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Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group L. Han, Ed. 3 Internet-Draft R. Li 4 Intended status: Informational A. Retana 5 Expires: 17 August 2022 Futurewei Technologies, Inc. 6 M. Chen 7 L. Su 8 China Mobile 9 N. Wang 10 University of Surrey 11 13 February 2022 13 Problems and Requirements of Satellite Constellation for Internet 14 draft-lhan-problems-requirements-satellite-net-02 16 Abstract 18 This document presents the detailed analysis about the problems and 19 requirements of satellite constellation used for Internet. It starts 20 from the satellite orbit basics, coverage calculation, then it 21 estimates the time constraints for the communications between 22 satellite and ground-station, also between satellites. How to use 23 satellite constellation for Internet is discussed in detail including 24 the satellite relay and satellite networking. The problems and 25 requirements of using traditional network technology for satellite 26 network integrating with Internet are finally outlined. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on 17 August 2022. 45 Copyright Notice 47 Copyright (c) 2022 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 52 license-info) in effect on the date of publication of this document. 53 Please review these documents carefully, as they describe your rights 54 and restrictions with respect to this document. Code Components 55 extracted from this document must include Revised BSD License text as 56 described in Section 4.e of the Trust Legal Provisions and are 57 provided without warranty as described in the Revised BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 4. Basics of Satellite Constellation . . . . . . . . . . . . . . 6 65 4.1. Satellite Orbit . . . . . . . . . . . . . . . . . . . . . 6 66 4.2. Coverage of LEO and VLEO Satellites and Minimum Number 67 Required . . . . . . . . . . . . . . . . . . . . . . . . 6 68 4.3. Real Deployment of LEO and VLEO for Satellite Network . . 9 69 5. Communications for Satellite Constellation . . . . . . . . . 10 70 5.1. Dynamic Ground-station-Satellite Communication . . . . . 11 71 5.2. Dynamic Inter-satellite Communication . . . . . . . . . . 12 72 5.2.1. Inter-satellite Communication Overview . . . . . . . 12 73 5.2.2. Satellites on Adjacent Orbit Planes with Same 74 Altitude . . . . . . . . . . . . . . . . . . . . . . 15 75 5.2.3. Satellites on Adjacent Orbit Planes with Different 76 Altitude . . . . . . . . . . . . . . . . . . . . . . 17 77 6. Use Satellite Network for Internet . . . . . . . . . . . . . 19 78 7. Problems and Requirements for Satellite Constellation for 79 Internet . . . . . . . . . . . . . . . . . . . . . . . . 22 80 7.1. Common Problems and Requirements . . . . . . . . . . . . 22 81 7.2. Satellite Relay . . . . . . . . . . . . . . . . . . . . . 25 82 7.2.1. One Satellite Relay . . . . . . . . . . . . . . . . . 25 83 7.2.2. Multiple Satellite Relay . . . . . . . . . . . . . . 26 84 7.3. Satellite Networking . . . . . . . . . . . . . . . . . . 28 85 7.3.1. L2 or L3 network . . . . . . . . . . . . . . . . . . 28 86 7.3.2. Inter-satellite-Link Lifetime . . . . . . . . . . . . 28 87 7.3.3. Problems for Traditional Routing Technologies . . . . 29 88 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 89 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 33 90 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33 91 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 92 11.1. Normative References . . . . . . . . . . . . . . . . . . 33 93 11.2. Informative References . . . . . . . . . . . . . . . . . 34 94 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 36 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 97 1. Introduction 99 Satellite constellation for Internet is emerging. Even there is no 100 constellation network established completely yet at the time of the 101 publishing of the draft (June 2021), some basic internet service has 102 been provided and has demonstrated competitive quality to traditional 103 broadband service. 105 This memo will analyze the challenges for satellite network used in 106 Internet by traditional routing and switching technologies. It is 107 based on the analysis of the dynamic characters of both ground- 108 station-to-satellite and inter-satellite communications and its 109 impact to satellite constellation networking. 111 The memo also provides visions for the future solution, such as in 112 routing and forwarding. 114 The memo focuses on the topics about how the satellite network can 115 work with Internet. It does not focus on physical layer technologies 116 (wireless, spectrum, laser, mobility, etc.) for satellite 117 communication. 119 2. Terminology 121 LEO Low Earth Orbit with the altitude from 180 km to 122 2000 km. 124 VLEO Very Low Earth Orbit with the altitude below 450 km 126 MEO Medium Earth Orbit with the altitude from 2000 km 127 to 35786 km 129 GEO Geosynchronous orbit with the altitude 35786 km 131 GSO Geosynchronous satellite on GEO 133 ISL Inter Satellite Link 135 ISLL Inter Satellite Laser Link 137 EIRP Effective isotropic radiated power 139 P2MP Point to Multiple Points 141 GS Ground Station, a device on ground connecting the 142 satellite. In the document, GS will hypothetically 143 provide L2 and/or L3 functionality in addition to 144 process/send/receive radio wave. It might be 145 different as the reality that the device to 146 process/send/receive radio wave and the device to 147 provide L2 and/or L3 functionality could be 148 separated. 150 SGS Source ground station. For a specified flow, a 151 ground station that will send data to a satellite 152 through its uplink. 154 DGS Destination ground station. For a specified flow, 155 a ground station that is connected to a local 156 network or Internet, it will receive data from a 157 satellite through its downlink and then forward to 158 a local network or Internet. 160 PGW Packet Gateway 162 UPF User Packet Function 164 PE router Provider Edge router 166 CE router Customer Edge router 168 P router Provider router 170 LSA Link-state advertisement 172 LSP Link-State PDUs 174 L1 Layer 1, or Physical Layer in OSI model [OSI-Model] 176 L2 Layer 2, or Data Link Layer in OSI model 177 [OSI-Model] 179 L3 Layer 3, or Network Layer in OSI model [OSI-Model], 180 it is also called IP layer in TCP/IP model 182 BGP Border Gateway Protocol [RFC4271] 184 eBGP External Border Gateway Protocol, two BGP peers 185 have different Autonomous Number 187 iBGP Internal Border Gateway Protocol, two BGP peers 188 have same Autonomous Number 190 IGP Interior gateway protocol, examples of IGPs include 191 Open Shortest Path First (OSPF [RFC2328]), Routing 192 Information Protocol (RIP [RFC2453]), Intermediate 193 System to Intermediate System (IS-IS [RFC7142]) and 194 Enhanced Interior Gateway Routing Protocol (EIGRP 195 [RFC7868]). 197 3. Overview 199 The traditional satellite communication system is composed of few GSO 200 and ground stations. For this system, each GSO can cover 42% Earth's 201 surface [GEO-Coverage], so as few as three GSO can provide the global 202 coverage theoretically. With so huge coverage, GSO only needs to 203 amplify signals received from uplink of one ground station and relay 204 to the downlink of another ground station. There is no inter- 205 satellite communications needed. Also, since the GSO is stationary 206 to the ground station, there is no mobility issue involved. 208 Recently, more and more LEO and VLEO satellites have been launched, 209 they attract attentions due to their advantages over GSO and MEO in 210 terms of higher bandwidth, lower cost in satellite, launching, ground 211 station, etc. Some organizations [ITU-6G][Surrey-6G][Nttdocomo-6G] 212 have proposed the non-terrestrial network using LEO, VLEO as 213 important parts for 6G to extend the coverage of Internet. SpaceX 214 has started to build the satellite constellation called StarLink that 215 will deploy over 10 thousand LEO and VLEO satellites finally 216 [StarLink]. China also started to request the spectrum from ITU to 217 establish a constellation that has 12992 satellites 218 [China-constellation]. European Space Agency (ESA) has proposed 219 "Fiber in the sky" initiative to connect satellites with fiber 220 network on Earth [ESA-HydRON]. 222 When satellites on MEO, LEO and VLEO are deployed, the communication 223 problem becomes more complicated than for GSO. This is because the 224 altitude of MEO/LEO/VLEO satellites are much lower. As a result, the 225 coverage of each satellite is much smaller than for GSO, and the 226 satellite is not relatively stationary to the ground. This will lead 227 to: 229 1. More satellites than GSO are needed to provide the global 230 coverage. Section 4.2 will analyze the coverage area, and the 231 minimum number of satellites required to cover the earth surface. 233 2. The point-to-point communication between satellite and ground 234 station will not be static. Mobility issue has to be considered. 235 Detailed analysis will be done in Section 5.1. 237 3. The inter-satellite communication is needed, and all satellites 238 need to form a network. details are described in Section 5.2. 240 In addition to above context, Section 7 will address the problem and 241 requirements when satellite constellation is joining Internet. 243 As the 1st satellite constellation company in history, the SpaceX/ 244 StarLink will be inevitably mentioned in the draft. But it must be 245 noted that all information about SpaceX/StarLink in the draft are 246 from public. Authors of the draft have no relationship or relevant 247 inside knowledge of SpaceX/Starlink. 249 4. Basics of Satellite Constellation 251 This section will introduce some basics for satellite such as orbit 252 parameters, coverage estimation, minimum number of satellite and 253 orbit plane required, real deployments. 255 4.1. Satellite Orbit 257 The orbit of a satellite can be either circular or ecliptic, it can 258 be described by following Keplerian elements [KeplerianElement]: 260 1. Inclination (i) 262 2. Longitude of the ascending node (Omega) 264 3. Eccentricity (e) 266 4. Semimajor axis (a) 268 5. Argument of periapsis (omega) 270 6. True anomaly (nu) 272 For a circular orbit, two parameters, Inclination and Longitude of 273 the ascending node, will be enough to describe the orbit. 275 4.2. Coverage of LEO and VLEO Satellites and Minimum Number Required 277 The coverage of a satellite is determined by many physical factors, 278 such as spectrum, transmitter power, the antenna size, the altitude 279 of satellite, the air condition, the sensitivity of receiver, etc. 280 EIRP could be used to measure the real power distribution for 281 coverage. It is not deterministic due to too many variants in a real 282 environment. The alternative method is to use the minimum elevation 283 angle from user terminals or gateways to a satellite. This is easier 284 and more deterministic. [SpaceX-Non-GEO] has suggested originally 285 the minimum elevation angle of 35 degrees and deduced the radius of 286 the coverage area is about 435km and 1230km for VLEO (altitude 287 335.9km) and LEO (altitude 1150km) respectively. The details about 288 how the coverage is calculated from the satellite elevation angle can 289 be found in [Satellite-coverage]. 291 Using this method to estimate the coverage, we can also estimate the 292 minimum number of satellites required to cover the earth surface. 294 It must be noted, SpaceX has recently reduced the required minimum 295 elevation angle from 35 degrees to 25 degrees. The following 296 analysis still use 35 degrees. 298 Assume there is multiple orbit planes with the equal angular interval 299 across the earth surface (The Longitude of the ascending node for 300 sequential orbit plane is increasing with a same angular interval). 301 Each orbit plane will have: 303 1. The same altitude. 305 2. The same inclination of 90 degree. 307 3. The same number of satellites. 309 With such deployment, all orbit planes will meet at north and south 310 pole. The density of satellite is not equal. Satellite is more 311 dense in the space above the polar area than in the space above the 312 equator area. Below estimations are made in the worst covered area, 313 or the area of equator where the satellite density is the minimum. 315 Figure 1 illustrates the coverage area on equator area, and each 316 satellite will cover one hexagon area. The figure is based on plane 317 geometry instead of spherical geometry for simplification, so, the 318 orbit is parallel approximately. 320 Figure 2 shows how to calculate the radius (Rc) of coverage area from 321 the satellite altitude (As) and the elevation angle (b). 323 x 324 | | 325 | | 326 x ********* 327 | * | * 328 | * | * 329 ********* x * 330 * | * | * 331 * | * | * 332 * x ********* 333 * | * | 334 * | * | 335 ********* x 336 | | 337 | orbit 2 ^ north 338 x | 339 | | 340 | | 341 orbit 1 +-------> east 343 Figure 1: Satellite coverage on ground 345 |<--- 2*Rc --->| 347 + Satellite 348 /| 349 / | 350 / | 351 / b | 352 /-\ + 353 / * | __Earth surface 354 / * | / 355 / *_----+----__ 356 + + 357 * * 358 * * 359 * 2*a * 360 * ___ * 361 *- -* 362 * * 363 * * 364 * Earth center 366 Figure 2: Satellite coverage estimation 368 x The vertical projection of satellite to Earth 370 Re The radius of the Earth, Re=6378(km) 371 As The altitude of a satellite 373 Rc The radius (arc length) of the coverage, or, the arc length of 374 hexagon center to its 6 vertices. Rc=Re*(a*pi)/180 376 a The cap angle for the coverage area (the RC arc). a = 377 arccos((Re/(Re+As))*cos(b))-b. 379 b The least elevation angle that a ground station or a terminal can 380 communicate with a satellite, b = 35 degree. 382 Ns The minimum number of satellites on one orbit plane, it is equal 383 to the number of the satellite's vertical projection on Earth, 384 so, Ns = 180/(a*cos(30)) 386 No The minimum number of orbit (with same inclination), it is equal 387 to the number of the satellite orbit's vertical projection, so, 388 No = 360/(a*(1+sin(30))) 390 For a example of two type of satellite LEO and VEO, the coverages are 391 calculated as in Table 1: 393 +============+=======+=======+========+========+ 394 | Parameters | VLEO1 | VLEO2 | LEO1 | LEO2 | 395 +============+=======+=======+========+========+ 396 | As(km) | 335.9 | 450 | 1100 | 1150 | 397 +------------+-------+-------+--------+--------+ 398 | a(degree) | 3.907 | 5.078 | 10.681 | 11.051 | 399 +------------+-------+-------+--------+--------+ 400 | Rc(km) | 435 | 565 | 1189 | 1230 | 401 +------------+-------+-------+--------+--------+ 402 | Ns | 54 | 41 | 20 | 19 | 403 +------------+-------+-------+--------+--------+ 404 | No | 62 | 48 | 23 | 22 | 405 +------------+-------+-------+--------+--------+ 407 Table 1: Satellite coverage estimation for 408 LEO and VLEO examples 410 4.3. Real Deployment of LEO and VLEO for Satellite Network 412 Obviously, the above orbit parameter setup is not optimal since the 413 sky in the polar areas will have the highest density of satellite. 415 In the real deployment, to provide better coverage for the areas with 416 denser population, to get redundance and better signal quality, and 417 to make the satellite distance within the range of inter-satellite 418 communication (2000km [Laser-communication-range]), more than the 419 minimum number of satellites are launched. For example, different 420 orbit planes with different inclination/altitude are used. 422 Normally, all satellites are grouped by orbit planes, each group has 423 a number of orbit planes and each orbit plane has the same orbit 424 parameters, so, each orbit in the same group will have: 426 1. The same altitude 428 2. The same inclination, but the inclination is less than 90 429 degrees. This will result in the empty coverage for polar areas 430 and better coverage in other areas. See the orbit picture for 431 phrase 1 for [StarLink]. 433 3. The same number of satellites 435 4. The same moving direction for all satellites 437 The proposed deployment of SpaceX can be seen in [SpaceX-Non-GEO] for 438 StarLink. 440 The China constellation deployment and orbit parameters can be seen 441 in [China-constellation]. 443 5. Communications for Satellite Constellation 445 Unlike the communication on ground, the communication for satellite 446 constellation is much more complicated. There are two mobility 447 aspects, one is between ground-station and satellite, another is 448 between satellites. 450 In the traditional mobility communication system, only terminal is 451 moving, the mobile core network including base station, front haul 452 and back haul are static, thus an anchor point, i.e., PGW in 4G or 453 UPF in 5G, can be selected for the control of mobility session. 454 Unfortunately, when satellite constellation joins the static network 455 system of Internet on ground, there is no such anchor point can be 456 selected since the whole satellite constellation network is moving. 458 Another special aspect that can impact the communication is that the 459 fast moving speed of satellite will cause frequent changes of 460 communication peers and link states, this will make big challenges to 461 the network side for the packet routing and delivery, session control 462 and management, etc. 464 5.1. Dynamic Ground-station-Satellite Communication 466 All satellites are moving and will lead to the communication between 467 ground station and satellite can only last a certain period of time. 468 This will greatly impact the technologies for the satellite 469 networking. Below illustrates the approximate speed and the time for 470 a satellite to pass through its covered area. 472 In Table 2, VLEO1 and LEO3 have the lowest and highest altitude 473 respectively, VLEO2 is for the highest altitude for VLEO. We can see 474 that longest communication time of ground-station-satellite is less 475 than 400 seconds, the longest communication time for VLEO ground- 476 station-satellite is less than 140 seconds. 478 The "longest communication time" is for the scenario that the 479 satellite will fly over the receiver ground station exactly above the 480 head, or the ground station will be on the diameter line of satellite 481 coverage circular area, see Figure 1. 483 Re The radius of the Earth, Re=6378(km) 485 As The altitude of a satellite 487 AL The arc length(in km) of two neighbor satellite on the same orbit 488 plane, AL=2*cos(30)*(Re+As)*(a*pi)/180 490 SD The space distance(in km) of two neighbor satellite on the same 491 orbir plane, SD=2*(Re+As)*sin(AL/(2*(Re+As))). 493 V the velocity (in m/s) of satellite, V=sqrt(G*M/(Re+As)) 495 G Gravitational constant, G=6.674*10^(-11)(m^3/(kg*s^2)) 497 M Mass of Earth, M=5.965*10^24 (kg) 499 T The time (in second) for a satellite to pass through its cover 500 area, or, the time for the station-satellite communication. T= 501 ALs/V 502 +============+=======+========+========+========+========+ 503 | Parameters | VLEO1 | VLEO2 | LEO1 | LEO2 | LEO3 | 504 +============+=======+========+========+========+========+ 505 | As(km) | 335.9 | 450 | 1100 | 1150 | 1325 | 506 +------------+-------+--------+--------+--------+--------+ 507 | a(degree) | 3.907 | 5.078 | 10.681 | 11.051 | 12.293 | 508 +------------+-------+--------+--------+--------+--------+ 509 | AL(km) | 793 | 1048 | 2415 | 2515 | 2863 | 510 +------------+-------+--------+--------+--------+--------+ 511 | SD(km) | 792.5 | 1047.2 | 2404 | 2503.2 | 2846.1 | 512 +------------+-------+--------+--------+--------+--------+ 513 | V(km/s) | 7.7 | 7.636 | 7.296 | 7.272 | 7.189 | 514 +------------+-------+--------+--------+--------+--------+ 515 | T(s) | 103 | 137 | 331 | 346 | 398 | 516 +------------+-------+--------+--------+--------+--------+ 518 Table 2: The time for the ground-station-satellite 519 communication 521 5.2. Dynamic Inter-satellite Communication 523 5.2.1. Inter-satellite Communication Overview 525 In order to form a network by satellites, there must be an inter- 526 satellite communication. Traditionally, inter-satellite 527 communication uses the microwave technology, but it has following 528 disadvantages: 530 1. Bandwidth is limited and only up to 600M bps 531 [Microwave-vs-Laser-communication]. 533 2. Security is a concern since the microwave beam is relatively wide 534 and it is easy for 3rd party to sniff or attack. 536 3. Big antenna size. 538 4. Power consumption is high. 540 5. High cost per bps. 542 Recently, laser is used for the inter-satellite communication, it has 543 following advantages, and will be the future for inter-satellite 544 communication. 546 1. Higher bandwidth and can be up to 10G bps 547 [Microwave-vs-Laser-communication]. 549 2. Better security since the laser beam size is much narrower than 550 microwave, it is harder for sniffing. 552 3. The size of optical lens for laser is much smaller than 553 microwave's antenna size. 555 4. Power saving compared with microwave. 557 5. Lower cost per bps. 559 The range for satellite-to-satellite communications has been 560 estimated to be approximately 2,000 km currently 561 [Laser-communication-range]. 563 From Table 2, we can see the Space Distance (SD) for some LEO 564 (altitude over 1100km) are exceeding the celling of the range of 565 laser communication, so, the satellite and orbit density for LEO need 566 to be higher than the estimation values in the Table 1. 568 Assume the laser communication is used for inter-satellite 569 communication, then we can analyze the lifetime of inter-satellite 570 communication when satellites are moving. The Figure 3 illustrates 571 the movement and relative position of satellites on three orbits. 572 The inclination of orbit planes is 90 degrees. 574 + North pole 575 /|\ 576 | s | 577 s | s 578 / s \ 579 s | s 580 | s1 | 581 s4 | s6 582 | s2 | -------- Equator 583 s5 | s7 584 | s3 | 585 s | s 586 \ s / 587 s | s 588 | s | 589 \|/ 590 + South pole 592 Figure 3: Satellite movement 594 There are four scenarios: 596 1. For satellites within the same orbit 597 The satellites in the same orbit will move to the same direction 598 with the same speed, thus the interval between satellites is 599 relatively steady. Each satellite can communicate with its front 600 and back neighbor satellite as long as satellite's orbit is 601 maintained in its life cycle. For example, in Figure 3, s2 can 602 communication with s1 and s3. 604 2. For satellites between neighbor orbits in the same group at 605 non-polar areas 606 The orbits for the same group will share the same orbit altitude 607 and inclination. So, the satellite speed in different orbit are 608 also same, but the moving direction may be same or different. 609 Figure 4 illustrates this scenario. When the moving direction is 610 the same, it is similar to the scenario 1, the relative position 611 of satellites in different orbit are relatively steady as long as 612 satellite's orbit is maintained in its life cycle. When the 613 moving direction is different, the relative position of 614 satellites in different orbit are un-steady, this scenario will 615 be analyzed in more details in Section 5.2.2. 617 3. For satellites between neighbor orbits in the same group at 618 polar areas 619 For satellites between neighbor orbits with the same speed and 620 moving direction, the relative position is steady as described in 621 #2 above, but the steady position is only valid at areas other 622 than polar area. When satellites meet in the polar area, the 623 relative position will change dramatically. Figure 5 shows two 624 satellites meet in polar area and their ISL facing will be 625 swapped. So, if the range of laser pointing angle is 360 degrees 626 and tracking technology supports, the ISL will not be flipping 627 after passing polar area; Otherwise, the link will be flipping 628 and inter-satellite communication will be interrupted. 630 4. For satellites between different orbits in the different group 631 The orbits for the different group will have different orbit 632 altitude, inclination and speed. So, the relative position of 633 satellite is not static. The inter-satellite communication can 634 only last for a while when the distance between two satellite is 635 within the limit of inter-satellite communication, that is 2000km 636 for laser [Laser-communication-range], this scenario will be 637 analyzed in more details in Section 5.2.3 638 i+N/2 i+1+N/2 i+2+N/2 639 / \ / \ / \ 640 / \ / \ / \ 641 S1 \ S2 \ S3 \ 642 / S4 / S5 / S6 643 / \ / \ / \ 644 / \ / \ / \ 645 i-1 i i+1 647 * The total number of orbit planes are N 648 * The number (i-1, i, i+1,...) represents the Orbit index 649 * The bottom numbers (i-1, i, i+1) are for orbit planes on 650 which satellites (S1, S2, S3) are moving from bottom to up. 651 * The top numbers (i+N/2, i+1+N/2, i+2+N/2) are for orbit 652 planes on which satellites (S4, S5, S6) are moving from up 653 to bottom. 655 Figure 4: Two satellites with same altitude and inclination (i) 656 move in the same or opposite direction 658 \ / 659 P3 P4 660 \ / 661 \/ 662 /\ 663 / \ 664 P1 P2 665 / \ 667 * Two satellites S1 and S2 are at position P1 and P2 at time T1 668 * S1's right facing ISL connected to S2's left facing ISL 669 * S1 and S2 move to the position P4 and P3 at time T2 670 * S1's left facing ISL connected to S2's right facing ISL 672 Figure 5: Two satellites meeting in the polar area will change 673 its facing of ISL 675 5.2.2. Satellites on Adjacent Orbit Planes with Same Altitude 677 For satellites on different orbit planes with same altitude, the 678 estimation of the lifetime when two satellite can communicate are as 679 follows. 681 Figure 6 illustrates a general case that two satellites move and 682 intersect with an angle A. 684 ^ V2 685 / 686 / 687 +- 688 / \ A 689 -------------+----+----> V1 690 / 691 / 693 Figure 6: Two satellites (speed vector V1 and V2) intersect with 694 angle A 696 More specifically, for orbit planes with the inclination angle i, 697 Figure 7 illustrates two satellites move in the opposite direction 698 and intersect with an angle 2*i. 700 ^ move from south to north 701 \ / 702 \ / 703 \ /\ 704 \/ | A = 2*i 705 /\ | 706 / \/ 707 / \ 708 / V move from north to south 710 Figure 7: Two satellites with same altitude and inclination (i) 711 intersect with angle A=2*i 713 Follows are the math to calculate the lifetime of communication. 714 Table 3 are the results using the math for two satellites with 715 different altitudes and different inclination angles. 717 Dl The laser communication limit, Dl=2000km 718 [Laser-communication-range] 720 A The angle between two orbit's vertical projection on Earth. 721 A=2*i 723 V1 The speed vector of satellite on orbit1 725 V2 The speed vector of satellite on orbit2 727 |V| the magnitude of the difference of two speed vector V1 and 728 V2, |V|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2). For 729 satellites with the same altitude and inclination angle i, V1=V2, 730 so, |V|=V1*sqrt(2-2*cos(2*i))=2V1*sin(i) 732 T The lifetime two satellites can communicate, or the time of two 733 satellites' distance is within the range of communication, T = 734 2*Dl/|V|. 736 +============+=======+=======+=======+======+=======+=======+ 737 | i (degree) | 80 | 80 | 65 | 65 | 50 | 50 | 738 +============+=======+=======+=======+======+=======+=======+ 739 | Alt (km) | 500 | 800 | 500 | 800 | 500 | 800 | 740 +============+=======+=======+=======+======+=======+=======+ 741 | |V| (km/s) | 14.98 | 14.67 | 13.79 | 13.5 | 11.66 | 11.41 | 742 +------------+-------+-------+-------+------+-------+-------+ 743 | T(s) | 267 | 273 | 290 | 296 | 343 | 350 | 744 +------------+-------+-------+-------+------+-------+-------+ 746 Table 3: The lifetime of communication for two LEOs (with 747 two altitudes and three inclination angles) 749 5.2.3. Satellites on Adjacent Orbit Planes with Different Altitude 751 For satellites on different orbit planes with different altitude, the 752 estimation of the lifetime when two satellite can communicate are as 753 follows. 755 Figure 8 illustrates two satellites (with the altitude difference Da) 756 move and intersect with an angle A. 758 ^ V2 759 / 760 / 761 -------+ / 762 Da /| +- 763 / |/ \ A 764 ----------/--+----+----> V1 765 / / 766 / 767 / 768 / 770 Figure 8: Satellite (speed vector V1 and V2, Altitude difference 771 Da) intersects with Angle A 773 Follows are the math to calculate the lifetime of communication 775 Dl The laser communication limit, Dl=2000km 776 [Laser-communication-range] 778 Da Altitude difference (in km) for two orbit planes 779 A The angle between two orbit's vertical projection on Earth 781 Vl The speed vector of satellite on orbit 1 783 V2 The speed vector of satellite on orbit 2 785 |V| the magnitude of the difference of two speed vector V1 and 786 V2, |v|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2) 788 T The lifetime two satellites can communicate, or the time of two 789 satellites' distance is within the range of communication, T = 790 2*sqrt(Dl^2-Da^2)/|V| 792 Using formulas above, below is the estimation for the life of 793 communication of two satellites when they intersect. Table 4 and 794 Table 5 are for two VLEOs with the difference of 114.1km for 795 altitude. (VLEO1 and VLEO2 on Table 2). Table 6 and Table 7 are for 796 two LEOs with the difference of 175km for altitude (LEO2 and LEO3 on 797 Table 2). 799 +============+=======+=======+ 800 | Parameters | VLEO1 | VLEO2 | 801 +============+=======+=======+ 802 | As(km) | 335.9 | 450 | 803 +------------+-------+-------+ 804 | V (km/s) | 7.7 | 7.636 | 805 +------------+-------+-------+ 807 Table 4: Two VLEO with 808 different altitude and 809 speed 811 +============+=======+=======+=======+========+========+========+ 812 | A (degree) | 0 | 10 | 45 | 90 | 135 | 180 | 813 +============+=======+=======+=======+========+========+========+ 814 | |V| (km/s) | 0.065 | 1.338 | 5.869 | 10.844 | 14.169 | 15.336 | 815 +------------+-------+-------+-------+--------+--------+--------+ 816 | T(s) | 61810 | 2984 | 680 | 368 | 282 | 260 | 817 +------------+-------+-------+-------+--------+--------+--------+ 819 Table 5: Two VLEO intersects with different angle and the life of 820 communication 822 +============+=======+=======+ 823 | Parameters | LEO1 | LEO2 | 824 +============+=======+=======+ 825 | As(km) | 1150 | 1325 | 826 +------------+-------+-------+ 827 | V (km/s) | 7.272 | 7.189 | 828 +------------+-------+-------+ 830 Table 6: Two LEO with 831 different altitude and 832 speed 834 +============+=======+=======+=======+========+========+========+ 835 | A (degree) | 0 | 10 | 45 | 90 | 135 | 180 | 836 +============+=======+=======+=======+========+========+========+ 837 | |V| (km/s) | 0.083 | 1.263 | 5.535 | 10.226 | 13.360 | 14.461 | 838 +------------+-------+-------+-------+--------+--------+--------+ 839 | T(s) | 47961 | 3155 | 720 | 390 | 298 | 276 | 840 +------------+-------+-------+-------+--------+--------+--------+ 842 Table 7: Two LEO intersects with different angle and the life 843 of communication 845 6. Use Satellite Network for Internet 847 Since there is no complete satellite network established yet, all 848 following analysis is based on the predictions from the traditional 849 GEO communication. The analysis also learnt how other type of 850 network has been used in Internet, such as Broadband access network, 851 Mobile access network, Enterprise network and Service Provider 852 network. 854 As a criteria to be part of Internet, any device connected to any 855 satellite should be able to communicate with any public IP4 or IPv6 856 address in Internet. There could be three types of methods to 857 deliver IP packet from source to destination by satellite: 859 1. Data packet is relayed between ground station and satellite. 860 For this method, there is no inter-satellite communication and 861 networking. Data packet is bounced once or couple times between 862 ground stations and satellites until the packet arrives at the 863 destination in Internet. 865 2. Data packet is delivered by inter-satellite networking. 866 For this method, the data packet traverses with multiple 867 satellites and inter-satellite networking is used to deliver the 868 packet to the destination in Internet. 870 3. Both satellite relay and inter-satellite networking are used. 871 For this method, the data packet is relayed in some segments and 872 traverse with multiple satellites in other segments. It is a 873 combination of the method 1 and method 2. 875 Using the above methods, follows are typical deployment scenarios 876 that a Satellite network is integrated with Internet: 878 1. The end user terminal access Internet through satellite relay 879 (Figure 9 for one satellite relay, Figure 10 for multiple 880 satellite relay). 882 2. The end user terminal access Internet through inter-satellite- 883 networking 884 (Figure 11). 886 3. The local network access Internet through satellite relay 887 (Figure 12 for one satellite relay, Figure 13 for multiple 888 satellite relay). 890 4. The local network access Internet through inter-satellite- 891 networking 892 (Figure 14). 894 5. The End user terminal or local network access Internet through 895 satellite network and Mobile Access Network, From mobile access 896 network to satellite network or From satellite network to mobile 897 access network, Satellite network includes inter satellite network 898 and relay network 899 (Figure 15 for mobile access network to satellite network, 900 Figure 16 for satellite netowk to mobile access network). 902 S1----\ /-----------\ 903 / \ / \ 904 T---GW--GS1--S2--GS2-------PE Internet + 905 \ / \ / 906 \---S3/ \-----------/ 908 Figure 9: End user terminal access Internet through one satellite 909 relay 911 S1----\ S4----\ /-----------\ 912 / \ / \ / \ 913 T---GW--GS1--S2--GS2---S5--GS3---PE Internet + 914 \ / \ / \ / 915 \---S3/ \---S6/ \-----------/ 917 Figure 10: End user terminal access Internet through multiple 918 satellite relay 920 S1-----S2-----S3--\ /----------\ 921 / \ / \ 922 T---GW--GS1--S4----S5---S6---GS2-------PE Internet + 923 \ / \ / 924 \---S7----S8----S9/ \----------/ 926 Figure 11: End user terminal access Internet through inter- 927 satellite-networking 929 /-----------\ S1----\ /-------\ 930 / \ / \ / \ 931 + Local network CE------GS1--S4--GS2-------PE Internet + 932 \ / \ / \ / 933 \-----------/ \---S7/ \-------/ 935 Figure 12: Local network access Internet through one satellite relay 937 /-----------\ S1----\ S4----\ /-------\ 938 / \ / \ / \ / \ 939 + Local network CE----GS1--S2--GS2--S5--GS3---PE Internet + 940 \ / \ / \ / \ / 941 \-----------/ \---S3/ \---S6/ \-------/ 943 Figure 13: Local network access Internet through multiple 944 satellite relay 946 /-----------\ S1-----S2-----S3---\ /------\ 947 / \ / \ / \ 948 + Local network CE------GS1--S4----S5---S6---GS2-------PE Internet+ 949 \ / \ / \ / 950 \-----------/ \---S7----S8----S9/ \------/ 952 Figure 14: Local network access Internet through inter-satellite- 953 networking 955 +--------------+ +-------------+ +---------+ +--------+ 956 | T or | |Mobile Access| |Satellite| |Internet| 957 | Local network+----+ Network +----+ Network +----+ | 958 +--------------+ +-------------+ +---------+ +--------+ 960 Figure 15: End user terminal or local network access Internet 961 through Mobile Access Network and Satellite Network 963 +--------------+ +---------+ +-------------+ +--------+ 964 | T or | |Satellite| |Mobile Access| |Internet| 965 | Local network+---+ Network +---+ Network +---+ | 966 +--------------+ +---------+ +-------------+ +--------+ 968 Figure 16: End user terminal or local network access Internet 969 through Satellite Network and Mobile Access Network 971 In above Figure 9 to Figure 16, the meaning of symbols are as 972 follows: 974 T The end user terminal 976 GW Gateway router 978 GS1, GS2, GS3 Ground station with L2/L3 routing/switch 979 functionality. 981 S1 to S9 Satellites 983 PE Provider Edge Router 985 CE Customer Edge Router 987 7. Problems and Requirements for Satellite Constellation for Internet 989 As described in Section 6, satellites in a satellite constellation 990 can either relay internet traffic or multiple satellites can form a 991 network to deliver internet traffic. More detailed analysis are in 992 following sub sections. There might have multiple solutions for each 993 method described in Section 6, following contexts only discuss the 994 most plausible solution from networking perspectives. 996 Section 7.1 will list the common problems and requirements for both 997 satellite relay and satellite networking. 999 Section 7.2 and Section 7.3 will describe key problems, requirement 1000 and potential solution from the networking perspective for these two 1001 cases respectively. 1003 7.1. Common Problems and Requirements 1005 For both satellite relay and satellite networking, satellite-ground- 1006 station must be used, so, the problems and requirements for the 1007 satellite-ground-station communication is common and will apply for 1008 both methods. 1010 When one satellite is communicating with ground station, the 1011 satellite only needs to receive data from uplink of one ground 1012 station, process it and then send to the downlink of another ground 1013 station. Figure 9 illustrates this case. Normally microwave is used 1014 for both links. 1016 Additionally, from the coverage analysis in Section 4.2 and real 1017 deployment in Section 4.3, we can see one ground station may 1018 communicate with multiple satellites. Similarly, one satellite may 1019 communicate with multiple ground stations. The characters for 1020 satellite-ground-station communication are: 1022 1. Satellite-ground-station communication is P2MP. 1023 Since microwave physically is the carrier of broadcast 1024 communication, one satellite can send data while multiple ground 1025 stations can receive it. Similarly, one ground station can send 1026 data and multiple satellites can receive it. 1028 2. Satellite-ground-station communication is in open space and 1029 not secure. 1030 Since electromagnetic fields for microwave physically are 1031 propagating in open space. The satellite-ground-station 1032 communication is also in open space. It is not secure naturally. 1034 3. Satellite-ground-station communication is not steady. 1035 Since the satellite is moving with high speed, from Section 5.1, 1036 the satellite-ground-station communication can only last a 1037 certain period of time. The communication peers will keep 1038 changing. 1040 4. Satellite-to-Satellite communication is not steady. 1041 For some satellites, even they are in the same altitude and move 1042 in the same speed, but they move in the opposite direction, from 1043 Section 5.2.2, the satellite-to-satellite communication can only 1044 last a certain period of time. The communication peers will keep 1045 changing. 1047 5. Satellite-to-Satellite distance is not steady. 1048 For satellites with the same altitude and same moving direction, 1049 even their relative position is steady, but the distance between 1050 satellites are not steady. This will lead to the inter- 1051 satellite-communication's bandwidth and latency keep changing. 1053 6. Satellite physical resource is limited. 1054 Due to the weight, complexity and cost constraint, the physical 1055 resource on a satellite, such as power supply, memory, link 1056 speed, are limited. It cannot be compared with the similar 1057 device on ground. The design and technology used should consider 1058 these factors and take the appropriate approach if possible. 1060 The requirements of satellite-ground-station communication are: 1062 R1. The bi-directional communication capability 1063 Both satellites and ground stations have the bi-directional 1064 communication capability 1066 R2. The identifier for satellites and ground stations 1067 Satellites and ground stations should have Ethernet and/or IP 1068 address configured for the device and each link. More detailed 1069 address configuration can be seen in each solution. 1071 R3. The capability to decide where the IP packet is forwarded to. 1072 In order to send Internet traffic or IP date to destination 1073 correctly, satellites and ground station must have Ethernet hub 1074 or switching or IP routing capability. More detailed capability 1075 can be seen in each solution. 1077 R4. The protocol to establish the satellite-ground-station 1078 communication. 1079 For security and management purpose, the satellite-ground-station 1080 communication is only allowed after both sides agree through a 1081 protocol. The protocol should be able to establish a secured 1082 channel for the communication when a new communication peer comes 1083 up. Each ground station should be able to establish multiple 1084 channels to communicate with multiple satellites. Similarly, 1085 each satellite should be able to establish multiple channels to 1086 communicate to multiple ground stations. 1088 R5. The protocol to discover the state of communication peer. 1089 The discover protocol is needed to detect the state of 1090 communication peer such as peer's identity, the state of the peer 1091 and other info of the peer. The protocol must be running 1092 securely without leaking the discovered info. 1094 R6. The internet data packet is forwarded securely. 1095 When satellite or ground station is sending the IP packet to its 1096 peer, the packet must be relayed securely without leaking the 1097 user data. 1099 R7. The internet data packet is processed efficiently on 1100 satellite 1101 Due to the resource constraint on a satellite, the packet may 1102 need more efficient mechanism to be processed on satellite. The 1103 process on satellite should be very minimal and offloaded to 1104 ground as much as possible. 1106 7.2. Satellite Relay 1108 One of the reasons to use satellite constellation for internet access 1109 is it can provide shorter latency than using the fiber underground. 1110 But using ISL for inter-satellite communication is the premise for 1111 such benefit in latency. Since the ISL is still not mature and 1112 adopted commercially, satellite relay is a only choice currently for 1113 satellite constellation used for internet access. In 1114 [UCL-Mark-Handley], detailed simulations have demonstrated better 1115 latency than fiber network by satellite relay even the ISL is not 1116 present. 1118 7.2.1. One Satellite Relay 1120 One satellite relay is the simplest method for satellite 1121 constellation to provide Internet service. By this method, IP 1122 traffic will be relayed by one satellite to reach the DGS and go to 1123 Internet. 1125 The solution option and associated requirements are: 1127 S1. The satellite only does L1 relay or the physical signal process. 1129 For this solution, a satellite only receives physical signal, amplify 1130 it and broadcast to ground stations. It has no further process for 1131 packet, such as L2 packet compositing and processing, etc. All 1132 packet level work is done only at ground station. The requirements 1133 for the solution are: 1135 R1-1. SGS and BGS are configured as IP routing node. Routing 1136 protocol is running in SGS and BGS 1137 SGS and BGS is a IP peer for a routing protocol (IGP or BGP). SGS 1138 will send internet traffic to DGS as next hop through satellite 1139 uplink and downlink. 1141 R1-2. DGS must be connected with Internet. 1142 DGS can process received packet from satellite and forward the 1143 packet to the destination in Internet. 1145 In addition to the above requirements, following problem should be 1146 solved: 1148 P1-1. IP continuity between two ground stations 1149 This problem is that two ground stations are connected by one 1150 satellite relay. Since the satellite is moving, the IP continuity 1151 between ground stations is interrupted by satellite changing 1152 periodically. Even though this is not killing problem from the 1153 view point that IP service traditionally is only a best effort 1154 service, it will benefit the service if the problem can be solved. 1155 Different approaches may exist, such as using hands off protocols, 1156 multipath solutions, etc. 1158 S2. The satellite does the L2 relay or L2 packet process. 1160 For this solution, IP packet is passing through individual satellite 1161 as an L2 capable device. Unlike in the solution S1, satellite knows 1162 which ground station it should send based on packet's destination MAC 1163 address after L2 processing. The advantage of this solution over S1 1164 is it can use narrower beam to communicate with DGS and get higher 1165 bandwidth and better security. The requirements for the solution 1166 are: 1168 R2-1. Satellite must have L2 bridge or switch capability 1169 In order to forward packet to properly, satellite should run some 1170 L2 process such as MAC learning, MAC switching. The protocol 1171 running on satellite must consider the fast movement of satellite 1172 and its impact to protocol convergence, timer configuration, table 1173 refreshment, etc. 1175 R2-2. same as R1-1 in S1 1177 R2-3. same as R1-2 in S1 1179 In addition to the above requirements, the problem P1-1 for S1 should 1180 also apply. 1182 7.2.2. Multiple Satellite Relay 1184 For this method, packet from SGS will be relayed through multiple 1185 intermediate satellites and ground station until reaching a DGS. 1187 This is more complicated than one satellite relay described in 1188 Section 7.2.1. 1190 One general solution is to configure both satellites and ground- 1191 stations as IP routing nodes, proper routing protocols are running in 1192 this network. The routing protocol will dynamically determine 1193 forwarding path. The obvious challenge for this solution is that all 1194 links between satellite and ground station are not static, according 1195 to the analysis in Section 5.1, the lifetime of each link may last 1196 only couple of minutes. This will result in very quick and constant 1197 topology changes in both link state and IP adjacency, it will cause 1198 the distributed routing algorithms may never converge. So this 1199 solution is not feasible. 1201 Another plausible solution is to specify path statically. The path 1202 is composed of a serials of intermediate ground stations plus SGS and 1203 DGS. This idea will make ground stations static and leave the 1204 satellites dynamic. It will reduce the fluctuation of network path, 1205 thus provide more steady service. One variant for the solution is 1206 whether the intermediate ground stations are connected to Internet. 1207 Separated discussion is as below: 1209 S1. Manual configuring routing path and table 1211 For this solution, the intermediate ground stations and DGS are 1212 specified and configured manually during the stage of network 1213 planning and provisioning. Following requirements apply: 1215 R1-1. Specify a path from SGS to DGS via a list of intermediate 1216 ground stations. 1217 The specified DGS must be connected with internet. Other 1218 specified intermediate ground stations does not have to 1220 R1-2. All Ground stations are configured as IP routing node. 1221 Static routing table on all ground stations must be pre- 1222 configured, the next hop of routes to Internet destination in any 1223 ground station is configured to going through uplink of satellite 1224 to the next ground station until reaching the DGS. 1226 R1-3. All Satellites are configured as either L1 relay or L2 1227 relay. 1228 The Satellite can be configured as L1 relay or L2 relay described 1229 in S1 and S2 respectively in Section 7.2.1 1231 In addition to the above requirements, the problem P1-1 in 1232 Section 7.2.1 should also apply. 1234 S2. Automatic decision by routing protocol. 1236 This solution is only feasible after the IP continuity problem (P1-1 1237 in Section 7.2.1) is solved. Following requirements apply: 1239 R2-1. All Ground stations are configured as IP routing node. 1240 Proper routing protocols are configured as well. 1241 The satellite link cost is configured to be lower than the ground 1242 link. In such a way, the next hop of routes for the IP forwarding 1243 to Internet destination in any ground station will be always going 1244 through the uplink of satellite to the next ground station until 1245 reaching the DGS. 1247 R2-2. All Satellites are configured as either L1 relay or L2 1248 relay. 1249 The Satellite can be configured as L1 relay or L2 relay described 1250 in S1 and S2 respectively in Section 7.2.1 1252 In addition to the above requirements, the problem P1-1 in 1253 Section 7.2.1 should also apply. 1255 7.3. Satellite Networking 1257 In the draft, satellite Network is defined as a network that 1258 satellites are inter-connected by inter-satellite links (ISL). One 1259 of the major difference of satellite network with the other type of 1260 network on ground (telephone, fiber, etc.) is its topology and links 1261 are not stationary, some new issues have to be considered and solved. 1262 Follows are the factors that impact the satellite networking. 1264 7.3.1. L2 or L3 network 1266 The 1st question to answer is should the satellite network be 1267 configured as L2 or L3 network? As analyzed in Section 4.2 and 1268 Section 4.3, since there are couple of hundred or over ten thousand 1269 satellites in a network, L2 network is not a good choice, instead, L3 1270 or IP network is more appropriate for such scale of network. 1272 7.3.2. Inter-satellite-Link Lifetime 1274 If we assume the orbit is circular and ignore other trivial factors, 1275 the satellite speed is approximately determined by the orbit altitude 1276 as described in the Section 5.1. The satellite orbit can determine 1277 if the dynamic position of two satellites is within the range of the 1278 inter-satellite communication. That is 2000km for laser 1279 communication [Laser-communication-range] by Inter Satellite Laser 1280 Link (ISLL). 1282 When two satellites' orbit planes belong to the same group, or two 1283 orbit planes share the same altitude and inclination, and when the 1284 satellites move in the same direction, the relative positions of two 1285 satellites are relatively stationary, and the inter-satellite 1286 communication is steady. But when the satellites move in the 1287 opposite direction, the relative positions of two satellites are not 1288 stationary, the communication lifetime is couple of minutes. The 1289 Section 5.2.2 has analyzed the scenario. 1291 When two satellites' orbit planes belong to the different group, or 1292 two orbit planes have different altitude, the relative position of 1293 two satellite are unstable, and the inter-satellite communication is 1294 not steady. As described in Section 5.2, The life of communication 1295 for two satellites depends on the following parameters of two 1296 satellites: 1298 1. The speed vectors. 1300 2. The altitude difference 1302 3. The intersection angle 1304 From the examples shown in Table 4 to Table 7, we can see that the 1305 lifetime of inter-satellite communication for the different group of 1306 orbit planes are from couple of hundred seconds to about 18 hours. 1307 This fact will impact the routing technologies used for satellite 1308 network and will be discussed in Section 7.3.3. 1310 7.3.3. Problems for Traditional Routing Technologies 1312 When the satellite network is integrated with Internet by traditional 1313 routing technologies, following provisioning and configuration (see 1314 Figure 17) will apply: 1316 1. The ground stations connected to local network and internet are 1317 treated as PE router for satellite network (called PE_GS1 and 1318 PE_GS2 in the following context), and all satellites are treated 1319 as P router. 1321 2. All satellites in the network and ground stations are configured 1322 to run IGP. 1324 3. The eBGP is configured between PE_GS and its peered network's PE 1325 or CE. 1327 The work on PE_GS1 are: 1329 * The local network routes are received at PE_GS1 from CE by eBGP. 1330 The routes are redistributed to IGP and then IGP flood them to all 1331 satellites. (Other more efficient methods, such as iBGP or BGP 1332 reflectors are hard to be used, since the satellite is moving and 1333 there is no easy way to configure a full meshed iBGP session for 1334 all satellites, or configure one satellite as BGP reflector in 1335 satellite network.) 1337 * The internet routes are redistributed from IGP to eBGP running on 1338 PE_GS1, and eBGP will advertise them to CE. 1340 The work on PE_GS2 are: 1342 * The Internet routes are received at PE_GS2 from PE by eBGP. The 1343 routes are redistributed to IGP and then IGP flood them to all 1344 satellites. (Similar as in PE_GS1, Other more efficient methods, 1345 such as iBGP or BGP reflector cannot be used.) 1347 * The local network routes are redistributed from IGP to eBGP 1348 running on PE_GS2, and eBGP will advertise them to Internet. 1350 /--------\ S1---S2----S3----\ /------\ 1351 / \ / IGP domain \ / \ 1352 + Local net CE--eBGP--PE_GS1---S4---S5---PE_GS2--eBGP--PE Internet + 1353 \ / \ / \ / 1354 \--------/ \---S6---S7---S8/ \------/ 1356 Figure 17: Local access Internet through inter-satellite-networking 1358 Local access Internet through inter-satellite-networking 1360 On PE-GS1, due to the fact that IGP link between PE_GS1 and satellite 1361 is not steady; this will lead to following routing activity: 1363 1. When one satellite is connecting with PE_GS1, the satellite and 1364 PE_GS1 form a IGP adjacency. IGP starts to exchange the link 1365 state update. 1367 2. The local network routes received by eBGP in PE_GS1 from CE are 1368 redistributed to IGP, and IGP starts to flood link state update 1369 to all satellites. 1371 3. Meanwhile, the Internet routes learnt from IGP in PE_GS1 will be 1372 redistributed to eBGP. eBGP starts to advertise to CE. 1374 4. Every satellite will update its routing table (RIB) and 1375 forwarding table (FIB) after IGP finishes the SPF algorithm. 1377 5. When the satellite is disconnecting with PE-GS1, the IGP 1378 adjacency between satellite and PE_GS1 is gone. IGP starts to 1379 exchange the link state update. 1381 6. The routes of local network and satellite network that were 1382 redistributed to IGP in step 2 will be withdrawn, and IGP starts 1383 to flood link state update to all satellites. 1385 7. Meanwhile, the Internet routes previously redistributed to eBGP 1386 in step 3 will also be withdrawn. eBGP starts to advertise route 1387 withdraw to CE. 1389 8. Every satellite will update its routing table (RIB) and 1390 forwarding table (FIB) after the SPF algorithm. 1392 Similarly on PE_GS2, due to the fact that IGP link between PE_GS2 and 1393 satellite is not steady; this will lead to following routing 1394 activity: 1396 1. When one satellite is connecting with PE_GS2, the satellite and 1397 PE_GS2 form a IGP adjacency. IGP starts to exchange the link 1398 state update. 1400 2. The Internet routes previously received by eBGP in PE_GS2 from PE 1401 are redistributed to IGP, IGP starts to flood the new link state 1402 update to all satellites. 1404 3. Meanwhile, the routes of local network and satellite network 1405 learnt from IGP in PE_GS2 will be redistributed to eBGP. eBGP 1406 starts to advertise to Internet peer PE. 1408 4. Every satellite will update its routing table (RIB) and 1409 forwarding table (FIB) after IGP finishes the SPF algorithm. 1411 5. When the satellite is disconnecting with PE-GS2, the IGP 1412 adjacency between satellite and PE_GS2 is gone. IGP starts to 1413 exchange the link state update. 1415 6. The internet routes previously redistributed to IGP in step 2 1416 will be withdrawn, and IGP starts to flood link state update to 1417 all satellites 1419 7. Meanwhile, the routes of local network and satellite network 1420 previously redistributed to eBGP in step 3 will also be 1421 withdrawn. eBGP starts to advertise route withdraw to PE. 1423 8. Every satellite will update its routing table (RIB) and 1424 forwarding table (FIB) after the SPF algorithm. 1426 For the analysis of detailed events above, the estimated time 1427 interval between event 1 and 5 for PE_GS1 and PE_GS2 can use the 1428 analysis in Section 5.1. For example, it is about 398s for LEO and 1429 103s for VLEO. Within this time interval, the satellite network 1430 including all satellites and two ground stations must finish the 1431 works from 1 to 4 for PE_GS1 and PE_GS2. The normal internet IPv6 1432 and IPv4 BGP routes size are about 850k v4 routes + 100K v6 routes 1433 [BGP-Table-Size]. There are couple critical problems associated with 1434 the events: 1436 P1. Frequent IGP update for its link cost 1437 Even for satellites in different orbit with the steady relative 1438 positions, the distance between satellites is keep changing. If 1439 the distance is used as the link cost, it means the IGP has to 1440 update the link cost frequently. This will make IGP keep running 1441 and update its routing table. 1443 P2. Frequent IGP flooding for the internet routes 1444 Whenever the IGP adjacency changes (step 1 and 5 for PE_GS2), it 1445 will trigger the massive IGP flooding for the link state update 1446 for massive internet routes learnt from eBGP. This will result in 1447 the IGP re-convergency, RIB and FIP update. 1449 P3. Frequent BGP advertisement for the internet routes 1450 Whenever the IGP adjacency changes (step 3 and 7 for PE_GS1), it 1451 will trigger the massive BGP advertisement for the internet routes 1452 learnt from IGP. This will result in the BGP re-convergency, RIB 1453 and FIB update. BGP convergency time is longer than IGP. The 1454 document [BGP-Converge-Time1] has shown that the BGP convergence 1455 time varies from 50sec to couple of hundred seconds. The analysis 1456 [BGP-Converge-Time2] indicated that per entry update takes about 1457 150us, and it takes o(75s) for 500k routes, or o(150s) for 1M 1458 routes. 1460 P4. More frequent IGP flooding and BGP update in whole satellite 1461 network 1462 To provide the global coverage, a satellite constellation will 1463 have many ground stations deployed. For example, StarLink has 1464 applied for the license for up to one million ground stations 1465 [StarLink-Ground-Station-Fcc], in which, more than 50 gateway 1466 ground stations (equivalent to the PE_GS2) have been registered 1467 [SpaceX-Ground-Station-Fcc] and deployed in U.S. 1468 [StarLink-GW-GS-map]. It is expected that the gateway ground 1469 station will grow quickly to couple of thousands 1470 [Tech-Comparison-LEOs]. This means almost each satellite in the 1471 satellite network would have a ground station connected. , Due to 1472 the fact that all satellites are moving, many IGP adjacency 1473 changes may occur in a shorter period of time described in 1474 Section 5.1 and result in the problem P1 and P2 constantly occur. 1476 P5. Service is not steady 1477 Due to the problems P1 to P3, the service provider of satellite 1478 constellation is hard to provide a steady service for broadband 1479 service by using inter-satellite network and traditional routing 1480 technologies. 1482 As a summary, the traditional routing technology is problematic for 1483 large scale inter-satellite networking for Internet. Enhancements on 1484 traditional technologies, or new technologies are expected to solve 1485 the specific issues associated with satellite networking. 1487 8. IANA Considerations 1489 This memo includes no request to IANA. 1491 9. Contributors 1493 10. Acknowledgements 1495 11. References 1497 11.1. Normative References 1499 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 1500 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 1501 DOI 10.17487/RFC4271, January 2006, 1502 . 1504 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 1505 DOI 10.17487/RFC2328, April 1998, 1506 . 1508 [RFC7142] Shand, M. and L. Ginsberg, "Reclassification of RFC 1142 1509 to Historic", RFC 7142, DOI 10.17487/RFC7142, February 1510 2014, . 1512 [RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453, 1513 DOI 10.17487/RFC2453, November 1998, 1514 . 1516 [RFC7868] Savage, D., Ng, J., Moore, S., Slice, D., Paluch, P., and 1517 R. White, "Cisco's Enhanced Interior Gateway Routing 1518 Protocol (EIGRP)", RFC 7868, DOI 10.17487/RFC7868, May 1519 2016, . 1521 11.2. Informative References 1523 [KeplerianElement] 1524 "Keplerian elements", 1525 . 1527 [GEO-Coverage] 1528 "Coverage of a geostationary satellite at Earth", 1529 . 1532 [Nttdocomo-6G] 1533 "NTTDPCOM 6G White Paper", 1534 . 1538 [ITU-6G] "ITU 6G vision", . 1541 [Surrey-6G] 1542 "Surrey 6G vision", 1543 . 1546 [OSI-Model] 1547 "OSI Model", . 1549 [StarLink] "Star Link", . 1551 [China-constellation] 1552 "China Constellation", . 1555 [ESA-HydRON] 1556 "HydRON: Fiber in the sky", 1557 . 1560 [SpaceX-Non-GEO] 1561 "FCC report: SPACEX V-BAND NON-GEOSTATIONARY SATELLITE 1562 SYSTEM", . 1565 [Satellite-coverage] 1566 Alan R.Washburn, Department of Operations Research, Naval 1567 Postgraduate School, "Earth Coverage by Satellites in 1568 Circular Orbit", 1569 . 1572 [Microwave-vs-Laser-communication] 1573 International Journal for Research in Applied Science and 1574 Engineering Technology (IJRASET), "Comparison of Microwave 1575 and Optical Wireless Inter-Satellite Links", 1576 . 1578 [Laser-communication-range] 1579 "Interferometric optical communications can potentially 1580 lead to robust, secure, and naturally encrypted long- 1581 distance laser communications in space by taking advantage 1582 of the underlying physics of quantum entanglement.", 1583 . 1587 [BGP-Table-Size] 1588 "BGP in 2020 - BGP table", 1589 . 1592 [BGP-Converge-Time1] 1593 "BGP in 2020 - BGP Update Churn", 1594 . 1596 [BGP-Converge-Time2] 1597 "Bringing SDN to the Internet, one exchange point at the 1598 time", 1599 . 1602 [StarLink-Ground-Station-Fcc] 1603 "APPLICATION FOR BLANKET LICENSED EARTH STATIONS", 1604 . 1606 [SpaceX-Ground-Station-Fcc] 1607 "List of SpaceX applications for ground stations", 1608 . 1611 [Tech-Comparison-LEOs] 1612 "A Technical Comparison of Three Low Earth Orbit Satellite 1613 Constellation Systems to Provide Global Broadband", 1614 . 1617 [StarLink-GW-GS-map] 1618 "StarLink gateway ground station map", 1619 . 1622 [UCL-Mark-Handley] 1623 "Using ground relays for low-latency wide-area routing in 1624 megaconstellations", 1625 . 1628 Appendix A. Change Log 1630 * Initial version, 07/03/2021 1632 * 01 version, 10/20/2021 1634 Authors' Addresses 1636 Lin Han (editor) 1637 Futurewei Technologies, Inc. 1638 2330 Central Expy 1639 Santa Clara, CA 95050, 1640 United States of America 1642 Email: lhan@futurewei.com 1643 Richard Li 1644 Futurewei Technologies, Inc. 1645 2330 Central Expy 1646 Santa Clara, CA 95050, 1647 United States of America 1649 Email: rli@futurewei.com 1651 Alvaro Retana 1652 Futurewei Technologies, Inc. 1653 2330 Central Expy 1654 Santa Clara, CA 95050, 1655 United States of America 1657 Email: alvaro.retana@futurewei.com 1659 Meiling Chen 1660 China Mobile 1661 32, Xuanwumen West 1662 BeiJing 100053 1663 China 1665 Email: chenmeiling@chinamobile.com 1667 Li Su 1668 China Mobile 1669 32, Xuanwumen West 1670 BeiJing 100053 1671 China 1673 Email: suli@chinamobile.com 1675 Ning Wang 1676 University of Surrey 1677 Guildford 1678 Surrey, GU2 7XH 1679 United Kingdom 1681 Email: n.wang@surrey.ac.uk