<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: arial,helvetica,sans-serif; font-size: 10pt; color: #000000'>TCP vs. UDP.<br><br><div><span name="x"></span><br><br>-----<br>Mike Hammett<br>Intelligent Computing Solutions<br>http://www.ics-il.com<br><br>Midwest-IX<br>http://www.midwest-ix.com<span name="x"></span><br></div><br><hr id="zwchr"><div style="color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From: </b>"K. Scott Helms" <kscott.helms@gmail.com><br><b>To: </b>"Job Snijders" <job@ntt.net><br><b>Cc: </b>"NANOG list" <nanog@nanog.org><br><b>Sent: </b>Tuesday, August 25, 2020 7:27:24 AM<br><b>Subject: </b>Re: TCP and UDP Port 0 - Should an ISP or ITP Block it?<br><br>Job,<br><br>Comcast is blocking it. From the table on that page.<br><br>"Port 0 is a reserved port, which means it should not be used by<br>applications. Network abuse has prompted the need to block this port."<br><br>"What about UDP IP fragmentation?"<br><br>I'm not sure I follow this. The IP packet will be fragmented with UDP<br>inside it. When the IP packet gets put together the UDP PDU will have<br>a port number. It's possible that some packet analyzers or network<br>gear will improperly "see" a partial UDP flow as port 0 but that's a<br>mischaracterization of the flow.<br><br><br>Scott Helms<br><br>Scott Helms<br><br><br><br>On Tue, Aug 25, 2020 at 8:17 AM Job Snijders <job@ntt.net> wrote:<br>><br>> On Tue, Aug 25, 2020 at 07:27:33AM -0400, K. Scott Helms wrote:<br>> > I think a fairly easy thing to do is see what other large retail ISPs<br>> > have done. Comcast, as an example, lists all of the ports they block<br>> > and 0 is blocked. I do recommend that port 0 be blocked by all of the<br>> > ISPs I work with and frankly Comcast's list is a pretty good one to<br>> > use in general, though you will get some pushback on things like SMTP.<br>> ><br>> > https://www.xfinity.com/support/articles/list-of-blocked-ports<br>><br>> I may be reading the table incorrectly, but it seems to me Comcast is<br>> *not* blocking UDP port 0 according to the above URL?<br>><br>> > Transit providers are a little bit different, but then again port 0 is<br>> > also different since AFAIK it's never had a legitimate use case. It's<br>> > always been a reserved port. I'd personally block it if I ran a<br>> > transit, but I'd be more willing to open it up for one of my large<br>> > customers (in a limited way) than I would on the retail side.<br>> ><br>> > https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml<br>><br>> What about UDP IP fragmentation?<br>><br>> Kind regards,<br>><br>> Job<br></div><br></div></body></html>