| Network Working Group | B. Williams |
| Internet-Draft | Akamai, Inc. |
| Intended status: Experimental | M. Boucadair |
| Expires: November 01, 2014 | France Telecom |
| D. Wing | |
| Cisco Systems, Inc. | |
| April 30, 2014 |
Experimental Option for TCP Host Identification
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.
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This Internet-Draft will expire on November 01, 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 host's identifier (HOST_ID) information to the receiver, which indicates that a solution using a TCP [RFC0793] option for this purpose could be applied with limited performance impact and a high success ratio.
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]. Specification of multiple option formats to serve the purpose of host identification increases the burden for potential implementers and presents interoperability challenges as well. This document defines a common TCP option format to meet the needs of all three of the above proposals.
The option defined in this document uses the TCP experimental option codepoint sharing mechanism defined in [RFC6994] and is intended to allow broad deployment of the mechanism on the public Internet in order to validate the utility of this option format for the required use cases.
Section 5 of this document discusses compatibility between this new TCP option and existing commonly deployed TCP options.
This memo focuses primarily on the carrier grade NAT (CGN), application proxy, and overlay network use cases described in [I-D.boucadair-intarea-host-identifier-scenarios]. This means that the option could either be applied to an individual TCP packet at the connection endpoint (e.g. an application proxy or a transport layer overlay network) or at an address-sharing middle box (e.g. a CGN or a network layer overlay network). See Section 4 below for additional details about the types of devices that may add the option to a TCP packet, as well as limitations on use of the option when it is to be inserted by an address-sharing middlebox, including issues related to packet fragmentation.
The receiver-side use cases considered by this memo include the following:
In all of the above cases, differentiation between address-sharing clients commonly needs to be performed by a network function that does not process the application layer protocol (e.g. HTTP) or the sercurity protocol (e.g. TLS), because the action needs to be performed prior to decryption or parsing the application layer. Due to this, a solution implemented within the application layer or security protocol cannot fully meet the receiver-side requirements. At the same time, as noted in [RFC6967], use of an IP option for this purpose has a low success rate. For these reasons, using a TCP option to deliver the host identifier has been selected as the most effective way to satisfy these specific use cases.
The extensive testing effort documented in [I-D.abdo-hostid-tcpopt-implementation] confirmed that a TCP option could be used for host identification purposes without significant disruption of TCP connectivity to legacy servers that do no support the option. It also showed how mechanisms available in existing TCP implementations could make use of such a TCP option for improved diagnostics and/or packet filtering.
Specification of the TCP option described in this memo will allow further experiments to be conducted in order to assess the viability of the option for the receiver-side use cases discussed above:
In particular, real-world deployment of the option is expected to provide opportunities for engagement with a broader range of both application and middleware implementations in order to develop a more complete picture of how well the option meets the use-case requirements.
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 included 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. |
[Note to RFC Editor: Please remove this section prior to publication.]
Clarification of arguments in favor of this approach.
Add discussion of important use cases.
Clarification of experiment goals and earlier test results.
Add note re: order of appearance.
Add discussion of experiment goals.
Limit external references to the earlier drafts.
Add guidance to limit the types of device that add the option.
Improve/correct discussion of TCP-AO and security.