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INTERNET-DRAFT                                                T. Herbert
Intended Status: Experimental                                     Google
Expires: June 2015                                      December 1, 2014


                   Remote checksum offload for VXLAN
                       draft-herbert-vxlan-rco-00


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Abstract

   This specification describes remote checksum offload for VXLAN.
   Remote checksum offload is a mechanism that provides checksum offload
   of transport checksums in encapsulated packets using rudimentary
   offload capabilities found in most Network Interface Card (NIC)
   devices. The outer UDP checksum is enabled on transmit and, with some
   additional meta data, a receiver is able to deduce the checksum to be
   set in an encapsulated packet. Effectively this offloads the
   computation of the inner checksum which can be a significant
   performance optimization. Enabling the UDP checksum has the
   additional advantage that it covers more of the packet including the
   IP pseudo header and virtual network identifier.


Table of Contents

   1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2 Remote checksum offload for VXLAN  . . . . . . . . . . . . . . .  3
     2.1 Header format  . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2 Transmitter operation  . . . . . . . . . . . . . . . . . . .  4
     2.3 Receiver operation . . . . . . . . . . . . . . . . . . . . .  4
   3  Security Considerations . . . . . . . . . . . . . . . . . . . .  6
   4  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  6
   5  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1  Normative References  . . . . . . . . . . . . . . . . . . .  6
     5.2  Informative References  . . . . . . . . . . . . . . . . . .  6
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  6























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1 Introduction

   Remote checksum offload is a mechanism that uses rudimentary NIC
   offload features to support offloading checksum calculation of
   encapsulated packets. The background and motivation for remote
   checksum offload is presented in [RCO].

   In this specification we describe remote checksum offload for VXLAN
   [RFC7348]. In this design the UDP [RFC0768] checksum is enabled on
   transmit, and optional data conveyed in the VXLAN header specifies
   the location of the checksum field being offloaded and its starting
   point for computation. Upon receipt, after the UDP checksum is
   verified, the receiver sets the offloaded checksum field per the
   computed packet checksum and the data in the header.

   This design should also be compatible with VXLAN-GPE [VXLANGPE].

2 Remote checksum offload for VXLAN

   This section describes remote checksum offload for VXLAN.

2.1 Header format

   The remote checksum data is encoded within the eight reserved bits of
   the VXLAN header that follow the VNI. A flag bit is allocated to
   indicate the presence of the remote checksum data.

   VXLAN header with remote checksum data:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|R|R|R|I|R|R|R|R|R|C|              Reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           VXLAN Network Identifier (VNI)      |O| Csum start  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o C bit: Remote checksum offload bit. When set indicates that the
     remote checksum offload data is present.

   o O bit: Offset bit. Indicates the checksum offset relative to
     checksum start. Two offsets are supported corresponding to TCP
     [RFC0793] and UDP [RFC0768].

        O = 1 indicates checksum offset is checksum start + 6 (UDP)

        O = 0 indicates checksum offset is checksum start + 16 (TCP)




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   o Csum start: Checksum start divided by two. Checksum start is
     relative to the the first byte of the encapsulated packet. Note
     that only even offsets are supported and that the maximum value is
     254. This typically refers to the offset of a transport header.

2.2 Transmitter 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.

   2) VXLAN header is added to the packet to do encapsulation. If the
      transport checksum is for UDP or TCP, checksum start is even, and
      checksum start relative to start of the payload is <= 254, then
      remote checksum offload may be used. To set remote checksum
      offload the C bit is set, the O bit is set for a UDP offset or
      cleared for a TCP offset, and checksum start value divided by two
      is set in the csum start field.

   3) Encapsulation layer arranges for NIC checksum offload of the outer
      UDP header checksum. This supersedes the settings to offload the
      inner packet's transport checksum.

   4) Packet is sent to the NIC. The NIC will perform transmit checksum
      offload and set the checksum field in the outer UDP header. The
      inner header and rest of the packet are transmitted without
      modification.

2.3 Receiver operation

   The typical actions a VXLAN receiver does to support remote checksum
   offload are:

   1) Receive packet and validate outer checksum following normal
      processing (ie. 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 or
      checksum-unnecessary conversion can be done.

   3) If the C bit is set, remote checksum offload is enabled. Checksum
      start is csum start value times two. If O bit is set then checksum
      offset is checksum start + 6, else it is checksum start + 16.




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   4) 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. The result is the deduced checksum to
      set in the checksum field of the encapsulated transport packet.

   5) Write the resultant checksum value into the packet at the offset
      provided by checksum offset.

   6) Adjust the packet checksum to account for changing the checksum
      field within the packet.

   7) 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.

   Steps 3,4,5, and 6 in pseudo code:

        packet_csum: checksum computed by receiver covering the start
          of the packet (outer IP header) to the end of the packet

        start_of_packet: memory address of start of packet

        offset_encap_payload: offset of encapsulation payload relative
          to start_of_packet

        csum_start: value of csum start field

        o_bit: value of the O bit

        checksum(start, len): function to compute checksum from start
          address for len bytes

        // Derive the start and offset values
        start = csum_start * 2
        if (o_bit)
          offset = start + 6
        else
          offset = start + 16

        // Compute packet checksum starting from checksum start value
        // (1's complement arithmetic)
        csum = packet_csum - checksum(start_of_packet,
                                      offset_encap_payload + start)

        // Set derived checksum in the checksum field
        old = *(start_of_packet + offset_encap_payload + offset)
        *(start_of_packet + offset_encap_payload + offset) = csum




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        // Adjust packet checksum (1's complement arithmetic)
        packet_csum += (csum - old)

3  Security Considerations

      Remote checksum offload should not impact protocol security.

4  IANA Considerations

      There are no IANA considerations in this specification. Remote
      checksum offload requires a one VXLAN reserved bit and use of the
      eight reserved bits after the VNI.

5  References

5.1  Normative References

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, August 2014, <http://www.rfc-
              editor.org/info/rfc7348>.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

5.2  Informative References

   [RCO]      Herbert T., "Remote checksum offload", draft-herbert-
              remotecsumoffload-00.

   [VXLANGPE] Quinn P. and et al.,  "Generic Protocol Extension for
              VXLAN", draft-quinn-vxlan-gpe-03.txt

Authors' Addresses


   Tom Herbert
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA
   EMail: therbert@google.com





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