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Real-time Transport Protocol

Real-time Transport Protocol. Kun-Ta Lee 2004.5.21 National Taipei University of Technology. Outline. Introduction Definitions RTP RTCP Translators and Mixers SSRC Identifier Allocation and Use Security Summary of Protocol Constants Reference. Introduction (1).

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Real-time Transport Protocol

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  1. Real-time Transport Protocol Kun-Ta Lee 2004.5.21 National Taipei University of Technology

  2. Outline • Introduction • Definitions • RTP • RTCP • Translators and Mixers • SSRC Identifier Allocation and Use • Security • Summary of Protocol Constants • Reference

  3. Introduction (1) • provides one-to-one or one-to-may delivery services for data whit real-time characteristics • does not guarantee delivery or prevent out-of-order delivery • receiver allow to reconstruct the sender’s packet sequence • RTP is primarily designed to satisfy the needs of multi-participant application • RTP control protocol (RTCP), to monitor the quality of service and to convey information about the participants in an on-going session

  4. Introduction (2) • RTP Use Scenarios • Simple Multicast Audio Conference • can change the encoding during a conference • allow the receivers to reconstruct the timing produced by the source • to know who is participating and how well they are receiving the audio data • sends the RTCP packet when it leaves the conference • Audio and Video Conference • no direct coupling at the RTP level between audio and video • participant can choose only one medium in the conference • using timing information carried in the RTCP packets can be achieved synchronization for audio and video session

  5. Introduction (3) • Mixers and Translators • Mixer (RTP-level relay): mixes reconstructed audio streams into a single stream and translates the audio encoding to a lower-bandwidth one and forwards he lower-bandwidth packet stream across the low-speed link. • Translator (RTP-level realy): translates different format Translator out-side Translator in-side Firewall

  6. Definitions (1) • RTP payload: the data transported by RTP in a packet • RTP packet: a data packet consisting of the fixed RTP header • RTCP packet: a control packet consisting of a fixed header part similar to that o RTP data packet, followed by structured elements that vary depending upon the RTCP packet type

  7. RTP RTCP Definitions (2) application media encapsulation UDP Source port + Dstination port + Message length + Checksum + UDP Data IPv4/IPv6 AAL5 Ethernet ATM

  8. Definitions (3) • Port: transport layer UDP port • Transport address: an IP address and a UDP port, packets are transmitted from a source transport address to a destination transport address • RTP Session • the session is defined by a particular pair of destination transport addresses (one network address plus a port pair for RTP and RTCP) • in a multimedia session, each medium is carried in a separate RTP session with its own RTCP packets. • Synchronization source (SSRC) • the source of s stream of RTP packets • 32-bit numeric • randomly chosen, unique • not use the same SSRC identifier for all the RTP sessions in a multimedia session • binding of the SSRC identifiers is provided through RTCP • Contributing source (CSRC): the source of a stream of RTP packets that has contributed to the combined stream produced by an RTP mixer

  9. Definitions (4) • End system: generates the content to be sent in RTP packets and/or consumes the content of received RTP packets • Mixer • receives RTP packet from one or more source • possibly changes the data format • combines the packets in some manner and forwards a new RTP packet • generate its own timing for the combined stream • new packets that from a mixer will be identified as having the mixer as their synchronization source • Translator • forwards RTP packets with their synchronization source identifier intact • convert encodings without mixing • replicate from multicast to unicast

  10. Definitions (5) end system SSRC=17 DVI4 192.35.149.52 GSM (Group Speciale Mobile) Mixer SSRC=5 SSRC=5 CSRC=1739 GSM translator 192.26.8.84 192.20.225.101 L16 end system SSRC=39 128.119.40.186

  11. Definitions (6) • Monitor • An application, that receives RTCP packets (reception report) sent by participants in an RTP session • estimates the current quality of service, fault diagnosis and long-term statistics • Non-RTP means • in addition to RTP to provide a usable service • example: negotiate the encryption algorithm, define dynamic mappings between RTP payload type values and payload formats they represent for formats that do not have a predefined payload type value • Simple applications, electronic mail or a conference database may also be used

  12. 24 0 8 16 32 RTP Packet (1) • first twelve octets are present in every RTP hacket • the list of CSRC identifiers is present only when inserted by a mix X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  13. 24 0 8 16 32 RTP Packet (2) • version (V): 2 bits • V=0 → “avt” audio tool. ; V=1 → first draft version of RTP • padding (P): 1 bit • the last octet of the padding contains a count of how many padding octets should be ignored X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  14. 24 0 8 16 32 RTP Packet (3) • extension (X): 1 bit • if the extension bit is set, the fixed header is followed by exactly one header extension X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  15. 24 0 8 16 32 RTP Packet (4) • CSRC count (CC): 4 bits • the CSRC count contains the number of CSRC identifiers that follow the fix header • Maker (M): 1 bit • intended to allow significant events such as frame boundaries to be marked in the packet stream X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  16. 24 0 8 16 32 RTP Packet (5) • payload type (PT): 7 bits • identifies the format of the RTP payload and determines its interpretation by the application • Default mappings for audio and video is specified in the companion profile Internet-Draft draft-ietf-avt-profile X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  17. 24 0 8 16 32 RTP Packet (6) • sequence number : 16 bits • increments by one for each RTP data packet sent • used by the receiver to detect packet loss and to restore packet sequence X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  18. 24 0 8 16 32 RTP Packet (7) • timestamp : 32 bits • timestamp reflects the sampling instant of the first octet in the RTP data packet X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  19. 24 0 8 16 32 RTP Packet (8) • SSRC : 32 bits • identifies the synchronization source • this identifier is chosen randomly • all RTP implementations must be prepared to detect and resolve collisions X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  20. 24 0 8 16 32 RTP Packet (9) • CSRC list: 0 to 15 itemse, 32 bits each • the CSRC list identifies the contributing source for the payload contained in this packet • if there are more than 15 contributing sources, only 15 may be identified • inserted by mixer X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. opt. header extension payload (audio, video, …) opt. 0x00

  21. 24 0 8 16 32 RTP Packet (10) • header extension • the first 16 bits of header extension are left open for distinguishing identifiers or parameters • 16-bit length field that counts the number of 32-bit words in the extension X CSRC count M payload type sequence number V2 P timestamp synchronization source identifier (SSRC) UDP packet contributing source identifier (CSRC) opt. defined by profile length opt. header extension payload (audio, video, …) opt. 0x00

  22. RTCP Functions • provide feedback on the quality of the data distribution • this feedback function is performed by the RTCP sender and receiver reports • RTCP carries a persistent transport-level identifier for an RTP source called the canonical name or CNAME • SSRC identifier may change if a conflict is discovered or a program is restarted • receivers require the CNAME to keep track of each participant • receivers require the CNAME to associate multiple data stream for a given participant in a set of related RTP sessions • each participant send its control packets to all the others, each can independently observe the number of participants • convey minimal session control information • for example participant identification to be displayed in the user interface

  23. RTCP Packet Format • SR: sender report, for transmission and reception statistics from participants that are active senders • RR: receiver report, for reception statistics from participants that are not active senders • SDES: source description items, including CNAME • BYE: indicates end of participation • APP: application specific functions

  24. 24 0 8 16 32 RTCP Packet - SR(first section - header) • version (V): 2 bits • V=0 → “avt” audio tool. ; V=1 → first draft version of RTP header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  25. 24 0 8 16 32 RTCP Packet - SR (first section - header) • padding (P): 1 bit • the last octet of the padding contains a count of how many padding octets should be ignored • In a compound RTCP packet, padding should only be required on the last individual packet header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  26. 24 0 8 16 32 RTCP Packet - SR (first section - header) • reception report count (RC): 5 bits • the number of reception report blocks contained in this packet header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  27. 24 0 8 16 32 RTCP Packet - SR (first section - header) • packet type (PT): 8 bits • the constant 200 to identify this as an SR packet header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  28. 24 0 8 16 32 RTCP Packet - SR (first section - header) • length: 16 bits • including the header and any padding • 1 unit = 32-bit header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  29. 24 0 8 16 32 RTCP Packet - SR (first section - header) • SSRC: 32 bits • synchronization source identifier for the originator of this SR packet header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  30. 24 0 8 16 32 RTCP Packet - SR (second section – sender info) • NTP timestamp: 64 bits • indicates the wallclock time when this report was sent header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  31. 24 0 8 16 32 RTCP Packet - SR (second section – sender info) • RTP timestamp: 32 bits header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  32. 24 0 8 16 32 RTCP Packet - SR (second section – sender info) • sender’s packer count: 32 bits • total number of RTP data packets transmitted by the sender since starting transmission up until the time this SR packet was generated header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  33. 24 0 8 16 32 RTCP Packet - SR (second section – sender info) • sender’s octet count: 32 bits • total number of payload octets (not including header or padding) transmitted in RTP data packets by the sender since starting transmission up until the time this SR packet was generated header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of first source) Report Block 2 ... profile-specific extensions

  34. RTCP Packet - SR(third section – reception report blocks) • contains 0 ~ more reception report blocks depending on the number of other source heard by this sender since the last report • conveys statistics on the reception of RTP packets from a single synchronization source • receivers do not carry over statistics when a source changes its SSRC identifier due to a collision

  35. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • SSRC_n (source identifier): 32 bits • the source’s SSRC to which the information in this reception report block pertains header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  36. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • fraction lost: 8 bits • the fraction of RTP data packets from source SSRC_n lost since the previous SR or RR packet was sent header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  37. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • cumulative number of packers lost: 24 bits • the total number of RTP data packets from source SSRC_n that have been lost since the beginning of reception header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  38. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • extended highest sequence number received: 32 bits • the low 16 bits contain highest sequence number received in an RTP data packer from source SSRC_n • the most significant 16 bits extend that sequence number with the corresponding count of sequence number cycles header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  39. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • Interarrival jitter: 32 bits • an estimate of the statistical variance of the RTP data packer interarrival time header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  40. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • Last SR timestamp (LSR): 32 bits • the middle 32 bits out of 64 in the NTP timestamp received as part of the most recent RTCP sender report packer form source SSRC_n • if no SR has been received yet, the field is set to zero header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  41. 24 0 8 16 32 RTCP Packet - SR(third section – reception report blocks) • Delay since last SR (DLSR): 32 bits • the delay, between receiving the last SR packer form source SSRC_N and sending this reception report block • if no SR has been received yet from SSRC_n, the field is set to zero header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  42. 24 0 8 16 32 RTCP Packet - SR(fourth section – extensions blocks) • profile-specific extensions • if there is additional information that should be reported regularly about the sender or receivers header RC PT=SR=200 length V2 P SSRC of sender NTP timestamp, most significant word NTP timestamp, least significant word sender info RTP tiemstamp sender’s packet count sender’s octet count SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  43. 24 0 8 16 32 RTCP Packet - RR • is the same as that of the SR packet except that the packer type field contains the constant 201 and the five words of sender information are omitted header RC PT=RR=201 length V2 P SSRC of sender SSRC_1 (SSRC of first source) fraction lost cumulative number of packets lost Report Block 1 extended highest sequence number received interarrival jitter last SR (LSR) Delay since last SR(DLSR) SSRC_2 (SSRC of second source) Report Block 2 ... profile-specific extensions

  44. 24 0 8 16 32 RTCP Packet – Source Description • version (V), padding (P), length: • as described for the SR packet • packet type (PT): 8 bits • contains the constant 202 to identify this as an RTCP SDES packet • source count (SC): 5 bits • the number of SSRC/CSRC chunks contained in this SDES packet header SC PT=SDES=202 length V2 P SSRC/CSRC_1 chunk 1 SDES items … SSRC/CSRC_2 chunk 2 SDES items …

  45. RTCP Packet – Source Description • some SDES itmes • CNAME • Name • Email • Phone • Loc • Tool • Note

  46. 24 0 8 16 32 RTCP Packet – Source Description item • type field: 8 bits • CNAME=1 • length field: 8 bits • not include type field and length field • user and domain name field: be no longer than 255 octets • examples: doe@sleepy.megacorp.com or doe@192.0.2.89 for a multi-user system • examples: sleepy.megacorp.com or 192.0.2.89 for the system with no user name CNAME=1 length User and domain name …

  47. 24 0 8 16 32 RTCP Packet – Source Description item • type field: 8 bits • NAME=2 • length field: 8 bits • not include type field and length field • common name of source field: be no longer than 255 octets • it may be any form desired by the user • examples: John Doe, Recycler, Megacorp NAME=2 length common name of source …

  48. 24 0 8 16 32 RTCP Packet – Source Description item • type field: 8 bits • EMAIL=3 • length field: 8 bits • not include type field and length field • email address of source field: be no longer than 255 octets • Email address is formatted according to RFC 822 • example, “ John.Doe@megacorp.com” EMAIL=3 length email address of source …

  49. 24 0 8 16 32 RTCP Packet – Source Description item • type field: 8 bits • PHONE=4 • length field: 8 bits • not include type field and length field • phone number of source field: be no longer than 255 octets • phone number PHONE=4 length Phone number of source …

  50. 24 0 8 16 32 RTCP Packet – Source Description item • type field: 8 bits • LOC=5 • length field: 8 bits • not include type field and length field • geographic location of site field: be no longer than 255 octets LOC=5 length geographic location of site

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