| Internet Engineering Task Force | N. Kuhn, Ed. |
| Internet-Draft | CNES |
| Intended status: Informational | E. Lochin, Ed. |
| Expires: May 16, 2019 | ISAE-SUPAERO |
| Nov 12, 2018 |
Network coding and satellites
draft-irtf-nwcrg-network-coding-satellites-02
This memo details a multi-gateway satellite system to identify multiple opportunities on how coding techniques could be deployed at a wider scale.
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Guaranteeing both physical layer robustness and efficient usage of the radio resource has been in the core design of SATellite COMmunication (SATCOM) systems. The trade-off often resided in how much redundancy a system adds to cope from link impairments, without reducing the good-put when the channel quality is high. There is usually enough redundancy to guarantee a Quasi-Error Free transmission. However, physical layer reliability mechanisms may not recover transmission losses (e.g. with a mobile user) and layer 2 (or above) re-transmissions induce 500 ms one-way delay with a geostationary satellite. Further exploiting coding schemes at higher OSI-layers is an opportunity for releasing constraints on the physical layer in such cases and improving the performance of SATCOM systems.
We have noticed an active research activity on coding and SATCOM in the past. That being said, not much has actually made it to industrial developments. In this context, this document aims at identifying opportunities for further usage of coding in these systems.
This document follows the taxonomy of coding techniques for efficient network communications [RFC8406].
The glossary of this memo extends the glossary of the taxonomy document [RFC8406] 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.
This section focuses on a generic description of the components composing a generic satellite system and their interaction. A high level description of a multi-gateway satellites network is provided. 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, the increase of the available capacity that is carried out to end users and the need for reliability results in the need for multiple gateways for one unique satellite platform.
In this context, Figure 1 shows an example of a multi-gateway satellite system. More details on a generic SATCOM ground segment architecture for a bi-directional Internet access can be found in [SAT2017]. We propose a multi-gateway system since some of the use-cases described in this document require multiple gateways. In a multi-gateway system, some elements may be centralized and/or gathered: the relevance of one approach compared to another depends on the deployment scenario. More information on these trade-off discussions can be found in [SAT2017].
It is worth noting that some functional blocks aggregate the traffic coming from multiple users. Even if coding schemes could be applied to any individual traffic, it could also work on an aggregate.
+---------------------+
| 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
+------------------+ +------------------+
| sat terminals | | sat terminals |
+------------------+ +------------------+
^ ^
| |
v v
+------------------+ +------------------+
| end user | | end user |
+------------------+ +------------------+
Figure 1: Data plane functions in a generic satellite multi-gateway system
Figure 2 presents the status of the reliability schemes deployment in satellite systems. The information is based on the taxonomy document [RFC8406] 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 within QUIC or other video streaming applications: this is not specific to SATCOM systems, but is relevant for broadband Internet access discussions.
X2 embodies the physical layer, applied to the PLFRAME, to optimize the satellite capacity usage: at the physical layer, FEC mechanisms can be exploited. This aspect is not in the scope of the WG according to the taxonomoy document [RFC8406].
+------+-------+---------+---------------+-------+ | | 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: Reliability schemes in current satellite systems
Reliability is an inherent part of the physical layer and usually achieved by using coding techniques. Based on public information, coding does not seem to be widely used at higher OSI layers, other than at the application layer.
This section details use-cases where 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 middleware (e.g. aggregation nodes) and packetization UDP/IP of Figure 2. Indeed, there are already lots of recovery mechanisms at the physical layer in currently deployed systems while E2E source coding are done at the application level. In a multi-gateway 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 coding operations could be done at the satellite level, thus reducing the end-to-end delay of the communication.
-X}- : traffic from satellite terminal X to the server
={X+Y= : traffic from X and Y combined transmitted from
the server to terminals X and Y
+-----------+ +-----+
|Sat term A |--A}-+ | |
+-----------+ | | | +---------+ +------+
^^ +--| |--A}--| |--A}--| |
|| | SAT |--B}--| Gateway |--B}--|Server|
===={A+B=========| |={A+B=| |={A+B=| |
|| | | +---------+ +------+
vv +--| |
+-----------+ | | |
|Sat term B |--B}-+ | |
+-----------+ +-----+
Figure 3: Network architecture for two way relay channel with NC
This use-case considers adding redundancy to a multicast 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 multicast flow (M) is forwarded to both satellite terminals A and B. However packet Ni (resp. Nj) get lost at terminal A (resp. B), and terminal A (resp. B) returns a negative acknowledgement Li (resp. Lj), indicating that the packet is missing. Then either the access gateway or the multicast server includes a repair packet (rather than the individual Ni and Nj packets) in the multicast flow to let both terminals recover from losses. This could be achieved by using NACK-Oriented Reliable Multicast (NORM) [RFC5740] in situations where a feedback is possible and desirable, or FLUTE/ALC [RFC6726] when it is not the case. Note that currently both NORM nor FLUTE/ALC are limited to block coding, none of them supporting sliding window encoding schemes [RFC8406].
-Li}- : packet indicated the loss of packet i of a multicast flow
={M== : multicast flow including the missing packets
+-----------+ +-----+
|Sat term A |-Li}-+ | |
+-----------+ | | | +---------+ +------+
^^ +-| |-Li}--| | |Multi |
|| | SAT |-Lj}--| Gateway |--|Cast |
===={M==========| |={M===| | |Server|
|| | | +---------+ +------+
vv +-| |
+-----------+ | | |
|Sat term B |-Lj}-+ | |
+-----------+ +-----+
Figure 4: Network architecture for a reliable multicast with NC
This use-case considers the use of multiple path management with coding at the transport level to increase the reliability and/or the total capacity (using multiple path does not guarantee an improvement of both the reliability and 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 TCPM working group [I-D.ietf-tcpm-converters].
To cope with packet loss (due to either end-user mobility or physical layer impairments), coding could be introduced in both the CPE and at the concentrator.
-{}- : bidirectional link
+-----+ +----------------+
+-{}-| SAT |-{}-| BACKBONE |
+------+ +------+ | +-----+ | +------------+ |
| End |-{}-| CPE |-{}-| | |CONCENTRATOR| |
| User | | | | +-----+ | +------------+ | +------+
+------+ +------+ |-{}-| DSL |-{}-| |-{}-|Data |
| +-----+ | | |Server|
| | | +------+
| +-----+ | |
+-{}-| LTE |-{}-| |
+-----+ +----------------+
Figure 5: Network architecture for an hybrid access using NC
This use-case considers the usage of 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. In these cases, Adaptative Coding and Modulation (ACM) may not adapt the modulation and coding accordingly and remaining errors could be recovered with higher layers redundancy packets. The 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.). Depending on the use-case (e.g. very high frequency bands, mobile users) or depending on the deployment use-cases (e.g. performance of the network between each individual block), the relevance of adding coding is different. Then, depending on the OSI level at which coding is applied, the impact on the satellite terminal is different: coding may be applied on IP packets or on layer-2 proprietary format packets.
-{}- : bidirectional link
+---------+ +---+ +--------+ +-------+ +--------+
|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, coding can be added to improve the reliability of the transmission and propose a seamless gateway handover. In this case, the coding could be applied at either the access gateway or the network function block. The entity responsible for taking the decision to change the communication gateway and changing the routes is the control plane manager; this entity exploits a management interface.
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 equipment might be communalised.
-{}- : bidirectional link
! : management interface
NC NC
+--------+ +-------+ +--------+
|Physical| |Access | |Network |
+-{}-|gateway |-{}-|gateway|-{}-|function|
| +--------+ +-------+ +--------+
| ! !
+---------+ +---+ +---------------+
|Satellite| |SAT| | Control plane |
|Terminal |-{}-| | | manager |
+---------+ +---+ +---------------+
| ! !
| +--------+ +-------+ +--------+
+-{}-|Physical|-{}-|Access |-{}-|Network |
|gateway | |gateway| |function|
+--------+ +-------+ +--------+
NC NC
Figure 7: Network architecture for dealing with gateway handover schemes with NC
SATCOM systems typically feature Performance Enhancement Proxy RFC 3135 which could be relevant to host coding mechanisms and thus support the use-cases that have been discussed in Section 4. PEP usually split TCP end-to-end connections and forward TCP packets to the satellite baseband gateway that deals with layer 2 and layer 1 encapsulations. Deploying coding schemes at the TCP level in these equipments could be relevant and independent from the specificities of a SATCOM link. That being said, we can notice a research issue in the recurrent trade-off in SATCOM systems that is related to the amount of reliability that you introduce in the first transmission to guarantee a better end-user QoE and the usage of the scarce resource.
Related to the foreseen virtualized network infrastructure, the coding schemes could be proposed as Virtual Network Function (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 coding in SATCOM extendable to other deployment scenarios [I-D.chin-nfvrg-cloud-5g-core-structure-yang]. As one example, the coding VNF functions deployment in a virtualized environment is presented in [I-D.vazquez-nfvrg-netcod-function-virtualization]. A research challenge would be the optimization of the NFV service function chaining, considering a virtualized infrastructure and other SATCOM specific functions, to guarantee an efficient radio resource utilization and easy to deploy SATCOM services.
In the context of deep-space communications, establishing communications from terrestrial gateways to satellite platforms can be a challenge. Indeed, 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. 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 re-transmissions. 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]. In this context, the use of erasure coding inside a Consultative Committee for Space Data Systems (CCSDS) architecture has been specified in [CCSDS-131.5-O-1].
In the context of the deep-space communications, coding could be improved by, e.g. using a feedback path: when a return path is available, on-the-fly schemes can be used to enable E2E reliable communication over DTN with adaptive re-encoding as proposed in [THAI15]. That being said, DTN can also be seen as an alternative solution to transfer the data between a central PEP and a remote PEP. In this case, the use-cases proposed in Section 4.4 would legitimate the usage of coding within the DTN stack to improve the channel utilization and the E2E transmission delay.
This document presents the current deployment of coding in some satellite telecommunications systems along with a discussion on the multiple opportunities to introduce these techniques at a wider scale.
Even if this document focuses on satellite systems, it is worth pointing out that the some scenarios proposed may be relevant to other wireless telecommunication systems. As one example, 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.
Many thanks to Tomaso de Cola, Vincent Roca, Lloyd Wood and Marie-Jose Montpetit for their help in writting this document.
This memo includes no request to IANA.
Security considerations are inherent to any access network. SATCOM systems introduce standard security mechanisms. In particular, there are some specificities related to the fact that all users under the coverage can record all the packets that are being transmitted, such as in LTE networks. On the specific scenario of NC in SATCOM systems, there are no specific security considerations.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |