Network Working Group E. Crabbe
Internet-Draft
Intended status: Standard Track L. Yong
Huawei USA
X. Xu
Huawei Technologies
T. Herbert
Google
Expires: August 2015 February 11, 2015
GRE-in-UDP Encapsulation
draft-ietf-tsvwg-gre-in-udp-encap-04
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. Usage restrictions
apply to GRE-in-UDP usage for traffic that is not congestion
controlled and to UDP zero checksum usage with IPv6.
Status of This Document
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 11, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................3
1.1. Applicability Statement...................................3
2. Terminology....................................................4
2.1. Requirements Language.....................................4
3. Encapsulation in UDP...........................................4
3.1. IP header.................................................7
3.2. UDP header................................................7
3.2.1. Source Port..........................................7
3.2.2. Destination port.....................................7
3.2.3. Checksum.............................................7
3.2.4. Length...............................................8
3.3. GRE header................................................8
4. UDP Checksum Handling..........................................8
4.1. UDP Checksum with IPv4....................................8
4.2. UDP Checksum with IPv6....................................9
4.2.1. Middlebox Considerations for IPv6 UDP Zero Checksums12
5. Encapsulation Process Procedures..............................12
5.1. Packet Fragmentation.....................................13
5.2. Differentiated services..................................13
6. Congestion Considerations.....................................14
7. Backward Compatibility........................................15
8. IANA Considerations...........................................16
9. Security Considerations.......................................16
9.1. Vulnerability............................................16
10. Acknowledgements.............................................17
11. Contributors.................................................17
12. References...................................................19
12.1. Normative References....................................19
12.2. Informative References..................................19
13. Authors' Addresses...........................................20
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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.
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 is 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 addresses,
or to tunnel IPv6 traffic over an IPv4 network, etc.
When encapsulating GRE in UDP, encapsulated traffic will be treated
as a UDP application, not as a GRE application, in an IP network.
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Thus GRE-in-UDP applications must meet UDP tunnel requirements as
specified in [RFC5405]. This may constrain GRE-in-UDP tunnel usage
in certain applications and/or environments. See Section 6.
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
end points 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.
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.
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.
How an encapsulator generates 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/UDP port (TBD)
(see Section 8).
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 4.
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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.
4. UDP Checksum Handling
4.1. 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. An
encapsulator MAY set the UDP checksum to zero for performance or
implementation considerations. The IPv4 header includes a checksum
which protects against mis-delivery of the packet due to corruption
of IP addresses. The UDP checksum potentially provides protection
against corruption of the UDP header, 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
already adequately 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
decapsularor 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
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decapsulator lacks the capability to verify a non-zero checksum, or
a packet with a zero-checksum was received and the decapsulator is
configured to disallow, the packet MUST be dropped and an event MAY
be logged.
4.2. 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, and so MUST
used with the following exceptions:
a. Use of GRE-in-UDP in networks 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. Use of GRE-in-UDP in networks 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. Use of GRE-in-UDP for traffic delivery for applications that
are tolerant of misdelivered 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.
For these exceptions, the UDP zero-checksum mode can be used.
However the use of the UDP zero-checksum mode must meet the
requirements specified in [RFC6935] and [RFC6936] as well at the
additional requirements stated below.
These exceptions may also be extended to the use of GRE-in-UDP
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).
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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 one or more of the exceptions for UDP zero-checksum mode
usage with GRE-in-UDP over IPv6.
The following additional requirements apply to 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. A decapsulator SHOULD only allow the use of UDP zero-checksum
mode for IPv6 on a single received UDP Destination Port. 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
d. By default a decapsulator MUST disallow receipt of GRE-in-UDP
packets with zero UDP checksums with IPv6. Zero checksums May
selectively be enabled for certain source address. A 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. An 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.
f. Any middlebox support for GRE-in-UDP with UDP zero-checksum mode
for IPv6 MUST comply with requirements 1 and 8-10 in Section 5 of
[RFC6936].[RFC6936].
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g. 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.
h. IPv6 traffic with zero UDP checksums MUST be actively monitored
for errors by the network operator.
i. The use a zero UDP checksum should present the equivalent risk of
undetected packet corruption when sending similar packet using
GRE-in-IPv6 without UDP and without GRE checksums.
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 a 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
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
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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 3.2 for further
middlebox discussion.
In summary, UDP zero-checksum mode for IPv6 is allowed to be used
with GRE-in-UDP when one of the three exceptions specified above
applies, provided that additional requirements stated above are
complied with. Otherwise the UDP checksum MUST be used for IPv6 as
specified in [RFC768] and [RFC2460].
4.2.1. Middlebox Considerations for IPv6 UDP Zero Checksums
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. 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].
5. Encapsulation Process Procedures
This GRE-in-UDP encapsulation allows packets to be forwarded through
"GRE-UDP tunnels". When performing GRE-in-UDP encapsulation by the
encapsulator, the entropy value would be 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.
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Upon receiving these UDP encapsulated packets, the decapsulator
would decapsulate them by removing the UDP 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.
5.1. Packet Fragmentation
Regarding Fragmentation, an encapsulator SHOULD perform
fragmentation [GREMTU] on a packet before encapsulation and factor
in both GRE and UDP header bytes in the effective Maximum
Transmission Unit (MTU) size. Not performing the fragmentation will
cause the packets exceeding network MTU size to be dropped or
fragmented in the network. An encapsulator MUST use the same source
UDP port for all packet fragments to ensure that the transit routers
will forward the packet fragments on the same path. An operator
should factor in the additional bytes of overhead when considering
an MTU size for the payload to reduce the likelihood of
fragmentation.
5.2. 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.
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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.
A major motivation for GRE-in-UDP encapsulation is to tunnel a
network protocol over IP network and improve the use of multipath
(such as ECMP) in cases where traffic is to traverse routers which
are able to hash on UDP Port and IP address. As such, in many cases
this may reduce the occurrence of congestion and improve usage of
available network capacity. However, it is also necessary to ensure
that the network, including applications that use the network,
responds appropriately in more difficult cases, such as when link or
equipment failures have reduced the available capacity.
The impact of congestion must be considered both in terms of the
effect on the rest of the network over which packets are sent in UDP
tunnels, and in terms of the effect on the flows that are sent by
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.
GRE encapsulation is widely used to carry a wide range of network
protocols and traffic. In many cases GRE encapsulation is used to
carry IP traffic. IP traffic is generally assumed to be congestion
controlled, and thus a tunnel carrying general IP traffic (as might
be expected to be carried across the Internet) generally does not
need additional congestion control mechanisms. As specified in RFC
5405:
"IP-based traffic is generally assumed to be congestion-controlled,
i.e., it is assumed that the transport protocols generating IP-based
traffic at the sender already employ mechanisms that are sufficient
to address congestion on the path. Consequently, a tunnel carrying
IP-based traffic should already interact appropriately with other
traffic sharing the path, and specific congestion control mechanisms
for the tunnel are not necessary."
For this reason, where GRE-in-UDP tunneling is used to carry IP
traffic that is known to be congestion controlled, the UDP tunnels
MAY be used within a single network or across multiple networks,
with cooperating network operators. Internet IP traffic is
generally assumed to be congestion-controlled.
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However, GRE-in-UDP tunneling can be also used to carry traffic that
is not necessarily congestion controlled. In such cases network
operators may avoid congestion by careful provisioning of their
networks, by rate limiting of user data traffic, and/or by using
Traffic Engineering tools to monitor the network segments and
dynamically steers traffic away from potentially congested links.
For this reason, where the GRE payload traffic is not congestion
controlled, GRE-in-UDP tunnels MUST only be used within a single
operator's network that utilizes careful 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.
As such, 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.
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).
o Use of a "Managed 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 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 allocation:
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
9. Security Considerations
9.1. Vulnerability
Neither UDP nor GRE encapsulation effects 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.
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
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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
presupposes that there is effective destination-based (or a
combination of source-based and destination-based) filtering at the
boundaries.
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
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Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: rcallon@juniper.net
John E. Drake
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
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12. References
12.1. Normative References
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[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.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC2983,
October 2000.
[RFC5405] Eggert, L., "Unicast UDP Usage Guideline for Application
Designers", RFC5405, November 2008.
[RFC6040] Briscoe, B., "Tunneling of Explicit Congestion
Notification", RFC6040, November 2010.
[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.
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[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.
[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.
[GREIPV6] Pignataro, C., el al, "IPv6 Support for Generic Routing
Encapsulation (GRE)", draft-ietf-intarea-gre-ipv6-02, work
in progress.
[GREMTU] Bonica, R., "A Fragmentation Strategy for Generic Routing
Encapsulation (GRE)", draft-ietf-intarea-gre-mtu, work in
progress.
[CB] Fairhurst, G., "Network Transport Circuit Breakers",
draft-fairhurst-tsvwg-circuit-breaker-01, 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
Tom Herbert
Google
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1600 Amphitheatre Parkway
Mountain View, CA
Email : therbert@google.com
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