Chapter 12:
Internetworking and the
Internet
Principles of Computer Networks
and Communications
M. Barry Dumas and Morris Schwartz
Objectives






Define and explain internetworks and intranets
Describe the Internet’s topology and explain why its
structure might be described as pseudo-hierarchical
Discuss the beginnings of the World Wide Web, its
evolution and its relation to the Internet
Describe Internet networking with the client/server model
Explain the composition of URLs and examine
addressing issues
Discuss issues associated with IPv4 addressing and the
move from IPv4 to IPv6
Chapter 12
Principles of Computer Networks and
Communications
2
Overview


Internetwork: a group of autonomous networks
Company internets and intranets
typically revolve around LANs


Creating an InterNetwork Requires paying attention to:





When varying locations are involved,
use WANs
For companies
that form
partnership
When these networks
use TCP/IP protocols,
they’re called extranets
Cost
Compatibility
Security
Reliability
The IPv4 system will soon be out of addresses

A move to IPv6 system is necessary
Chapter 12
Principles of Computer Networks and
Communications
3
Overview

Company intranet
 Company-owned, in-house network
 Uses TCP/IP protocols
 Designed only to be reached by authorized employees

Company extranet
 Company-owned, special outsider access to in-house network
 Uses TCP/IP protocols
 Connects between the owner company and networks of
“participating organizations” (e.g., suppliers, outsourcers, etc.)
Chapter 12
Principles of Computer Networks and
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4
History of the Internet Revisited

Usually traced back to its precursor, the ARPANET
project


Main concern—interconnecting independent (mainframe)
computers
Later concern—the development of a robust internetwork



That could keep military communications flowing
That could deal with complicated communications with
incompatible networks
Can be linked to the Advanced Research Projects
Agency (ARPA)

The U.S. response to the 1957 USSR launch
of the Sputnik
Chapter 12
Principles of Computer Networks and
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5
Internet Topology and Access
Compromises of millions
of interconnected hosts, l
LANS and WANS

Service providers


“The topology of the Internet . . . is a
pseudo-hierarchical structure based on links among
different levels of service providers.”
Organizations whose nodes and links supply all of the interconnections
Order of main hierarchy

International Internet service providers (IISPs) and
national service providers (NSPs) at the top



Most NSPs are also IISPs
Regional service providers (RSPs)
Local Internet service providers (ISPs) at the bottom
Many providers
connect directly
to each other,
whether at the
same or different
levels
Local providers offer dial-up access, bringing
the telephone system into the picture
Chapter 12
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Internet Topology and Access

National service providers (NSPs)



Form the Internet backbone that extends worldwide
Are private companies that own and maintain
the backbone networks
Basic global interconnections are provided by NSPs linked to
each other through network access points (NAPs)



NAPs are complex switching stations
NAPs are privately owned, usually by companies other than NSPs
Some NSPs bypass NAPs to link directly to each other using
peering points in their switching offices
Chapter 12
Peering points are like the point of presence POPs
which is the location of a node on a network that users
can connect to.
i.e telephone companies’ end offices
Principles of Computer Networks and
Communications
7
Internet Topology and Access

Regional service providers (RSPs)

Through routers



Connect hierarchically to NSPs
Connect directly to other RSPs
Local Internet service providers (ISPs)

Can link to NSPs, RSPs, and ISPs


The higher up on the hierarchy, the faster the links and
the greater their capacity
ISPs can support many connection types


Dial-up, cable modem, DSL, ATM, frame relay, Ethernet
Not all ISPs can support all types
Most individuals and businesses
use ISPs to connect
Chapter 12
Principles of Computer Networks and
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8
Basic Topology of the Internet
Some RSPs connect directly to
each other by routers
NSPs are linked to each other
by NAPs
Some NSPs connect directly to
each other by peering points
Fig. 12.1
Chapter 12
Principles of Computer Networks and
Communications
9
Internet2 and Abilene (a complete
separate entity)

Internet2 1995

Nonprofit development project



Purpose—to create advanced technologies and applications
that can be adopted by the Internet



Academic, industry, government partnership
Led by more than 200 universities (alliance)
Will eventually lead to the “Internet of the future”
Formation and constituency go back to its
predecessors
Abilene


High-speed wide-bandwidth
optical backbone network
Designed to support Internet2
Chapter 12
Principles of Computer Networks and
Communications
Abilene participants:
—Indiana University
—Juniper Networks
—Nortel Networks
—Qwest Communications in
partnership with Internet2
10
The World Wide Web aka “the Web”
An interface that allows us to access the Internet
the Web to the Internet is the same as the database application to a database
 Tim Berners-Lee in 1990



Wrote the first World Wide Web server: httpd
Created “WorldWideWeb”

Web interfaces:
Web browser software
Microsoft Internet Explorer
 Simplified the information-finding process on the Internet
Netscape Navigator
providing easy-to-use Web interfaces
Mozilla Firefox
Websites



the first client
a hypertext browser/editor


Collections of files (pages) organized by links
Via a structure called hypertext (that contains hyperlinks)
Hyperlinks are addresses that take us from page to page
and site to site, and make traversing the Internet straightforward
Chapter 12
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The Client/Server Model
Client Software requests services, Servers Software
provide services
 Name refers to the association between network entities



Client software requests services
Server software provides services
A software model, not a hardware model

Because it is software based, the client/server model provides a
flexible and scalable architecture


This explains its popularity
Different from master/slave relationship!


Server software in server/client model
does not control the network as in the case of the master slave
Servers and clients operate independently
Servers and clients
Chapter 12
Master Slave example is
the Mainframe computing
only join for the
request–response
12
relationship
The Client/Server Model

Client/Server—how different types of software
running on network devices interact

Examples



When you go to a website, your browser software (client)
requests Web pages from the site’s Web server software
(server)
You can download a file from an Internet server by using
an FTP (file transfer protocol) client that requests the file
from a server running FTP software (part of the TCP/IP
protocol suite)
An application can be both a client and a server

One time requesting services and
another time providing them
This is common in
peer-to-peer networks
Chapter 12
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The Challenge of Internetwork
Addressing
 Standardized protocols and procedures are key factors
in Internet success

To send a message, the system must



Resolve the location of the recipient machine
Distinguish it from all the devices on the Internet
Computers on a shared medium LAN (not an
internetwork) have unique flat physical addresses


Makes recipients easy to identify, but
Insufficient and impractical for internetworking!


Chapter 12
Addresses do not contain any location information
System would have to search every network in the internetwork
for the recipient machine
Principles of Computer Networks and
Communications
14
The Challenges of Internetwork
Addressing
solution

Hierarchical scheme

Different levels identify



A particular network of the internetwork
The physical machine address
Two architecture models

Open systems interconnection (OSI) model



The medium access control (MAC) sublayer of the data link layer
handles physical addresses
Network layer handles logical addresses
Transmission control protocol over Internet protocol (TCP/IP)
model

Follows the same pattern as OSI, but with possibly different labels


Chapter 12
OSI data link layer is the TCP/IP data link or link layer
OSI network layer is the TCP/IP network or Internet layer
Principles of Computer Networks and
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15
Hierarchical Addresses
(Reviewing from Chapter 6)

The postal system uses hierarchical addresses



Zip codes, states, cities, streets, names, etc.
Allows the post office to route mail in stages
Hierarchical network addresses similarly comprise
groupings/segment


Chapter 12
Allow the system to route messages to general areas,
particular networks and subnetworks, and finally the
destination machine
Addresses are constructed and routed in network layer (OSI)
or internetwork layer (TCP/IP)
Principles of Computer Networks and
Communications
16
Hierarchical Addresses

Physical address is different from the network address

Physical address—refers to a particular device


Network address—refers only to the network in which
the device resides


The physical address doesn’t change when the device is moved
The network address changes when the device is moved!
Analogy


An automobile VIN stays with the automobile (physical address)
if you move to a different state
The license plate (network address) changes to be
state-specific
Chapter 12
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Addressing in the Internet
“In 1983 ARPANET officially adopted TCP/IP as the
standard communications protocol.”



Replaced NCP (network control protocol)
Major step towards today’s Internet
Explains why the Internet uses TCP/IP model architecture

TCP/IP



OSI


Chapter 12
groups application functions into a single applications layer
Communications functions are in the other layers
Layers above transport focus on applications
Layers below session deal with communications
Principles of Computer Networks and
Communications
18
Model Architectures
Focused on
applications
Focused on
communications
Fig. 12.2
Chapter 12
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Communications
19
Addressing in the Internet

Internet protocol (IP) address


Used to identify a device for the Internet,
in the internet layer
Different from a medium access control
(MAC) address

IP address




MAC address

Chapter 12
Associated with a machine that may or may not be in a LAN
A logical address at the internet layer
May be changed without affecting the physical address
A physical address at the data link layer of a device on a LAN
Principles of Computer Networks and
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20
Addressing in the Internet

IP address

Can be

Static


Dynamic



Assigned and fixed on the device by a network administrator
Assigned to a device by a protocol process when the device
links (logs on) to the Internet
Dynamic IP addresses are recycled—released when a device
disconnects and available for assignment on another device
Is used by the Internet to route packets

To reach a device, there must be a mapping of its IP
address to its physical address
In other words, the IP address must be associated
with the device’s physical address
Chapter 12
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how to find the IP and the MAC address on your
computer


For the IP address:
 Run
 Cmd
 Ipconfig
MAC address
 open the network connections
 Select your LAN connections, right click, select status
 In the support tap click Details
 Your MAC is the physical address
Chapter 12
22
Addressing in the Internet
Mapping of its IP address to its physical
address
 There are several protocols to do this
mapping (i.e., IP address to physical
address)



Address resolution protocol (ARP) << Original
Reverse address resolution protocol (RARP) <<
companion of ARP
Dynamic host configuration protocol (DHCP) << new
)More about these in Chapter 13…)
Chapter 12
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The Domain Name System

Domain name


The alphabet version of an IP address on the Internet
Domain name system (DNS)


Used by the internet to translate a domain name or e-mail
address to an IP address
Every domain name and e-mail address



Is globally unique
Has a one-to-one relationship with a unique IP address
Resolving the domain name

The process where DNS translates a typed domain name into
an IP address that the Internet uses to route the transmission
For example, www.icann.org resolves (translates)
into dotted quad notation as 192.0.34.65
Translates into Binary 32 bits 4x8
Chapter 12
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The Domain Name System
The translation process is called resolving
the domain name, applies for e-mail as well

E-mail addresses



A computer program called a mail transfer agent sends e-mail
from one computer or mail server to another
These agents use the DNS to find out where to deliver the email
Smooth operations in the DNS



DNS is an interconnected hierarchical system of high-speed
servers running distributed domain name databases
For translation, this system simply searches its databases,
finds the IP address for the name, and relays it back
Centralized organization keeps the DNS up to date (new
additions or deletes)
Chapter 12
Domain name registries are responsible
Principles
of Computer
Networks
for distributing
domain
names
andandIP addresses
Communications
while ensuring their uniqueness
25
The Parts of a URL

Uniform resource locator (URL)

Is a symbolic meaning for specifying





a Web resource
The Web server on which the resource resides
The protocol that will be used to retrieve the resource
URL components are separated from each other by
forward slashes, dots, and sometimes colons
Easiest to interpret from right to left

The rightmost segment is called the top-level domain
(TLD)
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Top-Level Domains (TLDs)
Easier to interpret if starting from right to left
www.users.alvernia.edu

Five original TLDs





TLD
.com for commercial enterprises
.gov for government sites
.net for organizations providing network services
.mil for use by the military
.org for nonprofit organizations and those that do not fit other designations

Because .com, .org, and .net characteristics have blurred over time,
they are now referred to as generic TLDs (gTLDs)

TLD concept speeds up the searching process in the database
because each partition is relatively small
Chapter 12
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Domain and Sub-domain Names

Domain name
www.users.alvernia.edu




Also called second-level domain
To the left of the TLD, separated by a dot
Specifies a particular network, an autonomous system (AS)
within the Internet
Sub-domain name
www.users.alvernia.edu

Narrows the location of the resource server
Chapter 12
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URL Server

Server (host) name
www.users.alvernia.edu

Is located to the left of the sub-domain name
 Holds the requested resource
It is common practice to give the
name www to the server
that hosts Web documents
However, it is not required!
Chapter 12
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29
Domain Name and URL
Components
www is a server
at Baruch College
Combined domain name
.cuny.edu specifies a particular
network within the Internet
If you see a URL that ends after the TLD or after a subdirectory name,
the extension/index.htm or /index.html is assumed
Chapter 12
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Fig 12.3
30
Specifying the File on the Server

Domain names



Specify location of the server
Do not explicitly specify the file (Web page) on the server
Beyond domain names

We need the path to the file on the server


Path must include directories and the file name
Path information is appended to the right of the TLD by a slash (/)
Example
www.users.alvernia.edu/students/finalgrades/index.htm



Chapter 12
/students is the directory where Web files for students are stored
/finalgrades is the subdirectory where files specific to final grades
are stored
/index.htm specifies one particular file
Principles of Computer Networks and
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31
Specifying the File on the Server

.htm and .html

Indicate that the file is written in hypertext
markup language (HTML)
 Are default file names that are automatically
searched for if no file name is given
Any URL with nothing after
the TLD or a subdirectory name
assumes the extension
/index.htm or /index.html
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Specifying the File on the Server
The URL must inform the server of the protocol the
client will use in the interchange process
 Specifying the protocol in the URL


Leftmost segment of the URL defines actions taken
in response to particular requests
http:// is one of the most common Web protocols




Stands for hypertext transfer protocol
In a browser, sends a command to the site’s Web server to
download the page
Part of the application layer of the TCP/IP suite
A “stateless” protocol


Chapter 12
Each command is performed independently
Makes it difficult to create sites that interact with users
Principles of Computer Networks and
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33
The Http Protocol and “Cookies”

Software like Java is used to overcome
“stateless” protocol difficulties

Used to write very small text files (cookies)
to the client’s hard drive
Cookies contain “state” information
 Allow a server application to understand the http
requests that make up a continuous exchange
 http does not prevent unauthorized accessing see
next slide

Chapter 12
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34
Other Identifiers (common
protocols)

https://



ftp (file transfer protocol)




For sites that require secure transmissions, an s is added, indicating
encryption
Unreachable without appropriate passwords
Commonly employed protocol
Used for uploading and downloading files to and from ftp servers
ftp is typically in the server name, but not required
Country identifier


The country identification is part of the TLD, though separated from it
by a dot
For example, BBC News has a United Kingdom identifier
news.bbc.co.uk

When with the TLD, it is called a country code top-level domain (ccTLD)
There are more than 240 ccTLDs!
Chapter 12
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35
IPv4


IP addressing began with ARPANET
1981 IPv4 became the standard we use today


Hierarchical scheme
Classes of addresses
Three logical arrangements/splits of the bits reserved
for addresses

For few organizations needing many host addresses


For many companies with many more hosts


Many bits for network addresses, but also many for hosts
For the great many organizations with very few hosts

Chapter 12
Few bits for network addresses, many for hosts
Many bits for network addresses, few for hosts
This lead to the creation of 3 arrangements
called classes of address
36
IPv4 Classful Addressing

“Classful”—most widely used type of IPv4
 Consists of 32 bits
 Four 8-bit sections
 Makes





arranged in the dotted quad format
192 .0.34 .65
up three unicast classes
Unicast—from one source to one destination
Two-part addresses that split the 32-bits into network/host
Class A: 8 / 24
Class B: 16 / 16
Class C: 24 / 8
 Class
identifier bits (prefixes) are included in the
network address part of the split
Chapter 12
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37
Classful Addressing Prefixes

Prefixes

Identify class
 Are not part of the IP address
Class A is 0
Starting bit
 Class B is 10
 Class C is 110
 D (not classful) is 1110 used for multucasting
 E (not classful) is 1111 for Expermental

Chapter 12
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38
IPv4 Classful Addressing
These classes account for 87.5% of
potentially available addresses
(1st 8-bit section)
Number of
Networks
Number of
Hosts
A
0___
____
27 – 2 = 126
224 – 2 = 16,777,214
B
10 _ _
____
214 – 2 = 16,382
216 – 2 = 65,534
C
110 _
____
221 – 2 = 2,097,150
28 – 2 = 254
Class
Prefix
Table 12.1
Chapter 12
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39
IPv4 Non-Classful D and E

Two other categories of bits reserved for
addresses

D and E are not segmented into networks and hosts
 Both allow for 228 = 268,435,456 addresses

D

Multicasting


From a source to multiple destinations
E

Reserved for experimenting
Chapter 12
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40
Class A address
Network Address
32 bits
 First 8 left most for network address
 The other 24 bit for the host
 First left most bit used as a class identifier
 No address can be all 1’s or all 0’s
n
7
 For 8 bits 2
2 gives 128 address
 Without the address of all 1’s or 0’s we
get 126 network addresses

Chapter 12
Principles of Computer Networks and
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41
Class A address
host Address
24 bits
 First 24 right most for host address
 No address can be all 1’s or all 0’s
n
24
 For 24 bits 2
2 gives 16,777,216
address
 Without the address of all 1’s or 0’s we
get 17,777,214 host addresses
 Same calculations for class B and class C

Chapter 12
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42
Classful Addressing

An organization that applies for an IPv4 address


Receives a network address with a block of
host addresses
The size of this block is determined by class


If the organization can handle more addresses than it
actually uses, the other addresses associated with the
company’s block go unused
Significant limitation to classful addressing

It wastes a lot of addresses
Soon they will run out of addresses!
To forestall this, classless addressing was implemented
Chapter 12
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43
Classful Addresses, Networks,
Subnets, and Masks

Network ID

A company receives a network ID when a classful
network address is assigned



Network ID + host address all 0s = network address
Used by outside routers to direct IP packets addressed
to the company
Not assignable to any company host


No host ID can be all 0s
Logical IP networks

A company subdivides the classful network address
to organize its own hosts
Chapter 12
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44
Subnets and Masks

Subnets
Logical networks with their own subnet addresses
 Created by assigning hosts to groups with their own
subnet addresses
 Organized many ways—by building, floor, department, LAN
Major advantages: A single IP address can connect

a whole subnet to the Internet


Better control on subdividing and managing the network
Masks



Bit patterns applied to entire addresses to isolate their
components
Used to separate network, subnet, and host addresses
Have the same number of bits (arranged in dotted quad
segments) as the IP address, but only use 1s and 0s
Chapter 12
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45
Bitwise Multiplication and Masks

Bitwise multiplication of the address by the mask



Equivalent to applying the “and” operator
Captures address parts where mask bits are 1
and ignores where they are 0
Internet routers easily identify the IP address class
by finding bit patterns this way
Class B mask
Chapter 12
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Bitwise Multiplication and Masks

When the class is identified, a network default
mask is applied

Three default masks
Class A mask: 255.0.0.0
 Class B mask: 255.255.0.0
 Class C mask: 255.255.255.0

Chapter 12
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47
Bitwise Multiplication and Masks

In operation

After one of the three default masks is applied, the
network address is revealed

The network address is assigned to the edge router
of the organization

When a packet reaches any router, the appropriate
mask is applied


Chapter 12
If the network address it finds is not for that organization,
the packet is passed to the next hop router
If the network and router addresses match, a subnet mask
is applied
Principles of Computer Networks and
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48
Addressing in the Internet

Subnet address

Comprises the network address + subnet mask bits


The remaining host address bits are all 0s
The total number of bits in the combined network and
subnet addresses is indicated by a /n notation 130.57.110.9/19
at the end of the address
16 bits
Chapter 12
3 bits
Principles of Computer Networks and
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= 19 bits
49
Classless Addresses

A solution to the IP address shortage?

Classless addressing
All of IPv4’s address space of 32 bits would be
available without restriction
 Twice as many addresses could be created


But addressing hierarchy and restrictions
needed

Chapter 12
Otherwise, routers would be overwhelmed and
complicated
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Classless Addresses

Classless inter-domain routing (CIDR)

The compromise between classful and classless

Allows any number of leftmost bits to be assigned
as a network address



Addresses assigned based on the number of hosts a
network can support; no class designation
CIDR is not limited to network addresses of 8,16,or 24 bits
CIDR is NOT perfect

Chapter 12
Still wastes addresses, just not as many as classful
addressing
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CIDR, Subnetting, and Supernetting

Supernetting



CIDR’s hierarchical scheme that parallels subnetting
One key difference—it is applied to routing outside
of the organization (hence the name)
Is a method of route aggression



Chapter 12
A single high-level routing table entry represents many
lower-level routes
Internet backbone routers need fewer entries
More efficient, eases table size requirements
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IPv6
Even with CIDR, supernetting and subnetting, still
address shortage
IPV6 replaced IPV4
 Uses a 128-bit address sequence instead of 32

Provides IP header extensions

Adds quality of service (QoS) labeling to IP packets

Uses coloned octal, not dotted quad

Accommodates CIDR by adding a (an) /n
to the end of the address
Chapter 12
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IPv6

Uses a 128-bit address sequence instead of 32


increases the number of available IP addresses
allows for additional hierarchy levels that improve routing efficiency

Provides IP header extensions

Adds quality of service (QoS) labeling to IP packets

Uses coloned octal, not dotted quad

Accommodates CIDR by adding a /n to the end of the address
Chapter 12
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IPv6

Uses a 128-bit address sequence instead of 32

Provides IP header extensions

Improve privacy, authentication, and integrity

Adds quality of service (QoS) labeling to IP packets

Uses coloned octal, not dotted quad

Accommodates CIDR by adding a /n to the end of the address
Chapter 12
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IPv6

Uses a 128-bit address sequence instead of 32

Provides IP header extensions

Adds quality of service (QoS) labeling to IP packets

Specifies the level of service requests

Priority, real-time, normal handling

Uses coloned octal, not dotted quad

Accommodates CIDR by adding a /n to the end of the address
Chapter 12
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IPv6

Uses coloned octal, not dotted quad
Eight segments separated by colons
A1B9:CC5F:000D:0037:FF0E:3945:0000:2A4D
 Two bytes per segment
 Typically written in hexadecimal notation

Still 32 characters, one hexadecimal digit = 2 bytes
 Leading 0s in each section are eliminated for simplification
A1B9:CC5F:000D:0037:FF0E:3945:0000:2A4D
A1B9:CC5F:D:37:FF0E:3945:0:2A4D
 BUT, only one string of 0s can be removed in a given address

Chapter 12
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IPv6

Uses a 128-bit address sequence instead of 32

Provides IP header extensions

Adds quality of service (QoS) labeling to IP packets

Uses coloned octal, not dotted quad

Accommodates CIDR by adding a /n to the end of the address

Chapter 12
n is the number of bits in the CIDR prefix
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IPv4 Packet Headers
Fig. 12.4
IPv4
Chapter 12
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IPv6 Packet Headers
Fig. 12.4
IPv6
Chapter 12
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Moving from IPV4 to IPV6
A huge difference between both methods
 Has to be done gradually
 Three methods to allow gradual transition
and to permit functioning in mixed
environment

Chapter 12
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Methods for Moving from
IPv4 to IPv6
Dual stack
 What?

Stack—the IP protocols used by the network nodes (routers, hosts)
 Dual stack—nodes that contain the stacks for both IP versions

How?

The sender queries the DNS for an address




Pro


If the address is IPv4, the packet is sent as IPv4
If the address is IPv6, the packet is sent as IPv6
Once the change to IP6 is complete IP4 can be deleted
Network nodes accommodate both IPv4 and IPv6
Con

Each of the dual stack nodes must have an IPv4 address


Address scarcity is not alleviated
Processing through two stacks adds to switching time
Chapter 12
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Transitioning from IPv4 to IPv6
Both IPv4 and IPv6
addresses are
maintained
The sender uses whatever
packet format (i.e., IPv4 or IPv6)
is returned from the DNS server
for the destination node
Fig. 12.5A
Chapter 12
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Methods for Moving from
IPv4 to IPv6
Tunneling
 Why?


A packet from an IPv6 node or region of nodes (a cloud) may have to
travel across an IPv4 cloud to reach another IPv6 node
How?

An IPv4 tunnel is created for it to travel through




Pro


First it needs an IPv4 address from the IPv6 edge router at the IPv4/IPv6
border
The IPv6 router will encapsulate it into an IPv4 packet
At the other border, the IPv4 edge router will then decapsulate this packet
Avoids having to assign IPv4 addresses to IPv6-only nodes within a
capsule
Con

Additional processing at the borders
Chapter 12
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Transitioning from IPv4 to IPv6
An IPv4 header encapsulates IPv6 packets
while transiting through IPv4 regions
Chapter 12
Principles of Computer Networks and
Communications
Fig. 12.5B
65
Methods for Moving from
IPv4 to IPv6
Translation
 Why?

An IPv4-only host cannot understand packets from a IPv6-only host
 Tunneling will not help resolve this problem


How?


At the least, the edge router must translate the IPv6 header into an
IPv4 header
Pro


The packet is still IPv6 after the encapsulating header is removed
IPv4 hosts and IPv6 hosts can communicate
Con

Translation can be complicated!

Chapter 12
The end node processes can involve the IP protocols themselves
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