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I-D Internet-Draft The Internet Protocol, version 4 (IPv4) header includes a 16-bit Identification field in all packets, but this length is too small to ensure reassembly integrity even at moderate data rates in modern networks. Even for Internet Protocol, version 6 (IPv6), the 32-bit Identification field included when a Fragment Header is present may be smaller than desired for some applications. This specification addresses these limitations by defining an IPv6 Hop-by-Hop option for Identification Extension; it further defines control messaging services for fragmentation and reassembly congestion management.

The Internet Protocol, version 4 (IPv4) header includes a 16-bit Identification in all packets , but this length is too small to ensure reassembly integrity even at moderate data rates in modern networks . For Internet Protocol, version 6 (IPv6), the Identification field is only present in packets that include a Fragment Header where its standard length is 32-bits , but even this length may be too small for some applications. This document therefore defines a means to extend the IPv6 Identification length through the introduction of a new IPv6 Hop-by-Hop option. IPv6 Identification Extension may be useful for networks that engage fragmentation and reassembly at extreme data rates, or for cases when advanced packet Identification uniqueness assurance is critical. The specification further defines a messaging service for adaptive realtime response to congestion related to fragmentation and reassembly. Together, these extensions support robust fragmentation and reassembly services as well as packet Identification uniqueness for IPv6.

This document uses the term "IP" to refer generically to either protocol version (i.e., IPv4 or IPv6), and uses the term "IP ID" to refer generically to the IP Identification field whether in simple or extended form. The terms "Maximum Transmission Unit (MTU)", "Effective MTU to Receive (EMTU_R)", "Effective MTU to Send (EMTU_S)" and "Maximum Segment Lifetime (MSL)" are used exactly the same as for standard Internetworking terminology . The term MSL is equivalent to the term "maximum datagram lifetime (MDL)" defined in . The term "Packet Too Big (PTB)" refers to an IPv6 "Packet Too Big" message. The term "flow" refers to a sequence of packets sent from a particular source to a particular unicast, anycast or multicast destination . The term "source" refers to either the original end system that produces an IP packet or an encapsulation ingress intermediate system on the path. The term "destination" refers to either a decapsulation egress intermediate system on the path or the final end system that consumes an IP packet. The term "intermediate system" refers to a node on the path from the (original) source to the (final) destination that forward packets not addressed to itself. Intermediate systems that decrement the IP header TTL/Hop Limit are also known as "routers". The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Studies over many decades have shown that upper layer protocols often achieve greater performance by configuring segment sizes that exceed the path Maximum Transmission Unit (MTU). When the segment size exceeds the path MTU, IP fragmentation at some layer is a natural consequence. However, the 4-octet (32-bit) IPv6 Identification field may be too small to ensure reassembly integrity at sufficiently high data rates, especially when the source resets the starting sequence number often to maintain an unpredictable profile . This specification therefore proposes to fortify the IP ID by extending its length. A recent study proved that configuring segment sizes that cause IPv4 packets to exceed the path MTU (thereby invoking IPv4 fragmentation and reassembly) provides substantial performance increases at high data rates in comparison with using smaller segment sizes as long as fragment loss is negligible. This contradicts decades of unfounded assertions to the contrary and validates the Internet architecture which includes fragmentation and reassembly as core functions. An alternative to extending the IP ID was also examined in which IPv4 packets were first encapsulated in IPv6 headers then subjected to IPv6 fragmentation where a 4-octet Identification field already exists. While this IPv4-in-IPv6 encapsulation followed by IPv6 fragmentation also showed a performance increase for larger segment sizes in comparison with using MTU-sized or smaller segments, the magnitude of increase was significantly smaller than for invoking IP fragmentation directly without first applying encapsulation. Widely deployed implementations also often employ a common code base for both IPv4 and IPv6 fragmentation/reassembly since their algorithms are so similar. It therefore follows that IPv4 fragmentation and reassembly can support higher data rates than IPv6 when full (uncompressed) headers are used, while the rates may be comparable when IPv6 header compression is applied. In addition to accommodating higher data rates in the presence of fragmentation and reassembly, extending the IP ID can enable other important services. For example, an extended IP ID can support a duplicate packet detection service in which the network remembers recent IP ID values for a flow to aid detection of potential duplicates (note however that the network layer must not incorrectly flag intentional lower layer retransmissions as duplicates). An extended IP ID can also provide a packet sequence number that allows communicating peers to exclude any packets with IP ID values outside of a current sequence number window for a flow as potential spurious transmissions. For these reasons, it is clear that a robust IP fragmentation and reassembly service can provide a useful tool for performance maximization in the Internet and that an extended IP ID can provide greater uniqueness assurance. This document therefore presents a means to extend the IPv6 ID to better support these services.

For a standard 4-octet IPv6 Identification value, the source can simply include an ordinary IPv6 Fragment Header as specified in with the "Fragment Offset" field and "M" flag set either to values appropriate for a fragmented packet or the value '0' for an unfragmented packet. The source then includes a 4-octet Identification value for the packet. For an extended Identification, this specification permits the source to also include a new IPv6 ID Extension Hop-by-Hop option formatted as shown in :

The option is therefore identified by Option Type TBD1 (see: IANA Considerations) and the extended Identification is 4, 8, 12 or 16 octets in length with the portion encoded in the HBH option IPv6 ID Extension field as the most significant "Opt Data Len" octets and the portion encoded in the IPv6 Fragment Header Identification field as the least significant 4 octets. For IPv6 packets that include both an IPv6 ID Extension HBH option and a standard IPv6 Fragment Header, this document further defines a new coding for the 3 least significant ("flag") bits of the Fragment Header control field as shown in :

In this new coding, Bit 31 remains as the "More Fragments (MF)" flag, while bit 30 is re-defined as the "Permit Fragmentation (PF)" flag and bit 29 remains as a "Reserved Fragmentation (RF)" flag. For IPv6 packets that include the IPv6 ID Extension HBH option, when bit 30 of the Fragment Header control field is set to 0, network intermediate systems are not permitted to fragment the packet; otherwise, network fragmentation is permitted. Bit 29 is set to 0 on transmission and ignored on reception. Note: packets that include an IPv6 ID Extension HBH option with Opt Data Len set to 0 will still influence the fragmentation flags as above even though the IP ID "extension" is 0 octets in length.

When an IPv6 network intermediate system forwards a packet that includes both an IPv6 ID Extension HBH option and standard Fragment Header, it performs (further) fragmentation if the next hop link MTU is insufficient and if the "Permit Fragmentation (PF)" flag is set to '1' (see: ). The "Permit Fragmentation (PF)" flag for IPv6 therefore provides a "network fragmentation permitted" indication in the opposite sense of the IPv4 "Don't Fragment (DF)" flag. When PF is set to 1, IPv6 intermediate systems are permitted to apply fragmentation on packets that already include an IPv6 ID Extension HBH option and an IPv6 Fragment Header, but never to insert these headers themselves. This specification therefore updates by permitting network fragmentation for IPv6 under the above conditions.

The source must have assurance that each destination will recognize and accept the IPv6 ID Extension HBH option before it can assume the extended IP ID uniqueness properties. This can be through operational assurances or protocol message feedback. An operational assurance example is through application of a common encapsulation format shared by all nodes within a limited domain network for which recognition of the IPv6 ID Extension HBH option is mandatory (e.g., see: ). The scope of the extended IP ID uniqueness then applies only within the boundaries of the limited domain. For open Internetworks and/or other networks with no operational assurances, the source must instead receive continuous feedback proving that the destination recognizes and applies the IPv6 ID Extension HBH option as long as the source sends packets/fragments with extended IP IDs at high data rates. (Note that continuous feedback rather than a one-time test is necessary in case the destination is anycast and routing redirects traffic to a new anycast destination that does not recognize the option.) As an IPv6 HBH option, the highest-order 2 bits of the Option Type TBD1 for the IPv6 ID Extension HBH option MUST always encode '00' (skip over this option and continue processing the header) while the third-highest-order bit is normally set to '0' (Option Data does not change en route). To receive continuous feedback from the destination, the source can periodically set the third-highest-order bit to '1' in a packet/fragment to be used as a probe. If the source receives an ICMPv6 Parameter Problem message with Code TBD2 (see: IANA Considerations), it has assurance that the destination (still) correctly recognizes the option. Destinations that receive IPv6 packets/fragments with an IPv6 ID Extension HBH option simply ignore the option if they do not recognize it. Destinations that recognize the option instead examine the third-highest-order bit. If the bit is set to '1', the destination returns an ICMPv6 Parameter Problem message with Code TBD2 to the source. The destination then drops the packet/fragment if it includes an Authentication Header; otherwise it accepts the packet/fragment for further processing. Note: When the source sends a packet/fragment with both an Authentication Header and an IPv6 ID Extension HBH option with the third-highest-order bit set to '1', it prepares the packet as a NULL packet (e.g., one with final IPv6 Next Header field set to 'No Next Header', one with {TCP,UDP} port set to 'Discard', etc.). The destination will then unconditionally discard the packet (after returning an ICMPv6 Parameter Problem message if necessary) whether or not it recognizes the HBH option. Note: intermediate systems forward without dropping any IPv6 packets/fragments that contain an IPv6 ID Extension HBH option whether or not they recognize the option. Each packet/fragment that arrives at the destination will therefore always include the HBH option intended by the original source. Note: destinations that recognize and accept the IPv6 ID Extension HBH option are required to configure an EMTU_R of 65535 octets (see: ). Therefore, sources that receive ICMPv6 Parameter Problem messages with code TBD2 can also regard them as confirmation of the destination's EMTU_R.

When an intermediate system attempts to forward an IP packet that exceeds the next hop link MTU but for which fragmentation is forbidden, it returns a "Packet Too Big (PTB)" message to the source and discards the packet. This always results in wasted transmissions by the source and all intermediate systems on the path toward the one with the restricting link. Conversely, when network fragmentation is permitted the network will perform (further) fragmentation if necessary allowing the packet to reach the destination without loss due to a size restriction. This results in an internetwork that is adaptive to dynamic MTU fluctuations and not subject to wasted transmissions. While the fragmentation and reassembly processes eliminate wasted transmissions and support significant performance gains by accommodating upper layer protocol segment sizes that exceed the path MTU, the processes sometimes represent pain points that should be communicated to the source. The source should then take measures to reduce the size of the packets/fragments that it sends. The IPv6 PTB format includes a "Code" field set to the value '0' for ordinary PTB messages. The value '0' signifies a "classic" PTB and always denotes that the subject packet was unconditionally dropped due to a size restriction. For end systems and intermediate systems that recognize IP ID extensions according to this specification, the following additional PTB unused/Code values are defined: Sent by an intermediate system (subject to rate limiting) when it performs fragmentation on a packet with an extended IP ID. The intermediate system sends the PTB message while still fragmenting and forwarding the packet. The MTU field of the PTB message includes the maximum fragment size that can pass through the restricting link as an indication for the source to reduce its (source) fragment sizes. This size will often be considerably smaller than the current EMTU_R advertised by the destination. The same as for Code 1, except that the intermediate system drops the packet instead of fragmenting and forwarding. This message type represents a hard error that indicates loss. In one possible strategy, the intermediate system could begin sending Code 1 PTBs then revert to sending Code 2 PTBs if the source fails to reduce its fragment sizes. Sent by the destination (subject to rate limiting) when it performs reassembly on a packet with an extended IP ID during periods of reassembly congestion. The destination sends the PTB message while still reassembling and accepting the packet. The MTU field of the PTB message includes the largest possible EMTU_R value under current reassembly buffer congestion constraints as an indication for the source to begin sending smaller packets if necessary. This size will often be considerably larger than the path MTU and must be no smaller than the IPv6 minimum EMTU_R. The same as for Code 3, except that the destination drops the packet instead of reassembling and accepting. This message type represents a hard error that indicates loss. In one possible strategy, the destination could begin sending Code 3 PTBs then revert to dropping packets while sending Code 4 PTBs if the source fails to reduce its packet sizes.

IPv6 intermediate systems MUST forward without dropping packets that include an IPv6 ID Extension HBH option. Sources MUST include at most one IPv6 ID Extension HBH option plus at most one standard IPv6 Fragment Header in each packet. Intermediate systems and destinations SHOULD silently drop packets with multiples. Sources MUST transmit and destinations MUST process the octets of the extended IP ID in network byte order with the base IP ID field containing the least significant octets and the ID Extension field containing the most significant octets. Sources MUST include a constant IPv6 ID Extension HBH Option Data Length value of 0/4/8/12 octets for all packets sent to the same destination. Destinations MUST process all IPv6 ID Extension HBH options with Option Data Length values of 0/4/8/12 octets while silently dropping any packets with other values. Destinations MUST be capable of reassembling packets as large as the minimum Effective MTU to Receive (EMTU_R) specified for IPv6 (, section 5). Destinations that accept flows using extended IP IDs: MUST configure a minimum EMTU_R of 65535 octets, SHOULD advertise the largest possible EMTU_R in PTB messages and MAY advertise a reduced EMTU_R during periods of congestion. While a source has assurance that the destination(s) will recognize and correctly process extended IP IDs, it can continue to send fragmented or fragmentable packets as large as the EMTU_R at rates within the MSL/MDL wraparound threshold for the extended IP ID length; otherwise, the source honors the MSL/MDL threshold for the non-extended Identification field length . Note: IP fragmentation can only be applied for conventional packets as large as 65535 octets. IP parcels and Advanced Jumbos (AJs) provide a means for efficiently packaging and shipping multiple large segments or truly large singleton segments in packets that may exceed this size .

During the earliest days of internetworking, researchers asserted that fragmentation should be deemed "harmful" based on empirical observations in the ARPANET, DARPA Internet and other internetworks of the day . These assertions somehow inspired an engineering discipline known as "Path MTU Discovery" within a new community that evolved to become the Internet Engineering Task Force (IETF). In more recent times, the IETF amplified these assertions in "IP Fragmentation Considered Fragile" . Rather than encourage timely course corrections, however, the IETF somehow forgot that IP fragmentation and reassembly still serve as essential internetworking functions. This has resulted in a modern Internet where path MTU discovery (including its recent derivatives) provides a poor service for conventional packet sizes especially in dynamic networks with path MTU diversity. This document introduces a more robust solution based on a properly functioning IP fragmentation and reassembly service as intended in the original architecture. Although the IP fragmentation and reassembly services provide an appropriate solution for conventional packet sizes as large as 65535 octets, the services cannot be applied for IP parcels and AJs that exceed this size. Instead, modern path MTU discovery methods provide the only possible solution to accommodate these larger sizes. This means that a combined solution with fragmentation and reassembly applied for conventional packets and path MTU discovery applied for jumbos provides the necessary combination for Internetworking futures. This document therefore updates .

In progress.

The IANA is requested to assign a new IPv6 Hop-by-Hop Option type "TBD1" in the 'ipv6-parameters' registry (registration procedures IESG Approval, IETF Review or Standards Action). The option sets "act" to '00', "chg" to '0' and "rest" to TBD1 in a first line, and sets "act" to '00', "chg" to '1' and "rest" to TBD1 in the next line of the registry table. Both lines set "Description" to "IPv6 ID Extension" and "Reference" to this document [RFCXXXX]. The IANA is further requested to assign a new Code value "TBD2" in the "ICMPv6 Code Fields: Type 4 - Parameter Problem" table of the 'icmpv6-parameters' registry (registration procedures Standards Action or IESG Approval). The registration sets "Code" to TBD2 and "Name" to "Immutable option data may change" with "Reference" set to this document [RFCXXXX]. The IANA is further instructed to assign new Code values in the "ICMPv6 Code Fields: Type 2 - Packet Too Big" table of the 'icmpv6-parameters' registry (registration procedure is Standards Action or IESG Approval). The registry should appear as follows:

All aspects of IP security apply equally to this document, which does not introduce any new vulnerabilities. Moreover, when employed correctly the mechanisms in this document robustly address known IP reassembly integrity concerns and also provide an advanced degree of packet Identification uniqueness assurance.

This work was inspired by continued DTN performance studies. Amanda Baber, Tom Herbert, Bob Hinden and Eric Vyncke offered useful insights that helped improve the document. Honoring life, liberty and the pursuit of happiness.

For paths that reject packets that include certain IPv6 extension headers or options, an encapsulation service is necessary to avoid middlebox filtering. The source can elect to engage encapsulation if the first alternative IP ID extension method fails or advance immediately to engaging encapsulation if it has reason to believe there is better opportunity for success. The encapsulation employs UDP, IP and/or Ethernet headers recognized by intermediate/end systems within a limited domain . The OMNI specification defines an encapsulation format in which UDP/IP headers that use UDP port 8060 serve as a "Layer 2 (L2)" encapsulation for OMNI IPv6-encapsulated IP packets. The UDP header is then followed by a chain of IPv6 extension headers including a Fragment Header and a HBH header with an IPv6 ID Extension option which together form an extended IP ID. The extension header chain is then followed by an OMNI IPv6 encapsulation header in full/compressed form followed by any OMNI IPv6 extensions followed by the original IP packet as shown in :

The OMNI interface encapsulates each original IP packet in an IPv6 encapsulation header as an OMNI Adaptation Layer (OAL) encapsulation. The interface next encapsulates this "OAL packet" in UDP/IP headers as "L2" encapsulations. When the packet requires L2 fragmentation and/or any other extension header processing, the OMNI interface instead performs the following operations: If the L2 header is IPv4 or Ethernet, convert it to an IPv6 header while converting the IPv4/EUI source and destination addresses to IPv4-compatible IPv6 addresses per . Encapsulate the OAL packet in this L2 IPv6 header with any necessary IPv6 extension headers per , then perform normal extension header processing including fragmentation per . Each resulting IPv6 fragment will include both an IPv6 Fragment Header and HBH header with an IPv6 ID Extension option with the correct fragmentation parameters. For each fragment, insert a UDP header between the L2 IPv6 header and L2 IPv6 extension headers, then adjust the Next Header field of each successive extension header per . If the original L2 header was IPv4 or Ethernet, re-convert the IPv6 header back to IPv4/Ethernet. If the L2 header was IP, change {Protocol, Next Header} to '17' (UDP), set the remaining UDP/IP header fields to the correct values for each fragment, then transmit each fragment to the L2 destination. When the L2 destination receives these (concealed) fragments, it first notices the OMNI-encoded L2 IPv6 extension headers immediately following the L2 OMNI UDP header. The destination then removes the L2 UDP header and (for IPv4) also converts the L2 IPv4 header to IPv6. The destination then applies any necessary OMNI L2 IPv6 extension header processing, including reassembly. Following reassembly, the destination discards the L2 headers to arrive at the original OAL packet/fragment for further processing by the adaptation layer. For L2 encapsulations that do not include a UDP header (e.g., IP-only), these fragments will include the L2 IPv6 extension headers immediately after the L2 IP header. The L2 IP header must then set its IP {Protocol, Next Header} to the protocol number reserved for OMNI . For L2 encapsulations that do not include UDP/IP headers (e.g., Ethernet-only), these fragments will include the L2 IPv6 extension headers immediately after the true L2 header. The L2 header must then set its L2 type to the EtherType reserved for OMNI . Note: on the wire, these encapsulated IPv6 fragments will include an extended IP ID but will appear as ordinary packets to network middleboxes that inspect headers. This allows network middleboxes to make consistent forwarding decisions for each fragment of the same original OAL packet and without first attempting virtual fragment reassembly since each fragment appears as a whole packet. Note: the above procedures can also be applied to ordinary TCP/UDP datagrams. In that case, the L2 IPv6 extension headers are immediately followed by a TCP/UDP header instead of an OMNI IPv6 encapsulation header.

Although unicast flows are assumed throughout this document, similar considerations apply for flows in which the destination is a multicast group or an anycast address. In order to send fragmented/fragmentable packets with IP ID extensions to a multicast group, the source must have prior assurance that all group members will correctly recognize and process them. This assurance is normally through use of a UDP port number, IP protocol number and/or Ethernet type encapsulation for which extended IP ID processing is mandatory (see: ). When a source sends fragmented/fragmentable packets with extended IP IDs to a multicast group, the packets/fragments may be replicated in the network such that a single transmission may reach N destinations over as many as N different paths. Intermediate systems in each such path may return a Code 1/2 PTB message if (further) fragmentation is needed, and each such destination may return a Code 3/4 PTB message if it experiences reassembly congestion. While the source receives these PTB messages, it should reduce the fragment/packet sizes that it sends to the multicast group even if only one or a few paths or destinations are currently experiencing congestion. This means that transmissions to a multicast group will converge to the performance characteristics of the lowest common denominator group member destinations and/or paths. When a source sends fragmented/fragmentable packets with extended IP IDs to an anycast address, routing may direct initial fragments of the same packet to a first destination that configures the address while directing the remaining fragments to other destinations that configure the address. These wayward fragments will simply result in incomplete reassemblies at each such anycast destination which will soon purge the fragments from the reassembly buffer. The source will eventually retransmit, and all resulting fragments should eventually reach a single reassembly target. Note: the source must not send fragmented/fragmentable packets that include an extended IP ID to a multicast group or anycast address for which it does not have prior assurance that all potential recipients will recognize them. Otherwise, some recipients may correctly apply the IP ID extensions while others silently ignore them and may become subject to reassembly corruption.

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