Additional Considerations for UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets

Introduction specifies the UDP encapsulation of SCTP packets. To be able to adopt automatically to changes of the remote UDP encapsulation port number, it is updated when processing received packets. This includes automatic enabling and disabling of UDP encapsulation. Section 5.4 of describes the processing of received packets and requires the check of the verification tag before updating the remote UDP encapsulation port and the possible enabling or disabling of UDP encapsulation. basically misses a description of the handling of received packets where checking the verification tag is not possible. This includes packets for which no association can be found and packets containing an INIT chunk, since the verification tag of these packets is 0.

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 when, and only when, they appear in all capitals, as shown here.

Handling of Out of the Blue Packets If the processing of an out of the blue packet requires the sending of a packet in response according to the rules specified in Section 8.4 of , the following rules apply:

If the received packet was encapsulated in UDP, the response packets MUST also be encapsulated in UDP. The UDP source port and UDP destination port used for sending the response packet are the UDP destination port and UDP source port of the received packet.
If the received packet was not encapsulated in UDP, the response packet MUST NOT be encapsulated in UDP.

Please note that in these cases a check of the verification tag is not possible.

Handling of SCTP Packets Containing an INIT Chunk Matching an Existing Associations SCTP packets containing an INIT chunk have the verification tag 0 in the common header. Therefore the verification tag can't be checked. The following rules apply when processing the received packet:

The remote UDP encapsulation port for the source address of the received SCTP packet MUST NOT be updated if the encapsulation of outgoing packets is enabled and the received SCTP packet is encapsulated.
The UDP encapsulation for outgoing packets towards the source address of the received SCTP packet MUST NOT be enabled, if it is disabled and the received SCTP packet is encapsulated.
The UDP encapsulation for outgoing packets towards the source address of the received SCTP packet MUST NOT be disabled, if it is enabled and the received SCTP packet is not encapsulated.
If the UDP encapsulation for outgoing packets towards the source address of the received SCTP packet is disabled and the received SCTP packet is encapsulated, an SCTP packet containing an ABORT chunk MUST be sent. The ABORT chunk MAY include the error cause defined below indicating an "Restart of an Association with New Encapsulation Port". This packet containing the ABORT chunk MUST be encapsulated in UDP. The UDP source port and UDP destination port used for sending the packet containing the ABORT chunk are the UDP destination port and UDP source port of the received packet containing the INIT chunk.
If the UDP encapsulation for outgoing packets towards the source address of the received SCTP packet is disabled and the received SCTP packet is not encapsulated, the processing defined in MUST be performed. If a packet is sent in response, it MUST NOT be encapsulated.
If the UDP encapsulation for outgoing packets towards the source address of the received SCTP packet is enabled and the received SCTP packet is not encapsulated, an SCTP packet containing an ABORT chunk MUST be sent. The ABORT chunk MAY include the error cause defined below indicating an "Restart of an Association with New Encapsulation Port". This packet containing the ABORT chunk MUST NOT be encapsulated in UDP.
If the UDP encapsulation for outgoing packets towards the source address of the received SCTP packet is enabled and the received SCTP packet is encapsulated, but the UDP source port of the received SCTP packet is not equal to the remote UDP encapsulation port for the source address of the received SCTP packet, an SCTP packet containing an ABORT chunk MUST be sent. The ABORT chunk MAY include the error cause defined below indicating an "Restart of an Association with New Encapsulation Port". This packet containing the ABORT chunk MUST be encapsulated in UDP. The UDP source port and UDP destination port used for sending the packet containing the ABORT chunk are the UDP destination port and UDP source port of the received packet containing the INIT chunk.
If the UDP encapsulation for outgoing packets towards the source address of the received SCTP packet is enabled and the received SCTP packet is encapsulated and the UDP source port of the received SCTP packet is equal to the remote UDP encapsulation port for the source address of the received SCTP packet, the processing defined in MUST be performed. If a packet is sent in response, it MUST be encapsulated. The UDP source port and UDP destination port used for sending the packet containing the ABORT chunk are the UDP destination port and UDP source port of the received packet containing the INIT chunk.

The error cause indicating an "Restart of an Association with New Encapsulation Port" is defined by the following figure.

Restart of an Association with New Encapsulation Port error cause 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code = 14 | Cause Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Current Encapsulation Port | New Encapsulation Port | +-------------------------------+-------------------------------+

Cause Code: 2 bytes (unsigned integer): This field holds the IANA defined cause code for the "Restart of an Association with New Encapsulation Port" error cause. IANA is requested to assign the value 14 for this cause code.
Cause Length: 2 bytes (unsigned integer): This field holds the length in bytes of the error cause; the value MUST be 8.
Current Encapsulation Port: 2 bytes (unsigned integer): This field holds the remote encapsulation port currently being used for the destination address the received packet containing the INIT chunk was sent from. If the UDP encapsulation for destination address is currently disabled, 0 is used.
New Encapsulation Port: 2 bytes (unsigned integer): If the received SCTP packet containing the INIT chunk is encapsulated in UDP, this field holds the UDP source port number of the UDP packet. If the received SCTP packet is not encapsulated in UDP, this field is 0.

All transported integer numbers are in "network byte order" a.k.a., Big Endian.

Middlebox Considerations Middleboxes often use different timeouts for UDP based flows than for other flows. Therefore the HEARTBEAT.Interval parameter SHOULD be lowered to 15 seconds when UDP encapsulation is used.

IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document.] [NOTE to RFC-Editor: The requested values for the cause code are tentative and to be confirmed by IANA.] This document (RFCXXXX) is the reference for the registration described in this section. A new error cause code has to be assigned by IANA. This requires an additional line in the "Error Cause Codes" registry for SCTP: New entry in Error Cause Codes registry

Value	Cause Code	Reference
14	Restart of an Association with New Encapsulation Port	[RFCXXXX]

Security Considerations This document does not change the considerations given in . However, not following the procedures given in this document might allow an attacker to take over SCTP associations. The attacker needs only to share the IP address of an existing SCTP association. It should also be noted that if firewalls will be applied at the SCTP association level they have to take the UDP encapsulation into account.

Acknowledgments The authors wish to thank for the initial problem report. The authors wish to thank and for their invaluable comments. This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 644334 (NEAT). The views expressed are solely those of the author(s).