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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group L. Peterson 3 Internet-Draft Akamai Technologies, Inc. 4 Obsoletes: 3466 (if approved) B. Davie 5 Intended status: Informational VMware, Inc. 6 Expires: November 6, 2014 R. van Brandenburg, Ed. 7 TNO 8 May 5, 2014 10 Framework for CDN Interconnection 11 draft-ietf-cdni-framework-11 13 Abstract 15 This document presents a framework for Content Distribution Network 16 Interconnection (CDNI). The purpose of the framework is to provide 17 an overall picture of the problem space of CDNI and to describe the 18 relationships among the various components necessary to interconnect 19 CDNs. CDN Interconnection requires the specification of interfaces 20 and mechanisms to address issues such as request routing, 21 distribution metadata exchange, and logging information exchange 22 across CDNs. The intent of this document is to outline what each 23 interface needs to accomplish, and to describe how these interfaces 24 and mechanisms fit together, while leaving their detailed 25 specification to other documents. This document, in combination with 26 RFC 6707, obsoletes RFC 3466. 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 http://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 November 6, 2014. 45 Copyright Notice 47 Copyright (c) 2014 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 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.2. Reference Model . . . . . . . . . . . . . . . . . . . . . 5 65 1.3. Structure Of This Document . . . . . . . . . . . . . . . 9 66 2. Building Blocks . . . . . . . . . . . . . . . . . . . . . . . 9 67 2.1. Request Redirection . . . . . . . . . . . . . . . . . . . 9 68 2.1.1. DNS Redirection . . . . . . . . . . . . . . . . . . . 9 69 2.1.2. HTTP Redirection . . . . . . . . . . . . . . . . . . 11 70 3. Overview of CDNI Operation . . . . . . . . . . . . . . . . . 11 71 3.1. Preliminaries . . . . . . . . . . . . . . . . . . . . . . 13 72 3.2. Iterative HTTP Redirect Example . . . . . . . . . . . . . 14 73 3.3. Recursive HTTP Redirection Example . . . . . . . . . . . 19 74 3.4. Iterative DNS-based Redirection Example . . . . . . . . . 23 75 3.4.1. Notes on using DNSSEC . . . . . . . . . . . . . . . . 27 76 3.5. Dynamic Footprint Discovery Example . . . . . . . . . . . 28 77 3.6. Content Removal Example . . . . . . . . . . . . . . . . . 30 78 3.7. Pre-Positioned Content Acquisition Example . . . . . . . 31 79 3.8. Asynchronous CDNI Metadata Example . . . . . . . . . . . 32 80 3.9. Synchronous CDNI Metadata Acquisition Example . . . . . . 34 81 3.10. Content and Metadata Acquisition with Multiple Upstream 82 CDNs . . . . . . . . . . . . . . . . . . . . . . . . . . 36 83 4. Main Interfaces . . . . . . . . . . . . . . . . . . . . . . . 37 84 4.1. In-Band versus Out-of-Band Interfaces . . . . . . . . . . 38 85 4.2. Cross Interface Concerns . . . . . . . . . . . . . . . . 38 86 4.3. Request Routing Interfaces . . . . . . . . . . . . . . . 39 87 4.4. CDNI Logging Interface . . . . . . . . . . . . . . . . . 40 88 4.5. CDNI Control Interface . . . . . . . . . . . . . . . . . 42 89 4.6. CDNI Metadata Interface . . . . . . . . . . . . . . . . . 42 90 4.7. HTTP Adaptive Streaming Concerns . . . . . . . . . . . . 43 91 4.8. URI Rewriting . . . . . . . . . . . . . . . . . . . . . . 44 92 5. Deployment Models . . . . . . . . . . . . . . . . . . . . . . 45 93 5.1. Meshed CDNs . . . . . . . . . . . . . . . . . . . . . . . 46 94 5.2. CSP combined with CDN . . . . . . . . . . . . . . . . . . 47 95 5.3. CSP using CDNI Request Routing Interface . . . . . . . . 47 96 5.4. CDN Federations and CDN Exchanges . . . . . . . . . . . . 48 97 6. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 51 98 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52 99 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 52 100 9. Security Considerations . . . . . . . . . . . . . . . . . . . 53 101 9.1. Security of CDNI Interfaces . . . . . . . . . . . . . . . 54 102 9.2. Digital Rights Management . . . . . . . . . . . . . . . . 54 103 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 54 104 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 54 105 12. Informative References . . . . . . . . . . . . . . . . . . . 55 106 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 56 108 1. Introduction 110 This document provides an overview of the various components 111 necessary to interconnect CDNs, expanding on the problem statement 112 and use cases introduced in [RFC6770] and [RFC6707]. It describes 113 the necessary interfaces and mechanisms in general terms and outlines 114 how they fit together to form a complete system for CDN 115 Interconnection. Detailed specifications are left to other 116 documents. This document makes extensive use of message flow 117 examples to illustrate the operation of interconnected CDNs, but 118 these examples should be considered illustrative rather than 119 prescriptive. 121 [RFC3466] uses different terminology and models for "Content 122 Internetworking (CDI)". It is also less prescriptive in terms of 123 interfaces. To avoid confusion, this document obsoletes [RFC3466]. 125 1.1. Terminology 127 This document uses the core terminology defined in [RFC6707]. It 128 also introduces the following terms: 130 CDN-Domain: a host name (FQDN) at the beginning of a URL (excluding 131 port and scheme), representing a set of content that is served by a 132 given CDN. For example, in the URL http://cdn.csp.example/...rest of 133 url..., the CDN domain is cdn.csp.example. A major role of CDN- 134 Domain is to identify a region (subset) of the URI space relative to 135 which various CDN Interconnection rules and policies are to apply. 136 For example, a record of CDN Metadata might be defined for the set of 137 resources corresponding to some CDN-Domain. 139 Distinguished CDN-Domain: a CDN-Domain that is allocated by a CDN for 140 the purposes of communication with a peer CDN, but which is not found 141 in client requests. Such CDN-Domains may be used for inter-CDN 142 acquisition, or as redirection targets, and enable a CDN to 143 distinguish a request from a peer CDN from an end-user request. 145 Delivering CDN: the CDN that ultimately delivers a piece of content 146 to the end-user. The last in a potential sequence of downstream 147 CDNs. 149 Recursive CDNI Request Redirection: When an upstream CDN elects to 150 redirect a request towards a downstream CDN, the upstream CDN can 151 query the downstream CDN Request Routing system via the CDNI Request 152 Routing Redirection Interface (or use information cached from earlier 153 similar queries) to find out how the downstream CDN wants the request 154 to be redirected, which allows the upstream CDN to factor in the 155 downstream CDN response when redirecting the user agent. This 156 approach is referred to as "Recursive" CDNI Request Redirection. 157 Note that the downstream CDN may elect to have the request redirected 158 directly to a Surrogate inside the downstream CDN, to the Request 159 Routing System of the downstream CDN, to another CDN, or to whatever 160 system is necessary to handle the redirected request appropriately. 162 Iterative CDNI Request Redirection: When an upstream CDN elects to 163 redirect a request towards a downstream CDN, the upstream CDN can 164 base its redirection purely on a local decision (and without 165 attempting to take into account how the downstream CDN may in turn 166 redirect the user agent). In that case, the upstream CDN redirects 167 the request to the request routing system in the downstream CDN, 168 which in turn will decide how to redirect that request: this approach 169 is referred to as "Iterative" CDNI Request Redirection. 171 Synchronous CDNI operations: operations between CDNs that happen 172 during the process of servicing a user request, i.e. between the time 173 that the user agent begins its attempt to obtain content and the time 174 at which that request is served. 176 Asynchronous CDNI operations: operations between CDNs that happen 177 independently of any given user request, such as advertisement of 178 footprint information or pre-positioning of content for later 179 delivery. 181 Trigger Interface: a subset of the CDNI Control interface that 182 includes operations to pre-position, revalidate, and purge both 183 metadata and content. These operations are typically called in 184 response to some action (Trigger) by the CSP on the upstream CDN. 186 We also sometimes use uCDN and dCDN as shorthand for upstream CDN and 187 downstream CDN, respectively. 189 At various points in this document, the concept of a CDN footprint is 190 used. For a discussion on what constitutes a CDN footprint, the 191 reader is referred to 192 [I-D.ietf-cdni-footprint-capabilities-semantics]. 194 1.2. Reference Model 196 This document uses the reference model in Figure 1, which expands the 197 reference model originally defined in [RFC6707]. (The difference is 198 that the expanded model splits the Request Routing Interface into its 199 two distinct parts: the Request Routing Redirection interface and the 200 Footprint and Capabilities Advertisement interface, as described 201 below.) 202 -------- 203 / \ 204 | CSP | 205 \ / 206 -------- 207 * 208 * 209 * /\ 210 * / \ 211 ---------------------- |CDNI| ---------------------- 212 / Upstream CDN \ | | / Downstream CDN \ 213 | +-------------+ | | CI | | +-------------+ | 214 |******* Control |<======|====|=======>| Control *******| 215 |* +------*----*-+ | | | | +-*----*------+ *| 216 |* * * | | | | * * *| 217 |* +------*------+ | | LI | | +------*------+ *| 218 |* ***** Logging |<======|====|=======>| Logging ***** *| 219 |* * +-*-----------+ | | | | +-----------*-+ * *| 220 |* * * * | | | | * * * *| 221 .....*...+-*---------*-+ | | RI | | +-*---------*-+...*.*... 222 . |* * | |<======|====|=======>| | * *| . 223 . |* * | Req-Routing | | |FCI | | | Req-Routing | * *| . 224 . |* * *** |<======|====|=======>| |** * *| . 225 . |* * * +-------------+.| | | | +-------------+ * * *| . 226 . |* * * . | | | * * *| . 227 . |* * * +-------------+ |. | MI | | +-------------+ * * *| . 228 . |* * * | Distribution|<==.===|====|=======>| Distribution| * * *| . 229 . |* * * | | | . \ / | | | * * *| . 230 . |* * * |+---------+ | | . \/ | | +---------+| * * *| . 231 . |* * ***| +---------+| | ...Request......+---------+ |*** * *| . 232 . |* *****+-|Surrogate|***********************|Surrogate|-+***** *| . 233 . |******* +---------+| | Acquisition | |+----------+ *******| . 234 . | +-------------+ | | +-------*-----+ | . 235 . \ / \ * / . 236 . ---------------------- ---------*------------ . 237 . * . 238 . * Delivery . 239 . * . 240 . +--*---+ . 241 ...............Request............................| User |..Request.. 242 | Agent| 243 +------+ 245 <==> interfaces inside the scope of CDNI 247 **** and .... interfaces outside the scope of CDNI 249 Figure 1: CDNI Expanded Model and CDNI Interfaces 251 We note that while some interfaces in the reference model are "out of 252 scope" for the CDNI WG (in the sense that there is no need to define 253 new protocols for those interfaces) we still need to refer to them in 254 this document to explain the overall operation of CDNI. 256 We also note that, while we generally show only one upstream CDN 257 serving a given CSP, it is entirely possible that multiple uCDNs can 258 serve a single CSP. In fact, this situation effectively exists today 259 in the sense that a single CSP can currently delegate its content 260 delivery to more than one CDN. 262 The following briefly describes the five CDNI interfaces, 263 paraphrasing the definitions given in [RFC6707]. We discuss these 264 interfaces in more detail in Section 4. 266 o CDNI Control interface (CI): Operations to bootstrap and 267 parameterize the other CDNI interfaces, as well as operations to 268 pre-position, revalidate, and purge both metadata and content. 269 The latter subset of operations is sometimes collectively called 270 the "Trigger interface." 272 o CDNI Request Routing interface: Operations to determine what CDN 273 (and optionally what surrogate within a CDN) is to serve end- 274 user's requests. This interface is actually a logical bundling of 275 two separate but related interfaces: 277 * CDNI Footprint & Capabilities Advertisement interface (FCI): 278 Asynchronous operations to exchange routing information (e.g., 279 the network footprint and capabilities served by a given CDN) 280 that enables CDN selection for subsequent user requests; and 282 * CDNI Request Routing Redirection interface (RI): Synchronous 283 operations to select a delivery CDN (surrogate) for a given 284 user request. 286 o CDNI Metadata interface (MI): Operations to communicate metadata 287 that governs how the content is delivered by interconnected CDNs. 288 Examples of CDNI metadata include geo-blocking directives, 289 availability windows, access control mechanisms, and purge 290 directives. It may include a combination of: 292 * Asynchronous operations to exchange metadata that govern 293 subsequent user requests for content; and 295 * Synchronous operations that govern behavior for a given user 296 request for content. 298 o CDNI Logging interface (LI): Operations that allow interconnected 299 CDNs to exchange relevant activity logs. It may include a 300 combination of: 302 * Real-time exchanges, suitable for runtime traffic monitoring; 303 and 305 * Offline exchanges, suitable for analytics and billing. 307 The division between the sets of Trigger-based operations in the CDNI 308 Control interface and the CDNI Metadata interface is somewhat 309 arbitrary. For both cases, the information passed from the upstream 310 CDN to the downstream CDN can broadly be viewed as metadata that 311 describes how content is to be managed by the downstream CDN. For 312 example, the information conveyed by CI to pre-position, revalidate 313 or purge metadata is similar to the information conveyed by posting 314 updated metadata via the MI. Even the CI operation to purge content 315 could be viewed as a metadata update for that content: purge simply 316 says that the availability window for the named content ends now. 317 The two interfaces share much in common, so minimally, there will 318 need to be a consistent data model that spans both. 320 The distinction we draw has to do with what the uCDN knows about the 321 successful application of the metadata by the dCDN. In the case of 322 the CI, the downstream CDN returning a successful status message 323 guarantees that the operation has been successfully completed; e.g., 324 the content has been purged or pre-positioned. This implies that the 325 downstream CDN accepts responsibility for having successfully 326 completed the requested operation. In contrast, metadata passed 327 between CDNs via the MI carries no such completion guarantee. 328 Returning success implies successful receipt of the metadata, but 329 nothing can be inferred about precisely when the metadata will take 330 effect in the downstream CDN, only that it will take effect 331 eventually. This is because of the challenge in globally 332 synchronizing updates to metadata with end-user requests that are 333 currently in progress (or indistinguishable from currently being in 334 progress). Clearly, a CDN will not be viewed as a trusted peer if 335 "eventually" often becomes an indefinite period of time, but the 336 acceptance of responsibility cannot be as crisply defined for the MI. 338 Finally, there is a practical issue that impacts all of the CNDI 339 interfaces, and that is whether or not to optimize CDNI for HTTP 340 Adaptive Streaming (HAS). We highlight specific issues related to 341 delivering HAS content throughout this document, but for a more 342 thorough treatment of the topic, see [RFC6983]. 344 1.3. Structure Of This Document 346 The remainder of this document is organized as follows: 348 o Section 2 describes some essential building blocks for CDNI, 349 notably the various options for redirecting user requests to a 350 given CDN. 352 o Section 3 provides a number of illustrative examples of various 353 CDNI operations. 355 o Section 4 describes the functionality of the main CDNI interfaces. 357 o Section 5 shows how various deployment models of CDNI may be 358 achieved using the defined interfaces. 360 o Section 6 describes the trust model of CDNI and the issues of 361 transitive trust in particular that CDNI raises. 363 2. Building Blocks 365 2.1. Request Redirection 367 At its core, CDN Interconnection requires the redirection of requests 368 from one CDN to another. For any given request that is received by 369 an upstream CDN, it will either respond to the request directly, or 370 somehow redirect the request to a downstream CDN. Two main 371 mechanisms are available for redirecting a request to a downstream 372 CDN. The first leverages the DNS name resolution process and the 373 second uses application-layer redirection mechanisms such as the HTTP 374 302 or RTSP 302 redirection responses. While there exists a large 375 variety of application-layer protocols that include some form of 376 redirection mechanism, this document will use HTTP (and HTTPS) in its 377 examples. Similar mechanisms can be applied to other application- 378 layer protocols. What follows is a short discussion of both DNS- and 379 HTTP-based redirection, before presenting some examples of their use 380 in Section 3. 382 2.1.1. DNS Redirection 384 DNS redirection is based on returning different IP addresses for the 385 same DNS name, for example, to balance server load or to account for 386 the client's location in the network. A DNS server, sometimes called 387 the Local DNS (LDNS), resolves DNS names on behalf of an end-user. 388 The LDNS server in turn queries other DNS servers until it reaches 389 the authoritative DNS server for the CDN-Domain. The network 390 operator typically provides the LDNS server, although the user is 391 free to choose other DNS servers (e.g., OpenDNS, Google Public DNS). 393 This latter possibility is important because the authoritative DNS 394 server sees only the IP address of the DNS server that queries it, 395 not the IP address of the original end-user. 397 The advantage of DNS redirection is that it is completely transparent 398 to the end user; the user sends a DNS name to the LDNS server and 399 gets back an IP address. On the other hand, DNS redirection is 400 problematic because the DNS request comes from the LDNS server, not 401 the end-user. This may affect the accuracy of server selection that 402 is based on the user's location. The transparency of DNS redirection 403 is also a problem in that there is no opportunity to take the 404 attributes of the user agent or the URI path component into account. 405 We consider two main forms of DNS redirection: simple and CNAME- 406 based. 408 In simple DNS redirection, the authoritative DNS server for the name 409 simply returns an IP address from a set of possible IP addresses. 410 The answer is chosen from the set based on characteristics of the set 411 (e.g., the relative loads on the servers) or characteristics of the 412 client (e.g., the location of the client relative to the servers). 413 Simple redirection is straightforward. The only caveats are (1) 414 there is a limit to the number of alternate IP addresses a single DNS 415 server can manage; and (2) DNS responses are cached by downstream 416 servers so the TTL on the response must be set to an appropriate 417 value so as to preserve the fresheness of the redirection. 419 In CNAME-based DNS redirection, the authoritative server returns a 420 CNAME response to the DNS request, telling the LDNS server to restart 421 the name lookup using a new name. A CNAME is essentially a symbolic 422 link in the DNS namespace, and like a symbolic link, redirection is 423 transparent to the client; the LDNS server gets the CNAME response 424 and re-executes the lookup. Only when the name has been resolved to 425 an IP address does it return the result to the user. Note that DNAME 426 would be preferable to CNAME if it becomes widely supported. 428 One of the advantages of DNS redirection compared to HTTP redirection 429 is that it can be cached, reducing load on the redirecting CDN's DNS 430 server. However, this advantage can also be a drawback, especially 431 when a given DNS resolver doesn't strictly adhere to the TTL, which 432 is a known problem in some real world environments. In such cases, 433 an end-user might end up at a dCDN without first having passed 434 through the uCDN, which might be an undesirable scenario from a uCDN 435 point of view. 437 2.1.2. HTTP Redirection 439 HTTP redirection makes use of the redirection response of the HTTP 440 protocol (e.g.,"302" or "307"). This response contains a new URL 441 that the application should fetch instead of the original URL. By 442 changing the URL appropriately, the server can cause the user to 443 redirect to a different server. The advantages of HTTP redirection 444 are that (1) the server can change the URL fetched by the client to 445 include, for example, both the DNS name of the particular server to 446 use, as well as the original HTTP server that was being accessed; (2) 447 the client sends the HTTP request to the server, so that its IP 448 address is known and can be used in selecting the server; and (3) 449 other attributes (e.g., content type, user agent type) are visible to 450 the redirection mechanism. 452 Just as is the case for DNS redirection, there are some potential 453 disadvantages of using HTTP redirection. For example, it may affect 454 application behavior, e.g. web browsers will not send cookies if the 455 URL changes to a different domain. In addition, although this might 456 also be an advantage, results of HTTP redirection are not cached so 457 that all redirections must go through to the uCDN. 459 3. Overview of CDNI Operation 461 To provide a big picture overview of the various components of CDN 462 Interconnection, we walk through a "day in the life" of a content 463 item that is made available via a pair of interconnected CDNs. This 464 will serve to illustrate many of the functions that need to be 465 supported in a complete CDNI solution. We give examples using both 466 DNS-based and HTTP-based redirection. We begin with very simple 467 examples and then show how additional capabilities, such as recursive 468 request redirection and content removal, might be added. 470 Before walking through the specific examples, we present a high-level 471 view of the operations that may take place. This high-level overview 472 is illustrated in Figure 2. Note that most operations will involve 473 only a subset of all the messages shown below, and that the order and 474 number of operations may vary considerably, as the more detailed 475 examples illustrate. 477 The following shows Operator A as the upstream CDN (uCDN) and 478 Operator B as the downstream CDN (dCDN), where the former has a 479 relationship with a content provider and the latter being the CDN 480 selected by Operator A to deliver content to the end-user. The 481 interconnection relationship may be symmetric between these two CDN 482 operators, but each direction can be considered as operating 483 independently of the other so for simplicity we show the interaction 484 in one direction only. 486 End-User Operator B Operator A 487 | | | 488 | | | 489 | | [Async FCI Push] | (1) 490 | | | 491 | | [MI pre-positioning] | (2) 492 | | | 493 | CONTENT REQUEST | | 494 |-------------------------------------------------->| (3) 495 | | | 496 | | [Sync RI Pull] | (4) 497 | | | 498 | RI REPLY | | 499 |<--------------------------------------------------| (5) 500 | | | 501 | | | 502 | CONTENT REQUEST | | 503 |------------------------>| | (6) 504 | | | 505 | | [Sync MI Pull] | (7) 506 | | | 507 | | ACQUISITION REQUEST | 508 | X------------------------>| (8) 509 | X | 510 | X CONTENT DATA | 511 | X<------------------------| (9) 512 | | | 513 | CONTENT DATA | | 514 |<------------------------| | (10) 515 | | | 516 : : : 517 : [Other content requests] : 518 : : : 519 | | [CI: Content Purge] | (11) 520 : : : 521 | | [LI: Log exchange] | (12) 522 | | | 524 Figure 2: Overview of Operation 526 The operations shown in the Figure are as follows: 528 1. dCDN uses the FCI to advertise information relevant to its 529 delivery footprint and capabilities prior to any content 530 requests being redirected. 532 2. Prior to any content request, the uCDN uses the MI to pre- 533 position CDNI metadata to the dCDN, thereby making that metadata 534 available in readiness for later content requests. 536 3. A content request from a user agent arrives at uCDN. 538 4. uCDN may use the RI to synchronously request information from 539 dCDN regarding its delivery capabilities to decide if dCDN is a 540 suitable target for redirection of this request. 542 5. uCDN redirects the request to dCDN by sending some response 543 (DNS, HTTP) to the user agent. 545 6. The user agent requests the content from dCDN. 547 7. dCDN may use the MI to synchronously request metadata related to 548 this content from uCDN, e.g. to decide whether to serve it. 550 8. If the content is not already in a suitable cache in dCDN, dCDN 551 may acquire it from uCDN. 553 9. The content is delivered to dCDN from uCDN. 555 10. The content is delivered to the user agent by dCDN. 557 11. Some time later, perhaps at the request of the CSP (not shown) 558 uCDN may use the CI to instruct dCDN to purge the content, 559 thereby ensuring it is not delivered again. 561 12. After one or more content delivery actions by dCDN, a log of 562 delivery actions may be provided to uCDN using the LI. 564 The following sections show some more specific examples of how these 565 operations may be combined to perform various delivery, control and 566 logging operations across a pair of CDNs. 568 3.1. Preliminaries 570 Initially, we assume that there is at least one CSP that has 571 contracted with an upstream CDN (uCDN) to deliver content on its 572 behalf. We are not particularly concerned with the interface between 573 the CSP and uCDN, other than to note that it is expected to be the 574 same as in the "traditional" (non-interconnected) CDN case. Existing 575 mechanisms such as DNS CNAMEs or HTTP redirects (Section 2) can be 576 used to direct a user request for a piece of content from the CSP 577 towards the CSP's chosen upstream CDN. 579 We assume Operator A provides an upstream CDN that serves content on 580 behalf of a CSP with CDN-Domain cdn.csp.example. We assume that 581 Operator B provides a downstream CDN. An end user at some point 582 makes a request for URL 584 http://cdn.csp.example/...rest of url... 586 It may well be the case that cdn.csp.example is just a CNAME for some 587 other CDN-Domain (such as csp.op-a.example). Nevertheless, the HTTP 588 request in the examples that follow is assumed to be for the example 589 URL above. 591 Our goal is to enable content identified by the above URL to be 592 served by the CDN of operator B. In the following sections we will 593 walk through some scenarios in which content is served, as well as 594 other CDNI operations such as the removal of content from a 595 downstream CDN. 597 3.2. Iterative HTTP Redirect Example 599 In this section we walk through a simple, illustrative example using 600 HTTP redirection from uCDN to dCDN. The example also assumes the use 601 of HTTP redirection inside uCDN and dCDN; however, this is 602 independent of the choice of redirection approach across CDNs, so an 603 alternative example could be constructed still showing HTTP 604 redirection from uCDN to dCDN but using DNS for handling of request 605 inside each CDN. 607 We assume for this example that Operators A and B have established an 608 agreement to interconnect their CDNs, with A being upstream and B 609 being downstream. 611 The operators agree that a CDN-Domain peer-a.op-b.example will be 612 used as the target of redirections from uCDN to dCDN. We assume the 613 name of this domain is communicated by some means to each CDN. (This 614 could be established out-of-band or via a CDNI interface.) We refer 615 to this domain as a "distinguished" CDN-Domain to convey the fact 616 that its use is limited to the interconnection mechanism; such a 617 domain is never used directly by a CSP. 619 We assume the operators also agree on some distinguished CDN-Domain 620 that will be used for inter-CDN acquisition of CSP's content from 621 uCDN by dCDN. In this example, we'll use op-b-acq.op-a.example. 623 We assume the operators also exchange information regarding which 624 requests dCDN is prepared to serve. For example, dCDN may be 625 prepared to serve requests from clients in a given geographical 626 region or a set of IP address prefixes. This information may again 627 be provided out of band or via a defined CDNI interface. 629 We assume DNS is configured in the following way: 631 o The content provider is configured to make operator A the 632 authoritative DNS server for cdn.csp.example (or to return a CNAME 633 for cdn.csp.example for which operator A is the authoritative DNS 634 server). 636 o Operator A is configured so that a DNS request for 637 op-b-acq.op-a.example returns a request router in Operator A. 639 o Operator B is configured so that a DNS request for 640 peer-a.op-b.example/cdn.csp.example returns a request router in 641 Operator B. 643 Figure 3 illustrates how a client request for 645 http://cdn.csp.example/...rest of url... 647 is handled. 649 End-User Operator B Operator A 650 |DNS cdn.csp.example | | 651 |-------------------------------------------------->| 652 | | |(1) 653 |IPaddr of A's Request Router | 654 |<--------------------------------------------------| 655 |HTTP cdn.csp.example | | 656 |-------------------------------------------------->| 657 | | |(2) 658 |302 peer-a.op-b.example/cdn.csp.example | 659 |<--------------------------------------------------| 660 |DNS peer-a.op-b.example | | 661 |------------------------>| | 662 | |(3) | 663 |IPaddr of B's Request Router | 664 |<------------------------| | 665 | | | 666 |HTTP peer-a.op-b.example/cdn.csp.example | 667 |------------------------>| | 668 | |(4) | 669 |302 node1.peer-a.op-b.example/cdn.csp.example | 670 |<------------------------| | 671 |DNS node1.peer-a.op-b.example | 672 |------------------------>| | 673 | |(5) | 674 |IPaddr of B's Delivery Node | 675 |<------------------------| | 676 | | | 677 |HTTP node1.peer-a.op-b.example/cdn.csp.example | 678 |------------------------>| | 679 | |(6) | 680 | |DNS op-b-acq.op-a.example| 681 | |------------------------>| 682 | | |(7) 683 | |IPaddr of A's Request Router 684 | |<------------------------| 685 | |HTTP op-b-acq.op-a.example 686 | |------------------------>| 687 | | |(8) 688 | |302 node2.op-b-acq.op-a.example 689 | |<------------------------| 690 | |DNS node2.op-b-acq.op-a.example 691 | |------------------------>| 692 | | |(9) 693 | |IPaddr of A's Delivery Node 694 | |<------------------------| 695 | | | 696 | |HTTP node2.op-b-acq.op-a.example 697 | |------------------------>| 698 | | |(10) 699 | |Data | 700 | |<------------------------| 701 |Data | | 702 |<------------------------| | 704 Figure 3: Message Flow for Iterative HTTP Redirection 706 The steps illustrated in the figure are as follows: 708 1. A DNS resolver for Operator A processes the DNS request for its 709 customer based on CDN-Domain cdn.csp.example. It returns the IP 710 address of a request router in Operator A. 712 2. A Request Router for Operator A processes the HTTP request and 713 recognizes that the end-user is best served by another CDN, 714 specifically one provided by Operator B, and so it returns a 302 715 redirect message for a new URL constructed by "stacking" 716 Operator B's distinguished CDN-Domain (peer-a.op-b.example) on 717 the front of the original URL. (Note that more complex URL 718 manipulations are possible, such as replacing the initial CDN- 719 Domain by some opaque handle.) 721 3. The end-user does a DNS lookup using Operator B's distinguished 722 CDN-Domain (peer-a.op-b.example). B's DNS resolver returns the 723 IP address of a request router for Operator B. Note that if 724 request routing within dCDN was performed using DNS instead of 725 HTTP redirection, B's DNS resolver would also behave as the 726 request router and directly return the IP address of a delivery 727 node. 729 4. The request router for Operator B processes the HTTP request and 730 selects a suitable delivery node to serve the end-user request, 731 and returns a 302 redirect message for a new URL constructed by 732 replacing the hostname with a subdomain of the Operator B's 733 distinguished CDN-Domain that points to the selected delivery 734 node. 736 5. The end-user does a DNS lookup using Operator B's delivery node 737 subdomain (node1.peer-a.op-b.example). B's DNS resolver returns 738 the IP address of the delivery node. 740 6. The end-user requests the content from B's delivery node. In 741 the case of a cache hit, steps 6, 7, 8, 9 and 10 below do not 742 happen, and the content data is directly returned by the 743 delivery node to the end-user. In the case of a cache miss, the 744 content needs to be acquired by dCDN from uCDN (not the CSP). 745 The distinguished CDN-Domain peer-a.op-b.example indicates to 746 dCDN that this content is to be acquired from uCDN; stripping 747 the CDN-Domain reveals the original CDN-Domain cdn.csp.example 748 and dCDN may verify that this CDN-Domain belongs to a known peer 749 (so as to avoid being tricked into serving as an open proxy). 750 It then does a DNS request for an inter-CDN acquisition CDN- 751 Domain as agreed above (in this case, op-b-acq.op-a.example). 753 7. Operator A's DNS resolver processes the DNS request and returns 754 the IP address of a request router in operator A. 756 8. The request router for Operator A processes the HTTP request 757 from Operator B delivery node. Operator A request router 758 recognizes that the request is from a peer CDN rather than an 759 end-user because of the dedicated inter-CDN acquisition domain 760 (op-b-acq.op-a.example). (Note that without this specially 761 defined inter-CDN acquisition domain, operator A would be at 762 risk of redirecting the request back to operator B, resulting in 763 an infinite loop). The request router for Operator A selects a 764 suitable delivery node in uCDN to serve the inter-CDN 765 acquisition request and returns a 302 redirect message for a new 766 URL constructed by replacing the hostname with a subdomain of 767 the Operator A's distinguished inter-CDN acquisition domain that 768 points to the selected delivery node. 770 9. Operator A DNS resolver processes the DNS request and returns 771 the IP address of the delivery node in operator A. 773 10. Operator B requests (acquires) the content from Operator A. 774 Although not shown, Operator A processes the rest of the URL: it 775 extracts information identifying the origin server, validates 776 that this server has been registered, and determines the content 777 provider that owns the origin server. It may also perform its 778 own content acquisition steps if needed before returning the 779 content to dCDN. 781 The main advantage of this design is that it is simple: each CDN need 782 only know the distinguished CDN-Domain for each peer, with the 783 upstream CDN "pushing" the downstream CDN-Domain onto the URL as part 784 of its redirect (step 2) and the downstream CDN "popping" its CDN- 785 Domain off the URL to expose a CDN-Domain that the upstream CDN can 786 correctly process. Neither CDN needs to be aware of the internal 787 structure of the other's URLs. Moreover, the inter-CDN redirection 788 is entirely supported by a single HTTP redirect; neither CDN needs to 789 be aware of the other's internal redirection mechanism (i.e., whether 790 it is DNS or HTTP based). 792 One disadvantage is that the end-user's browser is redirected to a 793 new URL that is not in the same domain of the original URL. This has 794 implications on a number of security or validation mechanisms 795 sometimes used on endpoints. For example, it is important that any 796 redirected URL be in the same domain (e.g., csp.example) if the 797 browser is expected to send any cookies associated with that domain. 798 As another example, some video players enforce validation of a cross 799 domain policy that needs to accommodate the domains involved in the 800 CDN redirection. These problems are generally solvable, but the 801 solutions complicate the example, so we do not discuss them further 802 in this document. 804 We note that this example begins to illustrate some of the interfaces 805 that may be required for CDNI, but does not require all of them. For 806 example, obtaining information from dCDN regarding the set of client 807 IP addresses or geographic regions it might be able to serve is an 808 aspect of request routing (specifically of the CDNI Footprint & 809 Capabilities Advertisement interface). Important configuration 810 information such as the distinguished names used for redirection and 811 inter-CDN acquisition could also be conveyed via a CDNI interface 812 (e.g., perhaps the CDNI Control interface). The example also shows 813 how existing HTTP-based methods suffice for the acquisition 814 interface. Arguably, the absolute minimum metadata required for CDNI 815 is the information required to acquire the content, and this 816 information was provided "in-band" in this example by means of the 817 URI handed to the client in the HTTP 302 response. The example also 818 assumes that the CSP does not require any distribution policy (e.g. 819 time window, geo-blocking) or delivery processing to be applied by 820 the interconnected CDNs. Hence, there is no explicit CDNI Metadata 821 interface invoked in this example. There is also no explicit CDNI 822 Logging interface discussed in this example. 824 We also note that the step of deciding when a request should be 825 redirected to dCDN rather than served by uCDN has been somewhat 826 glossed over. It may be as simple as checking the client IP address 827 against a list of prefixes, or it may be considerably more complex, 828 involving a wide range of factors, such as the geographic location of 829 the client (perhaps determined from a third party service), CDN load, 830 or specific business rules. 832 This example uses the "iterative" CDNI request redirection approach. 833 That is, uCDN performs part of the request redirection function by 834 redirecting the client to a request router in the dCDN, which then 835 performs the rest of the redirection function by redirecting to a 836 suitable surrogate. If request routing is performed in the dCDN 837 using HTTP redirection, this translates in the end-user experiencing 838 two successive HTTP redirections. By contrast, the alternative 839 approach of "recursive" CDNI request redirection effectively 840 coalesces these two successive HTTP redirections into a single one, 841 sending the end-user directly to the right delivery node in the dCDN. 842 This "recursive" CDNI request routing approach is discussed in the 843 next section. 845 While the example above uses HTTP, the iterative HTTP redirection 846 mechanism would work over HTTPS in a similar fashion. In order to 847 make sure an end-user's HTTPS request is not downgraded to HTTP along 848 the redirection path, it is necessary for every request router along 849 the path from the initial uCDN Request Router to the final surrogate 850 in the dCDN to respond to an incoming HTTPS request with an HTTP 851 Redirect containing an HTTPS URL. It should be noted that using 852 HTTPS will have the effect of increasing the total redirection 853 process time and increasing the load on the request routers, 854 especially when the redirection path includes many redirects and thus 855 many TLS/SSL sessions. In such cases, a recursive HTTP redirection 856 mechanism, as described in an example in the next section, might help 857 to reduce some of these issues. 859 3.3. Recursive HTTP Redirection Example 861 The following example builds on the previous one to illustrate the 862 use of the request routing interface (specifically the CDNI Request 863 Routing Redirection interface) to enable "recursive" CDNI request 864 routing. We build on the HTTP-based redirection approach because it 865 illustrates the principles and benefits clearly, but it is equally 866 possible to perform recursive redirection when DNS-based redirection 867 is employed. 869 In contrast to the prior example, the operators need not agree in 870 advance on a CDN-Domain to serve as the target of redirections from 871 uCDN to dCDN. We assume that the operators agree on some 872 distinguished CDN-Domain that will be used for inter-CDN acquisition 873 of CSP's content by dCDN. In this example, we'll use 874 op-b-acq.op-a.example. 876 We assume the operators also exchange information regarding which 877 requests dCDN is prepared to serve. For example, dCDN may be 878 prepared to serve requests from clients in a given geographical 879 region or a set of IP address prefixes. This information may again 880 be provided out of band or via a defined protocol. 882 We assume DNS is configured in the following way: 884 o The content provider is configured to make operator A the 885 authoritative DNS server for cdn.csp.example (or to return a CNAME 886 for cdn.csp.example for which operator A is the authoritative DNS 887 server). 889 o Operator A is configured so that a DNS request for 890 op-b-acq.op-a.example returns a request router in Operator A. 892 o Operator B is configured so that a request for node1.op-b.example/ 893 cdn.csp.example returns the IP address of a delivery node. Note 894 that there might be a number of such delivery nodes. 896 Figure 3 illustrates how a client request for 898 http://cdn.csp.example/...rest of url... 900 is handled. 902 End-User Operator B Operator A 903 |DNS cdn.csp.example | | 904 |-------------------------------------------------->| 905 | | |(1) 906 |IPaddr of A's Request Router | 907 |<--------------------------------------------------| 908 |HTTP cdn.csp.example | | 909 |-------------------------------------------------->| 910 | | |(2) 911 | |RR/RI REQ cdn.csp.example| 912 | |<------------------------| 913 | | | 914 | |RR/RI RESP node1.op-b.example 915 | |------------------------>| 916 | | |(3) 917 |302 node1.op-b.example/cdn.csp.example | 918 |<--------------------------------------------------| 919 |DNS node1.op-b.example | | 920 |------------------------>| | 921 | |(4) | 922 |IPaddr of B's Delivery Node | 923 |<------------------------| | 924 |HTTP node1.op-b.example/cdn.csp.example | 925 |------------------------>| | 926 | |(5) | 927 | |DNS op-b-acq.op-a.example| 928 | |------------------------>| 929 | | |(6) 930 | |IPaddr of A's Request Router 931 | |<------------------------| 932 | |HTTP op-b-acq.op-a.example 933 | |------------------------>| 934 | | |(7) 935 | |302 node2.op-b-acq.op-a.example 936 | |<------------------------| 937 | |DNS node2.op-b-acq.op-a.example 938 | |------------------------>| 939 | | |(8) 940 | |IPaddr of A's Delivery Node 941 | |<------------------------| 942 | | | 943 | |HTTP node2.op-b-acq.op-a.example 944 | |------------------------>| 945 | | |(9) 946 | |Data | 947 | |<------------------------| 948 |Data | | 949 |<------------------------| | 951 Figure 4: Message Flow for Recursive HTTP Redirection 953 The steps illustrated in the figure are as follows: 955 1. A DNS resolver for Operator A processes the DNS request for its 956 customer based on CDN-Domain cdn.csp.example. It returns the IP 957 address of a Request Router in Operator A. 959 2. A Request Router for Operator A processes the HTTP request and 960 recognizes that the end-user is best served by another CDN-- 961 specifically one provided by Operator B--and so it queries the 962 CDNI Request Routing Redirection interface of Operator B, 963 providing a set of information about the request including the 964 URL requested. Operator B replies with the DNS name of a 965 delivery node. 967 3. Operator A returns a 302 redirect message for a new URL obtained 968 from the RI. 970 4. The end-user does a DNS lookup using the host name of the URL 971 just provided (node1.op-b.example). B's DNS resolver returns the 972 IP address of the corresponding delivery node. Note that, since 973 the name of the delivery node was already obtained from B using 974 the RI, there should not be any further redirection here (in 975 contrast to the iterative method described above.) 977 5. The end-user requests the content from B's delivery node, 978 potentially resulting in a cache miss. In the case of a cache 979 miss, the content needs to be acquired from uCDN (not the CSP.) 980 The distinguished CDN-Domain op-b.example indicates to dCDN that 981 this content is to be acquired from another CDN; stripping the 982 CDN-Domain reveals the original CDN-Domain cdn.csp.example, dCDN 983 may verify that this CDN-Domain belongs to a known peer (so as to 984 avoid being tricked into serving as an open proxy). It then does 985 a DNS request for the inter-CDN Acquisition "distinguished" CDN- 986 Domain as agreed above (in this case, op-b-acq.op-a.example). 988 6. Operator A DNS resolver processes the DNS request and returns the 989 IP address of a request router in operator A. 991 7. The request router for Operator A processes the HTTP request from 992 Operator B delivery node. Operator A request router recognizes 993 that the request is from a peer CDN rather than an end-user 994 because of the dedicated inter-CDN acquisition domain 995 (op-b-acq.op-a.example). (Note that without this specially 996 defined inter-CDN acquisition domain, operator A would be at risk 997 of redirecting the request back to operator B, resulting in an 998 infinite loop). The request router for Operator A selects a 999 suitable delivery node in uCDN to serve the inter-CDN acquisition 1000 request and returns a 302 redirect message for a new URL 1001 constructed by replacing the hostname with a subdomain of the 1002 Operator A's distinguished inter-CDN acquisition domain that 1003 points to the selected delivery node. 1005 8. Operator A recognizes that the DNS request is from a peer CDN 1006 rather than an end-user (due to the internal CDN-Domain) and so 1007 returns the address of a delivery node. (Note that without this 1008 specially defined internal domain, Operator A would be at risk of 1009 redirecting the request back to Operator B, resulting in an 1010 infinite loop.) 1012 9. Operator B requests (acquires) the content from Operator A. 1013 Operator A serves content for the requested CDN-Domain to dCDN. 1014 Although not shown, it is at this point that Operator A processes 1015 the rest of the URL: it extracts information identifying the 1016 origin server, validates that this server has been registered, 1017 and determines the content provider that owns the origin server. 1018 It may also perform its own content acquisition steps if needed 1019 before returning the content to dCDN. 1021 Recursive redirection has the advantage over iterative of being more 1022 transparent from the end-user's perspective, but the disadvantage of 1023 each CDN exposing more of its internal structure (in particular, the 1024 addresses of edge caches) to peer CDNs. By contrast, iterative 1025 redirection does not require dCDN to expose the addresses of its edge 1026 caches to uCDN. 1028 This example happens to use HTTP-based redirection in both CDN A and 1029 CDN B, but a similar example could be constructed using DNS-based 1030 redirection in either CDN. Hence, the key point to take away here is 1031 simply that the end user only sees a single redirection of some type, 1032 as opposed to the pair of redirections in the prior (iterative) 1033 example. 1035 The use of the RI requires that the request routing mechanism be 1036 appropriately configured and bootstrapped, which is not shown here. 1037 More discussion on the bootstrapping of interfaces is provided in 1038 Section 4 1040 3.4. Iterative DNS-based Redirection Example 1042 In this section we walk through a simple example using DNS-based 1043 redirection for request redirection from uCDN to dCDN (as well as for 1044 request routing inside dCDN and uCDN). As noted in Section 2.1, DNS- 1045 based redirection has certain advantages over HTTP-based redirection 1046 (notably, it is transparent to the end-user) as well as some 1047 drawbacks (notably the client IP address is not visible to the 1048 request router). 1050 As before, Operator A has to learn the set of requests that dCDN is 1051 willing or able to serve (e.g. which client IP address prefixes or 1052 geographic regions are part of the dCDN footprint). We assume 1053 Operator has and makes known to operator A some unique identifier 1054 that can be used for the construction of a distinguished CDN-Domain, 1055 as shown in more detail below. (This identifier strictly needs only 1056 to be unique within the scope of Operator A, but a globally unique 1057 identifier, such as an AS number assigned to B, is one easy way to 1058 achieve that.) Also, Operator A obtains the NS records for Operator 1059 B's externally visible redirection servers. Also, as before, a 1060 distinguished CDN-Domain, such as op-b-acq.op-a.example, must be 1061 assigned for inter-CDN acquisition. 1063 We assume DNS is configured in the following way: 1065 o The CSP is configured to make Operator A the authoritative DNS 1066 server for cdn.csp.example (or to return a CNAME for 1067 cdn.csp.example for which operator A is the authoritative DNS 1068 server). 1070 o When uCDN sees a request best served by dCDN, it returns CNAME and 1071 NS records for "b.cdn.csp.example", where "b" is the unique 1072 identifier assigned to Operator B. (It may, for example, be an AS 1073 number assigned to Operator B.) 1075 o dCDN is configured so that a request for "b.cdn.csp.example" 1076 returns a delivery node in dCDN. 1078 o uCDN is configured so that a request for "op-b-acq.op-a.example" 1079 returns a delivery node in uCDN. 1081 Figure 5 depicts the exchange of DNS and HTTP requests. The main 1082 differences from Figure 3 are the lack of HTTP redirection and 1083 transparency to the end-user. 1085 End-User Operator B Operator A 1086 |DNS cdn.csp.example | | 1087 |-------------------------------------------------->| 1088 | | |(1) 1089 |CNAME b.cdn.csp.example | | 1090 |<--------------------------------------------------| 1091 | | | 1092 |DNS b.cdn.csp.example | | 1093 |-------------------------------------------------->| 1094 | | |(2) 1095 |NS records for b.cdn.csp.example + | 1096 |Glue AAAA/A records for b.cdn.csp.example | 1097 |<--------------------------------------------------| 1098 | | | 1099 |DNS b.cdn.csp.example | | 1100 |------------------------>| | 1101 | |(3) | 1102 |IPaddr of B's Delivery Node | 1103 |<------------------------| | 1104 |HTTP cdn.csp.example | | 1105 |------------------------>| | 1106 | |(4) | 1107 | |DNS op-b-acq.op-a.example| 1108 | |------------------------>| 1109 | | |(5) 1110 | |IPaddr of A's Delivery Node 1111 | |<------------------------| 1112 | |HTTP op-b-acq.op-a.example 1113 | |------------------------>| 1114 | | |(6) 1115 | |Data | 1116 | |<------------------------| 1117 |Data | | 1118 |<------------------------| | 1120 Figure 5: Message Flow for DNS-based Redirection 1122 The steps illustrated in the figure are as follows: 1124 1. Request Router for Operator A processes the DNS request for CDN- 1125 Domain cdn.csp.example and recognizes that the end-user is best 1126 served by another CDN. (This may depend on the IP address of the 1127 user's local DNS resolver, or other information discussed below.) 1128 The Request Router returns a DNS CNAME response by "stacking" the 1129 distinguished identifier for Operator B onto the original CDN- 1130 Domain (e.g., b.cdn.csp.example). 1132 2. The end-user sends a DNS query for the modified CDN-Domain (i.e. 1133 b.cdn.csp.example) to Operator A's DNS server. The Request 1134 Router for Operator A processes the DNS request and return a 1135 delegation to b.cdn.csp.example by sending an NS record plus glue 1136 AAAA/A records pointing to Operator B's DNS server. (This extra 1137 step is necessary since typical DNS implementation won't follow 1138 an NS record when it is sent together with a CNAME record, 1139 thereby necessitating a two-step approach). 1141 3. The end-user sends a DNS query for the modified CDN-Domain (i.e., 1142 b.cdn.csp.example) to Operator B's DNS server, using the NS and 1143 AAAA/A records received in step 2. This causes B's Request 1144 Router to respond with a suitable delivery node. 1146 4. The end-user requests the content from B's delivery node. The 1147 requested URL contains the name cdn.csp.example. (Note that the 1148 returned CNAME does not affect the URL.) At this point the 1149 delivery node has the correct IP address of the end-user and can 1150 do an HTTP 302 redirect if the redirections in steps 2 and 3 were 1151 incorrect. Otherwise B verifies that this CDN-Domain belongs to 1152 a known peer (so as to avoid being tricked into serving as an 1153 open proxy). It then does a DNS request for an "internal" CDN- 1154 Domain as agreed above (op-b-acq.op-a.example). 1156 5. Operator A recognizes that the DNS request is from a peer CDN 1157 rather than an end-user (due to the internal CDN-Domain) and so 1158 returns the address of a delivery node in uCDN. 1160 6. Operator A serves content to dCDN. Although not shown, it is at 1161 this point that Operator A processes the rest of the URL: it 1162 extracts information identifying the origin server, validates 1163 that this server has been registered, and determines the content 1164 provider that owns the origin server. 1166 The advantages of this approach are that it is more transparent to 1167 the end-user and requires fewer round trips than HTTP-based 1168 redirection (in its worst case, i.e., when none of the needed DNS 1169 information is cached). A potential problem is that the upstream CDN 1170 depends on being able to learn the correct downstream CDN that serves 1171 the end-user from the client address in the DNS request. In standard 1172 DNS operation, uCDN will only obtain the address of the client's 1173 local DNS resolver (LDNS), which is not guaranteed to be in the same 1174 network (or geographic region) as the client. If not--e.g., the end- 1175 user uses a global DNS service--then the upstream CDN cannot 1176 determine the appropriate downstream CDN to serve the end-user. In 1177 this case, and assuming the uCDN is capable of detecting that 1178 situation, one option is for the upstream CDN to treat the end-user 1179 as it would any user not connected to a peer CDN. Another option is 1180 for the upstream CDN to "fall back" to a pure HTTP-based redirection 1181 strategy in this case (i.e., use the first method). Note that this 1182 problem affects existing CDNs that rely on DNS to determine where to 1183 redirect client requests, but the consequences are arguably less 1184 serious for CDNI since the LDNS is likely in the same network as the 1185 dCDN serves. 1187 As with the prior example, this example partially illustrates the 1188 various interfaces involved in CDNI. Operator A could learn 1189 dynamically from Operator B the set of prefixes or regions that B is 1190 willing and able to serve via the CDNI Footprint & Capabilities 1191 Advertisement interface. The distinguished name used for acquisition 1192 and the identifier for Operator B that is prepended to the CDN-Domain 1193 on redirection are examples of information elements that might also 1194 be conveyed by CDNI interfaces (or, alternatively, statically 1195 configured). As before, minimal metadata sufficient to obtain the 1196 content is carried "in-band" as part of the redirection process, and 1197 standard HTTP is used for inter-CDN acquisition. There is no 1198 explicit CDNI Logging interface discussed in this example. 1200 3.4.1. Notes on using DNSSEC 1202 Although it is possible to use DNSSEC in combination with the 1203 Iterative DNS-based Redirection mechanism explained above, it is 1204 important to note that the uCDN might have to sign records on the 1205 fly, since the CNAME returned, and thus the signature provided, can 1206 potentially be different for each incoming query. Although there is 1207 nothing preventing a uCDN from performing such on-the-fly signing, 1208 this might be computationally expensive. In the case where the 1209 number of dCDNs, and thus the number of different CNAMEs to return, 1210 is relatively stable, an alternative solution would be for the uCDN 1211 to pre-generate signatures for all possible CNAMEs. For each 1212 incoming query the uCDN would then determine the appropriate CNAME 1213 and return it together with the associated pre-generated signature. 1214 Note: In the latter case maintaining the serial and signature of SOA 1215 might be an issue since technically it should change every time a 1216 different CNAME is used. However, since in practice direct SOA 1217 queries are relatively rare, a uCDN could defer incrementing the 1218 serial and resigning the SOA until it is queried and then do it on- 1219 the-fly. 1221 Note also that the NS record and the glue AAAA/A records used in step 1222 2 in the previous section should generally be identical to those of 1223 their authoritative zone managed by Operator B. Even if they differ, 1224 this will not make the DNS resolution process fail, but the client 1225 DNS server will prefer the authoritative data in its cache and use it 1226 for subsequent queries. Such inconsistency is a general operational 1227 issue of DNS, but it may be more important for this architecture 1228 because the uCDN (operator A) would rely on the consistency to make 1229 the resulting redirection work as intended. In general, it is the 1230 administrator's responsibility to make them consistent. 1232 3.5. Dynamic Footprint Discovery Example 1234 There could be situations where being able to dynamically discover 1235 the set of requests that a given dCDN is willing and able to serve is 1236 beneficial. For example, a CDN might at one time be able to serve a 1237 certain set of client IP prefixes, but that set might change over 1238 time due to changes in the topology and routing policies of the IP 1239 network. The following example illustrates this capability. We have 1240 chosen the example of DNS-based redirection, but HTTP-based 1241 redirection could equally well use this approach. 1243 End-User Operator B Operator A 1244 |DNS cdn.csp.example | | 1245 |-------------------------------------------------->| 1246 | | |(1) 1247 | | RI REQ op-b.example | 1248 | |<------------------------| 1249 | | |(2) 1250 | | RI REPLY | 1251 | |------------------------>| 1252 | | |(3) 1253 |CNAME b.cdn.csp.example | | 1254 |NS records for b.cdn.csp.example | 1255 |<--------------------------------------------------| 1256 |DNS b.cdn.csp.example | | 1257 |------------------------>| | 1258 | |(2) | 1259 |IPaddr of B's Delivery Node | 1260 |<------------------------| | 1261 |HTTP cdn.csp.example | | 1262 |------------------------>| | 1263 | |(3) | 1264 | |DNS op-b-acq.op-a.example| 1265 | |------------------------>| 1266 | | |(4) 1267 | |IPaddr of A's Delivery Node 1268 | |<------------------------| 1269 | |HTTP op-b-acq.op-a.example 1270 | |------------------------>| 1271 | | |(5) 1272 | |Data | 1273 | |<------------------------| 1274 |Data | | 1275 |<------------------------| | 1277 Figure 6: Message Flow for Dynamic Footprint Discovery 1279 This example differs from the one in Figure 5 only in the addition of 1280 a FCI request (step 2) and corresponding response (step 3). The RI 1281 REQ could be a message such as "Can you serve clients from this IP 1282 Prefix?" or it could be "Provide the list of client IP prefixes you 1283 can currently serve". In either case the response might be cached by 1284 operator A to avoid repeatedly asking the same question. 1285 Alternatively, or in addition, Operator B may spontaneously advertise 1286 to Operator A information (or changes) on the set of requests it is 1287 willing and able to serve on behalf of operator A; in that case, 1288 Operator B may spontaneously issue RR/RI REPLY messages that are not 1289 in direct response to a corresponding RR/RI REQ message. (Note that 1290 the issues of determining the client's subnet from DNS requests, as 1291 described above, are exactly the same here as in Section 3.4.) 1293 Once Operator A obtains the RI response, it is now able to determine 1294 that Operator B's CDN is an appropriate dCDN for this request and 1295 therefore a valid candidate dCDN to consider in its Redirection 1296 decision. If that dCDN is selected, the redirection and serving of 1297 the request proceeds as before (i.e. in the absence of dynamic 1298 footprint discovery). 1300 3.6. Content Removal Example 1302 The following example illustrates how the CDNI Control interface may 1303 be used to achieve pre-positioning of an item of content in the dCDN. 1304 In this example, user requests for a particular content, and 1305 corresponding redirection of such requests from Operator A to 1306 Operator B CDN, may (or may not) have taken place earlier. Then, at 1307 some point in time, the uCDN (for example, in response to a 1308 corresponding Trigger from the Content Provider) uses the CI to 1309 request that content identified by a particular URL be removed from 1310 dCDN. The following diagram illustrates the operation. 1312 End-User Operator B Operator A 1313 | |CI purge cdn.csp.example/... 1314 | |<------------------------| 1315 | | | 1316 | |CI OK | 1317 | |------------------------>| 1318 | | | 1320 Figure 7: Message Flow for Content Removal 1322 The CI is used to convey the request from uCDN to dCDN that some 1323 previously acquired content should be deleted. The URL in the 1324 request specifies which content to remove. This example corresponds 1325 to a DNS-based redirection scenario such as Section 3.4. If HTTP- 1326 based redirection had been used, the URL for removal would be of the 1327 form peer-a.op-b.example/cdn.csp.example/... 1329 The dCDN is expected to confirm to the uCDN, as illustrated by the CI 1330 OK message, the completion of the removal of the targeted content 1331 from all the caches in dCDN. 1333 3.7. Pre-Positioned Content Acquisition Example 1335 The following example illustrates how the CI may be used to pre- 1336 position an item of content in the dCDN. In this example, Operator A 1337 uses the CDNI Metadata interface to request that content identified 1338 by a particular URL be pre-positioned into Operator B CDN. 1340 End-User Operator B Operator A 1342 | |CI pre-position cdn.csp.example/... 1343 | |<------------------------| 1344 | | |(1) 1345 | |CI OK | 1346 | |------------------------>| 1347 | | | 1348 | |DNS op-b-acq.op-a.example| 1349 | |------------------------>| 1350 | | |(2) 1351 | |IPaddr of A's Delivery Node 1352 | |<------------------------| 1353 | |HTTP op-b-acq.op-a.example 1354 | |------------------------>| 1355 | | |(3) 1356 | |Data | 1357 | |<------------------------| 1358 |DNS cdn.csp.example | | 1359 |--------------------------------------------->| 1360 | | |(4) 1361 |IPaddr of A's Request Router | 1362 |<---------------------------------------------| 1363 |HTTP cdn.csp.example| | 1364 |--------------------------------------------->| 1365 | | |(5) 1366 |302 peer-a.op-b.example/cdn.csp.example | 1367 |<---------------------------------------------| 1368 |DNS peer-a.op-b.example | 1369 |------------------->| | 1370 | |(6) | 1371 |IPaddr of B's Delivery Node | 1372 |<-------------------| | 1373 |HTTP peer-a.op-b.example/cdn.csp.example | 1374 |------------------->| | 1375 | |(7) | 1376 |Data | | 1377 |<-------------------| | 1379 Figure 8: Message Flow for Content Pre-Positioning 1381 The steps illustrated in the figure are as follows: 1383 1. Operator A uses the CI to request that Operator B pre-positions a 1384 particular content item identified by its URL. Operator B 1385 responds by confirming that it is willing to perform this 1386 operation. 1388 Steps 2 and 3 are exactly the same as steps 5 and 6 of Figure 3, only 1389 this time those steps happen as the result of the Pre-positioning 1390 request instead of as the result of a cache miss. 1392 Steps 4, 5, 6, 7 are exactly the same as steps 1, 2, 3, 4 of 1393 Figure 3, only this time Operator B CDN can serve the end-user 1394 request without triggering dynamic content acquisition, since the 1395 content has been pre-positioned in dCDN. Note that, depending on 1396 dCDN operations and policies, the content pre-positioned in the dCDN 1397 may be pre-positioned to all, or a subset of, dCDN caches. In the 1398 latter case, intra-CDN dynamic content acquisition may take place 1399 inside the dCDN serving requests from caches on which the content has 1400 not been pre-positioning; however, such intra-CDN dynamic acquisition 1401 would not involve the uCDN. 1403 3.8. Asynchronous CDNI Metadata Example 1405 In this section we walk through a simple example illustrating a 1406 scenario of asynchronously exchanging CDNI metadata, where the 1407 downstream CDN obtains CDNI metadata for content ahead of a 1408 corresponding content request. The example that follows assumes that 1409 HTTP-based inter-CDN redirection and recursive CDNI request-routing 1410 are used, as in Section 3.3. However, Asynchronous exchange of CDNI 1411 Metadata is similarly applicable to DNS-based inter-CDN redirection 1412 and iterative request routing (in which cases the CDNI metadata may 1413 be used at slightly different processing stages of the message 1414 flows). 1416 End-User Operator B Operator A 1417 | | | 1418 | |CI pre-position (Trigger)| 1419 | |<------------------------|(1) 1420 | | | 1421 | |CI OK | 1422 | |------------------------>|(2) 1423 | | | 1424 | |MI pull REQ | 1425 | |------------------------>|(3) 1426 | | | 1427 | |MI metadata REP |(4) 1428 | | | 1429 | | | 1430 | CONTENT REQUEST | | 1431 |-------------------------------------------------->|(5) 1432 | | | 1433 | | RI REQ | 1434 | |<------------------------|(6) 1435 | | | 1436 | | RI RESP | 1437 | |------------------------>|(7) 1438 | | | 1439 | CONTENT REDIRECTION | | 1440 |<--------------------------------------------------|(8) 1441 | | | 1442 | CONTENT REQUEST | | 1443 |------------------------>|(9) | 1444 | | | 1445 : : : 1446 | CONTENT DATA | | 1447 |<------------------------| |(10) 1449 Figure 9: Message Flow for Asynchronous CDNI Metadata 1451 The steps illustrated in the figure are as follows: 1453 1. Operator A uses the CI to Trigger to signal the availability of 1454 CDNI metadata to Operator B. 1456 2. Operator B acknowledges the receipt of this Trigger. 1458 3. Operator B requests the latest metadata from Operator A using 1459 the MI. 1461 4. Operator A replies with the requested metadata. This document 1462 does not constrain how the CDNI metadata information is actually 1463 represented. For the purposes of this example, we assume that 1464 Operator A provides CDNI metadata to Operator B indicating that: 1466 * this CDNI Metadata is applicable to any content referenced by 1467 some CDN-Domain. 1469 * this CDNI metadata consists of a distribution policy 1470 requiring enforcement by the delivery node of a specific per- 1471 request authorization mechanism (e.g. URI signature or token 1472 validation). 1474 5. A Content Request occurs as usual. 1476 6. A CDNI Request Routing Redirection request (RI REQ) is issued by 1477 operator A CDN, as discussed in Section 3.3. Operator B's 1478 request router can access the CDNI Metadata that are relevant to 1479 the requested content and that have been pre-positioned as per 1480 Steps 1-4, which may or may not affect the response. 1482 7. Operator B's request router issues a CDNI Request Routing 1483 Redirection response (RI RESP) as in Section 3.3. 1485 8. Operator B performs content redirection as discussed in 1486 Section 3.3. 1488 9. On receipt of the Content Request by the end user, the delivery 1489 node detects that previously acquired CDNI metadata is 1490 applicable to the requested content. In accordance with the 1491 specific CDNI metadata of this example, the delivery node will 1492 invoke the appropriate per-request authorization mechanism, 1493 before serving the content. (Details of this authorization are 1494 not shown.) 1496 10. Assuming successful per-request authorization, serving of 1497 Content Data (possibly preceded by inter-CDN acquisition) 1498 proceeds as in Section 3.3. 1500 3.9. Synchronous CDNI Metadata Acquisition Example 1502 In this section we walk through a simple example illustrating a 1503 scenario of Synchronous CDNI metadata acquisition, in which the 1504 downstream CDN obtains CDNI metadata for content at the time of 1505 handling a first request for the corresponding content. As in the 1506 preceding section, this example assumes that HTTP-based inter-CDN 1507 redirection and recursive CDNI request-routing are used (as in 1508 Section 3.3), but dynamic CDNI metadata acquisition is applicable to 1509 other variations of request routing. 1511 End-User Operator B Operator A 1512 | | | 1513 | CONTENT REQUEST | | 1514 |-------------------------------------------------->|(1) 1515 | | | 1516 | | RI REQ | 1517 | (2)|<------------------------| 1518 | | | 1519 | | MI REQ | 1520 | (3)|------------------------>| 1521 | | MI RESP | 1522 | |<------------------------|(4) 1523 | | | 1524 | | RI RESP | 1525 | |------------------------>|(5) 1526 | | | 1527 | | | 1528 | CONTENT REDIRECTION | | 1529 |<--------------------------------------------------|(6) 1530 | | | 1531 | CONTENT REQUEST | | 1532 |------------------------>|(7) | 1533 | | | 1534 | | MI REQ | 1535 | (8)|------------------------>| 1536 | | MI RESP | 1537 | |<------------------------|(9) 1538 | | | 1539 : : : 1540 | CONTENT DATA | | 1541 |<------------------------| |(10) 1543 Figure 10: Message Flow for Synchronous CDNI Metadata Acquisition 1545 The steps illustrated in the figure are as follows: 1547 1. A Content Request arrives as normal. 1549 2. An RI request occurs as in the prior example. 1551 3. On receipt of the CDNI Request Routing Request, Operator B's CDN 1552 initiates Synchronous acquisition of CDNI Metadata that are 1553 needed for routing of the end-user request. We assume the URI 1554 for the a Metadata server is known ahead of time through some 1555 out-of-band means. 1557 4. On receipt of a CDNI Metadata Request, Operator A's CDN 1558 responds, making the corresponding CDNI metadata information 1559 available to Operator B's CDN. This metadata is considered by 1560 operator B's CDN before responding to the Request Routing 1561 request. (In a simple case, the metadata could simply be an 1562 allow or deny response for this particular request.) 1564 5. Response to the RI request as normal. 1566 6. Redirection message is sent to the end user. 1568 7. A delivery node of Operator B receives the end user request. 1570 8. The delivery node Triggers dynamic acquisition of additional 1571 CDNI metadata that are needed to process the end-user content 1572 request. Note that there may exist cases where this step need 1573 not happen, for example because the metadata were already 1574 acquired previously. 1576 9. Operator A's CDN responds to the CDNI Metadata Request and makes 1577 the corresponding CDNI metadata available to Operator B. This 1578 metadata influence how Operator B's CDN processes the end-user 1579 request. 1581 10. Content is served (possibly preceded by inter-CDN acquisition) 1582 as in Section 3.3. 1584 3.10. Content and Metadata Acquisition with Multiple Upstream CDNs 1586 A single dCDN may receive end-user requests from multiple uCDNs. 1587 When a dCDN receives an end-user request, it must determine the 1588 identity of the uCDN from which it should acquire the requested 1589 content. 1591 Ideally, the acquisition path of an end-user request will follow the 1592 redirection path of the request. The dCDN should acquire the content 1593 from the same uCDN which redirected the request. 1595 Determining the acquisition path requires the dCDN to reconstruct the 1596 redirection path based on information in the end-user request. The 1597 method for reconstructing the redirection path differs based on the 1598 redirection approach: HTTP or DNS. 1600 With HTTP-redirection, the rewritten URI should include sufficient 1601 information for the dCDN to directly or indirectly determine the uCDN 1602 when the end-user request is received. The HTTP-redirection approach 1603 can be further broken-down based on the how the URL is rewritten 1604 during redirection: HTTP-redirection with or without Site 1605 Aggregation. HTTP-redirection with Site Aggregation hides the 1606 identity of the original CSP. HTTP-redirection without Site 1607 Aggregation does not attempt to hide the identity of the original 1608 CSP. With both approaches, the rewritten URI includes enough 1609 information to identify the immediate neighbor uCDN. 1611 With DNS-redirection, the dCDN receives the published URI (instead of 1612 a rewritten URI) and does not have sufficient information for the 1613 dCDN to identify the appropriate uCDN. The dCDN may narrow the set 1614 of viable uCDNs by examining the CDNI metadata from each to determine 1615 which uCDNs are hosting metadata for the requested content. If there 1616 is a single uCDN hosting metadata for the requested content, the dCDN 1617 can assume that the request redirection is coming from this uCDN and 1618 can acquire content from that uCDN. If there are multiple uCDNs 1619 hosting metadata for the requested content, the dCDN may be ready to 1620 trust any of these uCDNs to acquire the content (provided the uCDN is 1621 in a position to serve it). If the dCDN is not ready to trust any of 1622 these uCDNs, it needs to ensure via out of band arrangements that, 1623 for a given content, only a single uCDN will ever redirect requests 1624 to the dCDN. 1626 Content acquisition may be preceded by content metadata acquisition. 1627 If possible, the acquisition path for metadata should also follow the 1628 redirection path. Additionally, we assume metadata is indexed based 1629 on rewritten URIs in the case of HTTP-redirection and is indexed 1630 based on published URIs in the case of DNS-redirection. Thus, the RI 1631 and the MI are tightly coupled in that the result of request routing 1632 (a rewritten URI pointing to the dCDN) serves as an input to metadata 1633 lookup. If the content metadata includes information for acquiring 1634 the content, then the MI is also tightly coupled with the acquisition 1635 interface in that the result of the metadata lookup (an acquisition 1636 URL likely hosted by the uCDN) should serve as input to the content 1637 acquisition. 1639 4. Main Interfaces 1641 Figure 1 illustrates the main interfaces that are in scope for the 1642 CDNI WG, along with several others. The detailed specifications of 1643 these interfaces are left to other documents, but see [RFC6707] and 1644 [I-D.ietf-cdni-requirements] for some discussion of the interfaces. 1646 One interface that is not shown in Figure 1 is the interface between 1647 the user and the CSP. While for the purposes of CDNI that interface 1648 is out of scope, it is worth noting that it does exist and can 1649 provide useful functions, such as end-to-end performance monitoring 1650 and some forms of authentication and authorization. 1652 There is also an important interface between the user and the Request 1653 Routing function of both uCDN and dCDN (shown as the "Request" 1654 Interface in Figure 1). As we saw in some of the preceding examples, 1655 that interface can be used as a way of passing metadata, such as the 1656 minimum information that is required for dCDN to obtain the content 1657 from uCDN. 1659 In this section we will provide an overview of the functions 1660 performed by each of the CDNI interfaces and discuss how they fit 1661 into the overall solution. We also examine some of the design 1662 tradeoffs, and explore several cross-interface concerns. We begin 1663 with an examination of one such tradeoff that affects all the 1664 interfaces - the use of in-band or out-of-band communication. 1666 4.1. In-Band versus Out-of-Band Interfaces 1668 Before getting to the individual interfaces, we observe that there is 1669 a high-level design choice for each, involving the use of existing 1670 in-band communication channels versus defining new out-of-band 1671 interfaces. 1673 It is possible that the information needed to carry out various 1674 interconnection functions can be communicated between peer CDNs using 1675 existing in-band protocols. The use of HTTP 302 redirect is an 1676 example of how certain aspects of request routing can be implemented 1677 in-band (embedded in URIs). Note that using existing in-band 1678 protocols does not imply that the CDNI interfaces are null; it is 1679 still necessary to establish the rules (conventions) by which such 1680 protocols are used to implement the various interface functions. 1682 There are other opportunities for in-band communication beyond HTTP 1683 redirects. For example, many of the HTTP directives used by proxy 1684 servers can also be used by peer CDNs to inform each other of caching 1685 activity. Of these, one that is particularly relevant is the If- 1686 Modified-Since directive, which is used with the GET method to make 1687 it conditional: if the requested object has not been modified since 1688 the time specified in this field, a copy of the object will not be 1689 returned, and instead, a 304 (not modified) response will be 1690 returned. 1692 4.2. Cross Interface Concerns 1694 Although the CDNI interfaces are largely independent, there are a set 1695 of conventions practiced consistently across all interfaces. Most 1696 important among these is how resources are named, for exampmle, how 1697 the CDNI Metadata and Control interfaces identify the set of 1698 resources to which a given directive applies, or the CDNI Logging 1699 interface identifies the set of resources for which a summary record 1700 applies. 1702 While in the limit the CDNI interfaces could explicitly identify 1703 every individual resource, in practice, they name resource aggregates 1704 (sets of URIs) that are to be treated in a similar way. For example, 1705 URI aggregates can be identified by a CDN-Domain (i.e., the FQDN at 1706 the beginning of a URI) or by a URI-Filter (i.e., a regular 1707 expression that matches a subset of URIs contained in some CDN- 1708 Doman). In other words, CDN-Domains and URI-Filters provide a 1709 uniform means to aggregate sets (and subsets) of URIs for the purpose 1710 of defining the scope for some operation in one of the CDNI 1711 interfaces. 1713 4.3. Request Routing Interfaces 1715 The Request Routing interface comprises two parts: the Asynchronous 1716 interface used by a dCDN to advertize footprint and capabilities 1717 (denoted FCI) to a uCDN, allowing the uCDN to decide whether to 1718 redirect particular user requests to that dCDN; and the Synchronous 1719 interface used by the uCDN to redirect a user request to the dCDN 1720 (denoted RI). (These are somewhat analogous to the operations of 1721 routing and forwarding in IP.) 1723 As illustrated in Section 3, the RI part of request routing may be 1724 implemented in part by DNS and HTTP. Naming conventions may be 1725 established by which CDN peers communicate whether a request should 1726 be routed or content served. 1728 We also note that RI plays a key role in enabling recursive 1729 redirection, as illustrated in Section 3.3. It enables the user to 1730 be redirected to the correct delivery node in dCDN with only a single 1731 redirection step (as seen by the user). This may be particularly 1732 valuable as the chain of interconnected CDNs increases beyond two 1733 CDNs. For further discussion on the RI, see 1734 [I-D.ietf-cdni-redirection]. 1736 In support of these redirection requests, it is necessary for CDN 1737 peers to exchange additional information with each other, and this is 1738 the role of the FCI part of request routing. Depending on the 1739 method(s) supported, this might include: 1741 o The operator's unique id (operator-id) or distinguished CDN-Domain 1742 (operator-domain); 1744 o NS records for the operator's set of externally visible request 1745 routers; 1747 o The set of requests the dCDN operator is prepared to serve (e.g. a 1748 set of client IP prefixes or geographic regions that may be served 1749 by dCDN). 1751 o Additional capabilities of the dCDN, such as its ability to 1752 support different CDNI Metadata requests. 1754 Note that the set of requests that dCDN is willing to serve could in 1755 some cases be relatively static (e.g., a set of IP prefixes) which 1756 could be exchanged off-line, or might even be negotiated as part of a 1757 peering agreement. However, it may also be more dynamic, in which 1758 case the exchange supported by FCI would be be helpful. A further 1759 discussion of the Footprint & Capability Advertisement interface can 1760 be found in [I-D.ietf-cdni-footprint-capabilities-semantics]. 1762 4.4. CDNI Logging Interface 1764 It is necessary for the upstream CDN to have visibility into the 1765 delivery of content that it redirected to a downstream CDN. This 1766 allows the upstream CDN to properly bill its customers for multiple 1767 deliveries of content cached by the downstream CDN, as well as to 1768 report accurate traffic statistics to those content providers. This 1769 is one role of the LI. 1771 Other operational data that may be relevant to CDNI can also be 1772 exchanged by the LI. For example, dCDN may report the amount of 1773 content it has acquired from uCDN, and how much cache storage has 1774 been consumed by content cached on behalf of uCDN. 1776 Traffic logs are easily exchanged off-line. For example, the 1777 following traffic log is a small deviation from the Apache log file 1778 format, where entries include the following fields: 1780 o Domain - the full domain name of the origin server 1782 o IP address - the IP address of the client making the request 1784 o End time - the ending time of the transfer 1786 o Time zone - any time zone modifier for the end time 1788 o Method - the transfer command itself (e.g., GET, POST, HEAD) 1790 o URL - the requested URL 1792 o Version - the protocol version, such as HTTP/1.0 1794 o Response - a numeric response code indicating transfer result 1795 o Bytes Sent - the number of bytes in the body sent to the client 1797 o Request ID - a unique identifier for this transfer 1799 o User agent - the user agent, if supplied 1801 o Duration - the duration of the transfer in milliseconds 1803 o Cached Bytes - the number of body bytes served from the cache 1805 o Referer - the referrer string from the client, if supplied 1807 Of these, only the Domain field is indirect in the downstream CDN--it 1808 is set to the CDN-Domain used by the upstream CDN rather than the 1809 actual origin server. This field could then used to filter traffic 1810 log entries so only those entries matching the upstream CDN are 1811 reported to the corresponding operator. Further discussion of the LI 1812 can be found in [I-D.ietf-cdni-logging]. 1814 One open question is who does the filtering. One option is that the 1815 downstream CDN filters its own logs, and passes the relevant records 1816 directly to each upstream peer. This requires that the downstream 1817 CDN knows the set of CDN-Domains that belong to each upstream peer. 1818 If this information is already exchanged between peers as part of 1819 another interface, then direct peer-to-peer reporting is 1820 straightforward. If it is not available, and operators do not wish 1821 to advertise the set of CDN-Domains they serve to their peers, then 1822 the second option is for each CDN to send both its non-local traffic 1823 records and the set of CDN-Domains it serves to an independent third- 1824 party (i.e., a CDN Exchange), which subsequently filters, merges, and 1825 distributes traffic records on behalf of each participating CDN 1826 operator. 1828 A second open question is how timely traffic information should be. 1829 For example, in addition to offline traffic logs, accurate real-time 1830 traffic monitoring might also be useful, but such information 1831 requires that the downstream CDN inform the upstream CDN each time it 1832 serves upstream content from its cache. The downstream CDN can do 1833 this, for example, by sending a conditional HTTP GET request (If- 1834 Modified-Since) to the upstream CDN each time it receives an HTTP GET 1835 request from one of its end-users. This allows the upstream CDN to 1836 record that a request has been issued for the purpose of real-time 1837 traffic monitoring. The upstream CDN can also use this information 1838 to validate the traffic logs received later from the downstream CDN. 1840 There is obviously a tradeoff between accuracy of such monitoring and 1841 the overhead of the downstream CDN having to go back to the upstream 1842 CDN for every request. 1844 Another design tradeoff in the LI is the degree of aggregation or 1845 summarization of data. One situation that lends itself to 1846 summarization is the delivery of HTTP adaptive streaming (HAS), since 1847 the large number of individual chunk requests potentially results in 1848 large volumes of logging information. This case is discussed below, 1849 but other forms of aggregation may also be useful. For example, 1850 there may be situations where bulk metrics such as bytes delivered 1851 per hour may suffice rather than the detailed per-request logs 1852 outlined above. It seems likely that a range of granularities of 1853 logging will be needed along with ways to specify the type and degree 1854 of aggregation required. 1856 4.5. CDNI Control Interface 1858 The CDNI Control interface is initially used to bootstrap the other 1859 interfaces. As a simple example, it could be used to provide the 1860 address of the logging server in dCDN to uCDN in order to bootstrap 1861 the CDNI Logging interface. It may also be used, for example, to 1862 establish security associations for the other interfaces. 1864 The other role the CI plays is to allow the uCDN to pre-position, 1865 revalidate, or purge metadata and content on a dCDN. These 1866 operations, sometimes collectively called the Trigger interface, are 1867 discussed further in [I-D.ietf-cdni-control-triggers]. 1869 4.6. CDNI Metadata Interface 1871 The role of the CDNI Metadata interface is to enable CDNI 1872 distribution metadata to be conveyed to the downstream CDN by the 1873 upstream CDN. Such metadata includes geo-blocking restrictions, 1874 availability windows, access control policies, and so on. It may 1875 also include information to facilitate acquisition of content by dCDN 1876 (e.g., alternate sources for the content, authorization information 1877 needed to acquire the content from the source). For a full 1878 discussion of the CDNI Metadata Interface, see 1879 [I-D.ietf-cdni-metadata] 1881 Some distribution metadata may be partially emulated using in-band 1882 mechanisms. For example, in case of any geo-blocking restrictions or 1883 availability windows, the upstream CDN can elect to redirect a 1884 request to the downstream CDN only if that CDN's advertised delivery 1885 footprint is acceptable for the requested URL. Similarly, the 1886 request could be forwarded only if the current time is within the 1887 availability window. However, such approaches typically come with 1888 shortcomings such as inability to prevent from replay outside the 1889 time window or inability to make use of a downstream CDN that covers 1890 a broader footprint than the geo-blocking restrictions. 1892 Similarly, some forms of access control may also be performed on a 1893 per-request basis using HTTP directives. For example, being able to 1894 respond to a conditional GET request gives the upstream CDN an 1895 opportunity to influence how the downstream CDN delivers its content. 1896 Minimally, the upstream CDN can invalidate (purge) content previously 1897 cached by the downstream CDN. 1899 All of these in-band techniques serve to illustrate that uCDNs have 1900 the option of enforcing some of their access control policies 1901 themselves (at the expense of increased inter-CDN signaling load), 1902 rather than delegating enforcement to dCDNs using the MI. As a 1903 consequence, the MI could provide a means for the uCDN to express its 1904 desire to retain enforcement for itself. For example, this might be 1905 done by including a "check with me" flag in the metadata associated 1906 with certain content. The realization of such in-band techniques 1907 over the various inter-CDN acquisition protocols (e.g., HTTP) 1908 requires further investigation and may require small extensions or 1909 semantic changes to the acquisition protocol. 1911 4.7. HTTP Adaptive Streaming Concerns 1913 We consider HTTP Adaptive Streaming (HAS) and the impact it has on 1914 the CDNI interfaces because large objects (e.g., videos) are broken 1915 into a sequence of small, independent chunks. For each of the 1916 following, a more thorough discussion, including an overview of the 1917 tradeoffs involved in alternative designs, can be found in RFC 6983. 1919 First, with respect to Content Acquisition and File Management, which 1920 are out-of-scope for the CDNI interfaces but nontheless relevant to 1921 the overall operation, we assume no additional measures are required 1922 to deal with large numbers of chunks. This means that the dCDN is 1923 not explicitly made aware of any relationship between different 1924 chunks and the dCDN handles each chunk as if it were an individual 1925 and independent content item. The result is that content acquisition 1926 between uCDN and dCDN also happens on a per-chunk basis. This 1927 approach is in line with the recommendations made in RFC 6983, which 1928 also identifies potential improvements in this area that might be 1929 considered in the future. 1931 Second, with respect to Request Routing, we note that HAS manifest 1932 files have the potential to interfere with request routing since 1933 manifest files contain URLs pointing to the location of content 1934 chunks. To make sure that a manifest file does not hinder CDNI 1935 request routing and does not place excessive load on CDNI resources, 1936 the use of manifest files could either be limited to those containing 1937 relative URLs or the uCDN could modify the URLs in the manifest. Our 1938 approach for dealing with these issues is twofold. As a mandatory 1939 requirement, CDNs should be able to handle unmodified manifest files 1940 containing either relative or absolute URLs. To limit the number of 1941 redirects, and thus the load placed on the CDNI interfaces, as an 1942 optional feature uCDNs can use the information obtained through the 1943 CNDI Request Routing Redirection interface to modify the URLs in the 1944 manifest file. Since the modification of the manifest file is an 1945 optional uCDN-internal process, this does not require any 1946 standardization effort beyond being able to communicate chunk 1947 locations in the CDNI Request Routing Redirection interface. 1949 Third, with respect to the CDNI Logging interface, there are several 1950 potential issues, including the large number of individual chunk 1951 requests potentially resulting in large volumes of logging 1952 information, and the desire to correlate logging information for 1953 chunk requests that correspond to the same HAS session. For the 1954 initial CDNI specification, our approach is to expect participating 1955 CDNs to support per-chunk logging (e.g. logging each chunk request as 1956 if it were an independent content request) over the CDNI Logging 1957 interface. Optionally, the LI may include a Content Collection 1958 IDentifier (CCID) and/or a Session IDentifier (SID) as part of the 1959 logging fields, thereby facilitating correlation of per-chunk logs 1960 into per-session logs for applications benefiting from such session 1961 level information (e.g. session-based analytics). This approach is 1962 in line with the recommendations made in RFC 6983, which also 1963 identifies potential improvements in this area that might be 1964 considered in the future. 1966 Fourth, with respect to the CDNI Control interface, and in particular 1967 purging HAS chunks from a given CDN, our approach is to expect each 1968 CDN supports per-chunk content purge (e.g. purging of chunks as if 1969 they were individual content items). Optionally, a CDN may support 1970 content purge on the basis of a "Purge IDentifier (Purge-ID)" 1971 allowing the removal of all chunks related to a given Content 1972 Collection with a single reference. It is possible that this Purge- 1973 ID could be merged with the CCID discussed above for HAS Logging, or 1974 alternatively, they may remain distinct. 1976 4.8. URI Rewriting 1978 When using HTTP redirection, content URIs may be rewritten when 1979 redirection takes place within an uCDN, from an uCDN to a dCDN, and 1980 within the dCDN. In the case of cascaded CDNs, content URIs may be 1981 rewritten at every CDN hop (e.g., between the uCDN and the dCDN 1982 acting as the transit CDN, and between the transit CDN and the dCDN 1983 serving the request. The content URI used between any uCDN/dCDN pair 1984 becomes a common handle that can be referred to without ambiguity by 1985 both CDNs in all their inter-CDN communications. This handle allows 1986 the uCDN and dCDN to correlate information exchanged using other CDNI 1987 interfaces in both the downstream direction (e.g., when using the MI) 1988 and the upstream direction (e.g., when using the LI). 1990 Consider the simple case of a single uCDN/dCDN pair using HTTP 1991 redirection. We introduce the following terminology for content URIs 1992 to simplify the discussion: 1994 "u-URI" represents a content URI in a request presented to the 1995 uCDN; 1997 "ud-URI" is a content URI acting as the common handle across uCDN 1998 and dCDN for requests redirected by the uCDN to a specific dCDN; 2000 "d-URI" represents a content URI in a request made within the 2001 delegate dCDN. 2003 In our simple pair-wise example, the "ud-URI" effectively becomes the 2004 handle that the uCDN/dCDN pair use to correlate all CDNI information. 2005 In particular, for a given pair of CDNs executing the HTTP 2006 redirection, the uCDN needs to map the u-URI to the ud-URI handle for 2007 all MI message exchanges, while the dCDN needs to map the d-URI to 2008 the ud-URI handle for all LI message exchanges. 2010 In the case of cascaded CDNs, the transit CDN will rewrite the 2011 content URI when redirecting to the dCDN, thereby establishing a new 2012 handle between the transit CDN and the dCDN, that is different from 2013 the handle between the uCDN and transit CDN. It is the 2014 responsibility of the transit CDN to manage its mapping across 2015 handles so the right handle for all pairs of CDNs is always used in 2016 its CDNI communication. 2018 In summary, all CDNI interfaces between a given pair of CDNs need to 2019 always use the "ud-URI" handle for that specific CDN pair as their 2020 content URI reference. 2022 5. Deployment Models 2024 In this section we describe a number of possible deployment models 2025 that may be achieved using the CDNI interfaces described above. We 2026 note that these models are by no means exhaustive, and that many 2027 other models may be possible. 2029 Although the reference model of Figure 1 shows all CDN functions on 2030 each side of the CDNI interface, deployments can rely on entities 2031 that are involved in any subset of these functions, and therefore 2032 only support the relevant subset of CDNI interfaces. As already 2033 noted in Section 3, effective CDNI deployments can be built without 2034 necessarily implementing all the interfaces. Some examples of such 2035 deployments are shown below. 2037 Note that, while we refer to upstream and downstream CDNs, this 2038 distinction applies to specific content items and transactions. That 2039 is, a given CDN may be upstream for some transactions and downstream 2040 for others, depending on many factors such as location of the 2041 requesting client and the particular piece of content requested. 2043 5.1. Meshed CDNs 2045 Although the reference model illustrated in Figure 1 shows a 2046 unidirectional CDN interconnection with a single uCDN and a single 2047 dCDN, any arbitrary CDNI meshing can be built from this, such as the 2048 example meshing illustrated in Figure 11. (Support for arbitrary 2049 meshing may or may not be in the initial scope for the working group, 2050 but the model allows for it.) 2052 ------------- ----------- 2053 / CDN A \<==CDNI===>/ CDN B \ 2054 \ / \ / 2055 ------------- ----------- 2056 /\ \\ /\ 2057 || \\ || 2058 CDNI \==CDNI===\\ CDNI 2059 || \\ || 2060 \/ \/ \/ 2061 ------------- ----------- 2062 / CDN C \===CDNI===>/ CDN D \ 2063 \ / \ / 2064 ------------- ----------- 2065 /\ 2066 || 2067 CDNI 2068 || 2069 \/ 2070 ------------- 2071 / CDN E \ 2072 \ / 2073 ------------- 2075 ===> CDNI interfaces, with right-hand side CDN acting as dCDN 2076 to left-hand side CDN 2077 <==> CDNI interfaces, with right-hand side CDN acting as dCDN 2078 to left-hand side CDN and with left-hand side CDN acting 2079 as dCDN to right-hand side CDN 2081 Figure 11: CDNI Deployment Model: CDN Meshing Example 2083 5.2. CSP combined with CDN 2085 Note that our terminology refers to functional roles and not economic 2086 or business roles. That is, a given organization may be operating as 2087 both a CSP and a fully fledged uCDN when we consider the functions 2088 performed, as illustrated in Figure 12. 2090 ##################################### ################## 2091 # # # # 2092 # Organization A # # Organization B # 2093 # # # # 2094 # -------- ------------- # # ----------- # 2095 # / CSP \ / uCDN \ # # / dCDN \ # 2096 # | | | +----+ | # # | +----+ | # 2097 # | | | | C | | # # | | C | | # 2098 # | | | +----+ | # # | +----+ | # 2099 # | | | +----+ | # # | +----+ | # 2100 # | | | | L | | # # | | L | | # 2101 # | |*****| +----+ |===CDNI===>| +----+ | # 2102 # | | | +----+ | # # | +----+ | # 2103 # | | | | RR | | # # | | RR | | # 2104 # | | | +----+ | # # | +----+ | # 2105 # | | | +----+ | # # | +----+ | # 2106 # | | | | D | | # # | | D | | # 2107 # | | | +----+ | # # | +----+ | # 2108 # \ / \ / # # \ / # 2109 # -------- ------------- # # ----------- # 2110 # # # # 2111 ##################################### ################## 2113 ===> CDNI interfaces, with right-hand side CDN acting as dCDN 2114 to left-hand side CDN 2115 **** interfaces outside the scope of CDNI 2116 C Control component of the CDN 2117 L Logging component of the CDN 2118 RR Request Routing component of the CDN 2119 D Distribution component of the CDN 2121 Figure 12: CDNI Deployment Model: Organization combining CSP & uCDN 2123 5.3. CSP using CDNI Request Routing Interface 2125 As another example, a content provider organization may choose to run 2126 its own request routing function as a way to select among multiple 2127 candidate CDN providers; In this case the content provider may be 2128 modeled as the combination of a CSP and of a special, restricted case 2129 of a CDN. In that case, as illustrated in Figure 13, the CDNI 2130 Request Routing interfaces can be used between the restricted CDN 2131 operated by the content provider Organization and the CDN operated by 2132 the full CDN organization acting as a dCDN in the request routing 2133 control plane. Interfaces outside the scope of the CDNI work can be 2134 used between the CSP functional entities of the content provider 2135 organization and the CDN operated by the full CDN organization acting 2136 as a uCDN) in the CDNI control planes other than the request routing 2137 plane (i.e. Control, Distribution, Logging). 2139 ##################################### ################## 2140 # # # # 2141 # Organization A # # Organization B # 2142 # # # # 2143 # -------- ------------- # # ----------- # 2144 # / CSP \ / uCDN(RR) \ # # / dCDN(RR) \ # 2145 # | | | +----+ | # # | +----+ | # 2146 # | |*****| | RR |==========CDNI=====>| RR | | # 2147 # | | | +----+ | # RR # | +----+ | # 2148 # | | \ / # # | | # 2149 # | | ------------- # # |uCDN(C,L,D)| # 2150 # | | # # | +----+ | # 2151 # | | # # | | C | | # 2152 # | |*******************************| +----+ | # 2153 # | | # # | +----+ | # 2154 # | | # # | | L | | # 2155 # | | # # | +----+ | # 2156 # | | # # | +----+ | # 2157 # | | # # | | D | | # 2158 # | | # # | +----+ | # 2159 # \ / # # \ / # 2160 # -------- # # ----------- # 2161 # # # # 2162 ##################################### ################## 2164 ===> CDNI Request Routing Interface 2165 **** interfaces outside the scope of CDNI 2167 Figure 13: CDNI Deployment Model: Organization combining CSP and 2168 partial CDN 2170 5.4. CDN Federations and CDN Exchanges 2172 There are two additional concepts related to, but distinct from CDN 2173 Interconnection. The first is CDN Federation. Our view is that CDNI 2174 is the more general concept, involving two or more CDNs serving 2175 content to each other's users, while federation implies a multi- 2176 lateral interconnection arrangement, but other CDN interconnection 2177 agreements are also possible (e.g., symmetric bilateral, asymmetric 2178 bilateral). An important conclusion is that CDNI technology should 2179 not presume (or bake in) a particular interconnection agreement, but 2180 should instead be general enough to permit alternative 2181 interconnection arrangements to evolve. 2183 The second concept often used in the context of CDN Federation is CDN 2184 Exchange--a third party broker or exchange that is used to facilitate 2185 a CDN federation. Our view is that a CDN exchange offers valuable 2186 machinery to scale the number of CDN operators involved in a multi- 2187 lateral (federated) agreement, but that this machinery is built on 2188 top of the core CDNI interconnection mechanisms. For example, as 2189 illustrated in Figure 14, the exchange might aggregate and 2190 redistribute information about each CDN footprint and capacity, as 2191 well as collect, filter, and redistribute traffic logs that each 2192 participant needs for interconnection settlement, but inter-CDN 2193 request routing, inter-CDN content distribution (including inter-CDN 2194 acquisition) and inter-CDN control which fundamentally involve a 2195 direct interaction between an upstream CDN and a downstream CDN-- 2196 operate exactly as in a pair-wise peering arrangement. Turning to 2197 Figure 14, we observe that in this example: 2199 o each CDN supports a direct CDNI Control interface to every other 2200 CDN 2202 o each CDN supports a direct CDNI Metadata interface to every other 2203 CDN 2205 o each CDN supports a CDNI Logging interface with the CDN Exchange 2207 o each CDN supports both a CDNI Request Routing interface with the 2208 CDN Exchange (for aggregation and redistribution of dynamic CDN 2209 footprint discovery information) and a direct RI to every other 2210 CDN (for actual request redirection). 2212 ---------- --------- 2213 / CDN A \ / CDN B \ 2214 | +----+ | | +----+ | 2215 //========>| C |<==============CDNI============>| C |<==========\\ 2216 || | +----+ | C | +----+ | || 2217 || | +----+ | | +----+ | || 2218 || //=====>| D |<==============CDNI============>| D |<=======\\ || 2219 || || | +----+ | M | +----+ | || || 2220 || || | | /------------\ | | || || 2221 || || | +----+ | | +--+ CDN Ex| | +----+ | || || 2222 || || //==>| RR |<===CDNI==>|RR|<=======CDNI====>| RR |<====\\ || || 2223 || || || | +----+ | RR | +--+ | RR | +----+ | || || || 2224 || || || | | | /\ | | | || || || 2225 || || || | +----+ | | || +---+ | | +----+ | || || || 2226 || || || | | L |<===CDNI=======>| L |<=CDNI====>| L | | || || || 2227 || || || | +----+ | L | || +---+ | L | +----+ | || || || 2228 || || || \ / \ || /\ / \ / || || || 2229 || || || ----------- --||----||-- ----------- || || || 2230 || || || || || || || || 2231 || || || CDNI RR || || || || 2232 || || || || CDNI L || || || 2233 || || || || || || || || 2234 || || || ---||----||---- || || || 2235 || || || / \/ || \ || || || 2236 || || || | +----+ || | || || || 2237 || || \\=====CDNI==========>| RR |<=============CDNI========// || || 2238 || || RR | +----+ \/ | RR || || 2239 || || | +----+ | || || 2240 || || | | L | | || || 2241 || || | +----+ | || || 2242 || || | +----+ | || || 2243 || \\=======CDNI===========>| D |<=============CDNI===========// || 2244 || M | +----+ | M || 2245 || | +----+ | || 2246 \\==========CDNI===========>| C |<=============CDNI==============// 2247 C | +----+ | C 2248 \ CDN C / 2249 -------------- 2251 <=CDNI RR=> CDNI Request Routing Interface 2252 <=CDNI M==> CDNI Metadata Interface 2253 <=CDNI C==> CDNI Control Interface 2254 <=CDNI L==> CDNI Logging Interface 2256 Figure 14: CDNI Deployment Model: CDN Exchange 2258 Note that a CDN exchange may alternatively support a different set of 2259 functionality (e.g. Logging only, or Logging and full request 2260 routing, or all the functionality of a CDN including content 2261 distribution). All these options are expected to be allowed by the 2262 IETF CDNI specifications. 2264 6. Trust Model 2266 There are a number of trust issues that need to be addressed by a 2267 CDNI solution. Many of them are in fact similar or identical to 2268 those in a simple CDN without interconnection. In a standard CDN 2269 environment (without CDNI), the CSP places a degree of trust in a 2270 single CDN operator to perform many functions. The CDN is trusted to 2271 deliver content with appropriate quality of experience for the end 2272 user. The CSP trusts the CDN operator not to corrupt or modify the 2273 content. The CSP often relies on the CDN operator to provide 2274 reliable accounting information regarding the volume of delivered 2275 content. The CSP may also trust the CDN operator to perform actions 2276 such as timely invalidation of content and restriction of access to 2277 content based on certain criteria such as location of the user and 2278 time of day, and to enforce per-request authorization performed by 2279 the CSP using techniques such as URI signing. 2281 A CSP also places trust in the CDN not to distribute any information 2282 that is confidential to the CSP (e.g., how popular a given piece of 2283 content is) or confidential to the end user (e.g., which content has 2284 been watched by which user). 2286 A CSP does not necessarily have to place complete trust in a CDN. A 2287 CSP will in some cases take steps to protect its content from 2288 improper distribution by a CDN, e.g. by encrypting it and 2289 distributing keys in some out of band way. A CSP also depends on 2290 monitoring (possibly by third parties) and reporting to verify that 2291 the CDN has performed adequately. A CSP may use techniques such as 2292 client-based metering to verify that accounting information provided 2293 by the CDN is reliable. HTTP conditional requests may be used to 2294 provide the CSP with some checks on CDN operation. In other words, 2295 while a CSP may trust a CDN to perform some functions in the short 2296 term, the CSP is able in most cases to verify whether these actions 2297 have been performed correctly and to take action (such as moving the 2298 content to a different CDN) if the CDN does not live up to 2299 expectations. 2301 The main trust issue raised by CDNI is that it introduces transitive 2302 trust. A CDN that has a direct relationship with a CSP can now 2303 "outsource" the delivery of content to another (downstream) CDN. 2304 That CDN may in term outsource delivery to yet another downstream 2305 CDN, and so on. 2307 The top level CDN in such a chain of delegation is responsible for 2308 ensuring that the requirements of the CSP are met. Failure to do so 2309 is presumably just as serious as in the traditional single CDN case. 2310 Hence, an upstream CDN is essentially trusting a downstream CDN to 2311 perform functions on its behalf in just the same way as a CSP trusts 2312 a single CDN. Monitoring and reporting can similarly be used to 2313 verify that the downstream CDN has performed appropriately. However, 2314 the introduction of multiple CDNs in the path between CSP and end 2315 user complicates the picture. For example, third party monitoring of 2316 CDN performance (or other aspects of operation, such as timely 2317 invalidation) might be able to identify the fact that a problem 2318 occurred somewhere in the chain but not point to the particular CDN 2319 at fault. 2321 In summary, we assume that an upstream CDN will invest a certain 2322 amount of trust in a downstream CDN, but that it will verify that the 2323 downstream CDN is performing correctly, and take corrective action 2324 (including potentially breaking off its relationship with that CDN) 2325 if behavior is not correct. We do not expect that the trust 2326 relationship between a CSP and its "top level" CDN will differ 2327 significantly from that found today in single CDN situations. 2328 However, it does appear that more sophisticated tools and techniques 2329 for monitoring CDN performance and behavior will be required to 2330 enable the identification of the CDN at fault in a particular 2331 delivery chain. 2333 We expect that the detailed designs for the specific interfaces for 2334 CDNI will need to take the transitive trust issues into account. For 2335 example, explicit confirmation that some action (such as content 2336 removal) has taken place in a downstream CDN may help to mitigate 2337 some issues of transitive trust. 2339 7. IANA Considerations 2341 This memo includes no request to IANA. 2343 8. Privacy Considerations 2345 In general, a CDN has the opportunity to collect detailed information 2346 about the behavior of end-users e.g. by logging which files are being 2347 downloaded. While the concept of interconnected CDNs as described in 2348 this document doesn't necessarily allow any given CDN to gather more 2349 information on any specific user, it potentially facilitates sharing 2350 of this data by a CDN with more parties. As an example, the purpose 2351 of the CDNI Logging Interface is to allow a dCDN to share some of its 2352 log records with a uCDN, both for billing purposes as well as for 2353 sharing traffic statistics with the Content Provider on which behalf 2354 the content was delivered. The fact that the CDNI Interfaces provide 2355 mechanisms for sharing such potentially sensitive user data, shows 2356 that it is necessary to include in these interface appropriate 2357 privacy and confidentiality mechanisms. The definition of such 2358 mechanisms is dealt with in the respective CDN interface documents. 2360 9. Security Considerations 2362 While there are a variety of security issues introduced by a single 2363 CDN, we are concerned here specifically with the additional issues 2364 that arise when CDNs are interconnected. For example, when a single 2365 CDN has the ability to distribute content on behalf of a CSP, there 2366 may be concerns that such content could be distributed to parties who 2367 are not authorized to receive it, and there are mechanisms to deal 2368 with such concerns. Our focus in this section is on how CDN 2369 interconnection introduces new security issues not found in the 2370 single CDN case. For a more detailed analysis of the security 2371 requirements of CDNI, see section 9 of [I-D.ietf-cdni-requirements]. 2373 Many of the security issues that arise in CDNI are related to the 2374 transitivity of trust (or lack thereof) described in Section 6. As 2375 noted above, the design of the various interfaces for CDNI must take 2376 account of the additional risks posed by the fact that a CDN with 2377 whom a CSP has no direct relationship is now potentially distributing 2378 content for that CSP. The mechanisms used to mitigate these risks 2379 may be similar to those used in the single CDN case, but their 2380 suitability in this more complex environment must be validated. 2382 CDNs today offer a variety of means to control access to content, 2383 such as time-of-day restrictions, geo-blocking, and URI signing. 2384 These mechanisms must continue to function in CDNI environments, and 2385 this consideration is likely to affect the design of certain CDNI 2386 interfaces (e.g. metadata, request routing). For more information on 2387 URI signing in CDNI, see [I-D.leung-cdni-uri-signing]. 2389 Just as with a single CDN, each peer CDN must ensure that it is not 2390 used as an "open proxy" to deliver content on behalf of a malicious 2391 CSP. Whereas a single CDN typically addresses this problem by having 2392 CSPs explicitly register content (or origin servers) that are to be 2393 served, simply propagating this information to peer downstream CDNs 2394 may be problematic because it reveals more information than the 2395 upstream CDN is willing to specify. (To this end, the content 2396 acquisition step in the earlier examples force the dCDN to retrieve 2397 content from the uCDN rather than go directly to the origin server.) 2399 There are several approaches to this problem. One is for the uCDN to 2400 encode a signed token generated from a shared secret in each URL 2401 routed to a dCDN, and for the dCDN to validate the request based on 2402 this token. Another one is to have each upstream CDN advertise the 2403 set of CDN-Domains they serve, where the downstream CDN checks each 2404 request against this set before caching and delivering the associated 2405 object. Although straightforward, this approach requires operators 2406 to reveal additional information, which may or may not be an issue. 2408 9.1. Security of CDNI Interfaces 2410 It is noted in [I-D.ietf-cdni-requirements] that all CDNI interfaces 2411 must be able to operate securely over insecure IP networks. Since it 2412 is expected that the CDNI interfaces will be implemented using 2413 existing application protocols such as HTTP or XMPP, we also expect 2414 that the security mechanisms available to those protocols may be used 2415 by the CDNI interfaces. Details of how these interfaces are secured 2416 will be specified in the relevant interface documents. 2418 9.2. Digital Rights Management 2420 Issues of digital rights management (DRM, also sometimes called 2421 digital restrictions management) is often employed for content 2422 distributed via CDNs. In general, DRM relies on the CDN to 2423 distribute encrypted content, with decryption keys distributed to 2424 users by some other means (e.g. directly from the CSP to the end 2425 user.) For this reason, DRM is considered out of scope [RFC6707] and 2426 does not introduce additional security issues for CDNI. 2428 10. Contributors 2430 The following individuals contributed to this document: 2432 o Matt Caulfield 2434 o Francois le Faucheur 2436 o Aaron Falk 2438 o David Ferguson 2440 o John Hartman 2442 o Ben Niven-Jenkins 2444 o Kent Leung 2446 11. Acknowledgements 2448 The authors would like to thank Huw Jones and Jinmei Tatuya for their 2449 helpful input to this document. In addition, the authors would like 2450 to thank Stephen Farrell, Ted Lemon and Alissa Cooper for their 2451 reviews, which have helped to improve this document. 2453 12. Informative References 2455 [I-D.ietf-cdni-control-triggers] 2456 Murray, R. and B. Niven-Jenkins, "CDNI Control Interface / 2457 Triggers", draft-ietf-cdni-control-triggers-02 (work in 2458 progress), December 2013. 2460 [I-D.ietf-cdni-footprint-capabilities-semantics] 2461 Seedorf, J., Peterson, J., Previdi, S., Brandenburg, R., 2462 and K. Ma, "CDNI Request Routing: Footprint and 2463 Capabilities Semantics", draft-ietf-cdni-footprint- 2464 capabilities-semantics-02 (work in progress), February 2465 2014. 2467 [I-D.ietf-cdni-logging] 2468 Faucheur, F., Bertrand, G., Oprescu, I., and R. 2469 Peterkofsky, "CDNI Logging Interface", draft-ietf-cdni- 2470 logging-11 (work in progress), March 2014. 2472 [I-D.ietf-cdni-metadata] 2473 Niven-Jenkins, B., Murray, R., Watson, G., Caulfield, M., 2474 Leung, K., and K. Ma, "CDN Interconnect Metadata", draft- 2475 ietf-cdni-metadata-06 (work in progress), February 2014. 2477 [I-D.ietf-cdni-redirection] 2478 Niven-Jenkins, B. and R. Brandenburg, "Request Routing 2479 Redirection Interface for CDN Interconnection", draft- 2480 ietf-cdni-redirection-02 (work in progress), April 2014. 2482 [I-D.ietf-cdni-requirements] 2483 Leung, K. and Y. Lee, "Content Distribution Network 2484 Interconnection (CDNI) Requirements", draft-ietf-cdni- 2485 requirements-17 (work in progress), January 2014. 2487 [I-D.leung-cdni-uri-signing] 2488 Leung, K., Faucheur, F., Downey, B., Brandenburg, R., and 2489 S. Leibrand, "URI Signing for CDN Interconnection (CDNI)", 2490 draft-leung-cdni-uri-signing-05 (work in progress), March 2491 2014. 2493 [RFC3466] Day, M., Cain, B., Tomlinson, G., and P. Rzewski, "A Model 2494 for Content Internetworking (CDI)", RFC 3466, February 2495 2003. 2497 [RFC6707] Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content 2498 Distribution Network Interconnection (CDNI) Problem 2499 Statement", RFC 6707, September 2012. 2501 [RFC6770] Bertrand, G., Stephan, E., Burbridge, T., Eardley, P., Ma, 2502 K., and G. Watson, "Use Cases for Content Delivery Network 2503 Interconnection", RFC 6770, November 2012. 2505 [RFC6983] van Brandenburg, R., van Deventer, O., Le Faucheur, F., 2506 and K. Leung, "Models for HTTP-Adaptive-Streaming-Aware 2507 Content Distribution Network Interconnection (CDNI)", RFC 2508 6983, July 2013. 2510 Authors' Addresses 2512 Larry Peterson 2513 Akamai Technologies, Inc. 2514 8 Cambridge Center 2515 Cambridge, MA 02142 2516 USA 2518 Email: lapeters@akamai.com 2520 Bruce Davie 2521 VMware, Inc. 2522 3401 Hillview Ave. 2523 Palo Alto, CA 94304 2524 USA 2526 Email: bdavie@vmware.com 2528 Ray van Brandenburg (editor) 2529 TNO 2530 Brassersplein 2 2531 Delft 2612CT 2532 the Netherlands 2534 Phone: +31-88-866-7000 2535 Email: ray.vanbrandenburg@tno.nl