SCTP Padding Chunk: Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)
Two empty filler containers that let SCTP senders make a packet or an INIT chunk any size they need.
Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007, <https://www.rfc-editor.org/info/rfc4820>.
@misc{rfc4820,
series = {Request for Comments},
number = 4820,
howpublished = {RFC 4820},
publisher = {RFC Editor},
doi = {10.17487/RFC4820},
url = {https://www.rfc-editor.org/info/rfc4820},
author = {M. Tuexen and R. Stewart and P. Lei},
title = {{Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)}},
pagetotal = 6,
year = 2007,
month = mar,
} RFC text
Contents
Network Working Group M. Tuexen
Request for Comments: 4820 Muenster Univ. of Applied Sciences
Category: Standards Track R. Stewart
P. Lei
Cisco Systems, Inc.
March 2007
Padding Chunk and Parameter
for the Stream Control Transmission Protocol (SCTP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document defines a padding chunk and a padding parameter and
describes the required receiver side procedures. The padding chunk
is used to pad a Stream Control Transmission Protocol (SCTP) packet
to an arbitrary size. The padding parameter is used to pad an SCTP
INIT chunk to an arbitrary size.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Padding Chunk (PAD) . . . . . . . . . . . . . . . . . . . . . . 2
4. Padding Parameter (PAD) . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5.1. A New Chunk Type . . . . . . . . . . . . . . . . . . . . . 4
5.2. A New Parameter Type . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . . 5
1. Introduction
This document defines a padding chunk and a padding parameter and
describes the required receiver side procedures. The padding chunk
is used to pad an SCTP packet to an arbitrary size. The padding
parameter is used to pad an SCTP INIT chunk to an arbitrary size.
The usage of the PAD chunk for path MTU discovery is described in
PMTU [4]. The inappropriate usage of the PAD parameter or PAD chunk
can result in wasted bandwidth.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL", when they appear in this document, are to be interpreted
as described in RFC 2119 [1].
3. Padding Chunk (PAD)
This chunk is used to pad an SCTP packet. A PAD chunk can be used to
enlarge the packet by 4 to 65536 bytes in steps of 4 bytes. An SCTP
packet MAY contain multiple PAD chunks.
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x84 | Flags=0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ Padding Data /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1
Type: 1 byte (unsigned integer)
This value MUST be set to 0x84 for all PAD chunks.
Flags: 1 byte (unsigned integer)
This value SHOULD be set to zero on transmit and MUST be ignored
on receipt.
Length: 2 bytes (unsigned integer)
This value holds the length of the Padding Data plus 4.
Padding Data: n bytes (unsigned integer)
This holds the Padding Data. The Padding Data MUST be ignored by
the receiver.
The receiver of the PAD chunk MUST discard this chunk and continue
processing the rest of the chunks in the packet. Please note that
this is also the required processing behavior for any unknown chunk
having the same highest-order two bits of the type as the PAD chunk.
4. Padding Parameter (PAD)
This parameter is used to pad an INIT chunk. A PAD parameter can be
used to enlarge the INIT chunk by 4 bytes as the minimum to the
maximum size of the INIT chunk in steps of 4 bytes. An INIT chunk
MAY contain multiple PAD parameters.
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0x8005 | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Padding Data \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
Parameter Type: 2 bytes (unsigned integer)
This value MUST be set to 0x8005.
Parameter Length: 2 bytes (unsigned integer)
This value holds the length of the Padding Data plus 4.
The PAD parameter MAY be included only in the INIT chunk. It MUST
NOT be included in any other chunk. The receiver of the PAD
parameter MUST silently discard this parameter and continue
processing the rest of the INIT chunk. This means that the size of
the generated COOKIE parameter in the INIT-ACK MUST NOT depend on the
existence of the PAD parameter in the INIT chunk. A receiver of a
PAD parameter MUST NOT include the PAD parameter within any State
Cookie parameter it generates.
5. IANA Considerations
This document is the reference for all registrations described in
this section. All registrations have been listed in the document
available at sctp-parameters [3]. The changes are described below.
5.1. A New Chunk Type
A chunk type for the PAD chunk has been assigned by IANA. The value
has been assigned as described in Figure 1. The following has been
added to the "CHUNK TYPES" table of sctp-parameters [3]:
CHUNK TYPES
ID Value Chunk Type Reference
-------- ---------- ---------
132(0x84) Padding Chunk (PAD) [RFC4820]
5.2. A New Parameter Type
A parameter type has been assigned for the PAD parameter by IANA.
The value has been assigned as described in Figure 2. The following
has been added to the "CHUNK PARAMETER TYPES" table in sctp-
parameters [3]:
INIT Chunk Parameter Types
Chunk Parameter Type Value
-------------------- -----
Padding 32773(0x8005)
6. Security Considerations
This document does not add any additional security considerations to
the ones given in RFC 2960 [2].
7. Acknowledgments
The authors wish to thank Matthew J. Zekauskas and Lars Eggert for
their invaluable comments.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
8.2. Informative References
[3] "IANA registry",
<http://www.iana.org/assignments/sctp-parameters>.
[4] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
Authors' Addresses
Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
EMail: tuexen@fh-muenster.de
Randall R. Stewart
Cisco Systems, Inc.
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
EMail: rrs@cisco.com
Peter Lei
Cisco Systems, Inc.
955 Happfield Dr.
Arlington Heights, IL 60004
US
Phone: +1 773 695-8201
EMail: peterlei@cisco.com
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AI ELI5
On this page
§1Why Padding Is Needed
RFC 4820 defines a padding chunk and a padding parameter for the Stream Control Transmission Protocol (SCTP), together with the receiver-side procedures. The chunk pads an SCTP packet to an arbitrary size. The parameter pads an INIT chunk to an arbitrary size.
The RFC points to RFC 4821 for the use of the PAD chunk in path MTU discovery. It also warns that inappropriate use of the PAD parameter or PAD chunk can waste bandwidth.
Sometimes a sender wants its message to be a certain size, like testing how big a parcel the post office will carry. Padding is the empty filler that makes the parcel the right size.
§3The PAD Chunk
A PAD chunk can enlarge a packet by 4 to 65536 bytes, in steps of 4 bytes. A packet MAY contain several PAD chunks.
The chunk has a Type of 0x84 (1 byte), Flags (1 byte), a Length (2 bytes) and then the Padding Data. Flags SHOULD be zero on transmit and MUST be ignored on receipt. Length holds the length of the Padding Data plus 4.
The PAD chunk is a labeled bag of nothing. The label says how big the bag is, and you can stuff in as much empty space as you need.
§3.1What Receivers Do With a PAD Chunk
The Padding Data MUST be ignored by the receiver. The receiver of a PAD chunk MUST discard the chunk and continue processing the rest of the chunks in the packet.
The RFC notes that this is also the required behavior for any unknown chunk whose type has the same highest-order two bits as the PAD chunk. So a receiver that has never heard of PAD already handles it correctly.
When the receiver opens the bag of nothing, it throws the bag away and goes on reading the rest of the mail.
§4The PAD Parameter
The PAD parameter pads an INIT chunk. It can enlarge the INIT chunk by 4 bytes at minimum, up to the maximum size of the INIT chunk, in steps of 4 bytes. An INIT chunk MAY contain multiple PAD parameters.
The parameter has a 2-byte Parameter Type of 0x8005, a 2-byte Parameter Length (the Padding Data length plus 4), and the Padding Data. It MAY be included only in the INIT chunk and MUST NOT be included in any other chunk.
The receiver MUST silently discard it and continue with the rest of the INIT chunk. As a result, the size of the COOKIE parameter generated in the INIT-ACK MUST NOT depend on whether a PAD parameter was present, and the receiver MUST NOT include the PAD parameter in any State Cookie parameter it generates.
The PAD parameter is the same kind of filler, but it only goes inside the very first hello message. The receiver ignores it and does not copy it into the reply it hands back.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
PTYPE | Parameter Type | 2 bytes | 0x8005 | Set to 0x8005. |
PLENGTH | Parameter Length | 2 bytes | Length of the Padding Data plus 4. |
§5IANA Registrations
IANA assigned the chunk type value 132 (0x84) to the Padding Chunk (PAD) in the CHUNK TYPES table. It assigned the parameter type value 32773 (0x8005) to Padding in the INIT Chunk Parameter Types table. Both registrations reference RFC 4820.
The official number list now says that 0x84 means PAD chunk and 0x8005 means PAD parameter, so everyone uses the same numbers.
§6Security Considerations
The RFC states that it does not add any additional security considerations to the ones given in RFC 2960. The only risk it names elsewhere is wasted bandwidth when padding is used inappropriately.
Adding empty filler does not open any new doors for trouble. It can only waste space if someone overdoes it.