Network Working Group W. Simpson
Request for Comments: 1853 Daydreamer
Category: Informational October 1995

IP in IP Tunneling

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.

IESG Note:

Note that this memo is an individual effort of the author. This
document reflects a current informal practice in the internet. There
is an effort underway within the IETF Mobile-IP Working Group to
provide an appropriate proposed standard to address this issue.

Abstract

This document discusses implementation techniques for using IP
Protocol/Payload number 4 Encapsulation for tunneling with IP
Security and other protocols.

Table of Contents

1. IntrodUCtion .......................................... 2

2. Encapsulation ......................................... 3

3. Tunnel Management ..................................... 5
3.1 Tunnel MTU Discovery ............................ 5
3.2 Congestion ...................................... 6
3.3 Routing Failures ................................ 6
3.4 Other ICMP Messages ............................. 6

SECURITY CONSIDERATIONS ...................................... 7
REFERENCES ................................................... 7
ACKNOWLEDGEMENTS ............................................. 8
AUTHOR'S ADDRESS ............................................. 8

1. Introduction

The IP in IP encapsulation Protocol/Payload number 4 [RFC-1700] has
long been used to bridge portions of the Internet which have disjoint
capabilities or policies. This document describes implementation
techniques used for many years by the Amateur Packet Radio network
for joining a large mobile network, and also by early implementations
of IP Security protocols.

Use of IP in IP encapsulation differs from later tunneling techniques
(for example, protocol numbers 98 [RFC-1241], 94 [IDM91a], 53
[swIPe], and 47 [RFC-1701]) in that it does not insert its own
special glue header between IP headers. Instead, the original
unadorned IP Header is retained, and simply wrapped in another
standard IP header.

This information applies principally to encapsulation of IP version
4. Other IP versions will be described in separate documents.

2. Encapsulation

The encapsulation technique is fairly simple. An outer IP header is
added before the original IP header. Between them are any other
headers for the path, such as security headers specific to the tunnel
configuration.

The outer IP header Source and Destination identify the "endpoints"
of the tunnel. The inner IP header Source and Destination identify
the original sender and recipient of the datagram.

Each header chains to the next using IP Protocol values [RFC-1700].

+---------------------------+
Outer IP Header
+---------------------------+
Tunnel Headers
+---------------------------+ +---------------------------+
IP Header Inner IP Header
+---------------------------+ ====> +---------------------------+

IP Payload IP Payload

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

The format of IP headers is described in [RFC-791].

Type Of Service copied from the inner IP header. Optionally,
another TOS may be used between cooperating peers.

This is in keeping with the transparency principle
that if the user was eXPecting a given level of
service, then the tunnel should provide the same
service. However, some tunnels may be constructed
specifically to provide a different level of service
as a matter of policy.

Identification A new number is generated for each outer IP header.

The encapsulated datagram may have already been
fragmented, and another level of fragmentation may
occur due to the tunnel encapsulation. These tunnel
fragments will be reassembled by the decapsulator,
rather than the final destination.

Reserved
ignored (set to zero).

This unofficial flag has seen experimental use, and
while it remains in the inner IP header, does not
affect the tunnel.

Don't Fragment copied from the inner IP header. This allows the
originator to control the level of performance
tradeoffs. See "Tunnel MTU Discovery".

More Fragments set as required when fragmenting.

The flag is not copied for the same reason that a
separate Identification is used.

Time To Live the default value specified in the most recent
"Assigned Numbers" [RFC-1700]. This ensures that
long unanticipated tunnels do not interrupt the flow
of datagrams between endpoints.

The inner TTL is decremented once before
encapsulation, and is not affected by decapsulation.

Protocol the next header; 4 for the inner IP header, when no
intervening tunnel headers are in use.

Source an IP address associated with the interface used to
send the datagram.

Destination an IP address of the tunnel decapsulator.

Options not copied from the inner IP header. However, new
options particular to the path MAY be added.

Timestamp, Loose Source Route, Strict Source Route,
and Record Route are deliberately hidden within the
tunnel. Often, tunnels are constructed to overcome
the inadequacies of these options.

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.

3. Tunnel Management

It is possible that one of the routers along the tunnel interior
might encounter an error while processing the datagram, causing it to
return an ICMP [RFC-792] error message to the encapsulator at the IP
Source of the tunnel. 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 entire encapsulated header. Thus,
it is not generally possible for an encapsulating router to
immediately reflect an ICMP message from the interior of a tunnel
back to the originating host.

However, by carefully maintaining "soft state" about its tunnels, the
encapsulator can return accurate ICMP messages in most cases. The
router SHOULD maintain at least the following soft state information
about each tunnel:

- Reachability of the end of the tunnel.
- Congestion of the tunnel.
- MTU 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. 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 MTU is greater than the tunnel
MTU when Don't Fragment is set), the router sends an appropriate ICMP
error message back to the originator, but also forwards the datagram
into the tunnel. Forwarding the datagram despite returning the error
message ensures that changes in tunnel state will be learned.

Using this technique, the ICMP error messages from encapsulating
routers will not always match one-to-one with errors encountered
within the tunnel, but they will accurately reflect the state of the
network.

3.1. 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
[RFC-1191, RFC-1435] within their tunnels.

As a benefit of Tunnel MTU Discovery, any fragmentation which occurs
because of the size of the encapsulation header is done only once
after encapsulation. This prevents more than one fragmentation of a
single datagram, which improves processing efficiency of the path
routers and tunnel decapsulator.

3.2. Congestion

Tunnel soft state will collect indications of congestion, such as an
ICMP (Type 4) Source Quench in datagrams from the decapsulator
(tunnel peer). When forwarding another datagram into the tunnel,
it is appropriate to send Source Quench messages to the originator.

3.3. Routing Failures

Because the TTL is reset each time that a datagram is encapsulated,
routing loops within a tunnel are particularly dangerous when they
arrive again at the encapsulator. If the IP Source matches any of
its interfaces, an implementation MUST NOT further encapsulate.
Instead, the datagram is forwarded normally.

ICMP (Type 11) Time Exceeded messages report routing loops within the
tunnel itself. ICMP (Type 3) Destination Unreachable messages report
delivery failures to the decapsulator. This soft state MUST be
reported to the originator as (Type 3 Code 0) Network Unreachable.

3.4. Other ICMP Messages

Most ICMP error messages are not relevant to the use of the tunnel.
In particular, parameter problems are likely to be a result of
misconfiguration of the encapsulator, and MUST NOT be reported to the
originator.

Security Considerations

Security issues are briefly discussed in this memo. The use of
tunneling may obviate some older IP security options (labelling), but
will better support newer IP Security headers.

References

[IDM91a] Ioannidis, J., Duchamp, D., Maguire, G., "IP-based
protocols for mobile internetworking", Proceedings of
SIGCOMM '91, ACM, September 1991.

[RFC-791]
Postel, J., "Internet Protocol", STD 5, RFC791,
USC/Information Sciences Institute, September 1981.

[RFC-792]
Postel, J., "Internet Control Message Protocol", STD 5,
RFC792, USC/Information Sciences Institute, September
1981.

[RFC-1191]
Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,
DECWRL, Stanford University, November 1990.

[RFC-1241]
Mills, D., and R. Woodburn, "A Scheme for an Internet
Encapsulation Protocol: Version 1", UDEL, July 1991.

[RFC-1435]
Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC1435, FTP Software, March 1993.

[RFC-1700]
Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, USC/Information Sciences Institute, October 1994.

[RFC-1701]
Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC1701, October 1994.

[swIPe] Ioannidis, J., and Blaze, M., "The Architecture and
Implementation of Network-Layer Security Under Unix", Fourth
Usenix Security Symposium Proceedings, October 1993.

Acknowledgements

These implementation details of IP Tunneling are derived in large
part from independent work in 1990 by Phil Karn and the TCP-Group
hams using KA9Q NOS.

Special thanks to John Ioannidis (then of Columbia University) for
inspiration and experimentation which began this most recent round of
IP Mobility and IP Security development. Some of this text was
derived from [IDM91a] and [swIPe].

The chaining of headers was also described in "Simple Internet
Protocol", by Steve Deering (Xerox PARC).

The overall organization and some of this text was derived from
[RFC-1241], by David Mills (U Delaware) and Robert Woodburn (SAIC).

Some of the text on tunnel soft state was derived from "IP Address
Encapsulation (IPAE)", by Robert E. Gilligan, Erik Nordmark, and Bob
Hinden (all of Sun Microsystems).

Author's Address

Questions about this memo can also be directed to:

William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071