RFC 4460
Stream Control Transmission Protocol (SCTP) Specification Errata and Issues
2.10. Issues with Heartbeating and Failure Detection
2.10.1. Description of the Problem
Five basic problems have been discovered with the current heartbeat procedures: o The current specification does not specify that you should count a failed heartbeat as an error against the overall association. o The current specification is not specific as to when you start sending heartbeats and when you should stop. o The current specification is not specific as to when you should respond to heartbeats.
o When responding to a Heartbeat, it is unclear what to do if more
than a single TLV is present.
o The jitter applied to a heartbeat was meant to be a small variance
of the RTO and is currently a wide variance, due to the default
delay time and incorrect wording within the RFC.
2.10.2. Text Changes to the Document
---------
Old text: (Section 8.1)
---------
8.1 Endpoint Failure Detection
An endpoint shall keep a counter on the total number of consecutive
retransmissions to its peer (including retransmissions to all the
destination transport addresses of the peer if it is multi-homed).
If the value of this counter exceeds the limit indicated in the
protocol parameter 'Association.Max.Retrans', the endpoint shall
consider the peer endpoint unreachable and shall stop transmitting
any more data to it (and thus the association enters the CLOSED
state). In addition, the endpoint shall report the failure to the
upper layer, and optionally report back all outstanding user data
remaining in its outbound queue. The association is automatically
closed when the peer endpoint becomes unreachable.
The counter shall be reset each time a DATA chunk sent to that peer
endpoint is acknowledged (by the reception of a SACK), or a
HEARTBEAT-ACK is received from the peer endpoint.
--------- New text: (Section 8.1) --------- 8.1. Endpoint Failure Detection An endpoint shall keep a counter on the total number of consecutive retransmissions to its peer (this includes retransmissions to all the destination transport addresses of the peer if it is multi-homed), including unacknowledged HEARTBEAT Chunks. If the value of this counter exceeds the limit indicated in the protocol parameter 'Association.Max.Retrans', the endpoint shall consider the peer endpoint unreachable and shall stop transmitting any more data to it (and thus the association enters the CLOSED state). In addition, the endpoint MAY report the failure to the upper layer and optionally report back all outstanding user data remaining in its outbound queue. The association is automatically closed when the peer endpoint becomes unreachable. The counter shall be reset each time a DATA chunk sent to that peer endpoint is acknowledged (by the reception of a SACK), or a HEARTBEAT-ACK is received from the peer endpoint. --------- Old text: (Section 8.3) --------- 8.3 Path Heartbeat By default, an SCTP endpoint shall monitor the reachability of the idle destination transport address(es) of its peer by sending a HEARTBEAT chunk periodically to the destination transport address(es). --------- New text: (Section 8.3) --------- 8.3 Path Heartbeat By default, an SCTP endpoint SHOULD monitor the reachability of the idle destination transport address(es) of its peer by sending a HEARTBEAT chunk periodically to the destination transport address(es). HEARTBEAT sending MAY begin upon reaching the ESTABLISHED state and is discontinued after sending either SHUTDOWN or SHUTDOWN-ACK. A receiver of a HEARTBEAT MUST respond to a HEARTBEAT with a HEARTBEAT-ACK after entering the COOKIE-ECHOED state
(INIT sender) or the ESTABLISHED state (INIT receiver), up until reaching the SHUTDOWN-SENT state (SHUTDOWN sender) or the SHUTDOWN- ACK-SENT state (SHUTDOWN receiver). --------- Old text: (Section 8.3) --------- The receiver of the HEARTBEAT should immediately respond with a HEARTBEAT ACK that contains the Heartbeat Information field copied from the received HEARTBEAT chunk. --------- New text: (Section 8.3) --------- The receiver of the HEARTBEAT should immediately respond with a HEARTBEAT ACK that contains the Heartbeat Information TLV, together with any other received TLVs, copied unchanged from the received HEARTBEAT chunk. --------- Old text: (Section 8.3) --------- On an idle destination address that is allowed to heartbeat, a HEARTBEAT chunk is RECOMMENDED to be sent once per RTO of that destination address plus the protocol parameter 'HB.interval' , with jittering of +/- 50%, and exponential back-off of the RTO if the previous HEARTBEAT is unanswered. --------- New text: (Section 8.3) --------- On an idle destination address that is allowed to heartbeat, it is recommended that a HEARTBEAT chunk is sent once per RTO of that destination address plus the protocol parameter 'HB.interval', with jittering of +/- 50% of the RTO value, and exponential back-off of the RTO if the previous HEARTBEAT is unanswered.2.10.3. Solution Description
The above text provides guidance as to how to respond to the five issues mentioned in Section 2.10.1. In particular, the wording changes provide guidance as to when to start and stop heartbeating,
how to respond to a heartbeat with extra parameters, and it clarifies the error counting procedures for the association.2.11. Security interactions with firewalls
2.11.1. Description of the Problem
When dealing with firewalls, it is advantageous for the firewall to be able to properly determine the initial startup sequence of a reliable transport protocol. With this in mind, the following text is to be added to SCTP's security section.2.11.2. Text Changes to the Document
--------- New text: (no old text, new section added) --------- 11.4 SCTP Interactions with Firewalls It is helpful for some firewalls if they can inspect just the first fragment of a fragmented SCTP packet and unambiguously determine whether it corresponds to an INIT chunk (for further information, please refer to RFC1858). Accordingly, we stress the requirements, stated in 3.1, that (1) an INIT chunk MUST NOT be bundled with any other chunk in a packet, and (2) a packet containing an INIT chunk MUST have a zero Verification Tag. Furthermore, we require that the receiver of an INIT chunk MUST enforce these rules by silently discarding an arriving packet with an INIT chunk that is bundled with other chunks. --------- Old text: (Section 18) --------- 18. Bibliography [ALLMAN99] Allman, M. and Paxson, V., "On Estimating End-to-End Network Path Properties", Proc. SIGCOMM'99, 1999. [FALL96] Fall, K. and Floyd, S., Simulation-based Comparisons of Tahoe, Reno, and SACK TCP, Computer Communications Review, V. 26 N. 3, July 1996, pp. 5-21. [RFC1750] Eastlake, D. (ed.), "Randomness Recommendations for Security", RFC 1750, December 1994.
[RFC1950] Deutsch P. and J. Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, May 1996. [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, March 1997. [RFC2196] Fraser, B., "Site Security Handbook", FYI 8, RFC 2196, September 1997. [RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management Protocol", RFC 2522, March 1999. [SAVAGE99] Savage, S., Cardwell, N., Wetherall, D., and Anderson, T., "TCP Congestion Control with a Misbehaving Receiver", ACM Computer Communication Review, 29(5), October 1999. --------- New text: (Section 18) --------- 18. Bibliography [ALLMAN99] Allman, M. and Paxson, V., "On Estimating End-to-End Network Path Properties", Proc. SIGCOMM'99, 1999. [FALL96] Fall, K. and Floyd, S., Simulation-based Comparisons of Tahoe, Reno, and SACK TCP, Computer Communications Review, V. 26 N. 3, July 1996, pp. 5-21. [RFC1750] Eastlake, D. (ed.), "Randomness Recommendations for Security", RFC 1750, December 1994. [RFC1858] Ziemba, G., Reed, D. and Traina P., "Security Considerations for IP Fragment Filtering", RFC 1858, October 1995. [RFC1950] Deutsch P. and J. Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, May 1996. [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, March 1997. [RFC2196] Fraser, B., "Site Security Handbook", FYI 8, RFC 2196, September 1997. [RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management Protocol", RFC 2522, March 1999.
[SAVAGE99] Savage, S., Cardwell, N., Wetherall, D., and Anderson, T.,
"TCP Congestion Control with a Misbehaving Receiver", ACM
Computer Communication Review, 29(5), October 1999.
2.11.3. Solution Description
The above text, which adds a new subsection to the Security
Considerations section of RFC 2960 [5] makes clear that, to make
easier the interaction with firewalls, an INIT chunk must not be
bundled in any case with any other chunk that will silently discard
the packets that do not follow this rule (this rule is enforced by
the packet receiver).
2.12. Shutdown Ambiguity
2.12.1. Description of the Problem
Currently, there is an ambiguity between the statements in Sections
6.2 and 9.2. Section 6.2 allows the sending of a SHUTDOWN chunk in
place of a SACK when the sender is in the process of shutting down,
while section 9.2 requires that both a SHUTDOWN chunk and a SACK
chunk be sent.
Along with this ambiguity there is a problem wherein an errant
SHUTDOWN receiver may fail to stop accepting user data.
2.12.2. Text Changes to the Document
---------
Old text: (Section 9.2)
---------
If there are still outstanding DATA chunks left, the SHUTDOWN
receiver shall continue to follow normal data transmission procedures
defined in Section 6 until all outstanding DATA chunks are
acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data
from its SCTP user.
While in SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately
respond to each received packet containing one or more DATA chunk(s)
with a SACK, a SHUTDOWN chunk, and restart the T2-shutdown timer. If
it has no more outstanding DATA chunks, the SHUTDOWN receiver shall
send a SHUTDOWN ACK and start a T2-shutdown timer of its own,
entering the SHUTDOWN-ACK-SENT state. If the timer expires, the
endpoint must re-send the SHUTDOWN ACK.
--------- New text: (Section 9.2) --------- If there are still outstanding DATA chunks left, the SHUTDOWN receiver MUST continue to follow normal data transmission procedures defined in Section 6, until all outstanding DATA chunks are acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data from its SCTP user. While in SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately respond to each received packet containing one or more DATA chunks with a SHUTDOWN chunk and restart the T2-shutdown timer. If a SHUTDOWN chunk by itself cannot acknowledge all of the received DATA chunks (i.e., there are TSNs that can be acknowledged that are larger than the cumulative TSN, and thus gaps exist in the TSN sequence), or if duplicate TSNs have been received, then a SACK chunk MUST also be sent. The sender of the SHUTDOWN MAY also start an overall guard timer 'T5-shutdown-guard' to bound the overall time for shutdown sequence. At the expiration of this timer, the sender SHOULD abort the association by sending an ABORT chunk. If the 'T5-shutdown-guard' timer is used, it SHOULD be set to the recommended value of 5 times 'RTO.Max'. If the receiver of the SHUTDOWN has no more outstanding DATA chunks, the SHUTDOWN receiver MUST send a SHUTDOWN ACK and start a T2-shutdown timer of its own, entering the SHUTDOWN-ACK-SENT state. If the timer expires, the endpoint must re-send the SHUTDOWN ACK.2.12.3. Solution Description
The above text clarifies the use of a SACK in conjunction with a SHUTDOWN chunk. It also adds a guard timer to the SCTP shutdown sequence to protect against errant receivers of SHUTDOWN chunks.2.13. Inconsistency in ABORT Processing
2.13.1. Description of the Problem
It was noted that the wording in Section 8.5.1 did not give proper directions in the use of the 'T bit' with the Verification Tags.
2.13.2. Text changes to the document
--------- Old text: (Section 8.5.1) --------- B) Rules for packet carrying ABORT: - The endpoint shall always fill in the Verification Tag field of the outbound packet with the destination endpoint's tag value if it is known. - If the ABORT is sent in response to an OOTB packet, the endpoint MUST follow the procedure described in Section 8.4. - The receiver MUST accept the packet if the Verification Tag matches either its own tag, OR the tag of its peer. Otherwise, the receiver MUST silently discard the packet and take no further action. --------- New text: (Section 8.5.1) --------- B) Rules for packet carrying ABORT: - The endpoint MUST always fill in the Verification Tag field of the outbound packet with the destination endpoint's tag value, if it is known. - If the ABORT is sent in response to an OOTB packet, the endpoint MUST follow the procedure described in Section 8.4. - The receiver of a ABORT MUST accept the packet if the Verification Tag field of the packet matches its own tag OR if it is set to its peer's tag and the T bit is set in the Chunk Flags. Otherwise, the receiver MUST silently discard the packet and take no further action.2.13.3. Solution Description
The above text change clarifies that the T bit must be set before an implementation looks for the peer's tag.
2.14. Cwnd Gated by Its Full Use
2.14.1. Description of the Problem
A problem was found with the current specification of the growth and decay of cwnd. The cwnd should only be increased if it is being fully utilized, and after periods of underutilization, the cwnd should be decreased. In some sections, the current wording is weak and is not clearly defined. Also, the current specification unnecessarily introduces the need for special case code to ensure cwnd degradation. Plus, the cwnd should not be increased during Fast Recovery, since a full cwnd during Fast Recovery does not qualify the cwnd as being fully utilized. Additionally, multiple loss scenarios in a single window may cause the cwnd to grow more rapidly as the number of losses in a window increases [3].2.14.2. Text Changes to the Document
--------- Old text: (Section 6.1) --------- D) Then, the sender can send out as many new DATA chunks as Rule A and Rule B above allow. --------- New text: (Section 6.1) --------- D) When the time comes for the sender to transmit new DATA chunks, the protocol parameter Max.Burst SHOULD be used to limit the number of packets sent. The limit MAY be applied by adjusting cwnd as follows: if((flightsize + Max.Burst*MTU) < cwnd) cwnd = flightsize + Max.Burst*MTU Or it MAY be applied by strictly limiting the number of packets emitted by the output routine. E) Then, the sender can send out as many new DATA chunks as Rule A and Rule B allow.
--------- Old text: (Section 7.2.1) --------- o When cwnd is less than or equal to ssthresh an SCTP endpoint MUST use the slow start algorithm to increase cwnd (assuming the current congestion window is being fully utilized). If an incoming SACK advances the Cumulative TSN Ack Point, cwnd MUST be increased by at most the lesser of 1) the total size of the previously outstanding DATA chunk(s) acknowledged, and 2) the destination's path MTU. This protects against the ACK-Splitting attack outlined in [SAVAGE99]. --------- New text: (Section 7.2.1) --------- o When cwnd is less than or equal to ssthresh, an SCTP endpoint MUST use the slow start algorithm to increase cwnd only if the current congestion window is being fully utilized, an incoming SACK advances the Cumulative TSN Ack Point, and the data sender is not in Fast Recovery. Only when these three conditions are met can the cwnd be increased; otherwise, the cwnd MUST not be increased. If these conditions are met, then cwnd MUST be increased by, at most, the lesser of 1) the total size of the previously outstanding DATA chunk(s) acknowledged, and 2) the destination's path MTU. This upper bound protects against the ACK-Splitting attack outlined in [SAVAGE99]. --------- Old text: (Section 14) --------- 14. Suggested SCTP Protocol Parameter Values The following protocol parameters are RECOMMENDED: RTO.Initial - 3 seconds RTO.Min - 1 second RTO.Max - 60 seconds RTO.Alpha - 1/8 RTO.Beta - 1/4 Valid.Cookie.Life - 60 seconds Association.Max.Retrans - 10 attempts Path.Max.Retrans - 5 attempts (per destination address) Max.Init.Retransmits - 8 attempts HB.interval - 30 seconds
--------- New text: (Section 14) --------- 14. Suggested SCTP Protocol Parameter Values The following protocol parameters are RECOMMENDED: RTO.Initial - 3 seconds RTO.Min - 1 second RTO.Max - 60 seconds Max.Burst - 4 RTO.Alpha - 1/8 RTO.Beta - 1/4 Valid.Cookie.Life - 60 seconds Association.Max.Retrans - 10 attempts Path.Max.Retrans - 5 attempts (per destination address) Max.Init.Retransmits - 8 attempts HB.Interval - 30 seconds2.14.3. Solution Description
The above changes strengthen the rules and make it much more apparent as to the need to block cwnd growth when the full cwnd is not being utilized. The changes also apply cwnd degradation without introducing the need for complex special case code.2.15. Window Probes in SCTP
2.15.1. Description of the Problem
When a receiver clamps its rwnd to 0 to flow control the peer, the specification implies that one must continue to accept data from the remote peer. This is incorrect and needs clarification.2.15.2. Text Changes to the Document
--------- Old text: (Section 6.2) --------- The SCTP endpoint MUST always acknowledge the receipt of each valid DATA chunk.
--------- New text: (Section 6.2) --------- The SCTP endpoint MUST always acknowledge the reception of each valid DATA chunk when the DATA chunk received is inside its receive window. When the receiver's advertised window is 0, the receiver MUST drop any new incoming DATA chunk with a TSN larger than the largest TSN received so far. If the new incoming DATA chunk holds a TSN value less than the largest TSN received so far, then the receiver SHOULD drop the largest TSN held for reordering and accept the new incoming DATA chunk. In either case, if such a DATA chunk is dropped, the receiver MUST immediately send back a SACK with the current receive window showing only DATA chunks received and accepted so far. The dropped DATA chunk(s) MUST NOT be included in the SACK, as they were not accepted. The receiver MUST also have an algorithm for advertising its receive window to avoid receiver silly window syndrome (SWS), as described in RFC 813. The algorithm can be similar to the one described in Section 4.2.3.3 of RFC 1122. --------- Old text: (Section 6.1) --------- A) At any given time, the data sender MUST NOT transmit new data to any destination transport address if its peer's rwnd indicates that the peer has no buffer space (i.e., rwnd is 0, see Section 6.2.1). However, regardless of the value of rwnd (including if it is 0), the data sender can always have one DATA chunk in flight to the receiver if allowed by cwnd (see rule B below). This rule allows the sender to probe for a change in rwnd that the sender missed due to the SACK having been lost in transit from the data receiver to the data sender.
--------- New text: (Section 6.1) --------- A) At any given time, the data sender MUST NOT transmit new data to any destination transport address if its peer's rwnd indicates that the peer has no buffer space (i.e., rwnd is 0; see Section 6.2.1). However, regardless of the value of rwnd (including if it is 0), the data sender can always have one DATA chunk in flight to the receiver if allowed by cwnd (see rule B, below). This rule allows the sender to probe for a change in rwnd that the sender missed due to the SACK's having been lost in transit from the data receiver to the data sender. When the receiver's advertised window is zero, this probe is called a zero window probe. Note that a zero window probe SHOULD only be sent when all outstanding DATA chunks have been cumulatively acknowledged and no DATA chunks are in flight. Zero window probing MUST be supported. If the sender continues to receive new packets from the receiver while doing zero window probing, the unacknowledged window probes should not increment the error counter for the association or any destination transport address.This is because the receiver MAY keep its window closed for an indefinite time. Refer to Section 6.2 on the receiver behavior when it advertises a zero window. The sender SHOULD send the first zero window probe after 1 RTO when it detects that the receiver has closed its window and SHOULD increase the probe interval exponentially afterwards. Also note that the cwnd SHOULD be adjusted according to Section 7.2.1. Zero window probing does not affect the calculation of cwnd. The sender MUST also have an algorithm for sending new DATA chunks to avoid silly window syndrome (SWS) as described in RFC 813. The algorithm can be similar to the one described in Section 4.2.3.4 of RFC 1122.2.15.3. Solution Description
The above allows a receiver to drop new data that arrives and yet still requires the receiver to send a SACK showing the conditions unchanged (with the possible exception of a new a_rwnd) and the dropped chunk as missing. This will allow the association to continue until the rwnd condition clears.
2.16. Fragmentation and Path MTU Issues
2.16.1. Description of the Problem
The current wording of the Fragmentation and Reassembly forces an implementation that supports fragmentation to always fragment. This prohibits an implementation from offering its users an option to disable sends that exceed the SCTP fragmentation point. The restriction in RFC 2960 [5], Section 6.9, was never meant to restrict an implementations API from this behavior.2.16.2. Text Changes to the Document
--------- Old text: (Section 6.1) --------- 6.9 Fragmentation and Reassembly An endpoint MAY support fragmentation when sending DATA chunks, but MUST support reassembly when receiving DATA chunks. If an endpoint supports fragmentation, it MUST fragment a user message if the size of the user message to be sent causes the outbound SCTP packet size to exceed the current MTU. If an implementation does not support fragmentation of outbound user messages, the endpoint must return an error to its upper layer and not attempt to send the user message. IMPLEMENTATION NOTE: In this error case, the Send primitive discussed in Section 10.1 would need to return an error to the upper layer. --------- New text: (Section 6.1) --------- 6.9. Fragmentation and Reassembly An endpoint MAY support fragmentation when sending DATA chunks, but it MUST support reassembly when receiving DATA chunks. If an endpoint supports fragmentation, it MUST fragment a user message if the size of the user message to be sent causes the outbound SCTP packet size to exceed the current MTU. If an implementation does not support fragmentation of outbound user messages, the endpoint MUST return an error to its upper layer and not attempt to send the user message.
Note: If an implementation that supports fragmentation makes available to its upper layer a mechanism to turn off fragmentation it may do so. However, in so doing, it MUST react just like an implementation that does NOT support fragmentation, i.e., it MUST reject sends that exceed the current P-MTU. IMPLEMENTATION NOTE: In this error case, the Send primitive discussed in Section 10.1 would need to return an error to the upper layer.2.16.3. Solution Description
The above wording will allow an implementation to offer the option of rejecting sends that exceed the P-MTU size even when the implementation supports fragmentation.2.17. Initial Value of the Cumulative TSN Ack
2.17.1. Description of the Problem
The current description of the SACK chunk within the RFC does not clearly state the value that would be put within a SACK when no DATA chunk has been received.2.17.2. Text Changes to the Document
--------- Old text: (Section 3.3.4) --------- Cumulative TSN Ack: 32 bits (unsigned integer) This parameter contains the TSN of the last DATA chunk received in sequence before a gap. --------- New text: (Section 3.3.4) --------- Cumulative TSN Ack: 32 bits (unsigned integer) This parameter contains the TSN of the last DATA chunk received in sequence before a gap. In the case where no DATA chunk has been received, this value is set to the peer's Initial TSN minus one.
2.17.3. Solution Description
This change clearly states what the initial value will be for a SACK sender.2.18. Handling of Address Parameters within the INIT or INIT-ACK
2.18.1. Description of the Problem
The current description on handling address parameters contained within the INIT and INIT-ACK does not fully describe a requirement for their handling.2.18.2. Text Changes to the Document
--------- Old text: (Section 5.1.2) --------- C) If there are only IPv4/IPv6 addresses present in the received INIT or INIT ACK chunk, the receiver shall derive and record all the transport address(es) from the received chunk AND the source IP address that sent the INIT or INIT ACK. The transport address(es) are derived by the combination of SCTP source port (from the common header) and the IP address parameter(s) carried in the INIT or INIT ACK chunk and the source IP address of the IP datagram. The receiver should use only these transport addresses as destination transport addresses when sending subsequent packets to its peer. --------- New text: (Section 5.1.2) --------- C) If there are only IPv4/IPv6 addresses present in the received INIT or INIT ACK chunk, the receiver MUST derive and record all the transport addresses from the received chunk AND the source IP address that sent the INIT or INIT ACK. The transport addresses are derived by the combination of SCTP source port (from the common header) and the IP address parameter(s) carried in the INIT or INIT ACK chunk and the source IP address of the IP datagram. The receiver should use only these transport addresses as destination transport addresses when sending subsequent packets to its peer.
D) An INIT or INIT ACK chunk MUST be treated as belonging
to an already established association (or one in the
process of being established) if the use of any of the
valid address parameters contained within the chunk
would identify an existing TCB.
2.18.3. Solution description
This new text clearly specifies to an implementor the need to look
within the INIT or INIT ACK. Any implementation that does not do
this may (for example) not be able to recognize an INIT chunk coming
from an already established association that adds new addresses (see
Section 2.6) or an incoming INIT ACK chunk sent from a source address
different from the destination address used to send the INIT chunk.
2.19. Handling of Stream Shortages
2.19.1. Description of the Problem
The current wording in the RFC places the choice of sending an ABORT
upon the SCTP stack when a stream shortage occurs. This decision
should really be made by the upper layer, not the SCTP stack.
2.19.2. Text Changes to the Document
---------
Old text:
---------
5.1.1 Handle Stream Parameters
In the INIT and INIT ACK chunks, the sender of the chunk shall
indicate the number of outbound streams (OS) it wishes to have in
the association, as well as the maximum inbound streams (MIS) it
will accept from the other endpoint.
After receiving the stream configuration information from the other
side, each endpoint shall perform the following check: If the peer's
MIS is less than the endpoint's OS, meaning that the peer is
incapable of supporting all the outbound streams the endpoint wants
to configure, the endpoint MUST either use MIS outbound streams, or
abort the association and report to its upper layer the resources
shortage at its peer.
--------- New text: (Section 5.1.2) --------- 5.1.1. Handle Stream Parameters In the INIT and INIT ACK chunks, the sender of the chunk MUST indicate the number of outbound streams (OS) it wishes to have in the association, as well as the maximum inbound streams (MIS) it will accept from the other endpoint. After receiving the stream configuration information from the other side, each endpoint MUST perform the following check: If the peer's MIS is less than the endpoint's OS, meaning that the peer is incapable of supporting all the outbound streams the endpoint wants to configure, the endpoint MUST use MIS outbound streams and MAY report any shortage to the upper layer. The upper layer can then choose to abort the association if the resource shortage is unacceptable.2.19.3. Solution Description
The above changes take the decision to ABORT out of the realm of the SCTP stack and place it into the user's hands.2.20. Indefinite Postponement
2.20.1. Description of the Problem
The current RFC does not provide any guidance on the assignment of TSN sequence numbers to outbound messages nor reception of these messages. This could lead to a possible indefinite postponement.2.20.2. Text Changes to the Document
--------- Old text: (Section 6.1) --------- Note: The data sender SHOULD NOT use a TSN that is more than 2**31 - 1 above the beginning TSN of the current send window. 6.2 Acknowledgement on Reception of DATA Chunks
--------- New text: (Section 6.1) --------- Note: The data sender SHOULD NOT use a TSN that is more than 2**31 - 1 above the beginning TSN of the current send window. The algorithm by which an implementation assigns sequential TSNs to messages on a particular association MUST ensure that no user message that has been accepted by SCTP is indefinitely postponed from being assigned a TSN. Acceptable algorithms for assigning TSNs include (a) assigning TSNs in round-robin order over all streams with pending data; and (b) preserving the linear order in which the user messages were submitted to the SCTP association. When an upper layer requests to read data on an SCTP association, the SCTP receiver SHOULD choose the message with the lowest TSN from among all deliverable messages. In SCTP implementations that allow a user to request data on a specific stream, this operation SHOULD NOT block if data is not available, since this can lead to a deadlock under certain conditions. 6.2. Acknowledgement on Receipt of DATA Chunks2.20.3. Solution Description
The above wording clarifies how TSNs SHOULD be assigned by the sender.2.21. User-Initiated Abort of an Association
2.21.1. Description of the Problem
It is not possible for an upper layer to abort the association and provide the peer with an indication of why the association is aborted.2.21.2. Text changes to the document
Some of the changes given here already include changes suggested in Section 2.6 of this document.
--------- Old text: (Section 3.3.10) --------- Cause Code Value Cause Code --------- ---------------- 1 Invalid Stream Identifier 2 Missing Mandatory Parameter 3 Stale Cookie Error 4 Out of Resource 5 Unresolvable Address 6 Unrecognized Chunk Type 7 Invalid Mandatory Parameter 8 Unrecognized Parameters 9 No User Data 10 Cookie Received While Shutting Down Cause Length: 16 bits (unsigned integer) Set to the size of the parameter in bytes, including the Cause Code, Cause Length, and Cause-Specific Information fields Cause-specific Information: variable length This field carries the details of the error condition. Sections 3.3.10.1 - 3.3.10.10 define error causes for SCTP. Guidelines for the IETF to define new error cause values are discussed in Section 13.3.
--------- New text: (Section 3.3.10) --------- Cause Code Value Cause Code --------- ---------------- 1 Invalid Stream Identifier 2 Missing Mandatory Parameter 3 Stale Cookie Error 4 Out of Resource 5 Unresolvable Address 6 Unrecognized Chunk Type 7 Invalid Mandatory Parameter 8 Unrecognized Parameters 9 No User Data 10 Cookie Received While Shutting Down 11 Restart of an Association with New Addresses 12 User-Initiated Abort Cause Length: 16 bits (unsigned integer) Set to the size of the parameter in bytes, including the Cause Code, Cause Length, and Cause-Specific Information fields Cause-specific Information: variable length This field carries the details of the error condition. Sections 3.3.10.1 - 3.3.10.12 define error causes for SCTP. Guidelines for the IETF to define new error cause values are discussed in Section 13.3. --------- New text: (Note: no old text, new error added in Section 3.3.10) --------- 3.3.10.12. User-Initiated Abort (12) Cause of error -------------- This error cause MAY be included in ABORT chunks that are sent because of an upper layer request. The upper layer can specify an Upper Layer Abort Reason that is transported by SCTP transparently and MAY be delivered to the upper layer protocol at the peer.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=12 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Upper Layer Abort Reason /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
---------
Old text: (Section 9.1)
---------
9.1 Abort of an Association
When an endpoint decides to abort an existing association, it
shall send an ABORT chunk to its peer endpoint. The sender MUST
fill in the peer's Verification Tag in the outbound packet and
MUST NOT bundle any DATA chunk with the ABORT.
An endpoint MUST NOT respond to any received packet that contains
an ABORT chunk (also see Section 8.4).
An endpoint receiving an ABORT shall apply the special
Verification Tag check rules described in Section 8.5.1.
After checking the Verification Tag, the receiving endpoint shall
remove the association from its record and shall report the
termination to its upper layer.
---------
New text: (Section 9.1)
---------
9.1. Abort of an Association
When an endpoint decides to abort an existing association, it MUST
send an ABORT chunk to its peer endpoint. The sender MUST fill in
the peer's Verification Tag in the outbound packet and MUST NOT
bundle any DATA chunk with the ABORT. If the association is
aborted on request of the upper layer, a User-Initiated Abort
error cause (see 3.3.10.12) SHOULD be present in the ABORT chunk.
An endpoint MUST NOT respond to any received packet that contains
an ABORT chunk (also see Section 8.4).
An endpoint receiving an ABORT MUST apply the special Verification
Tag check rules described in Section 8.5.1.
After checking the Verification Tag, the receiving endpoint MUST
remove the association from its record and SHOULD report the
termination to its upper layer. If a User-Initiated Abort error
cause is present in the ABORT chunk, the Upper Layer Abort Reason
SHOULD be made available to the upper layer.
---------
Old text: (Section 10.1)
---------
D) Abort
Format: ABORT(association id [, cause code])
-> result
Ungracefully closes an association. Any locally queued user
data will be discarded and an ABORT chunk is sent to the peer.
A success code will be returned on successful abortion of the
association. If attempting to abort the association results
in a failure, an error code shall be returned.
Mandatory attributes:
o association id - local handle to the SCTP association
Optional attributes:
o cause code - reason of the abort to be passed to the peer.
---------
New text: (Section 10.1)
---------
D) Abort
Format: ABORT(association id [, Upper Layer Abort Reason])
-> result
Ungracefully closes an association. Any locally queued user
data will be discarded, and an ABORT chunk is sent to the peer.
A success code will be returned on successful abortion of the
association. If attempting to abort the association results
in a failure, an error code shall be returned.
Mandatory attributes:
o association id - Local handle to the SCTP association.
Optional attributes:
o Upper Layer Abort Reason - Reason of the abort to be passed
to the peer.
None.
---------
Old text: (Section 10.2)
---------
E) COMMUNICATION LOST notification
When SCTP loses communication to an endpoint completely (e.g., via
Heartbeats) or detects that the endpoint has performed an abort
operation, it shall invoke this notification on the ULP.
The following shall be passed with the notification:
o association id - local handle to the SCTP association
o status - This indicates what type of event has occurred; The
status may indicate a failure OR a normal termination
event occurred in response to a shutdown or abort
request.
The following may be passed with the notification:
o data retrieval id - an identification used to retrieve
unsent and unacknowledged data.
o last-acked - the TSN last acked by that peer endpoint;
o last-sent - the TSN last sent to that peer endpoint;
---------
New text: (Section 10.2)
---------
E) COMMUNICATION LOST notification
When SCTP loses communication to an endpoint completely (e.g., via
Heartbeats) or detects that the endpoint has performed an abort
operation, it shall invoke this notification on the ULP.
The following shall be passed with the notification:
o association id - Local handle to the SCTP association.
o status - This indicates what type of event has occurred; The
status may indicate that a failure OR a normal
termination event occurred in response to a shutdown
or abort request.
The following may be passed with the notification:
o data retrieval id - An identification used to retrieve unsent
and unacknowledged data.
o last-acked - The TSN last acked by that peer endpoint.
o last-sent - The TSN last sent to that peer endpoint.
o Upper Layer Abort Reason - The abort reason specified in
case of a user-initiated abort.
2.21.3. Solution Description
The above allows an upper layer to provide its peer with an
indication of why the association was aborted. Therefore, an
addition error cause was introduced.
2.22. Handling of Invalid Initiate Tag of INIT-ACK
2.22.1. Description of the Problem
RFC 2960 requires that the receiver of an INIT-ACK with the Initiate
Tag set to zero handles this as an error and sends back an ABORT.
But the sender of the INIT-ACK normally has no TCB, and thus the
ABORT is useless.
2.22.2. Text Changes to the Document
---------
Old text: (Section 3.3.3)
---------
Initiate Tag: 32 bits (unsigned integer)
The receiver of the INIT ACK records the value of the
Initiate Tag parameter. This value MUST be placed into
the Verification Tag field of every SCTP packet that the
INIT ACK receiver transmits within this association.
The Initiate Tag MUST NOT take the value 0. See Section 5.3.1
for more on the selection of the Initiate Tag value.
If the value of the Initiate Tag in a received INIT ACK chunk
is found to be 0, the receiver MUST treat it as an error and
close the association by transmitting an ABORT.
---------
New text: (Section 3.3.3)
---------
Initiate Tag: 32 bits (unsigned integer)
The receiver of the INIT ACK records the value of the
Initiate Tag parameter. This value MUST be placed into
the Verification Tag field of every SCTP packet that the
INIT ACK receiver transmits within this association.
The Initiate Tag MUST NOT take the value 0. See Section 5.3.1
for more on the selection of the Initiate Tag value.
If the value of the Initiate Tag in a received INIT ACK
chunk is found to be 0, the receiver MUST destroy the
association discarding its TCB. The receiver MAY send an
ABORT for debugging purpose.
2.22.3. Solution Description
The new text does not require that the receiver of the invalid INIT-
ACK send the ABORT. This behavior is in tune with the error case of
invalid stream numbers in the INIT-ACK. However, sending an ABORT
for debugging purposes is allowed.
2.23. Sending an ABORT in Response to an INIT
2.23.1. Description of the Problem
Whenever the receiver of an INIT chunk has to send an ABORT chunk in
response, for whatever reason, it is not stated clearly which
Verification Tag and value of the T-bit should be used.
2.23.2. Text Changes to the Document
---------
Old text: (Section 8.4)
---------
3) If the packet contains an INIT chunk with a Verification Tag
set to '0', process it as described in Section 5.1.
Otherwise,
--------- New text: (Section 8.4) --------- 3) If the packet contains an INIT chunk with a Verification Tag set to '0', process it as described in Section 5.1. If, for whatever reason, the INIT cannot be processed normally and an ABORT has to be sent in response, the Verification Tag of the packet containing the ABORT chunk MUST be the Initiate tag of the received INIT chunk, and the T-Bit of the ABORT chunk has to be set to 0, indicating that a TCB was destroyed. Otherwise,2.23.3. Solution Description
The new text stated clearly which value of the Verification Tag and T-bit have to be used.2.24. Stream Sequence Number (SSN) Initialization
2.24.1. Description of the Problem
RFC 2960 does not describe the fact that the SSN has to be initialized to 0, as required by RFC 2119.2.24.2. Text Changes to the Document
--------- Old text: (Section 6.5) --------- The stream sequence number in all the streams shall start from 0 when the association is established. Also, when the stream sequence number reaches the value 65535 the next stream sequence number shall be set to 0. --------- New text: (Section 6.5) --------- The stream sequence number in all the streams MUST start from 0 when the association is established. Also, when the stream sequence number reaches the value 65535 the next stream sequence number MUST be set to 0.
2.24.3. Solution Description
The 'shall' in the text is replaced by a 'MUST' to clearly state the required behavior.2.25. SACK Packet Format
2.25.1. Description of the Problem
It is not clear in RFC 2960 whether a SACK must contain the fields Number of Gap Ack Blocks and Number of Duplicate TSNs.2.25.2. Text Changes to the Document
--------- Old text: (Section 3.3.4) --------- The SACK MUST contain the Cumulative TSN Ack and Advertised Receiver Window Credit (a_rwnd) parameters. --------- New text: (Section 3.3.4) --------- The SACK MUST contain the Cumulative TSN Ack, Advertised Receiver Window Credit (a_rwnd), Number of Gap Ack Blocks, and Number of Duplicate TSNs fields.2.25.3. Solution Description
The text has been modified. It is now clear that a SACK always contains the fields Number of Gap Ack Blocks and Number of Duplicate TSNs.2.26. Protocol Violation Error Cause
2.26.1. Description of the Problem
There are many situations where an SCTP endpoint may detect that its peer violates the protocol. The result of such detection often results in the association being destroyed by the sending of an ABORT. Currently, there are only some error causes that could be used to indicate the reason for the abort, but these do not cover all cases.
2.26.2. Text Changes to the Document
Some of the changes given here already include changes suggested in Section 2.6 and 2.21 of this document. --------- Old text: (Section 3.3.10) --------- Cause Code Value Cause Code --------- ---------------- 1 Invalid Stream Identifier 2 Missing Mandatory Parameter 3 Stale Cookie Error 4 Out of Resource 5 Unresolvable Address 6 Unrecognized Chunk Type 7 Invalid Mandatory Parameter 8 Unrecognized Parameters 9 No User Data 10 Cookie Received While Shutting Down Cause Length: 16 bits (unsigned integer) Set to the size of the parameter in bytes, including the Cause Code, Cause Length, and Cause-Specific Information fields Cause-specific Information: variable length This field carries the details of the error condition. Sections 3.3.10.1 - 3.3.10.10 define error causes for SCTP. Guidelines for the IETF to define new error cause values are discussed in Section 13.3.
--------- New text: (Section 3.3.10) --------- Cause Code Value Cause Code --------- ---------------- 1 Invalid Stream Identifier 2 Missing Mandatory Parameter 3 Stale Cookie Error 4 Out of Resource 5 Unresolvable Address 6 Unrecognized Chunk Type 7 Invalid Mandatory Parameter 8 Unrecognized Parameters 9 No User Data 10 Cookie Received While Shutting Down 11 Restart of an Association with New Addresses 12 User Initiated Abort 13 Protocol Violation Cause Length: 16 bits (unsigned integer) Set to the size of the parameter in bytes, including the Cause Code, Cause Length, and Cause-Specific Information fields Cause-specific Information: variable length This field carries the details of the error condition. Sections 3.3.10.1 - 3.3.10.13 define error causes for SCTP. Guidelines for the IETF to define new error cause values are discussed in Section 13.3. --------- New text: (Note: no old text; new error added in section 3.3.10) --------- 3.3.10.13. Protocol Violation (13) Cause of error -------------- This error cause MAY be included in ABORT chunks that are sent because an SCTP endpoint detects a protocol violation of the peer that is not covered by the error causes described in 3.3.10.1 to 3.3.10.12. An implementation MAY provide additional information specifying what kind of protocol violation has been detected.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=13 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Additional Information /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.26.3. Solution Description
An additional error cause has been defined that can be used by an
endpoint to indicate a protocol violation of the peer.
2.27. Reporting of Unrecognized Parameters
2.27.1. Description of the Problem
It is not stated clearly in RFC 2960 [5] how unrecognized parameters
should be reported. Unrecognized parameters in an INIT chunk could
be reported in the INIT-ACK chunk or in a separate ERROR chunk, which
can get lost. Unrecognized parameters in an INIT-ACK chunk have to
be reported in an ERROR-chunk. This can be bundled with the COOKIE-
ERROR chunk or sent separately. If it is sent separately and
received before the COOKIE-ECHO, it will be handled as an OOTB
packet, resulting in sending out an ABORT chunk. Therefore, the
association would not be established.
2.27.2. Text Changes to the Document
Some of the changes given here already include changes suggested in
Section 2.2 of this document.
---------
Old text: (Section 3.2.1)
---------
00 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
10 - Skip this parameter and continue processing.
11 - Skip this parameter and continue processing but report the
unrecognized parameter in an 'Unrecognized Parameter Type' (in
either an ERROR or in the INIT ACK).
--------- New text: (Section 3.2.1) --------- 00 - Stop processing this SCTP chunk and discard it; do not process any further parameters within this chunk. 01 - Stop processing this SCTP chunk and discard it, do not process any further parameters within this chunk, and report the unrecognized parameter in an 'Unrecognized Parameter Type', as described in 3.2.2. 10 - Skip this parameter and continue processing. 11 - Skip this parameter and continue processing but report the unrecognized parameter in an 'Unrecognized Parameter Type', as described in 3.2.2. --------- New text: (Note: no old text; clarification added in Section 3.2) --------- 3.2.2. Reporting of Unrecognized Parameters If the receiver of an INIT chunk detects unrecognized parameters and has to report them according to Section 3.2.1, it MUST put the 'Unrecognized Parameter' parameter(s) in the INIT-ACK chunk sent in response to the INIT-chunk. Note that if the receiver of the INIT chunk is NOT going to establish an association (e.g., due to lack of resources), then no report would be sent back. If the receiver of an INIT-ACK chunk detects unrecognized parameters and has to report them according to Section 3.2.1, it SHOULD bundle the ERROR chunk containing the 'Unrecognized Parameter' error cause with the COOKIE-ECHO chunk sent in response to the INIT-ACK chunk. If the receiver of the INIT-ACK cannot bundle the COOKIE-ECHO chunk with the ERROR chunk, the ERROR chunk MAY be sent separately but not before the COOKIE-ACK has been received. Note: Any time a COOKIE-ECHO is sent in a packet, it MUST be the first chunk.2.27.3. Solution Description
The procedure of reporting unrecognized parameters has been described clearly.