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Network Working Group B. Aboba, Ed.
Request for Comments: 4840 E. Davies
Category: Informational D. Thaler
Internet Architecture Board
April 2007
Multiple Encapsulation Methods Considered Harmful
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes architectural and operational issues that
arise from link-layer protocols supporting multiple Internet Protocol
encapsulation methods.
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Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................3
1.2. Ethernet Experience ........................................4
1.2.1. IEEE 802.2/802.3 LLC Type 1 Encapsulation ...........6
1.2.2. Trailer Encapsulation ...............................7
1.3. PPP Experience ............................................10
1.4. Potential Mitigations .....................................10
2. Evaluation of Arguments for Multiple Encapsulations ............11
2.1. Efficiency ................................................11
2.2. Multicast/Broadcast .......................................12
2.3. Multiple Uses .............................................13
3. Additional Issues ..............................................15
3.1. Generality ................................................15
3.2. Layer Interdependence .....................................16
3.3. Inspection of Payload Contents ............................17
3.4. Interoperability Guidance .................................17
3.5. Service Consistency .......................................19
3.6. Implementation Complexity .................................19
3.7. Negotiation ...............................................19
3.8. Roaming ...................................................20
4. Security Considerations ........................................20
5. Conclusion .....................................................21
6. References .....................................................22
6.1. Normative Reference .......................................22
6.2. Informative References ....................................22
7. Acknowledgments ................................................25
Appendix A. IAB Members at the Time of This Writing ...............26
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1. Introduction
This document describes architectural and operational issues arising
from the use of multiple ways of encapsulating IP packets on the same
link.
While typically a link-layer protocol supports only a single Internet
Protocol (IP) encapsulation method, this is not always the case. For
example, on the same cable it is possible to encapsulate an IPv4
packet using Ethernet [DIX] encapsulation as defined in "A Standard
for the Transmission of IP Datagrams over Ethernet Networks"
[RFC894], the IEEE 802.2/802.3 LLC [IEEE-802.3.2002] Type 1
encapsulation defined in "Two Methods For The Transmission of IP
Datagrams over IEEE 802.3 Networks" [RFC948], or the IEEE 802
[IEEE-802.1A.1990] encapsulation defined in "A Standard for the
Transmission of IP Datagrams over IEEE 802 Networks" [RFC1042].
Historically, a further encapsulation method was used on some
Ethernet systems as specified in "Trailer Encapsulations" [RFC893].
Similarly, ATM (e.g., see [RFC2684]), the Point-to-Point Protocol
(PPP) [RFC1661], and IEEE 802.16 [IEEE-802.16e.2005] also support
multiple encapsulation mechanisms.
1.1. Terminology
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 RFC 2119 [RFC2119].
Broadcast domain
The set of all endpoints that receive broadcast frames sent by
an endpoint in the set.
Classification
As defined in [IEEE-802.16e.2005], the process by which a Medium
Access Control (MAC) Service Data Unit (SDU) is mapped into a
particular transport connection for transmission between MAC
peers.
Connection Identifier (CID)
In [IEEE-802.16e.2005] the connection identifier is a 16-bit
value that identifies a transport connection or an uplink
(UL)/downlink (DL) pair of associated management connections. A
connection is a unidirectional mapping between base station (BS)
and subscriber station (SS) MAC peers. Each transport
connection has a particular set of associated parameters
indicating characteristics such as the ciphersuite and quality-
of-service.
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Link
A communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer immediately below
IP.
Link Layer
The conceptual layer of control or processing logic that is
responsible for maintaining control of the link. The link-layer
functions provide an interface between the higher-layer logic
and the link. The link layer is the layer immediately below IP.
1.2. Ethernet Experience
The fundamental issues with multiple encapsulation methods on the
same link are described in [RFC1042] and "Requirements for Internet
Hosts -- Communication Layers" [RFC1122]. This section summarizes
the concerns articulated in those documents and also describes the
limitations of approaches suggested to mitigate the problems,
including encapsulation negotiation and use of routers.
[RFC1042] described the potential issues resulting from
contemporaneous use of Ethernet and IEEE 802.3 encapsulations on the
same physical cable:
Interoperation with Ethernet
It is possible to use the Ethernet link level protocol [DIX] on
the same physical cable with the IEEE 802.3 link level protocol.
A computer interfaced to a physical cable used in this way could
potentially read both Ethernet and 802.3 packets from the network.
If a computer does read both types of packets, it must keep track
of which link protocol was used with each other computer on the
network and use the proper link protocol when sending packets.
One should note that in such an environment, link level broadcast
packets will not reach all the computers attached to the network,
but only those using the link level protocol used for the
broadcast.
Since it must be assumed that most computers will read and send
using only one type of link protocol, it is recommended that if
such an environment (a network with both link protocols) is
necessary, an IP gateway be used as if there were two distinct
networks.
Note that the MTU for the Ethernet allows a 1500 octet IP
datagram, with the MTU for the 802.3 network allows only a 1492
octet IP datagram.
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When multiple IP encapsulation methods were supported on a given
link, all hosts could not be assumed to support the same set of
encapsulation methods. This in turn implied that the broadcast
domain might not include all hosts on the link. Where a single
encapsulation does not reach all hosts on the link, a host needs to
determine the appropriate encapsulation prior to sending. While a
host supporting reception of multiple encapsulations could keep track
of the encapsulations it receives, this does not enable initiation of
communication; supporting initiation requires a host to support
sending of multiple encapsulations in order to determine which one to
use. However, requiring hosts to send and receive multiple
encapsulations is a potentially onerous requirement. [RFC1122],
Section 2.3.3, notes the difficulties with this approach:
Furthermore, it is not useful or even possible for a dual-format
host to discover automatically which format to send, because of
the problem of link-layer broadcasts.
To enable hosts that only support sending and receiving of a single
encapsulation to communicate with each other, a router can be
utilized to segregate the hosts by encapsulation. Here only the
router needs to support sending and receiving of multiple
encapsulations. This requires assigning a separate unicast prefix to
each encapsulation, or else all hosts in the broadcast domain would
not be reachable with a single encapsulation.
[RFC1122], Section 2.3.3, provided guidance on encapsulation support:
Every Internet host connected to a 10Mbps Ethernet cable:
o MUST be able to send and receive packets using RFC-894
encapsulation;
o SHOULD be able to receive RFC-1042 packets, intermixed with
RFC-894 packets; and
o MAY be able to send packets using RFC-1042 encapsulation.
An Internet host that implements sending both the RFC-894 and the
RFC-1042 encapsulation MUST provide a configuration switch to select
which is sent, and this switch MUST default to RFC-894.
By making Ethernet encapsulation mandatory to implement for both send
and receive, and also the default for sending, [RFC1122] recognized
Ethernet as the predominant encapsulation, heading off potential
interoperability problems.
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1.2.1. IEEE 802.2/802.3 LLC Type 1 Encapsulation
Prior to standardization of the IEEE 802 encapsulation in [RFC1042],
an IEEE 802.2/802.3 LLC Type 1 encapsulation was specified in
[RFC948], utilizing 6 in the Source Service Access Point (SSAP) and
Destination Service Access Point (DSAP) fields of the IEEE 802.2
header. However, since the SSAP and DSAP fields are each only a
single octet, and the Ethertype values for IP, ARP [RFC826], and RARP
[RFC903] are greater than 1500, these values cannot be represented in
the SSAP and DSAP fields. As a result, the encapsulation described
in [RFC948] did not support protocols requiring distinct Ethertypes
such as ARP or RARP, and implementations typically included support
for alternatives to ARP such as the Probe [PROBE] protocol. Support
for ARP, RARP and other IP protocols utilizing distinct Ethertypes
was addressed in [RFC1042], which obsoleted [RFC948]. [RFC1042]
utilized the Sub-Network Access Protocol (SNAP) form of the IEEE
802.2 Logical Link Control (LLC) with the SSAP and DSAP fields set to
170, including support for the Ethertype field. As noted in
"Assigned Numbers" [RFC1010]:
At an ad hoc special session on "IEEE 802 Networks and ARP", held
during the TCP Vendors Workshop (August 1986), an approach to a
consistent way to send DoD-IP datagrams and other IP related
protocols on 802 networks was developed.
Due to some evolution of the IEEE 802.2 standards and the need to
provide for a standard way to do additional DoD-IP related
protocols (such as the Address Resolution Protocol (ARP) on IEEE
802 network, the following new policy is established, which will
replace the old policy (see RFC 960 and RFC 948 [108]).
The new policy is for the Internet community to use the IEEE 802.2
encapsulation on 802.3, 802.4, and 802.5 networks by using the
SNAP with an organization code indicating that the following 16
bits specify the EtherType code (where IP = 2048 (0800 hex), see
Ethernet Numbers of Interest).
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Header
...--------+--------+--------+
MAC Header| Length | 802.{3/4/5} MAC
...--------+--------+--------+
+--------+--------+--------+
| Dsap=K1| Ssap=K1| control| 802.2 SAP
+--------+--------+--------+
+--------+--------+---------+--------+--------+
|protocol id or org code =K2| Ether Type | 802.2 SNAP
+--------+--------+---------+--------+--------+
The total length of the SAP Header and the SNAP header is
8-octets, making the 802.2 protocol overhead come out on a nice
boundary.
K1 is 170. The IEEE likes to talk about things in little-endian
bit transmission order and specifies this value as 01010101. In
big-endian order, as used in Internet specifications, this becomes
10101010 binary, or AA hex, or 170 decimal.
K2 is 0 (zero).
The use of the IP LSAP (K1 = 6) is to be phased out as quickly as
possible.
Many of the issues involved in coexistence of the [RFC948] and
[RFC1042] encapsulations are similar to those described in Section
1.2. For example, due to use of different SSAP/DSAP values, the
broadcast domain might not include all hosts on the link, and a host
would need to determine the appropriate encapsulation prior to
sending. However, the lack of support for ARP within the [RFC948]
encapsulation created additional interoperability and implementation
issues. For example, the lack of support for ARP in [RFC948] implied
that implementations supporting both [RFC948] and [RFC894] or
[RFC1042] encapsulations would need to implement both ARP and an
alternative address resolution mechanism such as Probe. Also, since
the address resolution mechanism for [RFC948] implementations was not
standardized, interoperability problems would likely have arisen had
[RFC948] been widely implemented.
1.2.2. Trailer Encapsulation
As noted in "Trailer Encapsulations" [RFC893], trailer encapsulation
was an optimization developed to minimize memory-to-memory copies on
reception. By placing variable-length IP and transport headers at
the end of the packet, page alignment of data could be more easily
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maintained. Trailers were implemented in 4.2 Berkeley System
Distribution (BSD), among others. While, in theory, trailer
encapsulation could have been applied to the Ethernet [RFC894] or
IEEE 802 [RFC1042] encapsulations (creating four potential
encapsulations of IP!), in practice, trailer encapsulation was only
supported for Ethernet. A separate Ethertype was utilized in order
to enable IP packets in trailer encapsulation to be distinguished
from [RFC894] encapsulation. Since the [RFC948] encapsulation did
not support the Ethertype field (or ARP), this mechanism could not
have been used in [RFC948] implementations.
[RFC1122], Section 2.3.1, described the issues with trailer
encapsulation:
DISCUSSION
The trailer protocol is a link-layer encapsulation technique
that rearranges the data contents of packets sent on the
physical network. In some cases, trailers improve the
throughput of higher layer protocols by reducing the amount of
data copying within the operating system. Higher layer
protocols are unaware of trailer use, but both the sending and
receiving host MUST understand the protocol if it is used.
Improper use of trailers can result in very confusing symptoms.
Only packets with specific size attributes are encapsulated
using trailers, and typically only a small fraction of the
packets being exchanged have these attributes. Thus, if a
system using trailers exchanges packets with a system that does
not, some packets disappear into a black hole while others are
delivered successfully.
IMPLEMENTATION:
On an Ethernet, packets encapsulated with trailers use a
distinct Ethernet type [RFC893], and trailer negotiation is
performed at the time that ARP is used to discover the link-
layer address of a destination system.
Specifically, the ARP exchange is completed in the usual manner
using the normal IP protocol type, but a host that wants to
speak trailers will send an additional "trailer ARP reply"
packet, i.e., an ARP reply that specifies the trailer
encapsulation protocol type but otherwise has the format of a
normal ARP reply. If a host configured to use trailers
receives a trailer ARP reply message from a remote machine, it
can add that machine to the list of machines that understand
trailers, e.g., by marking the corresponding entry in the ARP
cache.
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Hosts wishing to receive trailers send trailer ARP replies
whenever they complete exchanges of normal ARP messages for IP.
Thus, a host that received an ARP request for its IP protocol
address would send a trailer ARP reply in addition to the
normal IP ARP reply; a host that sent the IP ARP request would
send a trailer ARP reply when it received the corresponding IP
ARP reply. In this way, either the requesting or responding
host in an IP ARP exchange may request that it receive
trailers.
This scheme, using extra trailer ARP reply packets rather than
sending an ARP request for the trailer protocol type, was
designed to avoid a continuous exchange of ARP packets with a
misbehaving host that, contrary to any specification or common
sense, responded to an ARP reply for trailers with another ARP
reply for IP. This problem is avoided by sending a trailer ARP
reply in response to an IP ARP reply only when the IP ARP reply
answers an outstanding request; this is true when the hardware
address for the host is still unknown when the IP ARP reply is
received. A trailer ARP reply may always be sent along with an
IP ARP reply responding to an IP ARP request.
Since trailer encapsulation negotiation depends on ARP, it can only
be used where all hosts on the link are within the same broadcast
domain. It was assumed that all hosts supported sending and
receiving ARP packets in standard Ethernet encapsulation [RFC894], so
that negotiation between Ethernet and IEEE 802 encapsulations was not
required, only negotiation between standard Ethernet [RFC894] and
trailer [RFC893] encapsulation. Had hosts supporting trailer
encapsulation also supported one or more IEEE 802 framing mechanisms,
the negotiation would have been complicated still further. For
example, since [RFC948] implementations did not support the Ethertype
field or ARP, the trailer negotiation mechanism could not have been
utilized, and additional difficulty would have been encountered in
distinguishing trailer encapsulated data frames from normally
encapsulated frames.
[RFC1122], Section 2.3.1, provided the following guidance for use of
trailer encapsulation:
The trailer protocol for link-layer encapsulation MAY be used, but
only when it has been verified that both systems (host or gateway)
involved in the link-layer communication implement trailers. If
the system does not dynamically negotiate use of the trailer
protocol on a per-destination basis, the default configuration
MUST disable the protocol.
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4.2BSD did not support dynamic negotiation, only configuration of
trailer encapsulation at boot time, and therefore [RFC1122] required
that the trailer encapsulation be disabled by default on those
systems.
1.3. PPP Experience
PPP can support both encapsulation of IEEE 802 frames as defined in
[RFC3518], as well as IPv4 and IPv6 [RFC2472] packets. Multiple
compression schemes are also supported.
In addition to PPP Data Link Layer (DLL) protocol numbers allocated
for IPv4 (0x0021), IPv6 (0x0057), and Bridging PDU (0x0031), the
following codepoints have been assigned:
o two for RObust Header Compression (ROHC) [RFC3095]:
ROHC small-CID (0x0003) and ROHC large-CID (0x0005)
o two for Van Jacobson compression [RFC1144]:
Compressed TCP/IP (0x002d) and Uncompressed TCP/IP (002f)
o one for IPv6 Header Compression [RFC2507]: (0x004f)
o nine for RTP IP Header Compression [RFC3544]:
Full Header (0x0061), Compressed TCP (0x0063), Compressed Non TCP
(0x0065), UDP 8 (0x0067), RTP 8 (0x0069), Compressed TCP No Delta
(0x2063), Context State (0x2065), UDP 16 (0x2067), and RTP 16
(0x2069)
Although PPP can encapsulate IP packets in multiple ways, typically
multiple encapsulation schemes are not operational on the same link,
and therefore the issues described in this document rarely arise.
For example, while PPP can support both encapsulation of IEEE 802
frames as defined in [RFC3518], as well as IPv4 and IPv6 [RFC2472]
packets, in practice, multiple encapsulation mechanisms are not
operational on the same link. Similarly, only a single compression
scheme is typically negotiated for use on a link.
1.4. Potential Mitigations
In order to mitigate problems arising from multiple encapsulation
methods, it may be possible to use switches [IEEE-802.1D.2004] or
routers, or to attempt to negotiate the encapsulation method to be
used. As described below, neither approach may be completely
satisfactory.
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The use of switches or routers to enable communication between hosts
utilizing multiple encapsulation methods is not a panacea. If
separate unicast prefixes are used for each encapsulation, then the
choice of encapsulation can be determined from the routing table. If
the same unicast prefix is used for each encapsulation method, it is
necessary to keep state for each destination host. However, this may
not work in situations where hosts using different encapsulations
respond to the same anycast address.
In situations where multiple encapsulation methods are enabled on a
single link, negotiation may be supported to allow hosts to determine
how to encapsulate a packet for a particular destination host.
Negotiating the encapsulation above the link layer is potentially
problematic since the negotiation itself may need to be carried out
using multiple encapsulations. In theory, it is possible to
negotiate an encapsulation method by sending negotiation packets over
all encapsulation methods supported, and keeping state for each
destination host. However, if the encapsulation method must be
dynamically negotiated for each new on-link destination,
communication to new destinations may be delayed. If most
communication is short, and the negotiation requires an extra round
trip beyond link-layer address resolution, this can become a
noticeable factor in performance. Also, the negotiation may result
in consumption of additional bandwidth.
2. Evaluation of Arguments for Multiple Encapsulations
There are several reasons often given in support of multiple
encapsulation methods. We discuss each in turn, below.
2.1. Efficiency
Claim: Multiple encapsulation methods allow for greater efficiency.
For example, it has been argued that IEEE 802 or Ethernet
encapsulation of IP results in excessive overhead due to the size of
the data frame headers, and that this can adversely affect
performance on wireless networks, particularly in situations where
support of Voice over IP (VoIP) is required.
Discussion: Even where these performance concerns are valid,
solutions exist that do not require defining multiple IP
encapsulation methods. For example, links may support Ethernet frame
compression so that Ethernet Source and Destination Address fields
are not sent with every packet.
It is possible for link layers to negotiate compression without
requiring higher-layer awareness; the Point-to-Point Protocol (PPP)
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[RFC1661] is an example. "The PPP Compression Control Protocol
(CCP)" [RFC1962] enables negotiation of data compression mechanisms,
and "Robust Header Compression (ROHC) over PPP" [RFC3241] and "IP
Header Compression over PPP" [RFC3544] enable negotiation of header
compression, without Internet-layer awareness. Any frame can be
"decompressed" based on the content of the frame, and prior state
based on previous control messages or data frames. Use of
compression is a good way to solve the efficiency problem without
introducing problems at higher layers.
There are also situations in which use of multiple encapsulations can
degrade performance or result in packet loss. The use of multiple
encapsulation methods with differing Maximum Transfer Units (MTUs)
can result in differing MTUs for on-link destinations. If the link-
layer protocol does not provide per-destination MTUs to the IP layer,
it will need to use a default MTU; to avoid fragmentation, this must
be less than or equal to the minimum MTU of on-link destinations. If
the default MTU is too low, the full bandwidth may not be achievable.
If the default MTU is too high, packet loss will result unless or
until IP Path MTU Discovery is used to discover the correct MTU.
Recommendation: Where encapsulation is an efficiency issue, use
header compression. Where the encapsulation method or the use of
compression must be negotiated, negotiation should either be part of
bringing up the link, or be piggybacked in the link-layer address
resolution exchange; only a single compression scheme should be
negotiated on a link. Where the MTU may vary among destinations on
the same link, the link-layer protocol should provide a per-
destination MTU to IP.
2.2. Multicast/Broadcast
Claim: Support for Ethernet encapsulation requires layer 2 support
for distribution of IP multicast/broadcast packets. In situations
where this is difficult, support for Ethernet is problematic and
other encapsulations are necessary.
Discussion: Irrespective of the encapsulation used, IP packets sent
to multicast (IPv4/IPv6) or broadcast (IPv4) addresses need to reach
all potential on-link receivers. Use of alternative encapsulations
cannot remove this requirement, although there is considerable
flexibility in how it can be met. Non-Broadcast Multiple Access
(NBMA) networks can still support the broadcast/multicast service via
replication of unicast frames.
Techniques are also available for improving the efficiency of IP
multicast/broadcast delivery in wireless networks. In order to be
receivable by any host within listening range, an IP
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multicast/broadcast packet sent as link-layer multicast/broadcast
over a wireless link needs to be sent at the lowest rate supported by
listeners. If the sender does not keep track of the rates negotiated
by group listeners, by default, multicast/broadcast traffic is sent
at the lowest supported rate, resulting in increased overhead.
However, a sender can also deliver an IP multicast/broadcast packet
using unicast frame(s) where this would be more efficient. For
example, in IEEE 802.11, multicast/broadcast traffic sent from the
Station (STA) to the Access Point (AP) is always sent as unicast, and
the AP tracks the negotiated rate for each STA, so that it can send
unicast frames at a rate appropriate for each station.
In order to limit the propagation of link-scope multicast or
broadcast traffic, it is possible to assign a separate prefix to each
host.
Unlike broadcasts, which are received by all hosts on the link
regardless of the protocol they are running, multicasts only need be
received by those hosts belonging to the multicast group. In wired
networks, it is possible to avoid forwarding multicast traffic on
switch ports without group members, by snooping of Internet Group
Management Protocol (IGMP) and Multicast Listener Discovery (MLD)
traffic as described in "Considerations for IGMP and MLD Snooping
Switches" [RFC4541].
In wireless media where data rates to specific destinations are
negotiated and may vary over a wide range, it may be more efficient
to send multiple frames via link-layer unicast than to send a single
multicast/broadcast frame. For example, in [IEEE-802.11.2003]
multicast/broadcast traffic from the client station (STA) to the
Access Point (AP) is sent via link-layer unicast.
Recommendation: Where support for link-layer multicast/broadcast is
problematic, limit the propagation of link-scope multicast and
broadcast traffic by assignment of separate prefixes to hosts. In
some circumstances, it may be more efficient to distribute
multicast/broadcast traffic as multiple link-layer unicast frames.
2.3. Multiple Uses
Claim: No single encapsulation is optimal for all purposes.
Therefore, where a link layer is utilized in disparate scenarios
(such as both fixed and mobile deployments), multiple encapsulations
are a practical requirement.
Discussion: "Architectural Principles of the Internet" [RFC1958],
point 3.2, states:
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If there are several ways of doing the same thing, choose one. If
a previous design, in the Internet context or elsewhere, has
successfully solved the same problem, choose the same solution
unless there is a good technical reason not to. Duplication of
the same protocol functionality should be avoided as far as
possible, without of course using this argument to reject
improvements.
Existing encapsulations have proven themselves capable of supporting
disparate usage scenarios. For example, the Point-to-Point Protocol
(PPP) has been utilized by wireless link layers such as General
Packet Radio Service (GPRS), as well as in wired networks in
applications such as "PPP over SONET/SDH" [RFC2615]. PPP can even
support bridging, as described in "Point-to-Point Protocol (PPP)
Bridging Control Protocol (BCP)" [RFC3518].
Similarly, Ethernet encapsulation has been used in wired networks as
well as Wireless Local Area Networks (WLANs) such as IEEE 802.11
[IEEE-802.11.2003]. Ethernet can also support Virtual LANs (VLANs)
and Quality of Service (QoS) [IEEE-802.1Q.2003].
Therefore, disparate usage scenarios can be addressed by choosing a
single encapsulation, rather than multiple encapsulations. Where an
existing encapsulation is suitable, this is preferable to creating a
new encapsulation.
Where encapsulations other than IP over Point-to-Point Protocol (PPP)
[RFC1661], Ethernet, or IEEE 802 are supported, difficulties in
operating system integration can lead to interoperability problems.
In order to take advantage of operating system support for IP
encapsulation over PPP, Ethernet, or IEEE 802, it may be tempting for
a driver supporting an alternative encapsulation to emulate PPP,
Ethernet, or IEEE 802 support. Typically, PPP emulation requires
that the driver implement PPP, enabling translation of PPP control
and data frames to the equivalent native facilities. Similarly,
Ethernet or IEEE 802 emulation typically requires that the driver
implement Dynamic Host Configuration Protocol (DHCP) v4 or v6, Router
Solicitation/Router Advertisement (RS/RA), Address Resolution
Protocol (ARP), or IPv6 Neighbor Discovery (ND) in order to enable
translation of these frames to and from native facilities.
Where drivers are implemented in kernel mode, the work required to
provide faithful emulation may be substantial. This creates the
temptation to cut corners, potentially resulting in interoperability
problems.
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For example, it might be tempting for driver implementations to
neglect IPv6 support. A driver emulating PPP might support only IP
Control Protocol (IPCP), but not IPCPv6; a driver emulating Ethernet
or IEEE 802 might support only DHCPv4 and ARP, but not DHCPv6, RS/RA,
or ND. As a result, an IPv6 host connecting to a network supporting
IPv6 might find itself unable to use IPv6 due to lack of driver
support.
Recommendation: Support a single existing encapsulation where
possible. Emulation of PPP, Ethernet, or IEEE 802 on top of
alternative encapsulations should be avoided.
3. Additional Issues
There are a number of additional issues arising from use of multiple
encapsulation methods, as hinted at in Section 1. We discuss each of
these below.
3.1. Generality
Link-layer protocols such as [IEEE-802.1A.1990] and [DIX] inherently
support the ability to add support for a new packet type without
modification to the link-layer protocol.
IEEE 802.16 [IEEE-802.16.2004] splits the Media Access Control (MAC)
layer into a number of sublayers. For the uppermost of these, the
standard defines the concept of a service-specific Convergence
Sublayer (CS). The two underlying sublayers (the MAC Common Part
Sublayer and the Security Sublayer) provide common services for all
instantiations of the CS.
While [IEEE-802.16.2004] defined support for the Asynchronous
Transfer Mode (ATM) CS and the Packet CS for raw IPv4, raw IPv6, and
Ethernet with a choice of six different classifiers,
[IEEE-802.16e.2005] added support for raw and Ethernet-framed ROHC
Enhanced Compressed RTP (ECRTP) compressed packets. As a result,
[IEEE-802.16e.2005] defines the ATM CS and multiple versions of the
Packet CS for the transmission of raw IPv4, raw IPv6, 802.3/Ethernet,
802.1Q VLAN, IPv4 over 802.3/Ethernet, IPv6 over 802.3/Ethernet, IPv4
over 802.1Q VLAN, IPv6 over 802.1Q VLAN, raw ROHC-compressed packets,
raw ECRTP-compressed packets, ROHC-compressed packets over
802.3/Ethernet. and ECRTP-compressed packets over 802.3/Ethernet.
As noted in [Generic], [IEEE-802.16.2004] appears to imply that the
standard will need to be modified to support new packet types:
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We are concerned that the 802.16 protocol cannot easily be
extendable to transport new protocols over the 802.16 air
interface. It would appear that a Convergence Sublayer is needed
for every type of protocol transported over the 802.16 MAC. Every
time a new protocol type needs to be transported over the 802.16
air interface, the 802.16 standard needs to be modified to define
a new CS type. We need to have a generic Packet Convergence
Sublayer that can support multi-protocols and which does not
require further modification to the 802.16 standard to support new
protocols. We believe that this was the original intention of the
Packet CS. Furthermore, we believe it is difficult for the
industry to agree on a set of CS's that all devices must implement
to claim "compliance".
The use of IP and/or upper-layer protocol specific classification and
encapsulation methods, rather than a 'neutral' general purpose
encapsulation, may give rise to a number of undesirable effects
explored in the following subsections.
If the link layer does not provide a general purpose encapsulation
method, deployment of new IP and/or upper-layer protocols will be
dependent on deployment of the corresponding new encapsulation
support in the link layer.
Even if a single encapsulation method is used, problems can still
occur if demultiplexing of ARP, IPv4, IPv6, and any other protocols
in use, is not supported at the link layer. While it is possible to
demultiplex such packets based on the Version field (first four bits
on the packet), this assumes that IPv4-only implementations will be
able to properly handle IPv6 packets. As a result, a more robust
design is to demultiplex protocols in the link layer, such as by
assigning a different protocol type, as is done in IEEE 802 media
where a Type of 0x0800 is used for IPv4, and 0x86DD for IPv6.
Recommendations: Link-layer protocols should enable network packets
(IPv4, IPv6, ARP, etc.) to be demultiplexed in the link layer.
3.2. Layer Interdependence
Within IEEE 802.16, the process by which frames are selected for
transmission on a connection identifier (CID) is known as
"classification". Fields in the Ethernet, IP, and UDP/TCP headers
can be used for classification; for a particular CS, a defined subset
of header fields may be applied for that purpose.
Utilizing IP and/or upper layer headers in link-layer classification
will almost inevitably lead to interdependencies between link-layer
and upper-layer specifications. Although this might appear to be
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desirable in terms of providing a highly specific (and hence
interoperable) mapping between the capabilities provided by the link
layer (e.g., quality-of-service support) and those that are needed by
upper layers, this sort of capability is probably better provided by
a more comprehensive service interface (Application Programming
Interface) in conjunction with a single encapsulation mechanism.
IPv6, in particular, provides an extensible header system. An
upper-layer-specific classification scheme would still have to
provide a degree of generality in order to cope with future
extensions of IPv6 that might wish to make use of some of the link
layer services already provided.
Recommendations: Upper-layer-specific classification schemes should
be avoided.
3.3. Inspection of Payload Contents
If a classification scheme utilizing higher-layer headers proposes to
inspect the contents of the packet being encapsulated (e.g., IEEE
802.16 IP CS mechanisms for determining the connection identifier
(CID) to use to transmit a packet), the fields available for
inspection may be limited if the packet is compressed or encrypted
before passing to the link layer. This may prevent the link layer
from utilizing existing compression mechanisms, such as Van Jacobson
Compression [RFC1144], ROHC [RFC3095][RFC3759], Compressed RTP (CRTP)
[RFC2508], Enhanced Compressed RTP (ECRTP) [RFC3545], or IP Header
Compression [RFC2507].
Recommendations: Link-layer classification schemes should not rely on
the contents of higher-layer headers.
3.4. Interoperability Guidance
In situations where multiple encapsulation methods are operational
and capable of carrying IP traffic, interoperability problems are
possible in the absence of clear implementation guidelines. For
example, there is no guarantee that other hosts on the link will
support the same set of encapsulation methods, or that if they do,
that their routing tables will result in identical preferences.
In IEEE 802.16, the Subscriber Station (SS) indicates the Convergence
Sublayers it supports to the Base Station (BS), which selects from
the list one or more that it will support on the link. Therefore, it
is possible for multiple CSes to be operational.
Note that IEEE 802.16 does not provide multiple encapsulation methods
for the same kind of data payload; it defines exactly one
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encapsulation scheme for each data payload. For example, there is
one way to encapsulate a raw IPv4 packet into an IEEE 802.16 MAC
frame, one encapsulation scheme for a raw IPv6 packet, etc. There is
also one way to encapsulate an Ethernet frame, even when there are
multiple possibilities for classifying an Ethernet frame for
forwarding over a connection identifier (CID). Since support for
multiple CSes enables IEEE 802.16 to encapsulate layer 2 frames as
well as layer 3 packets, IP packets may be directly encapsulated in
IEEE 802.16 MAC frames as well as framed with Ethernet headers in
IEEE 802.16 MAC frames. Where CSes supporting both layer 2 frames as
well as layer 3 packets are operational on the same link, a number of
issues may arise, including:
Use of Address Resolution Protocol (ARP)
Where both IPv4 CS and Ethernet CS are operational on the same
link, it may not be obvious how address resolution should be
implemented. For example, should an ARP frame be encapsulated
over the Ethernet CS, or should alternative mechanisms be used for
address resolution, utilizing the IPv4 CS?
Data Frame Encapsulation
When sending an IP packet, which CS should be used? Where
multiple encapsulations are operational, multiple connection
identifiers (CIDs) will also be present. The issue can therefore
be treated as a multi-homing problem, with each CID constituting
its own interface. Since a given CID may have associated
bandwidth or quality-of-service constraints, routing metrics could
be adjusted to take this into account, allowing the routing layer
to choose based on which CID (and encapsulation) appears more
attractive.
This could lead to interoperability problems or routing asymmetry.
For example, consider the effects on IPv6 Neighbor Discovery:
(a) If hosts choose to send IPv6 Neighbor Discovery traffic on
different CSes, it is possible that a host sending an IPv6
Neighbor Discovery packet will not receive a reply, even though
the target host is reachable over another CS.
(b) Where hosts all support the same set of CSes, but have different
routing preferences, it is possible for a host to send an IPv6
Neighbor Discovery packet over one CS and receive a reply over
another CS.
Recommendations: Given these issues, it is strongly recommended that
only a single kind of CS supporting a single encapsulation method
should be usable on a particular link.
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3.5. Service Consistency
If a link-layer protocol provides multiple encapsulation methods, the
services offered to the IP-layer and upper-layer protocols may differ
qualitatively between the different encapsulation methods. For
example, the 802.16 [IEEE-802.16.2004] link-layer protocol offers
both 'native' encapsulation for raw IPv4 and IPv6 packets, and
Ethernet encapsulation. In the raw case, the IP layer can be
directly mapped to the quality-of-service (QoS) capabilities of the
IEEE 802.16 transmission channels, whereas using the Ethernet
encapsulation, an IP-over-Ethernet CS has to be deployed to
circumvent the mapping of the IP QoS to the Ethernet header fields to
avoid the limitations of Ethernet QoS. Consequently, the service
offered to an application depends on the classification method
employed and may be inconsistent between sessions. This may be
confusing for the user and the application.
Recommendations: If multiple encapsulation methods for IP packets on
a single link-layer technology are deemed to be necessary, care
should be taken to match the services available between encapsulation
methods as closely as possible.
3.6. Implementation Complexity
Support of multiple encapsulation methods results in additional
implementation complexity. Lack of uniform encapsulation support
also results in potential interoperability problems. To avoid
interoperability issues, devices with limited resources may be
required to implement multiple encapsulation mechanisms, which may
not be practical.
When encapsulation methods require hardware support, implementations
may choose to support different encapsulation sets, resulting in
market fragmentation. This can prevent users from benefiting from
economies of scale, precluding some uses of the technology entirely.
Recommendations: Choose a single encapsulation mechanism that is
mandatory to implement for both sending and receiving, and make that
encapsulation mechanism the default for sending.
3.7. Negotiation
The complexity of negotiation within ARP or IP can be reduced by
performing encapsulation negotiation within the link layer.
However, unless the link layer allows the negotiation of the
encapsulation between any two hosts, interoperability problems can
still result if more than one encapsulation is possible on a given
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link. In general, a host cannot assume that all other hosts on a
link support the same set of encapsulation methods, so that unless a
link-layer protocol only supports point-to-point communication,
negotiation of multiple potential encapsulation methods will be
problematic. To avoid this problem, it is desirable for link-layer
encapsulation negotiation to determine a single IP encapsulation, not
merely to indicate which encapsulation methods are possible.
Recommendations: Encapsulation negotiation is best handled in the
link layer. In order to avoid dependencies on the data frame
encapsulation mechanism, it is preferable for the negotiation to be
carried out using management frames, if they are supported. If
multiple encapsulations are required and negotiation is provided,
then the negotiation should result in a single encapsulation method
being negotiated on the link.
3.8. Roaming
Where a mobile node roams between base stations or to a fixed
infrastructure, and the base stations and fixed infrastructure do not
all support the same set of encapsulations, then it may be necessary
to alter the encapsulation method, potentially in mid-conversation.
Even if the change can be handled seamlessly at the link and IP layer
so that applications are not affected, unless the services offered
over the different encapsulations are equivalent (see Section 3.5),
the service experienced by the application may change as the mobile
node crosses boundaries. If the service is significantly different,
it might even require 'in-flight' renegotiation, which most
applications are not equipped to manage.
Recommendations: Ensure uniformity of the encapsulation set
(preferably only a single encapsulation) within a given mobile
domain, between mobile domains, and between mobile domains and fixed
infrastructure. If a link layer protocol offers multiple
encapsulation methods for IP packets, it is strongly recommended that
only one of these encapsulation methods should be in use on any given
link or within a single wireless transmission domain.
4. Security Considerations
The use of multiple encapsulation methods does not appear to have
significant security implications.
An attacker might be able to utilize an encapsulation method that was
not in normal use on a link to cause a denial-of-service attack,
which would exhaust the processing resources of interfaces if packets
utilizing this encapsulation were passed up the stack to any
significant degree before being discarded.
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An attacker might be able to force a more cumbersome encapsulation
method between two endpoints, even when a lighter weight one is
available, hence forcing higher resource consumption on the link and
within those endpoints, or causing fragmentation. Since IP fragments
are more difficult to classify than non-fragments, this may result in
packet loss or may even expose security vulnerabilities [WEP].
If different methods have different security properties, an attacker
might be able to force a less secure method as an elevation path to
get access to some other resource or data. Similarly, if one method
is rarely used, that method is potentially more likely to have
exploitable implementation bugs.
Since lower-layer classification methods may need to inspect fields
in the packet being encapsulated, this might deter the deployment of
end-to-end security, which is undesirable. Where encryption of upper
layer headers (e.g., IPsec tunnel mode) is required, this may obscure
headers required for classification. As a result, it may be
necessary for all encrypted traffic to flow over a single connection.
5. Conclusion
The use of multiple encapsulation methods on the same link is
problematic, as discussed above.
Although multiple IP encapsulation methods were defined on Ethernet
cabling, recent implementations support only the Ethernet
encapsulation of IPv4 defined in [RFC894]. In order to avoid a
repeat of the experience with IPv4, for operation of IPv6 on IEEE
802.3 media, only the Ethernet encapsulation was defined in "A Method
for the Transmission of IPv6 Packets over Ethernet Networks"
[RFC1972], later updated in [RFC2464].
In addition to the recommendations given earlier, we give the
following general recommendations to avoid problems resulting from
use of multiple IP encapsulation methods:
When developing standards for encapsulating IP packets on a link-
layer technology, it is desirable that only a single encapsulation
method should be standardized for each link-layer technology.
If a link-layer protocol offers multiple encapsulation methods for
IP packets, it is strongly recommended that only one of these
encapsulation methods should be in use within any given link.
Where multiple encapsulation methods are supported on a link, a
single encapsulation should be mandatory to implement for send and
receive.
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6. References
6.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
6.2. Informative References
[DIX] Digital Equipment Corporation, Intel Corporation,
and Xerox Corporation, "The Ethernet -- A Local
Area Network: Data Link Layer and Physical Layer
(Version 2.0)", November 1982.
[Generic] Wang, L. et al, "A Generic Packet Convergence
Sublayer (GPCS) for Supporting Multiple Protocols
over 802.16 Air Interface", Submission to IEEE
802.16g: CB0216g_05_025r4.pdf, November 2005,
<http://www.ieee802.org/16/netman/contrib/
C80216g-05_025r4.pdf>.
[IEEE-802.1A.1990] Institute of Electrical and Electronics
Engineers, "Local Area Networks and Metropolitan
Area Networks: Overview and Architecture of
Network Standards", IEEE Standard 802.1A, 1990.
[IEEE-802.1D.2004] Institute of Electrical and Electronics
Engineers, "Information technology -
Telecommunications and information exchange
between systems - Local area networks - Media
access control (MAC) bridges", IEEE Standard
802.1D, 2004.
[IEEE-802.1Q.2003] IEEE Standards for Local and Metropolitan Area
Networks: Draft Standard for Virtual Bridged
Local Area Networks, P802.1Q-2003, January 2003.
[IEEE-802.3.2002] Institute of Electrical and Electronics
Engineers, "Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) Access Method and
Physical Layer Specifications", IEEE Standard
802.3, 2002.
[IEEE-802.11.2003] Institute of Electrical and Electronics
Engineers, "Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications",
IEEE Standard 802.11, 2003.
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[IEEE-802.16.2004] Institute of Electrical and Electronics
Engineers, "Information technology -
Telecommunications and information exchange
between systems - Local and metropolitan area
networks, Part 16: Air Interface for Fixed
Broadband Wireless Access Systems", IEEE Standard
802.16-2004, October 2004.
[IEEE-802.16e.2005] Institute of Electrical and Electronics
Engineers, "Information technology -
Telecommunications and information exchange
between systems - Local and Metropolitan Area
Networks - Part 16: Air Interface for Fixed and
Mobile Broadband Wireless Access Systems,
Amendment for Physical and Medium Access Control
Layers for Combined Fixed and Mobile Operation in
Licensed Bands", IEEE P802.16e, September 2005.
[PROBE] Hewlett Packard, "A Primer on HP Probe",
http://www.hp.com/rnd/support/manuals/pdf/
hp_probe.pdf, July 1993.
[RFC826] Plummer, D., "Ethernet Address Resolution
Protocol: Or converting network protocol
addresses to 48.bit Ethernet address for
transmission on Ethernet hardware", STD 37, RFC
826, November 1982.
[RFC893] Leffler, S. and M. Karels, "Trailer
encapsulations", RFC 893, April 1984.
[RFC894] Hornig, C., "A Standard for the Transmission of
IP Datagrams over Ethernet Networks", STD 41, RFC
894, April 1984.
[RFC903] Finlayson, R., Mann, T., Mogul, J., and M.
Theimer, "A Reverse Address Resolution Protocol",
STD 38, RFC 903, June 1984.
[RFC948] Winston, I., "Two Methods for the Transmission of
IP Datagrams over IEEE 802.3 Networks", RFC 948,
June 1985.
[RFC1010] Reynolds, J. and J. Postel, "Assigned Numbers",
RFC 1010, May 1987.
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[RFC1042] Postel, J. and J. Reynolds, "Standard for the
transmission of IP datagrams over IEEE 802
networks", STD 43, RFC 1042, February 1988.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October
1989.
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for
Low-Speed Serial Links", RFC 1144, February 1990.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC1958] Carpenter, B., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC1962] Rand, D., "The PPP Compression Control Protocol
(CCP)", RFC 1962, June 1996.
[RFC1972] Crawford, M., "A Method for the Transmission of
IPv6 Packets over Ethernet Networks", RFC 1972,
August 1996.
[RFC2472] Haskin, D. and E. Allen, "IP Version 6 over PPP",
RFC 2472, December 1998.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over
Ethernet Networks", RFC 2464, December 1998.
[RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP
Header Compression", RFC 2507, February 1999.
[RFC2508] Casner, S. and V. Jacobson, "Compressing
IP/UDP/RTP Headers for Low-Speed Serial Links",
RFC 2508, February 1999.
[RFC2615] Malis, A. and W. Simpson, "PPP over SONET/SDH",
RFC 2615, June 1999.
[RFC2684] Grossman, D. and J. Heinanen, "Multiprotocol
Encapsulation over ATM Adaptation Layer 5", RFC
2684, September 1999.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M.,
Fukushima, H., Hannu, H., Jonsson, L-E.,
Hakenberg, R., Koren, T., Le, K., Liu, Z.,
Martensson, A., Miyazaki, A., Svanbro, K.,
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Wiebke, T., Yoshimura, T., and H. Zheng, "RObust
Header Compression (ROHC): Framework and four
profiles: RTP, UDP, ESP, and uncompressed", RFC
3095, July 2001.
[RFC3241] Bormann, C., "Robust Header Compression (ROHC)
over PPP", RFC 3241, April 2002.
[RFC3518] Higashiyama, M., Baker, F., and T. Liao, "Point-
to-Point Protocol (PPP) Bridging Control Protocol
(BCP)", RFC 3518, April 2003.
[RFC3544] Koren, T., Casner, S., and C. Bormann, "IP Header
Compression over PPP", RFC 3544, July 2003.
[RFC3545] Koren, T., Casner, S., Geevarghese, J., Thompson,
B., and P. Ruddy, "Enhanced Compressed RTP (CRTP)
for Links with High Delay, Packet Loss and
Reordering", RFC 3545, July 2003.
[RFC3759] Jonsson, L-E., "RObust Header Compression (ROHC):
Terminology and Channel Mapping Examples", RFC
3759, April 2004.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management
Protocol (IGMP) and Multicast Listener Discovery
(MLD) Snooping Switches", RFC 4541, May 2006.
[WEP] Bittau, A., Handley, M., and J. Lackey, "The
Final Nail in WEP's Coffin", Proceedings of the
2006 IEEE Symposium on Security and Privacy, pp.
386-400.
7. Acknowledgments
The authors would like to acknowledge Jeff Mandin, Bob Hinden, Jari
Arkko, Max Riegel, Alfred Hoenes, and Phil Roberts for contributions
to this document.
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Appendix A. IAB Members at the Time of This Writing
Bernard Aboba
Loa Andersson
Brian Carpenter
Leslie Daigle
Elwyn Davies
Kevin Fall
Olaf Kolkman
Kurtis Lindqvist
David Meyer
David Oran
Eric Rescorla
Dave Thaler
Lixia Zhang
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax: +1 425 936 7329
Elwyn B. Davies
Consultant
Soham, Cambs
UK
EMail: elwynd@dial.pipex.com
Phone: +44 7889 488 335
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: dthaler@microsoft.com
Phone: +1 425 703 8835
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