Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Intended status: Standards Track March 5, 2010 Expires: September 6, 2010 The Subnetwork Encapsulation and Adaptation Layer (SEAL) draft-templin-intarea-seal-12.txt Abstract For the purpose of this document, a subnetwork is defined as a virtual topology configured over a connected IP network routing region and bounded by encapsulating border nodes. These virtual topologies may span 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. 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 September 6, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Templin Expires September 6, 2010 [Page 1] Internet-Draft SEAL March 2010 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. Templin Expires September 6, 2010 [Page 2] Internet-Draft SEAL March 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9 4. SEAL Protocol Specification . . . . . . . . . . . . . . . . . 10 4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 10 4.2. SEAL Header Format . . . . . . . . . . . . . . . . . . . . 12 4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 13 4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 13 4.3.2. Tunnel Interface Soft State . . . . . . . . . . . . . 15 4.3.3. Admitting Packets into the Tunnel . . . . . . . . . . 15 4.3.4. Mid-Layer Encapsulation . . . . . . . . . . . . . . . 16 4.3.5. SEAL Segmentation . . . . . . . . . . . . . . . . . . 17 4.3.6. Outer Encapsulation . . . . . . . . . . . . . . . . . 17 4.3.7. Probing Strategy . . . . . . . . . . . . . . . . . . . 18 4.3.8. Packet Identification . . . . . . . . . . . . . . . . 18 4.3.9. Sending SEAL Protocol Packets . . . . . . . . . . . . 18 4.3.10. Processing Raw ICMP Messages . . . . . . . . . . . . . 19 4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 19 4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 19 4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 20 4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 20 4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 21 4.5. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 22 4.5.1. Generating SCMP Messages . . . . . . . . . . . . . . . 23 4.5.2. Processing SCMP Messages . . . . . . . . . . . . . . . 25 4.6. TE Window Synchronization and Maintenance . . . . . . . . 26 5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 28 6. End System Requirements . . . . . . . . . . . . . . . . . . . 29 7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 29 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 9. Security Considerations . . . . . . . . . . . . . . . . . . . 29 10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 30 11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 30 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32 13.1. Normative References . . . . . . . . . . . . . . . . . . . 32 13.2. Informative References . . . . . . . . . . . . . . . . . . 32 Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 35 Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 35 Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 36 Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 37 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38 Templin Expires September 6, 2010 [Page 3] Internet-Draft SEAL March 2010 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 visible to the inner network 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 IPv4 address space is rapidly becoming depleted, there is a lesser-known but growing consensus that other IPv4 protocol limitations have already or may soon become problematic. First, the IPv4 header Identification field is only 16 bits in length, meaning that at most 2^16 unique packets with the same (source, destination, protocol)-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 for high data rate packet sources such as tunnel endpoints [RFC4963]. 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., Templin Expires September 6, 2010 [Page 4] Internet-Draft SEAL March 2010 in-the-network router fragmentation) to the potential for major integrity issues (e.g., mis-association of the fragments of multiple IP packets during reassembly [RFC4963]). 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 D). 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" [RFC2923]. 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 [I-D.ietf-tcpm-icmp-attacks]. 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. This behavior has been confirmed through documented studies showing clear evidence of path MTU discovery failures in the Internet today [TBIT][WAND]. The issues with both IPv4 fragmentation and this "classical" method of path MTU discovery are exacerbated further when IP tunneling is used [RFC4459]. For example, ingress tunnel endpoints (ITEs) may be required to forward encapsulated packets into the subnetwork on behalf of hundreds, thousands, or even more original sources in the end site. If the ITE allows IPv4 fragmentation on the encapsulated packets, persistent fragmentation could lead to undetected data corruption due to Identification field wrapping. If the ITE instead uses classical IPv4 path MTU discovery, it may be inconvenienced by excessive ICMP error messages coming from the subnetwork that may be either suspect or contain insufficient information for 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 Templin Expires September 6, 2010 [Page 5] Internet-Draft SEAL March 2010 ICMP error message into a suitable ICMP error message to return to the original source. Although recent works have led to the development of a robust end-to- end MTU determination scheme [RFC4821], this approach requires tunnels to present a consistent MTU the same as for ordinary links on the end-to-end path. Moreover, in current practice existing tunneling protocols mask the MTU issues by selecting a "lowest common denominator" MTU that may be much smaller than necessary for most paths and difficult to change at a later date. Due to these many consideration, a new approach to accommodate tunnels over links with diverse MTUs is necessary. 1.2. Approach For the purpose of this document, a subnetwork is defined as a virtual topology configured over a connected network routing region and bounded by encapsulating border nodes. Examples include the global Internet interdomain routing core, Mobile Ad hoc Networks (MANETs) and enterprise networks. Subnetwork border nodes forward unicast and multicast 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 tunneling network layer protocols (e.g., IP, OSI, etc.) over IP subnetworks that connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border nodes. It provides a modular specification designed to be tailored to specific associated tunneling protocols. A transport-mode of operation is also possible, and described in Appendix C. 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 specifically treats tunnels that traverse the subnetwork as unidirectional links that must support network layer services. As for any link, tunnels that use SEAL must provide suitable networking services including best-effort datagram delivery, integrity and consistent handling of packets of various sizes. As for any link whose media cannot provide suitable services natively, tunnels that use SEAL employ link-level adaptation functions to meet the legitimate expectations of the network layer service. As this is essentially a link level adaptation, SEAL is therefore permitted to alter packets within the subnetwork as long as it restores them to their original form when they exit the subnetwork. The mechanisms described within this document are designed precisely for this Templin Expires September 6, 2010 [Page 6] Internet-Draft SEAL March 2010 purpose. 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. This approach is in contrast to existing tunneling protocol practices which seek to avoid MTU issues by selecting a "lowest common denominator" MTU that may be overly conservative for many tunnels and difficult to change even when larger MTUs become available. The following sections provide the SEAL normative specifications, while the appendices present non-normative additional considerations. 2. Terminology and Requirements The following terms are defined within the scope of this document: subnetwork a virtual topology configured over a connected network routing region and bounded by encapsulating border nodes. Ingress Tunnel Endpoint a virtual interface over which an encapsulating border node (host or router) sends encapsulated packets into the subnetwork. Egress Tunnel Endpoint a virtual interface over which an encapsulating border node (host or router) receives encapsulated packets from the subnetwork. inner packet an unencapsulated network layer protocol packet (e.g., IPv6 [RFC2460], IPv4 [RFC0791], OSI/CLNP [RFC1070], etc.) before any mid-layer or outer encapsulations are added. Internet protocol numbers that identify inner packets are found in the IANA Internet Protocol registry [RFC3232]. mid-layer packet a packet resulting from adding mid-layer encapsulating headers to an inner packet. Templin Expires September 6, 2010 [Page 7] Internet-Draft SEAL March 2010 outer IP packet a packet resulting from adding an outer IP header to a mid-layer packet. packet-in-error the leading portion of an invoking data packet encapsulated in the body of an error control message (e.g., an ICMPv4 [RFC0792] error message, an ICMPv6 [RFC4443] error message, etc.). IP, IPvX, IPvY used to generically refer to either IP protocol version, i.e., IPv4 or IPv6. The following abbreviations correspond to terms used within this document and elsewhere in common Internetworking nomenclature: DF - the IPv4 header "Don't Fragment" flag [RFC0791] ETE - Egress Tunnel Endpoint HLEN - the sum of MHLEN and OHLEN ITE - Ingress Tunnel Endpoint MHLEN - the length of any mid-layer headers and trailers OHLEN - the length of the outer encapsulating headers and trailers, including the outer IP header, the SEAL header and any other outer headers and trailers. PTB - a Packet Too Big message recognized by the inner network layer, e.g., an ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4 "Fragmentation Needed" message [RFC0792], etc. S_MRU - the SEAL Maximum Reassembly Unit S_MSS - the SEAL Maximum Segment Size SCMP - the SEAL Control Message Protocol SEAL_ID - an Identification value, randomly initialized and monotonically incremented for each SEAL protocol packet SEAL_PORT - a TCP/UDP service port number used for SEAL SEAL_PROTO - an IPv4 protocol number used for SEAL Templin Expires September 6, 2010 [Page 8] Internet-Draft SEAL March 2010 TE - Tunnel Endpoint (i.e., either ingress or egress) 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]. When used in lower case (e.g., must, must not, etc.), these words MUST NOT be interpreted as described in [RFC2119], but are rather interpreted as they would be in common English. 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-intarea-vet] and the RANGER architecture [I-D.templin-ranger][I-D.russert-rangers]. SEAL can be used as a network sublayer for encapsulation of an inner packet within outer encapsulating headers. For example, for IPvX in IPvY encapsulation (e.g., as IPv4/SEAL/IPv6), the SEAL header appears as a subnetwork encapsulation as seen by the inner IP layer. SEAL can also be used as a sublayer within a UDP data payload (e.g., as IPv4/UDP/SEAL/IPv6 similar to Teredo [RFC4380]), where UDP encapsulation is typically used for operation over subnetworks that give preferential treatment to the "core" Internet protocols (i.e., TCP and UDP). The SEAL header is processed the same as for IPv6 extension headers, i.e., it is not part of the outer IP header but rather allows for the creation of an arbitrarily extensible chain of headers in the same way that IPv6 does. SEAL supports a segmentation and reassembly capability for adapting the network layer to the underlying subnetwork characteristics, where the Egress Tunnel Endpoint (ETE) determines how much or how little reassembly it is willing to support. In the limiting case, the ETE acts as a passive observer that simply informs the Ingress Tunnel Endpoint (ITE) of any MTU limitations and otherwise discards all packets that arrive as multiple fragments. This mode is useful for determining an appropriate MTU for tunnels between performance- critical routers connected to high data rate subnetworks such as the Internet DFZ, as well as for other uses in which reassembly would present too great of a burden for the routers or end systems. When the ETE supports reassembly, the tunnel can be used to transport packets that are too large to traverse the path without Templin Expires September 6, 2010 [Page 9] Internet-Draft SEAL March 2010 fragmentation. In this mode, the ITE determines the tunnel MTU based on the largest packet the ETE is capable of reassembling rather than on the MTU of the smallest link in the path. Therefore, SEAL can transport packets that are much larger than the underlying subnetwork links themselves can carry in a single piece. SEAL tunnels may be configured over paths that include not only ordinary physical links, but also virtual links that may include other SEAL tunnels. An example application would be linking two geographically remote supercomputer centers with large MTU links by configuring a SEAL tunnel across the Internet. A second example would be support for sub-IP segmentation over low-end 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 are anticipated, and will be identified as further experience is gained. 4. SEAL Protocol Specification The following sections specify the operation of the SEAL protocol. 4.1. Model of Operation SEAL is an encapsulation sublayer that supports a multi-level segmentation and reassembly capability for the transmission of unicast and multicast packets across an underlying IP subnetwork with heterogeneous links. First, the ITE can use IPv4 fragmentation to fragment inner IPv4 packets before SEAL encapsulation if necessary. 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 is used to detect and tune out any in-the-network fragmentation. SEAL-enabled ITEs encapsulate each inner packet in mid-layer headers and trailers, segment the resulting mid-layer packet into multiple segments if necessary, then append a SEAL header and (if necessary) a UDP header to each segment. The ITE then adds the outer encapsulation headers to each segment. For example, a single-segment inner IPv6 packet encapsulated in any mid-layer headers and trailers, the SEAL header, any outer headers and trailers and an outer IPv4 header would appear as shown in Figure 1: Templin Expires September 6, 2010 [Page 10] Internet-Draft SEAL March 2010 +--------------------+ ~ outer IPv4 header ~ +--------------------+ I ~ other outer hdrs ~ n +--------------------+ n ~ SEAL Header ~ e +--------------------+ +--------------------+ r ~ mid-layer headers ~ ~ mid-layer headers ~ +--------------------+ +--------------------+ I --> | | --> | | P --> ~ inner IPv6 ~ --> ~ inner IPv6 ~ v --> ~ Packet ~ --> ~ Packet ~ 6 --> | | --> | | +--------------------+ +--------------------+ P ~ mid-layer trailers ~ ~ mid-layer trailers ~ a +--------------------+ +--------------------+ c ~ outer trailers ~ k Mid-layer packet +--------------------+ e after mid-layer encaps. t Outer IPv4 packet after SEAL and outer encaps. Figure 1: SEAL Encapsulation - Single Segment In a second example, an inner IPv4 packet requiring three SEAL segments would appear as three separate outer IPv4 packets, where the mid-layer headers are carried only in segment 0 and the mid-layer trailers are carried in segment 2 as shown in Figure 2: +------------------+ +------------------+ ~ outer IPv4 hdr ~ ~ outer IPv4 hdr ~ +------------------+ +------------------+ +------------------+ ~ other outer hdrs ~ ~ outer IPv4 hdr ~ ~ other outer hdrs ~ +------------------+ +------------------+ +------------------+ ~ SEAL hdr (SEG=0) ~ ~ other outer hdrs ~ ~ SEAL hdr (SEG=2) ~ +------------------+ +------------------+ +------------------+ ~ mid-layer hdrs ~ ~ SEAL hdr (SEG=1) ~ | inner IPv4 | +------------------+ +------------------+ ~ Packet ~ | inner IPv4 | | inner IPv4 | | (Segment 2) | ~ Packet ~ ~ Packet ~ +------------------+ | (Segment 0) | | (Segment 1) | ~ mid-layer trails ~ +------------------+ +------------------+ +------------------+ ~ outer trailers ~ ~ outer trailers ~ ~ outer trailers ~ +------------------+ +------------------+ +------------------+ Segment 0 (includes Segment 1 (no mid- Segment 2 (includes mid-layer hdrs) layer encaps) mid-layer trails) Figure 2: SEAL Encapsulation - Multiple Segments Templin Expires September 6, 2010 [Page 11] Internet-Draft SEAL March 2010 The SEAL header itself is inserted according to the specific tunneling protocol. Examples include the following: o For simple encapsulation of an inner network layer packet within an outer IPvX header (e.g., [RFC1070][RFC2003][RFC2473][RFC4213], etc.), the SEAL header is inserted between the inner packet and outer IPvX headers as: IPvX/SEAL/{inner packet}. o For IPsec encapsulations [RFC4301], the SEAL header is inserted between the {AH,ESP} headers and outer IP headers as: IPvX/SEAL/ {AH,ESP}/{inner packet}. Here, the {AH, ESP} headers and trailers are seen as mid-layer encapsulations. o For encapsulations over transports such as UDP (e.g., [RFC4380]), the SEAL header is inserted between the outer transport layer header and the mid-layer packet, e.g., as IPvX/UDP/SEAL/{mid-layer packet}. Here, the UDP header is seen as an "other outer header". 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 TEs use the SEAL_ID for SEAL segmentation/ reassembly and protection against off-path attacks. The following sections specify the SEAL header format and SEAL-related operations of the ITE and ETE, respectively. 4.2. SEAL Header Format The SEAL 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|S|P|F|M|R| NEXTHDR/SEG | SEAL_ID (bits 48 - 32) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL_ID (bits 31 - 0) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: SEAL Header Format where the header fields are defined as: VER (2) a 2-bit version field. This document specifies Version 0 of the SEAL protocol, i.e., the VER field encodes the value 0. Templin Expires September 6, 2010 [Page 12] Internet-Draft SEAL March 2010 A (1) the "Acknowledge" bit. Set to 1 by the ETE to acknowledge a Synchronization event. S (1) the "Synchronize" bit. Set to 1 by the ITE to request Synchronization. P (1) the "Probe" bit. Set to 1 if the ITE wishes to receive an explicit acknowledgement from the ETE. 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. 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. R (1) a Reserved bit. 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.) SEAL_ID (48) a 48-bit Identification field. Setting of the various bits and fields of the SEAL header is specified in the following sections. Unless explicitly specified, each unspecified bit and field is assumed to be set to zero. 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 Layer 3 as the size for admission of inner packets into the tunnel. Since the tunnel interface may support a large set of ETEs that accept widely varying maximum packet sizes, however, a number of factors should be taken into consideration when selecting a tunnel interface MTU. Templin Expires September 6, 2010 [Page 13] Internet-Draft SEAL March 2010 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 ICMP Packet Too Big (PTB) message returned. When the 1500 byte packets sent by end systems incur additional encapsulation at an ITE, however, they may be dropped silently since the network may not always deliver the necessary PTBs [RFC2923]. The ITE should therefore set a tunnel virtual interface MTU of at least 1500 bytes plus extra room to accommodate any additional encapsulations that may occur on the path from the original source. 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. 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 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 outgoing interface and as such may select too large an initial size. This is not a problem for upper layers that use conservative initial maximum segment size estimates and/or when the tunnel interface can reduce the upper layer's maximum segment size (e.g., the size advertised in the TCP MSS option) based on the per-neighbor MTU. The inner network layer protocol consults the tunnel interface MTU when admitting a packet into the interface. For inner 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 IPv4 layer uses IPv4 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 inner packets, the ITE admits the packet if it is no larger than the tunnel interface MTU; otherwise, it drops the packet and sends a 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 network layer minimum MTU (e.g., 576 bytes for IPv4, 1280 bytes for IPv6, etc.). Templin Expires September 6, 2010 [Page 14] Internet-Draft SEAL March 2010 Note that when the tunnel interface sets an indefinite MTU the ITE unconditionally admits all packets into the interface without fragmentation. In light of the above considerations, it is RECOMMENDED that the ITE configure an indefinite MTU on the tunnel virtual interface and handle any per-neighbor MTU mismatches within the tunnel virtual interface (e.g., by reducing the size advertised in the TCP MSS option). 4.3.2. Tunnel Interface Soft State For each ETE, the ITE maintains soft state within the tunnel interface (e.g., in a neighbor cache) used to support inner fragmentation and SEAL segmentation for packets admitted into the tunnel interface. 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., AH, ESP, NULL, etc.) that must be added before SEAL segmentation. o an Outer Header Length (OHLEN); set to the length of the outer IP, SEAL and other outer encapsulation headers and trailers. o a total Header Length (HLEN); set to MHLEN plus OHLEN. o a SEAL Maximum Segment Size (S_MSS). The ITE initializes S_MSS to the underlying interface MTU if the underlying interface MTU can be determined (otherwise, the ITE initializes S_MSS to "infinity"). The ITE decreases or increased S_MSS based on any SCMP "MTU Report" messages received (see Section 4.5). o a SEAL Maximum Reassembly Unit (S_MRU). The ITE initializes S_MRU to "infinity" and decreases or increases S_MRU based on any SCMP MTU Report messages received (see Section 4.5). When (S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256) as the effective S_MRU value. Note that S_MSS and S_MRU include the length of the outer and mid- layer encapsulating headers and trailers (i.e., HLEN), since the ETE must retain the headers and trailers during reassembly. Note also that the ITE maintains S_MSS and S_MRU as 32-bit values such that inner packets larger than 64KB (e.g., IPv6 jumbograms [RFC2675]) can be accommodated when appropriate for a given subnetwork. 4.3.3. Admitting Packets into the Tunnel After the ITE admits an inner packet/fragment into the tunnel interface, it uses the following algorithm to determine whether the packet can be accommodated and (if so) whether (further) inner IP Templin Expires September 6, 2010 [Page 15] Internet-Draft SEAL March 2010 fragmentation is needed: o if the inner packet is unfragmentable (e.g., an IPv6 packet, an IPv4 packet with DF=1, etc.), and the packet is larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends a PTB message to the original source with an MTU value of (MAX(S_MRU, S_MSS) - HLEN); else, o if the inner packet is fragmentable (e.g., an IPv4 packet with DF=0), and the packet is larger than 1280 bytes, the ITE uses inner fragmentation to break the packet into fragments no larger than 1280 bytes; else, o the ITE processes the packet without inner fragmentation. In the above, the ITE must track whether the tunnel interface is using header compression. If so, the ITE must include the length of the uncompressed headers and trailers when calculating HLEN. Note also in the above that the ITE is permitted to admit inner packets into the tunnel that can be accommodated in a single SEAL segment (i.e., no larger than S_MSS) even if they are larger than the ETE would be willing to reassemble if fragmented (i.e., larger than S_MRU). When the ITE uses inner fragmentation, it can optionally use a "safe" fragment size of 1280 bytes for initial packets while probing in parallel for a larger fragment size that would still avoid outer IP fragmentation within the tunnel. If the ITE can determine a larger fragment size, it may use this larger size for inner fragmentation. If the inner packet is unfragmentable, and the packet will be sent in-the-clear with no mid-layer encryption, the ITE can instead employ a stateless strategy by simply encapsulating and sending the packet without regard to its length. The ITE can then translate any SCMP MTU Report messages it receives from the ETE into PTB messages to return to the original source (where the translation is based on the packet-in-error within the SCMP MTU Report message). In this method, the ITE need not maintain per-ETE S_MRU and S_MSS state. 4.3.4. Mid-Layer Encapsulation After inner IP fragmentation (if necessary), the ITE next encapsulates each inner packet/fragment in the MHLEN bytes of mid- layer headers and trailers. (For example, when IPsec ESP is used [RFC4301], the ITE performs the necessary security transformations on the inner packet/fragment then adds an ESP header and trailer.) The ITE then presents the mid-layer packet for SEAL segmentation and outer encapsulation. Templin Expires September 6, 2010 [Page 16] Internet-Draft SEAL March 2010 4.3.5. SEAL Segmentation After mid-layer encapsulation, 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 outside of the subnetwork. 4.3.6. Outer Encapsulation Following SEAL segmentation, the ITE next encapsulates each segment in a SEAL header formatted as specified in Section 4.2. For the first segment, the ITE sets F=1, then sets NEXTHDR to the Internet Protocol number of the encapsulated inner packet, and finally 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, since the initial segment encodes a NEXTHDR value and not a SEG value.) The ITE next encapsulates each segment in the requisite outer headers and trailers 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_PORT' in the outer destination service port field (e.g., when IP/UDP encapsulation is used). The ITE finally sets the P bit to 1 if necessary as specified in Section 4.3.7, sets the packet identification values as specified in Section 4.3.8 and sends the packets as specified in Section 4.3.9. Templin Expires September 6, 2010 [Page 17] Internet-Draft SEAL March 2010 4.3.7. Probing Strategy All SEAL encapsulated packets sent by the ITE are considered implicit probes, and will elicit SCMP MTU Report messages from the ETE (see: Section 4.5) with a new value for S_MSS if any IP fragmentation occurs in the path. Thereafter, the ITE can periodically reset S_MSS to a larger value (e.g., the underlying IP interface MTU) to detect path MTU increases. The ITE also sends explicit probes, periodically, to verify that the ETE is still reachable. The ITE sets P=1 in the SEAL header of a 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 NEXTHDR field to a value of "No Next Header" (see Section 4.7 of [RFC2460]). The probe will elicit an SCMP Neighbor Advertisement message from the ETE as an acknowledgement (see Section 4.5). Finally, the ITE MAY send "expendable" outer IP probe packets (see Section 4.3.9) as explicit probes in order to generate PTB messages from routers on the path to the ETE. In all cases, the ITE MUST be conservative in its use of the P bit in order to limit the resultant control message overhead. 4.3.8. Packet Identification The ITE maintains a randomly-initialized SEAL_ID value as per-ETE soft state (e.g., in the neighbor cache) and monotonically increments it for each successive SEAL protocol packet it sends to the ETE. For each successive SEAL protocol packet, the ITE writes the current SEAL_ID value into the header field of the same name in the SEAL header. 4.3.9. Sending SEAL Protocol Packets Following SEAL segmentation and encapsulation, the ITE sets DF=0 in the header of each outer IPv4 packet to ensure that they will be delivered to the ETE even if they are fragmented within the subnetwork. (The ITE can instead set DF=1 for "expendable" outer IPv4 packets (e.g., for NULL packets used as probes -- see Section 4.3.7), but these may be lost due to an MTU restriction). For outer IPv6 packets, the "DF" bit is always implicitly set to 1; hence, they will not be fragmented within the subnetwork. 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 September 6, 2010 [Page 18] Internet-Draft SEAL March 2010 4.3.10. 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; specific actions that the ITE may take in this case are out of scope. 4.4. ETE Specification 4.4.1. Reassembly Buffer Requirements The ETE SHOULD support IP-layer and SEAL-layer reassembly for inner packets of at least 1280 bytes in length and MAY support reassembly for larger inner packets; the ETE may optionally support no reassembly at all, but this may cause MTU underruns in some environments. The ETE must retain the outer IP, SEAL and other outer headers and trailers during both IP-layer and SEAL-layer reassembly for the purpose of associating the fragments/segments of the same packet, and must also configure a SEAL-layer reassembly buffer that is no smaller than the IP-layer reassembly buffer. Hence, the ETE: o SHOULD configure an outer IP-layer reassembly buffer size of at least (1280 + HELN) bytes. o MUST configure a SEAL-layer reassembly buffer size (i.e., S_MRU) that is no smaller than the IP-layer reassembly buffer size. o MUST be capable of discarding inner packets that require IP-layer or SEAL-layer reassembly and that are larger than (S_MRU - HLEN). The ETE can maintain S_MRU either as a single value to be applied for all ITEs, or as a per-ITE value. In that case, the ETE can manage each per-ITE S_MRU value separately (e.g., to reduce congestion caused by excessive segmentation from specific ITEs) but should seek to maintain as stable a value as possible for each ITE. Note that the ETE is permitted to accept inner packets that did not undergo IP-layer and/or SEAL-layer reassembly even if they are larger Templin Expires September 6, 2010 [Page 19] Internet-Draft SEAL March 2010 than (S_MRU - HELN) bytes. Hence, S_MRU is a maximum *reassembly* size, and may be less than the ETE is able to receive without reassembly. 4.4.2. IP-Layer Reassembly The ETE submits unfragmented SEAL protocol IP packets for SEAL-layer reassembly as specified in Section 4.4.3. The ETE instead performs standard IP-layer reassembly for multi-fragment SEAL protocol IP packets as follows. The ETE should maintain conservative IP-layer reassembly cache high- and low-water marks. 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 pending reassembly has arrived. Following successful IP-layer reassembly, the ETE submits the reassembled packet for SEAL-layer reassembly as specified in Section 4.4.3. 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 an SCMP MTU Report message back to the ITE with the MTU field set to S_MRU (see Section 4.5). When the ETE processes an IP fragment that would cause the reassembled outer packet to be larger than the IP- layer reassembly buffer following reassembly, it discontinues the reassembly and discards any further fragments. 4.4.3. SEAL-Layer Reassembly Following IP reassembly (if necessary), if the SEAL packet has an incorrect value in the SEAL header the ETE discards the packet and returns an SCMP "Parameter Problem" message (see Section 4.5). The ETE next submits single-segment mid-layer packets for decapsulation and delivery to upper layers as specified in Section 4.4.4. The ETE instead performs SEAL-layer reassembly for multi-segment mid-layer packets as follows. The ETE adds each segment of a multi-segment mid-layer packet to a SEAL-layer pending-reassembly queue according to the (Source, Destination, SEAL_ID)-tuple found in the outer IP and SEAL 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 segments. SEAL-layer reassembly requires the ETE to maintain a cache of recently received segments Templin Expires September 6, 2010 [Page 20] Internet-Draft SEAL March 2010 for a hold time that would allow for nominal inter-segment delays. When a SEAL reassembly times out, the ETE discards the incomplete reassembly and returns an SCMP "Time Exceeded" message to the ITE (see Section 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. 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. If the ETE accepts a new SEAL segment that would cause the reassembled outer packet to be larger than S_MRU following reassembly, it discontinues the reassembly and sends an SCMP MTU Report message with the MTU field set to S_MRU (see Section 4.5). After all segments are gathered, the ETE reassembles the packet by concatenating the segments encapsulated in the 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 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). Following successful SEAL-layer reassembly, the ETE submits the reassembled mid-layer packet for decapsulation and delivery to upper layers as specified in Section 4.4.4. 4.4.4. Decapsulation and Delivery to Upper Layers Following any necessary IP- and SEAL-layer reassembly, the ETE discards the outer headers and trailers and performs any mid-layer transformations (e.g., IPsec ESP) on the mid-layer packet. The ETE next discards the mid-layer headers and trailers, and delivers the inner packet to the upper-layer protocol indicated either in the SEAL NEXTHDR field or the next header field of the mid-layer packet (i.e., if the packet included mid-layer encapsulations). The ETE instead silently discards the inner packet if it was a NULL packet (see Section 4.3.9). Templin Expires September 6, 2010 [Page 21] Internet-Draft SEAL March 2010 4.5. The SEAL Control Message Protocol (SCMP) SEAL uses a companion SEAL Control Message Protocol (SCMP) that implements the same message format as the Internet Control Message Protocol for IPv6 (ICMPv6) [RFC4443]. SCMP messages are further identified by the NEXTHDR value '58' the same as for ICMPv6 messages, however the SCMP message is *not* immediately preceded by an inner IPv6 header. Instead, SCMP messages appear immediately following either the SEAL header or mid-layer header (i.e., if the packet included mid-layer encapsulations). Therefore, this differing header arrangement is the sole means by which TEs differentiate SCMP messages from ordinary ICMPv6 messages. Unlike ICMPv6 messages, SCMP messages are used only for the purpose of conveying information between TEs, i.e., they are used only for information sharing within the tunnel and not beyond the tunnel. SCMP messages use the same message types specified for ordinary ICMPv6 messages in [RFC4443][RFC4861]. SCMP can also be used to carry other ICMPv6 message types (e.g., [RFC4191], etc.) in manners that are outside the scope of this document. SCMP messages are formatted as shown in Figure 4: 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Message Body ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking SEAL data | ~ packet as possible without the SCMP ~ | packet exceeding 576 bytes (*) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ (*) the invoking SEAL packet segment (i.e., the "packet-in-error") is only included for SCMP messages sent in response to SEAL data Figure 4: SCMP Message Format As for ICMPv4 and ICMPv6 messages, the SCMP message begins with a 4 byte header that includes 8-bit Type and Code fields followed by a 16-bit Checksum field. The SCMP message header is followed by the message body which is followed by the leading portion of the invoking packet-in-error (when present) beginning with the packet's outer IP header. The Checksum is calculated the same as specified for ICMPv4 messages in [RFC0792], i.e., the checksum does not include a pseudo- Templin Expires September 6, 2010 [Page 22] Internet-Draft SEAL March 2010 header of the outer IP header since the SEAL_ID gives sufficient assurance against mis-delivery. 4.5.1. Generating SCMP Messages The TE prepares the SCMP message exactly as specified for the corresponding ICMPv6 message. If the SCMP message will include a packet-in-error, the TE includes the leading portion of the invoking SEAL data packet beginning with the outer IP header, followed by the SEAL header, etc., and extending to a length that would not cause the entire SCMP message to exceed 576 bytes. The TE then encapsulates the SCMP message in any mid-layer headers and trailers. For example, if the TE uses IPsec ESP it encapsulates the SCMP message directly within the mid-layer ESP headers and trailers, i.e., it does not encapsulate the SCMP message within an inner header. The TE next encapsulates the mid-layer packet in the SEAL header, any other outer headers and finally in the outer IP header. The SCMP message format is shown in Figure 5. +--------------------+ ~ outer IPv4 header ~ +--------------------+ ~ other outer hdrs ~ +--------------------+ ~ SEAL Header ~ S +--------------------+ +--------------------+ C ~ mid-layer headers ~ ~ mid-layer headers ~ M +--------------------+ +--------------------+ P --> ~ SCMP message header~ --> ~ SCMP message header~ --> +--------------------+ --> +--------------------+ M --> ~ SCMP message body ~ --> ~ SCMP message body ~ e --> +--------------------+ --> +--------------------+ s --> ~ packet-in-error ~ --> ~ packet-in-error ~ s +--------------------+ +--------------------+ a ~ mid-layer trailers ~ ~ mid-layer trailers ~ g +--------------------+ +--------------------+ e ~ outer trailers ~ SCMP Message +--------------------+ after mid-layer encaps. SCMP Message after SEAL and outer encaps. Figure 5: SCMP Message Encapsulation During outer encapsulation, the TE sets the outer IP destination and source addresses of the SCMP packet to the source and destination addresses (respectively) of the packet-in-error. If the destination address in the packet-in-error was multicast, the TE instead sets the Templin Expires September 6, 2010 [Page 23] Internet-Draft SEAL March 2010 outer IP source address of the SCMP packet to an address assigned to the underlying IP interface. The TE finally sets the NEXTHDR field in either the SEAL header or the mid-layer header (if present) to the value '58', i.e., the official IANA protocol number for the ICMPv6 protocol. 4.5.1.1. Generating SCMP MTU Report Messages An ETE generates an SCMP MTU Report message in the following cases: o Case 1: the ETE receives a SEAL data packet that would cause the reassembled outer packet to exceed S_MRU following reassembly. o Case 2: the ETE receives a SEAL data packet with P=1 in the SEAL header. o Case 3: the ETE receives the IP first fragment (i.e., one with MF=1 and Offset=0 in the IP header) of a fragmented SEAL data packet. The ETE prepares an SCMP MTU Report message the same specified for an ICMPv6 Packet Too Big message (see: [RFC4443], Section 3.2), and includes as much of the invoking SEAL data packet as possible in the packet-in-error field without the resulting SCMP packet exceeding 576 bytes. For Case 1 above, the ETE then writes the S_MRU value for this ITE in the MTU field and the value 0 in the Code field of the message. For Cases 2 and 3 above, the ETE instead writes the value 0 in the MTU field and the value 1 in the Code field of the message. The ETE then encapsulates the SCMP MTU Report message in any mid- layer and outer headers and trailers as shown in Figure 5 then sends the resulting SCMP message back to the ITE. After it sends the SCMP MTU Report message, the ETE next accepts or discards the SEAL data packet according to the specific case. For Case 1, the ETE discards the SEAL data packet and schedules any reassembly resources for deletion. For Cases 2 and 3, the ETE accepts the SEAL data packet even though it also returned an SCMP MTU Report message to the ITE. 4.5.1.2. Generating SCMP Destination Unreachable Messages An ETE generates an SCMP "Destination Unreachable - Communication with Destination Administratively Prohibited" message when it receives a SEAL packet with a SEAL_ID that is outside of the current window for this ITE (see: Section 4.6). The message is formatted the same as for ICMPv6 Destination Unreachable messages. Generation of SCMP Destination Unreachable messages with other codes Templin Expires September 6, 2010 [Page 24] Internet-Draft SEAL March 2010 is outside the scope of this document. 4.5.2. Processing SCMP Messages Each TE processes any SCMP messages it receives as long as it can verify that the message was sent from a legitimate tunnel far end. The TE can verify that the SCMP message came from a legitimate tunnel far end by checking that the SEAL_ID in the encapsulated packet-in- error corresponds to one of its recently-sent SEAL data packets. When the tunnel endpoints are synchronized, TE can also (or instead) check that the SEAL_ID in the SEAL header of the SCMP message is within the window of recently received packets from this tunnel far end (see Section 4.6). Each ITE maintains a window of outstanding SEAL_IDs of packets that it has recently sent to each ETE. For each SCMP message it receives, the ITE first verifies that the SEAL_ID encoded in the packet-in- error is within 32768 of the SEAL_ID of the most recent packet that it has sent to the ETE. The ITE then verifies that the checksum in the SCMP message header is correct. If the SEAL_ID is outside of the window and/or the checksum is incorrect, the ITE discards the message; otherwise, it processes the message the same as for ordinary ICMPv6 messages. 4.5.2.1. Processing SCMP MTU Report Messages An ITE may receive an SCMP MTU Report message after it sends a SEAL data packet (see: Section 4.5.1.1). When the ITE receives an SCMP MTU Report message, it processes the message as follows: For SCMP MTU Report messages with Code=0, the ITE records the value in the MTU field as the new S_MRU value for this ETE. The ITE then examines the packet-in-error to determine whether it can be translated into a PTB message to send back to the original source. If so, the ITE can optionally send a translated PTB message to the original source with MTU set to (S_MRU - HLEN). For SCMP MTU Report messages with Code=1, the ITE examines the IP header of the packet-in-error. If the packet-in-error is not an IP fragment, and if the packet-in-error length is greater than the current S_MSS value, the ITE records the length as the new S_MSS value in its soft state for this ETE. If the packet in-error is a first fragment, however, the ITE determines a new S_MSS value according to the packet-in-error length as follows: o If the length is no less than 1280, the ITE records the length as the new S_MSS value. Templin Expires September 6, 2010 [Page 25] Internet-Draft SEAL March 2010 o If the length is less than the current S_MSS value and also less than 1280, the ITE can discern that IP fragmentation is occurring but it cannot determine the true MTU of the restricting link due to the possibility that a router on the path is generating runt first fragments. In this latter case, the ITE must 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 in which the ITE sends SEAL data packets using a reduced S_MSS and receives additional SCMP MTU Report messages. During this process, it is essential that the ITE reduce S_MSS based on the first SCMP MTU Report message received under the current S_MSS size, and refrain from further reducing S_MSS until SCMP MTU Report messages pertaining to packets sent under the new S_MSS are received. Finally, the ITE examines the SEAL header of the packet-in-error to determine whether the message constitutes a reply to an explicit probe (see: Section 4.3.7) in order to facilitate neighbor unreachability detection "hints of forward progress". The ITE then discards the SCMP message. 4.5.2.2. Processing SCMP Destination Unreachable Messages An ITE may receive an SCMP "Destination Unreachable - Communication with Destination Administratively Prohibited" message after it sends a SEAL data packet. The ITE processes this message as an indication that it needs to (re)synchronize with the ETE (see: Section 4.6). Processing of SCMP Destination Unreachable messages with other codes is outside the scope of this document. 4.6. TE Window Synchronization and Maintenance SEAL Tunnel Endpoints (TEs) can optionally synchronize sequence numbers in an initial exchange that utilizes the IPv6 neighbor unreachability detection procedure and parallels the TCP 3-way handshake. Each ITE can then use the SEAL_ID in the packets it sends not only to support the segmentation and reassembly procedures, but also as a sequence number of packets that it has recently sent to the ETE. Similarly, each ETE can use the SEAL_ID in the packets it receives as a sequence number of packet that it has recently received from the ITE. This arrangement requires an initial synchronization of sequence numbers between tunnel endpoints as specified below. SEAL ITEs should be operationally configured to operate in either synchronized or unsynchronized fashion. When an ITE attempts to operate in unsynchronized fashion but the ETE requires synchronized Templin Expires September 6, 2010 [Page 26] Internet-Draft SEAL March 2010 operation, the ETE will return an SCMP "Destination Unreachable - Communication with Destination Administratively Prohibited" message (see Section 4.5). The ITE then verifies that the packet-in-error corresponds to a packet that it sent recently, and attempts to synchronize with the ETE so that future communications are not blocked. When an ITE needs to synchronize with a new ETE (i.e., one for which it has no neighbor cache entry), it first chooses a random 32-bit value. The ITE then creates an initial 48-bit sequence number (i.e., an initial "SEAL_ID(ITE)") with the random 32-bit value as the most significant 32-bits and the value 0 as the least significant 16 bits. The ITE then creates a neighbor cache entry for this ETE and records SEAL_ID(ITE) in the neighbor cache entry. Next, the ITE creates an SCMP "Neighbor Solicitation (NS)" message and writes the value SEAL_ID(ITE) in the SCMP message SEAL header. The ITE then sets the (A, S) bits in the SEAL header to (0, 1), then sends the NS message to the ETE. When the ETE receives the NS message, it notices that the (A, S) bits in the SEAL header are set to (0, 1), and considers the message as a potential window synchronization request. The ETE then chooses a random 48-bit value to use as its initial sequence number (i.e., an initial "SEAL_ID(ETE)") which it stores in a minimal temporary fast path data structure that caches only the IP source address of the SCMP message, SEAL_ID(ITE) and SEAL_ID(ETE). (For efficiency and security purposes, the data structure should be indexed, e.g., by a secret hash of the IP source address and SEAL_ID(ITE)). The ETE then creates an SCMP "Neighbor Advertisement (NA)" message that includes a Nonce option (see: [RFC3971], Section 5.3) that encodes the value SEAL_ID(ITE). The ETE then writes the value SEAL_ID(ETE) into the SEAL_ID field of the SCMP message SEAL header, sets the (A, S) bits in the SEAL header to (1, 1), and sends the NA message back to the ITE. When the ITE receives the NA, it notices that the (A, S) bits in the SEAL header are set to (1, 1) and considers the message as a potential window synchronization acknowledgement. The ITE then verifies that the value encoded in the Nonce option matches the SEAL_ID(ITE) in the neighbor cache entry. If so, the ITE records the value SEAL_ID(ETE) in the neighbor cache entry. (If instead the ITE does not receive a timely NA response, it retransmits the initial NS message for a total of 3 tries before giving up the same as for ordinary IPv6 neighbor unreachability detection.) After the ITE receives a matching NA message, it then uses SEAL_ID(ITE) as the SEAL _ID of subsequent SEAL packets that it sends to this ETE and uses SEAL_ID(ETE) as the SEAL_ID to match against subsequent SEAL packets that it receives from this ETE. Templin Expires September 6, 2010 [Page 27] Internet-Draft SEAL March 2010 After the ITE receives the NA message, it begins sending either unsolicited NA messages or ordinary data packets back to the ETE using SEAL_ID(ITE) as the initial sequence number and with the S bit set to 0. When the ETE receives these packets, it first checks its neighbor cache to see if there is a matching neighbor cache entry. If there is a neighbor cache entry, and the SEAL_ID in SEAL header of he packet is within +/- 32768 of the SEAL_ID recorded in the neighbor cache entry, the ETE accepts the packet and records this new SEAL_ID in the neighbor cache entry. If there is no neighbor cache entry, the ETE instead checks the fast path cache to see if the packet is a match for an in-progress window synchronization event. If the packet matches (i.e., if there is a fast path cache entry with a SEAL_ID (ITE) that matches the high-order 32 bits of the SEAL_ID in the packet header), the ETE accepts the packet and also creates a new neighbor cache entry. If there is no matching fast path cache entry, the ETE instead discards the packet. By maintaining the fast path cache, the ETE is able to mitigate buffer exhaustion attacks that may be launched by off-path attackers [RFC4987]. The ETE will receive positive confirmation that the synchronization request came from an on-path ITE after it receives the "third leg" of this three-way handshake as described above. The ITE and ETE should maintain neighbor cache entries as long as traffic is flowing through the tunnel, but should delete the neighbor cache entries after a nominal idle time (e.g., 30 seconds). The ETE should also purge fast-path cache entries for which no window synchronization messages are received within a nominal idle time (e.g., 5 seconds). After synchronization is complete, when a TE receives a SEAL packet it checks in its neighbor cache to determine whether the SEAL_ID is within the current window, and discards any packets that are outside the window. Since packets may be lost or reordered, and since SEAL presents only a best effort (i.e., and not reliable) link model, the TE should accept any packet with a SEAL_ID that is within +/- 32768 of the most recently received SEAL_ID. For this reason, the ITE must record the record the SEAL_ID of the most recently-received SEAL packet so that the window of SEAL_IDs advances with the flow of packets. 5. Link Requirements Subnetwork designers are expected to follow the recommendations in Section 2 of [RFC3819] when configuring link MTUs. Templin Expires September 6, 2010 [Page 28] Internet-Draft SEAL March 2010 6. 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 implement their own end-to-end MTU assurance, e.g., using Packetization Layer Path MTU Discovery per [RFC4821]. 7. 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. 8. 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. The IANA is instructed to establish a "SEAL Protocol" registry to record SEAL Version values. This registry should be initialized to include the initial SEAL Version number, i.e., Version 0. 9. Security Considerations Unlike IPv4 fragmentation, overlapping fragment attacks are not possible due to the requirement that SEAL segments be non- overlapping. This condition is naturally enforced due to the fact that each consecutive SEAL segment begins at offset 0 with respect to the previous SEAL segment. 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 SCMP 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 SCMP messages. The SEAL header is sent in-the-clear (outside of any IPsec/ESP encapsulations) the same as for the outer IP and other outer headers. In this respect, the threat model is no different than for IPv6 extension headers. As for IPv6 extension headers, the SEAL header is Templin Expires September 6, 2010 [Page 29] Internet-Draft SEAL March 2010 protected only by L2 integrity checks and is not covered under any L3 integrity checks. SCMP messages carry the SEAL_ID of the packet-in-error. Therefore, when an ITE receives an SCMP message it can unambiguously associate it with the SEAL data packet that triggered the error. Security issues that apply to tunneling in general are discussed in [I-D.ietf-v6ops-tunnel-security-concerns]. 10. 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 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 D of this document. 11. SEAL Advantages over Classical Methods The SEAL approach offers a number of distinct advantages over the classical path MTU discovery methods [RFC1191] [RFC1981]: Templin Expires September 6, 2010 [Page 30] Internet-Draft SEAL March 2010 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 may require several iterations of dropping packets and returning PTB messages until an acceptable path MTU value is determined. Under normal circumstances, SEAL determines the correct packet sizing parameters in a single iteration. 3. 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. 4. Using SEAL, ETEs encapsulate SCMP error messages in outer and mid-layer headers such that packet-filtering network middleboxes will not filter them the same as for "raw" ICMP messages that may be generated by an attacker. 5. The SEAL approach ensures that the tunnel either delivers or deterministically drops packets according to their size, which is a required characteristic of any IP link. 6. Most importantly, all SEAL packets have an Identification field that is sufficiently long to be used for duplicate packet detection purposes and to associate ICMP error messages with actual packets sent without requiring per-packet state; hence, SEAL avoids certain denial-of-service attack vectors open to the classical methods. 12. 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, Remi Despres, Ralph Droms, Aurnaud Ebalard, Gorry Fairhurst, Dino Farinacci, Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark Townsley, Ole Troan, 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 Templin Expires September 6, 2010 [Page 31] Internet-Draft SEAL March 2010 Deering on the MTUDWG mailing list in 1989. 13. References 13.1. 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. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [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. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. 13.2. Informative References [FOLK] Shannon, C., Moore, D., and k. claffy, "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-06 (work in progress), January 2010. [I-D.ietf-tcpm-icmp-attacks] Gont, F., "ICMP attacks against TCP", draft-ietf-tcpm-icmp-attacks-11 (work in progress), February 2010. Templin Expires September 6, 2010 [Page 32] Internet-Draft SEAL March 2010 [I-D.ietf-v6ops-tunnel-security-concerns] Hoagland, J., Krishnan, S., and D. Thaler, "Security Concerns With IP Tunneling", draft-ietf-v6ops-tunnel-security-concerns-01 (work in progress), October 2008. [I-D.russert-rangers] Russert, S., Fleischman, E., and F. Templin, "RANGER Scenarios", draft-russert-rangers-01 (work in progress), September 2009. [I-D.templin-intarea-vet] Templin, F., "Virtual Enterprise Traversal (VET)", draft-templin-intarea-vet-09 (work in progress), February 2010. [I-D.templin-ranger] Templin, F., "Routing and Addressing in Next-Generation EnteRprises (RANGER)", draft-templin-ranger-09 (work in progress), October 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. [RFC1070] Hagens, R., Hall, N., and M. Rose, "Use of the Internet as a subnetwork for experimentation with the OSI network layer", RFC 1070, February 1989. [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. [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. Templin Expires September 6, 2010 [Page 33] Internet-Draft SEAL March 2010 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. [RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-line Database", RFC 3232, January 2002. [RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, August 2002. [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. [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005. [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. [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. [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, August 2007. [RFC5445] Watson, M., "Basic Forward Error Correction (FEC) Schemes", RFC 5445, March 2009. [TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring Interactions Between Transport Protocols and Middleboxes", Templin Expires September 6, 2010 [Page 34] Internet-Draft SEAL March 2010 October 2004. [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.". [WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and Debugging Path MTU Discovery Failures", October 2005. Appendix A. Reliability Although a SEAL tunnel may span an arbitrarily-large subnetwork expanse, the IP layer sees the tunnel as a simple link that supports the IP service model. Since SEAL supports segmentation at a layer below IP, SEAL therefore presents a case in which the link unit of loss (i.e., a SEAL segment) is smaller than the end-to-end retransmission unit (e.g., a TCP segment). Links with high bit error rates (BERs) (e.g., IEEE 802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] to increase packet delivery ratios, while links with much lower BERs typically omit such mechanisms. Since SEAL tunnels may traverse arbitrarily- long paths over links of various types that are already either performing or omitting ARQ as appropriate, it would therefore often be inefficient to also require the tunnel to perform ARQ. When the SEAL ITE has knowledge that the tunnel will traverse a subnetwork with non-negligible loss due to, e.g., interference, link errors, congestion, etc., it can solicit Segment Reports from the ETE periodically 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. SEAL may also use alternate reliability mechanisms such as Forward Error Correction (FEC). A simple FEC mechanism may merely entail gratuitous retransmissions of duplicate data, however more efficient alternatives are also possible. Basic FEC schemes are discussed in [RFC5445]. The use of ARQ and FEC mechanisms for improved reliability are for further study. Appendix B. Integrity Each link in the path over which a SEAL tunnel is configured is Templin Expires September 6, 2010 [Page 35] Internet-Draft SEAL March 2010 responsible for link layer integrity verification for packets that traverse the link. As such, when a multi-segment SEAL packet with N segments is reassembled, its segments will have been inspected by N independent link layer integrity check streams instead of a single stream that a single segment SEAL packet of the same size would have received. Intuitively, a reassembled packet subjected to N independent integrity check streams of shorter-length segments would seem to have integrity assurance that is no worse than a single- segment packet subjected to only a single integrity check steam, since the integrity check strength diminishes in inverse proportion with segment length. In any case, the link-layer integrity assurance for a multi-segment SEAL packet is no different than for a multi- fragment IPv6 packet. Fragmentation and reassembly schemes must also consider packet- splicing errors, e.g., when two segments from the same packet are concatenated incorrectly, when a segment from packet X is reassembled with segments from packet Y, etc. The primary sources of such errors include implementation bugs and wrapping IP ID fields. In terms of implementation bugs, the SEAL segmentation and reassembly algorithm is much simpler than IP fragmentation resulting in simplified implementations. In terms of wrapping ID fields, when IPv4 is used as the outer IP protocol, the 16-bit IP ID field can wrap with only 64K packets with the same (src, dst, protocol)-tuple alive in the system at a given time [RFC4963] increasing the likelihood of reassembly mis-associations. However, SEAL ensures that any outer IPv4 fragmentation and reassembly will be short-lived and tuned out as soon as the ITE receives a Reassembly Repot, and SEAL segmentation and reassembly uses a much longer ID field. Therefore, reassembly mis-associations of IP fragments nor of SEAL segments should be prohibitively rare. Appendix C. Transport Mode SEAL can also be used in "transport-mode", e.g., when the inner layer comprises 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 IPv4/SEAL/TCP. In this sense, the "subnetwork" becomes the entire end-to-end path between the TCP peers and may potentially span the entire Internet. Section specifies 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. Templin Expires September 6, 2010 [Page 36] Internet-Draft SEAL March 2010 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 Section 4. Appendix D. Historic Evolution of PMTUD 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) 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 Templin Expires September 6, 2010 [Page 37] Internet-Draft SEAL March 2010 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. Author's Address Fred L. Templin (editor) Boeing Research & Technology P.O. Box 3707 Seattle, WA 98124 USA Email: fltemplin@acm.org Templin Expires September 6, 2010 [Page 38]