DATA COMMS-NETWORKS YR2 lecture 08 NAT & CLOUD.docx

DATA COMMUNICATION AND NETWORKS, YEAR2 –BIT/COMPS/TELECOMS 2017
PREPARED BY: DR MAURICE SESAY Page 1
NETWORK ADDRESS TRANSLATION (NAT)
When the Internet was becoming commercial in mid 1990s, it faced with two scalability
problems:
 Address exhaustion
 Routing table growth in the core networks.
Two “short” term solutions were created.
 Classless Inter Domain Routing (CIDR)
 Network Address Translation1(NAT)
At the borders of stub networks, a stub network does not carry transit traffic but only
provides access to users connected to it.
An extension to NAT is Network Address and Port Translation (NAPT). We lump both under
the simple term NAT. NATs let stub networks reuse IPv4 addresses from the range of private
addresses allocated by IANA:
The task of a NAT device is to provide a globally unique IP address to a host that wants to
access a server in the global address space. Usually, at the same time, the NAT maps the
source port the client is using to a dynamic port number. Since not all hosts are always active
and since one host does not need all 216 ports at the same time, many clients can share a
public IP address that is owned by the NAT.
Private Network
Private IP network is an IP network that is not directly connected to the Internet
 IP addresses in a private network can be assigned arbitrarily. Not registered and not
guaranteed to be globally unique
 Generally, private networks use addresses from the following experimental address
ranges (non-routable addresses):
Class A: 10.0.0.0 – 10.255.255.255
Class B: 172.16.0.0 – 172.31.255.255
Class C: 192.168.0.0 – 192.168.255.255

What is NAT
NAT is a router function where IP addresses (and possibly port numbers) of IP datagrams are
replaced at the boundary of a private network
• NAT is a method that enables hosts on private networks to communicate with hosts on the
Internet
• NAT is run on routers that connect private networks to the public Internet, to replace the IP
address-port pair of an IP packet with another IP address-port pair.
Basic Operation of NAT
NAT device has address translation table

Benefits of NAT and PAT
Pooling of IP addresses
Scenario: Corporate network has many hosts but only a small number of public IP addresses
NAT solution:
 Corporate network is managed with a private address space
 NAT device, located at the boundary between the corporate network and the public
Internet, manages a pool of public IP addresses
 When a host from the corporate network sends an IP datagram to a host in the public
Internet, the NAT device picks a public IP address from the address pool, and binds
this address to the private address of the host
Supporting Migration between Network Service Providers
Scenario: In CIDR, the IP addresses in a corporate network are obtained from the service
provider. Changing the service provider requires changing all IP addresses in the network.
NAT solution:
 Assign private addresses to the hosts of the corporate network
 NAT device has static address translation entries which bind the private address of a
host to the public address.
 Migration to a new network service provider merely requires an update of the NAT
device. The migration is not noticeable to the hosts on the network.
Note:

 The difference to the use of NAT with IP address pooling is that the mapping of
public and private IP addresses is static.
IP masquerading
Also called: Network address and port translation (NAPT), port address translation (PAT).
Scenario: Single public IP address is mapped to multiple hosts in a private network.
NAT solution:
 Assign private addresses to the hosts of the corporate network
 NAT device modifies the port numbers for outgoing traffic
Load balancing of servers
Scenario: Balance the load on a set of identical servers, which are accessible from a single IP
address
NAT solution:
 Here, the servers are assigned private addresses
 NAT device acts as a proxy for requests to the server from the public network
 The NAT device changes the destination IP address of arriving packets to one of the
private addresses for a server
 A sensible strategy for balancing the load of the servers is to assign the addresses of
the servers in a round-robin fashion.

Security Benefits of NAT and PAT
 Router cannot forward packets from Internet to an internal network unless a
NAT/PAT entry has been created in the router’s NAT table.
 In general, any connections not initiated from the inside are not allowed through the
PAT device
Concerns about NAT
Performance:
 Modifying the IP header by changing the IP address requires that NAT boxes
recalculate the IP header checksum
 Modifying port number requires that NAT boxes recalculate TCP checksum
Fragmentation
 Care must be taken that a datagram that is fragmented before it reaches the NAT
device, is not assigned a different IP address or different port numbers for each of the
fragments.
End-to-end connectivity:
 NAT destroys universal end-to-end reachability of hosts on the Internet.
 A host in the public Internet often cannot initiate communication to a host in a private
network.
 The problem is worse, when two hosts that are in a private network need to
communicate with each other.

IP address in application data:
 Applications that carry IP addresses in the payload of the application data generally
do not work across a private public network boundary.
 Some NAT devices inspect the payload of widely used application layer protocols
and, if an IP address is detected in the application-layer header or the application
payload, translate the address according to the address translation table.
CLOUD COMPUTING
What is Cloud Computing?
Cloud Computing Definitions
 Cloud computing is a model for enabling convenient, on-demand network access to a
shared pool of configurable computing resources (for example, networks, servers,
storage, applications, and services) that can be rapidly provisioned and released with
minimal management effort or service provider interaction.”
 Cloud computing is the delivery of computing as a service rather than a product,
whereby shared resources, software, and information are provided to computers and
other devices as a metered service over a network.

 Cloud computing provides computation, software, data access, and storage resources
without requiring cloud users to know the location and other details of the computing
infrastructure.” – Wiki
Characteristics of Cloud Computing
Elasticity and scalability: Expand and reduce resources according to your specific service
requirement.
Pay-per-use: Pay for cloud services only when you use them.
On-demand: Cloud services are invoked only when you need them. They are not permanent
parts of your IT infrastructure which is a significant advantage. No need to have dedicated
resources waiting to be used.
Resiliency: Completely isolate the failure of server and storage resources from cloud users.
Work is migrated to a different physical resource in the cloud with or without user awareness
and intervention.
Multitenancy: Can host the cloud services for multiple users with different requirements
within the same infrastructure.
Workload movement: Cloud-computing providers can migrate workloads across servers
both inside the data centre and across data centres.
Cloud Computing Context
Shifting from capital expenditures (CapEx, ie buying resources for internal IT) to an
operating expense (OpEx) model, where you pay for usage of resources.

Virtualization and Cloud Computing
Virtualization is the key to the success of CC.
 Virtualization software is used to run multiple Virtual Machines (VMs) on a single
physical server to provide the same functions as multiple physical machines.
 The software is known as a hypervisor, which performs the abstraction of the
hardware to the individual VMs.
 It was first invented and popularized by IBM in the 1960s for running multiple
software contexts on its mainframe computers.
Virtualization is not a prerequisite for cloud computing. However, virtualization provides a
valuable toolkit and enables significant flexibility in cloud-computing deployments.
Therefore, it is almost adopted by all cloud platforms.
Hypervisor
Hypervisor support the running of multiple VMs, schedule the VMs, provide a unified and
consistent access to the CPU, memory... resources on the physical machine.
 A VM runs an operating system and applications.
 The OS inside the VM may be virtualization-aware and require modifications. This
scheme known as para virtualization (as opposed to full virtualization).
Cloud Computing Models
Some popular models of cloud computing are offered today as services.
 Software as a Service (SaaS)
 Platform as a Service (PaaS)
 Infrastructure as a Service (IaaS)
Variations and add-ons:
 Data Storage as a Service (DaaS)
 Business Process as a Service (BPaaS)

Software as a Service
 Instead of obtaining desktop and/or licenses for software products, an enterprise can
obtain the same functions through a hosted service from a provider (known as SaaS
provider).
 The interface is usually through a web browser.
 Save the complexity of software installation, maintenance, upgrades, and patches.
 Services can be provided in a multitenant model.
 Examples: Google Docs, Webmail, Dropbox, Salesforce.com is an example of a SaaS
provider.
SaaS Pros and Cons
 Simple and easy access. Fire up a browser, log in, and go.
 The development, maintenance, updates, backups and so on are the responsibility of
the provider.
 Pay-per-use
Cons
 The development, backups, updates and so on are the responsibility of the provider.
You have no control.
 Data security
 Data portability
Platform as a Service
 Provide a software platform on which users can build their own applications and host
them on the PaaS provider’s infrastructure.
 It is used as a development framework to build, debug, and deploy applications.
 It often provides middleware-style services such as database and component services.
 The elasticity and scalability is guaranteed transparently by the PaaS platform.
Examples: Google GAE, Force.com from Salesforce.com

PaaS Pros and Cons
 Applications do not need to worry about the elasticity and scalability issues.
 Greater degree of user control than SaaS
 Pricing can be on a per-developer license and on a hosted-seats basis
Cons
 Applications need to follow specific API and be written in specific languages (likely
to change in the near future)
 The concerns about lock-in
 Not easy to migrate existing applications to a PaaS environment
Infrastructure as a Service
 An IaaS provider offers you “raw” computing, storage, and network infrastructure so
that you can load your own software, including operating systems and applications, on
to this infrastructure.
 Amazon Elastic Computing Cloud (EC2) service lets you rent servers with a certain
CPU speed, memory, and disk capacity along with the OS and applications that you
need.
 Pricing for the IaaS can be on a usage or subscription basis.
IaaS Pros and Cons
 Offers the greatest degree of control
 Infrastructure that can dynamically scaled
 Much less cost than having to build the infrastructure yourself
Cons
 Scaling and elasticity are yours not the provider’s responsibility.
 You need to know the resource requirements for your specific application to exploit
IaaS well.
 A mini do-it-yourself data center that you have to configure to get the job done.

Different Clouds
 Public clouds: Both the infrastructure and control of these clouds is with the service
provider.
 Private clouds: The cloud provider is responsible only for the infrastructure and not
for the control. A section of a shared data centres is partitioned for use by a specific
customer.
 Internal clouds: Cloud services are provided by the IT department of an enterprise
from the company’s own data centres.
 Better security and control
 Resiliency, scalability, and workload migration
When Does Cloud Computing Make Sense?
 For startup, you can focus on your core business without having to set up and
provision your IT infrastructure.
 As your company grows, the cloud-provided IT environment can scale along with it.
 When an IT department needs to “burst” to access additional IT resources to fulfill a
short-term requirement (known as cloud bursting).
 Consistent and universal access to services
 May prove to be good to the environment as well.
When not to use Clouds?
 Regulation and legal considerations may dictate that the enterprise house, secure, and
control data in a specific location or geographical area.
 Access to the data might need to be restricted to a limited set of internal applications.
 When application response time is critical, companies might be better off keeping
such demanding applications in house.

Cloud Comp: Gaps and Concerns
Security:
 A significant concern for enterprise IT managers Cloud provider must guarantee data
isolation and application security (and availability) through isolation across multiple
tenants.
 Authentication and authorization of cloud users and encryption of the “network pipe”
are other factors.
Network concerns:
What to do when cloud bursting is involved? Networking across multiple cloud data centers?
Cloud-to-cloud and Federation Concerns:
When an enterprise uses two separate cloud service providers, how do they
interoperate.
Legal and regulatory concerns:
Especially important for cases involving storing data in the cloud. It could be that the
laws governing the data are not the laws of the jurisdiction where the company is
located.

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