Internet Engineering Task Force T. Przygienda INTERNET DRAFT Redback 2 Apr 2002 Optional Checksums in ISIS Status of This Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract This draft describes an optional extension to IS-IS protocol, used today by several ISPs for routing within their clouds. IS-IS is an interior gateway routing protocol developed originally by OSI and used with IP extensions as IGP. IS-IS originally does not provide CSNP and PSNP checksums, relying on the underlying layers to verify the integrity of information provided. Experience with the protocol shows that this precondition does not always hold and scenarios can be imagined that impact protocol functionality. This document introduces a new optional TLV providing checksums. Przygienda et al. Expires 1 Sep 2002 [Page 1] ^L Internet Draft SNP Checksums 2 Apr 2002 1. Introduction IS-IS [ISO90 , Cal90a , Cal90b ] CSNPs and PSNPs and IIHs can be corrupted in case of faulty implementations of L2 hardware or lack of checksuming on a specific network technology. As a particularly ugly case, corruption of length and/or TLV length fields may lead to generation of extensive numbers of "empty" LSPs in the receiving node. Since we cannot rely on authentication as a checksum mechanism, this document proposes an optional TLV to add checksums to the elements. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [Bra97 ]. 2. TLV Description This optional TLV MAY BE included in all CSNP, PSNP and IIH packets and an implementation that implements optional checksums MUST accept PDUs if they do NOT contain the optional checksum. Implementations that receive an optional checksum TLV and support it MUST discard the PDU if the checksum is incorrect. An implementation that does NOT implement optional checksums MUST accept a PDU that contains the checksum TLV. An implementation that supports optional checksums and receives it within any other PDU than CSNP, PSNP or IIH MUST discard the PDU. Such an implementation MUST discard the PDU as well if more than one optional checksum TLVs are included within it. Additionally, any implementation supporting optional checksums MUST accept PDUs with an optional checksum with the value 0 and consider such a checksum as correct. 3. Checksum Computation The checksum is a fletcher checksum computed according to [ISO98 ], Annex C over the complete PDU. To compute the correct checksum, an implementation MUST add the optional checksum TLV to the PDU with the initial checksum value of 0 and compute the checksum over such a PDU. 4. Interaction with TLVs using PDU Data to Compute Signatures The implementation MUST either omit the optional checksum on an interface or send a 0 checksum value if it includes in the PDU signatures that provide equivalen or stronger functionality, such as HMAC or MD5. Otherwise an implementation that handles such Przygienda et al. Expires 1 Sep 2002 [Page 2] ^L Internet Draft SNP Checksums 2 Apr 2002 signatures but does not handle the optional checksums, may fail to compute the MD5 signature on the packet. Such a failure would be caused by the fact that MD5 is computed with checksum value set to 0 and only as final step the checksum value is being filled in. 5. TLV Format [Prz01 ] lists the according value of the TLV type and discusses issues surrounding the assignment of new TLV codepoints. 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TLV Type =12 | TLV Length =2 | Checksum (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 6. Acknowledgments Tony Li mentioned the original problem. Mike Shand provided comments. Somehow related problems with purging on LSP checksum errors have been observed by others before. Nischal Sheth spelled out the issues of interaction between MD5 and the optional checksums. 7. Security Consideration ISIS security applies to the work presented. No specific security issues as to the new element are known. References [Bra97] S. Bradner. Key Words for Use in RFCs to Indicate Requirement Levels, RFC 2119. Internet Engineering Task Force, March 1997. [Cal90a] R. Callon. OSI ISIS Intradomain Routing Protocol. RFC 1142, Internet Engineering Task Force, February 1990. [Cal90b] R. Callon. Use of OSI ISIS for Routing in TCP/IP and Dual Environments. RFC 1195, Internet Engineering Task Force, December 1990. Przygienda et al. Expires 1 Sep 2002 [Page 3] ^L Internet Draft SNP Checksums 2 Apr 2002 [ISO90] ISO. Information Technology - Telecommunications and Information Exchange between Systems - Intermediate System to Intermediate System Routing Exchange Protocol for Use in Conjunction with the Protocol for Providing the Connectionless-Mode Network Service. ISO, 1990. [ISO98] ISO. Information Technology - Protocol for Providing the Connectionless-Mode Network Service: Protocol Specification. ISO, 1998. [Prz01] T. Przygienda. Reserved TLV Codepoints in ISIS. work-in-progress, Internet Engineering Task Force, July 2001. 8. Authors' Addresses Tony Przygienda Redback 350 Holger Way San Jose, CA 95134-1362 (408) 548 9416 prz@redback.com 9. Full Copyright Statement Copyright (C) The Internet Society 2000. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. Przygienda et al. Expires 1 Sep 2002 [Page 4] ^L Internet Draft SNP Checksums 2 Apr 2002 The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Przygienda et al. Expires 1 Sep 2002 [Page 5]