Subtype Capability Exchange During MPTCP Handshake Huawei
jiao_kang2022@163.com
Huawei
No. 207, Jiufeng 3rd Road, East Lake High-tech Development Zone Wuhan China liangqiandeng@huawei.com
Huawei
D2-03,Huawei Industrial Base Shenzhen China dengshangling@huawei.com
Transport Area TCP Maintenance and Minor Extensions mptcp Multipath TCP provides the ability to simultaneously use multiple paths between peers. MPTCP protocol defines seven subtypes in MPTCP v0 and ten subtypes in MPTCP v1 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.
Introduction
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.
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. Overview MPTCP Subtypes
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.
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.
MPTCP Subtype Capability Exchange | | Determine and | Cache the capabilities | of Host A |<---------------------------------------------| | SYN/ACK + Host B Subtype Capability | Determine and Cache the | capabilities of Host B | | ACK | |--------------------------------------------->| | | | | | SYN + MP_JOIN | | |------------------------------->| | |<-------------------------------| | | SYN/ACK + MP_JOIN | | | | | | ACK + MP_JOIN | | |------------------------------->| | |<-------------------------------| | | ACK | | | | | Data Transmission(with subtype messages) | |<-------------------------------------------->| | | | | | Data Transmission(with subtype | | |<------------------------------>| | | messages) | | | | ]]>
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.
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.
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.
OptionSupported Format 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+---------------+-------+-------+---------------+ | Kind | Length |Subtype|Version|A|B|C|D|E|F|G|H| +---------------+---------------+-------+-------+---------------+ | Option Sender's Key (64 bits) | | (if option Length > 4) | | | +---------------------------------------------------------------+ | Option Receiver's Key (64 bits) | | (if option Length > 12) | | | +-------------------------------+-------------------------------+ | OptionSupported (32 bits) | +-------------------------------+-------------------------------+ | Data-Level Length (16 bits) | Checksum (16 bits, optional) | +-------------------------------+-------------------------------+
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.
Security Considerations To be added.
IANA Considerations To be added.
References Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Transmission Control Protocol TCP Extensions for Multipath Operation with Multiple Addresses TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failure. Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document defines an Experimental Protocol for the Internet community. TCP Extensions for Multipath Operation with Multiple Addresses TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and thus improve user experience through higher throughput and improved resilience to network failure. Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., a reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document specifies v1 of Multipath TCP, obsoleting v0 as specified in RFC 6824, through clarifications and modifications primarily driven by deployment experience. Informative References Writing I-Ds and RFCs using XML This memo presents a technique for using XML (Extensible Markup Language) as a source format for documents in the Internet-Drafts (I-Ds) and Request for Comments (RFC) series. This memo provides information for the Internet community.