INTERNET-DRAFT T. Herbert
Intended Status: Proposed Standard Facebook
Expires: December 2016 L. Yong
Huawei
F. Templin
Boeing
June 21, 2016
Extensions for Generic UDP Encapsulation
draft-herbert-gue-extensions-00
Abstract
This specification defines a set of the fundamental extensions for
Generic UDP Encapsulation (GUE). The extensions defined in this
specification are the security option, payload transform option,
checksum option, fragmentation option, and the remote checksum
offload option.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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Copyright and License Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. GUE header format with options . . . . . . . . . . . . . . . . 4
3. Checksum option . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. GUE checksum pseudo header . . . . . . . . . . . . . . . . 7
3.4. Checksum computation . . . . . . . . . . . . . . . . . . . 8
3.5. Transmitter operation . . . . . . . . . . . . . . . . . . 8
3.6. Receiver operation . . . . . . . . . . . . . . . . . . . . 8
3.7. Security Considerations . . . . . . . . . . . . . . . . . 9
4. Fragmentation option . . . . . . . . . . . . . . . . . . . . . 9
4.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Option format . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Fragmentation procedure . . . . . . . . . . . . . . . . . 12
4.5. Reassembly procedure . . . . . . . . . . . . . . . . . . . 14
4.6. Security Considerations . . . . . . . . . . . . . . . . . 16
5. Security and payload transform options . . . . . . . . . . . . 16
5.1. Security option format . . . . . . . . . . . . . . . . . . 16
5.2. Security option usage . . . . . . . . . . . . . . . . . . 17
5.2.1. Cookies . . . . . . . . . . . . . . . . . . . . . . . 17
5.2.2. Secure hash . . . . . . . . . . . . . . . . . . . . . 18
5.3. Payload Transform Option format . . . . . . . . . . . . . 18
5.3.1. Payload transform option usage . . . . . . . . . . . . 19
5.4. Operation of security mechanisms . . . . . . . . . . . . . 19
5.5. Considerations of Using Other Security Tunnel Mechanisms . 20
6. Remote checksum offload option . . . . . . . . . . . . . . . . 21
6.1. Option format . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Transmitter operation . . . . . . . . . . . . . . . . . . 22
6.3. Receiver operation . . . . . . . . . . . . . . . . . . . . 22
6.4. Security Considerations . . . . . . . . . . . . . . . . . 23
7. Processing order of options . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
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10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
Generic UDP Encapsulation [I.D.nvo3-gue] is a generic and extensible
encapsulation protocol. This specification defines a basic set of
extensions for GUE. These extensions are the security option, payload
transform option, checksum option, fragmentation option, and the
remote checksum offload option.
2. GUE header format with options
The general format of GUE with the options defined in this
specification 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 | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
|Ver|C| Hlen | Proto/ctype |V|SEC|K|F|T|R| Rsvd Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VNID (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Fragmentation (optional) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload transform (optional |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote checksum offload (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Private fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the UDP are described in [I.D.herbert-gue].
The GUE header consists of:
o Ver: Version. Set to 0x0 to indicate GUE encapsulation header.
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Note that version 0x1 does not allow options.
o C: Control bit. May be set or unset. GUE options can be used
with either control or data packets unless otherwise specified
in the option definition.
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. The length of the encapsulated
packet is determined from the UDP length and the Hlen:
encapsulated_packet_length = UDP_Length - 8 - GUE_Hlen.
o Proto/ctype: If the C bit is not set this indicates IP protocol
number for the packet in the payload, else if the C bit is set
this type of control message for the payload. The next header
begins at the offset provided by Hlen. When the fragmentation
option or payload transform option is used this field may be set
to protocol number 59 for a data message, or a value of 0 for a
control message, to indicate no next header for the payload.
o V: Indicates the network virtualization option (VNID) field is
present. The VNID option is described in [I.D.hy-nvo3-gue-4-
nvo].
o SEC: Indicates security option field is present. The security
option is described in section 5.
o K: Indicates checksum option field is present. The checksum
option is described in section 3.
o F: Indicates fragmentation option field is present. The
fragmentation option is described in section 4.
o T: Indicates transform option field is present. The transform
option is described in section 5.
o R: Indicates the remote checksum option field is present. The
remote checksum offload option is described in section 6.
o Private fields are described in [I.D.nvo3-gue].
3. Checksum option
The GUE checksum option provides a checksum that covers the GUE
header, a GUE pseudo header, and optionally part or all of the GUE
payload. The GUE pseudo header includes the corresponding IP
addresses as well as the UDP ports of the encapsulating headers. This
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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. In particular, the
GUE checksum can provide protection for some sensitive data, such as
the virtual network identifier ([I.D.hy-nvo3-gue-4-nvo]), which when
corrupted could lead to mis-delivery of a packet to the wrong virtual
network.
3.1. Option format
The presence of the GUE checksum option is indicated by the K bit in
the GUE header.
The format of the checksum option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Payload coverage |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields of the option are:
o Checksum: Computed checksum value. This checksum covers the GUE
header (including fields and private data covered by Hlen), the
GUE pseudo header, and optionally all or part of the payload
(encapsulated packet).
o Payload coverage: Number of bytes of payload to cover in the
checksum. Zero indicates that the checksum only covers the GUE
header and GUE pseudo header. If the value is greater than the
encapsulated payload length, the packet must be dropped.
3.2. Requirements
The GUE header checksum must be set on transmit when using a zero UDP
checksum with IPv6.
The GUE header checksum must be set when the UDP checksum is zero for
IPv4 if the GUE header includes data that when corrupted can lead to
misdelivery or other serious consequences, and there is no other
mechanism that provides protection (no security field for instance).
Otherwise the GUE header checksum should be used with IPv4 when the
UDP checksum is zero.
The GUE header checksum should not be set when the UDP checksum is
non-zero. In this case the UDP checksum provides adequate protection
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and this avoids convolutions when a packet traverses NAT that does
address translation (in that case the UDP checksum is required).
3.3. GUE checksum pseudo header
The GUE pseudo header checksum is included in the GUE checksum to
provide protection for the IP and UDP header elements which when
corrupted could lead to misdelivery of the GUE packet. The GUE pseudo
header checksum is similar to the standard IP pseudo header defined
in [RFC0768] and [RFC0793] for IPv4, and in [RFC2460] for IPv6.
The GUE pseudo header for IPv4 is:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source port | Destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The GUE pseudo header for IPv6 is:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source port | Destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that the GUE pseudo header does not include payload length or
protocol as in the standard IP pseudo headers. The length field is
deemed unnecessary because:
o If the length is corrupted this will usually be detected by a
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checksum validation failure on the inner packet.
o Fragmentation of packets in a tunnel should occur on the inner
packet before being encapsulated or GUE fragmentation (section
4) may be performed at tunnel ingress. GUE packets are not
expected to be fragmented when using IPv6. See RFC6936 for
considerations of payload length and IPv6 checksum.
o A corrupted length field in itself should not lead to
misdelivery of a packet.
o Without the length field, the GUE pseudo header checksum is the
same for all packets of flow. This is a useful property for
optimizations such as TCP Segment Offload (TSO).
3.4. Checksum computation
The GUE checksum is computed and verified following the standard
process for computing the Internet checksum [RFC1071]. Checksum
computation may be optimized per the mathematical properties
including parallel computation and incremental updates.
3.5. Transmitter operation
The procedure for setting the GUE checksum on transmit is:
1) Create the GUE header including the checksum and payload
coverage fields. The checksum field is initially set to zero.
2) Calculate the 1's complement checksum of the GUE header from
the start of the GUE header through the its length as indicated
in GUE Hlen.
3) Calculate the checksum of the GUE pseudo header for IPv4 or
IPv6.
4) Calculate checksum of payload portion if payload coverage is
enabled (payload coverage field is non-zero). If the length of
the payload coverage is odd, logically append a single zero
byte for the purposes of checksum calculation.
5) Add and fold the computed checksums for the GUE header, GUE
pseudo header and payload coverage. Set the bitwise not of the
result in the GUE checksum field.
3.6. Receiver operation
If the GUE checksum option is present, the receiver must validate the
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checksum before processing any other fields or accepting the packet.
The procedure for verifying the checksum is:
1) If the payload coverage length is greater than the length of
the encapsulated payload then drop the packet.
2) Calculate the checksum of the GUE header from the start of the
header to the end as indicated by Hlen.
3) Calculate the checksum of the appropriate GUE pseudo header.
4) Calculate the checksum of payload if payload coverage is
enabled (payload coverage is non-zero). If the length of the
payload coverage is odd logically append a single zero byte for
the purposes of checksum calculation.
5) Sum the computed checksums for the GUE header, GUE pseudo
header, and payload coverage. If the result is all 1 bits (-0
in 1's complement arithmetic), the checksum is valid and the
packet is accepted; otherwise the checksum is considered
invalid and the packet must be dropped.
3.7. Security Considerations
The checksum option is only a mechanism for corruption detection, it
is not a security mechanism. To provide integrity checks or
authentication of the GUE header, the GUE security option should be
used.
4. Fragmentation option
The fragmentation option allows an encapsulator to perform
fragmentation of packets being ingress to a tunnel. Procedures for
fragmentation and reassembly are defined in this section. This
specification adapts the procedures for IP fragmentation and
reassembly described in [RFC0791] and [RFC2460]. Fragmentation may be
performed on both data and control messages in GUE.
4.1. Motivation
This section describes the motivation for having a fragmentation
option in GUE.
MTU and fragmentation issues with In-the-Network Tunneling are
described in [RFC4459]. Considerations need to be made when a packet
is received at a tunnel ingress point which may be too large to
traverse the path between tunnel endpoints.
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There are four suggested alternatives in [RFC4459] to deal with this:
1) Fragmentation and Reassembly by the Tunnel Endpoints
2) Signaling the Lower MTU to the Sources
3) Encapsulate Only When There is Free MTU
4) Fragmentation of the Inner Packet
Many tunneling protocol implementations have assumed that
fragmentation should be avoided, and in particular alternative #3
seems preferred for deployment. In this case, it is assumed that an
operator can configure the MTUs of links in the paths of tunnels to
ensure that they are large enough to accommodate any packets and
required encapsulation overhead. This method, however, may not be
feasible in certain deployments and may be prone to misconfiguration
in others.
Similarly, the other alternatives have drawbacks that are described
in [RFC4459]. Alternative #2 implies use of something like Path MTU
Discovery which is not known to be sufficiently reliable. Alternative
#4 is not permissible with IPv6 or when the DF bit is set for IPv4,
and it also introduces other known issues with IP fragmentation.
For alternative #1, fragmentation and reassembly at the tunnel
endpoints, there are two possibilities: encapsulate the large packet
and then perform IP fragmentation, or segment the packet and then
encapsulate each segment (a non-IP fragmentation approach).
Performing IP fragmentation on an encapsulated packet has the same
issues as that of normal IP fragmentation. Most significant of these
is that the Identification field is only sixteen bits in IPv4 which
introduces problems with wraparound as described in [RFC4963].
The second possibility follows the suggestion expressed in [RFC2764]
and the fragmentation feature described in the AERO protocol
[I.D.templin-aerolink], that is for the tunneling protocol itself to
incorporate a segmentation and reassembly capability that operates at
the tunnel level. In this method fragmentation is part of the
encapsulation and an encapsulation header contains the information
for reassembly. This is different from IP fragmentation in that the
IP headers of the original packet are not replicated for each
fragment.
Incorporating fragmentation into the encapsulation protocol has some
advantages:
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o At least a 32 bit identifier can be defined to avoid issues of
the 16 bit Identification in IPv4.
o Encapsulation mechanisms for security and identification, such
as virtual network identifiers, can be applied to each segment.
o This allows the possibility of using alternate fragmentation and
reassembly algorithms (e.g. fragmentation with Forward Error
Correction).
o Fragmentation is transparent to the underlying network so it is
unlikely that fragmented packet will be unconditionally dropped
as might happen with IP fragmentation.
4.2. Scope
This specification describes the mechanics of fragmentation in
Generic UDP Encapsulation. The operational aspects and details for
higher layer implementation must be considered for deployment, but
are considered out of scope for this document. The AERO protocol
[I.D.templin-aerolink] defines one use case of fragmentation with
encapsulation.
4.3. Option format
The presence of the GUE fragmentation option is indicated by the F
bit in the GUE header.
The format of the fragmentation option 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment offset |Res|M| Orig-proto | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields of the option are:
o Fragment offset: This field indicates where in the datagram this
fragment belongs. The fragment offset is measured in units of 8
octets (64 bits). The first fragment has offset zero.
o Res: Two bit reserved field. Must be set to zero for
transmission. If set to non-zero in a received packet then the
packet MUST be dropped.
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o M: More fragments bit. Set to 1 when there are more fragments
following in the datagram, set to 0 for the last fragment.
o Orig-proto: The control type (when C bit is set) or the IP
protocol (when C bit is not set) of the fragmented packet.
o Identification: 40 bits. Identifies fragments of a fragmented
packet.
Pertinent GUE header fields to fragmentation are:
o C: This bit is set for each fragment based on the whether the
original packet being fragmented is a control or data message.
o Proto/ctype - For the first fragment (fragment offset is zero)
this is set to that of the original packet being fragmented
(either will be a control type or IP protocol). For other
fragments, this is set to zero for a control message being
fragmented, or to "No next header" (protocol number 59) for a
data message being fragmented.
o F bit - Set to indicate presence of the fragmentation option
field.
4.4. Fragmentation procedure
If an encapsulator determines that a packet must be fragmented (eg.
the packet's size exceeds the Path MTU of the tunnel) it should
divide the packet into fragments and send each fragment as a separate
GUE packet, to be reassembled at the decapsulator (tunnel egress).
For every packet that is to be fragmented, the source node generates
an Identification value. The Identification must be different than
that of any other fragmented packet sent within the past 60 seconds
(Maximum Segment Lifetime) with the same tunnel identification-- that
is the same outer source and destination addresses, same UDP ports,
same orig-proto, and same virtual network identifier if present.
The initial, unfragmented, and unencapsulated packet is referred to
as the "original packet". This will be a layer 2 packet, layer 3
packet, or the payload of a GUE control message:
+-------------------------------//------------------------------+
| Original packet |
| (e.g. an IPv4, IPv6, Ethernet packet) |
+------------------------------//-------------------------------+
Fragmentation and encapsulation are performed on the original packet
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in sequence. First the packet is divided up in to fragments, and then
each fragment is encapsulated. Each fragment, except possibly the
last ("rightmost") one, is an integer multiple of 8 octets long.
Fragments MUST be non-overlapping. The number of fragments should be
minimized, and all but the last fragment should be approximately
equal in length.
The fragments are transmitted in separate "fragment packets" as:
+--------------+--------------+---------------+--//--+----------+
| first | second | third | | last |
| fragment | fragment | fragment | .... | fragment |
+--------------+--------------+---------------+--//--+----------+
Each fragment is encapsulated as the payload of a GUE packet. This is
illustrated as:
+------------------+----------------+-----------------------+
| IP/UDP header | GUE header | first |
| header | w/ frag option | fragment |
+------------------+----------------+-----------------------+
+------------------+----------------+-----------------------+
| IP/UDP header | GUE header | second |
| header | w/ frag option | fragment |
+------------------+----------------+-----------------------+
o
o
+------------------+----------------+-----------------------+
| IP/UDP header | GUE header | last |
| header | w/ frag option | fragment |
+------------------+----------------+-----------------------+
Each fragment packet is composed of:
(1) Outer IP and UDP headers as defined for GUE encapsulation.
o The IP addresses and UDP destination port must be the same
for all fragments of a fragmented packet.
o The source port selected for the inner flow identifier must
be the same value for all fragments of a fragmented packet.
(2) A GUE header that contains:
o The C bit which is set to the same value for all the
fragments of a fragmented packet based on whether a control
message or data message was fragmented.
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o A proto/ctype. In the first fragment this is set to the
value corresponding to the next header of the original
packet and will be either an IP protocol or a control type.
For subsequent fragments, this field is set to 0 for a
fragmented control message or 59 (no next header) for a
fragmented data messages.
o The F bit is set and fragment option is present.
o Other GUE options. Note that options apply to the individual
GUE packet. For instance, the security option would be
validated before reassembly.
(3) The GUE fragmentation option. The option contents include:
o Orig-proto that identifies the first header of the original
packet.
o A Fragment Offset containing the offset of the fragment, in
8-octet units, relative to the start of the of the original
packet. The Fragment Offset of the first ("leftmost")
fragment is 0.
o An M flag value of 0 if the fragment is the last
("rightmost") one, else an M flag value of 1.
o The Identification value generated for the original packet.
(4) The fragment itself.
4.5. Reassembly procedure
At the destination, fragment packets are decapsulated and reassembled
into their original, unfragmented form, as illustrated:
+-------------------------------//------------------------------+
| Original packet |
| (e.g. an IPv4, IPv6, Ethernet packet) |
+------------------------------//-------------------------------+
The following rules govern reassembly:
The IP/UDP/GUE headers of each packet are retained until all
fragments have arrived. The reassembled packet is then composed
of the decapsulated payloads in the GUE fragments, and the
IP/UDP/GUE headers are discarded.
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When a GUE packet is received with the fragment option, the
proto/ctype in the GUE header must be validated. In the case
that the packet is a first fragment (fragment offset is zero),
the proto/ctype in the GUE header must equal the orig-proto
value in the fragmentation option. For subsequent fragments
(fragment offset is non-zero) the proto/ctype in the GUE header
must be 0 for a control message or 59 (no-next-hdr) for a data
message. If the proto/ctype value is invalid then for a received
packet it MUST be dropped.
An original packet is reassembled only from GUE fragment packets
that have the same outer Source Address, Destination Address,
UDP source port, UDP destination port, GUE header C bit, virtual
network identifier if present, orig-proto value in the
fragmentation option, and Fragment Identification. The protocol
type or control message type (depending on the C bit) for the
reassembled packet is the value of the GUE header proto/ctype
field in the first fragment.
The following error conditions may arise when reassembling fragmented
packets with GUE encapsulation:
If insufficient fragments are received to complete reassembly of
a packet within 60 seconds (or a configurable period) of the
reception of the first-arriving fragment of that packet,
reassembly of that packet must be abandoned and all the
fragments that have been received for that packet must be
discarded.
If the payload length of a fragment is not a multiple of 8
octets and the M flag of that fragment is 1, then that fragment
must be discarded.
If the length and offset of a fragment are such that the payload
length of the packet reassembled from that fragment would exceed
65,535 octets, then that fragment must be discarded.
If a fragment overlaps another fragment already saved for
reassembly then the new fragment that overlaps the existing
fragment MUST be discarded
If the first fragment is too small then it is possible that it
does not contain the necessary headers for a stateful firewall.
Sending small fragments like this has been used as an attack on
IP fragmentation. To mitigate this problem, an implementation
should ensure that the first fragment contains the headers of
the encapsulated packet at least through the transport header.
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A GUE node must be able to accept a fragmented packet that,
after reassembly and decapsulation, is as large as 1500 octets.
This means that the node must configure a reassembly buffer that
is at least as large as 1500 octets plus the maximum-sized
encapsulation headers that may be inserted during encapsulation.
Implementations may find it more convenient and efficient to
configure a reassembly buffer size of 2KB which is large enough
to accommodate even the largest set of encapsulation headers and
provides a natural memory page size boundary.
4.6. Security Considerations
Exploits that have been identified with IP fragmentation are
conceptually applicable to GUE fragmentation.
Attacks on GUE fragmentation can be mitigated by:
o Hardened implementation that applies applicable techniques from
implementation of IP fragmentation.
o Application of GUE security (section 5) or IPsec [RFC4301].
Security mechanisms can prevent spoofing of fragments from
unauthorized sources.
o Implement fragment filter techniques for GUE encapsulation as
described in [RFC1858] and [RFC3128].
o Do not accepted data in overlapping segments.
o Enforce a minimum size for the first fragment.
5. Security and payload transform options
The security option and the payload transform option are used to
provide security for the GUE headers and payload. The GUE security
option provides origin authentication and integrity protection of the
GUE header at tunnel end points to guarantee isolation between
tunnels and mitigate Denial of Service attacks. The payload transform
option provides a means to perform encryption and authentication of
the GUE packet that protects the payload from eavesdropping,
tampering, or message forgery.
5.1. Security option format
The presence of the GUE security option is indicated in the SEC flag
bits of the GUE header.
The format of the fragmentation option is:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields of the option are:
o Security (8, 16, or 32 octets). Contains the security
information. The specific semantics and format of this field is
expected to be negotiated between the two communicating nodes.
To provide security capability, the SEC flags MUST be set. Different
sizes are allowed to provide different methods and extensibility. The
use of the security field is expected to be negotiated out of band
between two tunnel end points.
The possible values in the SEC flags are:
o 00 - No security field
o 01 - 64 bit security field
o 10 - 128 bit security field
o 11 - 256 bit security field
5.2. Security option usage
The GUE security field should be used to provide integrity and
authentication of the GUE header. Security negotiation
(interpretation of security field, key management, etc.) is expected
to be negotiated out of band between two communicating hosts. Two
possible uses for this field are outlined below, a more precise
specification is deferred to other documents.
5.2.1. Cookies
The security field may be used as a cookie. This would be similar to
cookie mechanism described in L2TP [RFC3931], and the general
properties should be the same. The cookie may be used to validate the
encapsulation. The cookie is a shared value between an encapsulator
and decapsulator which should be chosen randomly and may be changed
periodically. Different cookies may used for logical flows between
the encapsulator and decapsulator, for instance packets sent with
different VNIs in network virtualization [I.D.hy-nvo3-gue-4-nvo]
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might have different cookies.
5.2.2. Secure hash
Strong authentication of the GUE header can be provided using a
secure hash. This may follow the model of the TCP authentication
option [RFC5925]. In this case the security field holds a message
digest for the GUE header (e.g. 16 bytes from MD5). The digest might
be done over static fields in IP and UDP headers per negotiation
(addresses, ports, and protocols). In order to provide enough
entropy, a random salt value in each packet might be added, for
instance the security field could be a 256 bit value that contains
128 bits of salt value, and a 128 bit digest value. The use of secure
hashes requires shared keys which are presumably negotiated and
rotated as needed out of band.
5.3. Payload Transform Option format
The presence of the GUE payload transform option is indicated by the
T bit in the GUE header.
The format of Payload Transform Field is:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Payload Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields of the option are:
Type: Payload Transform Type or Code point. Each payload transform
mechanism must have one code point registered in IANA. This
document specifies:
0x01: for DTLS [RFC6347]
0x80~0xFF: for private payload transform types
A private payload transform type can be used for
experimental purpose or vendor proprietary mechanisms.
Payload Type: Indicates the encrypted payload type. When payload
transform option is present, proto/ctype in the base header
should set to 59 ("No next header") for a data message and
zero for a control message. The payload type (IP protocol or
control message type) of the unencrypted payload must be
encoded in this field.
The benefit of this rule is to prevent a middle box from
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inspecting the encrypted payload according to GUE next
protocol. The assumption here is that a middle box may
understand GUE base header but does not understand GUE
option flag definitions.
Reserved field for DTLS type MUST set to Zero. For other
transformation types, the use of reserved field may be specified.
5.3.1. Payload transform option usage
The payload transform option provides a mechanism to transform or
interpret the payload of a GUE packet. The Type field describes the
format of the payload before transformation and Payload Type provides
the protocol of the packet after transformation. The payload
transformation option is generic so that it can have both security
related uses (such as DTLS) as well as non security related uses
(such as compression, CRC, etc.).
The payload of a GUE packet can be secured using Datagram Transport
Layer Security [RFC6347]. An encapsulator would apply DTLS to the GUE
payload so that the payload packets are encrypted and the GUE header
remains in plaintext. The payload transform option is set to indicate
that the payload should be interpreted as a DTLS record.
5.4. Operation of security mechanisms
GUE secure transport mechanisms apply to both IPv4 and IPv6 underlay
networks. The outer IP address MUST be tunnel egress IP address (dst)
and tunnel ingress IP address (src). The GUE security option provides
origin authentication and integrity to GUE based tunnel; GUE payload
encryption provides payload privacy over an IP delivery network or
Internet. The two functions are processed separately at tunnel end
points. A GUE tunnel can use both functions or use one of them.
When both encryption and security are required, an encapsulator must
perform payload encryption first and then encapsulate the encrypted
packet with security flag and payload transform flag set in GUE
header; the security option field must be filled according Section
5.2 above and the payload transform field must be filled according to
Section 5.3 above. The decapsulator must decapsulate the packet
first, then perform the authentication validation. If the validation
is successful, it performs the payload decryption according to the
encryption information in the payload encryption field in the header.
Else if the validation fails, the decapsulator must discard the
packet and may generate an alert to the management system. These
processing rules also apply when only one function, either encryption
or security, is enabled, except the other function is not performed
as stated above.
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If GUE fragmentation is used in concert with the GUE security option
and/or payload transform option, the security and playload
transformation are performed after fragmentation at the encapsulator
and before reassembly at the decapsulator.
In order to get flow entropy from the payload, an encapsulator must
determine the flow entropy value (e.g. a hash over the 4-tuple of a
TCP connection) before performing the payload encryption. The flow
entropy value can then be set in UDP src port of the GUE packet.
DTLS [RFC6347] provides packet fragmentation capability. To avoid
packets fragmentation being performed multiple times by an
encapsulator, an encapsulator SHOULD only perform the packet
fragmentation at part of the packet encapsulation process (e.g. using
the GUE fragmentation option), not in payload encryption process
(i.e. DTLS layer fragmentation should be avoided).
DTLS usage [RFC6347] is limited to a single DTLS session for any
specific tunnel encapsulator/decapsulator pair (identified by source
and destination IP addresses). Both IP addresses MUST be unicast
addresses - multicast traffic is not supported when DTLS is used. A
GUE tunnel decapsulator implementation that supports DTLS can
establish DTLS session(s) with one or multiple tunnel encapsulators,
and likewise a GUE tunnel encapsulator implementation can establish
DTLS session(s) with one or multiple decapsulators.
5.5. Considerations of Using Other Security Tunnel Mechanisms
GUE may rely on other secure tunnel mechanisms such as DTLS [RFC6347]
over the whole UDP payload for securing the whole GUE packet or IPsec
[RFC4301] to achieve the secure transport over an IP network or
Internet.
IPsec [RFC4301] was designed as a network security mechanism, and
therefore it resides at the network layer. As such, if the tunnel is
secured with IPsec, the UDP header would not be visible to
intermediate routers anymore in either IPsec tunnel or transport
mode. The big drawback here prohibits intermediate routers to perform
load balancing based on the flow entropy in UDP header. In addition,
this method prohibits any middle box function on the path.
By comparison, DTLS [RFC6347] was designed with application security
and can better preserve network and transport layer protocol
information than IPsec [RFC4301]. Using DTLS to secure the GUE
tunnel, both GUE header and payload will be encrypted. In order to
differentiate plaintext GUE header from encrypted GUE header, the
destination port of the UDP header between two must be different,
which essentially requires another standard UDP port for GUE with
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DTLS. The drawback on this method is to prevent a middle box
operation to GUE tunnel on the path.
Use of two independent tunnel mechanisms such as GUE and DTLS to
carry a network protocol over an IP network adds some overlap and
process complex. For example, fragmentation will be done twice.
As the result, a GUE tunnel SHOULD use the security mechanisms
specified in this document to provide secure transport over an IP
network or Internet when it is needed. GUE tunnels with the GUE
security options can be used as a secure transport mechanism over an
IP network and Internet.
6. Remote checksum offload option
Remote checksum offload is mechanism that provides checksum offload
of encapsulated packets using rudimentary offload capabilities found
in most Network Interface Card (NIC) devices. Many NIC
implementations can only offload the outer UDP checksum in UDP
encapsulation. Remote checksum offload is described in [UDPENCAP].
In remote checksum offload the outer header checksum, that is in the
outer UDP header, is enabled in packets and, with some additional
meta information, a receiver is able to deduce the checksum to be set
for an inner encapsulated packet. Effectively this offloads the
computation of the inner checksum. Enabling the outer checksum in
encapsulation has the additional advantage that it covers more of the
packet than the inner checksum including the encapsulation headers.
The remote offload checksum option should not be used when GUE
fragmentation is also being performed. In this case the offload of
the outer UDP checksum does not cover the whole transport segment so
remote checksum offload would not properly set the inner transport
layer checksum.
6.1. Option format
The presence of the GUE remote checksum offload option is indicated
by the R bit in the GUE header.
The format of remote checksum offload field is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum start | Checksum offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The fields of the option are:
o Checksum start: starting offset for checksum computation
relative to the start of the encapsulated payload. This is
typically the offset of a transport header (e.g. UDP or TCP).
o Checksum offset: Offset relative to the start of the
encapsulated packet where the derived checksum value is to be
written. This typically is the offset of the checksum field in
the transport header (e.g. UDP or TCP).
6.2. Transmitter operation
The typical actions to set remote checksum offload on transmit
are:
1) Transport layer creates a packet and indicates in internal
packet meta data that checksum is to be offloaded to the NIC
(normal transport layer processing for checksum offload). The
checksum field is populated with the bitwise not of the
checksum of the pseudo header or zero as appropriate.
2) Encapsulation layer adds its headers to the packet including
the offload meta data. The start offset and checksum offset are
set accordingly.
3) Encapsulation layer arranges for checksum offload of the outer
header checksum (e.g. UDP).
4) Packet is sent to the NIC. The NIC will perform transmit
checksum offload and set the checksum field in the outer
header. The inner header and rest of the packet are transmitted
without modification.
6.3. Receiver operation
The typical actions a host receiver does to support remote checksum
offload are:
1) Receive packet and validate outer checksum following normal
processing (e.g. validate non-zero UDP checksum).
2) Validate the checksum option. If checksum start is greater than
the length of the packet, then the packet must be dropped. If
checksum offset is greater then the length of the packet minus
two, then the packet must be dropped.
3) Deduce full checksum for the IP packet. If a NIC is capable of
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receive checksum offload it will return either the full
checksum of the received packet or an indication that the UDP
checksum is correct. Either of these methods can be used to
deduce the checksum over the IP packet [UDPENCAP].
4) From the packet checksum, subtract the checksum computed from
the start of the packet (outer IP header) to the offset in the
packet indicted by checksum start in the meta data. The result
is the deduced checksum to set in the checksum field of the
encapsulated transport packet.
In pseudo code:
csum: initialized to checksum computed from start (outer IP
header) to the end of the packet
start_of_packet: address of start of packet
encap_payload_offset: relative to start_of_packet
csum_start: value from meta data
checksum(start, len): function to compute checksum from start
address for len bytes
csum -= checksum(start_of_packet, encap_payload_offset +
csum_start)
4) Write the resultant checksum value into the packet at the
offset provided by checksum offset in the meta data.
In pseudo code:
csum_offset: offset of checksum field
*(start_of_packet + encap_payload_offset +
csum_offset) = csum
5) Checksum is verified at the transport layer using normal
processing. This should not require any checksum computation
over the packet since the complete checksum has already been
provided.
6.4. Security Considerations
Remote checksum offload allows a means to change the GUE payload
before being received at a decapsulator. In order to prevent misuse
of this mechanism, a decapsulator should apply security checks on the
GUE payload only after checksum remote offload has been processed.
7. Processing order of options
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Options must be processed in a specific order for both sending and
receive. Note that some options, such as the checksum option, depend
on other fields in the GUE header to be set.
The order of processing options to send a GUE packet are:
1) Set VNID option.
2) Fragment if necessary and set fragmentation option. VNID is
copied into each fragment. Note that if payload transformation
will increase the size of the payload that must be accounted
for when deciding how to fragment
3) Set Remote checksum offload.
4) Perform payload transform (potentially on each fragment) and
set payload transform option.
5) Set security option.
6) Calculate GUE checksum and set checksum option.
On reception the order of actions is reversed.
1) Verify GUE checksum.
2) Verify security option.
3) Perform payload transformation (i.e. decrypt payload)
4) Adjust packet for remote checksum offload.
5) Perform reassembly.
6) Receive on virtual network indicated by VNID.
Note that the relative processing order of private fields is
unspecified.
8. Security Considerations
Encapsulation of network protocol in GUE should not increase security
risk, nor provide additional security in itself. GUE requires that
the source port for UDP packets should be randomly seeded to mitigate
some possible denial service attacks.
If the integrity and privacy of data packets being transported
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through GUE is a concern, GUE security and payload encryption SHOULD
be used to remove the concern. If the integrity is the only concern,
the tunnel may consider use of GUE security only for optimization.
Likewise, if the privacy is the only concern, the tunnel may use GUE
encryption function only.
If GUE payload already provides secure mechanism, e.g., the payload
is IPsec packets, it is still valuable to consider use of GUE
security.
9. IANA Consideration
IANA is requested to assign flags for the extensions defined in this
specification. Specifically, an assignment is requested for the V,
SEC, K, F, T and R flags in the "GUE flag-fields" registry (proposed
in [I.D.nvo3-gue]).
10. References
10.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[I.D.nvo3-gue] T. Herbert, L. Yong, and O. Zia, "Generic UDP
Encapsulation" draft-ietf-nvo3-gue-03
10.2. Informative References
[RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the
Internet checksum", RFC1071, September 1988.
[RFC1624] Rijsinghani, A., Ed., "Computation of the Internet Checksum
via Incremental Update", RFC1624, May 1994.
[RFC1936] Touch, J. and B. Parham, "Implementing the Internet
Checksum in Hardware", RFC1936, April 1996.
[RFC4459] MTU and Fragmentation Issues with In-the-Network Tunneling.
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P. Savola. April 2006.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, DOI 10.17487/RFC4963,
July 2007, <http://www.rfc-editor.org/info/rfc4963>.
[RFC2764] B. Gleeson, A. Lin, J. Heinanen, G. Armitage, A. Malis, "A
Framework for IP Based Virtual Private Networks", RFC2764,
February 2000.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[RFC3931] Lau, J., Townsley, W., et al, "Layer Two Tunneling Protocol
- Version 3 (L2TPv3)", RFC3931, 1999
[RFC5925] Touch, J., et al, "The TCP Authentication Option", RFC5925,
June 2010.
[RFC6347] Rescoria, E., Modadugu, N., "Datagram Transport Layer
Security Version 1.2", RFC6347, 2012.
[I.D.hy-nvo3-gue-4-nvo] Yong, L., Herbert, T., "Generic UDP
Encapsulation (GUE) for Network Virtualization Overlay"
draft-hy-nvo3-gue-4-nvo-03
[I.D.draft-mathis-frag-harmful] M. Mathis, J. Heffner, and B.
Chandler, "Fragmentation Considered Very Harmful"
[I.D.templin-aerolink] F. Templin, "Transmission of IP Packets over
AERO Links" draft-templin-aerolink-62.txt
[UDPENCAP] T. Herbert, "UDP Encapsulation in Linux",
http://people.netfilter.org/pablo/netdev0.1/papers/UDP-
Encapsulation-in-Linux.pdf
Authors' Addresses
Tom Herbert
Facebook
1 Hacker Way
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Menlo Park, CA
USA
EMail: tom@herbertland.com
Lucy Yong
Huawei USA
5340 Legacy Dr.
Plano, TX 75024
USA
Email: lucy.yong@huawei.com
Fred L. Templin
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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