INTERNET-DRAFT T. Herbert
Intended Status: Experimental Google
Expires: May 2015 November 12, 2014
Remote checksum offload for encapsulation
draft-herbert-remotecsumoffload-01
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Abstract
This specification describes remote checksum offload, which 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 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 in an 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.
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 Protocol specific transmit offload . . . . . . . . . . . 4
2.3 Receive checksum offload . . . . . . . . . . . . . . . . . . 5
2.3.1 Checksum-complete . . . . . . . . . . . . . . . . . . . 5
2.3.2 Checksum-unnecessary . . . . . . . . . . . . . . . . . . 5
2.3.3 Checksum-unnecessary conversion . . . . . . . . . . . . 6
3 Remote checksum offload . . . . . . . . . . . . . . . . . . . . 6
3.1 Meta data format . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Transmitter operation . . . . . . . . . . . . . . . . . . . 6
3.3 Receiver operation . . . . . . . . . . . . . . . . . . . . . 7
3.4 Interaction with TCP segmentation offload . . . . . . . . . 8
4 Remote checksum offload for Generic UDP Encapsulation . . . . . 8
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 9
6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
7 References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Normative References . . . . . . . . . . . . . . . . . . . 10
7.2 Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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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,
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layer 4 packet length, and protocol. The checksum pseudo header is
defined in [RFC0768] and [RFC0793] for IPv4, and in [RFC2460] for
IPv6.
2.2 Transmit checksum offload
In transmit checksum offload, a host networking 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, 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 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. It is conceivable that a NIC could
provide offload for more checksums by defining more than one
checksum start, checksum offset pair in the transmit descriptor.
2.2.2 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
pseudo header checksum 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.
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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
is that the offload will obviate the need for the networking stack to
perform its own checksum calculation for 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.
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2.3.3 Checksum-unnecessary conversion
If a device returns checksum-unnecessary for a non-zero checksum, the
checksum-complete value can easily be derived as the bitwise "not" of
the pseudo header checksum. This is useful in the case that the
device has verified the outermost checksum of the packet, and there
are checksums in an encapsulated packet to be verified.
3 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 Meta data format
Remote checksum offload requires the sending of meta 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.
Remote checksum offload will typically be implemented as a remote
checksum option in the encapsulation headers. Any encapsulation
format that allows optional data for extensibility should be able to
support remote checksum offload. The format of the remote checksum
offload option is diagrammed below.
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 payload. This is
typically the offset of a transport header (e.g. UDP or TCP).
o Checksum offset: Offset where the derived checksum value is to
be written relative to the start of encapsulated payload. This
typically is the offset of the checksum field in the transport
header (e.g. UDP or TCP).
3.2 Transmitter operation
The typical actions to set remote checksum offload on transmit are:
1) Transport layer creates a packet and indicates in internal packet
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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
remote checksum offload option. The start offset and checksum
offset are set accordingly.
3) Encapsulation layer arranges for checksum offload of the outer
header checksum (e.g. UDP). 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 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 or
checksum-unnecessary conversion can be done.
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 remote checksum offload option.
The result is the deduced checksum to set in the checksum field of
the encapsulated transport packet.
4) Write the resultant checksum value into the packet at the offset
provided by checksum offset in the remote checksum offload option.
5) Adjust the packet checksum to account for changing the checksum
field within the packet.
6) 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, and 5 in pseudo code:
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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, csum_offset: values from remote checksum offload
option
checksum(start, len): function to compute checksum from start
address for len bytes
// Compute packet checksum starting from checksum start value
// (1's complement arithmetic)
csum -= checksum(start_of_packet,
offset_encap_payload + csum_start)
// Set derived checksum in the checksum field
old = *(start_of_packet + offset_encap_payload + csum_offset)
*(start_of_packet + offset_encap_payload + csum_offset) = csum
// Adjust packet checksum (1's complement arithmetic)
packet_csum += (csum - old)
3.4 Interaction with TCP segmentation offload
Remote checksum offload may be useful with TCP Segmentation Offload
(TSO) in order to avoid host checksum calculation at the receiver.
This can be implemented on a transmitter as follows:
1) Host stack prepares a large segment for transmission including
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 Remote checksum offload for Generic UDP Encapsulation
Remote checksum offload in Generic UDP Encapsulation [GUE] is
supported with the addition of a remote checksum option. The GUE
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header format below illustrates remote checksum option as a private
field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source port | Destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0x0|C| Hlen | Proto/ctype | Flags |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Private flags(optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum start | Checksum offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Private fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Pertinent fields are described below:
o Hlen: GUE header length. The offset of the encapsulated payload
is Hlen * 4 + 4.
o P bit: Set to one to indicate presence of private options
o R bit: Private flag bit that indicates presence of the remote
checksum option. Remote checksum offload is four bytes in length
o Checksum start: Offset of start of checksum computation for
remote checksum offload. This is relative to the encapsulated
payload whose offset is provided by Hlen.
o Checksum offset: Offset to write the checksum which is computed
by the receiver. This is relative to the encapsulated payload
whose offset is provided by Hlen.
5 Security Considerations
Remote checksum offload should not impact protocol security.
6 IANA Considerations
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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.
7 References
7.1 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.
[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.
7.2 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] Generic UDP Encapsulation draft-herbert-gue-02
Authors' Addresses
Tom Herbert
Google
1600 Amphitheatre Parkway
Mountain View, CA
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EMail: therbert@google.com
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