IP Parcels and Advanced Jumbos

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Table of Contents

1. Introduction

IP packets (both IPv4 [RFC0791] and IPv6 [RFC8200]) contain a single unit of transport layer protocol data which becomes the retransmission unit in case of loss. Transport layer protocols such as the Transmission Control Protocol (TCP) [RFC9293] and reliable transport protocol users of the User Datagram Protocol (UDP) [RFC0768] (including QUIC [RFC9000], LTP [RFC5326] and others) prepare data units known as segments which the network layer packages into individual IP packets each containing only a single segment. This document presents a new construct known as the IP Parcel which permits a single packet to include multiple segments. The parcel is essentially a "packet-of-packets" with the full {TCP,UDP}/IP headers appearing only once but with possibly multiple segments included.¶

Transport layer protocol entities form parcels by preparing a data buffer (or buffer chain) containing at most 64 consecutive transport layer protocol segments that can be broken out into individual packets and/or smaller sub-parcels if necessary. All segments except the final one must be equal in length and no larger than 65535 octets, while the final segment must be no larger than the others. The transport layer protocol entity then delivers the buffer(s), number of segments and non-final segment size to the network layer. The network layer next appends an Internet Checksum header and a Cyclic Redundancy Check (CRC) trailer to each segment, merges the segments into the parcel body, appends a {TCP,UDP} header and finally appends an IP header plus extensions that identify this as a parcel and not an ordinary packet.¶

The network layer then forwards each parcel over consecutive parcel-capable links in a path until they arrive at a node with a next hop link that does not support parcels, a parcel-capable link with a size restriction, or an ingress Overlay Multilink Network (OMNI) Interface [I-D.templin-intarea-omni] connection to an OMNI link that spans intermediate Internetworks. In the first case, the original source or next hop router applies packetization to break the parcel into individual IP packets. In the second case, the node applies network layer parcellation to form smaller sub-parcels. In the final case, the OMNI interface applies adaptation layer parcellation to form still smaller sub-parcels, then applies adaptation layer IPv6 encapsulation and fragmentation if necessary. The node then forwards the resulting packets/parcels/fragments to the next hop.¶

Following IPv6 reassembly if necessary, an egress OMNI interface applies adaptation layer reunification if necessary to merge multiple sub-parcels into a minimum number of larger (sub-)parcels then delivers them to the network layer which either processes them locally or forwards them via the next hop link toward the final destination. The final destination can then apply network layer (parcel-based) reunification or (packet-based) restoration if necessary to deliver a minimum number of larger (sub-)parcels to the transport layer. Reordering, loss or corruption of individual segments within the network is therefore possible, but most importantly the parcels delivered to the final destination's transport layer should be the largest practical size for best performance, and loss or receipt of individual segments (rather than parcel size) determines the retransmission unit.¶

This document further introduces an advanced jumbo service that provides essential extensions beyond the basic IPv6 jumbogram service defined in [RFC2675]. Advanced jumbos are defined for both IP protocol versions and provide end systems and routers with a more robust service when the transmission of truly large singleton segments is necessary.¶

The following sections discuss rationale for creating and shipping IP parcels and advanced jumbos as well as actual protocol constructs and procedures involved. IP parcels and advanced jumbos provide essential building blocks for improved performance, efficiency and integrity while encouraging larger Maximum Transmission Units (MTUs). These services should further inspire future innovation in applications, transport protocols, operating systems, network equipment and data links in ways that promise to transform the Internet architecture.¶

2. Terminology

The Oxford Languages dictionary defines a "parcel" as "a thing or collection of things wrapped in paper in order to be carried or sent by mail". Indeed, there are many examples of parcel delivery services worldwide that provide an essential transit backbone for efficient business and consumer transactions.¶

In this same spirit, an "IP parcel" is simply a collection of at most 64 transport layer protocol segments wrapped in an efficient package for transmission and delivery as a "packet-of-packets", with each segment preceded by an end-to-end integrity check to detect link errors. IP parcels are distinguished from ordinary packets and various forms of jumbograms through the constructs specified in this document.¶

The IP parcel construct is defined for both IPv4 and IPv6. Where the document refers to "IPv4 header length", it means the total length of the base IPv4 header plus all included options, i.e., as determined by consulting the Internet Header Length (IHL) field. Where the document refers to "IPv6 header length", however, it means only the length of the base IPv6 header (i.e., 40 octets), while the length of any extension headers is referred to separately as the "IPv6 extension header length". Finally, the term "IP header plus extensions" refers generically to an IPv4 header plus all included options or an IPv6 header plus all included extension headers.¶

The term "advanced jumbo" refers to a new type of IP jumbogram defined for both IP protocol versions and modeled from the basic IPv6 jumbogram construct defined in [RFC2675]. Advanced jumbos include a 32-bit Jumbo Payload Length field the same as for basic IPv6 jumbograms, but are differentiated from parcels and other jumbogram types by including an "Advanced Jumbo Type" value between 250 and 255 in the IP {Total, Payload} Length field and an end-to-end segment integrity check to detect link errors.¶

Where the document refers to "{TCP,UDP} header length", it means the length of either the TCP header plus options (20 or more octets) or the UDP header (8 octets). It is important to note that only a single IP header and a single full {TCP,UDP} header appears in each parcel regardless of the number of segments included. This distinction often provides a significant overhead savings advantage made possible only by IP parcels.¶

Where the document refers to checksum calculations, it means the standard Internet checksum unless otherwise specified. The same as for TCP [RFC9293], UDP [RFC0768] and IPv4 [RFC0791], the standard Internet checksum is defined as (sic) "the 16-bit one's complement of the one's complement sum of all (pseudo-)headers plus data, padded with zero octets at the end (if necessary) to make a multiple of two octets". A notional Internet checksum algorithm can be found in [RFC1071], while practical implementations require detailed attention to network byte ordering to ensure interoperability between diverse architectures.¶

The term Cyclic Redundancy Check (CRC) is used consistently with its application in widely deployed Internetworking services. The CRC32C [RFC3385] and CRC64E [ECMA-182] standards are selected for IP parcels according to non-final segment length "L" (see: Section 11). Advanced jumbos include a message digest calculated according to the MD5 [RFC1321], SHA1 [RFC3174] or US Secure Hash [RFC6234] algorithms. In all cases, the CRC or message digest is appended as a per-segment trailer arranged for transmission in network byte order according to standard Internetworking conventions.¶

The terms "application layer (L5 and higher)", "transport layer (L4)", "network layer (L3)", "(data) link layer (L2)" and "physical layer (L1)" are used consistently with common Internetworking terminology, with the understanding that reliable delivery protocol users of UDP are considered as transport layer elements. The OMNI specification further defines an "adaptation layer" logically positioned below the network layer but above the link layer (which may include physical links and Internet- or higher-layer tunnels). The adaptation layer is not associated with a layer number itself and is simply known as "the layer below L3 but above L2". A network interface is a node's attachment to a link (via L2), and an OMNI interface is therefore a node's attachment to an OMNI link (via the adaptation layer).¶

The term "parcel/jumbo-capable link/path" refers to paths that transit interfaces to adaptation layer and/or link layer media (either physical or virtual) capable of transiting {TCP,UDP}/IP packets that employ the parcel/jumbo constructs specified in this document. The source and each router in the path has a "next hop link" that forwards parcels/jumbos toward the final destination, while each router and the final destination has a "previous hop link" that accepts en route parcels/jumbos. Each next hop link must be capable of forwarding parcels/jumbos (after first applying parcellation if necessary) with segment lengths no larger than can transit the link. Currently only the OMNI link satisfies these properties, while other link types that support parcels/jumbos should soon follow.¶

The term "5-tuple" refers to a transport layer protocol entity identifier that includes the network layer (source address, destination address, source port, destination port, protocol number). The term "4-tuple" refers to a network layer parcel entity identifier that includes the adaptation layer (source address, destination address, Parcel ID, Identification).¶

The Internetworking term "Maximum Transmission Unit (MTU)" is widely understood to mean the largest packet size that can transit a single link ("link MTU") or an entire path ("path MTU") without requiring network layer IP fragmentation. If the MTU value returned during parcel path qualification is larger than 65535 (plus the length of the parcel headers), it determines the maximum-sized parcel or jumbo that can transit the link/path without requiring a router to perform packetization/parcellation. If the MTU is no larger than 65535, the value instead determines the "Maximum Segment Size (MSS)" for the leading portion of the path up to a router that cannot forward the parcel further. (Note that this size may still be larger than the MSS that can transit the remainder of the path to the final destination, which can only be determined through explicit MSS probing.)¶

The terms "packetization" and "restoration" refer to a network layer process in which the original source or a router on the path breaks a parcel out into individual IP packets that can transit the remainder of the path without loss due to a size restriction. The final destination then restores the combined packet contents into a parcel before delivery to the transport layer. In current practice, packetization/restoration can be considered as functional equivalents to the well-known Generic Segmentation/Receive Offload (GSO/GRO) services.¶

The terms "parcellation" and "reunification" refer to either network layer or adaptation layer processes in which the original source or a router on the path breaks a parcel into smaller sub-parcels that can transit the path without loss due to a size restriction. These sub-parcels are then reunified into larger (sub-)parcels before delivery to the transport layer. As a network layer process, the sub-parcels resulting from parcellation may only be reunified at the final destination. As an adaptation layer process, the resulting sub-parcels may be first reunified at an adaptation layer egress node then possibly further reunified by the network layer of the final destination.¶

The terms "fragmentation" and "reassembly" follow exactly from their definitions in the IPv4 [RFC0791] and IPv6 [RFC8200] standards. In particular, OMNI interfaces support IPv6 encapsulation and fragmentation as an adaptation layer process that can transit packets or (sub-)parcels of sizes that exceed the underlying Internetwork path MTU. OMNI fragmentation/reassembly occurs at a lower layer of the protocol stack than restoration and/or reunification and therefore provides a complimentary service.¶

"Automatic Extended Route Optimization (AERO)" [I-D.templin-intarea-aero] and the "Overlay Multilink Network Interface (OMNI)" [I-D.templin-intarea-omni] provide an adaptation layer framework for transmission of IP parcels and advanced jumbos over one or more concatenated Internetworks. AERO/OMNI will provide an operational environment for IP parcels beginning from the earliest deployment phases and extending indefinitely to accommodate continuous future growth. As more and more parcel/jumbo-capable links are deployed (e.g., in data centers, edge networks, space-domain, and other high data rate services) AERO/OMNI will continue to provide an essential service for Internetworking performance maximization.¶

The parcel sizing variables "J", "K", "L" and "M" are cited extensively throughout this document. "J" denotes the number of non-final segments included in the parcel, "K" is the length of the final segment, "L" is the length of each non-final segment and "M" is termed the "Parcel Payload Length".¶

3. Requirements

IP parcels and advanced jumbos are derived from the basic jumbogram specification found in [RFC2675], but the specifications in this document take precedence whenever they differ from the basic requirements. Most notably, IPv4 parcels use the IPv4 Probe MTU option [RFC1063] while IPv6 parcels and advanced jumbos may use one of either the IPv6 Minimum Path MTU [RFC9268] or basic IPv6 jumbogram [RFC2675] Hop-by-Hop option. IP parcels and advanced jumbos are further permitted to encode values other than 0 in the IP {Total, Payload} length field and they are not limited to packet sizes that exceed 65535 octets. (Instead, IP parcels can be as small as the packet headers plus a singleton segment while advanced jumbos can be as small as the headers plus a NULL payload.)¶

Each IPv4 parcel/advanced jumbo includes at most one Probe MTU option and each IPv6 parcel/advanced jumbo includes at most one IPv6 Minimum Path MTU or Jumbo Payload Hop-by-Hop option. Intermediate and end systems therefore silently drop any IP parcels/advanced jumbos that include multiple. For further Hop-by-Hop option considerations, see: [I-D.ietf-6man-hbh-processing]. For IPv6 extension header limits, see: [I-D.ietf-6man-eh-limits].¶

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.¶

4. Background and Motivation

Studies have shown that applications can improve their performance by sending and receiving larger packets due to reduced numbers of system calls and interrupts as well as larger atomic data copies between kernel and user space. Larger packets also result in reduced numbers of network device interrupts and better network utilization (e.g., due to header overhead reduction) in comparison with smaller packets.¶

A first study [QUIC] involved performance enhancement of the QUIC protocol [RFC9000] using the linux Generic Segment/Receive Offload (GSO/GRO) facility. GSO/GRO provides a robust service that has shown significant performance increases based on a multi-segment transfer capability between the operating system kernel and QUIC applications. GSO/GRO performs (virtual) fragmentation and reassembly at the transport layer with the transport protocol segment size limited by the path MTU (typically 1500 octets or smaller in today's Internet).¶

A second study [I-D.templin-dtn-ltpfrag] showed that GSO/GRO also improves performance for the Licklider Transmission Protocol (LTP) [RFC5326] used for the Delay Tolerant Networking (DTN) Bundle Protocol [RFC9171] for segments larger than the actual path MTU through the use of OMNI interface encapsulation and fragmentation. Historically, the NFS protocol also saw significant performance increases using larger (single-segment) UDP datagrams even when IP fragmentation is invoked, and LTP still follows this profile today. Moreover, LTP shows this (single-segment) performance increase profile extending to the largest possible segment size which suggests that additional performance gains are possible using (multi-segment) IP parcels that approach or even exceed 65535 octets in total length.¶

TCP also benefits from larger packet sizes and efforts have investigated TCP performance using jumbograms internally with changes to the linux GSO/GRO facilities [BIG-TCP]. The approach proposed to use the Jumbo Payload option internally and to allow GSO/GRO to use buffer sizes that exceed 65535 octets, but with the understanding that links that support jumbograms natively are not yet widely available. Hence, IP parcels provide a packaging that can be considered in the near term under current deployment limitations.¶

A limiting consideration for sending large packets is that they are often lost at links with MTU restrictions, and the resulting Packet Too Big (PTB) messages [RFC1191][RFC8201] may be lost somewhere in the return path to the original source. This path MTU "black hole" condition can degrade performance unless robust path probing techniques are used, however the best case performance always occurs when loss of packets due to size restrictions is minimized.¶

These considerations therefore motivate a design where transport protocols can employ segment sizes as large as 65535 octets (minus headers) while parcels that carry multiple segments may themselves be significantly larger. This would allow the receiving transport layer protocol entity to process multiple segments in parallel instead of one at a time per existing practices. Parcels therefore support improvements in performance, integrity and efficiency for the original source, final destination and networked path as a whole. This is true even if the network and lower layers need to apply packetization/restoration, parcellation/reunification and/or fragmentation/reassembly.¶

An analogy: when a consumer orders 50 small items from a major online retailer, the retailer does not ship the order in 50 separate small boxes. Instead, the retailer packs as many of the small items as possible into one or a few larger boxes (i.e., parcels) then places the parcels on a semi-truck or airplane. The parcels may then pass through one or more regional distribution centers where they may be repackaged into different parcel configurations and forwarded further until they are finally delivered to the consumer. But most often, the consumer will only find one or a few parcels at their doorstep and not 50 separate small boxes. This flexible parcel delivery service greatly reduces shipping and handling cost for all including the retailer, regional distribution centers and finally the consumer.¶

5. IP Parcel and Advanced Jumbo Link Service Model

The classical Internetworking link service model requires each link in the path to apply a link-layer frame integrity check often termed a "Frame Check Sequence (FCS)" or more commonly "CRC". The link near-end calculates and appends a CRC trailer to each packet pending transmission, and the link far-end verifies the CRC upon packet reception. If the CRC is incorrect, the link far-end unconditionally discards the packet. This process is repeated for each link in the path so that only packets that pass all link-layer checks are delivered to the final destination.¶

While this link service model has contributed to the unparalleled success of terrestrial Internetworks (including the global public Internet), new uses in which significant delays or disruptions can occur are not as well supported. For example, a path that contains links with significant bit errors may be unable to pass a majority percentage of packets since loss due to CRC failures can occur at any hop while each packet lost must be retransmitted. Especially with the advent of space-domain Internetworking, the long delays associated with interplanetary signal propagation can also often render any retransmissions useless especially when communications latency is critical.¶

IP parcels and advanced jumbos include an end-to-end CRC with each segment that is calculated and inserted by the original source and verified by the final destination. For each IP parcel or advanced jumbo admitted into a parcel-capable link, the link near-end applies its standard link-layer CRC upon transmission which the link far-end then verifies upon reception. Instead of unconditionally discarding frames with CRC errors, however, the link far-end delivers all frames to upper layers along with an error flag that is set if the link-layer CRC check failed or cleared otherwise.¶

For ordinary IP packets, each link along the path simply discards any that have incurred CRC errors according to current practice. For IP parcels and advanced jumbos received with link-layer CRC errors, each intermediate hop SHOULD and the final destination MUST first verify the parcel/jumbo header Checksum to protect against mis-delivery. Each intermediate hop then unconditionally forwards the parcel/advanced jumbo to the next hop.¶

The IP parcel/advanced jumbo segments may therefore acquire cumulative link errors along the path, but these will be detected by the per segment end-to-end CRC and/or Internet checksums performed by the final destination. The final destination in turn delivers each segment to the local transport layer along with an error flag that is set if an end-to-end CRC or Internet checksum error was detected (otherwise the flag is cleared). The error indication is then taken under advisement by the transport layer, which should consult any transport or higher-layer integrity checks to guide any corrective actions.¶

IP parcels and advanced jumbos therefore provide a revolutionary advancement for delay/disruption tolerance in air/land/sea/space mobile Internetworking applications. As the Internet continues to evolve from its more stable fixed terrestrial network origins to one where more and more nodes operate in the mobile edge, this new link service model relocates error detection and correction responsibilities from intermediate systems to the end systems that are uniquely capable of take corrective actions.¶

Note: IP parcels and advanced jumbos may already be compatible with widely-deployed link types such as 1/10/100-Gbps Ethernet. Each Ethernet frame is identified by a preamble followed by a Start Frame Delimiter (SFD) followed by the frame data itself followed by the FCS and finally an Inter Packet Gap (IPG). Since no length field is included, however, the frame can theoretically be allowed to extend as long as necessary for transmission of IP parcels and advanced jumbos that are much larger than the typical 1500 octet Ethernet MTU or even larger Ethernet "jumboframe" MTUs. Widely-deployed links may therefore already include all of the necessary features to natively support large IP parcels and advanced jumbos with no additional extensions, while operating systems may need to be modified to post larger receive buffers.¶

6. IP Parcel Formation

A transport protocol entity identified by its 5-tuple forms a parcel body by preparing a data buffer (or buffer chain) containing at most 64 transport layer protocol segments, with each TCP segment preceded by a 4-octet Sequence Number header and with each segment (plus Sequence Number) preceded by a 2-octet Internet Checksum header and followed by a 4-octet or 8-octet CRC trailer. All non-final segments MUST be equal in length while the final segment MUST NOT be larger and MAY be smaller. The number of non-final segments is represented as J; the total number of segments is therefore (J + 1).¶

The non-final segment size L is set to a 16-bit value that MUST be no smaller than 256 octets and SHOULD be no larger than 65535 octets minus the length of the {TCP,UDP} header (plus options), minus the length of the IP header (plus options/extensions), minus 2 octets for the Checksum header minus 4/8 octets for the CRC trailer (see: Appendix B). The final segment length K MUST NOT be larger than L but MAY be smaller. The transport layer protocol entity then presents the buffer(s) and size L to the network layer, noting that the combined buffer length(s) may exceed 65535 octets when there are sufficient segments of a large enough size.¶

If the next hop link is not parcel capable, the network layer performs packetization to package each segment as an individual IP packet as discussed in Section 7.1. If the next hop link is parcel capable, the network layer instead completes the parcel by appending an Internet Checksum header and CRC trailer to each segment then appending a single full {TCP,UDP} header (plus options) and a single full IP header (plus options/extensions). The network layer finally includes a specially-formatted Parcel Payload option as an extension to the IP header of each parcel prior to transmission over a network interface.¶

The Parcel Payload option format for both IP protocol versions appears as shown in Figure 1:¶

                   IPv4 Parcel Payload Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  option-type  | option-length |     Code      |     Check     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Index   |P|S|             Parcel Payload Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   IPv6 Parcel Payload Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |     Code      |     Check     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Index   |P|S|             Parcel Payload Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For IPv4, the network layer includes the Parcel Payload option as an IPv4 header option with option-type set to '0x0B' and option-length set to 8. The length also distinguishes this type from its obsoleted use as the IPv4 Probe MTU option [RFC1063]. The network layer next sets Parcel Payload Length to a 3-octet value M that encodes the length of the IPv4 header plus the length of the {TCP,UDP} header plus the combined length of all concatenated segments with their CRC (and for TCP also Sequence Number) headers. The network layer then sets the IPv4 header DF bit to 1 and Total Length field to the non-final segment size L.¶

For IPv6, the network layer includes the Parcel Payload option as an IPv6 Hop-by-Hop option with Option Type set to '0x30' and Opt Data Len set to 6. The length also distinguishes this type from its use as the IPv6 Minimum Path MTU Hop-by-Hop Option [RFC9268]. The network layer next sets Parcel Payload Length to a 3-octet value M that encodes the lengths of all IPv6 extension headers present plus the length of the {TCP,UDP} header plus the combined length of all concatenated segments with their CRC (and for TCP also Sequence Number) headers. The network layer then sets the IPv6 header Payload Length field to L.¶

For both IP protocol versions, the network layer then sets Index to an ordinal segment "Parcel Index" value between 0 and 63, sets the "(P)arcel" flag to 1 and sets the "More (S)egments" flag to 1 for non-final sub-parcels or 0 for the final (sub-)parcel. (Note that non-zero Index values identify the initial segment index in non-first sub-parcels of a larger original parcel while the value 0 denotes the first sub-parcel.) The network layer finally sets Code to 255 and sets Check to the same value that will appear in the IP header TTL/Hop Limit field on transmission. These values provide hop-by-hop assurance that previous hops correctly implement the parcel protocol without applying [RFC1063][RFC9268] processing.¶

Following this transport and network layer processing, {TCP,UDP}/IP parcels therefore have the structures shown in Figure 2:¶

        TCP/IP Parcel Structure            UDP/IP Parcel Structure
   +------------------------------+   +------------------------------+
   |                              |   |                              |
   ~    IP Hdr plus extensions    ~   ~    IP Hdr plus extensions    ~
   |                              |   |                              |
   +------------------------------+   +------------------------------+
   |                              |   |                              |
   ~   TCP header (plus options)  ~   ~         UDP header           ~
   |                              |   |                              |
   +------------------------------+   +------------------------------+
   |    Checksum 0 followed by    |   |    Checksum 0 followed by    |
   ~  Sequence Number 0 followed  ~   ~     Segment 0 (L octets)     ~
   ~    by Segment 0 (L octets)   ~   ~         followed by          ~
   |      followed by CRC 0       |   |            CRC 0             |
   +------------------------------+   +------------------------------+
   |    Checksum 1 followed by    |   |    Checksum 1 followed by    |
   ~  Sequence Number 1 followed  ~   ~     Segment 1 (L octets)     ~
   ~    by Segment 1 (L octets)   ~   ~         followed by          ~
   |      followed by CRC 1       |   |            CRC 1             |
   +------------------------------+   +------------------------------+
   ~             ...              ~   ~             ...              ~
   ~         More Segments        ~   ~         More Segments        ~
   ~             ...              ~   ~             ...              ~
   +------------------------------+   +------------------------------+
   |    Checksum J followed by    |   |    Checksum J followed by    |
   ~  Sequence Number J followed  ~   ~     Segment J (K octets)     ~
   ~    by Segment J (K octets)   ~   ~         followed by          ~
   |      followed by CRC J       |   |            CRC J             |
   +------------------------------+   +------------------------------+

The {TCP,UDP}/IP header is then followed by (J + 1) transport layer segments. For TCP, the TCP header Sequence Number field encodes the value 0, and each segment is preceded by its own 4-octet Sequence Number field with the 4-octet length included in L and K. Each segment (and its Sequence Number) is then preceded by a 2-octet Internet Checksum header and followed by a 4/8-octet CRC trailer but without the Checksum/CRC lengths included in L and K.¶

       /* L must be at least 256;
          T is temporary length;
          H is length of {TCP,UDP}/IP headers plus extensions;
          C is the combined length of the CRC and Checksum fields;
          integer arithmetic assumed.*/

       if ((L < 256) || ((T = (M - H)) <= 0))
           drop parcel;

       if ((J = (T / (L + C))) > 64)
           drop parcel;

       if ((K = (T % (L + C))) == 0) {
           J--; K = L;
       } else {
           if ((J > 63) || ((K -= C) <= 0))
               drop parcel;
       }

7. Transmission of IP Parcels

When the network layer of the source assembles a {TCP,UDP}/IP parcel it fully populates all IP header fields including the source address, destination address and Parcel Payload option as above. The source also sets IP {Total, Payload} Length to L (between 256 and 65535) to distinguish the parcel from other jumbogram types (see: Section 8).¶

The network layer of the source also maintains a randomly-initialized 4/8/12/16-octet (32/64/96/128-bit) (extended) Identification value for each destination expressed in an Identification Extension Option for the Internet Protocol and includes an Identification in each parcel (see: [I-D.templin-intarea-ipid-ext]). For each packet or parcel transmission, the source sets the (extended) Identification to the current cached value for this destination and increments the cached value by 1 (modulo 2**32/64/96/128) for each successive transmission. (The source can then reset the cached value to a new random number when necessary, e.g., to maintain an unpredictable profile.)¶

The network layer of the source finally presents the parcel to an interface for transmission to the next hop. For ordinary interface attachments to parcel-capable links, the source simply admits each parcel into the interface the same as for any IP packet where it may be forwarded by one or more routers over additional consecutive parcel-capable links possibly even traversing the entire forward path to the final destination. Note that any node in the path that does not recognize the parcel construct may either drop it and return an ICMP Parameter Problem message or (erroneously) attempt to forward it as an ordinary packet.¶

Most importantly, each parcel-capable link in the path forwards the parcel even if link CRC errors were detected since IP parcels and advanced jumbos include end-to-end CRC and Checksum integrity checks. This ensures that the majority of good data is delivered to the final destination instead of being discarded along with a small amount of errored data at each intermediate hop.¶

When the next hop link does not support parcels at all, or when the next hop link is parcel-capable but configures an MTU that is too small to pass the entire parcel, the source breaks the parcel up into individual IP packets (in the first case) or into smaller sub-parcels (in the second case). In the first case, the source can apply packetization using Generic Segment Offload (GSO), and the final destination can apply restoration using Generic Receive Offload (GRO) to deliver the largest possible parcel buffer(s) to the transport layer. In the second case, the source can apply parcellation to break the parcel into sub-parcels with each containing the same (extended) Identification value and with the S flag set appropriately. The final destination can then apply reunification to deliver the largest possible parcel buffer(s) to the transport layer. In all other ways, the source processes of breaking a parcel up into individual IP packets or smaller sub-parcels entail the same considerations as for a router on the path that invokes these processes as discussed in the following subsections.¶

Parcel probes that test the forward path's ability to pass parcels set a Path MTU (PMTU field) to a non-zero value as discussed in Section 7.6. Each router in the path then rewrites PMTU in a similar fashion as for [RFC1063][RFC9268]. Specifically, each router compares the parcel PMTU value with the next hop link MTU in the parcel path and MUST (re)set PMTU to the minimum value. The fact that the parcel transited a previous hop link provides sufficient evidence of forward progress (since parcel path MTU determination is unidirectional in the forward path only), but nodes can also include the previous hop link MTU in their minimum PMTU calculations in case the link may have an ingress size restriction (such as a receive buffer limitation). Each parcel also includes one or more transport layer segments corresponding to the 5-tuple for the flow, which may include {TCP,UDP} segment size probes used for packetization layer path MTU discovery [RFC4821][RFC8899]. (See: Section 7.6 for further details on parcel path probing.)¶

When a router receives an IP parcel it first compares Code with 255 and Check with the IP header TTL/Hop Limit; if either value differs, the router drops the parcel and returns a negative Jumbo Report (see: Section 7.5) subject to rate limiting. (Note that the IP parcel may also have been truncated in length by a previous-hop router that does not recognize the construct.) For all other intact IP parcels, the router next compares the value L with the next hop link MTU. If the next hop link is parcel capable but configures an MTU too small to admit a parcel with a single segment of length L the router returns a positive Jumbo Report (subject to rate limiting) with MTU set to the next hop link MTU. If the next hop link is not parcel capable and configures an MTU too small to pass an individual IP packet with a single segment of length L the router instead returns a positive Parcel Report (subject to rate limiting) with MTU set to the next hop link MTU. If the next hop link is parcel capable the router MUST reset Check to the same value that would appear in the IP header TTL/Hop Limit field upon transmission to the next hop.¶

If the router recognizes parcels but the next hop link in the path does not, or if the entire parcel would exceed the next hop link MTU, the router instead opens the parcel. The router then forwards each enclosed segment in individual IP packets or in a set of smaller sub-parcels that each contain a subset of the original parcel's segments. If the next hop link is via an OMNI interface, the router instead follows OMNI Adaptation Layer procedures. These considerations are discussed in detail in the following sections.¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |   Index   |P|S|      Fragment Offset    |Res|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                         Identification                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   IPv4 Parcel Probe Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  option-type  | option-length |     Code      |     Check     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Index   |P|S|             Parcel Payload Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Path MTU (PMTU)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   IPv6 Parcel Probe Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |     Code      |     Check     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Index   |P|S|             Parcel Payload Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Path MTU (PMTU)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   IPv4 Jumbo Reply MTU Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  option-type  | option-length |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               0                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path MTU (PMTU)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                         Identification                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   IPv6 Jumbo Reply MTU Option Format
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |  Option Type  |  Opt Data Len |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               0                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path MTU (PMTU)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                         Identification                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

8. Advanced Jumbos

This specification introduces an IP advanced jumbo service as an alternative to parcels and basic IPv6 jumbograms that also includes a path probing function based on the mechanisms specified in Section 7.6. The function employs an Advanced Jumbo Option with the same option type and length values as for the Parcel Payload/Probe options, but with the Parcel Index and Parcel Payload Length fields converted to a 32-bit Jumbo Payload Length field as shown in Figure 7:¶

                IPv4 Advanced Jumbo/Probe Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  option-type  | option-length |      Code     |     Check     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Jumbo Payload Length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                  Path MTU (PMTU) (Probes Only)                ~
   +- - - - - - - -+- - - - - - - -+- - - - - - - -+- - - - - - - -+

                IPv6 Advanced Jumbo/Probe Option Format
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |      Code     |     Check     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Jumbo Payload Length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                   Path MTU (PMTU) (Probes Only)               ~
   +- - - - - - - -+- - - - - - - -+- - - - - - - -+- - - - - - - -+

{TCP/UDP}/IP advanced jumbos/probes are formed the same as for parcels as shown in Figure 2 except that they include only a single segment ("Segment 0") preceded by a 2-octet Internet Checksum header and followed by an N-octet message digest trailer. Unlike IP parcels, TCP Advanced Jumbos do not include a Sequence Number for the (single) segment since the sequence number is coded in the TCP header the same as for an ordinary packet.¶

Advanced Jumbo implementations honor the message digest algorithms specified for MD5 [RFC1321], SHA1 [RFC3174] and the advanced US Secure Hash Algorithms [RFC6234] as selected by an Advanced Jumbo Type value between 250 and 255. The source includes a message digest according to an algorithm appropriate for the segment length while considering the error characteristics of the path. The destination verifies the digest according to the selected algorithm and uses local knowledge to determine whether the integrity check strength is sufficient to relax upper layer checking. Advanced Jumbo implementations MUST support the following:¶

   Type    Algorithm      Digest Length
   ----    ---------      -------------
    250          MD5          16 octets
    251         SHA1          20 octets
    252      SHA-224          28 octets
    253      SHA-256          32 octets
    254      SHA-384          48 octets
    255      SHA-512          64 octets

The source prepares an advanced jumbo/probe by first setting the IP {Total, Payload} Length field to an Advanced Jumbo Type value between 250 and 255 to distinguish this from a basic jumbogram or parcel. The source can begin by sending a Jumbo Probe to pre-qualify the path for advanced jumbos if necessary.¶

To prepare a Jumbo Probe that will trigger a Jumbo Report, the source can set {Protocol, Next Header} to {TCP,UDP}, set the {TCP,UDP} port to 9 (discard) and either include no octets beyond the {TCP,UDP} header or a single discard segment of the desired probe size immediately following the header. (The source can instead set the {TCP,UDP} port to the port number for a current data flow in order to receive IP Jumbo Reply MTU options in return packets as discussed in Section 7.6.) The source then sets Jumbo Payload Length to the length of the {TCP,UDP} header plus the length of the segment Checksum header and message digest trailer plus the discard segment plus the length of the full IP header for IPv4 or the extension headers for IPv6.¶

The source next sets the (extended) Identification the same as for an IP Parcel Probe, sets the Jumbo Probe PMTU to the next hop link MTU, then sets Code to 255 and Check to the next hop TTL/Hop Limit. The source then calculates the {TCP,UDP} Checksum based on the same pseudo header as for an ordinary parcel (see: Figure 11) but with the Parcel Index and Payload Length fields replaced with a 32-bit Jumbo Payload Length field and with the Segment Length replaced with an Advanced Jumbo Type value between 250 and 255. The source then calculates the checksum of the segment payload, writes the value into the segment Checksum header, then calculates the message digest over the length of the (single) segment beginning with the Checksum field and writes the value into the trailer. The source then sends the Jumbo Probe via the next hop link toward the final destination.¶

At each IP forwarding hop, the router examines Code and Check then drops the Jumbo Probe and returns a negative Jumbo Report if either value is incorrect. (Note that the Jumbo Probe may also have been truncated in length by a previous-hop router that does not recognize the construct.) For all other intact probes, if the next hop link is jumbo-capable the router compares PMTU to the next hop link MTU, resets PMTU to the minimum value, sets Check to the next hop TTL/Hop Limit then forwards the probe to the next hop. If the next hop link is not jumbo-capable, the router instead drops the probe and returns a negative Jumbo Report.¶

If the Jumbo Probe encounters an OMNI link, the OAL source can either drop the probe and return a negative Jumbo Report or forward the probe further toward the OAL destination using adaptation layer encapsulation. If the OAL source already knows the OAL path MTU for this OAL destination, it can encapsulate and forward the Jumbo Probe with PMTU set to the minimum of itself and the known value (minus the adaptation layer header size), and without adding any padding octets.¶

If the OAL path MTU is unknown, the OAL source can instead encapsulate the Jumbo Probe in an adaptation layer IPv6 header with an Advanced Jumbo option and with padding octets added beyond the end of the encapsulated Jumbo Probe to form an adaptation layer jumbogram as large as the minimum of PMTU and (2**24 - 1) octets (minus the adaptation layer header size) as a form of "jumbo-in-jumbo" encapsulation.¶

The OAL source then writes this size into the Jumbo Probe PMTU field and forwards the newly-created adaptation layer jumbogram toward the OAL destination. If the jumbogram somehow transits the path, the OAL destination then removes the adaptation layer encapsulation, discards the padding, then forwards the Jumbo Probe onward toward the final destination (with each hop reducing PMTU if necessary).¶

When a router on the path forwards a Jumbo Probe, it drops and returns a Jumbo Report if the next hop MTU is insufficient; otherwise, it forwards to the next hop toward the final destination. When the final destination receives the Jumbo Probe, it returns a Jumbo Report with the PMTU set to the maximum-sized jumbo that can transit the path.¶

When the Jumbo Probe reaches the final destination, the destination first examines the {TCP,UDP} port number. If the port number is 9 (discard), the destination returns a Jumbo Report UDP message; otherwise, the destination prepares an IP Jumbo Reply MTU option to include in a data packet on the return path to the original source. Detailed descriptions for these processes are found in Section 7.6.¶

After successfully probing the path, the original source can begin sending regular advanced jumbos by setting the IP {Total, Payload} Length field to an Advanced Jumbo Type value between 250 and 255, omitting the PMTU field and calculating the (TCP,UDP}/IP header checksum and per-segment Checksum header and message digest trailer the same as described for probes above. When the network layer of the final destination receives an advanced jumbo, it first verifies the integrity checks then delivers the data (along with a CRC/Checksum error flag) to the transport layer without returning a Jumbo Report. The source can continue to send advanced jumbos into the path with the possibility that the path may change. In that case, a router in the network may return an ICMP error, an ICMPv6 PTB, or a Jumbo Report if the path MTU decreases.¶

Note: when a source sends a jumbo probe into a new path that has not been probed previously, it should include enough padding payload so that the overall packet length is consistent with the value found in the IP {Total, Payload} Length field. This allows legacy routers on the path that do not recognize jumbos to see a length that is consistent with the value found in the IP header.¶

Note: If an OAL source can in some way determine that a very large packet is likely to transit the OAL path, it can encapsulate a Jumbo Probe to form an adaptation layer jumbogram even larger than (2**24 - 1) octets with the understanding that the time required to transit the path determines acceptable jumbogram sizes.¶

Note: The Jumbo Report message types returned in response to both Parcel and Jumbo Probes are one and the same, and signify that both parcels and advanced jumbos at least as large as the reported MTU can transit the path. However, only a Parcel Probe (i.e., and not a Jumbo Probe) may elicit a Parcel Report. This may indicate a preference to use Parcel Probes instead of Jumbo Probes for general-purpose path probing.¶

9. Minimal IPv6 Parcels/Advanced Jumbos

The basic IPv6 parcel and advanced jumbo constructs specified in the previous sections use the IPv6 Minimum Path MTU Hop-by-Hop option [RFC9268] initially to allow each hop to participate in path qualification. Once a path has been qualified to accept the basic constructs, however, the source can begin sending minimal IPv6 parcels or advanced jumbos that instead use the IPv6 Jumbo Payload Hop-by-Hop Option [RFC2675] to benefit from an 8-octet per packet savings as shown in Figure 9:¶

                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |  Option Type  |  Opt Data Len |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Option Data (first four octets)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In this format, the network layer includes the IPv6 minimal Parcel/Jumbo Option as an IPv6 Hop-by-Hop option with Option Type set to '0xC2' and Opt Data Len set to 4. For parcels, the first four octets of the Option Data are formatted exactly as shown in Figure 1 while for advanced jumbos the first four octets are exactly as shown in Figure 7. The network layer prepares all other aspects of IPv6 minimal parcels and advanced jumbos exactly the same as for the basic specifications found in previous sections except the option type/length are different and the Code/Check fields are omitted.¶

This implies that implementations that honor the basic IPv6 parcel and advanced jumbo formats and processing specified in the previous sections MUST also honor the IPv6 Minimal Parcel/Jumbo Option format specified above as an equivalent construct. Therefore, the Parcel/Jumbo probe results received for the basic formats also serve as probe results for the minimal format.¶

Since the minimal format does not include Code and Check fields, intermediate and end systems must monitor the lengths of minimal parcels and advanced jumbos they receive in case the path changes and an unqualified previous hop begins truncating them. In that case, the node MUST drop the packet and return a negative Jumbo Report to the source which must then re-initiate parcel/jumbo path probing.¶

10. OMNI IP Parcels/Advanced Jumbos

Network intermediate systems often drop IPv4 packets that contain IP header options unconditionally. This presents an obstacle to deploying new IPv4 options in the Internet, but may be less of a concern within some limited domain networks. As a first alternative, the source could encode IPv4 parcel and advanced jumbo options as IPv6 extension headers; for example, the source could set the IPv4 header Protocol to 0 and include an IPv6 Hop-by-Hop option immediately after the header. Since intermediate systems are also known to drop packets with IPv6 extension headers, however, the source could instead employ a second alternative more likely to provide service by concealing IPv6 options within the body of a protocol data unit such as UDP.¶

End systems and intermediate systems that recognize the OMNI protocol [I-D.templin-intarea-omni] can use the parcel, advanced jumbo and minimal parcel/jumbo formats specified in this document as native protocol extension headers coded within the body of the OMNI protocol data unit. This is true for both IPv6 and IPv4, where IPv4 parcels and advanced jumbos can use the same extension header formats defined for IPv6.¶

The section titled "OMNI L2 Extension Header Encapsulation" in [I-D.templin-intarea-omni] depicts protocol layering for encapsulation of IPv6 Extension Headers in IPv4 and IPv6 packets as shown in Figure 10:¶

   +---------------------------+
   |   L2 IP/Ethernet Header   |
   +---------------------------+
   | L2 UDP Header (port 8060) |
   +---------------------------+
   ~ L2 IPv6 Extension Headers ~
   +---------------------------+
   |   OAL IPv6 Encapsulation  |
   +---------------------------+
   ~    OAL IPv6 Extensions    ~
   +---------------------------+
   |                           |
   ~                           ~
   ~    Original IP Packet     ~
   ~                           ~
   |                           |
   +---------------------------+

In this encapsulation format, the IPv6 parcel, advanced jumbo and minimal parcel/jumbo extension headers specified in previous sections as well as the IPv6 (Extended) Fragment Header appear as IPv6 Extension Headers following the OMNI protocol UDP, IP or Ethernet header. The OMNI protocol requires each node to honor and implement the parcel and advanced jumbo constructs as specified in this document with reference to [I-D.templin-intarea-omni]. This includes the setting of the IP {Total, Payload} length fields as well as the settings of the parcel/jumbo options themselves.¶

Intermediate systems that do not recognize the OMNI protocol are likely to drop any OMNI packets that include parcel or advanced jumbo options, but they may instead forward the packet without updating the Code/Check values and/or while truncating the overall packet length. Intermediate systems and end systems that recognize OMNI therefore perform the checks specified in this document to determine whether previous path hops correctly process parcels and advanced jumbos.¶

Since parcel and advanced jumbo options are coded within the OMNI protocol data unit itself instead of as an IP header extension, network intermediate systems must also reset the OMNI protocol checksum if necessary when they alter the contents of an option (such as when resetting Path MTU or Check). For this reason, sources MAY disable the OMNI protocol checksum in path probes and SHOULD advance to using minimal parcels and advanced jumbos soon after probing the path to minimize intermediate system checksum interactions.¶

See: [I-D.templin-intarea-omni] for the full specification of OMNI L2 Extension Header encapsulation and processing. All parcel and advanced jumbo implementations that recognize the OMNI protocol are required to implement those portions of the OMNI specification.¶

11. Integrity

IP parcel and advanced jumbo integrity assurance responsibility is shared between lower layers of the protocol stack and the transport layer where more discrete compensations for lost or corrupted data recovery can be applied. In particular, intermediate system lower layers forward parcels or advanced jumbos with correct headers to the final destination transport layer even if there may have been cumulative bit errors incurred at intermediate hops. The destination is then responsible for its own integrity assurance.¶

The {TCP,UDP}/IP header plus each segment of a (multi-segment) IP parcel or advanced jumbo includes its own integrity checks. This means that IP parcels and advanced jumbos offer stronger and more discrete integrity checks for the same amount of transport layer protocol data compared to an individual IP packet or jumbogram. The {TCP,UDP} Checksum header integrity check SHOULD be verified at each hop for which a link layer CRC error is encountered to ensure that IP parcels and advanced jumbos with errored addressing information are detected. The per-segment Checksums and CRCs are set by the source and verified by the final destination. Note that each segment includes both checks since there will be many instances when errors missed by the CRC are detected by the Checksum [STONE].¶

IP parcels can range in length from as small as only the {TCP,UDP}/IP headers plus a single segment to as large as the headers plus (64 * 65535) octets, while advanced jumbos include only a single segment that can be as large as 2**32 octets (minus headers). Due to parcellation/packetization in the path, the segment contents of a received parcel may arrive in an incomplete and/or rearranged order with respect to their original packaging.¶

IP parcels and advanced jumbos include a separate 2-octet Internet Checksum header for each segment. The original source calculates the checksum for each segment beginning with the first octet of the per-segment Sequence Number for TCP or beginning with the first octet of the segment for UDP (noting that per-segment Checksum values of 0 still mean that the UDP checksum is disabled). The source extends the checksum calculation over the entire length of the segment (plus sequence number for TCP) but does not extend the calculation into the trailing CRC field.¶

IP parcels employ two different CRC types according to the non-final segment length "L". For values of L smaller than 8192 octets (8KB), the CRC32C specification is used [RFC3385] and the CRC is encoded in a 4 octet trailer. For larger L values, the CRC64E specification is used [ECMA-182] and the CRC is encoded in an 8 octet trailer. Advanced jumbos instead include an N-octet message digest trailer calculated per [RFC1321], [RFC3174] or [RFC6234] where N is determined according to the hash algorithm assigned to the Advanced Jumbo type (see: IANA Considerations).¶

When link layer CRC errors are detected, each network layer forwarding hop as well as the final destination SHOULD verify the IP parcel or advanced jumbo {TCP,UDP}/IP Checksum at its layer, since an errored header could result in mis-delivery. If a network layer protocol entity on the path detects an incorrect {TCP,UDP}/IP Checksum it should discard the entire IP parcel or advanced jumbo unless the header(s) can somehow first be repaired by lower layers.¶

To support the IP parcel and advanced jumbo {TCP,UDP}/IP header checksum calculation, the network layer uses modified versions of the {TCP,UDP}/IPv4 pseudo-header found in [RFC9293] [RFC0768], or the {TCP,UDP}/IPv6 pseudo-header found in Section 8.1 of [RFC8200]. Note that while the contents of the two IP protocol version-specific pseudo-headers beyond the address fields are the same, the order in which the contents are arranged differs and must be honored according to the specific IP protocol version as shown in Figure 11. This allows for maximum reuse of widely deployed code while ensuring interoperability.¶

                       IPv4 Parcel Pseudo-Header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      IPv4 Source Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    IPv4 Destination Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      zero     |  Next Header  |        Segment Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Index   |P|S|            Parcel Payload Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                       IPv6 Parcel Pseudo-Header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                      IPv6 Source Address                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                   IPv6 Destination Address                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Index   |P|S|            Parcel Payload Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Segment Length         |      zero     |  Next Header  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where the following fields appear in both pseudo-headers:¶

• Source Address is the 4-octet IPv4 or 16-octet IPv6 source address of the prepared parcel.¶

• Destination Address is the 4-octet IPv4 or 16-octet IPv6 destination address of the prepared parcel.¶

• zero encodes the constant value 0.¶

• Next Header is the IP protocol number corresponding to the transport layer protocol, i.e., TCP or UDP.¶

• Segment Length is the value that appears in the IP {Total, Payload} Length field of the prepared parcel.¶

• For IP parcels, [Index, P, S] is the combined 1-octet field and Parcel Payload Length is the 3-octet field that appear in the Parcel Payload Option fields of the same name. (For advanced jumbos, these two fields are replaced by a single 4-octet Jumbo Payload Option field.)¶

When the transport layer protocol entity of the source delivers a parcel body to the network layer, it presents the values L and J along with the (J + 1) segments in canonical order as a list of data buffers (and with each TCP segment preceded by a 4-octet Sequence Number field). When the network layer of the source accepts the parcel body from the transport layer protocol entity, it calculates the Internet checksum for each segment and writes the value in the per-segment Checksum header (or writes the value 0 when UDP checksums are disabled). The network layer then calculates the CRC for each segment beginning with the Checksum field, inserts the CRC result as a segment trailer in network byte order, then concatenates all segments and appends the necessary {TCP,UDP}/IP headers and extensions to form a parcel. The network layer then calculates the {TCP,UDP}/IP header checksum over the length of only the {TCP,UDP} headers plus IP pseudo header then forwards the parcel to the next hop without further processing.¶

When the network layer of the destination reunifies a parcel from one or more sub-parcels received from the source it first verifies the {TCP,UDP}/IP header checksum then verifies first the CRC and next the Checksum for each segment and marks any with incorrect integrity check values as errors. When the network layer restores a parcel from one or more individual {TCP,UDP}/IP packets received from the source, it instead marks the CRCs of each segment as correct since the individual packets were subject to CRC checks at each hop along the path. The network layer then verifies the Internet checksum of each individual packet (except when UDP checksums are disabled), restores the parcel, and delivers each parcel segment along with a CRC/Checksum error flag to the transport layer.¶

When the transport layer of the destination processes parcel or advanced jumbo segments, it can accept any with correct CRCs and Checksums while optionally applying additional higher-layer integrity checks. The transport layer can instead process any segments with CRC/Checksum errors by either discarding the entire segment or applying higher-layer integrity checks on the component elements of the segment to accept as many non-errored elements as possible. The transport layer can then either reconstruct from local information or request retransmission for any segment elements that may have been damaged in transit as necessary.¶

Note: when the destination network layer detects a per-segment CRC error, it immediately posts the segment plus an error code for delivery to the transport instead of continuing to verify the segment Checksum. Performing a second integrity check on a segment already determined to contain errors by a first check would serve no useful purpose.¶

Note: the source and destination network layers can often engage hardware functions to greatly improve CRC/Checksum calculation performance.¶

12. Implementation Status

Common widely-deployed implementations include services such as TCP Segmentation Offload (TSO) and Generic Segmentation/Receive Offload (GSO/GRO). These services support a robust service that has been shown to improve performance in many instances.¶

UDP/IPv4 parcels have been implemented in the linux-5.10.67 kernel and ION-DTN ion-open-source-4.1.0 source distributions. Patch distribution found at: "https://github.com/fltemplin/ip-parcels.git".¶

Performance analysis with a single-threaded receiver has shown that including increasing numbers of segments in a single parcel produces measurable performance gains over fewer numbers of segments due to more efficient packaging and reduced system calls/interrupts. For example, sending parcels with 30 2000-octet segments shows a 48% performance increase in comparison with ordinary IP packets with a single 2000-octet segment.¶

Since performance is strongly bounded by single-segment receiver processing time (with larger segments producing dramatic performance increases), it is expected that parcels with increasing numbers of segments will provide a performance multiplier on multi-threaded receivers in parallel processing environments.¶

13. IANA Considerations

The IANA is instructed to add a reference to this document ([RFCXXXX]) in the "MTUP - MTU Probe" and "MTUR - MTU Reply" entries in the "IP Option Numbers" section of the 'ip-parameters' registry.¶

The IANA is instructed to add a reference to this document ([RFCXXXX]) in the "Minimum Path MTU Hop-by-Hop Option" entry in the "Destination Options and Hop-by-Hop Options" section of the 'ipv6-parameters' registry.¶

The IANA is instructed to create and maintain a new registry titled "IP Parcel and Advanced Jumbo Formats and Types". For IPv4 parcels and Advanced Jumbos, the value in the 'option-length' field of Probe/Reply MTU options [RFC1063] serves as an "Option Format" code that distinguishes the various IPv4 option formats specified in this document. Initial values are given below:¶

   Value       Option Format                   Reference
   -----       -------------                   ---------
   4           Probe/Reply MTU                 [RFC1063]
   8           Parcel/Advanced Jumbo           [RFCXXXX]
   12          Parcel/Advanced Jumbo Probe     [RFCXXXX]
   16          Jumbo Reply MTU ( 4-octet ID)   [RFCXXXX]
   20          Jumbo Reply MTU ( 8-octet ID)   [RFCXXXX]
   24          Jumbo Reply MTU (12-octet ID)   [RFCXXXX]
   28          Jumbo Reply MTU (16-octet ID)   [RFCXXXX]
   3-7         Unassigned                      [RFCXXXX]
   9-11        Unassigned                      [RFCXXXX]
   13-15       Unassigned                      [RFCXXXX]
   17-19       Unassigned                      [RFCXXXX]
   21-23       Unassigned                      [RFCXXXX]
   25-27       Unassigned                      [RFCXXXX]
   29-253      Unassigned                      [RFCXXXX]
   254         Reserved for Experimentation    [RFCXXXX]
   255         Reserved by IANA                [RFCXXXX]

For IPv6 parcels and Advanced Jumbos, the value in the 'Opt Data Len' field of the IPv6 Minimum Path MTU Hop-by-Hop Option [RFC9268] serves as an "Option Format" code that distinguishes the various IPv6 option formats specified in this document. Initial values are given below:¶

   Value       Option Format                   Reference
   -----       -------------                   ---------
   4           IPv6 Minimum Path MTU           [RFC9268]
   6           Parcel/Advanced Jumbo           [RFCXXXX]
   10          Parcel/Advanced Jumbo Probe     [RFCXXXX]
   12          Jumbo Reply MTU ( 4-octet ID)   [RFCXXXX]
   16          Jumbo Reply MTU ( 8-octet ID)   [RFCXXXX]
   20          Jumbo Reply MTU (12-octet ID)   [RFCXXXX]
   24          Jumbo Reply MTU (16-octet ID)   [RFCXXXX]
   5           Unassigned                      [RFCXXXX]
   7-9         Unassigned                      [RFCXXXX]
   11          Unassigned                      [RFCXXXX]
   13-15       Unassigned                      [RFCXXXX]
   17-19       Unassigned                      [RFCXXXX]
   21-23       Unassigned                      [RFCXXXX]
   25-253      Unassigned                      [RFCXXXX]
   254         Reserved for Experimentation    [RFCXXXX]
   255         Reserved by IANA                [RFCXXXX]

For all Parcels/Advanced Jumbos and their corresponding probes, the IP {Total, Header} Length field encodes a "Jumbo Type" value instead of an ordinary total/payload length. Initial values are given below:¶

   Value      Jumbo Type                     Reference
   -----      ----------                     ---------
   0          Basic Jumbogram (IPv6 only)    [RFC2675]
   1          Reserved for Experimentation   [RFCXXXX]
   2          Reserved by IANA               [RFCXXXX]
   3-248      Unassigned                     [RFCXXXX]
   249        Advanced Jumbo / CRC128J       [RFCXXXX]
   250        Advanced Jumbo / MD5           [RFCXXXX]
   251        Advanced Jumbo / SHA1          [RFCXXXX]
   252        Advanced Jumbo / SHA-224       [RFCXXXX]
   253        Advanced Jumbo / SHA-256       [RFCXXXX]
   254        Advanced Jumbo / SHA-384       [RFCXXXX]
   255        Advanced Jumbo / SHA-512       [RFCXXXX]
   256-8192   IP Parcel / CRC32C             [RFCXXXX]
   8193-65535 IP Parcel / CRC64E             [RFCXXXX]

14. Security Considerations

In the control plane, original sources match any identifying information in received Parcel/Jumbo Reports and IP Jumbo Reply MTU options with their corresponding probes. If the information matches, the report is likely authentic. In environments where stronger authentication is necessary, nodes that send Parcel and/or Jumbo Reports can apply the message authentication services specified for AERO/OMNI.¶

In the data plane, multi-layer security solutions may be needed to ensure confidentiality, integrity and availability. Since parcels and advanced jumbos are defined only for TCP and UDP, IPsec-AH/ESP [RFC4301] cannot be applied in transport mode although they can certainly be used in tunnel mode at lower layers such as for transmission of parcels and advanced jumbos over OMNI link secured spanning trees, VPNs, etc. Since the network layer does not manipulate transport layer segments, parcels and advanced jumbos do not interfere with transport or higher-layer security services such as (D)TLS/SSL [RFC8446] which may provide greater flexibility in some environments.¶

IPv4 fragment reassembly is known to be dangerous at high data rates where undetected reassembly buffer corruptions can result from fragment misassociations [RFC4963]. IPv6 is less subject to these concerns when the 32-bit Identification field is managed responsibly. However, both IPv4 and IPv6 can robustly support high data rate reassembly using Identification Extension Options for the Internet Protocol [I-D.templin-intarea-ipid-ext].¶

IP parcels and advanced jumbos present a new link service model for the Internet in which intermediate systems may forward packets that incurred link errors and end systems are responsible for detecting any link errors incurred along the path. The destination end system in particular is uniquely positioned to verify and/or correct the integrity of any transport layer segments received. For this reason, transport layer protocols that use IP parcels and/or advanced jumbos should include higher layer error detection/correction codes in addition to the per-segment link error integrity checks.¶

The message digests included with Advanced Jumbos are used as integrity checks and must not be considered as security signatures in the absence of other supporting codes. Further security considerations related to IP parcels and Advanced Jumbos are found in the AERO/OMNI specifications.¶

15. Acknowledgements

This work was inspired by ongoing AERO/OMNI/DTN investigations. The concepts were further motivated through discussions with colleagues.¶

A considerable body of work over recent years has produced useful segmentation offload facilities available in widely-deployed implementations.¶

With the advent of networked storage, big data, streaming media and other high data rate uses the early days of Internetworking have evolved to accommodate the need for improved performance. The need fostered a concerted effort in the industry to pursue performance optimizations at all layers that continues in the modern era. All who supported and continue to support advances in Internetworking performance are acknowledged.¶

This work has been presented at working group sessions of the Internet Engineering Task Force (IETF). The following individuals are acknowledged for their contributions: Roland Bless, Scott Burleigh, Madhuri Madhava Badgandi, Joel Halpern, Tom Herbert, Andy Malis, Herbie Robinson, Bhargava Raman Sai Prakash.¶

Honoring life, liberty and the pursuit of happiness.¶

Appendix A. TCP Extensions for High Performance

TCP Extensions for High Performance are specified in [RFC7323], which updates earlier work that began in the late 1980's and early 1990's. These efforts determined that the TCP 16-bit Window was too small to accommodate sustained transmission at high data rates and devised a TCP Window Scale option to allow window sizes up to 2^30. The work also defined a Timestamp option used for round-trip time measurements and as a Protection Against Wrapped Sequences (PAWS) at high data rates. TCP users of IP parcels are strongly encouraged to adopt these measures.¶

Since TCP/IP parcels only include control bits for the first segment ("segment(0)"), nodes must regard all other segments of the same parcel as data segments. When a node breaks a TCP/IP parcel out into individual packets or sub-parcels, only the first packet/sub-parcel contains the original segment(0) and therefore only its TCP header retains the control bit settings from the original parcel TCP header. If the original TCP header included TCP options such as Maximum Segment Size (MSS), Window Scale (WS) and/or Timestamp, the node copies those same options into the options section of the new TCP header.¶

For all other packets/sub-parcels, the note sets all TCP header control bits to 0 as data segment(s). Then, if the original parcel contained a Timestamp option, the node copies the Timestamp option into the options section of the new TCP header. Appendix A of [RFC7323] provides implementation guidelines for the Timestamp option layout.¶

Appendix A of [RFC7323] also discusses Interactions with the TCP Urgent Pointer as follows: "if the Urgent Pointer points beyond the end of the TCP data in the current segment, then the user will remain in urgent mode until the next TCP segment arrives. That segment will update the Urgent Pointer to a new offset, and the user will never have left urgent mode". In the case of IP parcels, however, it will often be the case that the next TCP segment is included in the same (sub-)parcel as the segment that contained the urgent pointer such that the urgent pointer can be updated immediately.¶

Finally, if the parcel contains more than 65535 octets of data (i.e., spread across multiple segments), then the Urgent Pointer can be regarded in the same manner as for jumbograms as described in Section 5.2 of [RFC2675].¶

Appendix B. Extreme L Value Implications

For each parcel, the transport layer can specify any L value between 256 and 65535 octets. Transport protocols that send isolated control and/or data segments smaller than 256 octets should package them as ordinary packets or as the final segment of a parcel. It is also important to note that segments smaller than 256 octets are likely to include control information for which timely delivery rather than bulk packaging is desired. Transport protocol streams therefore often include a mix of (larger) parcels and (smaller) ordinary packets.¶

The transport layer should also specify an L value no larger than can accommodate the maximum-sized transport and network layer headers that the source will include without causing a single segment plus headers to exceed 65535 octets. For example, if the source will include a 28 octet TCP header plus a 40 octet IPv6 header with 24 extension header octets (plus 6/8 octets for the per-segment Checksum/CRC) the transport should specify an L value no larger than (65535 - 28 - 40 - 24 - 10) = 65433 octets.¶

The transport can specify still larger "extreme" L values up to 65535 octets, but the resulting parcels might be lost along some paths with unpredictable results. For example, a parcel with an extreme L value set as large as 65535 might be able to transit paths that can pass jumbograms natively but might not be able to transit a path that includes non-jumbo links. The transport layer should therefore carefully consider the benefits of constructing parcels with extreme L values larger than the recommended maximum due to high risk of loss compared with only minor potential performance benefits.¶

Parcels that include extreme L values larger than the recommended maximum and with a maximum number of included segments could also cause a parcel to exceed 16,777,215 (2**24 - 1) octets in total length. Since the Parcel Payload Length field is limited to 24 bits, however, the largest possible parcel is also limited by this size. See also the above risk/benefit analysis for parcels that include extreme L values larger than the recommended maximum.¶

Appendix C. Additional Parcel/Jumbo Probe Considerations

After sending a Parcel/Jumbo Probe, the source may receive a Parcel/Jumbo Report from either a router on the path or from the final destination itself. Alternatively, the source can shape its probes to request IP Jumbo Reply MTU options carried by ordinary data packets on the return path from the destination.¶

If a router or final destination receives a Parcel/Jumbo Probe but does not recognize the parcel/jumbo constructs, it will likely drop the probe without further processing and may return an ICMP error. The original source will then consider the probe as lost, but may attempt to probe again later, e.g., in case the path may have changed.¶

When the source examines the "packet in error" portion of a Parcel/Jumbo Report, it can easily match the Report against its recent transmissions if the (extended) Identification value is available. For each "packet in error" that does not include an (extended) Identification, the source can attempt to match based on any other identifying information; otherwise, it should discard the message.¶

If the source receives multiple Parcel/Jumbo Reports for a single parcel/jumbo sent into a given path, it should prefer any information reported by the final destination over information reported by a router. For example, if a router returns a negative report while the destination returns a positive report the latter should be considered as more-authoritative. For this reason, the source should provide a configuration knob allowing it to accept or ignore reports that originate from routers, e.g., according to the network trust model.¶

When a destination returns a Parcel/Jumbo Report, it can optionally pair the report with an ordinary data packet that it returns to the original source. For example, the OMNI specification includes a "super-packet" service that allows multiple independent IP packets to be encapsulated as a single adaptation layer packet. This is distinct from an IP parcel in that each packet member of the super-packet includes its own IP (and possibly other upper layer) header.¶

A source can request to receive two different types of parcel/jumbo path MTU feedback from the destination - a UDP encapsulated Parcel/Jumbo Report in response to a probe sent to port 9 (discard), or an ordinary data packet with an IP Jumbo Reply MTU option in response to a probe sent into an ordinary transport layer protocol flow. In some environments, one or both of these MTU feedback types may be erroneously dropped by a router along the return path. The source may therefore attempt to probe first using "method A", and then try again using "method B", e.g., if there is no response. In environments where ongoing transport protocol sessions are established, it is recommended that the source engage the IP Jumbo Reply MTU option as "method A".¶

Appendix D. Advanced Jumbo Cyclic Redundancy Check (CRC128J)

This section postulates a 128-bit Cyclic Redundancy Check (CRC) algorithm for Advanced Jumbos termed "CRC128J". An Advanced Jumbo Type value 249 is reserved for CRC128J, but at the time of this writing no algorithm specification exists. Future specifications of CRC128J may update this document and provide instructions for handling Advanced Jumbos with Type 249.¶

Appendix E. Change Log

<< RFC Editor - remove prior to publication >>¶

Changes from earlier versions:¶

• Submit for review.¶

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