INTERNET-DRAFT T. Herbert Intended Status: Informational Facebook February 29, 2016 Remote checksum offload for encapsulation draft-herbert-remotecsumoffload-02 Abstract This document describes remote checksum offload for encapsulation, which is a mechanism that provides checksum offload of encapsulated packets using rudimentary offload capabilities found in most Network Interface Card (NIC) devices. The outer header checksum e.g. that in UDP or GRE) is enabled in packets and, with some additional meta information, a receiver is able to deduce the checksum to be set for an inner encapsulated packet. Effectively this offloads the computation of the inner checksum. Enabling the outer checksum in encapsulation has the additional advantage that it covers more of the packet than the inner checksum including the encapsulation headers. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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 and License Notice Herbert Expires September 1, 2016 [Page 1] INTERNET DRAFT Remote Checksum Offload February 29, 2016 Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. 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Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Checksum offload background . . . . . . . . . . . . . . . . . . 3 2.1 The Internet checksum . . . . . . . . . . . . . . . . . . . 3 2.2 Transmit checksum offload . . . . . . . . . . . . . . . . . 4 2.2.1 Generic transmit offload . . . . . . . . . . . . . . . . 4 2.2.2 Local checksum offload . . . . . . . . . . . . . . . . . 4 2.2.3 Protocol specific transmit offload . . . . . . . . . . . 5 2.3 Receive checksum offload . . . . . . . . . . . . . . . . . . 5 2.3.1 CHECKSUM_COMPLETE . . . . . . . . . . . . . . . . . . . 6 2.3.2 CHECKSUM_UNNECESSARY . . . . . . . . . . . . . . . . . . 6 3.0 Remote checksum offload . . . . . . . . . . . . . . . . . . 6 3.1 Option format . . . . . . . . . . . . . . . . . . . . . . . 6 3.2 Transmit operation . . . . . . . . . . . . . . . . . . . . . 7 3.3 Receiver operation . . . . . . . . . . . . . . . . . . . . . 8 3.4 Interaction with TCP segmentation offload . . . . . . . . . 9 4 Security Considerations . . . . . . . . . . . . . . . . . . . . 9 5 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9 6 References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1 Normative References . . . . . . . . . . . . . . . . . . . 9 6.2 Informative References . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Herbert Expires September 1, 2016 [Page 2] INTERNET DRAFT Remote Checksum Offload February 29, 2016 1 Introduction Checksum offload is a capability of NICs where the checksum calculation for a transport layer packet (TCP, UDP, etc.) is performed by a device on behalf of the host stack. Checksum offload is applicable to both transmit and receive, where on transmit the device writes the computed checksum into the packet, and on receive the device provides the computed checksum of the packet or an indication that specific transport checksums were validated. This feature saves CPU cycles in the host and has become ubiquitous in modern NICs. A host may both source transport packets and encapsulate them for transit over an underlying network. In this case checksum offload is still desirable, but now must be done on an encapsulated packet. Many deployed NICs are only capable of providing checksum offload for simple TCP or UDP packets. Such NICs typically use protocol specific mechanisms where they must parse headers in order to perform checksum calculations. Updating these NICs to perform checksum offload for encapsulation requires new parsing logic which is likely infeasible or at cost prohibitive. In this specification we describe an alternative that uses rudimentary NIC offload features to support offloading checksum calculation of encapsulated packets. In this design, the outer checksum is enabled on transmit, and meta information indicating the location of the checksum field being offloaded and its starting point for computation are sent with a packet. On receipt, after the outer checksum is verified, the receiver sets the offloaded checksum field per the computed packet checksum and the meta data. 2 Checksum offload background In this section we provide some background into checksum offload operation. 2.1 The Internet checksum The Internet checksum [RFC0791] is used by several Internet protocols including IP [RFC1122], TCP [RFC0793], UDP [RFC0768] and GRE [RFC2784]. Efficient checksum calculation is critical to good performance [RFC1071], and the mathematical properties are useful in incrementally updating checksums [RFC1624]. An early approach to implementing checksum offload in hardware is described in [RFC1936]. TCP and UDP checksums cover a pseudo header which is composed of the source and destination addresses of the corresponding IP packet, upper layer packet length, and protocol. The checksum pseudo header Herbert Expires September 1, 2016 [Page 3] INTERNET DRAFT Remote Checksum Offload February 29, 2016 is defined in [RFC0768] and [RFC0793] for IPv4, and in [RFC2460] for IPv6. 2.2 Transmit checksum offload In transmit checksum offload, a host network stack defers the calculation and setting of a transport checksum in the packet to the device. A device may provide checksum offload only for specific protocols, or may provide a generic interface. In either case, support for only one offloaded checksum per packet is typical. When using transmit checksum offload, a host stack must initialize the checksum field in the packet. This is done by setting to zero (GRE) or to the bitwise not of the pseudo header (UDP or TCP). The device proceeds by computing the packet checksum from the start of the transport header through to the end of the packet. The bitwise not of the resulting value is written in the checksum field of the transport packet. 2.2.1 Generic transmit offload A device can provide a generic interface for transmit checksum offload. Checksum offload is enabled by setting two fields in the transmit descriptor for a packet: start offset and checksum offset. The start offset indicates the byte in the packet where the checksum calculation should start. The checksum offset indicates the offset in the packet where the checksum value is to be written. The generic interface is protocol agnostic, however only supports one offloaded checksum per packet. While it is conceivable that a NIC could provide offload for more checksums by defining more than one checksum start/offset pair in the transmit descriptor, a more general and efficient solution is Local Checksum Offload. 2.2.2 Local checksum offload Local Checksum Offload [LCO] (or LCO) is a technique for efficiently computing the outer checksum of an encapsulated datagram when the inner checksum is due to be offloaded. The ones-complement sum of a correctly checksummed TCP or UDP packet is equal to the sum of the pseudo header, since everything else gets 'cancelled out' by the checksum field. This property holds since the sum was complemented before being written to the checksum field. More generally, this holds in any case where the Internet one's complement checksum is used, and thus any checksum that generic transmit offload supports. That is, if we have set up transmit checksum offload with a start/offset pair, we know that after the device has filled in that checksum the one's complement sum from checksum start to the end of Herbert Expires September 1, 2016 [Page 4] INTERNET DRAFT Remote Checksum Offload February 29, 2016 the packet will be equal to whatever value is set in the checksum field beforehand. This property allows computing the outer checksum without considering at the payload per the algorithm: 1) Compute the checksum from the outer packet's checksum start offset to the inner packet's checksum start offset. 2) Add the bit-wise not of the pseudo header checksum for the inner packet. 3) The result is the checksum from the outer packet's start offset to the end of the packet. Taking into account the pseudo header for the outer checksum allows the outer checksum field to be set without offload processing. Step 1) requires that some checksum calculation is performed on the host stack, however this is only done over some portion of packet headers which is typically much smaller than the payload of the packet. LCO can be used for nested encapsulations; in this case, the outer encapsulation layer will sum over both its own header and the 'middle' header. Thus, if the device has the capability to offload an inner checksum in encapsulation, any number of outer checksums can be efficiently calculated using this technique. 2.2.3 Protocol specific transmit offload Some devices support transmit checksum offload for very specific protocols. For instance, many legacy devices can only perform checksum offload for UDP/IP and TCP/IP packets. These devices parse transmitted packets in order to determine the checksum start and checksum offset. They may also ignore the value in the checksum field by setting it to zero for checksum computation and computing the checksum of the pseudo header themselves. Protocol specific transmit offload is limited to the protocols a device supports. To support checksum offload of an encapsulated packet, a device must be a able to parse the encapsulation layer in order to locate the inner packet. 2.3 Receive checksum offload Upon receiving a packet, a device may perform a checksum calculation over the packet or part of the packet depending on the protocol. A result of this calculation is returned in the meta data of the receive descriptor for the packet. The host stack can apply the result in verifying checksums as it processes the packet. The intent Herbert Expires September 1, 2016 [Page 5] INTERNET DRAFT Remote Checksum Offload February 29, 2016 is that the offload will obviate the need for the networking stack to perform its own checksum calculation over the packet. There are two basic methods of receive checksum offload: CHECKSUM_COMPLETE and CHECKSUM_UNNECESSARY. 2.3.1 CHECKSUM_COMPLETE A device may calculate the checksum of a whole packet (layer 2 payload) and return the resultant value to the host stack. The host stack can subsequently use this value to validate checksums in the packet. As the packet is parsed through various layers, the calculated checksum is updated to correspond to each layer (subtract out checksum for preceding bytes for a given header). CHECKSUM_COMPLETE is protocol agnostic and does not require any protocol awareness in the device. It works for any encapsulation and supports an arbitrary number of checksums in the packet. 2.3.2 CHECKSUM_UNNECESSARY A device may explicitly validate a checksum in a packet and return a flag in the receive descriptor that a transport checksum has been verified (host performing checksum computation is unnecessary). Some devices may be capable of validating more than one checksum in the packet, in which case the device returns a count of the number verified. Typically, only a positive signal is returned, if the device was unable to validate a checksum it does not return any information and the host will generally perform its own checksum computation. If a device returns a count of validations, this must refer to consecutive checksums that are present and validated in a packet (checksums cannot be skipped). CHECKSUM_UNNECESSARY is protocol specific, for instance in the case of UDP or TCP a device needs to consider the pseudo header in checksum validation. To support checksum offload of an encapsulated packet, a device must be able to parse the encapsulation layer in order to locate the inner packet. 3.0 Remote checksum offload This section describes the remote checksum offload mechanism. This is primarily useful with UDP based encapsulation where the UDP checksum is enabled (not set to zero on transmit). The same technique could be applied to GRE encapsulation where the GRE checksum is enabled. 3.1 Option format Herbert Expires September 1, 2016 [Page 6] INTERNET DRAFT Remote Checksum Offload February 29, 2016 Remote checksum offload requires the sending of optional data with an encapsulated packet. This data is a pair of checksum start and checksum offset values. More than one offloaded checksum could be supported if multiple pairs are sent. The logical data format for remote checksum offload is: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum start | Checksum offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o Checksum start: starting offset for checksum computation relative to the start of the encapsulated packet. This is typically the offset of a transport header (e.g. UDP or TCP). o Checksum offset: Offset relative to the start of the encapsulated packet where the derived checksum value is to be written. This typically is the offset of the checksum field in the transport header (e.g. UDP or TCP). Support for remote checksum offload with specific encapsulation protocols is outside the scope of this document, however any encapsulation format that supports some reasonable form of optional meta data should be amenable. In Generic UDP Encapsulation [GUE] this would entail defining an optional field, in Geneve [GENEVE] a TLV would be defined, for NSH [NSH] the meta data can either be in a service header or within a TLV. In any scenario, what the offsets in the meta data are relative to must be unambiguous. 3.2 Transmit operation The typical actions to set remote checksum offload on transmit are: 1) Transport layer creates a packet and indicates in internal packet meta data that checksum is to be offloaded to the NIC (normal transport layer processing for checksum offload). The checksum field is populated with the bitwise not of the checksum of the pseudo header or zero as appropriate. 2) Encapsulation layer adds its headers to the packet including the offload meta data. The start offset and checksum offset are set accordingly. 3) Encapsulation layer arranges for checksum offload of the outer header checksum (e.g. UDP). Herbert Expires September 1, 2016 [Page 7] INTERNET DRAFT Remote Checksum Offload February 29, 2016 4) Packet is sent to the NIC. The NIC will perform transmit checksum offload and set the checksum field in the outer header. The inner header and rest of the packet are transmitted without modification. 3.3 Receiver operation The typical actions a host receiver does to support remote checksum offload are: 1) Receive packet and validate outer checksum following normal processing (e.g. validate non-zero UDP checksum). 2) Deduce full checksum for the IP packet. This is directly provided if device returns the packet checksum in CHECKSUM_COMPLETE. If the device returned CHECKSUM_UNNECESSARY, then the complete checksum can be trivially derived as either zero (GRE) or the bitwise not of the outer pseudo header (UDP). 3) From the packet checksum, subtract the checksum computed from the start of the packet (outer IP header) to the offset in the packet indicted by checksum start in the meta data. The result is the deduced checksum to set in the checksum field of the encapsulated transport packet. In pseudo code: csum: initialized to checksum computed from start (outer IP header) to the end of the packet start_of_packet: address of start of packet encap_payload_offset: relative to start_of_packet csum_start: value from meta data checksum(start, len): function to compute checksum from start address for len bytes csum -= checksum(start_of_packet, encap_payload_offset + csum_start) 4) Write the resultant checksum value into the packet at the offset provided by checksum offset in the meta data. In pseudo code: csum_offset: offset of checksum field *(start_of_packet + encap_payload_offset + csum_offset) = csum Herbert Expires September 1, 2016 [Page 8] INTERNET DRAFT Remote Checksum Offload February 29, 2016 5) Checksum is verified at the transport layer using normal processing. This should not require any checksum computation over the packet since the complete checksum has already been provided. 3.4 Interaction with TCP segmentation offload Remote checksum offload may be useful with TCP Segmentation Offload (TSO) in order to avoid host checksum calculations at the receiver. This can be implemented on a transmitter as follows: 1) Host stack prepares a large segment for transmission including adding of encapsulation headers and the remote checksum option which refers to the encapsulated transport checksum in the large segment. 2) TSO is performed by the device taking encapsulation into account. The outer checksum is computed and written for each packet. The inner checksum is not computed, and the encapsulation header (including checksum meta data) is replicated for each packet. 3) At the receiver remote checksum offload processing occurs as normal for each packet. 4 Security Considerations Remote checksum offload should not impact protocol security. 5 IANA Considerations There are no IANA considerations in this specification. The remote checksum offload meta data may require an option number or type in specific encapsulation formats that support it. 6 References 6.1 Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. Herbert Expires September 1, 2016 [Page 9] INTERNET DRAFT Remote Checksum Offload February 29, 2016 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. 6.2 Informative References [RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the Internet checksum", RFC1071, September 1988. [RFC1624] Rijsinghani, A., Ed., "Computation of the Internet Checksum via Incremental Update", RFC1624, May 1994. [RFC1936] Touch, J. and B. Parham, "Implementing the Internet Checksum in Hardware", RFC1936, April 1996. [GUE] Herbert, T., Yong, L, and Zia, O., "Generic UDP Encapsulation". draft-ietf-nvo3-gue-02 [GENEVE] Gross, J. and Gango, I., "Geneve: Generic Network Virtualization Encapsulation", draft-ietf-nvo3-geneve-01, January 1, 2016 [NSH] Quinn, P. and Elzur, U., "Network Service Header", draft- ietf-sfc-nsh-02.txt, January 19,2016 [LOC] Cree, E. Checksum Offloads in the Linux Networking Stack, Linux documentation: Documentation/networking/checksum-offloads.txt Authors' Addresses Tom Herbert Facebook 1 Hacker Way Menlo Park, CA US EMail: tom@herbertland.com Herbert Expires September 1, 2016 [Page 10]