| Internet-Draft | IP Parcels | December 2021 |
| Templin | Expires 21 June 2022 | [Page] |
IP packets (both IPv4 and IPv6) are understood to contain a unit of data which becomes the retransmission unit in case of loss. Upper layer protocols such as the Transmission Control Protocol (TCP) prepare data units known as "segments", with traditional arrangements including a single segment per packet. This document presents a new construct known as the "IP Parcel" which permits a single packet to carry multiple segments, essentially creating a "packet-of-packets". The parcel can be opened at middleboxes on the path with the included segments broken out into individual packets, then rejoined into one or more repackaged parcels to be forwarded further toward the final destination. Reordering of segments within parcels is unimportant; what matters is that the number of parcels delivered to the final destination should be kept to a minimum, and that loss or receipt of individual segments (and not parcel size) determines the retransmission unit.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 21 June 2022.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
IP packets (both IPv4 [RFC0791] and IPv6 [RFC8200]) are understood to contain a unit of data which becomes the retransmission unit in case of loss. Upper layer protocols such as the Transmission Control Protocol (TCP) [RFC0793] prepare data units known as "segments", with traditional arrangements including a single segment per packet. This document presents a new construct known as the "IP Parcel" which permits a single packet to carry multiple segments. This essentially creates a "packet-of-packets" with the IP layer headers appearing only once but with possibly multiple upper layer protocol segments.¶
Parcels are formed when an upper layer protocol entity (identified by the "5-tuple" source IP address/port number, destination IP address/port number and protocol number) prepares a buffer of data with the concatenation of up to 64 properly-formed segments that could stand alone if broken out into individual packets using a copy of the IP header. All segments except the final segment must be equal in size, while the final segment must not be larger than the others but may be smaller. Each non-final segment must be no larger than 65535 minus the length of the IP header plus extensions. The upper layer protocol entity then delivers the buffer and non-final segment size to the IP layer, which appends the necessary IP headers to identify this as a parcel and not an ordinary packet.¶
Each parcel can be opened at a first-hop middlebox on the path with the included segments broken out into individual packets, then rejoined into one or more parcels at a last-hop middlebox to be forwarded to the final destination. Reordering of segments within a parcel or even repackaging of parcels entirely is unimportant; what matters is that the number of parcels delivered to the final destination should be kept to a minimum, and that loss or receipt of individual segments (and not parcel size) determines the retransmission unit.¶
The following sections discuss rationale for creating and shipping parcels as well as the actual protocol constructs and procedures involved. It is expected that the parcel concept may drive future innovation in application, operating system, network equipment and data link design.¶
Studies have shown that applications that send and receive large packets can realize greater performance due to reduced numbers of system calls and interrupts as well as larger atomic data copies between kernel and user space. Within the network, large packets also result in reduced numbers of device interrupts and better network utilization in comparison with smaller packet sizes.¶
The issue with sending large packets is that they are often lost at links with smaller Maximum Transmission Units (MTUs), and the resulting Packet Too Big (PTB) message may be lost somewhere in the path back to the original source. This "Path MTU black hole" condition can cripple application performance unless also supplemented with robust path probing techniques, however the best case performance always occurs when no packets are lost due to size restrictions.¶
These considerations therefore motivate a design where the maximum segment size should be no larger than 65535 minus IP header sizes, while parcels that carry the segments may themselves be significantly larger. Then, even if a middlebox needs to open the parcels to deliver individual segments further toward final hops as separate IP packets, an important performance optimization for both the original source and final destination can be realized.¶
An analogy: when an end user orders 50 small items from a major online retailer, the retailer does not ship the order in 50 separate small boxes. Instead, the retailer puts as many of the small boxes as possible into one or a few larger boxes (or parcels) then puts these parcels on a semi-truck or airplane. The parcels arrive at a regional distribution center where they may be further redistributed into slightly smaller parcels that get delivered to the end user. But most often, the end user will only find one or a few parcels at his doorstep and not 50 individual boxes.¶
IP parcel formation is invoked by an upper layer protocol (identified by the 5-tuple as above) when it produces a data buffer containing the concatenation of up to 64 segments. All non-final segments MUST be equal in length while the final segment MUST NOT be larger but MAY be smaller. Each non-final segment MUST be no larger than 65535 minus the length of the IP header plus extensions. The application then presents the buffer and non-final segment size to the IP layer which appends a single IP header (plus any extension headers) before presenting the parcel to lower layers.¶
For IPv4, the IP layer prepares the parcel by appending an IPv4 header with a Jumbo Payload option (identified by option code TBD) formed as follows:¶
+--------+--------+--------+--------+--------+--------+ |000(TBD)|00000110| Jumbo Payload Length | +--------+--------+--------+--------+--------+--------+¶
where "Jumbo Payload Length" is a 32-bit unsigned integer value (in network byte order) set to the lengths of the IPv4 header plus all concatenated segments. The IP layer next sets the IPv4 header DF bit to 1, then sets the IPv4 header Total Length field to the length of the IPv4 header plus the length of the first segment only. Note that the IP layer can form true IPv4 jumbograms (as opposed to parcels) by instead setting the IPv4 header Total Length field to 0.¶
For IPv6, the IP layer forms a parcel by appending an IPv6 header with a Jumbo Payload option [RFC2675] the same as for IPv4 above where "Jumbo Payload Length" is set to the lengths of the IPv6 Hop-by-Hop Options header and any other extension headers present plus all concatenated segments. The IP layer next sets the IPv6 header Payload Length field to the lengths of the IPv6 Hop-by-Hop Options header and any other extension headers present plus the length of the first segment only. As with IPv4 the IP layer can form true IPv6 jumbograms (as opposed to parcels) by instead setting the IPv6 header Payload Length field to 0.¶
The IP layer next presents the parcel to the next lower layer. If the lower layer is the OMNI Adaptation Layer (OAL) [I-D.templin-6man-omni], the OAL source can open the parcel if necessary and forward each segment as an individual IP packet. These individual packets eventually arrive at the OAL destination which re-combines them into a new parcel or parcels to forward to the final destination. Details for OAL parcel forwarding are discussed in [I-D.templin-6man-omni].¶
If the lower layer is a true data link layer interface, however, the IP layer instead forwards the parcel according to the path MTU to either the first middlebox that configures an OAL layer or the final destination itself, whichever comes first. If the parcel is no larger than the path MTU, the IP layer simply forwards the parcel the same as it would an ordinary IP packet and processes any PTB messages that may be returned (but, see below for compatibility issues). If the parcel is larger than 65535 (minus encapsulation headers) and also larger than the path MTU, the IP layer instead discards the parcel and returns a packet size error to the upper layer protocol.¶
If the parcel is no larger than 65535 (minus encapsulation headers) but larger than the path MTU, the IP layer instead performs IP encapsulation with destination set to the IP address of the middlebox or final destination and (Payload Length / Total Length) set to the Jumbo Payload Length plus encapsulation header length then performs source-fragmentation on the encapsulated parcel the same as for an ordinary IP packet by generating IP fragments destined for the middlebox or final destination.¶
When the middlebox or final destination receives the fragments or whole parcels, it reassembles then discards the encapsulation headers if necessary then presents the parcel to the OAL in the middlebox case or the upper layer protocol in the final destination case.¶
Parcels can range in length from as small as the size of the IP headers plus a single octet to as large as the IP headers plus (64 * (2**16 minus headers)) octets. Although link layer integrity checks provide sufficient protection for contiguous blocks of data up to approximately 9KB, reliance on the presence of link-layer integrity checks may not be possible over links such as tunnels. Moreover, the segment contents of a received parcel may arrive in an incomplete and/or rearranged order with respect to their original packaging.¶
For these reasons, upper layers should include individual integrity checks with each segment included in the parcel with a strength compatible with the segment length. The integrity check should then be verified at the receiver on a per-segment basis, which discards any corrupted segments and considers them as a loss event.¶
Legacy networking gear that forwards parcels over ordinary data links may not recognize this new coding of the Jumbo Payload extension header and may act only on what is observed in the IPv4 Total Length or IPv6 Payload Length field. In that case, the legacy gear would likely forward the first segment of the parcel only while truncating the remainder since only the length of the first segment is included in the IP header.¶
In networks where compatibility is thought to be an issue, the original source can perform encapsulation on parcels uniformly whether or not fragmentation is required to ensure they are delivered to the OAL source or final destination (whichever comes first). In the same way the OAL destination can uniformly perform encapsulation to ensure that parcels are delivered to the final destination.¶
Section 3 of [RFC2675] provides a list of certain conditions to be considered as errors. In particular:¶
Implementations that obey this specification ignore these conditions and do not consider them as errors.¶
TBD.¶
The IANA is instructed to allocate a new IP option code in the 'ip option numbers' registry for the IPv4 Jumbo Payload option. The Copy and Class fields must both be set to 0, and the Number field must be set to 'TBD'.¶
Communications networking security is necessary to preserve confidentiality, integrity and availability.¶
This work was inspired by ongoing AERO/OMNI/DTN investigations. The concepts were further motivated through discussions on the intarea list.¶
A considerable body of work over recent years has produced useful "segmentation offload" facilities available in widely-deployed implementations.¶
.¶