Internet Draft T. Herbert
<draft-herbert-gue-03.txt> Google
Category: Standard track L. Yong
Expires September 2015 Huawei USA
O. Zia
Microsoft
March 6, 2015
Generic UDP Encapsulation
<draft-herbert-gue-03.txt>
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Abstract
This specification describes Generic UDP Encapsulation (GUE), which
is a scheme for using UDP to encapsulate packets of arbitrary IP
protocols for transport across layer 3 networks. By encapsulating
packets in UDP, specialized capabilities in networking hardware for
efficient handling of UDP packets can be leveraged. GUE specifies
basic encapsulation methods upon which higher level constructs, such
tunnels and overlay networks for network virtualization, can be
constructed. GUE is extensible by allowing optional data fields as
part of the encapsulation, and is generic in that it can encapsulate
packets of various IP protocols.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Packet formats . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. GUE header preamble . . . . . . . . . . . . . . . . . . . . 4
2.2. GUE header . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Flags and optional fields . . . . . . . . . . . . . . . . . 6
2.4 Private data . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Message types . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Control messages . . . . . . . . . . . . . . . . . . . . . 7
3.2. Data messages . . . . . . . . . . . . . . . . . . . . . . . 8
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Network tunnel encapsulation . . . . . . . . . . . . . . . 9
4.2. Transport layer encapsulation . . . . . . . . . . . . . . . 9
4.3. Encapsulator operation . . . . . . . . . . . . . . . . . . 9
4.4. Decapsulator operation . . . . . . . . . . . . . . . . . . 9
4.5. Router and switch operation . . . . . . . . . . . . . . . . 10
4.6. Middlebox interactions . . . . . . . . . . . . . . . . . . 10
4.7. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.8. Checksum Handling . . . . . . . . . . . . . . . . . . . . . 11
4.8.1. Checksum requirements . . . . . . . . . . . . . . . . . 11
4.8.2. GUE header checksum . . . . . . . . . . . . . . . . . . 11
4.8.3. UDP Checksum with IPv4 . . . . . . . . . . . . . . . . 11
4.8.4. UDP Checksum with IPv6 . . . . . . . . . . . . . . . . 12
4.9. MTU and fragmentation issues . . . . . . . . . . . . . . . 13
4.10 Congestion control . . . . . . . . . . . . . . . . . . . . 13
5. Inner flow identifier properties . . . . . . . . . . . . . . . 13
5.1. Flow classification . . . . . . . . . . . . . . . . . . . . 13
5.2. Inner flow identifier properties . . . . . . . . . . . . . 14
6. Motivation for GUE . . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 16
7.1. GUE security fields . . . . . . . . . . . . . . . . . . . . 17
7.2. GUE and IPsec . . . . . . . . . . . . . . . . . . . . . . . 17
8. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
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10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A: NIC processing for GUE . . . . . . . . . . . . . . . . 19
A.1. Receive multi-queue . . . . . . . . . . . . . . . . . . . . 20
A.2. Checksum offload . . . . . . . . . . . . . . . . . . . . . 20
A.2.1. Transmit checksum offload . . . . . . . . . . . . . . . 20
A.2.2. Receive checksum offload . . . . . . . . . . . . . . . 21
A.3. Transmit Segmentation Offload . . . . . . . . . . . . . . . 22
A.4. Large Receive Offload . . . . . . . . . . . . . . . . . . . 22
Appendix B: Privileged ports . . . . . . . . . . . . . . . . . . . 23
Appendix C: Inner flow identifier as a route selector . . . . . . 23
Appendix D: Hardware protocol implementation considerations . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
This specification describes Generic UDP Encapsulation (GUE) which is
a general method for encapsulating packets of arbitrary IP protocols
within User Datagram Protocol (UDP) [RFC0768] packets. Encapsulating
packets in UDP facilitates efficient transport across networks.
Networking devices widely provide protocol specific processing and
optimizations for UDP (as well as TCP) packets. Packets for atypical
IP protocols (those not usually parsed by networking hardware) can be
encapsulated in UDP packets to maximize deliverability and to
leverage flow specific mechanisms for routing and packet steering.
GUE provides an extensible header format for including optional data
in the encapsulation header. This data potentially covers items such
as virtual networking identifier, security data for validating or
authenticating the GUE header, congestion control data, etc. GUE also
allows private optional data in the encapsulation header. This
feature can be used by a site or implementation to define local
custom optional data, and allows experimentation of options that may
eventually become standard.
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2. Packet formats
A GUE packet is comprised of a UDP packet whose payload is a GUE
header followed by a payload which is either an encapsulated packet
of some IP protocol or a control message (like an OAM message). A GUE
packet has the general format:
+-------------------------------+
| |
| UDP/IP header |
| |
|-------------------------------|
| |
| GUE Header |
| |
|-------------------------------|
| |
| Encapsulated packet |
| or control message |
| |
+-------------------------------+
The GUE header is variable length as determined by the presence of
optional fields.
2.1. GUE header preamble
The first byte of the GUE header provides the GUE protocol version
number, indicator of a control or data message, and header length:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Ver|C| Hlen |
+-+-+-+-+-+-+-+-+
Contents are:
o Ver: GUE protocol version. The rest of the fields after the
preamble are defined based on the version. This field is two
bits allowing four possible values.
o Control flag: When set indicates a control message, not set
indicates a data message.
o Hlen: Length in 32-bit words of the GUE header, including
optional fields but not the first four bytes of the header.
Computed as (header_len - 4) / 4. All GUE headers are a multiple
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of four bytes in length. Maximum header length is 132 bytes.
2.2. GUE header
The header format for version 0x0 of GUE in UDP 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source port | Destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0x0|C| Hlen | Proto/ctype | Flags |E|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension flags (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Extension fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Private data (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the UDP header are:
o Source port (inner flow identifier): This should be set to a
value that represents the encapsulated flow. The properties of
the inner flow identifier are described below.
o Destination port: The GUE assigned port number, 6080.
o Length: Canonical length of the UDP packet (length of UDP header
and payload).
o Checksum: Standard UDP checksum.
The GUE header consists of:
o Preamble byte: Version number (0x0), C bit, and header length.
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o Proto/ctype: When the C bit is set this field contains a control
message type for the payload. When C bit is not set, the field
holds the IP protocol number for the encapsulated packet in the
payload. The control message or encapsulated packet begins at
the offset provided by Hlen.
o Flags. Header flags that may be allocated for various purposes
and may indicate presence of optional fields. Undefined header
flag bits must be set to zero on transmission.
o 'E' Extension flag. Indicates presence of extension flags option
in the optional fields.
o Fields: Optional fields whose presence is indicated by
corresponding flags.
o Extension flags: An optional field indicated by the E bit. This
field provides an additional set of 32 header bit flags for the
header.
o Extension fields: Optional fields whose presence is indicated by
corresponding extension flags.
o Private data: Optional private data. If private data is present
it immediately follows that last field present in the header.
The length of this data is determined by subtracting the
starting offset from the header length.
2.3. Flags and optional fields
Flags and associated optional fields are the primary mechanism of
extensibility in GUE. There are sixteens flag bits in the primary GUE
header with one being reserved to indicate that an optional extension
flags field is present. The extension flags field contains an
additional thirty-two flag bits.
A flag may indicate presence of optional fields. The size of an
optional field indicated by a flag must be fixed.
Flags may be paired together to allow different lengths for an
optional field. For example, if two flag bits are paired, a field may
possibly be three different lengths. Regardless of how flag bits may
be paired, the lengths and offsets of optional fields corresponding
to a set of flags must be well defined.
Optional fields are placed in order of the flags. New flags should be
allocated from high to low order bit contiguously without holes.
Flags allow random access, for instance to inspect the field
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corresponding to the Nth flag bit, an implementation only considers
the previous N-1 flags to determine the offset. Flags after the Nth
flag are not pertinent in calculating the offset of the Nth flag.
Flags (or paired flags) are idempotent such that new flags should not
cause reinterpretation of old flags. Also, new flags should not alter
interpretation of other elements in the GUE header nor how the
message is parsed (for instance, in a data message the proto/ctype
field always holds an IP protocol number as an invariant).
2.4 Private data
An implementation may use private data for its own use. The private
data immediately follows the last field in the GUE header and is not
a fixed length. This data is considered part of the GUE header and
must be accounted for in header length (Hlen). The length of the
private data must be a multiple of four and is determined by
subtracting the offset of private data in the GUE header from the
header length. Specifically:
Private_length = (Hlen * 4) - Length(flags)
Where "Length(flags)" returns the sum of lengths of all the optional
fields present in the GUE header. When there is no private data
present, length of the private data is zero.
The semantics and interpretation of private data are implementation
specific. A encapsulator and decapsulator MUST agree on the meaning
of private data before using it. The private data may be structured
as necessary, for instance it might contain its own set of flags and
optional fields.
If a decapsulator receives a GUE packet with private data, it MUST
validate the private data appropriately. If a decapsulator does not
expect private data from an encapsulator the packet MUST be dropped.
If a decapsulator cannot validate the contents of private data per
the provided semantics the packet MUST also be dropped. An
implementation may place security data in GUE private data which must
be verified for packet acceptance.
3. Message types
3.1. Control messages
Control messages are indicated in the GUE header when the C bit is
set. The payload is interpreted as a control message with type
specified in the proto/ctype field. The format and contents of the
control message are indicated by the type and can be variable length.
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Other than interpreting the proto/ctype field as a control message
type, the meaning and semantics of the rest of the elements in the
GUE header are the same as that of data messages. Forwarding and
routing of control messages should be the same as that of a data
message with the same outer IP and UDP header and GUE flags-- this
ensures that a control message can be created which follows the same
path as a data message.
Control messages can be defined for OAM type messages. For instance,
an echo request and corresponding echo reply message may be defined
to test for liveness.
3.2. Data messages
Data messages are indicated in GUE header with C bit not set. The
payload of a data message is interpreted as an encapsulated packet of
an IP protocol indicated in the proto/ctype field. The packet
immediately follows the GUE header.
Data messages are a primary means of encapsulation and can be used to
create tunnels for overlay networks.
4. Operation
The figure below illustrates the use of GUE encapsulation between two
servers. Sever 1 is sending packets to server 2. An encapsulator
performs encapsulation of packets from server 1. These encapsulated
packets traverse the network as UDP packets. At the decapsulator,
packets are decapsulated and sent on to server 2. Packet flow in the
reverse direction need not be symmetric; GUE encapsulation is not
required in the reverse path.
+---------------+ +---------------+
| | | |
| Server 1 | | Server 2 |
| | | |
+---------------+ +---------------+
| ^
V |
+---------------+ +---------------+ +---------------+
| | | | | |
| Encapsulator |-->| Layer 3 |-->| Decapsulator |
| | | Network | | |
+---------------+ +---------------+ +---------------+
The encapsulator and decapsulator may be co-resident with the
corresponding servers, or may be on separate nodes in the network.
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4.1. Network tunnel encapsulation
Network tunneling can be achieved by encapsulating layer 2 or layer 3
packets. In this case the encapsulator and decapsulator nodes are the
tunnel endpoints. These could be routers that provide network tunnels
on behalf of communicating servers.
4.2. Transport layer encapsulation
When encapsulating layer 4 packets, the encapsulator and decapsulator
should be co-resident with the servers. In this case, the
encapsulation headers are inserted between the IP header and the
transport packet. The addresses in the IP header refer to both the
endpoints of the encapsulation and the endpoints for terminating the
the transport protocol.
4.3. Encapsulator operation
Encapsulators create GUE data messages, set the source port to the
inner flow identifier, set flags and optional fields in the GUE
header, and forward packets to a decapsulator.
An encapsulator may be an end host originating the packets of a flow,
or may be a network device performing encapsulation on behalf of
servers (routers implementing tunnels for instance). In either case,
the intended target (decapsulator) is indicated by the outer
destination IP address.
If an encapsulator is tunneling packets, that is encapsulating
packets of layer 2 or layer 3 protocols (e.g. EtherIP, IPIP, ESP
tunnel mode), it should follow standard conventions for tunneling of
one IP protocol over another. Diffserv interaction with tunnels is
described in [RFC2983], ECN propagation for tunnels is described in
[RFC6040].
4.4. Decapsulator operation
A decapsulator performs decapsulation of GUE packets. A decapsulator
is addressed by the outer destination IP address of a GUE packet.
The decapsulator validates packets, including fields of the GUE
header. If a packet is acceptable, the UDP and GUE headers are
removed and the packet is resubmitted for IP protocol processing or
control message processing if it is a control message.
If a decapsulator receives a GUE packet with an unsupported version,
unknown flag, bad header length (too small for included optional
fields), unknown control message type, or an otherwise malformed
header, it must drop the packet and may log the event. No error
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message is returned back to the encapsulator. Note that set flags in
GUE that are unknown to a decapsulator MUST NOT be ignored. If a GUE
packet is received by a decapsulator with unknown flags, the packet
MUST be dropped.
4.5. Router and switch operation
Routers and switches should forward GUE packets as standard UDP/IP
packets. The outer five-tuple should contain sufficient information
to perform flow classification corresponding to the flow of the inner
packet. A switch should not normally need to parse a GUE header, and
none of the flags or optional fields in the GUE header should affect
routing.
A router should not modify a GUE header when forwarding a packet. It
may encapsulate a GUE packet in another GUE packet, for instance to
implement a network tunnel. In this case the router takes the role of
an encapsulator, and the corresponding decapsulator is the logical
endpoint of the tunnel.
4.6. Middlebox interactions
A middle box may interpret some flags and optional fields of the GUE
header for classification purposes, but is not required to understand
all flags and fields in GUE packets. A middle box should not drop a
GUE packet because there are flags unknown to it. The header length
in the GUE header allows a middlebox to inspect the payload packet
without needing to parse the flags or optional fields.
A middlebox may infer bidirectional connection semantics to a UDP
flow. For instance a stateful firewall may create a five-tuple rule
to match flows on egress, and a corresponding five-tuple rule for
matching ingress packets where the roles of source and destination
are reversed for the IP addresses and UDP port numbers. To operate in
this environment, a GUE tunnel must assume connected semantics
defined by the UDP five tuple and the use of GUE encapsulation must
be symmetric between both endpoints. The source port set in the UDP
header must be the destination port the peer would set for replies.
4.7. NAT
IP address and port translation can be performed on the UDP/IP
headers adhering to the requirements for NAT with UDP [RFC478]. In
the case of stateful NAT, connection semantics must be applied to a
GUE tunnel as described above.
When using transport mode encapsulation and traversing a NAT, the IP
addresses may be changed such that the pseudo header checksum used
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for checksum calculation is modified and the checksum will be found
invalid at the receiver. To compensate for this, A GUE option can be
added which contains the checksum over the source and destination
addresses when the packet is transmitted. Upon receiving this option,
the delta of the pseudo header checksum is computed by subtracting
the checksum over the source and and destination addresses from the
checksum value in the option. The resultant value is then added into
checksum calculation when validating the inner transport checksum.
4.8. Checksum Handling
This section describes the requirements around the UDP checksum and
GUE header checksum. Checksums are an important consideration in that
that they can provide end to end validation and protect against
packet mis-delivery. The latter is allowed by the inclusion of a
pseudo header that covers the IP addresses and UDP ports of the
encapsulating headers.
4.8.1. Checksum requirements
The potential for mis-delivery of packets due to corruption of IP,
UDP, or GUE headers must be considered. One of the following
requirements must be met:
o UDP checksums are enabled (for IPv4 or IPv6).
o The GUE header checksum is used.
o Zero UDP checksums are used in accordance with applicable
requirements in [GREUDP], [RFC6935], and [RFC6936].
4.8.2. GUE header checksum
The GUE header checksum provides a UDP-lite [RFC3828] type of
checksum capability as an optional field of the GUE header. The GUE
header checksum minimally covers the GUE header and a GUE pseudo
header. The GUE pseudo header includes the corresponding IP
addresses as well as the UDP ports of the encapsulating headers.
This checksum should provide adequate protection against address
corruption in IPv6 when the UDP checksum is zero. Additionally, the
GUE checksum provides protection of the GUE header when the UDP
checksum is set to zero with either IPv4 or IPv6. The GUE header
checksum is defined in [GUECSUM].
4.8.3. UDP Checksum with IPv4
For UDP in IPv4, the UDP checksum MUST be processed as specified in
[RFC768] and [RFC1122] for both transmit and receive. An
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encapsulator MAY set the UDP checksum to zero for performance or
implementation considerations. The IPv4 header includes a checksum
which protects against mis-delivery of the packet due to corruption
of IP addresses. The UDP checksum potentially provides protection
against corruption of the UDP header, GUE header, and GUE payload.
Enabling or disabling the use of checksums is a deployment
consideration that should take into account the risk and effects of
packet corruption, and whether the packets in the network are
already adequately protected by other, possibly stronger mechanisms
such as the Ethernet CRC. If an encapsulator sets a zero UDP
checksum for IPv4 it SHOULD use the GUE header checksum as described
in section 4.8.2.
When a decapsulator receives a packet, the UDP checksum field MUST
be processed. If the UDP checksum is non-zero, the decapsulator MUST
verify the checksum before accepting the packet. By default a
decapsulator SHOULD accept UDP packets with a zero checksum. A node
MAY be configured to disallow zero checksums per [RFC1122]; this may
be done selectively, for instance disallowing zero checksums from
certain hosts that are known to be sending over paths subject to
packet corruption. If verification of a non-zero checksum fails, a
decapsulator lacks the capability to verify a non-zero checksum, or
a packet with a zero-checksum was received and the decapsulator is
configured to disallow, the packet MUST be dropped and an event MAY
be logged.
4.8.4. UDP Checksum with IPv6
For UDP in IPv6, the UDP checksum MUST be processed as specified in
[RFC768] and [RFC2460] for both transmit and receive. Unlike IPv4,
there is no header checksum in IPv6 that protects against mis-
delivery due to address corruption. Therefore, when GUE is used over
IPv6, either the UDP checksum must be enabled or the GUE header
checksum must be used. An encapsulator MAY set a zero UDP checksum
for performance or implementation reasons, in which case the GUE
header checksum MUST be used or applicable requirements for using
zero UDP checksums in [GREUDP] MUST be met. If the UDP checksum is
enabled, then the GUE header checksum should not be used since it is
mostly redundant.
When a decapsulator receives a packet, the UDP checksum field MUST
be processed. If the UDP checksum is non-zero, the decapsulator MUST
verify the checksum before accepting the packet. By default a
decapsulator MUST only accept UDP packets with a zero checksum if
the GUE header checksum is used and is verified. If verification of
a non-zero checksum fails, a decapsulator lacks the capability to
verify a non-zero checksum, or a packet with a zero-checksum and no
GUE header checksum was received, the packet MUST be dropped and an
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event MAY be logged.
4.9. MTU and fragmentation issues
Standard conventions for handling of MTU (Maximum Transmission Unit)
and fragmentation in conjunction with networking tunnels
(encapsulation of layer 2 or layer 3 packets) should be followed.
Details are described in MTU and Fragmentation Issues with In-the-
Network Tunneling [RFC4459]
If a packet is fragmented before encapsulation in GUE, all the
related fragments must be encapsulated using the same source port
(inner flow identifier). An operator may set MTU to account for
encapsulation overhead and reduce the likelihood of fragmentation.
4.10 Congestion control
Per requirements of [RFC5405], if the IP traffic encapsulated with
GUE implements proper congestion control no additional mechanisms
should be required.
In the case that the encapsulated traffic does not implement any or
sufficient control, or it is not known rather a transmitter will
consistently implement proper congestion control, then congestion
control at the encapsulation layer must be provided. Note this case
applies to a significant use case in network virtualization in which
guests run third party networking stacks that cannot be implicitly
trusted to implement conformant congestion control.
Out of band mechanisms such as rate limiting, Managed Circuit
Breaker, or traffic isolation may used to provide rudimentary
congestion control. For finer grained congestion control that allow
alternate congestion control algorithms, reaction time within an
RTT, and interaction with ECN, in band mechanisms may warranted.
DCCP may be used to provide congestion control for encapsulated
flows. In this case, the protocol stack for an IP tunnel may be IP-
GUE-DCCP-IP. Alternatively, GUE can be extended to include
congestion control (related data carried in GUE optional fields).
Congestion control mechanisms for GUE will be elaborated in other
specifications.
5. Inner flow identifier properties
5.1. Flow classification
A major objective of using GUE is that a network device can perform
flow classification corresponding to the flow of the inner
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encapsulated packet based on the contents in the outer headers.
Hardware devices commonly perform hash computations on packet
headers to classify packets into flows or flow buckets. Flow
classification is done to support load balancing (statistical
multiplexing) of flows across a set of networking resources.
Examples of such load balancing techniques are Equal Cost Multipath
routing (ECMP), port selection in Link Aggregation, and NIC device
Receive Side Scaling (RSS). Hashes are usually either a three-tuple
hash of IP protocol, source address, and destination address; or a
five-tuple hash consisting of IP protocol, source address,
destination address, source port, and destination port. Typically,
networking hardware will compute five-tuple hashes for TCP and UDP,
but only three-tuple hashes for other IP protocols. Since the five-
tuple hash provides more granularity, load balancing can be finer
grained with better distribution. When a packet is encapsulated with
GUE, the source port in the outer UDP packet is set to reflect the
flow of the inner packet. When a device computes a five-tuple hash
on the outer UDP/IP header of a GUE packet, the resultant value
classifies the packet per its inner flow.
To support flow classification, the source port of the UDP header in
GUE is set to a value that maps to the inner flow. This is referred
to as the inner flow identifier. The inner flow identifier is set by
the encapsulator; it can be computed on the fly based on packet
contents or retrieved from a state maintained for the inner flow.
Examples of deriving an inner flow identifier are:
o If the encapsulated packet is a layer 4 packet, TCP/IPv4 for
instance, the inner flow identifier could be based on the
canonical five-tuple hash of the inner packet.
o If the encapsulated packet is an AH transport mode packet with
TCP as next header, the inner flow identifier could be a hash
over a three-tuple: TCP protocol and TCP ports of the
encapsulated packet.
o If a node is encrypting a packet using ESP tunnel mode and GUE
encapsulation, the inner flow identifier could be based on the
contents of clear-text packet. For instance, a canonical five-
tuple hash for a TCP/IP packet could be used.
5.2. Inner flow identifier properties
The inner flow identifier is the value set in the UDP source
port of a GUE packet. The inner flow identifier should adhere to
the following properties:
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o The value set in the source port should be within the ephemeral
port range. IANA suggests this range to be 49152 to 65535, where
the high order two bits of the port are set to one. This
provides fourteen bits of entropy for the inner flow identifier.
o The inner flow identifier should have a uniform distribution
across encapsulated flows.
o An encapsulator may occasionally change the inner flow
identifier used for an inner flow per its discretion (for
security, route selection, etc). Changing the value should
happen no more than once every thirty seconds.
o Decapsulators, or any networking devices, should not attempt any
interpretation of the inner flow identifier, nor should they
attempt to reproduce any hash calculation. They may use the
value to match further receive packets for steering decisions,
but cannot assume that the hash uniquely or permanently
identifies a flow.
o Input to the inner flow identifier is not restricted to ports
and addresses; input could include flow label from an IPv6
packet, SPI from an ESP packet, or other flow related state in
the encapsulator that is not necessarily conveyed in the packet.
o The assignment function for inner flow identifiers should be
randomly seeded to mitigate denial of service attacks. The seed
may be changed periodically.
6. Motivation for GUE
This section presents the motivation for GUE with respect to other
encapsulation methods.
A number of different encapsulation techniques have been proposed for
the encapsulation of one protocol over another. EtherIP [RFC3378]
provides layer 2 tunneling of Ethernet frames over IP. GRE [RFC2784],
MPLS [RFC4023], and L2TP [RFC2661] provide methods for tunneling
layer 2 and layer 3 packets over IP. NVGRE [NVGRE] and VXLAN
[RFC7348] are proposals for encapsulation of layer 2 packets for
network virtualization. IPIP [RFC2003] and Generic packet tunneling
in IPv6 [RFC2473] provide methods for tunneling IP packets over IP.
Several proposals exist for encapsulating packets over UDP including
ESP over UDP [RFC3948], TCP directly over UDP [TCPUDP], VXLAN, LISP
[RFC6830] which encapsulates layer 3 packets, and Generic UDP
Encapsulation for IP Tunneling (GRE over UDP)[GREUDP]. Generic UDP
tunneling [GUT] is a proposal similar to GUE in that it aims to
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tunnel packets of IP protocols over UDP.
GUE has the following discriminating features:
o UDP encapsulation leverages specialized network device
processing for efficient transport. The semantics for using the
UDP source port as an identifier for an inner flow are defined.
o GUE permits encapsulation of arbitrary IP protocols, which
includes layer 2 3, and 4 protocols. This potentially allows
nearly all traffic within a data center to be normalized to be
either TCP or UDP on the wire.
o Multiple protocols can be multiplexed over a single UDP port
number. This is in contrast to techniques to encapsulate
protocols over UDP using a protocol specific port number (such
as ESP/UDP, GRE/UDP, SCTP/UDP). GUE provides a uniform and
extensible mechanism for encapsulating all IP protocols in UDP
with minimal overhead (four bytes of additional header).
o GUE is extensible. New flags and optional fields can be defined.
o The GUE header includes a header length field. This allows a
network node to inspect an encapsulated packet without needing
to parse the full encapsulation header.
o Private data in the encapsulation header allows local
customization and experimentation while being compatible with
processing in network nodes (routers and middleboxes).
o GUE includes both data messages (encapsulation of packets) and
control messages (such as OAM).
7. Security Considerations
Encapsulation of IP protocols within GUE should not increase
security risk, nor provide additional security in itself. As
suggested in section 5 the source port for of UDP packets in GUE
should be randomly seeded to mitigate some possible denial
service attacks.
GUE is most useful when it is in the outermost header of a
packet which allows for flow hash calculation as well as making
GUE header data (such as virtual network identifier) visible to
switches and middleboxes. GUE must be amenable to encapsulating
(and being encapsulated within) IPsec. Also, we allow provisions
to secure the GUE header itself without external protocol.
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Security for Generic UDP Encapsulation is described in more
detail in [GUESEC].
7.1. GUE security fields
Security fields should be used to provide integrity and
authentication of the GUE header. Security negotiation
(algorithms, interpretation of security field, key management,
etc.) is expected to be done out of band between hosts.
7.2. GUE and IPsec
GUE may be used to encapsulate IPsec packets. This allows the
benefits of deriving a flow hash for the inner, potentially
encrypted, packet. In this case the protocol stack may be:
+-------------------------------+
| |
| UDP/IP header |
| |
|-------------------------------|
| |
| GUE Header |
| |
|-------------------------------|
| |
| ESP/AH/private security |
| |
|-------------------------------|
| |
| Encapsulated packet |
| |
+-------------------------------+
Note that IPsec would not cover the GUE header in this case
(does not authenticate it for instance). GUE security optional
fields may be used to provide authentication or integrity of the
GUE header.
8. IANA Consideration
A user UDP port number assignment for GUE has been assigned:
Service Name: gue
Transport Protocol(s): UDP
Assignee: Tom Herbert <therbert@google.com>
Contact: Tom Herbert <therbert@google.com>
Description: Generic UDP Encapsulation
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Reference: draft-herbert-gue
Port Number: 6080
Service Code: N/A
Known Unauthorized Uses: N/A
Assignment Notes: N/A
9. Acknowledgements
The authors would like to thank David Liu for valuable input on
this draft.
10. References
10.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434, October 1998.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983,
October 2000.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936,
April 2013.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
10.2. Informative References
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC 3948, January
2005.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830, January 2013.
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[RFC3378] Housley, R. and S. Hollenbeck, "EtherIP: Tunneling Ethernet
Frames in IP Datagrams", RFC 3378, September 2002.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed., "Encapsulating
MPLS in IP or Generic Routing Encapsulation (GRE)", RFC 4023, March
2005.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661,
August 1999.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram Protocol (UDP-
Lite)", RFC 3828, July 2004, <http://www.rfc-
editor.org/info/rfc3828>.
[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>.
[NVGRE] NVGRE: Network Virtualization using Generic Routing
Encapsulation draft-sridharan-virtualization-nvgre-03
[TCPUDP] Encapsulation of TCP and other Transport Protocols over UDP
draft-cheshire-tcp-over-udp-00
[GREUDP] Generic UDP Encapsulation for IP Tunneling draft-yong-tsvwg-
gre-in-udp-encap-02
[GUESEC] Yong, L., Herbert, T., "Generic UDP Encapsulation (GUE) for
Secure Transport", draft-hy-gue-4-secure-transport-00, work in
progress.
[GUT] Generic UDP Tunnelling (GUT) draft-manner-tsvwg-gut-02.txt
[REMCSUM] Remote Checksum Offload draft-herbert-remotecsumoffload-00
Appendix A: NIC processing for GUE
This appendix provides some guidelines for Network Interface Cards
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(NICs) to implement common offloads and accelerations to support GUE.
Note that most of this discussion is generally applicable to other
methods of UDP based encapsulation.
A.1. Receive multi-queue
Contemporary NICs support multiple receive descriptor queues (multi-
queue). Multi-queue enables load balancing of network processing for
a NIC across multiple CPUs. On packet reception, a NIC must select
the appropriate queue for host processing. Receive Side Scaling is a
common method which uses the flow hash for a packet to index an
indirection table where each entry stores a queue number. Flow
Director and Accelerated Receive Flow Steering (aRFS) allow a host to
program the queue that is used for a given flow which is identified
either by an explicit five-tuple or by the flow's hash.
GUE encapsulation should be compatible with multi-queue NICs that
support five-tuple hash calculation for UDP/IP packets as input to
RSS. The inner flow identifier (source port) ensures classification
of the encapsulated flow even in the case that the outer source and
destination addresses are the same for all flows (e.g. all flows are
going over a single tunnel).
By default, UDP RSS support is often disabled in NICs to avoid out of
order reception that can occur when UDP packets are fragmented. As
discussed above, fragmentation of GUE packets should be mitigated by
fragmenting packets before entering a tunnel, path MTU discovery in
higher layer protocols, or operator adjusting MTUs. Other UDP traffic
may not implement such procedures to avoid fragmentation, so enabling
UDP RSS support in the NIC should be a considered tradeoff during
configuration.
A.2. Checksum offload
Many NICs provide capabilities to calculate standard ones complement
payload checksum for packets in transmit or receive. When using GUE
encapsulation there are at least two checksums that may be of
interest: the encapsulated packet's transport checksum, and the UDP
checksum in the outer header.
A.2.1. Transmit checksum offload
NICs may provide a protocol agnostic method to offload transmit
checksum (NETIF_F_HW_CSUM in Linux parlance) that can be used with
GUE. In this method the host provides checksum related parameters in
a transmit descriptor for a packet. These parameters include the
starting offset of data to checksum, the length of data to checksum,
and the offset in the packet where the computed checksum is to be
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written. The host initializes the checksum field to pseudo header
checksum.
In the case of GUE, the checksum for an encapsulated transport layer
packet, a TCP packet for instance, can be offloaded by setting the
appropriate checksum parameters.
NICs typically can offload only one transmit checksum per packet, so
simultaneously offloading both an inner transport packet's checksum
and the outer UDP checksum is likely not possible. In this case
setting UDP checksum to zero (per above discussion) and offloading
the inner transport packet checksum might be acceptable.
If an encapsulator is co-resident with a host, then checksum offload
may be performed using remote checksum offload [REMCSUM]. Remote
checksum offload relies on NIC offload of the simple UDP/IP checksum
which is commonly supported even in legacy devices. In remote
checksum offload the outer UDP checksum is set and the GUE header
includes an option indicating the start and offset of the inner
"offloaded" checksum. The inner checksum is initialized to the pseudo
header checksum. When a decapsulator receives a GUE packet with the
remote checksum offload option, it completes the offload operation by
determining the packet checksum from the indicated start point to the
end of the packet, and then adds this into the checksum field at the
offset given in the option. Computing the checksum from the start to
end of packet is efficient if checksum-complete is provided on the
receiver.
A.2.2. Receive checksum offload
GUE is compatible with NICs that perform a protocol agnostic receive
checksum (CHECKSUM_COMPLETE in Linux parlance). In this technique, a
NIC computes a ones complement checksum over all (or some predefined
portion) of a packet. The computed value is provided to the host
stack in the packet's receive descriptor. The host driver can use
this checksum to "patch up" and validate any inner packet transport
checksum, as well as the outer UDP checksum if it is non-zero.
Many legacy NICs don't provide checksum-complete but instead provide
an indication that a checksum has been verified (CHECKSUM_UNNECESSARY
in Linux). Usually, such validation is only done for simple TCP/IP or
UDP/IP packets. If a NIC indicates that a UDP checksum is valid, the
checksum-complete value for the UDP packet is the "not" of the pseudo
header checksum. In this way, checksum-unnecessary can be converted
to checksum-complete. So if the NIC provides checksum-unnecessary for
the outer UDP header in an encapsulation, checksum conversion can be
done so that the checksum-complete value is derived and can be used
by the stack to validate an checksums in the encapsulated packet.
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A.3. Transmit Segmentation Offload
Transmit Segmentation Offload (TSO) is a NIC feature where a host
provides a large (>MTU size) TCP packet to the NIC, which in turn
splits the packet into separate segments and transmits each one. This
is useful to reduce CPU load on the host.
The process of TSO can be generalized as:
- Split the TCP payload into segments which allow packets with
size less than or equal to MTU.
- For each created segment:
1. Replicate the TCP header and all preceding headers of the
original packet.
2. Set payload length fields in any headers to reflect the
length of the segment.
3. Set TCP sequence number to correctly reflect the offset of
the TCP data in the stream.
4. Recompute and set any checksums that either cover the payload
of the packet or cover header which was changed by setting a
payload length.
Following this general process, TSO can be extended to support TCP
encapsulation in GUE. For each segment the Ethernet, outer IP, UDP
header, GUE header, inner IP header if tunneling, and TCP headers are
replicated. Any packet length header fields need to be set properly
(including the length in the outer UDP header), and checksums need to
be set correctly (including the outer UDP checksum if being used).
To facilitate TSO with GUE it is recommended that optional fields
should not contain values that must be updated on a per segment
basis-- for example the GUE fields should not include checksums,
lengths, or sequence numbers that refer to the payload. If the GUE
header does not contain such fields then the TSO engine only needs to
copy the bits in the GUE header when creating each segment and does
not need to parse the GUE header.
A.4. Large Receive Offload
Large Receive Offload (LRO) is a NIC feature where packets of a TCP
connection are reassembled, or coalesced, in the NIC and delivered to
the host as one large packet. This feature can reduce CPU utilization
in the host.
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LRO requires significant protocol awareness to be implemented
correctly and is difficult to generalize. Packets in the same flow
need to be unambiguously identified. In the presence of tunnels or
network virtualization, this may require more than a five-tuple match
(for instance packets for flows in two different virtual networks may
have identical five-tuples). Additionally, a NIC needs to perform
validation over packets that are being coalesced, and needs to
fabricate a single meaningful header from all the coalesced packets.
The conservative approach to supporting LRO for GUE would be to
assign packets to the same flow only if they have identical five-
tuple and were encapsulated the same way. That is the outer IP
addresses, the outer UDP ports, GUE protocol, GUE flags and fields,
and inner five tuple are all identical.
Appendix B: Privileged ports
Using the source port to contain an inner flow identifier value
disallows the security method of a receiver enforcing that the source
port be a privileged port. Privileged ports are defined by some
operating systems to restrict source port binding. Unix, for
instance, considered port number less than 1024 to be privileged.
Enforcing that packets are sent from a privileged port is widely
considered an inadequate security mechanism and has been mostly
deprecated. To approximate this behavior, an implementation could
restrict a user from sending a packet destined to the GUE port
without proper credentials.
Appendix C: Inner flow identifier as a route selector
An encapsulator generating an inner flow identifier may modulate the
value to perform a type of multipath source routing. Assuming that
networking switches perform ECMP based on the flow hash, a sender can
affect the path by altering the inner flow identifier. For instance,
a host may store a flow hash in its PCB for an inner flow, and may
alter the value upon detecting that packets are traversing a lossy
path. Changing the inner flow identifier for a flow should be subject
to hysteresis (at most once every thirty seconds) to limit the number
of out of order packets.
Appendix D: Hardware protocol implementation considerations
A low level protocol, such is GUE, is likely interesting to being
supported by high speed network devices. Variable length header (VLH)
protocols like GUE are often considered difficult to efficiently
implement in hardware. In order to retain the important
characteristics of an extensible and robust protocol, hardware
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vendors may practice "constrained flexibility". In this model, only
certain combinations or protocol header parameterizations are
implemented in hardware fast path. Each such parameterization is
fixed length so that the particular instance can be optimized as a
fixed length protocol. In the case of GUE this constitutes specific
combinations of GUE flags, fields, and next protocol. The selected
combinations would naturally be the most common cases which form the
"fast path", and other combinations are assumed to take the "slow
path".
In time, needs and requirements of the protocol may change which may
manifest themselves as new parameterizations to be supported in the
fast path. To allow allow this extensibility, a device practicing
constrained flexibility should allow the fast path parameterizations
to be programmable.
Authors' Addresses
Tom Herbert
Google
1600 Amphitheatre Parkway
Mountain View, CA
US
EMail: tom@herbertland.com
Lucy Yong
Huawei USA
5340 Legacy Dr.
Plano, TX 75024
US
Osama Zia
Microsoft
osamaz@microsoft.com
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