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<title>dordal</title>
<!-- 2019-01-19 Sat 21:44 -->
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<body>
<div id="content">
<h1 class="title">dordal</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#sec-1">1. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 1 An Overview of Networks</a></li>
<li><a href="#sec-2">2. Introduction</a></li>
<li><a href="#sec-3">3. 1.1. Layers</a></li>
<li><a href="#sec-4">4. 1.2. Data Rate, Throughput and Bandwidth</a></li>
<li><a href="#sec-5">5. 1.3. Packets</a></li>
<li><a href="#sec-6">6. 1.4. Datagram forwarding</a></li>
<li><a href="#sec-7">7. 1.5. Toplogy</a></li>
<li><a href="#sec-8">8. 1.5.1. Traffic engineering</a></li>
<li><a href="#sec-9">9. 1.6. Routing loops</a></li>
<li><a href="#sec-10">10. 1.7. Congestion</a></li>
<li><a href="#sec-11">11. 1.8. Packets again</a></li>
<li><a href="#sec-12">12. <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< 1 An Overview of Networks</a></li>
</ul>
</div>
</div>
<div id="outline-container-sec-1" class="outline-2">
<h2 id="sec-1"><span class="section-number-2">1</span> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 1 An Overview of Networks</h2>
</div>
<div id="outline-container-sec-2" class="outline-2">
<h2 id="sec-2"><span class="section-number-2">2</span> Introduction</h2>
<div class="outline-text-2" id="text-2">
<ul class="org-ul">
<li>chapter 1 is a summary of "essentials":
<ul class="org-ul">
<li>LAN
</li>
<li>IP
</li>
<li>TCP
</li>
<li>datagram forwarding
</li>
<li>packets
</li>
<li>congestion
</li>
<li>sliding windows
</li>
<li>firewalls
</li>
<li>network address translation
</li>
</ul>
</li>
<li>"LAN"
<ul class="org-ul">
<li>"physical" networks that provide the "connection" between different machines
</li>
</ul>
</li>
<li>"local"
</li>
<li>"IP"
<ul class="org-ul">
<li>a "layer"
</li>
<li>connects many LANs into the Internet
</li>
</ul>
</li>
<li>"TCP"
<ul class="org-ul">
<li>related to "transport", "connections", sending user data
</li>
</ul>
</li>
<li>"datagram forwarding"
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-3" class="outline-2">
<h2 id="sec-3"><span class="section-number-2">3</span> 1.1. Layers</h2>
<div class="outline-text-2" id="text-3">
<ul class="org-ul">
<li>"layers"
<ul class="org-ul">
<li>the entire topic of [starting with some data] and [sending it out] is complicated
</li>
<li>so, divide the complicated topic into different parts
</li>
<li>a "layer" represents one part
</li>
<li>one feature/function/aspect
</li>
</ul>
</li>
<li>"four-layer model"
<ul class="org-ul">
<li>(1) Application layer: creates some data
</li>
<li>(2) TCP layer
</li>
<li>(3) IP layer
</li>
<li>(4) LAN layer: actually delivers
</li>
</ul>
</li>
<li>5-layer model
<ul class="org-ul">
<li>divide LAN layer into "physical" LAN layer and "logical" LAN layer
</li>
</ul>
</li>
<li>7-layer model
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-4" class="outline-2">
<h2 id="sec-4"><span class="section-number-2">4</span> 1.2. Data Rate, Throughput and Bandwidth</h2>
<div class="outline-text-2" id="text-4">
<ul class="org-ul">
<li>"data rate"
<ul class="org-ul">
<li>the rate at which bits are transmitted
</li>
</ul>
</li>
<li>"Throughput"
<ul class="org-ul">
<li>the actual overall rate at which bits are transmitted
</li>
<li>things like competing traffic can affect the "Throughput"
</li>
</ul>
</li>
<li>"bandwidth"
<ul class="org-ul">
<li>can mean either "data rate" or "throughput"
</li>
<li>we use it mostly to mean "data rate"
</li>
</ul>
</li>
<li>"goodput"
<ul class="org-ul">
<li>the actual rate at which bits were received by destination application
</li>
</ul>
</li>
<li>kilobit
<ul class="org-ul">
<li>10<sup>3</sup> bits
</li>
</ul>
</li>
<li>megabit
<ul class="org-ul">
<li>10<sup>6</sup> bits
</li>
</ul>
</li>
<li>kbps and Mbps v. kB and MB
<ul class="org-ul">
<li>kB and MB denote data "VOLUMES" of 2<sup>10</sup> and 2<sup>20</sup> bytes
</li>
<li>the upper-case B denotes bytes
</li>
<li>newer abbreviation: KiB and MiB
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-5" class="outline-2">
<h2 id="sec-5"><span class="section-number-2">5</span> 1.3. Packets</h2>
<div class="outline-text-2" id="text-5">
<ul class="org-ul">
<li>"packet"
<ul class="org-ul">
<li>a unit of data that's transferred
</li>
<li>has header
</li>
</ul>
</li>
<li>"header"
<ul class="org-ul">
<li>meta-data portion of a packet
</li>
<li>fex: datagram forwarding: meta-data includes destination address
</li>
<li>fex: virtual-circuit forwarding: meta-data includes identifier for the connection
</li>
</ul>
</li>
<li>Single and multiple headers.png
<ul class="org-ul">
<li><img src="Single and multiple headers.png" alt="Single and multiple headers.png" />
</li>
</ul>
</li>
<li>"frames", "segments"
<ul class="org-ul">
<li>frames: packet at the LAN layer
</li>
<li>segments: packet at the Transport layer
</li>
</ul>
</li>
<li>packet size is limited
<ul class="org-ul">
<li>the type of LAN determines max size of a packet
</li>
<li>ethernet: 1500 bytes
</li>
</ul>
</li>
<li>one issue: how do I send a packet from a [large-packet LAN] to a [small-packet LAN]?
<ul class="org-ul">
<li>IP layer addresses this
</li>
</ul>
</li>
<li>each layer adds its own header
<ul class="org-ul">
<li>each layer adds its own header
</li>
<li>each header is about ~20 bytes
</li>
<li>so, in a 512-bytes-packet, it's not insignificant
</li>
</ul>
</li>
<li>"datagram forwarding network" and headers
<ul class="org-ul">
<li>the header will contain [destination address] and other delivery info]
</li>
<li>"routers" or "switches" try to ensure that the packet is delivered to the requested destination
</li>
</ul>
</li>
<li>packets and bytes
<ul class="org-ul">
<li>today, everyone agrees on 8-bit bytes
</li>
<li>in the past, this wasn't always so
</li>
<li>there's still some ambiguity:
</li>
<li>"binary integers" can be represented as a sequence of bytes in either "big-endian" or "little-endian" "byte order"
</li>
<li>RFC 1700: IP should use big-endian byte order
</li>
<li>so, big-endian byte order is aka "network byte order"
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-6" class="outline-2">
<h2 id="sec-6"><span class="section-number-2">6</span> 1.4. Datagram forwarding</h2>
<div class="outline-text-2" id="text-6">
<ul class="org-ul">
<li>"datagram-forwarding"
<ul class="org-ul">
<li>one way of delivering packets
</li>
<li>[packet headers] have a "destination address"
</li>
<li>"routers"/"switches" are to read this to send the packet to correct address
</li>
</ul>
</li>
<li>"forwarding table"
<ul class="org-ul">
<li>maps a [destination address] and [next<sub>hop]</sub>
</li>
</ul>
</li>
<li>"next<sub>hop</sub>"
<ul class="org-ul">
<li>the immediate "neighbor address" or "interface" toward the destination
</li>
</ul>
</li>
<li>"switch", "interface", and "neighbor"
</li>
<li>network diagram without a loop.png
<ul class="org-ul">
<li><img src="network diagram without a loop.png" alt="network diagram without a loop.png" />
</li>
</ul>
</li>
<li>"stateless forwarding"
<ul class="org-ul">
<li>forwarding tables don't have a "per-connection state"
</li>
</ul>
</li>
<li>[datagram forwarding] sometimes has other info in the header,
<ul class="org-ul">
<li>aside from destination address
</li>
<li>"quality of service information"
</li>
<li>there's a TON to transfer: use path A
</li>
<li>I need this transferred quick: use path B
</li>
</ul>
</li>
<li>"switches", "bridges" v. "routers"
<ul class="org-ul">
<li>switches/bridges work on the LAN layer
</li>
<li>routers work on the IP layer
</li>
</ul>
</li>
<li>"default entry" in a forwarding table
<ul class="org-ul">
<li>where to send a packet, if no match exists on the forwarding table
</li>
</ul>
</li>
<li>aside: virtual circuits, state-ful forwarding
<ul class="org-ul">
<li>different connections can be routed differently
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-7" class="outline-2">
<h2 id="sec-7"><span class="section-number-2">7</span> 1.5. Toplogy</h2>
<div class="outline-text-2" id="text-7">
<ul class="org-ul">
<li>network without a "loop"
<ul class="org-ul">
<li>there's only ONE path between any pair of nodes
</li>
<li>aka "acyclic", "tree"
</li>
<li>there's no "redundancy"
</li>
<li><img src="network diagram without a loop.png" alt="network diagram without a loop.png" />
</li>
</ul>
</li>
<li>network with a "loop"
<ul class="org-ul">
<li>a pair of nodes has more than 1 path
</li>
<li><img src="network diagram with a loop.png" alt="network diagram with a loop.png" />
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-8" class="outline-2">
<h2 id="sec-8"><span class="section-number-2">8</span> 1.5.1. Traffic engineering</h2>
<div class="outline-text-2" id="text-8">
<ul class="org-ul">
<li>"traffic engineering"
<ul class="org-ul">
<li>when there's redundant paths between 2 nodes,
</li>
<li>intentionally choosing 1 route over another
</li>
</ul>
</li>
<li>"quality of service information" in datagram forwarding
<ul class="org-ul">
<li>can do "traffic engineering"
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-9" class="outline-2">
<h2 id="sec-9"><span class="section-number-2">9</span> 1.6. Routing loops</h2>
<div class="outline-text-2" id="text-9">
<ul class="org-ul">
<li>"routing loop"
<ul class="org-ul">
<li>in a network with a loop, a packet might circulate endlessly
</li>
</ul>
</li>
<li>forwarding table is "distributed"
<ul class="org-ul">
<li>there is no "global authority" that does sanity checks on different forwarding tables
</li>
</ul>
</li>
<li>"linear routing loop"
<ul class="org-ul">
<li>a "routing loop" can happen in a network WITHOUT a loop too!
</li>
<li>2 nodes just send a packet back and forth infinitely
</li>
</ul>
</li>
<li>how to avoid routing loop?
<ul class="org-ul">
<li>maybe disallow a loop in the network "topology" altogether
</li>
<li>maybe forbid a switch send a packet "back out" the path ("interface") by which it arrived
</li>
<li>maybe have a "time to live" - a packet can only hop so many times before it is destroyed
</li>
</ul>
</li>
<li>"time to live" field, "TTL field"
</li>
<li>node D might decide that the best path to B is D-E-C-B
<ul class="org-ul">
<li>node E might decide that the best path to B is E-D-A-B
</li>
<li>D and E would send the packet back and forth to each other
</li>
<li>a linear loop arises
</li>
</ul>
</li>
<li>successful datagram routing requires cooperation (between nodes) and a consistent view of the network (held by different nodes)
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-10" class="outline-2">
<h2 id="sec-10"><span class="section-number-2">10</span> 1.7. Congestion</h2>
<div class="outline-text-2" id="text-10">
<ul class="org-ul">
<li>"congestion"
<ul class="org-ul">
<li>packets arrive faster than they can be sent out
</li>
<li>bottleneck, backlog
</li>
</ul>
</li>
<li>queue
<ul class="org-ul">
<li>when there's a backlog,
</li>
<li>a "queue" forms
</li>
<li>when the queue is full, packets will be "dropped"
</li>
<li>basically, ignore new packets
</li>
</ul>
</li>
<li>"knee"
<ul class="org-ul">
<li>the point where a queue is beginning to buidl up
</li>
<li>aka "contention"
</li>
</ul>
</li>
<li>"cliff"
<ul class="org-ul">
<li>the point where queue is full, and packets are being dropped
</li>
</ul>
</li>
<li>congestion doesn't necessarily mean "not enough bandwidth"
<ul class="org-ul">
<li>something about "network's feedback that the maximum transmission rate has been reached"
</li>
</ul>
</li>
</ul>
</div>
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<div id="outline-container-sec-11" class="outline-2">
<h2 id="sec-11"><span class="section-number-2">11</span> 1.8. Packets again</h2>
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<div id="outline-container-sec-12" class="outline-2">
<h2 id="sec-12"><span class="section-number-2">12</span> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< 1 An Overview of Networks</h2>
</div>
</div>
<div id="postamble" class="status">
<p class="author">Author: nugnoy</p>
<p class="date">Created: 2019-01-19 Sat 21:44</p>
<p class="creator"><a href="http://www.gnu.org/software/emacs/">Emacs</a> 25.2.1 (<a href="http://orgmode.org">Org</a> mode 8.2.10)</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
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