IPsec Extensions to Support Robust Header Compression over IPsec (ROHCoIPsec)Booz Allen Hamilton5220 Pacific Concourse Drive, Suite 200Los AngelesCA90045USertekin_emre@bah.comBooz Allen Hamilton13200 Woodland Park Dr.HerndonVA20171USchristou_chris@bah.comUniversitaet Bremen TZIPostfach 330440Bremen D-28334Germanycabo@tzi.org
Integrating ROHC with IPsec (ROHCoIPsec) offers the combined benefits
of IP security services and efficient bandwidth utilization. However,
in order to integrate ROHC with IPsec, extensions to the SPD and SAD are required.
This document describes the IPsec extensions required to support ROHCoIPsec.
Using IPsec ([IPSEC]) protection offers various security services for
IP traffic. However, these
benefits come at the cost of additional packet headers, which
increase packet overhead. As described in [ROHCOIPSEC], Robust
Header Compression (ROHC [ROHC]) can be used with IPsec to reduce the
overhead associated with IPsec-protected packets.
IPsec-protected traffic is carried over Security
Associations (SAs), whose parameters are negotiated on a case-by-case
basis. The Security Policy Database (SPD) specifies the services
that are to be offered to IP datagrams, and the parameters associated
with SAs that have been established are stored in the Security
Association Database (SAD). For ROHCoIPsec,
various extensions to the SPD and SAD that incorporate ROHC-relevant
parameters are required.
In addition, three extensions to IPsec processing are
required. First, a mechanism for identifying ROHC packets must be
defined. Second, a mechanism to ensure the integrity of the
decompressed packet is needed. Finally, the order of the inbound and outbound
processing must be enumerated when nesting IP Compression (IPComp [IPCOMP]), ROHC, and IPsec processing.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [BRA97].
The following subsections specify extensions to the SPD and the SAD
that MUST be supported for ROHCoIPsec. Appendix A provides an example
ASN.1 representation of the ROHC parameters that are included in the SPD.
In general, the SPD is responsible for specifying the security
services that are offered to IP datagrams. Entries in the SPD
specify how to derive the corresponding values for SAD entries. To
support ROHC, the SPD is extended to include per-channel ROHC
parameters. Together, the existing IPsec SPD parameters and the
ROHC parameters will dictate the security and header compression services that are provided to
packets.
The fields contained within each SPD entry are defined in RFC 4301 [IPSEC],
Section 4.4.1.2. To support ROHC, several processing info fields
are added to the SPD; these fields contain information regarding
the ROHC profiles and channel parameters supported by the local ROHC
instance.If the processing action associated with the selector sets is PROTECT, then the processing info must be extended with the following ROHC channel parameters:
MAX_CID: The field indicates the highest context ID that will be decompressed by the local decompressor.
MAX_CID MUST be at least 0 and at most 16383 (The value 0 implies having one context).
MRRU: The MRRU parameter indicates the size of the largest reconstructed unit (in octets) that
the local decompressor is expected to reassemble from ROHC segments. This size includes the CRC and the ROHC ICV.
NOTE: Since in-order delivery of ROHC packets cannot be guaranteed, the MRRU parameter SHOULD
be set to 0 (as stated in Section 5.2.5.1 of RFC 4995 [ROHC] and Section 6.1 of RFC 5225 [ROHCV2]), which indicates
that no segment headers are allowed on the ROHCoIPsec channel.
PROFILES: This field is a list of ROHC profiles supported by the local
decompressor. Possible values for this list
are contained in the ROHC Profile Identifiers registry [ROHCPROF].
In addition to these ROHC channel parameters, a ROHC Integrity Algorithm and a ROHC ICV Length field MUST be included within the SPD:
ROHC INTEGRITY ALGORITHM: This field is a list of integrity algorithms supported by the ROHCoIPsec instance. This will be used
by the ROHC process to ensure that packet headers are properly decompressed (see Section 4.2). Authentication algorithms that MUST be supported are specified in the "Authentication Algorithms" table in section 3.1.1 ("ESP Encryption and Authentication Algorithms") of RFC 4835 [CRYPTO-ALG] (or its successor). ROHC ICV LENGTH: This field specifies the length of the ICV that is used in conjunction with the ROHC Integrity Algorithm.
Several other ROHC channel parameters are omitted from the SPD,
because they are set implicitly. The omitted channel parameters are
LARGE_CIDS and FEEDBACK_FOR. The LARGE_CIDS channel parameter
MUST be set based on the value of MAX_CID (e.g. if MAX_CID is
<= 15, LARGE_CIDS is assumed to be 0). Finally, the ROHC FEEDBACK_FOR
channel parameter MUST be set to the ROHC channel associated
with the SA in the reverse direction. If an SA in the
reverse direction does not exist, the FEEDBACK_FOR channel parameter
is not set, and ROHC MUST NOT operate in bidirectional Mode.
Each entry within the SAD defines the parameters associated with each
established SA. Unless the "populate from packet" (PFP) flag is
asserted for a particular field, SAD entries are determined by the
corresponding SPD entries during the creation of the SA.
The data items contained within the SAD are defined in RFC 4301 [IPSEC],
Section 4.4.2.1. To support ROHC, the SAD must include a "ROHC Data Item"; this data item contains parameters used by ROHC instance. The ROHC Data Item exists for both inbound and outbound SAs.
The ROHC Data Item includes the ROHC channel parameters for the SA. These channel parameters
(i.e., MAX_CID, PROFILES, MRRU) are enumerated above in Section 3.1. For inbound SAs, the ROHC Data Item
MUST specify the ROHC channel parameters that are used by the local decompressor instance; conversely, for outbound SAs,
the ROHC Data Item MUST specify the ROHC channel parameters that are used by local compressor instance.
In addition to these ROHC channel parameters, the ROHC Data Item for both inbound and outbound SAs MUST include three
additional parameters. Specifically, these parameters store the integrity algorithm, the algorithm's respective key, and the ICV length that is used by the ROHC process
(see Section 3.2). The integrity algorithm and its associated key are used to calculate a ROHC ICV of the specified length; this ICV
is used to verify the packet headers post-decompression.
Finally, for inbound SAs, the ROHC Data Item MUST include a FEEDBACK_FOR parameter. The parameter is a
reference to a ROHC channel in the opposite direction (i.e., the outbound SA) between the same compression endpoints.
A ROHC channel associated with an inbound SA and a ROHC channel associated with an outbound SA MAY
be coupled to form a Bi-directional ROHC channel as defined in Section 6.1 and Section 6.2 in RFC 3759 [ROHC-TERM].
"ROHC Data Item" values
MAY be initialized manually (i.e., administratively configured for
manual SAs), or initialized via a key exchange protocol (e.g. IKEv2
[IKEV2]) that has been extended to support the signaling of ROHC
parameters [IKEV2EXT].
In order to demultiplex header-compressed from uncompressed traffic
on a ROHC-enabled SA, a "ROHC" value must be reserved in the IANA
Protocol Numbers registry. If an outbound packet has a compressed
header, the Next Header field of the security protocol header (e.g.,
AH [AH], ESP [ESP]) MUST be set to the "ROHC" protocol identifier. If
the packet header has not been compressed by ROHC, the Next Header
field does not contain the "ROHC" protocol identifier. Conversely,
for an inbound packet, the value of the security protocol Next
Header field MUST be checked to determine if the packet includes a
ROHC header, in order to determine if it requires ROHC decompression.
Use of the "ROHC" protocol identifier for purposes other than
ROHCOIPsec is currently not defined. Future protocols
that make use of the allocation (e.g., other applications of ROHC in multi-hop environments)
require specification of the logical compression channel between the ROHC
compressor and decompressor. In addition, these specifications will require
the investigation of the security considerations associated
with use of the "ROHC" protocol identifier outside the context of the next-header
field of security protocol headers.
Since ROHC is inherently a lossy compression algorithm, ROHCoIPsec MAY use an additional Integrity Algorithm (and respective
key) to compute a second Integrity Check Value (ICV) for the
uncompressed packet. This ICV MUST be computed over the uncompressed IP header, as well at the higher-layer headers and the packet payload. When computed, the ICV is appended to the ROHC-compressed packet. At the decompressor, the
decompressed packet (including the uncompressed IP header, higher-layer headers, and packet payload; but not including the authentication data) will be used with the integrity algorithm (and
its respective key) to compute a value that will be compared to the appended
ICV. If these values are not identical, the decompressed packet MUST
be dropped.
Figure 1 illustrates the composition of a ROHCoIPsec-processed IPv4 packet. In the example, TCP/IP compression is applied, and the packet is processed with tunnel mode ESP.Note: At the decompressor, the ROHC ICV field is not included in the calculation of the ROHC ICV.
In order to correctly verify the integrity of the decompressed packets, the processing steps for ROHCoIPsec MUST be implemented in a specific order, as given below.
For outbound packets that are processed by ROHC and IPsec-protected:
Compute an ICV for the uncompressed packet with the negotiated (ROHC) integrity algorithm and its respective key
Compress the packet headers (as specified by the ROHC process)
Append the ICV to the compressed packet
Apply AH or ESP processing to the packet, as specified in the appropriate SAD entry
For inbound packets that are to be decompressed by ROHC:
Apply AH or ESP processing, as specified in the appropriate SAD entry
Remove the ICV from the packet
Decompress the packet header(s)
Compute an ICV for the decompressed packet with the negotiated (ROHC) integrity algorithm and its respective key
Compare the computed ICV to the original ICV calculated at the compressor: if these two values differ, the packet MUST be dropped; otherwise resume IPsec processing
In certain scenarios, a ROHCoIPsec-processed packet may exceed the size of the IPsec tunnel MTU. RFC 4301 [IPSEC] currently stipulates the following for outbound
traffic that exceeds the SA PMTU:For ROHCoIPsec, Cases 1 and 3, and the post-encryption fragmentation for Case 2 are employed. However, since current
ROHC compression profiles do not support the compression of IP packet fragments, pre-encryption fragmentation is not compatible
with the current set of ROHC profiles. In place of pre-encryption fragmentation, ROHC segmentation MAY be used at the compressor to
divide the packet, where each segment conforms to the tunnel MTU. However, because in-order delivery of ROHC segments is
not guaranteed, the use of ROHC segmentation is not recommended.If the compressor determines that the compressed packet exceeds the tunnel MTU, ROHC segmentation MAY be applied to the compressed packet
before AH or ESP processing. This determination can be made by comparing the anticipated ROHCoIPsec packet size to the Path MTU data item
specified in the SAD entry. If the MRRU for the channel is non-zero, the compressor applies ROHC segmentation. If segmentation is applied, the process MUST account for the additional overhead imposed by IPsec
process (e.g., AH or ESP overhead, crypto synchronization data, the additional IP header, etc.) such that the final IPsec-processed segments are
less than the tunnel MTU. After segmentation, each ROHC segment is consecutively processed by the appropriate security protocol (e.g., AH, ESP) instantiated on the ROHC-enabled SA. Since ROHC segments are processed consecutively, the associated AH/ESP sequence number MUST be incremented by one for each segment transmitted over the ROHC channel. As such, after all ROHC segments receive AH/ESP processing, these segments can be identified (at the remote IPsec implementation) by a range of contiguous AH/ESP sequence numbers.For channels where the MRRU is non-zero, the ROHCoIPsec decompressor MUST re-assemble the ROHC segments that are received. To accomplish
this, the decompressor MUST identify the ROHC segments (as documented in Section 5.2.6 of RFC 4995 [ROHC]), and attempt reconstruction using the
ROHC segmentation protocol (Section 5.2.5 of RFC 4995 [ROHC]). To assist the reconstruction process, the AH/ESP sequence number SHOULD be used to identify segments that may have been subject to reordering. If
reconstruction fails, the packet MUST be discarded. As stated in Section 3.2.1, if the ROHC integrity algorithm is used to verify the decompression of packet headers, this ICV is appended to the compressed packet.
If ROHC segmentation is performed, the segmentation algorithm is executed on the compressed packet and the appended ICV. Note that the ICV is not appended to each ROHC segment.Under certain circumstances, IPsec implementations will not process
(or receive) unprotected ICMP messages, or they will not have a Path
MTU estimate value. In these cases, the IPsec implementation SHOULD
NOT attempt to segment the ROHC-compressed packet, as it does not have
full insight into the path MTU in the unprotected domain.
IPComp ([IPCOMP]) is another mechanism that can be implemented to
reduce the size of an IP datagram. If IPComp and ROHCoIPsec are
implemented in a nested fashion, the following steps MUST be followed for outbound and inbound packets.
For outbound packets that are to be processed by IPcomp and ROHC:
The ICV is computed for the uncompressed packet, and the appropriate ROHC compression profile is applied to the packet
IPComp is applied, and the packet is sent to the IPsec process
The security protocol is applied to the packet
Conversely, for inbound packets that are to be both ROHC- and IPcomp-decompressed:
A packet received on a ROHC-enabled SA is IPsec-processed
The datagram is decompressed based on the appropriate IPComp algorithm
The packet is sent to the ROHC module for header decompression and integrity verification
A ROHCoIPsec implementer should consider the strength of protection
provided by the integrity check algorithm used to verify decompressed headers. Failure to implement a
strong integrity check algorithm increases the probability for an
invalidly decompressed packet to be forwarded by a ROHCoIPsec device
into a protected domain.
The implementation of ROHCoIPsec may increase the susceptibility for traffic
flow analysis, where an attacker can
identify new traffic flows by monitoring the relative size of the
encrypted packets (i.e. a group of "long" packets, followed by a long
series of "short" packets may indicate a new flow for some ROHCoIPsec
implementations). To mitigate this concern, ROHC padding mechanisms
may be used to arbitrarily add padding to transmitted packets to
randomize packet sizes. This technique, however, reduces the overall
efficiency benefit offered by header compression.
IANA is requested to allocate one value within the "Protocol Numbers"
registry [PROTOCOL] for "ROHC". This value will be used to indicate
that the next level protocol header is a ROHC header.
The authors would like to thank Mr. Sean O'Keeffe, Mr. James Kohler,
Ms. Linda Noone of the Department of Defense, and Mr. A. Rich Espy of
OPnet for their contributions and support for developing this
document. The authors would also like to thank Mr. Yoav Nir, and Mr.
Robert A Stangarone Jr.: both served as committed document reviewers
for this specification.Finally, the authors would like to thank the following for their numerous reviews and comments to
this document:Mr. Magnus WesterlundDr. Stephen KentMr. Lars-Erik JonssonMr. Carl KnutssonMr. Pasi EronenDr. Jonah PezeshkiMr. Tero KivinenDr. Joseph TouchMr. Rohan JasaniThis appendix is included as an additional way to describe the ROHCoIPsec
parameters that are included in the IPsec SPD. It uses portions of the ASN.1
syntax provided in Appendix C of RFC 4301 [IPSEC]. In addition, several new
structures are defined.This syntax has been successfully compiled. However, it is merely illustrative
and need not be employed in an implementation to achieve compliance.The "Processing" data structure, defined in Appendix C of RFC 4301, is augmented to include
a ROHC parameters element as follows:The following data structures describe these ROHC parameters:Security Architecture for the Internet Protocol The RObust Header Compression (ROHC) FrameworkIP Payload Compression Protocol (IPComp)Key words for use in RFCs to Indicate Requirement LevelsRObust Header Compression Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-LiteInternet Key Exchange (IKEv2) ProtocolExtensions to IKEv2 to Support Robust Header Compression over IPsec (ROHCoIPsec)IP Authentication HeaderIP Encapsulating Security Payload (ESP)Integration of Header Compression over IPsec Security AssociationsRObust Header Compression (ROHC) Profile IdentifiersCryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)Robust Header Compression (ROHC): Terminology and Channel Mapping Examples"Assigned Internet Protocol Numbers", IANA registry at: http://www.iana.org/assignments/protocol-numbersIKEv2 Parameters, http://www.iana.org/assignments/ikev2-parameters