Network Working Group Magnus Westerlund INTERNET-DRAFT Ericsson Expires: April 2003 A transport independent bandwidth modifier for the Session Description Protocol. 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. 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 SAP, SIP and 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. A possible solution to this problem is proposed. Westerlund [Page 1] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 TABLE OF CONTENTS 1. Definitions.........................................................2 1.1. Glossary..........................................................2 1.2. Terminology.......................................................2 2. Introduction........................................................3 2.1. The Bandwidth Attribute...........................................3 2.1.1. Conference Total................................................3 2.1.2. Application Specific............................................3 2.1.3. RTCP Report bandwidth...........................................3 2.2. IPv6 and IPv4.....................................................4 3. The Bandwidth Signaling Problems....................................5 3.1.1. What IP version is used.........................................5 3.1.2. Converting bandwidth values.....................................5 3.2. Header Compression................................................6 3.3. Future development................................................6 4. Problem Scope.......................................................7 5. Requirements........................................................7 6. A Solution..........................................................7 6.1. The bandwidth modifier............................................7 6.2. Packet Rate parameter.............................................8 6.3. Converting to Transport Dependent values..........................8 6.4. ABNF definitions..................................................8 7. Security Consideration..............................................9 8. IANA Consideration..................................................9 9. Acknowledgments.....................................................9 10. Author's Addresses.................................................9 11. References........................................................10 11.1. Normative references............................................10 11.2. Informative References..........................................10 1. Definitions 1.1. Glossary SDP - Session Description Protocol SAP - Session Announcement Protocol SIP - Session Initiation Protocol RTSP - Real-Time Streaming Protocol 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 [7]. Westerlund [Page 2] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 2. Introduction Today the Session Description Protocol (SDP) [1] is used in several types of applications. The original application is session configuration for multicast sessions announced with Session Announcement Protocol (SAP) [2]. SDP is also a vital component in media negotiation for the Session Initiation Protocol (SIP) [3] by using the offer answer model [4]. The Real-Time Streaming Protocol (RTSP) [5] also makes use of SDP to declare what media and codec(s) a multi-media presentation consist of to the client. 2.1. The Bandwidth Attribute In SDP there exist a bandwidth attribute, which has a modifier used to specify what type of bandwidth the value refers to. The attribute has the following form: b=: Today there are four modifiers defined which are used for different purposes. 2.1.1. 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 so that it is possible to decide if this session can co-exist with any other sessions. Defined in the RFC 2327 [1]. 2.1.2. Application Specific The Application Specific 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 [6]. 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 the RFC 2327 [1]. 2.1.3. 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. Westerlund [Page 3] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 2.2. IPv6 and IPv4 Today there are two IP versions 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 IPv4 and IPv6 only hosts one most do translation. This translator can be for example a NAT-PT, 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 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 size results in that the bandwidth used for the IP layer is different. How big the difference is, depends on the packet rate. Westerlund [Page 4] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 3. 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. 3.1.1. 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 not clarified anywhere. 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. 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 cannot convert the bandwidth value, see section 3.1.2. In case 2 the client does not know that the server is in an IPv4 network and the bandwidth value is for not calculated with IPv6 overhead. In cases where a client reserve bandwidth for this flow, too little will be reserved, resulting in bad Quality of Service (QoS). 3.1.2. 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 Westerlund [Page 5] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 bandwidth value for IPv6 is "IPv4 bandwidth" + "packet rate" * 20 bytes. Where 20 bytes is the difference between IPv6 and IPv4 headers. As converting requires the packet rate for 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. 3.2. Header Compression Another mechanism that alters the actual overhead over links is header compression. Header compression uses the fact that most network protocol headers have either static or predictable values in their fields. This 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 certain but in most cases one can determine an average overhead for an 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. 3.3. 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, which is mostly used today. This results in even further numbers of possible transport combinations to calculate overhead for. Westerlund [Page 6] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 4. Problem Scope The problems described in chapter 3 does effect all the protocols that uses SDP to signal bandwidth parameters. In the MMUSIC WG there is work on a replacement of SDP called SDP-NG. That work SHOULD consider the problems outlined in this draft when designing solutions for specifying bandwidth in SDP-NG. 5. 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 combination of transport overhead. 6. A Solution 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 will hopefully be possible to clarify before the publishing of their specification. 6.1. The bandwidth modifier A new media level bandwidth modifier is defined: b=TIAS: The Transport Independent Application Specific Maximum bandwidth modifier (TIAS) has an integer bandwidth value in kbits 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 without counting IP or transport layer. For RTP based applications, TIAS gives the RTP "session bandwidth" as defined in section 6.2 of [6] with the following exception: - Only RTP header and payload SHALL be used in the calculation, i.e. the lower layers (IP/UDP) are excluded. The reason for not also excluding RTP from the value is that RTP will always be present for media transport using RTP. In the cases the RTP header is compressed it can be easily taken into account as long as the packet rate is known. Note: RTCP bandwidth is not included in the bandwidth value. In applications using RTCP the bandwidth used by RTCP is either 5% of Westerlund [Page 7] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 the RTP session bandwidth including lower layers or as specified by the RR and RS modifiers. 6.2. Packet Rate parameter To be able to calculate the bandwidth value including the actually used lower layers a packet rate parameter is also defined. The packet rate parameter is defined as: a=prate: The packet rate is a floating-point value for the streams packet rate. If the number of packets is variable the given value SHALL be the maximum the application can produce. The value is calculated by taking the maximum number of packets that are present within a 1 second window placed anywhere in the media stream during its life time. The "prate" attribute is a media level attribute. 6.3. Converting to Transport Dependent values When one want to convert the transport independent bandwidth value into one including the lower layers the following MUST be done. 1. Determine which lower layers that will be used and calculate the size of these headers (h-size). 2. Retrieve the packet rate from the SDP (prate). 3. Calculate the transport overhead by multiplying the header size with the packet rate (t-over = h-size * prate). 4. Round the transport overhead up to nearest integer (t-over = CEIL(t-over)). 5. Add the transport overhead to the TIAS bandwidth value (bandwidth = TIAS-value + t-over) 6.4. ABNF definitions This chapter defines in ABNF from RFC 2234 [8] the bandwidth modifier and the packet rate attribute. The bandwidth modifier: TIAS-bandwidth = "b" "=" "TIAS" ":" bandwidth-value bandwidth-value = 1*DIGIT The packet rate attribute: p-rate-def = "a" "=" "prate" ":" packet-rate CRLF Westerlund [Page 8] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 packet-rate = 1*DIGIT ["." 1*DIGIT] 7. 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. Due to these security risks it is 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 AMR encoded voice stream claiming to use 1000 kbps is not reasonable. 8. IANA Consideration This document register one new SDP media level attribute "prate", see section 6.2. A new bandwidth modifier "TIAS" is also registered in accordance with the rules requiring a standard tracks RFC. The modifier is defined in section 6.1. 9. Acknowledgments The author would like to thank Gonzalo Camarillo and Hesham Soliman for their work reviewing this document. 10. 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 [Page 9] INTERNET-DRAFT Bandwidth modifier for SDP okt 07, 2002 11. References 11.1. Normative references [1] M. Handley, V. Jacobson, "Session Description Protocol (SDP)", IETF RFC 2327, April 1998. [2] M. Handley et al., "Session Announcement Protocol", IETF RFC 2974, October 2000. [3] J. Rosenberg, et. al., "SIP: Session Initiation Protocol", IETF RFC 3261, June 2002. [4] J. Rosenberg, H. Schulzrine, "An Offer/Answer Model with Session Description Protocol (SDP)", IETF RFC 3164, June 2002. [5] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)", IETF RFC 2326, April 1998. [6] H. Schulzrinne, et. al., "RTP: A Transport Protocol for Real- Time Applications", IETF RFC 1889, January 1996. [7] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [8] D. Crocker and P. Overell, "Augmented BNF for syntax specifica- tions: ABNF," RFC 2234, Internet Engineering Task Force, Nov. 1997. 11.2. Informative References [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. This Internet-Draft expires in April 2003. Westerlund [Page 10]