Boeing Research & Technology

P.O. Box 3707 Seattle WA 98124 USA fltemplin@acm.org

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 intended uses. This specification addresses these limitations by defining both an IPv4 header option and an IPv6 Extended Fragment Header for Identification Extension.

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 . This document defines a new option for IPv4 that extends the Identification field to 32-bits (i.e., the same as for IPv6 packets that include a standard Fragment Header ) to support reassembly integrity at higher data rates. When an IPv4 packet includes this "Identification Extension" option, the value encoded in the IPv4 header Identification field represents the 2 least-significant octets while the option encodes the 2 most-significant octets of an extended 4-octet Identification. Hosts and routers that recognize the option employ it for packet identification purposes in general and to fortify the IPv4 reassembly procedure in particular. This specification also supports an "advanced" mode that extends the Identification field further for both IPv4 and IPv6. This format may be useful for networks that operate at extreme data rates, or for other cases when packet Identification uniqueness assurance is critical. Together, these extensions ensure robust reassembly integrity as well as packet Identification uniqueness for the Internet.

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. 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)" and "Maximum Segment Lifetime (MSL)" are used exactly the same as for standard Internetworking terminology. The term "Packet Too Big (PTB)" refers to either an IPv6 "Packet Too Big" or an IPv4 "Destination Unreachable - Fragmentation Needed" message.

Studies over many decades have shown that upper layer protocols can 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 2-octet (16-bit) IPv4 and 4-octet (32-bit) IPv6 Identification fields may be too short to support reassembly integrity at sufficiently high data rates. 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 a multiplicative performance increase 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 original design of the Internet 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 less 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 seems reasonable to conclude that IPv4 fragmentation and reassembly can support higher data rates than IPv6 when full (uncompressed) headers are used, while the rates may be similar when IPv6 header compression is applied. In addition to enabling 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 as potential spurious transmissions. These and other cases also hold true even if the source frequently resets its starting IP ID sequence numbers to maintain an unpredictable profile. 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 IP ID to better support these services.

IP ID extension for IPv4 is achieved by introducing a new IPv4 option. This new IPv4 ID Extension (IDEXT) Option begins with an option-type octet with "copied flag" set to '1', "option class" set to '00' and "option number" set to TBD. The option-type octet is followed immediately by an option-length octet set to the constant value '4'. The option-type is then followed by a 2-octet "ID Extension" field that (when combined with the 2 least-significant octets found in the IPv4 packet header Identification field) includes the 2 most-significant octets of an extended 4-octet (32-bit) IP ID for the packet. The option format is shown in :

When an IPv4 source node (i.e., an original source or an IPv4 encapsulation ingress) wishes to supply a 4-octet extended IP ID for the packet, it includes an IDEXT option in the IPv4 packet header options area, i.e., based on the same rules as for including any IPv4 option. The source next writes the 2 least-significant octets in the IPv4 header Identification field and writes the 2 most-significant octets in the "ID Extension" field. The source then applies source fragmentation if necessary while including the extended IP ID value. During fragmentation, the source copies the ID Extension option into each resulting fragment and sets or clears the "Don't Fragment (DF)" flag as desired. The source then forwards each packet/fragment to the next hop, where each successive IPv4 forwarding hop will direct them toward the final destination. If an IPv4 router on the path needs to apply network fragmentation, it copies the IDEXT option into each resulting fragment to provide the final destination with the correct reassembly context.

When an IPv4 source produces a sustained burst of IPv4 packets that use the same source address, destination address and protocol at extreme data rates (e.g., in excess of 1Tbps), or when the source plans to reset the IP ID starting sequence to a new pseudo-random value frequently, it can optionally extend the IP ID even further by supplying an 8-octet (64-bit), 12-octet (96-bit) or 16-octet (128-bit) value instead of a 2/4-octet value. To apply these longer extensions, the source includes an IDEXT option with option-type set to TBD the same as above, but with option-length ("optlen") set to '8', '12' or '16' instead of '4' as shown in :

The option will then include a 6, 10 or 14-octet ID Extension, with the most significant IP ID octets appearing in network byte order in the option-data and with the 2 least significant octets appearing in the IPv4 Identification field. For a 6-octet extension, the 8-octet IP ID can then fit properly within the longest word length for modern 64-bit architectures.

Techniques that improve IPv4 often also apply in a corresponding fashion for IPv6 (and vice-versa). The same is also true for IPv6 ID Extensions. For a simple 4-octet Identification value in IPv6, the original source can simply include a standard 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 advanced 4-octet Identification as well as for 8-octet or 12-octet Identification values, this document defines a new IPv6 Extended Fragment Header. The IPv6 Extended Fragment Header is identified by the Next Header type value 'TBD2' (see: IANA Considerations) and the format is shown in :

In the above format, the control values in the first four octets are interpreted exactly the same as for the standard IPv6 Fragment Header, while the Identification field is 12 octets in length and encodes a 96-bit value in network byte order. (When only a 32/64-bit Identification is needed, the original source sets the most significant 64/32 Identification bits to '0'.) For both the Standard and Extended IPv6 Fragment Header, this document further specifies a new coding for the 3 least significant ("flag") bits of the 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. When bit 30 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.

limits the use of the IPv4 ID field to only supporting the fragmentation and reassembly processes. When an IPv4 packet includes a TBD option, however, the source asserts that the IPv4 ID includes a well-managed extended-length value that can satisfy uniqueness properties useful for other purposes. This specification therefore updates by permitting use of the extended IPv4 ID for purposes other than to support fragmentation and reassembly.

specifies procedures for fragmenting and reassembling the constituent packets derived from IP parcels that have been opened somewhere along the path. Since each packet derived from the same parcel shares the same Identification value, an ancillary (Parcel) Index field is also necessary to differentiate the packets. The IPv6 Fragment Header includes an 8-bit Reserved field that can be re-purposed to encode an Index, but the IPv4 header does not provide sufficient space. With reference to and , this document therefore specifies the following IPv4 ID Parcel Index extension octet:

When the IPv4 TBD option-length is '5', the option-data includes 2 Identification extension octets followed by the Parcel Index extension octet. When option-length is '9', '13', or '17', the option-data instead includes 6, 10 or 14 Identification extension octets followed by the Parcel Index extension octet. The Parcel Index octet field names and descriptions appear in .

When an IPv6 network intermediate system forwards a packet that includes an (Extended) Fragment Header, it applies (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 (Extended) Fragment Header, but never to insert an (Extended) Fragment Header themselves. This specification therefore updates by permitting network fragmentation for IPv6 under the above conditions.

When a node forwards an IP packet that exceeds the next hop link MTU but for which fragmentation is forbidden, it drops the packet and returns a "Packet Too Big (PTB)" message to the original source . This always results in wasted transmissions by the source and all intermediate systems on the path toward the node with the restricting link. Conversely, when a packet includes an extended IP ID the network will perform (further) fragmentation if necessary allowing the packet to reach the final 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 retransmissions and can also support significant performance gains through transmissions of upper layer protocol segment sizes that exceed the path MTU, the processes sometimes represent pain points that should be communicated to the original source. The original source should then take measures to reduce the size of the packets or fragments that it sends. The IPv4 PTB format includes an "unused" field while the IPv6 PTB format includes a "Code" field, with both fields 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 IPv4 packets that include an IDEXT option and IPv6 packets that include an extended Fragment Header, other PTB unused/Code values are defined as follows: 1 - Sent by an intermediate system (subject to rate limiting) when it performs fragmentation on an IP packet with an extended IP ID (or a UDP port, IP protocol or Ethernet type known to accept these messages such as OMNI). The 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 begin applying source fragmentation. This size will often be considerably smaller than the current EMTU_R advertised by the final destination. 2 - 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. 3 - Sent by the final destination (subject to rate limiting) when it performs reassembly on a packet with an extended IP ID (or a UDP port, IP protocol or Ethernet type known to accept these messages such as OMNI). 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. This size will often be considerably larger than the path MTU and must be no smaller than the IP protocol version minimum EMTU_R. 4 - The same as for Code 3, except that the final destination drops the packet instead of reassembling and accepting. This message type represents a hard error that indicates loss. In one possible strategy, the final destination could begin sending Code 3 PTBs then revert to sending Code 4 PTBs if the source fails to reduce its packet sizes. 5 - Parcel Report - Sent by an intermediate system or final destination in response to an IP parcel that triggered the event (see: ). 6 - Jumbo Report - Sent by an intermediate system or final destination in response to an IP advanced jumbo that triggered the event (see: ). When an original source receives an authentic Code 1 or 2 PTB, it begins to perform source fragmentation using the fragment size found in the MTU field which may be smaller than the first hop link MTU. This reduces the burden on intermediate systems in the path which will see a reduced dependence on network fragmentation. When an original source receives a Code 3 or 4 PTB, it reduces the size of the packets it sends based on the EMTU_R size found in the MTU field which may be larger than the path MTU. This reduces the burden on the final destination which will see a reduced dependence on reassembly. When an original source that has no prior agreement with the final destination ceases to receive Code 3 or 4 PTB messages, it must assume that the destination no longer recognizes IP ID extensions and must then impose rate limiting based on the wraparound threshold for a non-extended Identification within the MSL . These rate limitations can be relaxed when the source can include an integrity check which the destination can verify. When an intermediate system or destination returns a Code 1-6 PTB, it prepares an ICMPv6 PTB message and with MTU set as discussed above. The node then writes its own IP address as the PTB source and writes the source address of the packet that invoked the report as the PTB destination (for IPv4, the node writes the PTB address as an IPv4-Compatible IPv6 address ). The node next copies as much of the leading portion of the invoking packet as possible (beginning with the IP header) into the "packet in error" field without causing the entire PTB (beginning with the IPv6 header) to exceed 512 octets in length. The node then sets the ICMPv6 Checksum field to 0 instead of calculating and setting a true checksum since the UDP checksum (see below) already provides an integrity check. Since IPv6 packets cannot transit IPv4 paths, and since middleboxes often filter ICMPv6 messages as they transit IPv6 paths, the node next wraps the ICMPv6 PTB message in UDP/IP headers of the correct IP version with the IP source and destination addresses copied from the PTB and with UDP port numbers set to the OMNI UDP port number . The node then calculates and sets the UDP Checksum (and for IPv4 clears the DF bit). The node finally sends the prepared PTB to the original source. Note: Original sources that send packets with extended IP IDs must be capable of accepting and processing these OMNI protocol UDP messages. A source that sends packets with extended IP IDs must therefore implement enough of the OMNI interface to be able to recognize and process these messages.

IPv4 routers MUST forward without dropping any packets with IPv4 option-type TBD while copying the option during network fragmentation, and IPv6 routers MUST forward without dropping any packets with IPv6 Next Header type TBD2. IPv6 sources MUST include at most one Standard or Extended Fragment Header in a single packet. IPv6 intermediate systems and destination MUST silently drop any packets that include multiple Fragment Headers whether Standard or Extended. Destinations that recognize IPv4 option-type TBD MUST accommodate packets that include all (advanced) extended IP ID formats based on any 4/8/12/16-octet value included by the source. 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. Implementations maintain the IP ID as a 16-octet (128-bit) integer with any most significant octets not included in an extension set to 0. Destinations MUST be capable of reassembling packets as large as the minimum Effective MTU to Receive (EMTU_R) specified for IPv4 (, Section 3.3.2) or IPv6 (, section 5). Destinations that recognize extended IP IDs MUST configure a minimum EMTU_R of 65535 octets and SHOULD advertise the largest possible EMTU_R in PTB messages, but they MAY dynamically advertise a reduced EMTU_R during periods of congestion (see: ). When a (source, destination) pair uses a UDP port number, IP protocol number and/or Ethernet type value for which extended IP ID implementation is mandatory (and for IPv4 when network fragmentation is disabled), the source can send fragmented packets at high data rates and the destination can silently reassemble unless/until it needs to assert an EMTU_R indication due to reassembly congestion. For other (source, destination) pairs, the destination must return a continuous stream of EMTU_R indications subject to rate limiting (see: ) while it continues to reassemble packets from the source. (Note that this latter option applies only for unicast destinations - see: for multicast/anycast considerations.) While a source has assurance that the destination(s) recognize and correctly process extended IP IDs, it can continue to send fragmented or fragmentable packets no larger than the EMTU_R at rates within the Maximum Segment Lifetime (MSL) wraparound threshold for the extended IP ID length; otherwise, the source must assume the MSL threshold for the non-extended Identification field length . (When the source includes sufficiently strong integrity checks that the destination(s) can use to detect reassembly errors, however, it can continue to send at rates that exceed the MSL wraparound threshold.) Extended IP ID formats supported by this specification include only the mandatory-to-implement (advanced) extended formats found in this document which are differentiated by the option-length value for IPv4 or the extension header type for IPv6. Future documents may specify additional extended IP ID formats. Note: IP fragmentation cannot be applied for packets larger than 65535 octets. IP parcels and advanced jumbos provide a means for efficiently packaging and shipping multiple large segments or truly large singleton segments in IP 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 observations somehow inspired a discipline known as "Path MTU Discovery" within a new community known as the Internet Engineering Task Force (IETF). In more recent times, the IETF amplified these assertions in "IP Fragmentation Considered Fragile" . Rather than encourage timely implementation improvements, however, the IETF somehow forgot that IP fragmentation and reassembly are essential internetworking functions. This has resulted in a modern Internet where path MTU discovery (including its recent derivatives) provides a poor service especially for 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. This document therefore updates .

In progress.

The IANA is requested to assign a new IPv4 Option named "IDEXT" in the "IP Option Numbers" table in the 'ip-parameters' registry (registration procedures not defined). The option sets "Copy" to '1', "Class" to '00' and "Number" to TBD. The IANA is further requested to assign a new Protocol Number TBD2 in the in the "Assigned Internet Protocol Numbers" table in the 'protocol-numbers' registry (registration procedures IESG Approval or Standards Action). The registry sets Decimal to "TBD2", Keyword to "IPv6-XFrag", Protocol to "Extended Fragment Header for IPv6" and IPv6 Extension Header to "Y". The IANA is instructed to assign new Code values in the "ICMPv6 Code Fields: Type 2 - Packet Too Big" table in the 'icmpv6-parameters' registry (registration procedure is Standards Action or IESG Approval). The registry should appear as follows:

(Note: this registry also defines the same above values for the "unused" field of ICMPv4 "Destination Unreachable - Fragmentation Needed" messages .)

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 a known IPv4 reassembly integrity concern and also provide an advanced degree of packet Identification uniqueness assurance.

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

Unicast destinations are assumed throughout this document, however the same considerations apply when the destination is a multicast group or an anycast address. In order to send fragmented/fragmentable packets with IP ID extensions (or IP fragmentation checksums) 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 for which extended IP ID processing is mandatory. When a source sends fragmented/fragmentable packets with extended IP IDs (or IP fragmentation checksums) 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. Each such path may return a Code 1/2 PTB message if it needs to perform (further) fragmentation, 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 must 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 (or IP fragmentation checksums) to an anycast address, routing may direct initial fragments of the same packet to a first node that configures the address while directing the remaining fragments to other nodes 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 (or IP fragmentation checksum) 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.

<< RFC Editor - remove prior to publication >> Differences from earlier versions: First draft publication.