Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Intended status: Standards Track June 12, 2009 Expires: December 14, 2009 The Subnetwork Encapsulation and Adaptation Layer (SEAL) draft-templin-intarea-seal-00.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 December 14, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract For the purpose of this document, subnetworks are defined as virtual topologies that span connected network regions bounded by encapsulating border nodes. These virtual topologies may span Templin Expires December 14, 2009 [Page 1] Internet-Draft SEAL June 2009 multiple IP and/or sub-IP layer forwarding hops, and can introduce failure modes due to packet duplication and/or links with diverse Maximum Transmission Units (MTUs). This document specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that accommodates such virtual topologies over diverse underlying link technologies. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 6 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 7 4. SEAL Protocol Specification (Version 0) . . . . . . . . . . . 8 4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 8 4.2. SEAL Header Format (Version 0) . . . . . . . . . . . . . . 10 4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 11 4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 11 4.3.2. Admitting Packets into the Tunnel Interface . . . . . 12 4.3.3. Inner Fragmentation and Segmentation . . . . . . . . . 12 4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 14 4.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 14 4.3.6. Packet Identification . . . . . . . . . . . . . . . . 15 4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 15 4.3.8. Processing Raw ICMPv4 Messages . . . . . . . . . . . . 15 4.3.9. Processing SEAL Errors . . . . . . . . . . . . . . . . 16 4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 17 4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 17 4.4.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 17 4.4.3. Sending SEAL Fragmentation Reports . . . . . . . . . . 18 4.4.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 18 4.4.5. Decapsulation and Delivery to Upper Layers . . . . . . 19 4.4.6. Generating SEAL Error Messages . . . . . . . . . . . . 19 5. SEAL Protocol Specification (Version 1) . . . . . . . . . . . 21 5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 21 5.2. SEAL Header Format (Version 1) . . . . . . . . . . . . . . 21 5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 22 5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 22 5.3.2. Admitting Packets into the Tunnel Interface . . . . . 22 5.3.3. Inner Fragmentation and Segmentation . . . . . . . . . 23 5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 23 5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 23 5.3.6. Packet Identification . . . . . . . . . . . . . . . . 23 5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 23 5.3.8. Processing Raw ICMPv4 Messages . . . . . . . . . . . . 23 5.3.9. Processing SEAL Errors . . . . . . . . . . . . . . . . 24 Templin Expires December 14, 2009 [Page 2] Internet-Draft SEAL June 2009 5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 24 5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 24 5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 24 5.4.3. Sending SEAL Fragmentation Reports . . . . . . . . . . 24 5.4.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 24 5.4.5. Decapsulation and Delivery to Upper Layers . . . . . . 24 5.4.6. Sending SEAL Error Messages . . . . . . . . . . . . . 24 6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 24 7. End System Requirements . . . . . . . . . . . . . . . . . . . 25 8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 25 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 10. Security Considerations . . . . . . . . . . . . . . . . . . . 25 11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 26 12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 26 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 14.1. Normative References . . . . . . . . . . . . . . . . . . . 28 14.2. Informative References . . . . . . . . . . . . . . . . . . 28 Appendix A. Reliability Extensions . . . . . . . . . . . . . . . 30 Appendix B. Transport Mode . . . . . . . . . . . . . . . . . . . 30 Appendix C. Historic Evolution of PMTUD . . . . . . . . . . . . . 31 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32 Templin Expires December 14, 2009 [Page 3] Internet-Draft SEAL June 2009 1. Introduction As Internet technology and communication has grown and matured, many techniques have developed that use virtual topologies (including tunnels of one form or another) over an actual network that supports the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual topologies have elements that appear as one hop in the virtual topology, but are actually multiple IP or sub-IP layer hops. These multiple hops often have quite diverse properties that are often not even visible to the endpoints of the virtual hop. This introduces failure modes that are not dealt with well in current approaches. The use of IP encapsulation has long been considered as the means for creating such virtual topologies. However, the insertion of an outer IP header reduces the effective path MTU as-seen by the IP layer. When IPv4 is used, this reduced MTU can be accommodated through the use of IPv4 fragmentation, but unmitigated in-the-network fragmentation has been found to be harmful through operational experience and studies conducted over the course of many years [FRAG][FOLK][RFC4963]. Additionally, classical path MTU discovery [RFC1191] has known operational issues that are exacerbated by in- the-network tunnels [RFC2923][RFC4459]. In the following subsections, we present further details on the motivation and approach for addressing these issues. 1.1. Motivation Before discussing the approach, it is necessary to first understand the problems. In both the Internet and private-use networks today, IPv4 is ubiquitously deployed as the Layer 3 protocol. The two primary functions of IPv4 are to provide for 1) addressing, and 2) a fragmentation and reassembly capability used to accommodate links with diverse MTUs. While it is well known that the addressing properties of IPv4 are limited (hence, the larger address space provided by IPv6), there is a lesser-known but growing consensus that other limitations may be unable to sustain continued growth. First, the IPv4 header Identification field is only 16 bits in length, meaning that at most 2^16 packets pertaining to the same (source, destination, protocol, Identification)-tuple may be active in the Internet at a given time. Due to the escalating deployment of high-speed links (e.g., 1Gbps Ethernet), however, this number may soon become too small by several orders of magnitude. Furthermore, there are many well-known limitations pertaining to IPv4 fragmentation and reassembly - even to the point that it has been deemed "harmful" in both classic and modern-day studies (cited above). In particular, IPv4 fragmentation raises issues ranging from minor annoyances (e.g., in-the-network router fragmentation) to the Templin Expires December 14, 2009 [Page 4] Internet-Draft SEAL June 2009 potential for major integrity issues (e.g., mis-association of the fragments of multiple IP packets during reassembly). As a result of these perceived limitations, a fragmentation-avoiding technique for discovering the MTU of the forward path from a source to a destination node was devised through the deliberations of the Path MTU Discovery Working Group (PMTUDWG) during the late 1980's through early 1990's (see Appendix C). In this method, the source node provides explicit instructions to routers in the path to discard the packet and return an ICMP error message if an MTU restriction is encountered. However, this approach has several serious shortcomings that lead to an overall "brittleness". In particular, site border routers in the Internet are being configured more and more to discard ICMP error messages coming from the outside world. This is due in large part to the fact that malicious spoofing of error messages in the Internet is made simple since there is no way to authenticate the source of the messages. Furthermore, when a source node that requires ICMP error message feedback when a packet is dropped due to an MTU restriction does not receive the messages, a path MTU-related black hole occurs. This means that the source will continue to send packets that are too large and never receive an indication from the network that they are being discarded. The issues with both IPv4 fragmentation and this "classical" method of path MTU discovery are exacerbated further when IP-in-IP tunneling is used. For example, site border routers that are configured as ingress tunnel endpoints may be required to forward packets into the subnetwork on behalf of hundreds, thousands, or even more original sources located within the site. If IPv4 fragmentation were used, this would quickly wrap the 16-bit Identification field and could lead to undetected data corruption. If classical IPv4 path MTU discovery were used instead, the site border router may be inconvenienced by excessive ICMP error messages coming from the subnetwork that may be either untrustworthy or insufficiently provisioned to allow translation into error messages to be returned to the original sources. The situation is exacerbated further still by IPsec tunnels, since only the first IPv4 fragment of a fragmented packet contains the transport protocol selectors (e.g., the source and destination ports) required for identifying the correct security association rendering fragmentation useless under certain circumstances. Even worse, there may be no way for a site border router that configures an IPsec tunnel to transcribe the encrypted packet fragment contained in an ICMP error message into a suitable ICMP error message to return to the original source. Due to these many limitations, a new approach Templin Expires December 14, 2009 [Page 5] Internet-Draft SEAL June 2009 to accommodate links with diverse MTUs is necessary. 1.2. Approach For the purpose of this document, subnetworks are defined as virtual topologies that span connected network regions bounded by encapsulating border nodes. Subnetworks in this sense correspond exactly to the "enterprise" abstraction defined in Virtual Enterprise Traversal (VET) [I-D.templin-autoconf-dhcp] and Routing and Addressing in Next-Generation EnteRprises (RANGER) [I-D.templin-ranger][I-D.russert-rangers]. Examples include the global Internet interdomain routing core, Mobile Ad hoc Networks (MANETs) and enterprise networks. Subnetwork border nodes forward unicast and multicast IP packets over the virtual topology across multiple IP and/or sub-IP layer forwarding hops that may introduce packet duplication and/or traverse links with diverse Maximum Transmission Units (MTUs). This document introduces a Subnetwork Encapsulation and Adaptation Layer (SEAL) for tunnel-mode operation of IP over subnetworks that connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border nodes. It provides a standalone specification designed to be tailored to specific associated tunneling protocols such as VET [I-D.templin-autoconf-dhcp], the Locator-Identifier Split Protocol (LISP) [I-D.ietf-lisp] and others. A transport-mode of operation is also possible, and described in Appendix B. SEAL accommodates links with diverse MTUs, protects against off-path denail-of-service attacks, and supports efficient duplicate packet detection through the use of a minimal mid-layer encapsulation. SEAL encapsulation introduces an extended Identification field for packet identification and a mid-layer segmentation and reassembly capability that allows simplified cutting and pasting of packets. Moreover, SEAL senses in-the-network IPv4 fragmentation as a "noise" indication that packet sizing parameters are "out of tune" with respect to the network path. As a result, SEAL can naturally tune its packet sizing parameters to eliminate the in-the-network fragmentation. SEAL encapsulation additionally includes a 2-bit version number to accommodate future protocol versions. This document specifies SEAL protocol versions 0 and 1. 2. Terminology and Requirements The terms "inner", "mid-layer", and "outer", respectively, refer to the innermost IP (layer, protocol, header, packet, etc.) before any Templin Expires December 14, 2009 [Page 6] Internet-Draft SEAL June 2009 encapsulation, the mid-layer IP (protocol, header, packet, etc.) after any mid-layer '*' encapsulation, and the outermost IP (layer, protocol, header, packet etc.) after SEAL/*/IPv4 encapsulation. The term "IP" used throughout the document refers to either Internet Protocol version (IPv4 or IPv6). Additionally, the notation IPvX/*/ SEAL/*/IPvY refers to an inner IPvX packet encapsulated in any mid- layer '*' encapsulations, followed by the SEAL header, followed by any outer '*' encapsulations, followed by an outer IPvY header, where the notation "IPvX" means either IP protocol version (IPv4 or IPv6). The following abbreviations correspond to terms used within this document and elsewhere in common Internetworking nomenclature: ITE - Ingress Tunnel Endpoint ETE - Egress Tunnel Endpoint PTB - an ICMPv6 "Packet Too Big", an ICMPv4 "Fragmentation Needed" or a SEAL "Fragmentation Report" message DF - the IPv4 header "Don't Fragment" flag MHLEN - the length of any mid-layer '*' headers and trailers OHLEN - the length of the outer encapsulating SEAL/*/IPv4 headers HLEN - the sum of MHLEN and OHLEN S_MRU - the per-ETE SEAL Maximum Reassembly Unit S_MSS - the SEAL Maximum Segment Size SEAL_ID - a 32-bit Identification value, randomly initialized and monotonically incremented for each SEAL protocol packet SEAL_PROTO - an IPv4 protocol number used for SEAL SEAL_PORT - a TCP/UDP service port number used for SEAL The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119]. 3. Applicability Statement SEAL was motivated by the specific case of subnetwork abstraction for Mobile Ad hoc Networks (MANETs); however, the domain of applicability Templin Expires December 14, 2009 [Page 7] Internet-Draft SEAL June 2009 also extends to subnetwork abstractions of enterprise networks, ISP networks, SOHO networks, the interdomain routing core, and many others. In particular, SEAL is a natural complement to the enterprise network abstraction manifested through the VET mechanism [I-D.templin-autoconf-dhcp], the RANGER architecture [I-D.templin-ranger][I-D.russert-rangers] and the LISP protocol [I-D.ietf-lisp]. Indeed, the term "subnetwork" within this document is used synonymously with the term "enterprise" that appears in these references. SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation (e.g., as IPv6/SEAL/IPv4), and appears as a subnetwork encapsulation as seen by the inner IP layer. SEAL can also be used as a sublayer for encapsulating inner IP packets within outer UDP/IPv4 headers (e.g., as IPv6/SEAL/UDP/IPv4) such as for the Teredo domain of applicability [RFC4380]. When it appears immediately after the outer IPv4 header, the SEAL header is processed exactly as for IPv6 extension headers. This document discusses the use of IPv4 as the outer encapsulation layer; however, the same principles apply when IPv6 is used as the outer layer. 4. SEAL Protocol Specification (Version 0) This section specifies the fully-functioned version of SEAL known as "SEAL Version 0", or "Classical SEAL". A minimal version of SEAL known as "SEAL Version 1", or "SEAL-lite", is specified in Section 5. 4.1. Model of Operation SEAL provides an encapsulation sublayer that supports the transmission of unicast and multicast packets across an underlying IP network. SEAL-enabled ITEs insert a SEAL header during the encapsulation of inner IP packets in mid-layer and outer encapsulating headers/trailers. For example, an inner IPv6 packet would appear as IPv6/*/SEAL/*/IPv4 after mid-layer and outer encapsulations, where '*' denotes zero or more additional encapsulation sublayers. SEAL-enabled ITEs add mid-layer '*' and outer SEAL/*/IPv4 encapsulations to the inner packets they inject into a subnetwork, where the outermost IPv4 header contains the source and destination addresses of the subnetwork entry/exit points (i.e., the ITE/ETE), respectively. ITEs encapsulate each inner IP packet in mid-layer and outer encapsulations as shown in Figure 1: Templin Expires December 14, 2009 [Page 8] Internet-Draft SEAL June 2009 +-------------------------+ | | ~ Outer */IPv4 headers ~ | | I +-------------------------+ n | SEAL Header | n +-------------------------+ +-------------------------+ e ~ Any mid-layer * headers ~ ~ Any mid-layer * headers ~ r +-------------------------+ +-------------------------+ | | | | I --> ~ Inner IP ~ --> ~ Inner IP ~ P --> ~ Packet ~ --> ~ Packet ~ | | | | P +-------------------------+ +-------------------------+ a ~ Any mid-layer trailers ~ ~ Any mid-layer trailers ~ c +-------------------------+ +-------------------------+ k ~ Any outer trailers ~ e +-------------------------+ t (After mid-layer encaps.) (After SEAL/*/IPv4 encaps.) Figure 1: SEAL Encapsulation where the SEAL header is inserted as follows: o For simple IPvX/IPv4 encapsulations (e.g., [RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between the inner IP and outer IPv4 headers as: IPvX/SEAL/IPv4. o For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the SEAL header is inserted between the {AH,ESP} header and outer IPv4 headers as: IPvX/*/{AH,ESP}/SEAL/IPv4. o For IP encapsulations over transports such as UDP, the SEAL header is inserted immediately after the outer transport layer header, e.g., as IPvX/*/SEAL/UDP/IPv4. SEAL-encapsulated packets include a 32-bit SEAL_ID formed from the concatenation of the 16-bit ID Extension field in the SEAL header as the most-significant bits, and with the 16-bit Identification value in the outer IPv4 header as the least-significant bits. (For tunnels that traverse middleboxes that might rewrite the IPv4 ID field, e.g., a Network Address Translator, the SEAL_ID is instead maintained only within the ID field in the SEAL header.) Routers within the subnetwork use the SEAL_ID for duplicate packet detection, and ITEs/ ETEs use the SEAL_ID for SEAL segmentation/reassembly and protection against off-path attacks. Templin Expires December 14, 2009 [Page 9] Internet-Draft SEAL June 2009 SEAL enables a multi-level segmentation and reassembly capability. First, the ITE can use IPv4 fragmentation to fragment inner IPv4 packets with DF=0 before SEAL encapsulation. Secondly, the SEAL layer itself provides a simple cutting-and-pasting capability for mid-layer packets to avoid IPv4 fragmentation on the outer packet. Finally, ordinary IPv4 fragmentation is permitted on the outer packet after SEAL encapsulation and used to detect and dampen any in-the- network fragmentation as quickly as possible. The following sections specify the SEAL header format and SEAL- related operations of the ITE and ETE, respectively. 4.2. SEAL Header Format (Version 0) The SEAL version 0 header is formatted as follows: 0 1 2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |VER|A|D|M| SEG | Next Header | ID Extension | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: SEAL Version 0 Header Format where the header fields are defined as: VER (2) a 2-bit value that encodes the SEAL protocol version number. This section describes Version 0 of the SEAL protocol, i.e., the VER field encodes the value '00'. A (1) the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes to receive an explicit acknowledgement from the ETE. D (1) the "Don't Fragment" bit. Copied from the D flag in the SEAL header of the inner packet if the inner packet is itself a SEAL/IP packet. Otherwise, set to 0 if the inner packet is an IPv4 packet with DF=0. Otherwise, set to 1. M (1) the "More Segments" bit. Set to 1 if this SEAL protocol packet contains a non-final segment of a multi-segment mid-layer packet. Templin Expires December 14, 2009 [Page 10] Internet-Draft SEAL June 2009 R (1) the "reserved" bit. Set to 0 for the purpose of this specification. SEG (2) a 2-bit segment number. Encodes a segment number between 0 - 3. Next Header (8) an 8-bit field that encodes an Internet Protocol number the same as for the IPv4 protocol and IPv6 next header fields. ID Extension (16) a 16-bit extension of the Identification field in the outer IPv4 header; encodes the most-significant 16 bits of a 32 bit SEAL_ID value. 4.3. ITE Specification 4.3.1. Tunnel Interface MTU The ITE configures a tunnel virtual interface over one or more underlying links that connect the border node to the subnetwork. The tunnel interface must present a fixed MTU to the inner IP layer (i.e., Layer 3) as the size for admission of inner IP packets into the tunnel. Since the tunnel interface may support a potentially large set of ETEs, however, care must be taken in setting a large- enough MTU for all ETEs while still upholding end system expectations. Due to the ubiquitous deployment of standard Ethernet and similar networking gear, the nominal Internet cell size has become 1500 bytes; this is the de facto size that end systems have come to expect will either be delivered by the network without loss due to an MTU restriction on the path or a suitable PTB message returned. However, the network may not always deliver the necessary PTBs, leading to MTU-related black holes [RFC2923]. The ITE therefore requires a means for conveying 1500 byte (or smaller) packets to the ETE without loss due to MTU restrictions and without dependence on PTB messages from within the subnetwork. In common deployments, there may be many forwarding hops between the original source and the ITE. Within those hops, there may be additional encapsulations (IPSec, L2TP, other SEAL encapsulations, etc.) such that a 1500 byte packet sent by the original source might grow to a larger size by the time it reaches the ITE for encapsulation as an inner IP packet. Similarly, additional encapsulations on the path from the ITE to the ETE could cause the encapsulated packet to become larger still and trigger in-the-network Templin Expires December 14, 2009 [Page 11] Internet-Draft SEAL June 2009 fragmentation. In order to preserve the end system expectations, the ITE therefore requires a means for conveying these larger packets to the ETE even though there may be links within the subnetwork that configure a smaller MTU. The ITE should therefore set a tunnel virtual interface MTU of 1500 bytes plus extra room to accommodate any additional encapsulations that may occur on the path from the original source (i.e., even if the path to the ETE does not support an MTU of this size). The ITE can set larger MTU values still, but should select a value that is not so large as to cause excessive PTBs coming from within the tunnel interface (see Sections 4.3.3 and 4.3.8). The ITE can also set smaller MTU values; however, care must be taken not to set so small a value that original sources would experience an MTU underflow. In particular, IPv6 sources must see a minimum path MTU of 1280 bytes, and IPv4 sources should see a minimum path MTU of 576 bytes. 4.3.2. Admitting Packets into the Tunnel Interface The inner IP layer consults the tunnel interface MTU when admitting a packet into the interface. For IPv4 packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet is larger than the tunnel interface MTU the inner IP layer uses IP fragmentation to break the packet into fragments no larger than the tunnel interface MTU. The ITE then admits each fragment into the tunnel as an independent packet. For all other packets, the ITE admits the packet if it is no larger than the tunnel interface MTU; otherwise, it drops the packet and sends an ICMP PTB error message to the source with the MTU value set to the tunnel interface MTU. The message must contain as much of the invoking packet as possible without the entire message exceeding the minimum IP MTU (i.e., 576 bytes for IPv4 and 1280 bytes for IPv6). 4.3.3. Inner Fragmentation and Segmentation For each ETE, the maintains soft state within the tunnel interface (e.g., in a destination cache) used to support inner fragmentation and/or SEAL segmentation. The soft state includes the following: o a Mid-layer Header Length (MHLEN); set to the length of any mid- layer '*' encapsulation headers and trailers (e.g., for '*' = AH, ESP, NULL, etc.). o an Outer Header Length (OHLEN); set to the length of the outer SEAL/*/IP encapsulation headers. Templin Expires December 14, 2009 [Page 12] Internet-Draft SEAL June 2009 o a total Header Lenght (HLEN); set to MHLEN plus OHLEN. o a SEAL Maximum Reassembly Unit (S_MRU); initialized to a value no larger than 2KB and used to determine the maximum-sized packet the ITE will require the ETE to reassemble. o a SEAL Maximum Segment Size (S_MSS); initialized to a value that is no larger than the maximum of (the underlying IPv4 interface MTU minus OHLEN) and S_MRU/4 bytes. The ITE decreases or increases S_MSS based on any Fragmentation Report messages received (see Section 4.3.9). After an inner packet/fragment has been admitted into the tunnel interface the ITE first determines whether the packet can be accommodated and (if so) whether inner IP fragmentation is needed. The ITE processes each inner packet/fragment as follows: o if the inner packet is the first IP fragment of a SEAL packet with D=1, and the packet is larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends a SEAL Fragmentation Report message to the original source with an MTU value of (MAX(S_MRU, S_MSS) - HLEN) the same as described in Section 4.4.3; else, o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1, and the packet is larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends an ICMP PTB message to the original source with an MTU value of (MAX(S_MRU, S_MSS) - HLEN) the same as described in Section 4.3.2; else, o if the inner packet is an IPv4 packet with DF=0, and the packet is larger than (S_MRU - HLEN), the ITE uses inner IP fragmentation to break the packet into fragments no larger than (S_MRU - HLEN); else, no inner fragmentation is required. Note that this final case would constitute a second instance of inner packet fragmentation, which implementations may elect to combine with the first instance specified in Section 4.3.2 above. The ITE next encapsulates each inner packet/fragment in the MHLEN bytes of mid-layer '*' headers and trailers. For each such resulting mid-layer packet of length 'M', if (S_MRU >= (M + OHLEN) > S_MSS), the ITE must perform SEAL segmentation. To do so, it breaks the mid- layer packet into N segments (N <= 4) that are no larger than (MIN(1KB, S_MSS) - OHLEN) bytes each. Each segment, except the final one, MUST be of equal length, while the final segment includes the remainder of the packet and MAY be of different length. The first byte of each segment MUST begin immediately after the final byte of the previous segment, i.e., the segments MUST NOT overlap. The ITE Templin Expires December 14, 2009 [Page 13] Internet-Draft SEAL June 2009 SHOULD generate non-final segments that are as large as possible (see above) and SHOULD generate the smallest number of segments possible, e.g., it SHOULD NOT generate 4 smaller segments when the packet could be accommodated with 2 larger segments. Note that this SEAL segmentation ignores the fact that the mid-layer packet may be unfragmentable. This segmentation process is a mid- layer (not an IP layer) operation employed by the ITE to adapt the mid-layer packet to the subnetwork path characteristics, and the ETE will restore the packet to its original form during reassembly. Therefore, the fact that the packet may have been segmented within the subnetwork is not observable outside of the subnetwork. 4.3.4. Encapsulation Following SEAL segmentation, the ITE encapsulates each segment in a SEAL header formatted as specified in Section 4.3.2 and sets VER='00' and R=0. For single-segment packets, the ITE sets (M=0; SEG=0) in the SEAL header; for N-segment mid-layer packets (N <= 4), the ITE sets (M=1; SEG=0) for the first segment, (M=1; SEG=1) for the second segment, etc., with the final segment setting (M=0; SEG=N-1). If the inner packet (i.e., before mid-layer encapsulation and SEAL segmentation) was also the first IP fragment of a SEAL packet, the ITE copies the D value that appeared in the inner SEAL header into the outer SEAL header of each segment. Otherwise, if the inner packet was an IPv4 packet with DF=0, the ITE sets D=0; otherwise, it sets D=1. The ITE also writes the Internet Protocol number corresponding to the mid-layer packet in the 'Next-Header' field of each segment. The ITE next encapsulates each segment in the requisite */IPv4 outer headers according to the specific encapsulation format (e.g., [RFC2003], [RFC4213], [RFC4380], etc.), except that it writes 'SEAL_PROTO' in the protocol field of the outer IPv4 header (when simple IPv4 encapsulation is used) or writes 'SEAL_PORT' in the outer destination service port field (e.g., when UDP/IPv4 encapsulation is used). The ITE finally sets the A bit as specified in Section 4.3.5 (if necessary), sets the packet identification values as specified in Section 4.3.5 and sends the packets as specified in Section 4.3.6. 4.3.5. Probing Strategy All SEAL packets sent by the ITE except those with (M=0; SEG!=0) are used as implicit probes, and will elicit a Fragmentation Report from an ETE/ITE if an MTU restriction is encountered. The ITE should additionally send explicit probes, periodically, to ping the ETE and to manage a window of SEAL_IDs of outstanding Templin Expires December 14, 2009 [Page 14] Internet-Draft SEAL June 2009 probes. The ITE sets A=1 in the SEAL header of a packet to be used as an explicit probe, where the probe can be either an ordinary data packet or a NULL packet created by setting the 'Next Header' field in the SEAL header to a value of "No Next Header" (see Section 4.7 of [RFC2460]). The ITE should further send probes, periodically, to detect S_MSS increases by resetting S_MSS to a larger value (e.g., the underlying IPv4 interface MTU minus OHLEN bytes), and/or by sending explicit probes that are larger than the current S_MSS. Finally, the ITE MAY send "expendable" probe packets with DF=1 in the outer IPv4 header (see Section 4.3.6) in order to generate ICMPv4 Fragmentation Needed messages from routers on the path to the ETE. 4.3.6. Packet Identification For the purpose of packet identification, the ITE maintains a SEAL_ID value as per-ETE soft state, e.g., in the IPv4 destination cache. The ITE randomly initializes SEAL_ID when the soft state is created and monotonically increments it for each successive SEAL protocol packet it sends to the ETE. For each packet, the ITE writes the least-significant 16 bits of the SEAL_ID value in the Identification field in the outer IPv4 header, and writes the most-significant 16 bits in the ID Extension field in the SEAL header. For SEAL encapsulations specifically designed for the traversal of IPv4 Network Address Translators (NATs) and other middleboxes that may rewrite the outer IPv4 ID field, the ITE instead writes SEAL_ID in the ID field of the SEAL header and writes a random 16-bit value in the Identification field in the outer IPv4 header. 4.3.7. Sending SEAL Protocol Packets Following SEAL segmentation and encapsulation, the ITE sets DF=0 in the outer IPv4 header of every SEAL packet it sends. For "expendable" packets (e.g., for NULL packets used as probes -- see Section 4.3.4), the ITE may instead set DF=1. The ITE then sends each outer packet that encapsulates a segment of the same mid-layer packet into the tunnel in canonical order, i.e., segment 0 first, followed by segment 1, etc. and finally segment N-1. 4.3.8. Processing Raw ICMPv4 Messages The ITE may receive "raw" ICMPv4 error messages from either the ETE or routers within the subnetwork that comprise an outer IPv4 header, followed by an ICMPv4 header, followed by a portion of the SEAL Templin Expires December 14, 2009 [Page 15] Internet-Draft SEAL June 2009 packet that generated the error (also known as the "packet-in- error"). For such messages, the ITE can use the SEAL ID encoded in the packet-in-error as a nonce to confirm that the ICMP message came from either the ETE or an on-path router. The ITE MAY process raw ICMPv4 messages as soft errors indicating that the path to the ETE may be failing. The ITE should specifically process raw ICMPv4 Protocol Unreachable messages as a hint that the ETE does not implement the SEAL protocol. 4.3.9. Processing SEAL Errors In addition to any raw ICMPv4 messages, the ITE may receive SEAL error messages from either the ETE or an intermediate ITE on the path to the ETE with 'SEAL_PORT' as the UDP destination port. The ITE must therefore monitor the 'SEAL_PORT' UDP port and process any messages that arrive on that port. Each SEAL error message is formatted as specified in Section 4.4.6. For each error message, the ITE can use the SEAL_ID as well as addresses, etc. encoded in the packet-in-error as nonces to confirm that the message came from a legitimate on-path source. The ITE can then verify that the SEAL_ID encoded in the packet-in-error is within the current window of transmitted SEAL_IDs for this ETE. If the SEAL_ID is outside of the window, the ITE discards the message; otherwise, it advances the window and processes the message. The ITE processes SEAL error messages other than IPv4 Fragmentation Reports according to [RFC0792] and [RFC4443]. (Processing considerations for additional error types may be specified in a future document.) For IPv4 Fragmentation Report messages, the ITE sets 'L' to the value encoded in the MTU field minus OHLEN. If (L > S_MSS), or if the packet-in-error is an IPv4 first-fragment (i.e., with MF=1; Offset=0) and (L >= (576 - OHLEN)), the ITE sets (S_MSS = L). Note that 576 in the above corresponds to the nominal minimum MTU for IPv4 links. When an ITE instead receives an IPv4 first-fragment packet-in-error with (L < (576 - OHLEN)), it discovers that IPv4 fragmentation is occurring in the network but it cannot determine the true MTU of the restricting link due to a router on the path generating runt first-fragments. The ITE should therefore search for a reduced S_MSS value through an iterative searching strategy that parallels (Section 5 of [RFC1191]). This searching strategy may require multiple iterations of sending SEAL packets using a reduced S_MSS and receiving additional Templin Expires December 14, 2009 [Page 16] Internet-Draft SEAL June 2009 Fragmentation Report messages, but it will soon converge to a stable value. During this process, it is essential that the ITE reduce S_MSS based on the first Fragmentation Report message received, and refrain from further reducing S_MSS until Fragmentation Report messages pertaining to packets sent under the new S_MSS are received. 4.4. ETE Specification 4.4.1. Reassembly Buffer Requirements ETEs must be capable of performing IPv4-layer reassembly for SEAL protocol outer IPv4 packets up to 2KB in length, and must also be capable of performing SEAL-layer reassembly for mid-layer packets up to (2KB - OHLEN). Note that the ETE must retain the SEAL/*/IPv4 header during both IPv4-layer and SEAL-layer reassembly for the purpose of associating the fragments/segments of the same packet. 4.4.2. IPv4-Layer Reassembly ETEs perform IPv4 reassembly as normal, and should maintain a conservative high- and low-water mark for the number of outstanding reassemblies pending for each ITE. When the size of the reassembly buffer exceeds this high-water mark, the ETE actively discards incomplete reassemblies (e.g., using an Active Queue Management (AQM) strategy) until the size falls below the low-water mark. The ETE should also use a reduced IPv4 maximum segment lifetime value (e.g., 15 seconds) as the time after which it will discard an incomplete IPv4 reassembly for a SEAL protocol packet. Finally, the ETE should also actively discard any pending reassemblies that clearly have no opportunity for completion, e.g., when a considerable number of new IPv4 fragments have been received before a fragment that completes a pending reassembly has arrived. After reassembly, the ETE either accepts or discards the reassembled packet based on the current status of the IPv4 reassembly cache (congested versus uncongested). The SEAL_ID included in the IPv4 first-fragment provides an additional level of reassembly assurance, since it can record a distinct arrival timestamp useful for associating the first-fragment with its corresponding non-initial fragments. The choice of accepting/discarding a reassembly may also depend on the strength of the upper-layer integrity check if known (e.g., IPSec/ESP provides a strong upper-layer integrity check) and/or the corruption tolerance of the data (e.g., multicast streaming audio/video may be more corruption-tolerant than file transfer, etc.). In the limiting case, the ETE may choose to discard all IPv4 reassemblies and process only the IPv4 first-fragment for Templin Expires December 14, 2009 [Page 17] Internet-Draft SEAL June 2009 SEAL-encapsulated error generation purposes (see the following sections). 4.4.3. Sending SEAL Fragmentation Reports When the ETE processes the IP first-fragment (i.e, one with MF=1 and Offset=0 in the IP header) of a fragmented SEAL packet that does not have (M=0; SEG!=0), it sends a Fragmentation Report message back to the ITE with the MTU field set to the length of the first-fragment. When an intermediate ITE on the path to the ETE is unable to accommodate a SEAL packet with D=1 (see Section 4.3.3), it drops the packet and also sends a Fragmentation Report back to the original ITE. Additionally, when the ETE processes a SEAL protocol packet with A=1 in the SEAL header following IP reassembly, it sends a Fragmentation Report message back to the ITE with the MTU value set to the IP length of the packet. Note therefore that when A=1, and IP reassembly was required, the ETE only sends a single Fragmentation Report message, i.e., it does not send two separate messages (one for the first-fragment and a second for the reassembled whole SEAL packet). The Fragmentation Report message is formatted as either an ICMPv4 Fragmentation Needed or an ICMPv6 Packet Too Big message, as specified in Section 4.4.6. 4.4.4. SEAL-Layer Reassembly Following IP reassembly of a SEAL packet with VER set to an unrecognized value or with R=1, the ETE generates an Parameter Problem message (with pointer set to the flags field in the SEAL header) as specified in Section 4.4.6, and discards the packet following SEAL reassembly. For all other SEAL packets, the ETE adds the packet to a SEAL-Layer pending-reassembly queue. The ETE performs SEAL-layer reassembly through simple in-order concatenation of the encapsulated segments from N consecutive SEAL protocol packets from the same mid-layer packet. SEAL-layer reassembly requires the ETE to maintain a cache of recently received segments for a hold time that would allow for reasonable inter- segment delays. The ETE uses a SEAL maximum segment lifetime of 15 seconds for this purpose, i.e., the time after which it will discard an incomplete reassembly. However, the ETE should also actively discard any pending reassemblies that clearly have no opportunity for completion, e.g., when a considerable number of new SEAL packets have been received before a packet that completes a pending reassembly has arrived. Templin Expires December 14, 2009 [Page 18] Internet-Draft SEAL June 2009 The ETE reassembles the mid-layer packet segments in SEAL protocol packets that contain segment numbers 0 through N-1, with M=1/0 in non-final/final segments, respectively, and with consecutive SEAL_ID values. That is, for an N-segment mid-layer packet, reassembly entails the concatenation of the SEAL-encapsulated segments with (SEG=0, SEAL_ID=i), followed by (SEG=1, SEAL_ID=((i + 1) mod 2^32)), etc. up to (SEG=(N-1), SEAL_ID=((i + N-1) mod 2^32)). (For SEAL encapsulations that use only an M-bit SEAL_ID value, the ETE instead uses mod 2^M arithmetic to associate the segments of the same packet.) 4.4.5. Decapsulation and Delivery to Upper Layers Following SEAL-layer reassembly, the ETE silently discards the reassembled packet if it was a NULL packet (see Section 4.3.4). In the same manner, the ETE silently discards any (reassembled) mid- layer packet larger than (2KB - OHLEN) that either experienced IPv4 fragmentation or did not arrive as a single SEAL segment. Next, the ETE begins the decapsulation process. During this process, if the ETE determines that the inner packet would cause an error message to be generated it prepares an error message sends it back to the ITE as specified in Section 4.4.6. The ETE then either accepts or drops the packet according to the type of error. Note that errors can occur through any stage of inner packet decapsulation, i.e., before, during or after decapsulation. For example, if IPv4 and IPv6 are used as the outer and inner IP protocols, respectively, the ETE may generate ICMPv4-formatted error messages before and during decapsulation, and it may generate ICMPv6- formatted error messages during and after decapsulation. This can be understood as a continuum along which the ETE transforms an IPv4 packet into an IPv6 packet, where the ETE must generate an error message that is appropriate for the particular point in the continuum at which the error occurs. In all cases, the packet-in-error includes all IP/*/SEAL/*IPv4 headers, i.e., even if the error occurred at the very last stage of decapsulation. Finally, if the packet is accepted, the ETE discards the outer */SEAL/*/IPv4 headers and delivers the inner packet to the upper- layer protocol indicated in the SEAL Next Header field. 4.4.6. Generating SEAL Error Messages The ETE or intermediate ITE reporting the error prepares the message as shown in Figure 3: Templin Expires December 14, 2009 [Page 19] Internet-Draft SEAL June 2009 +-------------------------+ - | | \ ~ Outer UDP/IP hdrs ~ | | (dport='SEAL_PORT') | | +--------+----------------+ | | Nxthdr | Reserved | | +--------+----------------+ | | ICMP Header | | +-------------------------+ > Up to 576 bytes for IPv4, | | > or 1280 bytes for IPv6 ~ IP/*/SEAL/*/IP ~ | ~ hdrs of packet/fragment ~ | | | | +-------------------------+ | | | | ~ Data of packet/fragment ~ | | | / +-------------------------+ - Figure 3: SEAL Error Message Format The error message consists of outer UDP/IP headers followed by a 32 bit shim header. The shim header includes an 8-bit "Next Header" field in bits 0 thru 7 and a 24-bit Reserved field in bits 8 thru 31. The shim header is followed by the body of an ICMP error message formatted exactly as specified for ICMPv4 [RFC0792] or [RFC4443]. The ETE/ITE reporting the error sets the outer IP destination and source addresses of the error message to the source and destination addresses (respectively) of the SEAL packet. If the destination address in the SEAL packet was multicast, the ETE/ITE instead sets the outer IP source address to an address assigned to the underlying IP interface. The ETE/ITE next sets the UDP destination port to 'SEAL_PORT'' and sets the UDP source port to a constant value of its choosing. It then sets the "Next Header" field to the IP protocol type of the header that follows (e.g., to the value '1' for an ICMPv4 message, the value '58' for an ICMPv6 message, etc.) and sets the Reserved field to 0. Associated tunneling mechanisms may instead set the Next-Header field to a different value (e.g., '59' for No-Next- Header) and define their own protocol specific coding in the Reserved field. The shim header is followed by an ICMP header of the correct IP protocol version and with fields filled out as specified in [RFC0792] or [RFC4443]. The ICMP header is followed by as much of the invoking packet as possible without the entire message exceeding the minimum Templin Expires December 14, 2009 [Page 20] Internet-Draft SEAL June 2009 IP MTU (i.e., 576 bytes for IPv4 and 1280 bytes for IPv6) . The ETE/ITE finally sends the error message to the original ITE. When the A bit in the packet/fragment is not set, the message is sent subject to rate limiting. 5. SEAL Protocol Specification (Version 1) This section specifies a minimal version of SEAL known as "SEAL Version 1", or "SEAL-lite". SEAL-lite observes the same protocol specifications as for Classical SEAL (see Section 4) with the exception that the ITE/ETE do not perform segmentation and reassembly. In particular, the ETE unilaterally drops any SEAL-lite packets that arrive as multiple IPv4 fragments and/or multiple SEAL segments. SEAL-lite can be considered for use by associated tunneling protocol specifications when it highly unlikely that "marginal" links will occur in any path, e.g., when it is known that the vast majority of links configure MTUs that are appreciably larger than 1500 bytes. SEAL-lite can also be used in instances when it is acceptable for the ITE to return ICMP PTB messages for packet sizes smaller than 1500 bytes. Finally, the use of SEAL-lite requires that the associated tunneling protocol specification either defines a next header field or ensures that the data immediately following the SEAL header is an IP header (i.e., either IPv4 or IPv6). The use of SEAL-lite must therefore be carefully examined in relation to the particular use case. With respect to Section 4, the SEAL-lite protocol corresponds to Classical SEAL as follows: 5.1. Model of Operation SEAL-lite follows the same model of operation as for Classical SEAL as described in Section 4.1 except as noted in the following sections. 5.2. SEAL Header Format (Version 1) The SEAL-lite header is formatted as follows: Templin Expires December 14, 2009 [Page 21] Internet-Draft SEAL June 2009 0 1 2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |VER|A|D| Reserved / Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: SEAL Version 1 Header Format where the header fields are defined as: VER (2) a 2-bit value that encodes the SEAL protocol version number. This section describes Version 1 of the SEAL protocol, i.e., the VER field encodes the value '01'. A (1) the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes to receive an explicit acknowledgement from the ETE. D (1) the "Don't Fragment" bit. Set to 1 if the inner packet is an IPv6 packet, an IPv4 packet with DF=1, or a SEAL packet with D=1. Set to 0 otherwise. Reserved (12 or fewer) a reserved field; used in a manner defined in the associated tunneling protocol specification. Identification (16 or more) an identification field; used either as an extension to the IPv4 ID field or as an independent Identification field as defined in the associated tunneling protocol specification. 5.3. ITE Specification 5.3.1. Tunnel Interface MTU SEAL-lite observes the Classical SEAL specification found in Section 4.3.1. 5.3.2. Admitting Packets into the Tunnel Interface SEAL-lite observes the Classical SEAL specification found in Section 4.3.2. Templin Expires December 14, 2009 [Page 22] Internet-Draft SEAL June 2009 5.3.3. Inner Fragmentation and Segmentation SEAL-lite observes the Classical SEAL specification found in Section 4.3.3, except that S_MRU is set to 0. The ITE must therefore break inner IP packets that are to undergo inner fragmentation into fragments that are no larger than would both provide a reasonably- large fragment size 'S' and avoid further fragmentation in the network. In that case, it is recommended that the ITE select an initial value for S between 1280 and (1500 - HLEN) unless it is known that all links in the path to the ETE configure an MTU that is significantly larger than this. 5.3.4. Encapsulation SEAL-lite observes the Classical SEAL specification found in Section 4.3.4, except that it uses the header format defined in this section and with the VER field set to '01'. SEAL-lite further uses the A and D bits the same as specified for Classical SEAL, but the Reserved and Identification fields are used in the manner specified by the associated tunneling protocol. 5.3.5. Probing Strategy SEAL-lite observes the Classical SEAL specification found in Section 4.3.5. 5.3.6. Packet Identification SEAL-lite observes the Classical SEAL soft state specifications found in Section 4.3.6, but configures and sets the Identification field in a manner specified by the associated tunneling protocol. As for the Classical SEAL specification in Section 4.3.6, SEAL-lite increments the Identification field modulo the field length for each consecutive SEAL packet. 5.3.7. Sending SEAL Protocol Packets SEAL-lite observes the Classical SEAL specification found in Section 4.3.7. 5.3.8. Processing Raw ICMPv4 Messages SEAL-lite observes the Classical SEAL specification found in Section 4.3.8. Templin Expires December 14, 2009 [Page 23] Internet-Draft SEAL June 2009 5.3.9. Processing SEAL Errors SEAL-lite observes the Classical SEAL specification found in Section 4.3.9. 5.4. ETE Specification 5.4.1. Reassembly Buffer Requirements SEAL-lite *does not* observe the Classical SEAL specification found in Section 4.4.1, i.e., it does not maintain a reassembly buffer for SEAL reassembly. 5.4.2. IP-Layer Reassembly SEAL-lite uses SEAL-protocol IP first-fragments solely for the purpose of generating fragmentation reports as specified in Section 4.4.2, but thereafter discards all SEAL-protocol IP fragments. 5.4.3. Sending SEAL Fragmentation Reports SEAL-lite observes the Classical SEAL specification found in Section 4.4.3. 5.4.4. SEAL-Layer Reassembly SEAL-lite observes the Classical SEAL error checking procedures in Section 4.4.4, i.e., SEAL-lite returns a Parameter Problem for SEAL packets with an unrecognized VER value. SEAL-lite *does not* observe the Classical SEAL reassembly procedures in Section 4.4.4; Instead, SEAL-lite discards all SEAL packets with (M!=0 || SEG!=0) following IP layer reassembly. 5.4.5. Decapsulation and Delivery to Upper Layers SEAL-lite observes the Classical SEAL specification found in Section 4.4.5. 5.4.6. Sending SEAL Error Messages SEAL-lite observes the Classical SEAL specification found in Section 4.4.6. 6. Link Requirements Subnetwork designers are expected to follow the recommendations in Templin Expires December 14, 2009 [Page 24] Internet-Draft SEAL June 2009 Section 2 of [RFC3819] when configuring link MTUs. 7. End System Requirements SEAL provides robust mechanisms for returning PTB messages; however, end systems that send unfragmentable IP packets larger than 1500 bytes are strongly encouraged to use Packetization Layer Path MTU Discovery per [RFC4821]. 8. Router Requirements IPv4 routers within the subnetwork are strongly encouraged to implement IPv4 fragmentation such that the first-fragment is the largest and approximately the size of the underlying link MTU, i.e., they should avoid generating runt first-fragments. 9. IANA Considerations The IANA is instructed to allocate an IP protocol number for 'SEAL_PROTO' in the 'protocol-numbers' registry. The IANA is instructed to allocate a Well-Known Port number for 'SEAL_PORT' in the 'port-numbers' registry. 10. Security Considerations Unlike IPv4 fragmentation, overlapping fragment attacks are not possible due to the requirement that SEAL segments be non- overlapping. An amplification/reflection attack is possible when an attacker sends IPv4 first-fragments with spoofed source addresses to an ETE, resulting in a stream of ICMPv4 Fragmentation Needed messages returned to a victim ITE. The encapsulated segment of the spoofed IPv4 first-fragment provides mitigation for the ITE to detect and discard spurious ICMPv4 Fragmentation Needed messages. The SEAL header is sent in-the-clear (outside of any IPsec/ESP encapsulations) the same as for the outer */IPv4 headers. As for IPv6 extension headers, the SEAL header is protected only by L2 integrity checks and is not covered under any L3 integrity checks. Templin Expires December 14, 2009 [Page 25] Internet-Draft SEAL June 2009 11. Related Work Section 3.1.7 of [RFC2764] provides a high-level sketch for supporting large tunnel MTUs via a tunnel-level segmentation and reassembly capability to avoid IP level fragmentation, which is in part the same approach used by tunnel-mode SEAL. SEAL could therefore be considered as a fully functioned manifestation of the method postulated by that informational reference. Section 3 of [RFC4459] describes inner and outer fragmentation at the tunnel endpoints as alternatives for accommodating the tunnel MTU; however, the SEAL protocol specifies a mid-layer segmentation and reassembly capability that is distinct from both inner and outer fragmentation. Section 4 of [RFC2460] specifies a method for inserting and processing extension headers between the base IPv6 header and transport layer protocol data. The SEAL header is inserted and processed in exactly the same manner. The concepts of path MTU determination through the report of fragmentation and extending the IP Identification field were first proposed in deliberations of the TCP-IP mailing list and the Path MTU Discovery Working Group (MTUDWG) during the late 1980's and early 1990's. SEAL supports a report fragmentation capability using bits in an extension header (the original proposal used a spare bit in the IP header) and supports ID extension through a 16-bit field in an extension header (the original proposal used a new IP option). A historical analysis of the evolution of these concepts, as well as the development of the eventual path MTU discovery mechanism for IP, appears in Appendix A of this document. 12. SEAL Advantages over Classical Methods The SEAL approach offers a number of distinct advantages over the classical path MTU discovery methods [RFC1191] [RFC1981]: 1. Classical path MTU discovery *always* results in packet loss when an MTU restriction is encountered. Using SEAL, IPv4 fragmentation provides a short-term interim mechanism for ensuring that packets are delivered while SEAL adjusts its packet sizing parameters. 2. Classical path MTU discovery requires that routers generate an ICMP PTB message for *all* packets lost due to an MTU restriction; this situation is exacerbated at high data rates and becomes severe for in-the-network tunnels that service many Templin Expires December 14, 2009 [Page 26] Internet-Draft SEAL June 2009 communicating end systems. Since SEAL ensures that packets no larger than S_MRU are delivered, however, it is sufficient for the ETE to return ICMP PTB messages subject to rate limiting and not for every packet-in-error. 3. Classical path MTU may require several iterations of dropping packets and returning ICMP PTB messages until an acceptable path MTU value is determined. Under normal circumstances, SEAL determines the correct packet sizing parameters in a single iteration. 4. Using SEAL, ordinary packets serve as implicit probes without exposing data to unnecessary loss. SEAL also provides an explicit probing mode not available in the classic methods. 5. Using SEAL, ETEs encapsulate ICMP error messages in an outer UDP/IP header such that packet-filtering network middleboxes will not filter them the same as for"raw" ICMP messages that may be generated by an attacker. 6. Most importantly, all SEAL packets have a 32-bit Identification value that can be used for duplicate packet detection purposes and to match ICMP error messages with actual packets sent without requiring per-packet state; hence, certain denial-of-service attack vectors open to the classical methods are eliminated. In summary, the SEAL approach represents an architecturally superior method for ensuring that packets of various sizes are either delivered or deterministically dropped. When end systems use their own end-to-end MTU determination mechanisms [RFC4821], the SEAL advantages are further enhanced. 13. Acknowledgments The following individuals are acknowledged for helpful comments and suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Aurnaud Ebalard, Gorry Fairhurst, DIno Farinacci, Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, Darrel Lewis, Joe Macker, Matt Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Margaret Wasserman, Magnus Westerlund, Robin Whittle, James Woodyatt, and members of the Boeing Research & Technology NST DC&NT group. Path MTU determination through the report of fragmentation was first proposed by Charles Lynn on the TCP-IP mailing list in 1987. Templin Expires December 14, 2009 [Page 27] Internet-Draft SEAL June 2009 Extending the IP identification field was first proposed by Steve Deering on the MTUDWG mailing list in 1989. 14. References 14.1. Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [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. 14.2. Informative References [FOLK] C, C., D, D., and k. k, "Beyond Folklore: Observations on Fragmented Traffic", December 2002. [FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", October 1987. [I-D.ietf-lisp] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "Locator/ID Separation Protocol (LISP)", draft-ietf-lisp-01 (work in progress), May 2009. [I-D.russert-rangers] Russert, S., Fleischman, E., and F. Templin, "RANGER Scenarios", draft-russert-rangers-00 (work in progress), May 2009. [I-D.templin-autoconf-dhcp] Templin, F., "Virtual Enterprise Traversal (VET)", draft-templin-autoconf-dhcp-38 (work in progress), April 2009. [I-D.templin-ranger] Templin Expires December 14, 2009 [Page 28] Internet-Draft SEAL June 2009 Templin, F., "Routing and Addressing in Next-Generation EnteRprises (RANGER)", draft-templin-ranger-07 (work in progress), February 2009. [MTUDWG] "IETF MTU Discovery Working Group mailing list, gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November 1989 - February 1995.". [RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP MTU discovery options", RFC 1063, July 1988. [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, October 1996. [RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A. Malis, "A Framework for IP Based Virtual Private Networks", RFC 2764, February 2000. [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. [RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, August 2002. [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, January 2004. [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, July 2004. [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. Templin Expires December 14, 2009 [Page 29] Internet-Draft SEAL June 2009 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, February 2006. [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the- Network Tunneling", RFC 4459, April 2006. [RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006. [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List, http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May 1987 - May 1990.". Appendix A. Reliability Extensions Future updates to this specification will examine improved reliability in the face of loss due to congestion, signal intermittence, etc. Automatic Repeat-ReQuest (ARQ) mechanisms are used to ensure reliable delivery between the endpoints of links [RFC3366] (e.g., on-link neighbors in an IEEE 802.11 network) as well as between the endpoints of an end-to-end transport (e.g., the endpoints of a TCP connection). However, ARQ mechanisms may not be ideally sutiable for all SEAL use cases, since retransmission of lost segments may require considerable state maintenance at the ITE and would result in considerable delay variance and packet reordering within the subnetwork. Alternate reliability mechanisms such as Forward Error Correction (FEC) may also be examined in future updates to this specification for the purpose of improved reliability. Such mechanisms may entail the ITE performing proactive transmissions of redundant data, e.g., by sending multiple copies of the same data. Signaling from the ETE (e.g., by sending Source Quench messages) may also be considered as a means for the ETE to dynamically inform the ITE of changing FEC conditions. Appendix B. Transport Mode SEAL can also be used in "transport-mode", e.g., when the inner layer Templin Expires December 14, 2009 [Page 30] Internet-Draft SEAL June 2009 includes upper-layer protocol data rather than an encapsulated IP packet. For instance, TCP peers can negotiate the use of SEAL for the carriage of protocol data encapsulated as TCP/SEAL/IPv4. In this sense, the "subnetwork" becomes the entire end-to-end path between the TCP peers and may potentially span the entire Internet. Sections 4 and 5 specify the operation of SEAL in "tunnel mode", i.e., when there are both an inner and outer IP layer with a SEAL encapsulation layer between. However, the SEAL protocol can also be used in a "transport mode" of operation within a subnetwork region in which the inner-layer corresponds to a transport layer protocol (e.g., UDP, TCP, etc.) instead of an inner IP layer. For example, two TCP endpoints connected to the same subnetwork region can negotiate the use of transport-mode SEAL for a connection by inserting a 'SEAL_OPTION' TCP option during the connection establishment phase. If both TCPs agree on the use of SEAL, their protocol messages will be carried as TCP/SEAL/IPv4 and the connection will be serviced by the SEAL protocol using TCP (instead of an encapsulating tunnel endpoint) as the transport layer protocol. The SEAL protocol for transport mode otherwise observes the same specifications as for Sections 4 and 5. Appendix C. Historic Evolution of PMTUD (Taken from "Neighbor Affiliation Protocol for IPv6-over-(foo)-over- IPv4"; written 10/30/2002): The topic of Path MTU discovery (PMTUD) saw a flurry of discussion and numerous proposals in the late 1980's through early 1990. The initial problem was posed by Art Berggreen on May 22, 1987 in a message to the TCP-IP discussion group [TCP-IP]. The discussion that followed provided significant reference material for [FRAG]. An IETF Path MTU Discovery Working Group [MTUDWG] was formed in late 1989 with charter to produce an RFC. Several variations on a very few basic proposals were entertained, including: 1. Routers record the PMTUD estimate in ICMP-like path probe messages (proposed in [FRAG] and later [RFC1063]) 2. The destination reports any fragmentation that occurs for packets received with the "RF" (Report Fragmentation) bit set (Steve Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal) 3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990) Templin Expires December 14, 2009 [Page 31] Internet-Draft SEAL June 2009 4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30, 1990) 5. Fragmentation avoidance by setting "IP_DF" flag on all packets and retransmitting if ICMPv4 "fragmentation needed" messages occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191] by Mogul and Deering). Option 1) seemed attractive to the group at the time, since it was believed that routers would migrate more quickly than hosts. Option 2) was a strong contender, but repeated attempts to secure an "RF" bit in the IPv4 header from the IESG failed and the proponents became discouraged. 3) was abandoned because it was perceived as too complicated, and 4) never received any apparent serious consideration. Proposal 5) was a late entry into the discussion from Steve Deering on Feb. 24th, 1990. The discussion group soon thereafter seemingly lost track of all other proposals and adopted 5), which eventually evolved into [RFC1191] and later [RFC1981]. In retrospect, the "RF" bit postulated in 2) is not needed if a "contract" is first established between the peers, as in proposal 4) and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on Feb 19. 1990. These proposals saw little discussion or rebuttal, and were dismissed based on the following the assertions: o routers upgrade their software faster than hosts o PCs could not reassemble fragmented packets o Proteon and Wellfleet routers did not reproduce the "RF" bit properly in fragmented packets o Ethernet-FDDI bridges would need to perform fragmentation (i.e., "translucent" not "transparent" bridging) o the 16-bit IP_ID field could wrap around and disrupt reassembly at high packet arrival rates The first four assertions, although perhaps valid at the time, have been overcome by historical events. The final assertion is addressed by the mechanisms specified in SEAL. Templin Expires December 14, 2009 [Page 32] Internet-Draft SEAL June 2009 Author's Address Fred L. Templin (editor) Boeing Research & Technology P.O. Box 3707 Seattle, WA 98124 USA Email: fltemplin@acm.org Templin Expires December 14, 2009 [Page 33]