| Internet Engineering Task Force | N. Kuhn, Ed. |
| Internet-Draft | CNES |
| Intended status: Informational | E. Lochin, Ed. |
| Expires: November 9, 2018 | ISAE-SUPAERO |
| May 8, 2018 |
Network coding and satellites
draft-kuhn-nwcrg-network-coding-satellites-04
This memo presents the current deployment of network coding in some satellite telecommunications systems along with a discussion on the multiple opportunities to introduce these techniques at a wider scale.
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Network coding schemes are inherent part of the satellite systems as the physical layer requires specific robustness to guarantee an efficient usage of the expensive radio resource. Further exploiting these schemes is an opportunity for a better end-user experience along with a better exploitation of the scarce resource.
In this context, this memo aims at:
The glossary of this memo is related to the network coding taxonomy document [I-D.irtf-nwcrg-network-coding-taxonomy].
The glossary is extended as follows:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
The objective of this section is to provide both a generic description of the components composing a generic satellite system and their interaction. It provides a high level description of a multi-gateway satellites network. There exist multiple SATCOM systems, such as those dedicated to broadcasting TV or to IoT applications: depending on the purpose of the SATCOM system, ground segments are specific. This memo lays on SATCOM systems dedicated to Internet access that follows the DVB-S2/RCS2 standards.
In this context, Figure 1 shows an example of a multigateway satellite system. More details on a generic SATCOM ground segment architecture for a bi-directional Internet access can be found in [SAT2017].
It is worth noting that some functional blocks aggregate the traffic coming from multiple users, allowing the deployment of network coding schemes.
+---------------------+
| Application servers |
+---------------------+
| | |
| | |
-----------------------------------
v v v v v v
+------------------+ +------------------+
| network function | | network function |
| (firewall, PEP) | | (firewall, PEP) |
+------------------+ +------------------+
| | | |
| | IP packets | |
v v v v
+------------------+ +------------------+
| access gateway | | access gateway |
+------------------+ +------------------+
| |
| BBFrames |
v v
+------------------+ +------------------+
| physical gateway | | physical gateway |
+------------------+ +------------------+
| |
| PLFrames |
v v
+------------------+ +------------------+
| outdoor unit | | outdoor unit |
+------------------+ +------------------+
| | | |
| | Satellite link | |
v v v v
+------------------+ +------------------+
| sat terminals | | sat terminals |
+------------------+ +------------------+
Figure 1: Data plane functions in a generic satellite multi-gateway system
Figure 2 presents the status of the network coding deployment in satellite systems. The information is based on the taxonomy document [I-D.irtf-nwcrg-network-coding-taxonomy] and the notations are the following: End-to-End Coding (E2E), Network Coding (NC), Intra-Flow Coding (IntraF), Inter-Flow Coding (InterF), Single-Path Coding (SP) and Multi-Path Coding (MP).
X1 embodies the source coding that could be used at application level for instance: for video streaming on a broadband access. X2 embodies the physical layer, applied to the PLFRAME, to optimize the satellite capacity usage. Furthermore, at the physical layer and when random accesses are exploited, FEC mechanisms are exploited.
+------+-------+---------+---------------+-------+ | | Upper | Middle | Communication layers | | | Appl. | ware | | + +-------+---------+---------------+-------+ | |Source | Network | Packetization | PHY | | |coding | AL-FEC | UDP/IP | layer | +------+-------+---------+---------------+-------+ |E2E | X1 | | | | |NC | | | | | |IntraF| X1 | | | | |InterF| | | | X2 | |SP | X1 | | | X2 | |MP | | | | | +------+-------+---------+---------------+-------+
Figure 2: Network coding and satellite systems
This section details use-cases where network coding schemes could improve the overall performance of a SATCOM system (e.g. considering a more efficient usage of the satellite resource, delivery delay, delivery ratio).
It is worth noting that these use-cases focus more on the middle ware (e.g. aggregation nodes) and packetization UDP/IP of Figure 2. Indeed, there are already lots of recovery mechanisms at the physical and access layers in currently deployed systems while E2E source coding are done at the application level. In a multigateway SATCOM Internet access, the specific opportunities are more relevant in specific SATCOM components such as the "network function" block or the "access gateway" of Figure 1.
This use-case considers a two-way communication between end users, through a satellite link. We propose an illustration of this scenario in Figure 3.
Satellite terminal A (resp. B) transmits a flow A (resp. B) to a server hosting NC capabilities, which forward a combination of the two flows to both terminals. This results in non-negligible bandwidth saving and has been demonstrated at ASMS 2010 in Cagliari [ASMS2010]. Moreover, with On-Board Processing satellite payloads, the network coding operations could be done at the satellite level, thus reducing the end-to-end delay of the communication.
+------------+ +-----+ +---------+
| Satellite | A | | A | |
| Terminal A |-->--| | |->---| | +------+
+------------+ | | |->---| | | |
|| A+B ->-| SAT | B | Gateway | | |
==================| | | |--|Server|
|| ->-| | | | | |
+------------+ B |>-| |=====| | | |
| Satellite |-->--| | | A+B | | +------+
| Terminal B | | | | |
+------------+ +-----+ +---------+
Figure 3: Network architecture for two way relay channel with NC
This use-case considers adding redundancy to a multi-cast flow depending on what has been received by different end-users, resulting in non-negligible scarce resource saving. We propose an illustration for this scenario in Figure 4.
A multi-cast flow (M) is forward to both satellite terminals A and B. On the uplink, terminal A (resp. B) does not acknowledge the packet Ni (resp. Nj) and either the access gateway or the multi-cast server includes the missing packets in the multi-cast flow so that the information transfer is reliable. This could be achieved by using NACK-Oriented Reliable Multicast (NORM) [RFC5740]. However, NORM does not consider other network coding schemes such as sliding window encoding described in [I-D.irtf-nwcrg-network-coding-taxonomy].
+------------+ +-----+ +---------+
| Satellite |NACK Ni | |NACK Ni| |
| Terminal A |-->--| | |->-----| | +------+
+------------+ | | |->-----| | | |
|| M ->-| SAT |NACK Nj| | |Multi |
==================| | | Gateway |--|Cast |
|| ->-| | | | |Server|
+------------+ |>-| |=======| | | |
| Satellite |-->--| | | M | | +------+
| Terminal B |NACK Nj | | | |
+------------+ +-----+ +---------+
Figure 4: Network architecture for a reliable multi-cast with NC
This use-case considers the use of multiple path management with network coding at the transport level to either increase the reliability or the total bandwidth. We propose an illustration for this scenario in Figure 5. This use-case is inspired from the Broadband Access via Integrated Terrestrial Satellite Systems (BATS) project and has been published as an ETSI Technical Report [ETSITR2017]. It is worth nothing that this kind of architecture is also discussed in the MPTCP working group [I-D.boucadair-mptcp-dhc].
To cope from packet loss (due to either end-user movements or physical layer impairments), network coding could be introduced in both the CPE and at the concentrator.
+-------------+ +----------------+
|->| SAT NETWORK |---| BACKBONE |
| +-------------+ | +------------+ |
+------+ | | |CONCENTRATOR| |
| CPE |-->-| +-----+ | +------------+ |
+------+ |->| DSL |-----------| |
| +-----+ | |
| | |
| +-----+ | |
|->| LTE |-----------| |
+-----+ +----------------+
Figure 5: Network architecture for an hybrid access using NC
This use-case considers the usage of network coding to overcome cases where the wireless link characteristics quickly change overtime and where the physical layer codes could not be made robust in time. This is particularly relevant when end users are moving and the channel shows important variations [IEEEVT2001].
The architecture is recalled in Figure 6. The network coding schemes could be applied at the access gateway or the network function block levels to increase the reliability of the transmission. This use-case is mostly relevant for when mobile users are considered or when the chosen band induce a required physical layer coding that may change over time (Q/V bands, Ka band, etc.).
+------------+ +-----+ +---------+ +--------+ +---------+
| Satellite | | SAT | | Physical| | Access | | Network |
| Terminal |->| |->| gateway |->| gateway|->| function|
+------------+ +-----+ +---------+ +--------+ +---------+
NC? NC NC? NC?
Figure 6: Network architecture for dealing with varying link characteristics with NC
This use-case considers the recovery of packets that may be lost during gateway handovers. Whether this is for off-loading one given equipment or because the transmission quality is not the same on each gateway, changing the transmission gateway may be relevant. However, if gateways are not properly synchronized, this may result in packet loss. During these critical phases, network coding can be added to improve the reliability of the transmission and propose a seamless gateway handover.
An example architecture for this use-case is showed in Figure 7. It is worth noting that depending on the ground architecture [I-D.chin-nfvrg-cloud-5g-core-structure-yang] [SAT2017], some equipments might be communalised.
+---------+ +--------+ +---------+
| Physical| | Access | | Network |
----->| gateway |->| gateway|->| function|
| +---------+ +--------+ +---------+
v | |
+------------+ +-----+ +-------------+
| Satellite | | SAT | | Switching |
| Terminal |->| | | Entity |
+------------+ +-----+ +-------------+
^ | |
| +---------+ +--------+ +---------+
----->| Physical| | Access | | Network |
| gateway |->| gateway|->| function|
+---------+ +--------+ +---------+
Figure 7: Network architecture for dealing with gateway handover schemes with NC
Establishing communications from terrestrial gateways to aerospace components is a challenge due to the distances involved. As a matter of fact, reliable end-to-end (E2E) communications over such links must cope with long delay and frequent link disruptions. Delay/Disruption Tolerant Networking [RFC4838] is a solution to enable reliable internetworking space communications where both standard ad-hoc routing and E2E Internet protocols cannot be used. DTN can also be seen as an alternative solution to cope with satellite communications usually managed by PEP. Therefore, the transport of data over such networks requires the use of replication, erasure codes and multipath protocol schemes [WANG05] [ZHANG06] to improve the bundle delivery ratio and/or delivery delay. For instance, transport protocols such as LTP [RFC5326] for long delay links with connectivity disruptions, use Automatic Repeat-reQuest (ARQ) and unequal error protection to reduce the amount of non-mandatory retransmissions. The work in [TOURNOUX10] proposed upon LTP a robust streaming method based on an on-the-fly coding scheme, where encoding and decoding procedures are done at the source and destination nodes, respectively. However, each link path loss rate may have various order of magnitude and re-encoding at an intermediate node to adapt the redundancy can be mandatory to prevent transmission wasting. This idea has been put forward in [I-D.zinky-dtnrg-random-binary-fec-scheme] and [I-D.zinky-dtnrg-erasure-coding-extension], where the authors proposed an encoding process at intermediate DTN nodes to explore the possibilities of Forward Error Correction (FEC) schemes inside the bundle protocol [RFC5050]. Another proposal is the use of erasure coding inside the CCSDS (Consultative Committee for Space Data Systems) architecture [COLA11]. The objective is to extend the CCSDS File Delivery Protocol (CFDP) [CCSDS-FDP] with erasure coding capabilities where a Low Density Parity Check (LDPC) [RFC6816] code with a large block size is chosen. Recently, on-the-fly erasure coding schemes [LACAN08] [SUNDARARAJAN08] [TOURNOUX11] have shown their benefits in terms of recovery capability and configuration complexity compared to traditional FEC schemes. Using a feedback path when available, on-the-fly schemes can be used to enable E2E reliable communication over DTN with adaptive re-encoding as proposed in [THAI15].
This section discusses the deployability of the use-cases detailed in Section 4.
SATCOM systems typically feature Proxy-Enhanced-Proxy RFC 3135 which could be relevant to host network coding mechanisms and thus support the use-cases that have been discussed in Section 4. In particular the discussion on how network coding can be integrated inside a PEP with QoS scheduler has been proposed in RFC 5865.
The generic architecture proposed in Figure 1 may be mapped to cellular networks as follows: the 'network function' block gather some of the functions of the Evolved Packet Core subsystem, while the 'access gateway' and 'physical gateway' blocks gather the same type of functions as the Universal Mobile Terrestrial Radio Access Network. This mapping extends the opportunities identified in this draft since they may be also relevant for cellular networks.
Related to the foreseen virtualized network infrastructure, the network coding schemes could be proposed as VNF and their deployability enhanced. The architecture for the next generation of SATCOM ground segments would rely on a virtualized environment. This trend can also be seen, making the discussions on the deployability of network coding in SATCOM extendable to other deployment scenarios [I-D.chin-nfvrg-cloud-5g-core-structure-yang]. As one example, the network coding VNF functions deployment in a virtualized environment is presented in [I-D.vazquez-nfvrg-netcod-function-virtualization].
Many thanks to Tomaso de Cola, Vincent Roca and Marie-Jose Montpetit.
Tomaso de Cola, Vincent Roca, Marie-Jose Montpetit.
This memo includes no request to IANA.
This document, by itself, presents no new privacy nor security issues.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |