Network Working Group Magnus Westerlund INTERNET-DRAFT Ericsson Category: Standards Track Expires: July 2003 A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP). Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract The existing Session Description Protocol (SDP) bandwidth modifiers and their values include the bandwidth needed also for the transport and IP layers. When using SDP in protocols like Session Announcement Protocol (SAP), Session Initiation Protocol (SIP) and Real-Time Streaming Protocol (RTSP) and the involved hosts reside in networks running different IP versions, the interpretation of what type of lower layers that is included is not clear. This documents defines a bandwidth modifier that does not include transport overhead, instead Westerlund [Page 1] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 an additional packet rate attribute is defined. The transport independent bit-rate value together with the packet rate can then be used to calculate the real bit-rate over the actually used transport. TABLE OF CONTENTS 1. Definitions.........................................................3 1.1. Glossary.......................................................3 1.2. Terminology....................................................3 2. Changes.............................................................3 3. Introduction........................................................3 3.1. The Bandwidth Attribute........................................3 3.1.1. Conference Total..........................................4 3.1.2. Application Specific Maximum..............................4 3.1.3. RTCP Report bandwidth.....................................4 3.2. IPv6 and IPv4..................................................4 4. The Bandwidth Signaling Problems....................................5 4.1. What IP version is used........................................5 4.2. Converting bandwidth values....................................6 4.3. Header Compression.............................................6 4.4. RTCP problems..................................................7 4.5. Future development.............................................7 5. Problem Scope.......................................................8 6. Requirements........................................................8 7. A Solution..........................................................8 7.1. Introduction...................................................8 7.2. The TIAS bandwidth modifier....................................8 7.2.1. Usage.....................................................8 7.2.2. Definition................................................9 7.2.3. Usage Rules..............................................10 7.3. Packet Rate parameters........................................10 7.4. Converting to Transport Dependent values......................11 7.5. Deriving RTCP bandwidth.......................................11 7.5.1. Motivation to being an acceptable solution...............11 7.6. ABNF definitions..............................................12 8. Protocol Interaction...............................................12 8.1. RTSP..........................................................12 8.2. SIP...........................................................12 8.3. SAP...........................................................13 9. Security Consideration.............................................13 10. IANA Consideration................................................13 11. Acknowledgments...................................................13 12. Author's Addresses................................................13 13. References........................................................14 13.1. Normative references.........................................14 13.2. Informative References.......................................14 14. IPR Notice........................................................15 15. Copyright Notice..................................................16 Westerlund Standards Track [Page 2] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 1. Definitions 1.1. Glossary RTSP - Real-Time Streaming Protocol, see [7]. SDP - Session Description Protocol, see [5]. SAP - Session Announcement Protocol, see [4]. SIP - Session Initiation Protocol, see [5]. TIAS - Transport Independent Application Specific maximum, a bandwidth modifier. 1.2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3]. 2. Changes [Note to RFC Editor: Remove this section when publishing] The following changes has been done to this version compared to draft-westerlund-mmusic-sdp-bwparam-01.txt: - Changed it to a working group document. - Removed "avgprate" as it is not useful. - Defined the attribute for session level. - Simplified bit-rate deriving for RTCP and explained why it does not cause serious problems. 3. Introduction Today the Session Description Protocol (SDP) [1] is used in several types of applications. The original application is session information and configuration for multicast sessions announced with Session Announcement Protocol (SAP) [4]. SDP is also a vital component in media negotiation for the Session Initiation Protocol (SIP) [5] by using the offer answer model [6]. The Real-Time Streaming Protocol (RTSP) [7] also makes use of SDP to declare what media and codec(s) a multi-media presentation consist of to the client. 3.1. The Bandwidth Attribute In SDP [1] there exist a bandwidth attribute, which has a modifier used to specify what type of bit-rate the value refers to. The attribute has the following form: b=: Westerlund Standards Track [Page 3] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 Today there are four modifiers defined which are used for different purposes. 3.1.1. Conference Total The Conference Total is indicated by giving the modifier "CT". The meaning of Conference total is to give a maximum bandwidth that a conference session will use. Its purpose is to decide if this session can co-exist with any other sessions. Defined in RFC 2327 [1]. 3.1.2. Application Specific Maximum The Application Specific maximum bandwidth is indicated by the modifier "AS". The interpretation of this attribute is depending on the application's notion of maximum bandwidth. For an RTP application this attribute is the RTP session bandwidth as defined in RFC 1889 [8]. The session bandwidth includes the bandwidth that the RTP data traffic will result in, including the lower layers down to IP layer. So the bandwidth is in most cases calculated over RTP payload, RTP header, UDP and IP. Defined in RFC 2327 [1]. 3.1.3. RTCP Report bandwidth Today there is a draft [9], currently in the RFC editors queue to become a Proposed Standard, which defines two new bandwidth modifiers. These modifiers "RS" and "RR", define the amount of bandwidth that is assigned for RTCP reports by active data senders respectively RTCP reports by receivers only. 3.2. IPv6 and IPv4 Today there are two IP versions 4 [15] and 6 [14] used in parallel on the Internet. This creates problems and there exist a number of possible transition mechanisms. ------------------ ---------------------- | IPv4 domain | | IPv6 Domain | | | ---------| | | | ---------- |-| NAT-PT |-| ---------- | | |Server A| | ---------| | |Client B| | | ---------- | | ---------- | ------------------ ---------------------- Figure 1. Translation between IPv6 and IPv4 addresses. - To achieve connectivity between an IPv4 only host and an IPv6 only host, translation is necessary. This translator can be for example Network Address Translation - Protocol Translation (NAT- PT) [13], see Figure 1. However to get connectivity for large number of protocols, Application Level Gateway (ALG) functionality is also required at the node. To be able to locate hosts through Westerlund Standards Track [Page 4] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 the translation node DNS ALG must be supported. - IPv6 nodes belonging to different domains running IPv6, but lacking IPv6 connectivity between them solves this by tunneling over the IPv4 net, see Figure 2. Basically the IPv6 packets are put as payload in IPv4 packets between the tunneling end-points at the edge of each IPv6 domain. --------------- --------------- --------------- | IPv6 domain | | IPv4 domain | | IPv6 Domain | | | |-------------| | | | ---------- |--||Tunnel ||--| ---------- | | |Server A| | |-------------| | |Client B| | | ---------- | | | | ---------- | --------------- --------------- --------------| Figure 2. Tunneling through a IPv4 domain IPv4 has minimal header size of 20 bytes. While the fixed part of the IPv6 header is 40 bytes. The difference in header sizes, results in that the bit-rate required for a certain IP layer is different. How big the difference is, depends on the packet rate and size difference of each packet. 4. The Bandwidth Signaling Problems When an application wants to use SDP to signal the bandwidth required for this application some problems becomes evident depending on the transport layers. 4.1. What IP version is used If one signals the bandwidth in SDP, with for example "b=AS:" for an RTP based application, one cannot know if the overhead is calculated for IPv4 or IPv6. An indication to which protocol has been used when calculating the bandwidth values is given by the "c=" connection data line. This line contains either a multicast group address or a unicast address of the data source or sink. The "c=" lines address type may be assumed to be of the same type as the one used in the bandwidth calculation. There seems to exist no specification pointing this out. In cases of SDP transported by RTSP this is even less clear. The normal usage for a unicast on-demand streaming session is to set the connection data address to a null address. This null address does have an address type, which could be used as an indication. However this is also not clarified anywhere. Westerlund Standards Track [Page 5] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 Figure 1, illustrates a connection scenario between a streaming server A and a client B over a translator here designated as a NAT- PT. When B receives the SDP from A over RTSP it will be very difficult for B to know what the bandwidth values in the SDP represent. The following possibilities exist: 1. The SDP is unchanged and "c=" null address is of type IPv4. The bandwidth value represents the bandwidth needed in an IPv4 network. 2. The SDP has been changed by the ALG. The "c=" address is changed to IPv6 type. The bandwidth value is unchanged. 3. The SDP is changed and both "c=" address type and bandwidth value is changed. Unfortunately, this can seldom be done, see 4.2. In case 1 the client can understand that the server is located in an IPv4 network and that it uses IPv4 overhead when calculating the bandwidth value. The client can almost never convert the bandwidth value, see section 4.2. In case 2 the client does not know that the server is in an IPv4 network and that the bandwidth value is not calculated with IPv6 overhead. In cases where a client reserve bandwidth for this flow, too little will be reserved, potentially resulting in bad Quality of Service (QoS). In case 3 everything works correctly. However this case will be very rare. If one tries to convert the bandwidth value without further information about the packet rate significant errors may be introduced into the value. 4.2. Converting bandwidth values If one would like to convert a bandwidth value calculated using IPv4 overhead to IPv6 overhead the packet rate is required. The new bandwidth value for IPv6 is normally "IPv4 bandwidth" + "packet rate" * 20 bytes. Where 20 bytes is the usual difference between IPv6 and IPv4 headers. The overhead difference may be other in cases when IPv4 options [15] or IPv6 extension headers [14] are used. As converting requires the packet rate of the stream, this is not possible in the general case. Many codecs has many possible packet rates. Therefore some extra information in the SDP will be required. The "a=ptime:" parameter may be a possible candidate. However this parameter is normally only used for audio codecs. Also its definition [1] is that it is only a recommendation, which the sender may disregard from. A better parameter is needed. 4.3. Header Compression Another mechanism that alters the actual overhead over links is header compression. Header compression uses the fact that most Westerlund Standards Track [Page 6] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 network protocol headers have either static or predictable values in their fields within a packet stream. Compression is normally only done on per hop basis, i.e. on a single link. The normal reason for doing header compression is that the link has fairly limited bandwidth and significant gain in throughput is achieved. There exist a couple of different header compression standard. For compressing IP headers only, there exist RFC 2507 [10]. For compressing packets with IP/UDP/RTP headers, CRTP [11] was created at the same time. More recently the Robust Header Compression (ROHC) working group has been developing a framework and profiles [12] for compressing certain combinations of protocols, like IP/UDP, IP/UDP/RTP. When using header compression the actual used overhead will be less deterministic, but in most cases an average overhead can be determined for a certain application. If a network node knows that some type of header compression is employed this can taken into consideration. To be able to do this with any accuracy the application and packet rate is needed. 4.4. RTCP problems When RTCP is used between host in IPv4 and IPv6 networks over an NAT- PT, similar problems exist. The RTCP traffic going from the IPv4 domain will result in a higher RTCP bit-rate than intended in the IPv6 domain due to the larger headers. This may result in up to 25% increase in required bandwidth for the RTCP traffic. The largest increase will be for small RTCP packets when the number of IPv4 hosts is much larger than the number of IPv6 hosts. Fortunately as RTCP has a limited bandwidth compared to RTP it will only result in a maximum of 1.75% increase of the total session bandwidth when RTCP bandwidth is 5% of RTP bandwidth. The RTCP randomization may easily result in short term effects of the same magnitude. The increase in bandwidth will in most cases be less. At the same time this results in unfairness in the reporting between an IPv4 and IPv6 node. The IPv6 node may report in the worst case with 25% longer intervals. These problems have been considered insignificant enough to not be worth any complex solutions. Therefore only a simple algorithm for deriving RTCP bandwidth is defined. 4.5. Future development Today there is work in IETF to design a new datagram transport protocol, which will be suitable to use for real-time media. This protocol is called the Datagram Congestion Control Protocol (DCCP). This protocol will most probably have another header size than UDP, Westerlund Standards Track [Page 7] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 which is mostly used today. This results in even further numbers of possible transport combinations to calculate overhead for. 5. Problem Scope The problems described in chapter 4 does effect all the protocols that uses SDP to signal bandwidth parameters including transport level bit-rates. In the MMUSIC WG there is work on a replacement of SDP called SDP-NG. That work is RECOMMENDED to consider the problems outlined in this draft when designing solutions for specifying bandwidth in SDP-NG. 6. Requirements A solution to the problems outlined in this draft should meet the following requirements: - The bandwidth value SHALL be given in a way so that it can be calculated for all possible combinations of transport overhead. 7. A Solution 7.1. Introduction This chapter describes a solution for the problems outlined in this document for the Application Specific (AS) bandwidth modifier. The CT is a session level modifier and cannot easily be dealt with. To address the problems with different overhead the CT value is RECOMMENDED to be calculated using reasonable worst case overhead. The RR and RS modifiers [9] will be used as defined and includes transport overhead. The small unfairness between hosts is deemed acceptable. 7.2. The TIAS bandwidth modifier 7.2.1. Usage A new bandwidth modifier is defined to be used for the following purposes: - Resource reservation. A single bit-rate can be enough to use for resource reservation. Some characteristics can be derived from the stream, codec type, etc. In some cases more information is needed, then another SDP parameter will be required. - Maximum media codec rate. With the definition below of "TIAS" the given bit-rate will mostly be from the media codec. Therefore it gives a good indication on the maximum codec bit-rate required to Westerlund Standards Track [Page 8] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 be supported by the decoder. - Communication bit-rate required for the stream. The "TIAS" value together with "maxprate" can be used to determine the maximum communication bit-rate the stream will require. By adding all maximum bit-rates from the streams in a session together, a receiver can determine if its communication resources are sufficient to handle the stream. For example a modem user can determine if the session fits his modems capabilities and the established connection. - Determine the RTP session bandwidth and derive the RTCP bandwidth. The derived transport dependent attribute will be the RTP session bandwidth in case of RTP based transport. The TIAS value can also be used to determine the RTCP bandwidth to use when using implicit allocation. RTP [8] defines that if not explicitly stated, additional bandwidth shall be used by RTCP equal to 5% of the RTP session bandwidth. The RTCP bandwidth can be explicitly allocated by using the RR and RS modifiers defined in [9]. 7.2.2. Definition A new session and media level bandwidth modifier is defined: b=TIAS: ; see section 7.6 for ABNF definition. The Transport Independent Application Specific Maximum (TIAS) bandwidth modifier has an integer bit-rate value in bits per second. A fractional bandwidth value SHALL always be rounded up to the next integer. The bandwidth value is the maximum needed by the application (session level) or media stream (media level) without counting IP or transport layers. On session level the TIAS value is simply the sum of all media stream's TIAS value. For RTP based media streams, TIAS on media level can be used to derive the RTP "session bandwidth" as defined in section 6.2 of [8]. However in the context of RTP transport the TIAS value is defined as: Only the RTP payload with its media SHALL be used in the calculation of the bit-rate, i.e. the lower layers (IP/UDP) and RTP headers are excluded. This will allow one to use the same value for both RTP based media transport as non-RTP transport and stored media. Note 1: The usage of bps is not in accordance with RFC 2327 [1]. This change has no implications on the parser, only the interpreter of the value must be aware. The change is done to allow for better resolution, and has also been used for the RR and RS bandwidth modifiers see [9]. Note 2: RTCP bandwidth is not included in the bandwidth value. In applications using RTCP, the bandwidth used by RTCP is either 5% of Westerlund Standards Track [Page 9] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 the RTP session bandwidth including lower layers or as specified by the RR and RS modifiers [9]. A definition on how to derive the RTCP bit-rate when using TIAS is present in chapter 7.5. 7.2.3. Usage Rules "TIAS" is primarily intended to be used at media level. The TIAS bandwidth attribute MAY be present on session level in the SDP. However, if present at session level it SHOULD be present also on media level. "TIAS" SHALL NOT be present on session level, unless the same transport is used for all media streams. The session level value if present MUST be the sum of all media levels. To allow for backwards compatibility towards users of SDP that does not implement "TIAS", it is RECOMMENDED to also include the "AS" modifier when using "TIAS". The presence of any value, even with problems is better than none. However an SDP user understanding TIAS when present SHOULD ignore the "AS" value and use TIAS instead. When using TIAS for an RTP transported stream(s) the "maxprate" attribute SHALL be included at the corresponding SDP level. 7.3. Packet Rate parameters To be able to calculate the bandwidth value including the actually used lower layers, a packet rate attribute is also defined. The session and media level maximum packet rate attribute is defined as: a=maxprate: ; see section 7.6 for ABNF definition. The is a floating-point value for the streams maximum packet rate. If the number of packets is variable the given value SHALL be the maximum the application, can produce in case of live stream, or for on-demand streams, have produced. The packet rate is calculated by adding together the number of packets sent within a 1 second long window. The maxprate is the largest value produced when the window is slide over the entire media stream. In cases that this can't be calculated, i.e. for example a live stream, a estimated value of the maximum rate the codec can produce for the given configuration and content SHALL be used. A session level value MUST be the sum of all media level packet rates. The session level value MAY only be given if all media streams uses the same transport. If that is not the case, the "maxprate" attribute MUST NOT be present at session level. If given at session level it SHOULD also be given at media level. "maxprate" SHOULD be included for all transports where a packet rate can be derived and TIAS is included. Westerlund Standards Track [Page 10] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 7.4. Converting to Transport Dependent values When converting the transport independent bit-rate value (bw-value) into a transport dependent value including the lower layers the following MUST be done: 1. Determine which lower layers that will be used and calculate the sum of the sizes of the headers in bits (h-size). In cases of variable header sizes the average size SHOULD be used. 2. Retrieve the maximum packet rate from the SDP (prate = maxprate). 3. Calculate the transport overhead by multiplying the header sizes with the packet rate (t-over = h-size * prate). 4. Round the transport overhead up to nearest integer in bits(t-over = CEIL(t-over)). 5. Add the transport overhead to the transport independent bit-rate value (bit-rate = bw-value + t-over) When the above calculation is performed using the "maxprate" the bit- rate value will be the absolute maximum the media stream will use over the transport used in the calculations. 7.5. Deriving RTCP bandwidth This chapter does not solve the fairness and possible bit-rate change introduced by IPv4 to IPv6 translation. These differences are considered small enough and a solution introduces code changes to the RTP/RTCP implementation. This chapter gives only a consistent way of calculating the bit-rate to assign to RTCP if not explicitly given. First the transport dependent RTP session bit-rate is calculated, in accordance with chapter 7.4, using the actual transport layers used at this end point. The RTCP bit-rate is then derived as usual based on the RTP session bandwidth, i.e. normally equal to 5% of the calculated value. 7.5.1. Motivation to being an acceptable solution. Giving the exact same bit-rate value to both the IPv4 and IPv6 host will result in that the IPv4 host has a higher RTCP sending rate. For a 100 bytes RTCP packet (including UDP/IPv4) the IPv4 sender has approximate 20 % higher rate. This rate falls with larger RTCP packets. For example, 300 bytes packets will only give the IPv4 host a 7% higher reporting rate. The above rule for deriving RTCP bandwidth, gives the same behavior as fixed assignment when the RTP session has traffic parameters giving a large TIAS/maxprate ratio. The two hosts will be fair when the TIAS/maxprate ratio is approximate 40 (bytes/packet) given 100 bytes RTCP packets. For a TIAS/maxprate ratio of 5 bytes/packet the IPv6 host will be allowed to send approximately 15-20 % more RTCP Westerlund Standards Track [Page 11] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 packets. The larger the RTCP packets become the more it will favor the IPv6 host in sending rate. The conclusions is that the within the normal useful combination of transport independent bit-rates and packet rates the difference in fairness between a host on different IP versions with different overhead is acceptable. For the 20 bytes difference in overhead between IPv4 and IPv6 headers the actually used RTCP bandwidth in a unicast connection case will not be larger than approximately 1% of the total session bandwidth. 7.6. ABNF definitions This chapter defines in ABNF from RFC 2234 [2] the bandwidth modifier and the packet rate attribute. The bandwidth modifier: TIAS-bandwidth-def = "b" "=" "TIAS" ":" bandwidth-value bandwidth-value = 1*DIGIT The maximum packet rate attribute: max-p-rate-def = "a" "=" "maxprate" ":" packet-rate CRLF packet-rate = 1*DIGIT ["." 1*DIGIT] 8. Protocol Interaction 8.1. RTSP The "TIAS", and "maxprate" can today be used with RTSP. To be able to calculate the transport dependent bandwidth, some of the transport header parameters will be required. There should be no problems for a client to calculate the required bandwidth(s) prior to a RTSP SETUP. The reason is that a client supports a limited number of transport setups. The one actually offered a server in a SETUP request will be dependent on the contents of the SDP description. The "m=" line will signal to the client the desired transport profile(s). 8.2. SIP The usage of "TIAS" together with "maxprate" should not be different from the handling of the "AS" modifier currently in use. The needed transport parameters will available in the transport field in the "m=" line. The address class can be determined from the "c=" field and the clients connectivity. Westerlund Standards Track [Page 12] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 8.3. SAP In the case of SAP all available information to calculate the transport dependent bit-rate should be present in the SDP. The "c=" information gives the used address family for the multicast. The transport layer, e.g. RTP/UDP, for each media is evident in the media line ("m=") and its transport field. 9. Security Consideration The bandwidth value that is supplied by the parameters defined here can, if not protected, be altered. By altering the bandwidth value one can fool a receiver to reserve either more or less bandwidth than actually needed. Reserving too much may result in unwanted expenses on behalf of user and also blocking of resources that other parties could have used. If to little bandwidth is reserved the receiving users quality MAY be effected. Trusting a to large TIAS value may also result in that the receiver will turn down the session due to insufficient communication and decoding resources. Due to these security risks it is STRONGLY RECOMMENDED that the SDP is authenticated so no tampering can be performed. It is also RECOMMENDED that any receiver of the SDP performs an analysis of the received bandwidth values so that they are reasonable and is what can be expected for the application. For example an single channel AMR encoded voice stream claiming to use 1000 kbps is not reasonable. 10. IANA Consideration This document register one new SDP session and media level attribute "maxprate", see section 7.3. A new SDP [1] bandwidth modifier (bwtype) "TIAS" is also registered in accordance with the rules requiring a standard tracks RFC. The modifier is defined in section 7.2. 11. Acknowledgments The author would like to thank Gonzalo Camarillo and Hesham Soliman for their work reviewing this document. The author would also like to thank all persons on the MMUSIC working group's mailing list that has commented on this specification. 12. Author's Addresses Magnus Westerlund Tel: +46 8 4048287 Ericsson Research Email: Magnus.Westerlund@ericsson.com Ericsson AB Torshamnsgatan 23 SE-164 80 Stockholm, SWEDEN Westerlund Standards Track [Page 13] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 13. References 13.1. Normative references [1] M. Handley, V. Jacobson, "Session Description Protocol (SDP)", IETF RFC 2327, April 1998. [2] D. Crocker and P. Overell, "Augmented BNF for syntax specifica- tions: ABNF," RFC 2234, Internet Engineering Task Force, Nov. 1997. [3] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. 13.2. Informative References [4] M. Handley et al., "Session Announcement Protocol", IETF RFC 2974, October 2000. [5] J. Rosenberg, et. al., "SIP: Session Initiation Protocol", IETF RFC 3261, June 2002. [6] J. Rosenberg, H. Schulzrine, "An Offer/Answer Model with Session Description Protocol (SDP)", IETF RFC 3164, June 2002. [7] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)", IETF RFC 2326, April 1998. [8] H. Schulzrinne, et. al., "RTP: A Transport Protocol for Real- Time Applications", IETF RFC 1889, January 1996. [9] S. Casner, "SDP Bandwidth Modifiers for RTCP Bandwidth", IETF WG draft, draft-ietf-avt-rtcp-bw-05.txt, November 2001, Work in progress [10] M. Degermark, B. Nordgren, S. Pink, "IP Header Compression", IETF RFC 2507, February 1999. [11] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for Low- Speed Serial Links", IETF RFC 2508, February 1999. [12] C. Bormann, et. al., "RObust Header Compression (ROHC): Framework and four profiles", IETF RFC 3095, July 2001. [13] Tsirtsis, G. and Srisuresh, P., "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, Internet Engineering Task Force, February 2000. [14] S. Deering and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, Internet Engineering Task Force, December 1998. [15] J. Postel, "internet protocol", RFC 791, Internet Engineering Task Force, September 1981. Westerlund Standards Track [Page 14] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 14. IPR Notice The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Westerlund Standards Track [Page 15] INTERNET-DRAFT Bandwidth modifier for SDP Jan. 22, 2003 15. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. This Internet-Draft expires in July 2003. Westerlund Standards Track [Page 16]