IPsec Extensions to Support Robust Header Compression over IPsec (ROHCoIPsec)
draft-ietf-rohc-ipsec-extensions-hcoipsec-03

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Abstract

Integrating ROHC with IPsec (ROHCoIPsec) offers the combined benefits of IP security services and efficient bandwidth utilization. However, extensions to the SPD and SAD are required in order to integrate ROHC with IPsec. This document describes the IPsec extensions required to support ROHCoIPsec.

Table of Contents

1.  Introduction
2.  Extensions to IPsec Databases
    2.1.  Security Policy Database (SPD)
    2.2.  Security Association Database (SAD)
3.  Extensions to IPsec Processing
    3.1.  Addition to the IANA Protocol Numbers Registry
    3.2.  Verifying the Integrity of Decompressed Packet Headers
        3.2.1.  ICV Computation and Integrity Verification
    3.3.  Nested IPComp and ROHCoIPsec Processing
4.  Security Considerations
5.  IANA Considerations
6.  Acknowledgments
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

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). To integrate ROHC and IPsec, 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 is required to ensure the integrity of the decompressed packet. Finally, the order of the inbound and outbound processing must be enumerated when nesting IP Compression (IPComp [IPCOMP]), ROHC, and IPsec processing.

2.  Extensions to IPsec Databases

The following subsections specify extensions to the SPD and the SAD to support ROHCoIPsec.

2.1.  Security Policy Database (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 must be extended to include per-channel ROHC parameters. Together, the existing IPsec SPD parameters and the ROHC parameters will dictate the services that are provided to packets protected by IPsec.

The fields contained within each SPD entry are defined in [IPSEC], Section 4.4.1.2. To support ROHC, several processing info fields must be added to the SPD; these fields contain information regarding the ROHC profiles and channel parameters supported by the local ROHC instance.

The SPD specifies what services are to be offered to IP datagrams, and in what fashion. To offer IP datagrams compression services, two per-channel configuration parameters are added to the SPD. Specifically, the following two parameters must be included if the processing info field in the SPD is set to PROTECT (suggested values for these parameters are consistent with [ROHCPPP]):

MAX_CID: The highest context ID number to be used by the compressor. MAX_CID must be at least 0 and at most 16383 (The value 0 implies having one context). The suggested value for MAX_CID is 15.

PROFILES: This indicates the ROHC profiles supported by the decompressor. The list of possible values this field may assume is defined in the [ROHCPROF] registry.

In addition to these ROHC channel parameters, a field within the SPD is required to store a list of integrity algorithms supported by the ROHCoIPsec instance:

INTEGRITY ALGORITHM: 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 3.2).

Several other ROHC channel parameters are omitted from the SPD, because they are set implicitly. The ROHC channel parameters are LARGE_CIDS, MRRU, and FEEDBACK_FOR. The LARGE_CIDS channel parameter is set implicitly, based on the value of MAX_CID (e.g. if MAX_CID is <= 15, LARGE_CIDS is assumed to be 0). Furthermore, the MRRU parameter must be set to 0; since packets may be reordered across a ROHCoIPsec channel, a compression session must not use segmentation. Finally, the ROHC FEEDBACK_FOR channel parameter is set implicitly to the ROHC channel associated with the SA in the reverse direction. If an SA in the reverse direction does not exist, ROHC must operate in the Unidirectional Mode.

2.2.  Security Association Database (SAD)

Each entry within the SAD defines the parameters associated with each established SA. Unless if 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 [IPSEC], Section 4.4.2.1. To support ROHC, this list of data items is augmented to include a "ROHC Data Item" field that defines the ROHC parameters. These parameters (i.e., MAX_CID, PROFILES) are enumerated above in Section 2.1. In addition, the FEEDBACK_FOR parameter is also included, which is associated with the SA in the reverse direction (this data item does not need to be included in the SPD, since its value is implicitly derived). Finally, two additional parameters are required to store the Integrity Algorithm and respective key that is to be used to ensure that packets are properly decompressed (see Section 3.2).

These "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 negotiation of ROHC parameters [IKEV2EXT].

3.  Extensions to IPsec Processing

3.1.  Addition to the IANA Protocol Numbers Registry

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, the Next Header field remains unaltered. Conversely, for an inbound packet, the value of the security protocol Next Header field is checked to determine if the packet includes a ROHC header.

3.2.  Verifying the Integrity of Decompressed Packet Headers

Since ROHC is inherently a lossy 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 is computed over the uncompressed IP header, as well at the higher-layer headers and the packet payload, and 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 ICV. If these values are not identical, the decompressed packet must be dropped by the decompressor.

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.

             BEFORE COMPRESSION AND APPLICATION OF ESP
             ----------------------------
       IPv4  |orig IP hdr  |     |      |
             |(any options)| TCP | Data |
             ----------------------------

	      AFTER ROHCOIPSEC COMPRESSION AND APPLICATION OF ESP
            ------------------------------------------------------
      IPv4  | new IP hdr  |     | Cmpr. |    | ROHC | ESP   | ESP|
            |(any options)| ESP | Hdr.  |Data| ICV  |Trailer| ICV|
            ------------------------------------------------------

Figure 1. Example of a ROHCoIPsec-processed packet.

Note: The authentication data should never be included in the calculation of the ICV.

3.2.1.  ICV Computation and Integrity Verification

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 to be processed by ROHC:

• An ICV is computed for the uncompressed packet via ROHCoIPsec's Integrity Algorithm (and respective key)
• The packet header(s) is(are) compressed by the ROHC process
• The ICV is appended to the end of the compressed packet
• The security protocol is applied to the packet

For inbound packets that are to be decompressed by ROHC:

• A packet received on a ROHC-enabled SA is IPsec-processed
• The packet header(s) is(are) decompressed by the ROHC process
• The decompressed packet is used with the Integrity Algorithm (and its respective key) to compute a value that is compared to the ICV (if these two values differ, the packet is dropped)

3.3.  Nested IPComp and ROHCoIPsec Processing

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 order of the outbound and inbound processing steps must be carefully enumerated.

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

4.  Security Considerations

A ROHCoIPsec implementer should consider the strength of protection provided by the integrity check algorithm used to verify the valid decompression of ROHC-compressed packets. Failure to implement a strong integrity check algorithm increases the probability of an invalidly decompressed packet to be forwarded by a ROHCoIPsec device into a protected domain. In general, if an integrity check algorithm is used with IPsec, it is recommended that the integrity check algorithm used by ROHC is at least the same strength.

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.

5.  IANA Considerations

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.

6.  Acknowledgments

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. In addition, the authors would like to thank Mr. Rohan Jasani for his valuable assistance. Finally, the authors would like to thank the following for their numerous reviews and comments to this document:

• Dr. Stephen Kent
• Mr. Lars-Erik Jonnson
• Mr. Pasi Eronen
• Mr. Tero Kivinen
• Dr. Joseph Touch

7.  References

7.1. Normative References

7.2. Informative References

Authors' Addresses

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