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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 6253 (Obsoleted by RFC 8002) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Host Identity Protocol T. Heer 3 Internet-Draft Albstadt-Sigmaringen University 4 Obsoletes: 6253 (if approved) S. Varjonen 5 Updates: 7401 (if approved) University of Helsinki 6 Intended status: Standards Track April 22, 2016 7 Expires: October 24, 2016 9 Host Identity Protocol Certificates 10 draft-ietf-hip-rfc6253-bis-08 12 Abstract 14 The Certificate (CERT) parameter is a container for digital 15 certificates. It is used for carrying these certificates in Host 16 Identity Protocol (HIP) control packets. This document specifies the 17 certificate parameter and the error signaling in case of a failed 18 verification. Additionally, this document specifies the 19 representations of Host Identity Tags in X.509 version 3 (v3). 21 The concrete use cases of certificates, including how certificates 22 are obtained, requested, and which actions are taken upon successful 23 or failed verification, are specific to the scenario in which the 24 certificates are used. Hence, the definition of these scenario- 25 specific aspects is left to the documents that use the CERT 26 parameter. 28 This document updates RFC7401 and obsoletes RFC6253. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on October 24, 2016. 47 Copyright Notice 48 Copyright (c) 2016 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 1. Introduction 63 Digital certificates bind pieces of information to a public key by 64 means of a digital signature, and thus, enable the holder of a 65 private key to generate cryptographically verifiable statements. The 66 Host Identity Protocol (HIP) [RFC7401] defines a new cryptographic 67 namespace based on asymmetric cryptography. The identity of each 68 host is derived from a public key, allowing hosts to digitally sign 69 data and issue certificates with their private key. This document 70 specifies the CERT parameter, which is used to transmit digital 71 certificates in HIP. It fills the placeholder specified in 72 Section 5.2 of [RFC7401], and thus, updates [RFC7401]. 74 1.1. Requirements Language 76 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 77 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 78 "OPTIONAL" in this document are to be interpreted as described in RFC 79 2119 [RFC2119]. 81 2. CERT Parameter 83 The CERT parameter is a container for certain types of digital 84 certificates. It does not specify any certificate semantics. 85 However, it defines supplementary parameters that help HIP hosts to 86 transmit semantically grouped CERT parameters in a more systematic 87 way. The specific use of the CERT parameter for different use cases 88 is intentionally not discussed in this document. Hence, the use of 89 the CERT parameter will be defined in the documents that use the CERT 90 parameter. 92 The CERT parameter is covered and protected, when present, by the HIP 93 SIGNATURE field and is a non-critical parameter. 95 The CERT parameter can be used in all HIP packets. However, using it 96 in the first Initiator (I1) packet is NOT RECOMMENDED because it can 97 increase the processing times of I1s, which can be problematic when 98 processing storms of I1s. Each HIP control packet MAY contain 99 multiple CERT parameters each carrying one certificate. These 100 parameters MAY be related or unrelated. Related certificates are 101 managed in CERT groups. A CERT group specifies a group of related 102 CERT parameters that SHOULD be interpreted in a certain order (e.g., 103 for expressing certificate chains). Ungrouped certificates exhibit a 104 unique CERT group field and set the CERT count to 1. CERT parameters 105 with the same group number in the CERT group field indicate a logical 106 grouping. The CERT count field indicates the number of CERT 107 parameters in the group. 109 CERT parameters that belong to the same CERT group MAY be contained 110 in multiple sequential HIP control packets. This is indicated by a 111 higher CERT count than the amount of CERT parameters with matching 112 CERT group fields in a HIP control packet. The CERT parameters MUST 113 be placed in ascending order, within a HIP control packet, according 114 to their CERT group field. CERT groups MAY only span multiple 115 packets if the CERT group does not fit the packet. A HIP packet MUST 116 NOT contain more than one incomplete CERT group that continues in the 117 next HIP control packet. 119 The CERT ID acts as a sequence number to identify the certificates in 120 a CERT group. The numbers in the CERT ID field MUST start from 1 up 121 to CERT count. 123 The CERT Group and CERT ID namespaces are managed locally by each 124 host that sends CERT parameters in HIP control packets. 126 0 1 2 3 127 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 128 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 129 | Type | Length | 130 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 131 | CERT group | CERT count | CERT ID | CERT type | 132 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 133 | Certificate / 134 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 135 / | Padding (variable length) | 136 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 138 Type 768 139 Length Length in octets, excluding Type, Length, and Padding 140 CERT group Group ID grouping multiple related CERT parameters 141 CERT count Total count of certificates that are sent, possibly 142 in several consecutive HIP control packets. 144 CERT ID The sequence number for this certificate 145 CERT Type Indicates the type of the certificate 146 Padding Any Padding, if necessary, to make the TLV a multiple 147 of 8 bytes. Any added padding bytes MUST be zeroed by 148 the sender, and their values SHOULD NOT be checked by 149 the receiver. 151 The certificates MUST use the algorithms defined in [RFC7401] as the 152 signature and hash algorithms. 154 The following certificate types are defined: 156 +--------------------------------+-------------+ 157 | CERT format | Type number | 158 +--------------------------------+-------------+ 159 | Reserved | 0 | 160 | X.509 v3 | 1 | 161 | Obsoleted | 2 | 162 | Hash and URL of X.509 v3 | 3 | 163 | Obsoleted | 4 | 164 | LDAP URL of X.509 v3 | 5 | 165 | Obsoleted | 6 | 166 | Distinguished Name of X.509 v3 | 7 | 167 | Obsoleted | 8 | 168 +--------------------------------+-------------+ 170 The next sections outline the use of Host Identity Tags (HITs) in 171 X.509 v3. X.509 v3 certificates and the handling procedures are 172 defined in [RFC5280]. The wire format for X.509 v3 is the 173 Distinguished Encoding Rules format as defined in [X.690]. 175 Hash and Uniform Resource Locator (URL) encoding (3) is used as 176 defined in Section 3.6 of [RFC7296]. Using hash and URL encodings 177 results in smaller HIP control packets than by including the 178 certificate(s), but requires the receiver to resolve the URL or check 179 a local cache against the hash. 181 Lightweight Directory Access Protocol (LDAP) URL encoding (5) is used 182 as defined in [RFC4516]. Using LDAP URL encoding results in smaller 183 HIP control packets but requires the receiver to retrieve the 184 certificate or check a local cache against the URL. 186 Distinguished Name (DN) encoding (7) is represented by the string 187 representation of the certificate's subject DN as defined in 188 [RFC4514]. Using the DN encoding results in smaller HIP control 189 packets, but requires the receiver to retrieve the certificate or 190 check a local cache against the DN. 192 3. X.509 v3 Certificate Object and Host Identities 194 If needed, HITs can represent an issuer, a subject, or both in X.509 195 v3. HITs are represented as IPv6 addresses as defined in [RFC7343]. 196 When the Host Identifier (HI) is used to sign the certificate, the 197 respective HIT SHOULD be placed into the Issuer Alternative Name 198 (IAN) extension using the GeneralName form iPAddress as defined in 199 [RFC5280]. When the certificate is issued for a HIP host, identified 200 by a HIT and HI, the respective HIT SHOULD be placed into the Subject 201 Alternative Name (SAN) extension using the GeneralName form 202 iPAddress, and the full HI is presented as the subject's public key 203 info as defined in [RFC5280]. 205 The following examples illustrate how HITs are presented as issuer 206 and subject in the X.509 v3 extension alternative names. 208 Format of X509v3 extensions: 209 X509v3 Issuer Alternative Name: 210 IP Address:hit-of-issuer 211 X509v3 Subject Alternative Name: 212 IP Address:hit-of-subject 214 Example X509v3 extensions: 215 X509v3 Issuer Alternative Name: 216 IP Address:2001:24:6cf:fae7:bb79:bf78:7d64:c056 217 X509v3 Subject Alternative Name: 218 IP Address:2001:2c:5a14:26de:a07c:385b:de35:60e3 220 Appendix A shows a full example X.509 v3 certificate with HIP 221 content. 223 As another example, consider a managed Public Key Infrastructure 224 (PKI) environment in which the peers have certificates that are 225 anchored in (potentially different) managed trust chains. In this 226 scenario, the certificates issued to HIP hosts are signed by 227 intermediate Certification Authorities (CAs) up to a root CA. In 228 this example, the managed PKI environment is neither HIP aware, nor 229 can it be configured to compute HITs and include them in the 230 certificates. 232 When HIP communications are established, the HIP hosts not only need 233 to send their identity certificates (or pointers to their 234 certificates), but also the chain of intermediate CAs (or pointers to 235 the CAs) up to the root CA, or to a CA that is trusted by the remote 236 peer. This chain of certificates SHOULD be sent in a CERT group as 237 specified in Section 2. The HIP peers validate each other's 238 certificates and compute peer HITs based on the certificate public 239 keys. 241 4. Revocation of Certificates 243 Revocation of X.509 v3 certificates is handled as defined in 244 Section 5 of [RFC5280] with two exceptions. First, any HIP 245 certificate serial number that appears on the CRL is treated as 246 invalid regardless of the reason code. Second, the certificateHold 247 is not supported. 249 5. Error Signaling 251 If the Initiator does not send all the certificates that the 252 Responder requires, the Responder may take actions (e.g. reject the 253 connection). The Responder MAY signal this to the Initiator by 254 sending a HIP NOTIFY message with NOTIFICATION parameter error type 255 CREDENTIALS_REQUIRED. 257 If the verification of a certificate fails, a verifier MAY signal 258 this to the provider of the certificate by sending a HIP NOTIFY 259 message with NOTIFICATION parameter error type INVALID_CERTIFICATE. 261 NOTIFICATION PARAMETER - ERROR TYPES Value 262 ------------------------------------ ----- 264 CREDENTIALS_REQUIRED 48 266 The Responder is unwilling to set up an association, 267 as the Initiator did not send the needed credentials. 269 INVALID_CERTIFICATE 50 271 Sent in response to a failed verification of a certificate. 272 Notification Data MAY contain CERT group and CERT ID octet 273 (in this order) of the CERT parameter that caused the 274 failure. 276 6. IANA Considerations 278 The following changes to the "HIP Certificate Types" registry should 279 be made. 281 The references should be updated from [RFC6253] to this document. 283 This document obsoleted the type numbers "2", "4", "6", "8" for 284 the SPKI certificates. 286 7. Security Considerations 287 Certificate grouping allows the certificates to be sent in multiple 288 consecutive packets. This might allow similar attacks, as IP-layer 289 fragmentation allows, for example, the sending of fragments in the 290 wrong order and skipping some fragments to delay or stall packet 291 processing by the victim in order to use resources (e.g., CPU or 292 memory). Hence, hosts SHOULD implement mechanisms to discard 293 certificate groups with outstanding certificates if state space is 294 scarce. 296 Although, CERT parameter is allowed in the first Initiator (I1) 297 packet it is NOT RECOMMENDED because it can increase the processing 298 times of I1s, which can be problematic when processing storms of I1s. 299 Furthermore, Initiator has to take into consideration that the 300 Responder can drop the CERT parameter in I1 without processing the 301 parameter. 303 Checking of the URL and LDAP entries might allow denial-of-service 304 (DoS) attacks, where the target host may be subjected to bogus work. 306 Security considerations for X.509 v3 are discussed in [RFC5280]. 308 8. Differences from RFC 6253 310 This section summarizes the technical changes made from [RFC6253]. 311 This section is informational, intended to help implementors of the 312 previous protocol version. If any text in this section contradicts 313 text in other portions of this specification, the text found outside 314 of this section should be considered normative. 316 The following changes have been made. 318 o Support for Simple Public Key Infrastructure (SPKI) certificates 319 has been removed. 321 9. Acknowledgements 323 The authors would like to thank A. Keranen, D. Mattes, M. Komu and T. 324 Henderson for the fruitful conversations on the subject. D. Mattes 325 most notably contributed the non-HIP aware use case in Section 3. 327 10. References 329 10.1. Normative References 331 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 332 Requirement Levels", BCP 14, RFC 2119, March 1997. 334 [RFC4514] Zeilenga, K., "Lightweight Directory Access Protocol 335 (LDAP): String Representation of Distinguished Names", RFC 336 4514, June 2006. 338 [RFC4516] Smith, M. and T. Howes, "Lightweight Directory Access 339 Protocol (LDAP): Uniform Resource Locator", RFC 4516, June 340 2006. 342 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 343 Housley, R., and W. Polk, "Internet X.509 Public Key 344 Infrastructure Certificate and Certificate Revocation List 345 (CRL) Profile", RFC 5280, May 2008. 347 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 348 Kivinen, "Internet Key Exchange Protocol Version 2 349 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 350 2014, . 352 [RFC7343] Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay 353 Routable Cryptographic Hash Identifiers Version 2 354 (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September 355 2014, . 357 [RFC7401] Moskowitz, R., Heer, T., Jokela, P., and T. Henderson, 358 "Host Identity Protocol Version 2 (HIPv2)", RFC 7401, 359 April 2015. 361 [X.690] ITU-T, , "Recommendation X.690 (2002) | ISO/IEC 362 8825-1:2002, Information Technology - ASN.1 encoding 363 rules: Specification of Basic Encoding Rules (BER), 364 Canonical Encoding Rules (CER) and Distinguished Encoding 365 Rules (DER)", July 2002. 367 10.2. Informative References 369 [RFC6253] Heer, T. and S. Varjonen, "Host Identity Protocol 370 Certificates", RFC 6253, DOI 10.17487/RFC6253, May 2011, 371 . 373 Appendix A. X.509 v3 certificate example 375 This section shows a X.509 v3 certificate with encoded HITs. 377 Certificate: 378 Data: 379 Version: 3 (0x2) 380 Serial Number: 12705268244493839545 (0xb0522e27291b2cb9) 381 Signature Algorithm: sha256WithRSAEncryption 382 Issuer: DC=Example, DC=com, CN=Example issuing host 383 Validity 384 Not Before: Feb 25 11:28:29 2016 GMT 385 Not After : Feb 24 11:28:29 2017 GMT 386 Subject: DC=Example, DC=com, CN=Example issuing host 387 Subject Public Key Info: 388 Public Key Algorithm: rsaEncryption 389 Public-Key: (2048 bit) 390 Modulus: 391 00:c9:b0:85:94:af:1f:3a:77:39:c9:d5:81:a5:ee: 392 d2:b5:6b:72:91:5d:22:2c:1e:59:e5:06:29:bd:a2: 393 19:f6:ac:ca:eb:f7:88:d8:54:55:41:01:58:d8:87: 394 64:d8:c8:cf:6e:c2:38:81:22:1a:ae:e9:a6:80:22: 395 03:ee:f3:1b:7e:68:11:e3:f4:7b:98:33:28:bf:40: 396 ec:4f:19:e8:10:8a:8b:07:60:f7:9f:e4:82:f8:a7: 397 58:04:3d:42:07:c8:34:ca:99:6d:11:eb:73:c1:d9: 398 96:93:55:e5:c7:ed:80:4f:8a:f2:1a:6f:83:c8:15: 399 a4:8f:b8:6a:fe:f3:4f:49:1a:5c:1f:89:bb:30:e6: 400 98:bc:ce:a3:a2:37:85:b1:79:1c:26:e6:44:0c:b9: 401 3e:d8:37:81:46:f4:02:25:46:a2:ea:da:25:5c:46: 402 a2:a3:c5:58:80:53:1f:c5:e5:11:a0:da:d8:f2:ad: 403 d6:98:d4:ce:55:35:cc:0b:d3:5b:09:48:ef:57:65: 404 80:cb:65:79:fd:cb:4d:5b:b3:8d:1a:ff:2a:58:3e: 405 96:65:10:3e:04:81:78:2b:d5:ca:89:78:ea:28:5c: 406 bc:02:4a:54:cd:aa:a9:99:8d:d6:39:e9:5e:a9:73: 407 1a:5d:93:55:39:9b:72:1a:c2:a0:1f:e3:4c:b0:41: 408 98:97 409 Exponent: 65537 (0x10001) 410 X509v3 extensions: 411 X509v3 Subject Alternative Name: 412 IP Address:2001:27:DCFC:CB8:F885:D53F:4E63:48B7 413 X509v3 Issuer Alternative Name: 414 IP Address:2001:2D:F878:64C1:67E3:9716:88BD:68E4 415 Signature Algorithm: sha256WithRSAEncryption 416 6d:e6:a9:a6:30:c4:ab:3e:86:39:1e:de:76:4d:4e:a4:2d:63: 417 4d:bb:41:bf:d3:0c:66:13:8b:4d:b2:50:59:36:fc:ae:42:9e: 418 c8:a0:41:1a:1c:94:56:05:28:82:34:4e:63:75:87:31:25:67: 419 36:a6:1a:0f:b8:f7:db:03:e7:dd:a6:9a:26:c4:68:e2:cf:59: 420 54:e6:ee:cc:a7:ce:fb:56:bf:31:60:f4:cb:e7:f0:0e:50:f8: 421 b7:c5:3c:1a:de:74:d0:aa:83:e5:15:25:b1:bf:be:a4:7f:af: 422 0a:de:08:09:0e:13:1d:2a:3b:1a:99:d9:af:10:fc:08:92:5f: 423 d8:d0:10:d6:b9:0c:86:da:85:3b:44:b5:97:90:10:02:4f:5a: 424 1f:ae:07:30:6b:f5:e6:12:93:72:e2:10:c9:8e:2c:00:8b:d6: 425 f0:05:c3:ff:91:24:69:6d:5b:5a:0c:40:28:01:f2:5b:45:b8: 426 9b:ae:9e:73:e9:dd:83:e0:85:d7:ad:6c:b1:81:ac:a0:30:37: 427 9d:60:bd:92:3b:d2:a1:21:87:8b:c4:d9:5a:5c:21:56:3e:02: 428 7e:f3:6f:a5:de:40:75:80:f5:41:68:5c:b2:61:fb:1d:9a:a5: 429 97:a8:d4:a9:82:45:86:79:3c:63:76:3d:fd:86:a0:f8:14:84: 431 55:c1:8c:fa 433 -----BEGIN CERTIFICATE----- 434 MIIDWTCCAkGgAwIBAgIJALBSLicpGyy5MA0GCSqGSIb3DQEBCwUAME0xFzAVBgoJ 435 kiaJk/IsZAEZFgdFeGFtcGxlMRMwEQYKCZImiZPyLGQBGRYDY29tMR0wGwYDVQQD 436 ExRFeGFtcGxlIGlzc3VpbmcgaG9zdDAeFw0xNjAyMjUxMTI4MjlaFw0xNzAyMjQx 437 MTI4MjlaME0xFzAVBgoJkiaJk/IsZAEZFgdFeGFtcGxlMRMwEQYKCZImiZPyLGQB 438 GRYDY29tMR0wGwYDVQQDExRFeGFtcGxlIGlzc3VpbmcgaG9zdDCCASIwDQYJKoZI 439 hvcNAQEBBQADggEPADCCAQoCggEBAMmwhZSvHzp3OcnVgaXu0rVrcpFdIiweWeUG 440 Kb2iGfasyuv3iNhUVUEBWNiHZNjIz27COIEiGq7ppoAiA+7zG35oEeP0e5gzKL9A 441 7E8Z6BCKiwdg95/kgvinWAQ9QgfINMqZbRHrc8HZlpNV5cftgE+K8hpvg8gVpI+4 442 av7zT0kaXB+JuzDmmLzOo6I3hbF5HCbmRAy5Ptg3gUb0AiVGouraJVxGoqPFWIBT 443 H8XlEaDa2PKt1pjUzlU1zAvTWwlI71dlgMtlef3LTVuzjRr/Klg+lmUQPgSBeCvV 444 yol46ihcvAJKVM2qqZmN1jnpXqlzGl2TVTmbchrCoB/jTLBBmJcCAwEAAaM8MDow 445 GwYDVR0RBBQwEocQIAEAJ9z8DLj4hdU/TmNItzAbBgNVHRIEFDAShxAgAQAt+Hhk 446 wWfjlxaIvWjkMA0GCSqGSIb3DQEBCwUAA4IBAQBt5qmmMMSrPoY5Ht52TU6kLWNN 447 u0G/0wxmE4tNslBZNvyuQp7IoEEaHJRWBSiCNE5jdYcxJWc2phoPuPfbA+fdppom 448 xGjiz1lU5u7Mp877Vr8xYPTL5/AOUPi3xTwa3nTQqoPlFSWxv76kf68K3ggJDhMd 449 KjsamdmvEPwIkl/Y0BDWuQyG2oU7RLWXkBACT1ofrgcwa/XmEpNy4hDJjiwAi9bw 450 BcP/kSRpbVtaDEAoAfJbRbibrp5z6d2D4IXXrWyxgaygMDedYL2SO9KhIYeLxNla 451 XCFWPgJ+82+l3kB1gPVBaFyyYfsdmqWXqNSpgkWGeTxjdj39hqD4FIRVwYz6 452 -----END CERTIFICATE----- 454 Appendix B. Change log 456 Contents of draft-ietf-hip-rfc6253-bis-00: 458 o RFC6253 was submitted as draft-RFC. 460 Changes from version 01 to 02: 462 o Updated the references. 464 Changes from version 02 to 03: 466 o Fixed the nits raised by the working group. 468 Changes from version 03 to 04: 470 o Added "obsoletes RFC 6253". 472 Changes from version 04 to 05: 474 o Updates to contact details. 476 o Correct updates and obsoletes headers. 478 o Removed the pre5378 disclaimer. 480 o Updated references. 482 o Removed the SPKI references from the document. 484 Changes from version 05 to 06: 486 o Addressed the Int-Dir review comments from Korhonen. 488 Changes from version 06 to 07: 490 o Addressed the GenArt, OPSdir, SecDir, and IANA comments. 492 Changes from version 07 to 08: 494 o Addresses one editorial nit for CERT group numbers. 496 Authors' Addresses 498 Tobias Heer 499 Albstadt-Sigmaringen University 500 Poststr. 6 501 72458 Albstadt 502 Germany 504 Email: heer@hs-albsig.de 506 Samu Varjonen 507 University of Helsinki 508 Gustaf Haellstroemin katu 2b 509 00560 Helsinki 510 Finland 512 Email: samu.varjonen@helsinki.fi