Previous: 00-Inspiration.html
document.querySelector('video').playbackRate = 1.2Our goal for today:
* get “feel” for big picture and terminology
* more depth, detail later in course
* approach: use Internet as example
Overview:
* what’s the Internet?
* what’s a protocol?
* network edge:
* hosts, access net, physical media
* network core:
* packet/circuit switching, Internet structure
* performance:
* loss, delay, throughput
* protocol layers, service models
* security
Billions of connected computing devices:
* hosts = end systems
* running network apps
* smart devices
Communication links:
* fiber, copper, radio, satellite
* transmission rate: bandwidth
Packet switches:
* forward packets (chunks of data)
* routers and switches
Inter-networked networks:
* Interconnected ISPs
Protocols:
* control sending, receiving of messages
* e.g., TCP, IP, HTTP, 802.11
Internet standards:
* RFC: Request for comments
* IETF: Internet Engineering Task Force
Infrastructure that provides services to
applications:
* Web, VoIP, email, games, e-commerce, social nets, …
Provides programming interface to apps:
* hooks that allow sending and receiving app programs to “connect” to
Internet
* provides service options, analogous to postal service
What do you do if you accidentally interrupt someone?
What are some other human protocols?
A network protocol is similar to a human protocol, except that the entities exchanging messages and taking actions are hardware or software components of some device (for example, computer, smartphone, tablet, router, or other network-capable device).
A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.
All communication activity in Internet governed by protocols, public or proprietary.
Arrival order packet joke!
is critical to good a make
Network edge:
* hosts: clients and servers
* servers often in data centers
Access networks, physical media:
* wired, wireless communication links
Network core:
* interconnected routers network of networks
Hosts = end systems, BOTH clients and servers
Routers = core
Q: How to connect end systems to edge router?
* residential access nets
* institutional access networks (school, company)
* mobile access networks
keep in mind:
* bandwidth (bits per second) of access network?
* shared or dedicated?
* use existing telephone line to central office DSLAM
* data over DSL phone line goes to Internet
* voice over DSL phone line goes to telephone net
* < 2.5 Mbps upstream transmission rate (typically < 1 Mbps)
* < 24 Mbps downstream transmission rate (typically < 10 Mbps)
frequency division multiplexing:
different channels transmitted in different frequency bands
* typically used in companies, universities, etc.
* 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates
* today, end systems typically connect into Ethernet switch
Two uses of the term:
Computer networks are often classified in function of the
geographical area that they cover
* LAN : a local area network typically interconnects
hosts that are up to a few or maybe a few tens of kilometers
apart.
* MAN : a metropolitan area network typically
interconnects devices that are up to a few hundred kilometers
apart
* WAN : a wide area network interconnect hosts that can
be located anywhere on Earth
* To allow any host to send messages to any other host in the network,
the easiest solution is to organize them as a full-mesh, with a direct
and dedicated link between each pair of hosts.
* Such a physical topology is sometimes used, especially when high
performance and high redundancy is required for a small number of
hosts.
* However, it has two major drawbacks.
* For a network containing n hosts, each host must have n-1 physical
interfaces.
* In practice, the number of physical interfaces on a node will limit
the size of a full-mesh network that can be built.
* The second possible physical organization, which is also used inside
computers to connect different extension cards, is the bus.
* In a bus network, all hosts are attached to a shared medium, usually a
cable through a single interface.
* When one host sends an electrical signal on the bus, the signal is
received by all hosts attached to the bus.
* A drawback of bus-based networks is that if the bus is physically cut,
then the network is split into two isolated networks.
* For this reason, bus-based networks are sometimes considered to be
difficult to operate and maintain, especially when the cable is long and
there are many places where it can break.
* Such a bus-based topology was used in early Ethernet networks.
* A third organization of a computer network is a star topology.
* In such topologies, hosts have a single physical interface and there
is one physical link between each host and the center of the star.
* The node at the center of the star can be either a piece of equipment
that amplifies an electrical signal, or an active device, such as a
piece of equipment that understands the format of the messages exchanged
through the network.
* Of course, the failure of the central node implies the failure of the
network.
* However, if one physical link fails (e.g. because the cable has been
cut), then only one node is disconnected from the network.
* In practice, star-shaped networks are easier to operate and maintain
than bus-shaped networks.
* Many network administrators also appreciate the fact that they can
control the network from a central point.
* Administered from a Web interface, or through a console-like
connection, the center of the star is a useful point of control
(enabling or disabling devices) and an excellent observation point
(usage statistics).
* A fourth physical organization of a network is the Ring
topology.
* Like the bus organization, each host has a single physical interface
connecting it to the ring.
* Any signal sent by a host on the ring will be received by all hosts
attached to the ring.
* From a redundancy point of view, a single ring is not the best
solution, as the signal only travels in one direction on the ring;
* thus if one of the links composing the ring is cut, the entire network
fails.
* In practice, such rings have been used in local area networks, but are
now often replaced by star-shaped networks.
* In metropolitan networks, rings are often used to interconnect
multiple locations.
* In this case, two parallel links, composed of different cables, are
often used for redundancy.
* With such a dual ring, when one ring fails all the traffic can be
quickly switched to the other ring.
* A fifth physical organization of a network is the tree.
* Such networks are typically used when a large number of customers must
be connected in a very cost-effective manner.
* Cable TV networks are often organized as trees.
* In practice, most real networks combine part of these
topologies.
* For example, a campus network can be organized as a ring between the
key buildings, while smaller buildings are attached as a tree or a star
to important buildings.
* An ISP network may have a full mesh of devices in the core of its
network, and trees to connect remote users.
Routing:
* determines source-destination route taken by packets
* a variety of routing algorithms
* operates on longer time-scales
Forwarding:
* move packets from router’s input to appropriate router output
* chooses instant path of packet
Ask:
Difference between routing and forwarding?
* In a network application, end systems exchange messages with each
other.
* Messages can contain anything the application designer wants.
* Messages may perform a control function (for example, the “Hi”
messages in our handshaking example in or can contain data, such as an
email message, a JPEG image, or an MP3 audio file.
* To send a message from a source end system to a destination end
system, the source breaks long messages into smaller chunks of data
known as packets.
* Between source and destination, each packet travels through
communication links and packet switches (for which there are two
predominant types, routers and link-layer switches).
In TV and radio transmission, broadcast is often used to indicate a technology that sends a video or radio signal to all receivers in a given geographical area. Broadcast is sometimes used in computer networks, but only in local area networks where the number of recipients is limited.
The first and most widespread transmission mode is called unicast. In
the unicast transmission mode, information is sent by one sender to one
receiver. The example shows a network with two types of devices: hosts
(drawn as computers) and intermediate nodes (drawn as cubes). Hosts
exchange information via the intermediate nodes. In the example below,
when host S uses unicast to send information, it sends it via three
intermediate nodes. Each of these nodes receives the information from
its upstream node or host, then processes and forwards it to its
downstream node or host. This is called store and forward and we will
see later that this concept is key in computer networks.
A second transmission mode is multicast transmission mode. This mode is
used when the same information must be sent to a set of recipients. It
was first used in LANs but later became supported in wide area networks.
When a sender uses multicast to send information to N receivers, the
sender sends a single copy of the information and the network nodes
duplicate this information whenever necessary, so that it can reach all
recipients belonging to the destination group.
The last transmission mode is the anycast transmission mode. It was
initially defined in RFC 1542. In this transmission mode, a set of
receivers is identified. When a source sends information towards this
set of receivers, the network ensures that the information is delivered
to one receiver that belongs to this set.
* Every end system has an address called an IP address.
* Source includes the destination’s IP address in the packet’s
header.
* Address has a hierarchical structure.
* router examines a portion of the packet’s destination address and
forwards the packet to best adjacent router.
* Each router has a forwarding table that maps destination addresses (or
portions of the destination addresses) to that router’s outbound
links.
* When a packet arrives at a router, the router examines the address and
searches its forwarding table, using this destination address, to find
the appropriate outbound link.
* Multiple routing protocols that are used to automatically develop the
forwarding tables themselves.
* E.g., determine the shortest path from each router to each destination
and use the shortest path results to configure the forwarding tables in
the routers.
* Takes L/R seconds to transmit (push out) L-bit packet into link at R
bps
* Store and forward: entire packet must arrive at router before it can
be transmitted on next link
* End-end delay = 2L/R (assuming zero propagation delay)
one-hop numerical example:
L = 7.5 Mb
R = 1.5 Mb/s
one-hop transmission delay = 5 sec
* If arrival rate (in bits) to link exceeds transmission rate of link
for a period of time, packets will queue, wait to be transmitted on
link.
* Packets can be dropped (lost) if memory (buffer) fills up
* end-end resources allocated to, reserved for “call” between source
& dest:
* in diagram, each link has four circuits.
* a call gets 2nd circuit in top link and 1st circuit in right
link.
* dedicated resources: no sharing
* circuit-like (guaranteed) performance
* circuit segment idle if not used by call (no sharing)
* commonly used in traditional telephone networks
How does this compare to packet switching?
A circuit in a link is implemented with either:
* frequency-division multiplexing (FDM) or
* time-division multiplexing (TDM).
Packet switching
* great for bursty data
* resource sharing
* simpler, no call setup (reserving a line)
* excessive congestion possible: packet delay and loss
* protocols needed for reliable data transfer, congestion control
* Q: How to provide circuit-like behavior?
* bandwidth guarantees needed for audio/video apps
* still an unsolved problem
* Q: human analogies of reserved resources (circuit switching) versus
on-demand allocation (packet-switching)?
+++++++++++++ Cahoot-01-1
Not just technical considerations: corporate, historical, collusive,
regulatory
++++++++++++++ Cahoot-01-2
How does loss and delay occur?
All delay is not the same.
If we let d_proc, d_queue, d_trans, and d_prop denote the
processing, queuing, transmission, and propagation delays, then the
total nodal delay is given by:
`d_nodal = d_proc + d_queue + d_trans + d_prop`Network load
The impact of increasing network load is super-linear on delay
R: link bandwidth (bps)
L: packet length (bits)
a: average packet arrival rate
La/R ~ 0: avg. queueing delay small
La/R -> 1: avg. queueing delay large
La/R > 1: more “work” arriving than can be serviced, average delay
infinite!
The traceroute program provides delay measurement from
source to router along end-end Internet path towards destination.
For all i:
* sends three packets that will reach router i on path towards
destination
* router i will return packets to sender
* sender times interval between transmission and reply.
(watch with Wireshark too):
~~~
$ traceroute mst.edu
$ traceroute www.mst.edu
$ traceroute efpl.ch
$ traceroute www.epfl.ch
~~~
Alternative fancier program:
$ mtr epfl.ch
With no place to store such a packet, a router will drop that packet; that is, the packet will be lost.
These could reasonably often have the same throughput, but which likely
has worse delay?
Bottleneck throughput limitations tend to be servers and local ISPs
(edge), not the backbone (core).
Analogy: horizontal layering of airline functionality
* layers:
* each layer implements a service
* via its own internal-layer actions
* relying on services provided by layer below
Why layering?
Internet protocol stack
Supporting network applications
* Network applications and their application-layer protocols reside
here
* Examples:
* HTTP Hypertext Transfer Protocol provides for Web
document request and transfer (example Wireshark
http://httpforever.com/)
* SMTP Simple Mail Transfer Protocol provides for the
transfer of e-mail messages
* FTP (which provides for the transfer of files between
two end systems).
* DNS provides translation of human-friendly names for
Internet end systems like https://www.ietf.org to a 32-bit network
address (ipv4). This is also done with the help of a specific
application-layer protocol, namely, the Domain Name System
(DNS).
* An application-layer protocol is distributed over multiple end
systems, with the application in one end system using the protocol to
exchange packets of information with the application in another end
system.
* Packet of information at application layer is a
message.
* Encryption often implemented at this layer (though it should at least
as low as the Network/IP-layer).
The upper layer of our architecture is the Application layer.
This layer includes all the mechanisms and data structures that are
necessary for the applications.
Our big-picture book calls these packets ADUs.
OS-Process to OS-process data transfer
* Transports application-layer messages between application
endpoints
* In the current Internet, there are two primary transport protocols,
TCP and UDP, either of which can transport application-layer
messages.
* TCP (Transmission Control Protocol) provides a
connection-oriented service to its applications.
* Guaranteed delivery of application-layer messages to the
destination
* Flow control (that is, sender/receiver speed matching).
* Congestion-control mechanism, so that a source throttles its
transmission rate when the network is congested.
* UDP (User Datagram Protocol) protocol provides a
connectionless service to its applications.
* No-frills service that provides no reliability, no flow control, and
no congestion control.
* Transport-layer packet is a segment.
04-NetworkData.html
06-NetworkControl.html
Routing of datagrams from source to
destination
* Responsible for moving network-layer packets known as
datagrams from one host to another.
* Transport-layer protocol (TCP or UDP) from source host passes a
transport-layer segment and a destination address to the network
layer
* Network layer then provides the service of delivering the segment to
the transport layer in the destination host.
* Network layer includes the celebrated IP, which
defines the fields in the datagram as well as how the end systems and
routers act on these fields.
* There is only one IP protocol (or two…), and all Internet components
that have a network layer must run it.
* Network layer also contains many routing protocols that determine the
routes
07-DataLink.html
08-Wireless.html
Data transfer between adjacent nodes
* At each node, the network layer passes the datagram down to the link
layer, which delivers the datagram to the next node along the
route.
* At this next node, the link layer passes the datagram up to the
network layer.
* Some link-layer protocols provide reliable delivery, from transmitting
node, over one link, to receiving node.
* Examples of link-layer protocols include: Ethernet, WiFi, the cable
access network’s DOCSIS protocol, and more
* A datagram may be handled by Ethernet on one link and by Wifi on the
next link.
* The network layer (above) will receive a different service from each
of the different link-layer protocols.
* link-layer packets are frames.
Bits “on the wire”
* The protocols in this layer are again link dependent and further
depend on the actual transmission medium of the link (for example,
twisted-pair copper wire, single-mode fiber optics).
* For example, Ethernet has many physical-layer protocols: one for
twisted-pair copper wire, another for coaxial cable, another for fiber,
and so on.
* In each case, a bit is moved across the link in a
different way.
An important point to note about the Physical layer is the service
that it provides. This service is usually an unreliable
connection-oriented service that allows the users of the Physical layer
to exchange bits. The unit of information transfer in the Physical layer
is the bit. The Physical layer service is unreliable because:
* the Physical layer may change, e.g. due to electromagnetic
interferences, the value of a bit being transmitted
* the Physical layer may deliver more bits to the receiver than the bits
sent by the sender
* the Physical layer may deliver fewer bits to the receiver than the
bits sent by the sender
Ask:
Does a packet go through all the layers in edge and the core?
What happens when the core meddles in layers higher than it is intended
to?
IP vs. OSI stacks
ISO/OSI reference model extras
Presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions
Session: synchronization, checkpointing, recovery of data exchange
Internet stack “missing” these layers.
These services, if needed, must be implemented in application.
Are they needed?
Most people talk about the OSI model as having 7
layers,
but they don’t mention layer 8, where most of the problems
actually occur…
Ask: What is layer 8??
Actual graph/tree of protocols
Application on top
Physical on the bottom
Encapsulating packets
TCP/IP suite
Left two are theoretical, while right is closer to the actual
system.
TCP/IP vs. OSI
OSI levels
++++++++++++++ Cahoot-01-3
In this graph, what is most central?
What is hardest to change?
What is easiest to innovate with?
Tunneling here is a very interesting way to visualize network
protocol interactions.
In some ways, this is the best way to make such a diagram:
+++++++++++++++++++ Cahoot-01-4
Demo:
~~~
# which ports are open?
nmap mst.edu
nmap epfl.ch
nmap www.epfl.ch
# Look at headers:
curl –head info.cern.ch
wget –server-response info.cern.ch
~~~
Denial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic
DDoS
IP spoofing: send packet with false source address
Next: 02-Application.html