| Network Working Group | B. Williams |
| Internet-Draft | Akamai, Inc. |
| Intended status: Experimental | M. Boucadair |
| Expires: August 16, 2014 | France Telecom |
| D. Wing | |
| Cisco Systems, Inc. | |
| February 12, 2014 |
Experimental Option for TCP Host Identification
draft-williams-exp-tcp-host-id-opt-02
Recent IETF proposals have identified benefits to more distinctly identifying the hosts that are hidden behind a shared address/prefix sharing device or application-layer proxy. Analysis indicates that the use of a TCP option for this purpose can be successfully applied to a broad range of use cases. This document describes a common experimental TCP option format for host identification.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on August 16, 2014.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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A broad range of issues associated with address sharing have been well documented in [RFC6269] and [I-D.boucadair-intarea-host-identifier-scenarios]. In addition, [RFC6967] provides analysis of various solutions to the problem of revealing the sending hosts's identifier (HOST_ID) information to the receiver, which indicates that a solution using a TCP [RFC0793] option for this purpose can be successfully applied to a broad range of use cases with limited performance impact.
Multiple recent Internet Drafts define TCP options for the purpose of host identification: [I-D.wing-nat-reveal-option], [I-D.abdo-hostid-tcpopt-implementation], and [I-D.williams-overlaypath-ip-tcp-rfc]. This document defines a common TCP option format to meet the needs of all three of the above proposals. This document is self-contained; required information to implement the proposed TCP option is specified in this memo.
The option defined in this document uses the TCP experimental option codepoint sharing mechanism defined in [RFC6994] and is intended to allow validation of this common option format in order to conduct more experimental work that will complement the experiment results already documented in [I-D.abdo-hostid-tcpopt-implementation].
In addition to the extensive testing effort documented in [I-D.abdo-hostid-tcpopt-implementation], further experiments will be conducted to assess the validity of the following points:
Section 5 of this document discusses compatibility between this new TCP option and existing commonly deployed TCP options.
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 [RFC2119].
When used for host identification, the TCP experimental option has the following format and content.
0 1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
| Kind | Length | ExID |
+--------+--------+--------+--------+
| Host ID ...
+--------+---
When multiple host identifiers are required (e.g. a list of IP addresses, an IP address and a port number), the HOST_ID option is included multiple times within the packet, once for each identifier. While this approach significantly increases option space utilization when multiple identifiers are required, cases where only a single identifier is required are more common and thus it is beneficial to optimize for those cases.
The HOST_ID option should only be added by the origin host or any device involved in the forwarding path that changes IP addresses and/or TCP port numbers (e.g., NAT44 [RFC3022], Layer-2 Aware NAT, DS-Lite AFTR [RFC6333], NPTv6 [RFC6296], NAT64 [RFC6146], Dual-Stack Extra Lite [RFC6619], TCP Proxy, etc.). The HOST_ID option MUST NOT be added or modified en-route by any device that does not modify IP addresses and/or TCP port numbers.
The information conveyed in the HOST_ID option SHOULD correlate to IP addresses and/or TCP port numbers change(s) (i.e., some of the IP address and /or port number bits are used to generate the HOST_ID).
Intermediary devices (e.g. address sharing device) SHOULD be configurable to enable including the HOST_ID TCP option. These devices MUST be configured with the type of information to populate the HOST_ID TCP option (e.g. certain bits of the source IPv6 address, the full source IPv6 address, certain bits of the source IPv4 address, the full source IPv4 address, the source port number, etc.).
The device may be configured to include multiple identifiers (e.g. both a source IP address and a source port number). In such case, the device MUST insert two instances of the HOST_ID option, each of which contains the appropriate information. Note, there is no need to signal the semantic of the included data as this specification assumes the service is aware of that information by out of band means (e.g. both the service and the address sharing device are managed by the same administrative entity).
When an intermediary device is configured to include the HOST_ID option, it MUST include the HOST_ID option in SYN messages. In addition, an intermediary device and a receiving end device MAY be configurable to allow inclusion of the HOST_ID option in additional messages in order to support the use of SYN cookies. For example:
The option SHOULD NOT be included in packets if the resulting packet would require local fragmentation. The option MUST NOT be include in packets when there is not enough space for at least one valid identifier of the configured type.
The device MUST be configured with the behavior to follow when a HOST_ID TCP option is already present in the message:
This section details how the HOST_ID option functions in conjunction with other TCP options.
TCP provides for a maximum of 40 octets for TCP options. As discussed in Appendix A of Multipath TCP (MPTCP) [RFC6824], a typical SYN from modern, popular operating systems contain several TCP options (MSS, window scale, SACK permitted, and timestamp) which consume 19-24 octets depending on word alignment of the options. The initial SYN from a multipath TCP client would consume an additional 16 octets.
HOST_ID needs at least 6 octets to be useful, so 9-21 octets are sufficient for many scenarios that benefit from HOST_ID. However, 4 octets are not enough space for the HOST_ID option. Thus, a TCP SYN containing all the typical TCP options (MSS, window Scale, SACK permitted, timestamp), and also containing multipath capable or multipath join), and also being word aligned, has insufficient space to also accommodate HOST_ID. This means something has to give. The choices are to avoid word alignment in that case (freeing 5 octets), remove a TCP option from the original TCP SYN, or avoid adding the HOST_ID option. We expect to learn from deployment experience during the experiment which of these options, or a combination of these options, is best.
The TCP-AO option [RFC5925] supports a "TCP option flag" to indicate whether TCP options other than TCP-AO are included in the MAC calculation (Section 3.1 of [RFC5925]). When the options are not included in the MAC calculation, the use of HOST_ID option does not interfere with TCP-AO option. However, because TCP-AO provides integrity protection of the source IP address, TCP-AO is broken in the presence of NAT.
Because TCP-AO is incompatible with address sharing, an experimental extension to TCP-AO (called TCP-AO-NAT) is introduced in [RFC6978]. Injecting a HOST_ID TCP option does not interfere with the use of TCP-AO-NAT if the TCP options are not included in the MAC calculation.
As specified in [RFC5925], TCP-AO must be the first TCP option processed on incoming segments.
Security (including privacy) considerations common to all HOST_ID solutions are discussed in [RFC6967]. These considerations should be taken into account.
The content of the HOST_ID option SHOULD NOT be used for purposes that require a trust relationship between the host and the server (e.g. billing and/or intrusion prevention) unless a mechanism outside the scope of this specification is used to ensure the necessary level of trust. When the receiving network uses the values provided by the option in a way that does not require trust (e.g. maintaining session affinity in a load-balancing system), then use of a mechanism to enforce the trust relationship might not be required.
Sending a TCP SYN across the public Internet necessarily discloses the public IP address of the sending host. When an intermediate address sharing device is deployed on the public Internet (see [I-D.boucadair-intarea-host-identifier-scenarios] for examples), anonymity of the hosts using the device will be increased, with hosts represented by multiple source IP addresses on the ingress side of the device using a single source IP address on the egress side. The HOST_ID TCP option removes that increased anonymity, taking information that was already visible in TCP packets on the public Internet on the ingress side of the address sharing device and making it available on the egress side of the device as well. In some cases, an explicit purpose of the address sharing device is anonymity, in which case use of the HOST_ID TCP option would be incompatible with the purpose of the device.
Use of the HOST_ID TCP option described here should follow the recommendations laid out in [RFC6967]. In particular:
This document specifies a new TCP option that uses the shared experimental options format [RFC6994], with ExID=0x0348 (840) in network-standard byte order. This ExID has already been registered with IANA.
Many thanks to J. Touch, M. Scharf, W. Eddy, T. Reddy, and Y. Nishida for their comments.
| [RFC0793] | Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |