33
44// //////////////////////////////////////////////////////////////////////////////
55// Company:
6- // Engineer: Chris Shucksmith
7- // Description:
8- // Pull out TCP payload bytes from packet-orientated TCP/IP frames.
9- // Follows a single TCP session/stream
6+ // Engineer: Chris Shucksmith
7+ // Description: Read-only TCP/IP decoder stack, 4x socket streams.
8+ //
9+ // Identify and classify TCP payload bytes from packet-orientated TCP/IP frames.
10+ // Follow up to four extenally configured TCP session/streams for sequence correctness
11+ //
12+ // Four sockets can be independantly followed, ABCD, documented here genericaly as N.
13+ //
14+ // If TCP packet with SYN flag set is seen matching the src/dest ip/port an initial sequence
15+ // number is loaded. TCP Payload bytes in-sequence from this are emitted guarded by
16+ // outDataMatchN flag. Out of sequence packets are emitted with the lower guard tcp_matchN
17+ // set, which indicates payload for the right TCP socket, but not in sequenece. A yet-lower
18+ // priority flag, outDataPayload indicates payload bytes belong *some* unclassified TCP session.
19+ //
20+ // Eg. For in-sequence payload bytes of TCPA, outDataMatchA and outDataPayload
21+ // are all asserted for every byte in the payload, while tcp_matchA is asserted from the
22+ // bytes that identidy the stream until the next stream is selected. Guard it by outDataPayload
23+ // if that is required.
24+ //
25+ // If the tcpN_{src/dest}_{port/ip} registers are modified to follow an established session
26+ // it is likely a SYN will not be seen. If the next packet should be used to seed the sequence
27+ // number, assert tcpN_resync for one cycle. This will be latched internally until such a
28+ // matching packet arrives. If the context of the packet should be used to determine if the
29+ // join is appropriate, based on the context within some higher level protocol, drive
30+ // tcp_rejoin within the payload to indicate the rejoin is permitted. Otherwise drive rejoin
31+ // with 1 to join at any packet boundary, or zero to join only at SYN packets.
32+ //
33+ // TCP sequence number is a modulo 32-bit counter increasing from the 'initial sequence number'
34+ // seeded from a random value. For a packet of payload length L and sequence number S, the next
35+ // packet will contain sequence number S+L.
36+ //
37+ // As mentioned previously, for packets in-sequence, outDataMatchN is asserted.
38+ // if the sequence number is below that expected, 'tcp_retran' is asserted, and the
39+ // stream sequence number unchanged. If the sequence number is above that expected,
40+ // 'tcp_gap' is asserted.
41+ //
42+ // TCP retransmission can be benign: packets with 'tcp_retran' can be ignored in the
43+ // hope that the retransmission will conclude with the expected packet. Such a scenario can
44+ // be caused by a lost-ACK on the reutrn path. A gap however requires active handling. If the
45+ // active host also experiances the gap and causes retransmission in the usual way (ack'ing a prev
46+ // sequence number) it is possible the sequence will be resumed autonoumously by that host's actions.
47+ // In an imperfect local network however it should be considered that the packet could be lost before
48+ // this decoder yet reached the active host. In this case, the gap will never be filled, ie. outDataMatchN
49+ // never resserted.
50+ //
51+ // Knowledge of the upper-level payload protocol is required to rejoin the stream at a
52+ // safe point, or to trigger the active host to reconnect, causing a SYN and reset of the sequence
53+ // number. By setting tcp_rejoin at a safe-point the logic is asserting to the stack that payload was
54+ // rejoined and that packets after that should be considered in sequence.
55+ //
56+ // There is a third classification for packets arriveing more than 1M bytes behind or 1M ahead of the
57+ // expected stream position, 'tcp_sequnknown' which is often found when looping a packet capture and
58+ // may require careful handling. It is not clear when this is asserted, if a retransmission or gap has
59+ // occured. Most likely ignoring the packet is the correct action to take, reusming from the next
60+ // safe-point.
61+ //
62+ //
63+ //
1064//
1165// //////////////////////////////////////////////////////////////////////////////
1266
@@ -21,6 +75,8 @@ module Tcp
2175 input dataValid,
2276 input [7 :0 ] data,
2377
78+ input rejoin, // assert during a payload, whilst tcp_matchN is asserted but not outDataMatchN to rejoin stream
79+
2480 // per session
2581 input [15 :0 ] tcpA_src_port, tcpB_src_port, tcpC_src_port, tcpD_src_port,
2682 input [31 :0 ] tcpA_src_ip, tcpB_src_ip, tcpC_src_ip, tcpD_src_ip,
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