Network Working Group F. Templin, Ed. Internet-Draft Boeing Phantom Works Intended status: Informational February 12, 2008 Expires: August 15, 2008 Subnetwork Encapsulation and Adaptation Layer draft-templin-seal-01.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 15, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract Subnetworks connect routers within a bounded region, and may also connect to other networks including the Internet. These routers forward unicast and multicast packets over paths that 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 supports simplified duplicate packet detection and accommodates links with diverse MTUs. Templin Expires August 15, 2008 [Page 1] Internet-Draft SEAL February 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 3 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 4 4. SEAL Protocol Specification . . . . . . . . . . . . . . . . . 5 4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 5 4.2. Packetization . . . . . . . . . . . . . . . . . . . . . . 6 4.2.1. Packet Size Considerations . . . . . . . . . . . . . . 6 4.2.2. Inner IPv4 Fragmentation . . . . . . . . . . . . . . . 7 4.2.3. SEAL Segmentation and Encapsulation . . . . . . . . . 7 4.2.4. Setting DF and Sending Packets . . . . . . . . . . . . 10 4.3. Reassembly . . . . . . . . . . . . . . . . . . . . . . . . 11 4.3.1. Reassembly Buffer Requirements . . . . . . . . . . . . 11 4.3.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 11 4.3.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 11 4.4. Generating Fragmentation Reports . . . . . . . . . . . . . 12 4.5. Receiving Fragmentation Reports . . . . . . . . . . . . . 13 4.6. S-MSS Probing . . . . . . . . . . . . . . . . . . . . . . 13 4.7. Processing ICMP PTBs . . . . . . . . . . . . . . . . . . . 14 5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 14 6. End System Requirements . . . . . . . . . . . . . . . . . . . 14 7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 15 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . . 16 Appendix A. Historic Evolution of PMTUD (written 10/30/2002) . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . . . 21 Templin Expires August 15, 2008 [Page 2] Internet-Draft SEAL February 2008 1. Introduction Mobile Ad-hoc Networks (MANETs) and other subnetworks connect routers on links with asymmetric reachability characteristics, and may also connect to other networks including the Internet. These routers forward unicast and multicast packets over paths 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]. The use of IPv4 encapsulation has long been considered as an alternative for introducing a well-behaved identification field useful for duplicate packet detection, such as required for Simplified Multicast Forwarding [I-D.ietf-manet-smf]. However, the 16-bit ID field in the outer IPv4 header supports only 2^16 distinct identification values and therefore does not provide sufficient space for robust duplicate packet detection over modern link technologies. Additionally, the insertion of an outer IPv4 header reduces the effective path MTU as-seen by the IP layer. 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][RFC2923][RFC4459][RFC4963]. This document proposes a Subnetwork Encapsulation and Adaptation Layer (SEAL) for the operation of IP over subnetworks that connect Ingress- and Egress Tunnel Endpoints (ITEs/ETEs). SEAL supports simple and robust duplicate packet detection, and accommodates links with diverse MTUs. SEAL additionally supports multiprotocol operation and provides extended quality of service for the protocols that use it. The SEAL protocol is specified in the following sections. 2. Terminology and Requirements The terminology of [RFC3819][RFC2501][I-D.ietf-autoconf-manetarch] is used in this document. The following abbreviations correspond to terms used within this document and elsewhere in common Internetworking nomenclature: MANET - Mobile Ad-hoc Network Subnetwork - a MANET or other network that connects (and is bounded by) ITEs and ETEs Templin Expires August 15, 2008 [Page 3] Internet-Draft SEAL February 2008 SEAL - Subnetwork Encapsulation and Adaptation Layer VET - Virtual EThernet ITE - Ingress Tunnel Endpoint ETE - Egress Tunnel Endpoint MTU - Maximum Transmission Unit S-MSS - SEAL Maximum Segment Size 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. Templin Expires August 15, 2008 [Page 4] Internet-Draft SEAL February 2008 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 ITR/ETR 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: +-------------------------+ | | ~ 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 Templin Expires August 15, 2008 [Page 5] Internet-Draft SEAL February 2008 and outer IPv4 headers as: IP/*/ESP/SEAL/IPv4. o For IP encapsulations over transports such as UDP (e.g., [RFC4380][I-D.farinacci-lisp]), the SEAL header is embedded in any middle- and outer-'*' encapsulations within the transport layer, 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 IPv4 and/or sub-IPv4 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 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 also 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 coming from within the subnetwork could be erroneous or maliciously fabricated. 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. Templin Expires August 15, 2008 [Page 6] Internet-Draft SEAL February 2008 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. 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, set to 1KB by default and configurable within the range of [128 - 2KB]. The ITE also institutes a [S-MSS - 2KB] segmentation region such that all inner packets within this size range are segmented into multiple SEAL packets. For 1500 byte and smaller inner packets/fragments, the 2KB upper bound allows for ~500 bytes of additional subnetwork encapsulation overhead on the path from the original source to the ITE. Similarly, the default 1KB lower bound allows ~500 bytes of additional encapsulation on the path between the ITE and ETE to accommodate each SEAL packet while avoiding IPv4 fragmentation along most paths within subnetwork that deploy 1500 byte links. 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 IP layer 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 2KB before any mid-layer '*' encapsulations. 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. 4.2.3. SEAL Segmentation and Encapsulation After inner IPv4 fragmentation, the ITE encapsulates the IPv4 packet/ fragment in any mid-layer '*' headers, then performs segmentation on this inner packet based on a segment size that is likely to avoid IPv4 fragmentation within the subnetwork. The ITE maintains a SEAL Maximum Segment Size (S-MSS) variable for each ETR as per-ETR IPv4 Templin Expires August 15, 2008 [Page 7] Internet-Draft SEAL February 2008 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 range [128, 2KB] based on static configuration and/or dynamic segment size probing. 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 in either minimal- or extended- formats according to the following figures: 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 A | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Minimal SEAL Header Format 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| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RSVD | Flow Label | Next Header B | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Extended 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, and must be zero. Templin Expires August 15, 2008 [Page 8] Internet-Draft SEAL February 2008 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' - an ordinary SEAL packet. '01' - a Fragmentation Report (FRAGREP). '10' - an implicit probe. '11' - an explicit probe. Segment (4) a 4-bit Segment number. Encodes a segment number between 0 - 15. Next Header A (8) an 8-bit field that encodes either an IP protocol number the same as for the IPv4 protocol and IPv6 next header fields, or the value zero. When Next Header A is non-zero, the SEAL header is in minimal format; otherwise, the SEAL header is in extended format. RSVD a 4-bit Reserved field, present only in extended format. Must be zero. Flow Label (20) a 20-bit flow label field, present only in extended format. Contains a 20-bit value corresponding to the inner packet during SEAL encapsulation. Next Header B (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='00' in the SEAL header of each segment if this segment is not to be used as an explicit or implicit probe. Otherwise, the ITE sets CTL='10' or '11' according to the type of probe desired (see: Section 4.6). The ITE next writes either the IP protocol number corresponding to Templin Expires August 15, 2008 [Page 9] Internet-Draft SEAL February 2008 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 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. Setting DF and 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 with an MTU value taken from the underlying interface minus the size of the encapsulation 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 or DF=1 in the outer IPv4 header of each SEAL packet according to the desired behavior as follows: o if the ITE is probing the path to the ETE, it MUST set DF=0 to allow the ETE to sense and report fragmentation. o if S-MSS=128, the ITE MUST set DF=0 in case any unavoidable in- the-network IPv4 fragmentation is required. o if the ITE has recently probed the path to the ETE, it MAY set DF=1 in subsequent SEAL packets until the next probing cycle. After setting the DF bits, 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. Templin Expires August 15, 2008 [Page 10] Internet-Draft SEAL February 2008 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 maintains a conservative high- and low-water mark for the number of outstanding reassemblies pending for each ITE. 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. The ETE otherwise performs IPv4 reassembly as-normal. Note that in the limiting case the ETE may choose to discard all reassemblies for packets that set CTL='1X' in the SEAL header and only perform reassembly for packets that set CTL='0X' in the SEAL header (see; Section 4.4). 4.3.3. SEAL-Layer Reassembly After 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 Templin Expires August 15, 2008 [Page 11] Internet-Draft SEAL February 2008 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. 4.4. Generating Fragmentation Reports When the ETE receives an IPv4 first fragment of a fragmented SEAL packet with (MF=1; Offset=0) in the IPv4 header and 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 an IPv4 first fragment of a SEAL packet with (MF=X; Offset=0) in the IPv4 header and CTL='11' in the SEAL header (see: Section 4.6). The FRAGREP message consists of an outer SEAL/*/IPv4 header followed by the first 68 bytes of the IPv4 first fragment. The ETE sets CTL='01' and Segment=0 in the SEAL header and sets the fields of the IPv4 header set 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='01'; Segment=0) | +-------------------------+ +-------------------------+ | | | | ~ IPv4 first fragment ~ ---> ~ First 68 bytes of IPv4 ~ ~ (R=1; Next Header!=0) ~ ---> ~ first fragment ~ | | | | +-------------------------+ +-------------------------+ Figure 4: Fragmentation Report (FRAGREP) Message Templin Expires August 15, 2008 [Page 12] Internet-Draft SEAL February 2008 4.5. Receiving Fragmentation Reports 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 corresponds to one of the SEAL packets that it actually sent to the ETE by examining the encapsulated 68 byte portion of the IPv4 first fragment. For a valid FRAGREP, if the length of the encapsulated IPv4 fragment ('LEN') minus ENCAPS is significantly larger than 128, the ITE sets S-MSS for this ETE to (LEN-ENCAPS); otherwise, it sets S-MSS = MIN(S- MSS/2, 128) . This limited halving procedure accounts for the possibility that the ETE received IPv4 first 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]. But, the limited halving procedure ensures that convergence will occur quickly even in extreme cases, while the correct MTU will be determined in a single iteration under normal circumstances in which routers produce large first fragments. Note that multiple FRAGREP messages may be received for SEAL packets that encapsulate segments of the same inner packet. In that case, the ITE should set S-MSS to the minimum length reported in all FRAGREP messages. If multiple FRAGREP messages corresponding to the same inner packet report small MTUs, however, the ITE should only halve the current S-MSS once; not multiple times. 4.6. S-MSS Probing When S-MSS is larger than 128, the ITE MUST probe the path to the ETE periodically to detect and dampen any in-the-network IPv4 fragmentation. The ITE performs implicit probing of the path by setting CLT='10' in the SEAL header and DF=0 in the IPv4 header of all SEAL packets of the same inner packet(s). If any in-the-network fragmentation occurs, the ITE will receive authentic FRAGREP messages from the ETE. The ITE can also send explicit probes to periodically probe for larger S-MSS values (to a maximum of 2KB) by sending single-segment SEAL packets with CTL='11' in the SEAL header and DF=0 in the IPv4 header. The ETE 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 MAY pad the length of SEAL packets used for explicit probing (to a maximum size of 2KB+ ENCAPS) if permitted by the specific */IPv4 encapsulation method. Templin Expires August 15, 2008 [Page 13] Internet-Draft SEAL February 2008 The ITE can optionally send intervening SEAL packets between probing intervals as passive probes by setting DF=0, or as non-probes by setting DF=1. When S-MSS=128, the ITE MUST set CTL='00' 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 managed subnetworks 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 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 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 15, 2008 [Page 14] Internet-Draft SEAL February 2008 7. Router Requirements IPv4 routers observe the requirements in [RFC1812]. However, when a router fragments an IPv4 datagram that may contain an encapsulated SEAL packet, the fragmentation MUST cause the first fragment to be at least 68 bytes (i.e., the minimum IPv4 MTU [RFC0791]). This document therefore updates [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 with CTL='1X' in the SEAL header, resulting in a stream of FRAGREP messages returned to a victim ITE. The encapsulated 68 byte 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 Templin Expires August 15, 2008 [Page 15] Internet-Draft SEAL February 2008 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. The extended SEAL header format was inspired by recent discussions. 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-autoconf-manetarch] Chakeres, I., Macker, J., and T. Clausen, "Mobile Ad hoc Network Architecture", draft-ietf-autoconf-manetarch-07 (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 Templin Expires August 15, 2008 [Page 16] Internet-Draft SEAL February 2008 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.". [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. [RFC2501] Corson, M. and J. Macker, "Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations", RFC 2501, January 1999. [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. Templin Expires August 15, 2008 [Page 17] Internet-Draft SEAL February 2008 [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.". 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 Templin Expires August 15, 2008 [Page 18] Internet-Draft SEAL February 2008 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 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].) Templin Expires August 15, 2008 [Page 19] Internet-Draft SEAL February 2008 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 15, 2008 [Page 20] 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. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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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 15, 2008 [Page 21]