| Internet-Draft | IPv6 Extended Fragment Header | December 2023 |
| Templin | Expires 8 June 2024 | [Page] |
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 Destination Options Extended Fragment Header option that includes a 64-bit Identification; it further defines control messaging services for fragmentation and reassembly congestion management.¶
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The Internet Protocol, version 4 (IPv4) header includes a 16-bit Identification in all packets [RFC0791], but this length is too small to ensure reassembly integrity even at moderate data rates in modern networks [RFC4963][RFC6864][RFC8900]. 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 [RFC8200], but even this length may be too small for some applications. This specification therefore defines a means to extend the IPv6 Identification length through the introduction of a new IPv6 Destination Options Extended Fragment Header option.¶
The IPv6 Extended Fragment Header option 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 [RFC1122]. The term MSL is equivalent to the term "maximum datagram lifetime (MDL)" defined in [RFC0791][RFC6864].¶
The term "Packet Too Big (PTB)" refers to an IPv6 "Packet Too Big" [RFC8201][RFC4443] message.¶
The term "flow" refers to a sequence of packets sent from a particular source to a particular unicast, anycast or multicast destination [RFC6437].¶
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 [RFC2119][RFC8174] 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 [RFC7739]. This specification therefore proposes to fortify the IP ID by extending its length.¶
A recent study [I-D.templin-dtn-ltpfrag] 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 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 IPv6 may perform better when 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 through the introduction of an IPv6 Extended Fragment Header Destination Option.¶
For a standard 4-octet IPv6 Identification, the source can simply include an ordinary IPv6 Fragment Header as specified in [RFC8200] 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 and/or for paths that do not recognize the standard IPv6 Fragment Header, this specification permits the source to instead include an IPv6 Extended Fragment Header in a Destination Options Header. The Extended Fragment Header must appear as the first option in a Destination Options Header that appears immediately following the Hop-by-Hop Options (if present) and immediately before the Routing Header (if present); see Section 4.1 of [RFC8200] for extension header order. The IPv6 Destination Options Header with Extended Fragment Header option is formatted as shown in Figure 1:¶
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Next Header (1)| Hdr Ext Len | Option Type | Opt Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Next Header (2)| Index |P|S| Fragment Offset |R|D|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+- Identification (64 bits) -+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header (1) for unfragmented packets, encodes protocol
number of header following the Destination
Options header. For fragmented packets,
encodes protocol number of the Routing Header
if present; otherwise, encodes "No Next Hdr".
Hdr Ext Len 8-bit value 1 (i.e., 2 units of 8 octets).
Encodes a larger value if the Destination
Options Header includes more options.
Option Type 8-bit value '001[TBD1]'.
Opt Data Len 8-bit value 12.
Next Header (2) a temporary copy of Next Header (1) cached
when the packet is subject to fragmentation.
Index/P/S a control octet that identifies the components
of an IP Parcel [I-D.templin-intarea-parcels].
Fragment Offset the same as the fragmentation offset field
of the standard IPv6 Fragment Header.
R/D/M fragmentation control flags: "(R)eserved",
"(D)on't Fragment" and "(M)ore Fragments".
Identification an 8-octet (64 bit) unsigned integer
Identification, in network byte order.
The Extended Fragment Header option is therefore identified by Option Type TBD1 (see: IANA Considerations) and the Identification field is 8 octets (64 bits) in length. The option may appear either in an unfragmented IPv6 packet or in one for which IPv6 fragmentation is applied (with the header appearing in each fragment).¶
The source applies fragmentation using the Extended Fragment Header destination option in the same fashion as for the standard IPv6 Fragment Header, except that the destination option itself is included in the Per-Fragment Headers - see: Section 4.5 of [RFC8200] and the standard IPv6 Fragment Header is not included. The source further sets the R/D/M fragmentation control flags the same as for IPv4 fragmentation as discussed in Section 5.¶
For each fragment produced during fragmentation, the source also caches the Next Header (1) value in the Next Header (2) field, then when no Routing Header is present resets the Next Header (1) field to "No Next Header". When a Routing Header is present, the source instead resets the Routing Header Next Header field to "No Next Header".¶
The destination reassembles the same as specified in Section 4.5 of [RFC8200]. Following reassembly, if the Routing Header is present the destination resets the Routing Header Next Header field to the value cached in the Extended Fragment Header. If no Routing Header is present, the destination instead resets the Destination Options Next Header field.¶
Intermediate systems that forward packets fragmented in this way will therefore interpret the data that follows the per-fragment headers as undefined data (by virtue of the "No Next Header" setting) unless they are configured to more deeply inspect the data content.¶
When an IPv6 network intermediate system forwards a packet that includes a Destination Options Header in the extension header order where the Extended Fragment Header is permitted to occur, the intermediate system can optionally examine the Destination Options Header to determine if the Extended Fragment Header is present.¶
Nodes interpret the IPv6 Extended Fragment Header R/D/M fragmentation control flags the same as for IPv4 fragmentation; specifically:¶
the source sets the "(R)eserved" flag to 0 on transmission and the destination ignores the flag on reception.¶
the source sets the "(D)on't Fragment" flag to 0 if network fragmentation is permitted; otherwise to 1.¶
the source (or a fragmenting intermediate system) sets the "(M)ore Fragments" flag to 0 for the final fragment or 1 for non-final fragments.¶
When an intermediate system forwards a packet with D=0 that is too large to traverse the next hop toward the destination, it can apply (further) fragmentation using the procedures discussed above, except that it only rewrites the Next Header fields if both Fragment Offset and M are 0. For this reason, the Extended Fragment Header option contents may change in the path; hence the option "chg" flag is 1.¶
This specification therefore updates [RFC8200] by permitting network fragmentation for IPv6 under the above conditions.¶
Destinations that do not recognize the Extended Fragment Header ignore the option and process the packet further according to the remaining extension header chain. For packets fragmented according to the Extended Fragment Header, the destination will simply discard the fragment since the Next Header field in the final per-fragment extension header will encode "No Next Header".¶
The source can therefore test whether a destination recognizes the Extended Fragment Header by occasionally sending a fragmented "probe" packet using the option. If the source receives an acknowledgement, it has assurance that the destination has successfully applied reassembly. The source should occasionally re-probe each destination in case routing redirects a flow to a different anycast destination.¶
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 [RFC4443][RFC8201] 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 intermediate systems may 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 more adaptive to dynamic MTU fluctuations and less 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 the Extended Fragment Header according to this specification, the following additional PTB unused/Code values are defined:¶
Note: sources that receive PTB messages with Code 1/2 from an intermediate system should immediately engage source fragmentation for future packets using a maximum fragment size no larger than the MTU advertised in the PTB messages. This not only eases the burden on intermediate systems but also ensures better performance by avoiding small fragment sizes that may result from intermediate system fragmentation.¶
All nodes that process an IPv6 Destination Options Header with Extended Fragment Header option observe the extension header limits specified in [I-D.ietf-6man-eh-limits]; the Extended Fragment Header further MUST appear as the first option in the first Destination Options Header.¶
Intermediate systems MUST forward without dropping IPv6 packets that include a Destination Options header with an Extended Fragment Header unless they detect a security policy threat through deeper inspection of the protocol data that follows.¶
Sources MUST include at most one IPv6 Standard or Extended Fragment Header in each IPv6 packet/fragment. Intermediate systems and destinations SHOULD silently drop packets/fragments with multiples. If the source includes an Extended Fragment Header, it must appear as the first option of a single Destination Options Header following the Hop-by-Hop Options Header (if present) and before the Routing Header (if present).¶
Destinations that accept flows using Extended Fragment Headers:¶
MUST configure an EMTU_R of 65535 octets or larger,¶
SHOULD advertise the largest possible receive packet size (i.e., as large as EMTU_R) in PTB messages, and¶
MAY advertise a reduced receive packet size in PTB messages during periods of congestion.¶
While a source has assurance that the destination(s) will recognize and correctly process the Extended Fragment Header, it can continue to send fragmented or fragmentable packets as large as the current receive packet size 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 [RFC6864].¶
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 [I-D.templin-intarea-parcels].¶
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 [KENT87]. 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" [RFC8900].¶
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 larger packets or for IP parcels and AJs of any size. Instead, appropriate path MTU probing methods provide the only possible solutions to accommodate these constructs. This means that a combined solution with fragmentation and reassembly applied for conventional packet sizes and path MTU probing applied for others provides the necessary combination for Internetworking futures. This document therefore updates [RFC8900].¶
In progress.¶
The IANA is requested to assign a new IPv6 Destination 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 '1', "rest" to TBD1, "Description" to "IPv6 Extended Fragment Header" and "Reference" 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:¶
Code Name Reference --- ---- --------- 0 PTB Hard Error [RFC4443] 1 (suggested) Fragmentation Needed (soft) [RFCXXXX] 2 (suggested) Fragmentation Needed (hard) [RFCXXXX] 3 (suggested) Reassembly Needed (soft) [RFCXXXX] 4 (suggested) Reassembly Needed (hard) [RFCXXXX]
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 [RFC4963] and also provide an advanced degree of packet Identification uniqueness assurance.¶
All normative security guidance on IPv6 fragmentation (e.g., processing of tiny first fragments, overlapping fragments, etc.) applies also to the fragments generated under the Extended Fragment Header.¶
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 [RFC8799].¶
The OMNI specification [I-D.templin-intarea-omni] 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 Destination Options header with Extended Fragment Header 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 Figure 3:¶
+---------------------------+ | L2 IP/Ethernet Header | +---------------------------+ | L2 UDP Header (port 8060) | +---------------------------+ ~ L2 IPv6 Extension Headers ~ +---------------------------+ | OMNI IPv6 Encapsulation | +---------------------------+ ~ OMNI IPv6 Extensions ~ +---------------------------+ | | ~ ~ ~ Original IP Packet ~ ~ ~ | | +---------------------------+
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 [I-D.templin-intarea-omni].¶
Encapsulate the OAL packet in this L2 IPv6 header with any necessary IPv6 extension headers per [I-D.templin-intarea-omni], then perform normal extension header processing including fragmentation according to the IPv6 Extended Fragment Header specification in this document.¶
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 [I-D.templin-intarea-omni].¶
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 [I-D.templin-intarea-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 [I-D.templin-intarea-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 IPv6 Extended Fragment Headers 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: Appendix A).¶
When a source sends fragmented/fragmentable packets with IPv6 Extended Fragment Headers 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 IPv6 Extended Fragment Headers 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.¶
<< RFC Editor - remove prior to publication >>¶
Differences from earlier versions:¶
First draft publication.¶