1.1How
does IP router deliver IP packets to an IP host?
First
we need to realize that IP router is a computer (host), which has
multiple network adapters, each associated with a different IP
address. These network adapters enable the router to be a physical
member of multiple subnets (e.g. LANs), thus enabling it to forward
IP packets from one subnet to another. You can think
of a router as an electronic version of an octopus, each one of its
legs is connected ("belongs") to a different subnet... Upon
receipt of an IP packet, the IP
router takes a look at the IP portion (header) of the packet
(datagram). In particular it examines the destination IP address
field in the IP header. It looks up its routing table (using the
destination IP address as a key) and finds the closets matching
entry. This entry tells the router what is the next hop to go to, and
via which network adapter (AKA interface) to do so. The next hop
could be as simple as a host on the neighbor subnet, which its IP
address is the one indicated in the IP packet (and thus is the final
destination of the packet), or it could be another router that needs
to take the packet further down the road.
In order to forward the packet to the next hop, the router needs to
discover the MAC
(Media Access Control) address of it by using ARP (see below). Then
it lets its MAC software and hardware (e.g. Ethernet module and
Ethernet network adapter) take care of the physical encapsulation and
delivery of the packet to the MAC hardware/software of the
destination (e.g. the Ethernet network adapter of the next hop). Note
that neighbor routers ALWAYS share at least one subnet (so they can
physically/directly forward packets to each other).
1.2What’s
the connection/relation between the Internet and IP?
IP
stands for Internet
Protocol. It is the basic protocol (set
of rules; language) that ALL computers on the Internet (must) use in
order to speak to each other. For instance, some computers on the
Internet might be capable of sending and receiving emails; Some
others might only be capable of downloading web pages. Still all of
them MUST be capable of speaking IP. Recall that email messages and
web pages are always encapsulated in IP packets, just like letters
are 'encapsulated' in postal envelops. Same for SIP messages...
1.3What’s
ARP?
ARP
stands for Address
Resolution
Protocol.
It is the protocol/process used to map IP addresses to MAC (Media
Access Control) addresses. When hosts want to communicate with each
other on the same segment of network cable (subnet) they need to know
the physical addresses (MAC addresses) of each other. To do so they
either broadcast ARP queries on the network segment or use ARP cache.
The input of the ARP query/cache is the IP address of the destination
host. The output is the MAC addresses of it.
1.4What’s the relation
between IP and Ethernet? How are IP datagrams and Ethernet frames
different from each other?
IP
packets (datagrams) are encapsulated in Ethernet frames. Computers
(hosts) that share network segment (IP subnet) can talk to each other
through their network interfaces (adapters). Every network adapter in
the world has its own unique Ethernet (MAC) address. The logic built
into it enables it to accept only frames whose destination MAC
address match its own MAC address (except for broadcast frames which
are always accepted). Perhaps now is the right time to realize that
when a web client sends a request to a web server for a web page, the
(HTTP) request is encapsulated in an IP packet. This IP packet may
travel many routers until it finally reaches its destination (the web
server). Every leap on its journey is consisted of ARP operation, MAC
(e.g. Ethernet) encapsulation and MAC delivery between two neighbor
network adapters (e.g. the adapters of two routers along the routing
path, or the adapters of the web client and the next hop router).
When a network adapter receives and accepts an Ethernet frame it
throws away the bits that constitute the Ethernet (MAC) header and
hands the remaining bits (IP header and its payload) up the stack to
the IP module. This is called decapsulation and stack propagation.
1.5Is TCP reliable? Is UDP
reliable?
Yes,
TCP is a reliable transport protocol. Its reliability is achieved by
means of acknowledgements and segments retransmission. UDP is not
reliable, but is very simple and is a good fit for packets that
contain real time media information, such as voice or video. Loosing
a single voice frame might not be that critical, but having to delay
the transmission of the next voice frame due to lack of
acknowledgement for its predecessor frame, might be critical and thus
makes TCP a bad choice for real time communication.
1.6What is
UDP?
UDP
stands for User
Datagram
Protocol.
It is the simple standardize way to encapsulate a message and
identify its application layer destination (by using port #). UDP
header contains the source and target ports, message length and
optional checksum.
1.7What is
TLS?
TLS
stands for Transport
Layer
Security.
It is a mechanism that provides hop to hop transport-layer security
over connection-oriented protocols (e.g. TCP); It provides data
integrity (no one has tampered with it), data confidentiality (no one
has seen it or was able to understand it) and authentication (i.e.,
the sender is who he claims he is). TLS
starts with a handshake phase that negotiates an encryption algorithm
(e.g., AES, IDEA) and keys, and authenticates the server to the
client (and vice versa) using certificates and trusted known CAs
(Certification Authorities). Once the handshake is complete and data
transmission begins, the data is encrypted using the keys and
algorithm negotiated during the handshake phase (Symmetric
cryptography).
1.8What is
port number? Why is it called port?
Port
number is a logical identifier for a sender or receiver application.
It has nothing to do with the physical port devices/connectors at the
back of the computer... It enables multiplexing of IP packets between
different applications sharing the same platform. In other words: A
computer can run several applications at the same time. Each one of
these may send/receive IP packets to/from different or identical
sources. Each one of the incoming IP packets will find its way (up in
the IP stack) to the correct waiting application based on the port
number associated with it. The port number is added to a message by
the application layer and is part of the transport header (e.g. UDP,
TCP).
1.9What is an
IP Switch? Is it the same as telephony switch?
IP
Switch is a new marketing buzz word. Normally it is used to describe
an IP telephony server. Another word which is used interchangeably
with the word 'IP switch' is Soft Switch. The IP switch (or Soft
Switch) is different from the legacy telephony switch by the strict
separation it keeps between the application logic (controller) unit
and the media switching unit. This is in contrast to the centric way
switching is done by circuit telephony switches.
1.10What is
DNS? Any relation between Internet domains and IP subnets?
DNS
Stands for Domain Name System. It enables us to use names which are
easy to remember (e.g. www.nba.com)
rather than long and meaningless IP addresses (e.g. 10.32.111.2).
Internet domain may contain many different IP subnets. Domain is a
logical term, and its geographical meaning might be loose in many
cases. For instance the domain 'intel.com' describes a virtual
network that is consisted of many subnets in different geographical
locations.
1.11What is
IP multicast? Do I need to add a network adapter to my PC to be able
to do multicast?
By Bryan McLaughlin -
Cisco: IP multicast is the ability for a host and the network to
enable delivery to a group of interested receivers. A host can
dynamically choose to listen/join a multicast group (using the same
network adapter). IP routers that support multicasting, take care of
duplicating the original IP packet and delivering the copies to all
listeners to the group. In short IP multicast enables an unlimited
number of hosts to receive a single data stream with 'no additional'
load on the source or the network
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