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     INTERNET-DRAFT                                           Bernard Aboba
     <draft-aboba-rtp-http-02.txt>                    Microsoft Corporation
                    Payload Format for HTTP Encoding in RTP
     1.  Status of this Memo
     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF),  its  areas,  and
     its  working groups.  Note that other groups may also distribute work-
     ing documents as Internet-Drafts.
     Internet-Drafts are draft documents valid for a maximum of six  months
     and  may  be updated, replaced, or obsoleted by other documents at any
     time. It is inappropriate to use Internet-Drafts as reference material
     or to cite them other than as ``work in progress.''
     To  learn  the  current status of any Internet-Draft, please check the
     ``1id-abstracts.txt'' listing contained in the Internet-Drafts  Shadow
     Directories   on   ds.internic.net   (US  East  Coast),  nic.nordu.net
     (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     aboba-rtp-http-02.txt>, and  expires July 1, 1997.  Please  send  com-
     ments to the authors.
     2.  Abstract
     This  document  specifies  a  payload  format for use in encoding HTTP
     within RTP. This payload format can be used for unreliable  multicast-
     ing of resources such as Web pages, stock tickers, etc.  As with other
     RTP applications, receiver feedback and group  membership  information
     is  provided  via RTCP.  This specification is not expected to be used
     with unicast, since unicast applications will instead  use  HTTP  over
     3.  Introduction
     3.1.  Purpose
     Considerable  interest  has  recently  arisen  in  the multicasting of
     resources residing on HTTP servers.  Many of these uses can be  satis-
     fied by unreliable multicast transport.
     In  this  context,  unreliable  refers  to applications where a repair
     mechanism is not required. These are typically applications where  the
     material is of time value (stock tickers), so that it makes more sense
     to wait for the resource to be re-multicast than to attempt to  repair
     it;  applications  in  which  an  alternative  means  is available for
     retrieving the resource (cache filling); applications in  which  error
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     INTERNET-DRAFT                                          5 January 1997
     correction  is  performed  at  the  datalink layer; or applications in
     which a separate error correction stream is transmitted along with the
     data, typically on a separate group.
     In  a cache filling application, there is a relatively small probabil-
     ity that a particular missing resource will be hit, and so it is often
     more costly to request a repair than to leave an incompletely received
     resource in the cache, where it may never be  requested.   Cache  hits
     when they do occur, will typically be spread out over time, and there-
     fore not synchronized to the original transmission. As a result,  such
     applications present no danger of a NAK implosion.
     The  encoding specified in this document is not appropriate for use in
     applications where reliable transmission is required.   Such  applica-
     tions  present  the  possibility of a NAK implosion or congestive col-
     lapse, and so must be carefully analyzed prior to deployment.
     3.2.  Requirements
     Before discussing the proposed HTTP encoding in RTP, it is  useful  to
     describe  the  requirements  for  unreliable multicast transmission of
          Source differentiation
          Resource demultiplexing
          Receiver reporting
          Sender reporting
          Layered encoding
          Ability to synchronize with other media
          Low overhead
     3.2.1.  Source differentiation
     Since mixing is not useful for transmission of resources, and allowing
     multiple  sources would make it difficult to maintain rate control, it
     is likely that only one source will be sending to a group at one time.
     Nevertheless, in the case where there is a handoff, it is necessary to
     be able to differentiate sources, since packets from the  two  sources
     may be intermingled.
     3.2.2.  Resource demultiplexing
     For  multicast resource transmission, it is not desirable to have each
     resource transmitted with a unique source ID. Resources are  typically
     of small size, and therefore the overhead of obtaining a source ID and
     setting up the transmission would be excessive. As a result,  multiple
     resources will typically be transmitted with a single source ID. Since
     it is possible for packets from one resource  to  become  intermingled
     with  another due to out of order delivery, it is necessary to be able
     to demultiplex resources within a single source ID.
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     INTERNET-DRAFT                                          5 January 1997
     3.2.3.  Receiver reporting
     Although the encoding described in this document is to  be  used  only
     for unreliable transmission, receiver feedback may still be desirable.
     Such feedback can be used to estimate listenership, packet loss rates,
     and receiver bandwidth availability.
     Typically,  receiver reporting information will be used both for engi-
     neering purposes (diagnosis of transmission problems) as well  as  for
     business  purposes  (listenership information). While receiver reports
     could be useful in allowing senders to adjust transmission parameters,
     typically  it  is more desirable to allow receiver-driven rate adapta-
     tion via layered encoding.
     By transmitting a resource on several groups, each starting  transmis-
     sion  with a different offset, the receiver may adjust their reception
     rate based on the available bandwidth. Typically, the group  transmis-
     sion  rates will be tailored to commonly available bandwidths, i.e. 10
     Kbps for 14.4 Kbps modems, 20 Kbps for 28.8 Kbps, 30 Kbps  for  single
     channel ISDN, etc.
     Sender-driven  transmission  rate  adjustment  appears to be useful in
     only a limited number of circumstances.  In cases where a small  frac-
     tion  of  listeners  are  experiencing  problems, it is undesirable to
     adjust the transmission rate; instead, the affected  receivers  should
     adjust  their  rate  by  leaving the higher bandwidth groups.  If this
     does not work, they should stop listening to  the  transmission  alto-
     A  circumstance  in  which  sender-driven transmission rate adjustment
     appears useful is in the case where the majority of listeners are only
     subscribed  to  the lowest transmission rate group, yet appear to lack
     the bandwidth to also join an error correction  group  appropriate  to
     their  packet  loss rate.  In this case the sender should back off the
     transmission rate on the lowest group to allow for  successful  recep-
     tion of the error correction information.
     As there are applications in which receiver feedback may not be feasi-
     ble or desirable (satellite transmission), it must be possible to turn
     off the receiver reporting mechanism if desired.
     3.2.4.  Sender reporting
     Just  as  receivers  may  wish  to provide feedback to senders, so may
     senders wish to provide instructions to receivers.  This  may  include
     information  about  the particular resource being transmitted (such as
     the resource length, related keywords, or URL), or information on  the
     status  of  the transmission itself, such as the highest offset trans-
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     INTERNET-DRAFT                                          5 January 1997
     3.2.5.  Layered encoding
     Layered multicasting appears to be essential for  multicast  transmis-
     sion  of resources, since it provides for receiver-driven rate adapta-
     tion, as well as for  transmission  of  error-correction  information.
     While layered encoding  was originally proposed for use in audio/video
     transmission, it can also be applied to data carousel style  transmis-
     sions.   In  this  application,  each of the layers transmits the same
     resource, beginning at a different offset. Receivers  with  sufficient
     bandwidth  may  then  subscribe  to  multiple  groups, and receive the
     resource more quickly.
     Layered encoding may also provide for error  correction,  by  allowing
     error  correction  information  to  be transmitted on separate groups.
     Receivers may then subscribe to these groups according to their  aver-
     age packet loss rate; receivers experiencing high loss rates will typ-
     ically join a higher bandwidth error correction  group.  In  order  to
     allow  for  the additional bandwidth of an error correction group, the
     sender transmission rate should be set appropriately.
     3.2.6.  Ability to synchronize with other media
     While most uses of unreliable resource multicasting do not have  real-
     time  requirements, this may not be true of all such applications. For
     example, it may be desirable to synchronize display of a resource with
     an  audio/video  stream, as in a  presentation or lecture, and in such
     an application it may be desirable to include an encoded clock.
     3.2.7.  Low Overhead
     Since resource multicasting will typically use a small MTU size  (i.e.
     536 octets), it is important that a low overhead encapsulation be cho-
     sen. In order to achieve this, the GET request must  not  be  sent  in
     every  packet. In addition, it may be desirable to support header com-
     3.3.  Why RTP?
     Given that RTP was originally created for  use  in  realtime  applica-
     tions,  it is not entirely obvious that it is the appropriate protocol
     to use for resource multicasting.  However, given that  this  applica-
     tion  appears to require receiver and sender reporting, layered encod-
     ing, and source and resource de-multiplexing, it  is  likely  that  an
     alternative  framing  will  end up more closely  resembling RTP than a
     simple UDP-based approach.
     Where an alternative encoding would be most likely to differ would  be
     in  its  reporting  mechanisms.  For example while the basic header of
     RMFP, described in [9], bears considerable  resemblance  to  RTP,  the
     sender  and  receiver  packet  formats  differ considerably from RTCP.
     However, given the current state of knowledge, it is  far  from  clear
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     INTERNET-DRAFT                                          5 January 1997
     that  the  reporting  needs of unreliable resource multicasting differ
     substantially from those provided by RTCP.
     Thus while it is not apparent that RTP is the ideal protocol  for  use
     in  this application, it appears to meet the application requirements.
     In addition, the behavior of RTP is well understood, and the  protocol
     provides  for functions such as RTP monitoring and header compression.
     3.4.  Overview
     Multicasting of HTTP payloads is useful in a variety of  applications,
     and  as  a result, several approaches have been taken. One of these is
     to put a single resource within a payload; another is to pack multiple
     resources in a payload. If multiple resources are placed within a sin-
     gle payload, this can be accomplished either via encapsulation or  via
     a packing method such as MHTML, defined in [5].
     This  document  specifies  encapsulation of a single resource per pay-
     load.  As defined in this specification, the HTTP payload consists  of
     a preamble header, a MIME-like header, and entity-body content. Infor-
     mation on the resource being transmitted (such as the URI and the off-
     set)  is  placed  in  the  preamble  header  so  as to avoid requiring
     receivers to parse MIME-headers in the HTTP payload in order to deter-
     mine what portions of a resource have been received. As a result, this
     specification does not propose any new  MIME  headers,  and  any  MIME
     headers  allowable in an HTTP GET response may be enclosed in the pay-
     This simplifies construction of unicast-multicast proxies,  since  the
     MIME-like  header  in the payload can be identical to that returned in
     the response to an HTTP-GET request.  Proxies  may  therefore  make  a
     request  for  a  URL, stuff the response into a payload, and multicast
     it.  This is an efficient process since the proxy  does  not  need  to
     parse the response or construct an MHTML package.
     4.  RTP header
     Rather  than  defining  a new profile, this specification assumes that
     HTTP payloads defined in [8] will be transmitted using the RTP profile
     defined in [3], that is the RTP profile for audio and video conferenc-
     ing.  Additional required parameters are accommodated  via  definition
     of  an  HTTP  payload format. As a result, there is no need for header
     extensions, SDES private extensions, new  sender  or  receiver  report
     fields,  new  RTCP  packet types, or changes in the reporting interval
     Nevertheless, some clarifications are useful in  describing  how  HTTP
     payload encoding is to be used with the profile defined in [3].
     Aboba                                                         [Page 5]

     INTERNET-DRAFT                                          5 January 1997
     4.1.  Extension bit
     Although  this  specification  does  not preclude use of header exten-
     sions, it does not define any such extensions, and thus it is expected
     that the extension bit will typically be cleared.
     4.2.  CSRC count
     Since HTTP payloads do not require mixing, there is no need for a con-
     tributing source field. As a result, the CSRC count field  is  set  to
     4.3.  Marker Bit
     For  use with HTTP payloads, a zero marker bit is used to indicate the
     last packet for a resource. The first packet  is  distinguished  by  a
     zero offset, and as a result, does not need to be marked.
     4.4.  Payload types
     A  dynamic payload type in the range of 96-128 is used. The binding of
     payload type and application is accomplished by non-RTP means, such as
     use of the "m=" and "a=" fields of the session description:
     m=data 32768 RTP/AVP 98
     a=rtpmap: 98 MHTTP
     4.5.  Port numbers
     Although  there  is  no official policy on this, current practice dic-
     tates usage of port ranges as follows:
     [0, 16384]     - lowest priority, unclassified
     [16384, 32768] - highest priority, i.e. audio
     [32768, 49152] - medium priority, i.e. whiteboard
     [49152, 65536] - low priority, i.e. video
     It is recommended that unreliable multicast HTTP use the medium prior-
     ity port range.
     4.6.  SSRC
     Unlike  with  A/V  payloads, a sender may use a single synchronization
     source for transmission of multiple HTTP payload streams. Thus a  JPEG
     file  may be transmitted with the same SSRC as an HTML file. This does
     not present a problem since the timestamp is used to uniquely identify
     resource streams.
     Aboba                                                         [Page 6]

     INTERNET-DRAFT                                          5 January 1997
     4.7.  Timestamp
     Since a single synchronization source is used for transmission of mul-
     tiple resources, an additional parameter is needed to identify packets
     within  the  same  resource. The URI was not felt to be sufficient for
     this purpose, since a given resource may be multicast in data-carousel
     style, changing with each transmission.
     As  a  result,  the RTP timestamp field is used for this purpose.  For
     use with HTTP payloads, the timestamp is constant for all packets that
     form  a  single transmission of a resource, and represents the time at
     which the sender began to transmit the resource.
     Due to out of order delivery it is  possible  that  packets  from  one
     resource will be intermingled with packets from another resource, sent
     to the same group and port. In this case the combination of  the  SSRC
     and timestamp can be used to demultiplex the resource stream.
     5.  HTTP Payload format
     HTTP  payloads  consist  of a preamble header, a MIME-like header, and
     entity-body content.
     The purpose of the preamble header  is  identify  the  resource  being
     sent.   Depending  on  the  caching implementation, partially received
     resources will either be discarded, or will  be  appropriately  marked
     for  storage  in  the  cache. This will allow the cache to request the
     missing portion of the resource if a cache hit occurs.
     Since this specification is primarily intended for use  in  situations
     where  a  local  repair  may  not be timely (such as with a frequently
     updated stock ticker), where the resource  will  be  re-multicast,  or
     where error correction information is transmitted on a separate group,
     no repair mechanism is required.
     While MIME-like headers could be used for resource identification  (to
     provide information such as the URI, referring URI, content length and
     content range), prepending of a binary header reduces  the  amount  of
     parsing  required  to  get  at this critical information. The use of a
     preamble header has the additional benefit of not requiring  MIME-like
     headers to be present other than in the first packet. Thus, subsequent
     packets will typically contain only the preamble header and body  con-
     Thus,  rather  than  adding  MIME-like  headers,  it  is intended that
     senders will simply retransmit the MIME-like header  obtained  in  the
     HTTP GET response. Similarly, it is expected that the entity-body con-
     tent will be retransmitted without modification.
     Aboba                                                         [Page 7]

     INTERNET-DRAFT                                          5 January 1997
     5.1.  Preamble header
     The preamble header is comprised of an 8 octet fixed  portion,  and  a
     single variable-length string.
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
     |                                                               |
     |                      Offset (8 octets)                        |
     |                                                               |
     |                       GET request...
     |   GET request....  |0|
     5.1.1.  Offset
     The offset field is 64-bits long. For a data packet, it represents the
     octet offset within the resource  identified  by  the  RTP  timestamp.
     Although  the  offset  field  will  typically increase with increasing
     sequence numbers, this need not always be the case, since portions  of
     a resource may be transmitted out of order. Receivers should therefore
     be prepared to handle packet ordering based on the offset rather  than
     the sequence number.
     While  the  offset information could have also been provided using the
     Content-range: HTTP response header, this would have  required  inser-
     tion  of  a  MIME-like  header within each packet, and parsing of this
     header by the receiver. It is felt that the offset mechanism  is  more
     5.1.2.  GET request
     The  GET  request  field is a null-terminated string, representing the
     full text of the GET request that caused the resource to be  transmit-
     ted.   At a minimum, the GET request, defined in [8], includes the GET
     method, the URI and the HTTP version.  While it  begins  on  a  32-bit
     boundary, the GET request field is not padded to such a boundary.
     Given the expected size of the GET request field, this variable-length
     string is expected to comprise the majority of the encapsulation over-
     head.  Given  that  the string may be 40 octets or larger, when the 40
     octets of IP/UDP/RTP header are added, the result may be 80 octets  or
     more  of  overhead. This level of overhead would be unacceptable if it
     were present in every packet.
     The GET request, offset, and packet length uniquely identify the  por-
     tion of the resource being transmitted. Since receivers may join late,
     or miss portions of  the  transmission,  receivers  must  be  able  to
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     INTERNET-DRAFT                                          5 January 1997
     quickly  bind  a  timestamp  to  a  GET  request  so that the incoming
     resource and missing portions can be identified.
     However, it is believed that this goal can be accomplished by  placing
     the  GET  request  within  the first packet of a series, and then only
     within every subsequent nth packet.  This  results  in  a  substantial
     reduction  in  overhead. For the purposes of this specification, it is
     suggested that n=4.
     For packets in which the GET request is not included,  a  single  null
     octet is used to indicate the missing field.
     6.  Resource length
     The resource length is not included in the preamble header. Typically,
     this information is included within the first packet via the  Content-
     length:  header.  Although  it is possible that the first and/or final
     packet will be lost, we do not believe that the resource length justi-
     fies  inclusion  within the preamble header. This is because it is not
     necessary to know the total length of the resource to make  a  partial
     GET  request  for  the  remainder  of  the resource should a cache hit
     Note that it is possible that the final  packets  of  a  resource  are
     lost.  In  this  case,  the  receiver  will  note  that it has not yet
     received a packet with the marker bit  indicating  completion  of  the
     resource  transmission, after waiting a suitable interval after recep-
     tion of the last packet. After this interval has expired, the receiver
     will write the incomplete resource out to the disk.
     7.  Non-RTP means
     In  addition to information transmitted within the RTP encoding, it is
     expected that receivers will make use of session information transmit-
     ted by non-RTP means.
     For  example,  the  existence  of  a  data-carousel  style  session is
     expected to be determined via SDP, transmitted in one of its  encapsu-
     lations, such as SAP. The SDP announcement will provide information on
     the bandwidth allocated to the session, as well as the  group  address
     (or  in the case of a multi-group session, addresses) allocated to the
     The SDP announcement will also be expected  to  indicate  whether  the
     data  carousel  transmission  provides  for  re-multicast  of the same
     resource (in which case receivers can make partial  GET  requests  for
     the  missing  portion),  or  whether  it  provides continually updated
     information (in which case receivers missing a portion of the resource
     should make a GET request for the entire resource).
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     INTERNET-DRAFT                                          5 January 1997
     8.  Layered encoding
     Reference  [4]  discusses use of RTP in layered multicasting, and pro-
     poses guidelines for group address and port  allocation,  as  well  as
     modifications to RTP semantics suitable for allocation of SSRC identi-
     fiers across layered streams. SDES packets are then sent only  on  the
     lowest layer. It is expected that these modifications, once available,
     should be applicable to transmission of layered HTTP payloads.
     9.  Acknowledgements
     Thanks to Peter Parnes of the University of Lulea,  and  Jim  Gemmell,
     Paul  Leach and Thomas Pfenning of Microsoft for useful discussions of
     this problem space.
     10.  References
     [1]  R. Braden.  "Requirements for Internet hosts  -  application  and
     support." STD 3, RFC 1123, IETF, October 1989.
     [2]   H.  Schulzrinne,  S. Casner, R. Frederick, V. Jacobson.  "RTP: A
     Transport Protocol for Real-Time Applications." RFC 1889,  GMD  Fokus,
     January 1996.
     [3]   H.  Schulzrinne.   "RTP  Profile for Audio and Video Conferences
     with Minimal Control." RFC 1890, GMD Fokus, January 1996.
     [4]  M. F. Speer, S. McCanne.   "RTP  Usage  with  Layered  Multimedia
     Streams."   draft-speer-avt-layered-video-01.txt,   Sun  Microsystems,
     LBNL, June 1996.
     [5]  J. Palme, A. Hopmann.  "MIME E-mail  Encapsulation  of  Aggregate
     Documents,   such  as  HTML  (MHTML)."   draft-ietf-mhtml-spec-03.txt,
     Stockholm University/KTH, ResNova Software, August, 1996.
     [6] E. Levinson.  "The MIME  Multipart/Related  Content-type."  draft-
     ietf-mhtml-related-00.txt, Xison, May, 1996.
     [7]  J. Palme.  "Sending HTML in E-mail, an informational supplement."
     draft-ietf-mhtml-info-03.txt, Stockholm University/KTH, August,  1996.
     [8]   R.  Fielding,  et al.  "Hypertext Transfer Protocol - HTTP/1.1."
     draft-ietf-http-v11-spec-07, UC Irvine, August, 1996.
     [9] J. Crowcroft, Z. Wang, A. Ghosh, C. Diot.  "RMFP: A Reliable  Mul-
     ticast  Framing Protocol." draft-crowcroft-rmfp-00.txt, UCL, November,
     [10] P. Parnes.  "RTP  Extension  for  Scalable  Reliable  Multicast."
     draft-parnes-rtp-ext-srm-01.txt, LuTH/CDT, November, 1996.
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     INTERNET-DRAFT                                          5 January 1997
     11.  Authors' Addresses
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     Phone: 206-936-6605
     EMail: bernarda@microsoft.com
     Aboba                                                        [Page 11]

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