Network Working Group B. Aboba, Ed. INTERNET-DRAFT Elwyn Davies Category: Informational D. Thaler <draft-iab-link-encaps-03.txt> Internet Architecture Board 1 October 2006 Multiple Encapsulation Methods Considered Harmful 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 April 1, 2007. Copyright Notice Copyright (C) The Internet Society (2006). All Rights Reserved. Abstract This document describes architectural and operational issues that arise from link layer protocols supporting multiple Internet Protocol encapsulation methods. IAB Informational [Page 1]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 Table of Contents 1. Introduction .......................................... 3 1.1 Terminology .................................... 3 1.2 Ethernet Experience ............................ 3 1.3 Trailer Encapsulation Experience ............... 5 1.4 Potential Mitigations .......................... 7 2. Evaluation of Arguments for Multiple Encapsulations ... 8 2.1 Efficiency ..................................... 8 2.2 Multicast/Broadcast ............................ 9 2.3 Multiple Uses .................................. 10 3. Additional Issues ..................................... 11 3.1 Generality ..................................... 12 3.2 Layer Interdependence .......................... 13 3.3 Inspection of Payload Contents ................. 13 3.4 Interoperability Guidance ...................... 13 3.5 Service Consistency ............................ 15 3.6 Implementation Complexity ...................... 15 3.7 Negotiation .................................... 16 3.8 Roaming ........................................ 16 4. Security Considerations ............................... 17 5. IANA Considerations ................................... 17 6. Conclusion ............................................ 17 7. References ............................................ 18 7.1 Informative References .......................... 18 Acknowledgments .............................................. 21 Appendix A - IAB Members ..................................... 21 Authors' Addresses ........................................... 21 Intellectual Property Statement .............................. 22 Disclaimer of Validity ....................................... 22 Copyright Statement .......................................... 22 IAB Informational [Page 2]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 1. Introduction This document describes architectural and operational issues arising from use of multiple ways of encapsulating IP packets on the same link. While typically a link layer protocol supports only a single Internet Protocol (IP) encapsulation method, this is not always the case. For example, on the same cable it is possible to encapsulate an IPv4 packet using Ethernet [DIX] encapsulation as defined in "A Standard for the Transmission of IP Datagrams over Ethernet Networks" [RFC894] or IEEE 802 [IEEE-802-1A.190] encapsulation as defined in "A Standard for the Transmission of IP Datagrams over IEEE 802 Networks" [RFC1042]. Historically, a further encapsulation method was used on some Ethernet systems as specified in "Trailer Encapsulations" [RFC893]. 1.1. Terminology Broadcast domain The set of all endpoints that receive broadcast frames sent by an endpoint in the set. Link A communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IP. Link Layer The conceptual layer of control or processing logic that is responsible for maintaining control of the link. The link layer functions provide an interface between the higher-layer logic and the link. The link layer is the layer immediately below IP. 1.2. Ethernet Experience The fundamental issues with multiple encapsulation methods on the same link are described in [RFC1042] and "Requirements for Internet Hosts -- Communication Layers" [RFC1122]. This section summarizes the concerns articulated in those documents and also describes the limitations of approaches suggested to mitigate the problems, including encapsulation negotiation and use of routers. [RFC1042] described the potential issues resulting from contemporaneous use of Ethernet and IEEE 802.3 encapsulation on the same physical cable: Interoperation with Ethernet It is possible to use the Ethernet link level protocol [DIX] on the same physical cable with the IEEE 802.3 link level protocol. A computer interfaced to a physical cable used in IAB Informational [Page 3]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 this way could potentially read both Ethernet and 802.3 packets from the network. If a computer does read both types of packets, it must keep track of which link protocol was used with each other computer on the network and use the proper link protocol when sending packets. One should note that in such an environment, link level broadcast packets will not reach all the computers attached to the network, but only those using the link level protocol used for the broadcast. Since it must be assumed that most computers will read and send using only one type of link protocol, it is recommended that if such an environment (a network with both link protocols) is necessary, an IP gateway be used as if there were two distinct networks. Note that the MTU for the Ethernet allows a 1500 octet IP datagram, with the MTU for the 802.3 network allows only a 1492 octet IP datagram. When multiple IP encapsulation methods were supported on a given link, all hosts could not be assumed to support the same set of encapsulation methods. This in turn implied that the broadcast domain might not include all hosts on the link. Where a single encapsulation does not reach all hosts on the link, a host needs to determine the appropriate encapsulation prior to sending. While a host supporting reception of multiple encapsulations could keep track of the encapsulations it receives, this does not enable initiation of communication; supporting initiation requires a host to support sending of multiple encapsulations in order to determine which one to use. However, requiring hosts to send and receive multiple encapsulations is a potentially onerous requirement. The use of multiple encapsulation methods with differing Maximum Transfer Units (MTUs) can degrade performance. This can result in differing MTUs for on-link destinations. If the link-layer protocol does not provide per-destination MTUs to the IP layer, it will need to use a default MTU; to avoid fragmentation this must be less than or equal to the minimum MTU of on-link destinations. If the default MTU is too low, the full bandwidth may not be achievable. If the default MTU is too high, packet loss will result unless or until IP Path MTU Discovery is used to discover the correct MTU. [RFC1122], Section 2.3.3 notes the difficulties with this approach: Furthermore, it is not useful or even possible for a dual-format host to discover automatically which format to send, because of IAB Informational [Page 4]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 the problem of link-layer broadcasts. To enable hosts that only support sending and receiving of a single encapsulation to communicate with each other, a router can be utilized to segregate the hosts by encapsulation. Here only the router needs to support sending and receiving of multiple encapsulations. This requires assigning a separate prefix to each encapsulation, or else all hosts in the broadcast domain would not be reachable with a single encapsulation. [RFC1122] Section 2.3.3 provided guidance on encapsulation support: Every Internet host connected to a 10Mbps Ethernet cable: o MUST be able to send and receive packets using RFC-894 encapsulation; o SHOULD be able to receive RFC-1042 packets, intermixed with RFC-894 packets; and o MAY be able to send packets using RFC-1042 encapsulation. An Internet host that implements sending both the RFC-894 and the RFC-1042 encapsulation MUST provide a configuration switch to select which is sent, and this switch MUST default to RFC- 894. By making Ethernet encapsulation mandatory to implement for both send and receive, and also the default for sending, [RFC1122] recognized Ethernet as the predominant encapsulation, heading off potential interoperability problems. 1.3. Trailer Encapsulation Experience As noted in "Trailer Encapsulations" [RFC893], trailer encapsulation was an optimization developed to minimize memory-to-memory copies on reception. By placing variable length IP and transport headers at the end of the packet, page alignment of data could be more easily maintained. Trailers were implemented in 4.2 Berkeley System Distribution (BSD) (among others). While in theory trailer encapsulation could have been applied to both Ethernet and IEEE 802 encapsulations (creating four potential encapsulations of IP!), in practice trailer encapsulation was only supported for Ethernet. A separate Ethertype was utilized in order to enable IP packets in trailer encapsulation to be distinguished from [RFC894] encapsulation. [RFC1122] Section 2.3.1 described the issues with trailer IAB Informational [Page 5]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 encapsulation: DISCUSSION The trailer protocol is a link-layer encapsulation technique that rearranges the data contents of packets sent on the physical network. In some cases, trailers improve the throughput of higher layer protocols by reducing the amount of data copying within the operating system. Higher layer protocols are unaware of trailer use, but both the sending and receiving host MUST understand the protocol if it is used. Improper use of trailers can result in very confusing symptoms. Only packets with specific size attributes are encapsulated using trailers, and typically only a small fraction of the packets being exchanged have these attributes. Thus, if a system using trailers exchanges packets with a system that does not, some packets disappear into a black hole while others are delivered successfully. IMPLEMENTATION: On an Ethernet, packets encapsulated with trailers use a distinct Ethernet type [RFC893], and trailer negotiation is performed at the time that ARP is used to discover the link- layer address of a destination system. Specifically, the ARP exchange is completed in the usual manner using the normal IP protocol type, but a host that wants to speak trailers will send an additional "trailer ARP reply" packet, i.e., an ARP reply that specifies the trailer encapsulation protocol type but otherwise has the format of a normal ARP reply. If a host configured to use trailers receives a trailer ARP reply message from a remote machine, it can add that machine to the list of machines that understand trailers, e.g., by marking the corresponding entry in the ARP cache. Hosts wishing to receive trailers send trailer ARP replies whenever they complete exchanges of normal ARP messages for IP. Thus, a host that received an ARP request for its IP protocol address would send a trailer ARP reply in addition to the normal IP ARP reply; a host that sent the IP ARP request would send a trailer ARP reply when it received the corresponding IP ARP reply. In this way, either the requesting or responding host in an IP ARP exchange may request that it receive trailers. This scheme, using extra trailer ARP reply packets rather than IAB Informational [Page 6]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 sending an ARP request for the trailer protocol type, was designed to avoid a continuous exchange of ARP packets with a misbehaving host that, contrary to any specification or common sense, responded to an ARP reply for trailers with another ARP reply for IP. This problem is avoided by sending a trailer ARP reply in response to an IP ARP reply only when the IP ARP reply answers an outstanding request; this is true when the hardware address for the host is still unknown when the IP ARP reply is received. A trailer ARP reply may always be sent along with an IP ARP reply responding to an IP ARP request. Since trailer encapsulation negotiation depends on ARP, it can only be used where all hosts on the link are within the same broadcast domain. It was assumed that all hosts supported sending and receiving ARP packets in standard Ethernet encapsulation [RFC894], so that negotiation between Ethernet and IEEE 802 encapsulation was not required, only negotiation between standard Ethernet [RFC894] and trailer [RFC893] encapsulation. Had hosts supporting trailer encapsulation also supported IEEE 802 framing, the negotiation would have been complicated still further. [RFC1122] Section 2.3.1 provided the following guidance for use of trailer encapsulation: The trailer protocol for link-layer encapsulation MAY be used, but only when it has been verified that both systems (host or gateway) involved in the link-layer communication implement trailers. If the system does not dynamically negotiate use of the trailer protocol on a per-destination basis, the default configuration MUST disable the protocol. 4.2BSD did not support dynamic negotiation, only configuration of trailer encapsulation at boot time, and therefore [RFC1122] required that the trailer encapsulation be disabled by default on those systems. 1.4. Potential Mitigations In order to mitigate problems arising from multiple encapsulation methods, it may be possible to use switches or routers, or to attempt to negotiate the encapsulation method to be used. As described below, neither approach is completely satisfactory. The use of switches or routers to enable communication between hosts utilizing multiple encapsulation methods is not a panacea. If separate prefixes are used for each encapsulation, then each encapsulation method can be treated as a separate interface with the choice of encapsulation determined from the routing table. However, IAB Informational [Page 7]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 if the same prefix is used for each encapsulation method, it is necessary to keep state for each destination host. In situations where multiple encapsulation methods are enabled on a single link, negotiation may be supported to allow hosts to determine how to encapsulate a packet for a particular destination host. Negotiating the encapsulation above the link layer is potentially problematic since the negotiation itself may need to be carried out using multiple encapsulations. In theory it is possible to negotiate an encapsulation method by sending negotiation packets over all encapsulation methods supported, and keeping state for each destination host. However, if the encapsulation method must be dynamically negotiated for each new on-link destination, communication to new destinations may be delayed. If most communication is short, and the negotiation requires an extra round trip beyond link-layer address resolution, this can become a noticeable factor in performance. Also, the negotiation may result in consumption of additional bandwidth. 2. Evaluation of Arguments for Multiple Encapsulations There are several reasons often given in support of multiple encapsulation methods. We discuss each in turn, below. 2.1. Efficiency Claim: Multiple encapsulation methods allow for greater efficiency. For example, it has been argued that IEEE 802 or Ethernet encapsulation of IP results in excessive overhead due to the size of the data frame headers, and that this can adversely affect performance on wireless networks, particularly in situations where support of Voice over IP (VOIP) is required. Discussion: Even where these performance concerns are valid, solutions exist that do not require defining multiple IP encapsulation methods. For example, links may support Ethernet frame compression so that Ethernet Source and Destination Address fields are not sent with every packet. It is possible for link layers to negotiate compression without requiring higher layer awareness; the Point-to-Point Protocol (PPP) [RFC1661] is an example. "The PPP Compression Control Protocol (CCP)" [RFC1962] enables negotiation of data compression mechanisms, and "Robust Header Compression (ROHC) over PPP" [RFC3241] and "IP Header Compression over PPP" [RFC3544] enable negotiation of header compression, without Internet layer awareness. Any frame can be "decompressed" based on the content of the frame, and prior state IAB Informational [Page 8]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 based on previous control messages or data frames. Use of compression is a good way to solve the efficiency problem without introducing problems at higher layers. While PPP supports multiple Data Link Layer (DLL) encapsulation mechanisms for IP packets corresponding to different compression schemes, in practice only a single compression scheme is negotiated for use on a link. In addition to DLL protocol numbers allocated for IPv4 (0x0021) and IPv6 (0x0057), two codepoints have been assigned for RObust Header Compression (ROHC) [RFC3095] (ROHC small-CID (0x0003), ROHC large-CIDE (0x0005)), two for Van Jacobson compression [RFC1144] (Compressed TCP/IP (0x002d), Uncompressed TCP/IP (002f)), one for IPv6 Header Compression [RFC2507] (0x004f) and nine codepoints for RTP IP Header Compression [RFC3544], (Full Header (0x0061), Compressed TCP (0x0063), Compressed Non TCP (0x0065), UDP 8 (0x0067), RTP 8 (0x0069), Compressed TCP No Delta (0x2063), Context State (0x2065), UDP 16 (0x2067), RTP 16 (0x2069)). Recommendation: Where encapsulation is an efficiency issue, use header compression. Where the encapsulation method, or the use of compression, must be negotiated, negotiation should either occur as part of bringing up the link, or be piggybacked in the link-layer address resolution exchange; only a single compression scheme should be negotiated on a link. Where the MTU may vary among destinations on the same link, the link layer protocol should provide a per destination MTU to IP. 2.2. Multicast/Broadcast Claim: Support for Ethernet encapsulation requires layer 2 support for distribution of IP multicast/broadcast packets. In order to be receivable by any host within listening range, a multicast/broadcast packet sent over a wireless link needs to be sent at the lowest rate supported by listeners. Since the sending host typically does not keep track of the rates negotiated by group listeners, by default the sending rate for multicast/broadcast traffic defaults to the lowest supported rate, resulting in greatly increased overhead. Therefore support for Ethernet is potentially problematic and other encapsulations are necessary. Discussion: Irrespective of the encapsulation used, IP packets sent to multicast (IPv4/IPv6) or broadcast addresses (IPv4) need to reach all potential on-link receivers. Use of alternative encapsulations cannot remove this requirement. In order to limit the propagation of link-scope multicast or broadcast traffic, it is possible to assign a separate prefix to each host. Unlike broadcasts, which are received by all hosts on the link IAB Informational [Page 9]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 regardless of the protocol they are running, multicasts only need be received by those hosts belonging to the multicast group. In wired networks, it is possible to avoid forwarding multicast traffic on switch ports without group members, by snooping of Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) traffic as described in "Considerations for IGMP and MLD Snooping Switches" [RFC4541]. In wireless media where data rates to specific destinations are negotiated and may vary over a wide range, it may be more efficient to send multiple frames via link layer unicast than to send a single multicast/broadcast frame. For example, in [IEEE-802.11] multicast/broadcast traffic from the client station (STA) to the Access Point (AP) is sent via link layer unicast. Recommendation: Where support for link layer multicast/broadcast is problematic, limit the propagation of link-scope multicast and broadcast traffic by assignment of separate prefixes to hosts. In some circumstances, it may be more efficient to distribute multicast/broadcast traffic as multiple link-layer unicast frames. 2.3. Multiple Uses Claim: No single encapsulation is optimal for all purposes. Therefore where a link layer is utilized in disparate scenarios (such as both fixed and mobile deployments), multiple encapsulations are a practical requirement. Discussion: "Architectural Principles of the Internet" [RFC1958] point 3.2 states: If there are several ways of doing the same thing, choose one. If a previous design, in the Internet context or elsewhere, has successfully solved the same problem, choose the same solution unless there is a good technical reason not to. Duplication of the same protocol functionality should be avoided as far as possible, without of course using this argument to reject improvements. Existing encapsulations have proven themselves capable of supporting disparate usage scenarios. For example, the Point-to-Point Protocol (PPP) has been utilized by wireless link layers such as GPRS, as well as in wired networks in applications such as "PPP over SONET/SDH" [RFC2615]. PPP can even support bridging, as described in "Point-to- Point Protocol (PPP) Bridging Control Protocol (BCP)" [RFC3518]. Similarly, Ethernet encapsulation has been used in wired networks as well as Wireless Local Area Networks (LANs) such as IEEE 802.11 IAB Informational [Page 10]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 [IEEE-802.11]. Ethernet can also support Virtual LANs (VLANs) and Quality of Service (QoS) [IEEE-802.1Q]. Therefore disparate usage scenarios can be addressed by choice of a single encapsulation, rather than multiple encapsulations. Where an existing encapsulation is suitable, this is preferable to creating a new encapsulation. Where encapsulations other than IP over Point-to-Point Protocol (PPP) [RFC1661], Ethernet or IEEE 802 are supported, difficulties in operating system integration can lead to interoperability problems. In order to take advantage of operating system support for IP encapsulation over PPP, Ethernet or IEEE 802, it may be tempting for a driver supporting an alternative encapsulation to emulate PPP, Ethernet or IEEE 802 support. Typically, PPP emulation requires that the driver implement PPP, enabling translation of PPP control and data frames to the equivalent native facilities. Similarly, Ethernet or IEEE 802 emulation typically requires that the driver implement Dynamic Host Configuration Protocol (DHCP)v4 or v6, Router Solicitation/Router Advertisement (RS/RA), Address Resolution Protocol (ARP) or IPv6 Neighbor Discovery (ND) in order to enable translation of these frames to and from native facilities. Where drivers are implemented in kernel mode, the work required to provide faithful emulation may be substantial. This creates the temptation to cut corners, potentially resulting in interoperability problems. For example, it might be tempting for driver implementations to neglect IPv6 support. A driver emulating PPP might support only IPCP, but not IPCPv6; a driver emulating Ethernet or IEEE 802 might support only DHCPv4 and ARP, but not DHCPv6, RS/RA or ND. As a result, an IPv6 host connecting to a network supporting IPv6 might find itself unable to use IPv6 due to lack of driver support. Recommendation: Support a single existing encapsulation where possible. Emulation of PPP, Ethernet or IEEE 802 on top of alternative encapsulations should be avoided. 3. Additional Issues There are a number of additional issues arising from use of multiple encapsulation methods, as hinted at in section 1. We discuss each of these below. IAB Informational [Page 11]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 3.1. Generality Link layer protocols such as [IEEE.802-1A.1990] and [DIX] inherently support the ability to add support for a new packet type without modification to the link layer protocol. As noted in [Generic], [IEEE-802.16] appears to imply that the standard will need to be modified to support new packet types: We are concerned that the 802.16 protocol cannot easily be extendable to transport new protocols over the 802.16 air interface. It would appear that a Convergence Sublayer is needed for every type of protocol transported over the 802.16 MAC. Every time a new protocol type needs to be transported over the 802.16 air interface, the 802.16 standard needs to be modified to define a new CS type. We need to have a generic Packet Convergence Sublayer that can support multi-protocols and which does not require further modification to the 802.16 standard to support new protocols. We believe that this was the original intention of the Packet CS. Furthermore, we believe it is difficult for the industry to agree on a set of CSes that all devices must implement to claim "compliance". The use of IP and/or upper layer protocol specific encapsulation methods, rather than a 'neutral' general purpose encapsulation may give rise to a number of undesirable effects explored in the following subsections. If the link layer does not provide a general purpose encapsulation method, deployment of new IP and/or upper layer protocols will be dependent on deployment of the corresponding new encapsulation support in the link layer. Even if a single encapsulation method is used, problems can still occur if de-multiplexing of ARP, IPv4, IPv6, and any other protocols in use, is not supported at the link layer. While is possible to demultiplex such packets based on the Version field (first four bits on the packet), this assumes that IPv4-only implementations will be able to properly handle IPv6 packets. As a result, a more robust design is to demultiplex protocols in the link layer, such as by assigning a different protocol type, as is done in IEEE 802 media where a Type of 0x0800 is used for IPv4, and 0x86DD for IPv6. Recommendations: Link layer protocols should enable network packets (IPv4, IPv6, ARP, etc.) to be de-multiplexed in the link layer. IAB Informational [Page 12]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 3.2. Layer Interdependence Standardizing IP and/or upper layer specific classification schemes in the link layer will almost inevitably lead to interdependencies between the two specifications. Although this might appear to be desirable in terms of providing a highly specific (and hence interoperable) mapping between the capabilities provided by the link layer (e.g., quality of service support) and those that are needed by the upper layer, this sort of capability is probably better provided by a more comprehensive service interface (Application Programming Interface) in conjunction with a single encapsulation mechanism. IPv6, in particular, provides an extensible header system. An upper layer specific classification scheme would still have to provide a degree of generality in order to cope with future extensions of IPv6 that might wish to make use of some of the link layer services already provided. Recommendations: Upper layer specific classification schemes should be avoided. 3.3. Inspection of Payload Contents If an IP or upper layer specific classification scheme proposes to inspect the contents of the packet being encapsulated (e.g., 802.16 IP CS mechanisms for determining the connection identifier (CID) to use to transmit a packet), the fields available for inspection may be limited if the packet is compressed or encrypted before passing to the link layer. This may prevent the link layer from utilizing existing compression mechanisms, such as Van Jacobson Compression [RFC1144], ROHC [RFC3095][RFC3759], Compressed RTP (CRTP) [RFC2508], Enhanced Compressed RTP (ECRTP) [RFC3545] or IP Header Compression [RFC2507]. Recommendations: Classification schemes should not rely on the contents of the layer 3 payload. 3.4. Interoperability Guidance In situations where multiple CSes are operational and capable of carrying IP traffic, interoperability problems are possible in the absence of clear implementation guidelines. For example, there is no guarantee that other hosts on the link will support the same set of CSes, or that if they do, that their routing tables will result in identical preferences. IEEE 802.16 [IEEE-802.16] splits the Media Access Control (MAC) layer into a number of sublayers. For the uppermost of these, the standard IAB Informational [Page 13]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 defines the concept of a service-specific Convergence Sublayer (CS). The two underlying sublayers (the MAC Common Part Sublayer and the Security Sublayer) provide common services for all instantiations of the CS. While [IEEE-802.16] defined support for the Asynchronous Transfer Mode (ATM) CS and the Packet CS, [IEEE-802.16e] added support for four new Convergence Sublayers. As a result, [IEEE-802.16e] defines the ATM CS, Packet CS, Ethernet CS, IPv4 CS and IPv6 CS as well as eight other Convergence Sublayers. In 802.16 the Mobile Station (MS) indicates the Convergence Sublayers it supports to the Base Station (BS), which selects from the list one or more that it will support on the link. Therefore it is possible for multiple CSes to be operational. However, IEEE 802.16 does not provide multiple encapsulation methods for the same kind of payload; it defines exactly one encapsulation scheme for each payload. For example, there is one way to encapsulate a raw IPv4 packet into an IEEE 802.16 MAC frame, one encapsulation scheme for a raw IPv6 packet, etc. There is also one way to encapsulate an Ethernet frame, even when there are multiple possibilities for classifying an Ethernet frame for forwarding over a Connection ID (CID). Since support for multiple CSes enables IEEE 802.16 to encapsulate layer 2 frames as well as layer 3 packets, IP packets may be directly encapsulated in IEEE802.16 MAC frames as well as framed with Ethernet headers in IEEE802.16 MAC frames. Where CSes supporting both layer 2 frames as well as layer 3 packets are operational on the same link, a number of issues may arise, including: Use of Address Resolution Protocol (ARP) Where both IPv4 CS and Ethernet CS are operational, it may not be obvious how address resolution should be implemented. For example, should an ARP frame be encapsulated over the Ethernet CS, or should alternative mechanisms be used for address resolution, utilizing the IPv4 CS? Data Frame Encapsulation When sending an IP packet, which CS should be used? Where multiple CSes are operational, the issue can be treated as a multi-homing problem, with each CS constituting its own interface. Since a given CS may have associated bandwidth or quality of service constraints, routing metrics could be adjusted to take this into account, allowing the routing layer to choose based on which CS appears more attractive. This could lead to interoperability problems or routing asymmetry. For example, consider the effects on IPv6 Neighbor Discovery: IAB Informational [Page 14]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 [a] If hosts choose to send IPv6 Neighbor Discovery traffic on different CSes, it is possible that a host sending an IPv6 Neighbor Discovery packet will not receive a reply, even though the target host is reachable over another CS. [b] Where hosts all support the same set of CSes, but have different routing preferences, it is possible for a host to send an IPv6 Neighbor Discovery packet over one CS and receive a reply over another CS. Recommendations: Given these issues, it is strongly recommended that only a single CS supporting a single encapsulation method be usable in a given circumstance. 3.5. Service Consistency If a link layer protocol provides multiple encapsulation methods, the services offered to the IP and upper layer protocols may differ qualitatively between the different encapsulation methods. For example, the 802.16 [IEEE-802.16] link layer protocol offers both 'native' encapsulation for raw IPv4 and IPv6 packets, and emulated Ethernet encapsulation. In the raw case, the IP layer has direct access to the quality of service (QoS) capabilities of the 802.16 transmission channels, whereas using the Ethernet encapsulation the IP QoS has first to be mapped through the rather more limited capabilities of Ethernet QoS. Consequently, the service offered to an application depends on the encapsulation method employed and may be inconsistent between sessions. This may be confusing for the user and the application. Recommendations: If multiple encapsulation methods for IP packets on a single link layer technology are deemed to be necessary, care should be taken to match the services available between encapsulation methods as closely as possible. 3.6. Implementation Complexity Support of multiple encapsulation methods results in additional implementation complexity. Lack of uniform encapsulation support also results in potential interoperability problems. To avoid interoperability issues, devices with limited resources may be required to implement multiple encapsulation mechanisms, which may not be practical. When encapsulation methods require hardware support, implementations may choose to support different encapsulation sets, resulting in market fragmentation. This can prevent users from benefiting from economies of scale, precluding some uses of the technology entirely. IAB Informational [Page 15]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 Recommendations: Choose a single mandatory to implement encapsulation mechanism for both sending and receiving, and make that encapsulation mechanism the default for sending. 3.7. Negotiation The complexity of negotiation within ARP or IP can be reduced by performing encapsulation negotiation within the link layer. However, unless the link layer allows the negotiation of the encapsulation between any two hosts, then interoperability problems can still result if more than one encapsulation is possible on a given link. In general, a host cannot assume that all other hosts on a link support the same set of encapsulation methods, so that unless a link layer protocol only supports point-to-point communication, negotiation of multiple potential encapsulation methods will be problematic. To avoid this problem, it is desirable for link layer encapsulation negotiation to determine a single IP encapsulation, not merely to indicate which encapsulation methods are possible. Recommendations: Encapsulation negotiation is best handled in the link layer. In order to avoid dependencies on the data frame encapsulation mechanism, it is preferable for the negotiation to be carried out using management frames, if they are supported. If multiple encapsulations are required and negotiation is provided, then the negotiation should result in a single encapsulation method being negotiated on the link. 3.8. Roaming Where a mobile node roams between base stations or to a fixed infrastructure and the base stations and fixed infrastructure do not all support the same set of encapsulations, then it may be necessary to alter the encapsulation method, potentially in mid-conversation. Even if the change can be handled seamlessly at the link and IP layer so that applications are not affected, unless the services offered over the different encapsulations are equivalent (see Section 3.5) the service experienced by the application may change as the mobile node crosses boundaries. If the service is significantly different, it might even require 'in-flight' renegotiation which most applications are not equipped to manage. Recommendations: Ensure uniformity of the encapsulation set (preferably only a single encapsulation) within a given mobile domain, between mobile domains, and between mobile domains and fixed infrastructure. If a link layer protocol offers multiple encapsulation methods for IP packets, it is strongly recommended that only one of these encapsulation methods should be in use on any given IAB Informational [Page 16]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 link or within a single wireless transmission domain. 4. Security Considerations The use of multiple encapsulation methods does not appear to have significant security implications. An attacker might be able to utilize an encapsulation method which was not in normal use on a link to cause a Denial of Service attack which would exhaust the processing resources of interfaces if packets utilizing this encapsulation were passed up the stack to any significant degree before being discarded. However, the use of encapsulation methods that need to inspect fields in the packet being encapsulated in order to provide service classification might deter the deployment of end-to-end security; this is undesirable. Similarly, if one method is rarely used, that method is potentially more likely to have exploitable implementation bugs. An attacker might be able to force a more cumbersome encapsulation method between two endpoints, even when a lighter weight one is available, hence forcing higher resource consumption on the link and within those endpoints. If different methods have different security properties, an attacker might be able to force a less secure method as an elevation path to get access to some other resource or data. Where lower layer classification is implemented, encryption of upper layer headers (e.g. IPsec tunnel mode), may obscure headers required for classification. As a result, it may be necessary for all encrypted traffic to flow over a single connection. 5. IANA Considerations This document has no actions for IANA. 6. Conclusion The use of multiple encapsulation methods on the same link is problematic, as discussed above. Although multiple IP encapsulation methods were defined on Ethernet cabling, recent implementations support only the Ethernet encapsulation of IPv4 defined in [RFC894]. In order to avoid a repeat of the experience with IPv4, for operation of IPv6 on IEEE 802.3 media, only the Ethernet encapsulation was defined in "A Method for the Transmission of IPv6 Packets over Ethernet Networks" IAB Informational [Page 17]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 [RFC1972], later updated in [RFC2464]. In addition to the recommendations given earlier, we give the following general recommendations to avoid problems resulting from use of multiple IP encapsulation methods: When developing standards for encapsulating IP packets on a link layer technology, it is desirable that only a single encapsulation method should be standardized for each link layer technology; If a link layer protocol offers multiple encapsulation methods for IP packets, it is strongly recommended that only one of these encapsulation methods should be in use within any given link or wireless transmission domain; Where multiple encapsulation methods are supported on a link, a single encapsulation should be mandatory to implement for send and receive. 7. References 7.1. Informative References [DIX] Digital Equipment Corporation, Intel Corporation, and Xerox Corporation, "The Ethernet -- A Local Area Network: Data Link Layer and Physical Layer (Version 2.0)", November 1982. [Generic] Wang, L. et al, "A Generic Packet Convergence Sublayer (GPCS) for Supporting Multiple Protocols over 802.16 Air Interface", Submission to IEEE 802.16g: CB0216g_05_025r4.pdf, November 2005, <http:// www.ieee802.org/16/netman/contrib/C80216g-05_025r4.pdf>. [IEEE-802.16] Institute of Electrical and Electronics Engineers, "Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks, Part 16: Air Interface for Fixed Broadband Wireless Access Systems", IEEE Standard 802.16-2004, October 2004. [IEEE-802.16e] Institute of Electrical and Electronics Engineers, "Information technology - Telecommunications and information exchange between systems - Local and Metropolitan Area Networks - Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, Amendment for Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed IAB Informational [Page 18]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 Bands", IEEE P802.16e, September 2005. [IEEE.802-1A.1990] Institute of Electrical and Electronics Engineers, "Local Area Networks and Metropolitan Area Networks: Overview and Architecture of Network Standards", IEEE Standard 802.1A, 1990. [IEEE.802-1D.1998] Institute of Electrical and Electronics Engineers, "Information technology - Telecommunications and information exchange between systems - Local area networks - Media access control (MAC) bridges", IEEE Standard 802.1D, 1998. [IEEE.802-3.1985] Institute of Electrical and Electronics Engineers, "Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications", IEEE Standard 802.3, 1985. [IEEE-802.11] Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE Standard 802.11, 2003. [RFC893] Leffler, S. and M. Karels, "Trailer encapsulations", RFC 893, April 1984. [RFC894] Hornig, C., "Standard for the transmission of IP datagrams over Ethernet networks", STD 41, RFC 894, April 1984. [RFC1042] Postel, J. and J. Reynolds, "Standard for the transmission of IP datagrams over IEEE 802 networks", STD 43, RFC 1042, February 1988. [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1958] Carpenter, B., "Architectural Principles of the Internet", RFC 1958, June 1996. [RFC1962] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962, June 1996. IAB Informational [Page 19]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 [RFC1972] Crawford, M., "A Method for the Transmission of IPv6 Packets over Ethernet Networks", RFC 1972, August 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, December 1998. [RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP Header Compression", RFC 2507, February 1999. [RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [RFC2615] Malis, A. and W. Simpson, "PPP over SONET/SDH", RFC 2615, June 1999. [RFC3095] Bormann, C., et. al, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP and uncompressed", RFC 3095, July 2001. [RFC3241] Bormann, C., "Robust Header Compression (ROHC) over PPP", RFC 3241, April 2002. [RFC3518] Higashiyama, M., Baker, F. and T. Liao, "Point-to-Point Protocol (PPP) Bridging Control Protocol (BCP)", RFC 3518, April 2003. [RFC3544] Koren, T., Casner, S. and C. Bormann, "IP Header Compression over PPP", RFC 3544, July 2003. [RFC3545] Koren, T., Casner, S., Geevarghese, J., Thompson, B., and P. Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering", RFC 3545, July 2003. [RFC3759] Jonsson, L-E., "RObust Header Compression (ROHC): Terminology and Channel Mapping Examples", RFC 3759, April 2004. [RFC4541] Christensen, M., Kimball, K. and F. Solensky, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541, May 2006. IAB Informational [Page 20]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 Acknowledgments The authors would like to acknowledge Jeff Mandin, Bob Hinden, Jari Arkko, and Phil Roberts for contributions to this document. Appendix A - IAB Members at the time of this writing Bernard Aboba Loa Andersson Brian Carpenter Leslie Daigle Elwyn Davies Kevin Fall Olaf Kolkman Kurtis Lindqvist David Meyer David Oran Eric Rescorla Dave Thaler Lixia Zhang Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: bernarda@microsoft.com Phone: +1 425 706 6605 Fax: +1 425 936 7329 Elwyn B. Davies Consultant Soham, Cambs UK Phone: +44 7889 488 335 EMail: elwynd@dial.pipex.com Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: dthaler@microsoft.com IAB Informational [Page 21]
INTERNET-DRAFT Multiple Encapsulation Methods Harmful 1 October 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. IAB Informational [Page 22]
