Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Intended status: Informational December 19, 2011 Expires: June 21, 2012 The Subnetwork Encapsulation and Adaptation Layer (SEAL) draft-templin-intarea-seal-42.txt Abstract For the purpose of this document, a subnetwork is defined as a virtual topology configured over a connected IP network routing region and bounded by encapsulating border nodes. These virtual topologies are manifested by tunnels that may span multiple IP and/or sub-IP layer forwarding hops, and can introduce failure modes due to packet duplication, packet reordering, source address spoofing and traversal of links with diverse Maximum Transmission Units (MTUs). This document specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that addresses these issues. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on June 21, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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 Templin Expires June 21, 2012 [Page 1] Internet-Draft SEAL December 2011 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 Simplified BSD License. Templin Expires June 21, 2012 [Page 2] Internet-Draft SEAL December 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9 4. SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 10 4.1. VET Interface Model . . . . . . . . . . . . . . . . . . . 10 4.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 10 4.3. SEAL Header and Trailer Format . . . . . . . . . . . . . . 12 4.4. ITE Specification . . . . . . . . . . . . . . . . . . . . 14 4.4.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14 4.4.2. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 15 4.4.3. Pre-Encapsulation . . . . . . . . . . . . . . . . . . 16 4.4.4. SEAL Encapsulation . . . . . . . . . . . . . . . . . . 17 4.4.5. Outer Encapsulation . . . . . . . . . . . . . . . . . 18 4.4.6. Path Probing and ETE Reachability Verification . . . . 19 4.4.7. Processing ICMP Messages . . . . . . . . . . . . . . . 19 4.4.8. IPv4 Middlebox Reassembly Testing . . . . . . . . . . 21 4.4.9. Stateful MTU Determination . . . . . . . . . . . . . . 22 4.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 23 4.5. ETE Specification . . . . . . . . . . . . . . . . . . . . 23 4.5.1. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 23 4.5.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 23 4.5.3. Decapsulation and Re-Encapsulation . . . . . . . . . . 24 4.6. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 25 4.6.1. Generating SCMP Error Messages . . . . . . . . . . . . 26 4.6.2. Processing SCMP Error Messages . . . . . . . . . . . . 28 5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 29 6. End System Requirements . . . . . . . . . . . . . . . . . . . 30 7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 30 8. Nested Encapsulation Considerations . . . . . . . . . . . . . 30 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 10. Security Considerations . . . . . . . . . . . . . . . . . . . 31 11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 31 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33 13.1. Normative References . . . . . . . . . . . . . . . . . . . 33 13.2. Informative References . . . . . . . . . . . . . . . . . . 33 Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 37 Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 37 Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 38 Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 38 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39 Templin Expires June 21, 2012 [Page 3] Internet-Draft SEAL December 2011 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 (also known as "tunneling") has long been considered as the means for creating such virtual topologies. However, the encapsulation headers often include insufficiently provisioned per-packet identification values. This can present issues for duplicate packet detection and detection of packet reordering within the subnetwork. IP encapsulation also allows an attacker to produce encapsulated packets with spoofed source addresses even if the source address in the encapsulating header cannot be spoofed. A denial-of-service vector that is not possible in non-tunneled subnetworks is therefore presented. Additionally, the insertion of an outer IP header reduces the effective path MTU visible to the inner network layer. When IPv6 is used as the encapsulation protocol, original sources will be informed of the MTU limitation through IPv6 path MTU discovery [RFC1981]. 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 IPv4 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, IP is ubiquitously deployed as the Layer 3 protocol. The primary functions of IP are to provide for routing, addressing, and a fragmentation and reassembly capability used to accommodate links with diverse MTUs. While it is well known that the IP address space is rapidly becoming depleted, there is a lesser-known but growing consensus that other IP protocol limitations have already or may soon Templin Expires June 21, 2012 [Page 4] Internet-Draft SEAL December 2011 become problematic. First, the Internet historically provided no means for discerning whether the source addresses of IP packets are authentic. This shortcoming is being addressed more and more through the deployment of site border router ingress filters [RFC2827], however the use of encapsulation provides a vector for an attacker to circumvent filtering for the encapsulated packet even if filtering is correctly applied to the encapsulation header. Secondly, the IP header does not include a well-behaved identification value unless the source has included a fragment header for IPv6 or unless the source permits fragmentation for IPv4. These limitations preclude an efficient means for routers to detect duplicate packets and packets that have been re-ordered within the subnetwork. For IPv4 encapsulation, when fragmentation is permitted the header includes a 16-bit Identification field, meaning that at most 2^16 unique packets with the same (source, destination, protocol)-tuple can be active in the Internet at the same time [I-D.ietf-intarea-ipv4-id-update]. (When middleboxes such as Network Address Translators (NATs) re-write the Identification field to random values, the number of unique packets is even further reduced.) Due to the escalating deployment of high-speed links, however, these numbers have 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 (see above). In particular, IPv4 fragmentation raises issues ranging from minor annoyances (e.g., in-the-network router fragmentation [RFC1981]) 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 have been known 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 trivial since there is no way to Templin Expires June 21, 2012 [Page 5] Internet-Draft SEAL December 2011 authenticate the source of the messages [RFC5927]. Furthermore, when a source node that requires ICMP error message feedback when a packet is dropped due to an MTU restriction does not receive the messages, a path MTU-related black hole occurs. This means that the source will continue to send packets that are too large and never receive an indication from the network that they are being discarded. This behavior has been confirmed through documented studies showing clear evidence of path MTU discovery failures in the Internet today [TBIT][WAND][SIGCOMM]. The issues with both IPv4 fragmentation and this "classical" method of IPv4 path MTU discovery are exacerbated further when IP tunneling is used [RFC4459]. For example, an ingress tunnel endpoint (ITE) may be required to forward encapsulated packets into the subnetwork on behalf of hundreds, thousands, or even more original sources within the end site that it serves. 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 must rely on ICMP error messages coming from the subnetwork that may be suspect, subject to loss due to filtering middleboxes, or insufficiently provisioned for translation into error messages to be returned to the original sources. Although recent works have led to the development of a robust end-to- end MTU determination scheme [RFC4821], they do not excuse tunnels from delivering path MTU discovery feedback when packets are lost due to size restrictions. 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. Therefore, 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. Example connected network routing regions include Mobile Ad hoc Networks (MANETs), enterprise networks and the global public Internet itself. Subnetwork border nodes forward unicast and multicast packets over the virtual topology across multiple IP and/or sub-IP layer forwarding hops that may introduce packet duplication and/or traverse links with diverse Maximum Transmission Units (MTUs). This document introduces a Subnetwork Encapsulation and Adaptation Layer (SEAL) for tunneling inner network layer protocol packets over Templin Expires June 21, 2012 [Page 6] Internet-Draft SEAL December 2011 IP subnetworks that connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border nodes. It provides a modular specification designed to be tailored to specific associated tunneling protocols. A transport-mode of operation is also possible, and described in Appendix C. SEAL provides a mid-layer encapsulation that accommodates links with diverse MTUs, and allows routers in the subnetwork to perform efficient duplicate packet and packet reordering detection. The encapsulation further ensures data origin authentication, packet header integrity and anti-replay in environments in which these functions are necessary. SEAL treats tunnels that traverse the subnetwork as ordinary links that must support network layer services. Moreover, SEAL provides dynamic mechanisms to ensure a maximal path MTU over the tunnel. This is in contrast to static approaches which avoid MTU issues by selecting a lowest common denominator MTU value that may be overly conservative for the vast majority of tunnel paths 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. IP used to generically refer to either Internet Protocol (IP) version, i.e., IPv4 or IPv6. 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. Templin Expires June 21, 2012 [Page 7] Internet-Draft SEAL December 2011 ETE Link Path a subnetwork path from an ITE to an ETE beginning with an underlying link of the ITE as the first hop. Note that, if the ITE's interface connection to the underlying link assigns multiple IP addresses, each address represents a separate ETE link path. inner packet an unencapsulated network layer protocol packet (e.g., IPv4 [RFC0791], OSI/CLNP [RFC0994], IPv6 [RFC2460], etc.) before any outer encapsulations are added. Internet protocol numbers that identify inner packets are found in the IANA Internet Protocol registry [RFC3232]. SEAL protocol packets that incur an additional layer of SEAL encapsulation are also considered inner packets. outer IP packet a packet resulting from adding an outer IP header (and possibly other outer headers) to a SEAL-encapsulated inner packet. packet-in-error the leading portion of an invoking data packet encapsulated in the body of an error control message (e.g., an ICMPv4 [RFC0792] error message, an ICMPv6 [RFC4443] error message, etc.). Packet Too Big (PTB) a control plane message indicating an MTU restriction (e.g., an ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4 "Fragmentation Needed" message [RFC0792], etc.). The following abbreviations correspond to terms used within this document and/or elsewhere in common Internetworking nomenclature: DF - the IPv4 header "Don't Fragment" flag [RFC0791] ETE - Egress Tunnel Endpoint HLEN - the length of the SEAL header plus outer headers ICV - Integrity Check Vector ITE - Ingress Tunnel Endpoint MTU - Maximum Transmission Unit SCMP - the SEAL Control Message Protocol SDU - SCMP Destination Unreachable message Templin Expires June 21, 2012 [Page 8] Internet-Draft SEAL December 2011 SPP - SCMP Parameter Problem message SPTB - SCMP Packet Too Big message SEAL - Subnetwork Encapsulation and Adaptation Layer TE - Tunnel Endpoint (i.e., either ingress or egress) VET - Virtual Enterprise Traversal The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" 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 originally motivated by the specific case of subnetwork abstraction for Mobile Ad hoc Networks (MANETs), however the domain of applicability also extends to subnetwork abstractions over enterprise networks, ISP networks, SOHO networks, the global public Internet itself, and any other connected network routing region. SEAL, along with the Virtual Enterprise Traversal (VET) [I-D.templin-intarea-vet] tunnel virtual interface abstraction, are the functional building blocks for the Internet Routing Overlay Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139] architectures. SEAL provides a network sublayer for encapsulation of an inner network layer packet within outer encapsulating headers. SEAL can also be used as a sublayer within a transport layer protocol data payload, where transport layer encapsulation is typically used for Network Address Translator (NAT) traversal as well as operation over subnetworks that give preferential treatment to certain "core" Internet protocols (e.g., TCP, UDP, etc.). The SEAL header is processed the same as for IPv6 extension headers, i.e., it is not part of the outer IP header but rather allows for the creation of an arbitrarily extensible chain of headers in the same way that IPv6 does. To accommodate MTU diversity, the Egress Tunnel Endpoint (ETE) acts as a passive observer that simply informs the Ingress Tunnel Endpoint (ITE) of any packet size limitations. This allows the ITE to return appropriate path MTU discovery feedback even if the network path Templin Expires June 21, 2012 [Page 9] Internet-Draft SEAL December 2011 between the ITE and ETE filters ICMP messages. SEAL further provides mechanisms to ensure data origin authentication, packet header integrity, and anti-replay. The SEAL framework is therefore similar to the IP Security (IPsec) Authentication Header (AH) [RFC4301][RFC4302], however it provides only minimal hop-by-hop authenticating services along a path while leaving full data integrity, authentication and confidentiality services as an end-to-end consideration. While SEAL performs data origin authentication, the origin site must also perform the necessary ingress filtering in order to provide full source address verification [I-D.ietf-savi-framework]. In many aspects, SEAL also very closely resembles the Generic Routing Encapsulation (GRE) framework [RFC1701]. SEAL can therefore be applied in the same use cases that are traditionally addressed by GRE, and can also provide additional capabilities as described in this document. 4. SEAL Specification The following sections specify the operation of SEAL: 4.1. VET Interface Model SEAL is an encapsulation sublayer used within VET non-broadcast, multiple access (NBMA) tunnel virtual interfaces. Each VET interface is configured over one or more underlying interfaces attached to subnetwork links. The VET interface connects an ITE to one or more ETE "neighbors" via tunneling across an underlying subnetwork, where the tunnel neighbor relationship may be either unidirectional or bidirectional. A unidirectional tunnel neighbor relationship allows the near end ITE to send data packets forward to the far end ETE, while the ETE only returns control messages when necessary. A bidirectional tunnel neighbor relationship is one over which both TEs can exchange both data and control messages. Implications of the VET unidirectional and bidirectional models are discussed in [I-D.templin-intarea-vet]. 4.2. SEAL Model of Operation SEAL-enabled ITEs encapsulate each inner packet in a SEAL header, any outer header encapsulations, and (in certain cases) a SEAL trailer as shown in Figure 1: Templin Expires June 21, 2012 [Page 10] Internet-Draft SEAL December 2011 +--------------------+ ~ outer IP header ~ +--------------------+ ~ other outer hdrs ~ +--------------------+ ~ SEAL Header ~ +--------------------+ +--------------------+ | | --> | | ~ Inner ~ --> ~ Inner ~ ~ Packet ~ --> ~ Packet ~ | | --> | | +--------------------+ +----------+---------+ . Trailer . +..........+ Figure 1: SEAL Encapsulation The ITE inserts the SEAL header according to the specific tunneling protocol. For simple encapsulation of an inner network layer packet within an outer IP header, the ITE inserts the SEAL header between the inner packet and outer IP headers as: IP/SEAL/{inner packet}. For encapsulations over transports such as UDP, the ITE inserts the SEAL header between the outer transport layer header and the inner packet, e.g., as IP/UDP/SEAL/{inner packet}. In that case, the UDP header is seen as an "other outer header" as depicted in Figure 1. In certain cases, the ITE also appends a 16-bit trailer at the end of the SEAL packet. In that case, the trailer is added after the final byte of the encapsulated packet and need not be aligned on an even word boundary. SEAL supports both "nested" tunneling and "re-encapsulating" tunneling. Nested tunneling occurs when a first tunnel is encapsulated within a second tunnel, which may then further be encapsulated within additional tunnels. Nested tunneling can be useful, and stands in contrast to "recursive" tunneling which is an anomalous condition incurred due to misconfiguration or a routing loop. Considerations for nested tunneling are discussed in Section 4 of [RFC2473]. Re-encapsulating tunneling occurs when a packet arrives at a first ETE, which then acts as an ITE to re-encapsulate and forward the packet to a second ETE connected to the same subnetwork. In that case each ITE/ETE transition represents a segment of a bridged path between the ITE nearest the source and the ETE nearest the destination. Combinations of nested and re-encapsulating tunneling are also naturally supported by SEAL. Templin Expires June 21, 2012 [Page 11] Internet-Draft SEAL December 2011 The SEAL ITE considers each {underlying interface, IP address} pair as the ingress attachment point to a subnetwork link path to the ETE. The ITE therefore maintains path MTU state on a per ETE link path basis, although it may instead maintain only the lowest-common- denominator values for all of the ETE's link paths in order to reduce state. Finally, the SEAL ITE ensures that the inner network layer protocol will see a minimum MTU of 1280 bytes over each ETE link path regardless of the outer network layer protocol version, i.e., even if a small amount of fragmentation and reassembly are necessary. 4.3. SEAL Header and Trailer Format The SEAL header is formatted as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |VER|C|A|R|I|V|T| NEXTHDR | PREFLEN | LINK_ID |LEVEL| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Integrity Check Vector (ICV) (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: SEAL Header Format VER (2) a 2-bit version field. This document specifies Version 0 of the SEAL protocol, i.e., the VER field encodes the value 0. C (1) the "Control/Data" bit. Set to 1 by the ITE in SEAL Control Message Protocol (SCMP) control messages, and set to 0 in ordinary data packets. A (1) the "Acknowledgement Requested" bit. Set to 1 by the ITE in SEAL data packets for which it wishes to receive an explicit acknowledgement from the ETE. R (1) the "Redirects Permitted" bit. For data packets, set to 1 by the ITE to inform the ETE that the source is accepting Redirects (see: [I-D.templin-intarea-vet]). Templin Expires June 21, 2012 [Page 12] Internet-Draft SEAL December 2011 I (1) the "Identification Included" bit. V (1) the "ICV included" bit. T (1) the "Trailer included" bit for IPv4 ETE link paths. Reserved for future use for IPv6 ETE link paths. 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. PREFLEN (8) an 8-bit field that encodes the length of the prefix to be applied to the source address of the inner packet. LINK_ID (5) a 5-bit link identification value, set to a unique value by the ITE for each link path over which it will send encapsulated packets to the ETE (up to 32 link paths per ETE are therefore supported). Note that, if the ITE's interface connection to the underlying link assigns multiple IP addresses, each address represents a separate ETE link path that must be assigned a separate LINK_ID. LEVEL (3) a 3-bit nesting level; use to limit the number of tunnel nesting levels. Set to an integer value up to 7 in the innermost SEAL encapsulation, and decremented by 1 for each successive additional SEAL encapsulation nesting level. Up to 8 levels of nesting are therefore supported. Identification (32) an optional 32-bit per-packet identification field; present when I==1. Set to a monotonically-incrementing 32-bit value for each SEAL packet transmitted to this ETE, beginning with 0. Integrity Check Vector (ICV) (32) an optional 32-bit header integrity check value; present when V==1. Covers the leading 128 bytes of the packet beginning with the SEAL header. The value 128 is chosen so that at least the SEAL header as well as the inner packet network and transport layer headers are covered by the integrity check. When (T==1), the SEAL encapsulation also includes a 16-bit trailing integrity check vector ("ICV2") formatted as follows: Templin Expires June 21, 2012 [Page 13] Internet-Draft SEAL December 2011 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ICV2(A) | ICV2(B) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: SEAL Trailer Format ICV2 (16) a 16-bit ICV2 value; present only when T==1. The value is calculated by the 8-bit Fletcher's algorithm given in [RFC1146], where the "A" result is placed in the most significant byte and the "B" result is placed in the least significant byte. 4.4. ITE Specification 4.4.1. Tunnel Interface MTU The tunnel interface must present a constant MTU value to the inner network layer as the size for admission of inner packets into the interface. Since VET NBMA tunnel virtual interfaces may support a large set of ETE link paths that accept widely varying maximum packet sizes, however, a number of factors should be taken into consideration when selecting a tunnel interface MTU. Due to the ubiquitous deployment of standard Ethernet and similar networking gear, the nominal Internet cell size has become 1500 bytes; this is the de facto size that end systems have come to expect will either be delivered by the network without loss due to an MTU restriction on the path or a suitable ICMP Packet Too Big (PTB) message returned. When large packets sent by end systems incur additional encapsulation at an ITE, however, they may be dropped silently within the tunnel since the network may not always deliver the necessary PTBs [RFC2923]. The ITE should therefore set a tunnel interface MTU of at least 1500 bytes plus extra room to accommodate any additional encapsulations that may occur on the path from the original source. The ITE can 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 inner network layer protocol consults the tunnel interface MTU when admitting a packet into the interface. For non-SEAL inner IPv4 packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet is larger than the tunnel interface MTU the inner IPv4 layer uses Templin Expires June 21, 2012 [Page 14] Internet-Draft SEAL December 2011 IPv4 fragmentation to break the packet into fragments no larger than the tunnel interface MTU. The ITE then admits each fragment into the interface as an independent packet. For all other inner packets, the inner network layer 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 contains as much of the invoking packet as possible without the entire message exceeding the network layer minimum MTU (e.g., 1280 bytes for IPv6, 576 bytes for IPv4, etc.). The ITE can alternatively set an indefinite MTU on the tunnel interface such that all inner packets are admitted into the interface regardless of their size. For ITEs that host applications that use the tunnel interface directly, this option must be carefully coordinated with protocol stack upper layers since some upper layer protocols (e.g., TCP) derive their packet sizing parameters from the MTU of the outgoing interface and as such may select too large an initial size. This is not a problem for upper layers that use conservative initial maximum segment size estimates and/or when the tunnel interface can reduce the upper layer's maximum segment size, e.g., by reducing the size advertised in the MSS option of outgoing TCP messages (sometimes known as "MSS clamping"). In light of the above considerations, the ITE should configure an indefinite MTU on tunnel *router* interfaces so that subnetwork adaptation is handled from within the interface. The ITE can instead set a finite MTU on tunnel *host* interfaces. 4.4.2. Tunnel Neighbor Soft State The tunnel virtual interface maintains a number of soft state variables for each ETE and for each ETE link path. When per-packet identification is required, the ITE maintains a per ETE window of Identification values for the packets it has recently sent to this ETE. The ITE then sets a variable "USE_ID" to TRUE, and includes an Identification in each packet it sends to this neighbor; otherwise, it sets USE_ID to FALSE. When data origin authentication and integrity checking is required, the ITE also maintains a per ETE integrity check vector (ICV) calculation algorithm and a symmetric secret key to calculate the ICV in each packet it will send to this ETE. The ITE then sets a variable "USE_ICV" to TRUE, and includes an ICV in each packet it sends to this ETE; otherwise, it sets USE_ICV to FALSE. Templin Expires June 21, 2012 [Page 15] Internet-Draft SEAL December 2011 For each ETE link path, the ITE must also account for encapsulation header lengths. The ITE therefore maintains the per ETE link path constant values "SHLEN" set to length of the SEAL header, "THLEN" set to the length of the outer encapsulating transport layer headers (or 0 if outer transport layer encapsulation is not used), "IHLEN" set to the length of the outer IP layer header, and "HLEN" set to (SHLEN+ THLEN+IHLEN). (The ITE must include the length of the uncompressed headers even if header compression is enabled when calculating these lengths.) In addition, the ETE maintains a constant value "MIN_MTU" set to (1280+HLEN) as well as a variable "PATH_MTU" initialized to the MTU of the underlying link. For ETE link paths that use IPv4 as the outer encapsulation protocol, the ITE also maintains the variables "USE_DF" set to FALSE, and "USE_TRAIL" set to TRUE if PATH_MTU is less than MIN_MTU (otherwise set to FALSE). The ITE may instead maintain the packet sizing variables and constants as per ETE (rather than per ETE link path) values. In that case, the values reflect the lowest-common-denominator MTU across all of the ETE's link paths. 4.4.3. Pre-Encapsulation For each inner packet admitted into the tunnel interface, if the packet is itself a SEAL packet (i.e., one with the port number for SEAL in the transport layer header or one with the protocol number for SEAL in the IP layer header) and the LEVEL field of the SEAL header contains the value 0, the ITE silently discards the packet. Otherwise, for IPv4 inner packets with DF==0 in the IPv4 header, if the packet is larger than 512 bytes and is not the first fragment of a SEAL packet (i.e., not a packet that includes a SEAL header) the ITE should fragment the packet into inner fragments no larger than 512 bytes unless it has operational assurance that the path can support a larger inner fragment size. The ITE then submits each inner fragment for SEAL encapsulation as specified in Section 4.4.4. For all other packets, if the packet is no larger than (MAX(PATH_MTU, MIN_MTU)-HLEN) for the corresponding ETE link path, the ITE submits it for SEAL encapsulation as specified in Section 4.4.4. Otherwise, the ITE sends a PTB error message toward the source address of the inner packet. To send the PTB message, the ITE first checks its forwarding tables to discover the previous hop toward the source address of the inner packet. If the previous hop is reached via the same tunnel interface, the ITE sends an SCMP PTB (SPTB) message to the previous Templin Expires June 21, 2012 [Page 16] Internet-Draft SEAL December 2011 hop (see: Section 4.6.1.1) with the MTU field set to (MAX(PATH_MTU, MIN_MTU)-HLEN). Otherwise, the ITE sends an ordinary PTB message appropriate to the inner protocol version with the MTU field set to (MAX(PATH_MTU, MIN_MTU)-HLEN). (NB: for IPv4 SEAL packets with DF==0, the ITE should set DF=1 and re-calculate the IPv4 header checksum before generating the PTB message in order to avoid bogon filters.) After sending the (S)PTB message, the ITE discards the inner packet. 4.4.4. SEAL Encapsulation The ITE next encapsulates the inner packet in a SEAL header formatted as specified in Section 4.3. The SEAL header includes an Identification field when USE_ID is TRUE, followed by an ICV field when USE_ICV is TRUE. When USE_TRAIL is TRUE, the ITE also leaves room for a trailing ICV2 field at the end of the packet. The ITE next sets C=0 in the SEAL header. The ITE also sets A=1 if necessary for ETE reachability determination (see: Section 4.4.6) or for stateful MTU determination (see Section 4.4.9). Otherwise, the ITE sets A=0. The ITE then sets R=1 if redirects are permitted (see: [I-D.templin-intarea-vet]) and sets PREFLEN to the length of the prefix to be applied to the inner source address. The ITE's claimed PREFLEN is subject to verification by the ETE; hence, the ITE must set PREFLEN to the exact prefix length that it is authorized to use. (Note that if this process is entered via re-encapsulation (see: Section 4.5.4), PREFLEN and R are instead copied from the SEAL header of the re-encapsulated packet. This implies that the PREFLEN and R values are propagated across a re-encapsulating chain of ITE/ETEs that must all be authorized to represent the prefix.) The ITE then sets LINK_ID to the value assigned to the underlying ETE link path, and sets NEXTHDR to the protocol number corresponding to the address family of the encapsulated inner packet. For example, the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets [RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213], '80' for encapsulated OSI/CLNP packets [RFC1070], etc. Next, if the inner packet is not itself a SEAL packet the ITE sets LEVEL to an integer value between 0 and 7 as a specification of the number of additional layers of nested SEAL encapsulations permitted. If the inner packet is a SEAL packet that is undergoing nested encapsulation, the ITE instead sets LEVEL to the value that appears in the inner packet's SEAL header minus 1. If the inner packet is undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL Templin Expires June 21, 2012 [Page 17] Internet-Draft SEAL December 2011 value from the SEAL header of the packet to be re-encapsulated. The ITE then sets the (I, V, T) flags and initializes any header extension fields as follows: o When USE_ID is TRUE, the ITE sets I=1 and writes a monotonically- increasing integer value for this ETE in the Identification field (beginning with 0 in the first packet transmitted). Otherwise, the ITE sets I=0. o When USE_ICV is TRUE, the ITE sets V=1 and initializes the ICV field to 0; otherwise, it sets V=0. o When USE_TRAIL is TRUE, the ITE sets T=1; otherwise, it sets T=0. When USE_TRAIL is TRUE, the next calculates the trailing ICV2 value using the 8-bit Fletcher checksum algorithm given in Appendix I of [RFC1146]. Beginning with the SEAL header, the ITE calculates the checksum over the entire packet then places the "A" result in the first byte of the trailing ICV2 field and places the "B" result in the second byte. When USE_ICV is TRUE, the ITE then calculates the packet header ICV value using an algorithm agreed on by the ITE and ETE. When data origin authentication is required, the algorithm uses a symmetric secret key so that the ETE can verify that the ICV was generated by the ITE. Beginning with the SEAL header, the ITE calculates the ICV over the leading 128 bytes of the packet (or up to the end of the packet if there are fewer than 128 bytes) and places result in the ICV field. (If the packet contains fewer than 128 bytes, the ITE does not include the trailing ICV2 field (if present) in the ICV calculation.) The ITE then adds the outer encapsulating headers and performs any necessary outer fragmentation as specified in Section 4.4.5. 4.4.5. Outer Encapsulation Following SEAL encapsulation, the ITE next encapsulates the packet in the requisite outer transport (when necessary) and IP layer headers. When a transport layer header is included, the ITE writes the port number for SEAL in the transport destination service port field and writes the protocol number of the transport protocol in the outer IP header protocol field. Otherwise, the ITE writes the protocol number for SEAL in the outer IP header protocol field. The ITE then sets the other fields of the outer transport and IP layer headers as specified in Sections 5.5.4 and 5.5.5 Templin Expires June 21, 2012 [Page 18] Internet-Draft SEAL December 2011 of[I-D.templin-intarea-vet]. If this process is entered via re- encapsulation (see: Section 4.5.4), the ITE instead follows the re- encapsulation procedures specified in Section 5.5.6 of [I-D.templin-intarea-vet]. For IPv4 ETE link paths, if USE_DF is FALSE the ITE sets DF=0 in the IPv4 header to allow the packet to be fragmented within the subnetwork if it encounters a restricting link; otherwise, the ITE sets DF=1. (For IPv6 ETE link paths, the "DF" flag is absent but implicitly set to 1. The packet therefore will not be fragmented within the subnetwork, since IPv6 deprecates in-the-network fragmentation.) Next, the ITE uses IP fragmentation if necessary to fragment the encapsulated packet into outer IP fragments that are no larger than PATH_MTU. By virtue of the pre-encapsulation packet size calculations specified in Section 4.4.3, fragmentation will therefore only occur for outer packets that are larger than PATH_MTU but no larger than MIN_MTU. (Note that, for IPv6, fragmentation must be performed by the ITE itself, while for IPv4 the fragmentation could instead be performed by a router in the ETE link path.) The ITE then sends each outer packet/fragment via the underlying link corresponding to LINK_ID. 4.4.6. Path Probing and ETE Reachability Verification All SEAL data packets sent by the ITE are considered implicit probes. SEAL data packets will elicit an SCMP message from the ETE if it needs to acknowledge a probe and/or report an error condition. SEAL data packets may also be dropped by either the ETE or a router on the path, which will return an ICMP message. The ITE can also send an SCMP Router/Neighbor Solicitation message to elicit an SCMP Router/Neighbor Advertisement response (see: [I-D.templin-intarea-vet]) as verification that the ETE is still reachable via a specific link path. The ITE processes ICMP messages as specified in Section 4.4.7. The ITE processes SCMP messages as specified in Section 4.6.2. 4.4.7. Processing ICMP Messages When the ITE sends SEAL packets, it may receive ICMP error messages [RFC0792][RFC4443] from an ordinary router within the subnetwork or from another ITE on the path to the ETE (i.e., in case of nested encapsulations). Each ICMP message includes an outer IP header, Templin Expires June 21, 2012 [Page 19] Internet-Draft SEAL December 2011 followed by an ICMP header, followed by a portion of the SEAL data packet that generated the error (also known as the "packet-in-error") beginning with the outer IP header. The ITE should process ICMPv4 Protocol Unreachable messages and ICMPv6 Parameter Problem messages with Code "Unrecognized Next Header type encountered" as a hint that the ETE does not implement the SEAL protocol. The ITE can also process other ICMP messages that do not include sufficient information in the packet-in-error as a hint that the ETE link path may be failing. Specific actions that the ITE may take in these cases are out of scope. For other ICMP messages, the ITE should use any outer header information available as a first-pass authentication filter (e.g., to determine if the source of the message is within the same administrative domain as the ITE) and discards the message if first pass filtering fails. Next, the ITE examines the packet-in-error beginning with the SEAL header. If the value in the Identification field (if present) is not within the window of packets the ITE has recently sent to this ETE, or if the value in the SEAL header ICV field (if present) is incorrect, the ITE discards the message. Next, if the received ICMP message is a PTB the ITE sets the temporary variable "PMTU" for this ETE link path to the MTU value in the PTB message. If PMTU==0, the ITE consults a plateau table (e.g., as described in [RFC1191]) to determine PMTU based on the length field in the outer IP header of the packet-in-error. (For example, if the ITE receives a PTB message with MTU==0 and length 1500, it can set PMTU=1450. If the ITE subsequently receives a PTB message with MTU==0 and length 1450, it can set PMTU=1400, etc.) If the ITE is performing stateful MTU determination for this ETE link path (see Section 4.4.9), the ITE next sets PATH_MTU=PMTU. If PMTU is less than MIN_MTU, the ITE sets PATH_MTU=PMTU whether or not stateful MTU determination is used (and for IPv4 also sets (USE_TRAIL=TRUE; USE_DF=FALSE)), then discards the message. If the ICMP message was not discarded, the ITE then transcribes it into a message to return to the previous hop. If the previous hop toward the inner source address within the packet-in-error is reached via the same tunnel interface the SEAL data packet was sent on, the ITE transcribes the ICMP message into an SCMP message. Otherwise, the ITE transcribes the ICMP message into a message appropriate for the inner protocol version. To transcribe the message, the ITE extracts the inner packet from Templin Expires June 21, 2012 [Page 20] Internet-Draft SEAL December 2011 within the ICMP message packet-in-error field and uses it to generate a new message corresponding to the type of the received ICMP message. For SCMP messages, the ITE generates the message the same as described for ETE generation of SCMP messages in Section 4.6.1. For (S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field. The ITE finally forwards the transcribed message to the previous hop toward the inner source address. 4.4.8. IPv4 Middlebox Reassembly Testing For IPv4 ETE link paths, the ITE can perform a qualification exchange to ensure that the subnetwork correctly delivers fragments to the ETE. This procedure can be used, e.g., to determine whether there are middleboxes on the path that violate the [RFC1812], Section 5.2.6 requirement that: "A router MUST NOT reassemble any datagram before forwarding it". The ITE should use knowledge of its topological arrangement as an aid in determining when middlebox reassembly testing is necessary. For example, if the ITE is aware that the ETE is located somewhere in the public Internet, middlebox reassembly testing should not be necessary. If the ITE is aware that the ETE is located behind a NAT or a firewall, however, then middlebox reassembly testing is recommended. The ITE can perform a middlebox reassembly test by selecting a data packet to be used as a probe. While performing the test with real data packets, the ITE should select only inner packets that are no larger than 1280 bytes for testing purposes so that the reassembled packet will not be discarded by the ETE. The ITE can also construct a NULL probe packet instead of using ordinary SEAL data packets. To generate a NULL probe packet, the ITE creates a packet buffer beginning with the same outer headers, SEAL header and an inner network layer header that would appear in an ordinary data packet. The ITE writes source address taken from the ITE's claimed prefix and a NULL destination address in the inner network layer header, then pads the packet with random data to a length that is at least 128 bytes but no more than 1280 bytes. The ITE then sets (C=0; R=0; T=0) in the SEAL header of the probe packet, writes the length of the ITE's claimed prefix in the PREFLEN field and sets the NEXTHDR field to the inner network layer protocol type. (The ITE may also set A=1 if it requires a positive acknowledgement; otherwise, it sets A=0.) Next, the ITE sets LINK_ID and LEVEL to the appropriate values for this ETE link path, sets Identification and I=1 (when USE_ID is TRUE), then finally calculates Templin Expires June 21, 2012 [Page 21] Internet-Draft SEAL December 2011 the ICV and sets V=1(when USE_ICV is TRUE). The ITE then encapsulates the probe packet in the appropriate outer headers, splits it into two outer IPv4 fragments, then sends both fragments over the same ETE link path. The ITE should send a series of probe packets (e.g., 3-5 probes with 1sec intervals between tests) instead of a single isolated probe in case of packet loss. If the ETE returns an SCMP PTB message with MTU != 0, then the ETE link path correctly supports fragmentation; otherwise, the ITE sets PATH_MTU=MIN_MTU and sets (USE_TRAIL=TRUE; USE_DF=FALSE). The ITE may instead enable stateful MTU determination for this ETE link path as specified in Section 4.4.9 to attempt to discover larger MTUs. NB: Examples of middleboxes that may perform reassembly include stateful NATs and firewalls. Such devices could still allow for stateless MTU determination if they gather the fragments of a fragmented IPv4 SEAL data packet for packet analysis purposes but then forward the fragments on to the final destination rather than forwarding the reassembled packet. 4.4.9. Stateful MTU Determination SEAL supports a stateless MTU determination capability, however the ITE may in some instances wish to impose a stateful MTU limit on a particular ETE link path. For example, when the ETE is situated behind a middlebox that performs IPv4 reassembly (see: Section 4.4.8) it is imperative that fragmentation of large packets be avoided. In other instances (e.g., when the ETE link path includes performance- constrained links), the ITE may deem it necessary to cache a conservative static MTU in order to avoid sending large packets that would only be dropped due to an MTU restriction somewhere on the path. To determine a static MTU value, the ITE can send a series of probe packets of various sizes to the ETE with A=1 in the SEAL header and DF=1 in the outer IP header. The ITE can then cache the size of the largest packet for which it receives a probe reply from the ETE as the PATH_MTU value this ETE link path. For example, the ITE could send NULL probe packets of 1500 bytes, followed by 1450 bytes, followed by 1400 bytes, etc. then set PATH_MTU for this ETE link path to the size of the largest probe packet for which it receives an SPTB reply message. While probing with NULL probe packets, the ITE processes any ICMP PTB message it receives as a potential indication of probe failure then discards the message. Templin Expires June 21, 2012 [Page 22] Internet-Draft SEAL December 2011 For IPv4 ETE link paths, if the largest successful probe is no larger than MIN_MTU the ITE then sets (USE_TRAIL=TRUE; USE_DF=FALSE); otherwise, the ITE sets (USE_TRAIL=FALSE; USE_DF=TRUE). 4.4.10. Detecting Path MTU Changes When stateful determination is used, the ITE can periodically reset PATH_MTU to the MTU of the underlying link and/or re-probe the path to determine whether PATH_MTU has increased. If the path still has a too-small MTU, the ITE will receive a PTB message that reports a smaller size. For IPv4 ETE link paths, when the path correctly implements fragmentation and USE_TRAIL is TRUE, the ITE can periodically reset USE_TRAIL=FALSE to determine whether the path still requires a trailing ICV2 field. If the ITE receives an SPTB message for an inner packet that is no larger than 1280 bytes (see: Section 4.6.1.1), the ITE should again set USE_TRAIL=TRUE. 4.5. ETE Specification 4.5.1. Tunnel Neighbor Soft State When data origin authentication and integrity checking is required, the ETE maintains a per-ITE ICV calculation algorithm and a symmetric secret key to verify the ICV. When per-packet identification is required, the ETE also maintains a window of Identification values for the packets it has recently received from this ITE. When the tunnel neighbor relationship is bidirectional, the ETE further maintains a per ETE link path mapping of outer IP and transport layer addresses to the LINK_ID that appears in packets received from the ITE. 4.5.2. IP-Layer Reassembly The ETE must maintain a minimum IP-layer reassembly buffer size of 1500 bytes for both IPv4 [RFC0791] and IPv6 [RFC2460]. The ETE should maintain conservative reassembly cache high- and low- water marks. When the size of the reassembly cache exceeds this high-water mark, the ETE should actively discard stale 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 arrived before a fragment that completes a pending reassembly arrives. Templin Expires June 21, 2012 [Page 23] Internet-Draft SEAL December 2011 The ETE processes non-SEAL IP packets as specified in the normative references, i.e., it performs any necessary IP reassembly then discards the packet if it is larger than the reassembly buffer size or delivers the (fully-reassembled) packet to the appropriate upper layer protocol module. For SEAL packets, the ITE performs any necessary IP reassembly until it has received at least the first 1280 bytes beyond the SEAL header or up to the end of the packet. The ETE then submits the (fully- or partially-reassembled) packet for SEAL decapsulation as specified in Section 4.5.3. 4.5.3. Decapsulation and Re-Encapsulation For each SEAL packet submitted for decapsulation, when I==1 the ETE first examines the Identification field. If the Identification is not within the window of acceptable values for this ITE, the ETE silently discards the packet. Next, if V==1 the ETE verifies the ICV value (with the ICV field itself reset to 0) and silently discards the packet if the value is incorrect. For IPv4, if T==1 and the packet is no larger than 1280 bytes the ITE next verifies the ICV2 value and silently discards the packet if the value is incorrect. (Note that the ITE must verify the ICV2 value even if the packet arrives unfragmented in case a middlebox is performing reassembly.) Next, if the packet arrived as multiple IPv4 fragments and T==0, the ETE sends an SPTB message back to the ITE with MTU set to the size of the largest fragment received minus HLEN (see: Section 4.6.1.1). Next, if the packet arrived as multiple IP fragments and the inner packet is larger than 1280 bytes, the ETE then silently discards the packet; otherwise, it continues to process the packet. Next, if there is an incorrect value in a SEAL header field (e.g., an incorrect "VER" field value), the ETE discards the packet. If the SEAL header has C==0, the ETE also returns an SCMP "Parameter Problem" (SPP) message (see Section 4.6.1.2). Next, if the SEAL header has C==1, the ETE processes the packet as an SCMP packet as specified in Section 4.6.2. Otherwise, the ETE continues to process the packet as a SEAL data packet. Next, if the SEAL header has A==1, the ETE sends an SPTB message back to the ITE with MTU=0 (see: Section 4.6.1.1). Finally, the ETE discards the outer headers and processes the inner Templin Expires June 21, 2012 [Page 24] Internet-Draft SEAL December 2011 packet according to the header type indicated in the SEAL NEXTHDR field. If the inner destination address of the packet is NULL the ETE silently discards the packet. Otherwise, if the next hop toward the inner destination address is via a different interface than the SEAL packet arrived on, the ETE discards the SEAL header and delivers the inner packet either to the local host or to the next hop interface if the packet is not destined to the local host. If the next hop is on the same interface the SEAL packet arrived on, however, the ETE submits the packet for SEAL re-encapsulation beginning with the specification in Section 4.4.3 above. In this process, the packet remains within the tunnel interface (i.e., it does not exit and then re-enter the interface); hence, the packet is not discarded if the LEVEL field in the SEAL header contains the value 0. 4.6. The SEAL Control Message Protocol (SCMP) SEAL provides a companion SEAL Control Message Protocol (SCMP) that uses the same message types and formats as for the Internet Control Message Protocol for IPv6 (ICMPv6) [RFC4443]. As for ICMPv6, each SCMP message includes a 32-bit header and a variable-length body. The TE encapsulates the SCMP message in a SEAL header and outer headers as shown in Figure 4: +--------------------+ ~ outer IP header ~ +--------------------+ ~ other outer hdrs ~ +--------------------+ ~ SEAL Header ~ +--------------------+ +--------------------+ | SCMP message header| --> | SCMP message header| +--------------------+ +--------------------+ | | --> | | ~ SCMP message body ~ --> ~ SCMP message body ~ | | --> | | +--------------------+ +--------------------+ SCMP Message SCMP Packet before encapsulation after encapsulation Figure 4: SCMP Message Encapsulation The following sections specify the generation, processing and relaying of SCMP messages. Templin Expires June 21, 2012 [Page 25] Internet-Draft SEAL December 2011 4.6.1. Generating SCMP Error Messages ETEs generate SCMP error messages in response to receiving certain SEAL data packets using the format shown in Figure 5: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type-Specific Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | As much of the invoking SEAL data packet as possible | ~ (beginning with the SEAL header) without the SCMP ~ | packet exceeding 576 bytes (*) | (*) also known as the "packet-in-error" Figure 5: SCMP Error Message Format The error message includes the 32-bit SCMP message header, followed by a 32-bit Type-Specific Data field, followed by the leading portion of the invoking SEAL data packet beginning with the SEAL header as the "packet-in-error". The packet-in-error includes as much of the invoking packet as possible extending to a length that would not cause the entire SCMP packet following outer encapsulation to exceed 576 bytes. When the ETE processes a SEAL data packet for which the Identification and ICV values are correct but an error must be returned, it prepares an SCMP error message as shown in Figure 5. The ETE sets the Type and Code fields to the same values that would appear in the corresponding ICMPv6 message [RFC4443], but calculates the Checksum beginning with the SCMP message header using the algorithm specified for ICMPv4 in [RFC0792]. The ETE next encapsulates the SCMP message in the requisite SEAL and outer headers as shown in Figure 4. During encapsulation, the ETE sets the outer destination address/port numbers of the SCMP packet to the values associated with the ITE and sets the outer source address/ port numbers to its own outer address/port numbers. The ETE then sets (C=1; A=0; R=0; T=0) in the SEAL header, then sets I, V, NEXTHDR, PREFLEN, and LEVEL to the same values that appeared in the SEAL header of the data packet. If the neighbor relationship between the ITE and ETE is unidirectional, the ETE next sets the LINK_ID field to the same value that appeared in the SEAL header of the data packet. Otherwise, the ETE sets the LINK_ID field to the Templin Expires June 21, 2012 [Page 26] Internet-Draft SEAL December 2011 value it would use in sending a SEAL packet to this ITE. When I==1, the ETE next sets the Identification field to an appropriate value for the ITE. If the neighbor relationship between the ITE and ETE is unidirectional, the ETE sets the Identification field to the same value that appeared in the SEAL header of the data packet. Otherwise, the ETE sets the Identification field to the value it would use in sending the next SEAL packet to this ITE. When V==1, the ETE then calculates and sets the ICV field the same as specified for SEAL data packet encapsulation in Section 4.4.4. Finally, the ETE sends the resulting SCMP packet to the ITE the same as specified for SEAL data packets in Section 4.4.5. The following sections describe additional considerations for various SCMP error messages: 4.6.1.1. Generating SCMP Packet Too Big (SPTB) Messages An ETE generates an SCMP "Packet Too Big" (SPTB) message when it receives a SEAL data packet that arrived as multiple outer IPv4 fragments and for which T==0. The ETE prepares the SPTB message the same as for the corresponding ICMPv6 PTB message, and writes the length of the largest outer IP fragment received minus HLEN in the MTU field of the message. The ETE also generates an SPTB message when it accepts a SEAL protocol data packet with A==1 in the SEAL header. The ETE prepares the SPTB message the same as above, except that it writes the value 0 in the MTU field. 4.6.1.2. Generating Other SCMP Error Messages An ETE generates an SCMP "Destination Unreachable" (SDU) message under the same circumstances that an IPv6 system would generate an ICMPv6 Destination Unreachable message. An ETE generates an SCMP "Parameter Problem" (SPP) message when it receives a SEAL packet with an incorrect value in the SEAL header. TEs generate other SCMP message types using methods and procedures specified in other documents. For example, SCMP message types used for tunnel neighbor coordinations are specified in VET [I-D.templin-intarea-vet]. Templin Expires June 21, 2012 [Page 27] Internet-Draft SEAL December 2011 4.6.2. Processing SCMP Error Messages An ITE may receive SCMP messages with C==1 in the SEAL header after sending packets to an ETE. The ITE first verifies that the outer addresses of the SCMP packet are correct, and (when I==1) that the Identification field contains an acceptable value. The ITE next verifies that the SEAL header fields are set correctly as specified in Section 4.6.1. When V==1, the ITE then verifies the ICV value. The ITE next verifies the Checksum value in the SCMP message header. If any of these values are incorrect, the ITE silently discards the message; otherwise, it processes the message as follows: 4.6.2.1. Processing SCMP PTB Messages After an ITE sends a SEAL data packet to an ETE, it may receive an SPTB message with a packet-in-error containing the leading portion of the packet (see: Section 4.6.1.1). For IP SPTB messages with MTU==0, the ITE processes the message as confirmation that the ETE received a SEAL data packet with A==1 in the SEAL header. The ITE then discards the message. For IPv4 SPTB messages with MTU != 0, the ITE instead processes the message as an indication of a packet size limitation as follows. If the inner packet is no larger than 1280 bytes, the ITE sets (USE_TRAIL=TRUE; USE_DF=FALSE). If the inner packet is larger than 1280 bytes, the ITE instead examines the SPTB message MTU field. If the MTU value is not less than (MIN_MTU-HLEN), the value is likely to reflect the true MTU of the restricting link on the path to the ETE; otherwise, a router on the path may be generating runt fragments. In that case, the ITE can consult a plateau table (e.g., as described in [RFC1191]) to rewrite the MTU value to a reduced size. For example, if the ITE receives an IPv4 SPTB message with MTU==256 and inner packet length 1500, it can rewrite the MTU to 1450. If the ITE subsequently receives an IPv4 SPTB message with MTU==256 and inner packet length 1450, it can rewrite the MTU to 1400, etc. If the ITE is performing stateful MTU determination for this ETE link path, it then writes the new MTU value in PATH_MTU. The ITE then checks its forwarding tables to discover the previous hop toward the source address of the inner packet. If the previous hop is reached via the same tunnel interface the SPTB message arrived on, the ITE relays the message to the previous hop. In order to relay the message, the first writes zero in the Identification and ICV fields of the SEAL header within the packet-in-error. The ITE next rewrites the outer SEAL header fields with values corresponding to the previous hop and recalculates the ICV using the ICV calculation parameters associated with the previous hop. Next, the Templin Expires June 21, 2012 [Page 28] Internet-Draft SEAL December 2011 ITE replaces the SPTB's outer headers with headers of the appropriate protocol version and fills in the header fields as specified in Sections 5.5.4-5.5.6 of [I-D.templin-intarea-vet], where the destination address/port correspond to the previous hop and the source address/port correspond to the ITE. The ITE then sends the message to the previous hop the same as if it were issuing a new SPTB message. (Note that, in this process, the values within the SEAL header of the packet-in-error are meaningless to the previous hop and therefore cannot be used by the previous hop for authentication purposes.) If the previous hop is not reached via the same tunnel interface, the ITE instead transcribes the message into a format appropriate for the inner packet (i.e., the same as described for transcribing ICMP messages in Section 4.4.7) and sends the resulting transcribed message to the original source. (NB: if the inner packet within the SPTB message is an IPv4 SEAL packet with DF==0, the ITE should set DF=1 and re-calculate the IPv4 header checksum while transcribing the message in order to avoid bogon filters.) The ITE then discards the SPTB message. 4.6.2.2. Processing Other SCMP Error Messages An ITE may receive an SDU message with an appropriate code under the same circumstances that an IPv6 node would receive an ICMPv6 Destination Unreachable message. The ITE either transcribes or relays the message toward the source address of the inner packet within the packet-in-error the same as specified for SPTB messages in Section 4.6.2.1. An ITE may receive an SPP message when the ETE receives a SEAL packet with an incorrect value in the SEAL header. The ITE should examine the SEAL header within the packet-in-error to determine whether a different setting should be used in subsequent packets, but does not relay the message further. TEs process other SCMP message types using methods and procedures specified in other documents. For example, SCMP message types used for tunnel neighbor coordinations are specified in VET [I-D.templin-intarea-vet]. 5. Link Requirements Subnetwork designers are expected to follow the recommendations in Section 2 of [RFC3819] when configuring link MTUs. Templin Expires June 21, 2012 [Page 29] Internet-Draft SEAL December 2011 6. End System Requirements End systems are encouraged to implement end-to-end MTU assurance (e.g., using Packetization Layer Path MTU Discovery per [RFC4821]) even if the subnetwork is using SEAL. 7. Router Requirements Routers within the subnetwork are expected to observe the router requirements found in the normative references, including the implementation of IP fragmentation and reassembly [RFC1812][RFC2460] as well as the generation of ICMP messages [RFC0792][RFC4443]. 8. Nested Encapsulation Considerations SEAL supports nested tunneling for up to 8 layers of encapsulation. In this model, the SEAL ITE has a tunnel neighbor relationship only with ETEs at its own nesting level, i.e., it does not have a tunnel neighbor relationship with other ITEs, nor with ETEs at other nesting levels. Therefore, when an ITE 'A' within an inner nesting level needs to return an error message to an ITE 'B' within an outer nesting level, it generates an ordinary ICMP error message the same as if it were an ordinary router within the subnetwork. 'B' can then perform message validation as specified in Section 4.4.7, but full message origin authentication is not possible. Since ordinary ICMP messages are used for coordinations between ITEs at different nesting levels, nested SEAL encapsulations should only be used when the ITEs are within a common administrative domain and/or when there is no ICMP filtering middlebox such as a firewall or NAT between them. An example would be a recursive nesting of mobile networks, where the first network receives service from an ISP, the second network receives service from the first network, the third network receives service from the second network, etc. NB: As an alternative, the SCMP protocol could be extended to allow ITE 'A' to return an SCMP message to ITE 'B' rather than return an ICMP message. This would conceptually allow the control messages to pass through firewalls and NATs, however it would give no more message origin authentication assurance than for ordinary ICMP messages. It was therefore determined that the complexity of extending the SCMP protocol was of little value within the context of the anticipated use cases for nested encapsulations. Templin Expires June 21, 2012 [Page 30] Internet-Draft SEAL December 2011 9. IANA Considerations The IANA is instructed to allocate a System Port number for "SEAL" in the 'port-numbers' registry for the TCP, UDP, DCCP and SCTP protocols. The IANA is further instructed to allocate an IP protocol number for "SEAL" in the "protocol-numbers" registry. Considerations for port and protocol number assignments appear in [RFC2780][RFC5226][RFC6335]. 10. Security Considerations SEAL provides a segment-by-segment data origin authentication and anti-replay service across the (potentially) multiple segments of a re-encapsulating tunnel. It further provides a segment-by-segment integrity check of the headers of encapsulated packets, but does not verify the integrity of the rest of the packet beyond the headers unless fragmentation is unavoidable. SEAL therefore considers full message integrity checking, authentication and confidentiality as end-to-end considerations in a manner that is compatible with securing mechanisms such as TLS/SSL [RFC5246]. An amplification/reflection/buffer overflow attack is possible when an attacker sends IP fragments with spoofed source addresses to an ETE in an attempt to clog the ETE's reassembly buffer and/or cause the ETE to generate a stream of SCMP messages returned to a victim ITE. The SCMP message ICV, Identification, as well as the inner headers of the packet-in-error, provide mitigation for the ETE to detect and discard SEAL segments with spoofed source addresses. The SEAL header is sent in-the-clear the same as for the outer IP and other outer headers. In this respect, the threat model is no different than for IPv6 extension headers. Unlike IPv6 extension headers, however, the SEAL header can be protected by an integrity check that also covers the inner packet headers. Security issues that apply to tunneling in general are discussed in [RFC6169]. 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. This Templin Expires June 21, 2012 [Page 31] Internet-Draft SEAL December 2011 capability was implemented in the first edition of SEAL, but is now deprecated. Section 3 of [RFC4459] describes inner and outer fragmentation at the tunnel endpoints as alternatives for accommodating the tunnel MTU. Section 4 of [RFC2460] specifies a method for inserting and processing extension headers between the base IPv6 header and transport layer protocol data. The SEAL header is inserted and processed in exactly the same manner. IPsec/AH is [RFC4301][RFC4301] is used for full message integrity verification between tunnel endpoints, whereas SEAL only ensures integrity for the inner packet headers. The AYIYA proposal [I-D.massar-v6ops-ayiya] uses similar means for providing message authentication and integrity. The concepts of path MTU determination through the report of fragmentation and extending the IPv4 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. An historical analysis of the evolution of these concepts, as well as the development of the eventual path MTU discovery mechanism, appears in Appendix D of this document. 12. Acknowledgments The following individuals are acknowledged for helpful comments and suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci, Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin Whittle, James Woodyatt, and members of the Boeing Research & Technology NST DC&NT group. Discussions with colleagues following the publication of RFC5320 have provided useful insights that have resulted in significant improvements to this, the Second Edition of SEAL. 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. Templin Expires June 21, 2012 [Page 32] Internet-Draft SEAL December 2011 13. References 13.1. Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. 13.2. Informative References [FOLK] Shannon, C., Moore, D., and k. claffy, "Beyond Folklore: Observations on Fragmented Traffic", December 2002. [FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", October 1987. [I-D.ietf-intarea-ipv4-id-update] Touch, J., "Updated Specification of the IPv4 ID Field", draft-ietf-intarea-ipv4-id-update-04 (work in progress), September 2011. [I-D.ietf-savi-framework] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, "Source Address Validation Improvement Framework", draft-ietf-savi-framework-05 (work in progress), July 2011. [I-D.massar-v6ops-ayiya] Massar, J., "AYIYA: Anything In Anything", Templin Expires June 21, 2012 [Page 33] Internet-Draft SEAL December 2011 draft-massar-v6ops-ayiya-02 (work in progress), July 2004. [I-D.templin-aero] Templin, F., "Asymmetric Extended Route Optimization (AERO)", draft-templin-aero-05 (work in progress), December 2011. [I-D.templin-intarea-vet] Templin, F., "Virtual Enterprise Traversal (VET)", draft-templin-intarea-vet-32 (work in progress), November 2011. [I-D.templin-ironbis] Templin, F., "The Internet Routing Overlay Network (IRON)", draft-templin-ironbis-09 (work in progress), November 2011. [MTUDWG] "IETF MTU Discovery Working Group mailing list, gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November 1989 - February 1995.". [RFC0994] International Organization for Standardization (ISO) and American National Standards Institute (ANSI), "Final text of DIS 8473, Protocol for Providing the Connectionless- mode Network Service", RFC 994, March 1986. [RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP MTU discovery options", RFC 1063, July 1988. [RFC1070] Hagens, R., Hall, N., and M. Rose, "Use of the Internet as a subnetwork for experimentation with the OSI network layer", RFC 1070, February 1989. [RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum options", RFC 1146, March 1990. [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, October 1994. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. Templin Expires June 21, 2012 [Page 34] Internet-Draft SEAL December 2011 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998. [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", RFC 2675, August 1999. [RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A. Malis, "A Framework for IP Based Virtual Private Networks", RFC 2764, February 2000. [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers", BCP 37, RFC 2780, March 2000. [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000. [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. [RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-line Database", RFC 3232, January 2002. [RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, August 2002. [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, July 2004. [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005. [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 2005. Templin Expires June 21, 2012 [Page 35] Internet-Draft SEAL December 2011 [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the- Network Tunneling", RFC 4459, April 2006. [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, August 2007. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5445] Watson, M., "Basic Forward Error Correction (FEC) Schemes", RFC 5445, March 2009. [RFC5720] Templin, F., "Routing and Addressing in Networks with Global Enterprise Recursion (RANGER)", RFC 5720, February 2010. [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010. [RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and Addressing in Networks with Global Enterprise Recursion (RANGER) Scenarios", RFC 6139, February 2011. [RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security Concerns with IP Tunneling", RFC 6169, April 2011. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, August 2011. [SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU Discovery Behavior", November 2010. [TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring Interactions Between Transport Protocols and Middleboxes", October 2004. Templin Expires June 21, 2012 [Page 36] Internet-Draft SEAL December 2011 [TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List, http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May 1987 - May 1990.". [WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and Debugging Path MTU Discovery Failures", October 2005. Appendix A. Reliability Although a SEAL tunnel may span an arbitrarily-large subnetwork expanse, the IP layer sees the tunnel as a simple link that supports the IP service model. Links with high bit error rates (BERs) (e.g., IEEE 802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] to increase packet delivery ratios, while links with much lower BERs typically omit such mechanisms. Since SEAL tunnels may traverse arbitrarily-long paths over links of various types that are already either performing or omitting ARQ as appropriate, it would therefore be inefficient to require the tunnel endpoints to also perform ARQ. Appendix B. Integrity The SEAL header includes an integrity check field that covers the SEAL header and at least the inner packet headers (or up to the end of the packet if IPv4 fragmentation is needed). This provides for header integrity verification on a segment-by-segment basis for a segmented re-encapsulating tunnel path. Fragmentation and reassembly schemes must also consider packet- splicing errors, e.g., when two fragments from the same packet are concatenated incorrectly, when a fragment from packet X is reassembled with fragments from packet Y, etc. The primary sources of such errors include implementation bugs and wrapping IPv4 ID fields. In particular, the IPv4 16-bit ID field can wrap with only 64K packets with the same (src, dst, protocol)-tuple alive in the system at a given time [RFC4963]. When the IPv4 ID field is re-written by a middlebox such as a NAT or Firewall, ID field wrapping can occur with even fewer packets alive in the system. When outer IPv4 fragmentation is unavoidable, SEAL therefore provides a trailing checksum as a first-pass filter to detect reassembly mis- associations. Any reassembly mis-associations not detected by the checksum will very likely be detected later by upper layer checksums. Templin Expires June 21, 2012 [Page 37] Internet-Draft SEAL December 2011 Appendix C. Transport Mode SEAL can also be used in "transport-mode", e.g., when the inner layer comprises upper-layer protocol data rather than an encapsulated IP packet. For instance, TCP peers can negotiate the use of SEAL (e.g., by inserting an unspecified 'SEAL_OPTION' TCP option during connection establishment) for the carriage of protocol data encapsulated as IP/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. If both TCPs agree on the use of SEAL, their protocol messages will be carried as IP/SEAL/TCP and the connection will be serviced by the SEAL protocol using TCP (instead of an encapsulating tunnel endpoint) as the transport layer protocol. The SEAL protocol for transport mode otherwise observes the same specifications as for Section 4. Appendix D. Historic Evolution of PMTUD The topic of Path MTU discovery (PMTUD) saw a flurry of discussion and numerous proposals in the late 1980's through early 1990. The initial problem was posed by Art Berggreen on May 22, 1987 in a message to the TCP-IP discussion group [TCP-IP]. The discussion that followed provided significant reference material for [FRAG]. An IETF Path MTU Discovery Working Group [MTUDWG] was formed in late 1989 with charter to produce an RFC. Several variations on a very few basic proposals were entertained, including: 1. Routers record the PMTUD estimate in ICMP-like path probe messages (proposed in [FRAG] and later [RFC1063]) 2. The destination reports any fragmentation that occurs for packets received with the "RF" (Report Fragmentation) bit set (Steve Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal) 3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990) 4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30, 1990) 5. Fragmentation avoidance by setting "IP_DF" flag on all packets and retransmitting if ICMPv4 "fragmentation needed" messages occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191] by Mogul and Deering). Option 1) seemed attractive to the group at the time, since it was Templin Expires June 21, 2012 [Page 38] Internet-Draft SEAL December 2011 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 June 21, 2012 [Page 39]