Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Intended status: Standards Track June 18, 2009 Expires: December 20, 2009 The Subnetwork Encapsulation and Adaptation Layer (SEAL) draft-templin-intarea-seal-03.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 20, 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 20, 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 . . . . . . . . . . . . . . . . . . . 8 4. SEAL with Traffic Engineering (SEAL-TE) Protocol Specification . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 9 4.2. SEAL Header Format (Version 0) . . . . . . . . . . . . . . 11 4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 12 4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 12 4.3.2. Admitting Packets into the Tunnel Interface . . . . . 13 4.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 14 4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 16 4.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 16 4.3.6. Packet Identification . . . . . . . . . . . . . . . . 17 4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 17 4.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 18 4.3.9. Processing SEAL Control Messages . . . . . . . . . . . 18 4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 20 4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 20 4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 20 4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 21 4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 22 4.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 22 5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol Specification . . . . . . . . . . . . . . . . . . . . . . . . 29 5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 29 5.2. SEAL Header Format (Version 1) . . . . . . . . . . . . . . 29 5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 30 5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 30 5.3.2. Admitting Packets into the Tunnel Interface . . . . . 30 5.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 30 5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 31 5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 31 5.3.6. Packet Identification . . . . . . . . . . . . . . . . 31 5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 31 5.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 31 Templin Expires December 20, 2009 [Page 2] Internet-Draft SEAL June 2009 5.3.9. Processing SEAL Control Messages . . . . . . . . . . . 31 5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 31 5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 31 5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 32 5.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 32 5.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 32 5.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 32 6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 32 7. End System Requirements . . . . . . . . . . . . . . . . . . . 32 8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 32 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 10. Security Considerations . . . . . . . . . . . . . . . . . . . 33 11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 33 12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 34 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35 14.1. Normative References . . . . . . . . . . . . . . . . . . . 35 14.2. Informative References . . . . . . . . . . . . . . . . . . 36 Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 38 Appendix B. Transport Mode . . . . . . . . . . . . . . . . . . . 39 Appendix C. Historic Evolution of PMTUD . . . . . . . . . . . . . 39 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41 Templin Expires December 20, 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]. The following subsections 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 20, 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. Templin Expires December 20, 2009 [Page 5] Internet-Draft SEAL June 2009 Due to these many limitations, a new approach 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 denial-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. 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 20, 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 Reassembly 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 SEAL Maximum Reassembly Unit S_MSS - the SEAL Maximum Segment Size S_CSS - the SEAL Clamped 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_CPORT - a TCP/UDP service port number used for SEAL control plane messaging SEAL_DPORT - a TCP/UDP service port number used for SEAL data plane messaging The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this Templin Expires December 20, 2009 [Page 7] Internet-Draft SEAL June 2009 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 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]. 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 specifies two variants of the SEAL protocol known as "SEAL with Traffic Engineering (SEAL-TE)" and "SEAL with Fragmentation Sensing (SEAL-FS)". SEAL-FS provides a minimal mechanism through which the egress tunnel endpoint (ETE) acts as a passive observer that simply informs the ingress tunnel endpoint (ITE) of any fragmentation. SEAL-FS therefore determines the tunnel MTU based on the MTU of the smallest link in the path. It is useful for determining an appropriate MTU for tunnels between performance- critical routers over robust links, as well as for other uses in which packet segmentation and reassembly would present too great of a burden for the routers or end systems. SEAL-TE is a functional superset of SEAL-FS, and requires that the tunnel endpoints support segmentation and reassembly of packets that are too large to traverse the tunnel without fragmentation. SEAL-TE determines the tunnel MTU based on the largest packet the ETE is capable of receiving rather than on the MTU of the smallest link in the path. Therefore, SEAL-TE can transport packets that are much larger than the underlying links themselves can carry in a single piece, i.e., even if IPv6 jumbograms are used [RFC2675]. SEAL-TE tunnels may be configured over paths that include only Templin Expires December 20, 2009 [Page 8] Internet-Draft SEAL June 2009 ordinary links, but they may also be configured over paths that include SEAL-FS tunnels or even other SEAL-TE tunnels. An example application would be linking two geographically remote supercomputer centers with large MTU links by configuring a SEAL_TE tunnel across the Internet. A second example would be support for large packet transfers over underprivileged links, i.e., especially over wireless transmission media such as IEEE 802.15.4, broadcast radio links in Mobile Ad-hoc Networks (MANETs), Very High Frequency (VHF) civil aviation data links, etc. Many other use case examples for both SEAL-FS and SEAL-TE are anticipated, and will be identified as further experience is gained. 4. SEAL with Traffic Engineering (SEAL-TE) Protocol Specification This section specifies the fully-functioned version of SEAL known as "SEAL with Traffic Engineering (SEAL-TE)"; a minimal version known as "SEAL with Fragmentation Sensing (SEAL-FS)" is specified in Section 5. SEAL-TE is a superset of SEAL-FS, and differs only in its segmentation and reassembly requirements. Both SEAL-TE and SEAL-FS have identical over-the-wire encapsulation profiles, and are distinguished simply by a version number in the SEAL header. The following sections therefore specify SEAL-TE, but use the simple term "SEAL" since the same formats and mechanisms apply also to SEAL-FS. 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 20, 2009 [Page 9] 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/IPvY encapsulations (e.g., [RFC2003][RFC2004][RFC2473][RFC4213]), the SEAL header is inserted between the inner and outer IP headers as: IPvX/SEAL/IPvY. o For tunnel-mode IPsec encapsulations, [RFC4301], the SEAL header is inserted between the {AH,ESP} header and outer IP headers as: IPvX/*/{AH,ESP}/SEAL/IPvY. 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/IPvY. SEAL-encapsulated packets include a SEAL_ID to uniquely identify each packet. 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. For IPv4, the SEAL_ID is 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 IPv6, the SEAL_ID is written into the 32-bit Identification field of the fragment header. For tunnels that traverse middleboxes that might rewrite the IP ID field, e.g., a Templin Expires December 20, 2009 [Page 10] Internet-Draft SEAL June 2009 Network Address Translator, the SEAL_ID is instead maintained only within the ID field in the SEAL header. SEAL enables a multi-level segmentation and reassembly capability. First, the ITE can use IPv4 fragmentation to fragment inner IPv4 packets before SEAL encapsulation. Secondly, the SEAL layer itself provides a simple cutting-and-pasting capability for mid-layer packets to avoid IP fragmentation on the outer packet. Finally, ordinary IP 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|I|F|M|RSV| NEXTHDR/SEG | 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. I (1) the "Information Request Solicit" bit. Set to 1 if the ITE wishes the ETE to initiate an Information Request. F (1) the "First Segment" bit. Set to 1 if this SEAL protocol packet contains the first segment (i.e., Segment #0) of a mid-layer packet. Templin Expires December 20, 2009 [Page 11] Internet-Draft SEAL June 2009 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. RSV (2) a 2-bit Reserved field. Set to 0 for the purpose of this specification. NEXTHDR/SEG (8) an 8-bit field. When 'F'=1, encodes the next header Internet Protocol number the same as for the IPv4 protocol and IPv6 next header fields. When 'F'=0, encodes a segment number of a multi-segment mid-layer packet. (The segment number 0 is reserved.) ID Extension (16) a 16-bit Identification extension field. 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 Templin Expires December 20, 2009 [Page 12] Internet-Draft SEAL June 2009 encapsulated packet to become larger still and trigger in-the-network 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. The ITE can alternatively set an indefinite MTU on the tunnel virtual interface such that all inner IP packets are admitted into the interface without regard to size. For ITEs that host applications, this option must be carefully coordinated with protocol stack upper layers, since some upper layer protocols (e.g., TCP) derive their packet sizing parameters from the MTU of the underlying interface and as such may select too large an initial size. This is not a problem for upper layers that use conservative initial estimates, e.g., when mechanisms such as Packetization Layer Path MTU Discovery [RFC4821] are used. 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). Note that when the tunnel interface sets an indefinite MTU all packets are unconditionally admitted into the interface without Templin Expires December 20, 2009 [Page 13] Internet-Draft SEAL June 2009 fragmentation. 4.3.3. Segmentation For each ETE, the ITE maintains soft state within the tunnel interface (e.g., in a destination cache) used to support inner fragmentation and 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.). MHLEN additionally includes 4 extra bytes for a trailing mid-layer checksum (see below). o an Outer Header Length (OHLEN); set to the length of the outer SEAL/*/IP encapsulation headers and trailers. o a total Header Length (HLEN); set to MHLEN plus OHLEN. o a SEAL Maximum Segment Size (S_MSS); initialized to a value that is no larger than the underlying IP interface MTU. The ITE decreases or increases S_MSS based on any SEAL Reassembly Report messages received (see Section 4.3.9). o a SEAL Clamped Segment Size (S_CSS); a value that is no larger than S_MSS and that would also be unlikely to incur fragmentation beyond the tunnel, (e.g., 576 bytes for IPv4 and 1280 bytes for IPv6). May be set to larger values only if there is high assurance that all links within the tunnel configure a larger MTU. The ITE decreases S_CSS in conjunction with S_MSS, but only increases S_CSS to "safe" values if S_MSS is increased. o a SEAL Maximum Reassembly Unit (S_MRU); initialized to the larger of S_MSS and the known or estimated Maximum Receive Unit (MRU) actually configured by the ETE (2KB minimum default). The ITE decreases or increases S_MRU based on any SEAL Reassembly Report messages received (see Section 4.3.9). When (S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256) as the effective S_MRU value. Note that here as well as in the SEAL control message protocol (see Section 4.4.5), S_MSS, S_CSS and S_MRU are maintained as 32-bit values specifically for the purpose of supporting jumbograms. After an inner packet/fragment has been admitted into the tunnel interface the ITE uses the following algorithm to determine whether the packet can be accommodated and (if so) whether inner IP fragmentation is needed: Templin Expires December 20, 2009 [Page 14] Internet-Draft SEAL June 2009 o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1, and the packet is larger than (S_MRU - HLEN), the ITE drops the packet and sends an ICMP PTB message to the original source with an MTU value of (S_MRU - HLEN) the same as described in Section 4.3.2; else, o if the inner packet is a SEAL IPv4 packet with DF=0, and the packet is larger than (S_MRU - HLEN), the ITE uses inner IPv4 fragmentation to break the packet into fragments no larger than (S_MRU - HLEN); else, o if the inner packet is a non-SEAL IPv4 packet with DF=0, the ITE uses inner IPv4 fragmentation to break the packet into fragments no larger than (S_CSS - HLEN); else, o the ITE processes the packet without inner fragmentation. (Note that in the above the ITE must also track whether the tunnel interface is using header compression on the inner */IP headers. If so, the ITE must include the length of the uncompressed */IP inner header when calculating the total length of the inner packet.) The ITE next encapsulates each inner packet/fragment in the MHLEN bytes of mid-layer '*' headers and trailers and reserves 4 bytes for a trailing checksum at the end of the mid-layer trailers. The ITE then calculates a checksum of the mid-layer packet using the 16-bit Fletcher Checksum algorithm specified in [RFC1146], Appendix II, and writes the 'A' portion as the most significant 16 bits and the 'B' portion as the least significant 16 bits. If the length of the resulting mid-layer packet plus OHLEN is greater than S_MSS, the ITE must additionally perform SEAL segmentation. To do so, it breaks the mid-layer packet into N segments (N <= 256) that are no larger than (S_MSS - OHLEN) bytes each. Each segment, except the final one, MUST be of equal 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 SHOULD generate the smallest number of segments possible, e.g., it SHOULD NOT generate 6 smaller segments when the packet could be accommodated with 4 larger segments. Note that this SEAL segmentation ignores the fact that the mid-layer packet may be unfragmentable outside of the subnetwork. 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 Templin Expires December 20, 2009 [Page 15] Internet-Draft SEAL June 2009 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 with VER='00' and RSV='00'. For the first segment, the ITE sets F=1 and sets NEXTHDR to the Internet Protocol number of the encapsulated packet; the ITE next sets M=1 if there are more segments or sets M=0 otherwise. For each non-initial segment of an N-segment mid-layer packet (N <= 256), the ITE sets (F=0; M=1; SEG=1) in the SEAL header of the first non- initial segment, sets (F=0; M=1; SEG=2) in the next non-initial segment, etc., and sets (F=0; M=0; SEG=N-1) in the final segment. (Note that the value SEG=0 is not used.) The ITE next encapsulates each segment in the requisite */IP outer headers according to the specific encapsulation format (e.g., [RFC2003], [RFC2473], [RFC4213], [RFC4380], etc.), except that it writes 'SEAL_PROTO' in the protocol field of the outer IP header (when simple IP encapsulation is used) or writes 'SEAL_DPORT' in the outer destination service port field (e.g., when UDP/IP 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 4.3.6 and sends the packets as specified in Section 4.3.7. Note that when IPv6 is used as the outer IP encapsulation layer, the ITE must insert an IPv6 fragment header with an Identification value set as described in Section 4.3.6. 4.3.5. Probing Strategy All SEAL packets sent by the ITE are considered implicit probes, and will elicit Reassembly Reports from the ETE with a new value for S_MSS if any IP fragmentation occurs in the path. Thereafter, the ITE may periodically reset S_MSS to a larger value (e.g., the underlying IP interface MTU minus OHLEN bytes) to detect path MTU increases. The ITE should additionally send explicit probes, periodically, to verify that the ETE is still reachable and to manage a window of SEAL_IDs. The ITE sets (A=1; F=1) in the SEAL header of a first- segment 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 to a value of "No Next Header" (see Section 4.7 of [RFC2460]). The probe will elicit a Reassembly Report from the ETE as an acknowledgement. Templin Expires December 20, 2009 [Page 16] Internet-Draft SEAL June 2009 The ITE can also send probes using non-initial SEAL segments to determine whether any of the preceding segments of the same SEAL packet are missing. The probe will elicit a Reassembly Report from the ETE with a Bitmap of received and missing segments. Finally, the ITE MAY send "expendable" probe packets (see Section 4.3.7) in order to generate ICMP PTB 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 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 outer IPv4 packet, the ITE writes the least-significant 16 bits of the SEAL_ID value into 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 each outer IPv6 packet, the ITE writes the entire SEAL_ID value into the Identification field in the IPv6 fragment header. For ITE->ETR tunnels specifically designed for the traversal of Network Address Translators (NATs) and other middleboxes that may rewrite the outer IP ID field, the ITE instead writes least significant bits of the SEAL_ID in the ID field of the SEAL header and writes a random value in the Identification field in the outer IP header. Since the ID field in the SEAL header is only 16 bits, however, the ITE must limit the rate at which it sends packets to avoid wrapping the ID field. Alternatively, the ITE and ETE can use SEAL-FS to obtain a larger ID field in the SEAL header (see Section 5.3.6). 4.3.7. Sending SEAL Protocol Packets Following SEAL segmentation and encapsulation, the ITE sets DF=0 for ordinary SEAL/*/IPv4 packets, but may set DF=1 for "expendable" SEAL/ */IPv4 packets (e.g., for NULL packets used as probes -- see Section 4.3.5). For SEAL/*/IPv6 packets, the "DF" bit is always implicitly set to 1, but when a fragment header is included a translating router on the path may still fragment the packet. The ITE 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. Templin Expires December 20, 2009 [Page 17] Internet-Draft SEAL June 2009 4.3.8. Processing Raw ICMP Messages The ITE may receive "raw" ICMP error messages [RFC0792][RFC4443] from either the ETE or routers within the subnetwork that comprise an outer IP header, followed by an ICMP header, followed by a portion of the SEAL packet that generated the error (also known as the "packet- in-error"). 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, and can use any additional information to determine whether to accept or discard the message. The ITE should specifically process raw ICMPv4 Protocol Unreachable messages and ICMPv6 Parameter Problem messages with Code "Unrecognized Next Header type encountered" as a hint that the ETE does not implement the SEAL protocol. 4.3.9. Processing SEAL Control Messages In addition to any raw ICMP messages, the ITE may receive UDP/IP SEAL control messages from the ETE formatted as specified in Section 4.4.5 and with 'SEAL_CPORT' as the UDP destination port. The ITE must therefore monitor the 'SEAL_CPORT' UDP port and process any messages that arrive on that port. For each control message, the ITE verifies the UDP checksum and discards the message if the checksum is incorrect. The ITE can then verify that the SEAL_ID 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 control messages as follows: 4.3.9.1. Reassembly Report (Type=0) When the ITE receives a Reassembly Report formatted as specified in Section 4.4.5.1, it processes the message according to the Code value as follows: 4.3.9.1.1. IP Fragmentation Experienced (Code=0) The ITE records the value in the S_MRU field in its soft state for this ETE and adjusts the S_MSS value in its soft state. If the S_MSS value in the Reassembly Report is greater than 576 (i.e., the nominal minimum MTU for IPv4 links), the ITE records this new value in its soft state. If the S_MSS value in the report is less than the current soft state value and also less than 576, the can discern that IP fragmentation is occurring but it cannot determine the true MTU of the restricting link due to a router on the path generating runt Templin Expires December 20, 2009 [Page 18] Internet-Draft SEAL June 2009 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 Reassembly 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 Reassembly Report message received, and refrain from further reducing S_MSS until SEAL Reassembly Report messages pertaining to packets sent under the new S_MSS are received. 4.3.9.1.2. Segment Acknowledged (Code=1) The ITE records the value in the S_MRU field in its soft state for this ETE. If the S_MSS value in the report is non-zero, the ITE also adjusts its S_MSS value the same as for an IP Fragmentation Experienced message (see Section 4.3.9.1.1). The ITE next examines the Bitmap field to determine which segments of this SEAL packet were received. The map is arranged with segment 0 represented as the most significant bit, segment 1 represented as the next most significant bit, etc., up to the segment that triggered the reassembly report (i.e., segment N) as the final bit. For example, when N=6 the bit map '0110011' means that segments 1, 2, 5 and 6 were received but segments 0, 3 and 4 were missing from the ETE's reassembly buffer. The ITE can then retransmit segments 0, 3 and 4 if it still has them in its cache, and if there is reason to believe the retransmissions may satisfy the pending reassembly. 4.3.9.1.3. Packet Too Big (Code=2) The ITE records the value in the S_MRU field in its soft state for this ETE. 4.3.9.1.4. Time Exceeded (Code=3) The ITE examines the time encoded in the Data field, and reduces its S_MRU estimate for this ETE if it there is significant evidence that large packets are timing out prior to SEAL reassembly completion. The ITE may log the event for network management purposes. 4.3.9.1.5. Checksum Incorrect (Code=4) The ITE may log the event for network management purposes. When sustained Checksum Incorrect messages are received from this ETE, the ITE may also benefit by adjusting its packet sizing parameters. Templin Expires December 20, 2009 [Page 19] Internet-Draft SEAL June 2009 4.3.9.2. Parameter Problem (Type=1) When the ITE receives a Parameter Problem message formatted as specified in Section 4.4.5.2, it examines the encapsulated SEAL header in the message to determine whether the header was corrupted or whether the header specified features that the ETE did not recognize. The ITE MAY log the event for network management purposes. For uncorrupted headers, the SHOULD adjust its SEAL header parameters in subsequent SEAL packets. 4.3.9.3. Information Request (Type=2) When the ITE receives an Information Request message formatted as specified in Section 4.4.5.5 and with a SEAL_ID that corresponds to a SEAL packet that it sent earlier with I=1, it sends an Information Reply as specified in Section 4.4.5.6. 4.4. ETE Specification 4.4.1. Reassembly Buffer Requirements ETEs must be capable of performing IP-layer reassembly for SEAL protocol IP packets up to 2KB in length, and must also be capable of performing SEAL-layer reassembly for mid-layer packets up to (2KB - OHLEN). Hence, ETEs: o MUST configure a reassembly buffer of at least 2KB o MAY configure a larger reassembly buffer o MUST be capable of discarding SEAL packets that are too large to reassemble Note that the ETE must retain the SEAL/*/IP header during both IP- layer and SEAL-layer reassembly for the purpose of associating the fragments/segments of the same packet. 4.4.2. IP-Layer Reassembly ETEs perform standard IP-layer reassembly for SEAL protocol IP fragments, and should maintain a conservative reassembly cache high- and low-water mark . When the size of the reassembly cache exceeds this high-water mark, the ETE should actively discard incomplete reassemblies (e.g., using an Active Queue Management (AQM) strategy) until the size falls below the low-water mark. The ETE should also actively discard any pending reassemblies that clearly have no opportunity for completion, e.g., when a considerable number of new fragments have been received before a fragment that completes a Templin Expires December 20, 2009 [Page 20] Internet-Draft SEAL June 2009 pending reassembly has arrived. 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, it sends a SEAL Reassembly Report message back to the ITE with the S_MSS field set to the length of the first-fragment and with the S_MRU field set to the size of the ETE's reassembly buffer (see Section 4.4.5). 4.4.3. SEAL-Layer Reassembly Following IP reassembly of a SEAL segment, the ETE adds the segment to a SEAL-Layer pending-reassembly queue according to the (Source, Destination, SEAL_ID)-tuple found in the outer SEAL/*/IP headers. The ETE performs SEAL-layer reassembly through simple in-order concatenation of the encapsulated segments of the same mid-layer packet from N consecutive SEAL packets. 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 (e.g., 15 seconds). When a SEAL reassembly times out, the ETE discards the incomplete reassembly and returns a Reassembly Report - Time Exceeded message to the ITE (see Section 4.4.5). As for IP- layer reassembly, the ETE should also maintain a conservative reassembly cache high- and low-water mark and should 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. When the ETE receives a SEAL packet with an incorrect value in the SEAL header, it discards the packet and returns a Parameter Problem message (see Section 4.4.5). If the ETE receives a SEAL packet for which a segment with the same (Source, Destination, SEAL_ID)-tuple is already in the queue, it must determine whether to accept the new segment and release the old, or drop the new segment. If accepting the new segment would cause an inconsistency with other segments already in the queue (e.g., differing segment lengths), the ETE drops the segment that is least likely to complete the reassembly. When the ETE adds a SEAL packet with A=1 and with SEG=N to a reassembly queue, it sends a Reassembly Report - Segment Acknowledged message back to the ITE as specified in Section 4.4.6 that contains a bitmask of this and all prior segments already in the queue beginning with segment 0 in the most-significant bit, segment 1 in the next bit, etc.,up to segment N in the final bit. For example, when N=4, the bitmask '01101' indicates that segments 1, 2 and 4 are in the queue while segments 0 and 3 are missing. After all segments are gathered, the ETE reassembles the mid-layer Templin Expires December 20, 2009 [Page 21] Internet-Draft SEAL June 2009 packet by concatenating the segments encapsulated in N consecutive SEAL packets beginning with the initial segment (i.e., SEG=0) and followed by any non-initial segments 1 through N-1. That is, for an N-segment mid-layer packet, reassembly entails the concatenation of the SEAL-encapsulated mid-layer packet segments with (F=1, M=1, SEAL_ID=j) in the first SEAL header, followed by (F=0, M=1, SEG=1, SEAL_ID=(j+1)) in the next SEAL header, followed by (F=0, M=1, SEG=2, SEAL_ID=(j+2)), etc., up to (F=0, M=0, SEG=(N-1), SEAL_ID=(j + N-1)) in the final SEAL header. (Note that modulo arithmetic based on the length of the SEAL_ID field is used). When the ETE determines that a mid-layer packet is too large to reassemble, it releases the reassembly queue resources and sends a Reassembly Report - Packet Too Big message back to the ITE with the S_MRU field set to the size of the ETE's reassembly buffer (see Section 4.4.5). 4.4.4. Decapsulation and Delivery to Upper Layers Following SEAL-layer reassembly, the ETE verifies the trailing checksum of the mid-layer packet using the algorithm in Section 4.3.3. If the checksum is incorrect, the ETE discards the packet and sends a Reassembly Report - Checksum Incorrect message to the ITE (see Section 4.4.5). If the reassembled mid-layer packet is larger than (S_MRU-OHLEN), the ETE discards the packet and sends a Reassembly Report - Packet Too Big message to the ITE (see Section 4.4.5). Otherwise, the ETE discards the outer and mid-layer headers and trailers, and delivers the inner packet to the upper-layer protocol indicated in the SEAL Next Header field. (If the reassembled packet if it was a NULL packet (see Section 4.3.4), the ETE instead silently discards the packet). 4.4.5. Sending SEAL Control Messages The ETE generates SEAL control messages in response to certain SEAL packets. SEAL control messages are formated much the same as for ICMPv4 [RFC0792] and ICMPv6 [RFC4443] messages, and are used for very similar purposes. The ETE prepares each control message as a UDP/IP packet as shown in Figure 3: Templin Expires December 20, 2009 [Page 22] 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ UDP/IP Headers (dport=SEAL_CPORT) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Message Body + | | +-+-+ ... Figure 3: SEAL Control Message Format The control message consists of outer UDP/IP headers followed by a 32-bit SEAL_ID followed by a 32-bit control field followed by the message body. When the ETE/ITE prepares a control message, it sets the outer IP destination and source addresses of the message to the source and destination addresses (respectively) of the SEAL packet that triggered the message. If the destination address in the 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_CPORT'' and sets the UDP source port to a constant value of its choosing. It then sets the SEAL_ID to the Identification value encoded in the SEAL packet that triggered the message. As for ICPMv4 and ICMPv6 messages, the SEAL control header includes an 8-bit Type field in bits 0 thru 7 and an 8-bit Code field in bits 8 thru 15. Unlike ICMPv4 and ICMPv6 messages, however, the control header does not include a checksum field (since the UDP header already contains a checksum) but instead includes a 16-bit Data field in bits 16 thru 31. The ETE/ITE sets the Type, Code, Data and Message body fields according to the specific SEAL control message type, then sends the message. The following types are currently defined; other values for Type will be recorded in the IANA registry for SEAL: 4.4.5.1. Reassembly Report (Type=0) The ETE generates a Reassembly Report to inform the ITE of various conditions encountered during SEAL-layer reassembly. The following values for Code are defined: Templin Expires December 20, 2009 [Page 23] Internet-Draft SEAL June 2009 o Code = 0 : IP Fragmentation Experienced o Code = 1 : Segment Acknowledged o Code = 2 : Packet Too Big o Code = 3 : Time Exceeded o Code = 4 : Checksum Incorrect (Other values for Code will be recorded in the IANA registry for SEAL.) The ETE prepares the Reassembly Report according to the Code as follows: 4.4.5.1.1. IP Fragmentation Experienced (Code=0) When the ITE receives an IP first-fragment of a SEAL packet that experienced outer IP fragmentation, it examines SEAL header and examines the IP reassembly buffer to assess the likelihood that reassembly will complete. If the 'A" bit is not set in the SEAL header, or if IP reassembly completion appears unlikely, the ETE uses the IP first-fragment to prepare a "Reassembly Report - IP Fragmentation Experienced" message with Type=0, Code=0, and Data=0. The message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0 | Code=0 | Data=0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL Header of Packet that Triggered the Report | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MSS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: IP Fragmentation Experienced Message Format The ETE unconditionally writes the size of its reassembly buffer (see Section 4.4.1) in the S_MRU field and writes the length of the first IP fragment in the S_MSS field. If the 'A' bit is set in the SEAL header and IP reassembly completion appears likely, the ETE should refrain from sending this message if possible and instead send a Segment Acknowledged message according to Templin Expires December 20, 2009 [Page 24] Internet-Draft SEAL June 2009 the next section. (Note that it is not an error for the ETE to generate both the IP Fragmentation Experienced and Segment Acknowledged messages for the same SEAL packet, however this may be inefficient in some instances.) 4.4.5.1.2. Segment Acknowledged (Code=1) When the ITE receives a SEAL segment following IP reassembly that has the 'A' bit set in the SEAL header, it prepares a "Reassembly Report - Segment Acknowledged" message with Type=0, Code=1, and Data=0. The message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0 | Code=1 | Data=0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL Header of Packet that Triggered the Report | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MSS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Bitmap | +-+-+ ... Figure 5: Segment Acknolwedged Message Format The ETE unconditionally writes the size of its reassembly buffer in the S_MRU field. If the Segment arrived as multiple IP fragments, the ETE also writes the length of the IP first-fragment in the S_MSS field; otherwise, it writes the value 0 in that field. The ETE next includes a Bitmap recording this and all preceding segments of the same SEAL packet that are already in its SEAL reassembly buffer. For each segment, the Bitmap records the value 1 if the segment is present and 0 if the segment is absent. The most significant bit of the Bitmap corresponds to segment 0, the next-most significant bit corresponds to segment 1, etc., and the least significant bit corresponds to the final segment. For example, when SEG=6 in the SEAL header, the bit map '0110011' means that segments 1, 2, 5 and 6 were received but segments 0, 3 and 4 were missing from the ETE's reassembly buffer. The Bitmap must include at least (SEG+ 1)-many bits, where SEG is at most 255. Templin Expires December 20, 2009 [Page 25] Internet-Draft SEAL June 2009 4.4.5.1.3. Packet Too Big (Code=2) The ETE generates a "Reassembly Report - Packet Too Big" message when it discards a SEAL packet that is too large for it to receive. The ETE sets Type=0, Code=2, and Data to 0 . The ETE then writes the SEAL header of segment 0 of the packet that generated the error into the first four bytes of the message body, then writes the size of its reassembly buffer in the S_MRU field. The message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0 | Code=2 | Data=0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL Header of First SEAL Segment of the Too-big Packet | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Packet Too Big Message Format 4.4.5.1.4. Time Exceeded (Code=3) The ETE generates a "Reassembly Report - Time Exceeded" message when it discards an incomplete SEAL reassembly buffer due to a reassembly timeout. The ETE sets Type=0, Code=3, and sets Data to the time in seconds from when the initial SEAL segment arrived until the reassembly time expired. The ETE finally writes the SEAL header of segment 0 of the packet that generated the error into the first four bytes of the message body. If segment 0 is unavailable, the ETE instead writes the SEAL header of the first available segment. The message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0 | Code=3 | Data=(Time, in Seconds) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL Header of First SEAL Segment in the Reassembly Buffer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Segment Acknolwedged Message Format Templin Expires December 20, 2009 [Page 26] Internet-Draft SEAL June 2009 4.4.5.1.5. Checksum Incorrect (Code=4) The ETE generates a "Reassembly Report - Checksum Incorrect" message when it reassembles a SEAL packet with an invalid checksum. The ETE sets Type=0, Code=4 and Data=0. The ETE finally writes the SEAL header of the first segment of the packet into the first four bytes of the message body. The message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0 | Code=4 | Data=0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL Header of First SEAL Segment | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Reassembly Checksum Failure Message Format 4.4.5.2. Parameter Problem (Type=1) The ETE generates a Parameter Problem message when it receives a SEAL packet with an invalid value in one of the SEAL header fields. The ETE sets Type=1 and Code=0, then sets data to the bit number of the SEAL header field that triggered the error (e.g., when Data=8, the parameter problem is specific to the NEXTHDR/SEG field). The ETE finally writes the SEAL header of the packet that generated the error into the first four bytes of the message body. The Parameter Problem message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=1 | Code=0 | Data=SEAL Header bit # | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL Header of Packet that Triggered the Parameter Problem | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Parameter Problem Message Format Other values for Code will be recorded in the IANA registry for SEAL. 4.4.5.3. Information Request (Type=2) The ETE generates an Information Request message when it receives a SEAL packet with I=1 in the SEAL header. The ETE sets Type=2 and sets Code/Data to values that are specific to the associated tunneling protocol (for example, the LISP protocol can use the Templin Expires December 20, 2009 [Page 27] Internet-Draft SEAL June 2009 Information Request message to request mapping updates). The message body further contains opaque data that is interpreted according to the Code/Data values. When Code=0, both Data and the Opaque Data are discarded upon receipt by the ETE. The information request message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=2 | Code=0 | Data=X | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque Data | +-+-+-+ ... Figure 10: Information Request Message Format Other values for Code will be recoded in the IANA registry for SEAL. After the ETE sends an Information Request message, it must retry until it receives a corresponding Information Reply (see Section 4.4.5.4). Note that while in this loop the ETE may receive further SEAL packets with I=1. In that case, the ETE should begin sending Information Requests specific to the SEAL_ID of the new packet and should not dwell on the old SEAL_ID. 4.4.5.4. Information Reply (Type=3) When the ETE sends an Information Request message to the ITE, the ITE responds by sending an Information Reply message back to the ETE with the IP source and destination address set to the destination and source address, the UDP destination port set to the UDP source port, and the SEAL-ID set to the same value that was present in the Information Request message. The ITE sets Type=3 and sets Code/Data to values that are specific to the associated tunneling protocol (for example, the LISP protocol can use the Information Reply message to encode mapping updates). The message body further contains opaque data that is interpreted according to the Code/Data values. When Code=0, both Data and the Opaque Data are discarded upon receipt by the ETE. The information reply message is formatted as follows: Templin Expires December 20, 2009 [Page 28] 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=3 | Code=0 | Data=X | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque Data | +-+-+-+ ... Figure 11: Information Reply Message Format Other values for Code will be recoded in the IANA registry for SEAL. 5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol Specification This section specifies a minimal version of SEAL known as "SEAL with Fragmentation Sensing (SEAL-FS)". SEAL-FS observes the same protocol specifications as for "SEAL with Traffic Engineering (SEAL-TE)" (see Section 4) with the exception that the ITE/ETE do not perform segmentation and reassembly. In particular, the ETE unilaterally drops any SEAL-FS packets that arrive as multiple IP fragments and/or multiple SEAL segments. SEAL-FS 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-FS 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-FS 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). With respect to Section 4, the SEAL-FS protocol corresponds to SEAL-TE as follows: 5.1. Model of Operation SEAL-FS follows the same model of operation as for SEAL-TE as described in Section 4.1 except as noted in the following sections. 5.2. SEAL Header Format (Version 1) The SEAL-FS header is formatted as follows: Templin Expires December 20, 2009 [Page 29] 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|I| Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: 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. I (1) the "Information Request Solicit" bit. Set to 1 if the ITE wishes the ETE to initiate an Information Request. Identification (28) a 28-bit identification field. 5.3. ITE Specification 5.3.1. Tunnel Interface MTU SEAL-FS observes the SEAL-TE specification found in Section 4.3.1. 5.3.2. Admitting Packets into the Tunnel Interface SEAL-FS observes the SEAL-TE specification found in Section 4.3.2. 5.3.3. Segmentation SEAL-FS observes the SEAL-TE specification found in Section 4.3.3, except that the inner fragmentation algorithm is adjusted to avoid all fragmentation and/or segmentation either within or beyond the tunnel as follows: o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1, and the packet is larger than (MIN(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 (MIN(S_MRU, S_MSS) - HLEN) the same as described in Section 4.3.2; else, Templin Expires December 20, 2009 [Page 30] Internet-Draft SEAL June 2009 o if the inner packet is an IPv4 packet with DF=0, and the packet is larger than (S_CSS - HLEN), the ITE uses inner IPv4 fragmentation to break the packet into fragments no larger than (S_CSS - HLEN); else, o the ITE processes the packet without inner fragmentation. 5.3.4. Encapsulation SEAL-FS observes the SEAL-TE 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-FS uses the A and I bits the same as specified for SEAL-TE. 5.3.5. Probing Strategy SEAL-FS observes the SEAL-TE specification found in Section 4.3.5. 5.3.6. Packet Identification SEAL-FS observes the SEAL-TE soft state specifications found in Section 4.3.6, but the SEAL_ID is treated as a 28-bit value that is written into the Identification field in the SEAL header. As for the SEAL-TE specification in Section 4.3.6, SEAL-FS increments the Identification field (modulo 28) for each consecutive SEAL packet. 5.3.7. Sending SEAL Protocol Packets SEAL-FS observes the SEAL-TE specification found in Section 4.3.7. 5.3.8. Processing Raw ICMP Messages SEAL-FS observes the SEAL-TE specification found in Section 4.3.8. 5.3.9. Processing SEAL Control Messages SEAL-FS observes the SEAL-TE specification found in Section 4.3.9. 5.4. ETE Specification 5.4.1. Reassembly Buffer Requirements SEAL-FS does not maintain a reassembly buffer for SEAL reassembly. Templin Expires December 20, 2009 [Page 31] Internet-Draft SEAL June 2009 5.4.2. IP-Layer Reassembly SEAL-FS uses SEAL-protocol IP first-fragments solely for the purpose of generating SEAL Reassembly Reports as specified in Section 4.4.2, but thereafter discards all SEAL-protocol IP fragments. 5.4.3. SEAL-Layer Reassembly SEAL-FS does not observe the SEAL-TE reassembly procedures in Section 4.4.3; Instead, the SEAL-FS ETE discards all SEAL packets with F=0 following IP layer reassembly, and may also return Reassembly Report - Packet Too Big messages when a packet that is too large to receive is discarded. As for SEAL-TE, SEAL-FS returns a Parameter Problem for SEAL packets with unrecognized values in the SEAL header. 5.4.4. Decapsulation and Delivery to Upper Layers SEAL-FS observes the SEAL-TE specification found in Section 4.4.4. 5.4.5. Sending SEAL Control Messages SEAL-FS observes the SEAL-TE specification found in Section 4.4.5. 6. Link Requirements Subnetwork designers are expected to follow the recommendations in 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. Templin Expires December 20, 2009 [Page 32] Internet-Draft SEAL June 2009 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 both 'SEAL_CPORT' and 'SEAL_DPORT' in the 'port-numbers' registry. The IANA is instructed to establish a "SEAL Control Protocol" registry to record SEAL control message Code and Type values. This registry should be initialized to include the Code and Type values defined in Section 4.4.5. 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 IP first-fragments with spoofed source addresses to an ETE, resulting in a stream of Reassembly Report messages returned to a victim ITE. The SEAL_ID in the encapsulated segment of the spoofed IP first- fragment provides mitigation for the ITE to detect and discard spurious Reassembly Reports. 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. 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. Templin Expires December 20, 2009 [Page 33] Internet-Draft SEAL June 2009 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, IP 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 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. Templin Expires December 20, 2009 [Page 34] Internet-Draft SEAL June 2009 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, Pascal Thubert, 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. 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. [RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum Templin Expires December 20, 2009 [Page 35] Internet-Draft SEAL June 2009 options", RFC 1146, March 1990. [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, 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, Templin Expires December 20, 2009 [Page 36] Internet-Draft SEAL June 2009 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. [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998. [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", RFC 2675, August 1999. [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. [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. Templin Expires December 20, 2009 [Page 37] Internet-Draft SEAL June 2009 [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. [RFC5445] Watson, M., "Basic Forward Error Correction (FEC) Schemes", RFC 5445, March 2009. [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 SEAL-TE presents a case in which the unit of loss (i.e., a SEAL segment) is smaller than the end-to-end retransmission unit (e.g., a TCP segment). SEAL-TE tunnels should therefore employ mechanisms to provide enhanced reliability for delivery of SEAL segments. Although a SEAL-TE tunnel may span an arbitrarily-large subnetwork expanse, the IP layer sees the tunnel as a simple link over which IP packets can be transmitted. Typical link layers use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] (e.g., between neighbors in an IEEE 802.11 network) to provide sufficiently reliable delivery. Link ARQ mechanisms are characterized by their persistence, i.e., their willingness to repeatedly retransmit frames to ensure reliable delivery. Therefore, when a SEAL-TE tunnel implements an ARQ mechanism it must carefully tune its degree of persistence to match the tunnel's delay and error characteristics. The SEAL-TE ITE may also tune its persistence based on differentiated link service classes and flows, however this may not be easily accommodated when inner packet headers cannot be inspected. The SEAL-TE ITE can solicit Reassembly Reports from the ETE to discover missing segments for retransmission within a single round- trip time, however retransmission of missing segments may require the ITE to maintain considerable state and may also result in considerable delay variance and packet reordering. As a result, the ETE may be required to maintain reassembly buffering proportional to the tunnel's (bandwidth*delay) product so that packets can be forwarded in the proper order. SEAL-TE may also use alternate reliability mechanisms such as Forward Templin Expires December 20, 2009 [Page 38] Internet-Draft SEAL June 2009 Error Correction (FEC). A simple FEC mechanism may merely entail gratuitous retransmissions, however more efficient alternatives are available. Basic FEC schemes are discussed in [RFC5445]. The use of ARQ and FEC mechanisms for improved reliability are for further study. Appendix B. Transport Mode SEAL can also be used in "transport-mode", e.g., when the inner layer 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: Templin Expires December 20, 2009 [Page 39] Internet-Draft SEAL June 2009 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) 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 Templin Expires December 20, 2009 [Page 40] Internet-Draft SEAL June 2009 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. 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 20, 2009 [Page 41]