SCION Extension Header Specification
This document contains the specification of the SCION extension headers. There are two extension headers defined, the Hop-by-Hop (HBH) Options Header and the End-to-End (E2E) Options Header. A SCION packet can have at most one of each. If both headers are present, the HBH options MUST come before the E2E options. The option header supports a variable number of type-length-value (TLV) encoded options.
Hop-by-Hop Options Header
The Hop-by-Hop Options header is used to carry optional information that may be
examined and processed by every node along a packet’s delivery path. The
Hop-by-Hop Options header is identified by a Next Header value of 200 in the
SCION common header and has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NextHdr | ExtLen | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- NextHdr
Unsigned 8-bit integer. Identifies the type of header immediately following the Hop-by-Hop Options header. Values of this field respect the Assigned SCION Protocol Numbers.
- ExtLen
Unsigned 8-bit integer. The value of this field is computed as the length of the complete Hop-by-Hop Options header in multiples of 4-bytes minus 1.
- Options
Variable-length field, of length such that the complete Hop-by-Hop Options header is an integer multiple of 4 bytes long. Contains one or more TLV-encoded options, as described below.
The Hop-by-Hop Options header is aligned to 4 bytes.
Assigned Option Types
The following option types are assigned for Hop-by-Hop options:
Decimal |
Option |
|---|---|
0 |
|
1 |
|
253 |
use for experimentation and testing |
254 |
use for experimentation and testing |
255 |
reserved |
End-to-End Options Header
The End-to-End Options header is used to carry optional information that may be
examined and processed by sender and/or receiver of the packet. The End-to-End
Options header is identified by a Next Header value of 201 in the SCION
common header and has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NextHdr | ExtLen | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- NextHdr
Unsigned 8-bit integer. Identifies the type of header immediately following the End-to-End Options header. Values of this field respect the Assigned SCION Protocol Numbers.
- ExtLen
Unsigned 8-bit integer. The value of this field is computed as the length of the complete End-to-End Options header in multiples of 4-bytes minus 1.
- Options
Variable-length field, of length such that the complete End-to-End Options header is an integer multiple of 4 bytes long. Contains one or more TLV-encoded options, as described below.
The End-to-End Options header is aligned to 4 bytes.
Assigned Option Types
The following option types are assigned for End-to-End options:
Decimal |
Option |
|---|---|
0 |
|
1 |
|
2 |
|
253 |
use for experimentation and testing |
254 |
use for experimentation and testing |
255 |
reserved |
TLV-encoded Options
The hbh and e2e options headers carry a variable number of options that are type-length-value (TLV) encoded in the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptType | OptDataLen | OptData |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- OptType
8-bit identifier of the type of option.
- OptDataLen
Unsigned 8-bit integer denoting the length of the
OptDatafield of this option in bytes.- OptData
Variable-length field. Option-Type specific data.
The sequence of options within a header must be processed strictly in the order they appear in the header; a receiver must not, for example, scan through the header looking for a particular kind of option and process that option prior to processing all preceding ones.
Individual options may have specific alignment requirements, to ensure that
multi-byte values within OptData fields fall on natural boundaries. The
alignment requirement of an option is specified using the notation xn+y, meaning
the OptType must appear at an integer multiple of x bytes from the start of
the header, plus y bytes. For example:
2n means any 2-bytes offset from the start of the header.
4n+2 means any 4-bytes offset from the start of the header, plus 2
bytes.
There are two padding options that are used when necessary to align subsequent options and to pad out the containing header to a multiple of 4 bytes in length. These padding options must be recognized by all SCION implementations:
Pad1 Option
Alignment requirement: none:
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
+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
Note
The format of the Pad1 option is a special case – it does not have length and value fields.
The Pad1 option is used to insert 1 byte of padding into the Options area of a header. If more than one byte of padding is required, the PadN option, described next, should be used, rather than multiple Pad1 options.
PadN Option
Alignment requirement: none:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | OptDataLen | OptData |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PadN option is used to insert two or more bytes of padding into the Options
area of a header. For N bytes of padding, the OptDataLen field contains the
value N-2, and the OptData consists of N-2 zero-valued bytes.