Re: Problem on SCTP

[Michael Tuexen]

Your router does NOT change any field in the SCTP packet, but the 
SCTP checksum was modified from
   Checksum: 0xbaea49e5 (not verified)
to
   Checksum: 0xa9a86d3f (not verified)
At least one of these is wrong. Read the tracefiles in wireshark and
enable checksum validation and wireshark will tell you which one is
correct. (That is why I asked for .pcap file instead of a .txt).

My guess is that the initial checksum is correct and your box middlebox
not only changes the destination address and ttl field and header
checksum in the IP-header (which is expected) but also incorrectly the
SCTP checksum. Since no field in the SCTP packet has changed, the checksum
must be the same.

Best regards
Michael
> On 13 Jan 2017, at 04:29, Sun Paul <paulrbk@xxxxxxxxx> wrote:
> 
> Frame 2: 98 bytes on wire (784 bits), 98 bytes captured (784 bits)
>    Encapsulation type: Ethernet (1)
>    Arrival Time: Jan  6, 2017 16:52:49.662321000 Malay Peninsula Standard Time
>    [Time shift for this packet: 0.000000000 seconds]
>    Epoch Time: 1483692769.662321000 seconds
>    [Time delta from previous captured frame: 0.000179000 seconds]
>    [Time delta from previous displayed frame: 0.000179000 seconds]
>    [Time since reference or first frame: 0.000179000 seconds]
>    Frame Number: 2
>    Frame Length: 98 bytes (784 bits)
>    Capture Length: 98 bytes (784 bits)
>    [Frame is marked: False]
>    [Frame is ignored: False]
>    [Protocols in frame: eth:ethertype:ip:sctp]
> Ethernet II, Src: Vmware_81:41:6b (00:50:56:81:41:6b), Dst:
> Vmware_81:a6:a3 (00:50:56:81:a6:a3)
>    Destination: Vmware_81:a6:a3 (00:50:56:81:a6:a3)
>        Address: Vmware_81:a6:a3 (00:50:56:81:a6:a3)
>        .... ..0. .... .... .... .... = LG bit: Globally unique
> address (factory default)
>        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
>    Source: Vmware_81:41:6b (00:50:56:81:41:6b)
>        Address: Vmware_81:41:6b (00:50:56:81:41:6b)
>        .... ..0. .... .... .... .... = LG bit: Globally unique
> address (factory default)
>        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
>    Type: IPv4 (0x0800)
> Internet Protocol Version 4, Src: 192.168.206.83, Dst: 192.168.206.66
>    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: 84
>    Identification: 0x0000 (0)
>    Flags: 0x02 (Don't Fragment)
>        0... .... = Reserved bit: Not set
>        .1.. .... = Don't fragment: Set
>        ..0. .... = More fragments: Not set
>    Fragment offset: 0
>    Time to live: 63
>    Protocol: SCTP (132)
>    Header checksum: 0x1d3d [validation disabled]
>        [Good: False]
>        [Bad: False]
>    Source: 192.168.206.83
>    Destination: 192.168.206.66
>    [Source GeoIP: Unknown]
>    [Destination GeoIP: Unknown]
> Stream Control Transmission Protocol, Src Port: 50001 (50001), Dst
> Port: 3868 (3868)
>    Source port: 50001
>    Destination port: 3868
>    Verification tag: 0x00000000
>    [Assocation index: 0]
>    Checksum: 0xa9a86d3f (not verified)
> 
>    INIT chunk (Outbound streams: 3000, inbound streams: 3000)
>        Chunk type: INIT (1)
>            0... .... = Bit: Stop processing of the packet
>            .0.. .... = Bit: Do not report
>        Chunk flags: 0x00
>        Chunk length: 52
>        Initiate tag: 0xe79f40cb
>        Advertised receiver window credit (a_rwnd): 62464
>        Number of outbound streams: 3000
>        Number of inbound streams: 3000
>        Initial TSN: 176990880
>        IPv4 address parameter (Address: 192.168.206.83)
>            Parameter type: IPv4 address (0x0005)
>                0... .... .... .... = Bit: Stop processing of chunk
>                .0.. .... .... .... = Bit: Do not report
>            Parameter length: 8
>            IP Version 4 address: 192.168.206.83
>        IPv4 address parameter (Address: 192.168.1.83)
>            Parameter type: IPv4 address (0x0005)
>                0... .... .... .... = Bit: Stop processing of chunk
>                .0.. .... .... .... = Bit: Do not report
>            Parameter length: 8
>            IP Version 4 address: 192.168.1.83
>        Supported address types parameter (Supported types: IPv6, IPv4)
>            Parameter type: Supported address types (0x000c)
>                0... .... .... .... = Bit: Stop processing of chunk
>                .0.. .... .... .... = Bit: Do not report
>            Parameter length: 8
>            Supported address type: IPv6 address (6)
>            Supported address type: IPv4 address (5)
>        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