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 RFC 1853 [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", RFC 1826, August 1995.
 [2] Atkinson, R., "IP Encapsulating Security Payload", RFC 1827,
 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", RFC 1435, March 1993.
 [6] Mankin, A., and K. Ramakrishnan, "Gateway Congestion Control
 Survey", RFC 1254, August 1991.
 [7] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
 November 1990.
 [8] Perkins, C., Editor, "IP Mobility Support", RFC 2002,
 October 1996.
 [9] Postel, J., Editor, "Internet Control Message Protocol", STD 5,
 RFC 792, September 1981.
 [10] Postel, J., Editor, "Internet Protocol", STD 5, RFC 791,
 September 1981.
 [11] Simpson, W., "IP in IP Tunneling", RFC 1853, 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