Network Working Group J. Vilhuber
INTERNET DRAFT Cisco Systems Inc.
Expire in December, 2004 January, 2003
IP header compression in IPsec ESP
<draft-vilhuber-hcoesp-01.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Distribution of this memo is unlimited.
Abstract
This draft outlines how to use IP Header compression over IP tunnels
[HCOIP] inside IPsec ESP [ESP].
Vilhuber [Page 1]
INTERNET DRAFT July, 2004 Expires December, 2004
1. Introduction
[HCOIP] defines a new IP protocol number (and IPv6 Next Header) value
for IP Header Compressed payloads for use in tunneling protocols over
IP. This draft outlines exactly how to encapsulate IP Header
Compressed packets into an ESP packet.
In this document, the key words "MAY", "MUST", "optional",
"recommended", "required", "SHOULD", and "SHOULD NOT", are to be
interpreted as described in [RFC2119].
2. IP header compressed packets in ESP
[HCOIP] defines a number-space for the "Header Compression Payload
Type" as well as a new IP protocol number, which can be used to
indicate that a packet inside ESP has been previously header
compressed.
Note that not all packets that fall under a certain ESP SA may be
header compressed. Whether a packet is header compressed or not
depends on whether the compressor has an empty slot for a flow, and
whether the packet is deemed compressible by the compressor. Hence,
we can not simply assume that all packets under an ESP SA with header
compression will be compressed. We need an explicit indication, hence
the need for the new IP protocol number in [HCOIP].
2.1. Order of processing of outbound packets
On outbound processing, the relevant SA bundle is found in whatever
manner the implementation uses. The SA bundle MUST indicate that
header compression needs to be attempted for this packet. The SA
should contain enough information to retrieve the relevant
compression context for this flow.
Header compression MUST be done before any other ESP or IPCOMP
processing. Any fragmentation decisions MUST be made on the result of
the header-compressed packet, rather than on the original (un-header-
compressed packet).
Original (pre-IPsec packet):
+-------------+
| IP | Data |
+-------------+
Header Compression is done, which removes the IP (and possibly other)
headers, replacing it with the appropriate compression context as
defined by [IPHC], [CRTP], and/or [ECRTP]:
+----------------------------+
| Comp ID and context | Data |
+----------------------------+
[HCOIP] header is prepended:
Vilhuber [Page 2]
INTERNET DRAFT July, 2004 Expires December, 2004
+---------------------------------+
| Comp Type | Comp context | Data |
+---------------------------------+
[ESP] is performed (including fragmentation decisions and possibly
[IPCOMP]) as usual, with next header set to IPHC:
+---------------------------------------------------------------------+
| IP | ESP-SPI | SEQ | IV | Comp Type | Comp context | Data | Trailer |
+---------------------------------------------------------------------+
An example of an ESP packet carrying a header compressed packet is as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| IV (depends on the cipher used; variable) | ^ ESP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload
| Comp Type | Header Compressed Data (variable) | | Data
+-+-+-+-+-+-+-+-+ + |
~ ~ |
| | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Padding (0-255 bytes) | v
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| | Pad Length | IPHC Proto |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (variable) |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Depending on the cipher used, the start of the ESP Payload data is
used as the IV. The length (and presence or absence) of the IV is
implicit knowledge, known to both sides.
"Comp Type" is the [HCOIP] header, and will be the first byte of the
plaintext payload. The rest of the Payload Data is the compressed
packet, the format of which conforms to the relevant RFC that covers
the type indicated in Comp Type (see [HCOIP] table 1).
2.2. Order of processing of inbound packets
On receiving an IPsec packet, the regular SA-lookup is used to
determine the SA bundle needed for decryption (decapsulation and
decompression). The SA bundle should carry enough information to
retrieve the decompression context.
Vilhuber [Page 3]
INTERNET DRAFT July, 2004 Expires December, 2004
If the receiver gets a packet with IP protocol IPHC, but the SA
bundle does not indicate that compression has been negotiated for
this SA, the packet MUST be dropped.
Since Header compression is the first thing done during
encryption/encapsulation, decompression is the last thing done. After
decapsulation and decryption (and maybe IPCOMP decompression), if the
resulting packet has a protocol type of IPHC, then the [HCOIP] header
is removed, and the packet along with the [HCOIP] type (from the
header), along with the decompression context stored with the IPsec
SA, MUST be handed to the decompressor.
After decompression, the decompressed packet MUST be checked against
the SADB as normal, and dropped, if the packet does not match the
SADB.
2.3. Decompressor Error handling
In the event that the decompressor has no appropriate compression
slot (as identified by the compression ID; see [IPHC]) for the packet
handed to it, the packet MUST be dropped. There is no error
indication that can be communicated to the peer.
In the event that the decompressor is out of sync with the compressor
(i.e. a decompression context for this Compression ID exists, but
packet loss has occurred), the decompressor may need to communicate a
CONTEXT_STATE packet back to the compressor. This packet MUST be sent
back through the IPsec Tunnel, i.e. must be encrypted and
encapsulated using the correct SA, i.e. the SA we would use to send
ESP packets to the peer. The out-of-sync packet MUST be dropped.
Should the Compressor receive a CONTEXT_STATE packet that has not
been authenticated via IPsec, then, as per [SECARCH], the packet MUST
be dropped and ignored.
On receipt of a valid CONTEXT_STATE packet, the receiver, who was the
sender of the packet that failed to decompress, will invalidate any
contexts listed in the CONTEXT_STATE packet, as per [RFC2507] (and
various addenda in [RFC2508] and [ECRTP]).
3. Security Considerations
3.1. Removing plain-text
Omitting the IP (as well as TCP or UDP/RTP) header removes a large
amount of known (or guess-able) plaintext from the to-be-encrypted
payload. While this can benefit security it still should not be
relied upon as a replacement for a strong cryptographic mechanism.
Vilhuber [Page 4]
INTERNET DRAFT July, 2004 Expires December, 2004
3.2. Active Attack Analysis
There is some concern about what an attacker can or can not do to the
decompressor, even when protected with ESP. We assume that an
attacker can not modify packets, because of ESP protection. For this
reason, it is recommended that ESP not be run without authentication,
even when esp-null is used.
Whether an attacker can look at the packets (i.e. a passive attacker)
has no immediate relevance to header compression.
An active attacker can drop packets, or insert fake ones. Fake ones
will be discarded by IPsec, but dropped packets can have an influence
on the decompressor as outlined in the following sections.
3.2.1 COMPRESSED_TCP, COMPRESSED_UDP, and COMPRESSED_RTP packets
DELTA fields depend on the number of packets we receive and send,
i.e. DELTA fields depend on the value of sent in the preceding
FULL_HEADER packet, and we increment the field value by a fixed,
known delta for each packet received. There are well-defined
algorithms that try to help detect dropped packets and correct in
those situations. However, to be conservative, we should assume that
dropped packets MAY influence DELTA fields (however, see below).
Likewise, any INFERRED fields that depend on DELTA fields could be
decompressed wrong (but most INFERRED fields do not, in fact, depend
on DELTA fields).
An attacker can NOT influence any NOCHANGE fields, as those are
explicitely copied from the compression context set up (and
refreshed) via FULL_HEADER packets, which can not be tampered with.
Examples of NOCHANGE fields are IP addresses (src and dst), protocol,
and src and dst ports. RANDOM fields are carried explicitely in the
compressed packet and thus can not be tampered with, either. Examples
of RANDOM fields are checksums.
An attacker can also not change any of the data in the packet by
selectively dropping packets, as the header compression mechanisms do
not affect the data.
So the best an attacker could do is influence DELTA fields, which are
generally sequence numbers, by selectively and intelligently dropping
packets. If the higher level protocol uses checksums (as TCP does,
and, mostly, UDP as well), then mis-decompression due to dropped
packets will be detected by the recipient of the mis-decompressed
packet.
For TCP packets, it is presumed that the end-host will detect and
discard any "off-by-k" sequence numbers via the TCP checksum. Neither
TCP nor UDP checksums cover anything in the IP header other than the
pseudo-header, which doesn't cover parts of the IPv4 header, nor
Vilhuber [Page 5]
INTERNET DRAFT July, 2004 Expires December, 2004
large parts of the IPv6 headers.
[CRTP] contains a 4 bit sequence number, to detect dropped packets
within a 16 packet window.
As long as the affected DELTA fields are covered by a higher level
checksum (i.e. UDP or TCP checksum), then attacking the data-stream
by selectively dropping some packets amounts to a denial of service
attack, which the attacker could perpetrate anyway, if he can drop
packets at will.
If the DELTA fields are not covered by a higher level i.e. UDP or
TCP) checksum, then these fields could be wrong after decompression
and the recipient may not notice. A new kind of HDRCKSUM similar to
the one defined in [ECRTP] should be devised to counteract this.
3.2.2 COMPRESSED_NON_TCP and COMPRESSED_TCP_NODELTA
As per [RFC2507], COMPRESSED_NON_TCP packets do not use differential
coding, and all fields are assumed to be NOCHANGE. If a NOCHANGE
field changes, a FULL_HEADER packet is sent, instead. Thus dropping
packets in this case has no effect on the values of the decompressed
packets.
COMPRESSED_TCP_NODELTA "is only sent in response to a header request
from the decompressor" [RFC2507]. Since there are no DELTA fields in
this packet type, dropping this packet (which, in any case is not
sent during normal operation) has no effect (except causing more
drops, i.e. more denial of service).
3.2.3 Future compressed headers
This analysis covers only known header-compression types known at the
time of this writing (see section 6. References). Any future new
compressed types of additional compressed headers should consider the
impact separately, following a similar analysis as in the previous
sections.
NOCHANGE and RANDOM fields can be safely ignored. They are safe.
INFERRED fields are safe as long as they do not depend on DELTA
fields. DELTA fields need to be considered on a case by case basis,
and should be covered by some checksum.
Checksums should never be optional. Alternatively, a scheme like
[ECRTP]'s HDRCKSUM should be used (see 3.2.3 as well).
3.2.3 Future work
As eluded to earlier, a simple way to fix this entire dilemma of
DELTA headers and decompression, is to define a checksum similar to
[ECRTP] HDRCKSUM, which covers the entire header that was compressed,
Vilhuber [Page 6]
INTERNET DRAFT July, 2004 Expires December, 2004
but none of the data, define the TCP and UDP checksums as INFERRED,
and carry ONLY the HDRCKSUM in the compressed packet. Since ESP
packets, when used with authentication, already verify that the data
hasn't been tampered with, we can re-calculate the TCP and UDP
checksums during decompression, as long as we have a way to verify
that the decompressed headers are exactly the same as they were prior
to compression. The HDRCKSUM gives us this assurance, and thus the
mechanism is secure.
The cost is extra processing at the decompressor (who needs to
calculate TCP or UDP checksums, which include the data of the
packet).
4. IANA Considerations
There are no IANA Considerations.
5. Acknowledgments
This document is derived in part from discussions with Cheryl Madson,
David McGrew, Mark Gillott, Patrick Ruddy, and Dan Wing.
6. References
[IPHC] Degermark, M., Nordgren, B. and S. Pink, "Header
Compression for IP", RFC 2507, February 1999.
[CRTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[ECRTP] Koren, Casner, Geevarghese, Thompson, Ruddy, "Compressing
IP/UDP/RTP headers on links with high delay, packet loss
and reordering", draft-ietf-avt-crtp-enhance-04.txt, work in
progress, February 2002
[ESP] Kent, S., Atkinson, R., "IP Encapsulating Security
Payload", RFC 2406, November 1998
[SECARCH] Kent, S., Atkinson, R., "Security Architecture for the Internet
Protocol", RFC 2401, November 1998
[HCOIP] Vilhuber, "IP header compression in IP tunneling protocols",
draft-vilhuber-hcoip-00.txt, work in progress, July, 2004
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997
[IPCOMP] Shacham, Monsour, Pereira, Thomas, "IP Payload Compression
Protocol (IPComp)", RFC 3173, September 2001
Vilhuber [Page 7]
INTERNET DRAFT July, 2004 Expires December, 2004
7. Editor's Address
Jan Vilhuber
<vilhuber@cisco.com>
Cisco Systems, Inc.
Vilhuber [Page 8]
Network Working Group J. Vilhuber
INTERNET DRAFT Cisco Systems Inc.
Expire in December, 2004 July, 2004
IP header compression in IP tunneling protocols
<draft-vilhuber-hcoip-01.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Distribution of this memo is unlimited.
Abstract
Bandwidth consumption of RTP flows can be reduced by tunneling and
compressing headers. This draft defines an IP protocol number and a
header which can be used to transport IP Header Compressed [IPHC],
Compressed RTP [CRTP], and Enhanced Compressed RTP [ECRTP] packets
over an arbitrary IP tunnel (IP-in-IP or ESP, for example) to reduce
bandwidth consumption for RTP flows.
Vilhuber [Page 1]
INTERNET DRAFT July, 2004 Expires December, 2004
1. Introduction
IP header compression [IPHC] and RTP compression [CRTP] can be used
to reduce bandwidth consumption, but are only defined for single hops
over connections with little to no loss and no packet reordering.
[ECRTP] extends the definition of IP header compression to be used
over lossy links with the possibility of packet reordering. I.e.
ECRTP can be used in protocols that run over the Internet at large.
In general, it turns out to be useful to carry IP Header Compressed
packets over an IP tunnel (IP-in-IP or IPSec tunnel mode, for
example), either because the combination (tunnel+HC) is smaller than
the original packet, or because the traffic is already flowing over
an existing tunnel, which we could take advantage of.
This draft recommends that ECRTP is used in the majority of these
cases, as it is expected that the underlying network does not meet
the criteria for reliable use of IPHC or CRTP. However, this draft
does not exclude IPHC and CRTP, as there may be situations where the
underlying network is well-known to be robust against loss and
reordering.
In this document, the key words "MAY", "MUST", "optional",
"recommended", "required", "SHOULD", and "SHOULD NOT", are to be
interpreted as described in [RFC2119].
2. IP header compression packet format
[IPHC], [CRTP], and [ECRTP] only define the compression mechanisms
and compressed packet formats, but leave defining the transport of
this compressed packet to the underlying transport mechanism.
In this vein, [PPPHC] extends the PPP Data Link Layer Protocol Field
values to include the needed Compression Payload Types. For exactly
the same reason, we need to define the Compression Payload Types used
when carrying a header-compressed packet over an IP tunnel in this
draft.
The types of Compression Payloads are scattered over [IPHC], [CRTP],
and [ECRTP]. The following table names the Compression Payload Type,
gives each a number, and specifies which document to look at for the
exact definition of the Compression Type.
Header Compression Payload Type Value Defined in
------------------------------------------------------------------
IPHC_FULL_HEADER 0 IPHC/CRTP
IPHC_COMPRESSED_NON_TCP 1 IPHC/CRTP
IPHC_COMPRESSED_TCP 2 IPHC
IPHC_COMPRESSED_TCP_NODELTA 3 IPHC
IPHC_CONTEXT_STATE 4 IPHC/ECRTP
IPHC_COMPRESSED_UDP_8 5 CRTP/ECRTP
IPHC_COMPRESSED_UDP_16 6 CRTP/ECRTP
IPHC_COMPRESSED_RTP_8 7 CRTP/ECRTP
Vilhuber [Page 2]
INTERNET DRAFT July, 2004 Expires December, 2004
IPHC_COMPRESSED_RTP_16 8 CRTP/ECRTP
Reserved by IANA 9-255
Table 1: Header Compression Payload Types
2.1. IP header compression packet format in IPv4
When carried over IPv4, IP header compressed packets will have the
following header prepended:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
! Comp Type !
+-+-+-+-+-+-+-+-+
Figure 1: IPv4 IP Header Compression Header
"Comp Type" is a 1 byte field that carries the "Header Compression
Payload Type" value (as defined in Table 1), that indicates the type
of compressed payload that follows the header.
Anything following the 1 byte Comp Payload type field is Compression
context and data, in accordance with the respective type, as defined
in the respective documents (see Table 1).
The IPv4 Protocol Number for IP Header compressed packets as defined
in this draft shall be XXX [TBD IANA].
2.2. IP header compression packet format in IPv6
When carried inside IPv6, an IP header compressed packet will be have
the IP Header Compression Header prepended.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Header | Comp Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: IPv6 IP Header Compression Header
The Next Header field is a regular IPv6 Next Header field.
"Comp Type" is a 1 byte field that carries the "Header Compression
Payload Type" value (as defined in Table 1), that indicates the type
of compressed payload that follows the header.
Anything following the 1 byte Comp Payload type field is Compression
context and data, in accordance with the respective type, as defined
in the respective documents (see Table 1).
The IPv6 Next Header value for IP header compressed packets as
Vilhuber [Page 3]
INTERNET DRAFT July, 2004 Expires December, 2004
defined in this draft shall be XXX [TBD IANA]
3. Security Considerations
This draft does not change the security of any protocol, as it merely
provides a mechanism to carry header-compressed packets within
another IP protocol.
That being said, this draft allows us to carry IP header compressed
packets inside IPsec ESP, which provides a way to carry header-
compressed packets over the Internet in a secure way.
When encryption is used, as in IPsec ESP tunnels, omitting the IP (as
well as TCP or UDP/RTP) header removes a large amount of known (or
guessable) plaintext from the to-be-encrypted payload. While this can
benefit security it still should not be relied upon as a replacement
for a strong cryptographic mechanism.
4. IANA Considerations
IANA is requested to create a new assignment registry for "Header
Compression Payload Type Values", initially allowing values in the
range 0 to 255 decimal.
Assignment of values in this field require:
- the identity of the assignee
- a brief description of the new Header Compression Payload type
- a reference to a stable document describing the Header
Compression Payload in detail.
During the first year of existence of this registry, IANA is
requested to refer all requests to the IETF <TBD> WG for review.
Subsequently, requests should be reviewed by the IETF <TBD> Area
Directors or by an expert that they designate.
If the number of assignments begins to approach 255, the <TBD> Area
Directors should be alerted.
5. Acknowledgments
This document is derived in part from discussions with Cheryl Madson,
Mark Gillott, Patrick Ruddy, and Dan Wing.
6. References
[IPHC] Degermark, M., Nordgren, B. and S. Pink, "Header
Compression for IP", RFC 2507, February 1999.
[CRTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[PPPHC] Engan, Casner, Bormann, "IP Header Compression over PPP",
RFC 2509, February 1999
Vilhuber [Page 4]
INTERNET DRAFT July, 2004 Expires December, 2004
[ECRTP] Koren, Casner, Geevarghese, Thompson, Ruddy, "Compressing
IP/UDP/RTP headers on links with high delay, packet loss
and reordering", draft-ietf-avt-crtp-enhance-04.txt, work in
progress, February 2002
[ESP] Kent, S., Atkinson, R., "IP Encapsulating Security
Payload", RFC 2406, November 1998
[IPIP] Perkins, "IP Encapsulation within IP", RFC 2003, October 1996
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997
7. Editor's Address
Jan Vilhuber
<vilhuber@cisco.com>
Cisco Systems, Inc.
Vilhuber [Page 5]