Internet Engineering Task Force M. Proshin Internet-Draft Ericsson Updates: 4960 (if approved) December 02, 2019 Intended status: Standards Track Expires: June 4, 2020 Retransmit bit for SCTP DATA, I-DATA and SACK draft-proshin-tsvwg-sctp-rtx-bit-02 Abstract This document defines a method which helps an SCTP sender to understand when a received SACK acknowledges the original transmission of a TSN or its retransmission. It is done by specifying a new bit, called Retransmit bit (R-bit), in the header of DATA, I-DATA and SACK chunks. The bit is used when a TSN is retransmitted and returned back in the acknowledgement. This document updates [RFC4960] if approved. 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 June 4, 2020. Copyright Notice Copyright (c) 2019 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 Proshin Expires June 4, 2020 [Page 1] Internet-Draft December 2019 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Updates in SCTP Chunks Header . . . . . . . . . . . . . . . . 3 3.1. R-bit in DATA Chunk Header . . . . . . . . . . . . . . . 3 3.2. R-bit in I-DATA Chunk Header . . . . . . . . . . . . . . 4 3.3. R-bit in SACK Chunk Header . . . . . . . . . . . . . . . 4 4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Sender Side Considerations . . . . . . . . . . . . . . . 6 4.3. Receiver Side Considerations . . . . . . . . . . . . . . 7 4.4. Processing of SACK with and without R-bit . . . . . . . . 7 5. R-bit vs Duplicate TSN for Detection of Spurious Retransmission . . . . . . . . . . . . . . . . . . . . . . . 8 6. Interoperability Considerations . . . . . . . . . . . . . . . 9 7. Socket API Considerations . . . . . . . . . . . . . . . . . . 9 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 10. Security Considerations . . . . . . . . . . . . . . . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 11.1. Normative References . . . . . . . . . . . . . . . . . . 11 11.2. Informative References . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction SCTP which is defined in [RFC4960] is a reliable message-oriented protocol. The SCTP sender splits user messages to DATA chunks and sends them to the receiver. The SCTP receiver uses the SACK chunk to acknowledge incoming data. The reliability in SCTP is achieved by the retransmission of DATA chunks which were not acknowledged. If a DATA chunk has been retransmitted at least once, at SACK reception SCTP cannot understand if the SACK was sent in response to the originally sent DATA or retransmitted one. Thus, due to that ambiguity, [RFC4960] prohibits making RTT measurements. Some other SCTP mechanisms such as loss recovery and congestion control are not accurate in that case either. This document describes a simple extension of the DATA and SACK chunks by a new bit, so called Retransmit bit (R-bit). The sender sets the R-bit in the DATA chunk header when it retransmits a DATA and the receiver sets it in the SACK chunk header when a DATA with Proshin Expires June 4, 2020 [Page 2] Internet-Draft December 2019 R-bit is acknowledged. The sender can now distinguish when a SACK acknowledges the originally sent DATA or retransmitted one. The extension requires support by the sender and the receiver. The mechanism described in this document is equally relevant for I-DATA chunk which is introduced in [RFC8260]. 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [[RFC8174]] when, and only when, they appear in all capitals, as shown here. 3. Updates in SCTP Chunks Header 3.1. R-bit in DATA Chunk Header Figure 1 describes the extended DATA chunk header. 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0 | Res |R|I|U|B|E| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier | Stream Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Protocol Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / User Data / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Extended DATA chunk The only difference between the DATA chunk in Figure 1 and the DATA chunk defined in [RFC4960] is the addition of the R-bit in the flags field of the DATA chunk header. [RFC4960] specified that bit as Reserved and that it should be set to 0 by the sender and ignored by the receiver. Proshin Expires June 4, 2020 [Page 3] Internet-Draft December 2019 3.2. R-bit in I-DATA Chunk Header Figure 2 describes the extended DATA chunk header. 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 64 | Res |R|I|U|B|E| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Protocol Identifier / Fragment Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / User Data / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Extended I-DATA chunk The only difference between the I-DATA chunk in Figure 2 and the I-DATA chunk defined in [RFC8260] is the addition of the R-bit in the flags field of the I-DATA chunk header. [RFC8260] specified that bit as Reserved and that it should be set to 0 by the sender and ignored by the receiver. 3.3. R-bit in SACK Chunk Header Figure 3 describes the extended SACK chunk header. Proshin Expires June 4, 2020 [Page 4] Internet-Draft December 2019 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 3 | Reserved |R| Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cumulative TSN Ack | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertised Receiver Window Credit (a_rwnd) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Gap Ack Blocks = N | Number of Duplicate TSNs = X | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Gap Ack Block #1 Start | Gap Ack Block #1 End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ ... \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Gap Ack Block #N Start | Gap Ack Block #N End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Duplicate TSN 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ ... \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Duplicate TSN X | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Extended SACK chunk The only difference between the SACK chunk in Figure 3 and the SACK chunk defined in [RFC4960] is the addition of the R-bit in the flags field of the SACK chunk header. [RFC4960] specified that bit as Reserved and that it should be set to 0 by the sender and ignored by the receiver. 4. Procedures 4.1. Negotiation R-bit MUST NOT be used unless both SCTP peers negotiated its support. The following new optional parameter is added to the INIT and INIT ACK chunks to negotiate R-bit support during association setup: Proshin Expires June 4, 2020 [Page 5] Internet-Draft December 2019 +----------------+-------------------------------------------+ | Parameter Type | Parameter Name | +----------------+-------------------------------------------+ | 0x8100 | Retransmit Bit Supported (RBIT-SUPPORTED) | +----------------+-------------------------------------------+ Table 1 The parameter format is the following: 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Parameter Type = 0x8100 | Parameter Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Format of RBIT-SUPPORTED Parameter Type: 2 bytes (unsigned integer) This value MUST be set to 0x8100 (33024). Parameter Length: 2 bytes (unsigned integer) This value MUST be set to 4. The RBIT-SUPPORTED parameter MAY be included once in the INIT or INIT ACK chunk if the sender wants to inform its peer that it supports R-bit. The new parameter type is encoded so that it requires the receiver to skip it and continue processing if the parameter is not recognized according to [RFC4960]. 4.2. Sender Side Considerations SCTP MUST NOT set the R-bit when it sends a DATA or I-DATA chunk first time. If R-bit support is negotiated as described in Section 4.1, SCTP SHOULD set the R-bit every time it retransmits a DATA or I-DATA chunk. This is regardless of if the chunk is retransmitted on the same path or on an alternative one. Note that it is possible that the same SCTP packet includes DATA or I-DATA chunks with and without the R-bit set in case when SCTP bundles chunks which are marked for retransmission with chunks which are sent first time. This is aligned with [RFC4960] which allows Proshin Expires June 4, 2020 [Page 6] Internet-Draft December 2019 bundling of DATA chunks marked for retransmission with new DATA chunks. IMPLEMENTATION NOTE: According to [RFC4960] new DATA chunks always follow DATA chunks marked for retransmission when bundled in one packet. 4.3. Receiver Side Considerations SCTP MUST NOT set the R-bit when it sends a SACK which acknowledges a DATA or I-DATA chunk without the R-bit set. The delay for a SACK without the R-bit set is defined according to [RFC4960]. When SCTP receives a packet with DATA or I-DATA chunk(s) with the R-bit set, it MUST immediately respond with a SACK with the R-bit set acknowledging only DATA or I-DATA chunks where the R-bit was set. If the packet also contains DATA or I-DATA chunk(s) without the R-bit set, SCTP MUST NOT acknowledge them in the same SACK chunk. TBD: SACK with the R-bit bundled with SACK without the R-bit? It may be useful. 4.4. Processing of SACK with and without R-bit If a DATA or I-DATA was retransmitted and the corresponding SACK is received, SCTP can distinguish if the SACK acknowledges the original transmission or retransmission by checking the R-bit in the SACK. SCTP mechanisms which can be improved by that information include, but are not limited to, the following: o RTO Calculation: [RFC4960] refers to Karn's algorithm and prohibits SCTP to make RTT measurements using packets that were retransmitted and for which it is ambiguous whether the reply was for the original transmission or retransmission(s). o Path Failure Detection: [RFC4960] specifies that the sender may choose not to clear the path error counter if there is undesirable ambiguity when a DATA is retransmitted on an alternative path. o SCTP-PF Operation in [RFC7829]: additionally to the path error counter case described in the previous bullet [RFC7829] also does not recommend to move a destination address in PF state back to the active state in case of ambiguity. o Detection of spurious retransmissions: using R-bit SCTP can detect spurious retransmissions. Namely, if a DATA was retransmitted and SACK acknowledging it does not include R-bit, it means that the retransmission was spurious. Note that this is valid even if a Proshin Expires June 4, 2020 [Page 7] Internet-Draft December 2019 DATA was retransmitted multiple times which makes this method more effective than detecting of spurious retransmissions based on DSACK. When a spurious retransmission is detected, SCTP implementation may: * Choose to revert the congestion control state. * Choose to adjust RTO settings such as the RTO.Min value to mitigate further spurious retransmissions. * Indicate the SCTP user. o SCTP latency of retransmitted data: If the original DATA is lost, the SCTP receiver will immediately acknowledge the retransmitted DATA. o Calculation of Maximum Ack Delay: SCTP implementations can support a technique for calculating of Maximum Ack Delay in run-time which is impossible to do properly in case of retransmissions. With R-bit SCTP can distinguish if the SACK acknowledges the original transmission or retransmission and can measure the delay even for a retransmitted DATA. o Measurement of packet loss: R-bit can be used for passive loss rate calculation. TBD: dup TSN but without R-bit: SACK loss or reordering: Can be used somehow? Note that this document does not solve the problem when the same DATA or I-DATA chunk is retransmitted multiple times. In that case, when SCTP receives a SACK without the R-bit set, it can ensure that the SACK acknowledges the original transmission but when SCTP receives a SACK with the R-bit set, it cannot distinguish which retransmission is actually acknowledged. Such limitation is not considered as severe because multiple retransmissions of the same DATA or I-DATA is a corner case and, if it happens, SCTP transmission is anyway inefficient. 5. R-bit vs Duplicate TSN for Detection of Spurious Retransmission The SACK chunk according to [RFC4960] contains the Duplicate TSN field which is used by the receiver to indicate TSNs received multiple times. This could happen due to spurious retransmissions or if packets were duplicated in the network between endpoints. The Duplicate TSN field in the SACK chunk can also be used by the sender to detect spurious retransmissions in some cases. However, the Proshin Expires June 4, 2020 [Page 8] Internet-Draft December 2019 mechanism based on the Duplicate TSN field would have serious limitations compared to the mechanism based on R-bit: o With R-bit the SCTP sender has an exclusive match between DATA and SACK while in case of the Duplicate TSN it is not guaranteed. Thus, if the original or retranmitted DATA is lost or one of the SACKs is lost or the packets were retransmitted, the SCTP sender cannot rely on the Duplicate TSN field. o Even in those cases where the sender could rely on the Duplicate TSN, it would need to wait the second SACK to detect the spurious retransmission, while with R-bit, the sender can detect it as soon as the first SACK is received. o In case of the Duplicate TSN the SCTP sender needs to keep information about the retransmitted TSN until the second SACK is received or during some time period which impacts memory usage and SCTP performance and complicates implementation. 6. Interoperability Considerations This document does not introduce any interoperability issues. Section 4.1 requires both ends to negotiate R-bit support before its usage. [RFC4960] requires the receiver of a DATA or SACK chunk with the R-bit set to ignore the bit if it is not recognized. [RFC8260] requires the receiver of an I-DATA chunk with the R-bit set to ignore the bit if it is not recognized. 7. Socket API Considerations This document does not address any changes to the socket API defined in [RFC6458]. 8. Acknowledgements TBD 9. IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document. ] IANA should assign 33024 (0x8100) as a new parameter type to SCTP. Proshin Expires June 4, 2020 [Page 9] Internet-Draft December 2019 Following the chunk flag registration procedure defined in [RFC6096], IANA should register a new bit, the R-bit, for the DATA chunk. The suggested value is 0x10 and the reference should be RFCXXXX. This requires an update of the "DATA Chunk Flags" registry for SCTP: +------------------+-----------------+-----------+ | Chunk Flag Value | Chunk Flag Name | Reference | +------------------+-----------------+-----------+ | 0x01 | E bit | [RFC4960] | | 0x02 | B bit | [RFC4960] | | 0x04 | U bit | [RFC4960] | | 0x08 | I bit | [RFC7053] | | 0x10 | R bit | RFCXXXX | | 0x20 | Unassigned | | | 0x40 | Unassigned | | | 0x80 | Unassigned | | +------------------+-----------------+-----------+ Table 2 Following the chunk flag registration procedure defined in [RFC6096], IANA should register a new bit, the R-bit, for the SACK chunk. The suggested value is 0x01 and the reference should be RFCXXXX. This requires an update of the "SACK Chunk Flags" registry for SCTP: +------------------+-----------------+-----------+ | Chunk Flag Value | Chunk Flag Name | Reference | +------------------+-----------------+-----------+ | 0x01 | R bit | RFCXXXX | | 0x02 | Unassigned | | | 0x04 | Unassigned | | | 0x08 | Unassigned | | | 0x10 | Unassigned | | | 0x20 | Unassigned | | | 0x40 | Unassigned | | | 0x80 | Unassigned | | +------------------+-----------------+-----------+ Table 3 Following the chunk flag registration procedure defined in [RFC6096], IANA should register a new bit, the R-bit, for the I-DATA chunk. The suggested value is 0x10 and the reference should be RFCXXXX. This requires an update of the "I-DATA Chunk Flags" registry for SCTP: Proshin Expires June 4, 2020 [Page 10] Internet-Draft December 2019 +------------------+-----------------+-----------+ | Chunk Flag Value | Chunk Flag Name | Reference | +------------------+-----------------+-----------+ | 0x01 | E bit | [RFC8260] | | 0x02 | B bit | [RFC8260] | | 0x04 | U bit | [RFC8260] | | 0x08 | I bit | [RFC8260] | | 0x10 | R bit | RFCXXXX | | 0x20 | Unassigned | | | 0x40 | Unassigned | | | 0x80 | Unassigned | | +------------------+-----------------+-----------+ Table 4 10. Security Considerations This document does not introduce any additional security considerations in addition to the ones described in [RFC4960] and [RFC8260]. 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, . [RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann, "Stream Schedulers and User Message Interleaving for the Stream Control Transmission Protocol", RFC 8260, DOI 10.17487/RFC8260, November 2017, . 11.2. Informative References [RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission Protocol (SCTP) Chunk Flags Registration", RFC 6096, DOI 10.17487/RFC6096, January 2011, . Proshin Expires June 4, 2020 [Page 11] Internet-Draft December 2019 [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. Yasevich, "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)", RFC 6458, DOI 10.17487/RFC6458, December 2011, . [RFC7053] Tuexen, M., Ruengeler, I., and R. Stewart, "SACK- IMMEDIATELY Extension for the Stream Control Transmission Protocol", RFC 7053, DOI 10.17487/RFC7053, November 2013, . [RFC7829] Nishida, Y., Natarajan, P., Caro, A., Amer, P., and K. Nielsen, "SCTP-PF: A Quick Failover Algorithm for the Stream Control Transmission Protocol", RFC 7829, DOI 10.17487/RFC7829, April 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Author's Address Maksim Proshin Ericsson Kistavaegen 25 Stockholm 164 80 Sweden Email: mproshin@tieto.mera.ru Proshin Expires June 4, 2020 [Page 12]