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Parse & encode the IPv6 fragment header in network byte order #149

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104 changes: 56 additions & 48 deletions etherparse/src/net/ipv6_fragment_header.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ pub struct Ipv6FragmentHeader {
pub fragment_offset: IpFragOffset,
/// True if more fragment packets will follow. False if this is the last packet.
pub more_fragments: bool,
/// Identifcation value generated by the source.
/// Identification value generated by the source.
pub identification: u32,
}

Expand Down Expand Up @@ -57,19 +57,9 @@ impl Ipv6FragmentHeader {
buffer
};

Ok(Ipv6FragmentHeader {
next_header: IpNumber(buffer[0]),
fragment_offset: unsafe {
// SAFE as the resulting number is guaranteed to have at most
// 13 bits.
IpFragOffset::new_unchecked(u16::from_be_bytes([
(buffer[2] >> 3) & 0b0001_1111u8,
((buffer[2] << 5) & 0b1110_0000u8) | (buffer[3] & 0b0001_1111u8),
]))
},
more_fragments: 0 != buffer[3] & 0b1000_0000u8,
identification: u32::from_be_bytes([buffer[4], buffer[5], buffer[6], buffer[7]]),
})
// SAFETY: `buffer` is exactly 8 bytes long, satisfying the length
// requirement of `from_slice_unchecked`.
Ok(unsafe { Ipv6FragmentHeaderSlice::from_slice_unchecked(&buffer) }.to_header())
}

/// Read an fragment header from the current reader position.
Expand All @@ -89,19 +79,9 @@ impl Ipv6FragmentHeader {
buffer
};

Ok(Ipv6FragmentHeader {
next_header: IpNumber(buffer[0]),
fragment_offset: unsafe {
// SAFE as the resulting number is guaranteed to have at most
// 13 bits.
IpFragOffset::new_unchecked(u16::from_be_bytes([
(buffer[2] >> 3) & 0b0001_1111u8,
((buffer[2] << 5) & 0b1110_0000u8) | (buffer[3] & 0b0001_1111u8),
]))
},
more_fragments: 0 != buffer[3] & 0b1000_0000u8,
identification: u32::from_be_bytes([buffer[4], buffer[5], buffer[6], buffer[7]]),
})
// SAFETY: `buffer` is exactly 8 bytes long, satisfying the length
// requirement of `from_slice_unchecked`.
Ok(unsafe { Ipv6FragmentHeaderSlice::from_slice_unchecked(&buffer) }.to_header())
}

/// Writes a given IPv6 fragment header to the current position.
Expand Down Expand Up @@ -168,18 +148,17 @@ impl Ipv6FragmentHeader {
/// sized byte array.
#[inline]
pub fn to_bytes(&self) -> [u8; 8] {
let fo_be: [u8; 2] = self.fragment_offset.value().to_be_bytes();
// fragment offset is the upper 13 bits, the more fragments flag is
// the least significant bit (bits 2..1 are reserved & left zero).
let fo_be: [u8; 2] = ((self.fragment_offset.value() << 3)
| if self.more_fragments { 1 } else { 0 })
.to_be_bytes();
let id_be = self.identification.to_be_bytes();
[
self.next_header.0,
0,
(((fo_be[0] << 3) & 0b1111_1000u8) | ((fo_be[1] >> 5) & 0b0000_0111u8)),
((fo_be[1] & 0b0001_1111u8)
| if self.more_fragments {
0b1000_0000u8
} else {
0
}),
fo_be[0],
fo_be[1],
id_be[0],
id_be[1],
id_be[2],
Expand Down Expand Up @@ -397,25 +376,18 @@ mod test {

// normal write
{
let fragment_offset_be = input.fragment_offset.value().to_be_bytes();
let fragment_offset_be = (
(input.fragment_offset.value() << 3) |
if input.more_fragments { 1u16 } else { 0u16 }
).to_be_bytes();
let id_be = input.identification.to_be_bytes();
assert_eq!(
&input.to_bytes(),
&[
input.next_header.0,
0,
(
(fragment_offset_be[0] << 3 & 0b1111_1000u8) |
(fragment_offset_be[1] >> 5 & 0b0000_0111u8)
),
(
(fragment_offset_be[1] & 0b0001_1111u8) |
if input.more_fragments {
0b1000_0000u8
} else {
0u8
}
),
fragment_offset_be[0],
fragment_offset_be[1],
id_be[0],
id_be[1],
id_be[2],
Expand All @@ -425,4 +397,40 @@ mod test {
}
}
}

/// Verify the exact RFC 8200 §4.5 wire layout: the 13 bit fragment offset
/// occupies the most significant bits of bytes 2..3 and the "more
/// fragments" flag is the least significant bit of byte 3.
#[test]
fn network_byte_order() {
use crate::ip_number::UDP;

// max offset (8191, all 13 bits set) with the more fragments flag
// set. The offset fills bytes 2..3 bits 15..3 and M is bit 0 (bits
// 2..1 are reserved & zero).
{
let header = Ipv6FragmentHeader::new(UDP, IpFragOffset::try_new(8191).unwrap(), true, 0);
let bytes = [UDP.0, 0, 0b1111_1111, 0b1111_1001, 0, 0, 0, 0];
assert_eq!(header.to_bytes(), bytes);
assert_eq!(Ipv6FragmentHeader::from_slice(&bytes).unwrap().0, header);
}

// max offset (8191, all 13 bits set) without the more fragments flag.
{
let header =
Ipv6FragmentHeader::new(UDP, IpFragOffset::try_new(8191).unwrap(), false, 0);
let bytes = [UDP.0, 0, 0b1111_1111, 0b1111_1000, 0, 0, 0, 0];
assert_eq!(header.to_bytes(), bytes);
assert_eq!(Ipv6FragmentHeader::from_slice(&bytes).unwrap().0, header);
}

// offset 1 with more fragments -> low byte holds the offset's least
// significant bit (bit 3) and M (bit 0).
{
let header = Ipv6FragmentHeader::new(UDP, IpFragOffset::try_new(1).unwrap(), true, 0);
let bytes = [UDP.0, 0, 0b0000_0000, 0b0000_1001, 0, 0, 0, 0];
assert_eq!(header.to_bytes(), bytes);
assert_eq!(Ipv6FragmentHeader::from_slice(&bytes).unwrap().0, header);
}
}
}
24 changes: 10 additions & 14 deletions etherparse/src/net/ipv6_fragment_header_slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,14 +66,13 @@ impl<'a> Ipv6FragmentHeaderSlice<'a> {
pub fn fragment_offset(&self) -> IpFragOffset {
unsafe {
// SAFETY: Safe as the resulting number is guaranteed to be only
// 13 bit long.
IpFragOffset::new_unchecked(u16::from_be_bytes([
// 13 bit long (the lower 3 bits are shifted away).
IpFragOffset::new_unchecked(
// SAFETY:
// Slice size checked to be at least 8 bytes in constructor.
(*self.slice.get_unchecked(2) >> 3) & 0b0001_1111u8,
((*self.slice.get_unchecked(2) << 5) & 0b1110_0000u8)
| (*self.slice.get_unchecked(3) & 0b0001_1111u8),
]))
u16::from_be_bytes([*self.slice.get_unchecked(2), *self.slice.get_unchecked(3)])
>> 3,
)
}
}

Expand All @@ -82,7 +81,7 @@ impl<'a> Ipv6FragmentHeaderSlice<'a> {
pub fn more_fragments(&self) -> bool {
// SAFETY:
// Slice size checked to be at least 8 bytes in constructor.
unsafe { 0 != *self.slice.get_unchecked(3) & 0b1000_0000u8 }
unsafe { 0 != *self.slice.get_unchecked(3) & 0b0000_0001u8 }
}

/// Identifcation value generated by the source
Expand Down Expand Up @@ -126,7 +125,7 @@ impl<'a> Ipv6FragmentHeaderSlice<'a> {
///
/// {
/// let slice = Ipv6FragmentHeaderSlice::from_slice(&[
/// 0, 0, 0, 0b1000_0000u8, // more_fragments set
/// 0, 0, 0, 0b0000_0001u8, // more_fragments set
/// 1, 2, 3, 4,
/// ]).unwrap();
/// assert!(slice.is_fragmenting_payload());
Expand All @@ -142,12 +141,9 @@ impl<'a> Ipv6FragmentHeaderSlice<'a> {
/// ```
#[inline]
pub fn is_fragmenting_payload(&self) -> bool {
// SAFETY:
// Slice size checked to be at least 8 bytes in constructor.
unsafe {
0 != *self.slice.get_unchecked(2) || 0 != (*self.slice.get_unchecked(3) & 0b1001_1111u8)
// exclude the reserved bytes
}
// decode via the accessors so the wire layout lives in one place
// (the reserved bits are inherently ignored as a result)
self.more_fragments() || 0 != self.fragment_offset().value()
}

/// Decode some of the fields and copy the results to a
Expand Down
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