LTP Fragmentation

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Table of Contents

1. Introduction

The Licklider Transmission Protocol (LTP) [RFC5326] provides a reliable datagram convergence layer for the Delay/Disruption Tolerant Networking (DTN) Bundle Protocol (BP) [RFC9171]. In common practice, LTP is often configured over the User Datagram Protocol (UDP) [RFC0768] and Internet Protocol (IP) [RFC0791] using the "socket" abstraction. LTP inherits its maximum segment size from the maximum-sized UDP/IP datagram (i.e., 64KB minus header sizes), however when that size exceeds the maximum transmission unit the path can support a service known as IP fragmentation must be engaged.¶

LTP breaks BP bundles into "blocks", then further breaks these blocks into "segments". The segment size is a configurable option and represents the largest atomic portion of data that LTP will require underlying layers to deliver as a single unit. The segment size is therefore also known as the "retransmission unit", since each lost segment must be retransmitted in its entirety. Experimental and operational evidence has shown that on robust networks increasing the LTP segment size (up to the maximum UDP/IP datagram size of slightly less than 64KB) can result in substantial performance increases over smaller segment sizes. However, the performance increases must be tempered with the amount of IP fragmentation invoked as discussed below.¶

When LTP presents a segment to the operating system kernel (e.g., via a sendmsg() system call), the UDP layer prepends a UDP header to create a UDP datagram. The UDP layer then presents the resulting datagram to the IP layer for packet framing and transmission over a networked path. The path is further characterized by the path Maximum Transmission Unit (Path-MTU) which is a measure of the smallest link MTU (Link-MTU) among all links in the path.¶

When LTP presents a segment to the kernel that is larger than the Path-MTU, the resulting UDP datagram is presented to the IP layer which in turn performs IP fragmentation to break the datagram into fragments that are no larger than the Path-MTU. For example, if the LTP segment size is 64KB and the Path-MTU is 1280 octets IP fragmentation results in 50+ fragments that are transmitted as individual IP packets. (Note that for IPv4 [RFC0791], fragmentation may occur either in the source host or in a router in the network path, while for IPv6 [RFC8200] only the source host may perform fragmentation.)¶

Each IP fragment is subject to the same best-effort delivery service offered by the network according to current congestion and/or link signal quality conditions; therefore, the IP fragment size becomes known as the "loss unit". Especially when the packet loss rate is non-negligible, however, performance can suffer dramatically when the loss unit is significantly smaller than the retransmission unit during periods of congestion. In particular, if even a single IP fragment of a fragmented LTP segment is lost then the whole LTP segment is considered lost and must be retransmitted in its entirety. Since LTP does not support flow control or congestion control, this can result in a cascading flood of redundant information when fragments are systematically lost in transit due to congestion or disruption.¶

This document discusses LTP interactions with IP fragmentation and mitigations for managing the amount of fragmentation employed. It further discusses methods for increasing LTP performance even when IP fragmentation is engaged.¶

2. Terminology

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.¶

3. IP Fragmentation Issues

IP fragmentation is a fundamental service of the Internet Protocol, yet it has long been understood that its use can be problematic in some environments. Beginning as early as 1987, "Fragmentation Considered Harmful" [FRAG] outlined multiple issues with the service including a performance-crippling condition that can occur at high data rates when the loss unit is considerably smaller than the retransmission unit during intermittent and/or steady-state loss conditions.¶

Later investigations also identified the possibility for undetected corruption at high data rates due to a condition known as "ID wraparound" when the 16-bit IP identification field (aka the "IP ID") increments such that new fragments overlap with existing fragments still alive in the network and with identical ID values [RFC4963][RFC6864]. Although this condition is most acute for the IPv4 protocol (and much less so for IPv6 where the IP ID is 32-bits in length), the IPv4 concerns along with the fact that IPv6 does not permit routers to perform "network fragmentation" have led many to discourage the use of fragmentation whenever possible.¶

Even in the modern era, investigators have seen fit to declare "IP Fragmentation Considered Fragile" in an Internet Engineering Task Force (IETF) Best Current Practice (BCP) reference [RFC8900]. Indeed, the BCP recommendations cite the Bundle Protocol LTP convergence layer as a user of IP fragmentation that depends on some of its properties to realize greater performance. However, the BCP summarizes by saying:¶

• "Rather than deprecating IP fragmentation, this document recommends that upper-layer protocols address the problem of fragmentation at their layer, reducing their reliance on IP fragmentation to the greatest degree possible."¶

This conclusion was based on the historical state of IP fragmentation and did not seem to consider the opportunity for forward-looking improvements. With the advent of "Identification Extension for the Internet Protocol" [I-D.templin-intarea-ipid-ext], however, the status of IP fragmentation may soon need to be recharacterized from "fragile" to "robust". We therefore next discuss our systematic approach to LTP fragmentation while considering IP fragmentation as a potentially useful tool for performance maximization.¶

4. LTP Fragmentation

In common LTP implementations over UDP/IP (e.g., the Interplanetary Overlay Network (ION)), performance is greatly dependent on the LTP segment size. This is due to factors including that larger segments reduce the number of segments LTP has to manage and that larger segments presented to UDP/IP as single units incur only a single system call with a single data copy from application to kernel space via the sendmsg() system call. Once inside the kernel, each segment incurs UDP/IP encapsulation and IP fragmentation.¶

During fragmentation, each fragment is transmitted immediately following the previous without delay so that the fragments appear as a "burst" of consecutive packets over the network path resulting in high network utilization during the burst period. Additionally, the use of IP fragmentation with a larger segment size conserves header framing octets since the upper layer headers only appear in the first IP fragment as opposed to appearing in all fragments.¶

Conventional wisdom has for many decades suggested that in order to avoid retransmission congestion (i.e., especially when fragment loss probability is non-negligible) the LTP segment size should be set to no larger than the Path-MTU. Assuming the minimum IPv4 Effective MTU to Receive (EMTU_R) of 576 octets, however, transmission of 64KB of data using a 576B segment size would require well over 100 independent sendmsg() system calls and data copies as opposed to just one when the largest segment size is used. This greatly reduces the theoretical bandwidth advantage offered by IP fragmentation bursts. Therefore, a means for providing the best aspects of both large segment fragment bursting and small segment retransmission efficiency would seem beneficial.¶

Common operating systems such as linux provide the sendmmsg() ("send multiple messages") system call that allows the LTP application to present the kernel with a vector of up to 1024 segments instead of just a single segment. This theoretically affords the bursting behavior of IP fragmentation coupled with the retransmission efficiency of employing small segment sizes. (Note that LTP receivers can also use the recvmmsg() ("receive multiple messages") system call to receive a vector of segments from the kernel in case multiple recent packet arrivals can be combined.)¶

A first approach to performance maximization therefore analyzed implementations of LTP that employ a large block size, a conservative segment size and a new configuration option known as the "Burst-Limit" which determines the number of segments that can be presented in a single sendmmsg() system call. When the implementation forwards an LTP block, it carves Burst-Limit-many segments from the block and presents the vector of segments to sendmmsg(). The kernel will prepare each segment as an independent UDP/IP packet and transmit them into the network as a burst in a fashion that parallels IP fragmentation. The loss unit and retransmission unit will be the same, therefore loss of a single segment does not result in a retransmission congestion event.¶

It should be noted that the Burst-Limit is bounded only by the LTP block size and not by the maximum UDP/IP datagram size. Therefore, each burst can in practice convey significantly more data than a single IP fragmentation event. It should also be noted that the segment size can still be made larger than the Path-MTU in low-loss environments without danger of triggering retransmission storms due to loss of IP fragments. This would result in combined large UDP/IP message transmission and IP fragmentation bursting for increased network utilization in more robust environments. Finally, both the Burst-Limit and UDP/IP message sizes need not be static values, and can be tuned to adaptively increase or decrease according to time varying network conditions.¶

5. Beyond "sendmmsg()"

In actual practice, implementation experience with the ION-DTN distribution along with two recent studies have demonstrated only very limited performance increases for employing sendmmsg() for transmission over UDP/IP sockets. A first study used sendmmsg() as part of an integrated solution to produce 1M packets per second assuming only raw data transmission conditions [MPPS], while a second study focused on performance improvements for the QUIC reliable transport service [QUIC]. In both studies, the use of sendmmsg() alone produced modest increases but complimentary enhancements were identified that when combined with sendmmsg() produced considerable additional increases.¶

In [MPPS], additional enhancements such as using recvmmsg() and configuring multiple receive queues at the receiver were introduced in an attempt to achieve greater parallelism and engage multiple processors and threads. However, the system was still limited to a single thread until multiple receiving processes were introduced using the "SO_REUSEPORT" socket option. By having multiple receiving processes (each with its own socket buffer), the performance advantages of parallel processing were employed to achieve the 1M packets per second goal.¶

In [QUIC], a new feature available in recent linux kernel versions was employed. The feature, known as "Generic Segmentation Offload (GSO) / Generic Receive Offload (GRO)" allows an application to provide the kernel with a "super-buffer" containing up to 64 separate upper layer protocol segments. When the application presents the super-buffer to the kernel, GSO segmentation then sends up to 64 separate UDP/IP packets in a burst. (Note that GSO requires each UDP/IP packet to be no larger than the Path-MTU so that receivers can invoke GRO without interactions with IP reassembly.) The GSO facility can be invoked by either sendmsg() (i.e., a single super-buffer) or sendmmsg() (i.e., multiple super-buffers), and the study showed a substantial performance increase over using just sendmsg() and sendmmsg() alone.¶

For LTP fragmentation, our ongoing efforts explore using these techniques in a manner that parallels the effort undertaken for QUIC. Using these higher-layer segmentation management facilities is consistent with the guidance in "IP Fragmentation Considered Fragile" that states:¶

• "Rather than deprecating IP fragmentation, this document recommends that upper-layer protocols address the problem of fragmentation at their layer, reducing their reliance on IP fragmentation to the greatest degree possible."¶

By addressing fragmentation at their layer, the LTP/UDP functions can then be tuned to minimize IP fragmentation in environments where it may be problematic or to adaptively engage IP fragmentation in environments where performance gains can be realized without risking sustained loss and/or data corruption.¶

6. Advanced LTP Performance Enhancement

Some modern operating systems include Generic Segment Offload (GSO) and Generic Receive Offload (GRO) services. For example, GSO/GRO support has been included in linux beginning with kernel version 4.18. Some network drivers and network hardware also support GSO/GRO at or below the operating system network device driver interface layer to provide benefits of delayed segmentation and/or early reassembly. The following sections discuss LTP interactions with GSO and GRO.¶

  /* Set LTP segment size */
  unsigned integer gso_size = SEGSIZE;
  ...
  /* Enable GSO for all messages sent on the socket */
  setsockopt(fd, SOL_UDP, UDP_SEGMENT, &gso_size, sizeof(gso_size)));
  ...
  /* Alternatively, set per-message GSO control */
  cm = CMSG_FIRSTHDR(&msg);
  cm->cmsg_level = SOL_UDP;
  cm->cmsg_type = UDP_SEGMENT;
  cm->cmsg_len = CMSG_LEN(sizeof(uint16_t));
  *((uint16_t *) CMSG_DATA(cm)) = gso_size;

  /* Enable GRO */
  unsigned integer use_gro = 1; /* boolean */
  setsockopt(fd, SOL_UDP, UDP_GRO, &use_gro, sizeof(use_gro)));
  ...
  /* Set per-message GRO control */
  cmsg->cmsg_len = CMSG_LEN(sizeof(int));
  *((int *)CMSG_DATA(cmsg)) = 0;
  cmsg->cmsg_level = SOL_UDP;
  cmsg->cmsg_type = UDP_GRO;
  ...
  /* Receive per-message GRO segment length */
  if ((segmentLength = *((int *)CMSG_DATA(cmsg))) <= 0)
       segmentLength = messageLength;

7. Implementation Status

Supporting code for invoking the sendmmsg() facility is included in the official ION source code distribution, beginning with release ion-4.0.1.¶

Working code for GSO/GRO has been incorporated into a pre-release of ION and scheduled for integration following the next major release.¶

An implementation of LTP/UDP/IP Parcels over OMNI interfaces is available in github. A multi-threaded LTP receiver implementation is currently under investigation.¶

8. IANA Considerations

This document introduces no IANA considerations.¶

9. Security Considerations

Communications networking security is necessary to preserve confidentiality, integrity and availability.¶

10. Acknowledgements

The NASA Space Communications and Networks (SCaN) directorate coordinates DTN activities for the International Space Station (ISS) and other space exploration initiatives.¶

Akash Agarwal, Madhuri Madhava Badgandi, Keith Philpott, Bill Pohlchuck, Vijayasarathy Rajagopalan, Bhargava Raman Sai Prakash and Eric Yeh are acknowledged for their significant contributions. Tyler Doubrava was the first to mention the "sendmmsg()" facility. Scott Burleigh provided review input, and David Zoller provided useful perspective.¶

Honoring life, liberty and the pursuit of happiness.¶

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