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TCP UDP Questions 2

July 30th, 2017 in ROUTE 300-101 Go to comments

Question 1


Unlike TCP which uses the sequence numbers to rearrange the segments when they arrive out of order, UDP just passes the received datagrams to the next OSI layer (the Session Layer) in the order in which they arrived.

Question 2

Question 3


In Asymmetric routing, a packet traverses from a source to a destination in one path and takes a different path when it returns to the source. This is commonly seen in Layer-3 routed networks.

Issues to Consider with Asymmetric Routing

Asymmetric routing is not a problem by itself, but will cause problems when Network Address Translation (NAT) or firewalls are used in the routed path. For example, in firewalls, state information is built when the packets flow from a higher security domain to a lower security domain. The firewall will be an exit point from one security domain to the other. If the return path passes through another firewall, the packet will not be allowed to traverse the firewall from the lower to higher security domain because the firewall in the return path will not have any state information. The state information exists in the first firewall.

Reference: http://www.cisco.com/web/services/news/ts_newsletter/tech/chalktalk/archives/200903.html

Specifically for TCP-based connections, disabling stateful TCP checks can help mitigate asymmetric routing. When TCP state checks are disabled, the ASA can allow packets in a TCP connection even if the ASA didn’t see the entire TCP 3-way handshake. This feature is called TCP State Bypass.

Reference: https://supportforums.cisco.com/document/55536/asa-asymmetric-routing-troubleshooting-and-mitigation

Note: The active/active firewall topology uses two firewalls that are both actively providing firewall services.

Question 4


A device that sends UDP packets assumes that they reach the destination. There is no mechanism to alert senders that the packet has arrived -> Answer A is not correct.

UDP throughput is not impacted by latency because the sender does not have to wait for the ACK to be sent back -> Answer B is not correct.

UDP does not negotiate how the connection will work, UDP just transmits and hopes for the best -> D is not correct.

Therefore only answer C is left.

Question 5


The command “show tcp brief numeric” displays a concise description of TCP connection endpoints.

Question 6

Question 7


TCP starvation/UDP dominance likely occurs if TCP-based applications is assigned to the same service-provider class as UDP-based applications and the class experiences sustained congestion.

TFTP (run on UDP port 69) and SNMP (runs on UDP port 161/162) are two protocols which run on UDP so they can cause TCP starvation.

Note: SMTP runs on TCP port 25; HTTPS runs on TCP port 443; FTP runs on TCP port 20/21


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