>Hi All,
>
>HostA----------Router1-----------Router2-----------HostB
> Ether Serial Ether
>
>If a packet is sent from HostA to HostB and the packet is lost between R1
>and R2 - who resends?
>
>I've seen this question before and thought the generally agreed upon answer
>was that Host A would resend with the reason being "The data-link layer
>protocols in use today on serial lines, including PPP, Frame Relay, and
>Cisco HDLC, detect errors but are not responsible for error correction or
>retransmission."
In general, I'd say Host A. The difficulty, however, is that there
is no absolute rule for what hosts do and what routers do. It's a
question of the specific protocols in use.
If, for example, the Ethernet between R1 and R2 were running X.25
(not a common configuration, but specified in ISO 8880), R1 would
retransmit.
I can come up with even weirder configurations involving multiple
levels of tunneling, but, if it is an IP environment, hosts are
architecturally responsible for the reliability of communications. I
used words very carefully there -- I'm not saying "reliable" but
simply the appropriate level of reliability. If, for example, the
hosts are running a UDP-based application such as streaming video, no
one will retransmit.
Again, the IP architecture assumes the default behavior of
retransmission, if done at all, is done in hosts. Routers retransmit
only when they are using data link protocols that specifically
retransmit (e.g., SDLC and LAP-B), or when they are acting as a host
(e.g., as a protocol converter or endpoint of a reliable tunnel).
>
>The reason I post this is I have found a contradictoring document and wanted
>some 2nd and 3rd opinions. The document in questions is the cramsession
>from BrainBuzz.com at the link:
>
>http://cramsession.brainbuzz.com/cramsession/cisco/ccie_written/
>
>The statement in question is at the top of Page 13. It states the the
>packet would be resent (actually rebroadcast) from Router B.
>
>I'm not using this guide to learn from as much to reinforce what I have
>learned and this statement is bringing up an old question.
>
>Thanks to all who answer,
>Sean
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