SCTP

class os_ken.lib.packet.sctp.cause_cookie_while_shutdown(length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Cookie Received While Shutting Down (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_invalid_param(length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Invalid Mandatory Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_invalid_stream_id(value=0, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Invalid Stream Identifier (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	stream id.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_missing_param(types=None, num=0, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Missing Mandatory Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
types	a list of missing params.
num	Number of missing params. (0 means automatically-calculate when encoding)
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_no_userdata(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for No User Data (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	the TSN of the DATA chunk received with no user data field.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_out_of_resource(length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Out of Resource (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_protocol_violation(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Protocol Violation (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Additional Information.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_restart_with_new_addr(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Restart of an Association with New Addresses (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	New Address TLVs.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_stale_cookie(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Stale Cookie Error (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Measure of Staleness.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_unrecognized_chunk(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Unrecognized Chunk Type (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Unrecognized Chunk.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_unrecognized_param(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Unrecognized Parameters (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Unrecognized Parameter.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_unresolvable_addr(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Unresolvable Address (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Unresolvable Address. one of follows: os_ken.lib.packet.sctp.param_host_addr, os_ken.lib.packet.sctp.param_ipv4, or os_ken.lib.packet.sctp.param_ipv6.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.cause_user_initiated_abort(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for User-Initiated Abort (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_abort

• os_ken.lib.packet.sctp.chunk_error

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Upper Layer Abort Reason.
length	length of this cause containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.chunk_abort(tflag=0, length=0, causes=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Abort Association (ABORT) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
tflag	'0' means the Verification tag is normal. '1' means the Verification tag is copy of the sender.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
causes	a list of derived classes of os_ken.lib.packet.sctp.causes.

class os_ken.lib.packet.sctp.chunk_cookie_ack(flags=0, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Cookie Acknowledgement (COOKIE ACK) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.chunk_cookie_echo(flags=0, length=0, cookie=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Cookie Echo (COOKIE ECHO) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
cookie	cookie data.

class os_ken.lib.packet.sctp.chunk_cwr(flags=0, length=0, low_tsn=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for CWR chunk (RFC 4960 Appendix A.).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
low_tsn	the lowest TSN.

class os_ken.lib.packet.sctp.chunk_data(unordered=0, begin=0, end=0, length=0, tsn=0, sid=0, seq=0, payload_id=0, payload_data=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Payload Data (DATA) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
unordered	if set to '1', the receiver ignores the sequence number.
begin	if set to '1', this chunk is the first fragment.
end	if set to '1', this chunk is the last fragment.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
tsn	Transmission Sequence Number.
sid	stream id.
seq	the sequence number.
payload_id	application specified protocol id. '0' means that no application id is identified.
payload_data	user data.

class os_ken.lib.packet.sctp.chunk_ecn_echo(flags=0, length=0, low_tsn=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for ECN-Echo chunk (RFC 4960 Appendix A.).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
low_tsn	the lowest TSN.

class os_ken.lib.packet.sctp.chunk_error(flags=0, length=0, causes=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Operation Error (ERROR) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
causes	a list of derived classes of os_ken.lib.packet.sctp.causes.

class os_ken.lib.packet.sctp.chunk_heartbeat(flags=0, length=0, info=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Heartbeat Request (HEARTBEAT) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
info	os_ken.lib.packet.sctp.param_heartbeat.

class os_ken.lib.packet.sctp.chunk_heartbeat_ack(flags=0, length=0, info=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Heartbeat Acknowledgement (HEARTBEAT ACK) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
info	os_ken.lib.packet.sctp.param_heartbeat.

class os_ken.lib.packet.sctp.chunk_init(flags=0, length=0, init_tag=0, a_rwnd=0, os=0, mis=0, i_tsn=0, params=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Initiation (INIT) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
init_tag	the tag that be used as Verification Tag.
a_rwnd	Advertised Receiver Window Credit.
os	number of outbound streams.
mis	number of inbound streams.
i_tsn	Transmission Sequence Number that the sender will use.
params	Optional/Variable-Length Parameters. a list of derived classes of os_ken.lib.packet.sctp.param.

class os_ken.lib.packet.sctp.chunk_init_ack(flags=0, length=0, init_tag=0, a_rwnd=0, os=0, mis=0, i_tsn=0, params=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Initiation Acknowledgement (INIT ACK) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
init_tag	the tag that be used as Verification Tag.
a_rwnd	Advertised Receiver Window Credit.
os	number of outbound streams.
mis	number of inbound streams.
i_tsn	Transmission Sequence Number that the sender will use.
params	Optional/Variable-Length Parameters. a list of derived classes of os_ken.lib.packet.sctp.param.

class os_ken.lib.packet.sctp.chunk_sack(flags=0, length=0, tsn_ack=0, a_rwnd=0, gapack_num=0, duptsn_num=0, gapacks=None, duptsns=None)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Selective Acknowledgement (SACK) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
tsn_ack	TSN of the last DATA chunk received in sequence before a gap.
a_rwnd	Advertised Receiver Window Credit.
gapack_num	number of Gap Ack blocks.
duptsn_num	number of duplicate TSNs.
gapacks	a list of Gap Ack blocks. one block is made of a list with the start offset and the end offset from tsn_ack. e.g.) gapacks = [[2, 3], [10, 12], [19, 21]]
duptsns	a list of duplicate TSN.

class os_ken.lib.packet.sctp.chunk_shutdown(flags=0, length=0, tsn_ack=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Shutdown Association (SHUTDOWN) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)
tsn_ack	TSN of the last DATA chunk received in sequence before a gap.

class os_ken.lib.packet.sctp.chunk_shutdown_ack(flags=0, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Shutdown Acknowledgement (SHUTDOWN ACK) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
flags	set to '0'. this field will be ignored.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.chunk_shutdown_complete(tflag=0, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Shutdown Complete (SHUTDOWN COMPLETE) chunk (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.sctp

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
tflag	'0' means the Verification tag is normal. '1' means the Verification tag is copy of the sender.
length	length of this chunk containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_cookie_preserve(value=0, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Cookie Preservative Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	Suggested Cookie Life-Span Increment (msec).
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_ecn(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for ECN Parameter (RFC 4960 Appendix A.).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init

• os_ken.lib.packet.sctp.chunk_init_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	set to None.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_heartbeat(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Heartbeat Info Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_heartbeat

• os_ken.lib.packet.sctp.chunk_heartbeat_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	the sender-specific heartbeat information.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_host_addr(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Host Name Address Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init

• os_ken.lib.packet.sctp.chunk_init_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	a host name that ends with null terminator.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_ipv4(value='127.0.0.1', length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for IPv4 Address Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init

• os_ken.lib.packet.sctp.chunk_init_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	IPv4 address of the sending endpoint.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_ipv6(value='::1', length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for IPv6 Address Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init

• os_ken.lib.packet.sctp.chunk_init_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	IPv6 address of the sending endpoint.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_state_cookie(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for State Cookie Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	the state cookie. see Section 5.1.3 in RFC 4960.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_supported_addr(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Supported Address Types Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	a list of parameter types. odd cases pad with 0x0000.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.param_unrecognized_param(value=None, length=0)

Stream Control Transmission Protocol (SCTP) sub encoder/decoder class for Unrecognized Parameter (RFC 4960).

This class is used with the following.

• os_ken.lib.packet.sctp.chunk_init_ack

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
value	the unrecognized parameter in the INIT chunk.
length	length of this param containing this header. (0 means automatically-calculate when encoding)

class os_ken.lib.packet.sctp.sctp(src_port=1, dst_port=1, vtag=0, csum=0, chunks=None)

Stream Control Transmission Protocol (SCTP) header encoder/decoder class (RFC 4960).

An instance has the following attributes at least. Most of them are same to the on-wire counterparts but in host byte order. __init__ takes the corresponding args in this order.

Attribute	Description
src_port	Source Port
dst_port	Destination Port
vtag	Verification Tag
csum	Checksum (0 means automatically-calculate when encoding)
chunks	a list of derived classes of os_ken.lib.packet.sctp.chunk.

classmethod parser(buf)

Decode a protocol header.

This method is used only when decoding a packet.

Decode a protocol header at offset 0 in bytearray buf. Returns the following three objects.

• An object to describe the decoded header.

• A packet_base.PacketBase subclass appropriate for the rest of the packet. None when the rest of the packet should be considered as raw payload.

• The rest of packet.

serialize(payload, prev)

Encode a protocol header.

This method is used only when encoding a packet.

Encode a protocol header. Returns a bytearray which contains the header.

payload is the rest of the packet which will immediately follow this header.

prev is a packet_base.PacketBase subclass for the outer protocol header. prev is None if the current header is the outer-most. For example, prev is ipv4 or ipv6 for tcp.serialize.