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| -rw-r--r-- | protocol.txt | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/protocol.txt b/protocol.txt new file mode 100644 index 0000000..fd07ced --- /dev/null +++ b/protocol.txt @@ -0,0 +1,58 @@ +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. |