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Networking-terminology.txt
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Terminology
===========
This writeup is meant to provide quick definitions for a number of networking
terms that we will use as we discuss networking security issues. The
writeup is structured into a number of topics:
General concepts
Layering
Terms associated with different layers
Terms associated with TCP
Protocols known by their acronyms
Notions particular to DNS
which in general reflects the order in which we discuss different facets
of networking in the overview lecture. Within each topic, terms are listed
alphabetically, except for "General concepts" they (basically) follow the
order of discussion in lecture.
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General concepts:
Protocol
How a communication between distinct parties is specified
and structured. This includes both its format (how to
determine the value of different groups of bits) and its
semantics (how to interpret what particular information
those values convey).
Header
The portion of a packet or a message that includes information
particular to a given protocol. The term refers to the
fact that generally headers come *before* the "data"
(or "payload") portion of the packet or message. Headers
for different protocols often come one after another, a
reflection of *layering*.
End system
A computer that originates or receives packets in
a network, but does not forward them on behalf of
other systems.
Host
This term is usually used as a synonym for "End system".
Sometimes, however, it is used as a synonym for the
related concept of "Node".
Node
A computer that's a member of a network. This can be
either an "end system" or a "router".
Packet
An atomic unit of structured communication. In lowest
level terms, a series of bits sent consecutively along
a network link. The bits a structured in terms of
headers (reflecting the management information associated
with different protocols) and data for those protocols,
also referred to as "payload".
Forwarding
Transmitting a packet or message along a single network
connection.
End-to-end
Considering (or implementing) a property all the way from
its ultimate source to its ultimate destination. Often
used in contrast to "Hop-by-hop". For example, end-to-end
protection against packets getting mangled during transmission
would be implemented by the sender including some sort of
checksum for a message, and only the ultimate receiver
testing to see whether the checksum validates, discarding
the message if it does not validate.
Layering
Building one protocol in terms of using the capabilities
and services provided by another protocol. This is
especially done in terms of a "protocol stack". See
below for a separate section on the names of standard
network layers.
Protocol stack
A suite of protocols for which those at each layer (see
below) uses the functionality provided to it by one or
more protocols at the next layer "below" (i.e., by the
protocol one level below).
Link
A communication path between/among two or more computers.
If specifically between two computers, the link is referred
to as a "point-to-point" link. If among more than two,
it is generally a "broadcast" link.
Subnetwork or Subnet
A network that uses a single physical technology. That
is, all of the nodes in the network can communicate with
one another using that technology.
Hop-by-hop
Considering (or implementing) a property in terms of
explicitly propagating it through each forwarding step
along a network path. For example, hop-by-hop protection
against packets getting mangled during transmission could
be implemented by each router validating a packet's checksum
and discarding the packet if the checksum fails to validate.
The router might also compute a new checksum prior to
forwarding the packet.
Routing
Used to both refer to the process of computing hop-by-hop
paths through a network, and to refer to the act of
forwarding packets along those paths.
Datagram
See "Packet" (synonym).
Message
A unit of communication between a sender and a receiver.
In some contexts, this is the same as "Packet" (see below),
but in other contexts, a message may be made up of multiple
packets. For example, a single Web request is one HTTP
"message", but may require a large number of TCP messages
(packets), each of which is sent using an IP packet.
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Layering:
General notion
The Internet's design is layered, with each layer scoped
to provide a particular general class of functionality.
Each layer is designed to use the functionality provided
by the layer immediately below it, and to make available
functionality for use by the layer immediately above it.
Physical layer
The lowest layer in the Internet's design ("layer 1").
Provides the service of transmitting individual bits over
a single physical link.
Link layer
Uses the physical layer to transmit collections of bits
that have been grouped (or "framed") together into individual
messages ("packets") for transmission across a single
subnetwork. Also referred to as "layer 2".
Internetwork layer or Network layer or IP layer (or just IP)
Uses the link layers provided by a series of network hops
to connect together multiple subnets and provide end-to-end
"internetwork" connectivity between nodes. As part of
providing this, introduces a notion of "global" (Internet-wide)
addresses. Also referred to as "layer 3".
IP = "Internet protocol", and refers to the Internet's
layer 3 protocol.
Transport layer
Uses IP to provide end-to-end communication between
processes. Can include a lot of additional end-to-end
functionality (such as reliability), though this is
not a fundamental requirement. Includes a notion of
"ports" to identify the processes associated with the
communication. Also referred to as "layer 4".
Application layer
Uses a transport-layer protocol to provide functional
end-to-end communication. All sorts of styles of
communication are possible. Examples are email (the
"SMTP" protocol), web ("HTTP"), Skype, and BitTorrent.
Also referred to as "layer 7 " (NOT "layer 5", due to
ancient history).
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Terms associated with different layers:
Address / IP address
A network-layer term that identifies a location in
the Internet. Addresses usually refer to individual
network nodes, though some nodes can have multiple
addresses, and some devices ("NATs") can remap
addresses of nodes
Best effort
The datagram delivery service provided by IP (the Internet's
network layer). Despite the name, it actually means
"IP will not callously throw away a packet without reason -
but it will give one try at forwarding/delivering a packet,
and if that fails, it will give up and throw it away".
Broadcast
The ability for a single message to be received by
every node on a network. Usually used in the context
of physical-layer or link-layer transmission, such
as electrical signals along a shared cable (physical
layer) or WiFi's everyone-receives-each-transmission
(link layer).
Port
A transport-layer notion that associates a 16-bit number
with a process at a given communication endpoint.
Internet transport-layer headers include a *source* port
(the number associated with the sender's process)
and a *destination* port (the number associated with
the receiver's process). Some ports are by convention
associated with specific application-layer protocols,
such as TCP's port 80 with the HTTP protocol used for
standard web access.
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Terms associated with TCP:
Acknowledgment
A statement from one host in a TCP connection to the other
host confirming the reception of either a portion of the
byte stream or a previous control message.
Byte Stream
An abstraction provided by the Internet's main transport
protocol, TCP. A byte stream is a sequence of bytes that
a sender transmits into the network and that subsequently
arrives in the same order, and with high reliability, at
the receiver.
Each TCP connection has *two* separate byte streams, one
from the host that initiated the connection (the "client")
to the host that accepted it (the "server"), and one
from the server to the client.
Control flags
Bits in the TCP header that indicate particular types
of information associated with managing the connection:
SYN Indicates setting up a new connection.
ACK Indicates that a TCP packet includes an acknowledgment
of some sort.
FIN Used to agree on terminating a connection.
RST Indicates an abrupt termination of a connection
because the host can no longer continue due to a
problem.
Initial sequence number or ISN
A 32-bit number that identifies the index associated
with the *beginning* of a byte stream.
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Protocols known by their acronyms (for most, the expansion of the acronym
isn't actually important, other than to help remember the acronym itself):
DHCP Dynamic Host Configuration Protocol
A layer-2 protocol for bootstrapping a host's network
setup. Won't be covered until the second lecture.
DNS Domain Name System
A layer-7 protocol used to map human-readable names like
www.google.com to Internet addresses. (Also provides other
forms of mapping that we will discuss as needed.)
HTTP Hypertext Transfer Protocol
The main (layer-7) protocol used for Web transfers/access.
IP Internet Protocol
The Internet's layer 3 protocol.
SMTP Simple Mail Transfer Protocol
The layer-7 protocol used for email. Won't be covered
until later in the class.
TCP Transmission Control Protocol
The Internet's most significant layer-4 protocol.
Used to provide reliable communication between hosts.
UDP User Datagram Protocol
A lightweight layer-4 protocol, used for communication
where simplicity outweighs the needs for more complex
services such as reliability or a "byte stream" abstraction.
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Notions particular to DNS:
Authoritative name server
The name server responsible for a subtree of the DNS naming
hierarchy. For example, there is a set of authoritative
name servers responsible for ".com", and a separate set
of authoritative name servers for ".google.com".
Name server
A server that accepts incoming DNS lookup requests
and returns DNS answers in reply.
Resolver
A server that a given client has been configured to use
to get DNS answers. Sometimes this refers to a process
running on the same computer as the client; other times
it refers to a process running on a separate computer,
such as an ISP's resolver, meaning the name server that
an ISP configures its clients to use.
Root
The topmost point in the DNS naming hierarchy, also referred
to as "." ("dot"). For example, both "com" and "org" are
names that can be looked up in ".".
Top-level domain or TLD
A DNS name that comes just below the root, such as "com"
or "org". These are often referred to with a leading
dot, such as "dot-com" or "dot-org".
TTL or Time To Live
For DNS, refers to how long a DNS reply may be cached
(in seconds). Note that TTL is used with a quite
different meaning in the IP protocol header, though
we will not encounter that use until later in the
course (if at all).