Editing Real-time Transport Protocol (section)

==Packet header==
RTP packets are created at the application layer and handed to the transport layer for delivery. Each unit of RTP media data created by an application begins with the RTP packet header.
{{APHD|start|title=RTP packet header}}
{{APHD|0|bits1=2|bits4=4|bits6=7|bits7=16|field1=Version|field2=P|field3=X|field4=CC|field5=M|field6=PT|field7=Sequence Number}}
{{APHD|4|bits1=32|field1=Timestamp}}
{{APHD|8|bits1=32|field1=SSRC Identifier}}
{{APHD|12|bits1=32|background1=linen|border1=bottom|field1=CSRC Identifier(s)}}
{{APHD|999|bits1=32|background1=linen|border1=top|field1=&vellip;}}
{{APHD|999|hoctets=12+4&times;CC|hbits=96+32&times;CC|bits1=16|bits2=16|background1=linen|background2=linen|field1=Profile-specific Extension Header ID|field2=Extension Header Length}}
{{APHD|999|hoctets=16+4&times;CC|hbits=128+32&times;CC|bits1=32|background1=linen|field1=Extension Data}}
{{APHD|999|bits1=32|background1=linen|border1=top|field1=&vellip;}}
{{APHD|end}}

The RTP header has a minimum size of 12 bytes. After the header, optional header extensions may be present. This is followed by the RTP payload, the format of which is determined by the particular class of application.<ref>{{harvnb|Peterson|Davie|2007|p=430}}</ref> The fields in the header are as follows:
;{{APHD|def|name=Version|length=2 bits|text=Indicates the version of the protocol. Current version is 2.<ref name="peterson_431">{{harvnb|Peterson|Davie|2007|p=431}}</ref>}}
;{{APHD|def|name=Padding|short=P|length=1 bit|text=Used to indicate if there are extra padding bytes at the end of the RTP packet. Padding may be used to fill up a block of certain size, for example as required by an encryption algorithm. The last byte of the padding contains the number of padding bytes that were added (including itself).{{Ref RFC|3550|rp=12}}<ref name="peterson_431"/>}}
;{{APHD|def|name=Extension|short=X|length=1 bit|text=Indicates presence of an ''Extension Header'' between the header and payload data. The extension header is application or profile specific.<ref name="peterson_431"/>}}
;{{APHD|def|name=CSRC Count|short=CC|length=4 bits|text=Contains the number of CSRC identifiers (defined below) that follow the SSRC (also defined below).{{Ref RFC|3550|rp=12}}}}
;{{APHD|def|name=Marker|short=M|length=1 bit|text=Signaling used at the application level in a profile-specific manner. If it is set, it means that the current data has some special relevance for the application.{{Ref RFC|3550|rp=13}}}}
;{{APHD|def|name=Payload Type|short=PT|length=7 bits|text=Indicates the format of the payload and thus determines its interpretation by the application. Values are profile specific and may be dynamically assigned.<ref>{{harvnb|Perkins|2003|p=59}}</ref>}}
;{{APHD|def|name=Sequence Number|length=16 bits|text=The sequence number is incremented for each RTP data packet sent and is to be used by the receiver to detect packet loss<ref name="Hardy_298"/> and to accommodate [[out-of-order delivery]]. The initial value of the sequence number should be randomized to make [[known-plaintext attack]]s on [[Secure Real-time Transport Protocol]] more difficult.{{Ref RFC|3550|rp=13}}}}
;{{APHD|def|name=Timestamp|length=32 bits|text=Used by the receiver to play back the received samples at appropriate time and interval. When several media streams are present, the timestamps may be independent in each stream.{{efn|{{IETF RFC|7273}} provides a means for signalling the relationship between media clocks of different streams.}} The granularity of the timing is application specific. For example, an audio application that samples data once every 125&nbsp;μs (8&nbsp;kHz, a common sample rate in digital telephony) would use that value as its clock resolution. Video streams typically use a 90&nbsp;kHz clock. The clock granularity is one of the details that is specified in the RTP profile for an application.<ref name="peterson_432">Peterson, p.[https://books.google.com/books?id=zGVVuO-6w3IC&pg=PA432 432]</ref>}}
;{{APHD|def|name=SSRC|length=32 bits|text=''Synchronization Source Identifier'' uniquely identifies the source of a stream. The synchronization sources within the same RTP session will be unique.{{Ref RFC|3550|rp=15}}}}
;{{APHD|def|name=CSRC|length=Variable (''CSRC Count'' &times; 32 bits)|text=''Contributing Source IDs'' enumerate contributing sources to a stream that has been generated from multiple sources.{{Ref RFC|3550|rp=15}}}}
;{{APHD|def|name=Header Extension|length=Variable|constraint=Exists when X=1|text=When ''Extension'' is true, this optional field contains:}}
:;{{APHD|def|name=Profile-specific Extension Header ID|length=16 bits|text=a profile-specific identifier}}
:;{{APHD|def|name=Extension Header Length|length=16 bits|text=indicates the length of the extension in 32-bit units, excluding the 32 bits of the extension header.}}
:;{{APHD|def|name=Extension Header Data|length=Variable|text=The data of the extension header.{{Ref RFC|3550|rp=18}}}}