Network Working Group C. Perkins
Request for Comment: 2003 IBM
Category: Standards Track October 1996

IP Encapsulation within IP

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

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 that
would otherwise not be selected by the (network part of the) IP
Destination Address field in the original IP header. Encapsulation
may serve a variety of purposes, sUCh as delivery of a datagram to a
mobile node using Mobile IP.

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 that
would otherwise not be selected based on the (network part of the) IP
Destination Address field in the original IP header. Once the
encapsulated datagram arrives at this intermediate destination node,
it is decapsulated, yielding the original IP datagram, which is then
delivered to the destination indicated by the original Destination
Address field. This use of encapsulation and decapsulation of a
datagram is frequently referred to as "tunneling" the datagram, and
the encapsulator and decapsulator are then considered to be the
"endpoints" of the tunnel.

In the most general tunneling case we have

source ---> encapsulator --------> decapsulator ---> destination

with the source, encapsulator, decapsulator, and destination being
separate nodes. The encapsulator node is considered the "entry

point" of the tunnel, and the decapsulator node is considered the
"exit point" of the tunnel. There in general may be multiple
source-destination pairs using the same tunnel between the
encapsulator and decapsulator.

2. Motivation

The Mobile IP working group has specified the use of encapsulation as
a way to deliver datagrams from a mobile node's "home network" to an
agent that can deliver datagrams locally by conventional means to the
mobile node at its current location away from home [8]. 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 of encapsulation include multicasting,
preferential billing, choice of routes with selected security
attributes, and general policy routing.

It is generally true that encapsulation and the IP loose source
routing option [10] 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
the IP source routing options.

- Current Internet routers exhibit performance problems when
forwarding datagrams that contain IP options, including the IP
source routing options.

- Many current Internet nodes process IP source routing options
incorrectly.

- Firewalls may exclude IP source-routed datagrams.

- 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 larger than source routed
datagrams.

- Encapsulation cannot be used unless it is known in advance that
the node at the tunnel exit point can decapsulate the datagram.

Since the majority of Internet nodes 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

To encapsulate an IP datagram using IP in IP encapsulation, an outer
IP header [10] is inserted before the datagram's existing IP header,
as follows:

+---------------------------+

Outer IP Header

+---------------------------+ +---------------------------+

IP Header IP Header

+---------------------------+ ====> +---------------------------+


IP Payload IP Payload


+---------------------------+ +---------------------------+

The outer IP header Source Address and Destination Address identify
the "endpoints" of the tunnel. The inner IP header Source Address
and Destination Addresses identify the original sender and recipient
of the datagram, respectively. The inner IP header is not changed by
the encapsulator, except to decrement the TTL as noted below, and
remains unchanged during its delivery to the tunnel exit point. 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 between the outer IP header and the inner IP header. Note
that the security options of the inner IP header MAY affect the
choice of security options for the encapsulating (outer) IP header.

3.1. IP Header Fields and Handling

The fields in the outer IP header are set by the encapsulator as
follows:

Version

4

IHL

The Internet Header Length (IHL) is the length of the outer IP
header measured in 32-bit Words [10].

TOS

The Type of Service (TOS) is copied from the inner IP header.

Total Length

The Total Length measures the length of the entire encapsulated
IP datagram, including the outer IP header, the inner IP
header, and its payload.

Identification, Flags, Fragment Offset

These three fields are set as specified in [10]. However, if
the "Don't Fragment" bit is set in the inner IP header, it MUST
be set in the outer IP header; if the "Don't Fragment" bit is
not set in the inner IP header, it MAY be set in the outer IP
header, as described in Section 5.1.

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 exit point.

Protocol

4

Header Checksum

The Internet Header checksum [10] of the outer IP header.

Source Address

The IP address of the encapsulator, that is, the tunnel entry
point.

Destination Address

The IP address of the decapsulator, that is, the tunnel exit
point.

Options

Any options present in the inner IP header are in general NOT
copied to the outer IP header. However, new options specific
to the tunnel path MAY be added. In particular, any supported
types of security options of the inner IP header MAY affect the
choice of security options for the outer header. 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.

When encapsulating a datagram, the TTL in the inner IP header is
decremented by one if the tunneling is being done as part of
forwarding the datagram; otherwise, the inner header TTL is not
changed during encapsulation. If the resulting TTL in the inner IP
header is 0, the datagram is discarded and an ICMP Time Exceeded
message SHOULD be returned to the sender. An encapsulator MUST NOT
encapsulate a datagram with TTL = 0.

The TTL in the inner IP header is not changed when decapsulating.
If, after decapsulation, the inner datagram has TTL = 0, the
decapsulator MUST discard the datagram. If, after decapsulation, the
decapsulator forwards the datagram to one of its network interfaces,
it will decrement the TTL as a result of doing normal IP forwarding.
See also Section 4.4.

The encapsulator may use any existing IP mechanisms appropriate for
delivery of the encapsulated payload to the tunnel exit point. In
particular, use of IP options is allowed, and use of fragmentation is
allowed unless the "Don't Fragment" bit is set in the inner IP
header. This restriction on fragmentation is required so that nodes
employing Path MTU Discovery [7] 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. Suppose a
datagram arrives at a router for forwarding, and the router

determines that the datagram has to be encapsulated before further
delivery. Then:

- If the IP Source Address of the datagram matches the router's own
IP address on any of its network interfaces, the router MUST NOT
tunnel the datagram; instead, the datagram SHOULD be discarded.

- If the IP Source Address of the datagram matches the IP address
of the tunnel destination (the tunnel exit point is typically
chosen by the router based on the Destination Address in the
datagram's IP header), the router MUST NOT tunnel the datagram;
instead, the datagram SHOULD be discarded.

See also Section 4.4.

4. ICMP Messages from within the Tunnel

After an encapsulated datagram has been sent, the encapsulator may
receive an ICMP [9] message from any intermediate router within the
tunnel other than the tunnel exit point. The action taken by the
encapsulator depends on the type of ICMP message received. When the
received message contains enough information, the encapsulator MAY
use the incoming message to create a similar ICMP message, to be sent
to the originator of the original unencapsulated IP datagram (the
original sender). This process will be referred to as "relaying" the
ICMP message from the tunnel.

ICMP messages indicating an error in processing a datagram include a
copy of (a portion of) the datagram causing the error. Relaying an
ICMP message requires that the encapsulator strip off the outer IP
header from this returned copy of the original datagram. For cases
in which the received ICMP message does not contain enough data to
relay the message, see Section 5.

4.1. Destination Unreachable (Type 3)

ICMP Destination Unreachable messages are handled by the encapsulator
depending upon their Code field. The model suggested here allows the
tunnel to "extend" a network to include non-local (e.g., mobile)
nodes. Thus, if the original destination in the unencapsulated
datagram is on the same network as the encapsulator, certain
Destination Unreachable Code values may be modified to conform to the
suggested model.

Network Unreachable (Code 0)

An ICMP Destination Unreachable message SHOULD be returned
to the original sender. If the original destination in
the unencapsulated datagram is on the same network as the
encapsulator, the newly generated Destination Unreachable
message sent by the encapsulator MAY have Code 1 (Host
Unreachable), since presumably the datagram arrived at the
correct network and the encapsulator is trying to create the
appearance that the original destination is local to that
network even if it is not. Otherwise, if the encapsulator
returns a Destination Unreachable message, the Code field MUST
be set to 0 (Network Unreachable).

Host Unreachable (Code 1)

The encapsulator SHOULD relay Host Unreachable messages to the
sender of the original unencapsulated datagram, if possible.

Protocol Unreachable (Code 2)

When the encapsulator receives an ICMP Protocol Unreachable
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 did not use protocol 4 in sending the datagram, it would
be meaningless to return Code 2 to that sender.

Port Unreachable (Code 3)

This Code should never be received by the encapsulator, since
the outer IP header does not refer to any port number. It MUST
NOT be relayed to the sender of the original unencapsulated
datagram.

Datagram Too Big (Code 4)

The encapsulator MUST relay ICMP Datagram Too Big messages to
the sender of the original unencapsulated datagram.

Source Route Failed (Code 5)

This Code SHOULD be handled by the encapsulator itself.
It MUST NOT be relayed to the sender of the original
unencapsulated datagram.

4.2. Source Quench (Type 4)

The encapsulator SHOULD NOT relay ICMP Source Quench messages to the
sender of the original unencapsulated datagram, but instead SHOULD
activate whatever congestion control mechanisms it implements to help
alleviate the congestion detected within the tunnel.

4.3. Redirect (Type 5)

The encapsulator MAY handle the ICMP Redirect messages itself. It
MUST NOT not relay the Redirect to the sender of the original
unencapsulated datagram.

4.4. Time Exceeded (Type 11)

ICMP Time Exceeded messages report (presumed) routing loops within
the tunnel itself. Reception of Time Exceeded messages by the
encapsulator MUST be reported to the sender of the original
unencapsulated datagram 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, then the datagram is considered
to have reached the correct network, but not the correct destination
node within that network.

4.5. Parameter Problem (Type 12)

If the Parameter Problem message points to a field copied from the
original unencapsulated datagram, the encapsulator MAY relay the ICMP
message to the sender of the original unencapsulated datagram;
otherwise, if the problem occurs with an IP option inserted by the
encapsulator, then the encapsulator MUST NOT relay the ICMP message
to the original sender. Note that an encapsulator following
prevalent current practice will never insert any IP options into the
encapsulated datagram, except possibly for security reasons.

4.6. Other ICMP Messages

Other ICMP messages are not related to the encapsulation operations
described within this protocol specification, and should be acted on
by the encapsulator as specified in [9].

5. Tunnel Management

Unfortunately, ICMP only requires IP routers to return 8 octets (64
bits) of the datagram beyond the IP header. This is not enough to
include a copy of the encapsulated (inner) IP header, so it is not
always possible for the encapsulator to relay the ICMP message from
the interior of a tunnel back to the original sender. However, by
carefully maintaining "soft state" about tunnels into which it sends,
the encapsulator can return accurate ICMP messages to the original
sender in most cases. The encapsulator SHOULD maintain at least the
following soft state information about each tunnel:

- MTU of the tunnel (Section 5.1)
- TTL (path length) of the tunnel
- Reachability of the end of the tunnel

The encapsulator 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
encapsulator checks the soft state for the tunnel. If the datagram
would violate the state of the tunnel (for example, the TTL of the
new datagram is less than the tunnel "soft state" TTL) the
encapsulator sends an ICMP error message back to the sender of the
original datagram, but also encapsulates the datagram and forwards it
into the tunnel.

Using this technique, the ICMP error messages sent by the
encapsulator 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 Datagram Too Big (Type 3, Code 4) messages reported to the
encapsulator. To support sending nodes which use Path MTU Discovery,

all encapsulator implementations MUST support Path MTU Discovery [5,
7] soft state within their tunnels. In this particular application,
there are several advantages:

- As a benefit of Path MTU Discovery within the tunnel, 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 decapsulator and the
routers within the tunnel.

- If the source of the unencapsulated datagram is doing Path MTU
Discovery, then it is desirable for the encapsulator to know
the MTU of the tunnel. Any ICMP Datagram Too Big messages from
within the tunnel are returned to the encapsulator, and as noted
in Section 5, it is not always possible for the encapsulator to
relay ICMP messages to the source of the original unencapsulated
datagram. By maintaining "soft state" about the MTU of the
tunnel, the encapsulator can return correct ICMP Datagram Too Big
messages to the original sender of the unencapsulated datagram to
support its own Path MTU Discovery. In this case, the MTU that
is conveyed to the original sender by the encapsulator 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.

- If the source of the original unencapsulated datagram is
not doing Path MTU Discovery, it is still desirable for the
encapsulator to know the MTU of the tunnel. In particular, it is
much better to fragment the original datagram when encapsulating,
than to allow the encapsulated datagram to be fragmented.
Fragmenting the original datagram can be done by the encapsulator
without special buffer requirements and without the need to
keep reassembly state in the decapsulator. By contrast, if
the encapsulated datagram is fragmented, then the decapsulator
must reassemble the fragmented (encapsulated) datagram before
decapsulating it, requiring reassembly state and buffer space
within the decapsulator.

Thus, the encapsulator SHOULD normally do Path MTU Discovery,
requiring it to send all datagrams into the tunnel with the "Don't
Fragment" bit set in the outer IP header. However there are problems
with this approach. When the original sender sets the "Don't
Fragment" bit, the sender can react quickly to any returned ICMP
Datagram Too Big error message by retransmitting the original
datagram. On the other hand, suppose that the encapsulator receives
an ICMP Datagram Too Big message from within the tunnel. In that
case, if the original sender of the unencapsulated datagram had not

set the "Don't Fragment" bit, there is nothing sensible that the
encapsulator can do to let the original sender know of the error.
The encapsulator MAY keep a copy of the sent datagram whenever it
tries increasing the tunnel MTU, in order to allow it to fragment and
resend the datagram if it gets a Datagram Too Big response.
Alternatively the encapsulator MAY be configured for certain types of
datagrams not to set the "Don't Fragment" bit when the original
sender of the unencapsulated datagram has not set the "Don't
Fragment" bit.

5.2. Congestion

An encapsulator might receive indications of congestion from the
tunnel, for example, by receiving ICMP Source Quench messages from
nodes within the tunnel. In addition, certain link layers and
various protocols not related to the Internet suite of protocols
might provide such indications in the form of a Congestion
Experienced [6] flag. The encapsulator SHOULD reflect conditions of
congestion in its "soft state" for the tunnel, and when subsequently
forwarding datagrams into the tunnel, the encapsulator SHOULD use
appropriate means for controlling congestion [3]; However, the
encapsulator SHOULD NOT send ICMP Source Quench messages to the
original sender of the unencapsulated datagram.

6. Security Considerations

IP encapsulation potentially reduces the security of the Internet,
and care needs to be taken in the implementation and deployment of IP
encapsulation. For example, IP encapsulation makes it difficult for
border routers to filter datagrams based on header fields. In
particular, the original values of the Source Address, Destination
Address, and Protocol fields in the IP header, and the port numbers
used in any transport header within the datagram, are not located in
their normal positions within the datagram after encapsulation.
Since any IP datagram can be encapsulated and passed through a
tunnel, such filtering border routers need to carefully examine all
datagrams.

6.1. Router Considerations

Routers need to be aware of IP encapsulation protocols in order to
correctly filter incoming datagrams. It is desirable that such
filtering be integrated with IP authentication [1]. Where IP
authentication is used, encapsulated packets might be allowed to
enter an organization when the encapsulating (outer) packet or the
encapsulated (inner) packet is sent by an authenticated, trusted
source. Encapuslated packets containing no such authentication
represent a potentially large security risk.

IP datagrams which are encapsulated and encrypted [2] might also pose
a problem for filtering routers. In this case, the router can filter
the datagram only if it shares the security association used for the
encryption. To allow this sort of encryption in environments in
which all packets need to be filtered (or at least accounted for), a
mechanism must be in place for the receiving node to securely
communicate the security association to the border router. This
might, more rarely, also apply to the security association used for
outgoing datagrams.

6.2. Host Considerations

Host implementations that are capable of receiving encapsulated IP
datagrams SHOULD admit only those datagrams fitting into one or more
of the following categories:

- The protocol is harmless: source address-based authentication is
not needed.

- The encapsulating (outer) datagram comes from an authentically
identified, trusted source. The authenticity of the source could
be established by relying on physical security in addition to
border router configuration, but is more likely to come from use
of the IP Authentication header [1].

- The encapuslated (inner) datagram includes an IP Authentication
header.

- The encapsulated (inner) datagram is addressed to a network
interface belonging to the decapsulator, or to a node with which
the decapsulator has entered into a special relationship for
delivering such encapsulated datagrams.

Some or all of this checking could be done in border routers rather
than the receiving node, but it is better if border router checks are
used as backup, rather than being the only check.

7. Acknowledgements

Parts of Sections 3 and 5 of this document were taken from portions
(authored by Bill Simpson) of earlier versions of the Mobile IP
Internet Draft [8]. The original text for section 6 (Security
Considerations) was contributed by Bob Smart. Good ideas have also
been included from RFC1853 [11], also authored by Bill Simpson.
Thanks also to Anders Klemets for finding mistakes and suggesting
improvements to the draft. Finally, thanks to David Johnson for
going over the draft with a fine-toothed comb, finding mistakes,
improving consistency, and making many other improvements to the
draft.

References

[1] Atkinson, R., "IP Authentication Header", RFC1826, August 1995.

[2] Atkinson, R., "IP Encapsulating Security Payload", RFC1827,
August 1995.

[3] Baker, F., Editor, "Requirements for IP Version 4 Routers", RFC
1812, June 1995.

[4] Gilligan, R., Nordmark, E., and B. Hinden, "IPAE: The SIPP
Interoperability and Transition Mechanism", Work in Progress.

[5] Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC1435, March 1993.

[6] Mankin, A., and K. Ramakrishnan, "Gateway Congestion Control
Survey", RFC1254, August 1991.

[7] Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,
November 1990.

[8] Perkins, C., Editor, "IP Mobility Support", RFC2002,
October 1996.

[9] Postel, J., Editor, "Internet Control Message Protocol", STD 5,
RFC792, September 1981.

[10] Postel, J., Editor, "Internet Protocol", STD 5, RFC791,
September 1981.

[11] Simpson, W., "IP in IP Tunneling", RFC1853, October 1995.

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
EMail: perk@watson.ibm.com

The working group can be contacted via the current chair:

Jim Solomon
Motorola, Inc.
1301 E. Algonquin Rd.
Schaumburg, IL 60196

Work: +1-847-576-2753
EMail: solomon@comm.mot.com