Internet Engineering Task Force C. Perkins
INTERNET DRAFT IBM
10 May 1996
IP Encapsulation within IP
draft-ietf-mobileip-ip4inip4-02.txt
Status of This Memo
This document is a submission by the Mobile-IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the mobile-ip@SmallWorks.com mailing list.
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Abstract
This document specifies a method by which an IP datagram may
be encapsulated (carried as payload) within an IP datagram.
Encapsulation is suggested as a means to alter the normal IP routing
for datagrams, by delivering them to an intermediate destination
which would not be otherwise selected by the (network part of the)
IP destination field. This may be done for any of a variety of
reasons, but is particular useful for adherence to the mobile-IP
specification.
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1. Introduction
This document specifies a method by which an IP datagram may
be encapsulated (carried as payload) within an IP datagram.
Encapsulation is suggested as a means to alter the normal IP routing
for datagrams, by delivering them to an intermediate destination
which would not be otherwise selected based the (network part of the)
IP destination field. The process of encapsulation and decapsulation
a datagram is frequently referred to as "tunneling" the datagram, and
the encapsulator and decapsulator are then considered to be the the
"endpoints" of the tunnel.
In the most general encapsulation case we have
source ---> encapsulator --------> decapsulator ---> destination
with these being separate machines. There may in general be multiple
source-destination pairs using the same tunnel.
2. Motivation
The mobile-IP working group has specified the use of encapsulation
as a way to deliver datagrams from a mobile host's "home network"
to an agent which can deliver datagrams to the mobile host by
conventional means [7]. The use of encapsulation may also be
desirable whenever the source (or an intermediate router) of an
IP datagram must influence the route by which a datagram is to be
delivered to its ultimate destination. Other possible applications
include preferential billing, choice of routes with selected security
attributes, and general policy routing.
It is generally true that encapsulation and source routing techniques
can be used in similar ways to affect the routing of a datagram, but
there are several technical reasons to prefer encapsulation:
- There are unsolved security problems associated with the use of
source routing.
- Current internet routers exhibit performance problems when
forwarding datagrams which use the IP source routing option.
- Too many internet hosts process source routing options
incorrectly.
- Firewalls may exclude source-routed datagrams.
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- Insertion of an IP source route option may complicate the
processing of authentication information by the source and/or
destination of a datagram, depending on how the authentication is
specified to be performed.
- It is considered impolite for intermediate routers to make
modifications to datagrams which they did not originate.
These technical advantages must be weighed against the disadvantages
posed by the use of encapsulation:
- Encapsulated datagrams typically are longer than source routed
datagrams.
- Encapsulation cannot be used unless it is known in advance that
the tunnel endpoint for the encapsulated datagram can decapsulate
the datagram.
Since the majority of internet hosts today do not perform well
when IP loose source route options are used, the second technical
disadvantage of encapsulation is not as serious as it might seem at
first.
3. IP in IP Encapsulation
An outer IP header is inserted before the datagram's IP header:
+---------------------------+
| Outer IP Header |
+---------------------------+ +---------------------------+
| IP Header | | IP Header |
+---------------------------+ ====> +---------------------------+
| | | |
| IP Payload | | IP Payload |
| | | |
+---------------------------+ +---------------------------+
The format of the IP header is described in RFC 791[9]. The outer
IP header source and destination addresses identify the "endpoints"
of the tunnel. The inner IP header source and destination addresses
identify the sender and recipient of the datagram. The inner IP
header is not changed by the node which encapsulates it, except
to decrement the TTL before delivery. The inner header remains
unchanged during its delivery to the tunnel endpoint. No change
to IP options in the inner header occurs during delivery of the
encapsulated datagram through the tunnel. If need be, other protocol
headers such as the IP Authentication header [1] may be inserted
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between the outer IP header and the inner IP header (also see
section 6.3).
3.1. IP Header Fields and Handling
Version
4
IHL
The Internet Header Length measures the length (in bytes) of
the outer IP header exclusive of its payload, but including any
options which the encapsulating agent may insert.
TOS
The type of service is copied from the inner IP header.
Total Length
The length measures the length of the outer IP header along
with its payload, that is to say (typically) the inner IP
header and the original datagram.
Identification, Flags, Fragment Offset
These three fields are set in accordance with the procedures
specified in [9]. The "Don't Fragment" bit in the outer IP
header is copied from the corresponding flag in the inner IP
header.
Time to Live
The Time To Live (TTL) field in the outer IP header is set to a
value appropriate for delivery of the encapsulated datagram to
the tunnel endpoint.
Protocol
The protocol field in the outer IP header is set to protocol
number 4 for the encapsulation protocol.
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Header Checksum
The Header Checksum is computed over the length (in bytes) of
the outer IP header exclusive of its payload, but including any
options which the encapsulating endpoint may insert.
Source Address
The IP address of the encapsulating agent, that is, the tunnel
starting point.
Destination Address
The IP address of the decapsulating agent, that is, the tunnel
completion point.
Options
not copied from the inner IP header. However, new options
particular to the path MAY be added. In particular, any
supported flavors of security options of the inner IP header
MAY affect the choice of security options for the tunnel. It
is not expected that there be a one-to-one mapping of such
options to the options or security headers selected for the
tunnel.
The inner TTL is decremented by one. If the resulting TTL is
0, the datagram is not tunneled. An encapsulating agent MUST
NOT encapsulate a datagram with TTL = 0 for delivery to a tunnel
endpoint. The TTL is not changed when decapsulating. If, after
decapsulation, the inner datagram has TTL zero, a tunnel endpoint
MUST discard the datagram. If the decapsulator forwards the datagram
to some network interface, it will decrement the TTL as a result of
doing normal IP forwarding. See also subsection 4.4.
The encapsulating agent is free to use any existing IP mechanisms
appropriate for delivery of the encapsulated payload to the tunnel
endpoint. In particular, this means that use of IP options and
fragmentation are allowed, unless the "Don't Fragment" bit is set in
the inner IP header. This is required so that hosts employing Path
MTU discovery [6] can obtain the information they seek.
3.2. Routing Failures
Routing loops within a tunnel are particularly dangerous when
they cause datagrams to arrive again at the encapsulator. If the
IP Source matches any of its interfaces, an implementation MUST
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NOT further encapsulate. If the IP Source matches the tunnel
destination, an implementation SHOULD NOT further encapsulate. See
also subsection 4.4.
4. ICMP messages from within the tunnel
After an encapsulated datagram has been sent, the encapsulating
agent may receive an ICMP [8] message from any intermediate router
within the tunnel, except for the tunnel endpoint. The action taken
by the encapsulating agent depends on the type of ICMP message
received. When the received message contains enough information, the
encapsulating agent may use the incoming message to create a similar
ICMP message, to be sent to the originator of the inner IP datagram.
This process will be referred to as "relaying" the ICMP message to
the source of the original unencapsulated datagram. Relaying an ICMP
message requires that the encapsulator must strip off the outer IP
header which it receives from the sender of the ICMP message. For
cases where the received message does not contain enough data, see
section 5.
4.1. Destination Unreachable (Type 3)
Destination Unreachable messages are handled depending upon their
type. The model suggested here allows the tunnel to "extend" a
network to include non-local (e.g., mobile) hosts. Thus, if the
original destination in the unencapsulated datagram is on the same
network as the encapsulating agent, certain Destination Unreachable
codes may be modified to conform to the suggested model.
Network Unreachable (Code 0)
A Destination Unreachable message may be returned to
the original sender. If the original destination in
the unencapsulated datagram is on the same network as
the encapsulating agent, the newly generated Destination
Unreachable message sent by the encapsulating agent MAY have
code 1 (Host Unreachable), since presumably the datagram
arrived to the correct network and the encapsulating agent is
trying to create the appearance that the original destination
is local even if it's not. Otherwise, the encapsulating agent
must return a Destination Unreachable with code 0 message to
the original sender.
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Host Unreachable (Code 1)
The encapsulating agent must relay Host Unreachable messages to
the source of the original unencapsulated datagram.
Protocol Unreachable (Code 2)
When the encapsulating agent receives a Protocol Unreachable
ICMP message, it should send a Destination Unreachable message
with code 0 or 1 (see the discussion for code 0) to the sender
of the original unencapsulated datagram. Since the original
sender might only rarely use protocol 4, it would be usually be
meaningless to return code 2 to that sender.
Port Unreachable (Code 3)
This code should never be received by the encapsulating
agent, since the outer IP header does not refer to any port
number. It must not be relayed to the source of the original
unencapsulated datagram.
Datagram Too Big (Code 4)
The encapsulating agent must relay Datagram Too Big messages to
the source of the original unencapsulated datagram.
Source Route Failed (Code 5)
This code should be treated by the encapsulating agent
itself. It must not be relayed to the source of the original
unencapsulated datagram.
4.2. Source Quench (Type 4)
The encapsulating agent SHOULD NOT relay Source Quench messages to
the source of the original unencapsulated datagram, but instead
activate whatever congestion control mechanisms it implements to
alleviate the congestion detected within the tunnel.
4.3. Redirect (Type 5)
The encapsulating agent may act on the Redirect message if it is
possible, but it should not relay the Redirect back to the source of
the datagram which was encapsulated.
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4.4. Time Exceeded (Type 11)
ICMP Time Exceeded messages report routing loops within the tunnel
itself. Reception of Time Exceeded messages by the encapsulator MUST
be reported to the originator as Host Unreachable (Type 3 Code 1).
Host Unreachable is preferable to Network Unreachable; since the
datagram was handled by the encapsulator, and the encapsulator is
often considered to be on the same network as the destination address
in the original unencapsulated datagram, the datagram is considered
to have reached the correct network, but not the correct destination
host within that network.
4.5. Parameter Problem (Type 12)
If the parameter problem points to a field copied from the original
unencapsulated datagram, the encapsulating agent may relay the ICMP
message to the source; otherwise, if the problem occurs with an IP
option inserted by the encapsulating agent, then the encapsulating
agent must not relay the ICMP message to the source. Note that an
encapsulating agent following prevalent current practice will never
insert any IP options into the encapsulated datagram, except possibly
for security reasons.
4.6. Other messages
Other ICMP messages are not related to the encapsulation operations
described within this protocol specification, and should be acted on
as specified in [8].
5. Tunnel Management
Unfortunately, ICMP only requires IP routers to return 8 bytes (64
bits) of the datagram beyond the IP header. This is not enough to
include the encapsulated header, so it is not always possible for the
home agent to immediately reflect the ICMP message from the interior
of a tunnel back to the originating host.
However, by carefully maintaining "soft state" about its tunnels,
the encapsulating router can return accurate ICMP messages in most
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cases. The router SHOULD maintain at least the following soft state
information about each tunnel:
- MTU of the tunnel (subsection 5.1)
- TTL (path length) of the tunnel
- Reachability of the end of the tunnel
The router uses the ICMP messages it receives from the interior of a
tunnel to update the soft state information for that tunnel. ICMP
errors that could be received from one of the routers along the
tunnel interior include:
- Datagram Too Big
- Time Exceeded
- Destination Unreachable
- Source Quench
When subsequent datagrams arrive that would transit the tunnel,
the router checks the soft state for the tunnel. If the datagram
would violate the state of the tunnel (such as, the TTL is less than
the tunnel TTL) the router sends an ICMP error message back to the
source, but also forwards the datagram into the tunnel.
Using this technique, the ICMP error messages sent by encapsulating
routers will not always match up one-to-one with errors encountered
within the tunnel, but they will accurately reflect the state of the
network.
Tunnel soft state was originally developed for the IP address
encapsulation (IPAE) specification [4].
5.1. Tunnel MTU Discovery
When the Don't Fragment bit is set by the originator and copied
into the outer IP header, the proper MTU of the tunnel will
be learned from ICMP (Type 3 Code 4) "Datagram Too Big" errors
reported to the encapsulator. To support originating hosts
which use this capability, all implementations MUST support Path
MTU Discovery [5, 6] within their tunnels. In this particular
application there are several advantages:
- As a benefit of Tunnel MTU Discovery, any fragmentation which
occurs because of the size of the encapsulation header is
performed only once after encapsulation. This prevents multiple
fragmentation of a single datagram, which improves processing
efficiency of the path routers and tunnel decapsulator.
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- If the source of the unencapsulated datagram is doing MTU
discovery then it is desirable for the encapsulator to know the
MTU to the decapsulator. If it doesn't know the MTU then it
can transfer the DF bit to the outer datagram; however, if that
triggers ICMP Datagram Too Big from within the tunnel (and hence
returned to the encapsulator) the encapsulator cannot always
return a correct ICMP response to the source unless it has kept
state information about recently sent datagrams. If the tunnel
MTU is returned to the source by the encapsulator in a Datagram
Too Big message, the MTU that is conveyed SHOULD be the MTU of
the tunnel minus the size of the encapsulating IP header. This
will avoid fragmentation of the original IP datagram by the
encapsulator, something that is otherwise certain to occur.
- If the source is not doing MTU discovery it is still desirable
for the encapsulator to know the MTU to the decapsulator. In
particular it is much better to fragment the inner datagram than
to allow the outer datagram to be fragmented. Fragmenting the
inner datagram can be done without special buffer requirements
and without the need to keep state in the decapsulator.
By contrast if the outer datagram is fragmented then the
decapsulator needs to keep state and buffer space on behalf of
the destination.
The encapsulator SHOULD in normal circumstances do MTU discovery
and try to send datagrams with the DF bit set. However there are
problems with this approach. When the source sets the DF bit it can
react quickly to resend the information if it gets a ICMP Datagram
Too Big. When the encapsulator gets a ICMP Datagram Too Big, but the
source had not set the DF bit, then there is nothing sensible that
the encapsulator can do to let the source know. The encapsulator MAY
keep a copy of the sent datagram whenever it tries increasing the MTU
- this will allow it to resend the datagram fragmented if it gets a
datagram too big response. Alternatively the encapsulator MAY be
configured for certain classes of input to not set the DF bit when
the source has not set the DF bit.
5.2. Congestion
Tunnel soft state will collect indications of congestion, such as
an ICMP (Type 4) Source Quench or a Congestion Experienced flag in
datagrams from the decapsulator (tunnel peer). When forwarding
another datagram into the tunnel, it is appropriate to use approved
means for controlling congestion [3]; Source Quench messages SHOULD
NOT be sent to the originator.
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6. Security Considerations
IP encapsulation potentially reduces the security of the Internet.
For this reason care needs to be taken in the implementation and
deployment.
Assume an organization has good physical control of a secure subset
of its network. Assume that the routers connecting that secure
network do not allow in datagrams with source addresses belonging to
interfaces on that secure network. In that situation it is possible
to safely deploy protocols within that network which depend on the
source address of datagrams for authentication purposes.
Networks with physical security can still be used to run protocols
which are convenient, but which have implementation or protocol bugs
which would make them dangerous to use if external sources have
access to the protocol. The external sources can be excluded using
router datagram filtering.
IP encapsulation protocols may allow datagrams to bypass the checks
in the border routers. There are two cases to consider:
- The case where the people controlling the border routers are
trying to protect inner machines from themselves.
- The case where the inner machine is looking after its own
defense.
An uncooperative inner machine cannot be protected by the border
router except by barring all packets to that machine. There is
nothing to stop encapsulated IP coming in to that inner machine in
otherwise harmless datagrams such as port 53 UDP datagrams (i.e.,
apparently DNS datagrams). So there is a strong case for placing
the security controls at the host rather than the router. However,
in situations where the administrative control of the inner machine
is cooperative but lacks thoroughness or competence, security can be
enhanced by also putting protection in the border routers.
6.1. Router Considerations
Routers need to be aware of IP encapsulation protocols so they can
correctly filter incoming datagrams.
Beyond that it is desirable that filtering be integrated with IP
authentication [1]. In the case of IP encapsulation this can have
2 forms: Encapsulation might be allowed (in some cases) as long
as the encapsulating datagrams authentically come from an expected
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encapsulator. Alternatively encapsulation might be allowed if the
encapsulated datagrams have authentication.
Data which is encapsulated and encrypted [2] may also pose a problem.
In this case the router can only filter the datagram if it knows
the security association. To allow this sort of encryption in
environments where all packets need to be filtered (or at least
accounted for) a mechanism must be in place for the receiving host
to securely communicate the association to the border router. This
might, more rarely, also apply to the association used for outgoing
datagrams.
6.2. Host Considerations
Receiving IP encapsulation software SHOULD classify incoming
datagrams and only allow datagrams fitting one of the following
categories:
- The protocol is harmless: source address based authentication is
not needed.
- The datagram can be trusted because of trust in the authentically
identified encapsulating host. That authentic identification
could come from physical security plus border router
configuration but is more likely to come from AH authentication.
- The inner datagram has AH authentication.
Some or all of this checking could be done in border routers rather
than the receiving host but it is better if border router checks are
used as backup rather than being the only check.
6.3. Using Security Options
The security options of the inner IP header MAY affect the choice of
security options for the encapsulating IP header.
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7. Acknowledgements
Parts of sections 3 and 5 of this document were taken from sections
(authored by Bill Simpson) of earlier versions of the mobile-IP
Internet Draft [7]. Good ideas have also been included from RFC
1853 [10], also authored by Bill Simpson. "Security Considerations"
(section 6) was largely contributed by Bob Smart. Thanks also to
Anders Klemets for finding mistakes and suggesting many improvements
to the draft.
References
[1] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
[2] R. Atkinson. IP Encapsulating Security Payload. RFC 1827,
August 1995.
[3] F. Baker, Editor. Requirements for IP Version 4 Routers. RFC
1812, June 1995.
[4] R. Gilligan, E. Nordmark, and B. Hinden. IPAE: The SIPP
Interoperability and Transition Mechanism. Internet Draft --
work in progress, March 1994.
[5] S. Knowles. IESG Advice from Experience with Path MTU
Discovery. RFC 1435, March 1993.
[6] J. Mogul and S. Deering. Path MTU Discovery. RFC 1191,
November 1990.
[7] C. Perkins, Editor. ietf-draft-mobileip-protocol-16.txt - work
in progress. IPv4 Mobility Support, April 1996.
[8] J. B. Postel, Editor. Internet Control Message Protocol. RFC
792, September 1981.
[9] J. B. Postel, Editor. Internet Protocol. RFC 791, September
1981.
[10] W. Simpson. IP in IP Tunneling. RFC 1853, October 1995.
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Author's Address
Questions about this memo can be directed to:
Charles Perkins
Room H3-D34
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1-914-784-7350
Fax: +1-914-784-6205
E-mail: perk@watson.ibm.com
The working group can be contacted via the current chairs:
Jim Solomon Tony Li
Motorola, Inc. cisco systems
1301 E. Algonquin Rd. 170 W. Tasman Dr.
Schaumburg, IL 60196 San Jose, CA 95134
Work: +1-847-576-2753 Work: +1-408-526-8186
E-mail: solomon@comm.mot.com E-mail: tli@cisco.com
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