Reconstructions of global temperature of the past 2000 years, using composite of different proxy methods
In the study of past climates ("paleoclimatology"), climate proxies are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions
over a longer fraction of the Earth's history. Reliable global records
of climate only began in the 1880s, and proxies provide the only means
for scientists to determine climatic patterns before record-keeping
began.
A large number of climate proxies have been studied from a
variety of geologic contexts. Examples of proxies include stable isotope
measurements from ice cores, growth rates in tree rings, species composition of sub-fossil pollen in lake sediment or foraminifera in ocean sediments, temperature profiles of boreholes, and stable isotopes and mineralogy of corals and carbonate speleothems.
In each case, the proxy indicator has been influenced by a particular
seasonal climate parameter (e.g., summer temperature or monsoon
intensity) at the time in which they were laid down or grew.
Interpretation of climate proxies requires a range of ancillary studies,
including calibration of the sensitivity of the proxy to climate and
cross-verification among proxy indicators.
Proxies can be combined to produce temperature reconstructions longer than the instrumental temperature record and can inform discussions of global warming
and climate history. The geographic distribution of proxy records, just
like the instrumental record, is not at all uniform, with more records
in the northern hemisphere.
In science, it is sometimes necessary to study a variable which
cannot be measured directly. This can be done by "proxy methods," in
which a variable which correlates with the variable of interest is
measured, and then used to infer the value of the variable of interest.
Proxy methods are of particular use in the study of the past climate,
beyond times when direct measurements of temperatures are available.
Most proxy records have to be calibrated against independent
temperature measurements, or against a more directly calibrated proxy,
during their period of overlap to estimate the relationship between
temperature and the proxy. The longer history of the proxy is then used
to reconstruct temperature from earlier periods.
In addition to oxygen isotopes, water contains hydrogen isotopes – 1H and 2H, usually referred to as H and D (for deuterium) – that are also used for temperature proxies. Normally, ice cores from Greenland are analyzed for δ18O and those from Antarctica for δ-deuterium. Those cores that analyze for both show a lack of agreement. (In the figure, δ18O is for the trapped air, not the ice. δD is for the ice.)
From 1989 to 1992, the European Greenland Ice Core Drilling Project drilled in central Greenland
at coordinates 72° 35' N, 37° 38' W. The ices in that core were 3840
years old at a depth of 770 m, 40,000 years old at 2521 m, and 200,000
years old or more at 3029 m bedrock. Ice cores in Antarctica can reveal the climate records for the past 650,000 years.
Dendroclimatology is the science of determining past climates from trees, primarily from properties of the annual tree rings.
Tree rings are wider when conditions favor growth, narrower when times
are difficult. Other properties of the annual rings, such as maximum
latewood density (MXD) have been shown to be better proxies than simple
ring width. Using tree rings, scientists have estimated many local
climates for hundreds to thousands of years previous. By combining
multiple tree-ring studies (sometimes with other climate proxy records),
scientists have estimated past regional and global climates (see Temperature record of the past 1000 years).
Fossil leaves
Paleoclimatologists
often use leaf teeth to reconstruct mean annual temperature in past
climates, and they use leaf size as a proxy for mean annual
precipitation. In the case of mean annual precipitation reconstructions, some researchers believe taphonomic processes cause smaller leaves to be overrepresented in the fossil record, which can bias reconstructions. However, recent research suggests that the leaf fossil record may not be significantly biased toward small leaves. New approaches retrieve data such as CO2 content of past atmospheres from fossil leaf stomata and isotope composition, measuring cellular CO2 concentrations. A 2014 study was able to use the carbon-13isotope ratios to estimate the CO2 amounts of the past 400 million years, the findings hint at a higher climate sensitivity to CO2 concentrations.
Boreholes
Borehole
temperatures are used as temperature proxies. Since heat transfer
through the ground is slow, temperature measurements at a series of
different depths down the borehole, adjusted for the effect of rising
heat from inside the Earth, can be "inverted"
(a mathematical formula to solve matrix equations) to produce a
non-unique series of surface temperature values. The solution is
"non-unique" because there are multiple possible surface temperature
reconstructions that can produce the same borehole temperature profile.
In addition, due to physical limitations, the reconstructions are
inevitably "smeared", and become more smeared further back in time. When
reconstructing temperatures around 1500 AD, boreholes have a temporal
resolution of a few centuries. At the start of the 20th century, their
resolution is a few decades; hence they do not provide a useful check on
the instrumental temperature record. However, they are broadly comparable.
These confirmations have given paleoclimatologists the confidence that
they can measure the temperature of 500 years ago. This is concluded by a
depth scale of about 492 feet (150 meters) to measure the temperatures
from 100 years ago and 1,640 feet (500 meters) to measure the
temperatures from 1,000 years ago.
Boreholes have a great advantage over many other proxies in that
no calibration is required: they are actual temperatures. However, they
record surface temperature not the near-surface temperature (1.5 meter)
used for most "surface" weather observations. These can differ
substantially under extreme conditions or when there is surface snow. In
practice the effect on borehole temperature is believed to be generally
small. A second source of error is contamination of the well by
groundwater may affect the temperatures, since the water "carries" more
modern temperatures with it. This effect is believed to be generally
small, and more applicable at very humid sites. It does not apply in ice cores where the site remains frozen all year.
More than 600 boreholes, on all continents, have been used as proxies for reconstructing surface temperatures. The highest concentration of boreholes exist in North America and Europe. Their depths of drilling typically range from 200 to greater than 1,000 meters into the crust of the Earth or ice sheet.
A small number of boreholes have been drilled in the ice sheets;
the purity of the ice there permits longer reconstructions. Central
Greenland borehole temperatures show "a warming over the last 150 years
of approximately 1°C ± 0.2°C preceded by a few centuries of cool
conditions. Preceding this was a warm period centered around A.D. 1000,
which was warmer than the late 20th century by approximately 1°C." A
borehole in the Antarctica icecap shows that the "temperature at A.D. 1
[was] approximately 1°C warmer than the late 20th century".
Borehole temperatures in Greenland were responsible for an
important revision to the isotopic temperature reconstruction, revealing
that the former assumption that "spatial slope equals temporal slope"
was incorrect.
Corals
Coral bleached due to changes in ocean water properties
Oceancoralskeletal
rings, or bands, also share paleoclimatological information, similarly
to tree rings. In 2002, a report was published on the findings of Drs.
Lisa Greer and Peter Swart, associates of University of Miami at the time, in regard to stable oxygen isotopes in the calcium carbonate
of coral. Cooler temperatures tend to cause coral to use heavier
isotopes in its structure, while warmer temperatures result in more
normal oxygen isotopes being built into the coral structure. Denser water salinity also tends to contain the heavier isotope. Greer's coral sample from the Atlantic Ocean
was taken in 1994 and dated back to 1935. Greer recalls her
conclusions, "When we look at the averaged annual data from 1935 to
about 1994, we see it has the shape of a sine wave. It is periodic and has a significant pattern of oxygen isotope composition that has a peak at about every twelve to fifteen years." Surface water temperatures
have coincided by also peaking every twelve and a half years. However,
since recording this temperature has only been practiced for the last
fifty years, correlation between recorded water temperature and coral
structure can only be drawn so far back.
Pollen grains
Pollen can be found in sediments. Plants produce pollen
in large quantities and it is extremely resistant to decay. It is
possible to identify a plant species from its pollen grain. The
identified plant community of the area at the relative time from that
sediment layer, will provide information about the climatic condition.
The abundance of pollen of a given vegetation
period or year depends partly on the weather conditions of the previous
months, hence pollen density provides information on short-term
climatic conditions. The study of prehistoric pollen is palynology.
Dinoflagellate cysts
Cyst of a dinoflagellate Peridinium ovatum
Dinoflagellates occur in most aquatic environments and during their
life cycle, some species produce highly resistant organic-walled cysts
for a dormancy period when environmental conditions are not appropriate
for growth. Their living depth is relatively shallow (dependent upon
light penetration), and closely coupled to diatoms on which they feed.
Their distribution patterns in surface waters are closely related to
physical characteristics of the water bodies, and nearshore assemblages
can also be distinguished from oceanic assemblages. The distribution of
dinocysts in sediments has been relatively well documented and has
contributed to understanding the average sea-surface conditions that
determine the distribution pattern and abundances of the taxa. Several studies, including and have compiled box and gravity cores in the North Pacific analyzing them
for palynological content to determine the distribution of dinocysts
and their relationships with sea surface temperature, salinity,
productivity and upwelling. Similarly, and
use a box core at 576.5 m of water depth from 1992 in the central Santa
Barbara Basin to determine oceanographic and climatic changes during
the past 40 kyr in the area.
Lake and ocean sediments
Similar to their study on other proxies, paleoclimatologists examine oxygen isotopes in the contents of ocean sediments. Likewise, they measure the layers of varve (deposited fine and coarse silt or clay) laminating lake sediments. Lake varves are primarily influenced by:
Summer temperature, which shows the energy available to melt seasonal snow and ice
Winter snowfall, which determines the level of disturbance to sediments when melting occurs
Rainfall
Diatoms, foraminifera, radiolarians, ostracods, and coccolithophores
are examples of biotic proxies for lake and ocean conditions that are
commonly used to reconstruct past climates. The distribution of the
species of these and other aquatic creatures preserved in the sediments
are useful proxies. The optimal conditions for species preserved in the
sediment act as clues. Researchers use these clues to reveal what the
climate and environment was like when the creatures died. The oxygen isotope ratios in their shells can also be used as proxies for temperature.
Water isotopes and temperature reconstruction
Ocean water is mostly H216O, with small amounts of HD16O and H218O, where D denotes deuterium, i.e. hydrogen with an extra neutron. In Vienna Standard Mean Ocean Water (VSMOW) the ratio of D to H is 155.76x10−6 and O-18 to O-16 is 2005.2x10−6. Isotope fractionation
occurs during changes between condensed and vapour phases: the vapour
pressure of heavier isotopes is lower, so vapour contains relatively
more of the lighter isotopes and when the vapour condenses the
precipitation preferentially contains heavier isotopes. The difference
from VSMOW is expressed as δ18O = 1000‰ ; and a similar formula for δD. δ values for precipitation are always negative.
The major influence on δ is the difference between ocean temperatures
where the moisture evaporated and the place where the final
precipitation occurred; since ocean temperatures are relatively stable
the δ value mostly reflects the temperature where precipitation occurs.
Taking into account that the precipitation forms above the inversion layer, we are left with a linear relation:
δ 18O = aT + b
This is empirically calibrated from measurements of temperature and δ as a = 0.67 ‰/°C for Greenland and 0.76 ‰/°C for East Antarctica. The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations. More recently, borehole thermometry has shown that for glacial-interglacial variations, a = 0.33 ‰/°C, implying that glacial-interglacial temperature changes were twice as large as previously believed.
A study published in 2017 called the previous methodology to
reconstruct paleo ocean temperatures 100 million years ago into
question, suggesting it has been relatively stable during that time,
much colder.
Membrane lipids
A novel climate proxy obtained from peat (lignites, ancient peat) and soils, membrane lipids known as glycerol dialkyl glycerol tetraether (GDGT) is helping to study paleo environmental factors, which control relative distribution of differently branched GDGT isomers. The study authors note, "These branched membrane lipids are produced by an as yet unknown group of anaerobic soil bacteria." As of 2018, there is a decade of research demonstrating that in mineral soils the degree of methylation of bacteria (brGDGTs), helps to calculate mean annual air temperatures. This proxy method was used to study the climate of the early Palaeogene,
at the Cretaceous–Paleogene boundary, and researchers found that annual
air temperatures, over land and at mid-latitude, averaged about
23–29 °C (± 4.7 °C), which is 5–10 °C higher than most previous
findings.
Pseudoproxies
The
skill of algorithms used to combine proxy records into an overall
hemispheric temperature reconstruction may be tested using a technique
known as "pseudoproxies". In this method, output from a climate model
is sampled at locations corresponding to the known proxy network, and
the temperature record produced is compared to the (known) overall
temperature of the model.
A nanolaser is a laser that has nanoscale
dimensions and it refers to a micro-/nano- device which can emit light
with light or electric excitation of nanowires or other nanomaterials
that serve as resonators. A standard feature of nanolasers includes their light confinement on a scale approaching or suppressing the diffraction limit of light. These tiny lasers can be modulated quickly and, combined with their small footprint, this makes them ideal candidates for on-chip optical computing.
History
Albert Einstein proposed the stimulated emission in 1916, which contributed to the first demonstration of laser in 1961.
From then on, people have been pursuing the miniaturization of lasers
for more compact size and less energy consumption all the time. Since
people noticed that light has different interactions with matter at the
nanoscale in the 1990s, significant progress has been made to achieve
the miniaturization of lasers and increase power conversion efficiency.
Various types of nanolasers have been developed over the past decades.
In the 1990s, some intriguing designs of microdisk laserand photonic crystal laser
were demonstrated to have cavity size or energy volume with
micro-/nano- diameters and approach the diffraction limit of light.
Photoluminescence behavior of bulk ZnO nanowires was first reported in
2001 by Prof. Peidong Yang from the University of California, Berkeley and it opened the door to the study of nanowire nanolasers. These designs still do not exceed the diffraction limit until the demonstration of plasmonic lasers or spasers.
David J. Bergman and Mark Stockman first proposed amplified surface plasmon waves by stimulated emission and coined the term spaser as "surface plasmon amplification by stimulated emission of radiation" in 2003. Until 2009, the plasmonic nanolasers or spasers were first achieved experimentally, which were regarded as the smallest nanolasers at that time.
While
sharing many similarities with standard lasers, nanolasers maintain
many unique features and differences from the conventional lasers due to
the fact that light interacts differently with matter at the nanoscale.
Mechanism
Similar to the conventional lasers, nanolasers also based on stimulated emission which was proposed by Einstein;
the main difference between nanolaser and the conventional ones in
mechanism is light confinement. The resonator or cavity plays an
important role in selecting the light with a certain frequency and the
same direction as the most priority amplification and suppressing the
other light to achieve the confinement of light. For conventional
lasers, Fabry–Pérot cavity
with two parallel reflection mirrors is applied. In this case, light
could be confined to a maximum of half its wavelength and such limit is
deemed the diffraction limit of light. To approach or decrease the diffraction limit of light, one way is to improve the reflectivity of gain medium,
such as using photonic bandgap and nanowires. Another effective way to
exceed the diffraction limit is to convert light into surface plasmons
in nanostructuralized metals, for amplification in cavity. Recently, new mechanisms of strong light confinement for nanolasers including parity–time symmetry, photonic topological insulators, and bound states in the continuum have been proposed.
Properties
Comparison
of nanolasers and macro lasers in properties. Compared with macro
lasers, nanolasers have decreased sizes, lower thresholds and
accelerated modulation speeds.
Compared with conventional lasers, nanolasers show distinct
properties and capabilities. The biggest advantages of nanolasers are
their ultra-small physical volumes to improve energy efficiencies,
decrease lasing thresholds, and achieve high modulation speeds.
Types of nanolasers
Microdisk laser
SEM image of microdisk laser with a whispering-gallery mode resonator.
A microdisk laser is a very small laser consisting of a disk with quantum well structures built into it. Its dimensions can exist on the micro-scale or nano-scale. Microdisk lasers use a whispering-gallery mode resonant cavity.
The light in cavity travels around the perimeter of the disk and the
total internal reflection of photons can result in a strong light
confinement and a high quality factor, which means a powerful ability of
the microcavity to store the energy of photons coupled into the cavity.
Photonic crystal laser
Photonic crystal lasers utilize periodic dielectric
structures with different refractive indices; light can be confined
with the use of a photonic crystal microcavity. In dielectric materials,
there is orderly spatial distribution. When there is a defect in the
periodic structure, the two-dimensional or three-dimensional photonic
crystal structure will confine the light in the space of the diffractive
limit and produce the Fano resonance
phenomenon, which means a high quality factor with a strong light
confinement for lasers. The fundamental feature of photonic crystals is
the photonic bandgap, that is, the light whose frequency falls in the
photonic band gap cannot propagate in the crystal structure, thus
resulting in a high reflectivity for incident light and a strong
confinement of light to a small volume of wavelength scale.
The appearance of photonic crystals makes the spontaneous emission in
the photon gap completely suppressed. But the high cost of photonic
crystal impedes the development and spreading applications of photonic
crystal lasers.
Semiconductor nanowire lasers have a quasi-one-dimensional structure
with diameters ranging from a few nanometers to a few hundred nanometers
and lengths ranging from hundreds of nanometers to a few microns. The
width of nanowires is large enough to ignore the quantum size effect, but they are high quality one-dimensional waveguides
with cylindrical, rectangular, trigonal, and hexagonal cross-sections.
The quasi-one-dimensional structure and high reflectivity of nanowire
laser makes it have good optical waveguide and the ability of light
confinement. Nanowire lasers are similar to Fabry–Pérot cavity in mechanism.
High reflectivity of nanowire and flat end facets of the wire
constitute a good resonant cavity, in which photons can be bound between
the two ends of the nanowire to limit the light energy to the axial
direction of the nanowire, thus meeting the conditions for laser
formation. Polygonal nanowires can form a nearly circular cavity in cross section that supports whispering-gallery mode.
Schematic
illustration of a plasmonic nanolaser. The process of lasing formation
includes energy transfer convert photons into surface plasmons.
Nanolaser based on surface plasmon is known as plasmonic nanolaser,
whose size far exceeds the diffraction limit of light. Especially, if a
plasmonic nanolaser is nanoscopic in three dimensions, it is also called
as spaser,
which is known to have the smallest cavity size and mode size. Design
of plasmonic nanolaser has become one of the most effective technology
methods for laser miniaturization at present. A little bit different from the conventional lasers, a typical configuration of plasmonic nanolaser includes a process of energy transfer to convert photons into surface plasmons. In plasmonic nanolaser or spaser, the exciton is not photons anymore but surface plasmon polariton. Surface plasmons are collective oscillations of free electrons on metal surfaces under the action of external electromagnetic fields.
According to their manifestations, the cavity mode in plasmonic
nanolasers can be divided into the propagating surface plasmon
polaritons (SPPs) and the non-propagating localized surface plasmons (LSPs).
Schematic of a SPP mode, where surface plasmon polaritons propagate along an interface between metal and dielectric.
SPPs are electromagnetic waves that propagate along the interface
between metal and medium, and their intensities decay gradually in the
direction perpendicular to the propagation interface. In 2008, Oulton
experimentally validated a plasma nanowire laser consisting of a thin
dielectric layer with a low reflectivity growing on a metal surface and a
gain layer with a high refractive index semiconductor nanowire.
In this structure, the electromagnetic field can be transferred from
the metal layer to the intermediate gap layer, so that the mode energy
is highly concentrated, thus greatly reducing the energy loss in the
metal.
Schematic
of configuration of a 3D spaser surrounded by a gain medium based on
localized surface plasmons. Metal core provides plasmon mode and surface
plasmon polaritons are formed on the surface of nanoshell with a
silicon dioxide doped with dye as gain medium.
The LSP mode exists in a variety of different metal nanostructures, such as metal nanoparticles (nanospheres, nanorods, nanocubes, etc.) and arrays of nanoparticles.
Unlike the propagating surface plasmon polaritons, the localized
surface plasmon does not propagate along the surface, but oscillates
back and forth in the nanostructure in the form of standing waves. When
light is incident to the surface of a metal nanoparticles, it causes a
real displacement of the surface charge relative to the ions. The
attraction between electrons and ions allows for the oscillation of
electrode cloud and the formation of local surface from polarization
excimer.
The oscillation of electrons is determined by the geometrical
boundaries of different metal nanoparticles. When its resonance
frequency is consistent with the incident electromagnetic field, it will
form the localized surface plasmon resonance. In 2009, Mikhail A.
Noginov of Norfolk State University in the United States successfully verified the LSPs-based nanolaser for the first time.
The nanolaser in this paper was composed of an Au core providing the
plasmon mode and a silicon dioxide doped with OG-488 dye providing the
gain medium. The diameter of the Au core was 14 nm, the thickness of the
silica layer was 15 nm, and the diameter of the whole device was only
44 nm, which was the smallest nanolaser at that time.
New types of nanolasers
In
addition, there have been some new types of nanolasers developed in
recent years to approach the diffraction limit. Parity-time symmetry is
related to a balance of optical gain and loss in a coupled cavity
system. When the gain–loss contrast and coupling constant between two
identical, closely located cavities are controlled, the phase transition
of lasing modes occurs at an exceptional point.
Bound states in the continuum laser confines light in an open system
via the elimination of radiation states through destructive interference
between resonant modes. A photonic topological insulator laser is based on topological
insulators optical mode, where the topological states is confined within
the cavity boundaries and they can be used for the formation of laser.
All of those new types of nanolasers have high quality factor and can
achieve cavity size and mode size approaching the diffraction limit of
the light.
Applications
Due
to the unique capabilities including low lasing thresholds, high energy
efficiencies and high modulation speeds, nanolasers show great
potentials for practical applications in the fields of materials characterization, integrated optical interconnects, and sensing.
Nanolasers for material characterization
The intense optical fields of such a laser also enable the enhancement effect in non-linear optics or surface-enhanced-raman-scattering (SERS).
Nanowire nanolasers can be capable of optical detection at the scale of
a single molecule with high resolution and ultrafast modulation.
Nanolasers for integrated optical interconnects
Internet is developing at an extremely high speed with large energy consumption for data communication. The high energy efficiency of nanolasers plays an important role in decreasing energy consumption for future society.
Plasmonic nanolaser sensors have recently been demonstrated that can detect specific molecules in air and be used for optical biosensors.
Molecules can modify the surface of metal nanoparticles and impact the
surface recombination velocity of gain medium of a plasmonic nanolaser,
which contributes to the sensing mechanism of plasmonic nanolasers.
Challenges
Although
nanolasers have shown great potential, there are still some challenges
towards the large-scale use of nanolasers, for example, electrically
injected nanolasers, cavity configuration engineering and metal quality
improvement.
For nanolasers, the realization of electrically injected or pumped
operation at room temperature is a key step towards its practical
application. However, most nanolaser are optically pumped and the
realization of electrically injected nanolasers is still a main
technical challenge at present.
Only a few studies have reported electrically injected nanolasers.
Moreover, it still remains a challenge to realize cavity configuration
engineering and metal quality improvement, which are crucial to satisfy
the high-performance requirement of nanolasers and achieve their
applications. Recently, nanolaser arrays show great potential to increase the power efficiency and accelerate modulation speed.
Communicating systems use well-defined formats for exchanging
various messages. Each message has an exact meaning intended to elicit a
response from a range of possible responses pre-determined for that
particular situation. The specified behavior is typically independent of
how it is to be implemented. Communication protocols have to be agreed upon by the parties involved. To reach an agreement, a protocol may be developed into a technical standard. A programming language describes the same for computations, so there is a close analogy between protocols and programming languages: protocols are to communication what programming languages are to computations. An alternate formulation states that protocols are to communication what algorithms are to computation.
Multiple protocols often describe different aspects of a single
communication. A group of protocols designed to work together is known
as a protocol suite; when implemented in software they are a protocol stack.
One of the first uses of the term protocol in a data-commutation context occurs in a memorandum entitled A Protocol for Use in the NPL Data Communications Network written by Roger Scantlebury and Keith Bartlett in April 1967.
On the ARPANET, the starting point for host-to-host communication in 1969 was the 1822 protocol, which defined the transmission of messages to an IMP. The Network Control Protocol (NCP) for the ARPANET was first implemented in 1970. The NCP interface allowed application software to connect across the ARPANET by implementing higher-level communication protocols, an early example of the protocol layering concept.
Networking research in the early 1970s by Robert E. Kahn and Vint Cerf led to the formulation of the Transmission Control Program (TCP). Its RFC675 specification was written by Cerf with Yogen Dalal and Carl Sunshine in December 1974, still a monolithic design at this time.
TCP software was redesigned as a modular protocol stack. Originally referred to as IP/TCP, it was installed on SATNET in 1982 and on the ARPANET in January 1983. The development of a complete protocol suite by 1989, as outlined in RFC1122 and RFC1123, laid the foundation for the growth of TCP/IP as a comprehensive protocol suite as the core component of the emerging Internet.
International work on a reference model for communication standards led to the OSI model, published in 1984. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard, the OSI model or the Internet protocol suite, would result in the best and most robust computer networks.
Concept
The
information exchanged between devices through a network or other media
is governed by rules and conventions that can be set out in
communication protocol specifications. The nature of communication, the
actual data exchanged and any state-dependent behaviors, is defined by these specifications. In digital computing systems, the rules can be expressed by algorithms and data structures. Protocols are to communication what algorithms or programming languages are to computations.
Operating systems usually contain a set of cooperating processes
that manipulate shared data to communicate with each other. This
communication is governed by well-understood protocols, which can be
embedded in the process code itself. In contrast, because there is no shared memory, communicating systems have to communicate with each other using a shared transmission medium. Transmission is not necessarily reliable, and individual systems may use different hardware or operating systems.
To implement a networking protocol, the protocol software modules
are interfaced with a framework implemented on the machine's operating
system. This framework implements the networking functionality of the
operating system. When protocol algorithms are expressed in a portable programming language the protocol software may be made operating system independent. The best-known frameworks are the TCP/IP model and the OSI model.
At the time the Internet was developed, abstraction layering
had proven to be a successful design approach for both compiler and
operating system design and, given the similarities between programming
languages and communication protocols, the originally monolithic
networking programs were decomposed into cooperating protocols. This gave rise to the concept of layered protocols which nowadays forms the basis of protocol design.
Systems typically do not use a single protocol to handle a
transmission. Instead they use a set of cooperating protocols, sometimes
called a protocol suite. Some of the best-known protocol suites are TCP/IP, IPX/SPX, X.25, AX.25 and AppleTalk.
The protocols can be arranged based on functionality in groups, for instance, there is a group of transport protocols.
The functionalities are mapped onto the layers, each layer solving a
distinct class of problems relating to, for instance: application-,
transport-, internet- and network interface-functions.
To transmit a message, a protocol has to be selected from each layer.
The selection of the next protocol is accomplished by extending the
message with a protocol selector for each layer.
Types
There are
two types of communication protocols, based on their representation of
the content being carried: text-based and binary.
Text-based
A text-based protocol or plain text protocol represents its content in human-readable format, often in plain text.
The immediate human readability stands in contrast to binary
protocols which have inherent benefits for use in a computer environment
(such as ease of mechanical parsing and improved bandwidth utilization).
Network applications have various methods of encapsulating data.
One method very common with Internet protocols is a text oriented
representation that transmits requests and responses as lines of ASCII
text, terminated by a newline character (and usually a carriage return
character). Examples of protocols that use plain, human-readable text
for its commands are FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), and the finger protocol.
Text-based protocols are typically optimized for human parsing
and interpretation and are therefore suitable whenever human inspection
of protocol contents is required, such as during debugging and during early protocol development design phases.
To be clear, all digital communication is fundamentally binary.
The "Text" based protocols mentioned here use only binary content, which
is made "humanly readable" by a text editor (or other such software).
Binary
A binary protocol utilizes all values of a byte, as opposed to a text-based protocol which only uses values corresponding to human-readable characters in ASCII
encoding. Binary protocols are intended to be read by a machine rather
than a human being. Binary protocols have the advantage of terseness,
which translates into speed of transmission and interpretation.
Binary have been used in the normative documents describing modern standards like EbXML, HTTP/2, HTTP/3 and EDOC. An interface in UML may also be considered a binary protocol.
Basic requirements
Getting
the data across a network is only part of the problem for a protocol.
The data received has to be evaluated in the context of the progress of
the conversation, so a protocol must include rules describing the
context. These kinds of rules are said to express the syntax of
the communication. Other rules determine whether the data is meaningful
for the context in which the exchange takes place. These kinds of rules
are said to express the semantics of the communication.
Messages are sent and received on communicating systems to
establish communication. Protocols should therefore specify rules
governing the transmission. In general, much of the following should be
addressed:
Data formats for data exchange
Digital message bitstrings are exchanged. The bitstrings are divided
in fields and each field carries information relevant to the protocol.
Conceptually the bitstring is divided into two parts called the header and the payload.
The actual message is carried in the payload. The header area contains
the fields with relevance to the operation of the protocol. Bitstrings
longer than the maximum transmission unit (MTU) are divided in pieces of appropriate size.
Address formats for data exchange
Addresses are used to identify both the sender and the intended
receiver(s). The addresses are carried in the header area of the
bitstrings, allowing the receivers to determine whether the bitstrings
are of interest and should be processed or should be ignored. A
connection between a sender and a receiver can be identified using an
address pair (sender address, receiver address). Usually, some address values have special meanings. An all-1s
address could be taken to mean an addressing of all stations on the
network, so sending to this address would result in a broadcast on the
local network. The rules describing the meanings of the address value
are collectively called an addressing scheme.
Address mapping
Sometimes protocols need to map addresses of one scheme on addresses
of another scheme. For instance, to translate a logical IP address
specified by the application to an Ethernet MAC address. This is
referred to as address mapping.
Routing
When systems are not directly connected, intermediary systems along the route
to the intended receiver(s) need to forward messages on behalf of the
sender. On the Internet, the networks are connected using routers. The
interconnection of networks through routers is called internetworking.
Detection of transmission errors
Error detection is necessary on networks where data corruption is
possible. In a common approach, a CRC of the data area is added to the
end of packets, making it possible for the receiver to detect
differences caused by corruption. The receiver rejects the packets on
CRC differences and arranges somehow for retransmission.
Acknowledgements
Acknowledgement of correct reception of packets is required for connection-oriented communication. Acknowledgments are sent from receivers back to their respective senders.
Loss of information - timeouts and retries
Packets may be lost on the network or be delayed in transit. To cope
with this, under some protocols, a sender may expect an acknowledgment
of correct reception from the receiver within a certain amount of time.
Thus, on timeouts, the sender may need to retransmit the information.
In case of a permanently broken link, the retransmission has no effect,
so the number of retransmissions is limited. Exceeding the retry limit
is considered an error.
Direction of information flow
Direction needs to be addressed if transmissions can only occur in one direction at a time as on half-duplex links or from one sender at a time as on a shared medium. This is known as media access control. Arrangements have to be made to accommodate the case of collision or contention where two parties respectively simultaneously transmit or wish to transmit.
Sequence control
If long bitstrings are divided into pieces and then sent on the
network individually, the pieces may get lost or delayed or, on some
types of networks, take different routes to their destination. As a
result, pieces may arrive out of sequence. Retransmissions can result in
duplicate pieces. By marking the pieces with sequence information at
the sender, the receiver can determine what was lost or duplicated, ask
for necessary retransmissions and reassemble the original message.
Flow control
Flow control is needed when the sender transmits faster than the
receiver or intermediate network equipment can process the
transmissions. Flow control can be implemented by messaging from
receiver to sender.
Queueing
Communicating processes or state machines employ queues (or
"buffers"), usually FIFO queues, to deal with the messages in the order
sent, and may sometimes have multiple queues with different
prioritization.
Protocol design
Systems engineering
principles have been applied to create a set of common network protocol
design principles. The design of complex protocols often involves
decomposition into simpler, cooperating protocols. Such a set of
cooperating protocols is sometimes called a protocol family or a
protocol suite, within a conceptual framework.
Communicating systems operate concurrently. An important aspect of concurrent programming
is the synchronization of software for receiving and transmitting
messages of communication in proper sequencing. Concurrent programming
has traditionally been a topic in operating systems theory texts.
Formal verification seems indispensable because concurrent programs are
notorious for the hidden and sophisticated bugs they contain. A mathematical approach to the study of concurrency and communication is referred to as communicating sequential processes (CSP). Concurrency can also be modeled using finite state machines, such as Mealy and Moore machines.
Mealy and Moore machines are in use as design tools in digital
electronics systems encountered in the form of hardware used in
telecommunication or electronic devices in general. The literature presents numerous analogies between computer
communication and programming. In analogy, a transfer mechanism of a
protocol is comparable to a central processing unit (CPU). The framework
introduces rules that allow the programmer to design cooperating
protocols independently of one another.
Layering
Figure 2. The TCP/IP model or Internet layering scheme and its relation to some common protocols.
In modern protocol design, protocols are layered to form a protocol
stack. Layering is a design principle that divides the protocol design
task into smaller steps, each of which accomplishes a specific part,
interacting with the other parts of the protocol only in a small number
of well-defined ways. Layering allows the parts of a protocol to be
designed and tested without a combinatorial explosion of cases, keeping each design relatively simple.
The communication protocols in use on the Internet
are designed to function in diverse and complex settings. Internet
protocols are designed for simplicity and modularity and fit into a
coarse hierarchy of functional layers defined in the Internet Protocol Suite. The first two cooperating protocols, the Transmission Control Protocol (TCP) and the Internet Protocol
(IP) resulted from the decomposition of the original Transmission
Control Program, a monolithic communication protocol, into this layered
communication suite.
The OSI model
was developed internationally based on experience with networks that
predated the internet as a reference model for general communication
with much stricter rules of protocol interaction and rigorous layering.
Typically, application software is built upon a robust data
transport layer. Underlying this transport layer is a datagram delivery
and routing mechanism that is typically connectionless
in the Internet. Packet relaying across networks happens over another
layer that involves only network link technologies, which are often
specific to certain physical layer technologies, such as Ethernet. Layering provides opportunities to exchange technologies when needed, for example, protocols are often stacked in a tunneling arrangement to accommodate the connection of dissimilar networks. For example, IP may be tunneled across an Asynchronous Transfer Mode (ATM) network.
Protocol layering
Figure
3. Message flows using a protocol suite. Black loops show the actual
messaging loops, red loops are the effective communication between
layers enabled by the lower layers.
Protocol layering forms the basis of protocol design. It allows the decomposition of single, complex protocols into simpler, cooperating protocols.
The protocol layers each solve a distinct class of communication
problems. Together, the layers make up a layering scheme or model.
Computations deal with algorithms and data; Communication involves protocols and messages; So the analog of a data flow diagram is some kind of message flow diagram.
To visualize protocol layering and protocol suites, a diagram of the
message flows in and between two systems, A and B, is shown in figure 3.
The systems, A and B, both make use of the same protocol suite. The
vertical flows (and protocols) are in-system and the horizontal message
flows (and protocols) are between systems. The message flows are
governed by rules, and data formats specified by protocols. The blue
lines mark the boundaries of the (horizontal) protocol layers.
Software layering
Figure
5: Protocol and software layering. The software modules implementing
the protocols are represented by cubes. The information flow between the
modules is represented by arrows. The (top two horizontal) red arrows
are virtual. The blue lines mark the layer boundaries.
The software supporting protocols has a layered organization and its relationship with protocol layering is shown in figure 5.
To send a message on system A, the top-layer software module
interacts with the module directly below it and hands over the message
to be encapsulated. The lower module fills in the header data in
accordance with the protocol it implements and interacts with the bottom
module which sends the message over the communications channel to the
bottom module of system B. On the receiving system B the reverse
happens, so ultimately the message gets delivered in its original form
to the top module of system B.
Program translation
is divided into subproblems. As a result, the translation software is
layered as well, allowing the software layers to be designed
independently. The same approach can be seen in the TCP/IP layering.
The modules below the application layer are generally considered
part of the operating system. Passing data between these modules is much
less expensive than passing data between an application program and the
transport layer. The boundary between the application layer and the
transport layer is called the operating system boundary.
Strict layering
Strictly adhering to a layered model, a practice known as strict layering, is not always the best approach to networking. Strict layering can have a negative impact on the performance of an implementation.
While the use of protocol layering is today ubiquitous across the
field of computer networking, it has been historically criticized by
many researchers
as abstracting the protocol stack in this way may cause a higher layer
to duplicate the functionality of a lower layer, a prime example being
error recovery on both a per-link basis and an end-to-end basis.
Design patterns
Commonly recurring problems in the design and implementation of communication protocols can be addressed by software design patterns.
Finite-state machine models are used to formally describe the possible interactions of the protocoland communicating finite-state machines.
Protocol development
For
communication to occur, protocols have to be selected. The rules can be
expressed by algorithms and data structures. Hardware and operating
system independence is enhanced by expressing the algorithms in a
portable programming language. Source independence of the specification
provides wider interoperability.
Protocol standards are commonly created by obtaining the approval or support of a standards organization,
which initiates the standardization process. The members of the
standards organization agree to adhere to the work result on a voluntary
basis. Often the members are in control of large market shares relevant
to the protocol and in many cases, standards are enforced by law or the
government because they are thought to serve an important public
interest, so getting approval can be very important for the protocol.
The need for protocol standards
The need for protocol standards can be shown by looking at what happened to the Binary Synchronous Communications (BSC) protocol invented by IBM.
BSC is an early link-level protocol used to connect two separate nodes.
It was originally not intended to be used in a multinode network, but
doing so revealed several deficiencies of the protocol. In the absence
of standardization, manufacturers and organizations felt free to enhance
the protocol, creating incompatible versions on their networks. In some
cases, this was deliberately done to discourage users from using
equipment from other manufacturers. There are more than 50 variants of
the original bi-sync protocol. One can assume, that a standard would
have prevented at least some of this from happening.
In some cases, protocols gain market dominance without going
through a standardization process. Such protocols are referred to as de facto standards. De facto standards are common in emerging markets, niche markets, or markets that are monopolized (or oligopolized).
They can hold a market in a very negative grip, especially when used to
scare away competition. From a historical perspective, standardization
should be seen as a measure to counteract the ill-effects of de facto
standards. Positive exceptions exist; a de facto standard operating
system like Linux does not have this negative grip on its market,
because the sources are published and maintained in an open way, thus
inviting competition.
International standards organizations are supposed to be more
impartial than local organizations with a national or commercial
self-interest to consider. Standards organizations also do research and
development for standards of the future. In practice, the standards
organizations mentioned, cooperate closely with each other.
Multiple standards bodies may be involved in the development of a
protocol. If they are uncoordinated, then the result may be multiple,
incompatible definitions of a protocol, or multiple, incompatible
interpretations of messages; important invariants in one definition
(e.g., that time-to-live values are monotone decreasing to prevent stable routing loops) may not be respected in another.
The standardization process
In
the ISO, the standardization process starts off with the commissioning
of a sub-committee workgroup. The workgroup issues working drafts and
discussion documents to interested parties (including other standards
bodies) in order to provoke discussion and comments. This will generate a
lot of questions, much discussion and usually some disagreement. These
comments are taken into account and a draft proposal is produced by the working group. After feedback, modification, and compromise the proposal reaches the status of a draft international standard, and ultimately an international standard. International standards are reissued periodically to handle the deficiencies and reflect changing views on the subject.
OSI standardization
A lesson learned from ARPANET,
the predecessor of the Internet, was that protocols need a framework to
operate. It is therefore important to develop a general-purpose,
future-proof framework suitable for structured protocols (such as
layered protocols) and their standardization. This would prevent
protocol standards with overlapping functionality and would allow clear
definition of the responsibilities of a protocol at the different levels
(layers). This gave rise to the Open Systems Interconnection model
(OSI model), which is used as a framework for the design of standard
protocols and services conforming to the various layer specifications.
In the OSI model, communicating systems are assumed to be
connected by an underlying physical medium providing a basic
transmission mechanism. The layers above it are numbered. Each layer
provides service to the layer above it using the services of the layer
immediately below it. The top layer provides services to the application
process. The layers communicate with each other by means of an
interface, called a service access point. Corresponding layers at each system are called peer entities.
To communicate, two peer entities at a given layer use a protocol
specific to that layer which is implemented by using services of the
layer below.
For each layer, there are two types of standards: protocol standards
defining how peer entities at a given layer communicate, and service
standards defining how a given layer communicates with the layer above
it.
In the OSI model, the layers and their functionality are (from highest to lowest layer):
The Application layer
may provide the following services to the application processes:
identification of the intended communication partners, establishment of
the necessary authority to communicate, determination of availability
and authentication of the partners, agreement on privacy mechanisms for
the communication, agreement on responsibility for error recovery and
procedures for ensuring data integrity, synchronization between
cooperating application processes, identification of any constraints on
syntax (e.g. character sets and data structures), determination of cost
and acceptable quality of service, selection of the dialogue discipline,
including required logon and logoff procedures.
The presentation layer
may provide the following services to the application layer: a request
for the establishment of a session, data transfer, negotiation of the
syntax to be used between the application layers, any necessary syntax
transformations, formatting and special purpose transformations (e.g.,
data compression and data encryption).
The session layer
may provide the following services to the presentation layer:
establishment and release of session connections, normal and expedited
data exchange, a quarantine service which allows the sending
presentation entity to instruct the receiving session entity not to
release data to its presentation entity without permission, interaction
management so presentation entities can control whose turn it is to
perform certain control functions, resynchronization of a session
connection, reporting of unrecoverable exceptions to the presentation
entity.
The transport layer
provides reliable and transparent data transfer in a cost-effective way
as required by the selected quality of service. It may support the
multiplexing of several transport connections on to one network
connection or split one transport connection into several network
connections.
The network layer
does the setup, maintenance and release of network paths between
transport peer entities. When relays are needed, routing and relay
functions are provided by this layer. The quality of service is
negotiated between network and transport entities at the time the
connection is set up. This layer is also responsible for network congestion control.
The data link layer
does the setup, maintenance and release of data link connections.
Errors occurring in the physical layer are detected and may be
corrected. Errors are reported to the network layer. The exchange of
data link units (including flow control) is defined by this layer.
The physical layer
describes details like the electrical characteristics of the physical
connection, the transmission techniques used, and the setup, maintenance
and clearing of physical connections.
In contrast to the TCP/IP layering scheme, which assumes a connectionless network, RM/OSI assumed a connection-oriented network.
Connection-oriented networks are more suitable for wide area networks
and connectionless networks are more suitable for local area networks.
Connection-oriented communication requires some form of session and
(virtual) circuits, hence the (in the TCP/IP model lacking) session
layer. The constituent members of ISO were mostly concerned with wide
area networks, so the development of RM/OSI concentrated on
connection-oriented networks and connectionless networks were first
mentioned in an addendum to RM/OSI and later incorporated into an update to RM/OSI.
At the time, the IETF had to cope with this and the fact that the Internet needed protocols that simply were not there. As a result, the IETF developed its own standardization process based on "rough consensus and running code". The standardization process is described by RFC2026.
Nowadays, the IETF has become a standards organization for the
protocols in use on the Internet. RM/OSI has extended its model to
include connectionless services and because of this, both TCP and IP
could be developed into international standards.
The wire image of a protocol is the information that a
non-participant observer is able to glean from observing the protocol
messages, including both information explicitly given meaning by the
protocol, but also inferences made by the observer. Unencrypted protocol metadata is one source making up the wire image, and side-channels including packet timing also contribute. Different observers with different vantages may see different wire images.
The wire image is relevant to end-user privacy and the extensibility of the protocol.
If some portion of the wire image is not cryptographically authenticated, it is subject to modification by intermediate parties (i.e., middleboxes), which can influence protocol operation.
Even if authenticated, if a portion is not encrypted, it will form part
of the wire image, and intermediate parties may intervene depending on
its content (e.g., dropping packets with particular flags). Signals
deliberately intended for intermediary consumption may be left
authenticated but unencrypted.
The wire image can be deliberately engineered, encrypting parts
that intermediaries should not be able to observe and providing signals
for what they should be able to. If provided signals are decoupled from the protocol's operation, they may become untrustworthy.
Benign network management and research are affected by metadata
encryption; protocol designers must balance observability for
operability and research against ossification resistance and end-user
privacy.
The IETF announced in 2014 that it had determined that large-scale
surveillance of protocol operations is an attack due to the ability to
infer information from the wire image about users and their behaviour, and that the IETF would "work to mitigate pervasive monitoring" in its protocol designs; this had not been done systematically previously. The Internet Architecture Board recommended in 2023 that disclosure of information by a protocol to the network should be intentional, performed with the agreement of both recipient and sender, authenticated to the degree possible and necessary, only acted upon to the degree of its trustworthiness, and minimised and provided to a minimum number of entities.
Engineering the wire image and controlling what signals are provided to
network elements was a "developing field" in 2023, according to the
IAB.
Protocol ossification is the loss of flexibility, extensibility and evolvability of network protocols. This is largely due to middleboxes
that are sensitive to the wire image of the protocol, and which can
interrupt or interfere with messages that are valid but which the
middlebox does not correctly recognize. This is a violation of the end-to-end principle. Secondary causes include inflexibility in endpoint implementations of protocols.
Ossification is a major issue in Internet
protocol design and deployment, as it can prevent new protocols or
extensions from being deployed on the Internet, or place strictures on
the design of new protocols; new protocols may have to be encapsulated in an already-deployed protocol or mimic the wire image of another protocol. Because of ossification, the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are the only practical choices for transport protocols on the Internet, and TCP itself has significantly ossified, making extension or modification of the protocol difficult.
Recommended methods of preventing ossification include encrypting protocol metadata, and ensuring that extension points are exercised and wire image variability is exhibited as fully as possible; remedying existing ossification requires coordination across protocol participants. QUIC is the first IETF transport protocol to have been designed with deliberate anti-ossification properties.
Taxonomies
Classification schemes for protocols usually focus on the domain of use and function. As an example of domain of use, connection-oriented protocols and connectionless protocols are used on connection-oriented networks and connectionless networks respectively. An example of function is a tunneling protocol,
which is used to encapsulate packets in a high-level protocol so that
the packets can be passed across a transport system using the high-level
protocol.
A layering scheme
combines both function and domain of use. The dominant layering schemes
are the ones developed by the IETF and by ISO. Despite the fact that
the underlying assumptions of the layering schemes are different enough
to warrant distinguishing the two, it is a common practice to compare
the two by relating common protocols to the layers of the two schemes. The layering scheme from the IETF is called Internet layering or TCP/IP layering. The layering scheme from ISO is called the OSI model or ISO layering.
In networking equipment configuration, a term-of-art distinction is often drawn: The term protocol strictly refers to the transport layer, and the term service refers to protocols utilizing a protocol
for transport. In the common case of TCP and UDP, services are
distinguished by port numbers. Conformance to these port numbers is
voluntary, so in content inspection systems the term service strictly refers to port numbers, and the term application is often used to refer to protocols identified through inspection signatures.
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