Add initial version of protocol description

1 files changed, 58 insertions, 0 deletions

diff --git a/protocol.txt b/protocol.txt
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index 0000000..fd07ced
--- /dev/null
+++ b/protocol.txt
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+The ICMP id and seqnum fields are unused in the protocol; the values are chosen
+by the client, with the restriction that the id field is constant over the
+entire communication. The server just mirrors the chosen values in the echo
+replies. All protocol information is contained in the payload part. In the
+following, 'seqnum' refers to the protocol sequence number, not the ICMP seqnum
+field.
+
+Each message containing data has a sequence number. This sequence number is in
+the seqnum range [0, SEQ_SIZE = 256). Windowing ARQ is applied for efficiency,
+with a window size of WIN_SIZE = 16. Each party has an independent seqnum
+counter. Only messages in the current window range may be sent.
+
+The first message byte is the message type. The message types are described
+below. All structs are packed.
+
+
+struct prot_data {
+  uint8_t type = 1;
+  uint8_t seqnum, rr;
+  uint32_t size;
+  uint8_t data[size];
+}
+
+Transmits application data, with piggy-back of a prot_rr message with nrej=0.
+
+- 'seqnum' is the sequence number of this message.
+- 'rr' is the expected next seqnum, as in the prot_rr message. This can shift
+  the window.
+- 'size' is the number of bytes of data sent in this packet.
+- 'data' contains the next-protocol-level data sent in this message.
+
+
+
+struct prot_rr {
+  uint8_t type = 2;
+  uint8_t rr, nrej;
+  uint8_t rej[nrej];
+}
+
+Acknowledgement of arrival of messages and indication of need of resending.
+'rr-1' should lie in the current window, and the seqnums in 'rej' should lie in
+the current window. All messages before seqnum 'rr' are acknowledged, and the
+recipient is asked to resend its messages with seqnums in 'rej'.
+
+- 'rr' is the expected next seqnum. Seqnums in window before this id are
+  considered successfully received.
+- 'nrej' is the number of seqnums in 'rej'.
+- 'rej' contains the seqnums that need to be resent.
+
+
+
+struct prot_rr_poll {
+  uint8_t type = 3;
+}
+
+Asks for a prot_rr message, because the sender has new data to send but thinks
+the window is full. The receiver of a prot_rr_poll should send its current
+prot_rr message.