Lab 2: the TCP receiver
Overview
In this lab, you will implement the TCPReceiver, the part of a TCP implementation that
handles the incoming byte stream.
The TCPReceiver receives messages from the peer's sender (via the receive() method) and turns them into calls to a Reassembler, which eventually writes to the incoming ByteStream. Applications read from this ByteStream.
Meanwhile, the TCPReceiver also generates messages that go back to the peer’s sender, via the send() method. These “receiver messages” are responsible for telling the sender:
- the index of the "first unassembled" byte, which is called the "acknowledgment number" or "ackno". This is the first byte that the receiver needs from the sender.
- the available capacity in the output ByteStream - the "window size".
Together, the ackno and window size describe describes the receiver’s window: a range of indexes that the TCP sender is allowed to send.
The TCP Receiver
In TCP,
acknowledgment means, "What's the index of the next byte that the receiver
needs so it can reassemble more of the ByteStream?" Flow control means,
"What range of indices is the receiver interested and willing to receive?" (a function of its available capacity).
This tells the sender how much it's allowed to send.
Translating between 64-bit indexes and 32-bit seqnos
In Reassembler, each individual byte has a 64-bit index.
IMPORTANT
In the TCP headers, however, space is precious, and each byte's index in the stream is represented not with a 64-bit index but with a 32-bit "sequence number" or "seqno". This adds three complexities:
- Your implementation needs to plan for 32-bit integers to wrap around. Streams in TCP can be arbitrarily long — there’s no limit to the length of a ByteStream that can be sent over TCP.
- TCP sequence numbers start at a random value: The first sequence number for a stream is a random 32-bit number called the Initial Sequence Number (ISN). 使用随机数的目的是improve robustness and avoid getting confused by old segments belonging to earlier connections.
- The logical beginning and ending each occupy one sequence number: The SYN (beginning-of-stream) and FIN (end-of-stream) control flags each occupy one sequence number.
Sequence numbers (seqnos) are transmitted in the header of each TCP segment.
Some terminology:
- absolute sequence number: always starts at zero and doesn’t wrap.
- stream index: an index for each byte in the stream, starting at zero.
Consider the byte stream cat:
| element | SYN | c | a | t | FIN |
|---|---|---|---|---|---|
| seqno | \(2^{32}-2\) | \(2^{32}-1\) | 0 | 1 | 2 |
| absolute seqno | 0 | 1 | 2 | 3 | 4 |
| stream index | 0 | 1 | 2 |
The figure shows the three different types of indexing involved in TCP:
| Sequence Numbers | Absolute Sequence Numbers | Stream Indices |
|---|---|---|
| Start at the ISN | Start at 0 | Start at 0 |
| Include SYN/FIN | Include SYN/FIN | Omit SYN/FIN |
| 32 bits, wrapping | 64 bits, non-wrapping | 64 bits, non-wrapping |
| "seqno" | "absolute seqno" | "stream index" |
Converting between sequence numbers and absolute sequence numbers is tricky.
To prevent these bugs systematically, we’ll represent sequence numbers with a custom type: Wrap32, and write the conversions between it and absolute sequence numbers (represented with uint64_t).
Wrap32 is an example of a wrapper type: a type that contains an inner type (in this case uint32_t) but provides a different set of functions/operators.