| Transport Area Working Group | M. Amend |
| Internet-Draft | E. Bogenfeld |
| Intended status: Experimental | Deutsche Telekom |
| Expires: May 7, 2020 | A. Brunstrom |
| A. Kassler | |
| Karlstad University | |
| V. Rakocevic | |
| City University of London | |
| November 04, 2019 |
DCCP Extensions for Multipath Operation with Multiple Addresses
draft-amend-tsvwg-multipath-dccp-03
DCCP 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 DCCP session could improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failure.
Multipath DCCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional DCCP to support multipath operation. The protocol offers the same type of service to applications as DCCP and it provides the components necessary to establish and use multiple DCCP flows across potentially disjoint paths.
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Multipath DCCP (MP-DCCP) is a set of extensions to regular DCCP [RFC4340], which enables a transport connection to operate across multiple paths simultaneously. DCCP multipath operations is suggested in the context of ongoing 3GPP work on 5G multi-access solutions [I-D.amend-tsvwg-multipath-framework-mpdccp] and for hybrid access networks [I-D.lhwxz-hybrid-access-network-architecture][I-D.muley-network-based-bonding-hybrid-access]. It can be applied for load-balancing, seamless session handover and aggregation purposes (referred to as steering, switching and splitting in 3GPP terminology [TR23.793]).
This document presents the protocol changes required to add multipath capability to DCCP; specifically, those for signaling and setting up multiple paths ("subflows"), managing these subflows, reassembly of data, and termination of sessions.
MP-DCCP operates at the transport layer and aims to be transparent to both higher and lower layers. It is a set of additional features on top of standard DCCP; Figure 1 illustrates this layering. MP-DCCP is designed to be used by applications in the same way as DCCP with no changes.
+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MP-DCCP |
+---------------+ + - - - - - - - + - - - - - - - +
| DCCP | |Subflow (DCCP) |Subflow (DCCP) |
+---------------+ +-------------------------------+
| IP | | IP | IP |
+---------------+ +-------------------------------+
Figure 1: Comparison of Standard DCCP and MP-DCCP Protocol Stacks
[Tbd], could be similar to [RFC6824]
Host A Host B
------------------------ ------------------------
Address A1 Address A2 Address B1 Address B2
---------- ---------- ---------- ----------
| | | |
| (DCCP flow setup) | |
|----------------------------------->| |
|<-----------------------------------| |
| | | |
| | (DCCP flow setup) | |
| |--------------------->| |
| |<---------------------| |
| merge individual DCCP flows to one multipath connection
| | | |
Figure 2: Example MP-DCCP Usage Scenario
Multipath DCCP is similar to Multipath TCP [RFC6824], in that it extends the related basic DCCP transport protocol [RFC4340] with multipath capabilities in the same way as Multipath TCP extends TCP [RFC0793]. However, mainly dominated by the basic protocols TCP and DCPP, the transport characteristics are different.
Table 1 compares the protocol characteristics of TCP and DCCP, which are by nature inherited by their respective multipath extensions. A major difference lies in the delivery of payload, which is for TCP an exact copy of the generated byte-stream. DCCP behaves contrary and does not guarantee to transmit any payload nor the order of delivery. Since this is mainly affecting the receiving endpoint of a TCP or DCCP communication, many similarities on sender side can be stated. Both transport protocols share the 3-way initiation of a communication and both exploit a congestion control to adapt to path characteristics.
| Feature | TCP | DCCP |
|---|---|---|
| Full-Duplex | yes | yes |
| Connection- Oriented | yes | yes |
| Header option space | 40 bytes | < 1008 bytes or PMTU |
| Data transfer | reliable | unreliable |
| Packet-loss handling | re- transmission | report only |
| Ordered data delivery | yes | no |
| Sequence numbers | one per byte | one per PDU |
| Flow control | yes | no |
| Congestion control | yes | yes |
| ECN support | yes | yes |
| Selective ACK | yes | depends on congestion control |
| Fix message boundaries | no | yes |
| Path MTU discovery | yes | yes |
| Fragmentation | yes | no |
| SYN flood protection | yes | no |
| Half-open connections | yes | no |
Consequently, the multipath features, shown in Table 2, are the same, supporting volatile paths, session handover and path aggregation capabilities. All of them profit by the existence of congestion control.
| Feature | MP-TCP | MP-DCCP |
|---|---|---|
| Volatile paths | yes | yes |
| Robust session establishment | no | yes |
| Data reassembly | yes | optional / modular |
| Expandability | limited by TCP header | flexible |
| Session handover | yes | yes |
| Path aggregation | yes | yes |
Therefore the sender logic is not much different between MP-DCCP and MPTCP, even if the multipath session initiation differs. MP-DCCP inherits a robust session establishment feature, which guarantees communication establishment if at least one functional path is available. MP-TCP relies on an initial path, which has to work; otherwise no communication can be established.
The receiver side for MP-DCCP has to deal with the unreliable transport character of DCCP and a possible re-assembly of the data stream. In practice, it is assumed that some sort of re-assembly has to be applied, even if DCCP and the order of delivery is unreliable by nature. Such re-assembly mechanisms have to account for the fact that packet loss may occur for any of the DCCP subflows. Another issue is the packet reordering introduced when a DCCP communication is split across paths with disjoint latencies. In theory, applications using DCCP certainly have to deal with packet reordering, since DCCP has no mechanisms to prevent it. However, in practice, without any multipath extension, packet reordering can be assumed to be very limited. Therefore most services on top of DCCP are not expecting massive packet reordering and degrades their performance if it happens anyway.
The receiving process for MP-TCP is on the other hand a simple "just wait" approach, since TCP guarantees reliable delivery.
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].
[Tbd], could be similar to [RFC6824]
The Multipath Capability for MP-DCCP can be negotiated with a new DCCP feature, as described in Section 3. Once negotiated, all subsequent MP-DCCP operations are signalled with a variable length multipath-related option, as described in Section 3.1.
The DCCP protocol feature list ([RFC4340] section 6.4) will be enhanced by a new Multipath related feature with Feature number 10, as shown in Figure 3.
Rec'n Initial
Number Meaning Rule Value Req'd
------ ------- ----- ----- -----
0 Reserved
1 Congestion Control ID (CCID) SP 2 Y
2 Allow Short Seqnos SP 0 Y
3 Sequence Window NN 100 Y
4 ECN Incapable SP 0 N
5 Ack Ratio NN 2 N
6 Send Ack Vector SP 0 N
7 Send NDP Count SP 0 N
8 Minimum Checksum Coverage SP 0 N
9 Check Data Checksum SP 0 N
10 Multipath Capable SP 0 N
11-127 Reserved
128-255 CCID-specific features
Figure 3: Proposed Feature Set
The DCCP protocol options ([RFC4340] section 5.8) will be enhanced by a new Multipath related variable-length option with option type 45, as shown in Figure 4.
Option DCCP-
Type Length Meaning Data?
---- ------ ------- -----
0 1 Padding Y
1 1 Mandatory N
2 1 Slow Receiver Y
3-31 1 Reserved
32 variable Change L N
33 variable Confirm L N
34 variable Change R N
35 variable Confirm R N
36 variable Init Cookie N
37 3-8 NDP Count Y
38 variable Ack Vector [Nonce 0] N
39 variable Ack Vector [Nonce 1] N
40 variable Data Dropped N
41 6 Timestamp Y
42 6/8/10 Timestamp Echo Y
43 4/6 Elapsed Time N
44 6 Data Checksum Y
45 variable Multipath Y
46-127 variable Reserved
128-255 variable CCID-specific options -
Figure 4: Proposed Option Set
[Tbd] On top it requires particular considerations for:
DCCP endpoints are multipath-disabled by default and multipath capability can be negotiated with the Multipath Capable Feature.
Multipath Capable has feature number 10 and is server-priority. It takes one-byte values. The first four bits are used to specify compatible versions of the MP-DCCP implementation. The following four bits are reserved for further use.
+--------+--------+--------+--------+-------- |00101101| Length | MP_OPT | Value(s) ... +--------+--------+--------+--------+-------- Type=45
Option
Type Length MP_OPT Meaning
---- ------ ------- -----
45 7 0 =MP_CONFIRM Confirm reception and processing of
an MP_OPT option
45 7 1 =MP_JOIN Join path to an existing MP-DCCP flow
45 3 2 =MP_FAST_CLOSE Close MP-DCCP flow
45 var 3 =MP_KEY Exchange key material for MP_HMAC
45 7 4 =MP_SEQ Multipath Sequence Number
45 23 5 =MP_HMAC HMA Code for authentication
45 12 6 =MP_RTT Transmit RTT values
45 TBD 7 =MP_ADDADDR TBD
45 TBD 8 =MP_REMOVEADDR TBD
45 TBD 9 =MP_PRIO TBD
Figure 5: MP_OPT Option Types
+--------+--------+--------+--------+--------+--------+--------+ |00101101| Length |00000000| List of options ... +--------+--------+--------+--------+--------+--------+--------+ Type=45 MP_OPT=0
MP_CONFIRM can be used to send confirmation of received and processed options. Confirmed options are copied verbatim and appended as List of options.
+--------+--------+--------+--------+--------+--------+--------+ |00101101|00001011|00000001| Path Token | +--------+--------+--------+--------+--------+--------+--------+ | Nonce | +--------+--------+--------+--------+ Type=45 Length=11 MP_OPT=1
The MP_JOIN option is used to add a new path to an existing MP-DCCP flow. The Path Token is the SHA-1 HASH of the derived key (d-key), which was previously exchanged with the MP_KEY option. MP_HMAC MUST be set when using MP_JOIN to provide authentication (See MP_HMAC for details). Also MP_KEY must be set to provide key material for authentication purposes.
+--------+--------+--------+ |00101101|00000011|00000010| +--------+--------+--------+ Type=45 Length=3 MP_OPT=2
MP_FAST_CLOSE terminates the MP-DCCP flow and all corresponding subflows.
+--------+--------+--------+--------+--------+--------+--------+ |00101101| Length |00000011|Key Type| Key Data +--------+--------+--------+--------+--------+--------+--------+ Type=45 MP_OPT=3
Option
Key Type Key Length Meaning
---- ------ -----
0 =Plain Text 8 Plain Text Key
1 =ECDHE-C25519-SHA256 32 ECDHE with SHA256 and Curve25519
2 =ECDHE-C25519-SHA512 32 ECDHE with SHA512 and Curve25519
3-255 Reserved
Figure 6: MP_KEY Key Types
The MP_KEY suboption is used to exchange key material between hosts. The Key Type field is used to specify the key type. Key types are shown in table Figure 6.
+--------+--------+--------+--------+--------+--------+--------+ |00101101|00000111|00000100| Multipath Sequence Number | +--------+--------+--------+--------+--------+--------+--------+ Type=45 Length=7 MP_OPT=4
The MP_SEQ option is used for end-to-end datagram-based sequence numbers of an MP-DCCP connection. The initial data sequence number (IDSN) SHOULD be set randomly.
+--------+--------+--------+--------+--------+--------+ |00101101|00000111|00000101| HMAC-SHA1 (20 bytes) ... +--------+--------+--------+--------+--------+--------+ Type=45 Length=23 MP_OPT=5
The MP_HMAC option is used to provide authentication for the MP_JOIN option. The HMAC is built using the derived key (d-key) calculated previously from the handshake key material exchanged with the MP_KEY option. The Message for the HMAC is the header of the MP_JOIN for which authentication shall be performed. By including a nonce in these datagrams, possible replay-attacks are remedied. t
+--------+--------+--------+--------+--------+--------+--------+ |00101101|00000111|00000110|RTT Type| RTT +--------+--------+--------+--------+--------+--------+--------+ | | Age | +--------+--------+--------+--------+--------+ Type=45 Length=12 MP_OPT=6
The MP_RTT option is used to transmit RTT values in milliseconds. Additionally, the age of the measurement is specified in milliseconds.
[TBD]
[TBD]
[TBD]
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| DCCP-Request + MP_CAPABLE |
|------- MP_KEY(Key-A) ------------------------------>|
|<---------------------- MP_KEY(Key-B) ---------------|
| DCCP-Response + MP_CAPABLE agreed |
| | |
| DCCP-Ack | |
|--------- MP_KEY(Key-A) + MP_KEY(Key-B) ------------>|
| | |
| |DCCP-Request + MP_CAPABLE |
| |--- MP_JOIN(TB,RA) ------------------->|
| |<------MP_JOIN(TB,RB) + MP_HMAC(A)-----|
| |DCCP-Response |
| | |
| |DCCP-Ack |
| |-------- MP_HMAC(B) ------------------>|
| |<--------------------------------------|
| |DCCP-ACK |
Figure 7: Example MP-DCCP Handshake
The basic initial handshake for the first flow is as follows:
The handshake for subsequent flows based on a successful initial handshake is as follows:
[Tbd]
[Tbd], should mention standardized technologies like [RFC5597] or [RFC6773] and U-DCCP [I-D.amend-tsvwg-dccp-udp-header-conversion]
This document is inspired by Multipath TCP [RFC6824] and some text passages for the -00 version of the draft are copied almost unmodified.
[Tbd], must include options for:
should include options carrying: