Network Working Group F. Templin, Ed. Internet-Draft Boeing Phantom Works Intended status: Informational February 14, 2008 Expires: August 17, 2008 Subnetwork Encapsulation and Adaptation Layer draft-templin-seal-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 17, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract Subnetworks are connected network regions bounded by border routers. These routers forward unicast and multicast packets over virtual links that are tunneled above another forwarding layer. These virtual links span multiple IP- and/or sub-IP layer forwarding hops which may cross links with diverse Maximum Transmission Units (MTUs) and introduce packet duplication. This document specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that accommodates diverse underlying link technologies. Templin Expires August 17, 2008 [Page 1] Internet-Draft SEAL February 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 4 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5 4. SEAL Protocol Specification . . . . . . . . . . . . . . . . . 6 4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 6 4.2. Packetization . . . . . . . . . . . . . . . . . . . . . . 8 4.2.1. Packet Size Considerations . . . . . . . . . . . . . . 8 4.2.2. Inner IPv4 Fragmentation . . . . . . . . . . . . . . . 9 4.2.3. SEAL Segmentation and Encapsulation . . . . . . . . . 9 4.2.4. Sending Packets . . . . . . . . . . . . . . . . . . . 11 4.3. Reassembly . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3.1. Reassembly Buffer Requirements . . . . . . . . . . . . 12 4.3.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 12 4.3.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 13 4.3.4. Reassembly Integrity Checks . . . . . . . . . . . . . 13 4.4. Generating Fragmentation Reports . . . . . . . . . . . . . 13 4.5. Receiving Fragmentation Reports . . . . . . . . . . . . . 14 4.6. S-MSS Probing and Setting DF . . . . . . . . . . . . . . . 15 4.7. Processing ICMP PTBs . . . . . . . . . . . . . . . . . . . 16 5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 16 6. End System Requirements . . . . . . . . . . . . . . . . . . . 16 7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 9. Security Considerations . . . . . . . . . . . . . . . . . . . 17 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 11.1. Normative References . . . . . . . . . . . . . . . . . . . 18 11.2. Informative References . . . . . . . . . . . . . . . . . . 18 Appendix A. Historic Evolution of PMTUD (written 10/30/2002) . . 20 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21 Intellectual Property and Copyright Statements . . . . . . . . . . 22 Templin Expires August 17, 2008 [Page 2] Internet-Draft SEAL February 2008 1. Introduction For the purpose of this document, subnetworks are defined as connected network regions bounded by border routers. Examples include the global Internet interdomain routing core, Mobile Ad Hoc Networks (MANETs) and enterprise networks. These subnetworks are manifested as virtual links that may span many underlying networks and traditional IP subnets, e.g., in the internal organization of an enterprise network. Subnetwork border routers forward unicast and multicast packets over virtual links that span multiple IP- and/or sub-IP layer forwarding hops, which may traverse links with diverse Maximum Transmission Units (MTUs) and may also introduce packet duplication due to temporal or persistent routing loops. It is also expected that these routers will support operation of the Internet protocols [RFC0791][RFC2460]. As internet technology and communication has grown and matured, many techniques have developed that use virtual topologies (frequently tunnels of one form or another) over an actual IP network. Those virtual topologies have elements which 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 which are often not even visible to the end-points of the virtual hop. This introduces many failure modes that are not dealt with well in current approaches. The use of IP encapsulation has long been considered as an alternative 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 shown to be harmful through operational experience and studies conducted over the course of many years [FRAG][FOLK][RFC2923][RFC4459][RFC4963]. This document proposes a Subnetwork Encapsulation and Adaptation Layer (SEAL) for the operation of IP over subnetworks that connect routers with Ingress- and Egress Tunnel Endpoints (ITEs/ETEs). SEAL supports simple and robust duplicate packet detection, and accommodates links with diverse MTUs. SEAL introduces a new encapsulation format that differs from existing encapsulations primarily in that it enables a mid-layer segmentation and reassembly capability that is distinct from IP fragmentation. This mid-layer segmentation allows an in-the-network cutting and pasting of packets that does not violate the IPv6 restriction of no in-the-network fragmentation, and also avoids the harmful effects of in-the-network Templin Expires August 17, 2008 [Page 3] Internet-Draft SEAL February 2008 IPv4 fragmentation. The SEAL protocol is specified in the following sections. 2. Terminology and Requirements The term subnetwork in this document refers to a connected network region bounded by border routers that connect over a virtual link manifested through tunneling that appears as a fully-connected shared link, or a "virtual ethernet". SEAL is the Subnetwork Encapsulation and Adaptation Layer that adapts this virtual ethernet to the underlying heterogeneous networking links and equipment. The terms "inner" and "outer" are used extensively throughout this document to respectively refer to the innermost {layer, protocol/ header, packet, etc.} *before* any encapsulation, and the outermost {layer, protocol, header, packet etc.} *after* any encapsulation. Between these inner and outer layers, there may also be mid-layer encapsulations, including the SEAL encapsulation. These mid-layer encapsulations are denoted as '*' (where '*' may signify NULL, a single mid-layer encapsulation, or multiple mid-layer encapsulations.) The notation IPvX/*/IPvY refers to an inner IPvX packet encapsulated in an outer IPvY header separated by any '*' mid-layer headers. The notation "IP" means either IP protocol version (IPv4 or IPv6). The following abbreviations correspond to terms used within this document and elsewhere in common Internetworking nomenclature: Subnetwork - a connected network region that is bounded by border routers SEAL - Subnetwork Encapsulation and Adaptation Layer MANET - Mobile Ad-hoc Network VET - Virtual EThernet ITE - Ingress Tunnel Endpoint ETE - Egress Tunnel Endpoint MTU - Maximum Transmission Unit S-MSS - SEAL Maximum Segment Size Templin Expires August 17, 2008 [Page 4] Internet-Draft SEAL February 2008 EMTU_R - Effective MTU to Receive PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "fragmentation needed" message DF - the IPv4 header Don't Fragment flag ENCAPS - the size of the outer encapsulating SEAL/*/IPv4 headers FRAGREP - a Fragmentation Report message SEAL packet - a segment of an inner packet encapsulated in outer SEAL/*/IPv4 headers SEAL ID - a 32-bit Identification value that is randomly initialized and monotonically incremented for each SEAL packet sent to an ETE Unfragmentable - an IPv4 packet with DF=1, or an IPv6 packet 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]. 3. Applicability Statement SEAL inserts an additional mid-layer encapsulation when IP/*/IPv4 encapsulation is used, and appears as a subnetwork encapsulation as seen by inner layers. While the SEAL approach was motivated by the specific use case of duplicate packet detection in MANETs, the domain of applicability is not limited to the MANET problem space and extends to other subnetwork uses such as tunneling across enterprise networks, the interdomain routing core, etc. SEAL can be used as a mid-layer encapsulation above an outer UDP/IPv4 encapsulation, however the technique of concatenating the 16-bit SEAL ID Extension and the IPv4 ID (i.e., co-mingling the two identifier spaces) will not work when there are network address translators (NATs) in the path that may re-write the IPv4 ID, e.g., such as for the Teredo domain of applicability [RFC4380]. Moreover, it may not be possible to expect non-initial IPv4 fragments to pass through NATs and firewalls in all cases. A variation of this proposal that maintains separate ID spaces for the SEAL ID and IPv4 ID and that operates in the presence of NATs and firewalls will be specified in a future version of this document. Templin Expires August 17, 2008 [Page 5] Internet-Draft SEAL February 2008 The current document version speaks exclusively to the use case of encapsulation over IPv4 as the outer layer, however the same principles apply when IPv6 is the outer layer. In-the-network fragmentation is not permitted for encapsulations over IPv6, however, so the "implicit" probing capabilities specified for IPv4 in this document are not available. Still, encapsulations over IPv6 can use "explicit" probing as well as the same architectural concepts as specified for IPv4 herein. A future version of this document will address the case of IPv6 as the outer encapsulation layer in more detail. For further study, SEAL may also be useful for "transport-mode" applications, e.g., when the inner packet encapsulates ordinary protocol data rather than an IP packet. 4. SEAL Protocol Specification 4.1. Model of Operation Ingres Tunnel Endpoints (ITEs) insert a SEAL header in the IP/*/ IPv4-encapsulated packets they inject into a subnetwork, where the outermost IPv4 header contains the source and destination addresses of the ITE/ETE subnetwork entry/exit points, respectively. SEAL defines a new IP protocol type and a new mid-layer encapsulation for both unicast and multicast inner packets. The ITE inserts a SEAL header during encapsulation as shown in Figure 1: Templin Expires August 17, 2008 [Page 6] Internet-Draft SEAL February 2008 +-------------------------+ | | ~ Outer */IPv4 headers ~ | | +-------------------------+ +-- SEAL Header --+ +-------------------------+ +-------------------------+ | | | | ~ Any mid-layer * headers ~ ~ Any mid-layer * headers ~ | | | | +-------------------------+ +-------------------------+ | | | | ~ Inner IP ~ ---> ~ Inner IP ~ ~ Packet ~ ---> ~ Packet ~ | | | | +-------------------------+ +-------------------------+ | Any mid-layer trailers | | Any mid-layer trailers | +-------------------------+ +-------------------------+ | Any outer trailers | +-------------------------+ Figure 1: SEAL Encapsulation where the SEAL header is inserted as follows: o For simple IP/IPv4 encapsulations (e.g., [RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between the inner IP and outer IPv4 headers as: IP/SEAL/IPv4. o For tunnel-mode IPsec/ESP encapsulations over IPv4, [RFC4301][RFC4303], the SEAL header is inserted between the ESP and outer IPv4 headers as: IP/*/ESP/SEAL/IPv4. o For IP encapsulations over transports such as UDP (e.g., [I-D.farinacci-lisp]), the SEAL header is inserted immediately after the outer transport layer header, e.g., as IP/*/SEAL/UDP/ IPv4. Encapsulation and tunneling establishes a virtual point-to-multipoint interface abstraction of the subnetwork. From a logical viewpoint, this interface appears as a Virtual EThernet (VET) [I-D.templin-autoconf-dhcp] that connects the ITE to all ETEs in the subnetwork as single-hop neighbors. From a physical perspective, however, packets sent over the VET interface may be forwarded across many IP and/or sub-IP layer subnetwork hops. SEAL-encapsulated packets include a 32-bit SEAL-ID formed from the concatenation of the 16-bit ID Extension field in the SEAL header as Templin Expires August 17, 2008 [Page 7] Internet-Draft SEAL February 2008 the most-significant bits and with the 16-bit ID value in the outer IPv4 header as the least-significant bits. Routers use the SEAL-ID for both duplicate packet detection within the subnetwork and also for multi-level segmentation and reassembly of large packets. SEAL enables a multi-level segmentation and reassembly capability. First, the ITE can use inner IPv4 fragmentation for fragmentable inner IPv4 packets before encapsulation to avoid lower-level segmentation and reassembly. Secondly, the SEAL layer itself provides a simple mid-layer cutting-and-pasting of inner packets without incurring IPv4 fragmentation on the outer packet. Finally, ordinary IPv4 fragmentation for the outer IPv4 packet after SEAL encapsulation is permitted under certain limited and carefully managed circumstances. 4.2. Packetization 4.2.1. Packet Size Considerations 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 be delivered by the network without loss due to an MTU restriction on the path, or a suitable ICMP PTB message returned. However, PTB messages are not delivered reliably, and any PTBs received could be erroneous or maliciously fabricated. (Indeed, in the case of treating the global Internet interdomain routing core as a subnetwork, the PTB messages could come from anywhere in the Internet.) The ITE therefore requires a means for conveying 1500 byte (or smaller) original packets over the VET interface without loss due to link MTU restrictions and/or triggering PTB messages from within the subnetwork. In common deployments, there may be many forwarding hops between the source and the ITE. Within those hops, there may be additional encapsulations (IPSec, L2TP, etc.) such that a 1500 byte original packet might grow to a larger size by the time it reaches the ITE. Similarly, additional encapsulations on the path from the ITE to the ETE could cause the packet to become larger still. In order to preserve the end system expectation of delivery for 1500 byte and smaller packets, the ITE therefore requires a means for conveying this larger packet over the VET interface even though there may be subnetwork links that configure a smaller MTU. The ITE upholds the 1500-byte-and-smaller packet delivery expectation by instituting a SEAL Maximum Segment Size (S-MSS) variable (suggested default 1KB) and configurable within the range of [128 - 2KB]. The ITE also institutes a segmentation region for packet sizes Templin Expires August 17, 2008 [Page 8] Internet-Draft SEAL February 2008 [S-MSS - 2KB] such that all inner packets within this size range are segmented into multiple SEAL packets while avoiding in-the-network IPv4 fragmentation. The ITE additionally admits all inner packets larger than 2KB into the VET interface as single-segment SEAL packets under the assumption that original sources that send packets larger than 1500 bytes are using an end-to-end MTU determination capability such as specified in [RFC4821]. 4.2.2. Inner IPv4 Fragmentation The IPv4 layer of a subnetwork border router that configures an ITE fragments inner IPv4 packets larger than 2KB and with the IPv4 Don't Fragment (DF) bit set to 0 into IPv4 fragments no larger than the minimum of 2KB and S-MSS. The IP layer then submits each inner IPv4 fragment to the ITE as an independent IP packet for encapsulation. Note that inner fragmentation may not be available for certain ITE types, e.g., for tunnel-mode IPsec. Any inner IPv4 fragments created in this fashion will be reassembled by the final destination. Inner IPv4 fragmentation is not performed for inner IPv4 packets larger than 2KB and with the DF bit set to 0. Instead, these packets are encapsulated by the ITE and sent as single segments as discussed in the following section. 4.2.3. SEAL Segmentation and Encapsulation After any inner IPv4 fragmentation, the ITE encapsulates IPv4 packets/fragments no larger than 2KB in any mid-layer '*' headers, then performs SEAL segmentation on this inner packet based on a segment size that is likely to avoid IPv4 fragmentation within the subnetwork. The ITE maintains S-MSS for each ETR, e.g., as per-ETR IPv4 destination cache soft state, including IPv4 multicast destinations. S-MSS SHOULD be initialized to 1KB by default, and MAY be changed to different values in the [128 - 2KB] range based on static configuration and/or dynamic segment size probing. Note that this SEAL segmentation ignores the DF bit in the inner IPv4 header or (in the case of IPv6) ignores the fact that the network is not permitted to perform IPv6 fragmentation, but this segmentation process is a mid-layer (not an IP layer) operation and is necessary to adapt the inner packet to the subnetwork path characteristics. Moreover, the inner packet will be restored to its original form when it is removed from the subnetwork by the ETE, therefore, the fact that the packet may have been segmented within the subnetwork is not observable outside of the subnetwork. Templin Expires August 17, 2008 [Page 9] Internet-Draft SEAL February 2008 The ITE MUST NOT break unfragmentable inner packets larger than 2KB into smaller segments, but rather MUST encapsulate them as a single segment SEAL packet. The ITE breaks inner packets no larger than 2KB into N segments (N <= 16) that are no larger than S-MSS bytes each, i.e., even if the inner packet is unfragmentable. Each segment except the final one MUST be of equal length, while the final segment MAY be of different 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 encapsulates each segment in a SEAL header formatted according to the following figure: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID Extension |R|M|CTL|Segment| Next Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Minimal SEAL Header Format where the header fields are defined as follows: ID Extension (16) a 16-bit extension of the 16-bit ID field in the outer IPv4 header; encodes the most-significant 16 bits of a 32 bit SEAL-ID value. R (1) Reserved. M (1) the "More Segments" bit. Set to 1 if this SEAL packet contains a non-final segment of a multi-segment inner packet. CTL (2) a 2-bit "Control" field that identifies the type of SEAL packet as follows: '00' - a Fragmentation Report (FRAGREP). '01' - a non-probe SEAL packet. '10' - an implicit probe. Templin Expires August 17, 2008 [Page 10] Internet-Draft SEAL February 2008 '11' - an explicit probe. Segment (4) a 4-bit Segment number. Encodes a segment number between 0 - 15. Next Header (8) an 8-bit field that encodes an IP protocol number the same as for the IPv4 protocol and IPv6 next header fields. For N-segment inner packets (N <= 16), the ITE selects a SEAL header format (minimal or extended) and encapsulates each segment in a header of the same format with (M=1; Segment=0) for the first segment, (M=1; Segment=1) for the second segment, etc., with the final segment setting (M=0; Segment=N-1). Note that single-segment inner packets instead set (M=0; Segment=0). During encapsulation, the ITE also sets CTL='01' in the SEAL header of each segment if this segment is not to be used as a probe. Otherwise, the ITE sets CTL='10' or '11 according to the type of probe (see: Section 4.6). The ITE next writes either the IP protocol number corresponding to the inner packet (minimal format) or the value zero (extended format) in 'Next Header A' in the SEAL header of each segment. When extended format is used, the ITE also writes a 20-bit flow label value corresponding to the inner packet into the Flow Label field and writes the IP protocol number corresponding to the inner packet in 'Next Header B'. The ITE then encapsulates the segment in the requisite */IPv4 outer headers. The ITE maintains a 32-bit SEAL-ID value as per-ETE soft state, e.g. in the IPv4 destination cache. The value is randomly-initialized when the soft state is created and monotonically incremented (modulo 2^32) for each successive SEAL packet sent to the ETE. For each SEAL packet, the ITE writes the least-significant 16 bits of the SEAL-ID value in the ID field in the outer IPv4 header, and writes the most- significant 16 bits in the ID Extension field in the SEAL header. The ITE finally sets other fields in the outer */IPv4 headers according to the specific encapsulation format (e.g., [RFC2003], [RFC4213], etc.). 4.2.4. Sending Packets For inner packets larger than 2KB, the ITE determines whether the size of the packet plus the size of the SEAL/*/IPv4 encapsulation headers is larger than the MTU of the underlying interface over which the tunnel is configured. If the packet is too large, the ITE discards it and sends an ICMP PTB message back to the original source Templin Expires August 17, 2008 [Page 11] Internet-Draft SEAL February 2008 with an MTU value taken from the underlying interface minus the size of the encapsulating headers. Otherwise, the ITE sets the Don't Fragment (DF) bit in the outer IPv4 header to DF=1. For inner packets that were no larger than 2KB before segmentation, the ITE sets DF=0 in the outer IPv4 header SEAL packets to be used as an implicit/explicit probes (as specified in Section 4.6) and MAY set DF=1 in the outer IPv4 header of other SEAL packets once the path has been probed. After setting DF, the ITE SHOULD send all SEAL packets that encapsulate segments of the same inner packet into the VET interface in canonical order, i.e., Segment 0 first, then Segment 1, etc. 4.3. Reassembly 4.3.1. Reassembly Buffer Requirements ETEs MUST be capable of using IPv4-layer reassembly to reassemble SEAL packets of at least (2KB+ENCAPS) bytes, i.e., ETEs MUST configure an IPv4 Effective MTU to Receive (EMTU_R) of at least (2KB+ ENCAPS). ETEs MUST also be capable of using SEAL-layer reassembly to reassemble inner packets of at least 2KB, i.e., ETEs MUST configure a SEAL EMTU_R of at least 2KB. 4.3.2. IPv4-Layer Reassembly The ETE performs IPv4 reassembly as-normal, and maintains a conservative high- and low-water mark for the number of outstanding reassemblies pending for each ITE as is common for widely deployed implementations. When the size of the reassembly buffer exceeds this high-water mark, the ETE actively discards incomplete reassemblies (e.g., using an Active Queue Management (AQM) strategy such as drop- eldest, Random Early Drop (RED), etc.) until the size falls below the low-water mark. After reassembly, the ETE either accepts or discards the reassembled SEAL packet based on the current status of the IPv4 reassembly cache (congested vs uncongested). The choice of accepting/discarding a reassembly may also depend on the strength of the upper-layer integrity check if known (e.g., IPSec/ESP provides a strong upper- layer integrity check) and/or the volatility of the data (e.g., multicast streaming audio/video). In the limiting case, the ETE may choose to discard all reassembled SEAL packets after sending Fragmentation Reports (see: Section 4.4). Templin Expires August 17, 2008 [Page 12] Internet-Draft SEAL February 2008 4.3.3. SEAL-Layer Reassembly After any IPv4-layer reassembly, the ETE performs SEAL-layer reassembly for N-segment inner packets through simple in-order concatenation of the encapsulated segments from N consecutive SEAL packets. These packets contain Segment numbers 0 through N-1, and with consecutive SEAL-ID values encoded in the 32-bit concatenation of the ID Extension field in the SEAL header and the ID field in the IPv4 header. That is, for an N-segment inner packet, inner packet reassembly entails the concatenation of the segments from SEAL packets with (Segment 0, SEAL-ID i), followed by (Segment 1, SEAL-ID ((i + 1) mod 2^32)), etc. up to (Segment N-1, SEAL-ID ((i + N-1) mod 2^32)). This requires the ETE to maintain a cache of recently received SEAL packets for a hold time that would allow for reasonable inter-segment delays. Rather than set an absolute hold time, the ETE must actively discard any pending reassemblies that appear to have no opportunity for completion, e.g., when a considerable number of SEAL packets have been received before a packet that completes the pending reassembly has arrived. This assumes that any packet reordering within the subnetwork will be on the order of a small number of positions and that any gross reordering will be short-lived in nature. 4.3.4. Reassembly Integrity Checks TBD - a future version of this draft may specify an integrity check vector, inserted by the ITE during encapsulation and used by the ETE to detect packet splicing errors during IPv4 reassembly. Such an integrity check capability is specified in [I-D.templin-inetmtu], and could lead to increased packet delivery ratios if used by SEAL. 4.4. Generating Fragmentation Reports When the ETE has received at least the leading 128 bytes (or up to the end) of a SEAL packet that was delivered as multiple IPv4 fragments and with CTL='1X" in the SEAL header, it generates a Fragmentation Report (FRAGREP) message to send back over the VET interface to the original source. The ETE also generates a FRAGREP for any SEAL packet with CTL='11' even if the packet was not fragmented. When the IPv4 reassembly cache is congested, convergence time may be improved if the ETE generates the FRAGREP even before the entire SEAL packet has been reassembled, since congestion-related loss may cause some fragments to be lost. The 128 byte FRAGREP size was chosen 1) to ensure that enough header bytes are included in order to provide sufficient information to the ITE, and 2) since RFC1812-compliant Templin Expires August 17, 2008 [Page 13] Internet-Draft SEAL February 2008 routers that fragment are permitted to create the smallest fragment as the initial fragment but should minimize the number of fragments. Thus, by reassembling at least 128 bytes the ITE is likely to receive a large enough fragment to determine a reasonable S-MSS estimate. The ETE prepares the FRAGREP message by encapsulating the leading 128 bytes of the fragmented SEAL packet in an outer SEAL/*/IPv4 header. The ETE sets the IPv4 length field in the encapsulated packet/ fragment to the length of the largest IPv4 fragment received, i.e., even if the largest fragment received was not the first fragment. The ETE next sets CTL='00' and Segment=0 in the SEAL header, and sets the fields of the outer */IPv4 headers according to the specific encapsulation type. In particular, the ETE sets the destination address of the FRAGREP to the source address that was included in the IPv4 first fragment, and sets the source address of the FRAGREP to the destination address that was included in the IPv4 first fragment. If the destination address in the first fragment was multicast, the ETE instead sets the source address of the FRAGREP to an address assigned to the underlying IPv4 interface. The FRAGREP message has the following format: +-------------------------+ | | ~ Outer */IPv4 headers ~ | | +-------------------------+ | SEAL Header | | (CTL='00', Segment=0) | +-------------------------+ | | ~ Up to 128 bytes of pkt, ~ ~ with IPv4 len set to ~ | len of largest fragment | | | +-------------------------+ Figure 3: Fragmentation Report (FRAGREP) Message 4.5. Receiving Fragmentation Reports FRAGREP messages are carried in SEAL packets that set (CTL='00'; Segment=0) in their SEAL headers. When the ITE receives a potential FRAGREP message, it first verifies that the message was formatted correctly by the ETE (per Section 4.4) and confirms that the FRAGREP matches one of the implicit/explicit probes that it actually sent to the ETE by examining the encapsulated IPv4 fragment, e.g., by Templin Expires August 17, 2008 [Page 14] Internet-Draft SEAL February 2008 examining the ID fields. If the FRAGREP matches one of the ITE's explicit probes, the ITE advances its window of outstanding implicit probes. For a valid FRAGREP, if the length field in the encapsulated IPv4 fragment contains a value larger than (128+ENCAPS), the ITE sets S-MSS for this ETE to this length minus ENCAPS; otherwise, it sets S-MSS = MIN(S-MSS/2, 128) . This limited halving procedure accounts for the possibility that the ETE received the leading 128 bytes of the fragmented SEAL packet in IPv4 fragments that were significantly smaller than the path MTU. In that case, convergence to an acceptable S-MSS size may require multiple iterations of sending SEAL packets and receiving FRAGREP messages in a manner that parallels classical path MTU discovery [RFC1191], albeit with all path MTU feedback coming from the ETE and not a network middlebox. But, the limited halving procedure ensures that convergence will occur quickly even in extreme cases, while the correct MTU will normally be determined in a single iteration since routers that use IPv4 fragmentation are recommended to produce the minimum number of fragments [RFC1812]. 4.6. S-MSS Probing and Setting DF When S-MSS is larger than 128, the ITE probes the path to the ETE to detect and dampen any in-the-network IPv4 fragmentation. The ITE sets CTL='10' in the SEAL header and DF=0 in the outer IPv4 header of SEAL packets to be used as implicit probes and will receive FRAGREP messages from the ETE if any in-the-network fragmentation occurs. The ITE must also send explicit probes periodically to maintain a "window" of outstanding implicit probes. This window allows the ITE validate any FRAGREPs it receives, since any FRAGREP received for an implicit probe that was sent prior to the last successful explicit probe, or at a later time than the next SEAL packet to be sent, must be invalid. The 32 bit SEAL-ID value reported in FRAGREP messages can be used as an index into the current implicit probe window. The ITE sends explicit probes by sending single-segment SEAL packets with CTL='11' in the SEAL header and DF=0 in the IPv4 header. The ITE can also probe for larger S-MSS values by sending explicit probes with trailing padding added to create a 2KB probe. When the ETE receives an explicit probe, it will return a FRAGREP message whether or not any in-the-network fragmentation occurs, which the ITE will process exactly as for any FRAGREP per Section 4.5. The ITE can optionally send intervening SEAL packets between explicit probing intervals as implicit probes by setting DF=0, or as classical Templin Expires August 17, 2008 [Page 15] Internet-Draft SEAL February 2008 path MTU discovery probes by setting DF=1. The choice of setting DF=0/1 is based on the subnetwork trust basis for receiving ICMP PTB messages, as discussed in Section 4.7. When S-MSS=128, the ITE MUST set CTL='01' in the SEAL header of each SEAL packet that is not being used as an explicit probe such that the ETE will not generate FRAGREPs for unavoidable in-the-network fragmentation. 4.7. Processing ICMP PTBs The ITE may receive ICMP PTB messages in response to any packets that were admitted into the VET interface with DF=1. The ITE may optionally ignore, log, or honor the messages according to the subnetwork trust basis. For example, ITEs connected to subnetworks managed under a single administrative domain may be configured to honor ICMP PTBs while ITEs connected to the global interdomain routing core may be configured to ignore/log them. When ICMP PTBs are honored, the ITE: o SHOULD send translated ICMP PTB messages back to the original source (if possible) for ICMP PTBs that correspond to SEAL packets that encapsulate a segment larger than 2KB. o SHOULD treat ICMP PTBs that correspond to SEAL packets that encapsulate segments no larger than 2KB as an indication to resume probing. 5. Link Requirements Subnetwork designers are strongly encouraged to follow the recommendations in [RFC3819] when configuring link MTUs. 6. End System Requirements SEAL is a router-to-router encapsulation protocol and therefore makes no requirements for end systems. However, end systems that send unfragmentable IP packets of 1501 bytes or larger are strongly encouraged to use Packetization Layer Path MTU Discovery per [RFC4821], since the network may not always be able to return useful ICMP PTB messages. Templin Expires August 17, 2008 [Page 16] Internet-Draft SEAL February 2008 7. Router Requirements IPv4 routers observe the requirements in [RFC1812]. 8. IANA Considerations A new IP protocol number for the SEAL protocol is requested. A new IPv4 site-scoped ALL_MANET_ROUTERS multicast group is requested. 9. Security Considerations Unlike IPv4 fragmentation, overlapping fragment attacks are not possible due to the requirement that SEAL segments be non- overlapping. An amplification/reflection attack is possible when an attacker sends spoofed IPv4 fragments to an ETE, resulting in a stream of FRAGREP messages returned to a victim ITE. The encapsulated segment of the spoofed IPv4 fragment provides mitigation for the ITE to detect and discard spurious FRAGREPs. The SEAL header is sent in-the-clear (outside of any IPsec/ESP encapsulations) the same as for the IPv4 header. As for IPv6 extension headers, the SEAL header is protected only by L2 integrity checks, and is not covered under any L3 integrity checks. 10. Acknowledgments Path MTU determination through the report of fragmentation experienced by the final destination was first proposed by Charles Lynn of BBN on the TCP-IP mailing list in May 1987. An historical analysis of the evolution of path MTU discovery appears in http://www.tools.ietf.org/html/draft-templin-v6v4-ndisc-01 and is reproduced in Appendix A of this document. This work was inspired in part by discussions on the IETF MANET and IRTF RRG mailing lists in the 12/07 - 01/08 timeframe, and the author acknowledges those who participated in the discussions. The work also draws on the earlier investigations of [I-D.templin-inetmtu] which acknowledges many who contributed to the effort. Jari Arkko and Joel Halpern provided useful comments that improved the document. Templin Expires August 17, 2008 [Page 17] Internet-Draft SEAL February 2008 11. References 11.1. Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [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. 11.2. 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.farinacci-lisp] Farinacci, D., "Locator/ID Separation Protocol (LISP)", draft-farinacci-lisp-05 (work in progress), November 2007. [I-D.ietf-manet-smf] Macker, J. and S. Team, "Simplified Multicast Forwarding for MANET", draft-ietf-manet-smf-06 (work in progress), November 2007. [I-D.templin-autoconf-dhcp] Templin, F., Russert, S., and S. Yi, "MANET Autoconfiguration", draft-templin-autoconf-dhcp-11 (work in progress), February 2008. [I-D.templin-inetmtu] Templin, F., "Simple Protocol for Robust IP/*/IP Tunnel Endpoint MTU Determination (sprite-mtu)", draft-templin-inetmtu-06 (work in progress), November 2007. [MTUDWG] "IETF MTU Discovery Working Group mailing list, gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November 1989 - February 1995.". Templin Expires August 17, 2008 [Page 18] Internet-Draft SEAL February 2008 [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. [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. [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. [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. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, February 2006. [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the- Network Tunneling", RFC 4459, April 2006. [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [TCP-IP] "TCP-IP mailing list archives, http://www-mice.cs.ucl.ac.uk/multimedia/mist/tcpip, May 1987 - May 1990.". Templin Expires August 17, 2008 [Page 19] Internet-Draft SEAL February 2008 Appendix A. Historic Evolution of PMTUD (written 10/30/2002) The topic of Path MTU discovery (PMTUD) saw a flurry of discussion and numerous proposals in the late 1980's through early 1990. The initial problem was posed by Art Berggreen on May 22, 1987 in a message to the TCP-IP discussion group [TCP-IP]. The discussion that followed provided significant reference material for [FRAG]. An IETF Path MTU Discovery Working Group [MTUDWG] was formed in late 1989 with charter to produce an RFC. Several variations on a very few basic proposals were entertained, including: 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 proposal (straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990) 4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30, 1990) 5. Fragmentation avoidance by setting "IP_DF" flag on all packets and retransmitting if ICMPv4 "fragmentation needed" messages occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191] by Mogul and Deering). Option 1) seemed attractive to the group at the time, since it was believed that routers would migrate more quickly than hosts. Option 2) was a strong contender, but repeated attempts to secure an "RF" bit in the IPv4 header from the IESG failed and the proponents became discouraged. 3) was abandoned because it was perceived as too complicated, and 4) never received any apparent serious consideration. Proposal 5) was a late entry into the discussion from Steve Deering on Feb. 24th, 1990. The discussion group soon thereafter seemingly lost track of all other proposals and adopted 5), which eventually evolved into [RFC1191] and later [RFC1981]. In retrospect, the "RF" bit postulated in 2) is not needed if a "contract" is first established between the peers, as in proposal 4) and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on Feb 19. 1990. These proposals saw little discussion or rebuttal, and were dismissed based on the following the assertions: Templin Expires August 17, 2008 [Page 20] Internet-Draft SEAL February 2008 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 leaving only the final to consider. But, [FOLK] has shown that IP_ID wraparound simply does not occur within several orders of magnitude the reassembly timeout window on high-bandwidth networks. (Authors 2/11/08 note: this final point was based on a loose interpretation of [FOLK], and is more accurately addressed in [RFC4963].) Author's Address Fred L. Templin (editor) Boeing Phantom Works P.O. Box 3707 Seattle, WA 98124 USA Email: fltemplin@acm.org Templin Expires August 17, 2008 [Page 21] Internet-Draft SEAL February 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Templin Expires August 17, 2008 [Page 22]