Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Intended status: Standards Track February 12, 2010 Expires: August 16, 2010 The Subnetwork Encapsulation and Adaptation Layer (SEAL) draft-templin-intarea-seal-09.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 August 16, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Templin Expires August 16, 2010 [Page 1] Internet-Draft SEAL February 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 8 4. SEAL with Segmentation and Reassembly (SEAL-SR) Protocol Specification . . . . . . . . . . . . . . . . . . . . . . . . 10 4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 10 4.2. SEAL-SR Header Format (Mode 1) . . . . . . . . . . . . . . 12 4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 13 4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 13 4.3.2. Admitting Packets into the Tunnel Interface . . . . . 14 4.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 14 4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 16 4.3.5. Probing Strategy and Information Exchanges . . . . . . 16 4.3.6. Packet Identification . . . . . . . . . . . . . . . . 17 4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 17 4.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 17 4.3.9. Processing SEAL Control Message Protocol (SCMP) Messages . . . . . . . . . . . . . . . . . . . . . . . 18 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 4.4.5. The SEAL Control Message Protocol (SCMP) . . . . . . . 21 5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol Specification . . . . . . . . . . . . . . . . . . . . . . . . 24 5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 25 5.2. SEAL-FS Header Format (Version 0) . . . . . . . . . . . . 25 5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 25 5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 25 5.3.2. Admitting Packets into the Tunnel Interface . . . . . 26 5.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 26 5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 26 Templin Expires August 16, 2010 [Page 2] Internet-Draft SEAL February 2010 5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 27 5.3.6. Packet Identification . . . . . . . . . . . . . . . . 27 5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 27 5.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 27 5.3.9. Processing SEAL Control Message Protocol (SCMP) Messages . . . . . . . . . . . . . . . . . . . . . . . 27 5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 27 5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 27 5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 27 5.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 27 5.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 28 5.4.5. Sending SEAL Control Message Protocol (SCMP) Messages . . . . . . . . . . . . . . . . . . . . . . . 28 6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 28 7. End System Requirements . . . . . . . . . . . . . . . . . . . 28 8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 28 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 10. Security Considerations . . . . . . . . . . . . . . . . . . . 29 11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 29 12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 30 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 14.1. Normative References . . . . . . . . . . . . . . . . . . . 31 14.2. Informative References . . . . . . . . . . . . . . . . . . 32 Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 34 Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 35 Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 36 Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 36 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38 Templin Expires August 16, 2010 [Page 3] Internet-Draft SEAL February 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 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 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 August 16, 2010 [Page 4] Internet-Draft SEAL February 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. The issues with both IPv4 fragmentation and this "classical" method of path MTU discovery are exacerbated further when IP-in-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 ICMP error message into a suitable ICMP error message to return to the original source. Templin Expires August 16, 2010 [Page 5] Internet-Draft SEAL February 2010 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 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 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 IP 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 IP services. As for any link, tunnels that use SEAL must provide suitable IP 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 IP 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 purpose. SEAL encapsulation introduces an extended Identification field for packet identification and a mid-layer segmentation and reassembly Templin Expires August 16, 2010 [Page 6] Internet-Draft SEAL February 2010 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 (ITE) a virtual interface over which an encapsulating border node (host or router) sends encapsulated packets into the subnetwork. Egress Tunnel Endpoint (ETE) a virtual interface over which an encapsulating border node (host or router) receives encapsulated packets from the subnetwork. inner packet an unencapsulated Layer 3 protocol packet before any mid-layer or outer encapsulations are added. Note that not only IPv6 and IPv4, but also any other Layer 3 protocol type packet (e.g., OSI/CLNP [RFC1070]) can appear as an inner packet. 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. outer IP packet a packet resulting from adding an outer IP header to a mid-layer packet. Templin Expires August 16, 2010 [Page 7] Internet-Draft SEAL February 2010 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: PTB - an ICMPv6 "Packet Too Big" [RFC4443]or an ICMPv4 "Fragmentation Needed" [RFC0792] message. DF - the IPv4 header "Don't Fragment" flag [RFC0791] MHLEN - the length of the UDP and SEAL encapsulation headers OHLEN - the length of the outer IP encapsulation header. HLEN - the sum of MHLEN and OHLEN S_MRU - the SEAL Maximum Reassembly Unit S_MSS - the SEAL Maximum Segment Size SCMP - the SEAL Control Message Protocol SEAL_ID - a 48-bit Identification value, randomly initialized and monotonically incremented for each SEAL protocol packet SEAL_PROTO - an IPv4 protocol number used for SEAL SEAL_PORT - a TCP/UDP service port number used for SEAL SEAL-FS - SEAL with Fragmentation Sensing SEAL-SR - SEAL with Segmentation and Reassembly 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 Templin Expires August 16, 2010 [Page 8] Internet-Draft SEAL February 2010 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 an IP protocol 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. This document specifies two modes of operation for the SEAL protocol known as "SEAL with Fragmentation Sensing (SEAL-FS)" and "SEAL with Segmentation and Reassembly (SEAL-SR)". 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-SR 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-SR 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-SR 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-SR tunnels may be configured over paths that include only ordinary links, but they may also be configured over paths that include SEAL-FS tunnels or even other SEAL-SR 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 sub-IP segmentation over low-end links, i.e., especially over wireless transmission media such as IEEE 802.15.4, broadcast radio links in Templin Expires August 16, 2010 [Page 9] Internet-Draft SEAL February 2010 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-SR are anticipated, and will be identified as further experience is gained. 4. SEAL with Segmentation and Reassembly (SEAL-SR) Protocol Specification This section specifies the fully-functioned mode of SEAL known as "SEAL with Segmentation and Reassembly (SEAL-SR)"; a minimal mode known as "SEAL with Fragmentation Sensing (SEAL-FS)" is specified in Section 5. SEAL-SR is a superset of SEAL-FS, and differs only in its segmentation and reassembly requirements. SEAL-SR and SEAL-FS are distinguished simply by a mode value in the SEAL header. The following sections therefore specify SEAL-SR, but use the simple term "SEAL" since the same formats and mechanisms apply also to SEAL-FS. 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 segment each inner 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 IP encapsulation header to each segment. For example, a single-segment inner IPv6 packet encapsulated in UDP/SEAL headers and an outer IPv4 header would appear as shown in Figure 1: Templin Expires August 16, 2010 [Page 10] Internet-Draft SEAL February 2010 I +--------------------+ n ~ outer IPv4 header ~ n +--------------------+ +--------------------+ e ~ UDP header ~ ~ UDP header ~ r +--------------------+ +--------------------+ ~ SEAL header ~ ~ SEAL header ~ I +--------------------+ +--------------------+ P | | | | v --> ~ inner IPv6 ~ --> ~ inner IPv6 ~ 6 --> ~ Packet ~ --> ~ Packet ~ | | | | P +--------------------+ +--------------------+ a c Mid-layer packet Outer IPv4 packet k after UDP/SEAL encaps. after outer encaps. e t Figure 1: SEAL Encapsulation - Single Segment In a second example, an inner IPv4 packet of length 'M' requiring three SEAL segments with segment length 'N' would appear as three separate outer IPv4 packets as shown in Figure 2: +------------------+ +------------------+ +------------------+ ~ outer IPv4 hdr ~ ~ outer IPv4 hdr ~ ~ outer IPv4 hdr ~ +------------------+ +------------------+ +------------------+ ~ UDP header ~ ~ UDP header ~ ~ UDP header ~ +------------------+ +------------------+ +------------------+ ~ SEAL hdr (SEG=0) ~ ~ SEAL hdr (SEG=1) ~ ~ SEAL hdr (SEG=2) ~ +------------------+ +------------------+ +------------------+ | inner IPv4 | | inner IPv4 | | inner IPv4 | ~ Packet Segment ~ ~ Packet Segment ~ ~ Packet Segment ~ | Bytes 0 to (N-1) | | Bytes N to (2N-1)| | Bytes 2N to M | +------------------+ +------------------+ +------------------+ Figure 2: SEAL Encapsulation - Multiple Segments In all cases, after SEAL segmentation the SEAL header is inserted immediately before each segment of the inner packet. When UDP encapsulation is used, the SEAL header in each segment is immediately preceded by a UDP header. Next, each such resulting mid-layer packet is encapsulated in an outer IPvX header. This implies that SEAL is not used with tunnel-mode IPsec [RFC4301], since tunnel-mode IPsec would place the {AH, ESP} header immediately before the outer IPvX header and with no intervening UDP/SEAL headers. Instead, SEAL expects that inner packets that require IPsec coverage will use transport-mode IPsec. Templin Expires August 16, 2010 [Page 11] Internet-Draft SEAL February 2010 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. The following sections specify the SEAL header format and SEAL-related operations of the ITE and ETE, respectively. 4.2. SEAL-SR Header Format (Mode 1) The SEAL mode 1 header (i.e., the SEAL-SR 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MODE|A|F|M|RSV| NEXTHDR/SEG | SEAL_ID (bits 48 - 32) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL_ID (bits 31 - 0) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: SEAL Mode 1 Header Format where the header fields are defined as: MODE (3) a 3-bit value that encodes the SEAL protocol mode. This section describes Mode 1 of the SEAL protocol, i.e., the MODE field encodes the value 1. A (1) the "Acknowledgement Requested" 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. RSV (2) a 2-bit Reserved field. Set to 0 for the purpose of this specification. Templin Expires August 16, 2010 [Page 12] Internet-Draft SEAL February 2010 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. 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. 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 (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 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 Templin Expires August 16, 2010 [Page 13] Internet-Draft SEAL February 2010 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 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. 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. 4.3.2. Admitting Packets into the Tunnel Interface 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 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 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 minimum IP MTU (e.g., 576 bytes for IPv4, 1280 bytes for IPv6, etc.). Note that when the tunnel interface sets an indefinite MTU all packets are unconditionally admitted into the interface without fragmentation. 4.3.3. Segmentation 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. The soft state includes the following: o a Mid-layer Header Length (MHLEN); set to the length of the SEAL header plus the length of the UDP header if UDP encapsulation is used. o an Outer Header Length (OHLEN); set to the length of the outer IP header. Templin Expires August 16, 2010 [Page 14] Internet-Draft SEAL February 2010 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 smallest MTU among the underlying IP interfaces. The ITE decreases or increases S_MSS based on any SEAL Control Message Protocol (SCMP) Fragmentation Report messages received (see Section 4.3.9). o a SEAL Maximum Reassembly Unit (S_MRU); initialized to "infinity", i.e., the largest-possible inner IP packet size. The ITE decreases or increases S_MRU based on any Packet Too Big messages received (see Section 4.3.9). When (S_MRU>((S_MSS-HLEN)*256))), the ITE uses ((S_MSS-HLEN)*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 and S_MRU are maintained as 32-bit values specifically for the purpose of supporting IPv6 jumbograms. In that case, the length of the inner IPv6 packet is determined through examining the Jumbo Payload Option [RFC2675]. 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 (further) inner IP fragmentation is needed: o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1, and the packet is larger than S_MRU, the ITE drops the packet and sends a PTB message to the original source with an MTU value of S_MRU the same as described in Section 4.3.2; else, o if the inner packet is an IPv4 packet with DF=0, and the packet is larger than MIN(S_MRU, (S_MSS - HLEN)), the ITE uses inner IPv4 fragmentation to break the packet into fragments no larger than MIN(S_MRU, (S_MSS - HLEN)); else, o if the inner packet is a non-IP protocol packet, packet sizing considerations specific to the inner protocol are observed; 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 headers. If so, the ITE must include the length of the uncompressed inner headers when calculating the total length of the inner packet.) The ITE next performs SEAL segmentation on each inner packet/fragment if necessary. If the length of the inner packet/fragment plus the length of the mid-layer and outer headers (i.e., HLEN) is greater Templin Expires August 16, 2010 [Page 15] Internet-Draft SEAL February 2010 than S_MSS, the ITE must first perform SEAL segmentation. To do so, it breaks the inner packet into N segments (N <= 256) that are no larger than (S_MSS - HLEN) 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 inner 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. 4.3.4. Encapsulation Following SEAL segmentation, the ITE encapsulates each segment in a SEAL header formatted as specified in Section 4.3.2 with MODE=1, RSV=0. 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 inner 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 and mid-layer 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_PORT' in the outer destination service port field (e.g., when IP/UDP encapsulation is used). The ITE finally sets the A bit as specified in Section 4.3.5, sets the packet identification values as specified in Section 4.3.6 and sends the packets as specified in Section 4.3.7. 4.3.5. Probing Strategy and Information Exchanges All SEAL encapsulated packets sent by the ITE are considered implicit probes, and will elicit SCMP Fragmentation Report messages from the ETE (see: Section 4.4.5) with a new value for S_MSS if any IP Templin Expires August 16, 2010 [Page 16] Internet-Draft SEAL February 2010 fragmentation occurs in the path. Thereafter, the ITE can 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 also 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 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 'Next Header' field to a value of "No Next Header" (see Section 4.7 of [RFC2460]). The probe will elicit an "Segment Acknowledged" message from the ETE as an acknowledgement. Finally, the ITE MAY send "expendable" outer IP probe packets (see Section 4.3.7) 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 A bit in order to limit the resultant control message overhead. 4.3.6. Packet Identification The ITE maintains a randomly-initialized 48-bit 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. 4.3.7. 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 4.3.5), 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. 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 Templin Expires August 16, 2010 [Page 17] Internet-Draft SEAL February 2010 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. 4.3.9. Processing SEAL Control Message Protocol (SCMP) Messages In addition to any raw ICMP messages, the ITE may receive SEAL Control Message Protocol (SCMP) messages from the ETE as specified in Section 4.4.5. These SCMP messages are identical to the ICMPv6 messages specified in [RFC4443][RFC4191][RFC4861] and other IPv6 specifications. In order to detect off-path spoofing attempts, the ITE maintains a window of outstanding SEAL_IDs of packets that it has sent recently. (The window may be maintained as a sliding time-based window or in some other manner specific to the implementation.) For each SCMP message, the ITE first verifies that the SEAL_ID in the packet-in-error is within the current window of transmitted SEAL_IDs for this ETE and also verifies that the checksum in the message 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. In addition to the currently-defined ICMPv6 message types, SEAL adds two new ICMPv6 message types (see: Section 4.4.5) which the ITE processes as follows: 4.3.9.1. Segment Acknowledged When the ITE sends SEAL a packet with the A bit set to 1 (see: Section 4.3.5), it may receive a "Segment Acknowledged" SCMP message from the ETE as an explicit acknowledgement that the ETE received the packet. The ITE uses this message to determine whether the ETE is still reachable, whether packets of a certain size are being delivered without loss due to an MTU restriction, etc. 4.3.9.2. Fragmentation Report When the ITE sends a SEAL data packet with DF=0 in the outer IPv4 header, the packet may be fragmented in the network on the path to the ETE. In that case, the ETE will return an SCMP "Fragmentation Templin Expires August 16, 2010 [Page 18] Internet-Draft SEAL February 2010 Report" message. When the ITE receives a Fragmentation Report message, it records the value in the S_MRU field in its soft state for this ETE. The ITE then adjusts the S_MSS value in its soft state. If the S_MSS value in the Fragmentation Report message 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 message is less than the current soft state value and also less than 576, the ITE 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 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 data packets using a reduced S_MSS and receiving additional Fragmentation Report messages, but it will soon converge to a stable value. During this process, it is essential that the ITE reduce S_MSS based on the first Fragmentation Report message received, and refrain from further reducing S_MSS until Fragmentation Report messages pertaining to packets sent under the new S_MSS are received. 4.4. ETE Specification 4.4.1. Reassembly Buffer Requirements ETEs MUST be capable of performing IP-layer reassembly for SEAL protocol IP packets of at least 2KB in length, and MUST also be capable of performing SEAL-layer reassembly for inner packets of at least (2KB -HLEN). Hence, ETEs: o MUST configure an outer IP reassembly buffer size of at least 2KB o MUST configure a SEAL layer reassembly buffer size (i.e., SEAL Maximum Reassembly Unit (S_MRU)) of at least 2KB-HLEN o MUST be capable of discarding SEAL packets that are larger than S_MRU The ETE can also maintain S_MRU as a per-ITE value that can be reduced if the current value becomes to too large, e.g., based on excessive reassembly timeouts. If so, the ETE SHOULD ensure that the per-ITE S_MRU converges to a stable value as quickly as possible. Note that the ETE must retain the outer IP, SEAL and other outer headers during both IP-layer and SEAL-layer reassembly for the purpose of associating the fragments/segments of the same packet. Templin Expires August 16, 2010 [Page 19] Internet-Draft SEAL February 2010 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 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 an SCMP Fragmentation 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 no more than the size of the SEAL layer 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 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 packets. SEAL-layer reassembly requires the ETE to maintain a cache of recently received segments 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 4.4.5). 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 an SCMP "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 Templin Expires August 16, 2010 [Page 20] Internet-Draft SEAL February 2010 reassembly. After all segments are gathered, the ETE reassembles the mid-layer packet by discarding the outer and mid-layer headers and 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). 4.4.4. Decapsulation and Delivery to Upper Layers Following IP- and SEAL-layer reassembly, if the reassembled SEAL packet is larger than S_MRU, the ETE discards the packet and sends a "Packet Too Big" message with the S_MRU field set to the maximum- sized inner packet it is willing to accept from this ITE (see Section 4.4.5). Next, the ETE discards the encapsulating headers, and delivers the inner packet to the upper-layer protocol indicated in the SEAL Next Header field. The ETE instead silently discards the inner packet if it was a NULL packet (see Section 4.3.4). 4.4.5. The SEAL Control Message Protocol (SCMP) SEAL provides a SEAL Control Message Protocol (SCMP) that is identical in nearly all respects to the Internet Control Message Protocol for IPv6 (ICMPv6). In particular, SCMP supports the same ICMPv6 messages specified in [RFC4443][RFC4191][RFC4861] and other IPv6 specifications in exactly the same format as specified in those documents. An ETE sends SCMP messages in response to certain SEAL data and SCMP messages it receives from the ITE. An ITE sends SCMP messages whenever it needs to inform the ETE of new information, e.g., new routing information. The SCMP message body is formatted the same as for ICMPv6 [RFC4443] messages, i.e., as shown in Figure 4: Templin Expires August 16, 2010 [Page 21] Internet-Draft SEAL February 2010 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 + | | Figure 4: SEAL Control Message Format As for ICMPv4 and ICMPv6 messages, the {ITE, ETE} prepares the message beginning with 8-bit Type and Code fields followed by a 16- bit Checksum field followed by the message body. When the outer IP protocol is IPv4, the Checksum is calculated exactly the same as specified for ICMPv4 messages in [RFC0792], i.e., the checksum does not include a pseudo-header of the outer IPv4 header. When the outer IP protocol is IPv6, the Checksum is calculated exactly the same as specified in Section 2.3 of [RFC4443], i.e., the checksum includes a pseudo-header of the outer IPv6 header. The {ITE, ETE} prepares the SCMP message body exactly as specified for ICMPv6 messages in their respective specifications. However, when the {ITE, ETE} includes the leading portion of the SEAL packet that triggered the SCMP message, it MUST include as much of the SEAL packet as possible without the total size of the SCMP message exceeding 576 bytes. The {ITE, ETE} then encapsulates the SCMP message body in an outer IP header, UDP header (if necessary) and SEAL header the same as for the encapsulation of an ordinary inner IP packet (see Section 4.3). During encapsulation, the {ITE, ETE} sets the outer IP destination and source addresses of the SCMP message to the source and destination addresses (respectively) of the invoking SEAL packet. If the destination address in the SEAL packet was multicast, the {ITE, ETE} instead sets the outer IP source address of the SCMP message to an address assigned to the underlying IP interface. The {ITE, ETE} finally sets the NEXTHDR field in the SCMP message SEAL header to the value '58', i.e., the official IANA protocol number for the ICMPv6 protocol. In addition to the ICMPv6 message types already specified in existing Internet standards, SEAL adds the following additional SCMP message types: Templin Expires August 16, 2010 [Page 22] Internet-Draft SEAL February 2010 4.4.5.1. Segment Acknowledged When an ETE receives a SEAL segment following IP reassembly that has the 'A' bit set in the SEAL header, it prepares a "Segment Acknowledged" message with Type=TBD and Code=0. The message body 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=TBD | Code=0 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Segment Acknowledged Message Format The ETE writes the maximum-sized inner packet it is willing to receive from this ITE in a 32-bit S_MRU field. The ETE then writes as much of the invoking packet in the reassembly buffer as possible at the end of the message body, adds the encapsulating headers, and sends the message to the ITE. 4.4.5.2. Fragmentation Report When an ETE receives an IP first fragment of a SEAL segment that experienced outer IP fragmentation, it uses the IP first fragment to prepare a "Fragmentation Report" message with Type=TBD and Code=0. The message body is formatted as follows: Templin Expires August 16, 2010 [Page 23] Internet-Draft SEAL February 2010 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=TBD | Code=0 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MSS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: IP Fragmentation Experienced Message Format The ETE writes the maximum-sized inner packet it is willing to receive from this ITE in the S_MRU field then writes the length of the first IP fragment in the S_MSS field. The ETE then writes as much of the invoking packet as possible at the end of the message body, adds the encapsulating headers, and sends the message to the ITE. 5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol Specification This section specifies a minimal mode of SEAL known as "SEAL with Fragmentation Sensing (SEAL-FS)". SEAL-FS observes the same protocol specifications as for "SEAL with Segmentation and Reassembly (SEAL-SR)" (see Section 4) except that 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 there is operational assurance that "marginal" links are rare, e.g., when it is known that the vast majority of links configure MTUs that are appreciably larger than a constant value 'M' (e.g., 1500 bytes). SEAL-FS can also be used in instances when it is acceptable for the ITE to return PTB messages for packet sizes smaller than 'M', however SEAL-SR should be used instead if excessive PTB messages would result. With respect to Section 4, the SEAL-FS protocol corresponds to SEAL-SR as follows: Templin Expires August 16, 2010 [Page 24] Internet-Draft SEAL February 2010 5.1. Model of Operation SEAL-FS follows the same model of operation as for SEAL-SR as described in Section 4.1 except as noted in the following sections. 5.2. SEAL-FS Header Format (Version 0) The SEAL-FS 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MODE|A| RSV | NEXTHDR | SEAL_ID (bits 47 - 32) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SEAL_ID (bits 31 - 0) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: SEAL Version 1 Header Format where the header fields are defined as: MODE (3) a 3-bit value that encodes the SEAL protocol mode. This section describes Mode 0 of the SEAL protocol, i.e., the MODE field encodes the value '0'. A (1) the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes to receive an explicit acknowledgement from the ETE. RSV (4) a 4-bit Reserved field. Set to 0 for the purpose of this specification. NEXTHDR (8) an 8-bit field that encodes the next header Internet Protocol number the same as for the IPv4 protocol and IPv6 next header fields. SEAL_ID (48) a 48-bit Identification field. 5.3. ITE Specification 5.3.1. Tunnel Interface MTU SEAL-FS observes the SEAL-SR specification found in Section 4.3.1. Templin Expires August 16, 2010 [Page 25] Internet-Draft SEAL February 2010 5.3.2. Admitting Packets into the Tunnel Interface SEAL-FS observes the SEAL-SR specification found in Section 4.3.2. 5.3.3. Segmentation SEAL-FS observes the SEAL-SR specification found in Section 4.3.3, except that the inner fragmentation algorithm is adjusted to avoid all outer IP fragmentation and SEAL segmentation within the tunnel. For this purpose, the SEAL-FS ITE maintains S_MSS as a value that would be unlikely to incur fragmentation within the tunnel, e.g., 576 bytes for IPv4 and 1280 bytes for IPv6. The ITE may also set S_MSS to a larger value if there is assurance that the vast majority of links that may occur within the tunnel configure a larger MTU, and/or may use explicit probes (e.g., dummy packets with the 'A' bit set in the SEAL header) to dynamically discover a larger S_MSS value. The ITE uses S_MRU and S_MSS in the following algorithm to determine when to discard, fragment or admit the inner packets into the tunnel without inner fragmentation: 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 a 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, o if the inner packet is an IPv4 packet with DF=0, and the packet is larger than MIN(S_MRU, (S_MSS - HLEN)), the ITE uses inner IPv4 fragmentation to break the packet into fragments no larger than MIN(S_MRU - (S_MSS - HLEN)); else, o the ITE admits the packet without inner fragmentation. If the inner packet is an IPv6 packet or an IPv4 packet with DF=1, the ITE can instead employ a stateless strategy by simply encapsulating and sending the packet as specified in Section 4.3.4 through 4.3.7. The ITE then translates any SCMP "Fragmentation Needed" and "Packet Too big" messages into PTB messages to return to the original source (where the translation is based on the encapsulated portion of the invoking packet at the end of the SCMP message). In this method, the ITE need not retain per-ETE S_MRU and S_MSS state. 5.3.4. Encapsulation SEAL-FS observes the SEAL-SR specification found in Section 4.3.4, except that it uses the header format defined in this section and Templin Expires August 16, 2010 [Page 26] Internet-Draft SEAL February 2010 with the MODE field set to '0'. SEAL-FS uses the C, A and I bits the same as specified for SEAL-SR. 5.3.5. Probing Strategy SEAL-FS observes the SEAL-SR specification found in Section 4.3.5. 5.3.6. Packet Identification SEAL-FS observes the SEAL-SR soft state specifications found in Section 4.3.6. 5.3.7. Sending SEAL Protocol Packets SEAL-FS observes the SEAL-SR specification found in Section 4.3.7. 5.3.8. Processing Raw ICMP Messages SEAL-FS observes the SEAL-SR specification found in Section 4.3.8. 5.3.9. Processing SEAL Control Message Protocol (SCMP) Messages SEAL-FS observes the SEAL-SR 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, but still maintains a value for S_MRU as the largest packet size the ETE is willing to receive. 5.4.2. IP-Layer Reassembly SEAL-FS uses SEAL-protocol IP first fragments solely for the purpose of generating SCMP Fragmentation Report messages as specified in Section 4.4.2, but otherwise discards all SEAL-protocol packets that arrived as multiple IP fragments. 5.4.3. SEAL-Layer Reassembly SEAL-FS does not observe the SEAL-SR reassembly procedures in Section 4.4.3, since SEAL-FS headers contain no segmentation and reassembly information. As for SEAL-SR, SEAL-FS returns a Parameter Problem for SEAL packets with unrecognized values in the SEAL header. Templin Expires August 16, 2010 [Page 27] Internet-Draft SEAL February 2010 5.4.4. Decapsulation and Delivery to Upper Layers SEAL-FS observes the SEAL-SR specification found in Section 4.4.4. 5.4.5. Sending SEAL Control Message Protocol (SCMP) Messages SEAL-FS observes the SEAL-SR 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. 9. IANA Considerations The IANA is instructed to allocate an IP protocol number for 'SEAL_PROTO' in the 'protocol-numbers' registry. The IANA is instructed to allocate a Well-Known Port number for 'SEAL_PORT' in the 'port-numbers' registry. The IANA is instructed to allocate new ICMPv6 Type values in the "icmpv6-parameters" registry for the SCMP "Segment Acknowledged" and "Fragmentation Report" messages specified in Section 4.4.5 The IANA is instructed to establish a "SEAL Protocol" registry to record SEAL Mode values. This registry should be initialized to include the Mode values defined in Sections 4.2 and 5.2, and the Code and Type values defined in Section 4.4.5. Templin Expires August 16, 2010 [Page 28] Internet-Draft SEAL February 2010 10. 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 wrt 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 protected only by L2 integrity checks and is not covered under any L3 integrity checks. SEAL control messages carry the SEAL_ID of the packet-in-error. Therefore, when an ITE receives a SEAL control message it can unambiguously associate the message with the data packet that triggered the error. Security issues that apply to tunneling in general are discussed in [I-D.ietf-v6ops-tunnel-security-concerns]. 11. Related Work Section 3.1.7 of [RFC2764] provides a high-level sketch for supporting large tunnel MTUs via a tunnel-level segmentation and reassembly capability to avoid IP level fragmentation, which is in part the same approach used by tunnel-mode SEAL. SEAL could therefore be considered as a fully functioned manifestation of the method postulated by that informational reference. Section 3 of [RFC4459] describes inner and outer fragmentation at the tunnel endpoints as alternatives for accommodating the tunnel MTU; however, the SEAL protocol specifies a mid-layer segmentation and reassembly capability that is distinct from both inner and outer fragmentation. Section 4 of [RFC2460] specifies a method for inserting and processing extension headers between the base IPv6 header and transport layer protocol data. The SEAL header is inserted and Templin Expires August 16, 2010 [Page 29] Internet-Draft SEAL February 2010 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. 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 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 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. 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 match ICMP error messages with actual packets sent without requiring per-packet state; hence, SEAL avoids certain denial-of-service attack vectors open to the Templin Expires August 16, 2010 [Page 30] Internet-Draft SEAL February 2010 classical methods. 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, Eliot Lear, Darrel Lewis, Joe Macker, Matt Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Margaret Wasserman, Magnus Westerlund, Robin Whittle, James Woodyatt, and members of the Boeing Research & Technology NST DC&NT group. Path MTU determination through the report of fragmentation was first proposed by Charles Lynn on the TCP-IP mailing list in 1987. 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 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. Templin Expires August 16, 2010 [Page 31] Internet-Draft SEAL February 2010 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-06 (work in progress), January 2010. [I-D.ietf-tcpm-icmp-attacks] Gont, F., "ICMP attacks against TCP", draft-ietf-tcpm-icmp-attacks-10 (work in progress), January 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 Templin Expires August 16, 2010 [Page 32] Internet-Draft SEAL February 2010 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. [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. [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. [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. [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. Templin Expires August 16, 2010 [Page 33] Internet-Draft SEAL February 2010 [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. [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. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 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 Although a SEAL-SR 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-SR supports segmentation at a layer below IP, SEAL-SR 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-SR tunnels may traverse arbitrarily-long paths over links of various types that are already either performing or omitting ARQ as appropriate, it would therefore be inefficient to also require the tunnel to perform ARQ in the general sense. Templin Expires August 16, 2010 [Page 34] Internet-Draft SEAL February 2010 When the SEAL-SR 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 Fragmentation 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-SR 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 responsible for first-pass 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 Templin Expires August 16, 2010 [Page 35] Internet-Draft SEAL February 2010 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 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 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. 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 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]) Templin Expires August 16, 2010 [Page 36] Internet-Draft SEAL February 2010 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 The first four assertions, although perhaps valid at the time, have been overcome by historical events. The final assertion is addressed by the mechanisms specified in SEAL. Templin Expires August 16, 2010 [Page 37] Internet-Draft SEAL February 2010 Author's Address Fred L. Templin (editor) Boeing Research & Technology P.O. Box 3707 Seattle, WA 98124 USA Email: fltemplin@acm.org Templin Expires August 16, 2010 [Page 38]