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network still remains that way. 19.2 Alleviating Routing Loops: Hop Limits on Packets If a packet is sent along a sequence of routers that are part of a routing loop, the packet
will remain in the network until the routing loop is eliminated. The typical time scales over
which routing protocols converge could be many seconds or even a few minutes, during
which these packets may consume signiﬁcant amounts of network bandwidth and reduce
the capacity available to other packets that can be sent successfully to other destinations.
To mitigate this (hopefully transient) problem, it is customary for the packet header to
include a hop limit. The source sets the “hop limit” ﬁeld in the packet’s header to some
value larger than the number of hops it believes is needed to get to the destination. Each
switch, before forwarding the packet, decrements the hop limit ﬁeld by 1. If this ﬁeld reaches
0, then it does not forward the packet, but drops it instead (optionally, the switch may send
a diagnostic packet toward the source telling it that the switch dropped the packet because
the hop limit was exceeded).
The forwarding process needs to make sure that if a checksum covers the hop limit
ﬁeld, then the checksum needs to be adjusted to reﬂect the decrement done to the hoplimit ﬁeld.1
Combining this information with the rest of the forwarding steps discussed in the previous lecture, we can summarize the basic steps done while forwarding a packet in a besteffort network as follows:
1. Check the hop-limit ﬁeld. If it is 0, discard the packet. Optionally, send a diagnostic packet toward the packet’s source saying “hop limit exceeded”; in response, the
source may decide to stop sending packets to that destination for some period of
2. If the hop-limit is larger than 0, then perform a routing table lookup using the destination address to determine the route for the packet. If no link is returned by the
lookup or if the link is considered “not working” by the switch, then discard the
packet. Otherwise, if the destination is the present node, then deliver the packet to
the appropriate protocol or application running on the node. Otherwise, proceed to
the next step.
3. Decrement the hop-limit by 1. Adjust the checksum (typically the header checksum)
if necessary. Enqueue the packet in the queue corresponding to the outgoing link
IP version 4 has such a header checksum, but IP version 6 dispenses with it, because higher-layer protocols
used to provide reliable delivery have a checksum that covers portions of the IP header. CHAPTER 19. NETWORK ROUTING - II 4 ROUTING AROUND FAILURES returned by the route lookup procedure. When this packet reaches the head of the
queue, the switch will send the packet on the link. 19.3 Neighbor Liveness: HELLO Protocol As mentioned in the previous lecture, determining which of a node’s neighbors is currently
alive and working is the ﬁrst step in any routing protocol. We now address thi...
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- Fall '13