[Tsvwg] draft-ietf-tsvwg-udp-lite and link CRC requirements
Dr G Fairhurst <gorry@erg.abdn.ac.uk> Wed, 13 November 2002 17:14 UTC
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Date: Wed, 13 Nov 2002 17:11:45 +0000
From: Dr G Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: [Tsvwg] draft-ietf-tsvwg-udp-lite and link CRC requirements
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I read draft-ietf-tsvwg-udp-lite last night, just to check I understood exactly what were the implications of using UDPLite. On page 5, in the section marked lower layer considerations, the current draft says: "Link layers that do not support partial checksums SHOULD detect errors in the entire frame". "In general, lower layers SHOULD detect errors at least in the sensitive part of the frame using strong error detection mechanisms, but do need not do so for the insensitive part." - Two points struck me: (i) Shouldn't this be more precise in exactly what should be covered. I think it shoudl be: IP header (including any options/extension headers) plus UDP Checksum Coverage - is that correct? (ii) This seems a VERY weak recommendation to use link CRC's to protect the protocol headers. I think this is a philosophically bad point to take. We know the UDP checksum is weak against certain patterns of errors and we do expect errors (otherwise why add UDPLite support to the link?), so why not at least say it MUST provide adequate protection (e.g., comparable to CRC-16 - or whatever). The link document from PILC made stronger recommendations: http://www.ietf.org/internet-drafts/draft-ietf-pilc-link-design-12.txt So.... was there a conscious decision to relax the link CRC requirement for UDPLite? If so, can it be explicitly expressed somewhere? Gorry Fairhurst ---- Section 8.3 of draft-ietf-pilc-link-design said (pasted here for convienence): "The TCP [RFC793], UDP [RFC768], ICMP, and IPv4 [RFC791] protocols all use the same simple 16-bit 1's complement checksum algorithm to detect corrupted packets. The IPv4 checksum protects only the IPv4 header, while the TCP, ICMP, and UDP checksums provide end-to-end error detection for both the transport pseudo header (including network and transport layer information) and the transport payload data. Protection of the data is optional for applications using UDP [RFC768]. The Internet checksum is not very strong from a coding theory standpoint, but it is easy to compute in software, and various proposals to replace the Internet checksums with stronger checksums have failed. However, it is known that undetected errors can and do occur in packets received by end hosts [SP2000]. To reduce processing costs, IPv6 has no IP header checksum. The destination host detects "important" errors in the IP header such as the delivery of the packet to the wrong destination. This is done by including the IP source and destination addresses (pseudo header) in the computation of the checksum in the TCP or UDP header, a practice already performed in IPv4. Errors in other IPv6 header fields may go undetected within the network; this was considered a reasonable price to pay for a considerable reduction in the processing required by each router, and it was assumed that subnetworks would use a strong link CRC. One way to provide additional protection for an IPv4 or IPv6 header is by the authentication and packet integrity services of the IP Security (IPSEC) protocol [RFC2401]. However, this may not be a choice available to the subnetwork designer. Most subnetworks implement error detection just above the physical layer. Packets corrupted in transmission are detected and discarded before delivery to the IP layer. A 16-bit cyclic redundancy check (CRC) is usually the minimum. This is significantly more robust against most patterns of errors than the 16-bit Internet checksum. The Point-to-Point Protocol [RFC1662] requires support of a 16-bit CRC for each link frame, with a 32-bit CRC as an option. (PPP is often used in conjunction with a dialup modem, which provides its own error control). Other subnetworks, including 802.3/Ethernet, AAL5/ATM, FDDI, Token Ring and PPP over SONET/SDH all use a 32-bit CRC. Many subnetworks can also use other mechanisms to enhance the error detection capability of the link CRC (e.g., FEC in dialup modems, mobile radio and satellite channels). Any new subnetwork designed to carry IP should therefore provide error detection for each IP packet that is at least as strong as the 32-bit CRC specified in [ISO3309]. While this will achieve a very low undetected packet error rate due to transmission errors, it will not (and need not) achieve a very low packet loss rate as the Internet protocols are better suited to dealing with lost packets than with corrupted packets [SRC81]." _______________________________________________ tsvwg mailing list tsvwg@ietf.org https://www1.ietf.org/mailman/listinfo/tsvwg
- [Tsvwg] draft-ietf-tsvwg-udp-lite and link CRC re… Dr G Fairhurst