IP/Network

 

 

 

SCTP

SCTP stands for Stream Control Trasport Protocol. As its name implies, it is a Trasport Protocol like TCP. It is similar to TCP in terms of fundamentals but added some features which would compensate the weak spot of TCP.  Watch videos in Reference Section if you want to learn further details on SCTP protocol in general.

SCTP in 4G/5G Network Architecture

In cellular communication system, SCTP is the protocol being used in the communication between eNB and MME(4G) and the protocol being used between gNB and AMF as illustrated below.

Structure of the protocol stack for these interface is illustrated as below.

If you take packet capture (e.g, Wireshark capture) over the interface, you would get the packet structure as below.

Packet Structure

Following is an example of IP header + SCTP packet.

Chunk Type

Followings are the list of values used as Chunk Type field.

Value

Abbreviation

Description

0

DATA

Payload data

1

INIT

Initiation

2

INIT ACK

Initiation acknowledgement

3

SACK

Selective acknowledgement

4

HEARTBEAT

Heartbeat request

5

HEARTBEAT ACK

Heartbeat acknowledgement

6

ABORT

Abort

7

SHUTDOWN

Shutdown

8

SHUTDOWN ACK

Shutdown acknowledgement

9

ERROR

Operation error

10

COOKIE ECHO

State cookie

11

COOKIE ACK

Cookie acknowledgement

12

ECNE

Explicit congestion notification echo (reserved)

13

CWR

Congestion window reduced (reserved)

14

SHUTDOWN COMPLETE

Shutdown complete

15

AUTH

Authentication chunk

1662

N/A

Reserved by IETF

63

IETF-defined chunk extensions

64

I-DATA

Payload data supporting packet interleaving

65126

N/A

Reserved by IETF

127

IETF-defined chunk extensions

128

ASCONF-ACK

Address configuration change acknowledgement

129

N/A

Unassigned[3]

130

RE-CONFIG

Stream reconfiguration

131

N/A

Unassigned[3]

132

PAD

Packet padding

133190

N/A

Reserved by IETF

191

IETF-defined chunk extensions

192

FORWARD-TSN

Increment expected TSN

193

ASCONF

Address configuration change

194

I-FORWARD-TSN

Increment expected TSN, supporting packet interleaving

195254

N/A

Reserved by IETF

255

IETF-defined chunk extensions

Chunk Flags

Followings are some of the chunk types. Which of these are used varies depending on Chunk Type.

  •   U (Unordered): Indicates that the chunk is to be delivered unordered to the receiving endpoint.
  •   B (Begin): Indicates that the chunk is the first of a multi-part message.
  •   E (End): Indicates that the chunk is the last of a multi-part message.
  •   I (Abort): Indicates that the sender has abandoned the transmission of the current message and is requesting an abort.
  •   T (Forward TSN Supported): Indicates support for the "Forward TSN" feature of SCTP.
  •   P (Padding): Indicates that the chunk contains padding to align the data on a 4-byte boundary.
  •   R (Reserved): Indicates that the flag bit is reserved for future use and must be set to zero.
  •   A (ASCONF Ack): Indicates that the chunk is an acknowledgment for an "Address Configuration" (ASCONF) chunk.

SCTP Establish

Like TCP, SCTP also requires the initial handshake to establish the connection between two end points. As shown below, SCTP goes through 4 step handshake whereas TCP goes through 3 way handshake. Following is SCTP initial setup process captured with Wireshark from Amarisoft Callbox.

(1) INIT

    Internet Protocol Version 4, Src: 10.0.0.185, Dst: 10.0.0.162

        0100 .... = Version: 4

        .... 0101 = Header Length: 20 bytes (5)

        Differentiated Services Field: 0x02 (DSCP: CS0, ECN: ECT(0))

            0000 00.. = Differentiated Services Codepoint: Default (0)

            .... ..10 = Explicit Congestion Notification: ECN-Capable Transport codepoint '10' (2)

        Total Length: 68

        Identification: 0x0000 (0)

        Flags: 0x40, Don't fragment

            0... .... = Reserved bit: Not set

            .1.. .... = Don't fragment: Set

            ..0. .... = More fragments: Not set

        Fragment Offset: 0

        Time to Live: 64

        Protocol: SCTP (132)

        Header Checksum: 0x24da [validation disabled]

        [Header checksum status: Unverified]

        Source Address: 10.0.0.185

        Destination Address: 10.0.0.162

    Stream Control Transmission Protocol, Src Port: 48041 (48041), Dst Port: 38412 (38412)

        Source port: 48041

        Destination port: 38412

        Verification tag: 0x00000000

        [Association index: 65535]

        Checksum: 0x0ffaab98 [unverified]

        [Checksum Status: Unverified]

        INIT chunk (Outbound streams: 8, inbound streams: 65535)

            Chunk type: INIT (1)

                0... .... = Bit: Stop processing of the packet

                .0.. .... = Bit: Do not report

            Chunk flags: 0x00

            Chunk length: 36

            Initiate tag: 0x8d10a6cf

            Advertised receiver window credit (a_rwnd): 106496

            Number of outbound streams: 8

            Number of inbound streams: 65535

            Initial TSN: 3757580128

            Supported address types parameter (Supported types: IPv4)

                Parameter type: Supported address types (0x000c)

                    0... .... .... .... = Bit: Stop processing of chunk

                    .0.. .... .... .... = Bit: Do not report

                Parameter length: 6

                Supported address type: IPv4 address (5)

                Parameter padding: 0000

            ECN parameter

                Parameter type: ECN (0x8000)

                    1... .... .... .... = Bit: Skip parameter and continue processing of the chunk

                    .0.. .... .... .... = Bit: Do not report

                Parameter length: 4

            Forward TSN supported parameter

                Parameter type: Forward TSN supported (0xc000)

                    1... .... .... .... = Bit: Skip parameter and continue processing of the chunk

                    .1.. .... .... .... = Bit: Do report

                Parameter length: 4

(2) INIT_ACK

    Internet Protocol Version 4, Src: 10.0.0.162, Dst: 10.0.0.185

        0100 .... = Version: 4

        .... 0101 = Header Length: 20 bytes (5)

        Differentiated Services Field: 0x02 (DSCP: CS0, ECN: ECT(0))

            0000 00.. = Differentiated Services Codepoint: Default (0)

            .... ..10 = Explicit Congestion Notification: ECN-Capable Transport codepoint '10' (2)

        Total Length: 292

        Identification: 0x0000 (0)

        Flags: 0x40, Don't fragment

            0... .... = Reserved bit: Not set

            .1.. .... = Don't fragment: Set

            ..0. .... = More fragments: Not set

        Fragment Offset: 0

        Time to Live: 64

        Protocol: SCTP (132)

        Header Checksum: 0x23fa [validation disabled]

        [Header checksum status: Unverified]

        Source Address: 10.0.0.162

        Destination Address: 10.0.0.185

    Stream Control Transmission Protocol, Src Port: 38412 (38412), Dst Port: 48041 (48041)

        Source port: 38412

        Destination port: 48041

        Verification tag: 0x8d10a6cf

        [Association index: 65535]

        Checksum: 0x30678470 [unverified]

        [Checksum Status: Unverified]

        INIT_ACK chunk (Outbound streams: 8, inbound streams: 8)

            Chunk type: INIT_ACK (2)

                0... .... = Bit: Stop processing of the packet

                .0.. .... = Bit: Do not report

            Chunk flags: 0x00

            Chunk length: 260

            Initiate tag: 0x790ad620

            Advertised receiver window credit (a_rwnd): 106496

            Number of outbound streams: 8

            Number of inbound streams: 8

            Initial TSN: 4169966867

            State cookie parameter (Cookie length: 228 bytes)

                Parameter type: State cookie (0x0007)

                    0... .... .... .... = Bit: Stop processing of chunk

                    .0.. .... .... .... = Bit: Do not report

                Parameter length: 232

                State cookie: 4124569d7e0548bdbfbc4c54e0af0c95e938d9f500000000000000000000000000000000

            ECN parameter

                Parameter type: ECN (0x8000)

                    1... .... .... .... = Bit: Skip parameter and continue processing of the chunk

                    .0.. .... .... .... = Bit: Do not report

                Parameter length: 4

            Forward TSN supported parameter

                Parameter type: Forward TSN supported (0xc000)

                    1... .... .... .... = Bit: Skip parameter and continue processing of the chunk

                    .1.. .... .... .... = Bit: Do report

                Parameter length: 4

(3) COOKIE_ECHO

    Internet Protocol Version 4, Src: 10.0.0.185, Dst: 10.0.0.162

        0100 .... = Version: 4

        .... 0101 = Header Length: 20 bytes (5)

        Differentiated Services Field: 0x02 (DSCP: CS0, ECN: ECT(0))

            0000 00.. = Differentiated Services Codepoint: Default (0)

            .... ..10 = Explicit Congestion Notification: ECN-Capable Transport codepoint '10' (2)

        Total Length: 264

        Identification: 0x0000 (0)

        Flags: 0x40, Don't fragment

            0... .... = Reserved bit: Not set

            .1.. .... = Don't fragment: Set

            ..0. .... = More fragments: Not set

        Fragment Offset: 0

        Time to Live: 64

        Protocol: SCTP (132)

        Header Checksum: 0x2416 [validation disabled]

        [Header checksum status: Unverified]

        Source Address: 10.0.0.185

        Destination Address: 10.0.0.162

    Stream Control Transmission Protocol, Src Port: 48041 (48041), Dst Port: 38412 (38412)

        Source port: 48041

        Destination port: 38412

        Verification tag: 0x790ad620

        [Association index: 65535]

        Checksum: 0xa00fe0f7 [unverified]

        [Checksum Status: Unverified]

        COOKIE_ECHO chunk (Cookie length: 228 bytes)

            Chunk type: COOKIE_ECHO (10)

                0... .... = Bit: Stop processing of the packet

                .0.. .... = Bit: Do not report

            Chunk flags: 0x00

            Chunk length: 232

            Cookie: 4124569d7e0548bdbfbc4c54e0af0c95e938d9f500000000000000000000000000000000

(4) COOKIE_ACK

    Internet Protocol Version 4, Src: 10.0.0.162, Dst: 10.0.0.185

        0100 .... = Version: 4

        .... 0101 = Header Length: 20 bytes (5)

        Differentiated Services Field: 0x02 (DSCP: CS0, ECN: ECT(0))

            0000 00.. = Differentiated Services Codepoint: Default (0)

            .... ..10 = Explicit Congestion Notification: ECN-Capable Transport codepoint '10' (2)

        Total Length: 36

        Identification: 0x0000 (0)

        Flags: 0x40, Don't fragment

            0... .... = Reserved bit: Not set

            .1.. .... = Don't fragment: Set

            ..0. .... = More fragments: Not set

        Fragment Offset: 0

        Time to Live: 64

        Protocol: SCTP (132)

        Header Checksum: 0x24fa [validation disabled]

        [Header checksum status: Unverified]

        Source Address: 10.0.0.162

        Destination Address: 10.0.0.185

    Stream Control Transmission Protocol, Src Port: 38412 (38412), Dst Port: 48041 (48041)

        Source port: 38412

        Destination port: 48041

        Verification tag: 0x8d10a6cf

        [Association index: 65535]

        Checksum: 0xa018cd9c [unverified]

        [Checksum Status: Unverified]

        COOKIE_ACK chunk

            Chunk type: COOKIE_ACK (11)

                0... .... = Bit: Stop processing of the packet

                .0.. .... = Bit: Do not report

            Chunk flags: 0x00

            Chunk length: 4

Videos

Reference