Transcript Flow control and TCP/IP

Flow control and
TCP/IP
Based on Computer Networks
and Internets, Comer
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Flow control
In networking, “flow control” refers to
adjusting the rate at which data is
transmitted from a source so that it does
not overwhelm the destination.
 Flow control is especially important in
cases when the source is faster than the
destination.

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An example

Imagine sending a large file from a computer
to a printer.
 The
computer can transmit much faster than the
printer can print; furthermore, the printer may be
shared and have other print jobs.
 The printer has a buffer but that buffer has a finite
size.

A common flow-control mechanism in a case
like this is xon-xoff.
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Xon-xoff

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In this case, the printer (destination) sends an
xoff message to the computer (source)
indicating that its buffer is full.
The computer (source) should then stop sending
data.
When the printer’s buffer is free and it is ready to
accept more data, the printer sends an xon
signal.
Flow control can be implemented in hardware or
software, or a combination of both.
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Xon-Xoff


The actual signal for X/on is the same bit
configuration as the ASCII Ctrl-Q keyboard
combination. The X/off signal is the Ctrl-S
character.
When you define your modem to your
computer's operating system, you may need
to specify the use of flow control with
X/on/Xoff or with CTS/RTS (Clear to
Send/Ready to Send). When sending binary
data, Xon/Xoff may not be recognized
because it is character-encoded.
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ASCII XON and XOFF
Dec
Oct
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Hex
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Choke packet



A specialized packet that is used for flow control
along a network.
A router detects congestion by measuring the
percentage of buffers in use, line utilization and
average queue lengths.
When it detects congestion, it sends choke packets
across the network to all the data sources associated
with the congestion. The sources respond by
reducing the amount of data they are sending.
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Balancing speed and
accuracy


Increasing various individual speeds might not
increase the speed of the network , particularly if it
overloads the destination or some routing device
along the way. This results in packets being dropped,
requiring retransmission. And the overall result can
be a reduction in speed.
Network administration can be a very delicate
balancing act.
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Congestion control and packet
delivery
If you slow down your transmission too
much, either the bandwidth is not being
used or it is being used by others.
 But if you send too many messages, you
will cause congestion, and you are
sending messages that will only have to
be retransmitted anyway.

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Protocol Techniques

Flow Control
 Recall
that a destination must
acknowledge the receipt of a packet (in
particular its Transport Layer).
 But
acknowledgements add to the traffic.
 There
are flow-control techniques (sliding
windows for example) specifying how one
acknowledges and how often.
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Congestion avoidance
Some packets contain “traffic reports,”
they have information about the status
of the traffic.
 Based on the status, the network may
inform senders.
 The network will either adjust the
transmission size, change the route or
change a window size used for
transmission.

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Unrestricted Protocol




Assumes the receiver has an unlimited capacity
to receive frames.
Sender and receiver use primitive calls to send
and receive.
Sender sends a frame. The arrival causes the
receiver to wake up and receive the frame.
Receiver extracts the packet and passes it up
the stack and then goes back into a wait state
until the next frame arrives.
In the Unrestricted Protocol, the sender does
not have to wait for acknowledgements to
send more packets.
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Stop and Go Protocol



Different from the Unrestricted Protocol in that
each time the destination receives a packet,
it replies with an acknowledgement.
Sender waits for the acknowledgement
before sending another frame.
The acknowledgement from the receiver
contains an error field structure called an ack.
 An
 An
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ack of 0, means there is no error.
ack of 1 means the packet contained an error.
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Fig. 16.7
Stop and go
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Sliding windows
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Sliding Window Protocols

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
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Compromise between Unrestricted and Stopand-go.
The source can send a number of packets
without waiting for acknowledgments, but
that number is limited.
The number of unacknowledged frames that
is allowed is called the “window.”
The size of the window is an important
network flow-control parameter.
Go back to Fig. 16-7
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Windowing
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Sliding Window Protocols

There are variations within a windowing
protocol.
 Go-back-n:
packets must be received in the
order they were transmitted. Receiver rejects
any packet other than “the next one.”
 Selective repeat: packets can arrive out of
order, the receiver holds them and orders
them before passing them to higher layer.
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Comparing Protocols
In Go-Back-N, if the timer set by sender
expires without an acknowledgement,
the protocol resends every outstanding
frame in the window.
 In Selective Repeat, if timer expires
only one packet must be sent. Allows for
out-of-order packets. But requires more
work on receiver’s part.

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Window announcements
Like Xon/Xoff, Window "announcements"
are messages sent by the receiving
system to the transmitting system.
 They acknowledge data receipt but also
inform the transmitting system of the
current buffer size.

 “I
got your message and I only have X
amount of space if you’re sending me more.”
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Piggyback
If the computers are having a two-way
conversation, then a computer can send
an acknowledge that it received
information along with information of its
own.
 This technique is called piggybacking.

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Bottleneck



A bottleneck is the slow part of a process – in
this case where packets tend to accumulate.
Bottlenecks affect network performance by
slowing down the flow of information
transmitted across networks.
TCP/IP connections were originally designed
to transmit only text files, and the proliferation
of bandwidth-intensive transmissions such as
high-resolution graphics has caused
bottlenecks in the process; therefore, the data
moves more slowly across networks.
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Various needs

No one network technology satisfies the
needs of all the users.
 Some
connect nearby computers (LAN)
 Some connect more distant computers (WAN)
 Some exchange data files (best with large packets
over connectionless)
 Some convey real-time video (best with small
packets over connection-oriented system)
 Some users have millions of dollars to spend
 Some users have a very limited budget
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Various Network Technologies

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Technologies emerged to fill these network niches.
But as “connectivity” grew in importance, one
needed the ability not only to connect computers
into a network, but also to connect networks into
an internet.
The networks can differ immensely (heterogeneity)
at the lowest level, but there must be some
overlying logic (software/protocol) providing the
homogeneity required for communication – we all
must follow the same set of rules.
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Universal Service
The goal of joining the various underlying
networks and having them work together
as if they were one uniform system is
called “universal service.”
 With Universal Service, a user at one
computer in any part of the organization
can exchange information with any other
user without changing computers.

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Heterogeneous Connections

Since specific information about the
transmission lines and other hardware are
needed at the Physical Layer and specific
information about the LAN protocol are need at
the Data-Link Layer (Ethernet, FDDI, ATM, etc.),
the homogenizing cannot occur until Layer 3 in
the OSI model.
 You
cannot use bridges (Layer 2) to connect
heterogeneous networks, but you can use routers
(Layer 3).
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Routers

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Routers forward packets from a source on one
network to a destination on another.
A router can connect different types of networks:
LAN-LAN, LAN-WAN, WAN-WAN.
A router can connect networks using different
protocols: e.g. Ethernet LAN to FDDI LAN.
If the frame formats and addressing schemes
differ on the networks connected by a router,
then software is needed to make this transition
possible.
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Virtual Network

A homogeneous high-level (Internet) software
gives the illusion of a homogenous network,
which at the lowest levels can be quite
heterogeneous. The logically homogenous
network is sometimes called a “virtual network.”
 Here

think of virtual as meaning software.
This is a version of information hiding: each
layer hides the details of its implementation.
Only the interface with adjacent layers is
exposed. Thus, the details of physical network
connections, physical address, and so on are
hidden from higher layers.
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clouds

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A similar notion is that of a “network cloud.” One
loses sight of information as it enters the cloud.
Especially in a connectionless scheme, the message
enters the network and takes an unknown path to its
destination. Furthermore, packets from the same
message may take different paths.
If the underlying network is heterogeneous, the
packets may be translated into other protocol
formats.
But when the message re-emerges from the cloud, it
appears to be the same message that entered the
cloud.
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Internet Protocols



TCP/IP is the most widely used internet
communication protocol suite (stack).
Originally the TCP/IP protocol suite was
designed to create a standard set of
procedures allowing individual computer
networks to connect to ARPANET.
Today it is used to connect computers to the
Internet (with a capital I).
 Small
i: two or more networks connected, capital I:
the global connection of networks.
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TCP/IP
 Transmission
 Control
 Protocol
 Internet
 Protocol
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Layering again
The TCP/IP is a reference model for
internet communication.
 Like the OSI model, it comes in layers.
 One difference is that it has fewer layers.
 The biggest difference is that TCP/IP is
widely implemented, whereas OSI has
become a “taxonomy” for discussing a
network but not a way of implementing
one.

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TCP/IP History


The early work on TCP/IP was done in the
1970’s much of it for ARPA (Advanced
Research Projects Agency).
The formal standards are laid out in IETF
RFC 791 and RFC 793.
 IETF
Internet Engineering Task Force
 RFC Request For Comments

TCP/IP gained its dominance in internet
protocols when it was included with a version
of the Unix operating system.
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IP Standards (RFC 791)
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TCP Standards (RFC 793)
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Layers

The Internet Layering Model (TCP/IP)
has 4 or 5 layers depending on whom
you ask, the layers are:
 Application
 Transport
 Internet
 Network
Interface
 (Physical) there must be a physical layer,
but it was not part of the original model
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Correspondence
The lowest layers of the TCP/IP are
pretty much equivalent to the lowest
layers of the OSI model.
 TCP/IP’s transport layer is somewhat
“thicker” (has more functions) than that
in the OSI model.
 The most notable difference is in the
upper layers, TCP/IP has one layer
roughly corresponding to OSI’s top
three layers.

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Correspondence
OSI Model
TCP/IP Model
Application
Presentation
Application
Session
Transport
Transport
Network
Internet
Data Link
Network Interface
Physical
(Hardware)
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TCP/IP’s
transport layer
is somewhat
thicker – has
more functions
than OSI’s.
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TCP/IP Layers

(Physical)
 corresponds

to OSI Layer 1
Network Layer
 specifies
how to organize data into frames and
how to transmit the frames over a network. (Data
Link Layer)

Internet
 specifies
the format of the packets sent across an
internet as well as forwarding. (Network Layer)

Transport
 ensures
reliable transfer (Transport Layer, some
Session)

Application
 specifies
how an application uses the internet
(Rest of Session, Presentation and Application)
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TCP/IP



TCP/IP is named after two parts of the
protocol stack: TCP (from the Transport
Layer) and IP (from the Internet Layer).
TCP/IP (Transmission Control
Protocol/Internet Protocol) is the basic
communication language or protocol of the
Internet.
TCP/IP communication is primarily point-topoint, meaning each communication is from
one point (or host computer) in the network to
another point or host computer.
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TCP

Transmission Control Protocol manages the
fragmenting of a message or file into smaller
packets that are transmitted over the Internet
and received by a TCP layer that reassembles
the packets into the original message.

TCP is responsible for the verification that
files were sent and received.
It makes the connectionless scheme that lies
underneath logically connection-oriented.

 Session
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verification not constant data rate.
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TCP Example


When an HTML file is sent to you from a Web
server, the TCP in that server breaks the file into
one or more packets, numbers the packets, and
then forwards each of them to the IP layer.
At the other end (the client program in your
computer), TCP reassembles the packets and
waits until they have arrived to forward them to
you as a single file.
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IP
Internet Protocol handles the adding
addresses (IP addresses) to the packets
and routing them, that is, IP’s job is to
get the packets where they’re going.
 The IP layer is connectionless
(sometimes also known as “stateless”),
different packets may travel differ routes,
and if a packet is dropped, it’s dropped.

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Delivering the packet

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Each packet is sent first to a gateway computer
(router) that understands a small portion of the
Internet.
The router reads the destination IP address and
forwards the packet to a neighboring router.
It repeats the process until one router
recognizes the packet as belonging to a
computer within its immediate neighborhood or
domain.
That gateway then forwards the packet directly
to the computer whose address is specified.
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Versions



The most commonly used version of IP today is
Internet Protocol Version 4 (IPv4).
However, IP Version 6 (IPv6) is also beginning
to be supported. IPv6 provides for much longer
addresses (128 bits) and therefore for the
possibility of many more Internet users.
IPv6 includes the capabilities of IPv4 and any
server that can support IPv6 packets can also
support IPv4 packets (backward compatibility).
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More on the model

TCP/IP uses the client/server model of
communication
a
computer user (a client) requests and is
provided a service (such as sending a Web
page) by another computer (a server) in
the network.

The other standard relationships are
master/slave (one program in charge of
all other programs) and peer-to-peer
(either of two programs is able to initiate
a transaction).
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Client Server Example
 To
check your bank account from your computer, a
client program in your computer forwards your
request to a server program at the bank.
 That program may in turn forward the request to
its own client program that sends a request to a
database server at another bank computer to
retrieve your account balance.
 The balance is returned back to the bank data
client, which in turn serves it back to the client in
your personal computer, which displays the
information for you.
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Daemon



In the usual client/server model, a program on
the server, sometimes called a daemon, is
activated and awaits client requests.
A daemon (pronounced DEE-muhn) runs
continuously and its job is to handle service
requests.
The daemon’s job is to receive the request, not
perform the service. It usually forwards the
request to the appropriate program.
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Daemon Example


Each server of pages on the Web has an
HTTPD or Hypertext Transfer Protocol daemon
that continually waits for requests to come in
from Web clients.
Your Web browser is a client program that
requests services (the sending of Web pages or
files) from a Web server (which technically is
called a Hypertext Transport Protocol or HTTP
server) in another computer somewhere on the
Internet.
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Application services

The services users are familiar with are in the
application layers. These include
 Hypertext
Transfer Protocol (HTTP) for transmitted
web-page documents.
 File Transfer Protocol (FTP) for transmitting other
files.
 Telnet which allows one to log on to a computer
remotely.
 Simple Mail Transfer Protocol (SMTP) for email.

These and other protocols are packaged
together with TCP/IP as a "suite."
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UDP





User Datagram Protocol
Like TCP sits on top of IP in the TCP/IP
model.
Unlike TCP it is connectionless.
UDP/IP provides very few error recovery
services, offering instead a direct (faster) way
to send and receive datagrams over an IP
network.
It's used primarily for broadcasting messages
over a network.
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Some other protocols

Other protocols are used by network host
computers for exchanging router
information. These include
 Internet
Control Message Protocol (ICMP)
 Interior Gateway Protocol (IGP)
 Exterior Gateway Protocol (EGP)
 Border Gateway Protocol (BGP).

(We’ll talk about these another time.)
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Intranet
Businesses liked the services (HTTP, FTP,
SMTP, telnet, etc.) offered by the TCP/IP
model but they did not necessarily want to
be connected to the Internet, so they
started setting up intranets.
 An intranet is a set-up like the Internet
(having browsers, email, etc.) but not
connected to the outside world.

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Intranet (cont.)



With “tunneling”, companies can send private
messages through the public network, using the
public network with special
encryption/decryption and other security
safeguards to connect one part of their intranet
to another.
Allows a public transmission line to be used as
part of a private network.
If a public line is used as part of a private
network, that network is called a Virtual Private
Network
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Intranet (Cont.)


Typically, larger enterprises allow users within
their intranet to access the public Internet
through firewall servers that have the ability to
screen messages in both directions so that
company security is maintained.
When part of an intranet is made accessible to
customers, partners, suppliers, or others outside
the company, that part becomes part of an
extranet.
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Extranet


A private network that uses TCP/IP and the
public telecommunication system to securely
share part of a business's information with
suppliers, vendors, customers, etc.
The required security and privacy are gained
by using firewall server management, the
issuance and use of digital certificates or
other means of user authentication,
encryption of messages, and the use of
virtual private networks (VPN) that tunnel
through the public network.
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Tunneling



Tunneling works by adding another protocol,
such as Microsoft’s Point-to-Point Tunneling
Protocol (PPTP) or Cisco’s or Layer Two
Forwarding (L2F).
This new protocol is embedded in the TCP/IP
packets.
This allows organizations to use the Internet to
transmit data across a virtual private network
(VPN).
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Other References
http://www.whatis.com
 http://www.webopedia.com
 TCP/IP Networking (James Martin and Joe
Leben)
 MCSE TCP/IP for Dummies, Cameron
Brandon

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