GRE-in-UDP Encapsulation
draft-ietf-tsvwg-gre-in-udp-encap-08
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Edward Crabbe , Lucy Yong , Xiaohu Xu , Tom Herbert | ||
| Last updated | 2015-12-21 (Latest revision 2015-10-16) | ||
| Replaces | draft-yong-tsvwg-gre-in-udp-encap | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-tsvwg-gre-in-udp-encap-08
Network Working Group E. Crabbe
Internet-Draft
Intended status: Standard Track L. Yong
Huawei USA
X. Xu
Huawei Technologies
T. Herbert
Google
Expires: April 2016 October 16, 2015
GRE-in-UDP Encapsulation
draft-ietf-tsvwg-gre-in-udp-encap-08
Abstract
This document describes a method of encapsulating network protocol
packets within GRE and UDP headers. In this encapsulation, the
source UDP port can be used as an entropy field for purposes of load
balancing, while the protocol of the encapsulated packet in the GRE
payload is identified by the GRE Protocol Type. This encapsulation
protocol can apply to IPv4 and IPv6 networks including the Internet.
When applying it to a well-managed operator network, the tunnel
implementation and usage can be less restrictive. The document
specifies the tunnel implementations under both network scenarios.
Status of This Document
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 16,2016.
Copyright Notice
Copyright (c) 2015 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ........................................... 3
1.1. Applicability Statement .............................. 3
1.1.1. Requirements for Default GRE-in-UDP Tunnel
Implementation over the Internet........................ 5
1.1.2. Requirements for Conditional GRE-in-UDP Tunnel
Implementation over a Well-Managed Operator Network..... 6
2. Terminology ............................................... 7
2.1. Requirements Language................................. 7
3. Encapsulation in UDP ...................................... 7
3.1. IP Header ........................................... 10
3.2. UDP Header .......................................... 10
3.2.1. Source Port .................................... 10
3.2.2. Destination Port ............................... 10
3.2.3. Checksum ....................................... 11
3.2.4. Length ......................................... 11
3.3. GRE Header .......................................... 11
4. Encapsulation Process Procedures ......................... 11
4.1. MTU and Fragmentation ............................... 12
4.2. Differentiated Services ............................. 13
5. UDP Checksum Handling ................................... 13
5.1. UDP Checksum with IPv4 .............................. 13
5.2. UDP Checksum with IPv6 .............................. 14
5.2.1. Middlebox Considerations ....................... 17
6. Congestion Considerations ................................ 17
7. Backward Compatibility .................................. 18
8. IANA Considerations ...................................... 19
9. Security Considerations .................................. 19
10. Acknowledgements ...................................... 21
11. Contributors ......................................... 21
12. References .............................................. 22
12.1. Normative References ............................... 22
12.2. Informative References ............................. 23
13. Authors' Addresses ...................................... 24
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1. Introduction
Load balancing, or more specifically statistical multiplexing of
traffic using Equal Cost Multi-Path (ECMP) and/or Link Aggregation
Groups (LAGs) in IP networks is a widely used technique for creating
higher capacity networks out of lower capacity links. Most existing
routers in IP networks are already capable of distributing IP
traffic flows over ECMP paths and/or LAGs on the basis of a hash
function performed on flow invariant fields in IP packet headers and
their payload protocol headers. Specifically, when the IP payload is
a User Datagram Protocol (UDP)[RFC768] or Transmission Control
Protocol (TCP) [RFC793] packet, router hash functions frequently
operate on the five-tuple of source IP address, destination IP
address, source port, destination port, and protocol/next-header
Several encapsulation techniques are commonly used in IP networks,
such as Generic Routing Encapsulation (GRE) [RFC2784], MPLS
[RFC4023] and L2TPv3 [RFC3931]. GRE is an increasingly popular
encapsulation choice. Unfortunately, use of common GRE endpoints may
reduce the entropy available for use in load balancing, especially
in environments where the GRE Key field [RFC2890] is not readily
available for use as entropy in forwarding decisions.
This document defines a generic GRE-in-UDP encapsulation for
tunneling network protocol packets across an IP network. The GRE
header provides payload protocol type as an EtherType in the
protocol type field [RFC2784][GREIPV6], and the UDP header provides
additional entropy by way of its source port. GRE-in-UDP offers the
additional possibility of using GRE across networks that might
otherwise disallow it; for instance GRE-in-UDP may be used to bridge
two islands where GRE is used natively across the Internet.
This encapsulation method requires no changes to the transit IP
network. Hash functions in most existing IP routers may utilize and
benefit from the use of a GRE-in-UDP tunnel without needing any
change or upgrade to their ECMP implementation. The encapsulation
mechanism is applicable to a variety of IP networks including Data
Center and wide area networks.
1.1. Applicability Statement
GRE encapsulation has been widely used for many applications. For
example, to redirect IP traffic to traverse a different path instead
of the default path in an operator network, to tunnel private
network traffic over a public network by use of public IP network
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addresses, to tunnel IPv6 traffic over an IPv4 network, tunnel
Ethernet traffic over IP networks [RFC7637], etc.
GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks including
the Internet. When using GRE-in-UDP encapsulation, encapsulated
traffic will be treated as a UDP application in an IP network. As
such, GRE-in-UDP tunnel needs to meet UDP application requirements
specified in [RFC5405bis], which requires additional tunnel
functions besides the packet encapsulation/decapsulation at the
tunnel endpoints. The required additional functions may be
simplified according to the network operation condition. For
example, if a GRE-in-UDP tunnel is used to carry IP payload only,
tunnel congestion control function is not necessary.
This document considers two network scenarios: 1) Use of GRE-in-UDP
in a general IP network including the Internet, where a default GRE-
in-UDP tunnel implementation specified in this draft can apply; 2)
Use of GRE-in-UDP in a well-managed operator IP network, where a
GRE-in-UDP tunnel implementation can be less restrictive than the
default implementation. The implementation for a well-managed
operator IP network is specified in this draft too and is referred
to as conditional GRE-in-UDP tunnel implementation in the remaining
document.
A well-managed operator IP network (referred to Operator Network in
the rest) is an IP network that meets at least one of following
conditions:
a. Under single administrative control (such as within a single
operator's network) where it is known (perhaps through knowledge
of equipment types and lower layer checks) that packet corruption
is exceptionally unlikely and where the operator is willing to
take the risk of undetected packet corruption.
b. Under single administrative control (such as within a single
operator's network) where it is judged through observational
measurements (perhaps of historic or current traffic flows that
use a non-zero checksum) that the level of packet corruption is
tolerably low and where the operator is willing to take the risk
of undetected packet corruption.
c. Carrying applications that are tolerant of mis-delivered or
corrupted packets (perhaps through higher layer checksum,
validation, and retransmission or transmission redundancy) where
the operator is willing to rely on the applications using the
tunnel to survive any corrupt packets.
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As a result, use of GRE-in-UDP within a well-managed operator
network, UDP zero-checksum in IPv6 may be used (see Section 5.2).
Another characteristic that a well-managed operator network often
has is a congestion control, i.e. the network is traffic-engineered
and/or operated to avoid congestion.
GRE-in-UDP tunnel implementation, either default or conditional,
does not have congestion control capability. Therefore, it limits
its usage for either tunneled traffic having congestion control
and/or a well-managed operator network that provides traffic-
engineering to avoid congestion.
As a result, default GRE-in-UDP tunnel implementation MUST NOT apply
to traffic that has no congestion control over the Internet;
conditional GRE-in-UDP tunnel implementation can apply to a well-
managed operator network that provides congestion control. (See
Section 6)
The following two sections summarize the requirements of GRE-in-UDP
tunnel implementation for a generic IP network including the
Internet and a well-managed operator network, respectively. The
networks can be IPv4 or Ipv6.
1.1.1. Requirements for Default GRE-in-UDP Tunnel Implementation
over the Internet
The following are the requirements for default GRE-in-UDP tunnel
implementation that can apply to an IP network including Internet.
1. SHOULD perform UDP checksum when over an IPv4 network.
2. MUST perform UDP checksum when over an IPv6 network.
3. IP-traffic can be assumed to be congestion-controlled; other
tunneled protocol/payload SHOULD implement an appropriate congestion
control method because the GRE/UDP tunnel does not itself provide
any congestion control. If GRE-in-UDP tunnel MUST NOT to traffic
that has no congestion control over the general Internet.
4. UDP src port that is used for flow entropy SHOULD be set to a UDP
ephemeral port (49152-65535).
5. For IPv6 delivery network, if IPv6 flow label load balancing is
supported [RFC4638], the flow entropy SHOULD also be placed in the
flow label field.
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6. If a tunnel ingress fragments the incoming packet (before
encapsulation), the UDP checksum MUST be used so that the receiving
endpoint can validate reassembly of the fragments, and the same src
UDP port SHOULD be used for all packet fragments to ensure that the
transit routers will forward the packet fragments on the same path.
7. If the incoming packet needs to be fragmented, it SHOULD be done
before the encapsulation [RFC7588] and calculate the size of
fragments based on the MTU and including the size of the UDP header.
1.1.2. Requirements for Conditional GRE-in-UDP Tunnel Implementation
over a Well-Managed Operator Network
The following are the requirements for conditional GRE-in-UDP tunnel
implementation that can apply to a well-managed IP network described
above.
1. When over an IPv4 network, SHOULD set UDP zero-checksum to
improve the tunnel performance.
2. When over an IPv6 network, MUST perform UDP checksum as default
but MAY be configured with UDP zero-checksum with additional
implementation requirements that are specified in Section 5.2.
3. A tunnel may encapsulate a protocol/payload that does not provide
congestion control if the delivery network is traffic-engineered
and/or operated by the network operator to avoid congestion, e.g.
use of pre-provision capacity or utilize a circuit breaker [CK].
4. UDP src port that is used for flow entropy SHOULD be set to a UDP
ephemeral port (49152-65535).
5. For IPv6 delivery network, if IPv6 flow label load balancing is
supported [RFC4638], the flow entropy SHOULD also be placed in the
flow label field.
6. If a tunnel ingress fragments the incoming packet (before
encapsulation), the UDP checksum MUST be used so that the receiving
endpoint can validate reassembly of the fragments, and the same src
UDP port SHOULD be used for all packet fragments to ensure that the
transit routers will forward the packet fragments on the same path.
7. If the incoming packet needs to be fragmented, it SHOULD be done
before the encapsulation [RFC7588] and calculate the size of
fragments based on the MTU and including the size of the UDP header.
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GRE-in-UDP encapsulation may be used to encapsulate already tunneled
traffic, i.e. tunnel-in-tunnel. The tunneled traffic may use GRE-in-
UDP or other tunnel encapsulation. In this case, GRE-in-UDP tunnel
endpoints treat other tunnel endpoints as of the end hosts for the
traffic and do not differentiate such end hosts from other end hosts.
2. Terminology
The terms defined in [RFC768][RFC2784] are used in this document.
Default GRE-in-UDP tunnel implementation: GRE-in-UDP tunnel
implementation that can apply to an IP network including the
Internet.
Conditional GRE-in-UDP tunnel implementation: GRE-in-UDP tunnel
implementation that can only apply to a well-managed operator
network that is defined in Section 1.1.
2.1. 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].
3. Encapsulation in UDP
GRE-in-UDP encapsulation format is shown as follows:
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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
IPv4 Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live |Protcol=17(UDP)| Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
UDP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port = XXXX | Dest Port = TBD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
GRE Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| |K|S| Reserved0 | Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum (optional) | Reserved1 (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 UDP+GRE Headers in IPv4
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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
IPv6 Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | NxtHdr=17(UDP)| Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Outer Source IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Outer Destination IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
UDP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port = XXXX | Dest Port = TBD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
GRE Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| |K|S| Reserved0 | Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum (optional) | Reserved1 (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 UDP+GRE Headers in IPv6
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The contents of the IP, UDP, and GRE headers that are relevant in
this encapsulation are described below.
3.1. IP Header
An encapsulator MUST encode its own IP address as the source IP
address and the decapsulator's IP address as the destination IP
address. The TTL field in the IP header MUST be set to a value
appropriate for delivery of the encapsulated packet to the peer of
the encapsulation.
3.2. UDP Header
3.2.1. Source Port
The UDP source port contains a 16-bit entropy value that is
generated by the encapsulator to identify a flow for the
encapsulated packet. The port value SHOULD be within the ephemeral
port range. IANA suggests this range to be 49152 to 65535, where the
high order two bits of the port are set to one. This provides
fourteen bits of entropy for the inner flow identifier. In the case
that an encapsulator is unable to derive flow entropy from the
payload header, it SHOULD set a randomly selected constant value for
UDP source port to avoid payload packet flow reordering, e.g. use of
the system time to yield a value that is the range of entropy values.
The source port value for a flow set by an encapsulator MAY change
over the lifetime of the encapsulated flow. For instance, an
encapsulator may change the assignment for Denial of Service (DOS)
mitigation or as a means to effect routing through the ECMP network.
An encapsulator SHOULD NOT change the source port selected for a
flow more than once every thirty seconds.
For IPv6 delivery network, if IPv6 flow label load balancing is
supported [RFC6438], the flow entropy SHOULD also be placed in the
flow label field.
How an encapsulator generates flow entropy from the payload is
outside the scope of this document.
3.2.2. Destination Port
The destination port of the UDP header is set the GRE-in-UDP port or
GRE-UDP-DTLS (TBD) (see Section 8).
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3.2.3. Checksum
The UDP checksum is set and processed per [RFC768] and [RFC1122] for
IPv4, and [RFC2460] for IPv6. Requirements for checksum handling and
use of zero UDP checksums are detailed in Section 5.
3.2.4. Length
The usage of this field is in accordance with the current UDP
specification in [RFC768]. This length will include the UDP header
(eight bytes), GRE header, and the GRE payload (encapsulated packet).
3.3. GRE Header
An encapsulator sets the protocol type (EtherType) of the packet
being encapsulated in the GRE Protocol Type field.
An encapsulator may set the GRE Key Present, Sequence Number Present,
and Checksum Present bits and associated fields in the GRE header as
defined by [RFC2784] and [RFC2890].
The GRE checksum MAY be enabled to protect the GRE header and
payload. An encapsulator SHOULD NOT enable both the GRE checksum and
UDP checksum simultaneously as this would be mostly redundant. Since
the UDP checksum covers more of the packet including the GRE header
and payload, the UDP checksum SHOULD have preference to using GRE
checksum. The GRE checksum SHOULD be used for the payload integrity
check when use of UDP zero-checksum.
An implementation MAY use the GRE keyid to authenticate the
encapsulator. (See Security Section) In this model, a shared value
is either configured or negotiated between an encapsulator and
decapsulator. When a decapsulator determines a presented keyid is
not valid for the source, the packet MUST be dropped.
Although GRE-in-UDP encapsulation protocol uses both UDP header and
GRE header, it is one tunnel encapsulation protocol. GRE and UDP
headers MUST be applied and removed as a pair at the encapsulation
and decapsulation points. This specification does not support UDP
encapsulation of a GRE header where that GRE header is applied or
removed at a network node other than the UDP tunnel ingress or
egress.
4. Encapsulation Process Procedures
The GRE-in-UDP encapsulation allows encapsulated packets to be
forwarded through "GRE-in-UDP tunnels". When performing GRE-in-UDP
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encapsulation by the encapsulator, the entropy value is generated by
the encapsulator and then be filled in the Source Port field of the
UDP header. The Destination Port field is set to a value (TBD)
allocated by IANA to indicate that the UDP tunnel payload is a GRE
packet. The Protocol Type header field in GRE header is set to the
EtherType value corresponding to the protocol of the encapsulated
packet.
Intermediate routers, upon receiving these UDP encapsulated packets,
could balance these packets based on the hash of the five-tuple of
UDP packets.
Upon receiving these UDP encapsulated packets, the decapsulator
would decapsulate them by removing the UDP and GRE headers and then
process them accordingly.
Note: Each UDP tunnel is unidirectional, as GRE-in-UDP traffic is
sent to the IANA-allocated UDP Destination Port, and in particular,
is never sent back to any port used as a UDP Source Port (which
serves solely as a source of entropy). This is at odds with a common
middlebox (e.g., firewall) assumption that bidirectional traffic
uses a common pair of UDP ports. As a result, arranging to pass
bidirectional GRE-in-UDP traffic through middleboxes may require
separate configuration for each direction of traffic.
GRE-in-UDP allows encapsulation of unicast, broadcast, or multicast
traffic. Entropy may 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 to the encapsulation and is otherwise outside the scope
of this document.
To provide entropy for ECMP, GRE-in-UDP does not rely on GRE keep-
alive. It is RECOMMENED no use of GRE keep-alive in the GRE-in-UDP
tunnel. This aligns with middlebox traversal guidelines in Section
3.5 of [RFC5405bis].
The procedures specified in this section apply to default GRE-in-UDP
tunnel implementation and conditional GRE-in-UDP tunnel
implementation.
4.1. MTU and Fragmentation
Regarding packet fragmentation, an encapsulator/decapsulator SHOULD
be compliant with [RFC7588]. For this case, the MTU is equal to the
PMTU associated with the path between the GRE ingress and the GRE
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egress nodes minus the GRE and UDP overhead. When applying payload
fragment, the UDP checksum MUST be used so that the receiving
endpoint can validate reassembly of the fragments; the same src UDP
port SHOULD be used for all packet fragments to ensure the transit
routers will forward the fragments on the same path. An operator
should factor in the additional bytes of overhead when considering
an MTU size for the payload to avoid the likelihood of fragmentation.
4.2. Differentiated Services
To ensure that tunneled traffic gets the same treatment over the IP
network, prior to the encapsulation process, an encapsulator should
process the payload to get the proper parameters to fill into the IP
header such as DiffServ [RFC2983]. Encapsulation end points that
support ECN must use the method described in [RFC6040] for ECN
marking propagation. This process is outside of the scope of this
document.
5. UDP Checksum Handling
5.1. 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. The IPv4
header includes a checksum which protects against mis-delivery of
the packet due to corruption of IP addresses. The UDP checksum
potentially provides protection against corruption of the UDP header,
GRE header, and GRE 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 protected by other, possibly stronger
mechanisms such as the Ethernet CRC.
When a decapsulator receives a packet, the UDP checksum field MUST
be processed. If the UDP checksum is non-zero, the decapsulator MUST
verify the checksum before accepting the packet. By default a
decapsulator SHOULD accept UDP packets with a zero checksum. A node
MAY be configured to disallow zero checksums per [RFC1122]; this may
be done selectively, for instance disallowing zero checksums from
certain hosts that are known to be sending over paths subject to
packet corruption. If verification of a non-zero checksum fails, a
decapsulator lacks the capability to verify a non-zero checksum, or
a packet with a zero-checksum was received and the decapsulator is
configured to disallow, the packet MUST be dropped and an event MAY
be logged.
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Default GRE-in-UDP tunnel implementation SHOULD perform UDP checksum.
Conditional GRE-in-UDP tunnel implementation MAY set UDP zero-
checksum.
5.2. UDP Checksum with IPv6
For UDP in IPv6, the UDP checksum MUST be processed as specified in
[RFC768] and [RFC2460] for both transmit and receive.
When UDP is used over IPv6, the UDP checksum is relied upon to
protect both the IPv6 and UDP headers from corruption. As such,
default GRE-in-UDP tunnel implementation MUST perform UDP checksum;
conditional GRE-in-UDP tunnel implementation MAY be configured with
the UDP zero-checksum mode when the tunnel is used in a well-managed
operator network and/or within a set of closely cooperating network
administrations (such as network operators who have agreed to work
together in order to jointly provide specific services).
As such, for IPv6, the UDP checksum for GRE-in-UDP MUST be used as
specified in [RFC768] and [RFC2460] for tunnels that span multiple
networks whose network administrations do not cooperate closely,
even if each non-cooperating network administration independently
satisfies the condition for UDP zero-checksum mode usage with GRE-
in-UDP over IPv6.
The use of the UDP zero-checksum mode must meet the requirements
specified in [RFC6935] and [RFC6936], which conducts the following
additional requirements for GRE-in-UDP tunnel implementation and use
of UDP zero-checksum mode for GRE-in-UDP over IPv6:
a. Use of the UDP checksum with IPv6 MUST be the default
configuration of all GRE-in-UDP implementations.
b. The GRE-in-UDP implementation MUST comply with all requirements
specified in Section 4 of [RFC6936] and with requirement 1
specified in Section 5 of [RFC6936].
c. The tunnel decapsulator SHOULD only allow the use of UDP zero-
checksum mode for IPv6 on a single received UDP Destination
Port regardless of the encapsulator. The motivation for this
requirement is possible corruption of the UDP Destination Port,
which may cause packet delivery to the wrong UDP port. If that
other UDP port requires the UDP checksum, the mis-delivered
packet will be discarded
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d. It is RECOMMENDED that UDP zero-checksum selectively be enabled
for certain source addresses. The tunnel decapsulator MUST
check that the source and destination IPv6 addresses are valid
for the GRE-in-UDP tunnel on which the packet was received if
that tunnel uses UDP zero-checksum mode and discard any packet
for which this check fails.
e. The tunnel encapsulator SHOULD use different IPv6 addresses for
each GRE-in-UDP tunnel that uses UDP zero-checksum mode
regardless of the decapsulator in order to strengthen the
decapsulator's check of the IPv6 source address (i.e., the same
IPv6 source address SHOULD NOT be used with more than one IPv6
destination address, independent of whether that destination
address is a unicast or multicast address). When this is not
possible, it is RECOMMENDED to use each source IPv6 address for
as few UDP zero-checksum mode GRE-in-UDP tunnels as is feasible.
Note that if UDP checksum is used, such restriction is not
necessary.
f. When any middlebox exists on the path of GRE-in-UDP tunnel, it
is RECOMMENDED to use the default mode, i.e. use UDP checksum,
to reduce the chance that the encapsulated packets to be
dropped.
g. Any middlebox for UDP zero-checksum mode for IPv6 MUST comply
with requirement 1 and 8-10 in Section 5 of [RFC6936]
h. Measures SHOULD be taken to prevent IPv6 traffic with zero UDP
checksums from "escaping" to the general Internet; see Section
6 for examples of such measures.
i. IPv6 traffic with zero UDP checksums MUST be actively monitored
for errors by the network operator. For example, Ethernet layer
packet error rate or probe packet error rate.
j. If a packet with a non-zero checksum is received, the checksum
MUST be verified before accepting the packet. This is
regardless of whether the tunnel encapsulator and decapsulator
have been configured with UDP zero-checksum mode.
The above requirements do not change either the requirements
specified in [RFC2460] as modified by [RFC6935] or the requirements
specified in [RFC6936].
The requirement to check the source IPv6 address in addition to the
destination IPv6 address, plus the strong recommendation against
reuse of source IPv6 addresses among GRE-in-UDP tunnels collectively
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provide some mitigation for the absence of UDP checksum coverage of
the IPv6 header. Additional assurance is provided by the
restrictions in the above exceptions that limit usage of IPv6 UDP
zero-checksum mode to well-managed networks for which GRE
encapsulated packet corruption has not been a problem in practice.
Hence GRE-in-UDP is suitable for transmission over lower layers in
the well-managed networks that are allowed by the exceptions stated
above and the rate of corruption of the inner IP packet on such
networks is not expected to increase by comparison to GRE traffic
that is not encapsulated in UDP. For these reasons, GRE-in-UDP does
not provide an additional integrity check except when GRE checksum
is used when UDP zero-checksum mode is used with IPv6, and this
design is in accordance with requirements 2, 3 and 5 specified in
Section 5 of [RFC6936].
GRE does not accumulate incorrect state as a consequence of GRE
header corruption. A corrupt GRE results in either packet discard or
forwarding of the packet without accumulation of GRE state. GRE
checksum MAY be used for protecting GRE header and payload. Active
monitoring of GRE-in-UDP traffic for errors is REQUIRED as
occurrence of errors will result in some accumulation of error
information outside the protocol for operational and management
purposes. This design is in accordance with requirement 4 specified
in Section 5 of [RFC6936].
The remaining requirements specified in Section 5 of [RFC6936] are
inapplicable to GRE-in-UDP. Requirements 6 and 7 do not apply
because GRE does not have a GRE-generic control feedback mechanism.
Requirements 8-10 are middlebox requirements that do not apply to
GRE-in-UDP tunnel endpoints, but see Section 5.2.1 for further
middle box discussion.
It is worth mentioning that the use of a zero UDP checksum should
present the equivalent risk of undetected packet corruption when
sending similar packet using GRE-in-IPv6 without UDP [GREIPV6] and
without GRE checksums.
In summary, conditional GRE-in-UDP tunnel implementation is allowed
to use UDP-zero-checksum mode for IPv6, when additional
implementation requirements stated above are provided. Otherwise the
UDP checksum MUST be used for IPv6 as specified in [RFC768] and
[RFC2460]. Use of GRE checksum favors non-use of the UDP checksum.
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5.2.1. Middlebox Considerations
IPv6 datagrams with a zero UDP checksum will not be passed by any
middlebox that validates the checksum based on [RFC2460] or that
updates the UDP checksum field, such as NATs or firewalls. Changing
this behavior would require such middleboxes to be updated to
correctly handle datagrams with zero UDP checksums. The GRE-in-UDP
encapsulation does not provide a mechanism to safely fall back to
using a checksum when a path change occurs redirecting a tunnel over
a path that includes a middlebox that discards IPv6 datagrams with a
zero UDP checksum. In this case the GRE-in-UDP tunnel will be black-
holed by that middlebox.
As such, when any middle box exists on the path of GRE-in-UDP tunnel,
it is RECOMMENDED to use the UDP checksum to reduce the chance that
the encapsulated packets to be dropped. Recommended changes to allow
firewalls, NATs and other middleboxes to support use of an IPv6 zero
UDP checksum are described in Section 5 of [RFC6936].
6. Congestion Considerations
Section 3.1.3 of [RFC5405] discussed the congestion implications of
UDP tunnels. As discussed in [RFC5405], because other flows can
share the path with one or more UDP tunnels, congestion control
[RFC2914] needs to be considered.
The impact of congestion must be considered both in terms of the
effect on the rest of the network of a UDP tunnel that is consuming
excessive capacity, and in terms of the effect on the flows using
the UDP tunnels. The potential impact of congestion from a UDP
tunnel depends upon what sort of traffic is carried over the tunnel,
as well as the path of the tunnel.
In many cases, GRE-in-UDP is used to carry IP traffic. IP traffic is
generally assumed to be congestion controlled, and thus a tunnel
carrying general IP traffic generally does not need additional
congestion control mechanisms.
However, GRE-in-UDP tunnel can be used in some cases to carry
traffic that is not necessarily congestion controlled. For example,
GRE-in-UDP may be used to carry MPLS that carries pseudowire or VPN
traffic where specific bandwidth guarantees are provided to each
pseudowire or to each VPN. In such cases, network operators may
avoid congestion by careful provisioning of their networks, by rate
limiting of user data traffic, and traffic engineer according to
path capacity. For this reason, GRE-in-UDP tunnel MUST be used
within a single operator's network that utilizes careful
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provisioning (e.g., rate limiting at the entries of the network
while over-provisioning network capacity) to ensure against
congestion, or within a limited number of networks whose operators
closely cooperate in order to jointly provide this same careful
provisioning.
Default GRE-in-UDP tunnel implementation can be used to carry IP
traffic that is known to be congestion controlled on the Internet.
Internet IP traffic is generally assumed to be congestion-controlled.
GRE-in-UDP MUST NOT be used over the general Internet, or over non-
cooperating network operators, to carry traffic that is not
congestion-controlled.
Conditional GRE-in-UDP tunnel implementation can be used within a
well-managed operator network to carry traffic that is not necessary
congestion controlled. Measures SHOULD be taken to prevent non-
congestion-controlled GRE-in-UDP traffic from "escaping" to the
general Internet, e.g.:
o Physical or logical isolation of the links carrying GRE-in-UDP
from the general Internet.
o Deployment of packet filters that block the UDP ports assigned
for GRE-in-UDP.
o Imposition of restrictions on GRE-in-UDP traffic by software
tools used to set up GRE-in-UDP tunnels between specific end
systems (as might be used within a single data center). For
examples, a GRE-in-UDP tunnel only carries IP traffic or a GRE-
in-UDP tunnel supports NVEGRE encapsulation only (Although the
payload type is Ethernet in NVGRE, NVGRE protocol mandates that
the payload of Ethernet is IP).
o Use of a "Circuit Breaker" for the tunneled traffic as described
in [CB].
7. Backward Compatibility
It is assumed that tunnel ingress routers must be upgraded in order
to support the encapsulations described in this document.
No change is required at transit routers to support forwarding of
the encapsulation described in this document.
If a router that is intended for use as a decapsulator does not
support or enable GRE-in-UDP encapsulation described in this
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document, it will not be listening on the destination port (TBD).
In these cases, the router will conform to normal UDP processing and
respond to an encapsulator with an ICMP message indicating "port
unreachable" according to [RFC792]. Upon receiving this ICMP
message, the node MUST NOT continue to use GRE-in-UDP encapsulation
toward this peer without management intervention.
8. IANA Considerations
IANA is requested to make the following allocations:
One UDP destination port number for the indication of GRE
Service Name: GRE-in-UDP
Transport Protocol(s): UDP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: GRE-in-UDP Encapsulation
Reference: [This.I-D]
Port Number: TBD
Service Code: N/A
Known Unauthorized Uses: N/A
Assignment Notes: N/A
One UDP destination port number for the indication of GRE with DTLS
Service Name: GRE-UDP-DTLS
Transport Protocol(s): UDP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: GRE-in-UDP Encapsulation with DTLS
Reference: [This.I-D]
Port Number: TBD
Service Code: N/A
Known Unauthorized Uses: N/A
Assignment Notes: N/A
9. Security Considerations
GRE-in-UDP encapsulation does not affect security for the payload
protocol. When using GRE-in-UDP, Network Security in a network is
mostly equivalent to that of a network using GRE.
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Datagram Transport Layer Security (DTLS) [RFC6347] can be used for
application security and can preserve network and transport layer
protocol information. Specifically, if DTLS is used to secure the
GRE-in-UDP tunnel, the destination port of the UDP header MUST be
set to an IANA-assigned value (TBD2) indicating GRE-in-UDP with DTLS,
and that UDP port MUST NOT be used for other traffic. The UDP
source port field can still be used to add entropy, e.g., for load-
sharing purposes. DTLS usage 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 GRE-in-UDP tunnel decapsulator implementation that supports
DTLS is expected to be able to establish DTLS sessions with multiple
tunnel encapsulators, and likewise an GRE-in-UDP tunnel encapsulator
implementation is expected to be able to establish DTLS sessions
with multiple decapsulators (although different source and/or
destination IP addresses may be involved -see Section 5.2 for
discussion of one situation where use of different source IP
addresses is important).
Use of ICMP for signaling of the GRE-in-UDP encapsulation capability
adds a security concern. Upon receiving an ICMP message and before
taking an action on it, the ingress MUST validate the IP address
originating against tunnel egress address and MUST evaluate the
packet header returned in the ICMP payload to ensure the source port
is the one used for this tunnel. The mechanism for performing this
validation is out of the scope of this document.
In an instance where the UDP source port is not set based on the
flow invariant fields from the payload header, a random port SHOULD
be selected in order to minimize the vulnerability to off-path
attacks. [RFC6056]. The random port may also be periodically changed
to mitigate certain denial of service attacks. How the source port
randomization occurs is outside scope of this document.
Using one standardized value in UDP destination port for an
encapsulation indication may increase the vulnerability of off-path
attack. To overcome this, an alternate port may be agreed upon to
use between an encapsulator and decapsulator [RFC6056]. How the
encapsulator end points communicate the value is outside scope of
this document.
This document does not require that decapsulator validates the IP
source address of the tunneled packets (with the exception that the
IPv6 source address MUST be validated when UDP zero-checksum mode is
used with IPv6), but it should be understood that failure to do so
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presupposes that there is effective destination-based (or a
combination of source-based and destination-based) filtering at the
boundaries.
Corruption of GRE header can cause a privacy and security concern
for some applications that rely on the key field for traffic
segregation. When GRE key field is used for privacy and security,
ether UDP checksum or GRE checksum SHOULD be used for GRE-in-UDP
with both IPv4 and IPv6, and in particular, when UDP zero-checksum
mode is used, GRE checksum SHOULD be used.
10. Acknowledgements
Authors like to thank Vivek Kumar, Ron Bonica, Joe Touch, Ruediger
Geib, Lar Edds, Lloyd, and many others for their review and valuable
input on this draft.
Thank the design team led by David Black (members: Ross Callon,
Gorry Fairhurst, Xiaohu Xu, Lucy Yong) to efficiently work out the
descriptions for the congestion considerations and IPv6 UDP zero
checksum.
11. Contributors
The following people all contributed significantly to this document
and are listed below in alphabetical order:
David Black
EMC Corporation
176 South Street
Hopkinton, MA 01748
USA
Email: david.black@emc.com
Ross Callon
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: rcallon@juniper.net
John E. Drake
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Juniper Networks
Email: jdrake@juniper.net
Gorry Fairhurst
University of Aberdeen
Email: gorry@erg.abdn.ac.uk
Yongbing Fan
China Telecom
Guangzhou, China.
Phone: +86 20 38639121
Email: fanyb@gsta.com
Adrian Farrel
Juniper Networks
Email: adrian@olddog.co.uk
Vishwas Manral
Hewlett-Packard Corp.
3000 Hanover St, Palo Alto.
Email: vishwas.manral@hp.com
Carlos Pignataro
Cisco Systems
7200-12 Kit Creek Road
Research Triangle Park, NC 27709 USA
EMail: cpignata@cisco.com
12. References
12.1. Normative References
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
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[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", RFC1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC2890, September 2000.
[RFC5405bis] Eggert, L., "Unicast UDP Usage Guideline for
Application Designers", draft-ietf-tsvwg-rfc5405bis, work
in progress.
[RFC6040] Briscoe, B., "Tunneling of Explicit Congestion
Notification", RFC6040, November 2010.
[RFC6347] Rescoria, E., Modadugu, N., "Datagram Transport Layer
Security Version 1.2", RFC6347, 2012.
[RFC6438] Carpenter, B., Amante, S., "Using the IPv6 Flow Label for
Equal Cost Multipath Routing and Link Aggregation in
tunnels", RFC6438, November, 2011.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 2013.
12.2. Informative References
[RFC792] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[RFC793] DARPA, "Transmission Control Protocol", RFC793, September
1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2914] Floyd, S.,"Congestion Control Principles", RFC2914,
September 2000.
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[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC2983,
October 2000.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
MPLS in IP or Generic Routing Encapsulation (GRE)", RFC
4023, March 2005.
[RFC6056] Larsen, M. and Gont, F., "Recommendations for Transport-
Protocol Port Randomization", RFC6056, January 2011.
[RFC6438] Carpenter, B., Amante, S., "Using the Ipv6 Flow Label for
Equal Cost Multipath Routing and Link Aggreation in
Tunnels", RFC6438, November 2011.
[RFC7588] Bonica, R., "A Fragmentation Strategy for Generic Routing
Encapsulation (GRE)", RFC7588, July 2015.
[RFC7637] Garg, P. and Wang, Y., "NVGRE: Network Virtualization
Using Generic Routing Encapsulation", RFC7637, September
2015.
[CB] Fairhurst, G., "Network Transport Circuit Breakers",
draft-ietf-tsvwg-circuit-breaker-04, work in progress.
[GREIPV6] Pignataro, C., Bonica, R., Krishnan, S., "IPv6 Support for
Generic Routing Encapsulation (GRE)", draft-ietf-intarea-
gre-ipv6, work in progress.
13. Authors' Addresses
Edward Crabbe
Email: edward.crabbe@gmail.com
Lucy Yong
Huawei Technologies, USA
Email: lucy.yong@huawei.com
Xiaohu Xu
Huawei Technologies,
Beijing, China
Email: xuxiaohu@huawei.com
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Tom Herbert
Google
1600 Amphitheatre Parkway
Mountain View, CA
Email : tom@herbertland.com
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