Joe, I strongly agree with what you said "A link MTU is different from a path MTU".
A Tunnel MTU is more like "path MTU" because - the Tunnel MTU is the minimum of all the links' MTU that the "tunnel" traverse, - the fragmented segments may be out of order when arriving at the egress node of the tunnel, whereas over a link, the fragmented frames always arrive at egress port in the same order (therefore the reassembly is much easier over a link). Another question, in Section 5.2: why you specifically discuss the "egress MTU" ? A tunnel MTU is determined by the minimum MTU of all the links' MTU that the tunnel traverse. The HSrc and HDst would have considered all the links' MTU when they choose the frame size. So the tunnel egress MTU would be big enough for the original packet size between HSrc & HDst. A tunnel MTU is a critical issue, not because of Tunnel being like a link", but - because the ingress adds the outer header encapsulation which may make the total frame size chosen by the original source to exceed the MTU of some intermediate links, - because the fragmented segments may be out of order when arriving at the egress node of the tunnel, whereas over a link the fragmented frames always arrive at egress port in the same order (therefore the reassembly is much easier over the link). A couple of minor typo: Section 3.6.1: "the term "inner")..", is the ")" extra? Section 4.2 first sentence: ".... It messages might be ...", do you mean "... its messages might be..." Linda From: Joe Touch [mailto:[email protected]] Sent: Thursday, October 06, 2016 4:10 PM To: Linda Dunbar <[email protected]>; [email protected] Cc: [email protected] Subject: Re: Questions to draft-intarea-tunnels-03 Hi, Linda, On 10/6/2016 1:34 PM, Linda Dunbar wrote: Joe and Mark, You said "Because tunnels are links, they are subject to the same issues as any link, e.g. MTU.." The MTU issue exist between any two points in a network. A link MTU is different from a path MTU, as explained in the document. The MTU issues of a link are different than those of a path, as described. The MTU issue for tunnel is more like the MTU issue between any two points in the network (i.e. traverse many links, and can change over time), less like the MTU for a single link because the single link MTU is only determined by the fixed TWO endpoints of the link, whereas the MTU of a tunnel (and a path) is impacted by many links in between, i.e. more dynamic and can change depending which path the packet traverse through. Both a link and tunnel may have multiple hops not visible to IP, each of which may have a different MTU; this is no different than an IP path MTU. The difference is that the link/tunnel hops are not visible to IP. In the section 1 of your draft, you stated: ".. tunnels emulate a link". To me, "tunnel" hide the inner addresses, but the packets can traverse many links. So Tunnels hardly emulate a link. There are many link layer technologies which use multiple link-layer hops and relaying, including Ethernet, ATM, SONET, etc. Joe
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