Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Intended status: Standards Track January 08, 2010 Expires: July 12, 2010 The Subnetwork Encapsulation and Adaptation Layer (SEAL) draft-templin-intarea-seal-08.txt Abstract 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. 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 July 12, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Templin Expires July 12, 2010 [Page 1] Internet-Draft SEAL January 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) . . . . . . . . . . . . . . 13 4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 14 4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14 4.3.2. Admitting Packets into the Tunnel Interface . . . . . 15 4.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 16 4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 17 4.3.5. Probing Strategy and Information Exchanges . . . . . . 18 4.3.6. Packet Identification . . . . . . . . . . . . . . . . 18 4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 19 4.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 19 4.3.9. Processing SEAL Control Messages . . . . . . . . . . . 20 4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 22 4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 22 4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 22 4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 23 4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 24 4.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 24 5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol Specification . . . . . . . . . . . . . . . . . . . . . . . . 31 5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 31 5.2. SEAL-FS Header Format (Version 0) . . . . . . . . . . . . 32 5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 32 5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 33 5.3.2. Admitting Packets into the Tunnel Interface . . . . . 33 5.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 33 5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 34 5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 34 Templin Expires July 12, 2010 [Page 2] Internet-Draft SEAL January 2010 5.3.6. Packet Identification . . . . . . . . . . . . . . . . 34 5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 34 5.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 34 5.3.9. Processing SEAL Control Messages . . . . . . . . . . . 34 5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 34 5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 34 5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 34 5.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 34 5.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 35 5.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 35 6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 35 7. End System Requirements . . . . . . . . . . . . . . . . . . . 35 8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 35 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 10. Security Considerations . . . . . . . . . . . . . . . . . . . 36 11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 36 12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 37 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38 14.1. Normative References . . . . . . . . . . . . . . . . . . . 38 14.2. Informative References . . . . . . . . . . . . . . . . . . 39 Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 41 Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 42 Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 42 Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 43 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 44 Templin Expires July 12, 2010 [Page 3] Internet-Draft SEAL January 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 July 12, 2010 [Page 4] Internet-Draft SEAL January 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 July 12, 2010 [Page 5] Internet-Draft SEAL January 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 IP over subnetworks that connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border nodes. It provides a standalone specification designed to be tailored to specific associated IP in 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 July 12, 2010 [Page 6] Internet-Draft SEAL January 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 IP packet an unencapsulated IP packet before any mid-layer or outer encapsulations are added. mid-layer packet a packet resulting from adding mid-layer encapsulating headers and trailers to an inner IP packet. outer IP packet a packet resulting from adding outer encapsulating headers and trailers to a mid-layer packet. 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 Templin Expires July 12, 2010 [Page 7] Internet-Draft SEAL January 2010 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 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 outer headers and trailers HLEN - the sum of MHLEN and OHLEN S_MRU - the SEAL Maximum Reassembly Unit S_MSS - the SEAL Maximum Segment Size SEAL_ID - a 32-bit Identification value, randomly initialized and monotonically incremented for each SEAL protocol packet SEAL_PROTO - an IPv4 protocol number used for SEAL SEAL_PORT - a TCP/UDP service port number used for SEAL 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 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 may also be useful Templin Expires July 12, 2010 [Page 8] Internet-Draft SEAL January 2010 as an adjunct mechanism for other tunneling protocols such as LISP[I-D.ietf-lisp]. SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation (e.g., as IPv4/SEAL/IPv6), and appears as a subnetwork encapsulation as seen by the inner IP layer. SEAL can also be used as a sublayer for encapsulating inner IPvX packets within outer IPvY/UDP headers (e.g., as IPv4/UDP/SEAL/IPv6) such as for the Teredo domain of applicability [RFC4380]. When it appears immediately after the outer IPv4 header, the SEAL header is processed exactly as for IPv6 extension headers, i.e., it is not part of the outer IPv4 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 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. Templin Expires July 12, 2010 [Page 9] Internet-Draft SEAL January 2010 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 encapsulate each inner IP packet in mid-layer headers and trailers, segment the resulting mid-layer packet if necessary, then append a SEAL header and outer encapsulating headers and trailers 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 July 12, 2010 [Page 10] Internet-Draft SEAL January 2010 +--------------------+ ~ outer IPv4 header ~ +--------------------+ ~ other outer hdrs ~ I +--------------------+ n | SEAL Header | n +--------------------+ +--------------------+ e ~ mid-layer headers ~ ~ mid-layer headers ~ r +--------------------+ +--------------------+ | | | | I --> ~ inner IPv6 ~ --> ~ inner IPv6 ~ P --> ~ Packet ~ --> ~ Packet ~ v | | | | 6 +--------------------+ +--------------------+ ~ mid-layer trailers ~ ~ mid-layer trailers ~ P +--------------------+ +--------------------+ a ~ outer trailers ~ c Mid-layer packet +--------------------+ k after mid-layer encaps. e Outer IPv4 packet t after SEAL and outer encaps. Figure 1: SEAL Encapsulation - Single Segment In a second example, an inner IPv6 packet requiring three SEAL segments would appear as three separate outer IPv4 packets (each with its own SEAL header) and with the mid-layer headers only occurring in segment 0 and the mid-layer trailers only appearing in segment 2 as shown in Figure 2: Templin Expires July 12, 2010 [Page 11] Internet-Draft SEAL January 2010 +------------------+ +------------------+ ~ 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 IPv6 ~ | | | | ~ Packet ~ ~ inner IPv6 ~ ~ inner IPv6 ~ | (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 The SEAL header itself is inserted according to the specific tunneling protocol. Examples include the following: o For simple IP in IP encapsulations (e.g., [RFC2003][RFC2004][RFC2473][RFC4213]), the SEAL header is inserted between the inner IPvY and outer IPvX headers as: IPvX/SEAL/IPvY. o For tunnel-mode IPsec encapsulations (e.g., [RFC4301]), the SEAL header is inserted between the {AH,ESP} header and outer IP headers as: IPvX/SEAL/{AH,ESP}/IPvY. Here, the {AH, ESP} headers and trailers are seen as mid-layer encapsulations. o For IP encapsulations over transports such as UDP (e.g., [RFC4380]), the SEAL header is inserted between the outer transport layer header and the inner IPvY header, e.g., as IPvX/ UDP/SEAL/IPvY. 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 {ITEs; ETEs} use the SEAL_ID for SEAL segmentation/reassembly and protection against off-path attacks. For IPv4 as the outer layer of encapsulation, the SEAL_ID is formed from the concatenation of the 16-bit ID Extension field in the SEAL header as the most-significant bits, and with the 16-bit Templin Expires July 12, 2010 [Page 12] Internet-Draft SEAL January 2010 Identification value in the outer IPv4 header as the least- significant bits. For IPv6 as the outer layer, the SEAL_ID is written into the 32-bit Identification field of the IPv6 fragment header. For tunnels that traverse middleboxes that might rewrite the IP ID field (e.g., a Network Address Translator) the SEAL_ID is instead maintained only within the ID extension field in the SEAL header and/or within additional mid-layer header fields. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |MOD|C|A|I|F|M|R| NEXTHDR/SEG | ID Extension | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: SEAL Mode 1 Header Format where the header fields are defined as: MOD (2) a 2-bit value that encodes the SEAL protocol mode. This section describes Mode 1 of the SEAL protocol, i.e., the MOD field encodes the value 1. C (1) the "Control" bit. Set to 1 in SEAL control messages, and set to 0 in SEAL data messages. A (1) the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes to receive an explicit acknowledgement from the ETE. I (1) the "Information Request Solicit" bit. Set to 1 if the ITE wishes the ETE to initiate an Information Request. F (1) the "First Segment" bit. Set to 1 if this SEAL protocol packet contains the first segment (i.e., Segment #0) of a mid-layer packet. Templin Expires July 12, 2010 [Page 13] Internet-Draft SEAL January 2010 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.) ID Extension (16) a 16-bit Identification extension field. 4.3. ITE Specification 4.3.1. Tunnel Interface MTU The ITE configures a tunnel virtual interface over one or more underlying links that connect the border node to the subnetwork. The tunnel interface must present a fixed MTU to the inner IP layer (i.e., Layer 3) as the size for admission of inner IP packets into the tunnel. Since the tunnel interface may support a potentially large set of ETEs, however, care must be taken in setting a large- enough MTU for all ETEs while still upholding end system expectations. Due to the ubiquitous deployment of standard Ethernet and similar networking gear, the nominal Internet cell size has become 1500 bytes; this is the de facto size that end systems have come to expect will either be delivered by the network without loss due to an MTU restriction on the path or a suitable ICMP Packet Too Big (PTB) message returned. However, the network may not always deliver the necessary PTBs, leading to MTU-related black holes [RFC2923]. The ITE therefore requires a means for conveying 1500 byte (or smaller) packets to the ETE without loss due to MTU restrictions and without dependence on PTB messages from within the subnetwork. In common deployments, there may be many forwarding hops between the original source and the ITE. Within those hops, there may be additional encapsulations (IPSec, L2TP, other SEAL encapsulations, etc.) such that a 1500 byte packet sent by the original source might grow to a larger size by the time it reaches the ITE for encapsulation as an inner IP packet. Similarly, additional encapsulations on the path from the ITE to the ETE could cause the encapsulated packet to become larger still and trigger in-the-network Templin Expires July 12, 2010 [Page 14] Internet-Draft SEAL January 2010 fragmentation. In order to preserve the end system expectations, the ITE therefore requires a means for conveying these larger packets to the ETE even though there may be links within the subnetwork that configure a smaller MTU. The ITE should therefore set a tunnel virtual interface MTU of 1500 bytes plus extra room to accommodate any additional encapsulations that may occur on the path from the original source (i.e., even if the path to the ETE does not support an MTU of this size). The ITE can set larger MTU values still, but should select a value that is not so large as to cause excessive PTBs coming from within the tunnel interface (see Sections 4.3.3 and 4.3.8). The ITE can also set smaller MTU values; however, care must be taken not to set so small a value that original sources would experience an MTU underflow. In particular, IPv6 sources must see a minimum path MTU of 1280 bytes, and IPv4 sources should see a minimum path MTU of 576 bytes. The ITE can alternatively set an indefinite MTU on the tunnel virtual interface such that all inner IP packets are admitted into the interface without regard to size. For ITEs that host applications, this option must be carefully coordinated with protocol stack upper layers, since some upper layer protocols (e.g., TCP) derive their packet sizing parameters from the MTU of the underlying interface and as such may select too large an initial size. This is not a problem for upper layers that use conservative initial estimates, e.g., when mechanisms such as Packetization Layer Path MTU Discovery [RFC4821] are used. 4.3.2. Admitting Packets into the Tunnel Interface The inner IP layer consults the tunnel interface MTU when admitting a packet into the interface. For IPv4 packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet is larger than the tunnel interface MTU the inner IP layer uses IP fragmentation to break the packet into fragments no larger than the tunnel interface MTU. The ITE then admits each fragment into the tunnel as an independent packet. For all other packets, the ITE admits the packet if it is no larger than the tunnel interface MTU; otherwise, it drops the packet and sends 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 (i.e., 576 bytes for IPv4 and 1280 bytes for IPv6). Note that when the tunnel interface sets an indefinite MTU all packets are unconditionally admitted into the interface without fragmentation. Templin Expires July 12, 2010 [Page 15] Internet-Draft SEAL January 2010 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 any mid- layer encapsulation headers and trailers (e.g., AH, ESP, NULL, etc.). 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); initialized to a value that is no larger than the underlying IP interface MTU. The ITE decreases or increases S_MSS based on any SEAL Reassembly Report messages received (see Section 4.3.9). o a SEAL 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 SEAL Reassembly Report messages received (see Section 4.3.9). When (S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256) as the effective S_MRU value. Note that here as well as in the SEAL control message protocol (see Section 4.4.5), S_MSS and S_MRU are maintained as 32-bit values specifically for the purpose of supporting jumbograms. After an inner packet/fragment has been admitted into the tunnel interface the ITE uses the following algorithm to determine whether the packet can be accommodated and (if so) whether inner IP fragmentation is needed: o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1, and the packet is larger than (S_MRU - HLEN), the ITE drops the packet and sends a PTB message to the original source with an MTU value of (S_MRU - HLEN) the same as described in Section 4.3.2; else, o if the inner packet is 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, Templin Expires July 12, 2010 [Page 16] Internet-Draft SEAL January 2010 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 encapsulates each inner packet/fragment in the MHLEN bytes of mid-layer headers and trailers. 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. 4.3.4. Encapsulation Following SEAL segmentation, the ITE encapsulates each segment in a SEAL header formatted as specified in Section 4.3.2 with MOD=1, C=0 and R=0. For the first segment, the ITE sets F=1, then sets NEXTHDR to the Internet Protocol number of the encapsulated 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.) The ITE next encapsulates each segment in the requisite outer IP and other 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 Templin Expires July 12, 2010 [Page 17] Internet-Draft SEAL January 2010 (e.g., when IP/UDP encapsulation is used). The ITE finally sets the A and/or I bits 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. Note that when IPv6 is used as the outer IP encapsulation layer, the ITE must insert an IPv6 fragment header with an Identification value set as described in Section 4.3.6. 4.3.5. Probing Strategy and Information Exchanges All SEAL packets sent by the ITE are considered implicit probes, and will elicit "Reassembly Report - Fragmentation Experienced" messages from the ETE 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 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 a "Reassembly Report - Segment Acknowledged" message from the ETE as an acknowledgement (see Section 4.4.5.1). 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. The ITE may in some cases also have out-of-band information to convey to the ETE. In that case, the ITE can set the I bit (i.e., the Information Request Solicit bit) in order to prompt the ETE to send an Information Request message (see: Section 4.4.5.5). In all cases, the ITE MUST be conservative in its use of the A and I bits in order to limit the resultant control message overhead. 4.3.6. Packet Identification For the purpose of packet identification, the ITE maintains a SEAL_ID value as per-ETE soft state, e.g., in the neighbor cache. The ITE randomly initializes SEAL_ID when the soft state is created, and monotonically increments it for each successive SEAL protocol packet it sends to the ETE. For each outer IPv4 packet, the ITE writes the least-significant 16 bits of the SEAL_ID value into the Identification field in the outer Templin Expires July 12, 2010 [Page 18] Internet-Draft SEAL January 2010 IPv4 header, and writes the most-significant 16 bits in the ID Extension field in the SEAL header. For each outer IPv6 packet, the ITE writes the entire SEAL_ID value into the Identification field in the IPv6 fragment header. For tunnels specifically designed for the traversal of Network Address Translators (NATs) (e.g., Teredo [RFC4380]) and other middleboxes that might rewrite the outer IP ID field, the ITE instead writes the least significant bits of the SEAL_ID in the ID field of the SEAL header and writes a random value in the Identification field in the outer IP header. The ITE can additionally write the high- order bits of the SEAL_ID (or, alternatively, the entire SEAL_ID) in a mid-layer header field, but in any case both the ITE and ETE must be aware of the manner in which the SEAL_ID is inserted. If only the least-significant bits of the SEAL_ID are included, the ITE must limit the rate at which it sends packets to avoid wrapping the ID field. 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, but when a fragment header is included a translating router on the path may still fragment the packet. The ITE sends each outer packet that encapsulates a segment of the same mid-layer packet into the tunnel in canonical order, i.e., segment 0 first, followed by segment 1, etc., and finally segment N-1. 4.3.8. Processing Raw ICMP Messages The ITE may receive "raw" ICMP error messages [RFC0792][RFC4443] from either the ETE or routers within the subnetwork that comprise an outer IP header, followed by an ICMP header, followed by a portion of the SEAL packet that generated the error (also known as the "packet- in-error"). The ITE can use the SEAL_ID encoded in the packet-in- error as a nonce to confirm that the ICMP message came from either the ETE or an on-path router, and can use any additional information to determine whether to accept or discard the message. The ITE should specifically process raw ICMPv4 Protocol Unreachable Templin Expires July 12, 2010 [Page 19] Internet-Draft SEAL January 2010 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 Messages In addition to any raw ICMP messages, the ITE may receive SEAL control messages from the ETE which have the C bit set to 1 in the SEAL header and are formatted as specified in Section 4.4.5. For each control message, the ITE verifies the checksum and discards the message if the checksum is incorrect. The ITE can then verify that the SEAL_ID in the invoking packet is within the current window of transmitted SEAL_IDs for this ETE. If the SEAL_ID is outside of the window, the ITE discards the message; otherwise, it advances the window and processes the message. The ITE processes SEAL control messages as follows: 4.3.9.1. Reassembly Report (Type=0) When the ITE receives a Reassembly Report formatted as specified in Section 4.4.5.1, it processes the message according to the Code value as follows: 4.3.9.1.1. Segment Acknowledged (Code=0) If the value in the S_MRU field is non-zero, the ITE records the value in its soft state for this ETE. 4.3.9.1.2. Fragmentation Experienced (Code=1) If the value in the S_MRU field is non-zero, the ITE records the value 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 Reassembly Report is greater than 576 (i.e., the nominal minimum MTU for IPv4 links), the ITE records this new value in its soft state. If the S_MSS value in the report is less than the current soft state value and also less than 576, the 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 packets using a reduced S_MSS and receiving additional Reassembly Report messages, but it will soon converge to a stable value. During this process, it is essential that the ITE reduce S_MSS based on the first Reassembly Report message received, and Templin Expires July 12, 2010 [Page 20] Internet-Draft SEAL January 2010 refrain from further reducing S_MSS until SEAL Reassembly Report messages pertaining to packets sent under the new S_MSS are received. 4.3.9.1.3. Packet Too Big (Code=2) If the value in the S_MRU field is non-zero, the ITE records the value in its soft state for this ETE. The ITE can then translate the message into a PTB message to return to the original source, where the translation is based on the encapsulated portion of the invoking packet at the end of the reassembly report message. 4.3.9.1.4. Time Exceeded (Code=3) If the value in the S_MRU field is non-zero, the ITE records the value in its soft state for this ETE. The ITE MAY then log the event for network management purposes. When excessive Time Exceeded messages are received from this ETE, the ITE should also reduce its S_MRU and/or S_MSS estimates. Unlike other SEAL control messages, the ETE does not necessarily generate the Time Exceeded message in synchronous response to the receipt of an invoking SEAL packet. The ITE must therefore consider as suspect any Time Exceeded messages that cannot be correlated with a recently sent SEAL packet. 4.3.9.2. Parameter Problem (Type=1) When the ITE receives a Parameter Problem message formatted as specified in Section 4.4.5.2, it examines the encapsulated SEAL header in the message to determine whether the header was corrupted or whether the header specified features that the ETE did not recognize. The ITE MAY log the event for network management purposes, and SHOULD adjust its SEAL header parameters in subsequent SEAL packets. 4.3.9.3. Information Request Solicit (Type=2) When the ITE receives an Information Request Solicit message formatted as specified in Section 4.4.5.3 and with a SEAL_ID that corresponds to a SEAL packet that it sent earlier, it sends an Information Request as specified in Section 4.4.5.4. 4.3.9.4. Information Request (Type=3) When the ITE receives an Information Request message formatted as specified in Section 4.4.5.4 and with a SEAL_ID that corresponds to a SEAL packet that it sent earlier with I=1, it sends an Information Reply as specified in Section 4.4.5.5. Templin Expires July 12, 2010 [Page 21] Internet-Draft SEAL January 2010 4.3.9.5. Information Reply (Type=4) When the ITE receives an Information Reply message formatted as specified in Section 4.4.5.5 and with a SEAL_ID that corresponds to a SEAL packet that it sent earlier, it processes any out-of-band data included in the reply. 4.4. ETE Specification 4.4.1. Reassembly Buffer Requirements ETEs must be capable of performing IP-layer reassembly for SEAL protocol IP packets up to 2KB in length, and must also be capable of performing SEAL-layer reassembly for mid-layer packets up to (2KB - OHLEN). Hence, ETEs: o MUST configure a reassembly buffer size (i.e., a SEAL Maximum Reassembly Unit (S_MRU)) of at least 2KB o MAY configure a larger S_MRU o MUST be capable of discarding SEAL packets that are too large to reassemble 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 and trailers during both IP-layer and SEAL-layer reassembly for the purpose of associating the fragments/segments of the same packet. 4.4.2. IP-Layer Reassembly ETEs perform standard IP-layer reassembly for SEAL protocol IP fragments, and should maintain a conservative reassembly cache high- and low-water mark. When the size of the reassembly cache exceeds this high-water mark, the ETE should actively discard incomplete reassemblies (e.g., using an Active Queue Management (AQM) strategy) until the size falls below the low-water mark. The ETE should also actively discard any pending reassemblies that clearly have no opportunity for completion, e.g., when a considerable number of new fragments have been received before a fragment that completes a pending reassembly has arrived. When the ETE processes the IP first-fragment (i.e., one with MF=1 and Templin Expires July 12, 2010 [Page 22] Internet-Draft SEAL January 2010 Offset=0 in the IP header) of a fragmented SEAL packet, it sends a "Reassembly Report - Fragmentation Experienced" 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 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 a "Reassembly Report - Time Exceeded" message to the ITE (see Section 4.4.5). As for IP-layer reassembly, the ETE should also maintain a conservative reassembly cache high- and low- water mark and should actively discard any pending reassemblies that clearly have no opportunity for completion, e.g., when a considerable number of new SEAL packets have been received before a packet that completes a pending reassembly has arrived. When the ETE receives a SEAL packet with an incorrect value in the SEAL header, it discards the packet and returns a "Parameter Problem" message (see Section 4.4.5). If the ETE receives a SEAL packet for which a segment with the same (Source, Destination, SEAL_ID)-tuple is already in the queue, it must determine whether to accept the new segment and release the old, or drop the new segment. If accepting the new segment would cause an inconsistency with other segments already in the queue (e.g., differing segment lengths), the ETE drops the segment that is least likely to complete the reassembly. After all segments are gathered, the ETE reassembles the mid-layer packet by discarding the outer 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 mid- layer packet segments with (F=1, M=1, SEAL_ID=j) in the first SEAL header, followed by (F=0, M=1, SEG=1, SEAL_ID=(j+1)) in the next SEAL header, followed by (F=0, M=1, SEG=2, SEAL_ID=(j+2)), etc., up to (F=0, M=0, SEG=(N-1), SEAL_ID=(j + N-1)) in the final SEAL header. (Note that modulo arithmetic based on the length of the SEAL_ID field is used). Templin Expires July 12, 2010 [Page 23] Internet-Draft SEAL January 2010 4.4.4. Decapsulation and Delivery to Upper Layers Following IP- and SEAL-layer reassembly, if the reassembled mid-layer packet is larger than (S_MRU-OHLEN), the ETE discards the packet and sends a "Reassembly Report - Packet Too Big" message with the S_MRU field set to the maximum-sized packet it is willing to accept from this ITE (see Section 4.4.5). Next, the ETE discards the outer and mid-layer headers and trailers, and delivers the inner packet to the upper-layer protocol indicated in the SEAL Next Header field. The ETE instead silently discards the inner packet if it was a NULL packet (see Section 4.3.4). 4.4.5. Sending SEAL Control Messages An ETE sends SEAL control messages in response to certain SEAL data packets and control messages received from the ITE. An ITE can also send SEAL control messages during an information exchange with an ETE. SEAL control messages are formatted much the same as for ICMPv4 [RFC0792] and ICMPv6 [RFC4443] messages, and are used for very similar purposes. The 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Outer, SEAL and Mid-Layer Headers ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Control Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Mid-Layer and Outer Trailers | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: SEAL Control Message Format As for ICPMv4 and ICMPv6 messages, the {ITE, ETE} prepares the message body beginning with 8-bit Type and Code fields followed by a 16-bit Checksum field. The Checksum field is followed by a variable- length control data field which is followed by as much of the Templin Expires July 12, 2010 [Page 24] Internet-Draft SEAL January 2010 invoking packet as possible without the entire message (including encapsulating headers and trailers) exceeding 576 bytes. As for ICMPv4 messages, the Checksum is the 16-bit ones's complement of the one's complement sum of the SEAL control message body starting with the Type field and ending with the final byte of the encapsulated invoking packet. For computing the checksum , the Checksum field is set to zero. After the {ITE, ETE} prepares the control message body, it encapsulates the body in outer, SEAL and mid-layer headers and trailers 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 message to the source and destination addresses (respectively) of the invoking packet. If the destination address in the packet was multicast, the {ITE, ETE} instead sets the outer IP source address to an address assigned to the underlying IP interface. The following SEAL control message types are currently defined; other values for Type will be recorded in the IANA registry for SEAL: 4.4.5.1. Reassembly Report (Type=0) An ETE generates a Reassembly Report to inform the ITE of various conditions encountered during outer IP and SEAL-layer reassembly. The following values for Code are currently defined (other values for Code will be recorded in the IANA registry for SEAL): o Code = 0 : Segment Acknowledged o Code = 1 : Fragmentation Experienced o Code = 2 : Packet Too Big o Code = 3 : Time Exceeded The ETE prepares the Reassembly Report according to the Code. In each case, the Reassembly Report includes an S_MRU value that denotes the maximum-sized packet the ETE is willing to receive from the ITE (normally set to the ETE's reassembly buffer size - see Section 4.4.1). The ETE MAY advertise different S_MRU values to different ITEs, but it SHOULD maintain a persistent value for each ITE that changes only very rarely (if at all). Reassembly Report formats for each Code are specified in the following sections: Templin Expires July 12, 2010 [Page 25] Internet-Draft SEAL January 2010 4.4.5.1.1. Segment Acknowledged (Code=0) When an ETE receives a SEAL segment following IP reassembly that has the 'A' bit set in the SEAL header, it prepares a "Reassembly Report - Segment Acknowledged" message with Type=0 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=0 | 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 packet it is willing to receive from this ITE in a 32-bit S_MRU field ( a value of zero in this field means that S_MRU is not specified in this message). 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 trailers, and sends the message to the ITE. 4.4.5.1.2. Fragmentation Experienced (Code=1) When an ETE receives an IP first-fragment of a SEAL packet that experienced outer IP fragmentation, it uses the IP first-fragment to prepare a "Reassembly Report - Fragmentation Experienced" message with Type=0 and Code=1. The message body is formatted as follows: Templin Expires July 12, 2010 [Page 26] Internet-Draft SEAL January 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=0 | Code=1 | 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 packet it is willing to receive from this ITE in the S_MRU field (a value of zero in this field means that S_MRU is not specified in this message) 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 trailers, and sends the message to the ITE. 4.4.5.1.3. Packet Too Big (Code=2) An ETE generates a "Reassembly Report - Packet Too Big" message when it discards a (reassembled) SEAL data packet that is larger than it is willing to receive from this ITE. The ETE sets Type=0 and Code=2. 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=0 | Code=2 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Packet Too Big Message Format The ETE writes the maximum-sized packet it is willing to receive from this ITE in the S_MRU field (a value of zero in this field means that S_MRU is not specified in this message). The ETE then writes as much Templin Expires July 12, 2010 [Page 27] Internet-Draft SEAL January 2010 of the invoking packet as possible at the end of the message body, adds the encapsulating headers and trailers, and sends the message to the ITE. 4.4.5.1.4. Time Exceeded (Code=3) An ETE generates a "Reassembly Report - Time Exceeded" message when it discards an incomplete SEAL reassembly buffer due to a reassembly timeout. The ETE sets Type=0 and Code=3. 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=0 | Code=3 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S_MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time (in milliseconds) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of most recent packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Time Exceeded Message Format The ETE writes the maximum-sized packet it is willing to receive from this ITE in the S_MRU field (a value of zero in this field means that S_MRU is not specified in this message) then writes the Time (in milliseconds) from when the first SEAL segment arrived until the SEAL reassembly timeout expired in the Time field. The ETE finally writes as much of the most recently received packet in the reassembly buffer as possible at the end of the message body, adds the encapsulating headers and trailers, and sends the message to the ITE. 4.4.5.2. Parameter Problem (Type=1) An ETE generates a "Parameter Problem" message when it receives a SEAL packet with an invalid value in the SEAL header. The ETE sets Type=1 and Code=0; other values for Code will be recorded in the IANA registry for SEAL. The message body is formatted as follows: Templin Expires July 12, 2010 [Page 28] Internet-Draft SEAL January 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=1 | Code=0 | Reserved=0 | Pointer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pointer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Parameter Problem Message Format The ETE writes the bit number of the SEAL header field that triggered the error in a 32-bit Pointer field. (For example, when the parameter problem is specific to the NEXTHDR/SEG field the ETE writes the value 8 in this field.) The ETE finally writes as much of the invoking packet as possible at the end of the message body, adds the encapsulating headers and trailers, and sends the message to the ITE. 4.4.5.3. Information Request Solicit (Type=2) An ETE generates an "Information Request Solicit" message when it receives a SEAL data packet with stale information and wishes to inform the ITE of new information. The ETE sets Type=2 and sets Code to a value specific to the associated tunneling protocol (for example, the tunneling protocol can use the Information Request Solicit message to initiate mapping updates). When the ETE sets Code=0 the Control Data field is NULL; other values for Code will be recorded in the IANA registry for SEAL, and other Control Data field formats will be specified by the associated tunneling protocol. The Information Request Solicit message is formatted as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=2 | Code=0 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Information Request Solicit Message Format The ETE writes as much of the invoking packet as possible at the end Templin Expires July 12, 2010 [Page 29] Internet-Draft SEAL January 2010 of the message body, adds the encapsulating headers and trailers, then sends the message to the ITE and listens for a corresponding Information Request (see Section 4.4.5.4). 4.4.5.4. Information Request (Type=3) An ITE generates an "Information Request" message when it receives an Information Request Solicit control message from an ETE. An ETE generates an Information Request message when it receives a SEAL data packet with I=1 in the SEAL header from an ITE. When an {ITE, ETE} generates an Information Request message, it sets Type=3 and sets Code to a value specific to the associated tunneling protocol (for example, the tunneling protocol can use the Information Request message to request mapping updates). When the {ITE, ETE} sets Code=0 the Control Data field is NULL; other values for Code will be recorded in the IANA registry for SEAL, and other Control Data field formats will be specified by the associated tunneling protocol. The Information Request message is formatted as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=3 | Code=0 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Information Request Message Format The {ITE, ETE} writes as much of the invoking packet as possible at the end of the message body, adds the encapsulating headers and trailers, then sends the Information Request message. The {ITE, ETE} MAY cache the SEAL_ID in the control message SEAL header so that it can be matched against a corresponding Information Reply (see Section 4.4.5.5). 4.4.5.5. Information Reply (Type=4) When an {ITE, ETE} receives an Information Request message, it responds by sending an "Information Reply" message. The {ITE, ETE} sets Type=4 and sets Code to a value specific to the associated tunneling protocol (for example, the tunneling protocol can use the Information Reply message to encode mapping updates). When Code=0 Templin Expires July 12, 2010 [Page 30] Internet-Draft SEAL January 2010 the Control Data field is NULL; other values for Code will be recorded in the IANA registry for SEAL, and other Control Data field formats will be specified by the associated tunneling protocol. The information reply message is formatted as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=4 | Code=0 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of invoking packet | ~ as possible without the message ~ | exceeding 576 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: Information Reply Message Format The {ITE, ETE} writes as much of the invoking packet as possible at the end of the message body, adds the encapsulating headers and trailers, then sends the Information Reply message. 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: 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. Templin Expires July 12, 2010 [Page 31] Internet-Draft SEAL January 2010 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |MOD|C|A|I| RSV | NEXTHDR | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13: SEAL Version 1 Header Format where the header fields are defined as: MOD (2) a 2-bit value that encodes the SEAL protocol mode. This section describes Mode 0 of the SEAL protocol, i.e., the MOD field encodes the value '0'. C (1) the "Control" bit. Set to 1 in SEAL control messages, and set to 0 in SEAL data messages. A (1) the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes to receive an explicit acknowledgement from the ETE. I (1) the "Information Request Solicit" bit. Set to 1 if the ITE wishes the ETE to initiate an Information Request. RSV (3) a 3-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. ID Extension (16) a 16-bit Identification extension field. 5.3. ITE Specification Templin Expires July 12, 2010 [Page 32] Internet-Draft SEAL January 2010 5.3.1. Tunnel Interface MTU SEAL-FS observes the SEAL-SR specification found in Section 4.3.1. 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. The ITE then 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 "Reassembly Report - Fragmentation Needed" and "Reassembly Report - 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 reassembly report message). In this method, the ITE need not retain per-ETE S_MRU and S_MSS state. Templin Expires July 12, 2010 [Page 33] Internet-Draft SEAL January 2010 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 with the MOD 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 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 SEAL Reassembly Reports 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. Templin Expires July 12, 2010 [Page 34] Internet-Draft SEAL January 2010 As for SEAL-SR, SEAL-FS returns a Parameter Problem for SEAL packets with unrecognized values in the SEAL header. 5.4.4. Decapsulation and Delivery to Upper Layers SEAL-FS observes the SEAL-SR specification found in Section 4.4.4. 5.4.5. Sending SEAL Control 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 establish a "SEAL Protocol" registry to record SEAL Mode values and SEAL control message Code and Type 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 July 12, 2010 [Page 35] Internet-Draft SEAL January 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 Reassembly Report messages returned to a victim ITE. The SEAL_ID in the encapsulated segment of the spoofed IP first- fragment provides mitigation for the ITE to detect and discard spurious Reassembly Reports. The SEAL header is sent in-the-clear (outside of any IPsec/ESP encapsulations) the same as for the outer IP and other outer headers and trailers. 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 Templin Expires July 12, 2010 [Page 36] Internet-Draft SEAL January 2010 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. 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. 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 classical methods. In summary, the SEAL approach ensures that packets of various sizes Templin Expires July 12, 2010 [Page 37] Internet-Draft SEAL January 2010 are either delivered or deterministically dropped. When end systems use their own end-to-end MTU determination mechanisms [RFC4821], the SEAL advantages are further enhanced. 13. Acknowledgments The following individuals are acknowledged for helpful comments and suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Aurnaud Ebalard, Gorry Fairhurst, Dino Farinacci, Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, 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 July 12, 2010 [Page 38] Internet-Draft SEAL January 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-05 (work in progress), September 2009. [I-D.ietf-tcpm-icmp-attacks] Gont, F., "ICMP attacks against TCP", draft-ietf-tcpm-icmp-attacks-07 (work in progress), December 2009. [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-05 (work in progress), December 2009. [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. [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. Templin Expires July 12, 2010 [Page 39] Internet-Draft SEAL January 2010 [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, October 1996. [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998. [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", RFC 2675, August 1999. [RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A. Malis, "A Framework for IP Based Virtual Private Networks", RFC 2764, February 2000. [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. [RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, August 2002. [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, January 2004. [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, July 2004. [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, February 2006. [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the- Network Tunneling", RFC 4459, April 2006. [RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, Templin Expires July 12, 2010 [Page 40] Internet-Draft SEAL January 2010 ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006. [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [RFC5445] Watson, M., "Basic Forward Error Correction (FEC) Schemes", RFC 5445, March 2009. [TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List, http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May 1987 - May 1990.". Appendix A. Reliability 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. 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 Reassembly 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]. Templin Expires July 12, 2010 [Page 41] Internet-Draft SEAL January 2010 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 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. Templin Expires July 12, 2010 [Page 42] Internet-Draft SEAL January 2010 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]) 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 Templin Expires July 12, 2010 [Page 43] Internet-Draft SEAL January 2010 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. Author's Address Fred L. Templin (editor) Boeing Research & Technology P.O. Box 3707 Seattle, WA 98124 USA Email: fltemplin@acm.org Templin Expires July 12, 2010 [Page 44]