Patch LWIP to allow RST for TIMEWAIT ports

Implementing only the protocol bug fix from RFC1337 causes issues when
the stack interacts with linux. Linux tries to use timestamps to reuse
a port combination, which LWIP does not implement. As a fallback they
use a RST packet but this doesn't work because of the RFC1337
implementation. Therefore, remove the RFC1337 behavior and restore the
original TCP RFC behavior.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
GitHub-Closes: #76

Author: jschlumpp

patches/0014-timewait-allow-rst.patch

@@ -0,0 +1,37 @@
+Don't use the naive RFC1337 mechanism
+
+Other than sending RST frames there is no other way to reuse port
+numbers in LWIP. Therefore, allow RST frames when they have the
+correct sequence number.
+
+diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
+--- a/src/core/tcp_in.c	2024-01-23 17:57:59.453522491 +0100
++++ b/src/core/tcp_in.c	2024-01-23 17:58:03.496864164 +0100
+@@ -744,6 +744,27 @@
+    *   acceptable since we only send ACKs)
+    * - second check the RST bit (... return) */
+   if (flags & TCP_RST) {
++    if (seqno == pcb->rcv_nxt) {
++      /* Go to closed state */
++      tcp_pcb_purge(pcb);
++      /* Remove it from the list */
++      struct tcp_pcb *prev = NULL;
++      struct tcp_pcb *pcbi = tcp_tw_pcbs;
++      while (pcbi != NULL) {
++        if (pcbi == pcb) {
++          if (prev != NULL) {
++            prev->next = pcb->next;
++          } else {
++            tcp_tw_pcbs = pcb->next;
++          }
++          pcbi = pcbi->next;
++          tcp_free(pcb);
++        } else {
++          prev = pcbi;
++          pcbi = pcbi->next;
++        }
++      }
++    }
+     return;
+   }
+