The Portland Timbers' perfect home record—the last remaining in MLS—has withstood visits from league powers F.C. Dallas and Real Salt Lake. Building on their 5-0-0 start at Jeld-Wen Field looks promising as D.C. United brings a 1-3-0 road record into town. Here are some story lines to keep your eye on in Sunday's match:

Familiar relations: Timbers defender Rodney Wallace and goalkeeper Troy Perkins both joined Portland from the United this offseason via trades. There aren't may secrets to soccer strategy, but both players have a familiarity with the opposing roster and would enjoy a strong performance against their former team, if Eric Brunner is any indication. Brunner's first goal of the MLS season came last weekend against the Columbus Crew, the team he played for before coming to Portland through the expansion draft. 

Prime scoring opportunity: D.C. United has allowed 18 goals this season, tied for the league-worst through 10 games. On the road things are even more grim—the United have struggled in MLS road games this season with a goal differential of minus 4.  Though they haven't lost a game in a month, they also haven't been on the road. United runs 4-3-1-2 formation regarded for its versatility at the defensive and offensive ends and has only allowed two goals in its past three games. But Portland's defense has been even better after conceding just one goal in the past three MLS contests. If D.C. abandons a conservative strategy, it could expose a porous back line.

Beware Charlie Davies: United's Charlie Davies may be the best player on the field Sunday. The left striker is a young American talent that was left off the 2010 World Cup roster after a severe car crash threatened to end his career. Davies is still a prime candidate for the United States' 2014 World Cup roster. He leads the United and is tied for 2nd in MLS with six goals scored thus far.

UPDATE: Davies has yet to return to practice after suffering a hamstring injury last week. His status for starting the game is now unlikely, although it is possible he will substitute in if his injury continues to heal.

Physical play in the box: Portland is one of four MLS teams that have not scored on a penalty kick and have allowed one penalty kick goal this season. The United lead MLS in both categories, having been awarded four and conceding another four.