Network Working Group B. Quinn Request for Comments: 4570 BoxnArrow.com Category: Standards Track R. Finlayson Live Networks, Inc. July 2006 Session Description Protocol (SDP) Source Filters Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractThis 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. 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., Session Announcement Protocol (SAP), SIP, Real Time Streaming Protocol (RTSP), email, and HTTP). 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 qualifies the session traffic by identifying the address (or FQDN) of legitimate sources (senders). The intent is for receivers to use the source and destination address pair(s) to filter traffic, so that applications receive only legitimate session traffic. Receiver applications are expected to 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). The "source-filter" attribute can appear at the session level and/or the media level. 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. RFC 2119 [REQMNT].
Note that the unicast source addresses specified by this attribute are those that are seen by a receiver. Therefore, if source addresses undergo translation en route from the original sender to the receiver - e.g., due to Network Address Translation (NAT) or some tunneling mechanism - then the SDP "source-filter" attribute, as presented to the receiver, will not be accurate unless the source addresses therein are also translated accordingly. The source-filter attribute has the following syntax: a=source-filter: <filter-mode> <filter-spec> The <filter-mode> is either "incl" or "excl" (for inclusion or exclusion, respectively). The <filter-spec> has four sub-components: <nettype> <address-types> <dest-address> <src-list> A <filter-mode> of "incl" means that an incoming packet is accepted only if its source address is in the set specified by <src-list>. A <filter-mode> of "excl" means that an incoming packet is rejected if its source address is in the set specified by <src-list>. The first sub-field, <nettype>, indicates the network type, since SDP is protocol independent. This document is most relevant to the value "IN", which designates the Internet Protocol. The second sub-field, <address-types>, identifies the address family, and for the purpose of this document may be either <addrtype> value "IP4" or "IP6". Alternately, when <dest-address> is an FQDN, the value MAY be "*" to apply to both address types, since either address type can be returned from a DNS lookup. The third sub-field, <dest-address>, 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, or the "*" wildcard to match any/all of the session's "connection-address" values. The fourth sub-field, <src-list>, 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 of this document.
section 3.2.4 for an example. When the <addrtype> value is the "*" wildcard, the <dest-address> MUST be either an FQDN or "*" (i.e., it MUST NOT be an IPv4 or IPv6 address). See section 3.2.6 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 <connection-address> 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 <connection-field> 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 <connection-field>. So, a media-level <source-filter> can reference a session-level <connection-field> value, and a session-level "source-filter" can be applied to all matching media-level <connection-field> values. See section 3.2.3 for an example. An SDP description MUST NOT contain more than one session-level "source-filter" attribute that covers the same destination address, or more than one media-level "source-filter" attribute that covers the same destination address.
There is no specified limit to the number of entries allowed in the <src-list>; 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. 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 192.0.2.10 to the multicast session address 220.127.116.11. <session-description> c=IN IP4 18.104.22.168/127 a=source-filter: incl IN IP4 22.214.171.124 192.0.2.10 <media-description 1>
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 an 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, an SDP description for a Source-Specific Multicast (SSM) RTP session SHOULD also include an a=rtcp-unicast ... attribute, as described in [RTCP-SSM] (section 10.1). This specifies that RTCP "Reception Report" (RR) packets are to be sent back via unicast.
126.96.36.199 or 188.8.131.52 multicast addresses is 192.0.2.10. Traffic sent from any other unicast source address should be discarded by the receiver. <session-description> a=source-filter: incl IN IP4 * 192.0.2.10 <media-description 1> c=IN IP4 184.108.40.206/127 <media-description 2> c=IN IP4 220.127.116.11/63
OFFER], because sets of source addresses do not represent 'capabilities' or 'limitations' of the offerer, and because the offerer does not, in general, have a priori knowledge of which IP source address(es) will be included in an answer. While an answerer may include the "source-filter" attribute in his/her answer (e.g., to designate a SSM session), the answerer SHOULD ignore any "source-filter" attribute that was present in the original offer. 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.
SDP] for security considerations specific to the Session Description Protocol in general. The central issue relevant to using 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 (i.e., use one other than its own) in datagrams that it sends. Proper router configuration, however, can reduce the likelihood of "spoofed" source addresses being sent to or from a network. Specifically, border routers are encouraged to filter traffic so that datagrams with invalid source addresses are not forwarded (e.g., routers drop datagrams if the source address is non-local) [FILTERING]. This, however, does not prevent IP source addresses from being spoofed on a Local Area Network (LAN). Also, as noted in section 3 above, tunneling or NAT mechanisms may require corresponding translation of the addresses specified in the SDP "source-filter" attribute, and furthermore, may cause a set of original source addresses to be translated to a smaller set of source addresses as seen by the receiver. Use of FQDNs for either <dest-address> or <src-list> 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 unsecured, it is conceivable that the DNS server could return illegitimate addresses. In addition, if source-filtering is implemented by sharing the source-filter information with network elements, then the security of the protocol(s) that are used for this (e.g., [IGMPv3]) becomes important, to ensure that legitimate traffic (and only legitimate traffic) is received. For these reasons, receivers SHOULD NOT treat the SDP "source-filter" attribute as being its sole mechanism for protecting the integrity of received content.
SDP] (Appendix B), the new attribute name "source-filter" has been registered with IANA, as follows: The following contact information shall be used for all registrations included here: Contact: Ross Finlayson email: finlayson (at) live555.com phone: +1-650-254-1184 SDP Attribute ("att-field"): Attribute name: source-filter Long form: Source Filter Type of name: att-field Type of attribute: Session level or media level Subject to charset: No Purpose: See this document Reference: This document Values: See this document, and registrations below [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [REQMNT] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[FILTERING] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000. [IGMPv1] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989. [IGMPv3] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [MSF-API] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface Extensions for Multicast Source Filters", RFC 3678, January 2004. [OFFER] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RTCP-SSM] Chesterfield, J., E. Schooler, J. Ott, "RTCP Extensions for Single-Source Multicast Sessions with Unicast Feedback", Work in Progress, October 2004. [SSM] Bhattacharyya, S., "An Overview of Source-Specific Multicast (SSM)", RFC 3569, July 2003.
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 "dest-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" ":" SP filter-mode SP filter-spec ; SP is the ASCII 'space' character ; (0x20, defined in [ABNF]). filter-mode = "excl" / "incl" ; either exclusion or inclusion mode. filter-spec = nettype SP address-types SP dest-address SP src-list ; nettype is as defined in [SDP]. address-types = "*" / addrtype ; "*" for all address types (both IP4 and IP6), ; but only when <dest-address> and <src-list> ; reference FQDNs. ; addrtype is as defined in [SDP]. dest-address = "*" / basic-multicast-address / unicast-address ; "*" applies to all connection-address values. ; unicast-address is as defined in [SDP]. src-list = *(unicast-address SP) unicast-address ; one or more unicast source addresses (in ; standard IPv4 or IPv6 ASCII-notation form) ; or FQDNs. ; unicast-address is as defined in [SDP]. basic-multicast-address = basic-IP4-multicast / basic-IP6-multicast / FQDN / extn-addr ; i.e., the same as multicast-address ; defined in [SDP], except that the ; /<ttl> and /<number of addresses> ; fields are not included. ; FQDN and extn-addr are as defined ; in [SDP].
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