INTERNET-DRAFT Bob Quinn Celox Networks; Ross Finlayson LIVE.COM Expires 15 August 2003 15 February 2003 Session Description Protocol (SDP) Source Filters Status of this Memo This document is an Internet-Draft and is subject to 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 to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 [REQMNT]. Abstract This document describes how to adapt the Session Description Protocol (SDP) to express one or more source addresses as a source filter for one or more destination "connection" addresses. It defines the syntax and semantics for an SDP "source-filter" attribute that may reference either IPv4 or IPv6 address(es) as either an inclusive or exclusive source list for either multicast or unicast destinations. In particular, an inclusive source-filter can be used to specify a Source-Specific Multicast (SSM) session. Receiver applications are expected use the SDP source-filter information to identify traffic from legitimate senders and discard traffic from illegitimate senders. Applications and hosts may also share the source-filter information with network elements (e.g., with routers using IGMPv3) so they can potentially perform the traffic filtering operation further "upstream," closer to the source(s). 1 Introduction The Session Description Protocol [SDP] provides a general-purpose format for describing multimedia sessions in announcements or invitations. SDP uses an entirely textual data format (the US-ASCII subset of [UTF-8]) to maximize portability among transports. SDP does not define a protocol, but only the syntax to describe a multimedia session with sufficient information to discover and participate in that session. Session descriptions may be sent using any number of existing application protocols for transport (e.g., SAP, SIP, RTSP, email, HTTP, etc.). Typically, session descriptions reference an IP multicast address for the "connection-address" (destination), though unicast addresses or fully qualified domain names (FQDNs) MAY also be used. The "source- filter" attribute defined in this document defines qualifies the session traffic by identifying the address (or FQDN) of legitimate source(s) (senders). The intent is for receivers to use the source and destination address pair(s) to filter traffic, so applications receive only legitimate session traffic. 1.1 Motivation The purpose of a source-filter is to help protect receivers from traffic sent from illegitimate source addresses. Filtering traffic can help to preserve content integrity and protect against denial of service (DoS) attacks. For multicast destination addresses, receiver applications MAY apply source-filters using the Multicast Source Filter APIs [MSF API]. Hosts are likely to implement these APIs using protocol mechanisms to convey the source filters to local multicast routers. Other "upstream" multicast routers MAY apply the filters and thereby provide more explicit multicast group management and efficient utilization of network resources. The protocol mechanisms to enable these operations are beyond the scope of this document, but their potential provided motivation for SDP source-filters. 2 The "source-filter" Attribute The SDP source-filter attribute does not change any existing SDP syntax or semantics, but defines a format for additional session description information. Specifically, source-filter syntax can prescribe one or more unicast addresses as either legitimate or illegitimate sources for any (or all) SDP session description "connection-address" field values. The source-filter attribute has the following syntax: a=source-filter: The is either "incl" or "excl" (for inclusion or exclusion, respectively). The has four sub-components: A of "incl" means that an incoming packet is accepted only if its source address is in the set specified by . A of "excl" means that an incoming packet is rejected if its source address is in the set specified by . The first sub-field indicates the network type, since SDP is protocol independent. This document is most relevant to the value "IN", which designates the Internet Protocol. Second sub-field identifies the address family and for the purpose of this document may be either values "IP4" or "IP6". Alternately, when is an FQDN, the value MAY be "*" to apply to both address types, since either address can be returned from a DNS lookup. The third sub-field is the destination address, which MUST correspond to one or more of the session's "connection-address" field values. It may be either a unicast or multicast address, an FQDN (fully-qualified domain name), or the "*" wildcard to match any/all of the session's "connection-address" values. The fourth sub-field is the list of source hosts/interfaces in the source-filter, and consists of one or more unicast addresses or FQDNs, separated by space characters. The format and content of these semantic elements are derived from and compatible with those defined in [SDP]. For more detail, see Appendix A in this document. 2.1 Processing Rules There are a number of details to consider when parsing the SDP source- filter syntax. The value in a "source-filter" attribute MUST correspond to an existing value in the session description. The only exception to this is when a "*" wildcard is used to indicate that the source-filter applies to all values. When the value is a multicast address, the field value MUST NOT include the sub-fields and from the value. The is implied, and all unicast addresses in the are valid sources for any of the multicast addresses in the address series implied by the . When the value is the "*" wildcard, the MUST be either an FQDN or "*" (i.e., it MUST NOT be an IPv4 or IPv6 address). See section 2.2.5 for an example. As has always been the case, the default behavior when a source-filter attribute is not provided in a session description is that all traffic sent to the specified value should be accepted (i.e., from any source address). The source-filter grammar does not include syntax to express either "exclude none" or "include all." Like the standard described in [SDP], the location of the "source-filter" attribute determines whether it applies to the entire session or only to a specific medium (i.e., "session-level" or "media-level"). A media-level source-filter will always completely override a session-level source-filter. A "source-filter" need not be located at the same hierarchy level as its corresponding . So, a media-level can reference a session-level value, and a session-level "source-filter" can be applied to all matching media- level values. See section 2.2.3 for an example. A SDP description MUST NOT contain more than one session-level "source-filter" attribute, nor more than one media-level "source-filter" attribute for the same medium. There is no specified limit to the number of entries allowed in the , however there are practical limits that should be considered. For example, depending on the transport to be used for the session description, there may be a limit to the total size of the session description (e.g., as determined by the maximum payload in a single datagram). Also, when the source-filter is applied to control protocols, there may be a limit to the number of source addresses that can be sent. These limits are outside the scope of this document, but should be considered when defining source-filter values for SDP. 2.2 Examples Here are a number of examples that illustrate how to use the source- filter attribute in some common scenarios. We use the following session description components as the starting point for the examples to follow. For each example, we show the source filter with additional relevant information, and provide a brief explanation. = v=0 o=The King s=Elvis Impersonation i=All Elvis, all the time u=http://www.example.com/ElvisLive/ t=0 0 a=recvonly = m=audio 54320 RTP/AVP 0 = m=video 54322 RTP/AVP 34 2.2.1 Source-Specific Multicast Example Multicast addresses in the Source-Specific Multicast [SSM] range require a single unicast sender address for each multicast destination, so the source-filter specification provides a natural fit. In this example, a session member should receive only traffic sent from 192.168.9.10 to the multicast session address 232.3.4.5. c=IN IP4 232.3.4.5/127 a=source-filter: incl IN IP4 232.3.4.5 192.168.9.10 This source filter example uses an inclusion list with a single multicast "connection-address" as the destination and single unicast address as the source. Note that the value of the connection-address matches the value specified in the connection-field. Also note that since the connection-field is located in the session- description section, the source-filter applies to all media. Furthermore, if the SDP description specifies a RTP session (e.g., its "m=" line(s) specify "RTP/AVP" as the transport protocol), then the "incl" specification will apply not only to RTP packets, but also to any RTCP packets that are sent to the specified multicast address. This means that, as a side effect of the "incl" specification, the only possible multicast RTCP packets will be "Sender Report" (SR) packets sent from the specified source address. Because of this, a SDP description for a Source-Specific Multicast (SSM) RTP session SHOULD also include a a=rtcp:unicast ... attribute, as described in [RTCP-SSM]. This specifies that RTCP "Reception Report" (RR) packets are to be sent back via unicast. 2.2.2 Unicast Exclusion Example Typically, an SDP session value is a multicast address, although it is also possible to use either a unicast address or FQDN. This example illustrates a scenario whereby a session description indicates the unicast source address 192.168.9.10 in an exclusion filter. In effect, this sample source-filter says, "host 192.168.10.11 destination should accept traffic from any sender *except* 192.168.9.10." c=IN IP4 192.168.10.11 a=source-filter: excl IN IP4 192.168.10.11 192.168.9.10 2.2.3 Multiple Session Address Example This source-filter example uses the wildcard "*" value for to correspond to any/all values. Hence, the only legitimate source for traffic sent to either 232.2.2.2 or 232.4.4.4 multicast addresses is 192.168.9.10. Traffic sent from any other unicast source address should be discarded by the receiver. a=source-filter: incl IN IP4 * 192.168.9.10 c=IN IP4 232.2.2.2/127 c=IN IP4 232.4.4.4/63 2.2.4 IPv6 Multicast Source-Filter Example This simple example defines a single session-level source-filter that references a single IPv6 multicast destination and source pair. The IP multicast traffic sent to FFOE::11A is valid only from the unicast source address 2001:210:1:2:240:96FF:FE25:8EC9 c=IN IP6 FF0E::11A/127 a=source-filter incl IN IP6 FF0E::11A 2001:210:1:2:240:96FF:FE25:8EC9 2.2.5 IPv4 and IPv6 FQDN Example This example illustrates use of the "*" wildcard, along with multicast and source FQDNs that may resolve to either an IPv6 or IPv4 address, or both. Although typically both the multicast and source addresses will be the same (either both IPv4 or IPv6), using the wildcard for addrtype in the source filter allows asymmetry between the two addresses (so an IPv4 source address may be used with an IPv6 multicast address). c=IN IP4 channel-1.example.com/127 c=IN IP6 channel-1.example.com/127 a=source-filter: incl IN * channel-1.example.com src-1.example.com 3 Interoperability Issues Defining a list of legitimate sources for a multicast destination address represents a departure from the Any-Source Multicast (ASM) model, as originally described in [IGMPv1]. The ASM model supports anonymous senders, and all types of multicast applications (e.g., many-to-many). Use of a source-filter excludes some (unknown or undesirable) senders, which lends itself more to one-to-many or few-to- few type multicast applications. Although these two models have contrasting operational characteristics and requirements, they can coexist on the same network using the same protocols. Use of source-filters do not corrupt the ASM semantics but provide more control for receivers, at their discretion. 4 Normative References [ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax Specifications: ABNF," RFC 2234, November 1997 [REQMNT] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels," BCP 14, RFC 2119, March 1997. [RTCP-SSM] J. Chesterfield, et al. "RTCP Extensions for Single-Source Multicast Sessions with Unicast Feedback," Work in progress, November 2002 [SDP] M. Handley, V. Jacobson, "SDP: Session Description Protocol," RFC 2327, April 1998 [UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode and ISO 10646," RFC 2044, October 1996 5 Informative References [CA-96.21] CERT Advisory CA-96.21, "TCP SYN Flooding and IP Spoofing Attacks," September 1996 [IGMPv1] S. Deering, "Host Extensions for IP Multicasting," RFC 1112 (STD 5), August 1989 [IGMPv3] B. Cain et al. "Internet Group Management Protocol, Version 3,", Work in progress, May 2002. [MSF API] D. Thaler, B. Fenner, B. Quinn, "Socket Interface Extensions for Multicast Source Filters," Work in progress, July 2002. [SSM] Bhattacharyya, S. et al., "An Overview of Source-Specific Multicast (SSM)", Work in progress, October 2002. 6 Security Considerations See [SDP] for security considerations specific to the Session Description Protocol in general. The central issue relevant to using unicast source address filters is the question of address authenticity. Using the source IP address for authentication is weak, since addresses are often dynamically assigned and it is possible for a sender to "spoof" its source address (use one other than its own) in datagrams it sends. Proper router configuration can reduce the likelihood of "spoofed" source addresses being sent to or from a network, however. Specifically, border routers are encouraged to filter traffic so datagrams with invalid source addresses are not forwarded (e.g., routers drop datagrams if the source address is non-local) [CA-96.21]. Use of FQDNs for either or values provides a layer of indirection that provides great flexibility. However, it also exposes the source-filter to any security inadequacies that the DNS system may have (if any). If unsecured, it is conceivable that the DNS server could return illegitimate addresses. 7 IANA Considerations As recommended by [SDP] (in Appendix B), the new "source-filter" attribute described in this document should be registered with IANA. 8 Acknowledgements The authors would like to thank Dave Thaler and Mark Handley, whose input provided much of the substance of this document. Also, Magnus Westerlund provided valuable feedback during editing. 9 Authors' Addresses Bob Quinn Celox Networks 2 Park Central Drive Southborough, MA 01772 phone: 508-305-7000 email: bquinn (at) celoxnetworks.com Ross Finlayson Live Networks, Inc. (LIVE.COM) 650 Castro St., suite 120-196 Mountain View, CA 94041 email: finlayson (at) live.com Appendix A: Source-Filter Attribute Syntax This appendix provides an Augmented BNF [ABNF] grammar for expressing an exclusion or inclusion list of one or more (IPv4 or IPv6) unicast source addresses. It is intended as an extension to the grammar for the Session Description Protocol, as defined in [SDP]. Specifically, it describes the syntax for the new "source-filter" attribute field, which MAY be either a session-level or media-level attribute. The "connection-address" value in each source filter field MUST match an existing connection-field value, unless the wildcard connection- address value "*" is specified. source-filter = "source-filter" ":" filter-mode filter-spec filter-mode = "excl" | "incl" ; either exclusion or inclusion mode filter-spec = nettype address-types dest-address src-list ; nettype is as defined in [SDP]. address-types = "*" | addrtype ; "*" for all address types (both IP4 and IP6), ; but only when and ; reference FQDNs. ; addrtype is as defined in [SDP]. dest-address = "*" | IP4-address | IP6-address | FQDN ; "*" applies to all connection-address values. ; IP4-address, IP6-address, FQDN are as defined ; in [SDP]. src-list = *(addr SP) addr ; one or more unicast source addresses (in standard ; IPv4 or IPv6 ASCII-notation form) or FQDNs. ; SP is the ASCII 'space' character (0x20). ; addr is as defined in [SDP]. Expires 15 August 2003 15 February 2003