Boeing Research & Technology

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I-D Internet-Draft IPv6 Parcels and Advanced Jumbos (AJs) present new data packaging constructs and a new link model for Internetworking. As is often the case, technologies developed in the IPv6 space can also be applied in IPv4 and vice-versa. This document presents the adaptations necessary to support Parcels and AJs in IPv4.

IPv6 Parcels and Advanced Jumbos (AJs) present new data packaging constructs and a new link model for Internetworking. As is often the case, technologies developed in the IPv6 space can also be applied in IPv4 and vice-versa. This document presents the differences that need to be addressed to adapt IPv6 Parcels and AJs to IPv4. All aspects of IPv6 Parcels and AJs, including the use of IPv6 extension headers and control messaging, apply also to IPv4. Only differences in the IP header format and some control option encapsulations need to be accounted for as discussed below. This document therefore specifies IPv4 parcels and AJs.

IPv4 parcels and AJs observe all requirements established for IPv6 including the use of IPv6 Hop-by-Hop Options headers. This means that nodes that recognize IPv4 parcels/AJs MUST recognize and correctly process IP protocol 0 (Hop-by-Hop) option headers the same as for IPv6 when they occur in an extension header chain following the IPv4 header but before the upper layer payload. When an IPv4 router or destination end system processes a parcel or AJ for which the IPv4 Protocol field encodes an unrecognized value (such as 0 for Hop-by-Hop Options), it drops the parcel/AJ and returns an ICMPv4 "Destination Unreachable - Protocol Unreachable" message . The source then regards any such messages as an advisory indication that parcels/AJs may not be supported along this path. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

All aspects of are imported as normative specifications for IPv4 parcels and AJs, with the exception of the following differences:

The IPv6 header includes a "Payload Length" field defined as the: "Length of the IPv6 payload, i.e., the rest of the packet following this IPv6 header, in octets". The IPv4 header instead includes a "Total Length" field defined as: "the length of the datagram, measured in octets, including internet header and data". These differences have no bearing for parcels/AJs, for which both the IPv6 Payload Length and IPv4 Total Length values carry identical codes. The same as for IPv6, IPv4 parcels encode the length "L" of the first segment in Total Length, while IPv4 AJs encode a jumbo type value in Total Length.

The IPv4 "Time To Live (TTL)" and IPv6 "Hop Limit" values are treated in exactly the same way in both protocol versions. In particular, the source sets the TTL/Hop Limit to an initial value and each router in the path to the destination decrements the TTL/Hop Limit by 1 when it forwards the parcel/AJ.

The same as for IPv6, the Parcel Payload Length field in the Parcel Payload Option and the Jumbo Payload Length field in the Jumbo Payload Option of IPv4 parcels/AJs encode the length of the IPv6 extension headers plus the length of the {TCP,UDP} header plus the combined length of all concatenated segments with their per-segment headers/trailers. Therefore, the length of the IPv4 header itself is not included in the Parcel/Jumbo Payload Length field the same as for IPv6. The IPv4 header length for IPv4 parcels and AJs is instead calculated from the IPv4 header Internet Header Length (IHL) field the same as for ordinary IPv4 packets.

Whenever an IPv4 address needs to be coded in an IPv6 address field, the address is coded as an IPv4-compatible IPv6 address as specified in .

When a node performs packetization on a {TCP,UDP}/IPv4 parcel, it inserts a Parcel Segment {TCP,UDP} option the same as for IPv6 . The IPv4 destination then performs restoration by gathering up IPv4 packets that arrive with the same upper layer 5-tuple and with Parcel Segment information including the same Identification. The Parcel Segment Index then determines the ordinal position of each packet segment to be concatenated into the restoration buffer, i.e., the same as for IPv6. (Note: if the IPv4 destination does not recognize the {TCP,UDP} Parcel Segment option, it simply processes the packet as a singleton IPv4 packet. This would result in correct behavior, but would fail to take advantage of Generic Receive Offload (GRO) benefits.)

When an IPv4 router receives an intact Parcel/Jumbo probe, it can return an immediate Parcel/Jumbo Reply if necessary in an ICMPv6 Packet Too Big (PTB) message prepared the same as for IPv6 but wrapped in UDP/IPv4 encapsulation headers. The Reply will then traverse the IPv4 network on the reverse path to the source. When an IPv4 destination receives a Parcel/Jumbo probe, it should instead return an MTU/Fragmentation Report option in any {TCP,UDP}/IPv4 packet to be returned to the source the same as for IPv6 . When the IPv4 source receives a {TCP,UDP} packet that includes an MTU/Fragmentation Report it matches the Identification value with one its recently-transmitted probes. If there is a match, the source then marks the destination as "Parcels/Jumbos supported" and records the MTU value found in the pseudo-option as the path MTU.

To support the IPv4 parcel/AJ header checksum calculation, the network layer uses modified versions of the {TCP,UDP}/IPv4 pseudo-header found in . 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. The IPv6 pseudo-header is found in , while the IPv4 pseudo-header is shown in . The similarities between the two pseudo-headers allows for maximal reuse of widely deployed code while ensuring interoperability.

Note: The same as for IPv6, the Index/P/S and Parcel Payload Length fields in the IPv4 parcel pseudo-header are replaced by the single 4-octet Jumbo Payload Length field in the IPv4 AJ pseudo-header.

An early prototype of UDP/IPv4 parcels (draft version -15) has been implemented relative to 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".

This document does not include any IANA instructions.

Security Considerations are the same as for IPv6 as found in .

This work was inspired by ongoing AERO/OMNI/DTN investigations. The concepts were further motivated through discussions with colleagues. Honoring life, liberty and the pursuit of happiness.

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