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The Most Common Message Body. ▫ Be session information describing the media to be exchanged between the parties. ▫ SDP, RFC (initial publication). atomcat, application/atomcat+xml, Atom Publishing Protocol, RFC atomsvc raudone.info2, application/raudone.info2+xml, OMA Download Agents, IANA: OMA Download Agents .dtb, application/x-dtbook+xml, Digital Talking Book, Wikipedia: EPUB sdp, application/sdp, Session Description Protocol, RFC IETF RFC – Replaces RFC • “The Session Initiation Protocol (SIP) is an . np Session Description Protocol. • IETF RFC
Microbial communities associated with surfaces, such as biofilms or microbial mats, are typically limited by the rate of transport of chemical substrates in the direction perpendicular to the surface, either through the diffusive boundary layer above the surface or through the community itself This mass transport limitation plays a fundamental role in the way that the community as a whole functions and is spatially organized.
As revealed by many confocal microscopy studies, the microscopic spatial organization of cells in such communities is complex and diverse; for example, migrating or nonmotile cells are loosely associated with each other or form tight multicellular consortia 13 , 34 , 49 , However, many microbial communities also exhibit distinctive spatial organization on a mesoscopic level, where the community structure changes from one dominant species or group to another dominant species or group over distances many times greater than a typical cell size.
This patchiness occurs in both vertical and horizontal directions. An illustrative example is the pronounced vertical layering in photosynthetic microbial mats with, e. The spatial scale on which this structural heterogeneity occurs depends on the cell density and activity, but it most crucially depends on the fluxes of available substrates that supply the community with energy, electron donors and acceptors, and nutrients.
In photosynthetic mats, distinct layers can be several tens or hundreds of micrometers 37 , 45 to several decimeters 78 thick, which is the scale at which the relevant substrates on which a particular community member or group depends e. To quantify this zonation or, in general, the three-dimensional patchiness of a microbial community on a mesoscale, the community is typically sectioned horizontally, and the pigments are extracted and quantified by high-performance liquid chromatography HPLC.
Additionally, spectroscopic measurements to quantify pigments in thin sections have also been described 4 , 59 , 60 , Although useful insights into the community structure have been obtained by using this sample-sectioning approach, the major disadvantages of the technique are that samples are destroyed, the native spatial organization of the microbial community is disturbed during fixation and preparation, and the spatial resolution is limited.
Less invasive, higher-resolution techniques are necessary to adequately describe small-scale heterogeneity in microbial communities.
Spectral imaging systems with a spatial resolution that could be adjusted to the relevant scale would clearly be a valuable alternative. This has been achieved to some extent with confocal laser scanning microscopy 34 , 50 , 55 , but such studies require hardware that is expensive and not portable.
The aim of the present work was to develop a measuring system that allows spectral imaging on spatial scales ranging from a few tens of micrometers to several tens of centimeters and thus facilitates minimally invasive identification and localization of pigments from the single-cell level up to the microbial community level.
The main requirements for the system were portability, flexibility, simplicity of use, and reliance on commercially available and relatively low-cost components. Below we provide a detailed description of our system's hardware and software and explain typical measurement procedures. We also suggest and describe, using examples, a number of applications for the system in microbial ecology.
Attributes may be defined to be used as "session-level" attributes, "media-level" attributes, or both. These are referred to as "media-level" attributes and add information about the media stream. Attribute fields can also be added before the first media field; these "session-level" attributes convey additional information that applies to the conference as a whole rather than to individual media; an example might be the conference's floor control policy.
Attribute fields may be of two forms: o property attributes. These are binary attributes, and the presence of the attribute conveys that the attribute is a property of the session. Thus receivers of session descriptions should be configurable in their interpretation of announcements in general and of attributes in particular.
However, when an attribute is defined, it can be defined to be charset-dependent, in which case it's value should be interpreted in the session charset rather than in ISO Unregistered attributes should begin with "X-" to prevent inadvertent collision with registered attributes. In either case, if an attribute is received that is not understood, it should simply be ignored by the receiver.
A media field also has several sub-fields: o The first sub-field is the media type. Currently defined media are "audio", "video", "application", "data" and "control", though this list may be extended as new communication modalities emerge e. The difference between "application" and "data" is that the former is a media flow such as whiteboard information, and the latter is bulk-data transfer such as multicasting of program executables which will not typically be displayed to the user.
The meaning of the transport port depends on the network being used as specified in the relevant "c" field and on the transport protocol defined in the third sub-field. Other ports used by the media application such as the RTCP port, see [ 2 ] should be derived algorithmically from the base media port. Note: For transports based on UDP, the value should be in the range to inclusive.
For RTP compliance it should be an even number. If a transport protocol is used over UDP to carry several distinct media types that need to be distinguished by a session directory, then specifying the transport protocol and media format separately is necessary.
RTP is an example of a transport-protocol that carries multiple payload formats that must be distinguished by the session directory for it to know how to start appropriate tools, relays, mixers or recorders. In addition, relays and monitoring tools that are transport-protocol-specific but format-independent are possible. Should other RTP profiles be defined in the future, their profiles will be specified in the same way.
When a list of payload formats is given, this implies that all of these formats may be used in the session, but the first of these formats is the default format for the session. Such formats should be defined in an additional specification document.
The encoding names in the RTP AV Profile do not specify unique audio encodings in terms of clock rate and number of audio channels , and so they are not used directly in SDP format fields.
Instead, the payload type number should be used to specify the format for static payload types and the payload type number along with additional encoding information should be used for dynamically allocated payload types.
This parameter may be omitted if the number of channels is one provided no additional parameters are needed. For video streams, no encoding parameters are currently specified.
Additional parameters may be defined in the future, but codecspecific parameters should not be added.
Parameters added to an rtpmap attribute should only be those required for a session directory to make the choice of appropriate media too to participate in a session. Codec-specific parameters should be added in other attributes. Up to one rtpmap attribute can be defined for each media format specified. Experimental encoding formats can also be specified using rtpmap. RTP formats that are not registered as standard format names must be preceded by "X-".
Note that RTP audio formats typically do not include information about the number of samples per packet. Since application writers may add new attributes as they are required, this list is not exhaustive. This is to enable a receiver to filter unwanted sessions by category.
It would probably have been a compulsory separate field, except for its experimental nature at this time. It is a session-level attribute, and is not dependent on charset.
This allows a receiver to select interesting session based on keywords describing the purpose of the session. It is a session-level attribute. This is probably only meaningful for audio data. It is a media attribute, and is not dependent on charset. It can be either a session or media attribute, and is not dependent on charset. This is necessary for interactive conferences with tools such as wb which defaults to receive only mode.
An example may be where a different unicast address is to be used for a traffic destination than for a traffic source. In such a case, two media descriptions may be use, one sendonly and one recvonly. It can be either a session or media attribute, but would normally only be used as a media attribute, and is not dependent on charset.
It specifies the orientation of a the whiteboard on the screen. It is a media attribute. Specifying the attribute type:H indicates that this loosely coupled session is part of a H. Specifying the attribute type:test is suggested as a hint that, unless explicitly requested otherwise, receivers can safely avoid displaying this session description to users.
The type attribute is a session-level attribute, and is not dependent on charset. If a more compact representation is required, other character sets may be used such as ISO for Northern European languages.
If the identifier is not recognised or not supported, all strings that are affected by it SHOULD be regarded as byte strings. Character sets requiring the use of these characters MUST define a quoting mechanism that prevents these bytes appearing within text fields. As a session level attribute, it specifies the language for the session description. As a media level attribute, it specifies the language for any media-level SDP information field associated with that media.
Multiple sdplang attributes can be provided either at session or media level if multiple languages in the session description or media use multiple languages, in which case the order of the attributes indicates the order of importance of the various languages in the session or media from most important to least important. In general, sending session descriptions consisting of multiple languages should be discouraged.
Instead, multiple descriptions should be sent describing the session, one in each language. However this is not possible with all transport mechanisms, and so multiple sdplang attributes are allowed although not recommended.
It is not dependent on the charset attribute. As a session level attribute, it specifies the default language for the session being described. As a media level attribute, it specifies the language for that media, overriding any session- level language specified. Multiple lang attributes can be provided either at session or media level if multiple languages if the session description or media use multiple languages, in which case the order of the attributes indicates the order of importance of the various languages in the session or media from most important to least important.
A lang attribute SHOULD be specified when a session is of sufficient scope to cross geographic boundaries where the language of recipients cannot be assumed, or where the session is in a different language from the locally assumed norm.
It is intended as a recommendation for the encoding of video data. It is a media attribute, is only defined for video media, and is not dependent on charset.
The intention of the quality attribute for video is to specify a non-default trade-off between frame-rate and still-image quality. For video, the value in the range 0 to 10, with the following suggested meaning: 10 - the best still-image quality the compression scheme can give. The format must be one of the formats specified for the media. Format-specific parameters may be any set of parameters required to be conveyed by SDP and given unchanged to the media tool that will use this format.
Communicating Conference Control Policy There is some debate over the way conference control policy should be communicated. In general, the authors believe that an implicit declarative style of specifying conference control is desirable where possible.
IP Multicast: PIM Configuration Guide, Cisco IOS Release 15S
This conference attribute conveys the conference control policy. If this is the case, then a conference attribute specifying external control might be set, and then one or more "media" fields might be used to specify the conference control tools and configuration data for those tools.
An example is an ITU H. In this document, only the declarative style of conference control declaration is specified. Other forms of conference control should specify an appropriate type attribute, and should define the implications this has for control media. Security Considerations SDP is a session description format that describes multimedia sessions.
A session description should not be trusted unless it has been obtained by an authenticated transport protocol from a trusted source. Many different transport protocols may be used to distribute session description, and the nature of the authentication will differ from transport to transport. One transport that will frequently be used to distribute session descriptions is the Session Announcement Protocol SAP. SAP provides both encryption and authentication mechanisms but due to the nature of session announcements it is likely that there are many occasions where the originator of a session announcement cannot be authenticated because they are previously unknown to the receiver of the announcement and because no common public key infrastructure is available.
On receiving a session description over an unauthenticated transport mechanism or from an untrusted party, software parsing the session should take a few precautions.
Session description contain information required to start software on the receivers system. Software that parses a session description MUST not be able to start other software except that which is specifically configured as appropriate software to participate in multimedia sessions.
As it is not always simple to tell whether a session is interactive or not, applications that are unsure should assume sessions are interactive. In this specification, there are no attributes which would allow the recipient of a session description to be informed to start multimedia tools in a mode where they default to transmitting.
Under some circumstances it might be appropriate to define such attributes.
MIME Types List
If this is done an application parsing a session description containing such attributes SHOULD either ignore them, or inform the user that joining this session will result in the automatic transmission of multimedia data. The default behaviour for an unknown attribute is to ignore it. For multicast sessions, it is likely that local administrators will apply their own policies, but the exclusive use of "local" or "site- local" administrative scope within the firewall and the refusal of the firewall to open a hole for such scopes will provide separation of global multicast sessions from local ones.
Using the terminology in the SDP specification BNF, they are "media", "proto", "fmt", "att-field", "bwtype", "nettype" and "addrtype".
The list of valid media names is the set of top-level MIME content types. The set of media is intended to be small and not to be extended except under rare circumstances.
The MIME subtype corresponds to the "fmt" parameter below. However, people will want to invent their own proprietary transport protocols. Some of these should be registered as a "fmt" using "udp" as the protocol and some of which probably can't be.
Where the protocol and the application are intimately linked, such as with the LBL whiteboard wb which used a proprietary and special purpose protocol over UDP, the protocol name should be "udp" and the format name that should be registered is "wb".
The rules for formats see below apply to such registrations.
Where the proprietary transport protocol really carries many different data formats, it is possible to register a new protocol name with IANA. Formats cover all the possible encodings that might want to be transported in a multimedia session. For RTP formats using a dynamic payload type number, the dynamic payload type number is given as the format and an additional "rtpmap" attribute specifies the format and parameters.
If there is an existing MIME type for this format, the MIME registration should be augmented to reference the transport specification for this media type. If there is not an existing MIME type for this format, and there exists no appropriate file format, this should be noted in the encoding considerations as "no appropriate file format".
When an attribute is registered, it must be accompanied by a brief specification stating the following: o contact name, email address and telephone number o attribute-name as it will appear in SDP o long-form attribute name in English o type of attribute session level, media level, or both o whether the attribute value is subject to the charset attribute. IANA will not sanity check such attribute registrations except to ensure that they do not clash with existing registrations.
Submitters of registrations should ensure that the specification is in the spirit of SDP attributes, most notably that the attribute is platform independent in the sense that it makes no implicit assumptions about operating systems and does not name specific pieces of software in a manner that might inhibit interoperability. New bandwidth specifiers may be registered with IANA. The submission MUST reference a standards-track RFC specifying the semantics of the bandwidth specifier precisely, and indicating when it should be used, and why the existing registered bandwidth specifiers do not suffice.
Whilst these are not normally the preserve of IANA, there may be circumstances when an Internet application needs to interoperate with a non- internet application, such as when gatewaying an internet telephony call into the PSTN.
The number of network types should be small and should be rarely extended. A new network type cannot be registered without registering at least one address type to be used with that network type.
A new network type registration MUST reference an RFC which gives details of the network type and address type and specifies how and when they would be used. An address type is only meaningful in the context of a network type, and any registration of an address type MUST specify a registered network type, or be submitted along with a network type registration.
Address types are not expected to be registered frequently. The registration itself should be sent to IANA. Attribute registrations should include the information given above. Other registrations should include the following additional information: o contact name, email address and telephone number o name being registered as it will appear in SDP o long-form name in English o type of name "media", "proto", "fmt", "bwtype", "nettype", or "addrtype" o a one paragraph explanation of the purpose of the registered name.
References [ 1 ] Mills, D.SAP Version 2 uses the well-known session directory multicast group This ensures that a figure containing a single image is parsed as a pandoc "implicit figure" i. For RTP compliance it should be an even number. Improve workaround for tables following headings. However, people will want to invent their own proprietary transport protocols.
The intention of the quality attribute for video is to specify a non-default trade-off between frame-rate and still-image quality. Internationalization only applies to free-text fields session name and background information , and not to SDP as a whole. It is a means to communicate the existence of a session, and is a means to convey sufficient information to enable joining and participating in the session.