Internet Area WG T. Herbert
Internet-Draft Quantonium
Intended status: Standard track L. Yong
Expires November 19, 2017 Huawei USA
O. Zia
Microsoft
May 18, 2017
Generic UDP Encapsulation
draft-ietf-intarea-gue-04
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Abstract
This specification describes Generic UDP Encapsulation (GUE), which
is a scheme for using UDP to encapsulate packets of different 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
as 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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology and acronyms . . . . . . . . . . . . . . . . . 5
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Base packet format . . . . . . . . . . . . . . . . . . . . . . 7
2.1. GUE version . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Version 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Header format . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Proto/ctype field . . . . . . . . . . . . . . . . . . . . . 9
3.2.1 Proto field . . . . . . . . . . . . . . . . . . . . . . 9
3.2.2 Ctype field . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Flags and extension fields . . . . . . . . . . . . . . . . 10
3.3.1. Requirements . . . . . . . . . . . . . . . . . . . . . 10
3.3.2. Example GUE header with extension fields . . . . . . . 11
3.4. Private data . . . . . . . . . . . . . . . . . . . . . . . 12
3.5. Message types . . . . . . . . . . . . . . . . . . . . . . . 12
3.5.1. Control messages . . . . . . . . . . . . . . . . . . . 12
3.5.2. Data messages . . . . . . . . . . . . . . . . . . . . . 13
3.6. Hiding the transport layer protocol number . . . . . . . . 13
4. Version 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Direct encapsulation of IPv4 . . . . . . . . . . . . . . . 14
4.2. Direct encapsulation of IPv6 . . . . . . . . . . . . . . . 14
5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Network tunnel encapsulation . . . . . . . . . . . . . . . 16
5.2. Transport layer encapsulation . . . . . . . . . . . . . . . 16
5.3. Encapsulator operation . . . . . . . . . . . . . . . . . . 16
5.4. Decapsulator operation . . . . . . . . . . . . . . . . . . 17
5.4.1. Processing a received data message . . . . . . . . . . 17
5.4.2. Processing a received control message . . . . . . . . . 18
5.5. Router and switch operation . . . . . . . . . . . . . . . . 18
5.6. Middlebox interactions . . . . . . . . . . . . . . . . . . 18
5.6.1. Inferring connection semantics . . . . . . . . . . . . 19
5.6.2. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.7. Checksum Handling . . . . . . . . . . . . . . . . . . . . . 19
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5.7.1. Requirements . . . . . . . . . . . . . . . . . . . . . 19
5.7.2. UDP Checksum with IPv4 . . . . . . . . . . . . . . . . 20
5.7.3. UDP Checksum with IPv6 . . . . . . . . . . . . . . . . 20
5.8. MTU and fragmentation . . . . . . . . . . . . . . . . . . . 21
5.9. Congestion control . . . . . . . . . . . . . . . . . . . . 21
5.10. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 22
5.11. Flow entropy for ECMP . . . . . . . . . . . . . . . . . . 22
5.11.1. Flow classification . . . . . . . . . . . . . . . . . 22
5.11.2. Flow entropy properties . . . . . . . . . . . . . . . 23
5.12 Negotiation of acceptable flags and extension fields . . . 24
6. Motivation for GUE . . . . . . . . . . . . . . . . . . . . . . 24
6.1. Benefits of GUE . . . . . . . . . . . . . . . . . . . . . . 24
6.2 Comparison of GUE to other encapsulations . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 27
8.1. UDP source port . . . . . . . . . . . . . . . . . . . . . . 27
8.2. GUE version number . . . . . . . . . . . . . . . . . . . . 28
8.3. Control types . . . . . . . . . . . . . . . . . . . . . . . 28
8.4. Flag-fields . . . . . . . . . . . . . . . . . . . . . . . . 28
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
10.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A: NIC processing for GUE . . . . . . . . . . . . . . . . 32
A.1. Receive multi-queue . . . . . . . . . . . . . . . . . . . . 32
A.2. Checksum offload . . . . . . . . . . . . . . . . . . . . . 33
A.2.1. Transmit checksum offload . . . . . . . . . . . . . . . 33
A.2.2. Receive checksum offload . . . . . . . . . . . . . . . 34
A.3. Transmit Segmentation Offload . . . . . . . . . . . . . . . 34
A.4. Large Receive Offload . . . . . . . . . . . . . . . . . . . 35
Appendix B: Implementation considerations . . . . . . . . . . . . 36
B.1. Priveleged ports . . . . . . . . . . . . . . . . . . . . . 36
B.2. Setting flow entropy as a route selector . . . . . . . . . 36
B.3. Hardware protocol implementation considerations . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
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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 the 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.
This document does not define any specific GUE extensions.
[GUEEXTENS] specifies a set of core extensions.
The motivation for the GUE protocol is described in section 6.
1.1. Terminology and acronyms
GUE Generic UDP Encapsulation
GUE Header A variable length protocol header that is composed
of a primary four byte header and zero or more four
byte words for optional header data
GUE packet A UDP/IP packet that contains a GUE header and GUE
payload within the UDP payload
Encapsulator A network node that encapsulates a packet in GUE
Decapsulator A network node that decapsulates and processes
packets encapsulated in GUE
Data message An encapsulated packet in the GUE payload that is
addressed to the protocol stack for an associated
protocol
Control message A formatted message in the GUE payload that is
implicitly addressed to the decapsulator to monitor
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or control the state or behavior of a tunnel
Flags A set of bit flags in the primary GUE header
Extension field
An optional field in a GUE header whose presence is
indicated by corresponding flag(s)
C-bit A single bit flag in the primary GUE header that
indicates whether the GUE packet contains a control
message or data message
Hlen A field in the primary GUE header that gives the
length of the GUE header
Proto/ctype A field in the GUE header that holds either the IP
protocol number for a data message or a type for a
control message
Private data Optional data in the GUE header that can be used for
private purposes
Outer IP header Refers to the outer most IP header or packet when
encapsulating a packet over IP
Inner IP header Refers to an encapsulated IP header when an IP
packet is encapsulated
Outer packet Refers to an encapsulating packet
Inner packet Refers to a packet that is encapsulated
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Base packet format
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 such as an OAM (Operations,
Administration, and Management) 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 extension fields.
2.1. GUE version
The first two bits of the GUE header contain the GUE protocol version
number. The rest of the fields after the GUE version number are
defined based on the version number. Versions 0 and 1 are described
in this specification; versions 2 and 3 are reserved.
3. Version 0
Version 0 of GUE defines a generic extensible format to encapsulate
packets by Internet protocol number.
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3.1. Header format
The header format for version 0 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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
| Length | Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
| 0 |C| Hlen | Proto/ctype | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Extensions Fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Private data (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the UDP header are:
o Source port: If connection semantics (section 5.6.1) are applied
to an encapsulation, this is set to the local source port for
the connection. When connection semantics are not applied, this
is set to a flow entropy value for use with ECMP (Equal-Cost
Mulit-Path [RFC2992]); the properties of flow entropy are
described in section 5.11.
o Destination port: If connection semantics (section 5.6.1) are
applied to an encapsulation, this is set to the destination port
for the tuple. If connection semantics are not applied this is
set to 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 (handling is described in
section 5.7).
The GUE header consists of:
o Ver: GUE protocol version (0).
o C: C-bit: When set indicates a control message, not set
indicates a data message.
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o Hlen: Length in 32-bit words of the GUE header, including
optional extension fields but not the first four bytes of the
header. Computed as (header_len - 4) / 4 where header_len is the
total header length in bytes. All GUE headers are a multiple of
four bytes in length. Maximum header length is 128 bytes.
o Proto/ctype: When the C-bit is set, this field contains a
control message type for the payload (section 3.2.2). When the
C-bit is not set, the field holds the Internet protocol number
for the encapsulated packet in the payload (section 3.2.1). 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 extension fields. Undefined header
flag bits MUST be set to zero on transmission.
o Extension Fields: Optional fields whose presence is indicated by
corresponding flags.
o Private data: Optional private data block (see section 3.4). If
the private block is present, it immediately follows that last
extension field present in the header. The private block is
considered to be part of the GUE header. The length of this data
is determined by subtracting the starting offset from the header
length.
3.2. Proto/ctype field
The proto/ctype fields either contains an Internet protocol number
(when the C-bit is not set) or GUE control message type (when the C-
bit is set).
3.2.1 Proto field
When the C-bit is not set, the proto/ctype field MUST contain an IANA
Internet Protocol Number. The protocol number is interpreted relative
to the IP protocol that encapsulates the UDP packet (i.e. protocol of
the outer IP header). The protocol number serves as an indication of
the type of the next protocol header which is contained in the GUE
payload at the offset indicated in Hlen. Intermediate devices MAY
parse the GUE payload per the number in the proto/ctype field, and
header flags cannot affect the interpretation of the proto/ctype
field.
When the outer IP protocol is IPv4, the proto field MUST be set to a
valid IP protocol number usable with IPv4; it MUST NOT be set to a
number for IPv6 extension headers or ICMPv6 options (number 58). An
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exception is that the destination options extension header using the
PadN option MAY be used with IPv4 as described in section 3.6. The
"no next header" protocol number (59) also MAY be used with IPv4 as
described below.
When the outer IP protocol is IPv6, the proto field can be set to any
defined protocol number except that it MUST NOT be set to Hop-by-hop
options (number 0). If a received GUE packet in IPv6 contains a
protocol number that is an extension header (e.g. Destination
Options) then the extension header is processed after the GUE header
is processed as though the GUE header is an extension header.
IP protocol number 59 ("No next header") can be set to indicate that
the GUE payload does not begin with the header of an IP protocol.
This would be the case, for instance, if the GUE payload were a
fragment when performing GUE level fragmentation. The interpretation
of the payload is performed through other means (such as flags and
extension fields), and intermediate devices MUST NOT parse packets
based on the IP protocol number in this case.
3.2.2 Ctype field
When the C-bit is set, the proto/ctype field MUST be set to a valid
control message type. A value of zero indicates that the GUE payload
requires further interpretation to deduce the control type. This
might be the case when the payload is a fragment of a control
message, where only the reassembled packet can be interpreted as a
control message.
Control messages will be defined in an IANA registry. Control message
types 1 through 127 may be defined in standards. Types 128 through
255 are reserved to be user defined for experimentation or private
control messages.
This document does not specify any standard control message types
other than type 0.
3.3. Flags and extension fields
Flags and associated extension fields are the primary mechanism of
extensibility in GUE. As mentioned in section 3.1, GUE header flags
indicate the presence of optional extension fields in the GUE header.
[GUEXTENS] defines a basic set of GUE extensions.
3.3.1. Requirements
There are sixteen flag bits in the GUE header. Flags may indicate
presence of an extension fields. The size of an extension field
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indicated by a flag MUST be fixed.
Flags can be paired together to allow different lengths for an
extension field. For example, if two flag bits are paired, a field
can possibly be three different lengths-- that is bit value of 00
indicates no field present; 01, 10, and 11 indicate three possible
lengths for the field. Regardless of how flag bits are paired, the
lengths and offsets of optional fields corresponding to a set of
flags MUST be well defined.
Extension fields are placed in order of the flags. New flags are to
be allocated from high to low order bit contiguously without holes.
Flags allow random access, for instance to inspect the field
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.
Random access of flags and fields permits processing of optional
extensions in an order that is independent of their position in the
packet. The processing order of extensions defined in [GUEEXTENS]
demonstrates this property.
Flags (or paired flags) are idempotent such that new flags MUST NOT
cause reinterpretation of old flags. Also, new flags MUST 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).
The set of available flags can be extended in the future by defining
a "flag extensions bit" that refers to a field containing a new set
of flags.
3.3.2. Example GUE header with extension fields
An example GUE header for a data message encapsulating an IPv4 packet
and containing the Group Identifier and Security extension fields
(both defined in [GUEXTENS]) is shown 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |0| 3 | 94 |1|0 0 1| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Security +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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In the above example, the first flag bit is set which indicates that
the Group Identifier extension is present which is a 32 bit field.
The second through fourth bits of the flags are paired flags that
indicate the presence of a Security field with seven possible sizes.
In this example 001 indicates a sixty-four bit security field.
3.4. 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 extension
fields present in the GUE header. When there is no private data
present, the length of the private data is zero.
The semantics and interpretation of private data are implementation
specific. The private data may be structured as necessary, for
instance it might contain its own set of flags and extension fields.
An encapsulator and decapsulator MUST agree on the meaning of private
data before using it. The mechanism to achieve this agreement is
outside the scope of this document but could include implementation-
defined behavior, coordinated configuration, in-band communication
using GUE control messages, or out-of-band messages.
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 if
present MUST be verified for packet acceptance.
3.5. Message types
3.5.1. Control messages
Control messages carry formatted data that are implicitly addressed
to the decapsulator to monitor or control the state or behavior of a
tunnel (OAM). For instance, an echo request and corresponding echo
reply message can be defined to test for liveness.
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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.
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 control messages can be created that follow the same
path as data messages.
3.5.2. Data messages
Data messages carry encapsulated packets that are addressed to the
protocol stack for the associated protocol. Data messages are a
primary means of encapsulation and can be used to create tunnels for
overlay networks.
Data messages are indicated in GUE header when the C-bit is not set.
The payload of a data message is interpreted as an encapsulated
packet of an Internet protocol indicated in the proto/ctype field.
The packet immediately follows the GUE header.
3.6. Hiding the transport layer protocol number
The GUE header indicates the Internet protocol of the encapsulated
packet. A protocol number is either contained in the Proto/ctype
field of the primary GUE header or in the Payload Type field of a GUE
Transform extension field (used to encrypt the payload with DTLS,
[GUEEXTENS]). If the transport protocol number needs to be hidden
from the network, then a trivial destination options can be used.
The PadN destination option [RFC2460] can be used to encode the
transport protocol as a next header of an extension header (and
maintain alignment of encapsulated transport headers). The
Proto/ctype field or Payload Type field of the GUE Transform field is
set to 60 to indicate that the first encapsulated header is a
destination options extension header.
The format of the extension header is below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | 2 | 1 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For IPv4, it is permitted in GUE to used this precise destination
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option to contain the obfuscated protocol number. In this case next
header MUST refer to a valid IP protocol for IPv4. No other extension
headers or destination options are permitted with IPv4.
4. Version 1
Version 1 of GUE allows direct encapsulation of IPv4 and IPv6 in UDP.
In this version there is no GUE header; a UDP packet carries an IP
packet. The first two bits of the UDP payload for GUE are the GUE
version and coincide with the first two bits of the version number in
the IP header. The first two version bits of IPv4 and IPv6 are 01, so
we use GUE version 1 for direct IP encapsulation which makes two bits
of GUE version to also be 01.
This technique is effectively a means to compress out the GUE header
when encapsulating IPv4 or IPv6 packets and there are no flags or
extension fields present. This method is compatible to use on the
same port number as packets with the GUE header (GUE version 0
packets). This technique saves encapsulation overhead on costly links
for the common use of IP encapsulation, and also obviates the need to
allocate a separate port number for IP-over-UDP encapsulation.
4.1. Direct encapsulation of IPv4
The format for encapsulating IPv4 directly 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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
| Length | Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
|0|1|0|0| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that 0100 value IP version field express the GUE version as 1
(bits 01) and IP version as 4 (bits 0100).
4.2. Direct encapsulation of IPv6
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The format for encapsulating IPv6 directly in UDP is demonstrated
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
| Source port | Destination port | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
| Length | Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
|0|1|1|0| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | NextHdr | Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Source IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that 0110 value IP version field expresses the GUE version as 1
(bits 01) and IP version as 6 (bits 0110).
5. Operation
The figure below illustrates the use of GUE encapsulation between two
hosts. Host 1 is sending packets to Host 2. An encapsulator performs
encapsulation of packets from Host 1. These encapsulated packets
traverse the network as UDP packets. At the decapsulator, packets are
decapsulated and sent on to Host 2. Packet flow in the reverse
direction need not be symmetric; GUE encapsulation is not required in
the reverse path.
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+---------------+ +---------------+
| | | |
| Host 1 | | Host 2 |
| | | |
+---------------+ +---------------+
| ^
V |
+---------------+ +---------------+ +---------------+
| | | | | |
| Encapsulator |-->| Layer 3 |-->| Decapsulator |
| | | Network | | |
+---------------+ +---------------+ +---------------+
The encapsulator and decapsulator may be co-resident with the
corresponding hosts, or may be on separate nodes in the network.
5.1. 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 hosts.
5.2. Transport layer encapsulation
When encapsulating layer 4 packets, the encapsulator and decapsulator
should be co-resident with the hosts. 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 transport
protocol. Note that the transport layer ports in the encapsulated
packet are independent of the UDP ports in the outer packet.
Details about performing transport layer encapsulation are discussed
in [TOU].
5.3. Encapsulator operation
Encapsulators create GUE data messages, set the fields of the UDP
header, set flags and optional extension fields in the GUE header,
and forward packets to a decapsulator.
An encapsulator can be an end host originating the packets of a flow,
or can be a network device performing encapsulation on behalf of
hosts (routers implementing tunnels for instance). In either case,
the intended target (decapsulator) is indicated by the outer
destination IP address and destination port in the UDP header.
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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 protocol over another. For instance, if an IP packet is being
encapsualated in GUE then diffserv interaction [RFC2983] and ECN
propagation for tunnels [RFC6040] SHOULD be followed.
5.4. 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 decapsulator receives a GUE packet with an unsupported version,
unknown flag, bad header length (too small for included extension
fields), unknown control message type, bad protocol number, an
unsupported payload type, or an otherwise malformed header, it MUST
drop the packet. Such events MAY be logged subject to configuration
and rate limiting of logging messages. No error message is returned
back to the encapsulator. Note that set flags in a GUE header 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.
5.4.1. Processing a received data message
If a valid data message is received, the UDP header and GUE header
are removed from the packet. The outer IP header remains intact and
the next protocol in the IP header is set to the protocol from the
proto field in the GUE header. The resulting packet is then
resubmitted into the protocol stack to process that packet as though
it was received with the protocol in the GUE header.
As an example, consider that a data message is received where GUE
encapsulates an IP packet. In this case proto field in the GUE header
is set 94 for IPIP:
+-------------------------------------+
| IP header (next proto = 17,UDP) |
|-------------------------------------|
| UDP |
|-------------------------------------|
| GUE (proto = 94,IPIP) |
|-------------------------------------|
| IP header and packet |
+-------------------------------------+
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The receiver removes the UDP and GUE headers and sets the next
protocol field in the IP packet to IPIP, which is derived from the
GUE proto field. The resultant packet would have the format:
+-------------------------------------+
| IP header (next proto = 94,IPIP) |
|-------------------------------------|
| IP header and packet |
+-------------------------------------+
This packet is then resubmitted into the protocol stack to be
processed as an IPIP packet.
5.4.2. Processing a received control message
If a valid control message is received, the packet MUST be processed
as a control message. The specific processing to be performed depends
on the ctype in the GUE header.
5.5. 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 router does not normally need to parse a GUE header, and
none of the flags or extension fields in the GUE header are expected
to affect routing. In cases where the outer five-tuple does not
provide sufficient entropy for flow classification, for instance UDP
ports are fixed to provide connection semantics (section 5.6.1), then
the encapsulated packet MAY be parsed to determine flow entropy.
A router MUST 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 (i.e. by encapsulating an IP packet with a
GUE payload in another IP packet as a GUE payload). In this case, the
router takes the role of an encapsulator, and the corresponding
decapsulator is the logical endpoint of the tunnel. When
encapsulating a GUE packet within another GUE packet, there are no
provisions to automatically GUE copy flags or fields to the outer GUE
header. Each layer of encapsulation is considered independent.
5.6. Middlebox interactions
A middle box MAY interpret some flags and extension fields of the GUE
header for classification purposes, but is not required to understand
any of the flags or extension fields in GUE packets. A middle box
MUST NOT drop a GUE packet merely because there are flags unknown to
it. The header length in the GUE header allows a middlebox to inspect
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the payload packet without needing to parse the flags or extension
fields.
5.6.1. Inferring connection semantics
A middlebox might infer bidirectional connection semantics for a UDP
flow. For instance, a stateful firewall might 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 should be configured to
assume connected semantics defined by the UDP five tuple and the use
of GUE encapsulation needs to be symmetric between both endpoints.
The source port set in the UDP header MUST be the destination port
the peer would set for replies. In this case the UDP source port for
a tunnel would be a fixed value and not set to be flow entropy as
described in section 5.11.
The selection of whether to make the UDP source port fixed or set to
a flow entropy value for each packet sent SHOULD be configurable for
a tunnel. The default MUST be to set the flow entropy value in the
UDP source port.
5.6.2. NAT
IP address and port translation can be performed on the UDP/IP
headers adhering to the requirements for NAT with UDP [RFC4787]. In
the case of stateful NAT, connection semantics MUST be applied to a
GUE tunnel as described in section 5.6.1. GUE endpoints MAY also
invoke STUN [RFC5389] or ICE [RFC5245] to manage NAT port mappings
for encapsulations.
5.7. Checksum Handling
The potential for mis-delivery of packets due to corruption of IP,
UDP, or GUE headers needs to be considered. Historically, the UDP
checksum would be considered sufficient as a check against corruption
of either the UDP header and payload or the IP addresses.
Encapsulation protocols, such as GUE, can be originated or terminated
on devices incapable of computing the UDP checksum for packet. This
section discusses the requirements around checksum and alternatives
that might be used when an endpoint does not support UDP checksum.
5.7.1. Requirements
One of the following requirements MUST be met:
o UDP checksums are enabled (for IPv4 or IPv6).
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o The GUE header checksum is used (defined in [GUEEXTENS]).
o Use zero UDP checksums. This is always permissible with IPv4; in
IPv6, they can only be used in accordance with applicable
requirements in [RFC8086], [RFC6935], and [RFC6936].
5.7.2. 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
encapsulator MAY set the UDP checksum to zero for performance or
implementation considerations. The IPv4 header includes a checksum
that 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 [GUEEXTENS] assuming there are no other mechanisms used
to protect the GUE packet.
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].
Configuration of zero checksums can be selective. For instance, zero
checksums might be disallowed from certain hosts that are known to
be traversing 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.
5.7.3. UDP Checksum with IPv6
In IPv6, there is no checksum in the IPv6 header that protects
against mis-delivery due to address corruption. Therefore, when GUE
is used over IPv6, either the UDP checksum or the GUE header
checksum SHOULD be used unless there are alternative mechanisms in
use that protect against misdelivery. The UDP checksum and GUE
header checksum SHOULD not be used at the same time since that would
be mostly redundant.
If neither the UDP checksum or the GUE header checksum is used, then
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the requirements for using zero IPv6 UDP checksums in [RFC6935] and
[RFC6936] MUST be met.
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.
5.8. MTU and fragmentation
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 UDP source
port. An operator SHOULD set MTU to account for encapsulation
overhead and reduce the likelihood of fragmentation.
Alternative to IP fragmentation, the GUE fragmentation extension can
be used. GUE fragmentation is described in [GUEEXTENS].
5.9. 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 whether a transmitter will
consistently implement proper congestion control, then congestion
control at the encapsulation layer MUST be provided per [RFC5405].
Note that 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 [CIRCBRK], or traffic isolation MAY be used to provide
rudimentary congestion control. For finer-grained congestion control
that allows alternate congestion control algorithms, reaction time
within an RTT, and interaction with ECN, in-band mechanisms might be
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warranted.
5.10. Multicast
GUE packets can be multicast to decapsulators using a multicast
destination address in the encapsulating IP headers. Each receiving
host will decapsulate the packet independently following normal
decapsulator operations. The receiving decapsulators need to agree
on the same set of GUE parameters and properties; how such an
agreement is reached is outside the scope of this document.
GUE allows encapsulation of unicast, broadcast, or multicast
traffic. Flow entropy (the value in the UDP source port) can be
generated from the header of encapsulated unicast or
broadcast/multicast packets at an encapsulator. The mapping
mechanism between the encapsulated multicast traffic and the
multicast capability in the IP network is transparent and
independent of the encapsulation and is otherwise outside the scope
of this document.
5.11. Flow entropy for ECMP
5.11.1. Flow classification
A major objective of using GUE is that a network device can perform
flow classification corresponding to the flow of the inner
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 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 and connection semantics are not
applied, the source port in the outer UDP packet is set to a flow
entropy value that corresponds to 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.
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Examples of deriving flow entropy for encapsulation are:
o If the encapsulated packet is a layer 4 packet, TCP/IPv4 for
instance, the flow entropy 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 flow entropy 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 flow entropy could be based on the contents
of the clear-text packet. For instance, a canonical five-tuple
hash for a TCP/IP packet could be used.
[RFC6438] discusses methods to compute and set flow entropy value for
IPv6 flow labels. Such methods can also be used to create flow
entropy values for GUE.
5.11.2. Flow entropy properties
The flow entropy is the value set in the UDP source port of a GUE
packet. Flow entropy in the UDP source port SHOULD adhere to the
following properties:
o The value set in the source port is within the ephemeral port
range (49152 to 65535 [RFC6335]). Since the high order two bits
of the port are set to one, this provides fourteen bits of
entropy for the value.
o The flow entropy has a uniform distribution across encapsulated
flows.
o An encapsulator MAY occasionally change the flow entropy used
for an inner flow per its discretion (for security, route
selection, etc). To avoid thrashing or flapping the value, the
flow entropy used for a flow SHOULD NOT change more than once
every thirty seconds (or a configurable value).
o Decapsulators, or any networking devices, SHOULD NOT attempt to
interpret flow entropy as anything more than an opaque value.
Neither should they attempt to reproduce the hash calculation
used by an encapasulator in creating a flow entropy value. They
MAY use the value to match further receive packets for steering
decisions, but MUST NOT assume that the hash uniquely or
permanently identifies a flow.
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o Input to the flow entropy calculation 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 flow entropy SHOULD be randomly
seeded to mitigate denial of service attacks. The seed SHOULD be
changed periodically.
5.12 Negotiation of acceptable flags and extension fields
An encapsulator and decapsulator need to achieve agreement about GUE
parameters will be used in communications. Parameters include GUE
version, flags and extension fields that can be used, security
algorithms and keys, supported protocols and control messages, etc.
This document proposes different general methods to accomplish this,
however the details of implementing these are considered out of
scope.
General methods for this are:
o Configuration. The parameters used for a tunnel are configured
at each endpoint.
o Negotiation. A tunnel negotiation can be performed. This could
be accomplished in-band of GUE using control messages or private
data.
o Via a control plane. Parameters for communicating with a tunnel
endpoint can be set in a control plane protocol (such as that
needed for nvo3).
o Via security negotiation. Use of security typically implies a
key exchange between endpoints. Other GUE parameters may be
conveyed as part of that process.
6. Motivation for GUE
This section presents the motivation for GUE with respect to other
encapsulation methods.
6.1. Benefits of GUE
* GUE is a generic encapsulation protocol. GUE can encapsulate
protocols that are represented by an IP protocol number. This
includes layer 2, layer 3, and layer 4 protocols.
* GUE is an extensible encapsulation protocol. Standardized
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optional data such as security, virtual networking identifiers,
fragmentation are being defined.
* For extensilbity, GUE uses flag fields as opposed to TLVs as
some other encapsulation protocols do. Flag fields are strictly
ordered, allow random access, and are efficient in use of header
space.
* GUE allows private data to be sent as part of the encapsulation.
This permits experimentation or customization in deployment.
* GUE allows sending of control messages such as OAM using the
same GUE header format (for routing purposes) as normal data
messages.
* GUE maximizes deliverability of non-UDP and non-TCP protocols.
* GUE provides a means for exposing per flow entropy for ECMP for
atypical protocols such as SCTP, DCCP, ESP, etc.
6.2 Comparison of GUE to other encapsulations
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 [RFC7637] 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
[RFC7348], LISP [RFC6830] which encapsulates layer 3 packets,
MPLS/UDP [RFC7510], and Generic UDP Encapsulation for IP Tunneling
(GRE over UDP)[RFC8086]. Generic UDP tunneling [GUT] is a proposal
similar to GUE in that it aims to 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 for flow entropy as input to ECMP are defined in
section 5.11.
o GUE permits encapsulation of arbitrary IP protocols, which
includes layer 2 3, and 4 protocols.
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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 extension 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).
o The flags-field model facilitates efficient implementation of
extensibility in hardware.
For instance a TCAM can be use to parse a known set of N flags
where the number of entries in the TCAM is 2^N.
By comparison, the number of TCAM entries needed to parse a set
of N arbitrarily ordered TLVS is approximately e*N!.
7. Security Considerations
There are two important considerations of security with respect to
GUE.
o Authentication and integrity of the GUE header.
o Authentication, integrity, and confidentiality of the GUE
payload.
GUE security is provided by extensions for security defined in
[GUEEXTENS]. These extensions include methods to authenticate the GUE
header and encrypt the GUE payload.
The GUE header can be authenticated using a security extension for an
HMAC. Securing the GUE payload can be accomplished use of the GUE
Payload Transform. This extension can be used to perform DTLS in the
payload of a GUE packet to encrypt the payload.
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A hash function for computing flow entropy (section 5.11) SHOULD be
randomly seeded to mitigate some possible denial service attacks.
8. IANA Considerations
8.1. UDP source port
A user UDP port number assignment for GUE has been assigned:
Service Name: gue
Transport Protocol(s): UDP
Assignee: Tom Herbert <tom@herbertland.com>
Contact: Tom Herbert <tom@herbertland.com>
Description: Generic UDP Encapsulation
Reference: draft-herbert-gue
Port Number: 6080
Service Code: N/A
Known Unauthorized Uses: N/A
Assignment Notes: N/A
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8.2. GUE version number
IANA is requested to set up a registry for the GUE version number.
The GUE version number is 2 bits containing four possible values.
This document defines version 0 and 1. New values are assigned in
accordance with RFC Required policy [RFC5226].
+----------------+-------------+---------------+
| Version number | Description | Reference |
+----------------+-------------+---------------+
| 0 | Version 0 | This document |
| | | |
| 1 | Version 1 | This document |
| | | |
| 2..3 | Unassigned | |
+----------------+-------------+---------------+
8.3. Control types
IANA is requested to set up a registry for the GUE control types.
Control types are 8 bit values. New values for control types 1-127
are assigned in accordance with RFC Required policy [RFC5226].
+----------------+------------------+---------------+
| Control type | Description | Reference |
+----------------+------------------+---------------+
| 0 | Need further | This document |
| | interpretation | |
| | | |
| 1..127 | Unassigned | |
| | | |
| 128..255 | User defined | This document |
+----------------+------------------+---------------+
8.4. Flag-fields
IANA is requested to create a "GUE flag-fields" registry to allocate
flags and extension fields used with GUE. This shall be a registry of
bit assignments for flags, length of extension fields for
corresponding flags, and descriptive strings. There are sixteen bits
for primary GUE header flags (bit number 0-15). New values are
assigned in accordance with RFC Required policy [RFC5226]. New flags
should be allocated from high to low order bit contiguously without
holes. [GUEXTENS] requests in initial set of flag assignments.
9. Acknowledgements
The authors would like to thank David Liu, Erik Nordmark, Fred
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Templin, Adrian Farrel, Bob Briscoe, and Murray Kucherawy for
valuable input on this draft.
10. References
10.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
10.17487/RFC0768, August 1980, <http://www.rfc-
editor.org/info/rfc768>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, DOI
10.17487/RFC1122, October 1989, <http://www.rfc-
editor.org/info/rfc1122>.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434, DOI
10.17487/RFC2434, October 1998, <http://www.rfc-
editor.org/info/rfc2434>.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC
2983, DOI 10.17487/RFC2983, October 2000, <http://www.rfc-
editor.org/info/rfc2983>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <http://www.rfc-editor.org/info/rfc6040>.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, DOI
10.17487/RFC6935, April 2013, <http://www.rfc-
editor.org/info/rfc6935>.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
<http://www.rfc-editor.org/info/rfc6936>.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
2006, <http://www.rfc-editor.org/info/rfc4459>.
10.2. Informative References
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
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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>.
[RFC7605] Touch, J., "Recommendations on Using Assigned Transport
Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
August 2015, <http://www.rfc-editor.org/info/rfc7605>.
[RFC7637] Garg, P., Ed., and Y. Wang, Ed., "NVGRE: Network
Virtualization Using Generic Routing Encapsulation", RFC
7637, DOI 10.17487/RFC7637, September 2015,
<http://www.rfc-editor.org/info/rfc7637>.
[RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
March 2017, <http://www.rfc-editor.org/info/rfc8086>.
[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
"Encapsulating MPLS in UDP", RFC 7510, DOI
10.17487/RFC7510, April 2015, <http://www.rfc-
editor.org/info/rfc7510>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, DOI
10.17487/RFC4340, March 2006, <http://www.rfc-
editor.org/info/rfc4340>.
[RFC4787] Audet, F., Ed., and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <http://www.rfc-editor.org/info/rfc4787>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008, <http://www.rfc-
editor.org/info/rfc5389>.
[RFC5285] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, DOI
10.17487/RFC5245, April 2010, <http://www.rfc-
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editor.org/info/rfc5245>.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, DOI
10.17487/RFC5405, November 2008, <http://www.rfc-
editor.org/info/rfc5405>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<http://www.rfc-editor.org/info/rfc6438>.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, DOI
10.17487/RFC2003, October 1996, <http://www.rfc-
editor.org/info/rfc2003>.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
3948, DOI 10.17487/RFC3948, January 2005, <http://www.rfc-
editor.org/info/rfc3948>.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830, DOI
10.17487/RFC6830, January 2013, <http://www.rfc-
editor.org/info/rfc6830>.
[RFC3378] Housley, R. and S. Hollenbeck, "EtherIP: Tunneling
Ethernet Frames in IP Datagrams", RFC 3378, DOI
10.17487/RFC3378, September 2002, <http://www.rfc-
editor.org/info/rfc3378>.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000, <http://www.rfc-
editor.org/info/rfc2784>.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed.,
"Encapsulating MPLS in IP or Generic Routing Encapsulation
(GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
<http://www.rfc-editor.org/info/rfc4023>.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, DOI 10.17487/RFC2661, August 1999,
<http://www.rfc-editor.org/info/rfc2661>.
[GUEEXTENS] Herbert, T., Yong, L., and Templin, F., "Extensions for
Generic UDP Encapsulation" draft-herbert-gue-extensions-00
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[GUE4NVO3] Yong, L., Herbert, T., Zia, O., "Generic UDP
Encapsulation (GUE) for Network Virtualization Overlay"
draft-hy-nvo3-gue-4-nvo-03
[GUESEC] Yong, L., Herbert, T., "Generic UDP Encapsulation (GUE) for
Secure Transport" draft-hy-gue-4-secure-transport-03
[TCPUDP] Chesire, S., Graessley, J., and McGuire, R.,
"Encapsulation of TCP and other Transport Protocols over
UDP" draft-cheshire-tcp-over-udp-00
[TOU] Herbert, T., "Transport layer protocols over UDP" draft-
herbert-transports-over-udp-00
[GUT] Manner, J., Varia, N., and Briscoe, B., "Generic UDP
Tunnelling (GUT) draft-manner-tsvwg-gut-02.txt"
[CIRCBRK] Fairhurst, G., "Network Transport Circuit Breakers",
draft-ietf-tsvwg-circuit-breaker-15
[LCO] Cree, E., https://www.kernel.org/doc/Documentation/
networking/checksum-offloads.txt
Appendix A: NIC processing for GUE
This appendix provides some guidelines for Network Interface Cards
(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 selects 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 is compatible with multi-queue NICs that support
five-tuple hash calculation for UDP/IP packets as input to RSS. The
flow entropy in the UDP 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).
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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 is be mostly avoided by
fragmenting packets before entering a tunnel, GUE fragmentation, path
MTU discovery in higher layer protocols, or operator adjusting MTUs.
Other UDP traffic might not implement such procedures to avoid
fragmentation, so enabling UDP RSS support in the NIC might 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 are of interest:
the encapsulated packet's transport checksum, and the UDP checksum in
the outer header.
A.2.1. Transmit checksum offload
NICs can 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
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.
If an encapsulator is co-resident with a host, then checksum offload
may be performed using remote checksum offload (described in
[GUEEXTENS]). 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
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the checksum from the start to end of packet is efficient if
checksum-complete is provided on the receiver.
Another alternative when an encapsulator is co-resident with a host
is to perform Local Checksum Offload [LCO]. In this method, the inner
transport layer checksum is offloaded and the outer UDP checksum can
be deduced based on the fact that the portion of the packet covered
by the inner transport checksum will sum to zero (or at least the bit
wise "not" of the inner pseudo header).
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 checksums in the encapsulated packet.
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.
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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 extension fields
do not contain values that need to be updated on a per segment basis.
For example, extension 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.
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.
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Appendix B: Implementation considerations
This appendix is informational and does not constitute a normative
part of this document.
B.1. Priveleged ports
Using the source port to contain a flow entropy 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.
B.2. Setting flow entropy as a route selector
An encapsulator generating flow entropy in the UDP source port could
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 flow entropy. For
instance, a host can store a flow hash in its PCB for an inner flow,
and might alter the value upon detecting that packets are traversing
a lossy path. Changing the flow entropy for a flow SHOULD be subject
to hysteresis (at most once every thirty seconds) to limit the number
of out of order packets.
B.3. Hardware protocol implementation considerations
Low level data path protocol, such is GUE, are often supported in
high speed network device hardware. 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
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".
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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
Quantonium
4701 Patrick Henry
Santa Clara, CA 95054
US
Email: tom@herbertland.com
Lucy Yong
Huawei USA
5340 Legacy Dr.
Plano, TX 75024
US
Email: lucy.yong@huawei.com
Osama Zia
Microsoft
1 Microsoft Way
Redmond, WA 98029
US
Email: osamaz@microsoft.com
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