Internet-Draft | Subtype-Cap Exchange | February 2021 |
Kang & Liang | Expires 10 August 2021 | [Page] |
- Workgroup:
- TCP Maintenance and Minor Extensions
- Internet-Draft:
- draft-kang-tcpm-subtype-capability-exchange-00
- Published:
- Intended Status:
- Informational
- Expires:
Subtype Capability Exchange During MPTCP Handshake
Abstract
Multipath TCP provides the ability to simultaneously use multiple paths between peers. MPTCP protocol defines seven subtypes in MPTCP v0 [RFC6824] and ten subtypes in MPTCP v1 [RFC8684] to differentiate message types and implement some additional functions during a session.¶
This draft proposes an enhancement to support Subtype Capability Exchange during MPTCP connection establishment in order to improve elastic scalability of MPTCP protocol. It includes: 1) requirements for which this kind of capability exchange during handshake is important for a MPTCP session; 2) a typical flow for Subtype Capability Exchange between peers; 3) a feasible solution on protocol design is suggested.¶
Status of This Memo
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 https://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 10 August 2021.¶
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
1. Introduction
1.1. Requirements Language
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 [RFC2119].¶
1.2. Background
Table 1 lists all subtypes that have been specified in current MPTCP versions. Besides version negotiation, MPTCP peers can not interact with each other on the granularity of subtype capability. This feature may cause inflexible protocol extension. For example, if a new message type A is added in future extension, a higher version should be released to import it and a new subtype may need to be allocated. Another case is that if a sender does not know the subtypes supported by a receiver in a MPTCP session, as a result, invalid data packets may been sent from the sender during data transmission and the receiver will discard it which causes system overhead on receiver side.¶
Value | Symbol | Name | MPTCPv0 | MPTCPv1 |
---|---|---|---|---|
0x0 | MP_CAPABLE | Multipath Capable | Supported | Supported |
0x1 | MP_JOIN | Join Connection | Supported | Supported |
0x2 | DSS | Data Sequence Signal (Data ACK and Data Sequence Mapping) | Supported | Supported |
0x3 | ADD_ADDR | Add Address | Supported | Supported |
0x4 | REMOVE_ADDR | Remove Address | Supported | Supported |
0x5 | MP_PRIO | Change Subflow Priority | Supported | Supported |
0x6 | MP_FAIL | Fallback | Supported | Supported |
0x7 | MP_FASTCLOSE | Fast Close | Supported | Supported |
0x8 | MP_TCPRST | Subflow Reset | / | Supported |
0xf | MP_EXPERIMENTAL | Reserved for Private Use | / | Supported |
This document suggests a new function of Subtype Capability Exchange during MPTCP handshake in the scenario that MPTCP peers in a session support same MPTCP protocol version but with different subtype sets.¶
2. One Typical Flow
Figure 1 illustrates a typical flow for this Subtype Capability Exchange during MPTCP connection setup. The field of Subtype Capability is used to indicate whether these subtypes are supported by the sender, for example, Host A Subtype Capabilities indicates the status of the subtypes on Host A and Host B Subtype Capabilities indicates that on Host B. Through the transmission of this information between both parties, a sender can determine whether a message can be properly processed by its receiver and only send the message that can be supported by the receiver during data transmission.¶
In practice, another possible implementation is as follows: after receiving the subtype capability information sent by Host A, Host B determines the common subtype sets supported by both parties, and returns this common subtype sets in the reponse. Host A caches this common subtype sets locally. In data transmission phase, Host A sends the specified subtype messages to Host B that are included in the common subtype sets. As an alternative solution, its protocol design on MPTCP will be considered and updated in later versions.¶
3. Protocol Implementation
This document describes one solution on the modifications to MPTCP protocol to support this mechanism.In this solution, MP_CAPABLE option is used and extended to add bits to carry subtype capabilities information. There should be other possible solutions that can be defined in subsequent discussions.¶
3.1. Carrying Subtype Capabilities in MP_CAPABLE Option
In Figure 2, a 32-bit "OptionSupported" is added to MP_CAPABLE option to indicate whether the subtypes are supported by the sender.¶
For MPTCP v1, ten subtypes has been defined and applied in practice. So the first 10-bits in OptionSupported field is used for indicating whether these subtypes is supported by sender. The order is listed below:¶
0: MP_CAPABLE¶
1: MP_JOIN¶
2: DSS¶
3: ADD_ADDR¶
4: REMOVE_ADDR¶
5: MP_PRIO¶
6: MP_FAIL¶
7: MP_FASTCLOSE¶
9: MP_TCPRST¶
10: MP_EXPERIMENTAL¶
11~31: Reserved for Future Use¶
Two values, that is 0 and 1, can be set to these bits in OptionSupported field. The value of 0 indicates that the sender does not support this subtype. The value of 1 indicates that the sender supports this subtype.¶
4. Security Considerations
To be added.¶
5. IANA Considerations
To be added.¶
6. References
6.1. Normative References
- [RFC0793]
- Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, , <https://www.rfc-editor.org/info/rfc793>.
- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- [RFC6824]
- Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, , <https://www.rfc-editor.org/info/rfc6824>.
- [RFC8684]
- Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C. Paasch, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, , <https://www.rfc-editor.org/info/rfc8684>.
6.2. Informative References
- [RFC2629]
- Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, DOI 10.17487/RFC2629, , <https://www.rfc-editor.org/info/rfc2629>.