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Implement reassembly of out-of-order TCP segments.
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@whitequark
whitequark committed Sep 22, 2017

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Learn about vigilant mode
1 parent af51f2a commit 78359e2
Showing 3 changed files with 167 additions and 107 deletions.
2 changes: 1 addition & 1 deletion README.md
View file
Original file line number Diff line number Diff line change
@@ -57,12 +57,12 @@ The TCP protocol is supported over IPv4. Server and client sockets are supported
* Header checksum is generated and validated.
* Maximum segment size is negotiated.
* Multiple packets are transmitted without waiting for an acknowledgement.
* Reassembly of out-of-order segments is supported, with no more than 4 missing sequence ranges.
* Lost packets are retransmitted with exponential backoff, starting at a fixed delay of 100 ms.
* Sending keep-alive packets is supported, with a configurable interval.
* Connection, retransmission and keep-alive timeouts are supported, with a configurable duration.
* After arriving at the TIME-WAIT state, sockets close after a fixed delay of 10 s.
* Urgent pointer is **not** supported; any urgent octets will be received alongside data octets.
* Reassembly of out-of-order segments is **not** supported.
* Silly window syndrome avoidance is **not** supported for either transmission or reception.
* Congestion control is **not** implemented.
* Delayed acknowledgements are **not** implemented.
180 changes: 111 additions & 69 deletions src/socket/tcp.rs
View file
Original file line number Diff line number Diff line change
@@ -7,7 +7,7 @@ use {Error, Result};
use phy::DeviceLimits;
use wire::{IpProtocol, IpAddress, IpEndpoint, TcpSeqNumber, TcpRepr, TcpControl};
use socket::{Socket, IpRepr};
use storage::RingBuffer;
use storage::{Assembler, RingBuffer};

pub type SocketBuffer<'a> = RingBuffer<'a, u8>;

@@ -171,6 +171,7 @@ pub struct TcpSocket<'a> {
debug_id: usize,
state: State,
timer: Timer,
assembler: Assembler,
rx_buffer: SocketBuffer<'a>,
tx_buffer: SocketBuffer<'a>,
/// Interval after which, if no inbound packets are received, the connection is aborted.
@@ -219,7 +220,7 @@ impl<'a> TcpSocket<'a> {
/// Create a socket using the given buffers.
pub fn new<T>(rx_buffer: T, tx_buffer: T) -> Socket<'a, 'static>
where T: Into<SocketBuffer<'a>> {
let rx_buffer = rx_buffer.into();
let (rx_buffer, tx_buffer) = (rx_buffer.into(), tx_buffer.into());
if rx_buffer.capacity() > <u16>::max_value() as usize {
panic!("buffers larger than {} require window scaling, which is not implemented",
<u16>::max_value())
@@ -229,8 +230,9 @@ impl<'a> TcpSocket<'a> {
debug_id: 0,
state: State::Closed,
timer: Timer::default(),
tx_buffer: tx_buffer.into(),
rx_buffer: rx_buffer.into(),
assembler: Assembler::new(rx_buffer.capacity()),
tx_buffer: tx_buffer,
rx_buffer: rx_buffer,
timeout: None,
keep_alive: None,
listen_address: IpAddress::default(),
@@ -332,6 +334,9 @@ impl<'a> TcpSocket<'a> {
fn reset(&mut self) {
self.state = State::Closed;
self.timer = Timer::default();
self.assembler = Assembler::new(self.rx_buffer.capacity());
self.tx_buffer.clear();
self.rx_buffer.clear();
self.keep_alive = None;
self.timeout = None;
self.listen_address = IpAddress::default();
@@ -345,8 +350,6 @@ impl<'a> TcpSocket<'a> {
self.remote_win_len = 0;
self.remote_mss = DEFAULT_MSS;
self.remote_last_ts = None;
self.tx_buffer.clear();
self.rx_buffer.clear();
}

/// Start listening on the given endpoint.
@@ -860,13 +863,14 @@ impl<'a> TcpSocket<'a> {
}
}

let payload_offset;
match self.state {
// In LISTEN and SYN-SENT states, we have not yet synchronized with the remote end.
State::Listen => (),
State::SynSent => (),
State::Listen | State::SynSent =>
payload_offset = 0,
// In all other states, segments must occupy a valid portion of the receive window.
_ => {
let mut send_challenge_ack = false;
let mut segment_in_window = true;

let window_start = self.remote_seq_no + self.rx_buffer.len();
let window_end = self.remote_seq_no + self.rx_buffer.capacity();
@@ -877,34 +881,22 @@ impl<'a> TcpSocket<'a> {
net_debug!("[{}]{}:{}: non-zero-length segment with zero receive window, \
will only send an ACK",
self.debug_id, self.local_endpoint, self.remote_endpoint);
send_challenge_ack = true;
segment_in_window = false;
}

if !((window_start <= segment_start && segment_start <= window_end) ||
if !((window_start <= segment_start && segment_start <= window_end) &&
(window_start <= segment_end && segment_end <= window_end)) {
net_debug!("[{}]{}:{}: segment not in receive window \
({}..{} not intersecting {}..{}), will send challenge ACK",
self.debug_id, self.local_endpoint, self.remote_endpoint,
segment_start, segment_end, window_start, window_end);
send_challenge_ack = true;
}

// For now, do not actually try to reassemble out-of-order segments.
if segment_start != window_start {
net_debug!("[{}]{}:{}: out-of-order SEQ ({} not equal to {}), \
will send challenge ACK",
self.debug_id, self.local_endpoint, self.remote_endpoint,
segment_start, window_start);
// Some segments between what we have last received and this segment
// went missing. Send a duplicate ACK; RFC 793 does not specify the behavior
// required when receiving a duplicate ACK, but in practice (see RFC 1122
// section 4.2.2.21) most congestion control algorithms implement what's called
// a "fast retransmit", where a threshold amount of duplicate ACKs triggers
// retransmission.
send_challenge_ack = true;
segment_in_window = false;
}

if send_challenge_ack {
if segment_in_window {
// We've checked that segment_start >= window_start above.
payload_offset = (segment_start - window_start) as usize;
} else {
// If we're in the TIME-WAIT state, restart the TIME-WAIT timeout, since
// the remote end may not have realized we've closed the connection.
if self.state == State::TimeWait {
@@ -1087,28 +1079,70 @@ impl<'a> TcpSocket<'a> {

if ack_len > 0 {
// Dequeue acknowledged octets.
debug_assert!(self.tx_buffer.len() >= ack_len);
net_trace!("[{}]{}:{}: tx buffer: dequeueing {} octets (now {})",
self.debug_id, self.local_endpoint, self.remote_endpoint,
ack_len, self.tx_buffer.len() - ack_len);
let acked = self.tx_buffer.dequeue_many(ack_len);
debug_assert!(acked.len() == ack_len);
self.tx_buffer.dequeue_many(ack_len);
}

// We've processed everything in the incoming segment, so advance the local
// sequence number past it.
if let Some(ack_number) = repr.ack_number {
// We've processed everything in the incoming segment, so advance the local
// sequence number past it.
self.local_seq_no = ack_number;
}

if repr.payload.len() > 0 {
// Enqueue payload octets, which are guaranteed to be in order.
let payload_len = repr.payload.len();
if payload_len == 0 { return Ok(None) }

println!("before1 {}", self.assembler);

// Try adding payload octets to the assembler.
match self.assembler.add(payload_offset, payload_len) {
Ok(()) => {
debug_assert!(self.assembler.total_size() == self.rx_buffer.capacity());
println!("after1 {}", self.assembler);

// Place payload octets into the buffer.
net_trace!("[{}]{}:{}: rx buffer: writing {} octets at offset {}",
self.debug_id, self.local_endpoint, self.remote_endpoint,
payload_len, payload_offset);
self.rx_buffer.get_unallocated(payload_offset, payload_len)
.copy_from_slice(repr.payload);
}
Err(()) => {
net_debug!("[{}]{}:{}: assembler: too many holes to add {} octets at offset {}",
self.debug_id, self.local_endpoint, self.remote_endpoint,
payload_len, payload_offset);
return Err(Error::Dropped)
}
}

println!("before2 {}", self.assembler);
if let Some(contig_len) = self.assembler.remove_front() {
println!("after2 {}", self.assembler);
debug_assert!(self.assembler.total_size() == self.rx_buffer.capacity());
// Enqueue the contiguous data octets in front of the buffer.
net_trace!("[{}]{}:{}: rx buffer: enqueueing {} octets (now {})",
self.debug_id, self.local_endpoint, self.remote_endpoint,
repr.payload.len(), self.rx_buffer.len() + repr.payload.len());
self.rx_buffer.enqueue_slice(repr.payload);
contig_len, self.rx_buffer.len() + contig_len);
self.rx_buffer.enqueue_many(contig_len);
}

Ok(None)
if self.assembler.is_empty() {
Ok(None)
} else {
// If the assembler isn't empty, some segments at the start of our window got lost.
// Send a reply acknowledging the data we already have; RFC 793 does not specify
// the behavior triggerd by such a reply, but RFC 1122 section 4.2.2.21 states that
// most congestion control algorithms implement what's called a "fast retransmit",
// where a threshold amount of duplicate ACKs triggers retransmission without
// the need to wait for a timeout to expire.
net_trace!("[{}]{}:{}: assembler: {}",
self.debug_id, self.local_endpoint, self.remote_endpoint,
self.assembler);
Ok(Some(self.ack_reply(ip_repr, &repr)))
}
}

fn timed_out(&self, timestamp: u64) -> bool {
@@ -1262,7 +1296,7 @@ impl<'a> TcpSocket<'a> {
}

if repr.payload.len() > 0 {
net_trace!("[{}]{}:{}: tx buffer: peeking at {} octets (from {})",
net_trace!("[{}]{}:{}: tx buffer: reading {} octets at offset {}",
self.debug_id, self.local_endpoint, self.remote_endpoint,
repr.payload.len(), self.remote_last_seq - self.local_seq_no);
} else {
@@ -2685,34 +2719,6 @@ mod test {
})));
}

#[test]
fn test_missing_segment() {
let mut s = socket_established();
send!(s, TcpRepr {
seq_number: REMOTE_SEQ + 1,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"abcdef"[..],
..SEND_TEMPL
});
recv!(s, [TcpRepr {
seq_number: LOCAL_SEQ + 1,
ack_number: Some(REMOTE_SEQ + 1 + 6),
window_len: 58,
..RECV_TEMPL
}]);
send!(s, TcpRepr {
seq_number: REMOTE_SEQ + 1 + 6 + 6,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"mnopqr"[..],
..SEND_TEMPL
}, Ok(Some(TcpRepr {
seq_number: LOCAL_SEQ + 1,
ack_number: Some(REMOTE_SEQ + 1 + 6),
window_len: 58,
..RECV_TEMPL
})));
}

#[test]
fn test_data_retransmit() {
let mut s = socket_established();
@@ -3013,6 +3019,7 @@ mod test {
fn test_zero_window_ack() {
let mut s = socket_established();
s.rx_buffer = SocketBuffer::new(vec![0; 6]);
s.assembler = Assembler::new(s.rx_buffer.capacity());
send!(s, TcpRepr {
seq_number: REMOTE_SEQ + 1,
ack_number: Some(LOCAL_SEQ + 1),
@@ -3042,6 +3049,7 @@ mod test {
fn test_zero_window_ack_on_window_growth() {
let mut s = socket_established();
s.rx_buffer = SocketBuffer::new(vec![0; 6]);
s.assembler = Assembler::new(s.rx_buffer.capacity());
send!(s, TcpRepr {
seq_number: REMOTE_SEQ + 1,
ack_number: Some(LOCAL_SEQ + 1),
@@ -3096,7 +3104,7 @@ mod test {
}

// =========================================================================================//
// Tests for timeouts
// Tests for timeouts.
// =========================================================================================//

#[test]
@@ -3193,7 +3201,7 @@ mod test {
}

// =========================================================================================//
// Tests for keep-alive
// Tests for keep-alive.
// =========================================================================================//

#[test]
@@ -3258,13 +3266,47 @@ mod test {
}

// =========================================================================================//
// Tests for packet filtering
// Tests for reassembly.
// =========================================================================================//

#[test]
fn test_out_of_order() {
let mut s = socket_established();
send!(s, TcpRepr {
seq_number: REMOTE_SEQ + 1 + 3,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"def"[..],
..SEND_TEMPL
}, Ok(Some(TcpRepr {
seq_number: LOCAL_SEQ + 1,
ack_number: Some(REMOTE_SEQ + 1),
..RECV_TEMPL
})));
assert_eq!(s.recv(10), Ok(&b""[..]));
send!(s, TcpRepr {
seq_number: REMOTE_SEQ + 1,
ack_number: Some(LOCAL_SEQ + 1),
payload: &b"abcdef"[..],
..SEND_TEMPL
});
recv!(s, [TcpRepr {
seq_number: LOCAL_SEQ + 1,
ack_number: Some(REMOTE_SEQ + 1 + 6),
window_len: 58,
..RECV_TEMPL
}]);
assert_eq!(s.recv(10), Ok(&b"abcdef"[..]));
}

// =========================================================================================//
// Tests for packet filtering.
// =========================================================================================//

#[test]
fn test_doesnt_accept_wrong_port() {
let mut s = socket_established();
s.rx_buffer = SocketBuffer::new(vec![0; 6]);
s.assembler = Assembler::new(s.rx_buffer.capacity());

let tcp_repr = TcpRepr {
seq_number: REMOTE_SEQ + 1,
92 changes: 55 additions & 37 deletions src/storage/assembler.rs
View file
Original file line number Diff line number Diff line change
@@ -3,14 +3,15 @@ use core::fmt;
/// A contiguous chunk of absent data, followed by a contiguous chunk of present data.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Contig {
hole_size: u32,
data_size: u32
hole_size: usize,
data_size: usize
}

impl fmt::Display for Contig {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
if self.has_hole() { write!(f, "({})", self.hole_size)?; }
if self.has_data() { write!(f, " {}", self.data_size)?; }
if self.has_hole() && self.has_data() { write!(f, " ")?; }
if self.has_data() { write!(f, "{}", self.data_size)?; }
Ok(())
}
}
@@ -20,11 +21,11 @@ impl Contig {
Contig { hole_size: 0, data_size: 0 }
}

fn hole(size: u32) -> Contig {
fn hole(size: usize) -> Contig {
Contig { hole_size: size, data_size: 0 }
}

fn hole_and_data(hole_size: u32, data_size: u32) -> Contig {
fn hole_and_data(hole_size: usize, data_size: usize) -> Contig {
Contig { hole_size, data_size }
}

@@ -36,23 +37,23 @@ impl Contig {
self.data_size != 0
}

fn total_size(&self) -> u32 {
fn total_size(&self) -> usize {
self.hole_size + self.data_size
}

fn is_empty(&self) -> bool {
self.total_size() == 0
}

fn expand_data_by(&mut self, size: u32) {
fn expand_data_by(&mut self, size: usize) {
self.data_size += size;
}

fn shrink_hole_by(&mut self, size: u32) {
fn shrink_hole_by(&mut self, size: usize) {
self.hole_size -= size;
}

fn shrink_hole_to(&mut self, size: u32) {
fn shrink_hole_to(&mut self, size: usize) {
assert!(self.hole_size >= size);

let total_size = self.total_size();
@@ -86,13 +87,13 @@ impl fmt::Display for Assembler {

impl Assembler {
/// Create a new buffer assembler for buffers of the given size.
pub fn new(size: u32) -> Assembler {
pub fn new(size: usize) -> Assembler {
let mut contigs = [Contig::empty(); CONTIG_COUNT];
contigs[0] = Contig::hole(size);
Assembler { contigs }
}

pub(crate) fn total_size(&self) -> u32 {
pub(crate) fn total_size(&self) -> usize {
self.contigs
.iter()
.map(|contig| contig.total_size())
@@ -107,13 +108,20 @@ impl Assembler {
self.contigs[self.contigs.len() - 1]
}

/// Remove a contig at the given index, and return a pointer to the the first empty contig.
/// Return whether the assembler contains no data.
pub fn is_empty(&self) -> bool {
!self.front().has_data()
}

/// Remove a contig at the given index, and return a pointer to the first contig
/// without data.
fn remove_contig_at(&mut self, at: usize) -> &mut Contig {
debug_assert!(!self.contigs[at].is_empty());

for i in at..self.contigs.len() - 1 {
self.contigs[i] = self.contigs[i + 1];
if self.contigs[i].is_empty() {
if !self.contigs[i].has_data() {
self.contigs[i + 1] = Contig::empty();
return &mut self.contigs[i]
}
}
@@ -139,7 +147,7 @@ impl Assembler {

/// Add a new contiguous range to the assembler, and return `Ok(())`,
/// or return `Err(())` if too many discontiguities are already recorded.
pub fn add(&mut self, mut offset: u32, mut size: u32) -> Result<(), ()> {
pub fn add(&mut self, mut offset: usize, mut size: usize) -> Result<(), ()> {
let mut index = 0;
while index != self.contigs.len() && size != 0 {
let contig = self.contigs[index];
@@ -172,8 +180,8 @@ impl Assembler {
// The range being added covers a part of the hole but not of the data
// in this contig, add a new contig containing the range.
self.contigs[index].shrink_hole_by(offset + size);
let empty = self.add_contig_at(index)?;
*empty = Contig::hole_and_data(offset, size);
let inserted = self.add_contig_at(index)?;
*inserted = Contig::hole_and_data(offset, size);
index += 2;
} else {
unreachable!()
@@ -192,30 +200,20 @@ impl Assembler {
Ok(())
}

/// Return `Ok(size)` with the size of a contiguous range in the front of the assembler,
/// or return `Err(())` if there is no such range.
pub fn front_len(&self) -> u32 {
let front = self.front();
if front.has_hole() {
0
} else {
debug_assert!(front.data_size > 0);
front.data_size
}
}

/// Remove a contiguous range from the front of the assembler and `Ok(data_size)`,
/// or return `Err(())` if there is no such range.
pub fn front_remove(&mut self) -> u32 {
/// Remove a contiguous range from the front of the assembler and `Some(data_size)`,
/// or return `None` if there is no such range.
pub fn remove_front(&mut self) -> Option<usize> {
let front = self.front();
if front.has_hole() {
0
None
} else {
let empty = self.remove_contig_at(0);
*empty = Contig::hole(front.data_size);
println!("{}", self);{
let last_hole = self.remove_contig_at(0);
last_hole.hole_size += front.data_size;
}println!("{}", self);

debug_assert!(front.data_size > 0);
front.data_size
Some(front.data_size)
}
}
}
@@ -225,8 +223,8 @@ mod test {
use std::vec::Vec;
use super::*;

impl From<Vec<(u32, u32)>> for Assembler {
fn from(vec: Vec<(u32, u32)>) -> Assembler {
impl From<Vec<(usize, usize)>> for Assembler {
fn from(vec: Vec<(usize, usize)>) -> Assembler {
let mut contigs = [Contig::empty(); CONTIG_COUNT];
for (i, &(hole_size, data_size)) in vec.iter().enumerate() {
contigs[i] = Contig { hole_size, data_size };
@@ -331,4 +329,24 @@ mod test {
assert_eq!(assr.add(2, 12), Ok(()));
assert_eq!(assr, contigs![(2, 12), (2, 0)]);
}

#[test]
fn test_empty_remove_front() {
let mut assr = contigs![(12, 0)];
assert_eq!(assr.remove_front(), None);
}

#[test]
fn test_trailing_hole_remove_front() {
let mut assr = contigs![(0, 4), (8, 0)];
assert_eq!(assr.remove_front(), Some(4));
assert_eq!(assr, contigs![(12, 0)]);
}

#[test]
fn test_trailing_data_remove_front() {
let mut assr = contigs![(0, 4), (4, 4)];
assert_eq!(assr.remove_front(), Some(4));
assert_eq!(assr, contigs![(4, 4), (4, 0)]);
}
}

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