A survey of different approaches to solve the packet reordering problem related to TCP.

The causes of packet reordering are:

• Packet-level multipath routing
• Route Fluttering: oscillates among a set of available routes
• Parallelism in Modern High-Speed Routers, e.g. packet striping and use of multiple ASICs in parallel
• Link-Layer Retransmissions, esp in wireless networks
• Router Forwarding Lulls: Some routers pause its forwarding activity for buffered packets when it processes a routing update

The impact of packet reordering to TCP are:

• Spurious retransmisions
• Keeping cwnd small
• Loss of self-clocking, i.e. induces bursts
• Interferes RTT and RTO estimation

The paper classify TCP reordering solutions into “ordinal approach” and “temporal approach”. The former is to adjust the TCP algorithms and the latter is to take time into account to solve the problem. There are several algorithms under the category of ordinal approach:

• Eifel algorithm: Use timestamp to help identify spurious retransmission and then restore the ssthresh and cwnd
• TCP-DOOR (Detection of Out-of-Order and Response): Extension of Eifel, which tailor for ad hoc networks that changes routes frequently. When out-of-order is detected, because in ad hoc network it is likely to have multiple route changes in a short time, it ceases the congestion control reaction for certain time. It also restore the congestion response state after it found the network stabilised again.
• DSACK is to exploit the SACK option in TCP so that duplicated transmission are reported by the receiver to the sender, and thus the sender can know about the spurious retransmission. This DSACK option helps detecting packet reordering and subsequent action can be taken.
• Lee-Park-Choi Sender Algorithm propose to increase the dupthresh logarithmically with the number of paths used
• Blanton-Allman algorithm proposes to change dupthresh dynamically, in 3 ways: (1) increase dupthresh by some constant every time a spurious retx is detected, (2) update dupthresh by taking the average of current dupthresh and the current length of reordering, (3) use EWMA to compute the reordering length
• RR-TCP improves over the Blanton-Allman algorithm such that a FFR Avoidance ratio is determined and such ratio in turn determines the value of dupthresh, so that the dupthresh can change for the same ratio. RR-TCP also propose to avoid Karn’s algorithm but instead, use timestamp to enhance the RTT estimation
• Leung-Ma algorithm extends Blanton-Allman algorithm by using EWMA plus the mean deviation of reordering length to set the dupthresh and found to have similar performance as RR-TCP but lower computational complexity
• RN-TCP (Reorder Notifying) asks the router to get involved so that when a packet is dropped, the sender is notified and thus TCP can avoid spurious retransmission

The temporal approach mentioned in the paper are:

• Lee-Park-Choi Receiver Algorithm to delay ACK for reordered packets but ACK immediately for retx packets
• TCP-DCR (Delayed Congestion Response) to hold off the congestion response for a time interval after the first dup ACK is received but send one new packet upon each dup ACK to maintain self-clocking
• TCP-PR is for network with persistent packet reordering, it retransmit only after a RTO, but the RTO is computed using non-smoothed, exponentially weighted maximum RTT so that RTT spikes can promptly reflected in the estimated RTT. The new RTT/RTO estimators are effective to tell if the reordering is caused by path delays

## Bibliographic data

@article{
title = "An Overview of Packet Reordering in TCP: Problems, Solutions, and Challenges",
author = "Ka-Cheong Leung and Victor O. K. Li and Daiqin Yang",
journal = "IEEE Transactions on Parallel and Distributed Systems",
volume = "18",
number = "4",
pages = "522--535",
month = "April",
year = "2007",
}