Internet Area WG J. Touch
Internet Draft USC/ISI
Intended status: Informational M. Townsley
Updates: 4459 Cisco
Expires: January 2017 July 6, 2016
IP Tunnels in the Internet Architecture
draft-ietf-intarea-tunnels-03.txt
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Abstract
This document discusses the role of IP tunnels in the Internet
architecture, in which IP datagrams are carried as payloads in non-
link layer protocols. It explains their relationship to existing
protocol layers and the challenges in supporting IP tunneling based
on the equivalence of tunnels to links.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................6
2.1. Key Words.................................................6
2.2. Terminology...............................................6
3. The Tunnel Model...............................................9
3.1. What is a tunnel?........................................10
3.2. View from the Outside....................................11
3.3. View from the Inside.....................................12
3.4. Location of the Ingress and Egress.......................12
3.5. Implications of This Model...............................13
3.6. Fragmentation............................................14
3.6.1. Outer Fragmentation.................................14
3.6.2. Inner Fragmentation.................................15
3.6.3. The necessity of Outer Fragmentation................16
4. IP Tunnel Requirements........................................16
4.1. Minimum MTU Considerations...............................17
4.2. Fragmentation............................................18
4.3. MTU discovery............................................21
4.4. IP ID exhaustion.........................................22
4.5. Hop Count................................................23
4.6. Signaling................................................24
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4.7. Relationship of Header Fields............................26
4.8. Congestion...............................................27
4.9. Checksums................................................27
4.10. Numbering...............................................27
4.11. Multicast...............................................28
4.12. Multipoint..............................................28
4.13. NAT / Load Balancing....................................29
4.14. Recursive tunnels.......................................29
5. Observations (implications)...................................29
5.1. Tunnel protocol designers................................29
5.2. Tunnel implementers......................................30
5.3. Tunnel operators.........................................30
5.4. Diagnostics..............................................30
5.5. For existing standards...................................31
5.5.1. Generic UDP Encapsulation (GUE - IP in UDP in IP)...31
5.5.2. Generic Packet Tunneling in IPv6....................31
5.5.3. Geneve (NVO3).......................................32
5.5.4. GRE (IP in GRE in IP)...............................33
5.5.5. IP in IP / mobile IP................................33
5.5.6. IPsec tunnel mode (IP in IPsec in IP)...............35
5.5.7. L2TP................................................36
5.5.8. L2VPN...............................................36
5.5.9. L3VPN...............................................36
5.5.10. LISP...............................................36
5.5.11. MPLS...............................................37
5.5.12. PWE................................................37
5.5.13. SEAL/AERO..........................................37
5.5.14. TRILL..............................................37
5.5.15. RTG DT encapsulations..............................38
5.6. For future standards.....................................38
6. Security Considerations.......................................39
7. IANA Considerations...........................................40
8. References....................................................40
8.1. Normative References.....................................40
8.2. Informative References...................................40
9. Acknowledgments...............................................44
APPENDIX A: Fragmentation efficiency.............................45
A.1. Selecting fragment sizes.................................45
A.2. Packing..................................................46
1. Introduction
The Internet is loosely based on the ISO seven layer stack, in which
data units traverse the stack by being wrapped inside data units one
layer down. A tunnel is a mechanism for transmitting data units
between endpoints by wrapping them as data units of the same or
higher layers, e.g., IP in IP (Figure 1) or IP in UDP (Figure 2).
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+----+----+--------------+
| IP'| IP | Data |
+----+----+--------------+
Figure 1 IP inside IP
+----+-----+----+--------------+
| IP'| UDP | IP | Data |
+----+-----+----+--------------+
Figure 2 IP in UDP in IP in Ethernet
This document focuses on tunnels that transit IP packets, i.e., in
which an IP packet is the payload of another protocol. Tunnels
provide a virtual link that can help decouple the network topology
seen by transiting packets from the underlying physical network
[To98][RFC2473]. Tunnels were critical in the development of
multicast because not all routers were capable of processing
multicast packets [Er94]. Tunnels allowed multicast packets to
transit between multicast-capable routers over paths that did not
support multicast. Similar techniques have been used to support other
protocols, such as IPv6 [RFC2460].
Use of tunnels is common in the Internet. The word "tunnel" occurs in
over 100 RFCs, and is supported within numerous protocols, including:
o IP in IP / mobile IP - IPv4 in IPv4 tunnels
[RFC2003][RFC2473][RFC5944]
o IP in IPv6 - IPv6 or IPv4 in IPv6 [RFC2473]
o IPsec - includes a tunnel mode to enable encryption or
authentication of the an entire IP datagram [RFC4301]
o Generic Router Encapsulation (GRE) - a shim layer for tunneling
any network layer in any other network layer, IP in GRE in IP
[RFC2784][RFC7588][RFC7676]
o Generic UDP Encapsulation (GUE) - IP in UDP (in IP)[He15]
o Automatic Multicast Tunneling (AMT) - IP in UDP for multicast
[RFC7450]
o L2TP - PPP over IP, to extend a subscriber's DSL/FTTH connection
from an access line provider to an ISP [RFC3931]
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o L2VPNs - provides a link topology different from that provided by
physical links [RFC4664]
o L3VPNs - provides a network topology different from that provided
by ISPs [RFC4176]
o LISP - reduces routing table load within an enclave of routers at
the expense of more complex ingress encapsulation tables [RFC6830]
o MPLS - IP over a circuit-like path in which identifiers are
rewritten on each hop, often used for traffic provisioning
[RFC3031]
o NVO3 - data center network sharing (which includes use of GUE,
above) [RFC7364]
o PWE3 - emulates wire-like services over packet-switched services
[RFC3985]
o SEAL/AERO -IP in IP tunneling with an additional shim header
designed to overcome the limitations of RFC2003 [RFC5320][Te16]
o TRILL - enables L3 routing (typically IS-IS) in an enclave of
Ethernet bridges [RFC5556][RFC6325]
The variety of tunnel mechanisms raises the question of the role of
tunnels in the Internet architecture and the potential need for these
mechanisms to have similar and predictable behavior. In particular,
the ways in which packet sizes (i.e., Maximum Transmission Unit or
MTU) mismatch and error signals (e.g., ICMP) are handled may benefit
from a coordinated approach.
Regardless of the layer in which encapsulation occurs, tunnels
emulate a link. The only difference is that a link operates over a
physical communication channel, whereas a tunnel operates over
software protocol layers. Because tunnels are links, they are subject
to the same issues as any link, e.g., MTU discovery, signaling, and
the potential utility of native support for broadcast and multicast
[RFC2460][RFC3819]. They have advantages over native links, being
potentially easier to reconfigure and control.
The first attempt to use large-scale tunnels transit multicast across
the Internet in 1988 lead to tunnel collapse. At the time, tunnels
were not implemented as encapsulation-based virtual links, but rather
as loose source routes on un-encapsulated IP datagrams [RFC1075].
Using encapsulation tunnels instead avoided that collapse [Er94] and
eventually to AMT [RFC7450].
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The remainder of this document describes the general principles of IP
tunneling and discusses the key considerations in the design of a
protocol that tunnels IP datagrams. It derives its conclusions from
the equivalence of tunnels and links. Note that all considerations
are in the context of existing standards and requirements.
2. Conventions used in this document
2.1. Key Words
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].
2.2. Terminology
This document uses the following terminology. These definitions are
given in the most general terms, but will be used primarily to
discuss IP tunnels in this document. They are presented in order from
most fundamental to those derived on earlier definitions:
o Messages: variable length data labeled with globally-unique
endpoint IDs, also known as a datagram for IP messages [RFC791].
o Network node (node): a device that can act as an endpoint or
forwarder. For datagrams (IP messages), these are hosts or
gateways/routers, respectively.
o Endpoint or host: a node that sources or sinks messages labeled
from/to its IDs, typically known as a host for both IP and higher-
layer protocol messages [RFC1122].
o Forwarder: a node that relays messages using destination IDs and
local context, also known as a gateway or router for IP messages
[RFC1812]. Note that most forwarders also act as endpoints when
they source or sink messages.
o Source (sender): the node that generates a message.
o Destination (receiver): the node that consumes a message.
o Link: a device (or medium) that transfers messages between nodes,
i.e., by which a message can traverse between nodes without being
processed by a forwarder. Note that the notion of forwarder is
relative to the layer at which message processing is considered
[To16].
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o Link interface (sometimes known as a network interface): a
location on a link co-located with a node where messages depart
onto that link or arrive from that link.
o Path: a sequence of one or more links or tunnels over which a
message can traverse between nodes (hosts or forwarders), which
may or may not involve being processed by a forwarder.
o Tunnel: a protocol mechanism that transits messages using
encapsulation to allow a path to appear as a single link. Note
that a protocol can be used to tunnel itself (IP over IP) and that
this includes the conventional layering of the ISO stack (i.e., by
this definition, Ethernet is a tunnel for IP). A tunnel can be
considered a virtual link.
o Ingress: the virtual link interface of a tunnel which receives
messages within a node, encapsulates them according to the tunnel
protocol, and transmits them into the tunnel. This is the tunnel
equivalent of the outgoing (departing) network interface of a
link. Note that the ingress virtual link interface and traffic
source node can be co-located.
o Egress: a virtual link interface that receives messages that have
finished transiting a tunnel and presents them to a node. This is
the tunnel equivalent of the incoming (arriving) network interface
of a link. The egress decapsulates messages for further transit to
the destination. Note that the egress virtual link interface and
traffic destination node can be co-located.
o Tunnel transit packet (TTP): the packet arriving at a node
connected to a tunnel that enters the ingress and exits the
egress, i.e., the packet carried over the tunnel. This is
sometimes known as the "tunneled packet", i.e., the packet carried
over the tunnel. This is the tunnel equivalent of a network layer
packet as it would traverse a link.
o Tunnel link packet (TLP): packets that traverse from ingress to
egress, in which resides all or part of a tunnel transit packet.
This is sometimes known as the "tunnel packet", i.e., the packet
of the tunnel itself. This is the tunnel equivalent of a link
layer packet as it would traverse a link.
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o Link MTU (LMTU): the largest message that can transit a link. It
typically does not include link-layer information, e.g., link
layer headers or trailers, i.e., it refers to the message that the
link can carry rather than the message as it appears on the link.
This is thus the largest network layer packet (including network
layer headers, e.g., IP datagram) that can transit a link. Note
that this need not be the native size of messages on the link,
i.e., the link may internally fragment and reassemble messages.
For IPv4, the smallest LMTU is 68 bytes [RFC791], and for IPv6 the
smallest LMTU is 1280 bytes [RFC2460].
o Path MTU (PMTU): the largest message that can transit a path.
Typically, this is the minimum of the link MTUs of the links of
the path, and represents the largest network layer message
(including network layer headers) that can transit a path. Note
that this is not the largest network packet that can be sent
between a source and destination; this is the largest network
network packet that can be sent without requiring reassembly at
the network layer of the destination.
o Reassembly MTU (RMTU): the largest message that can be reassembled
by a destination, which is not directly related to the link or
path MTU. Sometimes also referred to as "receiver MTU". For IPv4,
this is 576 bytes [RFC793] and for IPv6 it is 1500 bytes
[RFC2460]; note that in both cases, the size refers to the message
transferred at the network layer, which includes the network layer
headers.
o Tunnel MTU (TMTU): the largest message that can transit a tunnel,
i.e., this is the tunnel equivalent of a link MTU. Typically, this
is limited by the egress reassembly MTU. Note that this value may
have no relation to the path MTU between the tunnel ingress and
egress.
o Tunnel internal MTU (TIMTU): the largest message that a tunnel
egress can emit into a tunnel without requiring further
fragmentation to reach the tunnel egress. This the path MTU
between the ingress and egress.
o Egress reassembly MTU (ERMTU): the largest message that can be
reassembled by an egress. This is the size of the RMTU of a tunnel
minus the encapsulation overhead of that tunnel. Sometimes also
referred to as the "egress MTU".
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3. The Tunnel Model
A network architecture is an abstract description of a distributed
communications system, its components and their relationships, the
requisite properties of those components and the emergent properties
of the system that result [To03]. Such descriptions can help explain
behavior, as when the OSI seven-layer model is used as a teaching
example [Zi80]. Architectures describe capabilities - and, just as
importantly, constraints.
A network can be defined as a system of endpoints and relays
interconnected by communication paths, abstracting away issues of
naming in order to focus on message forwarding. To the extent that
the Internet has a single, coherent interpretation, its architecture
is defined by its core protocols (IP [RFC791], TCP [RFC793], UDP
[RFC768]) and messages, hosts, routers, and links [Cl88][To03], as
shown in Figure 3:
+------+ ------ ------ +------+
| | / \ / \ | |
| HOST |--+ ROUTER +--+ ROUTER +--| HOST |
| | \ / \ / | |
+------+ ------ ------ +------+
Figure 3 Basic Internet architecture
As a network architecture, the Internet is a system of hosts and
routers interconnected by links that exchange messages when possible.
"When possible" defines the Internet's "best effort" principle. The
limited role of routers and links represents the End-to-End Principle
[Sa84] and longest-prefix match enables hierarchical forwarding.
Although the definitions of host, router, and link seem absolute,
they are often relative as viewed within the context of one OSI
layer, each of which can be considered a distinct network
architecture. An Internet gateway is a Layer 3 router when it
transits IP datagrams but it acts as a Layer 2 host as it sources or
sinks Layer 2 messages on attached links to accomplish this transit
capability. In this way, a single node (Internet gateway) behaves as
different components (router, host) at different layers.
Even though a single node may have multiple roles - even concurrently
- at a given layer, each role is typically static and determined by
context. An Internet gateway always acts as a Layer 2 host and that
behavior does not depend on where the gateway is viewed from within
Layer 2. In the context of a single layer, a node's behavior is
modeled as a single component from all viewpoints in that layer.
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3.1. What is a tunnel?
A tunnel can be modeled as a link in another network
[To98][To01][To03]. In Figure 4, a source host (Hsrc) and destination
host (Hdst) communicating over a network M in which two routers (Ra
and Rd) are connected by a tunnel. Keep in mind that it is possible
that both network N and network M can both be components of the
Internet, i.e., there may be regular traffic as well as tunneled
traffic over any of the routers shown.
--_ --
+------+ / \ / \ +------+
| Hsrc |--+ Ra + -- -- + Rd +--| Hdst |
+------+ \ //\ / \ / \ /\\ / +------+
--/I \--+ Rb +--+ Rc +--/E \--
\ / \ / \ / \ /
\/ -- -- \/
<------ Network N ------->
<-------------------- Network M --------------------->
Figure 4 The big picture
The tunnel consists of two elements (ingress I, egress E), that lie
along a path connected by a (possibly different) network N.
Regardless of how the ingress and egress are connected, the tunnel
serves as a link to the nodes it connects (here, Ra and Rd).
IP packets arriving at the ingress are encapsulated to traverse
network N. We call these packets "tunnel transit packets" (TTPs)
because they will now transit the tunnel inside one or more "tunnel
link packets" (TLPs). TLPs use the source address of the ingress and
the destination address of the egress - using whatever address is
appropriate to the Layer at which the ingress and egress operate
(Layer 2, Layer 3, Layer 4, etc.). The egress decapsulates those
messages, which then continue on network M as if emerging from a
link. To tunnel transit packets, and to the routers the tunnel
connects (Ra and Rd), the tunnel acts as a link and the ingress and
egress act as network interfaces to that link.
The model of each component (ingress, egress) and the entire system
(tunnel) depends on the layer from which you view the tunnel. From
the perspective of the outermost hosts (Hsrc and Hdst), the tunnel
appears as a link between two routers (Ra and Rd). For routers along
the tunnel (e.g., Rb and Rc), the ingress and egress appear as the
endpoint hosts and Hsrc and Hdst are invisible.
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When the tunnel network (N) is implemented using the same protocol as
the endpoint network (M), the picture looks flatter (Figure 5), as if
it were running over a single network. However, note that this
appearance is incorrect - nothing has changed. From the perspective
of the endpoints, Rb and Rc and network N don't exist and aren't
visible, and from the perspective of the tunnel, network M doesn't
exist. The fact that network N and M use the same protocol, and may
traverse the same links is irrelevant.
--_ -- -- --
+------+ / \ /\ / \ / \ /\ / \ +------+
| Hsrc |--+ Ra +/I \--+ Rb +--+ Rc +--/E \+ Rd +--| Hdst |
+------+ \ / \ / \ / \ / \ / \ / +------+
-- \/ -- -- \/ --
<------ Network N ------->
<---------------------- Network M ----------------------->
Figure 5 IP in IP network picture
3.2. View from the Outside
From outside the tunnel, to network M, the entire tunnel acts as a
link (Figure 6). It may be numbered or unnumbered and the addresses
associated with the ingress and egress are irrelevant from outside.
--_ --
+------+ / \ / \ +------+
| Hsrc |--+ Ra +--------------------------+ Rd +--| Hdst |
+------+ \ / \ / +------+
-- --
Figure 6 Tunnels as viewed from the outside
A tunnel is effectively invisible to the network in which it resides,
except that it behaves exactly as a link. Consequently [RFC3819]
requirements for links supporting IP also apply to tunnels.
E.g., the IP datagram hop count (IPv4 Time-to-Live [RFC791] and IPv6
Hop Limit [RFC2460]) are decremented when traversing a router, not by
traversing a link - or thus a tunnel. Tunnels have a tunnel MTU - the
largest datagram that can transit, just as links have a corresponding
link MTU. A link MTU may not reflect the native link message sizes
(ATM AAL5 48 byte messages support a 9KB MTU) and the same is true
for a tunnel.
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3.3. View from the Inside
Within network N, i.e., from inside the tunnel itself, the ingress is
a source of tunnel link packets and the egress is a sink - both are
hosts on network N (Figure 7). Consequently [RFC1122] Internet host
requirements apply to ingress and egress nodes when Network N uses IP
(and thus the ingress/egress use IP encapsulation).
_ -- --
/\ / \ / \ /\
/I \--+ Rb +--+ Rc +--/E \
\ / \ / \ / \ /
\/ -- -- \/
<------ Network N ------->
Figure 7 Tunnels, as viewed from within the tunnel
Viewed from within the tunnel, the outer network (M) doesn't exist.
Tunnel link packets can be fragmented by the source (ingress) and
reassembled at the destination (egress), just as at any endpoint. The
path between ingress and egress may have a path MTU but the endpoints
can exchange messages as large as can be reassembled at the
destination (egress), i.e., an egress MTU. Information about the
network - i.e., regarding MTU sizes, network reachability, etc. - are
relayed from the destination (egress) and intermediate routers back
to the source (ingress), without regard for the external network (M).
3.4. Location of the Ingress and Egress
The ingress and egress are endpoints of the tunnel and the tunnel is
a link. The ingress and egress are thus link endpoints at the network
nodes the tunnel interconnects. Such link endpoints are typically
described as "network interfaces".
Tunnel interfaces may be physical or virtual. The interface may be
implemented inside the node where the tunnel attaches, e.g., inside a
host or router. The interface may also be implemented as a "bump in
the wire" (BITW), somewhere along a link between the two nodes the
link interconnects. IP in IP tunnels are often implemented as
interfaces, where IPsec tunnels are sometimes implemented as BITW.
These implementation variations determine only whether information
available at the link endpoints (ingress/egress) can be easily shared
with the connected network nodes.
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3.5. Implications of This Model
This approach highlights a few key features of a tunnel as a network
architecture construct:
o To the tunnel transit packets (TTPs), tunnels turn a network
(Layer 3) path into a (Layer 2) link
o To nodes the tunnel traverses, the tunnel ingress and egress act
as hosts that source and sink tunnel link packets (TLPs)
The consequences of these features are as follow:
o Like a link, a tunnel has an MTU defined by the reassembly MTU of
the receiving interface (egress).
o Like any other link, the MTU inside a tunnel are not relevant to
the transited traffic. There is no mechanism or protocol by which
they are measured or confirmed.
o Path MTU discovery in the network layer (i.e., outer network M)
has no direct relation to the MTU of the hops within the link
layer of the links (or thus tunnels) that connect its components.
o Hops remain defined as the number of routers encountered on a path
or the time spent at a router [RFC1812]. Hops are not decremented
solely by the transit of a link, e.g., a packet with a hop count
of zero should successfully transit a link (and thus a tunnel)
that connects two hosts. Routers, not links, alter hopcounts.
o The addresses of a tunnel ingress and egress correspond to link
layer addresses to the tunnel transit packet and outer network M.
Like point-to-point links, point-to-point tunnels can be
unnumbered in the network in which they reside (even though they
must have addresses in the network they transit).
o Like network interfaces, the ingress and egress are never a direct
source of ICMP messages but may provide information to their
attached host or router to generate those ICMP messages.
o Like network interfaces and links, two nodes may be connected by
any combination of tunnels and links, including multiple tunnels.
As with multiple links, existing routing determines which traffic
uses each link or tunnel.
These observations make it much easier to determine what a tunnel
must do to transit IP packets, notably it must satisfy all
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requirements expected of a link [RFC1122][RFC3819]. The consequence
of these observations are that tunnels are no different from links,
except only that a link has a physical instantiation.
3.6. Fragmentation
There are two places where fragmentation can occur in a tunnel,
called Outer Fragmentation and Inner Fragmentation. This document
assumes that only Outer Fragmentation is viable because it is the
only approach that works for IPv4 datagrams with DF=1 and for IPv6.
3.6.1. Outer Fragmentation
The simplest case is Outer Fragmentation, as shown in Figure 8. The
bottom of the figure shows the network topology, where packets start
at the source, enter the tunnel at the encapsulator, exit the tunnel
at the decapsulator, and arrive finally at the destination. The
packet traffic is shown above the topology, where the end-to-end
packets are shown at the top. The packets are composed of an inner
header (iH) and inner data (iD); the term "inner") is relative to the
tunnel, as will become apparent. When the packet (iH,iD) arrives at
the encapsulator, it is placed inside the tunnel packet structure,
here shown as adding just an outer header, oH, in step (a).
+----+----+ +----+----+
| iH | iD |------+ - - - - - - - - - - +------>| iH | iD |
+----+----+ | | +----+----+
v |
+----+----+----+ +----+----+----+
(a) | oH | iH | iD | | oH | iH | iD | (c)
+----+----+----+ +----+----+----+
| ^
| +----+----+-----+ |
(b1) +----- >| oH'| iH | iD1 |-------+
| +----+----+-----+ |
| |
| +----+-----+ |
(b2) +----- >| oH"| iD2 |------------+
+----+-----+
+-----+ +---+ +---+ +-----+
| | / \ ======================= / \ | |
| Src |=======| Enc |=======================| Dec |=======| Dst |
| | \ / ======================= \ / | |
+-----+ +---+ +---+ +-----+
Figure 8 Fragmentation of the outer packet
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When the encapsulated packet exceeds the tunnel MTU, the packet needs
to be fragmented. In this case we fragment the packet at the outer
header, with the fragments shown as (b1) and (b2). Note that the
outer header indicates fragmentation (as ' and "),the inner header
occurs only in the first fragment, and the inner data is broken
across the two packets. These fragments are reassembled at the
encapsulator in step (c), and the resulting packet is decapsulated
and sent on to the destination.
Outer fragmentation isolates Source and Destination from tunnel
encapsulation duties. This can be considered a benefit in clean,
layered network design, but also may result in complex decapsulator
design, especially where tunnels aggregate large amounts of traffic,
such as IP ID overload (see Sec. 4.4). Outer fragmentation is valid
for any tunnel encapsulation protocol that supports fragmentation
(e.g., IPv4 or IPv6), where the tunnel endpoints act as the host
endpoints of that protocol.
Along the tunnel, the inner header is contained only in the first
fragment, which can interfere with mechanisms that 'peek' into lower
layer headers, e.g., as for ICMP, as discussed in Sec. 4.6.
3.6.2. Inner Fragmentation
Inner Fragmentation distributes the impact of tunneling across both
the decapsulator and destination, and is shown in Figure 9; this can
be especially important when the tunnel aggregates large amounts of
traffic. However, this mechanism is thus valid only when the original
source packets can be fragmented on-path, e.g., as in IPv4 datagrams
with DF=0.
Again, the network topology is shown at the bottom of the figure, and
the original packets show at the top. Packets arrive at the
encapsulator, and are fragmented there based on the inner header into
(a1) and (a2). The fragments arrive at the decapsulator, which
removes the outer header and forwards the resulting fragments on to
the destination. The destination is then responsible for reassembling
the fragments into the original packet.
Along the tunnel, the inner headers are copied into each fragment,
and so are available to mechanisms that 'peek' into headers (e.g.,
ICMP, as discussed in Sec. 4.6). Because fragmentation happens on the
inner header, the impact of IP ID is reduced.
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+----+----+ +----+----+
| iH | iD |-------+- - - - - - - - - - - - - >| iH | iD |
+----+----+ | +----+----+
v ^
+----+-----+ +----+-----+ |
(a1) | iH'| iD1 | | iH'| iD1 |------+
+----+-----+ +----+-----+ |
|
+----+--- +----+-----+ |
(a2) | iH"| iD2 | | iH"| iD2 |------+
+----+-----+ +----+-----+
| ^
| +----+----+----- |
(b1) +----- >| oH | iH'| iD1 |-------+
| +----+----+-----+ |
| |
| +----+----+-----+ |
(b2) +----- >| oH | iH"| iD2 |-------+
+----+----+-----+
+-----+ +---+ +---+ +-----+
| | / \ ======================= / \ | |
| Src |=======| Enc |=======================| Dec |=======| Dst |
| | \ / ======================= \ / | |
+-----+ +---+ +---+ +-----+
Figure 9 Fragmentation of the inner packet
3.6.3. The necessity of Outer Fragmentation
Fragmentation is critical tunnels that support TTP packets for
protocols with minimum MTU requirements, while operating over tunnel
paths using protocols with minimum MTU requirements. Depending on the
amount of space used by encapsulation, these two minimums will
ultimately interfere, and the TTP will need to be fragmented to both
support a TTP minimum MTU while traversing tunnels with their own TLP
minimum MTUs.
Outer Fragmentation is the only solution that supports all IPv4 and
IPv6 traffic, because inner fragmentation is allowed only for IPv4
datagrams with DF=0. As a result, the remainder of this document
assumes Outer Fragmentation.
4. IP Tunnel Requirements
The requirements of an IP tunnel are defined by the requirements of
an IP link because both transit IP packets. A tunnel thus must
transit the IP minimum MTU, i.e., 68 bytes for IPv4 [RFC793] and 1280
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bytes for IPv6 [RFC2460] and a tunnel must support address resolution
when there is more than one egress.
The requirements of the tunnel ingress and egress are defined by the
network over which they exchange messages (tunnel link packets). For
IP-over-IP, this means that the ingress MUST NOT exceed the IPv4
Identification (fragment) field uniqueness requirements [RFC6864].
These requirements remain even though tunnels have some unique
issues, including the need for additional space for encapsulation
headers and the potential for tunnel MTU variation.
4.1. Minimum MTU Considerations
There are a variety of values of minimum MTU to consider, both in a
conventional network and in a tunnel as a link in that network. These
are indicated in Figure 10, an annotated variant of Figure 4.
(a) LMTU <->
(b) PMTU <------------------------------------>
(c) <-RMTU----------------------------------------------->
(d) TMTU <------------------------>
(e) TIMTU <---------------->
(f) ERMTU <------------------------>
--_ --
+------+ / \ / \ +------+
| Hsrc |--+ Ra + -- -- + Rd +--| Hdst |
+------+ \ //\ / \ / \ /\\ / +------+
--/I \--+ Rb +--+ Rc +--/E \--
\ / \ / \ / \ /
\/ -- -- \/
<------ Network N ------->
<-------------------- Network M --------------------->
Figure 10 The variety of MTU values
Consider the following example values. For IPv6, the minimum LMTU (a)
is 1280 bytes, which is also the minimum PMTU (b). The minimum RMTU
(c) is 1500 bytes, which is also the minimum MTU for endpoint-to-
endpoint communication. This means that IPv6 already assumes that
endpoint-to-endpoint communication may require source fragmentation
to transit IPv6-compatible links, even without considering tunnels.
The TMTU (d) is the tunnel equivalent of a LMTU, and thus also needs
to be 1280 bytes for IPv6. Assuming the links of a tunnel traverse
IPv6 hops (e.g., I to Rb, Rb to Rc, and Rc to E), the TIMTU (e) is
equivalent to the PMTU between I and E, which is 1280 - encaps (where
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"encaps" is the tunnel encapsulation overhead). This value is
insufficient to satisfy the requirement of an IPv6 link (which must
transit at least 1280 bytes unfragmented), but this is not a problem.
The TMTU (d) is not limited by TIMTU (e), but by ERMTU (f), the
tunnel equivalent of RMTU (c). For a tunnel using IPv6 over IPv6, the
ERMTU is the RMTU of tne underlying network N minus space for
encapsulation, i.e., 1500 - encaps bytes, and the tunnel is viable as
long as ERMTU >= 1280. Even though the tunnel will ultimately transit
ERMTU - encaps byte messages between the ingress and egress, each hop
within the tunnel transits only TIMTU - encaps byte messages. The
difference between TIMTU and ERMTU is the reason why the tunnel
ingresses need to support fragmentation and tunnel egresses need to
support reassembly. The high cost of fragmentation and reassembly is
why it is useful for applications to avoid sending messages too close
to the PMTU, even the PMTU at their own layer.
4.2. Fragmentation
A tunnel interacts with fragmentation in two different ways. As a
link in network M, it messages might be fragmented before they reach
the tunnel - i.e., at the TTP layer either during source
fragmentation (if generated at the same node as the ingress
interface) or forwarding fragmentation (for IPv4 DF=0 datagrams). In
addition, messages traversing the tunnel may require fragmentation by
the ingress - i.e., source fragmentation at the TLP layer by the
ingress. These two fragmentation operations are no more related than
are conventional IP fragmentation and ATM segmentation and
reassembly; one occurs at the network layer, the other at the
(virtual) link layer.
As with any link layer, a tunnel MTU (TMTU) is defined as the largest
message that can transit the tunnel. For a tunnel, this is the egress
reassembly MTU (ERMTU), which is the reassembly MTU (RMTU) of the
egress interface minus the space needed for the tunnel encapsulation
headers. This value must also satisfy the requirements of the IP
packets that the tunnel transits.
Note that many of the issues with tunnel fragmentation and MTU
handling were discussed in [RFC4459], but that document described a
variety of alternatives as if they were independent. This document
explains the combined approach that is necessary.
Like any other link, an IPv4 tunnel must transit 68 byte packets
without requiring source fragmentation [RFC791][RFC1122] and an IPv6
tunnel must transit 1280 byte packets without requiring source
fragmentation [RFC2460]. The tunnel MTU interacts with routers or
hosts it connects the same way as would a link MTU. In the following
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pseudocode, TTPsize is the size of the tunnel transit packet (TTP),
and ERMTU is the reassembly MTU of the egress. As with any link, the
link MTU (LMTU) is defined not by the native path of the link (or,
for a tunnel, the path MTU of encapsulated packets inside the tunnel)
but by the egress reassembly capability. This is because the ICMP
"packet too big" message indicates failure of a link to transit a
packet, not a preference for a size that matches that inside the
mechanism of the link. There is no ICMP message for "larger than I'd
like, but I can still transit it".
These rules apply at the host/router where the tunnel is attached,
i.e., at the network layer of the TTP (we assume that all tunnels,
including multipoint tunnels, have a single, uniform TMTU). These are
basic source fragmentation rules (or transit refragmentation for IPv4
DF=0 datagrams), and have no relation to the tunnel itself other than
to consider the TMTU as the effective LMTU of the next hop:
if (TTP > TMTU) then
if (TTP can be fragmented, e.g., IPv4 DF=0) then
split TTP into fragments of TMTU size
and send each fragment to the tunnel ingress
else
drop TTP and send ICMP "too big" to TTP source
endif
else
send TTP to the tunnel ingress
endif
These rules apply at the tunnel ingress, in its role as host on the
tunnel path, i.e., as source fragmentation of TLP messages (we assume
that all tunnels, even multipoint tunnels, have a single, uniform
TIMTU), where "encaps" is the encapsulation overhead:
if (TTP <= (TIMTU + encaps)) then
encapsulate the TTP and process as if arriving at the node
else
if ((TIMTU + encaps) < TTP <= (ERMTU - encaps)) then
fragment TTP into TIMTU chunks
encapslate each chunk and process as if arriving at the node
else
{never happens; host/router already dropped by now}
endif
endif
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There is one path above that never occurs - i.e., a network interface
should never receive a message larger than its MTU, and a tunnel
should thus never receive a message larger than its (ERMTU - encaps)
limit. A router attempting to process such a message would generate
an ICMP error (packet too big, fragmentation needed) and the packet
would already have been dropped before entering into this algorithm.
As an example, consider IPv4 over IPv6 or IPv6 over IPv6 tunneling,
where IPv6 encapsulation adds a 40 byte fixed header plus IPv6
options (i.e., IPv6 header extensions) of total size TOptSz. From
[RFC2460] it follows that the TMTU must be at least 1280 bytes and
the ERMTU must be at least 1500 - (40 + TOptSz) bytes. The TIMTU must
be a minimum of 1280 - (40 + TOptSz) bytes. Considering these minimum
values, the previous algorithm becomes:
if (TTP <= (1240 - TOptSz)) then
encapsulate the TTP and and process as if arriving at the node
else
if ((1240 - TOptSz) < TTP <= (1460 - TOptSz)) then
fragment TTP into (1240 - TOptSz) chunks
encapslate each chunk and process as if arriving at the node
else
{never happens; host/router already dropped by now}
endif
endif
This tunnel supports IPv6 transit only if TOptSize is smaller than
180 bytes, and supports IPv4 transit if TOptSize is smaller than 884
bytes. IPv6 TTPs of 1280 bytes may be guaranteed transit the outer
network (M) without needing fragmentation there but they may require
ongoing fragmentation and reassembly if the TMTU is not at least 1320
bytes.
When using IP directly over IP, the minimum ERMTU for IPv4 is 576
bytes and for IPv6 is 1500 bytes. This means that tunnels of IPv4-
over-IPv4, IPv4-over-IPv6, and IPv6-over-IPv6 are possible without
additional requirements, but this may involve ingress fragmentation
and egress reassembly. IPv6 cannot be tunneled directly over IPv4
without additional requirements, notably that the ERMTU is at least
1280 bytes. Fragmentation and reassembly cannot be avoided for IPv6-
over-IPv6 without similar requirements.
When ongoing ingress fragmentation and egress reassembly would be
prohibitive or costly, larger MTUs can be supported by design and
confirmed either out-of-band (by design) or in-band (e.g., using
PLPMTUD [RFC4821], as done in SEAL [RFC5320] and AERO [Te16]).
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Alternately, an ingress can encapsulate packets that fit and shut
down once fragmentation is needed, but it must not continue to
forward smaller packets while dropping larger packets that are still
within required limits.
4.3. MTU discovery
MTU discovery enables a network path to support a larger PMTU than it
can assume from the minimum requirements of protocol over which it
operates. A tunnel has two different LMTU-like values: TMTU and the
TIMTU.
There is temptation to optimize tunnel traversal so that packets are
not fragmented between ingress and egress, i.e., to attempt tune the
network PMTU to the TIMTU rather than the TMTU, to avoid ingress
fragmentation. This is hazardous for many reasons:
o The tunnel is capable of transiting packets as large as the ERMTU,
which is always at least as large as the TIMTU and typically is
larger.
o ICMP has only one type of error message regarding large packets -
"too big", i.e., too large to transit. There is no optimization
message of "bigger than I'd like, but I can deal with if needed".
o IP tunnels often involve some level of recursion, i.e.,
encapsulation over itself [RFC4459].
Recursive tunneling occurs whenever a protocol ends up encapsulated
in itself. This happens directly, as when IPv4 is encapsulated in
IPv4, or indirectly, as when IP is encapsulated in UDP which then is
a payload inside IP. It can involve many layers of encapsulation
because a tunnel provider isn't always aware of whether the packets
it transits are already tunneled.
Recursion is impossible when the tunnel transit packets are limited
to that of the native size of the TIMTU. Arriving tunnel transit
packets have a minimum supported size (1280 for IPv6) and the tunnel
PMTU has the same requirement; there would be no room for the
additional encapsulation headers. The result would be an IPv6 tunnel
that cannot satisfy IPv6 transit requirements.
It is more appropriate to require the tunnel to satisfy IP transit
requirements and enforce that requirement at design time or during
operation (the latter using PLPMTUD [RFC4821]). Conventional path MTU
discovery (PMTUD) relies on existing endpoint ICMP processing of
explicit negative feedback from routers along the path via "message
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to big" ICMP packets in the reverse direction of the tunnel
[RFC1191]. This technique is susceptible to the "black hole"
phenomenon, in which the ICMP messages never return to the source due
to policy-based filtering [RFC2923]. PLPMTUD requires a separate,
direct control channel from the egress to the ingress that provides
positive feedback; the direct channel is not blocked by policy
filters and the positive feedback ensures fail-safe operation if
feedback messages are lost [RFC4821].
4.4. IP ID exhaustion
In IPv4, the IP Identification (ID) field is a 16-bit value that is
unique for every packet for a given source address, destination
address, and protocol, such that it does not repeat within the
Maximum Segment Lifetime (MSL) [RFC791][RFC1122]. Although the ID
field was originally intended for fragmentation and reassembly, it
can also be used to detect and discard duplicate packets, e.g., at
congested routers (see Sec. 3.2.1.5 of [RFC1122]). For this reason,
and because IPv4 packets can be fragmented anywhere along a path, all
packets between a source and destination of a given protocol must
have unique ID values over a period of an MSL, which is typically
interpreted as two minutes (120 seconds). These requirements have
recently been somewhat relaxed in recognition of the primary use of
this field for reassembly and the need to handle only fragment
misordering at the receiver [RFC6864].
The uniqueness of the IP ID is a known problem for high speed nodes,
because it limits the speed of a single protocol between two
endpoints [RFC4963]. Although this suggests that the uniqueness of
the IP ID is moot, tunnels exacerbate this condition. A tunnel often
aggregates traffic from a number of different source and destination
addresses, of different protocols, and encapsulates them in a header
with the same ingress and egress addresses, all using a single
encapsulation protocol. The result is one of the following:
1. The IP ID rules are enforced, and the tunnel throughput is
severely limited.
2. The IP ID rules are enforced, and the tunnel consumes large
numbers of ingress/egress IP addresses solely to ensure ID
uniqueness.
3. The IP ID rules are ignored.
The last case is the most obvious solution, because it corresponds to
how endpoints currently behave. Fortunately, fragmentation is
somewhat rare in the current Internet at large, but it can be common
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along a tunnel. Fragments that repeat the IP ID risk being
reassembled incorrectly, especially when fragments are reordered or
lost. Reassembly errors are not always detected by other protocol
layers (see Sec. 4.9), and even when detected they can result in
excessive overall packet loss and can waste bandwidth between the
egress and ultimate packet destination.
4.5. Hop Count
This section considers the selection of the value of the hop count of
the tunnel link header, as well as the potential impact on the tunnel
transit header. The former is affected by the number of hops within
the tunnel. The latter determines whether the tunnel has visible
effect on the transit packet.
In general, the Internet hop count field is used to detect and avoid
forwarding loops that cannot be corrected without a synchronized
reboot. The IPv4 Time-to-Live (TTL) and IPv6 Hop Limit field each
serve this purpose [RFC791][RFC2460].
The IPv4 TTL field was originally intended to indicate packet
expiration time, measured in seconds. A router is required to
decrement the TTL by at least one or the number of seconds the packet
is delayed, whichever is larger [RFC1812]. Packets are rarely held
that long, and so the field has come to represent the count of the
number of routers traversed. IPv6 makes this meaning more explicit.
These hop count fields represent the number of network forwarding
elements traversed by an IP datagram. An IP datagram with a hop count
of zero can traverse a link between two hosts because it never visits
a router (where it would need to be decremented and would have been
dropped).
An IP datagram traversing a tunnel thus need not have its hopcount
modified, i.e., the tunnel transit header need not be affected. A
zero hop count datagram should be able to traverse a tunnel as easily
as it traverses a link. A router MAY be configured to decrement
packets traversing a particular link (and thus a tunnel), which may
be useful in emulating a path as if it had traversed one or more
routers, but this is strictly optional. The ability of the outer
network and tunnel network to avoid indefinitely looping packets does
not rely on the hop counts of the tunnel traversal packet and tunnel
link packet being related in any way at all.
The hop count field is also used by several protocols to determine
whether endpoints are "local", i.e., connected to the same subnet
(link-local discovery and related protocols [RFC4861]). A tunnel is a
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way to make a remote address appear directly-connected, so it makes
sense that the other ends of the tunnel appear local and that such
link-local protocols operate over tunnels unless configured
explicitly otherwise. When the interfaces of a tunnel are numbered,
these can be interpreted the same way as if they were on the same
link subnet.
4.6. Signaling
In the current Internet architecture, signaling goes upstream, either
from routers along a path or from the destination, back toward the
source. Such signals are typically contained in ICMP messages, but
can involve other protocols such as RSVP, transport protocol signals
(e.g., TCP RSTs), or multicast control or transport protocols.
A tunnel behaves like a link and acts like a link interface at the
nodes where it is attached. As such, it can provide information that
enhances IP signaling (e.g., ICMP), but itself does not directly
generate ICMP messages.
For tunnels, this means that there are two separate signaling paths.
The outer network M nodes can each signal the source of the tunnel
transit packets, Hsrc (Figure 11). Inside the tunnel, the inner
network N nodes can signal the source of the tunnel link packets, the
ingress I (Figure 12).
+--------+---------------------------+--------+
| | | |
v --_ -- v
+------+ / \ / \ +------+
| Hsrc |--+ Ra + -- -- + Rd +--| Hdst |
+------+ \ //\ / \ / \ /\\ / +------+
--/I \--+ Rb +--+ Rc +--/E \--
\ / \ / \ / \ /
\/ -- -- \/
<---- Network N ----->
<-------------------- Network M --------------------->
Figure 11 Signals outside the tunnel
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+-----+-------+------+
--_ | | | | --
+------+ / \ v | | | / \ +------+
| Hsrc |--+ Ra + -- -- + Rd +--| Hdst |
+------+ \ //\ / \ / \ /\\ / +------+
--/I \--+ Rb +--+ Rc +--/E \--
\ / \ / \ / \ /
\/ -- -- \/
<----- Network N ---->
<--------------------- Network M -------------------->
Figure 12 Signals inside the tunnel
These two signal paths are inherently distinct except where
information is exchanged between the network interface of the tunnel
(the ingress) and its attached node (Ra, in both figures).
It is always possible for a network interface to provide hints to its
attached node (host or router), which can be used for optimization.
In this case, when signals inside the tunnel indicate a change to the
tunnel, the ingress (i.e., the tunnel network interface) can provide
information to the router (Ra, in both figures), so that Ra can
generate the appropriate signal in return to Hsrc. This relaying may
be difficult, because signals inside the tunnel may not return enough
information to the ingress to support direct relaying to Hsrc.
In all cases, the tunnel ingress needs to determine how to relay the
signals from inside the tunnel into signals back to the source. For
some protocols this is either simple or impossible (such as for
ICMP), for others, it can even be undefined (e.g., multicast). In
some cases, the individual signals relayed from inside the tunnel may
result in corresponding signals in the outside network, and in other
cases they may just change state of the tunnel interface. In the
latter case, the result may cause the router Ra to generate new ICMP
errors when later messages arrive from Hsrc or other sources in the
outer network.
The meaning of the relayed information must be carefully translated.
In the case of soft or hard ICMP errors, the translation may be
obvious. ICMP "packet too big" messages from inside the tunnel might
update TIMTU at the ingress, but may have no effect on the tunnel as
visible to the router where it is attached (Ra).
In addition to ICMP, messages typically considered for translation
include Explicit Congestion Notification (ECN [RFC6040]) and
multicast (IGMP, e.g.).
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4.7. Relationship of Header Fields
Some tunnel specifications attempt to relate the fields of the tunnel
transit packet and tunnel link packet, i.e., the packet arriving at
the ingress and the encapsulation header. These two headers are
effectively independent and there is no utility in requiring their
contents to be related.
In specific, the encapsulation header source and destination
addresses are network endpoints in the tunnel network N, but have no
meaning in the outer network M, even when the tunneled packet
traverses the same network. The addresses are effectively
independent, and the tunnel endpoint addresses are link addresses to
the tunnel transit packet.
Because the tunneled packet uses source and destination addresses
with a separate meaning, it is inappropriate to copy or reuse the
IPv4 Identification or IPv6 Fragment ID fields of the tunnel transit
packet. These fields need to be generated based on the context of the
encapsulation header, not the tunnel transit header.
Similarly, the DF field need not be copied from the tunnel transit
packet to the encapsulation header of the tunnel link packet
(presuming both are IPv4). Path MTU discovery inside the tunnel does
not directly correspond to path MTU discovery outside the tunnel,
i.e., inside the tunnel it would update the TIMTU used for outer
fragmentation at the ingress, but has no effect on the TMTU reported
to the device where the ingress is attached as a network interface.
The same is true for most other fields. When a field value is
generated in the encapsulation header, its meaning should be derived
from what is desired in the context of the tunnel as a link. When
feedback is received from these fields, they should be presented to
the tunnel ingress and egress as if they were network interfaces. The
behavior of the node where these interfaces attach should be
identical to that of a conventional link.
There are exceptions to this rule that are explicitly intended to
relay signals from inside the tunnel to outside the tunnel. The
primary example is ECN [RFC6040], which copies the ECN bits from the
tunnel transit header to the tunnel link header during encapsulation
at the ingress and modifies the tunnel transit header at egress based
on a combination of the bits of the two headers. This is intended to
allow congestion notification within the tunnel to be interpreted as
if it were on the direct path. Other examples may involve the DSCP
flags. In both cases, it is assumed that the intent of copying values
on encapsulation and merging values on decapsulation has the effect
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of allowing the tunnel to act as if it participates in the same type
of network as outside the tunnel (network M).
4.8. Congestion
In general, tunnels carrying IP traffic need not react directly to
congestion any more than would any other link layer [RFC5405]. IP
traffic is not generally expected to be congestion reactive.
[text from David Black on ECN relaying?]
4.9. Checksums
IP traffic transiting a tunnel needs to expect a similar level of
error detection and correction as it would expect from any other
link. In the case of IPv4, there are no such expectations, which is
partly why it includes a header checksum [RFC791].
IPv6 omitted the header checksum because it already expects most link
errors to be detected and dropped by the link layer and because it
also assumes transport protection [RFC2460]. When transiting IPv6
over IPv6, the tunnel fails to provide the expected error detection.
This is why IPv6 is often tunneled over layers that include separate
protection, such as GRE [RFC2784].
The fragmentation created by the tunnel ingress can increase the need
for stronger error detection and correction, especially at the tunnel
egress to avoid reassembly errors. The Internet checksum is known to
be susceptible to reassembly errors that could be common [RFC4963],
and should not be relied upon for this purpose. This is why SEAL and
AERO include a separate checksum [RFC5320][Te16]. This requirement
can be undermined when using UDP as a tunnel with no UDP checksum (as
per [RFC6935][RFC6936]) when fragmentation occurs because the egress
has no checksum with which to validate reassembly. For this reason,
it is safe to use UDP with a zero checksum for atomic (non-
fragmented, non-fragmentable) tunnel link packets only; when used on
fragments, whether generated at the ingress or en-route inside the
tunnel, omission of such a checksum can result in reassembly errors
that can cause additional work (capacity, forwarding processing,
receiver processing) downstream of the egress.
4.10. Numbering
Tunnel ingresses and egresses have addresses associated with the
encapsulation protocol. These addresses are the source and
destination (respectively) of the encapsulated packet while
traversing the tunnel network.
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Tunnels may or may not have addresses in the network whose traffic
they transit (e.g., network M in Figure 4). In some cases, the tunnel
is an unnumbered interface to a point-to-point virtual link. When the
tunnel has multiple egresses, tunnel interfaces require separate
addresses in network M.
To see the effect of tunnel interface addresses, consider traffic
sourced at router Ra in Figure 4. Even before being encapsulated by
the ingress, that traffic needs a source IP network address that
belongs to the router. One option is to use an address associated
with one of the other interfaces of the router [RFC1122]. Another
option is to assign a number to the tunnel interface itself.
Regardless of which address is used, the resulting IP packet is then
encapsulated by the tunnel ingress using the ingress address as a
separate operation.
4.11. Multicast
[To be addressed]
Note that PMTU for multicast is difficult. PIM carries an option that
may help in the Population Count Extensions to PIM [RFC6807].
IMO, again, this is no different than any other multicast link.
4.12. Multipoint
Multipoint tunnels are tunnels with more than two ingress/egress
endpoints. Just as tunnels emulate links, multipoint tunnels emulate
multipoint links.
Multipoint links require a support for egress determination, just as
multipoint links do. This function is typically supported by ARP
[RFC826] or ARP emulation (e.g., LAN Emulation, known as LANE
[RFC2225]) for multipoint links. For multipoint tunnels, a similar
mechanism is required for the same purpose - to determine the egress
address for proper ingress encapsulation.
All multipoint systems - tunnels and links - might support different
MTUs between each ingress/egress (or link entrance/exit) pair. In
most cases, it is simpler to assume a uniform MTU throughout the
multipoint system, e.g., the minimum MTU supported across all
ingress/egress pairs. This applies to both the ERMTU and TIMETU (the
latter as used only by the ingress).
A multipoint tunnel MUST have support for broadcast and multicast, in
exactly the same way as this is already required for multipoint links
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[RFC3819]. Both modes can be supported either by a native mechanism
inside the tunnel or by emulation using serial replication at the
tunnel ingress, in the same way that links may provide the same
support either natively (e.g., via promiscuous or automatic
replication in the link itself) or network interface emulation (e.g.,
as for non-broadcast multiaccess networks, i.e., NBMAs).
4.13. NAT / Load Balancing
[To be addressed]
Talk about ECMP / LAG here
4.14. Recursive tunnels
[IS THIS REDUNDANT?]
The rules described in this document already support tunnels over
tunnels, sometimes known as "recursive" tunnels, in which IP is
transited over IP either directly or via intermediate encapsulation
(IP-UDP-IP).
There are known hazards to recursive tunneling, notably that the
independence of the tunnel transit header and tunnel link header hop
counts can result in a tunneling loop. Such looping can be avoided
when using direct encapsulation (IP in IP) by use of a header option
to track the encapsulation count and to limit that count [RFC2473].
This looping cannot be avoided when other protocols are used for
tunneling, e.g., IP in UDP in IP, because the encapsulation count may
not be visible where the recursion occurs.
5. Observations (implications)
[Leave this as a shopping list for now]
5.1. Tunnel protocol designers
Recursive tunneling + minimum MTU = frag/reassembly is inevitable, at
least to be able to split/join two fragments
Account for egress MTU/path MTU differences.
Include a stronger checksum.
Ensure the egress MTU is always larger than the path MTU.
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Ensure that the egress reassembly can keep up with line rate OR
design PLPMTUD into the tunneling protocol.
5.2. Tunnel implementers
Detect when the egress MTU is exceeded.
Detect when the egress MTU drops below the required minimum and shut
down the tunnel if that happens - configuring the tunnel down and
issuing a hard error may be the only way to detect this anomaly, and
it's sufficiently important that the tunnel SHOULD be disabled. This
is always better than blindly assuming the tunnel has been deployed
correctly, i.e., that the solution has been engineered.
Do NOT decrement the TTL as part of being a tunnel. It's always
already OK for a router to decrement the TTL based on different next-
hop routers, but TTL is a property of a router not a link.
5.3. Tunnel operators
Keep the difference between "enforced by operators" vs. "enforced by
active protocol mechanism" in mind. It's fine to assume something the
tunnel cannot or does not test, as long as you KNOW you can assume
it. When the assumption is wrong, it will NOT be signaled by the
tunnel. Do NOT decrement the TTL as part of being a tunnel. It's
always already OK for a router to decrement the TTL based on
different next-hop routers, but TTL is a property of a router not a
link.
Do NOT decrement the TTL as part of being a tunnel. It's always
already OK for a router to decrement the TTL based on different next-
hop routers, but TTL is a property of a router not a link.
>>>> PLPMTUD can give incorrect information during ECMP or LAG
5.4. Diagnostics
Some current implementations include diagnostics to support
monitoring the impact of tunneling, especially the impact on
fragmentation and reassembly resources, the status of path MTU
discovery, etc.
>> Because a tunnel ingress/egress is a network interface, it SHOULD
have similar resources as any other network interface. This includes
resources for packet processing as well as monitoring.
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5.5. For existing standards
5.5.1. Generic UDP Encapsulation (GUE - IP in UDP in IP)
[He15]
Consistent with this doc:
Inconsistent with this doc:
Imports RFC4459
Appears to allow both pre and post-encapsulation fragmentation
Recommendations:
Should not encourage pre-encaps fragmentation
See recommendations for RFC4459
5.5.2. Generic Packet Tunneling in IPv6
[RFC2473]
Consistent with this doc:
Considers the endpoints of the tunnel as virtual interfaces.
Considers the tunnel a virtual link.
Requires source fragmentation at the ingress and reassembly at the
egress.
Includes a recursion limit to prevent unlimited re-encapsulation.
Sets tunnel transit header hop limit independently.
Sends ICMPs back at the ingress based on the arriving tunnel
transit packet and its relation to the tunnel MTU (though it uses the
incorrect value of the tunnel MTU; see below).
Allows for ingress relaying of internal tunnel errors (but see
below; it does not discuss retaining state about these).
Inconsistent with this doc:
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Decrements the tunnel transit header by 1, i.e., incorrectly
assuming that tunnel endpoints occur at routers only and that the
tunnel, rather than the router, is responsible for this decrement.
This doc goes to pains to describe the decapsulation process as if
it were distinct from conventional protocol processing by the
receiver (when it should not be).
Copies traffic class from tunnel link to tunnel transit header (as
one variant).
Treats the tunnel MTU as the tunnel path MTU, rather than the
tunnel egress MTU.
Incorrectly fragments IPv4 DF=0 tunnel transit packets that arrive
larger than the tunnel MTU at the IPv6 layer; the relationship
between IPv4 and the tunnel is more complex (as noted in this doc).
Fails to retain state from the tunnel based on ingress receiving
ICMP messages from inside the tunnel, e.g., such as might cause
future tunnel transit packets arriving at the ingress to be discarded
with an ICMP error response rather than allowing them to proceed into
the tunnel.
Recommendation:
This doc should update 2473 for TTL decrement, tunnel MTU, and
fragmentation. Other issues are less critical.
5.5.3. Geneve (NVO3)
[RFC7364] info, [Gr16] stds - ISSUE US AS BCP; Gr16 should follow
Consistent with this doc:
Generation of the link header fields is not discussed and presumed
independent of transit packet.
Reportedly treats an ingress/egress as applying to multiple
tunnels, rather than considering them logically independent for each
tunnel. This appears to confuse implementation aggregation with
architecture.
Reportedly treats tunnels as supporting traffic for multiple
virtual networks, rather than considering them logically independent.
This appears to confuse implementation aggregation with architecture.
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Inconsistent with this doc:
Tries to match transit to tunnel path MTU rather than egress MTU.
Recommendation:
Gr16 should be updated to follow us
5.5.4. GRE (IP in GRE in IP)
IPv4 [RFC2784] stds, [RFC7588] info, [RFC7676] stds - NO CHANGES
Consistent with this doc:
Does not address link header generation.
Non-default behavior allows fragmentation of link packet to match
tunnel path MTU up to the limit of the egress MTU.
Default behavior sets link DF independently.
Shuts the tunnel down if the tunnel path MTU isn't >= 1280.
Inconsistent with this doc:
Based on tunnel path MTU, not egress MTU.
Claims that the tunnel (GRE) mechanism is responsible for
generating ICMP error messages.
Default behavior fragments transit packet (where possible) based
on tunnel path MTU (it should fragment based on egress MTU).
Default behavior does not support the minimum MTU of IPv6 when run
over IPv6.
Non-default behavior allows copying DF for IPv4 in IPv4.
Recommendations:
No changes - existing docs largely describe legacy deployment.
5.5.5. IP in IP / mobile IP
IPv4 [RFC2003] stds, [RFC4459] info:
Consistent with this doc:
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Generate link ID independently
Generate link DF independently when transit DF=0
Generate ECN/update ECN based on sharing info [RFC6040]
Set link TTL to transit to egress only (independently)
Do not decrement TTL on entry except when part of forwarding
Do not decrement TTL on exit except when part of forwarding
Options not copied, but used as a hint to desired services.
Generally treat tunnel as a link, e.g., for link-local.
Inconsistent with this doc
Set link DF when transit DF=1 (won't work unless I-E runs PLPMTUD)
Drop at egress if transit TTL=0 (wrong TTL for host-host tunnels)
Drop when transit source is router's IP (prevents tun from router)
Drop when transit source matches egress (prevents tun to router)
Use tunnel ICMPs to generate upper ICMPs, copying context (ICMPs
are now coming from inside a link!); these should be handled by
setting errors as a "network interface" and letting the attached
host/router figure out what to send.
Using tunnel MTU discovery to tune the transit packet to the
tunnel path MTU rather than egress MTU.
Recommendations:
IMO, ought to update 2003! (no "update" to informational), esp.
regarding TTL issues, transit source drop issues, and tunnel MTU.
IPv6 [RFC2473] std:
Consistent with this doc:
Doesn't discuss lots of header fields, but implies they're set
independently.
Sets link TTL independently.
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Inconsistent with this doc:
Tunnel issues ICMP PTBs.
ICMP PTB issued if larger then 1280 - header, rather than egress
reassembly MTU.
Fragments IPv6 over IPv6 fragments only if transit is <= 1280
(i.e., forces all tunnels to have a max MTU of 1280).
Fragments IPv4 over IPv6 fragments only if IPv4 DF=0
(misinterpreting the "can fragment the IPv4 packet" as permission to
fragment at the IPv6 link header)
Considers encapsulation a forwarding operation and decrements the
transit TTL.
Recommendation:
Should UPDATE 2473; tunnel should not issue PTBs (router should),
issue them correctly, fragment correctly, and not TTL decrement.
5.5.6. IPsec tunnel mode (IP in IPsec in IP)
[RFC4301] std
Consistent with this doc:
Most of the rules, except as noted below.
Inconsistent with this doc:
Writes its own header copying rules (Sec 5.1.2), rather than
referring to existing standards, but that makes sense for security
reasons.
Uses policy to set, clear, or copy DF (policy isn't the issue)
Intertwines tunneling with forwarding rather than presenting the
tunnel as a network interface; this can be corrected by using IPsec
transport mode with an IP-in-IP tunnel [RFC3884].
Recommendations:
None.
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5.5.7. L2TP
[RFC3931] std
Consistent with this doc:
Does not address most link headers, which are thus independent.
Inconsistent with this doc:
Manages tunnel access based on tunnel path MTU, instead of egress
MTU.
Refers to RFC2473 (IPv6 in IPv6), which is inconsistent with this
doc as noted above.
Recommendations:
Should update to use correct tunnel MTU.
5.5.8. L2VPN
[RFC4664]
Consistent with this doc:
Inconsistent with this doc:
Recommendations:
5.5.9. L3VPN
[RFC4176]
Consistent with this doc:
Inconsistent with this doc:
Recommendations:
5.5.10. LISP
[RFC6830]
Consistent with this doc:
Inconsistent with this doc:
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Recommendations:
5.5.11. MPLS
[RFC3031]
Consistent with this doc:
Inconsistent with this doc:
Recommendations:
5.5.12. PWE
[RFC3985]
Consistent with this doc:
Inconsistent with this doc:
Recommendations:
5.5.13. SEAL/AERO
[RFC5320][Te16]
Consistent with this doc:
Inconsistent with this doc:
Recommendations:
5.5.14. TRILL
[RFC5556][RFC6325]
Consistent with this doc:
Puts IP in Ethernet, so most of the issues don't come up.
Ethernet doesn't have TTL or fragment.
Rbridge (trill) TTL header is independent of transit packet.
Inconsistent with this doc:
None.
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Recommendations:
None.
5.5.15. RTG DT encapsulations
[No16], refers to NVO3 and other encapsulations
Includes info on tables for multipoint tunnels, additional info for
headers, etc.
Consistent with this doc:
Inconsistent with this doc:
Assumes MTU can be managed to avoid fragmentation. This is
impossible as long as any one layer is used recursively and that
layer includes a mandatory minimum MTU. A "trust but verify" policy
is better than assuming engineered MTU deployment is sufficient.
Relies on ICMP PTB to correct for tunnel path MTU issues.
Allows encaps protocols to not support fragmentation.
Recommendations:
That doc should refer to this regarding general tunneling issues,
including fragmentation, tunnel MTU, and TTL, including the "trust
but verify" issue for engineered MTU deployment.
All encaps protocols for IP over IP (eventually) MUST support
fragm.
5.6. For future standards
Larger IPv4 MTU (2K? or just 2x path MTU?) for reassembly
Always include frag support for at least two frags; do NOT try to
deprecate fragmentation.
Limit encapsulation option use/space.
Augment ICMP to have two separate messages: PTB vs P-bigger-than-
optimal
Include MTU as part of BGP as a hint - SB
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Hazards of multi-MTU draft-van-beijnum-multi-mtu-04
6. Security Considerations
Tunnels may introduce vulnerabilities or add to the potential for
receiver overload and thus DOS attacks. These issues are primarily
related to the fact that a tunnel is a link that traverses a network
path and to fragmentation and reassembly. ICMP signal translation
introduces a new security issue and must be done with care. ICMP
generation at the router or host attached to a tunnel is already
covered by existing requirements (e.g., should be throttled).
Tunnels traverse multiple hops of a network path from ingress to
egress. Traffic along such tunnels may be susceptible to on-path and
off-path attacks, including fragment injection, reassembly buffer
overload, and ICMP attacks. Some of these attacks may not be as
visible to the endpoints of the architecture into which tunnels are
deployed and these attacks may thus be more difficult to detect.
Fragmentation at routers or hosts attached to tunnels may place an
undue burden on receivers where traffic is not sufficiently diffuse,
because tunnels may induce source fragmentation at hosts and path
fragmentation (for IPv4 DF=0) more for tunnels than for other links.
Care should be taken to avoid this situation, notably by ensuring
that tunnel MTUs are not significantly different from other link
MTUs.
Tunnel ingresses emitting IP datagrams MUST obey all existing IP
requirements, such as the uniqueness of the IP ID field. Failure to
either limit encapsulation traffic, or use additional ingress/egress
IP addresses, can result in high speed traffic fragments being
incorrectly reassembled.
Tunnels are susceptible to attacks at both the inner and outer
network layers. The tunnel ingress/egress endpoints appear as network
interfaces in the outer network, and are as susceptible as any other
network interface. This includes vulnerability to fragmentation
reassembly overload, traffic overload, and spoofed ICMPs that
misreport the state of those interfaces. Similarly, the
ingress/egress appear as hosts to the path traversed by the tunnel,
and thus are as susceptible as any other host to attacks as well.
[management?]
[Access control?]
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describe relationship to [RFC6169] - JT (as per INTAREA meeting
notes, don't cover Teredo-specific issues in RFC6169, but include
generic issues here)
7. IANA Considerations
This document has no IANA considerations.
The RFC Editor should remove this section prior to publication.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[Cl88] Clark, D., "The design philosophy of the DARPA internet
protocols," Proc. Sigcomm 1988, p.106-114, 1988.
[Er94] Eriksson, H., "MBone: The Multicast Backbone,"
Communications of the ACM, Aug. 1994, pp.54-60.
[Gr16] Gross, J., et al., "Geneve: Generic Network Virtualization
Encapsulation," draft-ietf-nvo3-geneve-01, Jan. 2016.
[He15] Herbert, T., L. Yong, O. Zia, "Generic UDP Encapsulation,"
draft-ietf-nvo3-gue-04, Jul. 2016.
[No16] Nordmark, E. (Ed.), A. Tian, J. Gross, J. Hudson, L.
Kreeger, P. Garg, P. Thaler, T. Herbert, "Encapsulation
Considerations," draft-ietf-rtgwg-dt-encap-01, Mar. 2016.
[RFC768] Postel, J, "User Datagram Protocol," RFC 768, Aug. 1980
[RFC791] Postel, J., "Internet Protocol," RFC 791 / STD 5, September
1981.
[RFC793] Postel, J, "Transmission Control Protocol," RFC 793, Sept.
1981.
[RFC826] Plummer, D., "An Ethernet Address Resolution Protocol -- or
-- Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware," RFC 826,
Nov. 1982.
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[RFC1075] Waitzman, D., C. Partridge, S. Deering, "Distance Vector
Multicast Routing Protocol," RFC 1075, Nov. 1988.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers," RFC 1122 / STD 3, October 1989.
[RFC1191] Mogul, J., S. Deering, "Path MTU discovery," RFC 1191,
November 1990.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers," RFC
1812, June 1995.
[RFC2003] Perkins, C., "IP Encapsulation within IP," RFC 2003,
October 1996.
[RFC2225] Laubach, M., J. Halpern, "Classical IP and ARP over ATM,"
RFC 2225, Apr. 1998.
[RFC2460] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification," RFC 2460, Dec. 1998.
[RFC2473] Conta, A., "Generic Packet Tunneling in IPv6
Specification," RFC 2473, Dec. 1998.
[RFC2784] Farinacci, D., T. Li, S. Hanks, D. Meyer, P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March
2000.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery," RFC
2923, September 2000.
[RFC2473] Conta, A., S. Deering, "Generic Packet Tunneling in IPv6
Specification," RFC 2473, Dec. 1998.
[RFC3031] Rosen, E., A. Viswanathan, R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[RFC3819] Karn, P., Ed., C. Bormann, G. Fairhurst, D. Grossman, R.
Ludwig, J. Mahdavi, G. Montenegro, J. Touch, L. Wood,
"Advice for Internet Subnetwork Designers," RFC 3819 / BCP
89, July 2004.
[RFC3884] Touch, J., L. Eggert, Y. Wang, "Use of IPsec Transport Mode
for Dynamic Routing," RFC 3884, September 2004.
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[RFC3931] Lau, J., Ed., M. Townsley, Ed., I. Goyret, Ed., "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)," RFC 3931, March
2005.
[RFC3985] Bryant, S., P. Pate (Eds.), "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4176] El Mghazli, Y., Ed., T. Nadeau, M. Boucadair, K. Chan, A.
Gonguet, "Framework for Layer 3 Virtual Private Networks
(L3VPN) Operations and Management," RFC 4176, October 2005.
[RFC4301] Kent, S., and K. Seo, "Security Architecture for the
Internet Protocol," RFC 4301, December 2005.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling," RFC 4459, April 2006.
[RFC4664] Andersson, L., Ed., E. Rosen, Ed., "Framework for Layer 2
Virtual Private Networks (L2VPNs)," RFC 4664, September
2006.
[RFC4821] Mathis, M., J. Heffner, "Packetization Layer Path MTU
Discovery," RFC 4821, March 2007.
[RFC4861] Narten, T., E. Nordmark, W. Simpson, H. Soliman, "Neighbor
Discovery for IP version 6 (IPv6)," RFC 4861, Sept. 2007.
[RFC4963] Heffner, J., M. Mathis, B. Chandler, "IPv4 Reassembly
Errors at High Data Rates," RFC 4963, July 2007.
[RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
Adaptation Layer (SEAL)," RFC 5320, Feb. 2010.
[RFC5405] Eggert, L., G. Fairhurst, "Unicast UDP Usage Guidelines for
Application Designers," RFC 5405, Nov. 2008.
[RFC5556] Touch, J., R. Perlman, "Transparently Interconnecting Lots
of Links (TRILL): Problem and Applicability Statement," RFC
5556, May 2009.
[RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised"
RFC 5944, Nov. 2010.
[RFC6040] Briscoe, B., "Tunneling of Explicit Congestion
Notification," RFC 6040, Nov. 2010.
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[RFC6169] Krishnan, S., D. Thaler, J. Hoagland, "Security Concerns
With IP Tunneling," RFC 6169, Apr. 2011.
[RFC6325] Perlman, R., D. Eastlake, D. Dutt, S. Gai, A. Ghanwani,
"Routing Bridges (RBridges): Base Protocol Specification,"
RFC 6325, July 2011.
[RFC6807] Farinacci, D., G. Shepherd, S. Venaas, Y. Cai, "Population
Count Extensions to Protocol Independent Multicast (PIM),"
RFC 6807, Dec. 2012.
[RFC6830] Farinacci, D., V. Fuller, D. Meyer, D. Lewis, "The
Locator/ID Separation Protocol," RFC 6830, Jan. 2013.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field,"
Proposed Standard, RFC 6864, Feb. 2013.
[RFC6935] Eubanks, M., P. Chimento, M. Westerlund, "IPv6 and UDP
Checksums for Tunneled Packets," RFC 6935, Apr. 2013.
[RFC6936] Fairhurst, G., M. Westerlund, "Applicability Statement for
the Use of IPv6 UDP Datagrams with Zero Checksums," RFC
6936, Apr. 2013.
[RFC7364] Narten, T., Gray, E., Black, D., Fang, L., Kreeger, L., M.
Napierala, "Problem Statement: Overlays for Network
Virtualization", RFC 7364, Oct. 2014.
[RFC7450] Bumgardner, G., "Automatic Multicast Tunneling," RFC 7450,
Feb. 2015.
[RFC7588] Bonica, R., C. Pignataro, J. Touch, "A Widely-Deployed
Solution to the Generic Routing Encapsulation Fragmentation
Problem," RFC 7588, July 2015.
[RFC7676] Pignataro, C., R. Bonica, S. Krishnan, "IPv6 Support for
Generic Routing Encapsulation (GRE)," RFC 7676, Oct 2015.
[Sa84] Saltzer, J., D. Reed, D. Clark, "End-to-end arguments in
system design," ACM Trans. on Computing Systems, Nov. 1984.
[Te16] Templin, F., "Asymmetric Extended Route Optimization,"
draft-templin-aerolink-67, Jun. 2016.
[To01] Touch, J., "Dynamic Internet Overlay Deployment and
Management Using the X-Bone," Computer Networks, July 2001,
pp. 117-135.
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[To03] Touch, J., Y. Wang, L. Eggert, G. Finn, "Virtual Internet
Architecture," USC/ISI Tech. Report 570, Aug. 2003.
[To16] Touch, J., "Middleboxes Models Compatible with the
Internet," USC/ISI Tech. Report <TBD>, July 2016.
[To98] Touch, J., S. Hotz, "The X-Bone," Proc. Globecom Third
Global Internet Mini-Conference, Nov. 1998.
[Zi80] Zimmermann, H., "OSI Reference Model - The ISO Model of
Architecture for Open Systems Interconnection," IEEE Trans.
on Comm., Apr. 1980.
9. Acknowledgments
This document originated as the result of numerous discussions among
the authors, Jari Arkko, Stuart Bryant, Lars Eggert, Ted Faber, Gorry
Fairhurst, Dino Farinacci, Matt Mathis, and Fred Templin. It
benefitted substantially from detailed feedback from Toerless Eckert,
Vincent Roca, and Lucy Yong, as well as other members of the Internet
Area Working Group.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Email: touch@isi.edu
W. Mark Townsley
Cisco
L'Atlantis, 11, Rue Camille Desmoulins
Issy Les Moulineaux, ILE DE FRANCE 92782
Email: townsley@cisco.com
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APPENDIX A: Fragmentation efficiency
A.1. Selecting fragment sizes
There are different ways to fragment a packet. Consider a network
with an MTU as shown in Figure 13, where packets are encapsulated
over the same network layer as they arrive on (e.g., IP in IP). If a
packet as large as the MTU arrives, it must be fragmented to
accommodate the additional header.
X===========================X (MTU)
+----+----------------------+
| iH | DDDDDDDDDDDDDDDDDDDD |
+----+----------------------+
|
| X===========================X (MTU)
| +---+----+------------------+
(a) +->| H'| iH | DDDDDDDDDDDDDDDD |
| +---+----+------------------+
| |
| | X===========================X (MTU)
| | +----+---+----+-------------+
| (a1) +->| nH'| H | iH | DDDDDDDDDDD |
| | +----+---+----+-------------+
| |
| | +----+-------+
| (a2) +->| nH"| DDDDD |
| +----+-------+
|
| +---+------+
(b) +->| H"| DDDD |
+---+------+
|
| +----+---+------+
(b1) +->| nH'| H"| DDDD |
+----+---+------+
Figure 13Fragmenting via maximum fit
Figure 13 shows this process, using Outer Fragmentation as an example
(the situation is the same for Inner Fragmentation, but the headers
that are affected differ). The arriving packet is first split into
(a) and (b), where (a) is of the MTU of the network. However, this
tunnel then traverses over another tunnel, whose impact the first
tunnel ingress has not accommodated. The packet (a) arrives at the
second tunnel ingress, and needs to be encapsulated again, but
because it is already at the MTU, it needs to be fragmented as well,
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into (a1) and (a2). In this case, packet (b) arrives at the second
tunnel ingress and is encapsulated into (b1) without fragmentation,
because it is already below the MTU size.
In Figure 14, the fragmentation is done evenly, i.e., by splitting
the original packet into two roughly equal-sized components, (c) and
(d). Note that (d) contains more packet data, because (c) includes
the original packet header because this is an example of Outer
Fragmentation. The packets (c) and (d) arrive at the second tunnel
encapsulator, and are encapsulated again; this time, neither packet
exceeds the MTU, and neither requires further fragmentation.
X===========================X (MTU)
+----+----------------------+
| iH | DDDDDDDDDDDDDDDDDDDD |
+----+----------------------+
|
| X===========================X (MTU)
| +---+----+----------+
(c) +->| H'| iH | DDDDDDDD |
| +---+----+----------+
| |
| | X===========================X (MTU)
| | +----+---+----+----------+
| (c1) +->| nH | H'| iH | DDDDDDDD |
| +----+---+----+----------+
|
| +---+--------------+
(d) +->| H"| DDDDDDDDDDDD |
+---+--------------+
|
| +----+---+--------------+
(d1) +->| nH | H"| DDDDDDDDDDDD |
+----+---+--------------+
Figure 14 Fragmenting evenly
A.2. Packing
Encapsulating individual packets to traverse a tunnel can be
inefficient, especially where headers are large relative to the
packets being carried. In that case, it can be more efficient to
encapsulate many small packets in a single, larger tunnel payload.
This technique, similar to the effect of packet bursting in Gigabit
Ethernet (regardless of whether they're encoded using L2 symbols as
delineators), reduces the overhead of the encapsulation headers
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(Figure 15). It reduces the work of header addition and removal at
the tunnel endpoints, but increases other work involving the packing
and unpacking of the component packets carried.
+-----+-----+
| iHa | iDa |
+-----+-----+
|
| +-----+-----+
| | iHb | iDb |
| +-----+-----+
| |
| | +-----+-----+
| | | iHc | iDc |
| | +-----+-----+
| | |
v v v
+----+-----+-----+-----+-----+-----+-----+
| oH | iHa | iHa | iHb | iDb | iHc | iDc |
+----+-----+-----+-----+-----+-----+-----+
Figure 15 Packing packets into a tunnel
[NOTE: PPP chopping and coalescing?]
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