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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group B. Davie, Ed. 3 Internet-Draft Cisco Systems, Inc. 4 Intended status: Informational L. Peterson, Ed. 5 Expires: May 3, 2012 Verivue, Inc. 6 October 31, 2011 8 Framework for CDN Interconnection 9 draft-davie-cdni-framework-01 11 Abstract 13 This document presents a framework for Content Distribution Network 14 Interconnection (CDNI). The purpose of the framework is to provide 15 an overall picture of the problem space of CDNI and to describe the 16 relationships among the various components necessary to interconnect 17 CDNs. CDN Interconnection requires the specification of several 18 interfaces and mechanisms to address issues such as request routing, 19 metadata exchange, and the acquisition of content by one CDN from 20 another. The intent of this document is to outline what each 21 interface needs to accomplish, and to describe how these interfaces 22 and mechanisms fit together, while leaving their detailed 23 specification to other documents. 25 Status of this Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on May 3, 2012. 42 Copyright Notice 44 Copyright (c) 2011 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 61 1.2. Reference Model . . . . . . . . . . . . . . . . . . . . . 5 62 1.3. Structure Of This Document . . . . . . . . . . . . . . . . 8 63 2. Building Blocks . . . . . . . . . . . . . . . . . . . . . . . 8 64 2.1. Request Redirection . . . . . . . . . . . . . . . . . . . 8 65 2.1.1. DNS Redirection . . . . . . . . . . . . . . . . . . . 8 66 2.1.2. HTTP Redirection . . . . . . . . . . . . . . . . . . . 9 67 3. Overview of CDNI Operation . . . . . . . . . . . . . . . . . . 10 68 3.1. Preliminaries . . . . . . . . . . . . . . . . . . . . . . 12 69 3.2. HTTP Redirect Example . . . . . . . . . . . . . . . . . . 13 70 3.2.1. Comments on the example . . . . . . . . . . . . . . . 17 71 3.3. Recursive Redirection Example . . . . . . . . . . . . . . 18 72 3.3.1. Comments on the example . . . . . . . . . . . . . . . 22 73 3.4. DNS-based redirection example . . . . . . . . . . . . . . 22 74 3.4.1. Comments on the example . . . . . . . . . . . . . . . 25 75 3.5. Dynamic Footprint Discovery . . . . . . . . . . . . . . . 26 76 3.6. Content Removal . . . . . . . . . . . . . . . . . . . . . 28 77 3.7. Pre-Positioned Content Acquisition Example . . . . . . . . 28 78 3.8. Asynchronous CDNI Metadata Example . . . . . . . . . . . . 30 79 3.9. Synchronous CDNI Metadata Acquisition Example . . . . . . 32 80 4. Main Interfaces . . . . . . . . . . . . . . . . . . . . . . . 35 81 4.1. In-Band versus Out-of-Band Interfaces . . . . . . . . . . 35 82 4.2. Request Routing Interface . . . . . . . . . . . . . . . . 36 83 4.3. Logging Interface . . . . . . . . . . . . . . . . . . . . 37 84 4.4. Control Interface . . . . . . . . . . . . . . . . . . . . 39 85 4.5. Metadata Interface . . . . . . . . . . . . . . . . . . . . 39 86 5. Deployment Models . . . . . . . . . . . . . . . . . . . . . . 41 87 5.1. Meshed CDNs . . . . . . . . . . . . . . . . . . . . . . . 41 88 5.2. CSP combined with CDN . . . . . . . . . . . . . . . . . . 42 89 5.3. CSP using CDNI Request Routing Interface . . . . . . . . . 43 90 5.4. CDN Federations and CDN Exchanges . . . . . . . . . . . . 44 91 6. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 47 92 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48 93 8. Security Considerations . . . . . . . . . . . . . . . . . . . 48 94 8.1. Security of CDNI Interfaces . . . . . . . . . . . . . . . 49 95 8.2. Digital Rights Management . . . . . . . . . . . . . . . . 50 96 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 50 97 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 50 98 11. Informative References . . . . . . . . . . . . . . . . . . . . 50 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 51 101 1. Introduction 103 The interconnection of Content Distribution Networks (CDNs) is 104 motivated by several use cases, such as those described in 105 [I-D.ietf-cdni-use-cases]. The overall problem space for CDN 106 Interconnection is described in [I-D.ietf-cdni-problem-statement]. 107 The purpose of this document is to provide an overview of the various 108 components necessary to interconnect CDNs. CDN Interconnection 109 requires the specification of several interfaces and mechanisms to 110 address issues such as request routing, metadata exchange, and the 111 acquisition of content by one CDN from another. The intent of this 112 document is to describe how these interfaces and mechanisms fit 113 together, leaving their detailed specification to other documents. 114 We make extensive use of message flow examples to illustrate the 115 operation of interconnected CDNs, but these examples should be 116 considered illustrative rather than prescriptive. 118 1.1. Terminology 120 This document draws freely on the terminology defined in [RFC3466] 121 and [I-D.ietf-cdni-problem-statement]. 123 We also introduce the following terms: 125 CDN Domain: a host name (FQDN) at the beginning of a URL, 126 representing a set of content that is served by a given CDN. For 127 example, in the URL http://cdn.csp.com/...rest of url..., the CDN 128 domain is cdn.csp.com. 130 Distinguished CDN Domain: a CDN domain that is allocated by a CDN for 131 the purposes of communication with a peer CDN, but which is not found 132 in client requests. Such CDN domains may be used for inter-CDN 133 acquisition, or as redirection targets, and enable a CDN to 134 distinguish a request from a peer CDN from an end-user request. 136 Recursive CDNI request routing: When an Upstream CDN elects to 137 redirect a request towards a Downstream CDN, the Upstream CDN can 138 query the Downstream CDN Request Routing system via the CDNI Request 139 Routing interface (or use information cached from earlier similar 140 queries) to find out how the Downstream CDN wants the request to be 141 redirected, which allows the Upstream CDN to factor in the Downstream 142 CDN response when redirecting the user agent. This approach is 143 referred to as "recursive" CDNI request routing. Note that the 144 Downstream CDN may elect to have the request redirected directly to a 145 Surrogate inside the Downstream CDN, to the Request-Routing System of 146 the Downstream CDN, to another CDN, or to any other system that the 147 Downstream CDN sees as fit for handling the redirected request. 149 Iterative CDNI Request Routing: When an Upstream CDN elects to 150 redirect a request towards a Downstream CDN, the Upstream CDN can 151 base its redirection purely on a local decision (and without 152 attempting to take into account how the Downstream CDN may in turn 153 redirect the user agent). In that case, the Upstream CDN redirects 154 the request to the request routing system in the Downstream CDN, 155 which in turn will decide how to redirect that request: this approach 156 is referred to as "iterative" CDNI request routing. 158 Synchronous CDNI operations: operations between CDNs that happen 159 during the process of servicing a user request, i.e. between the time 160 that the user agent begins its attempt to obtain content and the time 161 at which that request is served. 163 Asynchronous CDNI operations: operations between CDNs that happen 164 independently of any given user request, such as advertisement of 165 footprint information or pre-positioning of content for later 166 delivery. 168 1.2. Reference Model 170 This document uses the reference model in Figure 1 as originally 171 created in [I-D.ietf-cdni-problem-statement]. 173 -------- 174 / \ 175 | CSP | 176 \ / 177 -------- 178 * 179 * 180 * /\ 181 * / \ 182 ---------------------- |CDNI| ---------------------- 183 / Upstream CDN \ | | / Downstream CDN \ 184 | +-------------+ | Control Interface| +-------------+ | 185 |******* Control |<======|====|========>| Control *******| 186 |* +------*----*-+ | | | | +-*----*------+ *| 187 |* * * | | | | * * *| 188 |* +------*------+ | Logging Interface| +------*------+ *| 189 |* ***** Logging |<======|====|========>| Logging ***** *| 190 |* * +-*-----------+ | | | | +-----------*-+ * *| 191 |* * * * | Request Routing | * * * *| 192 .....*...+-*---------*-+ | Interface | +-*---------*-+...*.*... 193 . |* * *** Req-Routing |<======|====|========>| Req-Routing *** * *| . 194 . |* * * +-------------+.| | | | +-------------+ * * *| . 195 . |* * * . CDNI Metadata | * * *| . 196 . |* * * +-------------+ |. Interface | +-------------+ * * *| . 197 . |* * * | Distribution|<==.===|====|========>| Distribution| * * *| . 198 . |* * * | | | . \ / | | | * * *| . 199 . |* * * |+---------+ | | . \/ | | +---------+| * * *| . 200 . |* * ***| +---------+| | ....Request......+---------+ |*** * *| . 201 . |* *****+-|Surrogate|************************|Surrogate|-+***** *| . 202 . |******* +---------+| | Acquisition | |+----------+ *******| . 203 . | +-------------+ | | +-------*-----+ | . 204 . \ / \ * / . 205 . ---------------------- ---------*------------ . 206 . * . 207 . * Delivery . 208 . * . 209 . +--*---+ . 210 ...............Request.............................| User |..Request.. 211 | Agent| 212 +------+ 214 <==> interfaces inside the scope of CDNI 216 **** interfaces outside the scope of CDNI 217 .... interfaces outside the scope of CDNI 219 Figure 1: CDNI Model and CDNI Interfaces 221 We note that while some interfaces in the reference model are "out of 222 scope" for the CDNI WG (in the sense that there is no need to define 223 new protocols for those interfaces) we still need to refer to them in 224 this document to explain the overall operation of CDNI. 226 We also note that, while we generally show only one uCDN serving a 227 given CSP, it is entirely possible that multiple uCDNs can serve a 228 single CSP. In fact, this situation effectively exists today in the 229 sense that a single CSP can connect to more than one CDN today. 231 Definitions of the four CDNI interfaces follow. More discussion of 232 these interfaces appears in Section 4. 234 o Control Interface: Operations to discover, initialize, and 235 parameterize the other CDNI interfaces. Once established, all 236 runtime control over CDNI behavior is under the purview of one of 237 these other interfaces. 239 o Request Routing Interface: Operations to determine what CDN (and 240 optionally what surrogate within a CDN) is to serve end-user's 241 requests. May include a combination of: 243 * Asynchronous operations to exchange routing information (e.g., 244 the network footprint served by a given CDN) that enables CDN 245 selection for subsequent user requests; and 247 * Synchronous operations to select a delivery CDN (surrogate) for 248 a given user request. 250 o Metadata Interface: Operations to communicate metadata that 251 governs the how content is delivered by interconnected CDNs. 252 Examples of CDNI metadata include geo-blocking directives, 253 availability windows, access control mechanisms, and purge 254 directives. May include a combination of: 256 * Asynchronous operations to exchange metadata that govern 257 subsequent user requests for content; and 259 * Synchronous operations that govern behavior for a given user 260 request for content. 262 o Logging Interface: Operations that allow interconnected CDNs to 263 exchange relevant activity logs. May include a combination of: 265 * Real-time exchanges, suitable for runtime traffic monitoring; 266 and 268 * Off-line exchanges, suitable for analytics and billing. 270 1.3. Structure Of This Document 272 The remainder of this document is organized as follows: 274 o Section 2 describes some essential building blocks for CDNI, 275 notably the various options for redirecting user requests to a 276 given CDN. 278 o Section 3 provides a number of illustrative examples of various 279 CDNI operations. 281 o Section 4 describes the functionality of the four main CDNI 282 interfaces. 284 o Section 5 shows how various deployment models of CDNI may be 285 achieved using the defined interfaces. 287 o Section 6 describes the trust model of CDNI and the issues of 288 transitive trust in particular that CDNI raises. 290 2. Building Blocks 292 2.1. Request Redirection 294 At its core, CDN Interconnection requires the redirection of requests 295 from one CDN to another. For any given request that is received by 296 an upstream CDN, it will either respond to the request directly, or 297 somehow redirect the request to a downstream CDN. Two main 298 mechanisms are available for redirecting a request to a downstream 299 CDN. The first leverages the DNS name resolution process and the 300 second uses in-protocol redirection mechanisms such as the HTTP 302 301 redirection response. We discuss these below as background before 302 discussing some examples of their use in Section 3. 304 2.1.1. DNS Redirection 306 DNS redirection is based on returning different IP addresses for the 307 same DNS name, for example, to balance server load or to account for 308 the client's location in the network. A DNS server, sometimes called 309 the Local DNS (LDNS), resolves DNS names on behalf of an end-user. 310 The LDNS server in turn queries other DNS servers until it reaches 311 the authoritative DNS server for the CDN-domain. The network 312 operator typically provides the LDNS server, although the user is 313 free to choose other DNS servers (e.g., OpenDNS, Google Public DNS). 315 The advantage of DNS redirection is that it is completely transparent 316 to the end user--the user sends a DNS name to the LDNS server and 317 gets back an IP address. On the other hand, DNS redirection is 318 problematic because the DNS request comes from the LDNS server, not 319 the end-user. This may affect the accuracy of server selection that 320 is based on the user's location. The transparency of DNS redirection 321 is also a problem in that there is no opportunity to modify the path 322 component of the URL being accessed by the client. We consider two 323 main forms of DNS redirection: simple and CNAME-based. 325 In simple DNS redirection, the authoritative DNS server for the name 326 simply returns an IP address from a set of possible IP addresses. 327 The answer is chosen from the set based on characteristics of the set 328 (e.g., the relative loads on the servers) or characteristics of the 329 client (e.g., the location of the client relative to the servers). 330 Simple redirection is straightforward. The only caveats are (1) 331 there is a limit to the number of delivery nodes a single DNS server 332 can manage; and (2) DNS responses are cached by downstream servers so 333 the TTL on the response must be set to an appropriate value so as to 334 preserve the timeliness of the redirection. 336 In CNAME-based DNS redirection, the authoritative server returns a 337 CNAME response to the DNS request, telling the LDNS server to restart 338 the name lookup using a new name. A CNAME is essentially a symbolic 339 link in the DNS namespace, and like a symbolic link, redirection is 340 transparent to the client--the LDNS server gets the CNAME response 341 and re-executes the lookup. Only when the name has been resolved to 342 an IP address does it return the result to the user. Note that DNAME 343 would be preferable to CNAME if it becomes widely supported. 345 2.1.2. HTTP Redirection 347 HTTP redirection makes use of the "302" redirection response of the 348 HTTP protocol. This response contains a new URL that the application 349 should fetch instead of the original URL. By changing the URL 350 appropriately, the server can cause the user to redirect to a 351 different server. The advantages of 302 redirection are that (1) the 352 server can change the URL fetched by the client to include, for 353 example, both the DNS name of the particular server to use, as well 354 as the original HTTP server that was being accessed; and (2) the 355 client sends the HTTP request to the server, so that its IP address 356 is known and can be used in selecting the server. 358 The disadvantages of HTTP redirection are (1) it is visible to the 359 application, so it requires application support and may affect the 360 application behavior (e.g., web browsers will not send cookies if the 361 URL changes to a different domain); (2) HTTP is a heavy-weight 362 protocol layered on TCP so it has relatively high overhead; and (3) 363 the results of HTTP redirection are not cached so that all 364 redirections must go through to the server. 366 3. Overview of CDNI Operation 368 To provide a big-picture overview of the various components of CDN 369 Interconnection, we walk through a "day in the life" of a content 370 item that is made available via a pair of interconnected CDNs. This 371 will serve to illustrate many of the functions that need to be 372 supported in a complete CDNI solution. We give examples using both 373 DNS-based and HTTP-based redirection. We begin with very simple 374 examples and then how additional capabilities, such as recursive 375 request redirection and content removal, might be added. 377 Before walking through some specific examples, we present a high- 378 level view of the operations that may take place. This high-level 379 overview is illustrated in Figure 2. Note that most operations will 380 involve only a subset of all the messages shown below, and that the 381 order and number of operations may vary considerably, as more 382 detailed examples illustrate below. 384 The following shows Operator A as the upstream CDN (uCDN) and 385 Operator B as the downstream CDN (dCDN), where the former has a 386 relationship with a content provider and the latter being the best 387 CDN to deliver content to the end-user. The interconnection 388 relationship may be symmetric between these two CDN operators, but 389 for simplicity we show the interaction in one direction only. 391 End-User Operator B Operator A 392 | | | 393 | | | 394 | | [Async Metadata Push] | (1) 395 | | | 396 | | [Async RRI Push] | (2) 397 | | | 398 | CONTENT REQUEST | | 399 |-------------------------------------------------->| (3) 400 | | | 401 | | [Sync RRI Pull] | (4) 402 | | | 403 | CONTENT REDIRECTION | | 404 |<--------------------------------------------------| (5) 405 | | | 406 | | | 407 | CONTENT REQUEST | | 408 |------------------------>| | (6) 409 | | | 410 | | [Sync Metadata Pull] | (7) 411 | | | 412 | | ACQUISITION REQUEST | 413 | X------------------------>| (8) 414 | X | 415 | X CONTENT DATA | 416 | X<------------------------| (9) 417 | | | 418 | CONTENT DATA | | 419 |<------------------------| | (10) 420 | | | 421 : : : 422 : [Other content requests ] : 423 : : : 424 | | [Content Purge] | (11) 425 : : : 426 | | [Logging exchange] | (12) 427 | | | 429 Figure 2: Overview of Operation 431 The operations shown in the Figure are as follows: 433 1. Prior to any content request, metadata may be asynchronously 434 pushed from uCDN to dCDN so that it is available in readiness 435 for later content requests. 437 2. dCDN may advertise information relevant to its delivery 438 capabilities (e.g. geographic footprint, reachable address 439 prefixes) prior to any content requests being redirected. 441 3. A content request from a user agent arrives at uCDN. 443 4. uCDN may synchronously request information from dCDN regarding 444 its delivery capabilities to decide if dCDN is a suitable target 445 for redirection of this request. 447 5. uCDN redirects the request to dCDN by sending some response 448 (DNS, HTTP) to the user agent. 450 6. The user agent requests the content from dCDN. 452 7. dCDN may synchronously request metadata related to this content 453 from uCDN, e.g. to decide whether to serve it. 455 8. If the content is not already in a suitable cache in dCDN, dCDN 456 may acquire it from uCDN. 458 9. The content is delivered to dCDN from uCDN. 460 10. The content is delivered to the user agent by dCDN. 462 11. Some time later, perhaps at the request of the CSP (not shown) 463 uCDN may instruct dCDN to purge the content to ensure it is not 464 delivered again. 466 12. After one or more content delivery actions by dCDN, a log of 467 delivery actions may be provided to uCDN. 469 The following sections show some more specific examples of how these 470 operations may be combined to perform various delivery, control and 471 logging operations across a pair of CDNs. 473 3.1. Preliminaries 475 Initially, we assume that there is at least one CSP that has 476 contracted with an upstream CDN (uCDN) to deliver content on its 477 behalf. We are not particularly concerned with the interface between 478 the CSP and uCDN, other than to note that it is expected to be the 479 same as in the "traditional" (non-interconnected) CDN case. Existing 480 mechanisms such as DNS CNAMEs or HTTP redirects (Section 2) can be 481 used to direct a user request for a piece of content from the CSP 482 towards the CSP's chosen upstream CDN. 484 We use the term "CDN-domain" to refer to the host name (a FQDN) at 485 the beginning of each URL. We assume Operator A provides an upstream 486 CDN that serves content on behalf of a CSP with CDN-domain 487 cdn.csp.com. We assume that Operator B provides a downstream CDN. 488 An end user at some point makes a request for URL 490 http://cdn.csp.com/...rest of url... 492 It may well be the case that cdn.csp.com is just a CNAME for some 493 other CDN-domain (such as csp.op-a.net). Nevertheless, the HTTP 494 request in the examples that follow is assumed to be for the example 495 URL above. 497 Our goal is to enable content identified by the above URL to be 498 served by the CDN of operator B. In the following sections we will 499 walk through some scenarios in which content is served, as well as 500 other CDNI operations such as the removal of content from a 501 downstream CDN. 503 3.2. HTTP Redirect Example 505 In this section we walk through a simple, illustrative example using 506 HTTP redirection from uCDN to dCDN. The example also assumes the use 507 of HTTP redirection inside uCDN and dCDN; however, this is 508 independent of the choice of redirection approach across CDNs, so an 509 alternative example could be constructed still showing HTTP 510 redirection from uCDN to dCDN but using DNS for handling of request 511 inside each CDN. 513 We assume for this example that Operators A and B have established an 514 agreement to interconnect their CDNs, with A being upstream and B 515 being downstream. (It is likely that the agreement would be made in 516 both directions, but we focus on just one here for clarity.) 518 The operators agree that a CDN-domain peer-a.op-b.net will be used as 519 the target of redirections from uCDN to dCDN. The name of this 520 domain must be communicated by some means to each CDN. (This could 521 be established out-of-band or via a CDNI interface.) We refer to 522 this domain as a "distinguished" CDN domain to convey the fact that 523 its use is limited to the interconnection mechanism; such a domain is 524 never embedded in URLs that end-users request. 526 The operators must also agree on some distinguished CDN-domain that 527 will be used for inter-CDN acquisition of CSP's content from uCDN by 528 dCDN. In this example, we'll use op-b-acq.op-a.net. 530 The operators must also exchange information regarding which requests 531 dCDN is prepared to serve. For example, dCDN may be prepared to 532 serve requests from clients in a given geographical region or a set 533 of IP address prefixes. This information may again be provided out 534 of band or via a defined interface. 536 DNS must be configured in the following way: 538 o The content provider must be configured to make operator A the 539 authoritative DNS server for cdn.csp.com (or to return a CNAME for 540 cdn.csp.com for which operator A is the authoritative DNS server). 542 o Operator A must be configured so that a DNS request for op-b- 543 acq.op-a.net returns a request router in Operator A. 545 o Operator B must be configured so that a DNS request for peer-a.op- 546 b.net/cdn.csp.com returns a request router in Operator B. 548 Figure 3 illustrates how a client request for 550 http://cdn.csp.com/...rest of url... 552 is handled. 554 End-User Operator B Operator A 555 |DNS cdn.csp.com | | 556 |-------------------------------------------------->| 557 | | |(1) 558 |IPaddr of A's Request Router | 559 |<--------------------------------------------------| 560 |HTTP cdn.csp.com | | 561 |-------------------------------------------------->| 562 | | |(2) 563 |302 peer-a.op-b.net/cdn.csp.com | 564 |<--------------------------------------------------| 565 |DNS peer-a.op-b.net | | 566 |------------------------>| | 567 | |(3) | 568 |IPaddr of B's Request Router | 569 |<------------------------| | 570 | | | 571 |HTTP peer-a.op-b.net/cdn.csp.com | 572 |------------------------>| | 573 | |(4) | 574 |302 node1.peer-a.op-b.net/cdn.csp.com | 575 |<------------------------| | 576 |DNS node1.peer-a.op-b.net| | 577 |------------------------>| | 578 | |(5) | 579 |IPaddr of B's Delivery Node | 580 |<------------------------| | 581 | | | 582 |HTTP node1.peer-a.op-b.net/cdn.csp.com | 583 |------------------------>| | 584 | |(6) | 585 | |DNS op-b-acq.op-a.net | 586 | |------------------------>| 587 | | |(7) 588 | |IPaddr of A's Request Router 589 | |<------------------------| 590 | |HTTP op-b-acq.op-a.net | 591 | |------------------------>| 592 | | |(8) 593 | |302 node2.op-b.acq.op-A.net 594 | |<------------------------| 595 | |DNS node2.op-b-acq.op-a.net 596 | |------------------------>| 597 | | |(9) 598 | |IPaddr of A's Delivery Node 599 | |<------------------------| 600 | | |(10) 601 | |Data | 602 | |<------------------------| 603 |Data | | 604 |<------------------------| | 606 Figure 3: Request Trace for HTTP redirection method 608 The steps illustrated in the figure are as follows: 610 1. A DNS resolver for Operator A processes the DNS request for its 611 customer based on CDN-domain cdn.csp.com. It returns the IP 612 address of a request router in Operator A. 614 2. A Request Router for Operator A processes the HTTP request and 615 recognizes that the end-user is best served by another CDN-- 616 specifically one provided by Operator B--and so it returns a 302 617 redirect message for a new URL constructed by "stacking" 618 Operator B's distinguished CDN-domain (peer-a.op-b.net) on the 619 front of the original URL. (Note that more complex URL 620 manipulations are possible, such as replacing the initial CDN- 621 domain by some opaque handle.) 623 3. The end-user does a DNS lookup using Operator B's distinguished 624 CDN-domain (peer-a.op-b.net). B's DNS resolver returns the IP 625 address of a request router for Operator B. Note that if request 626 routing within dCDN was performed using DNS instead of HTTP 627 redirection, B's DNS resolver would also behave as the request 628 router and directly return the IP address of a delivery node. 630 4. The request router for Operator B processes the HTTP request and 631 selects a suitable delivery node to serve the end-user request, 632 and returns a 302 redirect message for a new URL constructed by 633 replacing the hostname by a subdomain of the Operator B's 634 distinguished CDN-domain that points to the selected delivery 635 node. 637 5. The end-user does a DNS lookup using Operator B's delivery node 638 subdomain (node1.peer-a.op-b.net). B's DNS resolver returns the 639 IP address of the delivery node. 641 6. The end-user requests the content from B's delivery node. In 642 the case of a cache hit, steps 6, 7, 8, 9 and 10 below do not 643 happen, and the content data is directly returned by the 644 delivery node to the end-user. In the case of a cache miss, the 645 content needs to be acquired by dCDN from uCDN (not the CSP). 646 The distinguished CDN-domain peer-a.op-b.net indicates to dCDN 647 that this content is to be acquired from uCDN; stripping the 648 CDN-domain reveals the original CDN-domain cdn.csp.com and dCDN 649 may verify that this CDN-domain belongs to a known peer (so as 650 to avoid being tricked into serving as an open proxy). It then 651 does a DNS request for an inter-CDN acquisition CDN-domain as 652 agreed above (in this case, op-b-acq.op-a.net). 654 7. Operator A's DNS resolver processes the DNS request and returns 655 the IP address of a request router in operator A. 657 8. The request router for Operator A processes the HTTP request 658 from Operator B delivery node. Operator A request router 659 recognizes that the request is from a peer CDN rather than an 660 end-user because of the dedicated inter-CDN acquisition domain 661 (op-b-acq.op-a.net). (Note that without this specially defined 662 inter-CDN acquisition domain, operator A would be at risk of 663 redirecting the request back to operator B, resulting in an 664 infinite loop). The request router for Operator A selects a 665 suitable delivery node in uCDN to serve the inter-CDN 666 acquisition request and returns a 302 redirect message for a new 667 URL constructed by replacing the hostname by a subdomain of the 668 Operator A's distinguished inter-CDN acquisition domain that 669 points to the selected delivery node. 671 9. Operator A DNS resolver processes the DNS request and returns 672 the IP address of the delivery node in operator A. 674 10. Operator A serves content for the requested CDN-domain to dCDN. 675 Although not shown, it is at this point that Operator A 676 processes the rest of the URL: it extracts information 677 identifying the origin server, validates that this server has 678 been registered, and determines the content provider that owns 679 the origin server. It may also perform its own content 680 acquisition steps if needed before returning the content to 681 dCDN. 683 3.2.1. Comments on the example 685 The main advantage of this design is that it is simple: each CDN need 686 only know the distinguished CDN-domain for each peer, with the 687 upstream CDN "pushing" the downstream CDN-domain onto the URL as part 688 of its redirect (step 2) and the downstream CDN "popping" its CDN- 689 domain off the URL to expose a CDN-domain that the upstream CDN can 690 correctly process. Neither CDN needs to be aware of the internal 691 structure of the other's URLs. Moreover, the inter-CDN redirection 692 is entirely supported by a single HTTP redirect; neither CDN needs to 693 be aware of the other's internal redirection mechanism (i.e., whether 694 it is DNS or HTTP based). 696 One disadvantage is that the end-user's browser is redirected to a 697 new URL that is not in the same domain of the original URL. This has 698 implications on a number of security or validation mechanisms 699 sometimes used on endpoints. For example, it is important that any 700 redirected URL be in the same domain (e.g., csp.com) if the browser 701 is expected to send any cookies associated with that domain. As 702 another example, some video players enforce validation of a cross 703 domain policy that needs to allow for the domains involved in the CDN 704 redirection. These problems are generally soluble, but the solutions 705 complicate the example, so we do not discuss them further in this 706 version of the draft. 708 We note that this example begins to illustrate some of the interfaces 709 that may be required for CDNI, but does not require all of them. For 710 example, obtaining information from dCDN regarding the set of client 711 IP addresses or geographic regions it might be able to serve is an 712 aspect of the request routing interface. Important configuration 713 information such as the distinguished names used for redirection and 714 inter-CDN acquisition could also be conveyed via a CDNI interface 715 (e.g., perhaps the control interface). The example also shows how 716 existing HTTP-based methods suffice for the acquisition interface. 717 Arguably, the absolute minimum metadata required for CDNI is the 718 information required to acquire the content, and this information was 719 provided "in-band" in this example by means of the URI handed to the 720 client in the HTTP 302 response. Hence, there is no explicit 721 metadata interface invoked in this example. There is also no 722 explicit logging interface discussed in this example. 724 We also note that the step of deciding when a request should be 725 redirected to dCDN rather than served by uCDN has been somewhat 726 glossed over. It may be as simple as checking the client IP address 727 against a list of prefixes, or it may be considerably more complex, 728 involving a wide range of factors, such as the geographic location of 729 the client (perhaps determined from a third party service), CDN load, 730 or specific business rules. 732 This example uses the "iterative" CDNI request routing approach. 733 That is, uCDN performs part of the request routing function to 734 determine that dCDN should serve the request, and then redirects the 735 client to a request router in dCDN to perform the rest of the request 736 routing function. If request routing is performed in the dCDN using 737 HTTP redirection, this translates in the end-user experiencing two 738 successive HTTP redirections. By contrast, the alternative approach 739 of "recursive" CDNI request routing effectively coalesces these two 740 successive HTTP redirections into a single one, sending the end-user 741 directly to the right delivery node in the dCDN. This "recursive" 742 CDNI request routing approach is discussed in the next section. 744 3.3. Recursive Redirection Example 746 The following example builds on the previous one to illustrate the 747 use of the Request Routing interface to enable "recursive" CDNI 748 request routing. We build on the HTTP-based redirection approach 749 because it illustrates the principles and benefits clearly, but it is 750 equally possible to perform recursive redirection when DNS-based 751 redirection is employed. 753 In contrast to the prior example, the operators need not agree in 754 advance on a CDN-domain to serve as the target of redirections from 755 uCDN to dCDN. The operators still must agree on some distinguished 756 CDN-domain that will be used for inter-CDN acquisition of CSP's 757 content by dCDN. In this example, we'll use op-b-acq.op-a.net. 759 The operators must also exchange information regarding which requests 760 dCDN is prepared to serve. For example, dCDN may be prepared to 761 serve requests from clients in a given geographical region or a set 762 of IP address prefixes. This information may again be provided out 763 of band or via a defined protocol. 765 DNS must be configured in the following way: 767 o The content provider must be configured to make operator A the 768 authoritative DNS server for cdn.csp.com (or to return a CNAME for 769 cdn.csp.com for which operator A is the authoritative DNS server). 771 o Operator A must be configured so that a DNS request for op-b- 772 acq.op-a.net returns a request router in Operator A. 774 o Operator B must be configured so that a request for node1.op- 775 b.net/cdn.csp.com returns the IP address of a delivery node. Note 776 that there might be a number of such delivery nodes. 778 Figure 3 illustrates how a client request for 780 http://cdn.csp.com/...rest of url... 782 is handled. 784 End-User Operator B Operator A 785 |DNS cdn.csp.com | | 786 |-------------------------------------------------->| 787 | | |(1) 788 |IPaddr of A's Request Router | 789 |<--------------------------------------------------| 790 |HTTP cdn.csp.com | | 791 |-------------------------------------------------->| 792 | | |(2) 793 | |RRI REQ cdn.csp.com | 794 | |<------------------------| 795 | | | 796 | |RRI RESP node1.op-b.net | 797 | |------------------------>| 798 | | |(3) 799 |302 node1.op-b.net/cdn.csp.com | 800 |<--------------------------------------------------| 801 |DNS mode1.op-b.net | | 802 |------------------------>| | 803 | |(4) | 804 |IPaddr of B's Delivery Node | 805 |<------------------------| | 806 |HTTP node1.op-b.net/cdn.csp.com | 807 |------------------------>| | 808 | |(5) | 809 | |DNS op-b-acq.op-a.net | 810 | |------------------------>| 811 | | |(6) 812 | |IPaddr of A's Request Router 813 | |<------------------------| 814 | |HTTP op-b-acq.op-a.net | 815 | |------------------------>| 816 | | |(7) 817 | |302 node2.op-b.acq.op-A.net 818 | |<------------------------| 819 | |DNS node2.op-b-acq.op-a.net 820 | |------------------------>| 821 | | |(8) 822 | |IPaddr of A's Delivery Node 823 | |<------------------------| 824 | | |(9) 825 | |Data | 826 | |<------------------------| 827 |Data | | 828 |<------------------------| | 830 Figure 4: Request Trace for Recursive HTTP redirection method 832 The steps illustrated in the figure are as follows: 834 1. A DNS resolver for Operator A processes the DNS request for its 835 customer based on CDN-domain cdn.csp.com. It returns the IP 836 address of a Request Router in Operator A. 838 2. A Request Router for Operator A processes the HTTP request and 839 recognizes that the end-user is best served by another CDN-- 840 specifically one provided by Operator B--and so it queries the 841 CDNI Request Routing interface of Operator B, providing a set of 842 information about the request including the URL requested. 843 Operator B replies with the DNS name of a delivery node. 845 3. Operator A returns a 302 redirect message for a new URL obtained 846 from the Request Routing Interface. 848 4. The end-user does a DNS lookup using the host name of the URL 849 just provided (node1.op-b.net). B's DNS resolver returns the IP 850 address of the corresponding delivery node. Note that, since the 851 name of the delivery node was already obtained from B using the 852 CDNI Request Routing Interface, there should not be any further 853 redirection here (in contrast to the iterative method described 854 above.) 856 5. The end-user requests the content from B's delivery node, 857 potentially resulting in a cache miss. In the case of a cache 858 miss, the content needs to be acquired from uCDN (not the CSP.) 859 The distinguished CDN-domain op-b.net indicates to dCDN that this 860 content is to be acquired from another CDN; stripping the CDN- 861 domain reveals the original CDN-domain cdn.csp.com, dCDN may 862 verify that this CDN-domain belongs to a known peer (so as to 863 avoid being tricked into serving as an open proxy). It then does 864 a DNS request for the inter-CDN Acquisition "distinguished" CDN- 865 domain as agreed above (in this case, op-b-acq.op-a.net). 867 6. Operator A DNS resolver processes the DNS request and returns the 868 IP address of a request router in operator A. 870 7. The request router for Operator A processes the HTTP request from 871 Operator B delivery node. Operator A request router recognizes 872 that the request is from a peer CDN rather than an end-user 873 because of the dedicated inter-CDN acquisition domain (op-b- 874 acq.op-a.net). (Note that without this specially defined inter- 875 CDN acquisition domain, operator A would be at risk of 876 redirecting the request back to operator B, resulting in an 877 infinite loop). The request router for Operator A selects a 878 suitable delivery node in uCDN to serve the inter-CDN acquisition 879 request and returns a 302 redirect message for a new URL 880 constructed by replacing the hostname by a subdomain of the 881 Operator A's distinguished inter-CDN acquisition domain that 882 points to the selected delivery node. 884 8. Operator A recognizes that the DNS request is from a peer CDN 885 rather than an end-user (due to the internal CDN-domain) and so 886 returns the address of a delivery node. (Note that without this 887 specially defined internal domain, Operator A would be at risk of 888 redirecting the request back to Operator B, resulting in an 889 infinite loop.) 891 9. Operator A serves content for the requested CDN-domain to dCDN. 892 Although not shown, it is at this point that Operator A processes 893 the rest of the URL: it extracts information identifying the 894 origin server, validates that this server has been registered, 895 and determines the content provider that owns the origin server. 896 It may also perform its own content acquisition steps if needed 897 before returning the content to dCDN. 899 3.3.1. Comments on the example 901 Recursive redirection has the advantage over iterative of being more 902 transparent from the end-user's perspective, but the disadvantage of 903 each CDN exposing more of its internal structure (in particular, the 904 addresses of edge caches) to peer CDNs. By contrast, iterative 905 redirection does not require dCDN to expose the addresses of its edge 906 caches to uCDN. 908 This example happens to use HTTP-based redirection in both CDN A and 909 CDN B, but a similar example could be constructed using DNS-based 910 redirection in either CDN. Hence, the key point to take away here is 911 simply that the end user only sees a single redirection of some type, 912 as opposed to the pair of redirections in the prior (iterative) 913 example. 915 The use of the Request Routing Interface requires that interface to 916 be appropriately configured and bootstrapped, which is not shown 917 here. More discussion on the bootstrapping of interfaces is provided 918 in Section 4 920 3.4. DNS-based redirection example 922 In this section we walk through a simple example using DNS-based 923 redirection for request redirection from uCDN to dCDN (as well as for 924 request routing inside dCDN and uCDN). As noted in Section 2.1, DNS- 925 based redirection has certain advantages over HTTP-based redirection 926 (notably, it is transparent to the end-user) as well as some 927 drawbacks (notably the client IP address is not visible to the 928 request router). 930 As before, Operator A must learn the set of requests that dCDN is 931 willing or able to serve (e.g. which client IP address prefixes or 932 geographic regions are part of the dCDN footprint). Operator B must 933 have and make known to operator A some unique identifier that can be 934 used for the construction of a distinguished CDN domain, as shown in 935 more detail below. (This identifier strictly needs only to be unique 936 within the scope of Operator A, but a globally unique identifier, 937 such as an AS number assigned to B, is one easy way to achieve that.) 938 Also, Operator A must obtain the NS records for Operator B's 939 externally visible redirection servers. Also, as before, a 940 distinguished CDN-domain, such as op-b-acq.op-a.net, must be assigned 941 for inter-CDN acquisition. 943 DNS must be configured in the following way: 945 o The CSP must be configured to make Operator A the authoritative 946 DNS server for cdn.csp.com (or to return a CNAME for cdn.csp.com 947 for which operator A is the authoritative DNS server). 949 o When uCDN sees a request best served by dCDN, it returns CNAME and 950 NS records for "b.cdn.csp.com", where "b" is the unique identifier 951 assigned to Operator B. (It may, for example, be an AS number 952 assigned to Operator B.) 954 o dCDN must be configured so that a request for "b.cdn.csp.com" 955 returns a delivery node in dCDN. 957 o uCDN must be configured so that a request for "op-b-acq.op-a.net" 958 returns a delivery node in uCDN. 960 Figure 5 depicts the exchange of DNS and HTTP requests. The main 961 differences from Figure 3 are the lack of HTTP redirection and 962 transparency to the end-user. 964 End-User Operator B Operator A 965 |DNS cdn.csp.com | | 966 |-------------------------------------------------->| 967 | | |(1) 968 |CNAME b.cdn.csp.com | | 969 |NS records for b.cdn.csp.com | 970 |<--------------------------------------------------| 971 |DNS b.cdn.csp.com | | 972 |------------------------>| | 973 | |(2) | 974 |IPaddr of B's Delivery Node | 975 |<------------------------| | 976 |HTTP cdn.csp.com | | 977 |------------------------>| | 978 | |(3) | 979 | |DNS op-b-acq.op-a.net | 980 | |------------------------>| 981 | | |(4) 982 | |IPaddr of A's Delivery Node 983 | |<------------------------| 984 | |HTTP op-b-acq.op-a.net | 985 | |------------------------>| 986 | | |(5) 987 | |Data | 988 | |<------------------------| 989 |Data | | 990 |<------------------------| | 992 Figure 5: Request Trace for DNS-based Redirection Example 994 The steps illustrated in the figure are as follows: 996 1. Request Router for Operator A processes the DNS request for CDN- 997 domain cdn.csp.com and recognizes that the end-user is best 998 served by another CDN. (This may depend on the IP address of the 999 user's local DNS resolver, or other information discussed below.) 1000 The Request Router returns a DNS CNAME response by "stacking" the 1001 distinguished identifier for Operator B onto the original CDN- 1002 domain (e.g., b.cdn.csp.com), plus an NS record that maps 1003 b.cdn.csp.com to B's Request Router. 1005 2. The end-user does a DNS lookup using the modified CDN-domain 1006 (i.e., b.cdn.csp.com). This causes B's Request Router to respond 1007 with a suitable delivery node. 1009 3. The end-user requests the content from B's delivery node. The 1010 requested URL contains the name cdn.csp.com. (Note that the 1011 returned CNAME does not affect the URL.) At this point the 1012 delivery node has the correct IP address of the end-user and can 1013 do an HTTP 302 redirect if the redirections in steps 2 and 3 were 1014 incorrect. Otherwise B verifies that this CDN-domain belongs to 1015 a known peer (so as to avoid being tricked into serving as an 1016 open proxy). It then does a DNS request for an "internal" CDN- 1017 domain as agreed above (op-b-acq.op-a.net). 1019 4. Operator A recognizes that the DNS request is from a peer CDN 1020 rather than an end-user (due to the internal CDN-domain) and so 1021 returns the address of a delivery node in uCDN. 1023 5. Operator A serves content to dCDN. Although not shown, it is at 1024 this point that Operator A processes the rest of the URL: it 1025 extracts information identifying the origin server, validates 1026 that this server has been registered, and determines the content 1027 provider that owns the origin server. 1029 3.4.1. Comments on the example 1031 The advantages of this approach are that it is more transparent to 1032 the end-user and requires fewer round trips than HTTP-based 1033 redirection. A potential problem is that the upstream CDN depends on 1034 being able to learn the correct downstream CDN that serves the end- 1035 user from the client address in the DNS request. In standard DNS 1036 operation, uCDN will only obtain the address of the client's local 1037 DNS resolver (LDNS), which is not guaranteed to be in the same 1038 network (or geographic region) as the client. If not--e.g., the end- 1039 user uses a global DNS service--then the upstream CDN cannot 1040 determine the appropriate downstream CDN to serve the end-user. In 1041 this case, one option is for the upstream CDN to treat the end-user 1042 as it would any user not connected to a peer CDN. Another option is 1043 for the upstream CDN to "fall back" to a pure HTTP-based redirection 1044 strategy in this case (i.e., use the first method). Note that this 1045 problem affects existing CDNs that rely on DNS to determine where to 1046 redirect client requests, but the consequences are arguably less 1047 serious since the LDNS is likely in the same network as the dCDN 1048 serves. One approach to ensuring that the client's IP address prefix 1049 is correctly determined in such situations is described in 1050 [I-D.vandergaast-edns-client-subnet]. 1052 As with the prior example, this example partially illustrates the 1053 various interfaces involved in CDNI. Operator A could learn 1054 dynamically from Operator B the set of prefixes or regions that B is 1055 willing and able to serve via the request routing interface. The 1056 distinguished name used for acquisition and the identifier for 1057 Operator B that is prepended to the CDN domain on redirection are 1058 examples of information elements that might also be conveyed by CDNI 1059 interfaces (or, alternatively, statically configured). As before, 1060 minimal metadata sufficient to obtain the content is carried "in- 1061 band" as part of the redirection process, and standard HTTP is used 1062 for inter-CDN acquisition. There is no explicit logging interface 1063 discussed in this example. 1065 3.5. Dynamic Footprint Discovery 1067 There could be situations where being able to dynamically discover 1068 the set of requests that a given dCDN is willing and able to serve is 1069 beneficial. For example, a CDN might at one time be able to serve a 1070 certain set of client IP prefixes, but that set might change over 1071 time due to changes in the topology and routing policies of the IP 1072 network. The following example illustrates this capability. We have 1073 chosen the example of DNS-based redirection, but HTTP-based 1074 redirection could equally well use this approach. 1076 End-User Operator B Operator A 1077 |DNS cdn.csp.com | | 1078 |-------------------------------------------------->| 1079 | | |(1) 1080 | | RRI REQ op-b.net | 1081 | |<------------------------| 1082 | | |(2) 1083 | | RRI REPLY | 1084 | |------------------------>| 1085 | | |(3) 1086 |CNAME b.cdn.csp.com | | 1087 |NS records for b.cdn.csp.com | 1088 |<--------------------------------------------------| 1089 |DNS b.cdn.csp.com | | 1090 |------------------------>| | 1091 | |(2) | 1092 |IPaddr of B's Delivery Node | 1093 |<------------------------| | 1094 |HTTP cdn.csp.com | | 1095 |------------------------>| | 1096 | |(3) | 1097 | |DNS op-b-acq.op-a.net | 1098 | |------------------------>| 1099 | | |(4) 1100 | |IPaddr of A's Delivery Node 1101 | |<------------------------| 1102 | |HTTP op-b-acq.op-a.net | 1103 | |------------------------>| 1104 | | |(5) 1105 | |Data | 1106 | |<------------------------| 1107 |Data | | 1108 |<------------------------| | 1110 Figure 6: Request Trace for Dynamic Footprint Discovery Example 1112 This example differs from the one in Figure 5 only in the addition of 1113 a CDNI Request Routing Interface request (step 2) and corresponding 1114 response (step 3). The RRI Req could be a message such as "Can you 1115 serve clients from this IP Prefix?" or it could be "Provide the list 1116 of client IP prefixes you can currently serve". In either case the 1117 response might be cached by operator A to avoid repeatedly asking the 1118 same question. Alternatively, or in addition, Operator B may 1119 spontaneously advertise to Operator A information (or changes) on the 1120 set of requests it is willing and able to serve on behalf of operator 1121 A; in that case, Operator B may spontaneously issue RRI REPLY 1122 messages that are not in direct response to a corresponding RRI REQ 1123 message. (Note that the issues of determining the client's subnet 1124 from DNS requests, as described above, are exactly the same here as 1125 in Section 3.4.) 1127 Once Operator A obtains the RRI response, it is now able to determine 1128 that Operator B's CDN is an appropriate dCDN for this request and 1129 therefore a valid candidate dCDN to consider in its Redirection 1130 decision. If that dCDN is selected, the redirection and serving of 1131 the request proceeds as before (i.e. in the absence of dynamic 1132 footprint discovery). 1134 3.6. Content Removal 1136 The following example illustrates how the Metadata interface may be 1137 used to remove an item of content. In this example, user requests 1138 for a particular content, and corresponding redirection of such 1139 requests from Operator A to Operator B CDN, may (or may not) have 1140 taken place earlier. Then, at some point in time, the uCDN (for 1141 example, in response to a corresponding trigger from the Content 1142 Provider) uses the Metadata Interface to request that content 1143 identified by a particular URL be removed from dCDN. The following 1144 diagram illustrates the operation. 1145 End-User Operator B Operator A 1146 | |MI purge cdn.csp.com/... | 1147 | |<------------------------| 1148 | | |(1) 1149 | |MI OK | 1150 | |------------------------>| 1151 | | |(2) 1153 Figure 7: Request Trace for Content Removal 1155 The metadata interface is used to convey the request from uCDN to 1156 dCDN that some previously acquired content should be deleted. The 1157 URL in the request specifies which content to remove. This example 1158 corresponds to a DNS-based redirection scenario such as Section 3.4. 1159 If HTTP-based redirection had been used, the URL for removal would be 1160 of the form peer-a.op-b.net/cdn.csp.com/... 1162 The dCDN is expected to confirm to the uCDN, as illustrated by the MI 1163 OK message, the completion of the removal of the targeted content 1164 from all the caches in dCDN. 1166 3.7. Pre-Positioned Content Acquisition Example 1168 The following example illustrates how the metadata interface may be 1169 used to pre-position an item of content in the dCDN. In this 1170 example, Operator A uses the Metadata Interface to request that 1171 content identified by a particular URL be pre-positioned into 1172 Operator B CDN. 1174 End-User Operator B Operator A 1176 | |MI pre-position cdn.csp.com/... 1177 | |<------------------------| 1178 | | |(1) 1179 | |MI OK | 1180 | |------------------------>| 1181 | | | 1182 | |DNS op-b-acq.op-a.net | 1183 | |------------------------>| 1184 | | |(2) 1185 | |IPaddr of A's Delivery Node 1186 | |<------------------------| 1187 | |HTTP op-b-acq.op-a.net | 1188 | |------------------------>| 1189 | | |(3) 1190 | |Data | 1191 | |<------------------------| 1192 |DNS cdn.csp.com | | 1193 |-------------------------------------------------->| 1194 | | |(4) 1195 |IPaddr of A's Request Router | 1196 |<--------------------------------------------------| 1197 |HTTP cdn.csp.com | | 1198 |-------------------------------------------------->| 1199 | | |(5) 1200 |302 peer-a.op-b.net/cdn.csp.com | 1201 |<--------------------------------------------------| 1202 |DNS peer-a.op-b.net | | 1203 |------------------------>| | 1204 | |(6) | 1205 |IPaddr of B's Delivery Node | 1206 |<------------------------| | 1207 |HTTP peer-a.op-b.net/cdn.csp.com | 1208 |------------------------>| | 1209 | |(7) | 1210 |Data | | 1211 |<------------------------| | 1213 Figure 8: Request Trace for Content Pre-Positioning 1215 The steps illustrated in the figure are as follows: 1217 1. Operator A uses the Metadata Interface to request that Operator B 1218 pre-positions a particular content item identified by its URL. 1220 Operator B responds by confirming that it is willing to perform 1221 this operation. 1223 Steps 2 and 3 are exactly the same as steps 5 and 6 of Figure 3, only 1224 this time those steps happen as the result of the Pre-positioning 1225 request instead of as the result of a cache miss. 1227 Steps 4, 5, 6, 7 are exactly the same as steps 1, 2, 3, 4 of 1228 Figure 3, only this time Operator B CDN can serve the end-user 1229 request without triggering dynamic content acquisition, since the 1230 content has been pre-positioned in dCDN. Note that, depending on 1231 dCDN operations and policies, the content pre-positioned in the dCDN 1232 may be pre-positioned to all, or a subset of, dCDN caches. In the 1233 latter case, intra-CDN dynamic content acquisition may take place 1234 inside the dCDN serving requests from caches on which the content has 1235 not been pre-positioning; however, such intra-CDN dynamic acquisition 1236 would not involve the uCDN. 1238 3.8. Asynchronous CDNI Metadata Example 1240 In this section we walk through a simple example illustrating a 1241 scenario of asynchronously exchanging CDNI metadata, where the 1242 downstream CDN obtains CDNI metadata for content ahead of a 1243 corresponding content request. The example that follows assumes that 1244 HTTP-based inter-CDN redirection and recursive CDNI request-routing 1245 are used, as in Section 3.3. However, asynchronous exchange of CDNI 1246 Metadata is similarly applicable to DNS-based inter-CDN redirection 1247 and iterative request routing (in which cases the CDNI metadata may 1248 be used at slightly different processing stages of the message 1249 flows). 1251 End-User Operator B Operator A 1252 | | | 1253 | |MI push (cdn.csp.com/...,| 1254 | | distribution policy) | 1255 | |<------------------------|(1) 1256 | | | 1257 | | | 1258 | CONTENT REQUEST | | 1259 |-------------------------------------------------->| (2) 1260 | | | 1261 | |RRI REQ | 1262 | (3)|<------------------------| 1263 | | | 1264 | | | 1265 | |RRI RESP | 1266 | |------------------------>|(4) 1267 | | | 1268 | CONTENT REDIRECTION | | 1269 |<--------------------------------------------------| (5) 1270 | | | 1271 | CONTENT REQUEST | | 1272 |------------------------>| (6) | 1273 | | | 1274 : : : 1275 | CONTENT DATA | | 1276 |<------------------------| | (7) 1278 Figure 9: Request Trace for Asynchronous CDNI Metadata 1280 The steps illustrated in the figure are as follows: 1282 1. Operator A uses the Metadata Interface to asynchronously push 1283 CDNI metadata to Operator B. The present document does not 1284 constrain how the CDNI metadata information is actually 1285 represented. For the purposes of this example, we assume that 1286 Operator A provides CDNI metadata to Operator B indicating that: 1288 * this CDNI Metadata is applicable to any content referenced by 1289 "cdn.csp.com/op-b.net/..." (assuming HTTP redirection is used 1290 - it would be applicable to "cdn.csp.com/..." if DNS 1291 redirection were used as in Section 3.4). 1293 * this CDNI metadata consists of a distribution policy requiring 1294 enforcement by the delivery node of a specific per-request 1295 authorization mechanism (e.g. URI signature or token 1296 validation). 1298 2. A Content Request occurs as usual. 1300 3. A CDNI Request Routing Request (RRI REQ) is issued by operator A 1301 CDN, as discussed in Section 3.3. Operator B's request router 1302 can access the CDNI Metadata that are relevant to the requested 1303 content and that have been pre-positioned as per Step 1, which 1304 may or may not affect the response. 1306 4. Operator B's request router issues a CDNI Request Routing 1307 Response (RRI RESP) as in Section 3.3. 1309 5. Operator B performs content redirection as discussed in 1310 Section 3.3. 1312 6. On receipt of the Content Request by the end user, the delivery 1313 node detects that previously acquired CDNI metadata is applicable 1314 to the requested content. In accordance with the specific CDNI 1315 metadata of this example, the delivery node will invoke the 1316 appropriate per-request authorization mechanism, before serving 1317 the content. (Details of this authorization are not shown.) 1319 7. Assuming successful per-request authorization, serving of Content 1320 Data (possibly preceded by inter-CDN acquisition) proceeds as in 1321 Section 3.3. 1323 3.9. Synchronous CDNI Metadata Acquisition Example 1325 In this section we walk through a simple example illustrating a 1326 scenario of synchronous CDNI metadata acquisition, in which the 1327 downstream CDN obtains CDNI metadata for content at the time of 1328 handling a first request for the corresponding content. As in the 1329 preceding section, this example assumes that HTTP-based inter-CDN 1330 redirection and recursive CDNI request-routing are used (as in 1331 Section 3.3), but dynamic CDNI metadata acquisition is applicable to 1332 other variations of request routing. 1334 End-User Operator B Operator A 1335 | | | 1336 | |MI push (cdn.csp.com/...,| 1337 | | CDNI metadata acquisition info) 1338 | |<------------------------|(1) 1339 | | | 1340 : : : 1341 | CONTENT REQUEST | | 1342 |-------------------------------------------------->|(2) 1343 | | | 1344 | |RRI REQ | 1345 | (3)|<------------------------| 1346 | | | 1347 | |MI REQ | 1348 | (4)|------------------------>| 1349 | |MI RESP | 1350 | |<------------------------|(5) 1351 | | | 1352 | |RRI RESP | 1353 | |------------------------>|(6) 1354 | | | 1355 | | | 1356 | CONTENT REDIRECTION | | 1357 |<--------------------------------------------------|(7) 1358 | | | 1359 | CONTENT REQUEST | | 1360 |------------------------>| (8) | 1361 | | | 1362 | |MI REQ | 1363 | (9)|------------------------>| 1364 | |MI RESP | 1365 | |<------------------------|(10) 1366 | | | 1367 : : : 1368 | CONTENT DATA | | 1369 |<------------------------| | (11) 1371 Figure 10: Request Trace for Synchronous CDNI Metadata Acquisition 1373 The steps illustrated in the figure are as follows: 1375 1. Operator A initially uses the Metadata Interface to 1376 asynchronously push seed metadata to Operator B. For example, 1377 this seed information may include a URI indicating where CDNI 1378 Metadata can later be pulled from for some content set. (There 1379 are alternative ways that this seeding information may be 1380 provided, such as piggybacking on the CDNI RRI REQ message of 1381 Step 3.) 1383 2. A Content Request arrives as normal. 1385 3. A Request Routing Interface request occurs as in the prior 1386 example. 1388 4. On receipt of the CDNI Request Routing Request, Operator B's CDN 1389 initiates synchronous acquisition of CDNI Metadata that are 1390 needed for routing of the end-user request. The seeding 1391 information provided in Step 1 is used to determine how to 1392 obtain the metadata. Note that there may exist cases in which 1393 this step does not occur (e.g., because the CDNI metadata 1394 seeding information indicates CDNI metadata are not needed at 1395 that stage). 1397 5. On receipt of a CDNI Metadata MI Request, Operator A's CDN 1398 responds, making the corresponding CDNI metadata information 1399 available to Operator B's CDN. This metadata is considered by 1400 operator B's CDN before responding to the Request Routing 1401 request. (In a simple case, the metadata could simply be an 1402 allow or deny response for this particular request.) 1404 6. Response to the RRI request as normal. 1406 7. Redirection message is sent to the end user. 1408 8. A delivery node of Operator B receives the end user request. 1410 9. The delivery node triggers dynamic acquisition of additional 1411 CDNI metadata that are needed to process the end-user content 1412 request. Again the seeding information provided in Step 1 is 1413 used to determine how to acquire the needed CDNI metadata. Note 1414 that there may exist cases where this step need not happen, 1415 either because the metadata were already acquired previously, or 1416 because the seeding information indicates no metadata are 1417 required. 1419 10. Operator A's CDN responds to the CDNI Metadata Request and makes 1420 the corresponding CDNI metadata available to Operator B. This 1421 metadata influence how Operator B's CDN processes the end-user 1422 request. 1424 11. Content is served (possibly preceded by inter-CDN acquisition) 1425 as in Section 3.3. 1427 4. Main Interfaces 1429 Figure 1 illustrates the four main interfaces that are in scope for 1430 the CDNI WG, along with several others. The detailed specifications 1431 of these interfaces are left to other documents (mostly still to be 1432 written, but see [I-D.ietf-cdni-problem-statement] and 1433 [I-D.ietf-cdni-requirements] for some discussion of the interfaces). 1435 One interface that is not shown in Figure 1 is the interface between 1436 the user and the CSP. While for the purposes of CDNI that interface 1437 is out of scope, it is worth noting that it does exist and can 1438 provide useful functions, such as end-to-end performance monitoring 1439 and some forms of authentication and authorization. 1441 There is also an important interface between the user and the Request 1442 Routing function of both uCDN and dCDN. As we saw in some of the 1443 preceding examples, that interface can be used as a way of passing 1444 information such as the metadata that is required to obtain the 1445 content in dCDN from uCDN. 1447 In this section we will provide an overview of the functions 1448 performed by each of the CDNI interfaces and discuss how they fit 1449 into the overall solution. We also examine some of the design 1450 tradeoffs. We begin with an examination of one such tradeoff that 1451 affects all the interfaces - the use of in-band or out-of-band 1452 communication. 1454 4.1. In-Band versus Out-of-Band Interfaces 1456 Before getting to the individual interfaces, we observe that there is 1457 a high-level design choice for each, involving the use of existing 1458 in-band communication channels versus defining new out-of-band 1459 interfaces. 1461 It is possible that the information needed to carry out various 1462 interconnection functions can be communicated between peer CDNs using 1463 existing in-band protocols. The use of HTTP 302 redirect is an 1464 example of how certain aspects of request routing can be implemented 1465 in-band (embedded in URIs). Note that using existing in-band 1466 protocols does not imply that the CDNI interfaces are null; it is 1467 still necessary to establish the rules (conventions) by which such 1468 protocols are used to implement the various interface functions. 1470 There are other opportunities for in-band communication beyond HTTP 1471 redirects. For example, many of the HTTP directives used by proxy 1472 servers can also be used by peer CDNs to inform each other of caching 1473 activity. Of these, one that is particularly relevant is the If- 1474 Modified-Since directive, which is used with the GET method to make 1475 it conditional: if the requested object has not been modified since 1476 the time specified in this field, a copy of the object will not be 1477 returned, and instead, a 304 (not modified) response will be 1478 returned. 1480 4.2. Request Routing Interface 1482 We may think of the request routing interface as comprising two 1483 parts: the asynchronous advertisement of footprint and capabilities 1484 by a dCDN that allows a uCDN to decide whether to redirect particular 1485 user requests to that dCDN; and the synchronous operation of actually 1486 redirecting a user request. (These are somewhat analogous to the 1487 operations of routing and forwarding in IP.) 1489 As illustrated in Section 3, the synchronous part of the request 1490 routing interface may be implemented in part by DNS and HTTP. Naming 1491 conventions may be established by which CDN peers communicate whether 1492 a request should be routed or content served. 1494 In support of these exchanges, it is necessary for CDN peers to 1495 exchange additional information with each other. Depending on the 1496 method(s) supported, this includes 1498 o The operator's unique id (operator-id) or distinguished CDN-domain 1499 (operator-domain); 1501 o NS records for the operator's set of externally visible request 1502 routers; 1504 o The set of requests the dCDN operator is prepared to serve (e.g. a 1505 set of client IP prefixes or geographic regions that may be served 1506 by dCDN). 1508 Of these, the two operator identifiers are fixed, and can be 1509 exchanged off-line as part of a peering agreement. The NS records 1510 potentially change with some frequency, but an existing protocol-- 1511 DNS--can be used to dynamically track this information. That is, a 1512 peer can do a DNS lookup on operator-domain to retrieve the set of NS 1513 records corresponding to the peer's redirection service. 1515 The set of requests that dCDN is willing to serve could in some cases 1516 be relatively static (e.g., a set of IP prefixes) which could be 1517 exchanged off-line, or might even be negotiated as part of a peering 1518 agreement. However, it may also be more dynamic, in which case an 1519 explicit protocol for its exchange would be be helpful. 1521 A variety of options exist for the dCDN operator to advertise its 1522 footprint to uCDN. As discussed in 1524 [I-D.previdi-cdni-footprint-advertisement], footprint is comprised of 1525 two components: 1527 o a class of end user requests (represented, for example, by a set 1528 of IP prefixes, or a geographic region) that the dCDN is willing 1529 and able to serve directly, without use of another dCDN; 1531 o the connectivity of the dCDN to other CDNs that may be able to 1532 serve content to users on behalf of dCDN. 1534 [I-D.previdi-cdni-footprint-advertisement] describes an approach to 1535 advertising such footprint information asynchronously using BGP. In 1536 addition to this sort of information, a dCDN might also advertise 1537 "capabilities" such as the ability to handle certain types of content 1538 (e.g. specific streaming formats) or quality of service (QoS) 1539 capabilities. [I-D.xiaoyan-cdni-request-routing-protocol] describes 1540 an approach that exchanges CDN "capabilities" over HTTP, while 1541 [I-D.seedorf-alto-for-cdni] describes how ALTO [RFC5693] may be used 1542 to obtain request routing information. 1544 We also note that the Request Routing interface plays a key role in 1545 enabling recursive redirection, as illustrated in Section 3.3. It 1546 enables the user to be redirected to the correct delivery node in 1547 dCDN with only a single redirection step (as seen by the user). This 1548 may be particularly valuable as the chain of interconnected CDNs 1549 increases beyond two CDNs. 1551 4.3. Logging Interface 1553 It is necessary for the upstream CDN to have visibility into the 1554 delivery of content it originates to end-users connected to the 1555 downstream CDN. This allows the upstream CDN to properly bill its 1556 customers for multiple deliveries of content cached by the downstream 1557 CDN, as well as to report accurate traffic statistics to those 1558 content providers. This is one role of the Logging interface. 1560 Other operational data that may be relevant to CDNI can also be 1561 exchanged by the Logging interface. For example, dCDN may report the 1562 amount of content it has acquired from uCDN, and how much cache 1563 storage has been consumed by content cached on behalf of uCDN. 1565 Traffic logs are easily exchanged off-line. For example, the 1566 following traffic log is a small deviation from the Apache log file 1567 format, where entries include the following fields: 1569 o Domain - the full domain name of the origin server 1570 o IP address - the IP address of the client making the request 1572 o End time - the ending time of the transfer 1574 o Time zone - any time zone modifier for the end time 1576 o Method - the transfer command itself (e.g., GET, POST, HEAD) 1578 o URL - the requested URL 1580 o Version - the protocol version, such as HTTP/1.0 1582 o Response - a numeric response code indicating transfer result 1584 o Bytes Sent - the number of bytes in the body sent to the client 1586 o Request ID - a unique identifier for this transfer 1588 o User agent - the user agent, if supplied 1590 o Duration - the duration of the transfer in milliseconds 1592 o Cached Bytes - the number of body bytes served from the cache 1594 o Referrer - the referrer string from the client, if supplied 1596 Of these, only the Domain field is indirect in the downstream CDN--it 1597 is set to the CDN-domain used by the upstream CDN rather than the 1598 actual origin server. This field could then used to filter traffic 1599 log entries so only those entries matching the upstream CDN are 1600 reported to the corresponding operator. 1602 One open question is who does the filtering. One option is that the 1603 downstream CDN filters its own logs, and passes the relevant records 1604 directly to each upstream peer. This requires that the downstream 1605 CDN knows the set of CDN-domains that belong to each upstream peer. 1606 If this information is already exchanged between peers as part of the 1607 request routing interface, then direct peer-to-peer reporting is 1608 straightforward. If it is not available, and operators do not wish 1609 to advertise the set of CDN-domains they serve to their peers, then 1610 the second option is for each CDN to send both its non-local traffic 1611 records and the set of CDN-domains it serves to an independent third- 1612 party (i.e., a CDN Exchange), which subsequently filters, merges, and 1613 distributes traffic records on behalf of each participating CDN 1614 operator. 1616 A second open question is how timely traffic information should be. 1617 For example, in addition to off-line traffic logs, accurate real-time 1618 traffic monitoring might also be useful, but such information 1619 requires that the downstream CDN inform the upstream CDN each time it 1620 serves upstream content from its cache. The downstream CDN can do 1621 this, for example, by sending a conditional HTTP GET request (If- 1622 Modified-Since) to the upstream CDN each time it receives an HTTP GET 1623 request from one of its end-users. This allows the upstream CDN to 1624 record that a request has been issued for the purpose of real-time 1625 traffic monitoring. The upstream CDN can also use this information 1626 to validate the traffic logs received later from the downstream CDN. 1628 There is obviously a tradeoff between accuracy of such monitoring and 1629 the overhead of the downstream CDN having to go back to the upstream 1630 CDN for every request. 1632 Another design tradeoff in the Logging interface is the degree of 1633 aggregation or summarization of data. One situation that lends 1634 itself to summarization is the delivery of HTTP-based adaptive bit- 1635 rate video. Most schemes to deliver such video use a large number of 1636 relatively small HTTP requests (e.g. one request per 2-second chunk 1637 of video.) It may be desirable to aggregate logging information so 1638 that a single log entry is provided for the entire video rather than 1639 for each chunk. Note however that such aggregation requires a degree 1640 of application awareness in dCDN to recognize that the many HTTP 1641 requests correspond to a single video. 1643 Other forms of aggregation may also be useful. For example, there 1644 may be situations where bulk metrics such as bytes delivered per hour 1645 may suffice rather than the detailed per-request logs outlined above. 1646 It seems likely that a range of granularities of logging will be 1647 needed along with ways to specify the type and degree of aggregation 1648 required. 1650 4.4. Control Interface 1652 The control interface is primarily used for the bootstrapping of 1653 other interfaces. As a simple example, it could be used to provide 1654 the address of the logging server in dCDN to uCDN in order to 1655 bootstrap the logging interface. It may also be used, for example, 1656 to establish security associations for the other interfaces. We 1657 discuss the relationship between the Control and Metadata interfaces 1658 in the next section. 1660 4.5. Metadata Interface 1662 The role of the metadata interface is to enable CDNI distribution 1663 metadata to be conveyed to the downstream CDN by the upstream CDN. 1664 Such metadata includes geo-blocking restrictions, availability 1665 windows, access control policies, and so on. It may also include 1666 policy information such as the desire to pre-position content rather 1667 than fetch it on demand. 1669 Some metadata may be able to be conveyed using in-band mechanisms. 1670 For example, to inform the downstream CDN of any geo-blocking 1671 restrictions or availability windows, the upstream can elect to 1672 redirect a request to the downstream CDN only if that CDN's 1673 advertised delivery footprint is acceptable for the requested URL. 1674 Similarly, the request could be forwarded only if the current time is 1675 within the availability window. 1677 Similarly, some forms of access control may also be performed on a 1678 per-request basis using HTTP directives. For example, being able to 1679 respond to a conditional GET request gives the upstream CDN an 1680 opportunity to influence how the downstream CDN delivers its content. 1681 Minimally, the upstream CDN can invalidate (purge) content previously 1682 cached by the downstream CDN. 1684 Fine-grain control over how the downstream CDN delivers content on 1685 behalf of the upstream CDN is also possible. For example, by 1686 including the X-Forwarded-For HTTP header with the conditional GET 1687 request, the downstream CDN can report the end-user's IP address to 1688 the upstream CDN, giving it an opportunity to control whether the 1689 downstream CDN should serve the content to this particular end-user. 1690 The upstream CDN would communicate its directive through its response 1691 to the conditional GET. The downstream CDN can cache information for 1692 a period of time specified by the upstream CDN, thereby reducing 1693 control overhead. 1695 Thinking beyond what metadata operations can be done in-line, we note 1696 that all CDNs already export a "content purge" operation to their 1697 customers. The CDNI metadata interface could support a similar 1698 "content purge" API call. When a CSP invokes purge on the upstream 1699 CDN, that CDN in turn invokes purge on all downstream CDNs that might 1700 be caching the content. Of course, agreement as to the syntax and 1701 semantics of this call is required. 1703 One open question is how to distinguish between what functionality is 1704 supported by the Metadata interface and what functionality is 1705 supported by the Control interface. The approach taken in this 1706 document is to assume a minimal Control interface that is used to 1707 bootstrap the other interfaces. We assume all information that 1708 governs peer CDN behavior at the granularity of individual content 1709 items is exchanged via the Metadata interface. We note that some 1710 other documents have suggested that the purge operation should be 1711 part of the Control Interface. The authors' view is that purging a 1712 piece of content is just another form of metadata, similar to an 1713 availability window. In effect, a purge is equivalent to a statement 1714 that the availability window for that content has now expired. The 1715 timeliness requirements for purge operations may affect the detailed 1716 design of the metadata interface. 1718 5. Deployment Models 1720 In this section we describe a number of possible deployment models 1721 that may be achieved using the CDNI interfaces described above. We 1722 note that these models are by no means exhaustive, and that may other 1723 models may be possible. 1725 Although the reference model of Figure 1 shows all CDN functions on 1726 each side of the CDNI interface, deployments can rely on entities 1727 that are involved in any subset of these functions, and therefore 1728 only support the relevant subset of CDNI interfaces. As already 1729 noted in Section 3, effective CDNI deployments can be built without 1730 necessarily implementing all four interfaces. Some examples of such 1731 deployments are shown below. 1733 Note that, while we refer to upstream and downstream CDNs, this 1734 distinction applies to specific content items and transactions. That 1735 is, a given CDN may be upstream for some transactions and downstream 1736 for others, depending on many factors such as location of the 1737 requesting client and the particular piece of content requested. 1739 5.1. Meshed CDNs 1741 Although the reference model illustrated in Figure 1 shows a 1742 unidirectional CDN interconnection with a single uCDN and a single 1743 dCDN, any arbitrary CDNI meshing can be built from this, such as the 1744 example meshing illustrated in Figure 11. (Support for arbitrary 1745 meshing may or may not be in the initial scope for the working group, 1746 but the model allows for it.) 1747 ------------- ----------- 1748 / CDN A \<==CDNI===>/ CDN B \ 1749 \ / \ / 1750 ------------- ----------- 1751 /\ \\ /\ 1752 || \\ || 1753 CDNI \==CDNI===\\ CDNI 1754 || \\ || 1755 \/ \/ \/ 1756 ------------- ----------- 1757 / CDN C \===CDNI===>/ CDN D \ 1758 \ / \ / 1759 ------------- ----------- 1760 /\ 1761 || 1762 CDNI 1763 || 1764 \/ 1765 ------------- 1766 / CDN E \ 1767 \ / 1768 ------------- 1770 ===> CDNI interfaces, with right-hand side CDN acting as dCDN 1771 to left-hand side CDN 1772 <==> CDNI interfaces, with right-hand side CDN acting as dCDN 1773 to left-hand side CDN and with left-hand side CDN acting 1774 as dCDN to right-hand side CDN 1776 Figure 11: CDNI Deployment Model: CDN Meshing Example 1778 5.2. CSP combined with CDN 1780 Note that our terminology refers to functional roles and not economic 1781 or business roles. That is, a given organization may be operating as 1782 both a CSP and a fully-fledged uCDN when we consider the functions 1783 performed, as illustrated in Figure 12. 1785 ##################################### ################## 1786 # # # # 1787 # Organization A # # Organization B # 1788 # # # # 1789 # -------- ------------- # # ----------- # 1790 # / CSP \ / uCDN \ # # / dCDN \ # 1791 # | | | +----+ | # # | +----+ | # 1792 # | | | | C | | # # | | C | | # 1793 # | | | +----+ | # # | +----+ | # 1794 # | | | +----+ | # # | +----+ | # 1795 # | | | | L | | # # | | L | | # 1796 # | |*****| +----+ |===CDNI===>| +----+ | # 1797 # | | | +----+ | # # | +----+ | # 1798 # | | | | RR | | # # | | RR | | # 1799 # | | | +----+ | # # | +----+ | # 1800 # | | | +----+ | # # | +----+ | # 1801 # | | | | D | | # # | | D | | # 1802 # | | | +----+ | # # | +----+ | # 1803 # \ / \ / # # \ / # 1804 # -------- ------------- # # ----------- # 1805 # # # # 1806 ##################################### ################## 1808 ===> CDNI interfaces, with right-hand side CDN acting as dCDN 1809 to left-hand side CDN 1810 **** interfaces outside the scope of CDNI 1811 C Control component of the CDN 1812 L Logging component of the CDN 1813 RR Request Routing component of the CDN 1814 D Distribution component of the CDN 1816 Figure 12: CDNI Deployment Model: Organization combining CSP & uCDN 1818 5.3. CSP using CDNI Request Routing Interface 1820 As another example, a content provider organization may choose to run 1821 its own request routing function as a way to select among multiple 1822 candidate CDN providers; In this case the content provider may be 1823 modeled as the combination of a CSP and of a special, restricted case 1824 of a CDN. In that case, as illustrated in Figure 13, the CDNI 1825 Request Routing interface can be used between the restricted CDN 1826 operated by the content provider Organization and the CDN operated by 1827 the full-CDN organization acting as a dCDN in the request routing 1828 control plane. Interfaces outside the scope of the CDNI work can be 1829 used between the CSP functional entities of the content provider 1830 organization and the CDN operated by the full-CDN organization acting 1831 as a uCDN) in the CDNI control planes other than the request routing 1832 plane (i.e. Control, Distribution, Logging). 1834 ##################################### ################## 1835 # # # # 1836 # Organization A # # Organization B # 1837 # # # # 1838 # -------- ------------- # # ----------- # 1839 # / CSP \ / uCDN(RR) \ # # / dCDN(RR) \ # 1840 # | | | +----+ | # # | +----+ | # 1841 # | |*****| | RR |==========CDNI=====>| RR | | # 1842 # | | | +----+ | # RR # | +----+ | # 1843 # | | \ / # # | | # 1844 # | | ------------- # # |uCDN(C,L,D)| # 1845 # | | # # | +----+ | # 1846 # | | # # | | C | | # 1847 # | |*******************************| +----+ | # 1848 # | | # # | +----+ | # 1849 # | | # # | | L | | # 1850 # | | # # | +----+ | # 1851 # | | # # | +----+ | # 1852 # | | # # | | D | | # 1853 # | | # # | +----+ | # 1854 # \ / # # \ / # 1855 # -------- # # ----------- # 1856 # # # # 1857 ##################################### ################## 1859 ===> CDNI Request Routing interface 1860 **** interfaces outside the scope of CDNI 1862 Figure 13: CDNI Deployment Model: Organization combining CSP and 1863 partial CDN 1865 5.4. CDN Federations and CDN Exchanges 1867 There are two additional concepts related to, but distinct from CDN 1868 Interconnection. The first is CDN Federation. Our view is that CDNI 1869 is the more general concept, involving two or more CDNs serving 1870 content to each other's users, while federation implies a multi- 1871 lateral interconnection arrangement, but other CDN interconnection 1872 agreements are also possible (e.g., symmetric bilateral, asymmetric 1873 bilateral). An important conclusion is that CDNI technology should 1874 not presume (or bake in) a particular interconnection agreement, but 1875 should instead be general enough to permit alternative 1876 interconnection arrangements to evolve. 1878 The second concept often used in the context of CDN Federation is CDN 1879 Exchange--a third party broker or exchange that is used to facilitate 1880 a CDN federation. Our view is that a CDN exchange offers valuable 1881 machinery to scale the number of CDN operators involved in a multi- 1882 lateral (federated) agreement, but that this machinery is built on 1883 top of the core CDNI interconnection mechanisms. For example, as 1884 illustrated in Figure 14, the exchange might aggregate and 1885 redistribute information about each CDN footprint and capacity, as 1886 well as collect, filter, and re-distribute traffic logs that each 1887 participant needs for interconnection settlement, but inter-CDN 1888 request routing, inter-CDN content distribution (including inter-CDN 1889 acquisition) and inter-CDN control which fundamentally involve a 1890 direct interaction between an upstream CDN and a downstream CDN-- 1891 operate exactly as in a pair-wise peering arrangement. Turning to 1892 Figure 14, we observe that in this example: 1894 o each CDN supports a direct CDNI Control interface to every other 1895 CDN 1897 o each CDN supports a direct CDNI Metadata interface to every other 1898 CDN 1900 o each CDN supports a CDNI Logging interface with the CDN Exchange 1902 o each CDN supports both a CDNI request Routing interface with the 1903 CDN Exchange (for aggregation and redistribution of dynamic CDN 1904 footprint discovery information) and a direct CDNI Request Routing 1905 interface to every other CDN (for actual request redirection). 1907 ---------- --------- 1908 / CDN A \ / CDN B \ 1909 | +----+ | | +----+ | 1910 //========>| C |<==============CDNI============>| C |<==========\\ 1911 || | +----+ | C | +----+ | || 1912 || | +----+ | | +----+ | || 1913 || //=====>| D |<==============CDNI============>| D |<=======\\ || 1914 || || | +----+ | M | +----+ | || || 1915 || || | | /------------\ | | || || 1916 || || | +----+ | | +--+ CDN Ex| | +----+ | || || 1917 || || //==>| RR |<===CDNI==>|RR|<=======CDNI====>| RR |<====\\ || || 1918 || || || | +----+ | RR | +--+ | RR | +----+ | || || || 1919 || || || | | | /\ | | | || || || 1920 || || || | +----+ | | || +---+ | | +----+ | || || || 1921 || || || | | L |<===CDNI=======>| L |<=CDNI====>| L | | || || || 1922 || || || | +----+ | L | || +---+ | L | +----+ | || || || 1923 || || || \ / \ || /\ / \ / || || || 1924 || || || ----------- --||----||-- ----------- || || || 1925 || || || || || || || || 1926 || || || CDNI RR || || || || 1927 || || || || CDNI L || || || 1928 || || || || || || || || 1929 || || || ---||----||---- || || || 1930 || || || / \/ || \ || || || 1931 || || || | +----+ || | || || || 1932 || || \\=====CDNI==========>| RR |<=============CDNI========// || || 1933 || || RR | +----+ \/ | RR || || 1934 || || | +----+ | || || 1935 || || | | L | | || || 1936 || || | +----+ | || || 1937 || || | +----+ | || || 1938 || \\=======CDNI===========>| D |<=============CDNI===========// || 1939 || M | +----+ | M || 1940 || | +----+ | || 1941 \\==========CDNI===========>| C |<=============CDNI==============// 1942 C | +----+ | C 1943 \ CDN C / 1944 -------------- 1946 <=CDNI RR=> CDNI Request Routing interface 1947 <=CDNI M==> CDNI Metadata interface 1948 <=CDNI C==> CDNI Control interface 1949 <=CDNI L==> CDNI Logging interface 1951 Figure 14: CDNI Deployment Model: CDN Exchange 1953 Note that a CDN exchange may alternatively support a different set of 1954 functionality (e.g. Logging only, or Logging and full request 1955 routing, or all the functionality of a CDN including content 1956 distribution). All these options are expected to be allowed by the 1957 IETF CDNI specifications. 1959 6. Trust Model 1961 There are a number of trust issues that need to be addressed by a 1962 CDNI solution. Many of them are in fact similar or identical to 1963 those in a simple CDN without interconnection. In a standard CDN 1964 environment (without CDNI), the CSP places a degree of trust in a 1965 single CDN operator to perform many functions. The CDN is trusted to 1966 deliver content with appropriate quality of experience for the end 1967 user. The CSP trusts the CDN operator not to corrupt or modify the 1968 content. The CSP often relies on the CDN operator to provide 1969 reliable accounting information regarding the volume of delivered 1970 content. The CSP may also trust the CDN operator to perform actions 1971 such as timely invalidation of content and restriction of access to 1972 content based on certain criteria such as location of the user and 1973 time of day, and to enforce per-request authorization performed by 1974 the CSP using techniques such as URI signing. 1976 A CSP also places trust in the CDN not to distribute any information 1977 that is confidential to the CSP (e.g., how popular a given piece of 1978 content is) or confidential to the end user (e.g., which content has 1979 been watched by which user). 1981 A CSP does not necessarily have to place complete trust in a CDN. A 1982 CSP will in some cases take steps to protect its content from 1983 improper distribution by a CDN, e.g. by encrypting it and 1984 distributing keys in some out of band way. A CSP also depends on 1985 monitoring (possibly by third parties) and reporting to verify that 1986 the CDN has performed adequately. A CSP may use techniques such as 1987 client-based metering to verify that accounting information provided 1988 by the CDN is reliable. HTTP conditional requests may be used to 1989 provide the CSP with some checks on CDN operation. In other words, 1990 while a CSP may trust a CDN to perform some functions in the short 1991 term, the CSP is able in most cases to verify whether these actions 1992 have been performed correctly and to take action (such as moving the 1993 content to a different CDN) if the CDN does not live up to 1994 expectations. 1996 The main trust issue raised by CDNI is that it introduces transitive 1997 trust. A CDN that has a direct relationship with a CSP can now 1998 "outsource" the delivery of content to another (downstream) CDN. 1999 That CDN may in term outsource delivery to yet another downstream 2000 CDN, and so on. 2002 The top level CDN in such a chain of delegation is responsible for 2003 ensuring that the requirements of the CSP are met. Failure to do so 2004 is presumably just as serious as in the traditional single CDN case. 2005 Hence, an upstream CDN is essentially trusting a downstream CDN to 2006 perform functions on its behalf in just the same way as a CSP trusts 2007 a single CDN. Monitoring and reporting can similarly be used to 2008 verify that the downstream CDN has performed appropriately. However, 2009 the introduction of multiple CDNs in the path between CSP and end 2010 user complicates the picture. For example, third party monitoring of 2011 CDN performance (or other aspects of operation, such as timely 2012 invalidation) might be able to identify the fact that a problem 2013 occurred somewhere in the chain but not point to the particular CDN 2014 at fault. 2016 In summary, we assume that an upstream CDN will invest a certain 2017 amount of trust in a downstream CDN, but that it will verify that the 2018 downstream CDN is performing correctly, and take corrective action 2019 (including potentially breaking off its relationship with that CDN) 2020 if behavior is not correct. We do not expect that the trust 2021 relationship between a CSP and its "top level" CDN will differ 2022 significantly from that found today in single CDN situations. 2023 However, it does appear that more sophisticated tools and techniques 2024 for monitoring CDN performance and behavior will be required to 2025 enable the identification of the CDN at fault in a particular 2026 delivery chain. 2028 We expect that the detailed designs for the specific interfaces for 2029 CDNI will need to take the transitive trust issues into account. For 2030 example, explicit confirmation that some action (such as content 2031 removal) has taken place in a downstream CDN may help to mitigate 2032 some issues of transitive trust. 2034 7. IANA Considerations 2036 This memo includes no request to IANA. 2038 8. Security Considerations 2040 While there is a variety of security issues introduced by a single 2041 CDN, we are concerned here specifically with the additional issues 2042 that arise when CDNs are interconnected. For example, when a single 2043 CDN has the ability to distribute content on behalf of a CSP, there 2044 may be concerns that such content could be distributed to parties who 2045 are not authorized to receive it, and there are mechanisms to deal 2046 with such concerns. Our focus in this section is on how CDN 2047 interconnection introduces new security issues not found in the 2048 single CDN case. 2050 Many of the security issues that arise in CDNI are related to the 2051 transitivity of trust (or lack thereof) described in Section 6. As 2052 noted above, the design of the various interfaces for CDNI must take 2053 account of the additional risks posed by the fact that a CDN with 2054 whom a CSP has no direct relationship is now potentially distributing 2055 content for that CSP. The mechanisms used to mitigate these risks 2056 may be similar to those used in the single CDN case, but their 2057 suitability in this more complex environment must be validated. 2059 Another concern that arises in any CDN is that information about the 2060 behavior of users (what content they access, how much content they 2061 consume, etc.) may be gathered by the CDN. This risk certainly 2062 exists in inter-connected CDNs, but it should be possible to apply 2063 the same techniques to mitigate it as in the single CDN case. 2065 CDNs today offer a variety of means to control access to content, 2066 such as time-of-day restrictions, geo-blocking, and URI signing. 2067 These mechanisms must continue to function in CDNI environments, and 2068 this consideration is likely to affect the design of certain CDNI 2069 interfaces (e.g. metadata, request routing.) 2071 Just as with a single CDN, each peer CDN must ensure that it is not 2072 used as an "open proxy" to deliver content on behalf of a malicious 2073 CSP. Whereas a single CDN typically addresses this problem by having 2074 CSPs explicitly register content (or origin servers) that is to be 2075 served, simply propagating this information to peer downstream CDNs 2076 may be problematic because it reveals more information than the 2077 upstream CDN is willing to specify. (To this end, the content 2078 acquisition step in the earlier examples force the dCDN to retrieve 2079 content from the uCDN rather than go directly to the origin server.) 2081 There are several approaches to this problem. One is for the uCDN to 2082 encoded a signed token generated from a shared secret in each URL 2083 routed to a dCDN, and for the dCDN to validate the request based on 2084 this token. Another one is to have each upstream CDN advertise the 2085 set of CDN-domains they serve, where the downstream CDN checks each 2086 request against this set before caching and delivering the associated 2087 object. Although straightforward, this approach requires operators 2088 to reveal additional information, which may or may not be an issue. 2090 8.1. Security of CDNI Interfaces 2092 It is noted in [I-D.ietf-cdni-requirements] that all CDNI interfaces 2093 must be able to operate securely over insecure IP networks. Since it 2094 is expected that the CDNI interfaces will be implemented using 2095 existing application protocols such as HTTP or XMPP, we also expect 2096 that the security mechanisms available to those protocols may be used 2097 by the CDNI interfaces. Details of how these interfaces are secured 2098 will be specified in the relevant interface documents. 2100 8.2. Digital Rights Management 2102 Issues of digital rights management (DRM, also sometimes called 2103 digital restrictions management) is often employed for content 2104 distributed via CDNs. In general, DRM relies on the CDN to 2105 distribute encrypted content, with decryption keys distributed to 2106 users by some other means (e.g. directly from the CSP to the end 2107 user.) For this reason, DRM is considered out of scope for the CDNI 2108 WG [I-D.ietf-cdni-problem-statement] and does not introduce 2109 additional security issues for CDNI. 2111 9. Contributors 2113 The following individuals contributed to this document: 2115 o Francois le Faucheur 2117 o Ben Niven-Jenkins 2119 o David Ferguson 2121 o John Hartman 2123 10. Acknowledgements 2125 We thank Aaron Falk and Huw Jones for their helpful input to the 2126 draft. 2128 11. Informative References 2130 [I-D.ietf-cdni-problem-statement] 2131 Niven-Jenkins, B., Faucheur, F., and N. Bitar, "Content 2132 Distribution Network Interconnection (CDNI) Problem 2133 Statement", draft-ietf-cdni-problem-statement-01 (work in 2134 progress), October 2011. 2136 [I-D.ietf-cdni-requirements] 2137 Leung, K. and Y. Lee, "Content Distribution Network 2138 Interconnection (CDNI) Requirements", 2139 draft-ietf-cdni-requirements-01 (work in progress), 2140 October 2011. 2142 [I-D.ietf-cdni-use-cases] 2143 Bertrand, G., Emile, S., Watson, G., Burbridge, T., 2144 Eardley, P., and K. Ma, "Use Cases for Content Delivery 2145 Network Interconnection", draft-ietf-cdni-use-cases-00 2146 (work in progress), September 2011. 2148 [I-D.previdi-cdni-footprint-advertisement] 2149 Previdi, S., Faucheur, F., Faucheur, L., and J. Medved, 2150 "CDNI Footprint Advertisement", 2151 draft-previdi-cdni-footprint-advertisement-00 (work in 2152 progress), October 2011. 2154 [I-D.seedorf-alto-for-cdni] 2155 Seedorf, J., "ALTO for CDNi Request Routing", 2156 draft-seedorf-alto-for-cdni-00 (work in progress), 2157 October 2011. 2159 [I-D.vandergaast-edns-client-subnet] 2160 Contavalli, C., Gaast, W., Leach, S., and D. Rodden, 2161 "Client subnet in DNS requests", 2162 draft-vandergaast-edns-client-subnet-00 (work in 2163 progress), January 2011. 2165 [I-D.xiaoyan-cdni-request-routing-protocol] 2166 He, X., Li, J., Dawkins, S., and G. Chen, "Request Routing 2167 Protocol for CDN Interconnection", 2168 draft-xiaoyan-cdni-request-routing-protocol-00 (work in 2169 progress), October 2011. 2171 [RFC3466] Day, M., Cain, B., Tomlinson, G., and P. Rzewski, "A Model 2172 for Content Internetworking (CDI)", RFC 3466, 2173 February 2003. 2175 [RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic 2176 Optimization (ALTO) Problem Statement", RFC 5693, 2177 October 2009. 2179 Authors' Addresses 2181 Bruce Davie (editor) 2182 Cisco Systems, Inc. 2183 1414 Mass. Ave. 2184 Boxborough, MA 01719 2185 USA 2187 Email: bsd@cisco.com 2188 Larry Peterson (editor) 2189 Verivue, Inc. 2190 2 Research Way 2191 Princeton, NJ 2192 USA 2194 Phone: +1 978 303 8032 2195 Email: lpeterson@verivue.com