Network Working Group L. Wood Internet-Draft R. Asati Intended status: Experimental D. Floreani Expires: January 15, 2009 Cisco Systems July 14, 2008 Link properties advertisement from modem to router draft-wood-dna-link-properties-advertisement-01 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on January 15, 2009. Abstract Routers are increasingly connected to a variety of smart modems. Such a modem's incoming and outgoing link rates can be varied over time with adaptive coding and modulation to suit the channel characteristics. The link rate and conditions offered by the modem to connected devices therefore vary. For connected devices to condition traffic and get the most out of the modem's link capacity, they need to know the modem's link conditions. This document describes one simple method for the modem to advertise link rate and other characteristics, via UDP messages, and discusses alternative approaches to communicating this information. Wood, et al. Expires January 15, 2009 [Page 1] Internet-Draft Modem/router link properties July 2008 Table of Contents 1. Background and Introduction . . . . . . . . . . . . . . . . . 3 2. UDP messages providing link notifications . . . . . . . . . . 3 3. Other possible blocks . . . . . . . . . . . . . . . . . . . . 6 4. Router use of these notifications . . . . . . . . . . . . . . 7 5. Other approaches to the problem . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 9.1. Normative References . . . . . . . . . . . . . . . . . . . 9 9.2. Informative References . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Wood, et al. Expires January 15, 2009 [Page 2] Internet-Draft Modem/router link properties July 2008 1. Background and Introduction Consider a wireless modem connected to a router. The modem and router are connected via high-speed Ethernet, because Ethernet interfaces are plentiful and cheap. The router needs to know the link rate offered by the modem in order to set QoS and shaping usefully for that link; simply sending 10/100Mbps traffic to the modem and having the modem drop most of that traffic is not optimal. It is possible to configure QoS and shaping on the router's Ethernet interface to match the modem's link rate, effectively extending the modem's link an extra hop to the router. However, such a static configuration is only useful if the modem's link rate is also static. Many modern modems are able to vary their communications according to the channel characteristics they experience, or due to negotiation with the modem at the other end of the link. Examples include the Adaptive Coding and Modulation (ACM) and Variable Coding and Modulation (VCM) used in DVB-S2, and ADSL2's Seamless Rate Adaptation (SRA). Link characteristics may also vary due to layer-2 handovers, e.g. IEEE 802.21 media-independent handoffs. The router needs to learn about changes in modem link characteristics in order to vary its QoS and shaping configurations to match the current characteristics and get the most from the modem's link. The aim is to make the link between router and modem behave as an extension of the modem's own link. The simplest approach to this problem, taken by this draft, is to have the modem advertise its link conditions on its other local interfaces. We describe using UDP packets [RFC0768] sent to link- local multicast addresses to do this. This requires no explicit configuration setup to communicate with connected routers. UDP is well-understood and widely available, making deployment relatively easy for all types of modem and router. Updates are sent periodically or as notifications of link-layer events when they happen. This falls into the scope of DNA [Detecting Network Attachment] processes [RFC4957]. 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. [RFC2119] 2. UDP messages providing link notifications The modem sends UDP updates, about changing link and interface conditions, to connected routers, i.e. a link rate changes due to a Wood, et al. Expires January 15, 2009 [Page 3] Internet-Draft Modem/router link properties July 2008 coding change, or the link and its interface go up or down. As well as sending on event-triggered updates, the modem SHOULD send periodic advertisements about link conditions, in case new router devices have been connected on e.g. a broadcast Ethernet LAN. These updates are sent over both IPv4 and IPv6. This packet can include multiple Blocks containing different information, where each Block is structured as a Type/Length/Data field. This document defines a single Rate Block which has multiple Link Instance sub-block sections for each input or output interface, each identifying the input or output link interface, and describing the link capacity available (current/maximum/minimum rates in bps). The diagram below shows an example Rate Block with a single (Incoming) Link Instance. If a modem is both IPv4- and IPv6-capable over its link to the router, this information should be repeated in IPv4 and IPv6 packets received by the router. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP source port | UDP destination port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP length | UDP checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Rate Type Block ID (1) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + no. of links | link rate size| modem flags (15 bits unused)|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + unique modem interface identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + interface flags (28 bits unused) |4|6|U|I| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | current link rate (varies) - 32 bits is rate size of 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | minimum supported rate - 32 bits is rate size of 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | maximum supported rate - 32 bits is rate size of 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 address of modem's local link interface, if 4 flag set | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | IPv6 address of modem's local link interface, if 6 flag set | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Wood, et al. Expires January 15, 2009 [Page 4] Internet-Draft Modem/router link properties July 2008 The following fields are defined in this packet's Rate Block: +------------+------------------------------------------------------+ | Name | Value | +------------+------------------------------------------------------+ | Type Block | 1 for Rate Blocks. Permits future definition of | | ID | other Blocks conveying different types of | | | information. | | Length | Length of the Block data, until the next Block or | | | end of packet. | | no. of | total number of input or output interface sub-blocks | | links | listed in this Rate Block. | | link rate | The number of 32-bit words used for bps rate fields. | | size | A value of 1 equates to 32 bits, which can describe | | | rates up to 4Gibps, and is sufficient for most | | | current modems. This is the first item in the Rate | | | Block's Value data. | | modem | Flags that can describe properties of the modem. | | flags | Unused flag fields MUST be set to zero. One bit is | | | currently in use. | | S/A flag | indicates whether the Rate Block contains fields | | | describing Some modem interfaces (flag set to 1), or | | | All modem interfaces (0). Periodic messages SHOULD | | | describe All interfaces. Updates triggered by an | | | event on an interface, e.g. a link going down, where | | | nothing else has changed, would be a Some update | | | describing only that interface. If a complex modem | | | contains so many interfaces that MTU would be | | | exceeded by a single Rate Block, multiple packets | | | are sent with separate Rate Blocks with the Some | | | flag set. | | unique | Identifies the modem's local incoming or outgoing | | modem | link interface to disambiguate it from other links | | interface | offered by the modem. | | identifier | | | interface | A 32-bit field with flags describing each individual | | flags | link. Unused flag bits MUST be set to zero. Four | | | bits are currently in use. | | IPv4 flag | If set, an IPv4 address for the interface is | | | appended to the description. | | IPv6 flag | If set, an IPv6 address for the interface is | | | appended to the description. | | U/D flag | Indicates whether the link interface is currently Up | | | or Down, i.e. accepting traffic (up, value 1), or | | | not (down, value 0). | Wood, et al. Expires January 15, 2009 [Page 5] Internet-Draft Modem/router link properties July 2008 | I/O flag | Indicates whether the rate information given for the | | | interface describes the incoming rate to the modem | | | (and router) or the outgoing rate from the modem | | | (and router) over the modem's link to the remote | | | modem. Describing outgoing rates is most likely to | | | be useful for shaping traffic to be passed to the | | | modem. Incoming is 1, Outgoing is 0. | | Current | current offered link capacity in bps. When the link | | link rate | rate size is 1 this is a 32-bit word indicating the | | | range 0-4 Gibps. | | Min rate | minimum supported link capacity in bps, specified as | | | the current link rate is. | | Max rate | maximum supported link capacity in bps, specified as | | | the current link rate is. | | IPv4 | IPv4 address of modem, if present as indicated by | | address | the IPv4 flag. | | IPv6 | IPv6 address of modem, if present as indicated by | | address | the IPv6 flag. | +------------+------------------------------------------------------+ A bidirectional link on the modem will have incoming and outgoing interfaces that MUST share the same local identifier, and SHOULD share the same local IP address. These interfaces are identified in separate sub-blocks with the I/O flag set appropriately, so that any asymmetry can be described properly. This means that the common interface identifier would be repeated for each sub-block, where one sub-block describes describes Incoming information, and the other Outgoing information. If a link is down, the D flag is taken as 'zero bit rate' and the 'current' rate indicates the last rate before the modem took the link down. If the minimum and maximum rates are set to zero, this indicates a fixed-speed link whose rate ia never altered. An interface does not have to have any IP address. 3. Other possible blocks Other possible blocks, not yet defined here, could express measured error rate, current forward error-coding rate, latency (propagation delay, serialisation delay), link MTU size, indicate link-level security mechanisms in use, or provide authentication. The resource and relative link quality metrics defined in [RFC4938] may also be of use. Unlike [RFC4938], we deliberately define link capacity in exact bps rather than degraded approximate kbps, knowing that for very-low-rate modems, the exact bit rate matters for e.g. call admission control. The lowest link rate that we have encountered is 75bps for submarine applications. Wood, et al. Expires January 15, 2009 [Page 6] Internet-Draft Modem/router link properties July 2008 4. Router use of these notifications The router MUST be able to receive these UDP/IP packets sent to the "all routers" multicast address. Information from this message is used to construct or update the QoS and shaping policies applied on the interface for traffic directed to the leads to the modem's link. The router MAY use this information to update its routing table so that the routing information associated with a route through the modem is either deleted or added or updated with a new metric. Changes in routing table information are then propagated further. The modem MAY damp changes to Link Instance information, to prevent advertising transient changes, in line with [RFC4907]. The router MAY also damp responses to frequent changes in Link Instance information received, so that related QoS policies and routing information are not updated until a certain time period has elapsed. This would be useful in the case of a rapidly-varying link rate, where the router would stick to the minimum rate seen. The router may also use this information as input to e.g. Call Admission Control (CAC) functionality to reserve capacity for calls. This can deny registered applications such as telephony call setup etc. in the event of less-than-desired available link capacity through the modem's interface. 5. Other approaches to the problem The simplest approach to this problem is to have a fixed serial interface between modem and router, with the modem altering the serial rate clock to match the speed of its link, and the router measuring the rate of the received clock. However, fast serial interfaces are unfashionable, and Ethernet now dominates the world. We considered using ICMP [RFC0792] [RFC4443] to provide this rate information, using the framework for carrying extended information in ICMP messages [RFC4884]. However, this extended information can only be carried in Destination Unreachable and Time Exceeded ICMP messages (for both IPv4 and IPv6) and Parameter Problem (for IPv4 only) ICMP messages. These messages are responses to specific problems, and should not be overloaded for general advertisements. The appropriate ICMP message type would be the obsolete Information Request messge (type 15), though requests are also sent unsolicited in this use. Another factor in preferring UDP over ICMP is that sockets programming for UDP is easier than for ICMP, easing implementation. Other approaches to this problem have been proposed. Wood, et al. Expires January 15, 2009 [Page 7] Internet-Draft Modem/router link properties July 2008 The authors have outlined an approach leveraging the Access Node Control Protocol, used in the head-ends of DSL networks, within DSLAMs [I-D.floreani-ancp-wireless]. However, ANCP is not lightweight, and depends on GSMP, which depends on TCP. The ANCP workgroup is currently focused on the DSL headend, and has yet to extend beyond that environment, while this proposal is also useful at the tail-end in customer networks. Another approach aimed at improving communication between modems and routers is outlined in [RFC4938] and [I-D.bberry-rfc4938bis]. That approach assumes a PPPoE infrastructure. PPPoE is not always architecturally suitable to network needs, and requiring a global PPPoE infrastructure and introducing authentication dependencies for what was just a simple local modem-router problem is overkill. That approach may be suitable as an upgrade to existing PPPoE environments. Adoption of the metrics described in [RFC4938] but with communication separate from PPPoE could be very useful for a wider market, and would provide more information than the link rate information outlined in this draft. Ethernet pause frames have also been suggested as one way of slowing the Ethernet link to match the modem's link a hop further along [GePause2004]. This approach has the disadvantage of being tied to a particular layer-2 technology, while fine-tuning the pause frames to match the modem's offered link rate has the potential to introduce complex control loops and problems as the paused Ethernet rate approximates the modem link rate and interacts with QoS and shaping delay mechanisms on the router. DHCP is intended for address configuration of hosts, so is not considered suitable as a way of piggybacking this information. 6. Security Considerations Link instance advertisements should only be local to connecting devices, and should not be propagated further. As packets are sent only to link-local multicast addresses, TTL safeguards such as GTSM [RFC5082], which sets TTL to a hard-to-spoof 255, should be unnecessary. Deliberately setting a large TTL on any multicast packet would be unwise if it were to be propagated further. The decision to use this information more broadly feeds into routing metric updates and policy decisions there; taking the information beyond immediately-connected links becomes a routing problem, and has not been described here. Wood, et al. Expires January 15, 2009 [Page 8] Internet-Draft Modem/router link properties July 2008 Some form of authentication of the modem sender is required to prevent spoofing from other local devices. It should be possible to configure the UDP port number used by the router and modem, to avoid attacks on a well-known port assigned by IANA. 7. IANA Considerations IANA must allocate a UDP port for use. Packets are sent to the well-known link-local "all routers" multicast addresses (224.0.0.2 and ff02::2). No further addresses are needed. 8. Acknowledgements We thank our colleagues at Cisco Systems for vigorous discussion on this topic. (We briefly considered calling this UDP message format Modem To Router Link Advertisement, or MoToRoLA, but thought better of it. Suggestions for a better name are welcome.) 9. References 9.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 9.2. Informative References [GePause2004] Ge, A. and G. Chiruvolu, "Diffserv Compatible Extended Pause (DiffPause) for Fair Congestion Control in Metro- Ethernet", IEEE International Conference on Communications, vol. 2 pp. 1248-1252 , June 2004. [I-D.bberry-rfc4938bis] Berry, B., Ratliff, S., Paradise, E., Kaiser, T., and M. Adams, "PPP Over Ethernet (PPPoE) Extensions for Credit Flow and Link Metrics", draft-bberry-rfc4938bis-00 (work in progress), April 2008. Wood, et al. Expires January 15, 2009 [Page 9] Internet-Draft Modem/router link properties July 2008 [I-D.floreani-ancp-wireless] Floreani, D. and L. Wood, "Extension of ANCP for Satellite and Terrestrial Wireless Modem Networks", draft-floreani-ancp-wireless-00 (work in progress), June 2007. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. [RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "Extended ICMP to Support Multi-Part Messages", RFC 4884, April 2007. [RFC4907] Aboba, B., "Architectural Implications of Link Indications", RFC 4907, June 2007. [RFC4938] Berry, B. and H. Holgate, "PPP Over Ethernet (PPPoE) Extensions for Credit Flow and Link Metrics", RFC 4938, June 2007. [RFC4957] Krishnan, S., Montavont, N., Njedjou, E., Veerepalli, S., and A. Yegin, "Link-Layer Event Notifications for Detecting Network Attachments", RFC 4957, August 2007. [RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. Pignataro, "The Generalized TTL Security Mechanism (GTSM)", RFC 5082, October 2007. Authors' Addresses Lloyd Wood Cisco Systems 11 New Square Park, Bedfont Lakes Feltham, Middlesex TW14 8HA United Kingdom Phone: +44-20-8824-4236 Email: lwood@cisco.com Wood, et al. Expires January 15, 2009 [Page 10] Internet-Draft Modem/router link properties July 2008 Rajiv Asati Cisco Systems 7025-6 Kit Creek Road Research Triangle Park, North Carolina 27709-4987 United States Phone: +1 919 392 8558 Email: rajiva@cisco.com Daniel Floreani Cisco Systems Westpac House 91 King William Street Adelaide, South Australia 5000 Australia Phone: +61 8 8124 2206 Email: danielf@cisco.com Wood, et al. Expires January 15, 2009 [Page 11] Internet-Draft Modem/router link properties July 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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