the caller can access them 107/10000 · 107
7 ../../protocols/tcp.rst:19
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — remote computer access : :term:`telnet`, :term:`ssh`, :term:`X11`, :term:`VNC`, ... 83/10000 · 83
8 ../../exercises/routing-policies.rst:63 • ../../exercises/routing-policies.rst:96
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Consider the interdomain topology shown in the figure below. Assuming, that `AS1` advertises prefix ``2001:db8:1::/48``, `AS2` prefix ``2001:db8:2::/48``, ... compute the routing tables of the different ASes. 209/10000 · 209
8 ../../protocols/tcp.rst:20
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — non-interactive multimedia applications (flash, ...) 52/10000 · 52
9 ../../exercises/dns.rst:36
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `type` is the type of DNS record that is requested by the query such as `NS` for a nameserver, `A` for an IPv4 address, `AAAA` for an IPv6 address, `MX` for a mail relay, ... 174/10000 · 174
9 ../../protocols/rpc.rst:18
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — This model was developed with a single host in mind. How should it be modified if the caller and the callee are different hosts connected through a network ? Since the two hosts can be different, the two main problems are the fact they do not share the same memory and that they do not necessarily use the same representation for numbers, characters, ... Let us examine how the five steps identified above can be supported through a network. 441/10000 · 441
11 ../../protocols/http2.rst:62
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 630/10000 · 630
12 ../../protocols/ethernet.rst:81
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Ethernet frame format shown above is specified in [DIX]_. This is the format used to send both IPv4 :rfc:`894` and IPv6 packets :rfc:`2464`. After the publication of [DIX]_, the Institute of Electrical and Electronic Engineers (IEEE) began to standardize several Local Area Network technologies. IEEE worked on several LAN technologies, starting with Ethernet, Token Ring and Token Bus. These three technologies were completely different, but they all agreed to use the 48 bits MAC addresses specified initially for Ethernet [IEEE802]_ . While developing its Ethernet standard [IEEE802.3]_, the IEEE 802.3 working group was confronted with a problem. Ethernet mandated a minimum payload size of 46 bytes, while some companies were looking for a LAN technology that could transparently transport short frames containing only a few bytes of payload. Such a frame can be sent by an Ethernet host by padding it to ensure that the payload is at least 46 bytes long. However since the Ethernet header [DIX]_ does not contain a length field, it is impossible for the receiver to determine how many useful bytes were placed inside the payload field. To solve this problem, the IEEE decided to replace the `Type` field of the Ethernet [DIX]_ header with a length field [#ftypelen]_. This `Length` field contains the number of useful bytes in the frame payload. The payload must still contain at least 46 bytes, but padding bytes are added by the sender and removed by the receiver. In order to add the `Length` field without significantly changing the frame format, IEEE had to remove the `Type` field. Without this field, it is impossible for a receiving host to identify the type of network layer packet inside a received frame. To solve this new problem, IEEE developed a completely new sublayer called the Logical Link Control [IEEE802.2]_. Several protocols were defined in this sublayer. One of them provided a slightly different version of the `Type` field of the original Ethernet frame format. Another contained acknowledgments and retransmissions to provide a reliable service... In practice, [IEEE802.2]_ is never used to support IP in Ethernet networks. The figure below shows the official [IEEE802.3]_ frame format. 2223/10000 · 2223
13 ../../principles/sharing.rst:117
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Sharing resources is important to ensure that the network efficiently serves its user. In practice, there are many ways to share resources. Some resource sharing schemes consider that some users are more important than others and should obtain more resources. For example, on the roads, police cars and ambulances have priority. In some cities, traffic lanes are reserved for buses to promote public services, ... In computer networks, the same problem arise. Given that resources are limited, the network needs to enable users to efficiently share them. Before designing an efficient resource sharing scheme, one needs to first formalize its objectives. In computer networks, the most popular objective for resource sharing schemes is that they must be `fair`. In a simple situation, for example two hosts using a shared 2 Mbps link, the sharing scheme should allocate the same bandwidth to each user, in this case 1 Mbps. However, in a large networks, simply dividing the available resources by the number of users is not sufficient. Consider the network shown in the figure below where `A1` sends data to `A2`, `B1` to `B2`, ... In this network, how should we divide the bandwidth among the different flows ? A first approach would be to allocate the same bandwidth to each flow. In this case, each flow would obtain 5 Mbps and the link between `R2` and `R3` would not be fully loaded. Another approach would be to allocate 10 Mbps to `A1-A2`, 20 Mbps to `C1-C2` and nothing to `B1-B2`. This is clearly unfair. 1513/10000 · 1513
16 ../../protocols/http2.rst:79
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — To understand the benefits of pipelining, let us consider a simple but illustrative example. A client needs to retrieve 5 web objects that are each 100 bytes. The underlying transport connection has a 1 Gbps bandwidth but a one-way delay of 100 msec. A normal HTTP/1.x client would send the first request, wait 200 msec to receive the answer, then send another request... It would need one entire second to retrieve the five web objects. This is illustrated in the figure below. 478/10000 · 478
17 ../../exercises/tcp-2.rst:95
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — This TCP segment is sent immediately by the stack. The ``SYN`` flag is set and the dot next to the ``S`` character indicates that the ACK flag is also set. The SYN+ACK segment does not contain any data but its acknowledgment number is set to 1 (relative to the initial sequence number). For outgoing packets, packetdrill_ does not verify the value of the advertised window. In this line, it also accepts any TCP options (``<...>``). 432/10000 · 432
18 ../../protocols/rpc.rst:65
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — XDR also supports the definition of unions, structures, ... Additional details are provided in :rfc:`1832`. 107/10000 · 107
20 ../../protocols/congestion.rst:108
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The first difficulty in adding Explicit Congestion Notification (ECN) in TCP/IP network was to modify the format of the network packet and transport segment headers to carry the required information. In the network layer, one bit was required to allow the routers to mark the packets they forward during congestion periods. In the IP network layer, this bit is called the `Congestion Experienced` (`CE`) bit and is part of the packet header. However, using a single bit to mark packets is not sufficient. Consider a simple scenario with two sources, one congested router and one destination. Assume that the first sender and the destination support ECN, but not the second sender. If the router is congested it will mark packets from both senders. The first sender will react to the packet markings by reducing its transmission rate. However since the second sender does not support ECN, it will not react to the markings. Furthermore, this sender could continue to increase its transmission rate, which would lead to more packets being marked and the first source would decrease again its transmission rate, ... In the end, the sources that implement ECN are penalized compared to the sources that do not implement it. This unfairness issue is a major hurdle to widely deploy ECN on the public Internet [#fprivate]_. The solution proposed in :rfc:`3168` to deal with this problem is to use a second bit in the network packet header. This bit, called the `ECN-capable transport` (ECT) bit, indicates whether the packet contains a segment produced by a transport protocol that supports ECN or not. Transport protocols that support ECN set the ECT bit in all packets. When a router is congested, it first verifies whether the ECT bit is set. In this case, the CE bit of the packet is set to indicate congestion. Otherwise, the packet is discarded. This eases the deployment of ECN [#fecnnonce]_. 1893/10000 · 1893
20 ../../principles/sharing.rst:186
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — If `R1` has enough buffers, it will be able to absorb the load without having to discard packets. The packets sent by hosts `A` and `B` will reach their final destination `C`, but will experience a longer delay than when they are transmitting alone. The amount of buffering on the network node is the first parameter that a network operator can tune to control congestion inside his network. Given the decreasing cost of memory, one could be tempted to put as many buffers [#fbufferbloat]_ as possible on the network nodes. Let us consider this case in the network above and assume that `R1` has infinite buffers. Assume now that hosts `A` and `B` try to transmit a file that corresponds to one thousand packets each. Both are using a reliable protocol that relies on go-back-n to recover from transmission errors. The transmission starts and packets start to accumulate in `R1`'s buffers. The presence of these packets in the buffers increases the delay between the transmission of a packet by `A` and the return of the corresponding acknowledgment. Given the increasing delay, host `A` (and `B` as well) will consider that some of the packets that it sent have been lost. These packets will be retransmitted and will enter the buffers of `R1`. The occupancy of the buffers of `R1` will continue to increase and the delays as well. This will cause new retransmissions, ... In the end, only one file will be delivered (very slowly) to the destination, but the link `R1-R2` will transfer much more bytes than the size of the file due to the multiple copies of the same packets. This is known as the `congestion collapse` problem :rfc:`896`. Congestion collapse is the nightmare for network operators. When it happens, the network carries packets without delivering useful data to the end users. 1793/10000 · 1793
22 ../../principles/sharing.rst:190
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Congestion collapse is unfortunately not only an academic experience. Van Jacobson reports in [Jacobson1988]_ one of these events that affected him while he was working at the Lawrence Berkeley Laboratory (LBL). LBL was two network nodes away from the University of California in Berkeley. At that time, the link between the two sites had a bandwidth of 32 Kbps, but some hosts were already attached to 10 Mbps LANs. "In October 1986, the data throughput from LBL to UC Berkeley ... dropped from 32 Kbps to 40 bps. We were fascinated by this sudden factor-of-thousand drop in bandwidth and embarked on an investigation of why things had gotten so bad." This work lead to the development of various congestion control techniques that have allowed the Internet to continue to grow without experiencing widespread congestion collapse events. 839/10000 · 839
22 ../../protocols/http.rst:104
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The HTML document shown below is composed of two parts: a header, delineated by the `<head>` and `</head>` markers, and a body (between the `<body>` and `</body>` markers). In the example below, the header only contains a title, but other types of information can be included in the header. The body contains an image, some text and a list with three hyperlinks. The image is included in the web page by indicating its URI between brackets inside the `<img src="...">` marker. It is important to note that the image can reside on any server. The client will automatically download it when rendering the web page. The `<h1>...</h1>` marker is used to specify the first level of headings. The `<ul>` marker indicates an unnumbered list while the `<li>` marker indicates a list item. The `<a href="URI">text</a>` indicates a hyperlink. The `text` will be underlined in the rendered web page and the client will fetch the specified URI when the user clicks on the link. 965/10000 · 965